Code-Division Multiple Access Concept

CDMA is actually DSSS utilizing CDM. Like GSM, CDMA uses a dedicated frequency band for multiple simultaneous signal transmission, but what underlies this frequency use scheme is spread spectrum, which essentially spreads a single signal from a transmitter over the entire shared frequency band in such a way that signals will not interfere with each other, thanks to a spreading code assigned to each signal. A single data bit of 1 from a mobile station is mapped to a chip sequence that identifies the mobile station. For a data bit of 0, the complement of the chip sequence is used. The chip sequence is normally 64 or 128 chips long and is pairwise orthogonal, meaning that the normalized inner product (i.e., dot product) of any two distinct chip sequences (they are considered vectors of _ 1 and _ 1 in mathematical terms) is 0. After the mapping, multiple data bitstreams from different mobile stations are added linearly and transmitted. The intended receiver knows the chip sequence of the individual mobile station and uses it, along with the received aggregated bitstream, to compute data bits of that mobile station. The computation is quite straightforward: Simply compute the normalized inner product of the chip sequence of the desired mobile station and the received bitstream. In this way, the data bits sent by that mobile station will be recovered. Without knowing the correct chip sequence of a transmitter, the computation will yield some pseudorandom bits like noise. An implicit assumption of the decoding procedure is that the receiver and the transmitter are well synchronized in time, which allows the necessary computations for the correct portion of the transmitted bitstream. This is often done by utilizing a special synchronization bit sequence.

The chip sequences assigned to mobile stations can be generated by the Walsh code, an algorithm that produces mathematically orthogonal codes derived from the Walsh matrix. The Walsh-encoded chip sequences appear to be random noise to mobile terminals. Initially, the chip sequences are of equal length. To increase the number of usable chip sequences in the coding space, variable-length chip sequences have been devised and used in today’s CDMA systems. Interested readers can refer to A. J. Viterbi’s book “ CDMA — Principles of Spread Spectrum Communication ” for more details of CDMA codes.

Source of Information :  Elsevier Wireless Networking Complete 2010