Cooperative communications can significantly enhance the performance of wireless systems by exploiting the inherent spatial diversity gains. In this work, a decode-and-forward cooperative cellular system with spatial random users is investigated. Under a realistic channel model including path loss, shadowing and multipath fading, the users are selected as relays depending on their instantaneous SNR so that error propagation caused by detection errors at the relays can be mitigated. The locations of users are modeled as random point processes. Based on the theory of point processes, an analytic approach to system performance is developed to accommodate the random location of users as well as the underlying channel. The locations of the selected relays are investigated and the outage performance averaging over all possible user locations is analyzed. The accuracy of the analytical results is verified by Monte-Carlo simulations. Various issues, such as power allocation, are also investigated using the numerical results.

This paper investigates an opportunistic cooperative system with multiple relays. The locations of the relays are essentially random due to their unpredictable mobility and are thus assumed to form a spatial Poisson process. A general analytical approach to performance analysis is developed to accommodate the randomness of the locations as well as the underlying channels. The outage probability of the system is derived based on the theory of point processes. In particular, two relay selection criteria, namely the best forward channel selection and the best worse channel selection, are used as examples to illustrate the proposed approach. The accuracy of the analytical results is verified by Monte-Carlo simulations with various system configurations.

In this paper, partial data-dependent superimposed training based channel estimation for OFDM systems over doubly selective channels (DSCs) is addressed. Due to the presence of unknown data as interference, we first derive a minimum mean square error (MMSE) channel estimator by treating the effect of unknown data as noise. To further improve the performance, a novel iterative algorithm which jointly estimates channel and suppresses interference from data is proposed via variational inference approach. Simulation results show that the proposed algorithm converges after a few iterations. Furthermore, after convergence, the performance of the proposed channel estimator is very close to that with full training at high SNRs.

In this paper, robust transceiver design based on minimum-mean-square-error (MMSE) criterion for dual-hop amplify-and-forward MIMO relay systems is investigated. The channel estimation errors are modeled as Gaussian random variables, and then the effect are incorporated into the robust transceiver based on the Bayesian framework. An iterative algorithm is proposed to jointly design the precoder at the source, the forward matrix at the relay and the equalizer at the destination, and the joint design problem can be efficiently solved by quadratic matrix programming (QMP).

Semi-blind joint CFO, channel estimation and data detection for OFDM systems over doubly selective channels (DSCs) is investigated in this work. A joint iterative algorithm is developed based on the maximum a posteriori expectation-maximization (MAP-EM) algorithm. In addition, a novel algorithm is also proposed to obtain the initial estimates of CFO and channels. Simulation results show that the performance of the proposed CFO and channel estimators approaches to that of the estimators with full training at high SNRs. Moreover, after convergence, the performance of data detection is close to the ideal case with perfect CFO and channel state information.

In this paper, robust joint design of linear relay precoders and destination equalizers for amplify-and-forward (AF) MIMO relay systems under Gaussian channel uncertainties is investigated. After incorporating the channel uncertainties into the robust design based on the Bayesian framework, a closedform solution is derived to minimize the mean-square-error (MSE) of the received signal at the destination. The effectiveness of the proposed robust transceiver is verified by simulations.

Data detection for OFDM systems over unknown doubly selective channels (DSCs) and carrier frequency offset (CFO) is investigated. A semiblind iterative detection algorithm is developed based on the expectation-maximization (EM) algorithm. It iteratively estimates the CFO, channel and recovers the unknown data using only limited number of pilot subcarriers in one OFDM symbol. In addition, efficient initial CFO and channel estimates are also derived based on approximated maximum likelihood (ML) and minimum mean square error (MMSE) criteria respectively. Simulation results show that the proposed data detection algorithm converges in a few iterations and moreover, its performance is close to the ideal case with perfect CFO and channel state information.

This letter considers the multi-antenna cognitive radio (CR) network, which has a single secondary user (SU) and coexists with a primary network of multiple users. Our objective is to maximize the service probability of the SU, subject to the interference constraints on the primary users (PUs) in the form of probability. Exploiting imperfect channel state information (CSI), with its error modeled by added Gaussian noise, we address the optimization for the beamforming weights at the secondary transmitter. In particular, this letter devises an iterative algorithm that can efficiently obtain the robust optimal beamforming solution. For the case with one PU, we show that a much simpler algorithm based on a closed-form solution for the antenna weights of a given power can be presented. Numerical results reveal that the optimal solution for the constructed problem provides an effective means to tradeoff the performance between the PUs and the SU, bridging the non-robust and worst-case based systems.

In this paper, a joint channel estimation and data detection algorithm is proposed for OFDM systems under doubly selective channels (DSCs). After representing the DSC using Karhunen-Loe¿ve basis expansion model (K-L BEM), the proposed algorithm is developed based on the expectation-maximization (EM) algorithm. Basically, it is an iterative algorithm including two steps at each iteration. In the first step, the unknown coefficients in K-L BEM are first integrated out to obtain a function which only depends on data, and meanwhile, a maximum a posteriori (MAP) channel estimator is obtained. In the second step, data are directly detected by a novel approach based on the function obtained in the first step. Moreover, a Bayesian Cramer-Rao Lower Bound (BCRB) which is valid for any channel estimator is also derived to evaluate the performance of the proposed channel estimator. The effectiveness of the proposed algorithm is finally corroborated by simulation results.

In recent literature, an increasing radii algorithm (IRA) is introduced to decode the signals in multiple-input multiple-output (MIMO) systems. It is developed from the idea of sphere decoder (SD) and can achieve near-maximum likelihood (ML) decoding performance with relatively lower complexity than the SD by taking the noise statistics into consideration. In this paper, an improved IRA (IIRA) is proposed to further reduce the complexity. Apart from the noise statistics, channel information is taken into consideration as well. Additionally, a radii update scheme is introduced which enables the search space of the proposed algorithm to be further pruned. As a result, the proposed algorithm achieves a performance close to that of the IRA but with substantial computational savings. The effectiveness of the proposed decoder is verified by simulations.

Cooperative communications is a multiuser framework for wireless network environments. The collaboration among the users can exploit the inherent spatial diversity gains to thereby enhance the system performance significantly. Due to the mobility of users, especially in cellular systems, the exact locations of them are usually unknown a priori and hard/costly to discover. Therefore it is reasonable to model their locations as Bernoulli process. In this work, the uplink of a microcell in a decode-and-forward cooperative cellular system is investigated. The outage probability is derived based on a practical channel model including not only path loss but also shadowing and multipath fading effects. It is shown that considerable performance gains can be obtained.

In cognitive radio, it is crucial to control the interference from secondary users (SUs) to primary users (PUs). This paper studies the use of transmit beamforming in the cognitive secondary network for enhancing the performance of a SU while controlling the interference to the PUs. In particular, we propose to maximize the service probability of the SU with a number of probability constraints on the interference level at the PUs with the aid of imperfect channel state information (CSI). Modeling the CSI uncertainty as an additive Gaussian noise, it is shown that the optimum can be realized by second-order cone-programming (SOCP) in tandem with a one-dimensional search. Results reveal that the proposed approach provides a technique to tradeoff the performance between the PUs and the SU, making an analytical connection between non-robust and worst-case systems.

In this paper, a MIMO-OFDM system without cyclic prefix (CP) is considered and a two-step signal detection algorithm is proposed. The algorithm is based on some structural properties derived from shifting the received OFDM symbols. The first step cancels inter-carrier interference (ICI) and inter-symbol interference (ISI) with an equalizer designed using second-order statistics of the shifted received OFDM symbols. The second step detects the signals from the equalizer output in which the signals are still corrupted with multi- antenna interference (MAI). In the proposed algorithm, precise knowledge of the channel length is unnecessary and only one pilot OFDM symbol is utilized to estimate the required channel state information, assuming the number of transmit antennas is smaller than the number of subcarriers in one OFDM symbol. Simulation results show that the proposed algorithm achieves comparable performance to algorithms for MIMO-OFDM system with cyclic prefix and it is robust against channel length overestimation.

This letter addresses the problem of symbol error probability (SEP) analysis for maximum likelihood (ML) detection in multiple-input multiple-output (MIMO) multiplexing systems. A new analytical method is presented based on the total probability theorem. The effects of imperfect channel estimation and power allocation scheme are investigated. The accuracy of the proposed method is demonstrated by Monte-Carlo simulations. It is shown that the analytical results match quite well with the simulation ones irrespective of the signal-to noise- ratio (SNR).

This paper addresses the joint optimization of power control and receive beamforming vectors for a multiuser singleinput multiple-output (SIMO) antenna system in the uplink in which mobile users are single-antenna transmitters and the base station receiver has multiple antennas. Channel state information at the receiver (CSIR) is exploited but the CSIR is imperfect with its uncertainty being modeled as a random Gaussian matrix. Our objective is to devise an energy-efficient solution to minimize the individual users' transmit power while meeting the users' signal-to-interference plus noise ratio (SINR) constraints, under the consideration of CSIR and its error characteristics. This is achieved by solving a sum-power minimization problem, subject to a collection of users' outage probability constraints on their target SINRs. Regarding the signal power minus the sum of inter-user interferences (SMI) power as Gaussian, an iterative and convergent algorithm which is proved to reach the global optimum for the joint power allocation and receive beamforming solution, is proposed, though the optimization problem is indeed non-convex. A systematic scheme to detect feasibility and find a feasible initial solution, if there exists any, is also devised. Simulation results verify the use of Gaussian approximation and robustness of the proposed algorithm in terms of users' probability constraints, and indicate a significant performance gain as compared to the zero-forcing (ZF) and minimum mean square- error (MMSE) beamforming systems.

Wireless chip area networks (WCAN) signify a new development in wireless communications, where wireless interconnections among different cores within a chip (intrachip) or among different chips within a module (inter-chip) can be realized. In this paper, we analyze bit-error rate (BER) of an ultra-wideband (UWB) radio with binary phase-shift keying (BPSK) modulation over an inter-chip wireless radio channel. Specifically, a novel technique of dynamically shifting an integral window is proposed to reduce the effects of inter-symbol interference (ISI). BER expressions are analytically derived and verified by Monte Carlo simulations. Furthermore, by including both thermal and switching noise as the dominant noise sources, it is found that a BER less than 10^-6 is feasible for a link distance up to 252 mm at a data rate of up to 650 Mbps. Besides, the link margin analysis shows that a link margin of 28.22 dB could be obtained for the same distance and data rate.

Estimation of the channel state information (CSI) in quadratic form (i.e., quadratic channel estimation) in the downlink can be performed at the base station by using the relayed signals from the mobile users, which facilitates optimization with transmitter CSI. In this letter, the condition for the optimal training sequence for quadratic channel estimation in a multiuser multiple-input single-output (MISO) antenna system in the downlink is first obtained. The mean-square-error (MSE) in the CSI estimate is then analyzed. Based on the quadratic CSI estimates, a robust beamforming optimization algorithm to minimize the base station power while achieving individual users' quality-of-service (QoS) constraints, measured by the MSE in data reception, is proposed.

This letter proposes an analytical approach to evaluate the performance of MIMO-OFCDM systems with multicode transmission. Assuming zero-forcing successive interference cancellation (ZF-SIC) in the space domain and MMSE detection in the frequency domain, it is shown that at each step of SIC, the error events on multiple code channels are correlated to each other, which make the performance evaluation difficult due to the involvement of a complicated multivariate probability. By approximating the multivariate probability by a series of two-variate probabilities, the proposed analytical approach takes the correlation into account and provides accurate performance estimations. The analytical results are verified by simulations and shown to be more accurate than those where no correlation is considered.

The effectiveness of implementing bit-stream signal processing (BSSP) multiplier circuits in FPGAs, in terms of hardware resources and clock frequency, is presented. In particular, the result of realizing BSSP multipliers on FPGA architectures that utilize 6-input lookup tables (LUTs) is compared against architectures that utilize 4-input LUTs. It is found that architectures featuring 6-input LUTs suit well in BSSP applications where wide combinatorial paths are common. Furthermore, the performance of a BSSP multiplier is compared against conventional parallel multipliers in terms of LUT resource requirements. For a given resource requirement, it is found that an over-sampling ratio of less than 32 is required for a BSSP multiplier to outperform its parallel counterpart.

Quad-level bit-stream signal processing (BSSP) circuits are implemented and their performances are compared with previously published tri-level and bi-level BSSP implementations on FPGAs. BSSP refers to the process of performing computation directly on over-sampled delta-sigma modulated signals to eliminate the need of resource consuming decimators and interpolators. Quad-level BSSP offers better performance than their bi-and tri-level counterparts at the expense of higher resource utilization. Using a digital phase locked loop (DPLL) and a quadrature phase-shift keying (QPSK) demodulator as application examples, the effectiveness of quad-level BSSP on FPGAs is studied. The BSSP approach will be contrasted with conventional multi-bit implementations using built-in digital signal processing blocks in modern FPGAs.

This paper investigates the effects of channel estimation errors on zero-forcing (ZF) vertical Bell laboratories layered space time (V-BLAST) detection. An analytical method is presented to derive the symbol error probability (SEP) of the signals detected at each stage. The effects of imperfect channel estimation on the SEP performance of V-BLAST detection are then studied. It is shown that ZF-VBLAST detection is very sensitive to the channel estimation errors under high signal to noise ratio (SNR). It is also shown that when optimal ordering is adopted, the effects of channel estimation errors are more significant on the latter detection stages.

This paper addresses the problem of joint carrier frequency offset (CFO) and channel estimation in multiuser multiple-input multiple-output (MIMO) orthogonal frequency- division multiplexing (OFDM) systems. A joint CFO and channel estimator is derived based on the maximum likelihood (ML) criterion. A computationally efficient method using importance sampling technique is proposed to solve the highly demanding multi-dimensional exhaustive search required by the ML multi- CFO estimation. Simulation results verify the effectiveness of the proposed estimation scheme.

In (Dong and Ding, 2006), it has been proposed that channel estimates in quadratic form can be obtained at the base station by sending training sequences to the mobiles where the received signals are forwarded back to the base for channel estimation. In this paper, we first examine the optimal training sequence design for such quadratic channel estimation and then analyze the error bound and statistics of the channel estimates in quadratic form. With the analytical results, two problems for a multiple-input single-output (MISO) antenna system in the downlink are constructed and optimally solved: Power minimization with individual users' 1) worst-case signal-to-interference plus noise ratio (SINR) and 2) average mean-square-error (MSE) constraints, through optimal multiuser MISO beamforming and power allocation.

This paper investigates the performance of orthogonal frequency and code division multiplexing (OFCDM) systems with multiple input multiple output (MIMO) multiplexing (MIMO-OFCDM), where multi-code transmission is employed to increase the data rate. An analytical approach is presented to evaluate the performance of the system using joint space-frequency detection with zero-forcing successive interference cancellation (ZF-SIC) in the space domain and minimum mean square error (MMSE) detection in the frequency domain. It is shown that at each step of SIC, the decision errors on multiple code channels are correlated to each other. By taking the correlation between the errors into account, the proposed analytical approach provides a precise estimation on the performance of MIMO-OFCDM. Simulations demonstrate the accuracy of the proposed analytical approach.

This paper addresses the problem of performance analysis for maximum likelihood (ML) detection in two-input multiple-output multiplexing systems. A novel analytical method is presented to formulate the symbol error probability (SEP). Based on the total probability theory, the SEPs of the two transmitted signals are obtained in closed-form by solving the SEP equations. Both equal and unequal power allocations are investigated. The accuracy of the proposed method is verified by Monte-Carlo simulations. The proposed method can also be extended to systems with more than two inputs.

In this paper, an analytical approach to performance analysis of vertical Bell laboratories layered space time (V-BLAST) detection with optimal ordering for systems with two transmit antennas is presented. The post-detection signal-to-noise-ratio (SNR) at each stage is derived and the symbol error probabilities (SEP) of the signals are then given in closed-form. The analysis takes into account the effects of optimal ordering, imperfect channel estimation and error propagation, which were rarely considered in the literature due to the difficulties in evaluation. The accuracy of the analysis is demonstrated by Monte-Carlo simulations.

A novel subspace (SS) based blind channel estimation method for multi-input, multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems is proposed in this work. With an appropriate re-modulation on the received signal blocks, the SS method can be effectively applied to the cyclic prefix (CP) based MIMO-OFDM system when the number of the receive antennas is no less than the number of transmit antennas. These features show great compatibility with the coming fourth generation (4G) wireless communication standards as well as most existing single-input single-output (SISO) OFDM standards, thus allow the proposed algorithm to be conveniently integrated into practical applications. Compared with the traditional SS method, the proposed algorithm exhibits many advantages such as robustness to channel order over-estimation, capability of guaranteeing the channel identifiability etc. Analytical expressions for the mean-square error (MSE) and the approximated Cramer-Rao bound (ACRB) of the proposed algorithm are derived in closed forms. Various numerical examples are conducted to corroborate the proposed studies.

This letter addresses the problem of joint estimation of carrier frequency offset (CFO) and channel for OFDM systems in the presence of timing ambiguity. Based on two signal models for quasi-synchronized OFDM systems, two joint CFO and channel estimators are derived from the Maximum Likelihood (ML) criterion. The first estimator obtained is a joint estimator for all unknown parameters, which needs a multi-dimensional search. The second estimator has a reduced complexity, but its performance slightly degrades compared with the first one. Through MSE analyses, we find that when the number of subcarriers is large, the two estimators are equivalent.

This paper investigates the orthogonal frequency and code division multiplexing (OFCDM) systems with multiple input multiple output multiplexing (MIMO-OFCDM) and multicode transmission. Combining the iterative detection in the space domain and the hybrid multi-code interference (MDI) cancellation and minimum mean square error (MMSE) detection in the frequency domain, a joint iterative detection is proposed, which enables space and frequency diversity gains to be jointly exploited. Moreover, using a two-dimensional (2-D) averaging channel estimation algorithm, a close form expression is derived for the optimal power allocation between the pilot and all data channels that achieves the best system performance. It is shown that the optimal power ratio mainly depends on the channel estimation algorithm, the number of transmit antennas as well as the number of pilot and data symbols in a packet, but is not sensitive to the changes in signal-to-noise ratio (SNR) and diversity gains. Simulations are conducted to verify the derived optimal power ratio and study the performance of the proposed joint detection algorithm. It is shown that considerable improvement can be obtained when the number of loops in the joint iterative detection increases. Moreover, the system performance is enhanced significantly when the frequency domain spreading factor, NF, increases.

To provide high speed mobile services, orthogonal multi-code transmission is employed in the orthogonal frequency and code division multiplexing (OFCDM) system. However, in a wireless frequency selective channel either with or without Doppler shift, code orthogonality is distorted and multi-code interference (MCI) is caused. The variance of the MCI is dependent on which codes to be used. This paper investigates the code assignment in the OFCDM system, aiming to mitigate the MCI. The code assignment can be explored in terms of the code distance defined in this paper. Generally, the codes with large code distances to each other should be assigned with high priority. Moreover, to avoid large interference from the powerful code-multiplexed pilot channel, the time spreading code adjacent to the pilot code in the code tree (Adachi et al., 1998) is suggested not be used when the time domain spreading factor is large. The performance of the proposed non-sequential code assignment scheme is analytically evaluated. It has been shown that the system performance can be improved significantly by the non-sequential scheme under various channel conditions, especially when a low to medium system load is assumed.

This paper addresses the problem of optimal training for joint carrier frequency offset (CFO) and channel estimation in multiuser MIMO OFDM systems, where each user can utilize all available subcarriers. To choose the training sequence with the goal of providing the smallest estimation mean square error (MSE), the Cramer-Rao bound (CRB) is derived in close-form. However, little insight on the sequence selection can be obtained from the CRB directly due to its complicated dependence on training sequence and channel parameters. To proceed, asymptotic CRB, which has a much simpler dependence on the training, is derived. It is found that the optimal training sequences should be individually white and uncorrelated with each other. Simulation results illustrate the merits of the proposed training design.

This paper addresses the problem of joint estimation of carrier frequency offset (CFO) and frequency-selective channel in quasi-synchronized orthogonal frequency division multiplexing (OFDM) systems. Two equivalent signal models with embedded timing ambiguity are introduced first, and then two joint CFO and channel estimators are derived from the Maximum Likelihood (ML) criterion. The first estimator obtained is a joint estimator for timing, CFO and channel, which needs a two dimension search for parameter estimation. The second estimator has a reduced complexity, but its performance slightly degrades compared with the first estimator due to more unknown parameters required to be estimated. We analyze and compare the MSE performance of the two estimators and find that when the number of subcarriers is large, the two estimators are equivalent.

As a wireless access technique for future broadband mobile communications, orthogonal frequency and code division multiplexing (OFCDM) systems employ multi-code transmission to provide high speed mobile services. Since the multiple code channels are no longer orthogonal in the broadband wireless channel, multi-code interference (MCI) is caused. The variance of the MCI is dependent on which codes to be used. This paper investigates the code assignment in the multi-code OFCDM system, aiming to mitigate the MCI at the receiver. Since two-dimensional (2-D) spreading is employed in the OFCDM system, the code assignment is carried out in time and frequency domains, separately. In order to avoid large interference from the powerful code- multiplexed pilot channel, the time spreading code adjacent to the pilot code in the code tree is suggested not be used when the time domain spreading factor is large. The performance of the proposed code assignment scheme is analytically evaluated. It has been shown that the system performance can be improved significantly by the proposed scheme under various channel conditions, especially when a low to medium system load is assumed.

For orthogonal frequency division multiplexing (OFDM) systems, the frequency synchronization and channel estimation are always tightly coupled with timing synchronization. In this paper, we investigate the effects of timing offset on the performance of a recently proposed subspace based joint carrier frequency offset (CFO) and channel estimation scheme. It is found that this scheme is highly sensitive to timing offsets. An improved timing robust scheme is then proposed. In the proposed CFO estimation scheme, the orthogonality loss between two subspaces due to timing offset is restored by exploiting a reduced null-subspace. The missing taps in the original channel estimate are recovered by using an upper bound of the channel length instead of the actual value. The effectiveness of the proposed scheme is verified by simulations.

This paper presents an improved derivative method to provide good symbol synchronization in multi-path channels for orthogonal frequency division multiplexing (OFDM) systems. As the conventional derivative method (Willians et al., 2005), the improved derivative method can reduce the inter-symbol interference (ISI) caused by multi-path channels. Moreover, the proposed method is robust against the frequency offset and the filter length which are very selective to the conventional derivative method. Simulation results confirm that the improved derivative algorithm outperforms the maximum likelihood algorithm (ML) (Van De Veek et al., 1997) and the conventional derivative method.

In this letter, a Rake-based multiuser detection technique, consisting of multiuser single-path signal separation, time-delay estimation, and multipath combining, is proposed for quasi-synchronous spatial-division multiple-access (SDMA) systems. Time diversity is achieved for performance improvement. In addition, only the upper bounds of the channel length and the time delays are required. Simulation results verify the effectiveness of the proposed technique, as well as its robustness against overestimation of the maximum channel length and the maximum time delay

A semiblind method is proposed for simultaneously estimating the carrier frequency offsets (CFOs) and channels of an uplink multiuser multiple-input multiple-output (MIMO)–orthogonal frequency-division multiplexing (OFDM) system. By incorporating the CFOs into the transmitted symbols and channels, the MIMO–OFDM with CFO is remodeled into MIMO–OFDM without CFO. The known blind method for channel estimation [1] is then directly used for the remodeled system to obtain the shaped channels with an ambiguity matrix. A pilot OFDM block for each user is then exploited to resolve the CFOs and the ambiguity matrix. Two dedicated pilot designs, periodical and consecutive pilots, are discussed. Based on each pilot design and the estimated shaped channels, two methods are proposed to estimate the CFOs. As a result, based on the second-order statistics of the received signal and one pilot OFDM block, the CFOs and the channels are found simultaneously. Finally, a fast equalization method is given to recover the signals corrupted by the CFOs.

A semiblind method is proposed for simultaneously estimating the carrier frequency offsets (CFOs) and channels of an uplink multiuser multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) system. By incorporating the CFOs into the transmitted symbols and channels, the MIMO-OFDM with CFO is remodeled into an MIMO-OFDM without CFO. The known blind method for channel estimation (Zeng and Ng in 2004) (Y. H. Zeng and T. S. Ng, ldquoA semi-blind channel estimation method for multi-user multi-antenna OFDM systems,rdquo IEEE Trans. Signal Process., vol. 52, no. 5, pp. 1419-1429, May 2004.) is then directly used for the remodeled system to obtain the shaped channels with an ambiguity matrix. A pilot OFDM block for each user is then exploited to resolve the CFOs and the ambiguity matrix. Two dedicated pilot designs, periodical and consecutive pilots, are discussed. Based on each pilot design and the estimated shaped channels, two methods are proposed to estimate the CFOs. As a result, based on the second-order statistics (SOS) of the received signal and one pilot OFDM block, the CFOs and channels are found simultaneously. Finally, a fast equalization method is given to recover the signals corrupted by the CFOs.

In this paper, signal detection for MIMO-OFDM systems operating on quasi-static frequency selective fading channels with time offsets is considered. A channel equalizer is first proposed to cancel most inter-block and inter-symbol interferences, based on second order statistics of the received signals. The transmitted signals are then detected from the equalizer output with the aid of a few pilot symbols. In most scenarios, the number of pilot symbols required here is less than that required in the existing algorithms. In addition, perfect time synchronization, time offset and channel length estimations are not needed. The algorithm is applicable irrespective of whether the channel length is shorter than, equal to or longer than the cyclic prefix (CP) length. Simulations confirm the effectiveness of the proposed algorithm and verify its robustness against time offsets, namely, different time delays for different users.

This paper addresses the problem of maximizing the sum-rate with a total power constraint for a multiuser multiple-input multiple-output (MIMO) antenna system in the downlink using generalized beamforming. Channel state information at the transmitter (CSIT) and the receivers (CSIR) is assumed available. An iterative approach is proposed which alternates between optimizing the power allocation, transmit antenna vectors and receive antenna vectors using a combination of geometric programming (GP), signomial programming (SP), uplink-downlink duality and minimum mean-square-error (MMSE) receiver optimization. It is proved that the sum-rate in the proposed scheme has a monotonic increasing property and hence the algorithm is convergent. Simulation results show that this algorithm can significantly increase the sum-rate compared with a zero-forcing solution.

In this paper, a time domain equalization algorithm is proposed for single-TX multiple-RX OFDM systems over frequency selective fading channels. The algorithm, which cancels most of the intersymbol interference (ISI), utilizes the orthogonality of the IFFT matrix and the second order statistics of the received signals. The signals are then detected, with the aid of only four pilots, from the equalizer output. The number of pilots required in the proposed algorithm is less than that in existing algorithms and channel length estimation is not needed. In addition, the proposed algorithm is applicable to both the case where the channel length is shorter than or equal to the length of cyclic prefix (CP), and the case where the channel length is longer than the length of cyclic prefix which results in interblock interference (IBI). Simulation results confirm the effectiveness of the proposed algorithm in both cases and indicate that it is more practical as there is no restriction on the channel and CP lengths.

In this paper, we address the joint transmit and receive beamforming design of a 2-user (2, 2)1downlink system. Our aim is to minimize the aggregate mean-square-error (MSE) (or sum-MSE in short) subject to the total power constraint. By exploiting the uplink-downlink duality, the problem can be converted to the equivalent uplink system and so solved using semidefinite programming relaxation (SDPR). Due to the rank relaxation, however, SDPR will generally not give the optimal solution of the original problem. In particular, for some cases, we can show analytically that the exact optimal beamforming solution can be retrieved from the SDPR. For the other cases, a convergent iterative algorithm is proposed to provide a solution.

This paper presents two recently developed algorithms for efficient model order reduction. Both algorithms enable the fast solution of continuous-time algebraic Riccati equations (CAREs) that constitute the bottleneck in the passivity-preserving balanced stochastic truncation (BST). The first algorithm is a Smith-method-based Newton algorithm, called Newton/Smith CARE, that exploits low-rank matrices commonly found in physical system modeling. The second algorithm is a project-and-balance scheme that utilizes dominant eigenspace projection, followed by a simultaneous solution of a pair of dual CAREs through completely separating the stable and unstable invariant subspaces of a Hamiltonian matrix. The algorithms can be applied individually or together. Numerical examples show the proposed algorithms offer significant computational savings and better accuracy in reduced-order models over those from conventional schemes.

In this paper, signal detection with time delay estimation for quasi-synchronous MIMO systems on multipath channels is considered. The paper first formulates the problem by shifting each user's signals so that previous results on synchronized system for ISI cancellation based on second-order statistics of the received signals can be applied. The first step effectively reduces the quasi-synchronous system on the multipath channels to the quasi-synchronous system on single-path channels. In the second step, an equivalent model is constructed with the delay information embedded in the reduced channel model. A simultaneous time delay estimation and multiuser detection algorithm is then proposed. The proposed technique can easily be implemented in practical situations as perfect synchronization and knowledge of the channel length and the time delays are not required. Simulation results show that the proposed technique can achieve good performance and is not sensitive to overestimation of the maximum channel length and the maximum time delay.

Many known second-order statistics based blind algorithms for MIMO channel estimation are sensitive to channel order overestimations. To overcome this problem, an algorithm is proposed in Gazzah, H et al. (2002) for SIMO system only, and then a simple generalization of it to MIMO system is presented. In this paper, improvements and refinements on the algorithm are given, which makes the method robust to noise and round-off error. The method can give estimations of all channel impulse responses subject to a scalar matrix ambiguity when only an upper bound for all MIMO channel orders is known.

A semi-blind method is proposed for simultaneously estimating the carrier frequency offsets (CFOs) and channels of an uplink multi-user MIMO-OFDM system. By incorporating the CFOs into the transmitted symbols and channels, the MIMO-OFDM with CFO is re-modeled into a MIMO-OFDM without CFO. The known blind method for channel estimation (Y.H. Zeng and T.S. Ng, 2004) is then directly used for the re-modeled system to obtain the shaped channels with an ambiguity matrix. A dedicated pilot OFDM block for each user is then designed to resolve the CFOs and the ambiguity matrix. As a result, based on the second-order statistics (SOS) of the received signal and one pilot OFDM block, the CFOs and channels are found simultaneously

Cyclic prefixed single carrier (CP-SC) has emerged as a promising technique for wideband wireless communication. The cyclic prefix (CP) in CP-SC costs some bandwidth, but it cannot be used for channel estimation in fast-variant channel environment. In this letter, a new scheme is proposed for CP-SC which not only adds a CP but also a suffix. With the aid of the CP and suffix, channel responses can be resolved by the fast Fourier transform. The additional known symbols also ensure input symbols recoverable regardless of the channel null locations. Better synchronization is also possible.

A preprocessor for ultra-wideband (UWB) communications using impulse radio is proposed. The received UWB signal is divided into subband signals, which can be individually processed and recombined before demodulation of the signal. Perfect reconstruction of the pulses via analog subband processing is achieved. The division of subbands is flexible and the number depends on the number of frequency bands that requires processing. Only the subbands that require processing need to be digitized. The application of the preprocessor to interference cancellation is also demonstrated.

Based on the maximum-likelihood principle and the preamble structure of IEEE 802.11a wireless local area network (WLAN) standard, this paper proposes a new symbol synchronization algorithm for IEEE 802.11a WLANs over frequency-selective fading channels. In addition to the physical channel, the effects of filtering and unknown sampling phase offset are also considered. Loss in system performance due to synchronization error is used as a performance criterion. Computer simulations show that the proposed algorithm exhibits better performances than the simple correlation-based algorithms. When compared to the algorithm based on the generalized Akaike information criterion, the proposed algorithm presents comparable performance and exhibits reduced complexity.

A very fast Smith-method-based Newton algorithm is introduced for the solution of large-scale continuous-time algebraic Riccati equations (CAREs). When the CARE contains low-rank matrices, as is common in the modeling of physical systems, the proposed algorithm, called the Newton/Smith CARE or NSCARE algorithm, offers significant computational savings over conventional CARE solvers. The effectiveness of the algorithm is demonstrated in the context of VLSI model order reduction, wherein stochastic balanced truncation (SBT) is used to reduce large-scale passive circuits. It is shown that the NSCARE algorithm exhibits guaranteed quadratic convergence under mild assumptions. Moreover, two large-sized matrix factorizations and one large-scale singular value decomposition (SVD), necessary for SBT, can be omitted by utilizing the Smith method output in each Newton iteration, thereby significantly speeding up the model reduction process.

A subspace-based blind method is proposed for estimating the channel responses of a STC-OFDM system with two transmit antennas and one receive antenna. It gives estimations to the two channel responses subject to two ambiguity parameters. The method is valid whether the two channel transfer functions are coprime or not and does not require precise channel order information (only an upper bound for the orders is required). Furthermore, a method is presented to resolve the ambiguities by using two or more pilot symbols. Simulations show that the methods are effective and robust.

Motivated by the high capacity of multiple-input multiple-output (MIMO) broadcast channels, this paper investigates the use of only linear signal processing (and hence conventional codec) at the transmitter and receivers to realize the rich diversity (or capacity) of the channel. In particular, we combine a recently developed orthogonal space division multiplexing (OSDM) scheme and orthogonal frequency division multiplexing (OFDM) to decompose per-subcarrier channel into many uncoupled single-user spatial modes. Issue like channel mismatches arising from pilot-aided channel estimation and Doppler spread is examined, and a method exploiting the frequency correlation of the channel to reduce processing complexity is proposed. Simulation results show that for HiperLAN II, the scheme can achieve good performance with four simultaneous downlink users at Doppler spread of 40 Hz (or speed of 8.5 km/h at 5 GHz).

This paper's aim is to investigate the diversity performance of spatially correlated multiple-input multiple-output (MMO) broadcast channels where multiple communications occur simultaneously in the same frequency band and time slot from the base station (BS) to many mobile stations (MS). To deal with a broadcast system, we employ a previously developed orthogonal space division multiplexing (OSDM) method to decompose the channel into many uncoupled single-user systems so that the diversity can be readily characterized by the second-order statistics of the resultant channel coefficients. Simulation results reveal that good performance can still be achieved with correlation as high as 0.4. Most intriguingly, unlike single-user MIMO channels, the diversity is more sensitive to the spatial correlation on the transmitter than receiver sides.

Muquet et al. (2002) have proposed a subspace-based blind method for estimating the channel responses of cyclic-prefixed OFDM systems. Their proof for the channel identifiability uses the roots of channel transfer function and therefore depends on the channel order. When only an upper bound is known for the channel order, the proof fails. In this letter, a new proof for the identifiability is given which does not require knowledge of the precise channel order and can handle various choices of length of the OFDM block.

A pilot-tone-based maximum likelihood estimator (PBMLE) for carrier frequency offset (CFO) in orthogonal frequency-division multiplexing (OFDM) systems is proposed. To obtain a consistent estimate of the CFO over a frequency-selective fading channel, the proposed method employs a preamble comprising distinctively spaced pilot tones. As a result of this preamble configuration, a large estimation range equal to the bandwidth of the OFDM signal can be achieved. Different from previous ad hoc pilot-tone-based CFO estimators, the PBMLE exploits the relationship between the CFO and the periodogram of the preamble. Analysis shows that the proposed PBMLE is asymptotically unbiased and efficient. To realize this PBMLE in practice, a suboptimal estimator is also introduced, in which a zero-padded fast Fourier transform is invoked and the CFO estimation is split into two phases: coarse and fine estimation. Coarse estimation is obtained through the correlation between the received preamble and its original pattern, whereas fine estimation is obtained by exploiting the magnitude attenuation in the vicinities of those CFO-shifted pilot tones. Both analytical investigations and computer simulations indicate that the accuracy of this simplified suboptimal estimator is proportional to the oversize ratio of zero padding. When the oversize ratio is sufficiently high, the performance of the suboptimal estimator approaches that of the proposed PBMLE.

This paper addresses the problem of performing orthogonal space-division multiplexing (OSDM) for downlink, point-to-multipoint communications when multiple antennas are utilized at the base station (BS) and (optionally) all mobile stations (MS). Based on a closed-form antenna weight solution for single-user multiple-input multiple-output communications in the presence of other receiver points, we devise an iterative algorithm that finds the multiuser antenna weights for OSDM in downlink or broadcast channels. Upon convergence, each mobile user will receive only the desired activated spatial modes with no cochannel interference. Necessary and sufficient conditions for the existence of OSDM among the number of mobile users, the number of transmit antennas at the BS, and the number of receive antennas at the MS, are also derived. The assumption for the proposed method is that the BS knows the channels for all MS's and that the channel dynamics are quasi-stationary.

This paper derives and computes the probability of synchronization failure P/sub fail/ for IEEE 802.11a wireless LANs on frequency-flat Rician fading channels. For a frequency offset within /spl plusmn/232 kHz, it is shown that its effect on the synchronization performance is minor. The E/sub ds//N/sub 0/ ratios required to achieve P/sub fail/=10/sup -3/ and 10/sup -4/ are computed, where E/sub ds/ is the data-symbol energy. We find that E/sub ds//N/sub 0/ ratios over 20 dB are generally required for channels with Rician factors K/spl les/6 dB. In particular, E/sub ds//N/sub 0/ ratios that yield P/sub fail/=10/sup -4/ exceed 30 dB for K/spl les/4 dB.

Orthogonal space division multiplexing (OSDM) can be achieved by utilizing multiple antennas at the base station (BS) and all mobile stations (MS) (or multi-user MIMO) jointly. Recently, two iterative methods have been proposed to obtain the antenna weighting for realizing OSDM in the downlink (Kai-Kit Wong et al., IEEE Trans. Wireless Commun.; Zhengang Pan et al., IEEE Trans. Wireless Commun.). However, both suffer from high complexity if the number of BS antennas or users involved is large, as the computational complexity grows with the fourth power of the number of users. By allowing the signal streams for a user to be non-orthogonal, we propose a semi-OSDM system for complexity reduction. In the semi-OSDM system, the signals of multiple users are still orthogonal, but for each user, the multiple signaling streams are made triangular instead so that a simple backward substitution with symbol cancellation can be performed to maintain an accurate signal detection. Remarkably, the antenna weighting for semi-OSDM can be expressed in a closed form. Simulation results show that semi-OSDM has mild performance degradation, but it requires much lower computational complexity (by an order of magnitude) when compared with existing OSDM systems.

This paper presents a highly efficient geometrical approach for designing robust minimum variance (RMV) beamformers against uncertainties in the array steering vector. Instead of the conventional approach of modeling the uncertainty region by a convex closed space, the proposed algorithm exploits the optimization constraint and shows that optimization only needs to be done on the intersection of a hyperplane and a second-order cone (SOC). The problem can then be cast as a second-order cone programming (SOCP) problem so as to enjoy the high efficiency of a class of interior point algorithms. A general case of modeling the uncertainties of an array using complex-plane trapezoids is investigated. The efficiency and tightness of the proposed method over other schemes are demonstrated with numerical examples.

To overcome the limitation of previous feedback frequency synchronizers, which can only be applied to track small carrier frequency offset (CFO) in a fraction of the subchannel spacing, a blind CFO detector with large acquisition range is proposed in this paper. Based on the characteristics of CFO-induced complex weighting factors, a novel cost function of CFO is developed, and frequency synchronization is accomplished through the maximization of this cost function. To simplify the implementation of the proposed method, a low-complexity adaptive algorithm is introduced, which simply utilizes the FFT operation of the OFDM demodulator to generate the error signal for each iteration. Both analytical and computer simulation results are presented to demonstrate the performance of the proposed method.

In this paper, a pilot-tone-based maximum likelihood estimator (PTB-MLE) for carrier frequency offset (CFO) in orthogonal frequency division multiplexing (OFDM) systems is proposed. To obtain consistent estimate of the CFO over frequency-selective fading channels, the proposed method employs a preamble comprising distinctively-spaced pilot tones. As a result of this preamble configuration, a large estimation range equal to the bandwidth of OFDM signal can be achieved. Different from previous PTB estimators that are ad hoc, the PTB-MLE exploits the relationship between the CFO and the periodogram of the preamble. To realize this PTB-MLE in practice, a sub-optimal estimator based on zero-padded FFT is introduced as well. When the oversize ratio is sufficiently high, the sub-optimal estimator is asymptotically unbiased and its variance approaches the CRB.

The use of multiple-input multiple-output (MIMO) antennas for achieving diversity and/or support of multi-stream signaling has recently been demonstrated to gain lots of benefits including reduction in average power error probability and capacity enhancement. However, studies are limited to a point-to-point communication. In this paper, we address the optimization problem of enhancing the performance of a downlink multi-user system (i.e., point-to-multipoint communication) through joint use of multiple antennas at the base station (BS) and mobile stations (MS). The optimization is performed by finding the antenna weights at the BS and MS, jointly, to orthogonalize the multi-user multi-stream signals as well as maximizing the resultant channel gains of the diagonalized system for optimal performance.

This paper derives and computes the probability of synchronization failure, P/sub fail/, for IEEE 802.11a wireless LANs on frequency-flat Rician fading channels. For a frequency offset within /spl plusmn/232 kHz, it is shown that its effect on the synchronization performance is minor. The E/sub ds//N/sub 0/ ratios required to achieve P/sub fail/ = 10/sup -3/ and 10/sup -4/ are computed, where E/sub ds/ is the data symbol energy. We find that E/sub ds//N/sub 0/ ratio over 20dB are generally required for channels with Rician factors K /spl les/ 6 dB. In particular, E/sub ds//N/sub 0/ ratios that yields P/sub fail/ = 10/sup -4/ exceed 30 dB for K /spl les/ 4 dB.

The timing-synchronization performance of IEEE 802.11a wireless LANs on multipath Rician fading channels is investigated. Simulation results yield the following observations. A higher Rician factor gives a better performance. At low signal-to-noise conditions, the multipath-diversity gain enables an improvement in the synchronization performance but at high signal-to-noise conditions, irreducible probabilities of synchronization failure, which cannot be reduced even if the signal power is increased, occur. A more dispersive channel gives a poorer synchronization performance, and the performance is poor for Rayleigh fading environments.

In this paper, a novel carrier frequency offset (CFO) estimation algorithm is proposed for OFDM applications. The maximum likelihood estimator (MLE) for the CFO is derived, which reveals the relationship between the CFO and the periodogram of the received signal. Theoretical analysis shows that the proposed MLE is statistically efficient. To realize this MLE in practice, a sub-optimal estimator is also introduced in which zero-padded FFT is invoked for implementation. For the objectives of reducing the implementation complexity and broadening the estimation range, a preamble structure comprising nonuniformly-spaced pilot tones is presented. Based on this preamble, the CFO estimation is split into two phases: the coarse estimation is obtained through the correlation between the received spectrum and the original pattern of the preamble; whereas the fine estimation is obtained by comparing the relative magnitude attenuation in the vicinities of those CFO-shifted pilot tones. Both analytical investigations and computer simulations indicate that the accuracy of this simplified sub-optimal estimator is proportional to the oversize ratio of zero-padded FFT, and its estimation range is equal to the bandwidth of OFDM signal. When the oversize ratio is sufficiently high, the performance of the proposed sub-optimal estimator approaches that of the proposed MLE.

Bandpass sampling is a useful alternative for direct digital downconversion in software radio. It significantly relaxes the analog-to-digital converter (ADC) sampling rate requirement and facilitates the design goal of moving the ADC as close as possible to the antenna. This paper presents a modified interpretation to the graph of allowable sampling frequencies in uniform bandpass sampling. It is shown that the position and guard bands of a downconverted bandpass signal band are highly related to the order of the valid sampling range, called the wedge order. An efficient algorithm is then proposed, which significantly reduces the computational load in determining the valid sampling frequencies to downconvert multiple distinct bandpass signals. Conditions for the placement of bandpass signals to utilize a given sampled bandwidth are also discussed.

We proposed a novel interleaving technique, namely adaptive interleaving, which can break the bursty channel errors more effectively than traditional block interleaving. This new scheme resequences the transmitted symbols adaptively according to the instantaneous subcarrier channel state information. In this way, the bursty errors introduced by the channel can be broken more effectively, thus a better BER performance can be achieved. This technique is well-suited to OFDM systems because the CSI values of the whole frame could be estimated at once when transmitted symbols are framed in the frequency dimension. Computer simulation shows that significant SNR gains can be achieved, compared with traditional block interleaving.

In this paper, a new robust adaptive lattice-ladder filter for impulsive noise suppression is proposed. The filter is obtained by applying the non-linear filtering technique reported by Kim and Efron (1995) and the robust statistic approach to the gradient adaptive lattice filter. A systematic method is also developed to determine the corresponding threshold parameters for impulse suppression. Simulation results showed that the performance of the proposed algorithm is better than the conventional RLS, N-RLS, the gradient adaptive lattice normalised-LMS (GAL-NLMS), RMN and ATNA algorithms when the input and desired signals are corrupted by individual and consecutive impulses. The initial convergence, steady-state error, computational complexity and tracking capability of the proposed algorithm are also comparable to the conventional GAL-NLMS algorithm.

Implementation of maximum ratio transmission (MRT) systems is not a trivial task. Alternatively, switched-transmitter diversity (STD) systems are less difficult to implement, but their performance is suboptimal when compared to MRT systems. If the difference is not significant, implementation advantages of STD systems outweigh the minor performance loss so that these systems are desirable for practical implementations. This paper compares the performances between MRT and STD systems under Rayleigh-fading conditions for 2/spl times/1, 2/spl times/2 and 4/spl times/ (transmit antennas/spl times/receive antennas) configurations. It is found that the differences are only about 1 dB for the 2/spl times/1 and 2/spl times/2 configurations, indicating that implementation simplicity of a STD system can be utilized without sacrificing too much performance. However, the difference is around 3 dB for the 4/spl times/ configuration. The considerably larger loss prompts system designers to consider more critically on the tradeoffs between implementation aspects and performances of STD and MRT systems.

This paper proposes a practical linear software-radio architecture (dealing with linear modulations) that is suitable for multi-mode operation. In particular, it is shown how to integrate GMSK into the proposed architecture. Coherent and noncoherent detections of GMSK signals are detailed for the implementation of the proposed software radio.

Digital implementation of multitone (MT-) CDMA systems often involves considerable complexity. Assuming the spreading-sequence length, N, is a multiple of the number of subcarriers, M, this paper proposes a new digital technique that achieves low-implementation complexity. In particular, for the case of a rectangular chip waveform, it is shown that the MT-CDMA technique proposed earlier and the technique proposed in this paper require /spl alpha/+/spl alpha/log/sub 2//spl alpha/N and 1+(M/N)log/sub 2/M multiplications per chip, respectively, where /spl alpha//spl ges/1 is the number of samples per chip. The reduction in complexity is particularly considerable when N is large.

This paper proposes a recursive least M-estimate (RLM) algorithm for robust adaptive filtering in impulse noise. It employs an M-estimate cost function, which is able to suppress the effect of impulses on the filter weights. Simulation results showed that the RLM algorithm performs better than the conventional RLS, NRLS, and the OSFKF algorithms when the desired and input signals are corrupted by impulses. Its initial convergence, steady-state error, computational complexity, and robustness to sudden system change are comparable to the conventional RLS algorithm in the presence of Gaussian noise alone.

The performance of a coherent delay-locked tracking scheme for direct-sequence/spread-spectrum systems using half-sine or triangular chip waveforms for early and late despreading sequences is analyzed. The effect of band-limiting on the received signals is considered. Mean time to lose lock (MTLL) and root mean square (rms) tracking error of the delay-locked loop (DLL) are compared with that of a conventional DLL which uses rectangular chip despreading waveforms. Linear and nonlinear (based on the renewal process approach) analyses are employed to evaluate the performance of the DLL. Results show that the use of either the half-sine or triangular chip waveform reduces the rms tracking error and increases the MTLL considerably when the early-late spacing is approximately between 0.7-1.3 chip times. The results apply in particular to the commonly used DLL using one chip early-late spacing. Computer simulation for band-limited signals confirms the analytical results. The use of these despreading chip waveforms also reduces tracking offset in multipath environments.

This paper proposes a mathematical model for simulating a Nakagami-m (1960) fading channel, m<1. It is shown that the m fading process can be expressed as a product of a square-root-beta process and a complex Gaussian process. Extension of the proposed model to simulate correlated diversity channels is also given. Numerical results demonstrate that the statistical properties of the samples generated from the proposed model are close to the required ones.

In this paper, a robust M-estimate adaptive filter for impulse noise suppression is proposed. The objective function used is based on a robust M-estimate. It has the ability to ignore or down weight large signal error when certain thresholds are exceeded. A systematic method for estimating such thresholds is also proposed. An advantage of the proposed method is that its solution is governed by a system of linear equations. Therefore, fast adaptation algorithms for traditional linear adaptive filters can be applied. In particular, a M-estimate recursive least square (M-RLS) adaptive algorithm is studied in detail. Simulation results show that it is more robust against individual and consecutive impulse noise than the MN-LMS and the N-RLS algorithms. It also has fast convergence speed and a low steady state error similar to its RLS counterpart.

It is known that the multiple access interference (MAI) of a quasi-synchronous (QS) multicarrier direct signal code division multiple access (DS-CDMA) system can be substantially reduced by using signature sequences having optimized cross correlation at small shifts around the origin. This paper shows that the time-frequency cross correlation function rather than the usual (time-domain) cross correlation determines the MAI when the system is operated in the presence of carrier frequency offset (CFO), which arises due to the frequency-accuracy limit of the oscillator. Several known sets of sequences having optimized time-domain cross correlation are investigated for their MAI-minimization capabilities in the presence of CFO. It is found that: (i) a system using Walsh codes or Suehiro-Hatori (1985) polyphase sequences can be driven into outage as a result of significant worst-case MAI and (ii) it is possible to minimize the MAI for systems using preferentially phased Gold codes cyclic-shift m-sequences or Lin-Chang 9see ibid., vol.45, p.221-6, 1997) sequences only if the product of chip period and maximum frequency deviation is less than around 0.01. Implications of these findings to practical implementation of systems are discussed.

An asynchronous multicarrier (MC) direct-sequence (DS) code-division multiple-access (CDMA) scheme for the uplink of the mobile communication system operating in a frequency selective fading channel is analyzed. The bit error rate performance of the system with either equal gain combining or maximum-ratio combining is obtained. Numerical results indicate that the system performs better than that of the conventional DS-CDMA system and another MC-DS-CDMA system.

This paper addresses capacity enhancement of a band-limited direct-sequence code-division multiple-access system by using a weighted despreading function (WDF) in the receiver. An ideal Gaussian channel with perfect power control is assumed. The system performance is measured by the signal-to-interference-plus-noise ratio of the decision variable derived in the frequency domain, the bandwidth efficiency factor, the capacity enhancement factor, and the bit-error rate. It is shown that tuning a parameter of the WDF employed helps to partially flatten the in-band cross-spectrum of a pair of spreading and despreading functions. Numerical results show that the capacity of the proposed system improves over the conventional system using rectangular despreading function. To assess practical implications of the WDF receiver, the sensitivity to timing error is also analyzed.

This paper evaluates the performance of a direct-sequence code-division multiple-access system using coherent receivers in which the despreading sequences are weighted by adjustable chip waveforms. The chip weighting waveforms under consideration are designed for multiple-access interference (MAI) rejection. Assuming that the received chip waveforms are rectangular, new expressions for the signal-to-interference-plus-noise ratio (SINR) of the decision variable are derived when different weighted despreading sequences (WDSs) are used in the receiver. The novelty of the derived expressions is that each of the expressions, when the system parameters are given, is determined only by one parameter of the adjustable chip waveforms employed. As a result, we can simply tune the parameter to its optimal value in real-time for MAI rejection without knowing the other users' spreading codes, timing, and phase. The criterion for tuning the parameter is to maximize the SINR of the decision variable based on the relative strength between the additive Gaussian white noise and the MAI. Numerical results show that when the multiple-access interference is significant, the receivers using WDSs outperform significantly the conventional receiver using a rectangular despreading sequence. Brief analysis for bandlimited spreading signals is also provided to reveal the practical implications of the proposed technique.

This paper derives an infinite series and an integral for computing matched filter bounds for binary phase-shift keying (BPSK) communications over a multipath Rician-fading channel of discrete or continuous dispersion. Based on the derived bound we compare intrinsic frequency diversity (IFD) gains for channels of different Rician factors.

This paper addresses the effects of a weighted despreading sequence on the performance of a coherent receiver in a DS-CDMA system. The chip weighting waveforms used in the receiver are designed for the purpose of multiple access interference rejection by emphasizing the transitions of the received signal of interest. Since the resulting weighted despreading sequence is determined only by one parameter, this leads to easy tuning of the weighted despreading sequence in practice to achieve the best performance. The criterion of tuning the parameter is to maximize the derived signal to interference plus noise ratio of the decision variable based on the relative strength between white noise and multiple access interference. As a result, according to the proposed approach, we can reject the multiple access interference without knowing the co-user's spreading codes, timing, and phase since maximizing the signal to interference plus noise ratio in Gaussian noise minimizes the probability of error of the receiver.

This paper derives a Karhunen-Loeve (K-L) expansion of the time-varying output of a multipath Rayleigh fading wide-sense-stationary uncorrelated-scattering (WSSUS) channel. It is shown that under the same mean-squared error condition, the number of terms required by the truncated K-L expansion is less than that of the series expansion obtained by using the discrete-path approximation of the channel so that simulation using the K-L expansion is more efficient. This computational advantage becomes more significant as higher simulation accuracy is required. The derived K-L expansion is applied to develop an efficient simulation technique for digital transmission over a multipath Rayleigh fading WSSUS channel using an optimum receiver. We show that the proposed technique requires shorter computation time than two other known simulation techniques.

In this paper we present a fast algorithm to reduce the computational complexity of block motion estimation. The reduction is obtained from the use of a new hexagonal subsampling pattern and the domain decimation method introduced by Cheng and Chan (see Proc. IEEE ICASSP, vol.4, p.2313, 1996). The multiple candidates search method is also introduced to improve the robustness of the algorithm. Computer simulation shows that the performance is very close to that of the full search.

In this paper, a new buffer control algorithm for motion-compensated hybrid DPCM/DCT coding (like H.261 and MPEG-1 I pictures) is presented. The algorithm uses the bit allocation algorithm to determine the quantization scale factor of each macroblock under a given target bit rate. An important advantage of the algorithm is that it has precise control of the buffer and avoids buffer overflow events which is a severe problem in low bit rate video coders. Furthermore, the coder is able to allocate bits to the picture as a whole, resulting in better rate-distortion trade-off. Simulation results show that the H.261 coder, using the proposed algorithm, can achieve a higher PSNR and better visual quality than a codec using a conventional buffer control algorithm.

In cases where machines having bursty data are equally likely to transmit to one another, code-division multiple-access (CDMA) ALOHA which allows for an individual "virtual channel" for each receiving station may be a better multiple-access protocol than simple ALOHA. With the use of a "receiver-based code" multiple-access protocol, it is also possible for a station to listen to the channel of the intended receiver before transmission, and also abort transmission when it detects others transmitting on the same channel. This paper describes a model for a fully-connected, full duplex, and slotted CDMA ALOHA network where channel sensing and collision detection are used. The model is analyzed using a discrete time Markov chain and some numerical results are presented. For a system with a large number of users, where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system, and estimate the throughput as well as the delay performance of the system when it is stable. Finally, a comparison is made with a simple channel sense multiple-access with collision detection (CSMA-CD) network, showing that a substantial improvement in the performance is achieved by the proposed network.

This paper compares the parameter estimation accuracy of four adaptive algorithms for frequency estimation when applied to an IIR digital notch filter. All four algorithms were subjected to the same experimental conditions and the variance of parameter estimates are compared to the Cramer Rao Lower Bound. Results show that the RML yielded the most accurate parameter estimates although its computational burden is quite high. The AML produced good parameter estimates and it has the advantages of proven convergence properties as well as lower computational burden over the RML. For applications where the signal to noise ratio is moderate it is shown that the AGB algorithm may be suitable, particularly where minimal computational burden is desired.

This paper describes a model for a fully-connected, full duplex, and synchronized CDMA LAN where the receivers use the same maximal length sequence with different chip phase offset for receiving messages. The ALOHA medium access is used. The performance of the model is analyzed by considering the succession of states the system is in as a Markov process. The behaviour of a 5-station system is shown.

This paper proposes an efficient Monte Carlo method which reduces computation for digital communication simulations over a multipath Rayleigh fading, wide-sense-stationary uncorrelated-scattering (WSSUS) channel. An equivalent discrete-time channel representation, which can be realized by a FIR filter with time-variant tap gains, is employed. In the proposed method, the tap gains are generated from a linear transformation of a set of orthogonal zero-mean complex Gaussian random processes. By the central limit theorem, each random process is approximated by summing a finite number of randomly generated phasors (Monte Carlo approximation). When compared with the tap gain generation method, which approximates the physical channel by the Monte Carlo approximation first and then generates the tap gain values, the proposed method demonstrates a considerable reduction in the required simulation time as well as improved accuracy under similar conditions.

This paper derives closed-form formulas for statistical characterization of the time-variant tap gains of an equivalent discrete-time channel model, which bears an equivalent input/output relationship with a continuous-time, fading dispersive, wide sense stationary uncorrelated scattering (WSSUS) radio channel.>

The code phase assignment technique is proposed for indoor mobile and portable DS-CDMA communications. The aims are minimizing the multiple-access interference and increasing the system capacity through the use of optimized cyclic codes as signature sequences in a bit-synchronous environment. Simulation results on the average bit error rate are given. Significantly better performance over conventional DS-CDMA (using unoptimized signature sequences) is shown in the presence of larger number of users.>