Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks

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 repetition-based schemes. We discuss issues related to space-time code design for these protocols, emphasizing codes that readily allow for appealing distributed versions.     

A cross-layer protocol for exploiting cooperative diversity in multi-hop wireless ad hoc networks

Cooperative communication has emerged to reap the benefits of spatial diversity. To fully exploit cooperative diversity, we propose a medium access control and routing enabled cross-layer cooperative transmission (MACR-CCT) protocol for improving the performance in multi-hop wireless ad hoc networks (MWAN). Different from previous cooperative protocols that determine a receiver in one hop according to a non-cooperative routing protocol first and then select a cooperative relay, MACR-CCT selects the cooperative relay together with the receiver in one hop to exploit fully cooperative diversity, so that the receiver is selected for higher cooperative gain and closer distance to destination, and the relay is selected to achieve the better throughput performance while considering transmission error. Furthermore, considering that there are multiple source–destination pairs in MWAN, MACR-CCT takes interference mitigation into account to further improve network throughput when selecting the cooperative relay. Besides, we propose a theoretical model to analyze the throughput performance. Finally, we take advantage of simulation results to validate the effectiveness of our analytical model and show that our proposed MACR-CCT protocol can significantly outperform existing packet transmission mechanisms in terms of throughput and delay under the multi-hop multi-flow network scenario.     

Distributed topology management for wireless multimedia sensor networks: exploiting connectivity and cooperation

In this paper, we propose a distributed topology management algorithm, named T‐Must, which orchestrates coalition formation game between camera and scalar sensor (SS) nodes, for use in wireless multimedia sensor networks. In the proposed solution, connectivity among the peer camera sensor (CS) nodes is maintained, and coverage is ensured between them. Only the scalar data are not sufficient to describe an event in a particular monitored area. In many cases, multimedia data (specifically, video data) are required to provide more precise information about the event. As the CS nodes, which sense and transmit multimedia data, are costlier than the SS nodes, the former are deployed in the monitored area in lesser numbers compared to the latter ones. In case of CS nodes, power consumption due to sensing is also significant, similar to power consumption for the transmission and reception of packets. Therefore, in this work, in order to increase the network lifetime, topology is controlled by forming coalition between the CS and SS nodes. Upon occurrence of an event, the SS nodes send scalar data to their associated CS nodes. If the scalar data received from SS nodes cross a preconfigured threshold, the associated CS node in the coalition starts sensing the event, captures the video data, and forwards the video data toward other coalitions or sink. Copyright © 2014 John Wiley & Sons, Ltd.     

Decentralized multiuser diversity with opportunistic packet transmission in MIMO wireless sensor networks

In this study, we consider a single-hop wireless sensor network where both the sensor nodes and the controller node have multiple antennas. We focus on single beam opportunistic communication and propose a threshold-based medium access control (MAC) scheme for uplink packet transmission which exploits multiuser diversity gain without feedback in a decentralized manner. Packet transfer from sensor nodes to the controller node is initiated when the channel quality of any node exceeds the predefined threshold based on the effective signal-to-noise ratio (ESNR) measurements at the sensor nodes through linear combining techniques. The optimum threshold is determined to maximize the probability of successful packet transmission where only one sensor node transmits its packet in one time-slot. The proposed scheme trades the successful packet rate to increase the SNR of the successful packets assuming Rayleigh fading and collision-based reception model. Computer simulations confirm that proposed scheme has higher successful packet SNR compared to the simple time division multiple access (TDMA)-based MAC scheme with round-robin fashion. The use of multiple antennas at the sensor nodes can also improve the throughput of proposed scheme compared with our previous scheme without implementing the spatial diversity at the SNs.     

Antenna diversity in multiuser data networks

We consider the use of multiple antennas at the transmitter and/or the receiver to provide open-loop spatial diversity in a multiuser wireless data network. With channel quality information (CQI) available to the transmitter, and by always scheduling the transmission to the active user having the best channel conditions at the time of scheduling, another form of diversity, termed multiuser diversity, is obtained in a data system. This paper provides an analysis of the interaction between these two forms of diversity. From a network point of view, we prove that the asymptotic sum rate, in the limit of a large number of active homogeneous users and subject to the same average total transmit power, is inversely related to the number of transmit antennas for independent and identically distributed (i.i.d.) flat Rayleigh fading channels. In the case of i.i.d. flat Rician fading, the asymptotic sum rate also depends inversely on the number of transmit antennas, but directly on the number of receive antennas. Numerically, we show that the total diversity gain is also constrained by finite CQI quantization and channel fading statistics.     

A random beamforming technique in MIMO systems exploiting multiuser diversity

A random beamforming technique for multiple-input multiple-output (MIMO) systems that simultaneously obtains downlink multiuser diversity gain, spatial multiplexing gain and array gain by feeding back only effective signal-to-noise ratios (SNRs) is described. In addition, power control using waterfilling is employed to improve the throughput of our method in correlated channels. In a slow fading channel, we prove that the throughput of the proposed method converges to that of eigen beamforming when many users are in a cell. The number of users required to achieve capacity bound increases with the number of antennas and SNR was determined. However, the capacity bound is achieved even with a small number of users, e.g., 16 users in a cell, when the SNR is low, e.g., 0 dB, and the number of transmit and receive antenna is small, e.g., two. We also find that the effect of waterfilling is more noticeable in correlated channels.     

Distributed interference compensation for wireless networks

We consider a distributed power control scheme for wireless ad hoc networks, in which each user announces a price that reflects compensation paid by other users for their interference. We present an asynchronous distributed algorithm for updating power levels and prices. By relating this algorithm to myopic best response updates in a fictitious game, we are able to characterize convergence using supermodular game theory. Extensions of this algorithm to a multichannel network are also presented, in which users can allocate their power across multiple frequency bands.     

Distributed synchronization in wireless networks

Synchronization refers to the process of achieving and maintaining coordination among independent local clocks via the exchange of local time information. Different synchronization schemes differ in the way such information is encoded, exchanged, and processed by the clocks toward the end of overcoming the unavoidable nuisance effects of inaccurate clocks and propagation/processing delays. Wireless communications provide the natural platform for the exchange of local time information between synchronizing clocks. Conversely, synchronization of local clocks enables a wealth of signal processing and communication applications in wireless networks. It is this mutual link between synchronization and wireless networks, with emphasis on decentralized structures such as ad hoc and sensor networks, that constitutes the main subject of this article.     

Distributed simultaneous wireless information and power transfer in multiuser amplify‐and‐forward ad hoc wireless networks

This paper studies the problem of stable node matching for distributed simultaneous wireless information and power transfer in multiuser amplify‐and‐forward ad hoc wireless networks. Particularly, each source node aims to be paired with another node that takes the role of an amplify‐and‐forward relay to forward its signal to the destination, such that the achievable rate is improved, in return of some payment made to the relaying node. Each relaying node splits its received signal from its respective source into two parts: one for information processing and the other for energy harvesting. In turn, a matching‐theoretic solution based on the one‐to‐one stable marriage matching game is studied, and a distributed polynomial‐time complexity algorithm is proposed to pair each source node with its best potential relaying node based on the power‐splitting ratios, such that their utilities or payments are maximized while achieving network stability. For comparison purposes, an algorithm to enumerate all possible stable matchings is also devised to study the impact of different matchings on the source and relay utilities. Simulation results are presented to validate the proposed matching algorithm and illustrate that it yields sum‐utility and sum‐payment that are closely comparable to those of centralized power allocation and node pairing, with the added merits of low complexity, truth telling, and network stability.     

多天线无线数据通信系统中多用户分集的研究

研究当接收天线不少于发送天线时多输入多输出(MIMO)系统的多用户分集能力。首先从理论上分析了发送天线个数等于1和2时最大似然接收和迫零接收系统的平均吞吐量和调度增益,以及仿真分析了发送天线个数大于2时系统性能。理论分析和仿真表明：在多用户的MIMO系统中,接收的平均信噪比、用户个数、收发天线个数、接收机的结构等对于多用户分集有很大的影响。当发送天线个数为1时,接收天线较少(1,2,3)和平均信噪比为．10dB时调度增益很大,但调度增益随着天线个数和发送功率增大急剧下降。和最大似然接收相比,迫零接收具有更大的多用户分集增益,因此迫零接收机的吞吐量可以很容易超过最大似然接收机。     

Multi-radio diversity in wireless networks

This paper describes the Multi-Radio Diversity (MRD) wireless system, which uses path diversity to improve loss resilience in wireless local area networks (WLANs). MRD coordinates wireless receptions among multiple radios to improve loss resilience in the face of path-dependent frame corruption over the radio. MRD incorporates two techniques to recover from bit errors and lower the loss rates observed by higher layers, without consuming much extra bandwidth. The first technique is frame combining, in which multiple, possibly erroneous, copies of a given frame are combined together in an attempt to recover the frame without retransmission. The second technique is a low-overhead retransmission scheme called request-for-acknowledgment (RFA), which operates above the link layer and below the network layer to attempt to recover from frame combining failures. We present an analysis that determines how the parameters for these algorithms should be chosen. We have designed and implemented MRD as a fully functional WLAN infrastructure based on 802.11a. We evaluate the MRD system under several different physical configurations, using both UDP and TCP, and measured throughput gains up to 3× over single radio communication schemes employing 802.11’s autorate adaptation scheme. Computer and Communication Sciences, EPFL, Switzerland. Allen Miu received his Ph.D. degree at the Massachusetts Institute of Technology in 2006 and is currently a wireless systems architect at Ruckus Wireless, Inc. He received his S.M. in Computer Science from MIT and a B.Sc. with highest honors in Electrical Engineering and Computer Science from the University of California at Berkeley. He previously worked on the Cricket Indoor Location System and was a research intern at Microsoft Research, Redmond in 2000 and Hewlett-Packard Laboratories, Palo Alto in 2002. His research interests include wireless networks, location systems, mobile computing, and embedded systems. Hari Balakrishnan is an Associate Professor in the EECS Department and a member of the Computer Science and Artificial Intelligence Laboratory (CSAIL) at MIT. His research interests is in the area of networked computer systems. In addition to many widely cited papers, several systems developed as part of his research are available in the public domain. He received a Ph.D. in Computer Science from the University of California at Berkeley in 1998 and a B.Tech. from the Indian Institute of Technology (Madras) in 1993. His honors include an Alfred P. Sloan Research Fellowship (2002), an NSF CAREER Award (2000), the ACM doctoral dissertation award for his work on reliable data transport over wireless networks (1998), and seven award-winning papers at various top conferences and journals, including the IEEE Communication Society’s William R. Bennett Prize (2004). He has also received awards for excellence in teaching and research at MIT (Spira, Junior Bose, and Harold Edgerton faculty achievement awards). C. Emre Koksal received his B.S. degree in Electrical Engineering from the Middle East Technical University, Ankara in 1996. He received his S.M. and Ph.D. degrees from MIT in Electrical Engineering and Computer Science in 1998 and 2002 respectively. He was a postdoctoral fellow in the Networks and Mobile Systems Group in the Computer Science and Artificial Intelligence Laboratory at MIT until 2003. Since then he has been a senior researcher jointly in the Laboratory for Computer Communications and the Laboratory for Information Theory at EPFL, Switzerland. His general areas of interest are wireless communications, computer networks, information theory, stochastic processes and financial economics. He also has a certificate on Financial Technology from the Sloan School of Management at MIT.     

Packet level performance analysis of a packet scheduler exploiting multiuser diversity

In this paper, we consider a packet scheduler exploiting multiuser diversity in a wireless network with AMC (Adaptive Modulation and Coding), and propose a packet level performance model based on the effective bandwidth theory to investigate the joint effect of the AMC and the multiuser diversity scheduling on the scheduler performance. Based on our performance model and analysis, we investigate the packet level performance behavior of the scheduler. Numerical studies are also provided to show the usefulness of our packet level performance model in the design of the packet scheduler.     

Distributed cooperative MAC for multihop wireless networks

This article investigates distributed cooperative medium access control protocol design for multihop wireless networks. Cooperative communication has been proposed recently as an effective way to mitigate channel impairments. With cooperation, single-antenna mobile terminals in a multi-user environment share antennas from other mobiles to generate a virtual multipleantenna system that achieves more reliable communication with a higher diversity gain. However, more mobiles conscribed for one communication inevitably induces complex medium access interactions, especially in multihop wireless ad hoc networks. To improve the network throughput and diversity gain simultaneously, we investigate the issues and challenges in designing an efficient MAC scheme for such networks. Furthermore, based on the IEEE 802.11 DCF, a cross-layer designed cooperative MAC protocol is proposed. The MAC scheme adapts to the channel condition and payload length.     

Distributed power control algorithms for wireless networks

Power control has been shown to be an effective way to increase capacity in wireless systems. In previous work on power control, it has been assumed that power levels can be assigned from a continuous range. In practice, however, power levels are assigned from a discrete set. In this work, we consider the minimization of the total power transmitted over given discrete sets of available power levels subject to maintaining an acceptable signal quality for each mobile. We have developed distributed iterative algorithms for solving a more general version of this integer programming problem, which is of independent interest, and have shown that they find the optimal solution in a finite number of iterations which is polynomial in the number of power levels and the number of mobiles     

Distributed learning in wireless sensor networks

This paper discusses nonparametric distributed learning. After reviewing the classical learning model and highlighting the success of machine learning in centralized settings, the challenges that wireless sensor networks (WSN) pose for distributed learning are discussed, and research aimed at addressing these challenges is surveyed.     

The use of multiuser detectors for multicasting in wireless ad hocCDMA networks

In this paper, we address the issue of performance of linear multiuser detectors for a multicasting application in an ad hoc wireless network. Using a code-division multiple-access (CDMA) framework, we demonstrate how capacity results for multiuser detectors can be adapted to do session admission control for the multicasting problem. We then develop a multicast routing algorithm for ad hoc wireless networks. Using the session admission control mechanism and the multicast routing algorithm, we evaluate the performance of three different linear multiuser detectors for the multicasting application     

Resource allocation for downlink multiuser video transmission over wireless lossy networks

Demand for multimedia services, such as video streaming over wireless networks, has grown dramatically in recent years. The downlink transmission of multiple video sequences to multiple users over a shared resource-limited wireless channel, however, is a daunting task. Among the many challenges in this area are the time-varying channel conditions, limited available resources, such as bandwidth and power, and the different transmission requirements of different video content. This work takes into account the time-varying nature of the wireless channels, as well as the importance of individual video packets, to develop a cross-layer resource allocation and packet scheduling scheme for multiuser video streaming over lossy wireless packet access networks. Assuming that accurate channel feedback is not available at the scheduler, random channel losses combined with complex error concealment at the receiver make it impossible for the scheduler to determine the actual distortion of the sequence at the receiver. Therefore, the objective of the optimization is to minimize the expected distortion of the received sequence, where the expectation is calculated at the scheduler with respect to the packet loss probability in the channel. The expected distortion is used to order the packets in the transmission queue of each user, and then gradients of the expected distortion are used to efficiently allocate resources across users. Simulations show that the proposed scheme performs significantly better than a conventional content-independent scheme for video transmission.     

Cooperative diversity schemes for asynchronous wireless networks

We construct cooperative diversity coding schemes that mitigate the effects of symbol asynchronicity among network users. We do so by modifying, at the expense of implementation practicality, the signaling complexity of well behaved existing schemes. The modification allows the same good performance (DMT optimality) in the presence of synchronicity, and almost-surely permits full-diversity gains for any event of symbol asynchronicity. This work was in part carried out while Petros Elia was at the University of Southern California. This research is supported in part by NSF-ITR CCR-0326628.     

Design of a fair scheduler exploiting multiuser diversity with feedback information reduction

In this letter, we propose a new downlink fair scheduling scheme exploiting the multiuser diversity to enhance the transmission capacity. In the proposed scheme, only the MSs (mobile stations) whose normalized SNR (signal-to-noise ratio) values are larger than a given threshold feedback one-bit information to the BS (base station). As a result, while achieving the strict fairness, the proposed scheme can efficiently utilize the spectrum by reducing the considerable amount of the feedback information, compared to the proportional fair scheduling scheme where all the MSs feed back the normalized SNR values to the BS. Numerical studies show that the transmission capacity in the proposed scheme with a suitable value of the threshold is very close to that in the proportional fair scheduling scheme.