Michael R. Souryal

Performance of Amplify-and-Forward and Decode-and-Forward Relaying in Rayleigh Fading with Turbo Codes

Cooperative transmission, in which a source and relay cooperate to send a message to a destination, can provide spatial diversity against fading in wireless networks. We derive analytical expressions for the error probability of amplify-and-forward (AF), decode-and-forward (DF), and a new hybrid AF/DF relaying protocol, for systems using strong forward error correction in quasi-static Rayleigh fading channels, and these expressions are shown to compare favorably with simulation results using turbo codes. Analytical results include an exact expression for the distribution of the SNR in AF transmission. For the protocols that achieve diversity (AF, adaptive DF, and hybrid AF/DF), the optimum position of the relay is midway between the source and destination, implying that mutual relaying (or partnering) to a common destination is suboptimal

Adaptive modulation with imperfect channel information in OFDM

Adaptive modulation in OFDM is a means by which the OFDM transmitter adapts the subchannel bit and power allocation to the amplitude response of the frequency selective channel for improved performance in high data rate wireless communications. Previous studies demonstrating the performance gains of adaptive modulation have assumed the availability of perfect channel state information. This paper examines the impact on performance of an adaptive OFDM system due to imperfect channel information arising from realistic channel estimators and time-varying channels. The latter is of particular relevance to mobile wireless applications, for which OFDM has received much attention. The performance of a typical channel estimator is found to be sufficient to justify the use of adaptive modulation in wireless OFDM systems, but in mobile situations the effect of outdated channel information in the presence of quickly time-varying channels can be significant. The use of channel prediction is shown to mitigate the impact of outdated channel information and improve performance when the Doppler spread, or mobile velocity, is high.

Ad hoc, multihop CDMA networks with route diversity in a Rayleigh fading channel

In an ad hoc multihop CDMA network, fading can adversely affect the expected progress per hop of a packet. However, the effects of fading can be mitigated through route diversity, where a given source-destination pair has multiple possible paths through which to route packets. First, the distribution of the interference power is derived using a previously proposed model, and with Rayleigh fading the distribution is shown to be the same as for the non fading case except for a scaling factor. Expressions for the expected progress per hop are derived for the cases with and without next-hop route diversity. We find that the expected progress per hop in a Rayleigh fading channel can be significantly improved with low-order route diversity.

Cooperative turbo coding with time-varying rayleigh fading channels

In a cooperative transmission system, a source cooperates with a relay to transmit a message to a local destination. This paper proposes an adaptive cooperative protocol utilizing turbo codes and studies the trade-off between spatial and temporal diversity in increasingly mobile channels. While the advantage of cooperation is most pronounced in slow fading channels—on the order of 8-12 dB for the system considered here—the benefit gradually decreases with increasing mobility and vanishes at a normalized Doppler spread of 10 –2. Nevertheless, for typical wireless packet data services, cooperative coding can provide appreciable gain even in moderately mobile environments (e.g., up to 50 km/h). While the protocol implicitly adapts to varying channel conditions, adaptivity based on explicit knowledge of channel state information is also considered, and we find that the adaptive choice of two relays reduces the power requirement in half.

Adaptive modulation in ad hoc DS/CDMA packet radio networks

This paper investigates the benefit of adaptive modulation based on channel state information (CSI) in DS/CDMA multihop packet radio networks. By exploiting varying channel conditions on different links, adaptive modulation can be used in ad hoc networks to provide upper layers with higher capacity links over which to route traffic. Performance is evaluated in terms of the information efficiency, a new progress-related measure for multihop networks. Three types of adaptivity are analyzed, differing in the level of CSI available: (i) full knowledge of the SIR at the receiver, (ii) knowledge of only the signal attenuation due to fading, and (iii) knowledge of only the slow fading component of the signal attenuation. The effect on performance of imperfect channel information is also investigated. Sample results are given for interference-limited networks experiencing fourth-power path loss with distance, Rayleigh fading, and lognormal shadowing.

Non-Coherent Amplify-and-Forward Generalized Likelihood Ratio Test Receiver

This paper proposes a simple non-coherent amplify- and-forward receiver for the relay channel and evaluates its diversity performance for Rayleigh fading channels. We use the generalized likelihood ratio test to obtain the decision rule in closed form, independent of the fading distribution. The receiver is developed for M-ary orthogonal signals and multiple relays. The only side information required is the average noise energy at the receiver; no statistical knowledge of the channel gains is needed. We develop closed-form upper and lower bounds on the probability of error of this receiver for the case of binary signaling with a single relay and show that this receiver achieves near-full diversity, with the probability of error decreasing with increasing signal-to-noise ratio (SNR) as log2 (SNR) /SNR2 for large SNR. Additional results obtained by simulation demonstrate increasing diversity gain with additional relays.

Exploiting multiuser diversity in reservation random access

In this paper, we study the uplink of a cellular network where reservation random access schemes such as reservation slotted ALOHA and reservation slotted nonpersistent ISMA are used for multiple access. When channel state information is not available to users when they contend for channel access, we find that capture effect inherent in wireless communications can exploit the channel variations and reserve users with favorable channels for data transmission. We investigate two channel models where in the first model, channel variations come from time-varying Nakagami-m fading and in the second model, mobile users have both time-varying fading and path loss

User-cooperative transmission with channel feedback in slow fading environment

User-cooperative transmission exploits spatial diversity created by antenna sharing in a wireless network. The paper investigates the performance of narrowband, slowly fading and delay-limited cooperation systems where perfect channel state information (CSI) is available at cooperative transmitters. Power control algorithms which attempt to minimize the outage probability under short-term and long-term power constraints are developed. We show that significant improvement of outage probability is achieved through exploiting the CSI of forward channels.

Interference model for ad hoc DS/CDMA packet radio networks with variable coherent modulation

This paper presents a model for the multiple access interference (MAI) in an ad hoc DS/CDMA packet radio network in which multiple modulation schemes are used. One application of this model is in the performance analysis of ad hoc networks using adaptive modulation, where each node may be using a different modulation scheme depending on channel conditions or rate requirements. Previous work has shown that the MAI in a random access packet radio network can be characterized by an /spl alpha/-stable distribution. This model is extended to account for systems using two-dimensional constellations and is compared with simulation results to validate the model. In particular, the accuracy of the model is examined for low processing gain, and a previously used assumption of independent shadowing on different links is tested in a correlated shadowing environment.

Diversity Performance of a Practical Non-Coherent Detect-and-Forward Receiver

We propose a non-coherent receiver for the fixed detect-and-forward relay channel and derive a closed-form tight upper bound on its bit error probability in Rayleigh fading channels. Using this upper bound, we show that the receiver achieves nearly full, second-order diversity, and the gap from full diversity is quantified. While the general receiver depends on the source-relay channel statistics, we show that similar diversity performance can be achieved when only first-order statistics of the source-destination channel are known. The receiver is derived using the generalized likelihood ratio test to eliminate dependence on the channel gains, applies to M-ary orthogonal signal sets, and is independent of the fading distribution. The results demonstrate that low-complexity transceivers, such as those used in some sensor networks, can benefit from cooperative diversity.