Matthew C. Valenti

Asynchronous cooperative diversity

Cooperative diversity, which employs multiple nodes for the simultaneous relaying of a given packet in wireless ad hoc networks, has been shown to be an effective means of improving diversity, and, hence, mitigating the detrimental effects of multipath fading. However, in previously proposed cooperative diversity schemes, it has been assumed that coordination among the relays allows for accurate symbol-level timing synchronization at the destination and orthogonal channel allocation, which can be quite costly in terms of signaling overhead in mobile ad hoc networks, which are often defined by their lack of a fixed infrastructure and the difficulty of centralized control. In this paper, cooperative diversity schemes are considered that do not require symbol-level timing synchronization or orthogonal channelization between the relays employed. In the process, a novel minimum mean-squared error (MMSE) receiver is designed for combining disparate inputs in the multiple-relay channel. Outage probability calculations and simulation results demonstrate the not unexpected significant performance gains of the proposed schemes over single-hop transmission, and, more importantly, demonstrate performance comparable to schemes requiring accurate symbol-level synchronization and orthogonal channelization

Iterative channel estimation and decoding of pilot symbol assisted turbo codes over flat-fading channels

A method for coherently detecting and decoding turbo-coded binary phase shift keying (BPSK) signals transmitted over frequency-flat fading channels is discussed. Estimates of the complex channel gain and variance of the additive noise are derived first from known pilot symbols and an estimation filter. After each iteration of turbo decoding, the channel estimates are refined using information fed back from the decoder. Both hard-decision and soft-decision feedback are considered and compared with three baseline turbo-coded systems: (1) a BPSK system that has perfect channel estimates; (2) a system that uses differential phase shift keying and hence needs no estimates; and (3) a system that performs channel estimation using pilot symbols but has no feedback path from decoder to estimator. Performance can be further improved by borrowing channel estimates from the previously decoded frame. Simulation results show the influence of pilot symbol spacing, estimation filter size and type, and fade rate. Performance within 0.49 and 1.16 dB of turbo-coded BPSK with perfect coherent detection is observed at a bit-error rate of 10/sup -4/ for normalized fade rates of f/sub d/T/sub s/=0.005 and f/sub d/T/sub s/=0.02, respectively.

Communication delays in wide area measurement systems

This paper provides a tutorial introduction to phasor measurement units (PMU) when applied in a power system environment, provides an overview of communication alternatives for wide area measurement systems (WAMS), and computes the delay budget for each type of communication link. The goal of this study is to provide data regarding the communication delay that can be incorporated into the analysis and simulation of WAMS.

The UMTS Turbo Code and an Efficient Decoder Implementation Suitable for Software-Defined Radios

This paper provides a description of the turbo code used by the UMTS third-generation cellular standard, as standardized by the Third-Generation Partnership Project (3GPP), and proposes an efficient decoder suitable for insertion into software-defined radio architectures or for use in computer simulations. Because the decoder is implemented in software, rather than hardware, single-precision floating-point arithmetic is assumed and a variable number of decoder iterations is not only possible but desirable. Three twists on the well-known log-MAP decoding algorithm are proposed: (1) a linear approximation of the correction function used by the max* operator, which reduces complexity with only a negligible loss in BER performance; (2) a method for normalizing the backward recursion that yields a 12.5% savings in memory usage; and (3) a simple method for halting the decoder iterations based only on the log-likelihood ratios.

Distributed turbo codes: towards the capacity of the relay channel

A novel coding technique is proposed for the relay channel. The source broadcasts a recursive convolutional code to both relay and destination. After detecting the data broadcasted by the source, the relay interleaves and re-encodes the message prior to forwarding it to the destination. Because the destination receives both codes in parallel, a distributed turbo code is embedded in the relay channel. Simulation results show that the proposed code performs close to the information-theoretic bound on outage event probability of decode-and-forward relaying.

Iterative demodulation and decoding of turbo-coded M-ary noncoherent orthogonal modulation

This paper considers bit-interleaved coded modulation (BICM) with a turbo channel code and M-ary orthogonal modulation. The BICM signal is iteratively demodulated and decoded in a noncoherent fashion. A soft demodulator suitable for noncoherent orthogonal modulation is presented, and the convergence of the iterative receiver is analyzed through extrinsic information transfer charts. The demodulator can work either with or without fading amplitude estimates. Extensive simulation results are presented for the well-known cdma-2000 turbo code, and the results are compared with the corresponding channel capacities, which are computed using a Monte Carlo technique. The results indicate gains of up to 1 dB relative to noniterative BICM can be achieved with the iterative receiver.

On the throughput of Bluetooth data transmissions

Analytical expressions for the throughput (in kbit/s) as a function of channel symbol signal-to-noise ratio (E/sub s//N/sub 0/) are derived for the six Bluetooth ACL packets that use automatic repeat request (ARQ). The analysis is exact under the assumptions that the outer CRC code provides perfect error detection and that the channel remains stationary for the duration of each packet. Using an expression for noncoherent correlated (h<0.5) full response FSK signals, numerical results are provided for AWGN and quasi-static Rayleigh fading channels. These curves are an appropriate benchmark against which practical demodulators and custom error control techniques may be compared.

The Outage Probability of a Finite Ad Hoc Network in Nakagami Fading

An ad hoc network with a finite spatial extent and number of nodes or mobiles is analyzed. The mobile locations may be drawn from any spatial distribution, and interference-avoidance protocols or protection against physical collisions among the mobiles may be modeled by placing an exclusion zone around each radio. The channel model accounts for the path loss, Nakagami fading, and shadowing of each received signal. The Nakagami m-parameter can vary among the mobiles, taking any positive value for each of the interference signals and any positive integer value for the desired signal. The analysis is governed by a new exact expression for the outage probability, defined to be the probability that the signal-to-interference-and-noise ratio (SINR) drops below a threshold, and is conditioned on the network geometry and shadowing factors, which have dynamics over much slower timescales than the fading. By averaging over many network and shadowing realizations, the average outage probability and transmission capacity are computed. Using the analysis, many aspects of the network performance are illuminated. For example, one can determine the influence of the choice of spreading factors, the effect of the receiver location within the finite network region, and the impact of both the fading parameters and the attenuation power laws.

Performance of turbo codes in interleaved flat fading channels with estimated channel state information

This paper investigates the performance of a turbo coded system transmitting over correlated flat fading channels with channel interleaving and both perfect and estimated channel state information. We show that channel interleaving is necessary on a correlated fading channel, and that the performance degrades as the product of fading bandwidth and symbol duration (BT/sub s/) decreases. We consider both Rayleigh and Rician fading, and compare the performance of log-maximum a posteriori (log-MAP) and soft output Viterbi algorithm (SOVA) based decoding algorithms. It is shown that for fading channels, the performance of the log-MAP algorithm is considerably superior to that of the SOVA algorithm. A simple method for estimating the fading amplitudes and noise variance is proposed, and the impact of its use is investigated. It is shown that estimating the fades degrades the performance only slightly, and estimating the noise variance does not noticeably affect the performance.

Coded transmit macrodiversity: block space-time codes over distributed antennas

This paper considers the combination of space-time codes and macrodiversity to the cellular downlink. The antenna array used by the space-time code is comprised of the antennas of two or three geographically separated base stations. For ease of exposition and to reduce algorithmic complexity, we have restricted our attention to space-time block codes. Simulation results indicate a significant improvement in energy efficiency at remote locations when the proposed system is used.