cdma notes for hussain personal references

8/12/2019 Cdma Notes for Hussain personal references

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1. Handoffs in CDMACDMA systems support handoffs of the mobile from one cell to another while the mobile is

in the Idle state, the Access state, or the Traffic Channel state:

1. Idle Transition from one cell to another while in the Idle state must be a hard

handoff.

2. Access Handoffs during Access are permitted only in TIA/EIA-95, but not in IS-

95A.

3. Traffic The in-traffic transition from one cell to another can be either a soft handoff

or a hard handoff.

1.1 Idle Handoff

While in the Idle state, the mobile may move from one cell to another. Idle handoff arises

from the transition between any two cells. Idle handoff is initiated by the mobile when it

measures a Pilot signal significantly stronger (3 dB) than the current serving Pilot.

1.3.1 Types of Handoff:

1.3.1.1 Mobile assisted Soft Handoff Soft handoff is the process of establishing a link with a target cell before breaking the link

with a serving cell. Mobiles continuously search for Pilot Channels on the current frequency,

to detect potential candidates for handoff. Both Cells must be on the Same Frequency The mobile typically contains only one RF receiver. Therefore soft handoff requires that both

the serving and the target cells be transmitting on the same CDMA frequency. All Cells Deliver Vocoded Frames to the BSC Each Base Transceiver Station (BTS) participating in a soft handoff transmits identical

frames. The mobile combines these frames and then forwards a single frame to the vocoder.

On the Reverse link, each BTS independently decodes and then delivers vocoded frames tothe Base Station Controller (BSC). 1.3.1.2 Softer Handoff Softer handoff is a soft handoff between two sectors of the same cell. Signals received by

different sectors can be combined by the rake receiver in the BTS. It should be noted,

however, that only one voice frame is eventually forwarded to the BSC. Softer handoff

enables greater efficiency in the use of hardware since only one Channel element is used to

support such a handoff. 1.3.1.3 Soft-Softer Handoff :


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Multiple cells and sectors may be involved in a handoff in a variety of ways. The figure

depicts a scenario where a mobile is in softer handoff with two sectors of one cell and is also

in soft handoff with another cell. The BSC will receive a vocoded frame from each cell and

choose the error-free one.

1.3.1.4 Hard handoff A hard handoff entails a brief disconnection from a current serving cell prior to establishing a

connection with a target cell. Hard handoffs can occur for several reasons.

The figure illustrates a hard handoff from a CDMA system to an analog system. Hard

handoffs, however, may also occur between CDMA cells. CDMA-to-CDMA hard handoffs

are due to frequency mismatches, frame offset misalignment, or disjoint cells (cells served by

different BSCs).

2. Pilot Searching Process

The Mobile Searches for Strong Pilot Signals

The searching process is continuous and is conducted not only to find handoff candidates, but

also to identify usable multi path arrivals from the serving cell.

The Mobile Reports

The handoff process is mobile-assisted, meaning that when the mobile detects a Pilot of

sufficient strength, it reports the event to the Base Station.

The Base Station Directs

When the Base Station receives a report from the mobile, a handoff decision is made and

directions are sent to the mobile to perform the handoff.

3. Pilot Set Pilots are grouped into four sets, which prioritize them and increase the efficiency of

searching. The searching process is not standardized, but generally Pilots are searched in the

following order:

Active Set Pilot Channels associated with forward Traffic Channels currently assigned to

the mobile. This is a search for additional multi paths of the same Pilot Channels.

Candidate Set Pilot Channels whose strength, as measured by the mobile, exceeds a given

threshold.

Neighbor Set Pilot Channels transmitted by cells in the vicinity of the cells currently

transmitting to the mobile. The contents of the Neighbor Set are normally configured by the

system operator, by means of the Neighbor List Message.

Remaining Set All other Pilot Channels that are possible within the current system.


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This search is conducted to allow the system to configure itself as well as to account for

special coverage spots within the cell.

Search Windows

The system operator determines the size of the search windows used by the mobile.Searching over a window of chips accommodates unpredictable changes in propagation delaydue to varying multipath conditions and propagation delay differences between the serving cells and other cells that may be useful in the future.

Multi path Arrivals:

The figure depicts the signals arriving from three different cells. The horizontal axis is time,

in PN chips. The vertical axis is the Pilot signal-to-noise ratio, E c/I0, in dB.

4. Handoff Signaling Messages

Pilot Strength Measurement Message (PSMM)

Handoff Direction Message (HDM)

Handoff Completion Message (HCM)

5. Handoff Signaling Parameters :

Pilot Detection Threshold :( T_ADD)
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Any Pilot that is strong but is not in the Handoff Direction Message is a source ofinterference. This Pilot should be immediately moved into the active set for handoff to avoidvoice/data degradation or a possible drop call. T_ADD affects the percentage of MS inhandoff. It should be low enough to quickly add useful Pilots and high enough to avoid falsealarms due to noise.

Pilot Drop Threshold :( T_DROP) This affects the percentage of MS in handoff. It should be low enough to avoid dropping agood Pilot that goes into a short fade. It should be high enough not to remove quickly usefulPilots in the active or candidate state. Pilot Drop Timer :( T_TDROP) This is a timer. Whenever the strength of a Pilot in the active set falls below a value ofT_DROP, a timer is started by the MS. If the Pilot strength goes back above T_DROP, thetimer is reset; otherwise the timer expires when a T_TDOP has elapsed since the Pilotstrength has fallen below T_DROP. Every MS maintain a handoff drop timer for each Pilot inthe Active and Candidate Sets. The Comparison Threshold: T_COMP An additional parameter, T_COMP, is used to control handoff signaling. When the strengthof a new Pilot exceeded the strength of the current serving Pilot by the amount of thecomparison threshold, the MS will signal the BTS.

The Mobile Adjusts Pilot Priorities as Necessary

When the strength of a Pilot rises above T_ADD, the mobile will autonomously add that Pilot

to its Candidate Set and signal the system by sending a PSMM. If the system directs the

mobile to handoff, the new Pilot will be added to the mobiles Active Set. When the strength

of the Pilot falls below T_DROP for a sufficient period of time (T_TDROP), the mobilesignals the Base Station with a PSMM.

6. Transition between Pilot Sets

This graph illustrates the soft handoff process. The steps shown in this diagram are: 1. Pilot 2>T_ADD.MS sends PSMM (Pilot Strength Measurement Message) and addsPilot 2 to the Candidate Set. 2. Pilot 2>Pilot1+T_COMP*0.5. MS sends another PSMM. BTS decides to add Pilot2 to the Active Set and sets up the soft handoff.

3. MS receives message and moves Pilot 2 to the Active Set. 4. Pilot 1


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5. Drop timer expires. MS sends PSMM indicating that Pilot 1 should be dropped. 6. MS receives message indication that Pilot 1 has been dropped and moves Pilot 1 tothe Candidate Set. 7. Pilot 1< T_DROP. MS starts a new drop timer. 8. Drop timer expires. MS moves Pilot 1 to the Neighbor Set.

7. Handoff Problems: Window Dropped Calls Calls are dropped during Hand off is due to:

Calls often drop when strong neighbors suddenly appear outside the neighbor

search window and cannot be used to establish soft handoff.

Neighbor Search Window SRCH_WIN_N should be set to a width at least twice

the propagation delay between any site and its most distant neighbor site

Remaining Search Window SRCH_WIN_R should be set to a width at least twice

the propagation delay between any site and another site which might deliver

occasional RF into the service area

CDM Signal Generation Process

A CDMA signal is generated by spreading the code symbols by a wideband code sequence

that is produced at a rate of 1.2288 Mcps (Megachips per second). This spreading of the

information signal provides a substantial gain that is referred to as Processing Gain . The
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Processing Gain is defined as the ratio of the bandwidth to the information rate (W/R).

Processing Gain:

Processing gain is the ratio of chip rate to the bit rate.

The processing gain in IS-95 system is 128, about 21dB. The processing gain is calculated as follows:

10*log 10128=21db

The Bit

A bit is the fundamental unit of information: a single binary digit. Analog information is

encoded into a sequence of binary digits (A/D conversion). Both user data and error detection

code digits are considered bits. The bit rate (bits per second) is a measure of the volume of

information being transmitted.

The Code Symbol

In cdmaOne systems, a code symbol is the output of the coding process (Forward Error

Correction [FEC]). Each bit produces several code symbols. The symbol rate is a measure of

the redundancy introduced by the FEC scheme. Each symbol is also a single binary digit. The Chip

The output digits of a spreading code generator are commonly termed chips . A chip is also a

single binary digit. Several chips are used to spread a single code symbol. The chip rate is a

measure of the amount of spreading performed.

Bits, symbols, and chips all look the same: a single binary digit. What distinguishes one from

another is their relationship to the information signal.

Pseudorandom Noise (PN) Codes
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PN codes are deterministic codes that mimic randomness properties. The state of the code

resembles the outcome of tossing a two- sided coin with 1 and 0, rather than heads or

tails However, if the current state and the generating function of the PN code are known,

the future state of the code can be predicted. The two short codes and one long code used incdmaOne systems are time-synchronized to midnight, January 6, 1980 (GPS time). In

cdmaOne systems, all Base Stations and all mobiles use the same three PN sequences.

Note: For CDMA2000 Spreading Rate 3, the short code length is 3 times the short code

length given above or 3 x 2 15 in length. Spreading Rate 3 is discussed in the CDMA2000

Overview section of this course.

Types of PN Codes:

1. Short PN code 2. Long PN code

1: Two Short Codes (215 = 32,768)

o Short code is a PN sequence with period of 215 chips

Termed I and Q codes (different taps)

Used for Quadrature Spreading

Unique offsets serve as identifiers for a Cell or a Sector

Minimum PN sequence offset used is 64 chips, that is, 512 PN offsets areavailable to identify the CDMA sectors (215/64=512).

Repeat every 26.67 msec (at a clock rate of 1.2288Mcps)

2: One Long Code (242 = 4400 Billion)

In a reverse direction, different long codes are used for the information sent by different users

and these are known to the base station and these users. Thus, the base station can identify

different mobile stations.

o The long code is a PN sequence with period of 242-1chips
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o Repeats every 41 days (at a clock rate of 1.2288Mcps)

o Long PN codes have a length of 2 44 chips.

o The functions of a long code:

Scramble the forward CDMA channel Control the insertion of power control bit

Spread the information on the reverse CDMA channel to identify the mobile stations

Autocorrelation of a Pseudorandom Noise Code

PN sequences have an important property: time-shifted versions of the same PN sequence

have very little correlation with each other. Autocorrelation is the measure of correlation

between a PN code and a time-shifted version of the same code. The figure shows the

autocorrelation1 function, and it is clear that it is a two-valued function. As long as the timeshift is greater than the chip time, correlation is very small. The channelization of users in the

Reverse link is accomplished by assigning them different time shifted versions of the long

code, thus making them uncorrelated with each other. This property is then exploited to

separate subscribers signals in the BTS rece ivers.

PN Code Generation:

PN codes are generated from prime polynomials using modulo_2 arithmetic. The state

machines generating these codes are very simple and consist of shift registers and XOR gates.
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Shift Registers:

PN codes are maximum length. In general, if there are N shift registers (N = number of shift

registers), the length of the PN code is equal to 2 N-1.

In this example, the number of distinct states in the shift registers is 2 3-1=7.

PN Offset (Masking)

Masking provides the shift in time for PN codes. Different masks correspond to different time

shifts. In cdmaOne systems, Electronic Serial Numbers (ESN) are used as masks for users on

the Traffic Channels.
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Sequence Produced by a Masked Generator:

This example illustrates how a mask produces the same original sequence shifted in time.

The content of the 3-digit mask determines the offset of the sequence. Masking is used to produce offsets in both the short codes and the long code. The offsets of the short PN codes

are used to uniquely identify the Forward Channels of individual sectors or cells. The offsets

of the Long PN code are used to separate code channels in the reverse direction.

Walsh CodesOrthogonal codes are easily generated by starting with a seed of 0, repeating the 0

horizontally and vertically, and then complementing the 1 diagonally. This process is to be
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continued with the newly generated block until the desired codes with the proper length are

generated. Sequences created in this way are referred as Walsh code.

The Walsh code is used to separate the user in the forward CDMA link. In any given sector,

each forward code channel is assigned a distinct Walsh code.

64-order Walsh function is used as a spreading function and each Walsh code is

orthogonal to other.

A Walsh can be presented by Wim where ith (row) is the position and m is the

order. For example, W24 means 0101 code in W4 matrix

In forward direction, each symbol is spread with Walsh code

Walsh code is used to distinguish the user in forward link

For IS95A/B, in the reverse, every 6 symbols correspond to one Walsh code. For

example, if the symbol input is 110011,the output after spreading is W5164(110011=51).

For CDMA2000, in the reverse, Walsh function is used to define the type of

channel (RC 3-9)

Orthogonal Functions:

Orthogonal functions (that is, signals or sequences) have zero cross-correlation. Zero

correlation is obtained if the product of two signals, summed over a period of time, is zero.

For the special case of binary sequences, the values 0 and 1 may be viewed as havingopposite polarity. Thus when the product (XORing in this case) of two binary sequences

results in an equal number of 1s and 0s, the cross-correlation is zero.

Creating Orthogonal Functions:

Orthogonal codes are easily generated by starting with a seed of 0, repeating the 0

horizontally and vertically, and then complementing the 0 diagonally. This process is

continued with the newly generated block until the desired codes with the proper length are

generated. Sequences created in this way are referred to as Walsh codes .
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Variable Walsh codes:

Enhancements of CDMA2000 include the use of Walsh spreading factor to attain high data

rates on the forward link. Variable Walsh spreading uses the tree structure for recursively

constructing Walsh codes of the longer lengths. Higher data rates for the user can beobtained by using the shorter Walsh code. However, use of one of the shorter codes

precludes using any longer code that is derived from it.

What is GSMGSM (Global System for Mobile communications) is an open, digital cellular technology used fortransmitting mobile voice and data services. GSM is a standard set developed by the EuropeanTelecommunications Standards Institute (ETSI) to describe technologies for second generation (2G ) digital cellular networks.

What does GSM offer?

GSM supports voice calls and data transfer speeds of up to 9.6 kbps, together with the transmissionof SMS (Short Message Service).

GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in
http://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/2Ghttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/Cellular_networkhttps://sites.google.com/site/the4gtelecom/walsh-code/variable%20wash%20codes.JPG?attredirects=0https://sites.google.com/site/the4gtelecom/walsh-code/orthogonal%20function.JPG?attredirects=0https://sites.google.com/site/the4gtelecom/walsh-code/variable%20wash%20codes.JPG?attredirects=0https://sites.google.com/site/the4gtelecom/walsh-code/orthogonal%20function.JPG?attredirects=0http://en.wikipedia.org/wiki/Cellular_networkhttp://en.wikipedia.org/wiki/2G


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the US. GSM services are also transmitted via 850MHz spectrum in Australia, Canada and manyLatin American countries. The use of harmonised spectrum across most of the globe, combined withGSMs international roaming capability, allows travelers to access the same mobile services at homeand abroad. GSM enables individuals to be reached via the same mobile number in up to 219countries.

Terrestrial GSM networks now cover more than 90% of the worlds population. GSM satellite roaminghas also extended service access to areas where terrestrial coverage is not available.

cdma notes for hussain personal references

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