In ad-hoc networks, mobile nodes are limited by a range of radio coverage and have an irregular source of power due to their battery. In ad-hoc networks, there are a lot of situations that all mobile nodes need to agree on their key not at the same time but in part and then merge themselves subsequently. This is because ad-hoc networks have specific features such as mobility and allow various conditions during configuration. In this thesis, we propose MCP (Merging Clusters Protocol), a simple key agreement scheme that can effectively deal with merging different adjacent clusters in mobile ad-hoc networks. When nodes of each cluster have already agreed on their own group keys and intend to merge themselves for further secure communications, MCP can be used in an efficient and secure way. In addition, it can be utilized for efficient group key agreement in a large ad-hoc network. We analyze the security and efficiency of MCP and discuss the experimental results according to practical implementation scenarios.

We propose a robust iterative multiuser receiver for decoding convolutional coded code-division multiple access (CDMA) signals in both Gaussian and non-Gaussian channel noise. The receiver is derived from a modified maximum a-posteriori (MAP) algorithm called the max-log-MAP algorithm for robustness against erroneous channel variance estimation. Furthermore, the effect of destructive outliers arising from impulsive noise is mitigated in the proposed receiver by incorporating the robust Huber penalty function into the multiuser detector. The proposed receiver is shown to perform satisfactorily over Gaussian and non-Gaussian impulsive channels. In every iteration, cumulative improvement in the quality of the a-posteriori probabilities is also demonstrated.

Internet access traffic follows hourly patterns that depend on various factors, such as the periods users stay on-line at the access point (e.g. at home or in the office) or their preferences for applications. The clustering of Internet users may provide important information for traffic engineering and billing. For example, it can be used to set up service differentiation according to hourly behavior, resource optimization based on multi-hour routing and definition of tariffs that promote Internet access in low busy hours. In this work, we propose a methodology for clustering Internet users with similar patterns of Internet utilization, according to their hourly traffic utilization. The methodology resorts to three statistical multivariate analysis techniques: cluster analysis, principal component analysis and discriminant analysis. The methodology is illustrated through measured data from two distinct ISPs, one using a CATV access network and the other an ADSL one, offering distinct traffic contracts. Principal component analysis is used as an exploratory tool. Cluster analysis is used to identify the relevant Internet usage profiles, with the partitioning around medoids and Ward's method being the preferred clustering methods. For the two data sets, these methods lead to the choice of 3 clusters with different hourly traffic utilization profiles. The cluster structure is validated through discriminant analysis. It is also evaluated in terms of several characteristics of the user traffic not used in the cluster analysis, such as the type of applications, the amount of downloaded traffic, the activity duration and the transfer rate, resulting in coherent outcomes.

Bursty traffic is dominant in modern communication networks and keeps the call-level QoS assessment an open issue. ON-OFF traffic models are commonly used to describe bursty traffic. We propose an ON-OFF traffic model of a single link which accommodates service-classes of finite population (f-ON-OFF). Calls compete for the available link bandwidth under the complete sharing policy. Accepted calls enter the system via state ON and then may alternate between ON-OFF states. When a call is transferred to state OFF it releases the bandwidth held in state ON, while when a call tries to return to state ON, it re-requests its bandwidth. If it is available a new ON-period (burst) begins; otherwise the call remains in state OFF (burst blocking). We prove that the proposed f-ON-OFF model has a product form solution, and we provide an accurate recursive formula for the call blocking probabilities calculation. For the burst blocking probabilities calculation we propose an approximate but robust formula. In addition, we show the relation between the f-ON-OFF model and other call-level loss models. Furthermore, we generalize the f-ON-OFF model to include service-classes of both finite and infinite population. Simulation results validate our analytical methodology.

Burst assembly at edge nodes is an important issue for the Optical Burst Switching (OBS) networks because it has a great impact on the traffic characteristics. We analyze the assembled traffic of the Science Information Network (SINET) by using the Fractional Brownian Motion (FBM) model. The analytical and simulation results show that existing assembly schemes cannot avoid increasing the burstiness, which will deteriorate the network performance. Here, burstiness is defined as the variance of the bitrate in small timescales. Therefore, we address the issue of how to reduce the burstiness of the assembled network traffic. Firstly, a sliding window-based assembly algorithm is introduced to reduce the burstiness of assembled traffic by transmitting bursts at an average rate in a small timescale. Next, an advanced timer-based assembly algorithm is introduced, by which the traffic rate is smoothed out by restricting the burst length to a threshold. The simulation results show that both the sliding window-based and advanced timer-based assembly algorithms perform better than existing assembly algorithms do in terms of the burst loss ratio. The simulation also indicates that the advanced timer-based assembly algorithm performs better in terms of the edge buffering delay than the sliding window-based assembly algorithm does.

A novel Adaptive Resource-based Probabilistic Search algorithm (ARPS) for P2P networks is proposed in this paper. ARPS introduces probabilistic forwarding for query messages according to the popularity of the resource being searched. A mechanism is introduced to estimate the popularity and adjust the forwarding probability accordingly such that a tradeoff between search performance and cost can be made. Using computer simulations, we compare the performance of ARPS with several other search algorithms. It is shown that ARPS performs well under various P2P scenarios. ARPS guarantees a success rate above a certain level under all circumstances, and enjoys high and popularity-invariant search success rate. Furthermore, ARPS adapts well to the variation of popularity, resulting in high efficiency and flexibility.

In wireless ad hoc networks, network services are provided through the cooperation of all nodes. Albeit that good teamwork could smoothly run a mobile network, selfish node behaviors would probably cause it to break down. Some examples of these selfish node behaviors would include, "listening only" for saving energy or "receiving the valuable" without forwarding the packets to others. To cope with this problem, we propose PDM, a price-demand function based pricing model, to restrains the selfish behaviors of mobile nodes. PDM is based on the packet sending requirements of the source nodes and the forwarding cost of relay nods. Using this pricing methodology, the packet forwarding activities will be profitable for the relay node and further stimulate cooperation in the network. In particular, the new model enjoys the merit of giving relay nodes no reason to dishonestly report their forwarding costs, because an honest cost claim has proven to be an optimal strategy for relay nodes. Furthermore, our new model uses a price-demand function to reflect the relationship between the service demand of the source nodes and the service supply of the relay nodes. As a consequence, our approach reduces the source nodes' payments to send packets, and at the same time guarantees that the packets sent by the source nodes are delivered to the destination.

Several structured peer-to-peer networks have been created to solve the scalability problem of previous peer-to-peer systems such as Gnutella and Napster. These peer-to-peer networks which support distributed hash table functionality construct a sort of structured overlay network, which can cause a topology mismatch between the overlay and the underlying physical network. To solve this mismatch problem, we propose a topology-aware hierarchical overlay framework for DHTs. The hierarchical approach for the overlay is based on the concept that the underlying global Internet is also a hierarchical architecture, that is, a network of networks. This hierarchical approach for the overlay puts forth two benefits: finding data in a physically near place with a high probability, and smaller lookup time. Our hierarchical overlay framework is different from other hierarchical architecture systems in a sense that it provides a specific self-organizing grouping algorithm. Our additional optimization schemes complete the basic algorithm which constructs a hierarchical structure without any central control.

We study the problem of optimizing admission control policies in mobile multimedia cellular networks when predictive information regarding movement is available and we evaluate the gains that can be achieved by making such predictive information available to the admission controller. We consider a general class of prediction agents which forecast the number of future handovers and we evaluate the impact on performance of aspects like: whether the prediction refers to incoming and/or outgoing handovers, inaccurate predictions, the anticipation of the prediction and the way that predictions referred to different service classes are aggregated. For the optimization process we propose a novel Reinforcement Learning approach based on the concept of afterstates. The proposed approach, when compared with conventional Reinforcement Learning, yields better solutions and with higher precision. Besides it tackles more efficiently the curse of dimensionality inherent to multimedia scenarios. Numerical results show that the performance gains measured are higher when more specific information is provided about the handover time instants, i.e. when the anticipation time is deterministic instead of stochastic. It is also shown that the utilization of the network is maintained at very high values, even when the highest improvements are observed. We also compare an optimal policy obtained deploying our approach with a previously proposed heuristic prediction scheme, showing that plenty of room for technological innovation exists.

A method for flexibly allocating and reallocating optical access network (OAN) resources, including fibers and equipment, using the constraint satisfaction problem (CSP) is described. OAN resource allocation during service delivery provisioning involves various input conditions and allocation sequences, so an OAN resource allocation method has to support various workflow patterns. Furthermore, exception processing, such as reallocating OAN resources once they are allocated, is inevitable, especially during the spread of service using optical fiber and during the deployment of an optical access network. However, it is almost impossible to describe all workflow patterns including exception processes. Improving the efficiency of these exception processes, as well as that of the typical processes, is important for reducing the service delivery time. Describing all these patterns and process flows increases development cost. The CSP can be used to search for solutions without having to fix the process sequence and input conditions beforehand. We have formulated the conditions for OAN resource allocation and reallocation as a CSP. Use of this method makes it possible to handle various allocation workflow patterns including exception processes. Evaluation of the solution search time demonstrated its feasibility.

The performance of TCP data transmission deteriorates significantly when a TCP connection traverses a heterogeneous network consisting of wired and wireless links. This is mainly because of packet losses caused by the high bit error rate of wireless links. We proposed receiver-based ACK splitting mechanism in [1]. It is a new mechanism to improve the performance of TCP over wired and wireless heterogeneous networks. Our mechanism employs a receiver-based approach, which does not need modifications to be made to the sender TCP or the base station. It uses the ACK-splitting method for increasing the congestion window size quickly in order to restrain the throughput degradation caused by packet losses due to the high bit error rate of wireless links. In this paper, we develop a mathematical analysis method to derive the throughput of a TCP connection, with/without our mechanism, which traverses wired and wireless heterogeneous networks. By using the analysis results, we evaluate the effectiveness of our mechanism in the network where both of packet losses due to network congestion and those caused by the high bit error rate of wireless links take place. Through An evaluation of the proposed method shows that it can give a good estimation of TCP throughput under the mixture networks of wired/wireless links. We also find that the larger the bandwidth of the wireless link is, the more effective our mechanism becomes, therefore, the mechanism's usability will increase in the future as wireless networks become faster.

Adaptive hybrid genetic algorithm concatenated with improved parallel interference cancellation, i.e. adaptive hybrid genetic algorithm parallel interference cancellation (AHGAPIC) was proposed. A study is conducted on the application of AHGAPIC to soft decoding high rate multi-user detection with diversity reception for dual-rate wideband DS-CDMA spread spectrum communications, aiming to mitigate the effect of multiple access interference. The relevant research has revealed that the local search capability of hybrid genetic algorithm (HGA) is still not good enough. Therefore, first, two evolutionary operations, i.e. inversion and insertion are merged into HGA to constitute a novel algorithm. With its moderate local search capability, this new algorithm can search for the global optimum region according to the information entropy, and then it is made adaptively vary its probabilities of crossover and mutation depending on the fitness values of the solutions to form the adaptive hybrid genetic algorithm (AHGA). Second, AHGA is utilized to effectively identify the better and better binary string to maximize the log-likelihood function of dual-rate multi-user detection. As AHGA converges to the optimum region, the control factor of the improved parallel interference cancellation (IPIC) detector is set to be the ratio of the average fitness value to the maximum fitness value of the population of AHGA. Finally, equipped with both the control factor and the binary string with the maximum fitness value as the initial data, the IPIC detector can rapidly find out the approximately optimum soft decoding vector. Then, it can obtain the approximately global optimum estimate point on the basis of the soft decoding rule, corresponding to the transmitted data bits. A lower bound of computational complexity has been achieved through simulations and qualitative analyses. The property of the proposed algorithm to converge rapidly leads to lower computational complexity. Emulation results have shown that the AHGAPIC soft decoding high rate multi-user detector is superior to other suboptimum detectors considered in this paper in terms of two points. They are the mitigation of multiple access interference and the resistance to near-far effects. Its performance is close to the sequential group optimum multi-user detector but with a shorter time delay.

This paper considers a blind DS-CDMA data and channel estimation algorithm using a uniform circular array. The channels are assumed to be sparse and static during a short packet transmission period. The channel estimates for different users yield the explicit estimates of the angle and time of arrivals, which are used for radiolocation. Our algorithm employs three approaches to solve the problem. The generalized successive interference (GSIC) algorithm is used to eliminate the multiple access interference. Matching pursuit (MP) is applied to enforce the channel sparsity constraint. Per survivor processing (PSP) is then employed to jointly estimate the channel parameters and data symbols. By successfully incorporating them, we present the GSIC/MP/PSP algorithm. Its performance is demonstrated by computer simulations and compared to the GSIC/MP algorithm which requires training sequences.

This paper investigates system performance for an MC-CDMA (multi-carrier coded-division multiple-access) system, in which an operating environment with both single-cell and multiple-cell configurations and correlated-Nakagami-m statistics for fading channels are adopted. It is worthwhile noting that applying the joint characteristic function to determine the jpdf (joint probability density function) with a generalized Laguerre polynomial yields a simpler method. The traditional difficult methods for explicitly obtaining the jpdf are avoided for the sake of simplification. Some new closed-form formulas for average BER (bit-error rate) with statistical calculation of MAI (multiple-access interference) for MC-CDMA system operation in multi-cell environments were obtained in this study. For achieving accuracy in the derived formulas, an example of an MC-CDMA system with a dual-receiver branch operating in a multiple-cell structure with 12 surrounding cells is presented.

The objective of this paper is to develop the theoretical foundation to the pilot-assisted channel estimation using delay-time domain windowing for the coherent detection of OFDM signals. The pilot-assisted channel estimation using delay-time domain windowing is jointly used with polynomial interpolation, decision feedback and Wiener filter. A closed-form BER expression is derived. The impacts of the delay-time domain window width, multipath channel decay factor, the maximum Doppler frequency are discussed. The theoretical analysis is confirmed by computer simulation.

In orthogonal frequency-division multiplex access (OFDMA) uplink, the carrier-frequency offsets (CFOs) between the multiple transmitters and the receiver introduce inter-carrier interference (ICI) and severely degrade the performance. In this paper, based on the perfect estimation of each user's CFO, we propose two low-complexity iterative algorithms to cancel ICI due to CFOs, which are denoted as the basic algorithm and the improved algorithm with decision-feedback equalization (DFE), respectively. For the basic one, two theorems are proposed that yield a sufficient condition for the convergence of iterations. Moreover, the interference-power-evolution (IPE) charts are proposed to evaluate the convergence behavior of this interference cancellation algorithm. Motivated by the IPE chart, the procedure of DFE is introduced into the iterations, which is the basic idea of the improved algorithm. For this improved algorithm, the error-propagation effect are analyzed and suppressed by an efficient stopping criterion. From IPE charts and simulation results, it can be easily observed that the basic algorithm has the same capability of ICI cancellation as the linear optimal minimum mean square error (MMSE) method, but offers lower complexity, while the improved algorithm with DFE outperforms the MMSE method in terms of the bit-error rate (BER) performance.

Frequency-domain equalization (FDE) based on minimum mean square error (MMSE) criterion can replace the conventional rake combining to significantly improve the bit error rate (BER) performance in a frequency-selective fading channel. MMSE-FDE requires an accurate estimate of the channel transfer function and the signal-to-noise power ratio (SNR). Direct application of pilot-assisted channel estimation (CE) degrades the BER performance, since the frequency spectrum of the pilot chip sequence is not constant over the spreading bandwidth. In this paper, we propose a pilot-assisted decision feedback frequency-domain MMSE-CE. The BER performance with the proposed pilot-assisted MMSE-CE in a frequency-selective Rayleigh fading channel is evaluated by computer simulation. It is shown that MMSE-CE always gives a good BER performance irrespective of the choice of the pilot chip sequence and shows a high tracking ability against fading. For a spreading factor SF of 16, the E_b/N₀ degradation for BER=10^-4 with MMSE-CE from the ideal CE case is as small as 0.9 dB (including an E_b/N₀ loss of 0.28 dB due to the pilot insertion).

In this paper, a non-data aided minimum mean square error (MMSE) receiver with enhanced multiple access interference (MAI) suppression is proposed for direct-sequence code-division multiple-access (DS-CDMA) systems over a multipath fading channel. The design of the proposed receiver is via the following procedure: First, an adaptive correlator is constructed based on the linearly constrained minimum variance (LCMV) criterion to collect each multipath signal and suppress MAI blindly. A maximum ratio combiner is then utilized to coherently combine the correlator outputs. With a set of judicious chosen weight vectors, effective diversity combining can successfully suppress MAI and the desired signals can be effectively retained. Finally, further performance improvement against the finite data sample effect is achieved using a decision-aided scheme in which the channel response is obtained by the decision data and incorporated with the MMSE method to compute the refined weight vector. Performance analysis based on the output signal-to-interference-plus-noise ratio (SINR) is done to examine the efficacy of the proposed non-data aided MMSE receiver, which can offer the similar results as those of the MMSE receiver with the channel estimation correctly obtained beforehand. Computer simulation results then confirm correctness of the analysis results and demonstrate that the proposed blind receiver can successfully resist MAI as well as the finite data sample effect, and significantly outperform than the conventional blind receivers.

Orthogonal frequency division multiplexing (OFDM) is one of the promising transmission techniques for next generation mobile communication systems. Accurate channel estimation is essential for coherent OFDM signal transmission. So far, many pilot-assisted channel estimation schemes have been proposed. In the case of packet transmission, each received packet can be repeatedly processed by decision feedback to improve the channel estimation accuracy, resulting in a decision directed block iterative channel estimation (DD-BICE). However, decision feedback of erroneously detected data symbols degrades the packet error rate (PER) or bit error rate (BER) performance. In this paper, theoretical analysis is presented for the DD-BICE taking into account the decision feedback errors assuming quadrature phase shift keying (QPSK) data modulation. A 2-dimensional (2D) averaging filter is used for reducing the negative impact of decision feedback errors. The impacts of 2D averaging filter and antenna diversity reception are discussed and the validity of the theoretical analysis is confirmed by computer simulation.

Recent theoretical and experimental studies indicate that spatial multiplexing (SM) systems have enormous potential for increasing the capacity of corresponding multiple input multiple output (MIMO) channels in rich scattering environments. In this paper, we propose a new recursion based algorithm for Bell Labs layered space time (BLAST) signal detection in SM systems. The new algorithm uses an inflated recursion in the initialization and a deflated recursion in the iteration stage: as a result, the complexity is greatly reduced and the irregularity issues are completely avoided. Compared with the conventional fastest recursive approach, the complexity of our proposal is lower by a factor of 2 and it is also very implementation friendly.

Multicast delivery in heterogeneous wireless networks requires careful coordination, in order to take full advantage of the resources such an interworking network environment can offer. Effective coordination, however, may require interworking signaling from coordinating network entities to receivers of a multicast service. Scalable delivery of such signaling is of great importance, since a large number of receivers may be interested in a multicast service. This paper therefore investigates the use of a multicast signaling channel (MSCH) to carry such interworking signaling in a scalable manner. Applications of interworking signaling for multicast service delivery in heterogeneous wireless networks are presented, motivating the need for an MSCH. Then a comparative study is performed analysing potential benefits of employing an MSCH for signaling message delivery compared to conventional unicast signaling. The analysis reveals that the benefits of the MSCH depend mainly on the selection of an appropriate signaling network to carry the MSCH and also on efficient addressing of a subset of receivers within the MSCH. Based on the findings, guidelines for the selection of a suitable signaling network are provided. Furthermore a novel approach is proposed that allows efficient addressing of a subset of receivers within a multicast group. The approach minimizes the required signaling load on the MSCH by reducing the size of the required addressing information. This is achieved by an aggregation of receivers with common context information. To demonstrate the concept, a prototype of the MSCH has been developed and is presented in the paper.

This paper deals with adaptive array beamforming based on stochastic gradient descent independent component analysis (ICA) for suppressing interference with robust capabilities. The approach first uses estimates of the interested source directions to construct the multiple regularized constraints, which form an efficient ICA-based beamformer to achieve fast convergence and more robust capabilities than existing MCMV and ESB beamformers. In conjunction with the regularization parameters of the high-order derivative constraints, the width of the main beam for remaining the desired signal and the depth of nulls for suppressing interferers can be adjusted. Several computer simulation examples are provided for illustration and comparison.

Target shape estimation with UWB pulse radars is a promising imaging technique for household robots. We have already proposed a fast imaging algorithm, SEABED, that is based on a reversible transform BST (Boundary Scattering Transform) between the received signals and the target shape. However, the target image obtained by SEABED deteriorates in a noisy environment because it utilizes a derivative of received data. In this paper, we propose a robust imaging method with an envelope of circles. We clarify by numerical simulation that the proposed method can realize a level of robust and fast imaging that cannot be achieved by the original SEABED.

A novel type Brillouin optical time-domain analysis (BOTDA), called double-pulse BOTDA (DP-BOTDA), is proposed for measuring distributed strain and temperature in a fiber with a centimeter spatial resolution. The DP-BOTDA system transmits a double-pulsed light instead of a conventional single-pulsed light into a fiber to interact with a counter-propagating continuous-wave light through the induced acoustic wave in the fiber. The interference between acoustic waves induced by the front and rear pulses of the double-pulsed light produces broad but oscillatory Brillouin gain spectra that make it possible to measure the Brillouin frequency shift accurately despite the very narrow pulse width. Our numerical simulation, which includes an estimation of the signal-to-noise ratio of the system, shows that it is possible to measure the distributed Brillouin frequency shift with a spatial resolution of 4 cm and accuracies of 1-2 MHz for a 5-km long fiber.

We developed a comprehensive simulation system for evaluating satellite-based navigation services in highly built-up areas; the system can accommodate Global Positioning System (GPS) multipath effects, as well as line-of-sight (LOS) and dilution of position (DOP) issues. For a more realistic simulation covering multipath and diffracted signal propagations, a 3D-ray tracing method was combined with a satellite orbit model and three-dimensional (3D) geographic information system (GIS) model. An accuracy estimation model based on a 3D position determination algorithm with a theoretical delay-locked loop (DLL) correlation computation could measure the extent to which multipath mitigation improved positioning accuracy in highly built-up areas. This system could even capture the multipath effect from an invisible satellite, one of the greatest factors in accuracy deterioration in highly built-up areas. Further, the simulation results of satellite visibility, DOP, and multipath occurrence were mapped to show the spatial distribution of GPS availability. By using object-oriented programming, our simulation system can be extended to other global navigation satellite systems (GNSSs) simply by adding the orbital information of the corresponding GNSS satellites. We demonstrated the applicability of our simulation system in an experimental simulation for Shinjuku, an area of Tokyo filled with skyscrapers.

As semiconductor processing technology advances, complex, high density circuits can be integrated in a chip. However, increasing energy consumption is becoming one of the most important limiting factors. Power estimation at the early stage of design is essential since design changes at later stages may significantly lengthen the design period and increase the costs. For efficient power estimation, we analyze the "key" control signals of a digital circuit and develop power models for several operational modes. The trade-off between accuracy and complexity can be made by choosing the number and the complexity of the power models. When compared with those of logic simulation based estimation, experimental results show that 13 to 15 times faster power estimation with an estimation error of about 5% is possible. We have also developed new logic-level power modeling techniques in which logic gates are levelized and several levels are selected to build power model tables. This table based method shows significant improvement in estimation accuracy and a slight improvement in efficiency when compared to a well-known previous method. The average estimation error has been reduced from 13.3% to 3.8%.

This letter proposes a new algorithm for the check node update in the decoding of low-density parity-check (LDPC) codes. The proposed algorithm is based on a new approximation formula of standard sum-product algorithm (SPA) which can reduce the approximation error of min-sum algorithm (MSA) and has almost the same performance as sum-product algorithm (SPA) under both floating precision operation and fixed-point operation. Besides, the new approximation formula can be implemented in simple structures competitive with MSA.

Motion estimation (ME) is a computation intensive procedure in H.264. In ME for variable block sizes, an effective scan ordering method has been devised for early termination of absolute difference computation when the termination does not affect the performance. The new ME circuit with effective scan ordering can reduce the amount of computation by 70% compared to JM8.2 and by 30% compared to the disable approximation unit (DAU) approach.

This letter proposes a vertical handoff scheme for integrated WLAN and UMTS that use the mobile Internet Protocol (IP) to reduce the packet loss caused by the ping-pong effect for high mobility users. The simulation results show that the proposed scheme efficiently increases the throughput of high mobility users.

The frequency hopping (FH) based ultra-wideband (UWB) communication system divides its available frequency spectrum into several sub-bands, which leads to inherent disparities between carrier frequencies of each sub-band. Since the propagation loss is proportional to the square of the transmission frequency, the propagation loss on the sub-band having the highest carrier frequency is much larger than that on the sub-band having the lowest carrier frequency, resulting in disparities between received signal powers on each sub-band, which in turn leads to a bit error rate (BER) degradation in the FH UWB system. In this paper we propose an adaptive receiver for FH based UWB communications, where the integration time is adaptively adjusted relative to the hopping carrier frequency, which reduces the disparity between the received signal energies on each sub-band. Such compensation for lower received powers on sub-bands having higher carrier frequency leads to an improvement on the total average BER of the entire FH UWB communication system. We analyze the performance of the proposed reception scheme in Nakagami fading channels, and it is shown that the performance gain provided by the proposed receiver is more significant as the Nakagami fading index m increases (i.e., better channel conditions).

In this letter, we discuss the problem of receive antenna selection in the downlink of multiuser multiple-input multiple-output (MIMO) systems with Tomlinson-Harashima precoding (THP), where the number of receivers is assumed equal to that of transmit antennas. Based on the criterion of maximum system sum-capacity, a per-layer receive antenna selection scheme is proposed. This scheme, which selects one receive antenna for each receiver, can well exploit the nonlinear and successive characteristics of THP. Two models are established for the proposed per-layer scheme and the conventional per-user scheme. Both the theoretical analysis and simulation results indicate that the proposed scheme can greatly improve the equivalent channel power gains and the system sum-capacity.

This paper proposes a co-channel interference (CCI) suppression scheme employing a frequency-domain nulling filter and turbo equalizer for single-carrier uplink time division multiple access (TDMA) systems. In the proposed scheme, after the received signal is transformed into a frequency-domain signal via fast Fourier transform (FFT), CCI from an adjacent cell is suppressed by the nulling filter. Moreover, the proposed scheme employs a soft canceller and minimum mean square error (SC/MMSE) based turbo equalizer to suppress the performance degradation due to inter-symbol interference (ISI) caused by the nulling filter as well as the ISI induced by fading channel. Computer simulation confirms that the proposed scheme is effective in suppression of CCI compared to the conventional linear frequency-domain equalizer.

In this work particle swarm optimization (PSO) aided with radial basis functions (RBF) has been suggested to carry out multiuser detection (MUD) for synchronous direct sequence code division multiple access (DS-CDMA) systems. The performance of the proposed algorithm is compared to that of other standard suboptimal detectors and genetic algorithm (GA) assisted MUD. It is shown to offer better performance than the others especially if there are many users.

The subspace algorithm can be utilized for the blind detection of space-time block codes (STBC) without knowledge of channel state information (CSI) both at the transmitter and receiver. However, its performance degrades when the channels are correlated. In this letter, we analyze the impact of channel correlation from the orthogonality loss between the transmit signal subspace (TSS) and the statistical noise subspace (SNS). Based on the decoding property of the subspace algorithm, we propose a revised detection in favor of the channel correlation matrix (CCM) only known to the receiver. Then, a joint transmit-receive preprocessing scheme is derived to obtain a further performance improvement when the CCM is available both at the transmitter and receiver. Analysis and simulation results indicate that the proposed methods can significantly improve the blind detection performance of STBC over the correlated channels.

Cooperative communication enables single antenna to realize a virtual multiple antenna by sharing their antennas. Therefore, it offers transmit diversity. In this letter, we apply pre-coding scheme to the transmit symbols. Although pre-coding has shortcomings, it is possible to employ Space-Time Block Codes (STBC) with single antenna and achieve high gain.

In this letter, a lattice-reduction-aided (LRA) minimum mean square error (MMSE) Tomlinson-Harashima precoding (THP) is proposed for multiple input multiple output (MIMO) systems. The extended channel is exploited to develop the LRA MMSE-THP based on the lattice reduction method. Simulation results show that the proposed scheme significantly outperforms the conventional MMSE THP and the LRA zero-forcing (ZF) THP and achieves full diversity order.

LDCs system with finite-rate error-free feedback is proposed in this letter. The optimal transmission codeword is selected at the receiver and the codeword index is sent to the transmitter. A simple random search algorithm is introduced for codebook generation. Moreover, the max-min singular value criterion is adopted for codeword selection. Simulation results showed that, with only 3-4 feedback bits, the low-complexity Zero-Forcing receiver can approach the Maximum-Likelihood (ML) performance.

An adaptive subcarrier allocation (SA) algorithm is proposed for both the enhancement of system capacity and the practical implementation in a clustered OFDM system. The proposed algorithm is based on the two dimensional comparison of the channel gain in both rows and columns of the channel matrix to achieve higher system capacity. Simulation results demonstrate that the proposed algorithm outperforms the SA algorithm based on only one dimensional comparison in terms of system capacity, and furthermore, it performs as well as the optimal SA algorithm at relatively low computational cost.

We analytically prove that the error in the channel idle time-based collision probability estimation in face of non-saturated stations is bounded by 2/(CW_min+1) in the IEEE 802.11 wireless LANs (WLANs). This work explicitly quantifies the impact of non-saturation, and the result vindicates the use of the estimation technique in real-life IEEE 802.11 WLANs, in such applications as the acknowledgement-based link adaptation and the throughput optimization through contention window size adaptation.