Abstract—We develop and analyze low-complexity cooperative diversity protocols that combat fading induced by multipath propagation in wireless networks. The underlying techniques exploit space diversity available through cooperating terminals’ relaying signals for one another. We outline several strategies employed by the cooperating radios, including fixed relaying schemes such as amplify-and-forward and decode-and-forward, selection relaying schemes that adapt based upon channel measurements between the cooperating terminals, and incremental relaying schemes that adapt based upon limited feedback from the destination terminal. We develop performance characterizations in terms of outage events and associated outage probabilities, which measure robustness of the transmissions to fading, focusing on the high signal-to-noise ratio (SNR) regime. Except for fixed decode-and-forward, all of our cooperative diversity protocols are efficient in the sense that they achieve full diversity (i.e., second-order diversity in the case of two terminals), and, moreover, are close to optimum (within 1.5 dB) in certain regimes. Thus, using distributed antennas, we can provide the powerful benefits of space diversity without need for physical arrays, though at a loss of spectral efficiency due to half-duplex operation and possibly at the cost of additional receive hardware. Applicable to any wireless setting, including cellular or ad hoc networks—wherever space constraints preclude the use of physical arrays—the performance characterizations reveal that large power or energy savings result from the use of these protocols. Index Terms—Diversity techniques, fading channels, outage probability, relay channel, user cooperation, wireless networks. I.

How much information can be carried over a wireless network with a multiplicity of nodes? What are the optimal strategies for information transmission and cooperation among the nodes? We obtain sharp information theoretic scaling laws under some conditions.

In [1], Gupta and Kumar determined the capacity of wireless networks under certain assumptions, among them point-to-point coding, which excludes for example multi-access and broadcast codes. In this paper, we consider essentially the same physical model of a wireless network under a different traffic pattern, namely the relay traffic pattern, but we allow for arbitrarily complex network coding. In our model, there is only one active source/destination pair, while all other nodes assist this transmission. We show code constructions leading to achievable rates and derive upper bounds from the max-flow min-cut theorem. It is shown that lower and upper bounds meet asymptotically as the number of nodes in the network goes to infinity, thus proving that the capacity of the wireless network with n nodes under the relay traffic pattern behaves like log n bits per second. This demonstrates also that network coding is essential: under the point-topoint coding assumption considered in [1], the achievable rate is constant, independent of the number of nodes.

Abstract — We develop and analyze space-time coded cooperative diversity protocols for combating multipath fading across multiple protocol layers in a wireless network. The protocols exploit spatial diversity available among a collection of distributed terminals that relay messages for one another in such a manner that the destination terminal can average the fading, even though it is unknown a priori which terminals will be involved. In particular, a source initiates transmission to its destination, and many relays potentially receive the transmission. Those terminals that can fully decode the transmission utilize a space-time code to cooperatively relay to the destination. We demonstrate that these protocols achieve full spatial diversity in the number of cooperating terminals, not just the number of decoding relays, and can be used effectively for higher spectral efficiencies than repetitionbased schemes. We discuss issues related to space-time code design for these protocols, emphasizing codes that readily allow for appealing distributed versions. I.

Abstract—The capacity of a particular large Gaussian relay network is determined in the limit as the number of relays tends to infinity. Upper bounds are derived from cut-set arguments, and lower bounds follow from an argument involving uncoded transmission. It is shown that in cases of interest, upper and lower bounds coincide in the limit as the number of relays tends to infinity. Hence, this paper provides a new example where a simple cut-set upper bound is achievable, and one more example where uncoded transmission achieves optimal performance. The findings are illustrated by geometric interpretations. The techniques developed in this paper are then applied to a sensor network situation. This is a network joint source–channel coding problem, and it is well known that the source–channel separation theorem does not extend to this case. The present paper extends this insight by providing an example where separating source from channel coding does not only lead to suboptimal performance—it leads to an exponential penalty in performance scaling behavior (as a function of the number of nodes). Finally, the techniques developed in this paper are extended to include certain models of ad hoc wireless networks, where a capacity scaling law can be established: When all nodes act purely as relays for a single source–destination pair, capacity grows with the logarithm of the number of nodes. Index Terms—Capacity, CEO problem, joint source–channel coding, network, relay, sensor network, separation theorem, uncoded transmission. I.

Abstract not found

Abstract — In this paper we study the broadcast capacity of multihop wireless networks which we define as the maximum rate at which broadcast packets can be generated in the network such that all nodes receive the packets successfully within a given time. To asses the impact of topology and interference on the broadcast capacity we employ the Physical Model and Generalized Physical Model for the channel. Prior work was limited either by density constraints or by using the less realistic but manageable Protocol model [1], [2]. Under the Physical Model, we find that the broadcast capacity is within a constant factor of the channel capacity for a wide class of network topologies. Under the Generalized Physical Model, on the other hand, the network configuration is divided into three regimes depending on how the power is tuned in relation to network density and size and in which the broadcast capacity is asymptotically either zero, constant or unbounded. As we show, the broadcast capacity is limited by distant nodes in the first regime and by interference in the second regime. In the second regime, which covers a wide class of networks, the broadcast capacity is within a constant factor of the bandwidth. I.

Wireless networks contain an inherent distributed spatial diversity that can be exploited by the use of relaying. Relay networks take advantage of the broadcast-oriented nature of radio and require node-based, rather than link-based, protocols. Prior work on relay networks has studied performance limits either with unrealistic assumptions, complicated protocols, or only a single relay. In this paper, a practical approach to networks comprising multiple relays operating over orthogonal time slots is proposed based on a generalization of hybrid-ARQ. In contrast with conventional hybrid-ARQ, retransmitted packets do not need to come from the original source radio but could instead be sent by relays that overhear the transmission. An information theoretic framework is exposed that establishes the performance limits of such systems in a block fading environment, and numerical results are presented for some representative topologies and protocols. The results indicate a significant improvement in the energy-latency tradeoff when compared with conventional multihop protocols implemented as a cascade of point-to-point links.

Abstract—Recently, several landmark wireless sensor network deployment studies clearly demonstrated a large discrepancy between experimentally observed communication properties and properties produced by widely used simulation models. Our first goal is to provide sound foundations for conclusions drawn from these studies by extracting the relationship between pairs of location (e.g distance) and communication properties (e.g. reception rate) using non-parametric statistical techniques and by calculating intervals of confidence for all claims. The objective is to determine not only the most likely value of one feature for an alternate given feature value, but also to establish a complete characterization of the relationship by providing a probability density function (PDF). The PDF provides the likelihood that any particular value of one feature is associated with a given value of another feature. Furthermore, we study not only individual link properties, but also their correlation with respect to common transmitters and receivers and their geometrical location. The second objective is to develop a series of wireless network simulation environments that produce networks of an arbitrary size and under arbitrary deployment rules with realistic communication properties. For this task we use an iterative improvement-based optimization procedure to generate instances of the network that are statistically similar to empirically observed networks. We evaluate the accuracy of the conclusions drawn using the proposed model and therefore comprehensiveness of the considered properties on a set of standard communication tasks, such as connectivity maintenance and routing. Index terms: sensor networks, wireless channel modeling, simulations, network measurements, experimentation with real networks/testbeds, statistics. I.

The transmission capacity of a wireless ad hoc network can be defined as the maximum allowable area spectral efficiency such that the outage probability does not exceed some specified threshold. This work studies the improvement in transmission capacity obtainable with successive interference cancellation (SIC), an important receiver technique that has been shown to achieve the capacity of several classes of multiuser channels, but has not been carefully evaluated in the context of an ad hoc wireless network. This paper develops closedform bounds for the transmission capacity of CDMA ad hoc networks with SIC receivers, for both perfect and imperfect interference cancellation. In addition to providing the first closedform capacity results for SIC in ad hoc networks (or, to our knowledge, any type of multiuser detection), design-relevant insights are made possible. In particular, although the capacity gain from perfect SIC is very large, any imperfections in the interference cancellation rapidly degrade its usefulness. More encouragingly from a receiver complexity standpoint, due to the geographic properties of ad hoc networks, only a few – often just one – interfering nodes need to be cancelled in order to get the vast majority of the available performance gain.