Chapter 7

Multiple Division Techniques

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OutlineFrequency Division Multiple Access (FDMA)Time Division Multiple Access (TDMA)Code Division Multiple Access (CDMA)Comparison of FDMA, TDMA, and CDMAWalsh Codes

Near-far ProblemTypes of InterferencesAnalog and Digital SignalsBasic 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) 2

Frequency Division Multiple Access (FDMA)

User 1

User 2

User n

…

Time

Frequency

• Single channel per carrier

• All first generation systems use FDMA

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Time Division Multiple Access (TDMA)

Use

r 1

Use

r 2

Use

r n…

Time

Frequency

• Multiple channels per carrier

• Most of second generation systems use TDMA

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Code Division Multiple Access (CDMA)

Use

r 1

Time

Frequency

• Allowing several transmitters to send information simultaneously over a singlecommunication channel (multiple access.)

• Users share bandwidth by using code sequences that are orthogonal to each other (Eachuser is associated with a different code)

• Some second generation systems use CDMA & Most of third generation systems useCDMA

Use

r 2

Use

r n

Code

...

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Types of Channels

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

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

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Types of Channels (Cont’d)

MS BS

f1’

f2’

fn’

f ’

f

…

Reverse channel (Uplink)

Forward channels

(Downlink)

f1

f2

fn

…

Control channels

Traffic channels

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FDMA

MS #1

MS #2

MS #n

BS

f1’

f2’

fn’

f1

f2

fn

…

……

Reverse channels

(Uplink)

Forward channels

(Downlink)

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FDMA: Channel Structure

1 2 3 … NFrequency

Total Bandwidth W=NWc

Guard Band Wg

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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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TDMA: Channel Structure

… 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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TDMA: Frame Structure (Cont’d)

…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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TDMA: Frame Structure (Cont’d)

…Time

Frequency

#1 #2 #n #1 #2 #n… …#1 #2 #n

Frame FrameFrame

Head DataGuard time

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Code Division Multiple Access (CDMA)

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 (Walsh Codes )Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes (Orthogonal codes have zero cross-correlation)

Frequency f

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Example.Letb = (1, -1, -1, 1)c = (-1, -1, 1, 1c .b = 1×(-1) + (-1)× (-1) +(-1) x 1+ 1 x 1 = 0then b and c are orthogonal.

Definition. A finite subset M of F is said to be anorthogonal code if any two vectors in M , calledcodewords, are orthogonal.

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

Orthogonal Codes

Comparisons of FDMA, TDMA, and CDMA (Example)

Operation FDMA TDMA CDMAAllocated Bandwidth 12.5 MHz 12.5 MHz 12.5 MHzFrequency reuse 7 7 1Required channel BW 0.03 MHz 0.03 MHz 1.25 MHzNo. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10Channels/cell 416/7=59 416/7=59 12.5/1.25=10Control channels/cell 2 2 2Usable channels/cell 57 57 8Calls per RF channel 1 4* 40**

Voice channels/cell 57x1=57 57x4=228 8x40=320Sectors/cell 3 3 3Voice calls/sector 57/3=19 228/3=76 320Capacity vs FDMA 1 4 16.8

* Depends on the number of slots ** Depends on the number of codesDelay ? ? ?

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Comparisons of FDMA, TDMA, and CDMA

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Power

Direct Sequence Spread Spectrum for CDMA

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

Transmitter Receiver

A spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power.

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In Direct-Sequence CDMA, the user signal is multipliedby a pseudo-noise code sequence of high bandwidth.This code sequence is also called the chip sequence. Theresulting coded signal is transmitted over the radiochannel.

Direct Sequence Spread Spectrum for CDMA

Digital signal

Hopping Pattern

Spreading signal Digital signal

Spreading Despread

Frequency Frequency Frequency

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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Orthogonal codes are easily generated by startingwith a seed of 0, repeating the 0 horizontally andvertically, and then complementing the 1 diagonally.This process is to be continued with the newlygenerated block until the desired codes with theproper length are generated. Sequences created inthis way are referred as “Walsh” code.

Walsh Codes (Orthogonal Codes)Based on Hadamard Matrix

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1. The code length is the size of the matrix.2. Each row is one Walsh code of the size N, for example W0=00, W1=01 taken from 2x 2 matrix.3. A base station can communicate on up to 64 channels. It has one pilotsignal, one synch channel and 8 paging channels. The remaining are usedfor traffic (54).4. The communications between the mobile and the base station takes place using specific channels.

The forward channel (from base station to mobile) is made up of the following channels:Pilot channel (always uses Walsh code W0), establish the contact with the MS.Paging channel(s) (use Walsh codes W1-W7), name of the network, ring tone , messageSync channel (always uses Walsh code W32), time of the day, GPSTraffic channels (use Walsh codes W8-W31 and W33- W63)

The reverse channel (from mobile to base station) is made up of the following channels:Access channelTraffic channel

Walsh Codes

MS1MS2 BS

Distance Distance0d2 d1

Received signal strength

MS1MS2 BS

Near-far Problem

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• The closer transmitters use less power so that the SNRfor all transmitters at the receiver is roughly the same.

• The receiver will receive more power from the nearertransmitter. This makes the farther transmitter voice moredifficult to understand. Since one transmission's signal isthe other's noise the signal-to-noise ratio (SNR) for thefarther transmitter is much lower.

• In CDMA systems, this is commonly solved bydynamic output power adjustment of the transmitters.

Near-far Problem

Frequency

Baseband signal

Frequency

Interference baseband signals

Spreading signal

Frequency

Despread signal

Interference signals

Interference in spread spectrum system in CDMA

Types of Interference in CDMA

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Adjacent Channel Interference in CDMA

f1 f2

Channel1 Channel2

Frequency

Power

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Power Control in CDMA

Pr Pt

= 1

⎝⎜⎜⎛

⎠⎟⎟⎞4πdf

cα

Controlling transmitted power

Pt = Transmitted powerPr = Received power in free spaced = Distance between receiver and transmitterf = Frequency of transmissionc = Speed of lightα = Attenuation constant (2 to 4)

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Modulation

Why need modulation?Small antenna size

Antenna size is inversely proportional to frequency

e.g., 3 kHz 50 km antenna

3 GHz 5 cm antennaLimits noise and interference, e.g., FM (Frequency Modulation)Multiplexing techniques,

e.g., FDM, TDM, CDMA

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Analog and Digital SignalsAnalog 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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Hearing, Speech, and Voice-band Channels

Voice-grade Telephone channel

Human hearing

Human 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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Amplitude Modulation (AM)

Message signalx(t)

Carrier signal

AM signals(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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Frequency Modulation (FM)

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 signal s(t)

Time

Time

Time

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Frequency Shift Keying (FSK)

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

Message signalx(t)

Carrier signal 2for message signal ‘0’

Carrier signal 1for message signal ‘1’

FSK signals(t)

1 0 1 1 0 1

Time

Time

Time

Time

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Phase Shift Keying (PSK)• Use alternative sine wave phase to encode bits

Carrier signal

Carrier signal)2sin( ππ +tfc

Message signalx(t)

)2sin( tfcπ

1 0 1 1 0 1

PSK signals(t)

Time

Time

Time

Time

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QPSK Signal Constellation

Q

I0,01,1

0,1

1,0

Q

I01

(a) BPSK (b) QPSK

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All Possible State Transitions in π/4 QPSK

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Quadrature Amplitude Modulation (QAM)

Combination of AM and PSKTwo carriers out of phase by 90 deg are amplitude modulated

Rectangular constellation of 16QAM

I

Q

0000010011001000

0001010111011001

0011011111111011

0010011011101010

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