白皮书
Code division multiple access (CDMA) communications systems allow the transmission of multiple channels of information on the same frequency at the same time. Unlike in traditional communications systems where an information symbol is represented directly using some form of carrier modulation, in the CDMA system, each information symbol is modulated by a binary signaling (spreading) code. Each information channel is assigned a unique spreading code with known length such that the information symbol rate of each channel is converted or "spread" to a common higher modulation rate (chip rate) for the particular system. The individual channels can then be summed to create a composite signal, which is transmitted by the communication system. The spreading codes chosen for each channel are orthogonal, I.e. the dot product of any two timealigned codes is zero. If the received composite signal is despread with one of the codes used in the spreading process and the result then demodulated, the output is the original symbol data that was spread by the transmitter using that specific code. The use of different length spreading codes (spreading factors) allows for channels with different symbol data rates to be spread up to the common system chip rate.
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