This invention relates to a demodulator for a code-division multiple-access (CDMA) communication system of the block-spreading type.
CDMA is a spread-spectrum system employed in personal communication systems and other mobile communication systems, in which multiple stations transmit simultaneously over the same frequency band. In a block-spreading CDMA system, data to be transmitted are divided for modulation into P-bit blocks. Each block is converted to a Q-chip codeword, which is then further modulated by a spreading code to generate a baseband transmit signal. (P and Q are positive integers.) The Q-chip codewords are mutually orthogonal and are the same at all transmitting stations, but each station uses a different spreading code. A receiver demodulates the data transmitted from a particular station by, for example, multiplying the incoming baseband signal by that station's spreading code, correlating each resulting block with all possible codewords, and selecting the codeword that gives the highest correlation.
It is known that the channel capacity of such a system can be improved if the receiver takes steps to cancel interference between different transmitting stations. One interference-canceling system relies on the fact that signals from different transmitting stations usually arrive with different strengths. The receiver first demodulates the strongest signal, then cancels it out as interference and demodulates the strongest remaining signal, continuing in this way until all signals have been demodulated. Unfortunately, this system does not work well when several signals arrive with substantially the same strength.
Another possible system demodulates the signals from all transmitting stations in parallel, subtracting the estimated interference generated by each station from the signals of all the other stations. The process can be iterated to obtain increasingly accurate estimates of the transmitted signals and their interference. This system appears to work well in theory, but when its actual behavior is simulated it performs poorly, because it fails to take maximum advantage of the available interference information. The initial estimates of interference from a given station, for example, are always derived from a signal from which interference has not yet been removed.