Code Division Multiple ma

8/3/2019 Code Division Multiple ma

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Code Division Multiple Access(CDMA)


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Code Division Multiple Access(CDMA) What is CDMA

Introduction to Spread Spectrum Communications

Direct Sequence Spread Spectrum (DSSS) CDMA Algorithm

Advantages of CDMA

CDMA Vs TDMA Vs FDMA

Sites and Books


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What is CDMA CDMA : Code Division Multiple Access is a channel

access method

CDMA employs spreadspectrum (SS) technology and aspecial coding scheme (where each transmitter is assigned a

code) to allow multiple users to be multiplexed over the

same physical channel

A conflict-free protocol that allows overlap transmission,

both in frequency division and time division techniques


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Multiple Access Schemes


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Introduction to spread spectrum Spectrum , refers to the range of frequencies over which

electromagnetic radio frequencies signals can be sent Spread Spectrum, refers Spreading the bandwidth to transmit data.

There are three ways to spread the bandwidth of the signal: Frequency hopping : The signal is rapidly switched between

different frequencies within the hopping bandwidth pseudo-randomly, and the receiver knows before hand where to find thesignal at any given time.

Time hopping : The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the

burst. Direct sequence : The digital data is directly coded at a much higher

frequency. The code is generated pseudo-randomly, the receiverknows how to generate the same code, and correlates the received

signal with that code to extract the data.


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Direct Sequence Spread Spectrum (DSSS) Chippingsequence Code given to user DSSS takes user bit stream and XOR with chipping

sequence. Chip- is smaller pulses of Chipping sequence Spreading factor : s = tb/tc tb = User bit time

tc = Chip duration Example : Chip sequences

11,110,1110,11101,1110010,10110111000 called Bankerscode


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Direct Sequence Spread Spectrum (DSSS)


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CDMA Algorithm Each station is assigned m bit unique code called chip

sequence say 8 bit Consider 4 stations A : 0 0 0 1 1 0 1 1 ( 1 1 1 +1 +1 1 +1 +1) B : 0 0 1 0 1 1 1 0 ( 1 1 +1 1 +1 +1 +1 1) C : 0 1 0 1 1 1 0 0 ( 1 +1 1 +1 +1 +1 1 1) D : 0 1 0 0 0 0 1 0 ( 1 +1 1 1 1 1 +1 1)

(Binary chips) (Bipolar chip) All chip sequence are pairwise orthogonal Orthogonal chip sequences are generated using Walsh

method


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CDMA Algorithm cont..- - 1 - C S1 = ( 1 +1 1 +1 +1 +1 1 1)

- 1 1 - B+C S2 = ( 2 0 0 0 +2 +2 0 2)

1 0 - - A+B S3 = ( 0 0 2 +2 0 2 0 +2)1 0 1 - A+B+C S4 = ( 1 +1 3 +3 +1 1 1 +1)

1 1 1 1 A+B+C+D S5 = ( 4 0 2 0 +2 0 +2 2)

1 1 0 1 A+B+C+D S6 = ( 2 2 0 2 0 2 +4 0 )

Examples of transmission


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CDMA Algorithm cont.. To Transmit bit 1 by B and Bit 1 C

S2 = ( 2 0 0 0 +2 +2 0 2)

S2 = ( 1 1 +1 1 +1 +1 +1 1) +( 1 +1 1 +1 +1 +1 1 1)

To recover the signal

Suppose S = transmitted chip sequence

Receiver have to extract bit sent by station C then

S.C = (A + B+C).C = A.C +B.C + C.C = 0 + 0 + 1


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CDMA Algorithm cont..Code

Time

FreqABC

D


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CDMA Algorithm cont..S4.C = ( 1 +1 3 +3 +1 1 1 +1) .

( 1 +1 1 +1 +1 +1 1 1)

= ( +1+1 +3 +3 +1 1+1 1)/8 = 1

S1 . C = (1+1+1+1+1+1+1)/8 = 1

S2 . C = (2+0+0+2+2+0+2)/8 = 1

S3 . C = (0+0+2+2+0-2+0-2)/8 = 0

S4 . C = (1+1+3+3+1-1+1-1)/8 = 1

S5 . C = (4+0+2+0+2+0-2+2)/8 = 1

S6 . C = (2-2+0-2+0-2-4+0)/8 = -1

Recovery of station C signal


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Advantages of CDMA No of users increased

Soft Handover

Flexible Used in military applications as provides security for data

Universal frequency reuse

Fast and accurate power control

Rake receiver


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CDMA VS TDMA VS FDMACDMA TDMA FDMA

Idea Spread spectrumusing codes

Time slots Frequency bands

Format Flexibility Flexible Flexible Rigid

Multiple Fading Rake Receiver Better than FDMA Different frequencies somore

Capacity 108 32 6

Handoff Soft Hard Hard

Signal separation Code and Specialreceivers

Synchronization intime domain

Filtering in frequencydomain

Implementation

complexity

Most Complex Complex Simple


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References http://www.telecomspace.com/cdma.html

http://en.wikipedia.org/wiki/Code_division_multiple_access

http://www.complextoreal.com/CDMA.pdf http://www.bee.net/mhendry/vrml/library/cdma/cdma.htm

http://www.umtsworld.com/technology/cdmabasics.htm

Computer Networks by Tenenbaum

Mobile Communications by J Schiller


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ThankYou

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Code Division Multiple ma