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Direct Sequence CDMA

In Direct Sequencespread spectrumtransmission, the user data signal is multiplied by acode

sequence.Mostly, binary sequences are used. The duration of an element in the code is called the

"chip time". The ratio between the user symbol time and the chip time is called the spread factor.

The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the

user data.

Figure: A DS-CDMA signal is generated by multiplication of a user data signal by a code sequence.

Source: Jack Glas, T.U. Delft.

In the receiver, the received signal is again multiplied by the same (synchronized) code. This

operation removes the code, so we recover the transmitted user data.

Different CDMA users use different codes. In this

example the receiver sees the signal from user 1,

while the signal from user 2 is heavily attenuated by

the correlator (multiplier and integrator) in the

receiver.

A CDMA receiver can retrieve the wanted signal by multiplying the receive signal with the same code

as the one used during transmission. We find:

where c1is the code sequence used by user 1, Tcis the chip duration, tdis a common time offset,

shared between transmitter and receiver and Nis the length of the code sequence.. Note that the

receive code must be perfectly time aligned with the transmit code.

Popularcode sequencesare

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Maximum length or Pseudo Noise (PN) sequences. Walsh Hadamard Codes. Gold codes, and Kasami codes.

Spreading gain

Figure: DS-CDMA signal with a jamming signal

Source: Jack Glas, T.U. Delft.

As despreading is the same operation as spreading a possible narrowband jammer signal in the radio

channel is spread before the detector. Thus, the jam signal is attenuated by the spread factor

("spreading gain").

Near-Far Effect

A major difficulty in Direct Sequence transmission is the Near-Far effect. If more than one user is

active, the incoming interference power is suppressed by the cross correlation between the code of

the reference user and the code of the interferer. In the event that the interferer is closer to the

receiver then the reference user, the interference components may not be sufficiently attenuated by

the despreading process. Incellular CDMAsystems, (adaptive)power controlis needed to avoid this

problem

InherentDiversity

In a multipathfadingchannel, delayed reflections interfere with the direct signal. However, a DS-

CDMA signal suffering from multipathdispersioncan be detected by arakereceiver. This receiver

optimally combines signals received over multiple paths.

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