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Communications, IET

Issue 7 • Date August 2008

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Displaying Results 1 - 14 of 14
  • Frequency domain simulator for mobile radio channels and for IEEE 802.16-2004 standard using measured channels

    Page(s): 869 - 877
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (668 KB)  

    Two radio channel simulators based on the time variant frequency transfer function have been implemented using digital signal processing techniques in SIMULINK. The first simulator uses a two ray channel model to determine the number of taps per coherent bandwidth of the channel by comparing the bit error rate (BER) with the tapped delay line model for differential quadrature phase shift keying. The results show that ten taps per coherent bandwidth are appropriate for a very close approximation of the channel. The second simulator utilises real channel measurements to estimate the BER for the IEEE 802.16 -2004 wireless metropolitan area network orthogonal frequency division-multiplexing standard with 256 carriers. Channel measurements at three frequencies in the 2-6-GHz frequency band in rural/semi rural environment demonstrate the simulator and relate the BER performance of the standard to the frequency selectivity of the channel. View full abstract»

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  • Measurements of radio channels and bit error rate estimation of IEEE802.16 standard in semi-rural environment at three frequencies in the 2-6 GHz frequency band

    Page(s): 878 - 885
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1308 KB)  

    Simultaneous measurements with 10 MHz bandwidth at 2.5, 3.5 and 5.8 GHz were performed in a rural/semi-rural environment in the UK. The measurements were processed to generate power delay profiles to estimate the root mean square delay spread of the channel. The frequency range of the orthogonal frequency division multiplexing (OFDM) symbol bandwidth that has dropped below a predefined level, termed herein as average fade bandwidth, and the corresponding level crossing are employed to quantify the severity of frequency selectivity. The channel data were used to estimate the bit error rate, for the 256 carrier-OFDM IEEE802.16 standard using a frequency domain channel simulator especially designed for the study. It was found that the performance of quasi-stationary wireless broadband systems depends mainly upon the frequency selectivity and the channel coding rate with 1/2 rate coding giving a superior performance to 3/4 rate coding. Puncturing was found to weaken the capability of forward error correction coding in the presence of series of deep fades in the channel transfer function. View full abstract»

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  • An efficient joint channel estimation and decoding algorithm for turbo-coded space-time orthogonal frequency division multiplexing receivers

    Page(s): 886 - 894
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (253 KB)  

    The challenging problem in the design of digital receivers of today's and future high-speed, high data-rate wireless communication systems is to implement the optimal decoding and channel estimation processes jointly in a computationally feasible way. Without realising such a critical function perfectly at receiver, the whole system will not work properly within the desired performance limits. Unfortunately, direct implementation of such optimal algorithms is not possible mainly due to their mathematically intractable and computationally prohibitive nature. A novel algorithm that reaches the performance of the optimal maximum a posteriori (MAP) algorithm with a feasible computational complexity is proposed. The algorithm makes use of a powerful statistical signal processing tool called the expectation maximisation (EM) technique. It iteratively executes the MAP joint channel estimation and decoding for space'time block-coded orthogonal frequency division multiplexing systems with turbo channel coding in the presence of unknown wireless dispersive channels. The main novelty of the work comes from the facts that the proposed algorithm estimates the channel in a non-data-aided fashion and therefore except a small number of pilot symbols required for initialisation, no training sequence is necessary. Also the approach employs a convenient representation of the discrete multipath fading channel based on the Karhunen'Loeve (KL) orthogonal expansion and finds MAP estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MAP estimator. Moreover, optimal rank reduction is achieved by exploiting the optimal truncation property of the KL expansion resulting in a smaller computational load on the iterative estimation approach. View full abstract»

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  • Coding techniques for link adaptation in multicarrier systems

    Page(s): 895 - 902
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (380 KB)  

    A novel technique for improving coding and diversity gains in orthogonal frequency division multiplexing (OFDM) systems is proposed. Multidimensional symbol design based on complex field codes with interleaving across frequency has been known for some time now. However, such symbols cannot be concatenated to convolutional codes owing to the prohibitive complexity of decoding. A novel way of designing multidimensional symbols that allow to concatenate them to convolutional codes while maintaining a low decoding complexity is shown. The proposed multidimensional symbols are based on tailbiting convolutional codes and the design of codes is discussed with desirable properties. Also the design of bit interleaved coded modulation-type and trellis-coded modulation-type codes over these multidimensional symbols is shown. Simulations show that the proposed coding scheme provides significant performance and/or complexity improvements over existing alternatives and also provides more degrees of freedom for channel-based link adaptation. View full abstract»

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  • Average symbol error probability of arbitrary rectangular quadrature amplitude modulation in generalised shadowed fading channels

    Page(s): 903 - 908
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (170 KB)  

    A closed-form approximation of average symbol error probability of arbitrary rectangular quadrature amplitude modulation in a gamma-shadowed Nakagami fading radio channel is derived. The final result is obtained on the basis of an exponential approximation of the Gaussian Q-function. It is shown that the derived formula can also be used for error probability estimation in Nakagami-log-normal radio channels for a wide range of shadowing spread values. The numerical aspects of calculation are discussed. View full abstract»

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  • Predictive closed-loop power control scheme with comb-type sample arrangement for code division multiple access cellular networks

    Page(s): 909 - 917
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (209 KB)  

    The authors present and analyse a predictive closed-loop power control (CLPC) scheme, which employs a comb-type sample arrangement to effectively compensate multiple power control group delays over mobile fading channels. The authors consider both least squares and recursive least squares filters in our CLPC scheme. The effects of channel estimation error, prediction filter error and power control bit transmission error on the performance of the proposed CLPC method along with competing non-predictive and predictive CLPC schemes are thoroughly investigated. The results clearly indicate the superiority of the proposed scheme with its improved robustness under non-ideal conditions. Furthermore, the authors carry out a Monte-Carlo simulation study of a 5%5 square grid cellular network and evaluate the user capacity. Capacity improvements up to 90% are observed for a typical cellular network scenario. View full abstract»

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  • Modelling the desynchronisation of hidden nodes in IEEE 802.11 wireless networks

    Page(s): 918 - 927
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (347 KB)  

    An analytically solvable mathematical model is presented for an IEEE 802.11 network, where two terminals, both visible to an access point but not to each other, contend for the channel. This situation, known as the hidden node topology, has been identified as a key reason for the degradation of the protocol's performance. It is shown that the renewal theory, the analytical tool used traditionally, is not suitable for this topology and a discrete time Markov chain is introduced for the modelling of the channel contention. The model permits the accurate computation of several key performance metrics and is in agreement with simulation results over a variety of scenarios. View full abstract»

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  • Error performance of general order selection in correlated nakagami fading channels

    Page(s): 928 - 934
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (354 KB)  

    A procedure for determining the probability distribution of the rth order statistic, Gr:L, r=1, 2, ==, L, among a set of L correlated Nakagami diversity branch gains G 1, G 2, , GL has been described in David and Nagaraja (2003) and Elkashlan et al. (2008). The results are used to evaluate the bit error rate (BER) of general order selection (GOS), a diversity method in which the rth order branch is selected for transmission, over correlated Nakagami fading branches. GOS can be used to improve system throughput and provide various levels of services, both of which are highly desirable in high-speed communication systems. Numerical and simulation results are presented and used to illustrate the effects of fading correlation on the BER associated with the rth order gain branch. View full abstract»

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  • Analysis of an authenticated identity-based multicast scheme

    Page(s): 935 - 937
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (110 KB)  

    In IEE Proceedings Communications, 2005, 152, (6), an efficient authenticated identity-based multicast scheme from bilinear pairing was proposed. Wang proved that the scheme is secure against the adversary who can forge a ciphertext, but it is shown that their scheme is not secure against an inside forger. View full abstract»

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  • Precoded orthogonal frequency division multiplexing with hidden pilots for ultrawideband wireless communications

    Page(s): 938 - 950
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (993 KB)  

    The authors propose a new hidden pilot scheme equipped with precoding and its application to orthogonal frequency division multiplexing-ultra-wideband (OFDM-UWB) systems. The proposed scheme can be thought of as an improvement over conventional hidden pilot schemes. By carefully designing precoder and hidden pilot from the view point of frequency diversity, channel estimation and the peak-to-average power ratio (PAPR), more frequency diversity gain and reduced PAPR can be achieved. In addition, the authors can support more pilots to estimate a channel providing mitigated self-interference between data symbol and hidden pilot with almost no loss of bandwidth efficiency in OFDM-based UWB communication systems. The authors show improved performance of the proposed scheme over the multiband OFDM scheme through simulations in a realistic UWB channel environment. View full abstract»

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  • Dynamic multicast routing and wavelength assignment using generic graph model for wavelength-division-multiplexing networks

    Page(s): 951 - 959
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (814 KB)  

    The problem of dynamic multicast routing and wavelength assignment (MC-RWA) in the wavelength-routed wavelength-division-multiplexing networks is addressed. Current solutions to this problem always rely on homogeneous network constructions. However, future backbone networks tend to be heterogeneous in nature. Thus, the dynamic MC-RWA problem should be studied in a more realistic situation by considering the heterogeneity of network structures. A new graph model is proposed for the MC-RWA problem. This model is based on layered auxiliary graph which is generic and able to support various node architectures and heterogeneous network structures. Based on this graph model, the dynamic MC-RWA problem can be simply solved by an efficient multicast tree algorithm on various light-splitting and wavelength-conversion scenarios. In general, this graph model provides a universal platform to study different aspects of the dynamic MC-RWA as well as related problems. View full abstract»

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  • Improved regular and semi-random rate-compatible low-density parity-check codes with short block lengths

    Page(s): 960 - 971
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (302 KB)  

    Powerful rate-compatible codes are essential for achieving high throughput in hybrid automatic repeat request (ARQ) systems for networks utilising packet data transmission. The paper focuses on the construction of efficient rate-compatible low-density parity-check (RC-LDPC) codes over a wide range of rates. Two LDPC code families are considered; namely, regular LDPC codes which are known for good performance and low error floor, and semi-random LDPC codes which offer performance similar to regular LDPC codes with the additional property of linear-time encoding. An algorithm for the design of punctured regular RC-LDPC codes that have low error floor is presented. Furthermore, systematic algorithms for the construction of semi-random RC-LDPC codes are proposed based on puncturing and extending. The performance of a type-ll hybrid ARQ system employing the proposed RC-LDPC codes is investigated. Compared with existing hybrid ARQ systems based on regular LDPC codes, the proposed ARQ system based on semi-random LDPC codes offers the advantages of linear-time encoding and higher throughput. View full abstract»

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  • Flooding-limited for multi-constrained quality-of-service routing protocol in mobile ad hoc networks

    Page(s): 972 - 981
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (361 KB)  

    Multi-constrained quality-of-service (QoS) routing is used to find routes in a network to satisfy multiple independent QoS constraints. This problem is considered to be NP-complete, and most existing QoS routing algorithms are based on maintaining a global network state at every node. A multi-constrained, flooding-limited, QoS routing method to deal with limited available resources and minimum computation in a dynamic environment is proposed. The solution is based on decomposition of a routing area and a restriction in the exchange of routing information. It reduces the size of the control messages, restricts the amount of routing information, minimises the overhead from the flooding of control traffic and decreases the complexity of path selection. It is also proved that the flooding-limited-path heuristic can achieve very high performance by maintaining entries in each node, which indicates that the performance of the limited-path heuristic is not sensitive to the number of constraints. Simulation results show that this protocol provides better performance than other protocols, especially with regards to end-to-end delay, throughput and packet loss. View full abstract»

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  • Iterative (turbo) expectation - maximisation-based time and frequency synchronisation for multiple-input multiple-output-orthogonal frequency-division multiplexing systems

    Page(s): 982 - 993
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (608 KB)  

    An iterative expectation-maximisation time-frequency synchronisation algorithm joint with channel estimation for multiple-input multiple-output-orthogonal frequency-division multiplexing (OFDM) systems in frequency-selective fading channels is addressed. The receiver iterates between detection and estimation stages. For each iteration, the expectation of ODFM symbols is calculated first by using a posteriori probabilities provided by maximum a posteriori (MAP) decoder, and second, a proposed metric is maximised to obtain both frequency offset and symbol timing. The channel can be identified by means of these estimates. This algorithm can work in transmission mode, and thus, can be used to estimate the residual errors or track the change of the parameters. The performance of the proposed synchronisation approach, in terms of bit-error rate and mean-square error, is shown. View full abstract»

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IET Communications covers the theory and practice of systems, networks and applications involving line, mobile radio, satellite and optical technologies for telecommunications, and Internet and multimedia communications.

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