wireless networks ch6

7/30/2019 Wireless Networks Ch6

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Ali BAZZI

Chapter 6

Multiple Division techniques


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Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Comparison of FDMA, TDMA, and CDMA Walsh Codes Near-far Problem Types of Interferences Analog and Digital Signals Basic Modulation Techniques

Amplitude Modulation (AM) Frequency Modulation (FM) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Phase Shift Keying (QPSK) Quadrature Amplitude Modulation (QAM)


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User 1

User 2

User n

Time

Frequency

Single channel per carrier All first generation systems use FDMA


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User1

User2

Usern

Time

Frequency

Multiple channels per carrier Most of second generation systems use TDMA


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User1

Time

Frequency

Users share bandwidth by using code sequences that are orthogonal to each other Some second generation systems use CDMA Most of third generation systems use CDMA

User2

User

n

Code

...


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Control channel Forward (Downlink) control channel Reverse (Uplink) control channel

Traffic channel Forward traffic (traffic or information) channel Reverse traffic (traffic or information) channel


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MS BS

f1

f2

fn

f

f

Reverse channel (Uplink)

Forward channels

(Downlink)

f1

f2

fn

Control channels

Traffic channels


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MS #1

MS #2

MS #n

BS

f1

f2

fn

f1

f2

fn

Reverse channels

(Uplink)

Forward channels

(Downlink)


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1 2 3

N

Frequency

Total Bandwidth W=NWc

Guard Band Wg

4

Sub Band Wc

Frequency

Protecting bandwidth

f1 f2 fn

f1 f2 fn

Reverse channels Forward channels


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MS #1

MS #2

MS #n

BS

Reverse channels

(Uplink)

Forward channels

(Downlink)

t

Frequency f

#1 #1

Frame

Slot

#1

#1

Frame

t

Frequency f

Frame Frame

t#2

#2

t#n

#n

#2 #2

t

#n #n

t

TDMA


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t

f

#1

#2

#n

#1

#2

#n

(a). Forward channel

#1

#2

#n

Frame FrameFrame

t

f

#1

#2

#n

#1

#2

#n

(b). Reverse channel

#1

#2

#n

Frame FrameFrame


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Time

Frequency

f = f

#1

#2

#n

#1

#2

#n

Forward

channel

Reverse

channel

#1

#2

#n

Forward

channel

Frame Frame

#1

#2

#n

Reverse

channel

Channels in Simplex Mode


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Time

Frequency

#1

#2

#n

#1

#2

#n #

1#2

#n

Frame FrameFrame

Head Data

Guardtime


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MS #1

MS #2

MS #n

BS

C1

C2

Cn

C1

C2

Cn

Reverse channels

(Uplink)

Forward channels

(Downlink)

Frequency f

Note: Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes, Ci

x Cj = 0, i.e., Ci and Cj are orthogonal codes

Frequency f


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Operation FDMA TDMA CDMA

Allocated Bandwidth 12.5 MHz 12.5 MHz 12.5 MHz

Frequency reuse 7 7 1

Required channel BW 0.03 MHz 0.03 MHz 1.25 MHz

No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10

Channels/cell 416/7=59 416/7=59 12.5/1.25=10

Control channels/cell 2 2 2

Usable channels/cell 57 57 8

Calls per RF channel 1 4* 40**

Voice channels/cell 57x1=57 57x4=228 8x40=320

Sectors/cell 3 3 3

Voice calls/sector 57/3=19 228/3=76 320

Capacity vs FDMA 1 4 16.8

* Depends on the number of slots ** Depends on the number of codes

Delay ? ? ?


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Problem of radio transmission: frequency dependent fading can wipe outnarrow band signals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using aspecial code

protection against narrow band interference

protection against narrowband interference Side effects:

coexistence of several signals without dynamic coordination tap-proof

Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal signal

f f

power

power


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MERITS OF SPREAD SPECTRUM

" Because Spread Spectrum signals are noise-like, they are hardto detect.

" Spread Spectrum signals are harder to jam (interfere with) thannarrowband signals

" Spread Spectrum transmitters use similar transmit power levelsto narrow band transmitters.

" Because Spread Spectrum signals are so wide, they transmit ata much lower spectral power density, measured in Watts perHertz, than narrowband transmitters.


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MERITS OF SPREAD SPECTRUM

spread spectrum signals are hard to exploit or spoof.

Signal exploitation is the ability of an enemy (or anon-network member) to listen in to a network anduse information from the network without being avalid network member or participant.

Spoofing is the act of falsely or maliciouslyintroducing misleading or false traffic or messages toa network.


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dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signal

broadband interference

narrowband interference

dP/df


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(i) Original signal to be transmitted.(ii) The sender spreads the signal and converts the narrow-band

signal to broadband (Power level can be much lower

without losing data)

(iii) During transmission, narrow and broadband noise getsadded.

(iv) The receiver despreads the given signal, narrow bandinterference is spread, leaving the broadband as it is.

(v) Receiver applies a band pass filter cutting off left & right ofnarrow band signal.


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frequency

channelquality

1 2

34

5 6

narrow bandsignal

guard space

narrowband channels


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In the figure above,

Frequencies 1,2 and 5 have reasonably good quality ofservice.

Frequencies 3 & 4 are of very narrow band and they canget corrupted.

Spread Spectrum can help in such a situation.


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22

22

2

frequency

channelquality

1

spreadspectrum

spread spectrum channels


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All narrow band signals are spread into broadband signalsusing the same frequency range

To separate the Channels, CDM is used. Each channel is allocated its own code which the receivers

know.

Because of secret code, spread spectrum acts as a securityprotection.


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Digital signal

s(t)

Code

c(t)

Spreading signal

m(t)

Code

c(t)

Digital signal

s(t)

Spreading Despread

Frequency Frequency Frequency

Power Power Power

Transmitter Receiver

Direct Sequence Spread Spectrum for CDMA


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XOR of the signal with pseudo-random number (chippingsequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal

Advantages reduces frequency selective

fading

in cellular networks base stations can use the

same frequency range several base stations can

detect and recover the signal

soft handover Disadvantages

precise power control necessary

user data

chippingsequence

resulting

signal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit period

tc: chip period


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tc = Chip Period

tb = Bit Period

Spreading factor s = tb/tc

s*original bandwidth is the new bandwidth.

It determines the BW of the resulting signal


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Most civil applications need a spreading factor of 10 to100.

Military applications use a speeding factor of around10,000.


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- Barker Codes are used as a pseudo random numbers(chipping codes). Ex:

- 10110111000 (used in 802.11 wireless LANS)- 11- 110- 1110- 11101- 1110010- 1111100110101


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Let the code to be transmitted be 110.

Let the Chip Barker Code be

10110111000

Hence the transmitted code is:

11111111111 11111111111 00000000000 XOR

10110111000 10110111000 10110111000

- 01001000111 01001000111 10110111000


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At the Receiver : The transmitted signal is XORed with the same

chip sequence.

01001000111 01001000111 10110111000

XOR 10110111000 10110111000 10110111000

Resulting in :

11111111111 11111111111 00000000000

This is the original signal 110


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Digital signal

Hopping Pattern

Spreading signal Digital signal

Spreading Despread

Frequency FrequencyFrequency

Power Power Power

Hopping Pattern

Transmitter Receiver

Concept of Frequency Hopping Spread Spectrum


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Time

Frequency

An Example of Frequency Hopping Pattern


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Wal (0, t) t

Wal (1, t) t

Wal (2, t) t

Wal (3, t) t

Wal (4, t) t

Wal (5, t) t

Wal (6, t) t

Wal (7, t) t


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MS1MS2 BS

Distance Distance0

d2 d1

Received signal strength

MS1MS2 BS

Near-far Problem


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Frequency

Baseband signal

Frequency

Interference baseband signals

Spreading signal

Frequency

Despread signal

Interferencesignals

Interference in spread spectrum system in CDMA

Types of Interference in CDMA


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f1 f2

Channel1 Channel2

Frequency

Power


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Power Control in CDMA

PrPt

=1

4df

c

Controlling transmitted power affects the CIR

Pt= Transmitted powerPr= Received power in free space

d = Distance between receiver and transmitter

f = Frequency of transmission

c = Speed of light

= Attenuation constant (2 to 4)


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Why need modulation? Small antenna size

Antenna size is inversely proportional to frequency

e.g., 3 kHz 50 km antenna

3 GHz 5 cm antenna

Limits noise and interference,e.g., FM (Frequency Modulation)

Multiplexing techniques,e.g., FDM, TDM, CDMA


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Analog Signal (Continuous signal)

Digital Signal (Discrete signal)

Time

Amplitude

Time

Amplitude

1 1 1 10 0

Bit

+

_0

0

S(t)


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Voice-grade

Telephone channel

Human hearingHuman speech

Frequency (Hz)

Frequency (Hz)

Pass band

Frequency cutoff point

Guard band Guard band

100

0 200 3,500 4,000

10,000..


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Message signal

x(t)

Carrier signal

AM signal

s(t)

Amplitude of carrier signal is varied as the message signal to be transmitted.

Frequency of carrier signal is kept constant.

Time

Time

Time


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FM integrates message signal with carrier signal by varying the instantaneous

frequency. Amplitude of carrier signal is kept constant.

Carrier signal

Message signalx(t)

FM signals(t)

Time

Time

Time


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1/0 represented by two different frequencies slightly offset from carrier frequency

Message signal

x(t)

Carrier signal 2for message signal 0

Carrier signal 1

for message signal 1

FSK signals(t)

1 0 1 1 0 1

Time

Time

Time

Time


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Use alternative sine wave phase to encode bits

Carrier signal

Carrier signal

)2sin( +tfc

Message signal

x(t)

)2sin( tfc

1 0 1 1 0 1

PSK signal

s(t)

Time

Time

Time

Time


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Q

I

0,01,1

0,1

1,0

Q

I

01

(a) BPSK (b) QPSK


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Combination of AM and PSK

Two carriers out of phase by 90 deg are amplitude modulated

Rectangular constellation of 16QAM

I

Q

0000010011001000

0001010111011001

0011011111111011

0010011011101010


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special pre-computation avoids sudden phase shiftsMSK (Minimum Shift Keying)

bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency,

original or inverted is chosen

the frequency of one carrier is twice the frequency of the other Equivalent to offset QPSK even higher bandwidth efficiency using a Gaussian low-pass filter

GMSK (Gaussian MSK), used in GSM


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If Even and Odd bits are both zero :

- f2 is inverted.

If Even bit is 1 and odd bit is zero:

- Lower frequency f1 is inverted.

If Even bit is zero and the odd bit is 1:

- f1 is taken without phase change, as is.

If Both even and odd bits are 1 :

- frequency f2 is taken as is.


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data

even bits

odd bits

1 1 1 1 000

t

lowfrequency

highfrequency

MSKsignal

biteven 0 1 0 1

odd 0 0 1 1

signal h n n hvalue - - + +

h: high frequencyn: low frequency

+: original signal

-: inverted signal

No phase shifts!

wireless networks ch6

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