Recently space division multiple access (SDMA) assisted multiple-input–multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems invoking multiuser detection (MUD) techniques have attracted substantial research interest, which is capable of exploiting both transmitter multiplexing gain and receiver diversity gain. A new scheme referred to here as slowsubcarrierhopping (SSCH) assisted multiuser SDMA-OFDM, is proposed. It is shown that, with the aid of the so-called uniform SSCH (USSCH) pattern, the multiuser interference (MUI) experienced by the high-throughput SDMA-OFDM system can be effectively suppressed, resulting in a significant performance improvement. In the investigations conducted, the proposed USSCH-aided SDMA-OFDM system was capable of outperforming a range of SDMA-OFDM systems considered, including the conventional SDMA-OFDM system dispensing with the employment of frequency-hopping techniques. For example, at an Eb/N0 value of 12 dB, the proposed USSCH/SDMA-OFDM system reduced the bit error ratio (BER) by about three orders of magnitude, in comparison to the conventional SDMA-OFDM system, while maintaining a similar computational complexity.
We study the multiuser diversity in both multiple-access channel (MAC) and broadcast channel (BC) in multiple-input multiple-output (MIMO) systems, where more than one users are communicating with the base station simultaneously. We consider multi-user scheduling with linear receivers for uplink MIMO MAC and linear precoders for downlink MIMO BC. In each time slot, a group of users is selected for either transmission or reception. A selection criterion is presented to optimise the detection signal-to-noise ratio. We show that the linear processings achieve asymptotically the same detection signal-to-noise ratio for downlink and uplink. This makes it convenient to use a single scheduling algorithm in both downlink and uplink. Furthermore, we present a performance analysis of the proposed scheduling algorithm. Both the analysis and simulations have shown that the scheduling algorithm can provide a significant gain in capacity and error performance for both MIMO BC and MIMO MAC with linear processing.
The error probability of maximum-likelihood (ML) soft-decision decoded binary block codes rarely accepts exact closed forms. In addition, for long codes ML decoding becomes prohibitively complex. Nevertheless, bounds on the performance of ML decoded systems provide insight into the effect of system parameters on the overall system performance in addition to a measure of efficiency of the sub-optimum decoding methods used in practice. In the article, a comprehensive study of a number of lower and upper bounds on the error probability of ML decoding of binary codes over AWGN channel is provided. Bounds considered here are bounds based on the so-called Bonferroni-type inequalities and bounds developed primarily in the light of the geometrical structure of the underlying signal constellations. The interrelationships among the bounds are explored and current tightest bounds at different noise levels are pointed out.
Multipath effects in complex environments can result in distortion and time elongation of received UWB pulses. These effects have been analysed using a ray tracing channel model with a PPM-TH modulation scheme and a RAKE receiver architecture. The resultant BER has been calculated using both this model and a generalised statistical model from the literature. Results indicate that the late arrival multipath components allowed for in the ray tracing model have a significant effect on system performance for bit rates of the order 100 Mbit/s. The generalised channel model is shown to be in general agreement with the ray tracing approach for low bit rate systems but somewhat optimistic for high bit rates in a complex multipath scenario.
It is shown how to construct an algorithm to search for binary idempotents that may be used to construct binary LDPC codes. The algorithm, which allows control of the key properties of sparseness, code rate and minimum distance, is constructed in the Mattson-Solomon domain. Examples are given of the codes constructed that include equivalent codes to the Euclidean and Projective Geometry codes in addition to some new codes. Codes having cycles of length 4 can also be constructed and are demonstrated to have good performance under iterative decoding.
The authors present the asymptotic bit error rate (BER) performance analysis of a system, over a Rayleigh fading channel that is based on the combination of low density parity check (LDPC)-based space-time (ST) codes and multi-carrier code division multiple access (MC-CDMA) techniques. MC-CDMA systems have the benefits of both orthogonal frequency division multiplexing (OFDM) and CDMA techniques in fading channels. Additionally, the LDPC-based ST codes not only increase the capacity of the system by using MIMO techniques, but also increase the coding gain owing to the use of powerful LDPC codes. First, the downlink of the MC-CDMA system with MIMO channel is analysed, and then the LDPC is applied to the results to complete the analysis. The simulation results are also compared with those previously obtained using turbo codes instead of the LDPC codes.
Narrowband path loss models are not applicable to the link budget calculation of ultra wideband (UWB) signals. In the paper, a rigorous analysis reveals the power loss behaviour of short-range UWB pulse systems when operated in a rich multi-path environment. A novel breakpoint is derived that is within the vicinity of the transmitter, and is shown to influence the UWB link budget considerably. Furthermore, it is shown that the power delay profile does not generally follow a simple negative exponential dependency. Also, the effects of perfect and imperfect correlator onto the overall UWB pulse power loss are considered.
A novel approach for improving the performance of the soft-output Viterbi algorithm (SOVA) when applied to turbo decoding is proposed. Based upon a modified two-step scaling factor approach for the decoder's extrinsic information, it is shown that the proposed technique reduces significantly the error floor present in previous SOVA-based turbo decoding techniques. Various computer simulated bit error rate (BER) performance evaluation results for binary phase shift keying (BPSK) signals transmitted over the additive white Gaussian noise (AWGN) and flat Rician fading channels clearly indicate that for large interleaver sizes and high numbers of decoding iterations no error floor is observed for BERs as low as 10-6
The paper presents a novel approach to estimating the multi-path channel of orthogonal frequency-division multiplexing (OFDM)/spatial-diversity multiple-access (SDMA) systems. Having OFDM/SDMA symbols transmitted not aligned with base station (BS) timing, demodulated signals may severely suffer from an inter-OFDM-symbol interference (ISI), namely the misalignment interference. Consequently, the interference degrades the channel estimation extensively. By appropriately designing pilots, the misalignment effects on the channel estimation can fully be cancelled and, the channel is optimally estimated in the minimum mean square error (MMSE) sense by a new estimation scheme. Owing to the interference cancellation, the new scheme is more robust against timing offsets (TO) and gives a better performance in estimating the SDMA channel than the existing approaches.
The authors propose a novel use of complete complementary (CC) sequences for increased spectral efficiency in a multiple-input–multiple-output (MIMO) code division multiple access (CDMA) system. The new method overcomes many of the problems and limitations persistent in single-input–single-output (SISO) and proves that under the proposed MIMO CC-CDMA system, the capacity for the number of supported users increases linearly by the number of transmitters. The paper also shows that the MIMO CC-CDMA system demonstrates a superior performance over those using traditional Walsh spreading sequences (Walsh-CDMA). Results include the bit error rate comparison for CC-CDMA frequency selective fading channels and that of the Walsh-CDMA under flat fading channels.
Recently, the added-hyperplane (AHP) bound was proposed on the foundation of the tangential sphere bound (TSB) of Poltyrev. AHP utilises a Bonferroni-type inequality (known as the Hunter bound) together with the Gallager first bounding technique (GFBT) and is tighter than TSB; however, it suffers from a performance-degrading overhead. Another inequality from the Hunter-bound family is applied to the GFBT and a novel technique has been proposed to waive the need for global geometrical properties of the code, removing the aforementioned overhead. Also, a star-structured graph is proposed as the corresponding spanning tree for the Hunter bound. The improved tangential sphere bound (ITSB) is tighter than TSB and AHP and does not impose any overhead or extra optimisation. ITSB is thus the tightest upper bound on the performance of linear binary block codes over AWGN channel. ITSB is then applied to different block (slow) fading channels as well as low-density parity-check codes.
Recently, the single-carrier transmission with frequency domain equalisatlon has attracted much attention for reasons of being able to obtain an excellent performance even in strong frequency selective channels. In the paper, the authors evaluate the performance of direct sequence - ultra wideband (DS-UWB) with frequency domain channel estimation (FDCE) and equalisation (FDE) in the UWB multipath channel and compare it to that of conventional DS-UWB with time domain channel estimation (TDCE) and RAKE receiver. The authors purpose is to compare the performances of both schemes under Fair conditions. In particular, the authors focus on the computational complexity of frequency domain process (FDP) that uses FDCE and FDE and compare it to that of time domain process (TDP) that uses TDCE and RAKE receiver. The authors simulation results show that when E-b/N-0 is high, DS-UWB with FDP has significantly less computational complexity and achieves a better performance than DS-UWB with TDP; conversely when E-b/N-0 is low, DS-UWB with TDP has significantly more computational complexity and achieves a better performance than DS-UWB with FDP.
The optimal channel estimation approach for multiple-input multiple-output (MIMO) systems, which runs filters for all possible symbol combinations, requires exponentially growing resources with time and number of transmit antennas. The conventional suboptimal channel estimation approaches are in a decision-directed manner, i.e. estimating the channels using the symbols already detected. The drawback of this kind of method is that possible symbol-detection errors are not fully accounted for in the channel estimation. In the paper, a sub-optimal joint channel tracking and symbol detection method is proposed based on probabilistic data association (PDA) and generalised pseudo Bayesian (GPB) algorithms. The PDA principle is applied to reduce the model size at every time instant and the first order GPB algorithm (GPB 1) is used to control the size of the filtering tree by combining the estimation result from different models at every time instant. Simulation results demonstrate that the proposed multiple model channel estimation algorithm (PDAMM) performs better than the conventional decision based single model channel estimation algorithm (PDAKal).
The authors have previously implemented a hard-decision decoder and have shown with simulation results that Hermitian codes perform well over the AWGN and Rayleigh fading channel, owing to their large code lengths. However, further increases in coding gain can be achieved by using a combined error and erasure decoding algorithm instead. In the paper a combined error and erasure decoding algorithm for Hermitian codes is implemented and simulation results over the AWGN and Rayleigh fading channel for the (64,49,10), (64,44,15) and (512,314,171) Hermitian codes are presented. The results show only a small increase in performance over the AWGN channel but more significant coding gains can be achieved over a Rayleigh fading channel.
Under perfect synchronisation conditions, communication systems using multiple transmit and receive antennas, known as the multiple input multiple output (MIMO) technique, along with the multicarrier transmission technique can achieve very high transmission data rates, bandwidth efficiency and robustness to the channel frequency selective fading. In a practical system, however, the presence of a carrier frequency offset (CFO), due to lack of perfect synchronisation, causes a severe degradation in the overall system performance. A low-cost blind CFO estimator is proposed for the downlink of the MIMO multicarrier system, which is based on the null subcarriers concept and Taylor's series expansion. The theoretical mean squared error (MSE) of the proposed CFO estimator is also derived. It is shown in particular, that the computational complexity of the proposed estimator is significantly low, yet its performance is good.
Performance evaluation methods that have been used so far failed to capture the real characteristics of network traffic. This is especially true when the service times are general such as the variable bit rate (VBR) video,, which is proven to be self-similar. In the paper, diffusion approximation methods are proposed to analyse a dynamic scheduler for self-similar VBR video traffic in asynchronous transfer mode (ATM) networks. The proposed scheme employs a Coupled queueing system that has been used extensively in the modelling Of Computer and Communication systems. Diffusion approximations methods are used to decouple a queueing system which represents an ATM network node into separate G/G/1 queues. Real MPEG video traces are used in the discrete event simulation. Results are compared with the approximation, and are found to work very well under different traffic conditions.
The resilient packet ring (RPR), defined under IEEE 802.17, has been proposed as a high-speed backbone technology for metropolitan area networks. RPR is introduced to mitigate the underutilisation and unfairness problems associated with the current technologies SONET and Ethernet, respectively. The key performance objectives of RPR are to achieve high bandwidth utilisation, optimum spatial reuse on the dual rings, and fairness. The RPR standard implements three traffic classes: Class A, Class B, and Class C. The RPR MAC has one queue for each traffic class. A potential performance limitation is associated with the head-of-line blocking. When the MAC uses a single FIFO to buffer frames awaiting access, a packet that is traversing through a congestion point may block transmission of other packets destined to a point before the congestion. The use of virtual destination queues (VDQs) to avoid the head-of-line blocking is introduced. Different bandwidth allocation policies are discussed to assign rates to VDQs. Finally, a bandwidth allocation policy is proposed, which would achieve the maximum utilisation at a very low complexity.
New Bonferroni-type lower bounds on the word error probability of uncoded systems are developed. The new family of bounds is based on a recent Bonferroni inequality proposed by Cohen and Merhav. These novel tight bounds are developed for optimal maximum a posteriori (MAP) coherent detectors with nonuniform signalling over additive white Gaussian noise channel. The results are compared to the state-of-the-art KAT lower bounds and it is shown that the superiority of one bound to another is dependent on the signal constellation, the amount of nonuniformity of the Bernoulli source to be communicated, and the SNR range of interest. For instance, for smaller deviations from the uniform case, which are in fact more plausible, and at low SNRs, the new bounds are tighter than KAT lower bounds for all the constellations studied.
Improving the performance of the traditional TCP in wireless IP communications has been an active research area. The significant cause of packet losses in such heterogenous networks is no longer limited to network congestion. The performance degradation of TCP in wireless and wired-wireless hybrid networks is mainly due to its lack of ability to differentiate the packet losses caused by network congestions from the losses caused by wireless link errors. New variants of TCP Vegas and TCP Reno named Snug-Vegas and Snug-Reno, respectively, are proposed. By using random-loss indications marked by base stations, Snug-Vegas and Snug-Reno may detect random packet losses precisely. Through the packet loss differentiation, Snug-Vegas and Snug-Reno react appropriately to the losses, and based on the simulation results, it is seen that the throughput of connection over heterogeneous networks can be significantly improved.
The authors consider an integrated switching element with a shared buffer memory and a constant hardware delay. This hardware delay is caused by the hardware operations required to process the routing information of incoming cells. A general uncorrelated cell arrival process in the switch, an independent and uniform routing process of cells from the inlets to the outlets of the switch and a first-come-first-served queueing discipline are assumed. The performance of the switching element is evaluated by means of an analytical technique based on an extensive use of probability generating functions. Explicit expressions for the probability generating functions, the mean values, the variances and the tail probabilities of the occupancy and the cell delay of the switch are obtained. Numerical examples show that the hardware delay has an important impact on the switch performance.
