METHOD AND APPARATUS FOR INCREASING SPECTRAL EFFICIENCY OF CDMA SYSTEMS USING DIRECT SEQUENCE SPREAD SPECTRUM SIGNALS

摘要

An improved modulator and demodulator for use in a spread spectrum system. The modulator can process two data channels (I and Q) at nominal data rate or one high speed channel demultiplexed into two nominal rate data channels. The modulator includes one pseudorandom noise (PN) sequence generator that generates two orthogonal PN sequences. The two PN sequences are provided to a switch. Each channel includes a demultiplexer, one output of which controls which PN sequence is provided by the switch. The other demultiplexer output is then modulo-2 added (exclusive-OR'd) with the chosen PN sequence, and the modulo-2 sum is provided to the rest of the transmitter. The demodulator processes the two I and Q channels and includes a PN sequence generator providing replicas of the two PN sequences. Each channel is divided into subchannels, each subchannel being multiplied by one of the PN sequences and the results integrated. A channel decision circuit compares the two integrated results and chooses the subchannel with the largest integrator result magnitude. The channel decision circuit provides to a multiplexer a control signal based on which subchannel integrator result was greater. Decision circuits in each subchannel determine whether the bits in each subchannel are high or low and provides this signal to the inputs of a switch. The switch outputs the bit from the subchannel with the greater integrator result. The multiplexer combines the switch output and the control signal and outputs a signal at the nominal data rate. If there was one high speed input, the outputs of the two channels can be multiplexed into one high speed output. Such an apparatus and method improves over the prior art by doubling spectral efficiency, thus increasing the number of users or the data rate with no increase in bandwidth used.