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Online ISSN 1751-8636 Print ISSN 1751-8628

IET Communications

Volume 3, Issue 11, November 2009

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Volume 3, Issue 11

November 2009

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    • Fast antenna subset selection algorithms for multiple-input multiple-output relay systems
      • Author(s): G. Zhang ; G. Li ; J. Qin
      • Source: IET Communications, Volume 3, Issue 11, p. 1693 –1703
      • DOI:  10.1049/iet-com.2009.0013
      • Type: Article
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      • p. 1693 –1703 (11)
        The antenna subset selection technique balances the performance and hardware cost in the multiple-input multiple-output (MIMO) systems, and the problems on the antenna selection in MIMO relay systems have not been fully solved. This paper considers antenna selection on amplify-and-forward (AF) and decode-and-forward (DF) MIMO relay systems to maximise capacity. Since the optimal antenna selection algorithm has high complexity, two fast algorithms are proposed. The selection criterion of the algorithm for AF relay is to maximise a lower bound of the capacity, but not the exact capacity. This criterion reduces algorithmic complexity. The algorithm for DF relay is an extension of an existing antenna subset selection algorithm for one-hop MIMO systems. The authors show the derivations of the algorithms in detail, and analyse their complexity in terms of numbers of complex multiplications. Simulation results show that the proposed algorithms for both cases achieve comparable performance to the optimal algorithm under various conditions, and have decreased complexity.
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      Virtual multiple input multiple output in multiple high-altitude platform constellations
      • Author(s): T. Celcer ; T. Javornik ; M. Mohorcic ; G. Kandus
      • Source: IET Communications, Volume 3, Issue 11, p. 1704 –1715
      • DOI:  10.1049/iet-com.2008.0741
      • Type: Article
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      • p. 1704 –1715 (12)
        Multiple-input multiple-output (MIMO) technology, which makes use of the spatial dimension by utilising multiple antennas at the transmitter and receiver, has proved an efficient solution for providing higher data throughput and/or link reliability in wireless systems. In this study, the authors investigate a virtual MIMO (V-MIMO) technique, based on a constellation of multiple high-altitude platforms (HAPs), providing broadband wireless access to high-speed trains under predominantly line-of-sight (LOS) propagation conditions. They analyse the performance of transmit diversity based on space–time block coding (STBC), in particular Alamouti and extended Alamouti schemes, using fixed wide-lobe receive antennas, and compare it to the reference receive diversity scheme based on best HAP selection that requires highly directional and steerable antennas. Simulation results for different diversity schemes are based on the calculation of carrier to interference-plus-noise ratio (CINR) along representative railway lines in a realistic terrain configuration. The CINR levels obtained are mapped onto transmission modes specified by standards developed for two wireless systems operating at distinct frequency bands, the DVB-S2 assumed for operation in mm-wave bands and the IEEE 802.16e (mobile WiMAX) for operation at 3.5 GHz. Simulation results show that the use of transmit diversity schemes in a multiple HAP constellation decreases the link outage and also increases the average spectral efficiency, but requires a marked increase in transmit power, especially for the system operating in mm-wave bands.
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      Analytical models of blocking probability for multi-granularity cross-connect-based optical networks
      • Author(s): Y. Zhao ; J. Zhang ; D. Han ; X. Zhang ; Y. Yao ; W. Gu ; Y. Ji
      • Source: IET Communications, Volume 3, Issue 11, p. 1716 –1723
      • DOI:  10.1049/iet-com.2008.0673
      • Type: Article
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      • p. 1716 –1723 (8)
        Multi-granularity optical cross-connect (MG-OXC)-based optical network is a promising optical network architecture as it is capable of flexible switching at different granularity levels. In MG-OXC-based optical networks, wavelength conversion (WC) capability and the number of usable add/drop ports of the nodes are two key factors affecting its performance. Two analytical models of blocking probability for MG-OXC-based optical networks both without WC capability and with sparse WC capability are proposed, exploiting Erlang's loss formula and birth–death process. Based on the models and simulation, the impact of WC capability and the number of add/drop ports on the blocking probability are investigated. Three kinds of granularities (i.e. fibre, waveband and wavelength) are considered in MG-OXC nodes to reduce the complexity and size of switch fabric. Both the analytical and simulation results are given on two network topologies under dynamic traffic patterns. Simulation results show that the proposed models are accurate and effective for the analysis of blocking probability in MG-OXC-based optical networks.
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      Optimisation of limited feedback design for heterogeneous users in multi-antenna downlinks
      • Author(s): W. Xu ; C. Zhao ; Z. Ding
      • Source: IET Communications, Volume 3, Issue 11, p. 1724 –1735
      • DOI:  10.1049/iet-com.2008.0726
      • Type: Article
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      • p. 1724 –1735 (12)
        The authors study the problem of limited channel information feedback in multiple-input single-output (MISO) broadcast channels serving heterogeneous users. The heterogeneous users have different feedback channel capacities because of their physical constraints and limitations. Our objective is to design an optimised limited feedback strategy for multiple users by using a set of multi-resolution codebooks such that, under varying and quantised channel information feedback, the multiuser system can maximise its achievable sum rate. By exploiting the receive antenna combining technique, the authors further generalise the proposed scheme to the multiuser beamforming case with multi-antenna users. Finally, the authors verify the proposed scheme by numerical methods. Simulation results show that the sum rate performance for heterogeneous users can be effectively improved by using the proposed limited feedback scheme, which is optimised according to only statistic channel information of users.
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      Assigning channels by link directionality in a medium access control protocol for IEEE 802.11 ad hoc networks
      • Author(s): P.C. Ng ; D.J. Edwards ; S.C. Liew
      • Source: IET Communications, Volume 3, Issue 11, p. 1736 –1746
      • DOI:  10.1049/iet-com.2008.0522
      • Type: Article
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      • p. 1736 –1746 (11)
        This study attempts to exploit the potential of link directionality to increase the achievable capacities of ad hoc networks. When an IEEE 802.11 ad hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2C. However, most of the dual-channel 802.11 protocols proposed in the literature appear only to be able to achieve less than 60% of the 2C targeted capacity. The authors thus propose a link-directionality-based dual-channel medium access control protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. The main idea is to assign channels according to link directionality to allow a link to transmit simultaneously within the carrier-sensing region of another link provided that these transmissions do not interfere with each other. Simulations show that our proposed scheme can achieve more than 85% of our targeted capacities, 0.85×2C=1.7C, in large-scale random topologies. In lattice and irregular topologies, the throughput is boosted up to 2.83C and 2.13C, respectively. An approach for capacity analysis is also introduced to determine the throughput improvements that can be achieved by our proposed protocol. We believe using link directionality for channel allocations is a key step that yields significant potential for multiplying the capacity of ad hoc networks.
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      Beamforming technique to solve the hidden beam problem in wireless communication systems
      • Author(s): D. Arora  and  P. Agathoklis
      • Source: IET Communications, Volume 3, Issue 11, p. 1747 –1756
      • DOI:  10.1049/iet-com.2008.0362
      • Type: Article
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      • p. 1747 –1756 (10)
        A new method to deal with the hidden beam problem in carrier sensing medium access (CSMA)-based systems is proposed. The method is based on the modification of the beamformer weights that reduce null depths of any given beam. The proposed technique is applied to uniformly spaced linear antenna arrays with broadsided and endfired beams to planar beams and to Dolph–Chebychev beams. The throughput of non-persistent CSMA systems using a uniformly spaced broadsided linear antenna array is evaluated and the performance of the proposed technique is compared with that of a original beam in terms of reducing the hidden beam problem. The proposed methodology is shown to be effective in solving the hidden beam problem in a CSMA-based system by reducing the null depths and is shown to yield higher throughput than the original beam.
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      High rate open-loop MIMO multi-user downlink transmission scheme based on blind precoding quasi-orthogonal space time block code
      • Author(s): Y. Han ; S.H. Ting ; Y.L. Guan
      • Source: IET Communications, Volume 3, Issue 11, p. 1757 –1768
      • DOI:  10.1049/iet-com.2008.0544
      • Type: Article
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      • p. 1757 –1768 (12)
        The authors first propose a new class of full-rate full-diversity quasi-orthogonal space time block code (QO-STBC), namely QO-STBC with blind precoding (BP-QO-STBC). Based on this code, a novel open-loop multiple-input multiple-output (MIMO) multi-user downlink transmission scheme is derived. Presuming M blocks of BP-QO-STBC (which could be of different code lengths) are transmitted from the access point simultaneously, thus achieving a multiplexing gain of M, we show that the multiplexed signal can be separated by any user with only mR≥M receive antennas, which is much smaller than that required by conventional linear schemes. The proposed scheme is extendable by simple iterative construction to an arbitrary number of transmit antennas. By exploiting the special structure of BP-QO-STBC, we derived two decoding algorithms with different levels of complexities, namely an optimal maximum likelihood decoder that achieves the lowest decoding complexity known for all full-rate full-diversity QO-STBC and a suboptimal linear decoder with even lower complexity. These two decoders combined with the simple construction method of BP-QO-STBC provide flexible tradeoffs between complexity and performance, as well as between diversity and multiplexing gains. A comparison between the proposed scheme and random beamforming in terms of bit error rate and achievable sum throughput is shown via simulation, and the results support our proposed scheme as a better practical candidate for high-speed wireless network.
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      Performance analyses of selection combining diversity receiver over α–μ fading channels in the presence of co-channel interference
      • Author(s): S.R. Panić ; M.Č. Stefanović ; A.V. Mosić
      • Source: IET Communications, Volume 3, Issue 11, p. 1769 –1777
      • DOI:  10.1049/iet-com.2009.0023
      • Type: Article
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      • p. 1769 –1777 (9)
        An approach to the performance analyses of dual selection combining (SC) diversity receiver over correlated α–μ fading channels with the arbitrary parameters is presented. Fading between the diversity branches and interferers is correlated and distributed with α–μ distribution. Infinite series expressions are obtained for the output SIR's probability density function (PDF), cumulative distribution function (CDF) and an important measure of the system's performances, the outage probability (OP). An average error probability is efficiently evaluated for coherent and non-coherent modulation schemes such as coherent frequency-shift keying (CFSK), coherent phase-shift keying (CPSK), binary frequency-shift keying (BFSK) and binary differentially phase-shift keying (BDPSK). Numerical results are presented to show the effects of various parameters such as fading severity, input SIR unbalance and the level of correlation between received desired signals and interferences on system's performances.
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      Diversity analysis of smart relaying over Nakagami and Hoyt generalised fading channels
      • Author(s): N.H. Vien ; H.H. Nguyen ; T. Le-Ngoc
      • Source: IET Communications, Volume 3, Issue 11, p. 1778 –1789
      • DOI:  10.1049/iet-com.2008.0354
      • Type: Article
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      • p. 1778 –1789 (12)
        Signal transmission with the help of relay(s) in wireless networks can achieve spatial diversity without the need of having multiple attennas at the source and/or destination. Among various signal processing techniques proposed for the relays, the adaptive decode-and-forward (DF) relaying strategy, recently proposed by Wang et al. and generally referred to as smart relaying, has been shown to achieve the maximal spatial diversity even when imperfect detection is committed at the relays. The work by Wang et al., however, only considers Rayleigh fading channels. This paper extends the diversity analysis of the smart relaying technique to the important Nakagami and Hoyt generalised fading channels. Performance analysis proves that, at high signal-to-noise ratio, the maximal diversity order achieved by the smart relaying system under the Nakagami channel is mSD+min{mSR, mRD}, where mSR, mRD and mSD are the fading figures of the source–relay (S–R), relay–destination (R–D) and source–destination (S–D) links. Under the Hoyt fading channel, the diversity order is 2. The obtained results on the diversity order are shown to be insensitive to the quality of the R–D feedback channel.
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      Turbo channel estimation and equalisation for a superposition-based cooperative system
      • Author(s): Y. Gong ; Z. Ding ; T. Ratnarajah ; C.F.N. Cowan
      • Source: IET Communications, Volume 3, Issue 11, p. 1790 –1799
      • DOI:  10.1049/iet-com.2008.0730
      • Type: Article
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      • p. 1790 –1799 (10)
        This study investigates the superposition-based cooperative transmission system. In this system, a key point is for the relay node to detect data transmitted from the source node. This issued was less considered in the existing literature as the channel is usually assumed to be flat fading and a priori known. In practice, however, the channel is not only a priori unknown but subject to frequency selective fading. Channel estimation is thus necessary. Of particular interest is the channel estimation at the relay node which imposes extra requirement for the system resources. The authors propose a novel turbo least-square channel estimator by exploring the superposition structure of the transmission data. The proposed channel estimator not only requires no pilot symbols but also has significantly better performance than the classic approach. The soft-in-soft-out minimum mean square error (MMSE) equaliser is also re-derived to match the superimposed data structure. Finally computer simulation results are shown to verify the proposed algorithm.
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      Turbo codes with symmetric and asymmetric component codes defined over finite fields of integers
      • Author(s): J. da Silva Barros  and  R. Baldini Filho
      • Source: IET Communications, Volume 3, Issue 11, p. 1800 –1807
      • DOI:  10.1049/iet-com.2009.0047
      • Type: Article
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      • p. 1800 –1807 (8)
        Binary asymmetric turbo codes and non-binary turbo codes have been proposed to improve the bit error rate (BER) performance of parallel concatenated coding schemes. Both strategies have certain advantages that can be exploited when they are put together. This paper investigates turbo codes based on two component recursive systematic convolutional (RSC) codes defined over a finite field of integers. Symmetric and asymmetric non-binary turbo codes are obtained and their BER performance in both the ‘waterfall’ and the ‘error-floor’ regions is analysed. The results show good performance improvements when compared to binary and quaternary turbo codes with same throughput.
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      Iterative bit and power allocation for multi-cell orthogonal frequency division multiplexing with minimum variance distortionless response beamforming
      • Author(s): Y.-F. Chen ; C.-Y. Lin ; L.-Y. Chen
      • Source: IET Communications, Volume 3, Issue 11, p. 1808 –1819
      • DOI:  10.1049/iet-com.2008.0494
      • Type: Article
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      • p. 1808 –1819 (12)
        This paper develops bit and power allocation schemes with beamforming for multi-cell orthogonal frequency division multiplexing (OFDM) systems on uplink. The model of the multi-cell channel with frequency reuse is considered. The transmit signal from each mobile causes interference to the received signals of other base stations. The schemes aim to minimise the total mobile transmit power while satisfying the required data rate and the bit error rate (BER) of each mobile. The proposed schemes offer better performance than that of the fixed bit allocation method. The proposed distributed allocation scheme reduces computational complexity compared to the proposed centralised multi-user greedy method with insignificant performance degradation. The simulation results are presented to demonstrate the efficacy of the proposed schemes.
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