A Class of Errorless Codes for Over-loaded Synchronous Wireless and Optical CDMA Systems

摘要

In this paper we introduce a new class of codes for over-loaded synchronouswireless and optical CDMA systems which increases the number of users for fixednumber of chips without introducing any errors. Equivalently, the chip rate canbe reduced for a given number of users, which implies bandwidth reduction fordownlink wireless systems. An upper bound for the maximum number of users for agiven number of chips is derived. Also, lower and upper bounds for the sumchannel capacity of a binary over-loaded CDMA are derived that can predict theexistence of such over-loaded codes. We also propose a simplified maximumlikelihood method for decoding these types of over-loaded codes. Although ahigh percentage of the over-loading factor degrades the system performance innoisy channels, simulation results show that this degradation is notsignificant. More importantly, for moderate values of Eb/N0 (in the range of6-10 dB) or higher, the proposed codes perform much better than the binaryWelch bound equality sequences.