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Selected Areas in Communications, IEEE Journal on

Issue 6 • Date August 2008

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Displaying Results 1 - 22 of 22
  • Table of contents

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  • Design Tradeoffs and Hardware Architecture for Real-Time Iterative MIMO Detection using Sphere Decoding and LDPC Coding

    Page(s): 1003 - 1014
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (581 KB) |  | HTML iconHTML  

    We explore the performance and hardware complexity tradeoffs associated with performing iterative multiple- input multiple-output (MIMO) detection using a sphere decoder and a low-density parity-check (LDPC) decoder. Iterations are performed both within the LDPC decoder as well as via an outer iteration loop through which refined soft information is fed back from the LDPC decoder to a MIMO detector. A hardware architecture and associated implementation results on Xilinx Virtex-5 field programmable gate array for a 4 x 4 QPSK MIMO system are presented. The system offers a performance improvement of approximately 1 dB over systems without the outer iteration loop, and provides an information bit throughput that ranges from 60 to 300 megabits per second when a length 1944 rate 1/2 LDPC code is used. View full abstract»

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  • On the Design of Linear Transceivers for Multiuser Systems with Channel Uncertainty

    Page(s): 1015 - 1024
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (464 KB) |  | HTML iconHTML  

    We consider the design of linear transceivers for multiuser communication systems in the presence of uncertain channel state information (CSI), with an emphasis on downlink systems with a single antenna at each receiver. For systems with uplink-downlink reciprocity, we consider a stochastic model for the channel uncertainty, and we propose an efficient algorithm for the joint design of the linear preceding matrix at the base station and the equalizing gains at the receivers so as to minimize the average mean-square-error (MSE) over the channel uncertainty. The design is based on a generalization, derived herein, of the MSE duality between the broadcast and multiple access channels (MAC) to scenarios with uncertain CSI, and on a convex formulation for the design of robust transceivers for the dual MAC. For systems in which quantized channel feedback is employed, we consider a deterministically-bounded model for the channel uncertainty, and we study the design of robust downlink transceivers that minimize the worst- case MSE over all admissible channels. While we show that the design problem is NP-hard, we also propose an iterative local optimization algorithm that is based on efficiently-solvable convex conic formulations. Our framework is quite flexible, and can incorporate different bounded uncertainty models as well as a variety of power constraints. In particular, we study a "system-wide" uncertainty model, and although the resulting design problem is still NP hard, it does result in a significantly simpler iterative local design algorithm than the "per-user" uncertainty model. Our approaches to the minimax design for the downlink can be extended to the uplink, and we provide explicit formulations for the resulting uplink designs. Simulation results indicate that the proposed approaches to robust linear transceiver design can significantly reduce the sensitivity of the downlink to uncertain CSI, and can provide improved performance over that of existing robust de- - signs. View full abstract»

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  • Robust Power Allocation Schemes for Multibeam Opportunistic Transmission Strategies Under Quality of Service Constraints

    Page(s): 1025 - 1034
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (582 KB) |  | HTML iconHTML  

    Scheduling in a Broadcast (BC) channel based on partial Channel State Information at the Transmitter (CSIT) is carried out in an opportunistic way, where several orthogonal beams are randomly generated at the Base Station transmitter to simultaneously deliver several users with their intended data. The paper presents a power allocation over the transmitting beams, where a minimum rate per user restriction is required for each scheduled user, standing as a potential Quality of Service (QoS) indicator for the system behaviour. However, in practical wireless scenarios the CSIT is imperfect due to non-accurate estimation, so that robust schemes are required to meet the system demands. Based on the allowed system outage in the QoS achievement, different robust power allocation schemes are proposed, which are efficiently solved through convex optimization tools. The presented strategies are later compared via simulations for the different scenarios and the system specifications. View full abstract»

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  • Guest Editorial: MIMO Systems and Applications: Field Experience, Practical Aspects, Limitations and Challenges

    Page(s): 841 - 844
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    The 17 papers in this special issue focus on MIMO systems and applications: field experience, practical aspects, limitations and challenges. The papers are summarized here. View full abstract»

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  • Field Experiments on MIMO Multiplexing with Peak Frequency Efficiency of 50 Bit/Second/Hz Using MLD Based Signal Detection for OFDM High-Speed Packet Access

    Page(s): 845 - 856
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (614 KB) |  | HTML iconHTML  

    This paper presents indoor and field experimental results on extremely high-speed packet transmissions of 4.92 Gbps in a 100 -MHz channel bandwidth, i.e., the frequency efficiency of approximately 50 bit/second/Hz, in the downlink OFDM radio access. The required received signal-to-noise power ratio (SNR) is less than 30dB, which is almost the upper limit in cellular environments near a cell site even with a light channel load, with the aim of application to future universal broadband packet radio access. We apply 12-by-12 MIMO multiplexing using the maximum likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS),64QAM data modulation, and Turbo coding with the coding rate of R = 8/9 to achieve an extremely high frequency efficiency level. In the field experiments conducted in the Yokosuka Research Park (YRP) district of Yokosuka city, we show that the measured throughput of 4.92 Gbps is achieved at the average received SNR per receiver antenna of approximately 28.0 and 28.5 dB when the respective receiver antenna separation is 40 cm (6.2lambda) and 10 cm (1.5lambda) and the transmitter antenna separation is 70 cm (10.9lambda) at the average speed of 10 km/h under non-line-of-sight (NLOS) conditions. View full abstract»

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  • FPGA Implementation of an Iterative Receiver for MIMO-OFDM Systems

    Page(s): 857 - 866
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (539 KB) |  | HTML iconHTML  

    Today iterative receivers have proved their efficiency in cancelling interference within the field of wireless communications. However their complexity is often seen as a brake for their use in real systems. In this paper an efficient iterative receiver real-time implementation for a 4 times 4 MIMO system is presented. An architecture of MMSE iterative receiver for MIMO-OFDM systems is proposed to limit latency and complexity due to iterative process: MMSE equalization implementation is realized using CORDIC operators; the scheduling between MIMO detection and channel decoding is optimized and specific interleaving functions are introduced to reduce latency and accelerate the convergence process. The implemented receiver is integrated in a real-time FPGA testbench and compared in terms of complexity and performance with a non iterative solution. View full abstract»

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  • Transmitter Noise Effect on the Performance of a MIMO-OFDM Hardware Implementation Achieving Improved Coverage

    Page(s): 867 - 876
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (594 KB) |  | HTML iconHTML  

    This paper presents analysis of performance measurements from a MIMO-OFDM IEEE 802.11n hardware implementation at 5.2 GHz using four transmitters and four receivers. Two spatial multiplexing systems are compared; one which uses a zero-forcing (ZF) detector and the other a list sphere detector (LSD). We show that the measured results do not align with standard prediction based on simulation assuming uncorrelated receiver noise. We show that the discrepancy can be explained by the inclusion of transmitter noise into the channel model. This effect is not included in existing MIMO-OFDM channel models. The measured results from our hardware implementation show successful packet transmission at 600 Mb/s with 15 bits/s/Hz spectral efficiency at 73% coverage for ZF and 84% coverage for LSD with an average receiver signal to noise ratio (SNR) of 26 dB. View full abstract»

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  • A Real-Time 4-Stream MIMO-OFDM Transceiver: System Design, FPGA Implementation, and Characterization

    Page(s): 877 - 889
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (575 KB) |  | HTML iconHTML  

    When designing complex communication systems, such as MIMO-OFDM transceivers, prototypes have become an important tool for understanding the implementation trade-offs and the system behavior. This paper presents a real-time FPGA prototype for a 4-stream MIMO-OFDM transceiver capable of transmitting 216 Mbit/s in 20 MHz bandwidth. The paper covers all parts of the system from RF to channel decoding and considers both algorithm and implementation aspects. In particular, we discuss the initial parameter estimation, channel estimation, MIMO detection, parameter tracking, and channel decoding. FPGA implementation results are reported along with measurements that demonstrate the throughput of spatial multiplexing with four spatial streams. View full abstract»

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  • Performance of Spatial Division Multiplexing MIMO with Frequency Domain Packet Scheduling: From Theory to Practice

    Page(s): 890 - 900
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (742 KB) |  | HTML iconHTML  

    This paper addresses the performance of spatial division multiplexing (SDM) multiple-input multiple-output (MIMO) techniques together with frequency domain packet scheduling (FDPS) in both theory and practice. We start with a theoretical analysis under some ideal assumptions to derive the performance bounds of SDM-FDPS. To facilitate the analysis, a unified SINR concept is utilized to make a fair comparison of MIMO schemes with different number of spatial streams. The effect of packet scheduling is included in the post-scheduling SINR distribution using an analytical model. Based on that, the performance bounds are obtained with a more realistic SINR to throughput mapping metric. The system-level performance of SDM-FDPS has been evaluated under practical constraints using detailed simulations based on the UTRAN long term evolution (LTE) downlink cellular system framework. The purpose is to investigate the impact of realistic factors on performance. Results confirm that the combination of SDM and FDPS can increase the spectral efficiency significantly, particularly in a micro-cell scenario, and up to 30%-60% gain is observed over 1times2 with FDPS depending on the traffic models considered. Finally, the more practical simulation results are compared against the theoretical performance bounds. A performance loss is seen in the simulations due to realistic coding/modulation, impact of frequency selectivity, signalling constraints, imperfect channel quality indicator (CQI), etc. View full abstract»

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  • Multiple Frequency Offset Estimation for the Downlink of Coordinated MIMO Systems

    Page(s): 901 - 912
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (535 KB) |  | HTML iconHTML  

    We consider downlink MIMO beamforming from several coordinated basestations (BSs), and the associated problem of in dependent carrier frequency offsets (CFOs) at the BSs which cause accumulated phase errors to compromise beamforming accuracy. Correction of the CFOs requires estimation of their values, so our topic is multiple CFO estimation, a little-explored area. We present a robust and easily generalized estimator that accounts for the training sequence (TS) correlations caused by the CFOs, and show that it meets the Cramer-Rao lower bound (CRLB) at moderate signal-to-noise ratios (SNRs). The performance of the estimator is contingent upon TSs short enough to ensure convexity of the log-likelihood over the allowable CFO ranges. For combinations of TS length and CFO range that violate this constraint, we present two suboptimal estimators based on segmentation of the TS, both of which also meet the CRLB at moderate to high SNRs. View full abstract»

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  • Cross-Layer Optimization for MIMO-Based Wireless Ad Hoc Networks: Routing, Power Allocation, and Bandwidth Allocation

    Page(s): 913 - 926
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (612 KB) |  | HTML iconHTML  

    MIMO-based communications systems have great potential to improve network capacity for wireless ad hoc networks. Due to unique physical layer characteristics associated with MIMO, network performance is tightly coupled with mechanisms at physical, link, and routing layers. So far, research on MIMO-based wireless ad hoc networks is still in its infancy and few results are available. In this paper, we consider the problem of jointly optimizing power and bandwidth allocation at each node and multi-hop/multi-path routing in a MIMO-based wireless ad hoc network. We develop a solution procedure to this cross-layer optimization problem and use simulations to validate the efficacy of this solution. View full abstract»

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  • Space-Time Codes for MIMO Systems with Non-Collocated Transmit Antennas

    Page(s): 927 - 937
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB) |  | HTML iconHTML  

    We consider space-time coding methods for cooperative narrowband and wideband downlink transmission from multiple base stations. The communication channels in multi base-station signaling differ from those involving collocated transmit-antenna systems, and, if properly used, can provide improved resistance to shadowing and extended range. One challenge, however, that arises in this context is asynchronous reception of the signal elements. In the narrowband case the proposed designs generate asynchrony robust space-time block codes (STBCs) via transformations of existing orthogonal STBCs. Subject to a maximum allowable relative delay between signals from distinct transmit antennas, the resulting received signal can be equivalently modeled as arising from synchronous STBC transmission with orthogonal or diagonal STBCs, implying that these designs provide full space diversity with low-complexity decoding in asynchronous settings. We also describe three system approaches for wideband transmission with throughput diversity receiver-complexity trade-offs. Common to all these systems are elements such as OFDM- type signaling, bit-interleaved coded modulation, and iterative decoding. The lowest data-rate lowest complexity system employs an inner orthogonal space-time block code with embedded OFDM-type transmission, while the highest data-rate highest complexity scheme does not employ an inner code. The latter group of schemes can also conveniently provide high data rates at the cost of reduced space diversity. All schemes can be readily extended to provide flexible unequal error protection for media transport. View full abstract»

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  • Space-Time Codes for MIMO Ultra-Wideband Communications and MIMO Free-Space Optical Communications with PPM

    Page(s): 938 - 947
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (470 KB) |  | HTML iconHTML  

    In this paper, we consider the problem of space-time (ST) coding with pulse position modulation (PPM). While all the existing ST block codes necessitate rotating the phase or amplifying the amplitude of the transmitted symbols, the proposed scheme can be associated with unipolar PPM constellations without introducing any additional constellation extension. In other words, full transmit diversity can be achieved while conveying the information only through the time delays of the modulated signals transmitted from the different antennas. The absence of phase rotations renders the proposed scheme convenient for low- cost carrier-less multiple-input-multiple-output (MIMO) time- hopping ultra-wideband (TH-UWB) systems and for MIMO free-space optical (FSO) communications with direct detection. In particular, we propose two families of minimal-delay ST block codes that achieve a full transmit diversity order with PPM. Designate by n the number of transmit antennas and by M the number of modulation positions. For a given set of values of (n, m), the first family of codes achieves a rate of 1 symbol per channel use (PCU) which is the highest possible achievable rate when no constellation extensions are introduced. The second family of codes can be applied with a wider range of (n, m) at the expense of a reduced rate given by: R=1/n+n-1/n log2(M-1)/n log2(M). View full abstract»

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  • MIMO-OFDM Beamforming for Improved Channel Estimation

    Page(s): 948 - 959
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (561 KB) |  | HTML iconHTML  

    The MIMO-OFDM beamforming design problem is addressed from a system level standpoint. A beamforming method is proposed which helps improve the receiver channel estimation performance without degrading any benefit of a conventional beamformer. To that end, the smoothed singular value decomposition (SSVD) algorithm is first developed to get "close" effective channels after beamforming for two adjacent subcarriers. Based on the SSVD algorithm, the frequency smoothed beamformer (FSB) design is then derived, in which smooth effective channels across all subcarriers are generated and thus the receiver can apply interpolation and smoothing to improve the channel estimation performance. The close singular value problem is discussed. Statistical characteristics of the effective channel are analyzed, which is used to design channel estimation for the beamformed channel. Simulation results show that the FSB design is efficient in the IEEE 802.11n setting. View full abstract»

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  • A Pragmatic PHY Abstraction Technique for Link Adaptation and MIMO Switching

    Page(s): 960 - 971
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (691 KB) |  | HTML iconHTML  

    MIMO (multiple input multiple output) techniques are widely employed to improve the performance of wireless systems. These techniques are used to overcome multipath fading and/or improve the peak throughput of wireless systems. It is well known that there is a fundamental tradeoff between diversity gain and multiplexing gain [1]. Orthogonal space time codes such as the Alamouti code (also known as space time block codes (STBC)) exploit multiple antennas as a diversity source, and thus improve packet error rate (PER) and the average throughput. However, space time block codes are not designed to increase the peak data rate of the system. On the other hand, spatial multiplexing (SM) techniques offer higher peak throughput by transmitting parallel streams of data from different antennas. In order to successfully decode the parallel streams, the channel must exhibit a small eigenvalue spread. Otherwise, the streams will interfere with one another and it is difficult to decode the information data. The performance improvement from SM is therefore highly dependent on the channel characteristics. It is possible to use multiple encoders and rate control per layer to improve the SM performance. However, there are instances of channel where STBC is more beneficial than SM with a single encoder. In order to resolve the shortcomings of STBC and SM, a hybrid technique can be applied where depending on the instantaneous channel conditions either STBC or SM is selected. This technique is commonly referred to as adaptive MIMO switching (AMS) [5]-[11]. One important aspect of the technique is the switching criteria, namely the PHY abstraction method which estimates a packet error event as a function of the instantaneous channel condition, transmission profiles, and receiver characteristics. We show that the proposed algorithm outperforms other techniques such as determinant and Demmel condition number based techniques in flat fading channel. In selective fading channels where a code- - word sees a finite number of multiple channel qualities, estimating the resulting PER is very challenging. Existing techniques rely on an estimate of the average of the channel qualities seen by each codeword. In this paper, we propose a PHY abstraction and switching algorithm hereby referred to by weighted sum of instantaneous qualities (WSIQ) whereby the channel qualities are ordered in order to reduce the variance of channel qualities and then a weighted sum of the qualities is applied. The weighting vector is chosen to minimize the variance of the linear sum. We have provided simulations to show the superiority of WSIQ even when channel statistics are unknown. In addition, computationally efficient techniques suited for practical implementation are proposed. View full abstract»

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  • A Novel Approach to MIMO Transmission Using a Single RF Front End

    Page(s): 972 - 980
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (701 KB) |  | HTML iconHTML  

    In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants. View full abstract»

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  • Quantized Multi-Rank Beamforming for MIMO-OFDM Systems

    Page(s): 981 - 992
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (666 KB) |  | HTML iconHTML  

    We consider the sum-rate maximization via linear preceding in downlink MIMO-OFDM systems with quantized feedback. We address the preceding codebook design based on the capacity measure by introducing a new distance metric. We propose a codebook structure and its associated design algorithm that allows for significant reduction in the memory requirement and computational complexity in real-time system implementation. We then provide a system design approach comprising of four main ingredients: (i) a multi-rank beamforming (MRBF) scheme, (ii) an efficient CQI-based precoder selection algorithm, (iii) reduced feedback strategies, and (iv) novel channel quality indicator (CQI) combining. Our simulation results show that the proposed MRBF scheme can approach the precoding upper bounds with relatively few feedback bits. Moreover, with the same number of bits, the proposed scheme simultaneously achieves higher throughput and lower computational complexity in comparison to the other existing precoding schemes. View full abstract»

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  • A Detection Algorithm for Multi-Input Multi-Output (MIMO) Transmission using Poly-Diagonalization and Trellis Decoding

    Page(s): 993 - 1002
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (551 KB) |  | HTML iconHTML  

    A MIMO detection algorithm utilizing poly- diagonalization and tail-biting trellis is proposed. Linear MIMO detection, such as zero-forcing or minimum mean squared error (MMSE) equalization, is basically a channel diagonalization technique, where interferences from other data streams are decoupled for the separate decoding. It is well known, however, that such decoders suffer from noise enhancement, which causes considerable performance degradation. In this paper, we propose poly-diagonalization of the channel matrix in zero-forcing and MMSE senses. The idea behind poly-diagonalization is to allow interferences partially in order to alleviate the noise enhancement and, then, to use post trellis decoding for the joint detection utilizing the poly-diagonal structure of the effective channel. Under the proposed framework, the zero-forcing and the MMSE equalizer can be regarded as special cases of poly-diagonalization, i.e., of the first order. The proposed scheme can provide a tradeoff between the complexity and performance by choosing an appropriate order of poly-diagonalization. According to the simulation results, considerable gain can be obtained even with the second order, i.e., bi-diagonalization, for which the decoding complexity is far less than that of the maximum likelihood decoding. View full abstract»

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  • IEEE Communications Society 2008 Board of Governors

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  • Table of contents [continued]

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Aims & Scope

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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Editor-in-Chief
Muriel Médard
MIT