ABSTRACT Because of the very low signal duty cycles, synchronization is the most critical issue in ultra wideband (UWB) impulse radio (IR) systems. Some effective synchronization schemes like a symbol‐differential (SD) IR‐UWB receiver have been proposed to synchronize received signals rapidly. Yet, SD IR‐UWB receiver is unsuitable for operation in multi‐user environment because of multiple access interference (MAI). By taking advantage of frame‐differential IR‐UWB receivers, we propose a parallel frame‐differential (PFD) IR‐UWB receiver to do so. Our proposed PFD IR‐UWB receiver manifests better immunity against message passing interface and MAI than the SD IR‐UWB. Based on this PFD IR‐UWB receiver, uncertain (search) regions are limited to one frame duration without any symbol‐level synchronization process. Performance of PFD and SD receivers are compared by computer simulations, showing that the proposed PFD receiver not only achieves significant bit error rate performance but also better and more robust results than the SD receiver in this literature. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, we use the space time coding method in AF (amply-and-forward) cooperative network with two relay node by properly arranging the relay node working state. After deriving the SNR (signal to noise ratio) at the destination node, we show that the physical meaning of the SNR is equivalent to the SNR in the case of a resource transmitting via two paths while the destination received two additional noises coming from the relay node. In order to reach the maxima channel capacity, the optimal relay node position problem has been discussed in this paper under Rayleigh fading channel assumption.

In this paper, we propose an adaptive power allocation algorithm for multi-relay amplify-and-forward cooperative orthogonal frequency division multiplexing (OFDM) systems. However, Conventional power resource allocation schemes based on equal distribution of bandwidth and/or power may not be efficient given the constrained power and bandwidth at the relay node. In this paper, consider a total relay power constraint, we propose an optimal power allocation scheme to maximum the receiver signal-to-noise ratio. Lagrange is used to solve the formulated problem. Numerical results demonstrate the effectiveness of the proposed scheme, and shed interesting throughput-fairness trade-off in the resource allocation problem. Moreover, it is also verified that the proposed transmission scheme outperforms other transmission schemes without considering non-linear distortion.

In the past decades, cooperative communications technologies have gained significant attention in wireless networks. The cooperative scheme leads to longer transmission time which can considerably degrade the performance. Thus, there is always a tradeoff between saturation throughput and saturation delay. In order to obtain the optimal system performance, we utilize the cost function to tradeoff the system performance. In this paper, we evaluate the saturation throughput and saturation delay of the Markov chain model with direct/cooperative transmission strategy. The cost function is introduced to tradeoff the system performance to determine the optimal strategy for adopting the cooperative transmission.

ABSTRACT Multicarrier code division multiple access (MC‐CDMA), is a promising multiplexing technique for future communication systems. In this study, we employ the well‐known Walsh‐Hadamard spreading codes for synchronous downlink transmission of MC‐CDMA systems. The spreading codes allow that the frequency diversity to be efficiently exploited. However, multipath propagation may cause orthogonality among users is distorted, and this distortion produces multiple access interference (MAI). To eliminate this effect, we propose a pre‐filtering‐based MC‐CDMA system which uses a pre‐filtering technique at the transmitter and an equal gain combining (EGC) scheme at the receivers, respectively. Our proposed pre‐filtering technique transforms the transmitted signals so that the MAI can be eliminated, and the EGC scheme weights the signals received from all subcarriers so that channel distortions can be compensated. Furthermore, the proposed technique can calculate the transmitted power over all subcarriers to satisfy the required quality of service of each user and archive MAI‐free. In this paper, performance in terms of bit error rate is analyzed; in comparison with the EGC, orthogonal restoring combining, and maximal ratio combining schemes at receiver, respectively. Copyright © 2011 John Wiley & Sons, Ltd.

ABSTRACT The high peak‐to‐average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems not only increases the complexity of the analog‐to‐digital (A/D) and digital‐to‐analog (D/A) converters but also reduces the efficiency of the radio frequency (RF) power amplifier. In this paper, we present a data position permutation (DPP) method, which is based on a selected mapping (SLM) scheme, for reducing the PAPR in OFDM systems. The candidate signal on each branch of the SLM scheme is generated by permuting the position and rotating the phase of the original data. In addition, a modified DPP method with lower bit error rate (BER) is proposed. The simulation results show that the proposed method provides better performance with regard to complexity, spectrum efficiency, and BER as compared to that of the SLM‐based dummy sequence insertion (SLM‐DSI) method. Copyright © 2008 John Wiley & Sons, Ltd.

This study applies the expectation-maximisation (EM) algorithm to direct sequence ultra-wideband (DS-UWB) systems, combining channel estimation with data detection in iterative steps. UWB systems often suffer from multipath interference because of obstacles in indoor environments. This study employed a maximal-ratio combining (MRC) Rake receiver for its ability to exploit the gain in multipath diversity provided by multipath propagation when the channel coefficient is estimated correctly. Simulation results demonstrate that the channel coefficient captured by the MRC Rake receiver can be estimated efficiently using the proposed EM algorithm. The proposed system is capable of attaining practical convergence within a few iterations.

ABSTRACT In this contribution, a novel particle swarm optimization (PSO)‐based multi‐user detector (MUD) aided time‐hopping ultra‐wide band (TH‐UWB) system has been investigated in the multi‐path channel model. In this approach, the PSO‐based MUD employs the output of the Rake receiver as its initial value to search for the best solution which results in a formulated optimization mechanism. By taking advantage of the heuristic values and the collective intelligence of PSO technique, the proposed detector offers almost the same bit error rate (BER) performance as the full‐search‐based optimum MUD does, while greatly reducing the potentially computational complexity. Simulation results have been provided to examine the evolutionary behavior and the detection performance of the proposed PSO‐based MUD in both the additive white Gaussian noise and the multi‐path fading channel. Copyright © 2011 John Wiley & Sons, Ltd.

ABSTRACT One of the most important functions in IEEE 802.11e is enhanced distributed coordination function (EDCF). EDCF is an enhanced version of IEEE 802.11 distributed coordination function (DCF) which provides a priority scheme by differentiating the inter‐frame space and the initial window size. We proposed a modified Markov chain model to study the EDCF priority scheme. We have theoretically analyzed the saturation throughput and delay by using the modified Markov chain model in non‐ideal channel scenario. We also have compared it with the Markov chain model that exists under ideal channel scenario. Simulations are also conducted to validate analytical results. Our study shows that the modified model under ideal channel scenario is better than that in channel error scenario. Copyright © 2011 John Wiley & Sons, Ltd.

The proposed scheme utilizes local information and a predictive model to provide feedback control commands for power adjustments. A sufficient condition that ensures system stability is obtained. In addition, the bound of the received carrier-to-interference ratio (CIR) is derived in the presence of short-term fading. The bound of the received CIR is shown to be a function of the power control step size and the target CIR. Simulation results were obtained to verify the theoretical derivations.

It is well known that orthogonal frequency division multiplexing (OFDM) is robust to frequency-selective fading in wireless channels. However, it is sensitive to carrier frequency offset (CFO) which results in inter-carrier interference (ICI) and then degrades the transmission performance significantly. In this paper, a resource allocation scheme for OFDM systems in the presence of ICI is proposed. The aim of the proposed scheme is to maximize the system throughput under a total transmission power constraint. By allocating the resource appropriately, the effect of ICI can be reduced without any ICI cancellation scheme at the receiver, resulting in the increase of system throughput. Numerical results show that the proposed scheme has better throughput as compared to the adaptive subcarrier bandwidth method with equal power allocation or power allocation method with conventional water-filling algorithm.

Synchronization poses a major challenge to ultra wideband (UWB) systems due to the low signal duty cycles in UWB. This study develops an effective synchronization scheme for frame-differential IR-UWB receivers to improve the synchronization speed. The proposed parallel search mechanism reduces the search region of the symbol boundaries to only a single frame duration. Moreover, only one delay element is needed in each branch, since a shared looped delay-line (SLD) is also proposed to lower the implementation complexity of the parallel search mechanism. Simulations and performance analysis show that the proposed scheme achieves a lower mean square error and a higher probability of detection than other alternatives.

Abstract Power control plays a vital role in the operation of cellular systems. It has been shown that a power control mechanism with distributed fixed‐step control algorithm using a single bit can maintain the received carrier‐to‐interference ratio (CIR) within a desired range in long‐term fading channels. However, as far as the short‐term fading is concerned, whether such a power control algorithm remains convergent is not clear. In this work, a simple received CIR model is adopted to derive the condition that ensures system stability for short‐term fading channels. The bounds of the received CIR are also obtained. Furthermore, the effects of the power control step size and the time required for convergence are also analyzed. Copyright © 2007 John Wiley & Sons, Ltd.

A convenient signalling scheme, called orthogonal on–off (O3), together with a simple one-shot linear decorrelating detector (LDD) and a RAKE bank, is adopted in asynchronous ultra-wideband (UWB) systems. The performance of the proposed system is evaluated over multipath fading channels. This investigation also analyses mathematically the effect of the maximal-ratio combining diversity technique on the O3UWB LDD performance over lognormal fading channels. The analytical and simulation results indicate that the diversity technique significantly improves the performance of the O3UWB LDD scheme over fading channels, and the proposed receiver still demonstrates a better near–far-resistant property than a conventional UWB receiver.

A new family of two-dimensional (2-D) codes constructed by combining frequency-hop and time-spreading codes is presented. The proposed codes are designed to overcome the limitation imposed on the maximum permissible number of simultaneous users in optical code-division multiple-access (OCDMA) systems. To increase the data transmission rate, the 2-D codes employ a M-ary signalling scheme. The proposed family of codes has the favourable characteristics of a zero autocorrelation constraint and a cross-correlation constraint of one. The performance of the 2-D code is analysed in terms of the bit error rate (BER) and is compared with that of a conventional OCDMA system using optical orthogonal code (OOC). The numerical and simulation results reveal that the proposed system outperforms conventional OCDMA systems both in terms of the BER and the data transmission rate. In addition, the system capacity is sufficient to ensure reliable communication (BER≤10−9) in local area networks.

This study presents a novel multiuser detector for DS-CDMA systems, based on a particle swarm optimization (PSO) algorithm. The investigation demonstrates that the proposed detector can compromise the global and local exploration abilities and perform a near optimal multiuser detection. Simulation results reveal that the proposed algorithm outperforms GA-based multiuser detection schemes, and can attain a near-optimal performance.

Channel predictor implemented by discrete Fourier transform (DFT) is considered, which predict the channel gain of each receiving path by adaptive filters. A fuzzy logic controlled step size could overcome the drawback of conventional least-mean-square error (LMS) algorithm. In this paper, LMS algorithm with fuzzy updated step size is proposed to realize the adaptive channel prediction in orthogonal frequency division multiplexing (OFDM) systems. Simulation results show that as compared to conventional LMS algorithm, the proposed algorithm can achieve a faster convergence rate and lower steady-state mean square error. The proposed algorithm can also achieve more robust tracking ability in the various channel environments.

A distributed multi-step power control algorithm is proposed for cellular networks. The proposed scheme utilizes local information to feedback control command for power adjustment. The sufficient condition that ensures system stability is obtained. In addition, general formula for the bound of received CIR is derived in the presence of short-term fading. The bound of received CIR is shown to be a function of number of power control steps, step size, and dead factor. Simulation results are obtained to verify the theoretical derivations

We propose a fuzzy step size least-mean-square (LMS) algorithm, in which an appropriate step size is selected by a fuzzy inference system (FIS) to achieve faster convergence rate and lower steady-state fluctuation. An adaptive fuzzy sequential partial update scheme is developed to reduce system complexity without trading off bit-error rate (BER) and convergence/tracking performance. Simulations of a DS-CDMA interference suppression receiver illustrate the robust convergence and tracking behavior of the proposed LMS-based approaches with various fuzzy input vectors and fuzzy sets. The performance advantages of the proposed algorithms over other LMS algorithms in multipath Rayleigh-fading channels are investigated as well.

Power control plays a vital role in the operation of the cellular mobile systems. In the open literature, it has shown that the received signal to interference ratio (SIR) can be manipulated within a desirable range when the path gain is almost constant during a number of consecutive power controls. However, when the short-term fading is considered, whether the existing power control algorithms remain a convergence result is not clear. In this paper, we deal with the same power control issue with short-term fading taken into account. By using a fixed-step power control algorithm and a simple received SIR model, we derive a necessary condition that is required for the system to be stable. Under such a condition, we discuss the effects of power control step size on the variation of target SIR.

In the partial parallel interference cancellation (PPIC) scheme for direct-sequence code-division multiple-access (DS-CDMA) communication systems, the partial cancellation weight (PCW) does not give us any idea about the correctness of the bit-decision in the previous stage. However, the PCW can be updated by the least-mean-square (LMS) algorithm and may turn into the negative side as the bit-decision error occurs in the previous stage. Therefore, in this paper, we investigate the property of the PCW based on the LMS algorithms. Furthermore, with the maximum a posteriori probability (MAP) detection criterion, an optimal threshold for the adaptation PCW is found to judge the correctness of the bit-decisions and get a mount performance improvement.

In direct-sequence code-division multiple-access (DS-CDMA) communication systems, the partial cancellation weight (PCW) in the partial parallel interference cancellation (PPIC) multiuser detection scheme does not give us any idea about the correctness of the bit-decision in the previous stage. Therefore, in this study, we investigate the statistical behavior of the adaptation PCW based on the least mean square (LMS) algorithm in which the adaptation PCW of the last chip in a symbol period reveals the information of the correctness of the bit-decision. Then, an adaptive decision-threshold obtained by maximum a posteriori probability (MAP) detection criterion is proposed to judge the correctness of the bit-decision in the previous stage and the bit-inversion (BI) procedure is proposed to invert the incorrect bit-decision. Furthermore, after BI procedure, a full PIC is performed to cancel the multiple access interference (MAI). Simulation results show that the proposed three-stage BI-based full PIC (BI-FPIC) multiuser detector can reach the performance of single-user-bound over flat fading channels under perfect channel estimation.

A noncollision packet reservation multiple access with dynamic allocation (NC-PRMA/DA) scheme is proposed and investigated as a suitable candidate protocol for wireless multimedia communications. Access requests of the existing users in NC-PRMA/DA are conveyed to the base station in a noncollision manner by using a time-frequency signaling scheme. In addition, the scheduling NC-PRMA/DA (SNC-PRMA/DA) scheme, which is NC-PRMA/DA with packet scheduling, is also presented. Mixed voice, variable-rate video, and data traffic is used to demonstrate the efficiency of the proposed schemes for a single-cell environment. Simulation results show that our proposed NC-PRMA/DA and SNC-PRMA/DA protocols outperform the existing PRMA/DA protocol.

Power Control is essential in the design of DS-CDMA techniques. The optimum power control (OPC) methods with carrier-to-interference ratio (CIR) balancing have been identified as eigenvalue problems. For the CDMA cellular system, Wu (see IEEE Trans. Veh. Technol., vol.48, p.571-5, 1999) formulated the CIR-balanced power control problem to an eigenvalue problem based on a newly proposed link-gain matrix. However, the size of the matrix is proportional to the number of active mobiles in a CDMA cellular system. Therefore the implementation will be infeasible due to the exceedingly large size. We present a novel link-gain matrix, whose size is only proportional to the number of cells of a whole CDMA system. Based on the proposed link-gain matrix, the CIR-balanced power control in a CDMA cellular system is formulated as an eigenvalue problem and the corresponding implementation algorithm is presented as well that is optimum in the sense that the interference probability is minimized. As a result of the small size of link-gain matrix, the proposed OPC algorithm has better convergence performance than Wu's. The numerical results indicate that our proposed OPC method achieves the same performance as Wu's regardless of the pretty small size of the proposed link-gain matrix.

A joint code division multiple access and noncollision packet reservation multiple access (CDMA/NC-PRMA) technique is proposed and investigated as an uplink protocol for the third-generation (3G) mobile systems. Being the underlying time division multiple access (TDMA) architecture of the CDMA transmissions, NC-PRMA enables the base station (BS) to have a centralized control over the slot allocation policy. In order to reduce the multiple access interference (MAI) variation in a CDMA transmission, two different slot assignment schemes, referred to as load-balancing (LB) and power-grouping (PG) schemes, are proposed and evaluated. Simulation results show that considerable improvement can be achieved over the joint CDMA/PRMA scheme, in which the MAI variation is reduced by way of a dynamic permission probability for contending terminals. Especially when an imperfect power control mechanism is considered, the proposed PG assignment scheme achieves significant performance advantages.

An approach to computing the symbol error probability of fast frequency hopped M-ary frequency shift keying (FFH-MFSK) systems with majority vote is introduced. The error performances of an FFH-MFSK system threatened by a multitone jammer are analyzed. First, we consider the case where the source data rate is fixed and the hopping rate is variable. Then, a fixed hopping rate case for satellite communications is considered; for which we derive the maximum possible throughputs for various jamming to signal power ratios.

Power assignment schemes are man-made method used to enhance the capture effect of contention-based multiple access systems while Rayleigh fading, near/far effect and shadow fading are natural factors that cause the capture effect in radio communications. The combined impact of natural and man-made capture effects on the performance of a cellular system with finite population is studied. A Markov model is used to carry out an analysis of the system throughput and delay. Some numerical calculations are used to demonstrate the degree of performance improvement.

A new protocol called non-collision packet reservation multiple access (NC-PRMA) is introduced. Two kinds of duplexing schemes, frequency division duplexing (FDD) and shared time division duplexing (STDD), are taken into account. For the FDD scheme, each speech terminal is modeled as a two-state Markov process, whereas for the STDD scheme, each two-way conversation is characterized by a three-state Markov process. After modeling the offered traffic, the system performance measures, including channel throughput, mean access delay, and packet dropping probability are evaluated. In particular, the system capacities of both schemes are emphasized. The system capacity, denoted by M/sub 0.01/ is defined as the maximum number of conversations which can be supported under the constraint of packet dropping probability being no more than 1%. It is shown that the values of M/sub 0.01/ for the FDD and STDD schemes are about 38 and 45, respectively. This indicates that the STDD scheme achieves a significant improvement in system capacity relative to the FDD scheme. In addition, the numerical and simulation results show that our analytical approach yields reasonable accurate results.

The packet reservation multiple access (PRMA) protocol has been proposed for cellular wireless voice communications in the literature. The study results show that the PRMA system has a stability problem in a heavy load condition. This problem is very similar to that of the ALOHA system. Hence, the PRMA system requires a congestion control mechanism to overcome the stability problem. Focusing on this problem, a modified version, which is referred to as non-collision PRMA, is proposed. By assigning a pair of dedicated control minislots in the call set-up phase for each terminal, the contention mechanism required for reservation in the PRMA protocol is replaced by an individual reservation procedure. As expected, packet collision never occurs. The study results also show that the overall performance of the NC-PRMA system is superior to that of the PRMA system.

A packet reservation multiple access (PRMA) protocol was proposed for wireless communications. It can improve the system capacity by extracting the active terminals while they are in a silent state. Nanda, Goodman and Timor (see IEEE Trans. VT. vol.40, no.3, p.584-98, 1991) have analyzed the system by using the equilibrium point analysis (EPA). However, this method possibly generates multiple stable equilibrium points in high load conditions. The exact behaviour of the system can not be determined uniquely. Thus, we proposed a two-dimensional Markov chain model to approximately analyze the PRMA protocol. After the steady state probabilities are found, the channel throughput can be uniquely determined.

The effect of multipath interference on the performance of packet radios is investigated. The protocols considered include pure ALOHA and slotted ALOHA. Signal reception with and without capture effect is considered in the analysis. Mathematical expressions of system throughput and packet delay have been successfully obtained for each protocol. Numerical calculations are used to demonstrate when the multipath interference can be ignored and when it can be disastrous.>