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无线通信系统第5章

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Multiple Access Techniques for Wireless Communications

2011-6-15

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Duplexing

Frequency domain techniques FDD Time domain techniques TDD

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TDD system

Mobile Station Transmitter Receiver BPF F1

Base Station Transmitter BPF F1 Synchronous Switches Receiver

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Multiple Access
– Frequency (f) – Time (t) – Code (c) – Space (si) – Packet Radio Goal: multiple use of a shared medium Multiple Access Techniques PR FDMA TDMA CDMA SDMA

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FDMA

CDMA TDMA

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Multiple ascess technologies used in different wireless systems

Cellular Systems
AMPS ( Advanced Mobile Phone system ) GSM ( Global System for Mobile ) IS – 95 ( U.S Narrowband Spread Spectrum )

MA Technique FDMA / FDD TDMA / FDD CDMA / FDD CDMA / FDD,CDMA/TDD

WCDMA

TD-SCDMA Cdma2000

CDMA / TDD CDMA / FDD

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FDMA

Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time
? Each user is allocated a unique frequency band or channel. These channels are assigned on demand to users who request service. ? ? The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ carrier. ISI is low since the symbol time is large compared to average delay spread No equalization is required.

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FDMA

? Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes (such as synchronization and framing bits) as compared to TDMA. ? The FDMA mobile unit uses duplexers. ? FDMA requires tight RF filtering to minimize adjacent channel interference. ? Nonlinear Effects in FDMA: Intermodulation frequency

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Number Of Channel Supported By FDMA System

Bg
Bt
N= B t ? 2B g Bc

Bg

Eg. B t = 1 2 .5 M H z
B g = 10 KH z B c = 30 KH z ? (1 2 .5 × 1 0 6 ) ? 2 (1 0 × 1 0 3 ) ? ? = 416 N = ? 30 × 103
9

B g → G uard Band B c → C hannelB andw idth

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TDMA

TDMA systems divide the radio spectrum into time slots and each user is allowed to either transmit or receive in each time slots. Each user occupies a cyclically repeating time slots. Disadvantages: ? precise synchronization necessary

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TDMA

Utilized only in digital systems!!! Number of time slots (TS) depends on technology

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TS1

TS2

TS3

TS4

TS5

TS6

Call 1 Call 2 Call 3 BS Antenna

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TDMA

TDMA shares the single carrier frequency with several users, where each user makes use of non-overlapping timeslots. nonData Transmission for user of TDMA system is discrete bursts ? The result is low battery consumption, power save. ? Handoff process is simpler, since it is able to listen for other base stations during idle time slots. Since different slots are used for T and R, duplexers are not required. Equalization is required, since transmission rates are generally very high as compared to FDMA channels.

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Frame Efficiency

Traffic bits / frame ηf = Total Numberof bits / frame
= (1 ? bOH / bT ) × 100% (bT ? bOH ) = × 100% bT
Number of channels in TDMA system

m = Maximum number of TDMA users supported on each radio channel Bguard = Guard band to present user at the edge of the band from 'bleeding over' to an adjacent radio service
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TDMA&FDMA

Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM

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Example 1
If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel, find (a) the time duration of a bit, (b) the time duration of a slot, (c) the time duration of a frame, and (d) how long must a user occupying a single time slot wait between two successive transmissions. Solution
(a) The time duration of a bit, (b) The time duration of a slot, Tslot = 156.25 x Tb = 0.577 ms. (c) The time duration of a frame, Tf= 8 x Tslot= 4.615 ms. (d) A user has to wait 4.615 ms, the arrival time of a new frame, for its next transmission.

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SSMA

Spread Spectrum modulation spreads signal bandwidth to a bandwidth that is several orders of magnitude wider than the signal bandwidth Two spectrum spread methods
?Frequency ?Direct

Hopping Spread Spectrum

Sequence Spread Spectrum

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Spread Spectrum

Shannon’s capacity equation

C = Blog 2 (1 + S / N )

Power Spectral Density (PSD)

Narrowband Waveform

Spread Waveform Noise Level Frequency
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FHMA

Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence

Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS

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DSSS (CDMA)

Each channel has a unique code All channels use the same spectrum at the same time –bandwidth efficient –no coordination and synchronization necessary Implemented using spread spectrum technology

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CDMA
Direct Sequence (DS)
Power Density

How to spread

user data
TIME Modulation (primary modulation) Base-band Frequency data rate

Spreading (secondary modulation)

spreading sequence (spreading code)

Power Density

10110100

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Radio Frequency

Tx 21

Demodulating

you know the correct spreading sequence (code) ,
Power Density

received signal
TIME
10110100 01001011 10110100

spreading sequence (spreading code) Radio Frequency

10110100 10110100 10110100

gathering energy !

you can find the spreading timing which gives the maximum detected power, and
00000000 11111111 00000000

Accumulate for one bit duration

Demodulated data
0 1 0 Base-band Frequency

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? Unlike TDMA or FDMA, CDMA has a soft capacity limit. ?Self-jamming is a problem in CDMA system. ?The near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. ?CDMA power control ?Since CDMA uses co-channel cells, it can use macroscopic spatial diversity to provide soft handoff. ?A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal .
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Received multipath signal

Received signal consists of a sum of delayed (and weighted) replicas of transmitted signal.

Signal replicas: same signal at different delays, with different amplitudes and phases

:

0110110000111
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Packet Radio

Packet Radio
?High spectral efficiency ?Collisions ?Service:DATA

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Vulnerable period Normalized channel traffic

Normalized throughput

Possion process

(λ t ) n ? λ t Pr(n) = e n!
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Pure aloha
? Transmit whenever a message is ready ? Retransmit when there is a collision

R=0.5 Tmax=0.1839

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Slotted aloha
rTime

is divided into equal time slots rTransmit only at the beginning of a time slot rAvoid partial collisions rIncrease delay, and require synchronization

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CSMA
? non-persistent CSMA ? 1-persistent CSMA p-persistent CSMA ? CSMA/CD reasy in wired LANs: measure signal strengths, compare transmitted, received signals rdifficult in wireless LANs: receiver shut off while transmitting ? CSMA/CA

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Capacity of Cellular Systems

Channel capacity for a radio system is defined as the maximum number of channels or users that can be provided in a fixed frequency band spectrum efficiency of wireless system. For a Cellular System m = Bt / (BC * N) Bt = Total allocated spectrum for the system BC = Channel bandwidth N= Number of cells in frequency reuse pattern

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FDMA

C = I 6 × D?n Do =Distance from desired base station to mobile

Do ? n

( I) (C I )
C

min

= 1 = 1

6

( D)
R Q)
?n

?n

min

6(

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When n = 4, m =

Bt

? ? ?Bc ?( 2 / 3 ) × C I ? ?

( )

? min ? ?

(1/ 2)

? ? ?

radio channels / cells

Typical Values of C

( I)

min

= 18 dB for Analog FM and12 dB for Digital

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TDMA
C = Eb R b I = IoBc where Io = Interference power /Hz

C′ = E b R ′ b I′ = IoB′ c where R′ = Transmission rate of TDMA system b R b = Transmission rate of FDMA system Eb = Energy per bit

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Capacity of Digital Cellular CDMA

Capacity of FDMA and TDMA system is bandwidth limited.Capacity of CDMA system is interference limited. The link performance of CDMA increases as the number of users decreases.

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W/R processing gain
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Antenna Sectorization A cell site with 3 antennas,each having a beamwidth of 120 degree.This increase the capacity by a factor of 3 . Monitoring or Voice activity: Each transmitter is switched off during period of no voice activity. Voice activity is denoted by a factor a

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Example 2
If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eb/ No is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using (a) omni directional base station antennas and no voice activity detection (b) 3 sectors at base station and a = 3/8. Assume the system is interference limited. α = 0.

therefore N=3*35.7 =107 users/ cell.
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