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This paper investigates the benefit of adaptive modulation based on channel state information (CSI) in direct-sequence/ code-division multiple-access (DS/CDMA) multihop packet radio networks. By exploiting varying channel conditions, adaptive modulation can be used in ad hoc networks to provide upper layers with higher capacity links over which to relay traffic. Using the /spl alpha/-stable interference model, the distribution of the signal-to-interference ratio (SIR) is obtained for a slotted system of randomly, uniformly distributed nodes using multilevel coherent modulation schemes. Performance is evaluated in terms of the information efficiency, which is a new progress-related measure for multihop networks. Three types of adaptivity are analyzed, differing in the level of CSI available: 1) full knowledge of the SIR at the receiver; 2) knowledge of only the signal attenuation due to fading; and 3) knowledge of only the slow fading component of the signal attenuation. The effect of imperfect channel information is also investigated. Sample results are given for interference-limited networks experiencing fourth-power path loss with distance, Ricean fading, and lognormal shadowing.

In [1], we investigated the use of multi-hop relays (a.k.a. breadcrumbs) to extend the range of public safety communications in indoor environments where the radio propagation conditions are harsh or wireless communications are limited by distance. In this work, we consider the impact of interference on the reliability of the breadcrumb system. We develop a simulation model for the system that considers the incident area floor plan, the characteristics of radio propagation within the incident area, emergency responders' mobility, and the distributed functionalities of the breadcrumb system. We then evaluate the performance of the breadcrumb system in the presence of interference showing that it is diminished. Consequently, we consider the use of the cognitive radio technology to enable breadcrumbs to detect interference, to identify vacant channels, and to dynamically and autonomously adapt their channel selection to enhance the system performance. We develop the functionalities of cognitive radio in our simulation, re-evaluate the system performance in the presence of interference, and show that employing cognitive radios alleviates the impact of interference. We also highlight the overhead that the cognitive radio functionalities may have under favorable network conditions.

Using cooperative transmission, two or more single- antenna users can share their antennas to achieve spatial diversity in a slow fading channel. One relaying protocol that achieves diversity, amplify-and-forward (AF), is striking in its simplicity, but prior analysis has been concerned with an idealized version of AF. In practice, the signal received by the relay must be quantized and stored in finite memory before retransmission. This paper examines a quantize-and-forward (QF) relaying approach that is amenable to implementation on resource-constrained relays. We describe QF relaying with M-ary phase shift keying (PSK) and derive the maximum likelihood-based soft-decision metric for this scheme. When each M-PSK channel symbol is quantized with q bits at the relay, simulation results show that quantizing with q = 1 + log2 M bits (i.e., only one extra bit per symbol) provides comparable performance in Rayleigh fading to the idealized (unquantized) AF protocol as well as to an adaptive decode- and-forward protocol at frame error rates of practical interest. Furthermore, this performance is achieved without requiring channel decoding or channel state information at the relay (i.e., using only non-coherent detection at the relay). The proposed QF scheme allows the use of resource-limited relays (with low processing power and low memory) to achieve cooperative diversity.

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.

This paper derives new soft decision metrics for coded orthogonal signaling in symmetric /spl alpha/-stable noise, which has been used to model impulsive noise. In addition to the optimum metrics for Gaussian (/spl alpha/=2) noise and Cauchy (/spl alpha/=1) noise, a class of generalized likelihood ratio (GLR) metrics with lower side information requirements is derived. Through numerical results for a turbo code example, the Cauchy decoder is found to be robust for a wide range of /spl alpha/, and GLR metrics are found which provide performance gains relative to the Gaussian metric, but with lower complexity and less a priori information.

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

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.

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.

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 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.

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.