Research Overviews

Steve Weller

Low-density parity-check (LDPC) codes: from principles to practice

University of Newcastle

Abstract: Low-density parity-check (LDPC) codes are powerful forward error-correction codes, first proposed in the early 1960s, then largely neglected for over 35 years. Today, design techniques for LDPC
codes exist which enable the construction of codes which approach the capacity of several classical memoryless channels to within hundredths of a decibel. So rapid has progress been in this area that coding
theory today is in many ways unrecognisable from its state just a decade ago. Whatever the future holds for coding theory, it is difficult to imagine that codes defined on sparse graphs, together with their associated iterative decoding algorithms, will not play a central role.

But coding practice is also being impacted on by LDPC codes, with such codes currently being evaluated for wireless local- and metropolitan-area networks, longhaul optical links, magnetic and optical recording, and deep-space communications, and with good reasons: in addition to their capacity-approaching performance, the Tanner graph representation of LDPC codes is highly suggestive of very large-scale integration (VLSI) implementations; the sum-product algorithm so closely associated with LDPC codes handles soft (probabilistic) information in an entirely natural way; and the wide range of code design methods produces an almost limitless range of code rates and blocklengths.

This talk aims to give an overview of the principles of LDPC codes, their design and decoding, and prospects for wide-scale deployment in future communication systems.

Kim Blackmore

The price of mobility in AdHoc networks

Australian National University

Abstract: Ad-Hoc networks do not rely on any fixed infrastructure - all nodes are of equal status and network communications are established according to rules which apply uniformly to all nodes. Each node can act as a host as well as a router.

While Ad-Hoc networks may be comprised of static nodes, we are interested in Mobile Ad-Hoc networks, with wireless communications between nodes. For such networks, all nodes are assumed to move constantly, so the propagation channel between two nodes varies considerably. No compensation for channel failure due to excessive distance is possible, so the communications link between any two nodes in the network may disappear. The network interconnection topology changes constantly.

In networks with fixed interconnection topology, it is possible for each node (or some centralized controller) to exactly determine the topology and use this knowledge to optimally route packets between sending and receiving nodes. Cellular networks rely on the fixed topology between the base stations, with movement of nodes between cells introducing the need for additional location management and handoff management. Channel impairment can be compensated to ensure reliable transmission between any mobile node and some base station (fixed node) at all times.

On the other hand, in Ad-Hoc networks there is no centralised controller, and the interconnection topology changes constantly, so employing traditional routing would require constant recalculation of optimal routes. Moreover, the process of ensuring each node has an accurate knowledge of the current topology is extremely expensive, and potentially impossible. Routing protocols for Ad-Hoc networks need to be designed to allow for the dynamic nature of the network topology.

In this talk I will survey the routing protocols that have been proposed for Ad-Hoc networks. I will also discuss the simulation techniques used to evaluate routing protocols, and the assumptions about mobility that are embedded in the simulations.
 

Research Overview 3

John Ness

Wireless communications: taking theory to practice

EM Solutions

Abstract: Free space digital microwave links are both competitive and complementary to guided wave fibre optic and coaxial cable communications networks. "Free space" is not always free in that substantial licence fees based on frequency, bandwidth and geography may apply. There is continual commercial pressure to reduce costs, improve performance and maximise spectral efficiency. This paper will describe the key technical and commercial factors that EM Solutions has encountered over a 5 year period in upgrading it's microwave link products from 2mb/s to 100mb/s in frequency bands from 5GHz to 18GHz. The modulation has changed from simple low cost FSK to 32QAM and this has had major impacts on cost, hardware and propagation considerations. From a strictly engineering view point the performance of a digital link can be assessed by how closely the system approaches the theoretical limits for a given type of modulation. From a commercial view, factors such as   hardware/software failure rates, propagation reliability, network monitoring and support are critical and these influence the design and manufacture of links. How these considerations feed into the design and influence performance measures will be outlined.

The next major challenge is to develop a point to multipoint (PTMP) architecture for wireless and local area broadband networks using as much of the existing design as possible. The technical challenges to convert to point to multipoint and what has been achieved will be summarised.