Near-far Resistance Research Articles

GMM-MUD: An Effective Multiuser Detection Algorithm for DS-UWB-Based Space Formation Flying Systems

In this paper, an effective multiuser detection (MUD) is proposed for direct sequence ultrawideband- (DS-UWB-) based space formation flying systems. The proposed method called GMM-MUD is based on Gaussian mixture models (GMMs) to suppress multiple access interference. The GMM describes probability distributions of the hypothesis testing problem which is used for bit classification. To reveal the difference between correct bits and error bits, the preprocessing operation applies a mapping function based on optimal multiuser detection. The parameters of GMM are estimated by using expectation-maximization (EM) algorithm. EM algorithm employs iterative operation to simplify the complexity of maximum likelihood estimation method and considers the mapping values of received bits as the observations. Simulation results demonstrate that the proposed GMM-MUD algorithm achieves good performances in terms of bit error rate performance, user capacity, and near-far resistance. Moreover, the computational complexity is low enough for space formation flying applications.

IA-OPD: An Optimized Orthogonal Pulse Design Scheme for Waveform Division Multiple Access UWB Systems

A new design scheme of orthogonal pulses is proposed for waveform division multiple access ultra-wideband (WDMA-UWB) systems. In order to achieve WDMA and to improve user capacity, the proposed method, termed as interference alignment based orthogonal pulse design (IA-OPD), employs combined orthogonal wavelet functions in the pulse design. The combination coefficients are optimized by using interference alignment. Due to the reciprocity between transmitted and local template signals, the iterative process based on maximum signal to interference plus noise ratio (Max-SINR) criterion can be used to solve the optimization problem in interference alignment. Numerical results demonstrate that the optimized orthogonal pulses provide excellent performances in terms of multiple access interference suppression, user capacity and near-far resistance without using any multiuser detection techniques. Thus, the IA-OPD scheme can be used to efficiently design a large number of orthogonal pulses for multiuser WDMA-UWB systems with low computational complexity and simple transceiver structure.

A hybrid multiuser detector based on MMSE and AFSA for TDRS system forward link.

This study mainly focuses on multiuser detection in tracking and data relay satellite (TDRS) system forward link. Minimum mean square error (MMSE) is a low complexity multiuser detection method, but MMSE detector cannot achieve satisfactory bit error ratio and near-far resistance, whereas artificial fish swarm algorithm (AFSA) is expert in optimization and it can realize the global convergence efficiently. Therefore, a hybrid multiuser detector based on MMSE and AFSA (MMSE-AFSA) is proposed in this paper. The result of MMSE and its modified formations are used as the initial values of artificial fishes to accelerate the speed of global convergence and reduce the iteration times for AFSA. The simulation results show that the bit error ratio and near-far resistance performances of the proposed detector are much better, compared with MF, DEC, and MMSE, and are quite close to OMD. Furthermore, the proposed MMSE-AFSA detector also has a large system capacity.

A Multiuser Detector Based on Artificial Bee Colony Algorithm for DS-UWB Systems

Artificial Bee Colony (ABC) algorithm is an optimization algorithm based on the intelligent behavior of honey bee swarm. The ABC algorithm was developed to solve optimizing numerical problems and revealed premising results in processing time and solution quality. In ABC, a colony of artificial bees search for rich artificial food sources; the optimizing numerical problems are converted to the problem of finding the best parameter which minimizes an objective function. Then, the artificial bees randomly discover a population of initial solutions and then iteratively improve them by employing the behavior: moving towards better solutions by means of a neighbor search mechanism while abandoning poor solutions. In this paper, an efficient multiuser detector based on a suboptimal code mapping multiuser detector and artificial bee colony algorithm (SCM-ABC-MUD) is proposed and implemented in direct-sequence ultra-wideband (DS-UWB) systems under the additive white Gaussian noise (AWGN) channel. The simulation results demonstrate that the BER and the near-far effect resistance performances of this proposed algorithm are quite close to those of the optimum multiuser detector (OMD) while its computational complexity is much lower than that of OMD. Furthermore, the BER performance of SCM-ABC-MUD is not sensitive to the number of active users and can obtain a large system capacity.

Multiple phase detector of M-ary phase shift keying symbols in code division multiple access systems

A novel iterative technique, the phase descent search (PDS) algorithm, for M-ary phase shift keying (M-PSK) symbols detection was proposed. This technique constrained the solution to have a unit magnitude and it was based on coordinate descent iterations where coordinates were the unknown symbol phases. The PDS algorithm, together with a descent local search (also implemented as a version of the PDS algorithm), was used multiple times with different initializations in a proposed multiple phase detector; the solution with the minimum cost was then chosen as the final solution. The simulation results show that for highly loaded multiuser scenarios, the proposed technique has a detection performance that is close to the single-user bound. The results also show that the multiple phase detector allows detection in highly overloaded scenarios and it exhibits near-far resistance. In particular, the detector has a performance that is significantly better, and complexity that is significantly lower, than that of the detector based on semi-definite relaxation.

Robust multi-user detection based on quantum bee colony optimisation

To resolve local convergent difficulty of combinatorial optimisation algorithm, a quantum bee colony optimisation (QBCO) that employs novel evolutionary quantum equations is proposed. The proposed QBCO algorithm applies quantum coding and quantum rotation mechanism to evolutionary process of bee colony, which is a simple and effective discrete optimisation algorithm. Then, the proposed quantum bee colony optimisation algorithm is used to solve robust multi-user detection problem of code division multiple access (CDMA) system in the presence of impulsive noise. Furthermore, by hybridising the stochastic Hopfield neural network and quantum bee colony optimisation, the quantum state and measure state of the quantum bee are co-evolutionary in design of robust multi-user detection. The new multi-user detection algorithm can search global optimal solution in faster convergence rate. Simulation results for CDMA system are provided to show that the designed robust detectors are superior to some previous detectors in bit error rate (BER), multiple access interference and near-far resistance.

Estimating Multiple Frequency-Hopping Signal Parameters via Sparse Linear Regression

Frequency hopping (FH) signals have well-documented merits for commercial and military applications due to their near-far resistance and robustness to jamming. Estimating FH signal parameters (e.g., hopping instants, carriers, and amplitudes) is an important and challenging task, but optimum estimation incurs an unrealistic computational burden. The spectrogram has long been the starting non-parametric estimator in this context, followed by line spectra refinements. The problem is that hop timing estimates derived from the spectrogram are coarse and unreliable, thus severely limiting performance. A novel approach is developed in this paper, based on sparse linear regression (SLR). Using a dense frequency grid, the problem is formulated as one of under-determined linear regression with a dual sparsity penalty, and its exact solution is obtained using the alternating direction method of multipliers (ADMoM). The SLR-based approach is further broadened to encompass polynomial-phase hopping (PPH) signals, encountered in chirp spread spectrum modulation. Simulations demonstrate that the developed estimator outperforms spectrogram-based alternatives, especially with regard to hop timing estimation, which is the crux of the problem.

Population Declining Ant Colony Optimization Multiuser Detection in Asynchronous CDMA Communications

In this paper we present a population declining ant colony optimization (PDACO) multiuser detector for asynchronous CDMA communications. Ant colony optimization (ACO) algorithms have already been used in multiuser detection in CDMA systems; however, as the pheromone accumulates, we may not get a global optimum because it stops searching early. PDACO can enlarge searching range through increasing the initial population of the ant colony, and the population declines in successive iterations. So, the performance of PDACO is superior with the same computational complexity. PDACO is applied to multiuser detection in asynchronous CDMA systems over slowly multipath Rayleigh-fading channels in this paper. Via computer simulations it is shown that the performance of PDACO detector is much better in bit-error rate and near-far effect resistance than conventional detector, ACO detector and genetic algorithm detector, and is close the optimal multiuser detector.

Performance analysis of narrowband interference mitigation and near-far resistance scheme for GNSS receivers

This paper presents a method to mitigate the interferences against pseudolite-type and continuous-wave interferences in a global navigation satellite system (GNSS) environment. Ground-based pseudolites can transmit GPS-liked signals and augment GNSS constellations to enhance the accuracy, integrity, and availability. Unfortunately, a nearby pseudolite may induce a strong interference to GPS receivers. As a result, the receiver may fail to acquire and track signals from satellites. In order to account for such a problem, a mitigation scheme that combines the use of frequency domain excision method and direct reconstruction method is proposed to effectively nullify continuous-wave and pseudolite-type interferences, so that the GPS signals can still be tracked in the presence of strong interferences. Experiment results show that with the removal of interferences, the measured narrow-to-wide power ratio for all satellites can be enhanced. A sensitivity analysis is then conducted to assess the performance of the proposed algorithm in the presence of errors in the estimation of the signal power, Doppler frequency and code delay. Finally, the problem of combined interferences of pseudolites and continuous-wave sources is investigated both theoretically and experimentally. It is shown that the method is applicable to mitigate narrowband continuous-wave interferences and pseudolite-type interferences.

Fixed-point blind adaptive multiuser detection algorithm for IR UWB systems in multipath channel

Using the hypothesis that data transmitted by different users are statistically independent of each other, this paper proposes a fixed-point blind adaptive multiuser detection algorithm for Time-Hopping (TH) Impulse Radio (IR) Ultra Wide Band (UWB) systems in multipath channel, which is based on Independent Component Analysis (ICA) idea. The proposed algorithm employs maximizing negentropy criterion to separate the data packets of different users. Then the user characteristic sequences are utilized to identify the data packet order of the desired user. This algorithm only needs the desired user’s characteristic sequence instead of channel information, power information and time-hoping code of any user. Due to using hypothesis of statistical independence among users, the proposed algorithm has the outstanding Bit Error Rate (BER) performance and the excellent ability of near-far resistance. Simulation results demonstrate that this algorithm has the performance close to that of Maximum-Likelihood (ML) algorithm and is a suboptimum blind adaptive multiuser detection algorithm of excellent near-far resistance and low complexity.

On Minimal Eigenvalues of a Class of Tridiagonal Matrices

It is known that the worst case near-far resistance of optimum multiuser detectors for asynchronous Gaussian multiple-access channels can be expressed in terms of a class of block-tridiagonal matrices, and the minimal eigenvalues of such a class of block-tridiagonal matrices serve as a good measure of the worst case near-far resistance. In this paper, we focus on the two-user scenario where each block-tridiagonal matrix under consideration is a tridiagonal matrix. We derive closed-form expressions for the minimal eigenvalues of such a class of tridiagonal matrices in terms of the largest real solution of a trigonometric equation in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[0,pi]</i> . We also obtain lower bounds and upper bounds on the minimal eigenvalues which improve on previously known results in the literature.

LMSE-based parameter acquisition for multicarrier CDMA systems

An acquisition scheme, based on the least mean-squared error (LMSE), for the symbol timing, carrier phases, and multipath gains for multicarrier code-division multiple-access (MC-CDMA) systems is proposed. A new coefficient called the quasi-mean-squared accuracy coefficient (QMSAC) is introduced. This allows the search for multiple optimal parameters to be split into simpler single-parameter estimates. The symbol timing is acquired with a comparison of the QMSACs followed by a linear interpolation scheme or a search of a minimizing variable. Carrier phases and multipath gain coefficients are then estimated via some algebraic computations based only on the estimated symbol timing and the QMSACs. The spreading sequence of the desired user is the only prior knowledge required for the proposed approach. Simulation results demonstrate the near-far resistance of the approach and show that the error performance of minimum mean-squared error (MMSE) detection with acquired parameters is only a few decibels worse for the considered channel than the performance obtained with perfect knowledge about the channel impulse response.

Performance Analysis of a De-correlated Modified Code Tracking Loop for Synchronous DS-CDMA System under Multiuser Environment

This paper presents the performance analysis of a De-correlated Modified Code Tracking Loop (D-MCTL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environment. Previous studies have shown that the imbalance of multiple access interference (MAI) in the time lead and time lag portions of the signal causes tracking bias or instability problem in the traditional correlating tracking loop like delay lock loop (DLL) or modified code tracking loop (MCTL). In this paper, we exploit the de-correlating technique to combat the MAI at the on-time code position of the MCTL. Unlike applying the same technique to DLL which requires an extensive search algorithm to compensate the noise imbalance which may introduce small tracking bias under low signal-to-noise ratio (SNR), the proposed D-MCTL has much lower computational complexity and exhibits zero tracking bias for the whole range of SNR, regardless of the number of interfering users. Furthermore, performance analysis and simulations based on Gold codes show that the proposed scheme has better mean square tracking error, mean-time-to-lose-lock and near-far resistance than the other tracking schemes, including traditional DLL (T-DLL), traditional MCTL (T-MCTL) and modified de-correlated DLL (MD-DLL).

A Modified Decorrelated Delay Lock Loop for Synchronous DS-CDMA Systems

This paper presents a modified decorrelated delay lock loop (MD-DLL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environments. The structure of the MD-DLL is similar to that of a traditional delay lock loop (T-DLL), aside from the fact that the despreading signals in T-DLL are replaced by a linear combination of all the users' code sequences with various weightings and timing offsets. The basic principles of MD-DLL are given as follows: 1) to use the decorrelating technique of a multiuser detector to remove the multiple-access interference at the on-time code position instant and 2) to choose a proper timing offset for the code sequences to reduce the weighting imbalance between the two arms of the tracking loop and suppress noise enhancement. The proposed MD-DLL scheme solves the tracking bias problems in a decorrelated delay lock loop under a low-signal-to-noise-ratio (SNR) condition. Computer simulations based on Gold code sequences are used to verify the findings. It is shown that MD-DLL achieves a better and stable S-curve with negligible tracking bias, better rms tracking error and mean-time-to-lose-lock performance under high-SNR conditions, and stronger near-far resistance than T-DLL.

Minimum BER Receiver Filters with Block Memory for Uplink DS-CDMA Systems

The problem of synchronous multiuser receiver design in the case of direct-sequence single-antenna code division multiple access (DS-CDMA) uplink networks is studied over frequency selective fading channels. An exact expression for the bit error rate (BER) is derived in the case of BPSK signaling. Moreover, an algorithm is proposed for finding the finite impulse response (FIR) receiver filters with block memory such that the exact BER of the active users is minimized. Several properties of the minimum BER FIR filters with block memory are identified. The algorithm performance is found for scenarios with different channel qualities, spreading code lengths, receiver block memory size, near-far effects, and channel mismatch. For the BPSK constellation, the proposed FIR receiver structure with block memory has significant better BER with respect to and near-far resistance than the corresponding minimum mean square error (MMSE) filters with block memory.

Ant colony optimisation-based multiuser detection for direct-sequence CDMA systems with diversity reception

An ant colony optimisation (ACO)-based multiuser detector assisted by spatial diversity reception using direct-sequence CDMA is proposed. By taking advantage of heuristic values and the collective intelligence of ACO, the proposed detector offers almost the same bit error rate (BER) performance as the full-search-based optimum multiuser detector does, while greatly reducing the computational complexity. The near-far resistance of the ACO-based multiuser detector is also examined. The BER performance for a ten-user system deteriorates scarcely when the interfering users' energy is 12 dB higher than that of the desired user.

Multiuser detection for DS-CDMA systems in non-Gaussian channels

An adaptive multi-user detector was developed for direct-sequence code division multiple access (DS-CDMA) systems corrupted by non-Gaussian channel noise, which can be quite detrimental to the performance of the multi-user detectors based on classical white Gaussian assumption. This receiver simultaneously combats multipleaccess interference (MAI) and non-Gaussian impulsive noise. The channel parameters are estimated and transmitted signals are jointly detected by a simple recursive algorithm derived from the EM/SAGE algorithm. Analytical and simulation results show that the proposed technique is robust with wider applicability than conventional multi-user detectors in terms of near-far resistance and bit-error ratio (BER) when either MAI or non-Gaussian impulsive noise is dominant.

Decision Aided Hybrid MMSE/SIC Multiuser Detection: Structure and AME Performance Analysis

This paper presents a simple, yet effective hybrid of the minimum mean square error (MMSE) multi-user detection (MUD) and successive interference cancellation (SIC) for direct-sequence code division multiple access (DS-CDMA) systems. The proposed hybrid MUD first divides the users into groups, with each group consisting of users with a close power level. The SIC is then used to distinguish users among different groups, while the MMSE MUD is used to detect signals within each group. To further improve the performance impaired by the propagation errors, an information reuse scheme is also addressed, which can be used in conjunction with the hybrid MMSE/SIC MUD to adequately cancel the multiple access interferences (MAIs) so as to attain more accurate detections. Furthermore, the asymptotic multiuser efficiency (AME), a measure to characterize the near-far resistance capability, is also conducted to provide further insights into the new detectors. Furnished simulations, in both additive white Gaussian noise (AWGN) channels and slow flat Rayleigh fading channels, show that the performances of the proposed hybrid MMSE/SIC detectors, with or without the decision aided scheme, are superior to that of the SIC and, especially, the one with decision aided is close to that of the MMSE MUD but with substantially lower computational complexity.

Multi-User Detection Scheme Based on Hidden Training Sequence for DS-UWB Systems

SUMMARY In this letter, we propose a multi-user detection scheme based on a hidden training sequence for DS-UWB systems. The hidden training sequence, which uses a fraction of the informative sequence’s transmitting power as training information, is utilized for the receiver filter adaptation and channel estimation. By using this, the proposed scheme offers increased bandwidth efficiency (no period dedicated for training) and also shows reasonably good performance and near-far resistance in single

Near-far resistance of optimum and suboptimum CDMA detectors under multipath

Near-far resistance is one of the most important performance measures in a multiuser communication system, especially in a code division multiple access (CDMA) system. The optimum multiuser detector can achieve the optimum near-far resistance. However, due to its high computational complexity, the optimum detector is impractical to implement. Nevertheless, computation of the near-far resistance of the optimum multiuser detector is still important since it is the least upper bound on the near-far resistance of any detector, as well as a benchmark for relative performance of any suboptimum detector. Lupas et al. and Zvonar et al. provided the computation of the optimum near-far resistance in the asynchronous AWGN and unipath channel case. In this paper, we extend the above results to the asynchronous multipath channel case. We derive the optimum near-far resistance under the assumption of a common identifiability condition in the blind multiuser detection/equalization literature. We also derive the ideal near-far resistance of three typical methods in blind multiuser detection/equalization (subspace, linear prediction, and constrained optimization) and compare them with that of the optimum detector. Two methods to enhance near-far resistance of suboptimum detectors are also presented. Furthermore, one possible cause that may affect the near-far resistance in a practical setting is analyzed. The computational complexity of the proposed methods is analyzed and compared. Simulations confirm the theoretical findings.