DS-CDMA

In Direct Sequence spread spectrum transmission, the user data signal is multiplied by a code 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.A DS-CDMA signal is generated by multiplication of a user data signal by a code sequence.

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. A CDMA receiver can retrieve the wanted signal by multiplying the receive signal with the same code as the one used during transmission.

Popular code sequences are:

• • Maximum length or Pseudo Noise (PN) sequences.

  • Walsh Hadamard Codes.

  • Gold codes

  • Kasami codes.

As de-spreading 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. In cellular CDMA systems, (adaptive) power control is needed to avoid this problem