Protocol Coding through Reordering of User Resources: Applications and Capacity Results

TL;DR

This paper explores how to embed additional information into existing communication systems by reordering user resources, enabling secondary channels that enhance system capacity with minimal changes.

Contribution

It introduces models for secondary communication channels based on resource reordering, relating them to channels with causal CSIT, and discusses capacity-achieving strategies and practical applications.

Findings

Capacity of secondary channels can be computed under primary system restrictions.

Reordering user resources creates new communication opportunities without hardware changes.

Strategies for coding over secondary channels are proposed and analyzed.

Abstract

While there are continuous efforts to introduce new communication systems and standards, it is legitimate to ask the question: how can one send additional bits by minimally changing the systems that are already operating? This is of a significant practical interest, since it has a potential to generate additional value of the systems through, for example, introduction of new devices and only a software update of the access points or base stations, without incurring additional cost for infrastructure hardware installation. The place to look for such an opportunity is the communication protocol and we use the term *protocol coding* to refer to strategies for sending information by using the degrees of freedom available when one needs to decide the actions taken by a particular communication protocol. In this paper we consider protocol coding that gives a rise to *secondary communication channels*, defined by combinatorial ordering of the user resources (packets, channels) in a primary (legacy) communication system. We introduce communication models that enable us to compute the capacity of such secondary channels under suitable restrictions imposed by the primary systems. We first show the relation to the capacity of channels with causal channel state information at the transmitter (CSIT), originally considered by Shannon. By using the specific communication setup, we develop an alternative framework for achieving the capacity and we discuss coding strategies that need to be used over the secondary channels. We also discuss some practical features of the secondary channels and their applications that add value to the existing wireless systems.