Cross-layer design for communication systems

Abstract

Recent years have witnessed a widespread proliferation of wireless communication networks around the world. Wireless networks, and wireless communications in general, present several engineering challenges that were not present in their predecessor wired networks. At the same time, wireless networks---in particular ad-hoc wireless networks---offer certain modalities of communications that were just not possible in the wired networks. Such peculiarities of wireless communication networks are ushering in new paradigms for communication protocol design that better address the challenges and opportunities created by the wireless medium. This thesis looks at one such emerging paradigm termed as cross-layer design. The main idea behind cross-layer design is to allow enhanced dependence and information sharing between the different layers of the protocol stack. This is in contrast with the layered architectures that have been the cornerstone of data network design and development. In this thesis, we attempt to understand the cross-layer design methodology in more detail. We take stock of the existing work in this area, distill some key insights and spell out some of the open challenges. After discussing in detail about the different aspects of cross-layer design, we present an instance of cross-layer design involving the link layer and the physical layer. In particular, we study the design of physical layer for a point-to-point communication system with the link layer average service time as our metric of interest. We come up with necessary and sufficient conditions on the parameters of specific physical layer processes like forward error correction and digital modulation such that the link layer average service time is favorably affected. We also study the impact of physical layer processing on the link layer average delay (sum of the average service time and the average queueing delay), assuming a Poisson arrival process. Finally, we focus on forward error correction and merge the necessary and sufficient condition for improving the link layer average service time mentioned above with the Varshamov-Gilbert (VG) bound and the Sphere-packing bound, which are well-known coding theoretic results. Doing so enables us to study the existence of Service-Time Improving (STI) codes. By STI codes, we mean forward error correcting codes that reduce the average service time with respect to uncoded transmission for a fixed symbol rate and constellation size. We also explore the asymptotic case of large packet length and determine sufficient conditions for the existence of STI codes in this regime using the asymptotic form of the VG bound and the channel capacity theorem. In summary, this thesis starts with a qualitative exploration of the various facets of cross-layer design. To the best of our knowledge, the methodology of cross-layer design has not been looked at so closely elsewhere. We then move on to apply some of the ideas to a specific scenario of a point-to-point communication system. Quantitative guidelines for physical layer processing with a higher layer metric in mind are developed for the system under consideration. These guidelines can be of practical importance. The application of coding theory ideas yields results that are more theoretical in nature but represent the application of ideas from two different disciplines---queueing theory and coding theory---in solving a communications problem.