Two-layer Coded Channel Access with Collision Resolution: Design and Analysis

TL;DR

This paper introduces a two-layer coding scheme for multi-user shared medium access that improves collision resolution and throughput by combining outer error-correcting codes with an inner collision-resolution code, analyzed for different code types.

Contribution

It presents a novel two-layer coding architecture with optimized parameters for enhanced collision resolution and throughput in shared communication channels.

Findings

LDPC codes enable joint optimization for maximum throughput.

Algebraic codes approximate erasure correction capabilities for large user numbers.

Proposed schemes operate closer to capacity than existing methods.

Abstract

We propose a two-layer coding architecture for communication of multiple users over a shared slotted medium enabling joint collision resolution and decoding. Each user first encodes its information bits with an outer code for reliability, and then transmits these coded bits with possible repetitions over transmission time slots of the access channel. The transmission patterns are dictated by the inner collision-resolution code and collisions with other users' transmissions may occur. We analyze two types of codes for the outer layer: long-blocklength LDPC codes, and short-blocklength algebraic codes. With LDPC codes, a density evolution analysis enables joint optimization of both outer and inner code parameters for maximum throughput. With algebraic codes, we invoke a similar analysis by approximating their average erasure correcting capability while assuming a large number of active transmitters. The proposed low-complexity schemes operate at a significantly smaller gap to capacity than the state of the art. Our schemes apply both to a multiple access scenario where number of users within a frame is known a priori, and to a random access scenario where that number is known only to the decoder. In the latter case, we optimize an outage probability due to the variability in user activity.