Abstract
AbstractAlthough the conventional spread‐spectrum communication system has the advantage of asynchronous multiple‐access, the number of simultaneously communicable channels is small due to the interchannel interference and the efficiency of frequency utilization is very low. to deal with this problem, this paper proposes a new synchronous spread‐spectrum multiplex communication system employing a modified M‐sequence. the modified M‐sequence is derived by giving an appropriate dc offset to an M‐sequence so that the sidelobe of its autocorrelation function becomes zero. In this system bitshifted modified M‐sequences are used as spreading codes for each channel and the synchronization among them is maintained. Let N be the length of the spreading code. If a bandwidth N times as large as that required for one channel transmission is used, and the product of the band‐limiting characteristics and the power spectral density of the spreading code satisfies Nyquist's first criterion, multiplex transmission of N channels is realized without interchannel interference. It is proved that equally chosen filters for the transmitter and the receiver to realize the transmission bandwidth restriction maximize the SN ratio due to noise in the transmission channel and the value becomes equal to that in the one‐channel transmission. Therefore, the same number of channels as in TDM is achieved with the same SN ratio and the efficiency of frequency utilization is improved greatly. As for synchronization, almost the same bit synchronization error as in TDM is allowed.
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