Bai 7_Da truy cap

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CHAPTER 7:CODE DIVISION MULTIPLE ACCESS

ng L KhoaEmail: [email protected]

Facuty of Electronics & Telecommunications, HCMUS1

Outline

This Class: CDMA and its capacity

FHSSDSSSBluetoothCDMA capacity

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Spread Spectrum Access

SSMA uses signals that have transmissionbandwidth that is several orders of magnituedlarger than minimum required RF bandwidth.Provides

Immunity to multipath interferenceRobust multiple access.

Two techniquesFrequency Hopped Multiple Access (FHMA)Direct Sequence Multiple Access (DSMA)

Also called Code Division Multiple Access CDMA

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Frequency Hopping (FHMA)

Digital muliple access techniqueA wideband radio channel is used.

Same wideband spectrum is usedThe carrier frequency of users are varied in a pseudo-random fashion.

Each user is using a narrowband channel (spectrum) at a s pecificinstance of time.The random change in frequency make the change of using thesame narrowband channel very low.

The sender receiver change frequency (calling hopping) using the samepseudo-random sequence, hence they are synchronized .

Rate of hopping versus Symbol rateIf hopping rate is greather: Called Fast Frequency Hopping One bit transmitted in multiple hops.

If symbol rate is greater: Called Slow Frequency Hopping Multiple bits are transmitted in a hopping period GSM and Bluetooth are example systems

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Case Study - Bluetooth

Uses Frequency Hopping in cell (piconet) over a 79 MHzwideband radio channel.Uses 79 narrowband channels (carrier frequencies) to hopthrough.

Freq (f) = 2402+k MHz, k = 0,...,78Channel spacing is 1 MHz (narrowband channel bandwidth)Wideband spectrum width = 79 MHz.Hopping Rate = 1600 Hops/SecondHopping sequence is determined by Bluetooth Hardware addressand Clocks that are syncrozied between sender and receiver

0 1 2 3 77 78.....

79-Hop System

A hop sequence could be: 7,1,78,67,0, 56,39,.......

79 MHZ

1 MHZ

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Case Study: Bluetooth P iconet and FHSS

S S

S

M

Each node is classified as master or slave.Master defines a piconet (a cell). Maximum 7 slaves can be connected toa master. Master coordinates access to the the media.

All traffic has to go over master.Slaves can not talk to each-otherdirectly.

Range = 10mRaw Data-rate: 1 Mbps/piconet

Radio channel used by devices ina piconet is 79MHz channel, whichIs frequency hopped: hoppingthough 789 channels.Hoprate = 1600 hops/sec

FHSS

Picocell

All slaves and the master hops according to the same hopping sequence .The hopping sequence is determined by the clock and BT_address of the master.

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Case Study: Bluetooth Scatternet and FHSS

S S

S

M1FHSS

Piconet

S

SS

M2

Piconet

Piconet can be combinedinto scatternets.

Red slave acts as abridge between twopiconets.FHSS

Each piconet uses FHSS with differenthopping sequences (masters are different).

This prevents interference between piconets.

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Case Study: Bluetooth - Media access in a piconet

S2 S3

S1

MFHSS

PiconetInside a piconet, access to the

frequency hopped radio channelis coordinated using time division multiple access : TDMA/TDD.

Slot duration = 1/1600 sec = 625 s

M-S1 S1 -M M-S2 S2 -M M-S3 S3 -M M-S1 S1 -M

0 1 2 3 4 5 6 7 ..

In an even slot , master transmits to aslave.In an odd slot , the slave that is addressedin the previous master-to-slave slot transmits.

slot time=625 s

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Code Division Multiple Access (CDMA)

In CDMA, the narrowband message signal is multiplied by a very largebandwidth signal called spreading signal (code) before modulation andtransmission over the air. This is called spreading .CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is amore general term.Message consists of symbols

Has symbol period and hence, symbol rateSpreading signal (code) consists of chips

Has Chip period and and hence, chip rateSpreading signal use a pseudo-noise (PN) sequence (a pseudo-randomsequence)PN sequence is called a codewordEach user has its own cordwordCodewords are orthogonal. (low autocorrelation)Chip rate is oder of magnitude larger than the symbol rate.

The receiver correlator distinguishes the senders signal by examining thewideband signal with the same time-synchronized spreading codeThe sent signal is recovered by despreading process at the receiver.

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CDMA Advantages

Low power spectral density.Signal is spread over a larger frequency bandOther systems suffer less from the transmitter

Interference limited operationAll frequency spectrum is used

PrivacyThe codeword is known only between the sender and receiver. Hence otherusers can not decode the messages that are in transit

Reduction of multipath affects by using a larger spectrumRandom access possible

Users can start their transmission at any timeCell capacity is not concerete fixed like in TDMA or FDMA systems. Has softcapacityHigher capacity than TDMA and FDMANo frequency managementNo equalizers neededNo guard time neededEnables soft handoff

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CDMA Principle

1

0

1

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

Chip period

One bit period (symbol period)

Data

CodedSignal

Input to the modulator (phase modulation)

Represent bit 1 with +1Represent bit 0 with -1

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Processing Gain

u Main parameter of CDMA is the processing gain t hat is defined as:

R

B

R

BG chipspread p ==

Gp: processing gainBspread : PN code rateBchip : Chip rateR: Data rate

u IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100.

l 1.228 Mhz chipping ratel 1.25 MHz spread bandwidth

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Near Far Problem and Power Control

At a receiver, the signals may come fromvarious (multiple sources.

The strongest signal usually captures themodulator. The other signals areconsidered as noise

Each source may have different distancesto the base station

In CDMA, we want a base station toreceive CDMA coded signals from variousmobile users at the same time. Therefore the receiver power at the base

station for all mobile users should beclose to eacother.

This requires power control at themobiles.

Power Control : Base station monitors theRSSI values from different mobiles andthen sends power change commands tothe mobiles over a forward channel. Themobiles then adjust their transmit power.

B

M

MM

M

p r(M)

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DSSS Transmitter

+

PN CodeGenerator

Chip Clock

BasebandBPF

Oscillatorfc

Message s ss (t)m(t)

p(t)TransmittedSignal

)2cos()()(2

)( += t f t pt mT E

t s cs

sss

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DSSS Receiver

)(t sss )(t p

)(t mIF Wideband

Filter

PN CodeGenerator

Phase Shift KeyingDemodulator

SynchronizationSystem

ReceivedData

ReceivedDSSS Signalat IF

)(1 t s

)2cos()(2

)(1 += t f t mT E

t s cs

s

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Spectra of Received Signal

FrequencyFrequency

SpectralDensity

SpectralDensity

Signal

Interference

Interference

Signal

Output of Wideband filter Output of Correlator afterdispreading,

Input to Demodulator

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CDMA Example

R

A B

Receiver (a base station)

Transmitter (a mobile) TransmitterCodeword=010011 Codeword=101010

Data=1011 Data=0010

Data transmitted from A and B is multiplexed using CDMA and codewords.The Receiver de-multiplexes the data using dispreading.

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CDMA Example transmission from two sources

1 0 1 0 1 01 0 1 0 1 01 0 1 0 1 0

CodeData

0 1 0 0 1 1 0 1 0 0 1 10 1 0 0 1 10 1 0 0 1 1

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

0 0 1 0

1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 00 1 0 1 0 1

1 0 1 1

1 0 1 1 0 0

TransmittedA+B

Signal

A Data

ACodeword

B Data

BCodeword

CodeData A Signal

B Signal

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CDMA Example recovering signal A at the receiver

0 1 0 0

A+BSignal

received

ACodeword

atreceiver

CodeB)(A +

IntegratorOutput

ComparatorOutput

Take the inve rse of this to o btain A

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CDMA Example recovering signal B at the receiver

1 1 0 1

A+BSignal

received

BCodeword

atreceiver

CodeB)(A +

IntegratorOutput

ComparatorOutput

Take the inverse of this to obtain B

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CDMA Example using wrong codeword at the receiver

X 0 1 1Noise

A+BSignal

received

WrongCodeword

Used atreceiver

IntegratorOutput

ComparatorOutput

Wrong codeword will not be able to deco de the original data!

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Hybrid Spread Spectrum Techniques

FDMA/CDMAAvailable wideband spectrum is frequency divided intonumber narrowband radio channels. CDMA isemployed inside each channel.

DS/FHMAThe signals are spread using spreading codes (directsequence signals are obtained), but these signal are

not transmitted over a constant carrier frequency; theyare transmitted over a frequency hopping carrierfrequency.

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Hybrid Spread Spectrum Techniques

Time Division CDMA (TCDMA)Each cell is using a different spreading code (CDMAemployed between cells) that is conveyed to themobiles in its range.Inside each cell (inside a CDMA channel), TDMA isemployed to multiplex multiple users.

Time Division Frequency HoppingAt each time slot, the user is hopped to a newfrequency according to a pseudo-random hoppingsequence.Employed in severe co-interference and multi-pathenvironments.

Bluetooth and GSM are using this technique.

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Capacity of CDMA Systems

Uplink Single-cell System Model

...

.

.

.

...

.

.

.

User 1

User 2

User k

User K u

User n

Assumptions Total active users K uThe intra-cell MAI can be

modeled as AWGN Per fect power control isassumed Random sequences

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Capacity of CDMA SystemsCoarse estimate of the reverse link (uplink)capacity

Assumptions: Single Cell. The interference caused by other users in the cell can be

modeled as AWGN. Perfect power control is used, i.e. the received power of each

user at the base station is the same.If the received power of each user is Ps watts, and thebackground noise can be ignored (ex: microcells), then the totalinterference power (MAI) at the output of the desired usersdetector is

where Ku is the total number of equal energy users in the cell.Suppose each user can operate against Gaussian noise at a bit-energy-to-noise density level of Eb/Io. Let W be the entire spreadbandwidth, then the interference spectral density can beexpressed as:

( ) su PK I 1

)( / 0 sided one HzWattsW I

I =25


Interferencelimited

Also, the bit energy Eb is

Thus,b

sb R

P E =

0b

b

bb

0

su IE

RWREWI

PI

1K =

==

{ Now, if we consider the factors of voice activity (G v),

sectorizedantenna gain (G A), and other-cell interface factor (f), where

Gv 1/v = 2.67GA (three sectors) 2.4f = (Interference form other cells)/(Interference from given cell)

0.6

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In this case, Ku can be approximated by

Ex: If Gv 2.67, GA 2.4, f 0.6

If (Eb/Io) required is 6 dB (i.e. Eb/Io = 4)

which will be larger than the TDMA or FDMA systems in the cellular environment.

( )f 1GG

IE

RWK Av

0b

bu +

( )( )ob

bu I E

RW K

4

bu R

W K

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SDMA

use spot beam antennasthe different beam area can use TDMA,

FDMA, CDMAsectorized antenna can be thought of as a

SDMAadaptive antennas can be used in the future

(simultaneously steer energyin the direction of many users)

spot beamantenna

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Projects and Homework

9.39.49.59.79.109.129.179.19

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