Bounds on the Sum Capacity of Synchronous Binary CDMA Channels

TL;DR

This paper derives lower and conjectured upper bounds on the sum capacity of binary CDMA channels with noise, showing that the system can support many users without significant capacity loss under low noise conditions.

Contribution

It introduces a family of lower bounds for noisy binary CDMA sum capacity and a conjectured upper bound, linking them to system parameters and noise levels.

Findings

Lower bounds are tight and approach noiseless capacity as noise diminishes.

Supports the feasibility of overloaded CDMA with many users at low noise levels.

Asymptotic bounds align with statistical physics-based formulas for large systems.

Abstract

In this paper, we obtain a family of lower bounds for the sum capacity of Code Division Multiple Access (CDMA) channels assuming binary inputs and binary signature codes in the presence of additive noise with an arbitrary distribution. The envelope of this family gives a relatively tight lower bound in terms of the number of users, spreading gain and the noise distribution. The derivation methods for the noiseless and the noisy channels are different but when the noise variance goes to zero, the noisy channel bound approaches the noiseless case. The behavior of the lower bound shows that for small noise power, the number of users can be much more than the spreading gain without any significant loss of information (overloaded CDMA). A conjectured upper bound is also derived under the usual assumption that the users send out equally likely binary bits in the presence of additive noise with an arbitrary distribution. As the noise level increases, and/or, the ratio of the number of users and the spreading gain increases, the conjectured upper bound approaches the lower bound. We have also derived asymptotic limits of our bounds that can be compared to a formula that Tanaka obtained using techniques from statistical physics; his bound is close to that of our conjectured upper bound for large scale systems.