Thomas E. Fuja

LDPC block and convolutional codes based on circulant matrices

A class of algebraically structured quasi-cyclic (QC) low-density parity-check (LDPC) codes and their convolutional counterparts is presented. The QC codes are described by sparse parity-check matrices comprised of blocks of circulant matrices. The sparse parity-check representation allows for practical graph-based iterative message-passing decoding. Based on the algebraic structure, bounds on the girth and minimum distance of the codes are found, and several possible encoding techniques are described. The performance of the QC LDPC block codes compares favorably with that of randomly constructed LDPC codes for short to moderate block lengths. The performance of the LDPC convolutional codes is superior to that of the QC codes on which they are based; this performance is the limiting performance obtained by increasing the circulant size of the base QC code. Finally, a continuous decoding procedure for the LDPC convolutional codes is described.

A Network Coding Approach to Cooperative Diversity

This paper proposes a network coding approach to cooperative diversity featuring the algebraic superposition of channel codes over a finite field. The scenario under consideration is one in which two ldquopartnersrdquo - node A and node B - cooperate in transmitting information to a single destination; each partner transmits both locally generated information and relayed information that originated at the other partner. A key observation is that node B already knows node As relayed information (because it originated at node B) and can exploit that knowledge when decoding node As local information. This leads to an encoding scheme in which each partner transmits the algebraic superposition of its local and relayed information, and the superimposed codeword is interpreted differently at the two receivers i.e., at the other partner and at the destination node, based on their different a priori knowledge. Decoding at the destination is then carried out by iterating between the codewords from the two partners. It is shown via simulation that the proposed scheme provides substantial coding gain over other cooperative diversity techniques, including those based on time multiplexing and signal (Euclidean space) superposition.

Bandwidth- and power-efficient routing in linear wireless networks

The goal of this paper is to establish which practical routing schemes for wireless networks are most suitable for power-limited and bandwidth-limited communication regimes. We regard channel state information (CSI) at the receiver and point-to-point capacity-achieving codes for the additive white Gaussian noise (AWGN) channel as practical features, interference cancellation (IC) as possible, but less practical, and synchronous cooperation (CSI at the transmitters) as impractical. We consider a communication network with a single source node, a single destination node, and N-1 intermediate nodes placed equidistantly on a line between them. We analyze the minimum total transmit power needed to achieve a desired end-to-end rate for several schemes and demonstrate that multihop communication with spatial reuse performs very well in the power-limited regime, even without IC. However, within a class of schemes not performing IC, single-hop transmission (directly from source to destination) is more suitable for the bandwidth-limited regime, especially when higher spectral efficiencies are required. At such higher spectral efficiencies, the gap between single-hop and multihop can be closed by employing IC, and we present a scheme based upon backward decoding that can remove all interference from the multihop system with an arbitrarily small rate loss. This new scheme is also used to demonstrate that rates of O(logN) are achievable over linear wireless networks even without synchronous cooperation.

Optical orthogonal codes with unequal auto- and cross-correlation constraints

An optical orthogonal code (OOC) is a collection of binary sequences with good auto- and cross-correlation properties; they were defined by Salehi and others as a means of obtaining code-division multiple access on optical networks. Up to now, all work on OOCs have assumed that the constraint placed on the autocorrelation and that placed on the cross-correlation are the same. We consider-codes for which the two constraints are not equal. Specifically we develop bounds on the size of such OOCs and demonstrate constriction techniques for building them. The results demonstrate that a significant increase in the code size is possible by letting the autocorrelation constraint exceed the cross-correlation constraint. These results suggest that for a given performance requirement the optimal OOC may be one with unequal constraints. This paper also views OOCs with unequal auto- and cross-correlation constraints as constant-weight unequal error protection (UEP) codes with two levels of protection. The bounds derived are interpreted from this viewpoint. >

The Design and Performance of Distributed LT Codes

This paper describes techniques to decompose LT codes (a class of rateless erasure-correcting codes) into distributed LT (DLT) codes. DLT codes can be used to independently encode data from multiple sources in a network in such a way that, when the DLT-encoded packets are combined at a common relay, the resulting bit stream (called a modified LT (MLT) code) has a degree distribution approximating that of an LT code, with simulations indicating comparable performance. In essence, DLT codes are designed so that the final stage of encoding for erasure correction can be carried out by a low-complexity relay that selectively xors the bit streams generated at each source and transmits the result to the sink. This paper presents results for two-source and four-source networks. It is shown that, when the relay-to-sink link is the bottleneck, the DLT/MLT approach can yield substantial performance benefits compared with a competing strategy wherein each of the sources uses its own independent LT encoder and the resulting bit streams are time-multiplexed through the relay.

A comparative study of signal processing techniques for clustering microsensor data (a first step towards an artificial nose)

Microsensor technology has progressed to the point where it is now feasible to place several hundred sensors on a computer chip. Such a sensor array can potentially be used in many applications including detecting hazardous chemical emissions, food processsing, and fire detection. This paper addresses an important aspect involved in microsensor applications, namely how the sensor signals are processed. The problem treated involves classifying whether a sensed signal is generated by one of four chemicals. Two broad approaches to processing the sensor signals are discussed, one based on classical signal processing approaches, and one based on a model of how the olfactory system in animals functions. The classical approaches used include: Gram Schmidt orthogonalization, fast Fourier transforms, and Haar wavelets. For the experimental signals treated, the classical approaches give superior results compared to those produced by the olfactory model.

Nested Codes with Multiple Interpretations

This paper proposes a new approach to channel code design for wireless network applications. The resulting nested codes can be decoded at different effective rates by different receivers-rates that depend on the prior knowledge possessed by each receiver; we say these codes have multiple interpretations. We have identified several applications in wireless networks where this property is useful. Specific nested code constructions as well as efficient soft and hard decision decoding algorithms are described. The concept of a nested code with multiple interpretations provides flexibility in the design of error protection schemes for multi-terminal wireless networks.

On the optimum number of hops in linear wireless networks

We consider a wireless communication system with a single source node, a single destination node, and multiple relay nodes placed equidistantly between them. We limit our analysis to the case of coded TDMA multihop transmission, i.e., the nodes do not cooperate and do not try to access the channel simultaneously. Given a global constraint on bandwidth, we determine the number of hops that achieves a desired end-to-end rate with the least total transmission power. Furthermore, we examine how the optimum number of hops changes when an end-to-end delay constraint is introduced using the sphere-packing bound and computer simulations. The analysis demonstrates that the optimum number of hops depends on the end-to-end rate and the path-loss exponent. Specifically, we show the existence of an asymptotic per-link spectral efficiency, which is the preferred spectral efficiency in TDMA multihop transmission.

LDPC codes over rings for PSK modulation

This paper describes the design and analysis of low-density parity-check (LDPC) codes over rings and shows how these codes, when mapped onto appropriate signal constellations, can be used to effect bandwidth-efficient modulation. Specifically, LDPC codes are constructed over the integer rings /spl Zopf//sub m/ and G/sub m//sup 2/ and mapped onto phase-shift keying (PSK)-type signal sets to yield geometrically uniform signal space codes. This paper identifies and addresses the design issues that affect code performance. Examples of codes over /spl Zopf//sub 8/ and G/sub 64/ mapped onto 8-ary and 64-ary signal sets at a spectral efficiency of 1.5 and 2.0 bits per second per hertz (b/s/Hz) illustrate the approach; simulation of these codes over the additive white Gaussian noise (AWGN) channel demonstrates that this approach is a good alternative to bandwidth-efficient techniques based on binary LDPC codes-e.g., bit-interleaved coded modulation.

Mobile Relaying: Coverage Extension and Throughput Enhancement

This paper presents a quantitative study of the benefits that mobile relays can provide to the wireless infrastructure namely, extension of base station coverage and enhancement of wireless connection throughput. The end user can choose to connect directly to a base station, or, as an alternative, to establish a two-hop link using a relay. Relay locations are modelled as realizations of a two-dimensional Poisson process with random motion, and as such their availability to forward messages received from a base station or from an end user is analyzed. Two important performance metrics are derived for out-of-coverage end users: the probability of establishing a route and the expected duration that a route or connection can be sustained. For an end user within the coverage area, the maximum and average throughput gains that can be achieved using mobile relays are derived. These results provide insight into the benefits mobile relays can offer in terms of improving connectivity or throughput.