Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.      

Proposal of an interference cancellation receiver with lowcomplexity for DS/CDMA mobile communication systems

The main targets of any direct-sequence code division multiple-access (DS/CDMA) mobile communication system are to overcome the multipath fading influences as well as the near/far effect and to increase the capacity. Many optimal and suboptimal detection approaches have been proposed and analyzed in the literature. Unfortunately, most of them share the drawback of requiring a complex implementation and do not represent a practical solution. This paper proposes a simple interference cancellation receiver for applications in DS/CDMA uplink communications. This receiver allows users to overcome the near/far effect and to enhance the system capacity. Differently from previous methods, the interference cancellation is performed on a one-shot basis. The performance of the proposed interference cancellation receiver is derived through computer simulations. However, a suitable analytical approach is also presented in the appendix in order to evaluate the bit error rate (BER) performance in the particular case of synchronous users and the transmission channel being affected only by additive white Gaussian noise (AWGN). The good behavior of the proposed approach is demonstrated by means of comparisons in terms of the BER performance and implementation complexity with the classical RAKE receiver and different multiuser receivers previously proposed in the literature on this subject     

Group-blind intersymbol multiuser detection for downlink CDMA with multipath

Group-blind multiuser detectors for uplink code-division multiple-access (CDMA) were recently developed by Wang and Host-Madsen. These detectors make use of the spreading sequences of known users to construct a group constraint to suppress the intracell interference. However, such techniques demand the estimation of the multipath channels and the delays of the known users. In this paper, several improved blind linear detectors are developed for CDMA in fading multipath channels. The proposed detectors utilize the correlation information between consecutively received signals to generate the corresponding group constraint. It is shown that by incorporating this group constraint, the proposed detectors can provide different performance gains in both uplink and downlink environments. Compared with the previously reported group-blind detectors, our new methods only need to estimate the multipath channel of the desired user and do not require the channel estimation of other users. Simulation results demonstrate that the proposed detectors outperform the conventional blind linear multiuser detectors.     

MAI and ICI of Asynchronous Uplink MC-CDMA With Frequency Offset

In this paper, we analyze the performance of random spreading code-division multiple-access (CDMA) systems with a multicarrier in asynchronous uplink channels. We first derive the probability density function (pdf) of the multiple-access interference (MAI) plus noise and then extend the results to orthogonal frequency division multiplexing (OFDM) with intercarrier interference (ICI) that is caused by a frequency offset. We obtain the pdf of the MAI and the ICI plus noise under Rayleigh and frequency-selective fading as a function of the number of users and the spreading factor, as well as the number of subcarriers and the frequency offset. The bit-error-rate (BER) analysis shows that the power penalty from the frequency offset increases with the system loading. We develop the Gaussian approximation that provides an accurate estimation of the BER with reduced computational complexity comparing to the direct calculations using the pdf.     

Multilevel trellis-coded modulations for mobile multimedia satellite systems using synchronous CDMA

In this paper the performance of multilevel trellis-coded modulations (MTCMs) for satellite mobile multimedia systems using synchronous CDMA is investigated. A model for the multiple-access interference arising from the non-orthogonality of the spreading waveforms is developed for different modulation and spreading schemes and transmission over the mobile satellite channel. Using this model, the performance of MTCMs for synchronous CDMA is analysed and compared with the Ungerboeck and Schegel–Costello trellis-coded modulation (TCM) designs. The impact on the transmission of synchronization errors between the different users is presented and the power control requirements are derived. © 1998 John Wiley & Sons, Ltd.     

Group-blind multiuser detection for uplink CDMA

Previously developed blind techniques for multiuser detection in code division multiple access (CDMA) systems lead to several near-far resistant adaptive receivers for demodulating a given user's data with the prior knowledge of only the spreading sequence of that user. In the CDMA uplink, however, typically the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper, group-blind techniques are developed for multiuser detection in such scenarios. These new techniques make use of the spreading sequences and the estimated multipath channels of all known users to suppress the intracell interference, while blindly suppressing the intercell interference. Several forms of group-blind linear detectors are developed based on different criteria. Moreover, group-blind multiuser detection in the presence of correlated noise is also considered. In this case, two receiving antennas are needed for channel estimation and signal separation. Simulation results demonstrate that the proposed group-blind linear multiuser detection techniques offer substantial performance gains over the blind linear multiuser detection methods in a CDMA uplink environment     

Interference-Resilient Block-Spreading CDMA With Minimum-MAI Sequence Design

Code-division multiple-access (CDMA) schemes based on block spreading implement the spreading of entire data blocks rather than single symbols, thus achieving a higher robustness against the frequency selectivity of the channel and allowing the use of efficient modulation/equalization schemes operating in the frequency domain (FD). In this paper, we present a new block CDMA (B-CDMA) system where a single cyclic prefix (CP) is used at the end of each spread block. This provides a higher spectral efficiency with respect to existing schemes. By observing that complete orthogonality among users is achievable only for half-loaded systems on dispersive channels, we introduce new criteria for the design of spreading and despreading sequences, which aim at minimizing the mean-square error at the output of the despreader. For the equalization of the received signal, we propose an iterative block decision feedback equalizer, which iterates between equalization and decoding. Equalization filters are designed to minimize the mean-square error and take into account the residual interference due to the nonorthogonality of the spreading sequences. The performance of B-CDMA is evaluated in an uplink wireless scenario and compared to existing CDMA schemes.     

Interference suppression for space-time coded CDMA via decision-feedback equalization

A single-user receiver structure is proposed for space-time coded code-division multiple-access (CDMA) downlink in a multiuser frequency-selective channel. This structure is a two-dimensional (2-D) decision-feedback equalizer (2D-DFE) whose filters are optimized based on the MMSE criterion to mitigate noise, intersymbol interference (ISI), and multiuser interference (MUI) with a moderate complexity. By modeling the spreading codes of the interfering users as random sequences, system performance was evaluated using the Gaussian approximation. Two models for the desired user's spreading sequence have been considered and compared. Our numerical results show that in both cases the 2D-DFE exhibits significant performance improvement over the standard space-time coded RAKE, especially in interference-limited conditions. It is also observed that the gain obtained by using DFE in a MISO channel is less that in a SISO channel and this problem can be solved by providing diversity at the receiver.     

A concatenated multitone multiple-antenna air-interface for the asynchronous multiple-access channel

In this paper, we present a novel transmission technology approach for application in the asynchronous multiple-access wireless channel (uplink). It is based on the concatenation of an inner filtered multitone (FMT) modulator with transmission over multiple antennas, and an outer space-time cyclically prefixed discrete multitone (ST-CP-DMT) modulator. The inner modulator is used to efficiently realize frequency-division multiplexing of the users by partitioning the FMT subchannels among them. The outer modulator copes with the residual subchannel intersymbol interference and it further implements a form of transmit diversity. Frequency and space diversity is exploited via direct sequence (DS) data spreading across the DMT tones. The design parameters are flexible and are chosen to make the proposed air-interface robust to the time and frequency offsets between uplink users, as well as to the channel time-frequency selectivity. Furthermore, the ST-CP-DMT modulator with DS spreading provides sensible performance gains. In particular, it yields a diversity gain for the users that transmit at low rate and occupy a fraction of the overall spectrum.     

A novel transmitter design for GLSFBC–OFDM–CDMA communication systems

Relying on mutual orthogonality between subcarriers of different users in orthogonal frequency-division multiple access (OFDMA) systems and mutual orthogonality between spreading codes in code-division multiple-access (CDMA) systems, a novel transmitter design is proposed for group layered space–frequency block code (GLSFBC)–OFDM–CDMA communication systems over frequency-selective fading channels. The proposed method is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on space–frequency block coding) results in space–frequency diversity, and the lower level (based on CDMA) handles multiuser interference. The new approach only needs one receive antenna to distinguish multiple users and suppress group interference simultaneously, so the complexity of the receiver decreases remarkably. Simulation results confirm the validity of the proposed technique.     

A dynamic CDMA network for multicore systems

Code-division multiple-access (CDMA) is a data transmission method based on the spreading code technology, wherein multiple data streams share the same physical medium with no interference. A novel architecture for on-chip communication networks based on this approach is devised. The proposed design allows sharing coding resources among network?s users through the use of dynamic assignment of spreading codes. Data transmission latency is reduced by adopting a parallel structure for the coding/decoding circuitry. A 14-node CDMA network based on the proposed architecture is synthesised using 65 nm ST technology library. Performance analysis reveals that the proposed approach achieves significantly lower data packet latency compared to both conventional CDMA and packet switched network-on-chip implementations. Large area and power savings compared to existing approaches are also obtained.     

Constrained Tensor Modeling Approach to Blind Multiple-Antenna CDMA Schemes

In this paper, we consider an uplink multiple-antenna code-division multiple-access (CDMA) system linking several multiple-antenna mobile users to one multiple-antenna base station. For this system, a constrained third-order tensor decomposition is introduced for modeling the multiple-antenna transmitter as well as the received signal. The constrained structure of the proposed tensor decomposition is characterized by two constraint matrices that have a meaningful physical interpretation in our context. They can be viewed as canonical allocation matrices that define the allocation of users' data streams and spreading codes to the transmit antennas. The distinguishing features of the proposed tensor modeling with respect to the already existing tensor-based CDMA models are: i) it copes with multiple transmit antennas and spreading codes per user and ii) it models several spatial spreading/multiplexing schemes with multiple spreading codes by controlling the constrained structure of the tensor signal model. A systematic design procedure for the canonical allocation matrices leading to a unique blind symbol (or joint blind symbol-code) recovery is proposed which allows us to derive a finite set of multiple-antenna schemes for a fixed number of transmit antennas. Identifiability of the proposed tensor model is discussed, and a blind multiuser detection receiver based on the alternating least squares algorithm is considered for performance evaluation of several multiple-antenna CDMA schemes derived from the constrained modeling approach.     

海面的红外被动成像

本文在研究海面的红外成像特性中,建立了基于刚体的双尺度成像模型,从被动成像方面讨论了海面辐射特性的变化规律.该模型仿真结果很好地说明了风向对海面红外被动成像特性的影响.     

A Cyclodespreader Architecture for Detecting Multiple Downlink Aperiodic CDMA Signals

Although a code-division multiple-access (CDMA) system is spectrally efficient and has some immunity against intentional reception, its capacity and performance are generally limited by multiple access interference (MAI) caused by other users. For an aperiodic CDMA system, the spreading sequences span multiple symbol intervals, and the cross-correlations between these sequences, which are a measure of the MAI, are periodic over multiple symbols. In this paper, we present a receiver with a parallel architecture that converts an aperiodic CDMA sequence into a piecewise periodic sequence in each arm. We refer to this receiver as a cyclodespreader because the despreader exploits the cyclostationary property of an aperiodic signal. As a result, the transmitted data can be detected separately in each arm of the receiver using low-complexity conventional algorithms proposed for periodic CDMA systems. The goal of the receiver is to decode several signals received from different cochannel base stations. The performance of the system is evaluated using real aperiodic CDMA signals, and it is compared to that of a conventional matched filter (MF) receiver using the number of correctly decoded messages as the performance measure.     

Sensitivity of multiple-access techniques to narrow-bandinterference

This paper investigates the sensitivity of several multiple-access techniques to narrow-band interference. The analysis covers time-division multiple access (TDMA), code-division multiple access (CDMA), and orthogonal frequency-division multiple access (OFDMA). The study is carried out under the assumption that all the considered multiple-access systems occupy the same total bandwidth, and the bit rates of all active users are identical. A major finding of this study is that CDMA with pseudonoise spreading sequences is more sensitive to narrow-band interference than TDMA. We point out that the signal-to-jammer power ratio at the decision device input is in fact identical for both multiple-access techniques, but the amplitude distribution of the jammer term at the threshold detector input is more favorable to TDMA, which turns out to be more robust in terms of bit-error rate. Another finding is that in terms of sensitivity to narrow-band interference, orthogonal CDMA (OCDMA) is closer to TDMA than to CDMA with pseudonoise sequences, because the degradation is not the same for all users. Finally, we discuss the relationship of OCDMA and TDMA and highlight the superiority, in terms of capacity over the narrow-band interference channel, of TDMA to the other multiple-access techniques considered in this paper     

Combined code reuse scheme with two‐dimensional OVSF codes assignment algorithm for uplink multi‐user/multi‐rate block spread multi‐cellular CDMA

The two‐dimensional (2D) block spread code division multiple access (CDMA) can avoid the uplink multiple‐access interference with low‐complexity single‐user detection in a slow fading channel and, therefore, is very attractive. In the 2D spreading, orthogonal variable spreading factor (OVSF) is used for spreading; an important problem is how to efficiently assign the limited resource of OVSF codes to users with different data rates, while meeting the requirement of quality of service in a multi‐cell environment. In this paper, it is shown that the code reuse can improve the code reuse efficiency and the proposed code reuse scheme combined with code assignment algorithm can allow flexible multi‐rate uplink transmission. The computer simulation confirms that the proposed code assignment algorithm improves the code reuse efficiency while achieving lower blocking probability than traditional CDMA. Copyright © 2012 John Wiley & Sons, Ltd.     

Linear receivers for the multiple-input multiple-output multiple-access channel

We consider a multiuser multiple-input multiple-output (MIMO) communication system using code-division multiple access (CDMA) and multiuser detection to discriminate the different users. Our focus is on the CDMA uplink of a frequency-nonselective Rayleigh fading channel. We study two types of receivers: joint receivers, which address simultaneously both spatial and multiple-access interference; and separate receivers, addressing the two types of interference individually. This approach allows assessing the benefits of adding MIMO processing capabilities to existing multiuser single-input single-output systems. For both receiver types, we analyze solutions based on linear (matched filter, decorrelator, minimum mean-square error) and maximum-likelihood receivers. For all the receivers considered, we provide closed-form expressions (as expectations of given functions) of the resulting pairwise error probabilities. Performance results are obtained in terms of frame-error rate versus E/sub b//N/sub 0/, following two different approaches. An analytic approach using large-system asymptotic methods, whereby the system parameters (number of users and antennas, spreading gain) are assumed to grow to infinity with finite limiting ratios. A computer-simulation approach is used to illustrate the differences between asymptotic and simulation results.     

Blind turbo multiuser detection for long-code multipath CDMA

We consider the problem of demodulating and decoding multiuser information symbols in an uplink asynchronous coded code-division multiple-access (CDMA) system employing long (aperiodic) spreading sequences, in the presence of unknown multipath channels, out-cell multiple-access interference (OMAI), and narrow-band interference (NBI). A blind turbo multiuser receiver, consisting of a novel blind Bayesian multiuser detector and a bank of MAP decoders, is developed for such a system. The effect of OMAI and NBI is modeled as colored Gaussian noise with some unknown covariance matrix. The main contribution of this paper is to develop blind Bayesian multiuser detectors for long-code multipath CDMA systems under both white and colored Gaussian noise. Such detectors are based on the Bayesian inference of all unknown quantities. The Gibbs sampler, a Markov chain Monte Carlo procedure, is then used to calculate the Bayesian estimates of the unknowns. The blind Bayesian multiuser detector computes the a posteriori probabilities of the channel coded symbols, which are differentially encoded before being sent to the channel. Being soft-input soft-output in nature, the proposed blind Bayesian multiuser detectors and the MAP decoders can iteratively exchange the extrinsic information to successively refine the performance, leading to the so-called blind turbo multiuser receiver     

Output MAI distributions of linear MMSE multiuser receivers inDS-CDMA systems

Multiple-access interference (MAI) in a code-division multiple-access (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. We study the behavior of the output MAI of the minimum mean-square error (MMSE) receiver employed in the uplink of a direct-sequence (DS)-CDMA system. We focus on imperfect power-controlled systems with random spreading, and establish that in a synchronous system (1) the output MAI of the MMSE receiver is asymptotically Gaussian, and (2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of single-user Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signal-to-interference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system     

Optimal resource allocation in multiservice CDMA networks

This paper addresses the problem of dynamic resource allocation in a multiservice direct-sequence code-division multiple-access (DS-CDMA) wireless network supporting real-time (RT) and nonreal-time (NRT) communication services. For RT users, a simple transmission power allocation strategy is assumed that maximizes the amount of capacity available to NRT users without violating quality of service requirements of RT users. For NRT users, a joint transmission power and spreading gain (transmission rate) allocation strategy, obtained via the solution of a constrained optimization problem, is provided. The solution maximizes the aggregate NRT throughput, subject to peak transmission power constraints and the capacity constraint imposed by RT users. The optimization problem is solved in a closed form, and the resulting resource allocation strategy is simple to implement as a hybrid CDMA/time-division multiple-access strategy. Numerical results are presented showing that the optimal resource allocation strategy can offer substantial performance gains over other conventional resource allocation strategies for DS-CDMA networks.