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

TL;DR

This paper introduces a new class of errorless codes for over-loaded synchronous wireless and optical CDMA systems, allowing more users without errors, reducing bandwidth, and improving performance at moderate noise levels.

Contribution

The paper presents a novel class of errorless codes for over-loaded CDMA systems, with bounds on user capacity and a simplified decoding method, outperforming existing sequences at certain noise levels.

Findings

Increased number of users without errors for fixed chips

Derived bounds for user capacity and channel capacity

Simulation shows improved performance at moderate Eb/N0

Abstract

In this paper we introduce a new class of codes for over-loaded synchronous wireless and optical CDMA systems which increases the number of users for fixed number of chips without introducing any errors. Equivalently, the chip rate can be reduced for a given number of users, which implies bandwidth reduction for downlink wireless systems. An upper bound for the maximum number of users for a given number of chips is derived. Also, lower and upper bounds for the sum channel capacity of a binary over-loaded CDMA are derived that can predict the existence of such over-loaded codes. We also propose a simplified maximum likelihood method for decoding these types of over-loaded codes. Although a high percentage of the over-loading factor degrades the system performance in noisy channels, simulation results show that this degradation is not significant. More importantly, for moderate values of Eb/N0 (in the range of 6-10 dB) or higher, the proposed codes perform much better than the binary Welch bound equality sequences.