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Communications, IET

Issue 9 • Date June 14 2012

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Displaying Results 1 - 18 of 18
  • Distributed beamforming in cognitive relay networks with partial channel state information

    Page(s): 1011 - 1018
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (492 KB)  

    In this study, the authors consider the problem of distributed relay beamforming in an underlay-based cognitive relay network in which a secondary network transmits its data using multiple relays under an amplify-and-forward protocol simultaneously with a primary network over the same spectrum. For such a network, assuming that the partial channel state information (CSI) is available, the authors study two different beamforming approaches to improve performance of the secondary transmissions while ensuring the quality-of-service of the primary transmissions. In the first approach, the authors obtain the beamforming weights as well as the transmit power of the secondary source such that the signal-to-interference plus noise ratio (SINR) at the secondary destination is maximised whereas the interference power from the secondary to the primary network is kept below a predefined threshold. In the second approach, the beamforming weights are obtained through minimising the received interference power at the primary receiver subject to a constraint on SINR at the secondary destination. The authors prove that both proposed approaches result in closed-form solutions. Simulation results show that as the uncertainty in CSI decreases, the proposed schemes achieve better performances. In addition, increasing the number of relays improves the overall system performance. View full abstract»

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  • Discrete rate maximisation power allocation with enhanced bit error ratio

    Page(s): 1019 - 1024
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (485 KB)  

    This study aims to maximise the rate over a multiple-in multiple-out (MIMO) link using incremental power and bit allocation. Two different schemes, greedy power allocation (GPA) and greedy bit allocation (GBA), are addressed and compared with the standard uniform power allocation (UPA). The design is constrained by the target bit error ratio (BER), the total power budget and fixed discrete modulation orders. The authors demonstrate through simulations that GPA outperforms GBA in terms of throughput and power conservation, whereas GBA is advantageous when a lower BER is beneficial. Once the design constraints are satisfied, remaining power is utilised in two possible ways, leading to improved performance of GPA and UPA algorithms. This redistribution is analysed for fairness in BER performance across all active subchannels using a bisection method. View full abstract»

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  • D-ODMRP: a destination-driven on-demand multicast routing protocol for mobile ad hoc networks

    Page(s): 1025 - 1031
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (449 KB)  

    This article proposes a destination-driven on-demand multicast routing protocol (D-ODMRP) to improve the multicast forwarding efficiency in mobile ad hoc networks (MANETs). In D-ODMRP, the path from the multicast source to a multicast destination tends to use those paths passing through another multicast destination. If such multiple paths are available, the one leading to the least extra cost is preferred. This destination-driven strategy is introduced into the on-demand construction process of a multicast forwarding structure in a popular multicast protocol ODMRP. Simulation results show that D-ODMRP can significantly improve the forwarding efficiency as compared with ODMRP. Moreover, the destination-driven strategy can also be introduced into other existing multicast routing protocols for MANETs. View full abstract»

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  • Adaptive threshold to mitigate the effect of cross-modulation interference and synchronisation errors in energy detection pulse position modulation ultra-wideband systems

    Page(s): 1032 - 1037
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (434 KB)  

    In conventional energy detection (ED) pulse position modulation (PPM) systems, the optimal decision threshold is 0. However, when cross-modulation interference (CMI) or synchronisation errors occur, the optimal threshold deviates from 0 and the bit error rate (BER) performance is degraded greatly. An adaptive threshold is developed to replace 0 threshold to find the optimal threshold. The research results show that this adaptive threshold effectively mitigates the effect of cross modulation interference (CMI) or synchronisation errors and achieves BER improvement. View full abstract»

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  • Efficient joint source–channel decoding of multi-state Markov sequences

    Page(s): 1038 - 1044
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (234 KB)  

    In this study, joint source-channel decoding for non-binary source samples is conducted. The non-binary source samples can be modelled as the output of a multi-state Markov chain (MC). As the source samples are directly transmitted after channel coding without source compression, the transmitted signals can be highly correlated. At the receiver, the multi-state MC module can be designed to exploit the statistical correlation of source samples to improve the error correcting performance. However, as the number of states is increased, the multi-state MC module requires high computational complexity. To alleviate this problem, a simplified MC module is proposed. In the simplified MC module, the multi-state MC is replaced with multiple number of two-state MCs each of which exploits bit-level correlation of samples. Simulation results demonstrate that the simplified MC module can lead to competitive reduction in the required signal-to-noise ratio in comparison with the multi-state MC module with reduced computational complexity. View full abstract»

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  • Throughput analysis of shared-memory crosspoint buffered packet switches

    Page(s): 1045 - 1053
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (596 KB)  

    This study presents a theoretical throughput analysis of two buffered-crossbar switches, called shared-memory crosspoint buffered (SMCB) switches, in which crosspoint buffers are shared by two or more inputs. In one of the switches, the shared-crosspoint buffers are dynamically partitioned and assigned to the sharing inputs, and memory is sped up. In the other switch, inputs are arbitrated to determine which of them accesses the shared-crosspoint buffers, and memory speedup is avoided. SMCB switches have been shown to achieve a throughput comparable to that of a combined input-crosspoint buffered (CICB) switch with dedicated crosspoint buffers to each input but, with less memory than a CICB switch. The two analysed SMCB switches use random selection as the arbitration scheme. The authors modelled the states of the shared-crosspoint buffers of the two switches using a Markov-modulated process and prove that the throughput of the proposed switches approaches 100% under independent and identically distributed uniform traffic. In addition, the authors provide numerical evaluations of the derived formulas to show how the throughput approaches asymptotically to 100%. View full abstract»

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  • Diversity of multi-hop cluster-based routing with arbitrary relay selection

    Page(s): 1054 - 1060
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (484 KB)  

    Clustered multi-hop wireless networks have attracted significant attention for their robustness to fading, hierarchical structure, and ability to exploit the broadcast nature of the wireless channel. The authors propose an opportunistic routing (or relay selection) algorithm for such networks. In contrast to the majority of existing approaches to routing in clustered networks, the algorithm presented in this study only requires channel state information in the final hop, which is shown to be crucial for reaping the diversity offered by the channel. In addition to exploiting the available diversity, our simple cross-layer algorithm has the flexibility to simultaneously satisfy an additional routing objective such as the maximisation of network lifetime. The authors demonstrate through analysis and simulation that our proposed routing algorithm attains full diversity under certain conditions on the cluster sizes, and its diversity is equal to the diversity of more complicated approaches that require full channel state information. View full abstract»

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  • Analysis of switch diversity combining over two wave with diffuse power fading

    Page(s): 1061 - 1067
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (306 KB)  

    The two wave with diffused power (TWDP) fading model is a very useful fading model characterised by two specular waves in the presence of other diffused waves and is applicable to both the Rayleigh and Rician fading channels as special cases. The performance of switch diversity with binary modulation schemes over the TWDP fading channel is examined. The four types of switch diversity investigated in this study comprise of the switch-and-stay combining, switch-and-examine combining, switch-and-examine combining with post-selection and scan-and-wait combining. The expressions of the average output signal-to-noise ratio (SNR) at the receiver and bit-error rate (BER) are derived and presented. The pre-determined SNR thresholds of the switch diversity over the TWDP fading channel are optimised to obtain the minimum BER. Analytical results are validated by simulation. View full abstract»

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  • Ultra-wideband pulse shaping: bypassing the inherent limitations of the Gaussian monocycle

    Page(s): 1068 - 1074
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (598 KB)  

    UWB impulse radio transmissions are power limited as they must respect a frequency mask with low total permitted power and severe isolation requirements for the GPS band. Shaping of the short UWB impulse maximizes transmit power subject to these constraints. We have previously published results for intricate pulse shaping in the optical domain that achieves record levels of power transmission. The most successful pulse shaping techniques in the electrical domain have been achieved with derivatives of the Gaussian pulse shape. The Gaussian monocycle is a low performance choice in terms of transmit power, but nonetheless the subject of intense research activity. In this paper we compare and contrast these UWB pulses, and quantitatively compare their performance in realistic systems (specific RF and antenna hardware). There is the perception that failings in the pulse shape (especially for the monocycle) can be compensated by simple highpass filter and the bandpass nature of the UWB antenna. We fabricate and characterize two UWB antenna designs available in the literature, as well as one commercial antenna. We use our optical pulse shaping device to experimentally investigate the three different pulse shapes and measure their transmitted power spectral density with each of the UWB antennas. We find that the monocycle is significantly less powerful than the others. The fine pulse shaping available in optical processing provides 1.7 to 2.9 dB gain over the electrical processing methods, depending on the antenna used. View full abstract»

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  • Practical robust uplink pilot time interval optimisation scheme for time-division duplex multiple-input-single-output beamforming system

    Page(s): 1075 - 1081
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (517 KB)  

    Focusing on a time-division duplex (TDD) multiple-input-single-output (MISO) beamforming system, this study investigates the robust uplink pilot time interval (UPTI) design to overcome the impact of delay and channel estimation error. In TDD beamforming systems, the base station estimates the downlink (DL) channel state information (CSI) by exploiting the received uplink (UL) pilots and the channel reciprocity. Then, utilising the estimated DL CSI, the beamforming vector in the DL transmission is generated. Owing to the constraints of the TDD frame structure and the UL pilot overhead, there inevitably exist delay and estimation error between the estimated and the actual DLCSI, which would degrade system performance. In this study, the upper bound is first derived on ergodic rate for TDD MISO beamforming systems with channel estimation error and delay. Then, with any one given setting of the normalised UL pilot overhead, the optimal robust UPTI is designed, which maximises the upper bound on ergodic rate in the worst case of the CSI delay (i.e. in the case of maximum delay). Simulation results validate that the designed optimal robust UPTI can not on I y maximise the upper bound on ergodic rate but also be adaptive to the variation of channel conditions very well. View full abstract»

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  • Complex-valued pipelined decision feedback recurrent neural network for non-linear channel equalisation

    Page(s): 1082 - 1096
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1049 KB)  

    A novel complex-valued non-linear equaliser-based pipelined decision feedback recurrent neural network (CPDFRNN) is proposed in this study for non-linear channel equalisation in wireless communication systems. The CPDFRNN with low computational complexity, a modular structure comprising a number of modules that are interconnected in a chained form, is an extension of the recently proposed real-valued pipelined decision feedback recurrent neural equalisers. Each module is implemented by a small-scale complex-valued decision feedback recurrent neural network (CDFRNN). Moreover, a decision feedback part in each module can overcome the unstable characteristic of the complex-valued recurrent neural network (CRNN). To suit the modularity of the CPDFRNN, an adaptive amplitude complex-valued real-time recurrent learning (CRTRL) algorithm is presented. Simulations demonstrate that the CPDFRNN equaliser using the amplitude CRTRL algorithm with less computational complexity not only eliminates the adverse effects of the nesting architecture, but also provides a superior performance over the CRNN and CDFRNN equalisers for non-linear channels in wireless communication systems. View full abstract»

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  • Power saving for cooperative spectrum sharing-based cognitive radios under primary user short-term rate protection

    Page(s): 1097 - 1103
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (333 KB)  

    In cooperative spectrum sharing-based cognitive radio systems, the secondary user (SU) acts as a cooperator of the primary user (PU) to assist the PU's transmission and is allowed to simultaneously transmit at the same spectrum only if it is not harmful to the primary transmission. Here the authors studies a power minimising problem of the SU with real-time services for this system under constraints on the SU's quality-of-service requirement and guaranteeing the PU's instantaneous rate unchanged if the SU allocates part of its power for its own transmissions. Superposition coding (SC) is further employed by the SU to eliminate the interference caused by the PU. The problem is formulated as an optimisation problem. Owing to its high complexity, this problem is then converted into a single variable optimisation problem, and a greedy-based power minimisation method with low complexity is thus proposed. Numerical comparison shows that the proposed method can lead to reductions in the secondary transmission power of about 30 and 14% against for the non-SC one and the equal bit allocation-based one, respectively. View full abstract»

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  • Throughput analysis of cooperative spectrum sensing in Rayleigh-faded cognitive radio systems

    Page(s): 1104 - 1110
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (382 KB)  

    In a cognitive radio (CR) network, cooperative spectrum sensing is a viable sensing technique to enhance spectral utilisation efficiency of secondary users (SUs) while ensuring the quality of service (QoS) of primary users (PUs). Intuitively, the more SUs are involved in sensing, the more sensing accuracy the CR can achieve, whereas the more sensing overhead the SUs consume, the less throughput the CR network can achieve. In this study, the authors investigate overhead-throughput trade-off over Rayleigh-fading channels in a cooperative CR network that consists of a number of the SUs employing energy detectors and a single decision fusion centre. Considering the trade-off, the authors prove that there is an optimal set of the sensing length and the number of SUs that maximise the throughput of an SU network. They further extend their analysis to a two-stage cooperative sensing mechanism where the second-stage fine sensing is triggered whenever any SU reports the presence of a PU after the first-stage detection. Numerical results showed that compared with the single-stage sensing, the two-stage sensing scheme achieves higher throughput via a reduction of the false alarm probability. View full abstract»

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  • Efficient VLSI implementation of soft-input soft-output fixed-complexity sphere decoder

    Page(s): 1111 - 1118
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (325 KB)  

    Fixed-complexity sphere decoder (FSD) is one of the most promising techniques for the implementation of multiple-input multiple-output (MIMO) detection, with relevant advantages in terms of constant throughput and high flexibility of parallel architecture. The reported works on FSD are mainly based on software level simulations and a few details have been provided on hardware implementation. The authors present the study based on a four-nodes-per-cycle parallel FSD architecture with several examples of VLSI implementation in 4×4 systems with both 16-quadrature amplitude modulation (QAM) and 64-QAM modulation and both real and complex signal models. The implementation aspects and details of the architecture are analysed in order to provide a variety of performance-complexity trade-offs. The authors also provide a parallel implementation of log-likelihood-ratio (LLR) generator with optimised algorithm to enhance the proposed FSD architecture to be a soft-input soft-output (SISO) MIMO detector. To the authors best knowledge, this is the first complete VLSI implementation of an FSD based SISO MIMO detector. The implementation results show that the proposed SISO FSD architecture is highly efficient and flexible, making it very suitable for real applications. View full abstract»

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  • Multi-user diversity with two-step channel state information feedback

    Page(s): 1119 - 1125
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (265 KB)  

    This study proposes the performance of a scheduling scheme using two-step quantised channel state information (CSI) feedback. Considering fixed and random access networks, the results are obtained under both short- and long-term power constraints and for both single- and multi-layer transmission techniques. Further, the authors evaluate the asymptotic performance of the proposed scheme and show that, while the users scheduling is based on quantised CSI feedback in the first round, the second round of partial CSI feedback can be provided by either channel quantisation or hybrid automatic repeat request feedback, leading to the same forward channel average rate. In comparison with the case of no CSI feedback, the results indicate substantial average rate increment with limited number of feedback bits. Also, the average rate increases via successive partial CSI feedback when compared with one-step CSI feedback schemes. View full abstract»

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  • Improved multicast key management of Chinese wireless local area network security standard

    Page(s): 1126 - 1130
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (191 KB)  

    Multicasting is an important business in the field of the wireless local area network (WLAN), because the access point (AP) usually has to send the same message to each station (STA) of a specific group, and broadcasting the message to this group is one of the most efficient ways of communication. Chinese WLAN security standard, called WLAN Authentication and Privacy Infrastructure (WAPI), has taken secure multicasting into account, and proposed a Multicast Key Management Protocol (MKMP), in which the multicast session key (MSK) is distributed to each STA over the secure unicast channel built between STA and AP one by one. It is clear that the MSK distribution is very inefficient in performance, especially when the number of STAs is very large. In this study, a new MSK distribution protocol is proposed, and it can be used to substitute the original protocol in WAPI. Analyses show that the proposed protocol can achieve needed security requirements, and is more efficient than the original one in WAPI. Now, WAPI has been in the process of ISO/IEC standard building, and thus the authors think that their proposal can ameliorate WAPI largely and promote its ISO/IEC standard building. View full abstract»

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  • Improved two-stage CMA-based blind receivers for joint equalisation and multiuser detection

    Page(s): 1131 - 1136
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (428 KB)  

    In direct sequence-code division multiple access (DS-CDMA) systems over frequency-selective channels, blind joint equalisation and multiuser detection has been the focus subject of research by many researchers. In this scheme, only the desired user's spreading code and timing information are available to the receiver. The two-stage constant modulus algorithm (CMA)-based receiver proposed, implements an adaptive blind joint equalisation and multiuser detection without explicit channel estimation. The first stage of the algorithm can be seen as a linear combination of blind adaptive minimum output energy (MOE) detectors based on total data correlation matrix. In this study, the authors adopt a MOE detector based on interference correlation matrix instead of total data correlation matrix to improve the performance of the first-stage detector, thus achieving an improved two-stage CMA-based blind joint equalisation and multiuser detection algorithm. In addition, the convergence of the proposed adaptive algorithm is analysed. Both the analysis and computer simulation verify the performance improvement of our proposed algorithm. View full abstract»

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  • Timing synchronisation for orthogonal frequency division multiplexing using improved cross-correlation

    Page(s): 1137 - 1143
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (399 KB)  

    A timing synchronisation method for orthogonal frequency division multiplexing (OFDM) using improved cross-correlation (ICC) is proposed in this study. The proposed method is derived in terms of maximum-a-posteriori (MAP) criterion over an additive white Gaussian noise (AWGN) channel. In this way, the proposed ICC method is able to take into consideration the surrounded data signals, which is not considered in the conventional cross-correlation (CCC) method and thus the timing accuracy is improved accordingly. Compared to the CCC method, the ICC method only appends a correction term to the cross-correlator output, which can be easily implemented using an iterative method. Therefore the complexity of the proposed ICC method has no significant increase. Simulation results show that the proposed ICC method significantly outperforms the CCC based timing method. View full abstract»

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