Lund University Publications
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Lund University Lund University Publications2000-01-01T00:00+00:001dailyEnergy-Efficient Cooperative Relaying over Fading Channels with Simple Relay Selection
https://lup.lub.lu.se/search/publication/0ac45314-d919-4f3e-9d73-d8d4201969b6
Madan, R.Mehta, N.B.Molisch, AndreasZhang, Jin2008We consider a cooperative wireless network where a set of nodes cooperate to relay in parallel the information from a source to a destination using a decode-and-forward approach. The source broadcasts the data to the relays, some or all of which cooperatively beamform to forward the data to the destination. We generalize the standard approaches for cooperative communications in two key respects: (i) we explicitly model and factor in the cost of acquiring channel state information (CSI), and (ii) we consider more general selection rules for the relays and compute the optimal one among them. In particular, we consider simple relay selection and outage criteria that exploit the inherent diversity of relay networks and satisfy a mandated outage constraint. These criteria include as special cases several relay selection criteria proposed in the literature. We obtain expressions for the total energy consumption for general relay selection and outage criteria for the non-homogeneous case, in which different relay links have different mean channel power gains, and the homogeneous case, in which the relay links statistics are identical. We characterize the structure of the optimal transmission scheme. Numerical results show that the cost of training and feedback of CSI is significant. The optimal strategy is to use a varying subset (and number) of relay nodes to cooperatively beamform at any given time. Depending on the relative location of the relays, the source, and the destination, numerical computations show energy savings of about 16% when an optimal relay selection rule is used. We also study the impact of shadowing correlation on the energy consumption for a cooperative relay network.http://lup.lub.lu.se/record/5336037http://dx.doi.org/10.1109/TWC.2008.06090scopus:50049094366engIEEE Transactions on Wireless Communications; 7(8), pp 3013-3025 (2008)ISSN: 1536-1276Electrical Engineering, Electronic Engineering, Information EngineeringEnergy-Efficient Cooperative Relaying over Fading Channels with Simple Relay Selectioncontributiontojournal/articleinfo:eu-repo/semantics/articletextApplying antenna selection in WLANs for achieving broadband multimedia communications
https://lup.lub.lu.se/search/publication/8e6dbaac-bea1-4fbb-8b66-b373fae6ea0a
Zhang, HongyuanMolisch, AndreasZhang, Jin2006A combination of orthogonal frequency division multiplexing (OFDM) and Multiple-input-multiple-output(MIMO) systems appears to be a promising solution for the PHY layer of indoor multimedia transmission via wireless Local Area Networks (WLANs). Antenna selection is an excellent way of reducing the hardware costs of MIMO-OFDM systems while retaining high performance. This paper addresses two major practical concerns for the application of antenna selection: antenna selection training protocol design, and calibration to solve RF imbalance. We present novel solutions that are especially suitable for slowly time-varying environments, e.g., indoor scenarios, sports stadiums, and shopping malls. Specifically, a low Doppler spread associated with such environments enables us to train all antenna subsets by multiple training packets transmitted in burst; consequently antenna selection techniques can be accommodated in the emerging standards with minimum modifications. In order to deal with RF imbalance, we propose a novel calibration procedure that reduces the performance degradations. Both numerical and analytical approaches are used to verify the effectiveness of the proposed solutions, which make antenna selection more easily adaptable for high-throughput WLAN systems. Our solutions have been accommodated in the current draft of the IEEE 802.11n standard for high-throughput WLANs.http://lup.lub.lu.se/record/685532http://dx.doi.org/10.1109/TBC.2006.884831wos:000242390500006scopus:33845603462engIEEE Transactions on Broadcasting; 52(4), pp 475-482 (2006)ISSN: 0018-9316Electrical Engineering, Electronic Engineering, Information Engineeringantenna selectionMIMO-OFDMRF imbalanceWLANApplying antenna selection in WLANs for achieving broadband multimedia communicationscontributiontojournal/articleinfo:eu-repo/semantics/articletextApproximating a sum of random variables with a lognormal
https://lup.lub.lu.se/search/publication/f90bc38c-e15d-41ca-9fde-2379e07b81cf
Mehta, Neelesh B.Wu, JingxianMolisch, AndreasZhang, Jin2007A simple, novel, and general method is presented in this paper for approximating the sum of independent or arbitrarily correlated lognormal random variables (RV) by a single lognormal RV. The method is also shown to be applicable for approximating the sum of lognormal-Rice and Suzuki RVs by a single lognormal RV. A sum consisting of a mixture of the above distributions can also be easily handled. The method uses the moment generating function (MGF) as a tool in the approximation and does so without the extremely precise numerical computations at a large number of points that were required by the previously proposed methods in the literature. Unlike popular approximation methods such as the Fenton-Wilkinson method and the Schwartz-Yeh method, which have their own respective short-comings, the proposed method provides the parametric flexibility to accurately approximate different portions of the lognormal sum distribution. The accuracy of the method is measured both visually, as has been done in the literature, as well as quantitatively, using curve-fitting metrics. An upper bound on the sensitivity of the method is also provided.http://lup.lub.lu.se/record/645754http://dx.doi.org/10.1109/TWC.2007.051000wos:000247984000039scopus:34547485292engIEEE Transactions on Wireless Communications; 6(7), pp 2690-2699 (2007)ISSN: 1536-1276Electrical Engineering, Electronic Engineering, Information Engineeringmoment generating functioncharacteristic functionmoment methodslognormal-Rice distributionSuzuki distributionlognormal distributioncorrelationapproximation methodsco-channelinterferenceApproximating a sum of random variables with a lognormalcontributiontojournal/articleinfo:eu-repo/semantics/articletextPerformance of fountain codes in collaborative relay networks
https://lup.lub.lu.se/search/publication/809f9edf-a02f-429b-b708-190518b7ae86
Molisch, AndreasMehta, Neelesh B.Yedidia, Jonathan S.Zhang, Jin2007Cooperative communications, where parallel relays forward information to a destination node, can greatly improve the energy efficiency and latency in ad-hoc networks. However, current networks do not fully exploit its potential as they only use traditional energy-accumulation, which is often used in conjunction with repetition coding or cooperative space-time codes. In this paper, we show that the concept of mutual-information -accumulation can be realized with the help of fountain codes, and leads to a lower energy expenditure and a lower transmission time than energy accumulation. We then provide an analysis of the performance of mutual information accumulation in relay networks with N relay nodes. We first analyze the quasi-synchronuous scenario where the source stops transmitting and the relay nodes start transmitting after L relay nodes have successfully decoded the source data. We show that an optimum L exists, and is typically on the order of 3 or 4. We also give closed-form equations for the energy savings that can be achieved by the use of mutual-information-accumulation at the receiver. We then analyze and provide bounds for an alternate scenario where each relay node starts its transmission to the destination as soon as it has decoded the source data, independent of the state of the other relay nodes. This approach further reduces the transmission time, because the transmission by the relay nodes helps the other relay nodes that are still receiving.http://lup.lub.lu.se/record/974355http://dx.doi.org/10.1109/TWC.2007.060232wos:000250765800030scopus:36249012581engIEEE Transactions on Wireless Communications; 6(11), pp 4108-4119 (2007)ISSN: 1536-1276Electrical Engineering, Electronic Engineering, Information Engineeringnetworksradiofountain codecooperative communicationsenergy accumulationrelaytransmit energy minimizationPerformance of fountain codes in collaborative relay networkscontributiontojournal/articleinfo:eu-repo/semantics/articletextEnergy-Efficient Decentralized Cooperative Routing in Wireless Networks
https://lup.lub.lu.se/search/publication/4264a745-116d-49c0-bf37-13df6490bb1a
Madan, RiteshMehta, Neelesh B.Molisch, AndreasZhang, Jin2009Wireless adhoc networks transmit information from a source to a destination via multiple hops in order to save energy and, thus, increase the lifetime of battery-operated nodes. The energy savings can be especially significant in cooperative transmission schemes, where several nodes cooperate during one hop to forward the information to the next node along a route to the destination. Finding the best multi-hop transmission policy in such a network which determines nodes that are involved in each hop, is a very important problem, but also a very difficult one especially when the physical wireless channel behavior is to be accounted for and exploited. We model the above optimization problem for randomly fading channels as a decentralized control problem - the channel observations available at each node define the information structure, while the control policy is defined by the power and phase of the signal transmitted by each node. In particular, we consider the problem of computing an energy-optimal cooperative transmission scheme in a wireless network for two different channel fading models: (i) slow fading channels, where the channel gains of the links remain the same for a large number of transmissions, and (ii) fast fading channels, where the channel gains of the links change quickly from one transmission to another. For slow fading, we consider a factored class of policies (corresponding to local cooperation between nodes), and show that the computation of an optimal policy in this class is equivalent to a shortest path computation on an induced graph, whose edge costs can be computed in a decentralized manner using only locally available channel state information (CSI). For fast fading, both CSI acquisition and data transmission consume energy. Hence, we need to jointly optimize over both these; we cast this optimization problem as a large stochastic optimization problem. We then jointly optimize over a set of CSI functions of the local channel states, and a corresponding factored class of control policies corresponding to local cooperation between nodes with a local outage constraint. The resulting optimal scheme in this class can again be computed efficiently in a decentralized manner. We demonstrate significant energy savings for both slow and fast fading channels through numerical simulations of randomly distributed networks.http://lup.lub.lu.se/record/1401851http://dx.doi.org/10.1109/TAC.2009.2012979wos:000264397200008scopus:63449103477eng54(3), pp 512-527 (2009)ISSN: 0018-9286Electrical Engineering, Electronic Engineering, Information Engineeringmultiple output (MIMO) systemsmultiple inputAd hoc networkschannel state information (CSI)Energy-Efficient Decentralized Cooperative Routing in Wireless Networkscontributiontobookanthology/conferenceinfo:eu-repo/semantics/conferencePapertextAsynchronous interference mitigation in cooperative base station systems
https://lup.lub.lu.se/search/publication/eb0db84d-f362-4267-857d-65df905657f2
Zhang, HongyuanMehta, Neelesh B.Molisch, AndreasZhang, JinDai, Huaiyu2008Cooperative transmission by base stations (BSs) can significantly improve the spectral efficiency of multiuser, multicell, multiple input multiple output (MIMO) systems. We show that contrary to what is often assumed in the literature, the multiuser interference in such systems is fundamentally asynchronous. Intuitively, perfect timing-advance mechanisms can at best only ensure that the desired signal components - but not also the interference components - are perfectly aligned at their intended mobile stations. We develop an accurate mathematical model for the asynchronicity, and show that it leads to a significant performance degradation of existing designs that ignore the asynchronicity of interference. Using three previously proposed linear precoding design methods for BS cooperation, we develop corresponding algorithms that are better at mitigating the impact of the asynchronicity of the interference. Furthermore, we also address timing-advance inaccuracies (jitter), which are inevitable in a practical system. We show that using jitter-statistics-aware precoders can mitigate the impact of these inaccuracies as well. The insights of this paper are critical for the practical implementation of BS cooperation in multiuser MIMO systems, a topic that is typically oversimplified in the literature.http://lup.lub.lu.se/record/1199891http://dx.doi.org/10.1109/TWC.2008.060193wos:000252374600023scopus:38349163612engIEEE Transactions on Wireless Communications; 7(1), pp 155-165 (2008)ISSN: 1536-1276Electrical Engineering, Electronic Engineering, Information Engineeringmultiuser MIMOmean square errorprecodinglinearjitterbase station cooperationinterference leakagetiming-advancespectral efficiencyAsynchronous interference mitigation in cooperative base station systemscontributiontojournal/articleinfo:eu-repo/semantics/articletext