cdma

7/4/2007 UTL Technologies Ltd ©

CDMA


Multiple Access Methods

•FDMA (Frequency Division Multiple

Access )

•TDMA (Time Division Multiple Access )

•CDMA (Code Division Multiple Access )


FDMA

• Frequency Division Multiple Access

(FDMA)

• – An approach to sharing a channel by

separating the simultaneous users in

frequency


FDMA

f

c

t


Analog Cellular Systems (FDMA Systems)

• AMPS (Analog Mobile Phone Service)

• TACS (Total Access Communication System)

• NMTS (Nordic Mobile Telephone System)


Time division multiple access (TDMA)

• – Approach for allotting single-channel

usage amongst many users, by

dividing the channel into slots of time

during which each user has access to

the medium.

TDMA


TDMA

f

t

c

2.17.1


TDMA/FDD

f

c

2.18.1

t


CDMA

Code Division Multiple Access (CDMA)

• – Spread spectrum technique using

high-speed pseudo random (PN) codes

to scramble data words and spread

spectral occupancy for added

robustness.


CDMA

f

t

c


CDMASpreading and De-Spreading

• The base band signal to be transmitted is

multiplied by the user specific unique code

• The code word is of the order of 64 bits( to

enable generation of a large number of

combinations)

• Thus, the communication demands a larger

spectrum spread than a typical FDMA

UTL Technologies Ltd ©7/4/2007

Defining Spread Spectrum

A complete definition to Spread Spectrum, is the one given

by Haykins..

Spread Spectrum is a means of transmission in which

the data sequences occupy a minimum bandwidth necessary

to send it on bandwidth which is excess to it.

Spread Spectrum is accomplished before transmission

through the use of a code that is independent of data

sequences .

The same code is used at the receiver to despread the

received signal so that the original data sequence may be

recovered


Advantages of Spread Spectrum

• Multi path access: Number of users uses a common channel for communication.

• Random access possibilities: Users can start their transmission at any arbitrary time.

• Privacy due to unknown random codes: The applied codes are - in principle - unknown to a hostile user. This means that it is hardly possible to detect the message of an other user.


• Concept of Spread Spectrum

When the information bearing signal and a PN sequence is multiplied at a multiplier we obtain the desired modulation. The question is how do we get the increase spectrum? This is a simple Fourier Transform property.

Multiplication in time domain is convolution in frequency domain. Hence by multiplying a narrow band information signal and a wide-band code sequence, the multiplied signal will have the spectrum similar to the wide-band PN code sequence.


• How Spread Spectrum gives secure communication?

The spread signal gets the characteristics of the PN code sequence. This signal appears noise like to a receiver that has no idea about the spreading code.


Spread Spectrum Technique

Encoding Encoding Channel DecodingModulator

PseudoRandomGenerator

PseudoRandomGenerator

De-modulator

SpreadingCode

SpreadingCode

InputData

OutputData


DSSS: Transmitter

Modulator(BPSK)

PseudoNoiseBit

Source

Spread SpectrumSignal

SpreaderDataInput

(Binary)


0

1

1 1 1

0 0 0

1

1 1 1 1 1 1 1

0 0 0 0 0 0 0 0 0 00 00

A

B

C

A = Data Input

B = PN Bit Stream

C= A + B

Direct Sequence Spread Spectrum : Transmission

0 0

1

0

0 1 0 1 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 1

1 0 1 0 1 0 0 1

1 0 1 0 0 1 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 0 1 1 0 1

0

0 0 0 00 00 0 00 000 0

1 1 1 1 11 1 1 1 1 1 1 1


DSSS:Receiver

Demodulator(BPSK)

PseudoNoiseBit

Source

Spread SpectrumSignal

Despreader

DataOutput(Binary)


0

1

1 1 1

0 0 0

1

1 1 1 1 1 1 1

0 0 0 0 0 0 0 0 0 00 00

A

B

C

C = Recd.Signal

B = PN Bit Stream

A = C + B

Direct Sequence Spread Spectrum : Reception

0 0

1

0

0 1 0 1 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 1

1 0 1 0 0 1 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 0 1 1 0 1

0 0 0 00 00 0 00 000 0

1 1 1 1 11 1 1 1 1 1 1 1

1 0 1 0 1 0 0 1 0


Quadrature Phase Shift Keying (QPSK)

A

B

C

D

E

F

Carrier

Sin Wct

Cos Wct

Pi/2

2 bit Serial to parallel converter

Summer


Point A : Binary input Data Stream at the rate of R=1/Tb, where Tb is the duration of an individual data bit.

Point B and C : The serial input is converted into two channels with alternate bits going to B and C respectively. Thus the data rate is halved. B and C are known as in-phase and Quadrature phase channels respectively.

The Data bit 1 is represented by a level +1 and the bit 0 represented by -1.

Point D : The data bits from B are multiplied on one part of the carrier. Sin(Wct).

Point E : The data bits from C are multiplied (modulated) on the other part of the carrier, which is shifted in phase by pi/2.


For example, if two adjacent bits are represented by +1, -1.

The output at D will be [+1][SinWct] and that at E will be[-1][SinWct+pi/2] and the summer output at F will be

[Sin(Wct)]+[Sin(Wct)+(pi/2)-(pi)]=Sin (Wct)+Sin[(Wct)-pi/2]=

2 sin[(Wct)-(pi/4)]Cos(pi/4).i.e., the two adjacent bits 1, 0 represented by (1,-1) now combined and denoted as a phase shift of the carrier by –pi/4.

The following table gives all the phase shift values for a combination of two bits taken at a time.


-Pi/4 (as per example above)01

-3Pi/400

3Pi/410

Pi/411

Phase Shift of the CarrierCB

I

Q11 State01 State

10 State00 State

QPSK


Offset Quadrature Phase Shift Keying (OQPSK)

A

B

C

D

E

F

Carrier

Sin Wct

Cos Wct

Pi/2

2 bit Serial to parallel converter

Summer

Delay

OQPSK differs from the conventional QPSK in that prior to carrier multiplication, a delay of a half-bit interval (with respect to the I path) is

placed in the Q path.


Basic Model and Concepts

pj

psTransmitter

Receiver

Jammer

Wss

Bit Rate = Rb

Wss = Bandwidth of the spread signal

Rb= Bit rate of base band signalPs = Signal powerPj = Jammer power


• E b =Ps/Rb and Nj = Pj/Wss.

• E b/Nj= (Wss/Rb)/ (Pj/Ps), where Wss/Rb is called processing gain (PG)

• Capacity of CDMA system :

• The capacity of a CDMA system depends on• E b/No• Frequency Reuse• Voice Activity Factor• Sectorization.For M users, we have the basic capacity equation as

M-1=[PG]/[E b/No], where PG is the processing gain Wss/Rb


• Considering voice activity factor (v), frequency reuse advantage (f), and the sectorization advantage (s), the capacity equation gets modified as:

M = [PG] * [E b/No] -1 * (1/v)*f*s.

Ex: If Rb=9600 bps, Rc=1.2288Mcps, v=40%, f=0.65 and s=2.55, find the capacity?


Features of CDMA

• 1. High Capacity :•More channels per carrier as against 8 in GSM.• Better Traffic Handling capacity• Use of vocoders increases capacity• Simpler frequency planning / frequency re-use.


2. Lower Transmit Power :

The reduction in required Eb/No means that the MS has to Transmit less power. This means that the cost of the MS comes down and the battery life increases. This also results in minimum interference, which means more users can talk simultaneously. Hence, more capacity.

3. Improved privacy :

The PN sequence operation, wide band signalling and certain addressee specific protection features provide very good security to the users.


4. Better performance in Fading/ interference prone environment :

This is derived from the basic concept of spread spectrum technique which separates signals based on specific PN sequences. The system has inherent “multipath diversity”features.

5. Improved capabilities:• Variable rate vocoders for different grades of service.

• Interfaces to ISDN /Wireless PBXS/PCN/Cellular.

• Local loop applications in the PSTN.


GSM Vs. CDMA(Frequency Reuse)

GSM CDMA


Models of Spread Spectrum CDMA Systems

There are basically 4 models of SS-CDMA Systems:

1. One User – One path

2. One User – Many paths

3. Many Users – One path

4. Many Users – Many paths


One user – one path Model

1. The base band signal of User “j” is denoted by …. Bj(t).

2. The PN sequence used for spreading the spectrum is … Cj(t).

3. The transmitted spectrum for user “j” is ….. Bj(t) X Cj(t).

4. At the receiver, the signal is received after a delay Tj. i.e., Bj(t-Tj) X Cj(t-Tj).

5. We generate the PN sequence locally, with an arbitrary delay : Cj(t-T).

6. The receiver output is then described by :Bj(t-Tj) X Cj(t-Tj)X Cj(t-T).


7. If the local delay T is equal to the propagation delay Tj, then the receiver output becomes:

Bj(t-Tj) X Cj(t-Tj)X Cj(t-Tj)= Bj(t-Tj) i.e. at the receiver output we get back the base band delayed by Tj.

8. If the delay T generated locally at the receiver is not equal to Tj, then the receiver output becomes :

Bj(t-Tj) X Cj(t-Tj)X Cj(t-T)= Bj(t-Tj) Cj(t-Tq). This means that the receive output is still a “SPREAD” data.

9. Thus, all other signals arriving at the receiver appear as spread data, excepting the desired data which got decoded as Bj(t-Tj). The other signals appear as noise.

10. Here, we have taken that there is only one direct path between the transmitter and the receiver and the effects of multipath are not considered.


One user – Many path Model

• The signals arrive at the receiver through a number of paths, generated by reflections / scattering from buildings, trees, mountains, water etc.

• Therefore, the input to the receiver always has 2 components:a. A direct path component given by : Bj(t-Tj) X Cj(t-Tj).b. A multi path component given by : Bj(t-T jm) X Cj(t-Tjm).

Where Tjm is the multipath delay for the signal from user J.


Suppose, the locally generated PN Sequence (at the receiver) is delayed by Tj. Let us look at the receiver now:

• Receiver input : Bj(t-T jm) X Cj(t-Tjm).

• Locally generated PN Code: Cj(t-Tj).

• Receiver output : Direct path component + Multipath component .

= [ Bj(t-Tj) X Cj(t-Tj) + Bj(t-T jm) X Cj(t-Tjm)] . Cj(t-Tj).

= Bj(t-Tj) + [ Bj(t-T jm) X Cj(t-Tjm) X Cj(t-Tj) ]

The term inside the bracket is the interference signal appearing as a spread multipath signal.


The spread interference or the multipath signal has a (t-Tjm) component.

If (t-Tjm) is less than one chip (one bit ) duration of Cj, the multipath component cannot be resolved and the demodulated output is distorted.

However, if (t-Tjm) is greater than one chip period, then it could be resolved. This could be done by introducing a second receiver which has its local PN generator delayed by Tjm.

i.e., the second receiver is synchronized with the multipath component. One of the most popular type of such receivers is RAKE receiver.


Many users – Multi path Model In a real life situation, there would be many users operating at the same frequency, at the same time. Each User would be spreading the signal with a unique PN Sequence.

The signals sent by each user undergo multipath propagation and hence would have direct path and multipath components at the receiver. The desired signal is separated by the receiver by using a local PN sequence generator as explained in the above model.

Also, if the time difference (t-Tj) is greater than one chip duration, then for each user, the multipath component could be resolved by using RAKE receiver arrangement.


However, for each user, there would be additional interference signals in the form of signals from other users operating at the same frequency.

If there are 3 users p, q and r.

Then for user p, the receiver will have :

• A direct path component given by Bp(t-Tp)

• Spread interference signals pertaining to pq and pr.

Components pq and pr are interfering signals from users q and r received at p.

In other words, pq and pr are the co channel interference for the desired signals. i.e., Bp(t-Tp).


Rake Receiver

The rake receiver is a technique which uses several base band correlators to individually process multi path signal components. The outputs from the different correlators are combined to achieve improved reliability and performance.


• RAKE RECEIVER• A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" each delayed slightly in order to tune in to the individual multipath components.

• Each component is decoded independently, but at a later stage combined in order to make the most use of the different transmissioncharacteristics of each transmission path.

• This could very well result in higher signal-to-noise ratio (or Eb/N0) in a multipath environment.


• The rake receiver is so named because of its analogous function to a garden rake, each finger collecting bit or symbol energy similarly to how tines on a rake collect leaves.

• Rake receivers are common in a wide variety of CDMA and W-CDMA radio devices such as mobile phones and wireless LAN equipment.

• RAKE RECEIVER Schemetic is shown below.


CDMA ARCHITECTURE


BSC

WPT 0

WPT1

TAP

WPT 2

BTS

TE1

TE2

MSC

OS AUX IWF

MSC

DMH

HLR

ACEIR

VLR

Other

VLR

A

O XL

Ai

P

E

F C B Mi

D

Um A bis

I

I

PSPDN

PSTN

ISDN

PLMN

TE2

TE2

TE2

TE1

TA

W R

S

R

R

DCE

G

D

H

MS

CDMA IS-95A ARCHITECTURE

TE2

Sm

Rm

Rm

WPT : Wireless Personal Terminal

DCE : Data communication equipment

TE : Terminal Equipment

Base Station subsystem

External network


• The reference model elements are :

Mobile Station :

The MS terminates with radio path on the user side and enables the user to gain access to services from the network.

It can be a stand alone device or other device like PC, fax machine etc.

Base station Subsystem:

Base station terminates the radio path and connects to the mobile switching center (MSC). The base station subsystem is divided in to BTS and BSC.


• BTS (Base Transceiver System):

BTS terminate the radio path on the network side. It may be co-located with BSC or independently placed.

The BTS contains the RF components that provide the air interface for a particular cell. This is the part of the CDMA network, which communicates with the MS. The antenna is included as part of the CDMA.

BTS includes Transceivers, base band sections, digital signal processing cards, power cards, Transmission cards, Antenna and filter sections etc.


• BSC (Base station controller):

It is the control management for one or more BTS. BSC exchanges information between BTS and MSC.

The BTSs and BSC may either be located at the same cell site “co-located”, or located at different sites “Remote”. In reality most BTSs will be remote, as there are many more BTSs than BSCs in a network.

Another BSS configuration is the daisy chain. A BTS need not communicate directly with the BSC

which controls it, it can be connected to the BSC via a chain of BTSs.


Daisy chaining reduces the amount of cabling required to set up a network as a BTS can be connected to its nearest BTS rather than all the way to the BSC. Problems may arise when chaining BTSs, due to the

transmission delay through the chain.

The length of the chain must, therefore, be kept sufficiently short to prevent the round trip speech delay becoming too long.


• Network switching subsystem:

Mobile Switching Center:

It is an automatic system interfaces with the user traffic from the wireless network or other wireless networks.

Depending upon the function of MSC, it can be classified into different ways.

1. Provides radio contact in a call2. Controls BTS’s adjacent to location of a MS.3. Accepts a call or a hand off


4. Directing an incoming a call towards MS.

5. Currently providing service to a call.

6. Providing only trunk connections7. Selected for Handoff8. Providing service to MS.

• DMH (Data Message Handler) :

Uses to collect billing data.


Home Location Register (HLR):

It manages mobile subscriber’s information by maintaining all subscriber’s information.

The information is like ESN, IMSI, User Profiles, and current location etc.

It can be co-located or placed independently. One HLR servers multiple MSC’s.

Visitor Location Register (VLR):

It is linked to one or more MSC. It dynamically stores subscriber’s information (ESN, User profiles)


• These informations are obtained from HLR. When a roaming MS enters a new service area, MSC informs the associated VLR about the MS by querying the HLR after MS goes through a registration procedure.

• Authentication Center :It manages the information associated with

the individual subscriber. It can be located with in an HLR, or MSC or located independently of both.


• Equipment Identity Register:

Provides information about the mobile station for record purpose.

• Inter working function :Enables MSC to communicate with other

networks.

• External Networks:They can be PSTN, ISDN, PLMN, PSPDN.


• Operation subsystem (OSS) :

Responsible for overall management. Some of the functions of OSS are

• Event/Alarm Management.

• Fault Management.

• Performance Management.

• Configuration Management.

• Security Management.


Interfaces between various elements of the system

• BS to MSC (A-interface) :

This interface between the base station and the MSC, supports signalling and traffic (both voice and data).

A-interface protocols have been defined using SS 7, ISDN BRI/PRI and frame relay transport (TIA IS-634).


• BTS to BSC interface : Abis if the base station is segmented into a BTS and BSC, this internal interface is defined.

• MSC to PSTN interface : Ai. This is an analog interface, using either DTMF or MF signaling.

• MSC to VLR : B-interface. This interface is in the TIA-IS-45 protocol specification.

• MSC to HLR : C-interface. This interface is in the TIA-IS-45 protocol specification.

• HLR to VLR : D-interface. It is the signaling interface between HLR and VLR based on SS7.

• MSC to ISDN : Di interface. This interface is the digital interface to the PSTN and is a T1 interface.


• MSC to MSC : E interface. This interface is the traffic and signaling interface between wireless networks.

• MSC to EIR : F interface.• VLR to VLR : G interface. When communication is needed between VLR, this interface is used.

• HLR to AC : H interface.• DMH to MSC : I interface.• MSC to IWF : I interface, this interface is defined by the interworking function.

• MSC to PLMN : Mi interface. It is for another wireless network.

• MSC to OS : O interface.


• MSC to PSPDN : Pi interface. It is defined by the packet network.

• BS to MS : Um interface. This is an air interface.

• MSC to AUX: X interface. This depends on the auxiliary equipment connected to MSC

• PSTN to DCE : W interface. This is the outside the scope of PCS and is defined within the PSTN system.


CDMA Subscriber UnitArchitecture


Transmitter Section

Speech

Source Encode

Channel Encode

Multiple Access

Modulate Transmitter

Receiver Section

Source Decode

Channel Decode

Multiple Access

Demodulate Receiver

Reconstructed Speech


Voice Coding:Variable Rate Coding :Codecs used in CDMA systems are• CELP (Code Excited Linear Predictor)• EVRC (Enhanced Variable Rate Codec)(20 msec of speech is taken at a time for voice coding).

Vocoders are source dependent coding systems. They analyse the voice signal at the transmitter end, and transmit parameters derived from the analysis. The receiver then synthesizes the voice signals according to the parameters received.


Channel coding : Channel coding maps the input data into coded symbols containing more number of bits than the original signal. Different types of Channel coding are

1. Block coding , as the name implies, code an information sequence one block at a time.

2. Convolution coding has a memory property.

3. Turbo Coding is the most recent coding system which is used in 3G systems. The principle used is based on convolution coding but, the method is used for high speed wireless communication requirements.


After this coding, bit output from the vocoder is grouped in terms of their probability to errors and are appropriately treated with FEC codes. The output coded symbols are then spread over several data frames by the method of interleaving the data along with the FEC codes. This is done to minimize the loss of data during transmission.


CODES IN CDMA


EX-OR FUNCTION

011

101

110

000

Y=A BBA

A

B

Y

EX-OR Function Truth Table

EX-OR gate produces HIGH output, if odd number of inputs or HIGH, in all other cases, the output will be LOW.


PN SEQUENCE

PN SEQUENCE :

A Pseudo Random Noise sequence is one in which the bits appear in a random manner with a specified length and the pattern is repeated for subsequent sequences.

PN sequence is the best choice as it appears as noise to all other users excepting the desired receiver.


Requirements of a PN sequence:

• Be easy to generate

• Have random properties

• Have long periods

• Be difficult to reconstruct from a short segment.


PN SEQUENCE GENERATION

• Example : Suppose we have 4 digit words.

The natural sequence is from 0000 to 1111. Purely random sequence could be a series of 15 word sets, with the combination of words in each set being random

The PN sequence is important because, the receiver needs a replica of the transmitted sequence to de spread the signals.

The PN sequence has a random set of words which repeat after a specific sequence length.


Example of a PN sequence :

• Consider a 4 bit sequence.. 0001.

EX-OR

1 2 3 4

0 0 0 1

The first 4 bit sequence is 0001.

The next 4 bit sequence will be 1000, 1100, 1110, 1111, 0111, 1011, 0101, 1010, 1101, 0110, 0011, 1001, 0100, 0010.

The next 4 bit after this again starts with 0001.


Example for PN Sequence generator is (SSRG)

SIMPLE SHIFT REGISTER GENERATOR.

• A generic form of a simple shift register generator is shown below.

1 2 3 4 5 6… ……… N

Feed back logic-exclusive OR circuits


Properties of PN sequences:PN Sequences exhibit the following properties:

• The maximal length of the sequence is 2n-1, where n is the number of stages in the shift register.

• The number of 1’s will be 2(n-1) and that of 0’s will be 2(n-1) -1. i.e., the number of 1’s will be one more than the number of 0’s.

• If a maximal SRG sequence is added to a phase shift (time shift) of it, then the resulting sequence is another phase shift of the original sequence. This is called the “shift and add”property of SSRGs.


A 4 stage SSRG type PN sequence generator

EX-OR

1 2 3 4

0 0 0 1

0001

1000

1100

1110

1111

0111

1011

0101

1010

1101

0110

0011

1001

0100

0010

• The output sequence is : 1000 1111 0101 100

• It is 15 bits Long.i.e., L= 2n-1

• There are 8 ones and 7 zeroes

• In any period, half the run of continuous 1s or 0s are of length1. One fourth run are of length 2, one eighth are of length 3, and so on.

• Shift and Add property : If the PN sequence is shifted in time, the resulting sequence is another shift of the original sequence itself if the shift is by one bit/chip, then the original pattern repeats after 2n-1 sequence


• Example :Code : 100011110101100 100011110101100 100011110101100

Shift 1: 000111101011001 000111101011001 000111101011001

……………………………………………………………………………………………..

XOR : 100100011110101 100100011110101 100100011110101

The original code is marked with white digits in the result of XOR. By shifting the sequence successively by one bit, it can be verified that the original sequence repeats after 15 sequences (2n-1).

Example 2:

For the same tappings, change the initial seeding of the shift register or feedback points and obtain the output sequence. What are your observations, on the new sequence?


Alternate configuration of the 4 stage SSRG

EX-OR

1 2 3 4

0 0 0 1

0001

1000

0100

0010

1001

1100

0110

1011

0101

1010

1101

1110

1111

0111

0011

……..

0001

Out put sequence : 1000 1001 1010 111.


Auto Correlation

• This describes the extent of likeness between a random variable and its time shifted version, it can be defined by a simplified formula:

Auto Correlation =∫x(t) * x(t-T) dT

For PN sequences this could be written as :

Auto correlation = 0∫T C J (t) * C J(t-T)dT.

The Auto correlation function of the PN sequence has a positive value = 2n -1 for zero time shift instances and has a small negative value at other instances, when the shift in time is equal to or more than one chip duration.


Cross Correlation

• Cross Correlation defines the likeness between two different random variables and could be described by :

Cross Correlation =∫x(t) * y(t-T) dT

For a PN sequence, this could be re written as :

Cross correlation = 0∫T C J (t) * C k(t-T)dT.

The cross correlation function will have very small negative values.


• Auto correlation of PN sequences :Consider the original sequence

1000 1111 0101 100. This is compared with receive sequences with time shifts T0, T1, T2 etc.

T0 means zero time shift or exact synchronization between transmitted and receive sequences.

To calculate the auto correlation values, we give a mark +1 for every bit that matches with the reference (original) sequence and -1 for every bit mismatch.


• Ref Code : 100011110101100 Auto correlation

TS 0 : 100011110101100 15

TS 1 : 000111101011001 -1

TS 2 : 001111010110010 -1

TS 3 : 011110101100100 -1 and so on….

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-1

1

3

2

4

5

6

8

7

9

10 11 12 13 14 0

10

11

12

13

14

15

Auto

Correlation

value

t


Cross correlation of PN sequences:

• If we change the feed back tappings for our 4 stage SSRG, we get an entirely different PN sequence. Let us take the feed back from the 3rd and 4th stage outputs with the initial loading as 0001, from left to right (as shown in the slide above).

• The new PN sequence is : 1000 1001 1010 111We take a time shifted code from the

original sequence 1000 1111 0101 100 and compare that with the new code generated.


• This would give us the cross correlation values between the 2 different PN sequences.

• It is also possible to have the comparison between the first code and a time shifted version of the second.

• It is observed that the cross correlation values vary from -5 to +7, depending upon the extent of similarity between the 2 sequences.

• The comparison between the 2 PN sequences is given below and the cross correlation function is presented pictorially in the coming slide.


• Ref Code : 100010011010111 cross corelation

TS 0 : 100011110101100 -1

TS 1 : 000111101011001 -1

TS 2 : 001111010110010 -1

TS 3 : 011110101100100 -5

TS 4 : 111101011001000 -5

TS 5 : 111010110010001 +1

TS 6 : 110101100100011 -5

TS 7 : 101011001000111 +2

2nd code


• Ref Code : 100010011010111 cross corelation

TS 08 : 010110010001111 +1

TS 09 : 101100100011110 -1

TS 10 : 011001000111101 -5

TS 11 : 110010001111010 +1

TS 12 : 100100011110101 +7

TS 13 : 001000111101011 -1

TS 14 : 010001111010110 -1

-------------------------------------------------------

TS 0 : 100011110101100 -1

Note that the cross correlation values vary from -5 to +7.

2nd code

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-6

-4

-5

-3

-2

-1

0

1

10 11 12 13 14 0

2

3

4

5

6

7

cross

Correlation

value

t

8

1 2 3 4


Classification of PN Codes :

PN codes are classified into 2 types.

1. PN Long code2. PN Short codePN Long Code : It is used for Mobile

Identification in reverse link and data scrambling in forward link

PN short code : It is used for base station identification in forward link and orthogonal modification in reverse link.


• PN long code : It is having 42 bit length (242) and generated at a rate of 1.2288 Mcps. The

code sequence repeats after 41.2 days.

242=4.39X1012.

Chip Duration = 1/(1.2288X106)

= 0.8138 µ Sec.

Hence code duration = 4.39X1012X 0.8138X10-6

= 3.579 X 106 sec

= (3.579X106)/(24X60X60)= 41.2 days.


• In forward link, long code is used to encrypt user ID, in reverse link long code is used (ESN – Electronic serial number). The code mask is done by using ESN.

• Both Base station and respective user have knowledge of PN sequence at any given instant of time as a specified private code mask is exchanged between them.


PN short code :

• This sequence uses 15 bits and code is generated at 1.2288 Mcps. The code repeats with a period of 26.67 m sec.

215 = 32.768X103

Chip duration = 1/(1.2288X106) = 0.8138 µ Sec

Hence, the code duration

= 32.768X103X 0.8138X10-6

= 26.67 m sec.


• The code is used by Base station and is used for final spreading of signal in the forward channel and is also transmitted as a pilot sequence by Base Station.

• All the base station use the same short code by distinct off-set time for identification.

i.e., PN offsets are used for BTS identity.


ORTHOGONAL CODES• Two codes are said to be orthogonal, if the product (Ex-or) of the two codes produces equal number of 1s and 0s.

Ex : if A= 111010 and B=011001, then Xoring

A and B gives the answer as 100011, which has three number of 1s and three number of 0s.

Conventionally, 1 is mapped as +1 and 0 as -1.

Example for orthogonal codes are “WALSH CODES”.


WALSH CODES

• Most commonly used orthogonal codes in CDMA systems

• A set of length ‘n’ consists of ‘n’ rows of nXn Walsh matrix.

• W1= (0).

• In general, W2n= [ Wn, Wn, Wn, Wn ], where ‘n’ is the dimension of the matrix. The over score denotes the logical NOT of the bits in the matrix.


• The matrix has the property that every row is orthogonal to every other row and also to the logical NOT of every other row.

W2x2 = 0 0

0 1

W4x4 = 0 0 0 0

0 1 0 1

0 0 1 1

0 1 1 0


W2nx2n = wn wn

wn wn

0 0 0

0 1

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

1 1

1 0

1 1

1 0

1 1

1 0

0 0

0 1

Walsh Code Generation :


HAND OFFS IN CDMA


Handoff Process in CDMA

• What is hand off?

In cellular telecommunications, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another.


• In a cellular telephone network, handoff is the transition for any given user of signal transmission from one base station to a geographically adjacent base station as the user moves around. In an ideal cellular telephone network, each end user's telephone set or modem (the subscriber's hardware) is always within range of a base station. The region covered by each base station is known as its cell. The size and shape of each cell in a network depends on the nature of the terrain in the region, the number of base stations, and the transmit/receive range of each base station. In theory, the cells in a network overlap; for much of the time, a subscriber's hardware is within range of more than one base station. The network must decide, from moment to moment, which base station will handle the signals to and from each and every subscriber's hardware.


• Each time a mobile or portable cellular subscriber passes from one cell into another, the network automatically switches coverage responsibility from one base station to another. Each base-station transition, as well as the switching process or sequence itself, is called handoff.

In a properly functioning network, handoff occurs smoothly, without gaps in communications and without confusion about which base station should be dealing with the subscriber. Subscribers to a network need not do anything to make handoff take place, nor should they have to think about the process or about which base station is dealing with the signals at any given moment.


Base Station

Base Station

Control

Equipment

Network InterfacePSTN

Forward

Rever

se


“Handoff”

Base Station

Base Station

Base Station

Cell

Boundary

Control


Types of Hand off• Soft Hand off• Soft Soft Hand off• Softer Hand off• Hard Hand off

All types of soft hand offs are called as make before break connections, and these are done only between cells having the same CDMA channel (frequency) assignments.

Hard hand off is called as break before make connection and these are done between cells having different CDMA channel frequencies.


• Soft Hand off :This is the condition when two cells in a

mobile network are in simultaneous communication with the mobile receiver.

This situation will continue till one of the pilot signals from a particular base station falls below a chosen threshold level.

From this, the mobile will be transferred to the remaining base station.


• Soft Soft Hand off :The situation here is the same as that

described above. However, the number of cells involved in this case are 3. This number is dictated by the provision of rake receiver in the mobile. This has 3 fingers for the separate reception and processing of three stations at a given time.


• Softer Hand off :This hand off procedure is similar to

the soft hand off. But, comes into picture when the mobile transition is from one sector to another in the same cell area. The mobile is served by two sectors simultaneously till one drops off.


• Hard Hand off :Needing frequency transition

This is similar to GSM. This occurs only when the mobile transit from one area to another where the allocated / used channel frequency happens to be different.


Pilot sets in Hand off Procedure

• Hand offs are done by measuring pilot signal strengths from neighboring cells.

• Measurement message sent to base station when a pilot of adequate strength, not belonging to the mobile’s current assignment of traffic channels is detected.

• Hand off process is dependent on pilot searches.


Pilots can be grouped into 4 sets.

1. ACTIVE SET

2. CANDIDATE SET

3. NEIGHBOURING SET

4. REMAINING SET


1. Active Set :

The pilots associated with the forward traffic channels assigned to the mobile station.

2. Candidate Set :

These pilots which are NOT in the current Active Set but have sufficient signal strength to qualify for hand-off.


3. Neighbour Set :

This contains a list of neighbouring pilots whose signal strengths are strong enough to make them candidate pilots. This list is entered through the data base. Even if a pilot from an adjoining cell is very strong, the mobile will not look at if it is NOT mentioned in the neighbor list.

4. Remaining Set :

The set of all pilots other than the three types mentioned above.


Remaining SetNeighbour Set

Candidate Set

ACTIVE SET


Parameters used in Hand-Offs

The following Parameters are very important in Hand-off situations:

1. T_ADDs :

Suggested value : -13 dB

(Range : -31.5 to 0dB).

T_ADD is the Pilot Detection Threshold.


2. T_COMPs :

Suggested Value : 2.5 dB

( Range : 0 to 7.5 dB )

This is the ACTIVE SET Vs CANDIDATE SET COMPARISION THRESHOLD.


3. T_DROPs :

Suggested Value : -15 dB

( Range : -31.5 to 0 dB)

This is the PILOT DROP THRESHOLD.

4. T_TDROPs :

Suggested Value : 2 seconds

( Range : 0 to 15 seconds )

This is the PILOT DROP TIMER VALUE.


Neighbour Set Remaining Set

P exceeds T_ADD

1. Move the pilot to candidate set

2. Send Measurement report to base station

P exceeds an active pilot by T_Comp X0.5

No YES

Keep pilot in candidate set

Send measurement report

Receive H/O Direction message

Move Pilot to Active set

Send Hand off completion Message

yes

No

Search for Pilot


POWER CONTROL IN CDMA


MS1

MS2

d

12

1

21

==

=

R

R

RR

P

P

I

C

PP

P

f1.25 MHz

1RP

2RP

1TP

2TP

Ideal CDMA System

MS1

MS2

d

( )

16

1

16

1

2/

1

2

1

21

142

==

⋅≈

⋅=

R

R

RR

RR

P

P

I

C

PP

Pd

P

P

f1.25 MHz

12 16RR

PP =

1TP

2TP

d/2

Near-Far Effect


What is Power Control?

Power control refers to the strategies or techniques required in order to adjust, correct and manage the power from the BS/MS in both directions (i.e. uplink and downlink) in an efficient manner.


Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users.

The power of each individual user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.


• To illustrate how power control is essential in CDMA, we consider a single cell that has two users.

• User 2 is much closer to the base station than user 1. If there is no power control, both users would transmit a fixed amount of power pt.

• However, because of the difference in distance, the received power from user 2 or pr2 would be much larger than the received power from user1 or pr1.

• This would be a great disadvantage.


Power control is implemented to overcome the near-far problem and to improve the quality and reduce the interference.

Power control is where the transmit power from each user is controlled such that the received power of each user at the base station is equal to one another.


User 2

User 1

pr1

pt

pt

pr2

A base station with two users, each user is transmitting to the base station a fixed amount of power pt


User 2

User 1

pr1

pt1

pt2

pr2

A base station with two users, each user is transmitting to the base station with different powers, depending on the distance from the base station. (power control)


Solution for Near far problem• One Problem that has to be immediately solved in power control is the initial mobile transmit power. Before the mobile establishes contact with the base station, the mobile cannot be power controlled by the base station.

when the mobile first attempts to access the base station, what power level should the mobile use to transmit? At this point, the base station has not yet made contact with the mobile user, and the base station has no idea as to the location of the mobile user.


• There are two options for this….The first option is that the mobile can

attempt to access the base station with a high transmit power.

Such high power increases the probability that the base station will receive that mobile’s access request. But, the disadvantage of a high initial transmit power is that such high power represents interference to other users currently served by that cell.


The second option is that the mobile can request access from the base station with a low transmit power.

such low power decreases the probability that the base station will receive the mobile’s access request.

But, the advantage is that the mobile won’t cause much interference to other users.


ACCESS PROBES

Access probes are a series of transmissions of progressively higher power. The mobile transmits its first access probe at a relatively low power, then it will wait for a response back from the base station.

If after a random time interval the mobile does not receive an acknowledgement from the base station, then the mobile transmits a second access probe at a slightly higher power.


The process repeats until the mobile receives an acknowledgement back from the base station.

The power difference between the current access probe and the previous access probe is called an access probe correction.

The step size for a single access probe correction is specified by the system parameter PWR_STEP.

Pt initial = -Pr -73 +NOM_PWR + INIT_PWR

Where NOM_PWR = normal power

INIT_PWR = Initial power.


Initial transmit power

Mobile Transmit Power

First access Probe correction

Second access Probe correction

TimeRandom Time intervals

Fig : A series of access probes by the mobile to access the system.


TYPES OF POWER CONTROL

Basically, there are 2 methods of power control available in cdma

1. Open Loop Power Control :

This is purely a mobile unit function. It gives open estimation. This is done only during the initial stage as soon as the mobile is turned ON.

After the call is established and mobile moves around within a cell, the path losses between MS and BS will continue to change.


As a result the received power at mobile will change and open loop power control will continue to monitor mobile received power and adjusts the mobile transmit power according to the following equation.

Pt initial= - Pr -73 +NOM_PWR + INIT_PWR +sum of all access probes correction.


2. closed loop power control :

This involves both base station and the mobile unit and gives a closed loop power correction.

BS continuously monitors Eb/No on reverse link. If Eb/No is very high, the BS commands the mobile to decrease the transmit power of mobile.

If Eb/No is low, i.e., below a threshold value, it commands the mobile to increase transmit power of mobile.


The mobile should use the power level it receives from the base station to estimate how much to initially transmit.

In other words, if the mobile sees a strong signal from the base station, then it assumes that the base station is nearby and thus transmits initially at a relatively low level.

If the mobile sees a weak signal from the base station, then it assumes that the base station is far away and thus transmits initially at a relatively high level.


CHANNELS IN CDMA


• CDMA channels can be divided in to two types of links.

1. Forward Link : It is from BTS to Mobile Station.

Frequency Range : 869 – 894 MHz.

2. Reverse Link : It is from Mobile Station to BTS.

Frequency Range : 824 - 849 MHz.


IS-95 Standard

• Forward Link– Pilot Channel

– Sync Channel

– Paging Channels (max. 7)

– Traffic Channels

• Reverse Link– Access Channels

– Traffic Channels


Forward CDMA Channel• Forward link consists of up to 64 logical channels (code channels)

• The code channels are distinguished by a set of 64 Walsh functions

• Walsh function code number zero is always reserved as the pilot

• Short Code (with period 215, 27.667 ms at 1.2288 MHz)

– spreading the CDMA Forward Channel

– used in conjunction with the Long Code for spreading the CDMA Reverse Channel

• Long Code (with period 242 - 1)

– spreading the CDMA Reverse Channel

– Long Code Mask serves as a reverse link address


Logical Forward CDMA Channel

Forward CDMA Channel

Pilot

Chan

Sync

Chan

W0 W32

Paging

Ch 1

W1

...Up

to W7

Paging

Ch 7

W8

...Up

to

Traffic

Ch N

Traffic

Ch 1

Traffic DataMobile Power

Control Sub-Channel

...

W31

Traffic

Ch 24Up

to

...

W63

Traffic

Ch 55

W33

Traffic

Ch 25


CDMA Channel Structure (Forward Link)

0- Pilot Channel

Paging Channel

Paging Channel

Traffic Channel

Synchronisation Channel

Traffic Channel

Traffic Channel

Traffic Channel

Traffic Channel

1

0

7

8

9

31

32

33

63

1.2288 MHz

W0

W1

W7

W32

W8

W31

W33

W63


Pilot Channel

• Unmodulated signal

• Unique per sector/cell• Signal level is 4~6 dB higher than traffic channel

• Perfect phase/time/signal strength reference for MS


• Used in initial system acquisition and handoff for MS

• DS at 1.2288 Mcps• Every cell uses the same PN sequence (period 2 15 or 26.67ms) and is identified by a pre-defined offset (64 x n chips)

• QPSK modulation


(All 0’s)

Walsh Function

W0 @ 1.2288Mcps BBF

BBF

To QPSK Modulator

A

B

I- Pilot PN Sequence @1.2288MCPS

Q- Pilot PN Sequence @1.2288MCPS

PILOT CHANNEL


carrier

Pi/2

A

B

D

C

ESUMMER

Sin Wc t

Cos Wc t

QPSK MODULATOR


• The PILOT Channel is identified by Walsh function 0 (W0). The Channel itself contains no base band information. The base band is stream of zeroes, that are spread by Walsh function zero, which is also a sequence of all zeroes.

• The resulting sequence still all zeroes, is then spread are multiplied by a pair of quadrature sequence.

• The pilot channel is effectively PN-Sequence itself.


• Both Walsh function zero and PN-Sequence are running at a rate of 1.2288 Mcps.

• The pilot channel is transmitted continuously by Base Station Sector.

• It provides the mobile with timing and phase reference.

• The mobile measurements of signal to noise ratio of pilot channel also gives an indication of which is strongest serving sector of that mobile.

• Pilot power is 4 to 6 dB higher than traffic channel.


BBF

BBF

To QPSK Modulator

A

B



Block interleaver

Symbol Repetition

Convolutional Encoder R=1/2

Sync channel bits

1.2kbps 2.4kbps 4,8kbps 4,8kbps

Walsh Function

W32 @ 1.2288Mcps

1.2288 Mcps

SYNC CHANNEL


carrier

Pi/2

A

B

D

C

ESUMMER

Sin Wc t

Cos Wc t

QPSK MODULATOR


Sync Channel

• Sync channel operates at a fixed bit rate of 1200 bps.

• Used for the mobile to get timing and long code references.

• It is not scrambled

• It does not carry the power control bits• Uses the same pilot PN offset as the pilot channel


• Unlike the pilot channel, the sync channel carries base band information.

• The information contained in sync channel message that notifies the mobile of important information about system synchronization and parameters.

• To convey pilot PN sequence offset, time of day, and long code state to allow immediate sync of MS to the network


• The sync data is passed through convolutional coder.

• Output of convolutional coder is 2400 bps and this is passed through code repetition circuit to get 4800 bps.

• The output of repetition circuit is then block interleaved and subjected to direct sequence spreading using Walsh code 32and then given to Quadrature spreading circuit.


Sync ch super frame

Sync ch frame

Sync ch frame

Sync ch frame

SOM

SOM

SOM

1 0 0

Sync ch super frame

Sync ch frame

Sync ch frame

Sync ch frame

SOM

SOM

SOM

0 0 0

-------------

------

---

80 msec, 96 bits

Sync channel message capsule = 96 X Ns, Ns is the no of super frames needed.,

Sync channel message Padding bits

MSG_LENGTH Message Body CRC

8 bits 2-1146 bits 30 bits

SOM = Start Of Message

SYNC CHANNEL FRAME STRUCTURE

26.66 msec

+ Padding bits if needed


• Sync channel is sent in super frames of 96 bits in 80 msec duration.

• Each super frame has 3 frames of 26.66.. Msec each.

• A number of super frames form a sync channel message capsule.

• A capsule has a message length indicator, Message body and CRC, Message length in octets.

SYNC CHANNEL FRAME STRUCTURE


• It also has padding bits set to 0; the number of padding bits is to make the total number of bits equal to an integer multiple of 96 bits.

• The beginning of a super frame has an SOM bit set to 1 for the first frame and set to 0 for other frames.


Sync Channel Frame Structure

• Sync channel message has length of 96 x N bits

• Sent in N super frames:

– 1 super frame (96 bits, 80 ms) = 3 sync channel frames

– 1 frame (32 bits, 26.67 ms) = 1-bit SOM + 31-bit data

– SOM = 1 : start of message

Message length (8 bits)

Data (2 to 1146 bits)CRC

(10bits)Padding bits

all 0s…


• Sync Channel Message contains:– system identification (SID) and network identification (NID)

– PN sequence offset and long code state

– system time, leap seconds, offset from UTC, etc.

– paging channel data rate (PRAT).


• Only the Sync Channel Message is sent on the sync channel.

• MS – obtains information from Sync Channel Message

– adjusts its timing to normal system timing

– begins monitoring its Paging Channel


Paging Channel

• Data rate: 4800, 9600 bps• One 9600 bps Paging Channel can support 180 pages/sec.

• Does not carry power control sub channels• Use the same PN sequence offset as the pilot channel

• Up to 7 paging channels are possible, the first one taking Walsh code number W1


• Paging Channel conveys four major types of messages:

– overhead

– paging

– order

– channel assignment

• Configuration of the system is conveyed in four overhead messages:

– System Parameter Message

– Access Parameter Message

– Neighbor List Message

– CDMA Channel List Message


Overhead Messages

• System Parameter Message:

– configuration of the Paging Channel

– registration parameters

– parameters to aid pilot acquisition

• Access Parameter Message

– configuration of the Access Channel

– control parameters used to stabilize the Access Channel


• Neighbor List Message– time offset of the pilot

– basic neighbor configuration

• CDMA Channel List Message– CDMA frequency assignment that contain Paging Channels


Paging Channel Messages

• Page Message: – contains pages to one or more mobile stations.

• Order Message: – a broad class of messages used to control a particular MS.

• Channel Assignment Message:

– let BS to assign a MS to the traffic channel

– change Paging Channel Assignment

– direct the MS to use the analog FM system


BBF

BBF

A

B



Block interleaver

Symbol Repetition


4.8kbps 9.6 kbps 19.2ksps

Walsh Function

W1-7 @ 1.2288Mcps

1.2288 Mcps

PAGING CHANNEL

Long code Generator

Decimator 64:1

Long code Mask for paging channel

19.2ksps

19.2ksps


carrier

Pi/2

A

B

D

C

ESUMMER

Sin Wc t

Cos Wc t

QPSK MODULATOR


• Like Sync channel, paging channel also carries base band information. But, unlike sync channel, the paging channel transmits at a higher rate.

• It can transmit at either 4.8 or 9.6 kbps.

• PRAT field in the sync message inform the mobile of data rate of paging channel. Once the mobile acquires timing & synchronization using sync channel, the mobile begins to monitor paging channel.

• Although there can be up to 7 paging channels per sector, each mobile only monitors one paging channel.


• From the above figure, the base band information is error protected then if the data rate is at 4.8kbps, the bits are repeated once, after that interleaving will be taken place.

• The data is first scrambled by a decimated long PN sequence, then it is spread by a specific Walsh function and further spread by short PN sequence assigned to serving sector.


Max. Paging Channel Slot Cycle (80 msec)

PCH slot 0

PCH slot 1

PCH slot 2

PCH slot 2047

PCH slot 3 ----------------------------------------------

PCH frame

PCH frame

PCH frame

PCH frame

PCH half frame

PCH half frame

PCH half frame

PCH half frame

PCH half frame

PCH half frame

PCH half frame

PCH half frame

SCI

SCI

SCI

SCI

SCI

SCI

SCI

SCI

192 bits 20 msec

96bits 10msec

SCI : Synchronised Capsule Indicator

PAGING CHANNEL FRAME STRUCTURE


Paging Channel Frame Structure

• Synchronized paging channel message has length of 47 x N or 95 x N bits

• Sent in N paging channel slots:

– 1 slot (80 ms) = 8 paging channel half-frames

– 1 half-frame (10 ms, 48 or 96 bits) = 1-bit SCI + 47 (or 95)-bit data

– SCI = 1 : start of a paging channel message

Message length (8 bits)

Data (2 to 1146 bits)CRC

(10bits)Padding bits

all 0s…


• Message contains:– system parameters

– access parameter (for access channel)

– channel assignment

– TMSI (temporary MS identification) assignment


BBF

BBF

A

B



Block interleaver

Symbol Repetition


1.2 kbps

2.4 kbps

4.8 kbps

9.6 kbps

2.4 kbps 19.2ksps

Walsh Function

@ 1.2288Mcps

1.2288 Mcps

FORWARD TRAFFIC CHANNEL

Long code Generator

Decimator 64:1

Long code Mask Soecific user ESN

19.2ksps

19.2ksps

Decimator 24:1

mux

PCB 80bps

1.2288 MCps


• The forward traffic channel at variable spreads up to 9600 bps with one pilot channel, one sync channel and seven paging channels.

• We can have a maximum of 55 traffic channels.

• From the figure of forward traffic channel, the signals are passed through a convolutional encoder R=1/2. for every input bit, we get 2 bits at output. The symbol repeater after the convolutional encoder repeats the symbol twice for 4800 bps speed, 4 times for 2400 bps and 8 times at 1200 bps.


• The block interleaver output is scrambled using a decimator. The scrambling is done by modulo 2 addition (Ex-or) of interleaved output with binary value of long code chip. The resulting PN Sequence is same as long code as 1.2288 MHz clock rate where only the first output of every 64kbps is used for scrambling data.

• The scrambled data (19.2kbps) is then passed through multiplexers, whereas power control bits are added. For scrambling power control bits, the output of 64:1 decimator to produce a bit rate of 800 bps.


• The multiplexer output is then subjected to orthogonal modulation by Walsh codes and then are quadrature spread.

• The Walsh code table is same as in case of reverse channel. The major difference is that in forward channel, the 64 bit Walsh code is send for every code symbol, whereas in reverse channel, the Walsh code is send for every 6 symbols (chips).


• In forward traffic channel, there are 2 types of rate sets.

• Rate set 1 supports 4 data rates. They are 9.6,4.8,2.4 and 1.2 kbps.

• Rate set 2 supports the data rates of 14.4, 7.2, 3.6, 1.8 kbps.


12 8172

192 bits (20 m sec)

9600 bps frame structure

8 880

96 bits (20 m sec)


840

48 bits (20 m sec)


816

24 bits (20 m sec)


F

F

T

T

TF : Frame quality indicator (CRC)

T : Encoder Tail Bits


Forward Link Channel Parameters

Channel Sync Paging Traffic

Data rate 1200 4800 9600 1200 2400 4800 9600 bps

Code repetition 2 2 1 8 4 2 1

Modulation symbol rate 4800 19,200 19,200 19,200 19,200 19,200 19,200 sps

PN chips/modulation symbol 256 64 64 64 64 64 64

PN chips/bit 1024 256 128 1024 512 256 128


CDMA Channel Structure (Reverse Link)

Access Channel

Traffic Channel

Traffic Channel

Traffic Channel

Traffic Channel

2

1

32

1

2

3

62

1.2288 MHz

Access Channel

Access Channel


Logical Reverse CDMA Channel

Reverse CDMA Channel

Access

Ch 1

Access

Ch n... Traffic

Ch 1

Traffic

Ch m

Address by Long Code PNs

n ≤ 32 m ≤ 62

…


Access Channel

• Access Channel provides communications from MS to BS when MS is not using a Traffic Channel.

• All Access Channel use 4800 bps mode

• Access Channel Message:– call origination

– response to pages

– orders

– registrations


Access Channels

• To access the system, respond the page, make call origination and process other messages between the MS and the BS

• 4.8 kbps slotted random access channel

• MS is identified by orthogonal long code


BBF

BBF

A

B



Block interleaver

Symbol Repetition


4.8kbps 14.4 kbps

28.8ksps

1.2288 Mcps

ACCESS CHANNEL

Long code GeneratorLong code Mask

for paging channel

28.8ksps D

64-ary Orthogonal modulator

To QPSK Modulator


• The Access channel is used by the mobile to communicate with the base station when the mobile doesn’t have a traffic channel assigned.

• The mobile uses this channel to make call originations and respond to pages and orders.

• The base band data rate of the access channel is fixed at 4.8 kbps.


• The base band information is first error protected by an R=1/3 convolutional encoder. The lower encoding rate makes error protection more robust on the reverse link.

• The symbol repetition function repeats the symbol once, giving a code symbol rate of 28.8ksps.

• The data is then interleaved. Following interleaving, the data is coded by a 64-ary orthogonal modulator.


• The orthogonal modulated date is then spread by the long PN sequence. The long PN sequence is running at 1.2288Mcps, and the bandwidth of the data after spreading is 1.2288 Mcps.

• The data is further scrambled in the in phase and quadrature phase paths by the short PN sequences defined .

• Because the reverse link uses OQPSK modulation, the data in the Q path is delayed by one half a PN Chip.


BBF

A

B



Block interleaver

Symbol Repetition


Reverse Traffic Channel

Long code GeneratorLong code Mask

D

64-ary Orthogonal modulator

To QPSK Modulator

BBF

Data Burst Randomizer

1.2kbps

2.4 kbps

4.8kbps

9.6kbps

3.6kbps

7.2 kbps

14.4kbps

28.8kbps

28.8ksps 28.8ksps

1.2288 MCps


• The reverse traffic channel is used to transmit user data and voice. Signaling messages are also sent over traffic channel structure of reverse channel and is similar to that of access channel.

• The major difference is that reverse traffic channel contains a burst randomizer. The orthogonally modulated data is fed to data burst randomizer. The function of data burst randomizer is to take advantage of voice activity factor of reverse link.

• There are two rate sets. Rate set1 has 1.2,2.4,4.8 and 9.6 kbps rates where as rate set 2 has 1.8,3.6,7.2 and 14.4 kbps rates.


CDMA CALL PROCESSING


To process a call, MS goes through the following stages to get traffic channel.

• SYSTEM INITIALISATION STATE• SYSTEM IDLE STATE• SYSTEM ACCESS STATE• TRAFFIC CHANNEL STATE


Tune to CDMA Carrier

Mobile acquires pilot channel and sync channel

Mobile acquires paging channel and monitors for messages

Mobile sends messages on access channel, BS sends messages on paging channel.

Speech communication on forward and reverse traffic channel, Power control on forward link

System initialization state

Idle state

Access State

Traffic channel state

CALL PROCESSING


CALL PROCESSING STATE:

• In system initialization state the mobile acquires a pilot channel by searching the PN off sets and selects the strongest pilot signal.

• Then it acquires synchronization channel using W32 and detecting timing offset of pilot channel. Then mobile acquires the system configuration and timing information.

• Then the MS enters the system idle state where it monitors the paging channel. Now the mobile can receive necessary message to initiate or receive a call.


• If a call is placed or received, MS enters the system access state exchanging necessary parameters. MS transmits its response on access channel and BS transmits on paging channel.

• On access, the MS enters traffic channel state and communicate. It also transmits control messages replacing speech by signaling.

• Power control messages are sent on forward link channel.


SYSTEM INITIALIZATION STATE

Power up and mobile enters

initialization state

System determination sub state

Pilot acquisition sub state

Sync channel acquisition sub state

Timing change sub

state

CDMA system is selected

Pilot channel is acquired

Sync channel is acquired

System timing is acquired

Enter idle stateAnalog

initialization

Begin analog mode


• System determination sub state :

Mobile can select analog or CDMA, if the system selects CDMA, the mobile sets the CDMA channel parameters to channel number.

• Pilot channel acquisition sub state:Mobile acquires pilot channel of CDMA

system and tunes to CDMA channel and sets its code channel for the pilot channel and searches for pilot channel within 20 msec, the mobile should acquire pilot channel, else mobile enters system determination sub state indicating failure.


• Synchronization acquisition sub state :Mobile acquires the sync channel and

obtains system configuration and timing information for CDMA system and sets its code channel for sync channel if mobile does not receive a sync channel message within 21 msec, mobile enters the sync determination sub state with a protocol mismatch indication.

If it receives within 21 msec, mobile stores system configuration and timing information.


• Timing change sub state :

If a mobile synchronizes its long code timing of system timing to those of CDMA system after receiving and processing sync messages.

After sync channel is acquired and sync message is received, it stores the following information like protocol revision message, system identification, network identification, pilot PN offset, system time, long code at system time, paging channel data rate etc..


CDMA initialization

Acquires primary paging channel (W1)

• monitors paging channel

•Authentication

•Idle hand off

•Page Response

•Mobile origination

•Receives an incoming call

•Registration

Idle State


Idle State :

Mobile monitors paging channel. Mobile can receive messages from base station to initiate or receive a call.

It can also initiate a registration process, message transmission. After entering idle state, mobile sets its Walsh code to the primary paging channel and sets its rate to the rate obtained from sync message.

Paging channel is subdivided into 80 msec slots are called paging channel slots.


• In slotted mode, the mobile monitors only certain assigned slots. When paging channel is not monitored, mobile can stop or reduce its processing activities to save battery power.

• In the non slotted mode, paging and control data can be received on any of this slot. The mobile monitors all the slots on continuous basis.


• Idle Hand off :Idle hand off or change of paging channel

occurs when a mobile has moved from coverage of one base station to coverage of another, during idle state. Hand off should occur when it detects new pilot that is stronger than current pilot.

Pilots are identified by short PN offsets. These pilot sets are maintained by active, candidate, neighbor, remaining. If neighbor set or remaining set is stronger than active set and candidate set, pilot hand off is performed in the non slotted mode. On receiving new message, from new paging channel, it resumes operations.


SYSTEM ACCESS STATE:

It includes following sub states.

1. Update overhead information

2. Mobile State origination Attempt

3. Page Response4. Registration Access sub state5. Mobile Station Transmission


Traffic channel sub state:Speech communication takes

associated with control messages.

There are 5 sub states in this.

1. Traffic Channel initialization :In this state, mobile verifies it can

receive forward traffic channel and starts transmit on reverse link.

2. Waiting for order sub state:

In this, MS waits for an alert with information message.


3. Waiting for MS answer sub state:

In this state, the MS waits for user to answer the call.

4. Conversation sub state:Here, the MS exchange primary traffic

data packets with base station.

5. Release Sub state:In this case, MS disconnects a call.


CDMA REGISTRATION

Registration is a process in which the MS tells the base station about the whereabouts. It notifies base station of its location status, identification, slot cycle and other characteristics.

Purpose of registration is to enable the base station to page the mobile in case of an incoming call.


CDMA supports 9 types of registration.

1. Power up registration : The mobile registers, when it powers up.

2. Power down registration : The mobile registers, when it powers down informing the system that it is no longer active.

3. Time based registration : the mobile registers at regular intervals of time. Its use also alerts the system to automatically deregister mobile stations that did not perform a successful power down registration.

4. Distance based registration : Mobile performs a registration when distance between current base station and the base station which it has previous registered exceeds the threshold.


5. Zone based registration : Mobile registers when it enters a new zone.

6. Parameter changed registration : Mobile does a registration when it enters a new system or some of its stored parameters change.

7. Ordered registration : mobile registers when a base station asks for it.

8. Implicit registration : When a MS sends a successful origination message or page response message, base station can ask a mobile of its location.

9. Traffic channel registration : Base station tells the mobile, it is registered.

Only power up, power down registrations are presently supported.


CALL PROCESSING (Mobile Station to Base Station)

Sends origination message Receives origination messageAccess Channel

Receive Paging Channel Sets up Traffic Channel, starts sending null traffic data

Paging Channel

Sets up reverse traffic chAcquires reverse traffic Ch

Rev traffic ChStarts sending Traffic Ch preamble

Fwd traffic ChReceives Ack from BS

Starts sending null traffic data

Rev traffic Ch

Sends BS Acknowledgement

Receives data from Mobile

Sends origination continuation message

Rev traffic Ch Receives the message

Receives ring back tone Fwd traffic ChSends Alert with info msg

Ring Stops Fwd traffic ChCalled subscriber answers, alert tone off.

Conversation Conversation Fwd/Rev traffic Ch

Mobile originated callMS BTS


CALL PROCESSING (Base Station to Mobile Station)

Sends page message Receives page messagePaging Channel

Receive Paging Response Sends paging Response message

Access Channel

Sets up traffic chReceives traffic data frames

Paging ChStarts sending null Traffic data

Rev traffic ChSends channel assignment

Acquires rev tfc ch sends bs ack order

Fwd traffic Ch

Sets up reverse traffic ch, sends traffic channel preamble

Process primary traffic data

Sends alert with info msg Fwd traffic Ch Starts ringing, user answers, stops ringing

Receives connect order Rev traffic ChSends connect order

Conversation Conversation Fwd/Rev traffic Ch

Mobile Terminated callBS MS


Mobile initiated call disconnect

MS

Detects user initiated , sends Release order

Enters system determination sub state, of mobile station initialization state

Sends release order

Rev Tfc Ch

Fwd Tfc Ch

BS


Base Station initiated call disconnect

BS

Detects user initiated ,

sends Release order

Enters system determination

sub state, of mobile station

initialization state

Fwd Tfc Ch

Rev Tfc Ch

MS


CALL PROCESSING DURING SOFT HAND OFF

Mobile Station Base Station

< User conversation using A > < User conversation using A >

Pilot B level exceeds T_ADD sends “pilot strength Measurement Report”

A receives measurement report, B starts sending data

Receives hand off direction message ( Starts using active

set A&B)

acquires reverse traffic channel, A and B send hand off direction message to use A&B

Sends hand off completion message

A&B receive completion message

Hand off drop timer for pilot A expires

A&B receive measurement report

Continued….

RTCH

RTCH

RTCH

FTCH


CALL PROCESSING DURING SOFT HAND OFF

Mobile Station Base Station

Mobile sends measurement report

A&B send hand off direction message to use pilot B only

Receives hand off direction message

Sends hand off completion message

• A&B receive completion message

• A stops transmitting on fwd ch and receiving on rev traffic channel

< User conversation using B > < User conversation using B >

RTCH

FTCH

RTCH

F/RTCH


AUTHENICATION


Authentication is a process by which Base station confirms the identity of mobile station.

There is 128 bit data called Shared Secret Data (SSD), which is stored in semi permanent memory of mobile.

We can say authentication is successful only when MS and BS pocess same SSD.


Authentication parameters

The parameters of Authentication are as follows:

1. Random challenge number (RAND)

2. Electronic Serial Number (ESN)

3. Mobile Identification Number (MIN)

4. Shared Secret Data (SSD)5. Call history parameters (count s.p)


1. Random challenge Number(RAND) :

It is a 32 bit sequence send by Base Station. This is sent on access parameter in paging channel used along SSD and other parameters for authenticating mobile.


2. Electronic serial Number (ESN) :

It is a 32 bit sequence that uniquely defines mobile. Its bits (0 to 17) are for serial number. Bits (18 to 23) are reserved, and remaining bits (24 to 31) are manufacturer code.

01718232431

ReservedManufacturer code

Serial number


3. Mobile Identification Number (MIN) :

It is a 34 bit sequence. First 24 bits (LSBs) are called MIN1 and remaining bits (MSBs) are called MIN2.

MIN 1MIN 2

0232433


4. Shared Secret Data (SSD) :

It is 128 bit data stored in MS, similar to K i in GSM. The first subset of 64 bits are called SSD-A, and are used for authentication purpose.

The next 64 bits are called SSD-B, and are used for supporting ciphering procedure.

SSD – B (64 bits)SSD – A (64 bits)


5. Call History parameters (Count s.p):

This is a modulo-64 count held in a semi permanent memory of mobile and is updated upon receipt of a parameter update order from Base Station.


Authentication procedure

Field AUTH in the system parameter message is set to 1 for enabling standard Authentication mode. Mobile uses Random number (RAND), ESN, MIN, SSD-A, MIN-1 data for AUC process.

It runs Authentication procedure to generate an 18 bit long AUC signature, through the AUTHR field in registration message.


The mobile sends AUTHR and RAND C (8 MSB) of RAND to Base Station. Base Station compares the RAND C received from mobile with its internally stored value of RAND, infact it is derived from RAND C coming from mobile.

Base Station also retrieves the ESN & MIN of mobile from its data base based on count value received from mobile.

It runs authentication process locally by using the internally stored SSD-A and generates its own AUTH R, AUTHRbase


If AUC response AUTHRmobile matches AUC response of base AUTHRbase , then Authentication is successful.

If AUC fails, then Base Station may either do a Unique Challenge response or initiate an “SSD Update”.


SSD-AMIN1ESNRAND SSD-AMIN1ESNRAND

Authentication algorithm


AUTHRmobile

(18 Bits)

AUTHRbase

(18 Bits)

AUTHRm=AUTHRb

?

RANDCRANDC

YES NO

Authentication Successful

Perform Unique Challenge response or SSD Update

Procedure

Mobile Station End Base Station End


Unique Challenge response :

Initiated by Base Station in the event of unsuccessful authentication attempt.

This can be done either on paging or access or forward or reverse Traffic channel. The base Station sends to MS an Authentication Challenge message.

It generates 24 bit data called RANDU and sends it on challenge message. The mobile sets AUC parameters using 24 MSBs of RANDU and 8 MSBs of MIN2 in its RAND field.


The mobile performs an AUC procedure and returns AUTHR to Base Station. The base station also does a similar calculation using internal parameters including SSD-A.

If comparison fails, then Base Station may either deny further access to mobile or drop the call in progress or initiates an SSD procedure.


SSD-AMIN1ESNRANDU



AUTHRmobile

(18 Bits)

AUTHRbase

(18 Bits)

AUTHRm=AUTHRb

?

YES NO

Authentication Successful

Deny access or drop call in progress or initiate SSD update.


MIN2SSD-AMIN1ESNRANDU MIN2


SSD Update Procedure:

When Authentication fails, initiated by Base station, as SSD update procedure is used along with mobile specific data and authentication key.

The authentication key of mobile is 64 bits long and is unique to mobile. It is known only to mobile and HLR, similar to Ki in GSM.

The Base Station sends an SSD Update message either on paging channel or forward traffic channel . It generates RAND, SSD number and sends it on SSD update message.


Both mobile and B.S performs AUC procedure to get AUTH values and these are compared. For its comparison the BS sends its AUTHbs through a BS challenge confirmation order.

If comparison match then mobile performs an SSD update procedure at end of which it sends an SSD update conformation order to BS. It also sets the SSD-A, SSD-B values to new values. Base Station also sets its corresponding new values.


If comparison fails, then mobile discards the new values of SSD-A, SSD-B and sends an SSD update rejection order to Base Station.

Again if Mobile doesn’t receive Base Station confirmation order with in a time limit set by timer (10sec), new values are discarded and update procedure is terminated.


A – Key (64 bits)

ESN

SSD - Generation SSD Generation

AUTHBSm=AUTHBSb

?

YES

SSD Update confirmation Order

SSD Update Rejection Order


ESNRANDSSD (56 bits)

A – Key (64 bits)

RANDSSD (56 bits)

SSD – A new

SSD – B new

SSD – A new

SSD – B new

AUC process

AUC processRAND BS BS

challenge order

BS challenge confirmation


CDMA 2000


• CDMA2000 is the collective name for a group of standards that are based on technology enhancements to the 2G CDMA technology, CDMAone (IS95a).

• They have been designed to offer operators 3G data rate performance on their existing CDMAone networks, with minimal infrastructure changes.

• As all the CDMA2000 technologies can meet the requirements outlined in the IMT-2000 proposals of what data rate performance constitute a 3G system, they are being marketed as 3G networks.


CDMA2000 1xRTT, the core CDMA2000 wireless air interface standard, is known by many terms: 1x, 1xRTT, or IS-2000.

The designation "1xRTT" (1 times Radio Transmission Technology) is used to identify the version of CDMA2000 radio technology that operates in a pair of 1.25-MHz radio channels (one times 1.25 MHz, as opposed to three times 1.25 MHz in 3xRTT).

1xRTT almost doubles voice capacity over IS-95 networks. Although capable of higher data rates, most deployments have limited the peak data rate to 144 kbit/s. While 1xRTT officially qualifies as 3G technology, 1xRTT is considered by some to be a 2.5G(or sometimes 2.75G) technology.

This has allowed it to be deployed in 2G spectrum in some countries which limit 3G systems to certain bands.


• 1xRTT:CDMAone system enhancement with

twice the number of Walsh Codes, coupled improved power control, voice codecs and other system improvements to give a effective data rate in the range of 40-70kbps.


• 1xEV-DO:Enhancement to 1xRTT system that

can support asymmetric non-real-time data packet services of up to 2.4Mbps in the forward link and 153.3Kbps in the reverse link. The system requires the network operator to dedicate a separate RF carrier for the data channel.


• 1xEV-DV:Enhancement to 1xRTT system that

allows support of voice and packet-data services on the same 1.25MHz spectrum of a single RF carrier. Theoretically, providing a peak data rate of 5Mbps and an average throughput of 1.2Mbps.


• 3xRTT: Uses three 1x CDMA channels (of

1.25MHz) grouped together to provide data rates of 384Kbps with a peak of 2Mbps. As this is less than 1xEV-DV and uses much more spectrum, many have questioned whether 3xRTT will be developed, or may be modified to provide even higher data rate services.


Changes Needed in CDMA2000 Compared to CDMA (IS-95)

• 1. The MSC-VLR and HLR (All the circuit switched Core networks) need to software upgrades in order to support the authentication and authorization of the packet data network. Note, that it is still the CS-CN (Circuit Switched Core Network), which authenticates and authorizes the wireless access of the user during packet session initiation. The MSC-VLR/HLR are updated with the Packet data user profile information. The information is then downloaded from the HLR to the VLR of the associated network switch during the successful registration process.


• 2. HW upgrade in the BTS – A new digital processing HW “CDMA2000 Chipset” which is a Multimode Channel Element card + SW upgrade.

• 3. New MS with new digital processing (“CDMA2000 Chipset”) that supports radio channels to carry voice, Circuit switched data along with the packetdata services that requires low and high data rates.

• 4. SW update in the BSC in order to support IP routing.

• 5. Apart from routing the Time division multiplexing (TDM) traffic to the circuit switched platform, the BSC routes the packet to and from the PDSN.

• 6. A new Core network for CDMA2000 – PS-CN.


Roaming in CDMA network (RELIANCE)


• Reliance Infocomm has launched international roaming services that will allow Reliance IndiaMobile(RIM) customers to roam internationally on GSM networks across 300 mobile networks in 172 countries.

• RIM customers traveling abroad can subscribe to the service at Reliance WebWorlds. A SIM Card will be provided to the customer. This SIM card when used with a GSM handset will enable a RIM customer to receive all calls made to his/her number as well as make outgoing calls while travelling in any of the 172 countries. RIM customers using Removable User Identification (RUIM) Card enabled handsets such as GTran GCP 4020 and Telson TWC 1150 can activate International roaming feature on their RUIM card and use it with a GSM Handset.


• International Roaming facility will allow a RIM subscriber to receive all his incoming calls and SMS messages sent on the RIM number in India on his GSM SIM card/RUIM card provided by Reliance.

• "It is for the first time that a global roaming facility has been made available by a CDMA operator on GSM networks on such a large scale. With this unique roaming service, Reliance India Mobile is adding another dimension to its services, where RIM customers can get the dual benefit of receiving calls or SMS outside India forwarded from their existing number and getting a consolidated bill for their global roaming as well as well as regular calls at their billing address in India," said S P Shukla, President - Wireless Products & Services, Reliance Infocomm Ltd.


• A monthly rental of Rs 99 will be charged for the service. Charges for calls made or received while roaming shall be as levied by the foreign network selected by the customer per standard global roaming norms. The SIM will be issued free of charge and the RUlM card is already available with the customer. A roaming deposit will also be charged depending upon the creditworthiness of individual customers for availing International Roaming.

• "The facility will be initially offered through Reliance WebWorlds in top 9 cities which include Mumbai, Delhi, Kolkata, Chennai, Banga!ore, Hyderabad, Ahmedabad, Jaipur and Pune. Other major locations would be covered over the next few weeks," added Shukla.


CDMA2000 1xRTT, the core CDMA2000 wireless air interface standard, is known by many terms: 1x, 1xRTT, or IS-2000.

The designation "1xRTT" (1 times Radio Transmission Technology) is used to identify the version of CDMA2000 radio technology that operates in a pair of 1.25-MHz radio channels (one times 1.25 MHz, as opposed to three times 1.25 MHz in 3xRTT).

1xRTT almost doubles voice capacity over IS-95 networks. Although capable of higher data rates, most deployments have limited the peak data rate to 144 kbit/s. While 1xRTT officially qualifies as 3G technology, 1xRTT is considered by some to be a 2.5G(or sometimes 2.75G) technology.

This has allowed it to be deployed in 2G spectrum in some countries which limit 3G systems to certain bands.


The main differences between IS-95 and IS-2000 signaling are: 64 more traffic channels on the forward link that are orthogonal to the original set.

Some changes were also made to the data link layer to accommodate the greater use of data services—IS-2000 has media and link access control protocols and QoS control.

In IS-95, none of these were present, and the data link layer basically consisted of "best effort delivery" (which is still used for voice).


• In India, BSNL, Reliance and Tata Teleservices are major wireless services providers on CDMA 2000 1x.

• In Indonesia, Mobile-8 is the major mobile wireless service provider on CDMA 2000 1x (with EVDO in Western Java). The other CDMA providers are fixed wireless (such as Bakrie Telkom, Telkom Flexi, and Indosat's Starone).

• In People's Republic of China, China Unicom is the major mobile wireless service provider on CDMA 2000 1x.

• In Bangladesh, Pacific Telecom's CityCell is the only CDMA mobile connection.

• In Venezuela, Movilnet and movistar are the country's wireless services providers on CDMA 2000 1x.

• In Pakistan, PTCL , World Call ,GoCdma provides cdma connections..


• CDMA2000 3x

• CDMA2000 3x utilizes a pair of 3.75-MHz radio channels (i.e., 3 X 1.25 MHz) to achieve higher data rates. The 3x version of CDMA2000 is sometimes referred to as Multi-Carrier or MC. The 3x version of CDMA2000 has not been deployed and is not under development at present.


CHANNELS


Forward Link Channels in CDMA 2000

BTS MOBILE STATION

F_PICH

F_PCH

F_SYNC

F_FCH

F_SCH

F_QPCH


F_PICH :

This channel is continuously broadcast through out the cell in order to provide timing and phase information.

F_PICH is shared by all traffic channels and is used for detecting multipath rays so that RAKE fingers efficiently assigned to the strongest multi path, estimating channel gain and phase.


F_PCH :

A CDMA 2000 system can have multiple paging channels per base station. This channel carries overhead messages, pages, acknowledgements, channel assignments, status requests and updates from the BTS to the MS.

F_SYNC:

Used by mobiles operating within the coverage area of the base station to acquire initial time synchronization.


F_FCH : (Fundamental channel)

Basic Low rate channel used for voice, low data rate, signaling, and power control.

F_SCH : (supplemental channel)

High rate data channel.

F_QPCH : (quick paging channel)

Used for alerting mobiles in standby mode that a page is coming on the paging channel.


Reverse link Channels in CDMA 2000

BTS MOBILE STATION

R_PICH

R_ACH

R_DCCH

R_FCH

R_SCH


R_PICH : Pilot Channel

Pilot used for power control.

R_ACH : Access Channel

Access the channel

R_DCCH : Dedicated Control Channel

Signaling channel.

R_FCH : Fundamental Channel

basic low rate channel used for voice, low rate data, signaling and power control.

R_SCH : Supplemental Channel

High data channel.


Additional CDMA 2000 feature

• Improved spreading: Hybrid QPSK, complex spreading method, used to improve power efficiency.

• Fast Power Control in Forward Link : Improved multipath resistance and orthogonality.

• Turbo codes: Variation of convolutional coding, supplemental channels for higher data rates.

• More Walsh codes : 128.


Goals of IMT – 2000 :

• Global Standard• High speed packet data services.

2 Mbps for stationary use.

384 kbps for pedestrian use.

144 kbps for vehicular use.

• 3G applications enabled by cdma 2000.

Packet based services like, wireless internet, e-mail, e-commerce and multimedia. Location based services, longer battery life.


Outline

• cdma2000 network architecture

• Call processing states and call flows

• CDMA evolution

• Essential elements in a CDMA system

• Power Control

• Mobility management

– Handoffs

– Registration

– Roaming

• Radio Admission Control


Network Architecture

Ericsson

Black

Mountain

UCSD

MSC

BSC

PSTN Packet Network

PDSN


Call Processing - Pilot

First MS monitors Pilot channel for

– Initial acquisition

– Channel estimation

– Detection of multipaths for rake receiver

– Handoffs

Pilot Ch


Call Processing - Sync

Pilot channel is transmitted at all times by the base station. MS uses it to lock to Synch Channel to

• Synchronize to CDMA system time

• Obtain configuration parameters such as

– Protocol Revision (P-REV)

– Network Identifier (NID)

– Pilot PN offsetLong-code state

– Paging channel data rate

Sync Ch


Call Processing - Paging

MS decodes the Paging Channel with the information received from the Sync Channel. Paging channel provides

– Overhead messages: systems parameter, access parameter, neighborlist, channel list

– Mobile directed messages: page request, SMS Paging Ch


Call Processing – Access

MS uses Access channel to originate a call or to respond to a page request.

Access Channel is used in a random access fashion.

Access Ch


Call Processing - Traffic

• Base station assigns a forward and reverse traffic channel to the mobile when it is in conversation

• Traffic Channel conveys signaling and traffic information

• When MS is on traffic channel it no longer listens to paging channel or uses the access channel


Mobile Station States

Power Up

Initialization

State

Access

State

Traffic

State

Synchronization

Paging Loss

Call origination

or page response

Page response completed

End of call

Idle

State


CDMA Evolution (1/3)

• IS-95A (2G)– First CDMA protocol, published in May’99

– 14.4/9.6 kbps circuit/packet data

• IS-95B (2.5G)– Most analog information is removed

– Some technical corrections

– New Capabilities, such as higher data rate

– 64 kbps packet data



• CDMA2000 1X– High speed data (144 kbps packet data with Mobile IP)

– Coding (Turbo) and Modulation (Hybrid QPSK)

– New dedicated and common channels

– Enhanced Power Control

– Reverse link detection

– Forward link modulation



• 1X EV-DO (1xRTT Evolution for high-speed integrated Data Only)

– The objective is to provide the largest practical number of users to run high-speed packet data applications

– 2.4 Mbps packet data

• 1X EV-DV (1xRTT Evolution for high-speed integrated Data and Voice)

– Voice and High Speed Data mixed on one carrier

– Backward-compatible with CDMA2000 1X

– 3.1 Mbps packet data


Multiple Access Methods

Dedicated band during entire call

Certain frequency, time-slotted

Each user transmits at the same time,

at the same frequency with a unique

code


Frequency Re-use Patterns

FDMA and TDMA vs. CDMA

A

A

A

A

A

A A

A

A

A

A

A

A

D

C

G

B

E F

E

G

F

B

A


Channelization

• Channelization is provided by orthogonal Walsh codes

• cdma2000 uses variable length Walsh codes for supplemental channel data services

• Walsh codes can be of length 8, 16, 32, 64, and 128


Walsh Codes

• Walsh codes are orthogonal to each other

• The shorter the code the higher the data rate since the chip rate is kept constant

1

10 11

1001 1010 1100 1111


A Code Channel Example

Forward Traffic Channel Generation

Encoder and

Repetition

Block

Interleaver

Long Code

PN GeneratorDecimator

MU

X

Decimator

Power Control bit

Wt1.2288 Mcps

19.2 ksps

800 Hz

1.2288 Mcps


Cell Separation

• Walsh code spreading is followed by quadrature spreading using PN chips with time offsets

• Adjacent cells have different PN offsets.

• This prevents interference since time shifted PN sequences are orthogonal to each other

I-PN

Q-PN

Wt Baseband

Filter

BasebandFilter

sin wot

cos wot


Use of Multipath in CDMA Systems

• FDMA/TDMA (narrow-band)– multipath hurts

– equalizers are used to cancel multipath

• CDMA (wide-band)– can discriminate between the multipath arrivals

– Rake receivers are used to combine multipath signals to reduce error rate at the receiver


Near – Far Problem

A user near the base station would jam the user far from the base station


Power Control – Motivation

• Overcomes near-far problem

• CDMA wouldn’t work without it

• Copes with path loss and fading


Power Control – Algorithm

• Capacity is maximized

– By having each user transmitting just sufficient SNR to maintain a target FER

• Open Loop Estimate

– Initial transmit power level for the mobile is determined by the received pilot strength

• Closed Loop Power Control– Base station controls the power level on the mobile by the received quality information.


Mobility management

A CDMA system provides mobility:

• Handoff – continuity of the service across adjacent cells

• Registration – locating the mobile user

• Roaming – continuity of the service across different service providers


Handoff• Handoffs between cells are supported while the mobile is in traffic or idle

• MS continuously keeps searching for new cells as it moves across the network

• MS maintains active set, neighbor set, and remaining set as well as candidate set

• There are 4 types of handoffs:

– Idle Handoff

– Access Handoff

– Soft/Softer Handoff

– Hard Handoff


Handoff Parameters (1/2)

• If a pilot strength (P) is greater than T_ADD it will be added into the candidate set

• If pilot strength is less than T_DROP for T_TDROP, it will be moved from active set to neighbor set

• If the new pilot strength is T_COMP units greater current pilots a Pilot Strength Measurement Message will be send


Handoff Parameters (2/2)

T_DROP

T_ADD

Pilot Strength

TimeT_TDROP

Cell B

Cell A


Soft Handoff

Ericsson

Black

Mountain

UCSD

MSC

BSC

PDSN

Both cells have the same frequency


Soft Handoff

• Make-before-break• Both cells are at the same frequency

• Reduces number of call drops

• Increases the overall capacity• Mobile transmit power is reduced

• Voice quality near the cell boundaries are improved

• MS reports the SNR of the candidate sets


Soft Handoff Gain

Power

(dBm)

Distance

Cell A

Cell B

Total at MS


Hard Handoff

• Break-before-make

• Handoff between different frequencies, non-synchronized or disjoint cells which are controlled by different BSCs


Registration

• It is sufficient to know the cell or the region that a MS is active for routing purposes

• Mobile station identifier, desired paging slot cycle, and registration type is conveyed

• Cell/LAC based paging is preferred to flood paging


Registration Types

• Autonomous Registration: power-up, power-down, timer-based, distance-based, zone-based registration.

• Parameter-change registration

• Ordered registration• Implicit registration

• Traffic channel registration


Roaming

• Users that are outside their home area can receive service from another system by paying some additional charges

• Mobile station can be:

– Home state (not roaming)

– Network roaming

– System roaming

Network 1

Network 2

Network 3System


Radio Admission Control

• cdma2000 allocates resources dynamically

• Admission control is important to ensure quality of service for the existing users when new resources are requested

• A new request can be call setup, supplemental channel set-up, handoff, data rate change

• Available Walsh codes, residual power in the forward and reverse links are considered before granting a request


Summary (1/2)

• cdma2000 supports both voice and data services in the same carrier

• provides enhanced voice capacity– Forward link

• Fast power control in forward/reverse links• Lower code rates•New code channels

– Reverse Link

• Coherent detection


Summary 2/2

• Higher data rates: 1x up to 153.6 kbps and 1x EV-DV up to 3.09 Mbps

• Battery life is improved

• Introduction of Turbo codes provides better link quality for supplemental channels


THANK YOU…..

cdma

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