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Selected Areas in Communications, IEEE Journal on

Issue 2 • Date February 2007

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Displaying Results 1 - 25 of 32
  • IEEE Journal on Selected Areas in Communications

    Page(s): c1
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  • IEEE Communications Society

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  • Finite-SNR diversity-multiplexing tradeoffs in fading relay channels

    Page(s): 245 - 257
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    We analyze the diversity-multiplexing tradeoff in a fading relay channel at finite signal-to-noise ratios (SNRs). In this framework, the rate adaptation policy is such that the target system data rate is a multiple of the capacity of an additive white Gaussian noise (AWGN) channel. The proportionality constant determines how aggressively the system scales the data rate and can be interpreted as a finite-SNR multiplexing gain. The diversity gain is given by the negative slope of the outage probability with respect to the SNR. Finite-SNR diversity performance is estimated using a constrained max-flow min-cut upper bound on the relay channel capacity. Moreover, the finite-SNR diversity-multiplexing tradeoff is characterized for three practical decode and forward half-duplex cooperative protocols with different amounts of broadcasting and simultaneous reception. For each configuration, system performance is computed as a function of SNR under a system-wide power constraint on the source and relay transmissions. Our analysis yields the following findings; (i) improved multiplexing performance can be achieved at any SNR by allowing the source to transmit constantly, (ii) both broadcasting and simultaneous reception are desirable in half-duplex relay cooperation for superior diversity-multiplexing performance, and (iii) the diversity-multiplexing tradeoff at finite-SNR is impacted by the power partitioning between the source and the relay terminals. Finally, we verify our analytical results by numerical simulations. View full abstract»

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  • Asymptotically optimal cooperative wireless networks with reduced signaling complexity

    Page(s): 258 - 267
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    This paper considers an orthogonal amplify-and-forward (OAF) protocol for cooperative relay communication over Rayleigh-fading channels in which the intermediate relays are permitted to linearly transform the received signal and where the source and relays transmit for equal time durations. The diversity-multiplexing gain (D-MG) tradeoff of the equivalent space-time channel associated to this protocol is determined and a cyclic-division-algebra-based D-MG optimal code constructed. The transmission or signaling alphabet of this code is the union of the QAM constellation and a rotated version of QAM. The size of this signaling alphabet is small in comparison with prior D-MG optimal constructions in the literature and is independent of the number of participating nodes in the network. View full abstract»

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  • Cooperative lattice coding and decoding in half-duplex channels

    Page(s): 268 - 279
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    We propose novel lattice coding/decoding schemes for half-duplex outage-limited cooperative channels. These schemes are inspired by the cooperation protocols of Azarian et al. and enjoy an excellent performance-complexity tradeoff. More specifically, for the. relay channel, we first use our lattice coding framework to generalize Yang and Belfiore implementation of the non-orthogonal amplify and forward cooperation protocol. This generalization is shown to offer significant performance gains while keeping the decoding complexity manageable. We then devise a novel variant of the dynamic decode and forward protocol, along with a lattice-coded implementation, which enjoys a near-optimal diversity-multiplexing tradeoff with a low encoding/decoding complexity. Finally, for the cooperative multiple-access channel, we present a lattice-coded implementation of the non-orthogonal amplify and forward protocol and demonstrate its excellent performance-complexity tradeoff. Throughout the paper, we establish the performance gains of our proposed protocols via a comprehensive simulation study. View full abstract»

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  • Low density parity check codes for the relay channel

    Page(s): 280 - 291
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    We propose Low Density Parity Check (LDPC) code designs for the half-duplex relay channel. Our designs are based on the information theoretic random coding scheme for decode-and-forward relaying. The source transmission is decoded with the help of side information in the form of additional parity bits from the relay. We derive the exact relationships that the component LDPC code profiles in the relay coding scheme must satisfy. These relationships act as constraints for the density evolution algorithm which is used to search for good relay code profiles. To speed up optimization, we outline a Gaussian approximation of density evolution for the relay channel. The asymptotic noise thresholds of the discovered relay code profiles are a fraction of a decibel away from the achievable lower bound for decode-and-forward relaying. With random component LDPC codes, the overall relay coding scheme performs within 1.2 dB of the theoretical limit. View full abstract»

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  • Statistical channel knowledge-based optimum power allocation for relaying protocols in the high SNR regime

    Page(s): 292 - 305
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    We are concerned with transmit power optimization in a wireless relay network with various cooperation protocols. With statistical channel knowledge (in the form of knowledge of the fading distribution and the path loss information across all the nodes) at the transmitters and perfect channel state information at the receivers, we derive the optimal power allocation that minimizes high signal-to-noise ratio (SNR) approximations of the outage probability of the mutual information (MI) with amplify-and-forward (AF), decode-and-forward (DF) and distributed space-time coded (DSTC) relaying protocols operating over Rayleigh fading channels. We demonstrate that the high SNR approximation-based outage probability expressions are convex functions of the transmit power vector, and the nature of the optimal power allocation depends on whether or not a direct link between the source and the destination exists. Interestingly, for AF and DF protocols, this allocation depends only on the ratio of mean channel power gains (i.e., the ratio of the source-relay gain to the relay-destination gain), whereas with a DSTC protocol this allocation also depends on the transmission rate when a direct link exists. In addition to the immediate benefits of improved outage behavior, our results show that optimal power allocation brings impressive coding gains over equal power allocation. Furthermore, our analysis reveals that the coding gain gap between the AF and DF protocols can also be reduced by the optimal power allocation. View full abstract»

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  • Lifetime maximization via cooperative nodes and relay deployment in wireless networks

    Page(s): 306 - 317
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    Extending lifetime of battery-operated devices is a key design issue that allows uninterrupted information exchange among distributed nodes in wireless networks. Cooperative communications has recently emerged as a new communication paradigm that enables and leverages effective resource sharing among cooperative nodes. In this paper, a general framework for lifetime extension of battery-operated devices by exploiting cooperative diversity is proposed. The framework efficiently takes advantage of different locations and energy levels among distributed nodes. First, a lifetime maximization problem via cooperative nodes is considered and performance analysis for M-ary PSK modulation is provided. With an objective to maximize the minimum device lifetime under a constraint on bit-error-rate performance, the optimization problem determines which nodes should cooperate and how much power should be allocated for cooperation. Since the formulated problem is NP hard, a closed-form solution for a two-node network is derived to obtain some insights. Based on the two-node solution, a fast suboptimal algorithm is developed for multi-node scenarios. Moreover, the device lifetime is further improved by a deployment of cooperative relays in order to help forward information of the distributed nodes in the network. Optimum location and power allocation for each cooperative relay are determined with an aim to maximize the minimum device lifetime. A suboptimal algorithm is developed to solve the problem with multiple cooperative relays and cooperative nodes. Simulation results show that the minimum device lifetime of the network with cooperative nodes improves 2 times longer than the lifetime of the non-cooperative network. In addition, deploying a cooperative relay in a proper location leads up to 12 times longer lifetime than that of the non-cooperative network. View full abstract»

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  • Energy aware power allocation strategies for multihop-cooperative transmission schemes

    Page(s): 318 - 327
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    This paper is focused on the optimization of transmitted power in a cooperative decoded relaying scheme for nodes belonging to the single primary route towards. a destination. The proposed transmission protocol, referred to as Multihop Cooperative Transmission Chain (MCTC), is based on the linear combination of copies of the same message by multiple previous terminals along the route in order to maximize the multihop diversity. Power allocations among transmitting nodes in the route can be obtained according to the average (not instantaneous) node-to-node path attenuation using a recursive power assignment. The latter can be employed locally on each node with limited signalling exchange (for fixed or nomadic terminals) among nodes. In this paper the power assignments for the MCTC strategy employing conventional linear combining schemes at receivers (i.e., selection combining, maximal ratio combining and equal gain combining) have been derived analytically when the power optimization is constrained to guarantee the end-to-end outage probability. In particular, we show that the power assignment that minimize the maximum spread of received power (min-max strategy) can efficiently exploit the multihop diversity. In addition, for ad hoc networks where the energy of each node is an issue, the MCTC protocol with the min-max power assignment increases considerably the network lifetime when compared to non-cooperative multihop schemes View full abstract»

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  • Joint optimization of relay strategies and resource allocations in cooperative cellular networks

    Page(s): 328 - 339
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    This paper considers a wireless cooperative cellular data network with a base station and many subscribers in which the subscribers have the ability to relay information for each other to improve the overall network performance. For a wireless network operating in a frequency-selective slow-fading environment, the choices of relay node, relay strategy, and the allocation of power and bandwidth for each user are important design parameters. The design challenge is compounded further by the need to take user traffic demands into consideration. This paper proposes a centralized utility maximization framework for such a network. We show that for a cellular system employing orthogonal frequency-division multiple-access (OFDMA), the optimization of physical-layer transmission strategies can be done efficiently by introducing a set of pricing variables as weighting factors. The proposed solution incorporates both user traffic demands and the physical channel realizations in a cross-layer design that not only allocates power and bandwidth optimally for each user, but also selects the best relay node and best relay strategy (i.e. decode-and-forward vs. amplify-and-forward) for each source-destination pair View full abstract»

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  • CoopMAC: A Cooperative MAC for Wireless LANs

    Page(s): 340 - 354
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    Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation View full abstract»

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  • Cooperative Relay Service in a Wireless LAN

    Page(s): 355 - 368
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    As a family of wireless local area network (WLAN) protocols between physical layer and higher layer protocols, IEEE 802.11 has to accommodate the features and requirements of both ends. However, current practice has addressed the problems of these two layers separately and is far from satisfactory. On one end, due to varying channel conditions, WLANs have to provide multiple physical channel rates to support various signal qualities. A low channel rate station not only suffers low throughput, but also significantly degrades the throughput of other stations. On the other end, the power saving mechanism of 802.11 is ineffective in TCP-based communications, in which the wireless network interface (WNI) has to stay awake to quickly acknowledge senders, and hence, the energy is wasted on channel listening during idle awake time. In this paper, considering the needs of both ends, we utilize the idle communication power of the WNI to provide a Cooperative Relay Service (CRS) for WLANs with multiple channel rates. We characterize energy efficiency as energy per bit, instead of energy per second. In CRS, a high channel rate station relays data frames as a proxy between its neighboring stations with low channel rates and the Access Point, improving their throughput and energy efficiency. Different from traditional relaying approaches, CRS compensates a proxy for the energy consumed in data forwarding. The proxy obtains additional channel access time from its clients, leading to the increase of its own throughput without compromising its energy efficiency. Extensive experiments are conducted through a prototype implementation and ns-2 simulations to evaluate our proposed CRS. The experimental results show that CRS achieves significant performance improvements for both low and high channel rate stations View full abstract»

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  • Grouping and partner selection in cooperative wireless networks

    Page(s): 369 - 378
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    Various results to date have demonstrated the advantages of one or several relay nodes assisting transmissions in a wireless network. In many practical scenarios, not all nodes in the network are simultaneously involved in every transmission; therefore, protocols are needed to form groups or subsets of nodes for the purposes of cooperation. We consider this problem in the context of regenerative nodes and non-altruistic cooperation (all nodes have data of their own to transmit). For a network-wide diversity advantage, the protocol must provide each transmitting node with enough "partners" that can decode its message with high-enough probability. Assuming that the nodes cannot communicate their control decisions (distributed scenario), and that each node chooses to help n other nodes, we point out a simple, static selection strategy that guarantees diversity n+1 for all transmissions. We then consider centralized control strategies and study the additional gains that arise from a central control, under various amounts of information being available to the central controller. View full abstract»

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  • Spectral efficient protocols for half-duplex fading relay channels

    Page(s): 379 - 389
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    We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss View full abstract»

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  • Optimal relay functionality for SNR maximization in memoryless relay networks

    Page(s): 390 - 401
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    We explore the SNR-optimal relay functionality in a mernoryless relay network, i.e. a network where, during each channel use, the signal transmitted by a relay depends only on the last received symbol at that relay. We develop a generalized notion of SNR for the class of memoryless relay functions. The solution to the generalized SNR optimization problem leads to the novel concept of minimum mean squared uncorrelated error (MMSUE) estimation. For the elemental case of a single relay, we show that MMSUE estimate is a scaled version of the MMSE estimate. This scheme, that we call estimate and forward (EF), performs better than the best of amplify and forward (AF) and demodulate and forward (DF) in both parallel and serial relay networks. We determine that AF is near-optimal at low transmit power in a parallel network, while DF is near-optimal at high transmit power in a serial network. For hybrid networks that contain both serial and parallel elements, the advantage of EF over the best of AF and DF is found to be significant. Error probabilities are provided to substantiate the performance gain obtained through SNR optimality. We also show that, for Gaussian inputs, AF, DF and EF are identical View full abstract»

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  • Cooperative distributed MIMO channels in wireless sensor networks

    Page(s): 402 - 414
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    The large number of network nodes and the energy constraints make Wireless Sensor Networks (WSN) one of the most important application fields for Cooperative Diversity. Node cooperation increases the spatial diversity of wireless channels and, thus, reduces the transmitted power. In this paper, we propose a multi-hop WSN with nodes grouped in cooperative clusters that exploits transmit and receive cooperation among cluster nodes. Multi-hop transmission is carried out by concatenating single cluster-to-cluster hops, where every cluster-to-cluster link is defined as a cooperative distributed multiple-input-multiple-output (MIMO) channel. Transmit diversity is exploited through a time-division, decoder-and-forward, relaying scheme based upon two time slots: the Intracluster Slot, used for data sharing within the cluster, and the Intercluster Slot, used for transmission between clusters. At the receiver side, a distributed reception protocol is devised based upon a Selection Diversity algorithm. The proposed multi-hop cooperative WSN is optimally designed for minimum end-to-end outage probability by deriving the optimum time and power allocated on the intracluster and intercluster slots of every single hop, given a per-link energy constraint. A simplified suboptimum resource allocation is also proposed, which performs close to the optimal policy. Results show that the proposed scheme achieves diversity equal to the equivalent MIMO system and significantly reduces energy consumption with respect to. the non-cooperative channel View full abstract»

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  • Multi-source cooperation with full-diversity spectral-efficiency and controllable-complexity

    Page(s): 415 - 425
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    A general framework is developed for multi-source cooperation (MSC) protocols to improve diversity and spectral efficiency relative to repetition based alternatives that rely on single-source cooperation. The novel protocols are flexible to balance tradeoffs among diversity, spectral efficiency and decoding-complexity. Users are grouped in clusters and follow a two-phase MSC protocol which involves time division multiple access (TDMA) to separate users within a cluster, and code division multiple access (CDMA) used to separate clusters. An attractive protocol under the general MSC framework relies on distributed complex field coding (DCFC) to enable diversity order equal to the number of users per cluster. Cluster separation based on orthonormal spreading sequences leads to spectral efficiency 1/2. When the number of clusters exceeds the amount of spreading, spectral efficiency can be enhanced without sacrificing diversity, at the expense of controllable increase in complexity. Simulations corroborate our analytical claims View full abstract»

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  • Partially-coherent distributed space-time codes with differential encoder and decoder

    Page(s): 426 - 433
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    Distributed space-time coding is a mean of achieving diversity through cooperative communication in a wireless relay network. In this paper, we consider a transmission protocol that follows a two-stage model: transmission from source to relays in the first stage, followed by a simple relaying technique from relays to destination. The relays transmit a vector which is a transformation of the received vector by a relay-specific unitary transformation. We assume that the relays do not have any channel information, while the destination has only a partial-channel knowledge, by which we mean that destination knows only the relay-to-destination channel. For such a setup, we derive a Chernoff bound on the pairwise error probability and propose code design criteria. A second contribution is the differential encoding and decoding scheme for this setup, which is different from the existing ones. Furthermore, differential codes from cyclic division algebra are proposed that achieve full diversity. For our setup with two relays, a Generalized PSK code is shown to achieve full diversity, for which the decoding complexity is independent of code size View full abstract»

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  • Collaborative decoding in bandwidth-constrained environments

    Page(s): 434 - 446
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    We present a cooperative communication scheme in which a group of receivers can collaborate to decode a message that none of the receivers can individually decode. The receivers act as a virtual antenna array in which the combining must be performed over bandwidth-constrained links. The proposed approach is targeted at systems in which the cooperative information must be digitized, such as for wireless or wired links that are constrained to use digital modulation. In such systems, previously proposed schemes such as amplify-and-forward would require that a large amount of information be exchanged when there are many collaborating nodes. The approach presented in this paper, called improved least-reliable bits (I-LRB) collaborative decoding, provides a higher level of adaptation than previously proposed cooperative schemes. The I-LRB scheme utilizes reliability information and information about competing paths in soft-input, soft-output (SISO) decoders to adaptively select the amount of information that is needed to correct a particular part of a message, as well as which bits should be exchanged. Simulation results show that the proposed approach offers a significant performance advantage over a constrained-overhead, incremental form of maximal ratio combining (MRC) View full abstract»

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  • Joint optimization of relay-precoders and decoders with partial channel side information in cooperative networks

    Page(s): 447 - 458
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    We jointly optimize the relay-precoders and decoders with full or partial channel side information (CSI) in a cooperative network. Specifically, three different CSI assumptions are considered: 1) full CSI at the destination terminal and the relay terminals; 2) full CSI at the destination terminal and partial CSI at the relay terminals; 3) partial CSI at the destination terminal and the relay terminals. We show that, under the assumption of full CSI at the destination terminal and the relay terminals, the optimum relay-precoder is the cooperative transmission beamforming and the optimum decoder is a maximum ratio combiner. Under the two partial CSI assumptions, the optimum relay-precoders and decoders work in a fashion of channel selection. It is demonstrated that the proposed optimum relay-precoders and decoders improve the performance considerably View full abstract»

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  • Energy efficiency of dense wireless sensor networks: to cooperate or not to cooperate

    Page(s): 459 - 470
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    Decentralized detection in a network of wireless sensor nodes involves the fusion of information about a phenomenon of interest (PoI) from geographically dispersed nodes. In this paper, we investigate the problem of binary decentralized detection in a dense and randomly deployed wireless sensor network (WSN), whereby the communication channels between the nodes and the fusion center are bandwidth-constrained. We consider a scenario in which sensor observations, conditioned on the alternate hypothesis, are independent but not identically distributed across the sensor nodes. We compare two different fusion architectures, namely, the parallel fusion architecture (PFA) and the cooperative fusion architecture (CFA), for such bandwidth-constrained WSNs, where each sensor node is restricted to send a I-bit information to the fusion center. For each architecture, we derive expression for the probability of decision error at the fusion center. We propose a consensus flooding protocol for CFA and analyze its average energy consumption. We analyze the effects of PoI intensity, realistic link models, consensus flooding protocol, and network connectivity on the system reliability and average energy consumption for both fusion architectures. We demonstrate that a trade-off exists among spatial diversity gain, average energy consumption, delivery ratio of the consensus flooding protocol, network connectivity, node density, and Poll intensity in CFA. We then provide insight into the design of cooperative WSNs. View full abstract»

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  • Cooperative routing for distributed detection in large sensor networks

    Page(s): 471 - 483
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    In this paper, the detection of a correlated Gaussian field using a large multi-hop sensor network is investigated. A cooperative routing strategy is proposed by introducing a new link metric that characterizes the detection error exponent. Derived from the Chernoff information and Schweppe's likelihood recursion, this link metric captures the contribution of a given link to the decay rate of error probability and has the form of the capacity of a Gaussian channel with the sender transmitting the innovation of its measurement. For one-dimensional Gauss-Markov fields, the link metric can be represented explicitly as a function of the link length. Cooperative routing is achieved using the Kalman data aggregation and shortest path routing. Numerical simulations show that cooperative routing can be significantly more energy efficient than noncooperative routing for the same detection performance View full abstract»

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  • On broadcasting with cooperative diversity in multi-hop wireless networks

    Page(s): 484 - 496
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    Cooperative diversity facilitates spatio-temporal communications without requiring the deployment of physical antenna arrays. While physical layer studies on cooperative diversity have been extensive, higher layer protocols which translate the achievable reduction in the SNR per bit for a given target BER, into system wide performance enhancements are yet to mature. The challenge is that appropriate higher layer functions are needed in order to enable cooperative diversity at the physical layer. We focus on network-wide broadcasting with the use of cooperative diversity in ad hoc networks. We design a novel distributed network-wide broadcasting protocol that takes into account the physical layer dependencies that arise with cooperative diversity. We perform extensive simulations that show that our protocol can outperform the best of the noncooperative broadcasting protocols by: (a) achieving up to a threefold increase in network coverage and, (b) by decreasing the latency incurred during the broadcast by about 50%. We also construct an analytical model that captures the behavior of our protocol. Furthermore, we show that computing the optimal solution to the cooperative broadcast problem is NP-complete and construct centralized approximation algorithms. Specifically, we construct an O(N epsi)-approximation algorithm with a computational complexity of O(N4/epsi); we also construct a simpler greedy algorithm.. The costs incurred with these algorithms serve as benchmarks with which one can compare that achieved by any distributed protocol View full abstract»

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  • On the power efficiency of cooperative broadcast in dense wireless networks

    Page(s): 497 - 507
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    A fundamental problem in large scale wireless networks is the energy efficient broadcast of source messages to the whole network. The energy consumption increases as the network size grows, and the optimization of broadcast efficiency becomes more important. In this paper, we study the optimal power allocation problem for cooperative broadcast in dense large-scale networks. In the considered cooperation protocol, a single source initiates the transmission and the rest of the nodes retransmit the source message if they have decoded it reliably. Each node is allocated an-orthogonal channel and the nodes improve their receive signal-to-noise ratio (SNR), hence the energy efficiency, by maximal-ratio combining the receptions of the same packet from different transmitters. We assume that the decoding of the source message is correct as long as the receive SNR exceeds a predetermined threshold. Under the optimal cooperative broadcasting, the transmission order (i.e., the schedule) and the transmission powers of the source and the relays are designed so that every node receives the source message reliably and the total power consumption is minimized. In general, finding the best scheduling in cooperative broadcast is known to be an NP-complete problem. In this paper, we show that the optimal scheduling problem can be solved for dense networks, which we approximate as a continuum of nodes. Under the continuum model, we derive the optimal scheduling and the optimal power density. Furthermore, we propose low-complexity, distributed and power efficient broadcasting schemes and compare their power consumptions with those-of-a traditional noncooperative multihop transmission. View full abstract»

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IEEE Journal on Selected Areas in Communications focuses on all telecommunications, including telephone, telegraphy, facsimile, and point-to-point television, by electromagnetic propagation.

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Muriel Médard
MIT