Errorless Codes for Over-loaded CDMA with Active User Detection

TL;DR

This paper introduces a new class of errorless codes for over-loaded synchronous CDMA systems that increase user capacity, support active user detection, and demonstrate robustness against noise through a feasible ML decoding scheme.

Contribution

The paper proposes a novel coding scheme for over-loaded CDMA that supports active user detection and provides bounds on user capacity with practical decoding algorithms.

Findings

Supports more users without errors at fixed spreading factors

Provides an ML decoder that is computationally feasible

Demonstrates robustness against additive noise in simulations

Abstract

In this paper we introduce a new class of codes for over-loaded synchronous wireless CDMA systems which increases the number of users for a fixed number of chips without introducing any errors. In addition these codes support active user detection. We derive an upper bound on the number of users with a fixed spreading factor. Also we propose an ML decoder for a subclass of these codes that is computationally implementable. Although for our simulations we consider a scenario that is worse than what occurs in practice, simulation results indicate that this coding/decoding scheme is robust against additive noise. As an example, for 64 chips and 88 users we propose a coding/decoding scheme that can obtain an arbitrary small probability of error which is computationally feasible and can detect active users. Furthermore, we prove that for this to be possible the number of users cannot be beyond 230.