Adaptive space-time feedforward/feedback detection for high datarate CDMA in frequency-selective fading

We investigate linear and nonlinear space-time minimum mean-square-error (MMSE) multiuser detectors for high data rate wireless code-division multiple-access (CDMA) networks. The centralized reverse-link detectors comprise a space-time feedforward filter and a multiuser feedback filter which processes the previously detected symbols of all in-sector users. The feedforward filter processes chip-rate samples from a bank of chip-matched filters which operate on the baseband outputs from an array of antennas. We present an adaptive multiuser recursive least squares (RLS) algorithm which determines the MMSE adjusted filter coefficients with less complexity than individual adaptation for each user. We calculate the outage probabilities and isolate the effects of antenna, diversity, and interference suppression gains for linear and nonlinear filtering and for CDMA systems with varying levels of system control (e.g., timing control, code assignment, cell layout). For eight users transmitting uncoded 2-Mb/s quadrature phase-shift keying with a spreading gain of eight chips per symbol over a fading channel with a multipath delay spread of 1.25 μs, the performance of a three-antenna feedforward/feedback detector was within 1 dB (in signal-to-noise ratio per antenna) of ideal detection in the absence of interference. By training for 10% of a 5-ms frame, RLS adaptation enabled the same detector to suffer less than a 0.5-dB penalty due to the combined effects of imperfect coefficients and error propagation. The advantage of nonlinear feedforward/feedback detection over linear feedforward detection was shown to be significantly larger for a CDMA system with enhanced system control     

Adaptive techniques for multiuser CDMA receivers

A number of CDMA receivers have been proposed that cover the whole spectrum of performance/complexity from the simple matched filter to the optimal Viterbi (1995) processor. Adaptive solutions, in particular, have the potential of providing the anticipated multiuser detection (MD) performance gains with a complexity that would be manageable for third generation systems. Our goal, in this article, is to provide an overview of previous work in MD with an emphasis on adaptive methods. We start with (suboptimal) linear receivers and discuss the data-aided MMSE receiver. Blind (nondata-aided) implementations are also reviewed together with techniques that can mitigate possible multipath effects and channel dispersion. In anticipation of those developments, appropriate discrete-time (chip rate) CDMA models are reviewed, which incorporate asynchronism and channel dispersion. For systems with large spreading factors, the convergence and tracking properties of conventional adaptive filters may be inadequate due to the large number of coefficients which must be estimated. In this context, reduced rank adaptive filtering is discussed. In this approach, the number of parameters is reduced by restricting the receiver tap vector to belong to a carefully chosen subspace. In this way the number of coefficients to be estimated is significantly reduced with minimal performance loss     

Ratioed power and rate control for CDMA wireless networks

We propose in this paper the ratioed power and rate control (RPRC) algorithm to satisfy the requirements of both the data rate and the signal quality. During the execution of the RPRC algorithm, only the representative user in each cell adjusts its power level according to certain distributed power control algorithm, and the ratio of the power level for the non‐representative user to that for the representative user is kept constant. When the RPRC algorithm is finished, the power levels for all users remain unchanged and the transmission rate is determined by the available rate. We show that the RPRC algorithm can be used for allocating the bandwidth of each cell and achieving the required rate of each user. Furthermore, simulation results reveal that the RPRC algorithm results in larger throughput and finds a feasible power set faster than the other algorithm. Copyright © 2008 John Wiley & Sons, Ltd.     

A data transmission scheme for CDMA wireless networks based onIS-95

Data integration into the existing cellular voice systems is an attractive approach since it can reduce an initial implementation cost of the data network and can provide voice/data integrated services. In this paper, we propose a data transmission scheme for code-division multiple-access (CDMA) cellular voice systems. The CDMA systems under consideration include IS-95 digital cellular systems using the 800-900-MHz band and J-STD-008 personal communications systems using the 1.8-2.0-GHz band. The proposed protocol and control schemes are designed for real systems based on IS-95 rather than conceptual systems and require no modifications to the existing voice mobile stations (MSs). Using computer simulation, the performance of voice and data users is evaluated. As a result, we show that the proposed scheme well meets our design goals, which are that the data transmission scheme should not only provide a good data performance, but also have little effect on the voice performance     

ARC: an integrated admission and rate control framework for competitive wireless CDMA data networks using noncooperative games

The competition among wireless data service providers brings in an option for the unsatisfied customers to switch their providers, which is called churning. The implementation of Wireless Local Number Portability (WLNP) is expected to further increase the churn rate (the probability of users switching the provider). However, the existing resource management algorithms for wireless networks fail to fully capture the far-reaching impact of this unforeseen competitiveness. From this perspective, we first formulate noncooperative games between the service providers and the users. A user's decision to leave or join a provider is based on a finite set of strategies. A service provider can also construct its game strategy set so as to maximize their utility (revenue) considering the churn rate. Based on the game theoretic framework, we propose an integrated admission and rate control (ARC) framework for CDMA-based wireless data networks. The admission control is at the session (macro) level while the rate control is at the link layer packet (micro) level. Two admission control modes are considered - one-by-one mode and batch processing mode, in which multiple users are admitted at a time. We show that: 1) for the one-by-one mode, the Nash equilibrium using pure strategy can be established for both under-loaded and fully-loaded systems and 2) for batch processing mode, there is either an equilibrium in pure strategy or a dominant strategy exists for the service provider. Therefore, the providers have clearly defined admission criteria as outcome of the game. Users are categorized into multiple classes and offered differentiated services based on the price they pay and the service degradation they can tolerate. We show that the proposed ARC framework significantly increases the provider's revenue and also successfully offers differentiated QoS to the users.     

Information-guided channel-hopping for high data rate wireless communication

In this letter, we introduce information-guided channel-hopping, a new scheme for high data rate communication over Rayleigh fading channel using multiple transmit antennas. This scheme is proposed based on the fact that the independence character of multi-channel can be used as an additional information transmitting channel, and the maximum likelihood decoding can be achieved in a simple way by decoupling of the signals conveyed by different methods. The analysis results prove that the capacity behavior of this scheme is better than that of the space-time block coding for more than two transmit antennas.     

Power and rate control with outage constraints in CDMA wireless networks

A radio power control strategy to achieve maximum throughput for the up-link of CDMA wireless systems with variable spreading factor is investigated. The system model includes slow and fast fading, rake receiver, and multi-access interference caused by users with heterogeneous data sources. The quality of the communication is expressed in terms of outage probability, while the throughput is defined as the sum of the users' transmit rates. The outage probability is accounted for by resorting to a lognormal approximation. A mixed integer-real optimization problem P1, where the objective function is the throughput under outage probability constraints, is investigated. Problem P1 is solved in two steps: firstly, we propose a modified problem P2 to provide feasible solutions, and then the optimal solution is obtained with an efficient branch-and-bound search. Numerical results are presented and discussed to assess the validity of our approach.     

Adaptive ordering for imperfect successive decision feedback multiuser detection

A method for selecting the order in which the users are detected in communication systems employing adaptive successive decision feedback multiuser detection is proposed. Systems employing channel coding without the assumption of perfect decision feedback are analyzed. The method is based on the mean squared error (MSE) measurements during a training period for each user. The analysis' shows that the method delivers BER performance improvement relative to other previously proposed ordering methods.     

Distributed approaches for exploiting multiuser diversity in wireless networks

In wireless fading channels, multiuser diversity can be exploited by scheduling users to transmit when their channel conditions are favorable. This leads to a sum throughput that increases with the number of users and, in certain cases, achieves capacity. However, such scheduling requires global knowledge of every user's channel gain, which may be difficult to obtain in some situations. This paper addresses contention-based protocols for exploiting multiuser diversity with only local channel knowledge. A variation of the ALOHA protocol is given in which users attempt to exploit multiuser diversity gains, but suffer contention losses due to the distributed channel knowledge. The growth rate of the sum throughput for this protocol is characterized in a backlogged system under both short-term and long-term average power constraints. A simple "fixed-rate" system is shown to be asymptotically optimal and to achieve the same growth rate as in a system with an optimal centralized scheduler. Moreover, asymptotically, the fraction of throughput lost due to contention is shown to be 1/e. Also, in a system with random arrivals and an infinite user population, a variation of this ALOHA protocol is shown to be stable for any total arrival rate, given that users can estimate the backlog.     

Adaptive Bayesian multiuser detection for synchronous CDMA withGaussian and impulsive noise

We consider the problem of simultaneous parameter estimation and data restoration in a synchronous CDMA system in the presence of either additive Gaussian or additive impulsive white noise with unknown parameters. The impulsive noise is modeled by a two-term Gaussian mixture distribution. Bayesian inference of all unknown quantities is made from the superimposed and noisy received signals. The Gibbs sampler (a Markov chain Monte Carlo procedure) is employed to calculate the Bayesian estimates. The basic idea is to generate ergodic random samples from the joint posterior distribution of all unknown and then to average the appropriate samples to obtain the estimates of the unknown quantities. Adaptive Bayesian multiuser detectors based on the Gibbs sampler are derived for both the Gaussian noise synchronous CDMA channel and the impulsive noise synchronous CDMA channel. A salient feature of the proposed adaptive Bayesian multiuser detectors is that they can incorporate the a priori symbol probabilities, and they produce as output the a posteriori symbol probabilities. (That is, they are “soft-input soft-output” algorithms.) Hence, these methods are well suited for iterative processing in a coded system, which allows the adaptive Bayesian multiuser detector to refine its processing based on the information from the decoding stage, and vice versa-a receiver structure termed the adaptive turbo multiuser detector     

Integrating voice and data applications in T/CDMA architectures

This letter analyzes the behavior of two time-code division architectures serving voice and data applications on the uplink of a cellular radio system. Both solutions attribute speech users one code over a timeslot, whereas data users are granted several codes over the same timeslot or a single code over several time slots. Call admission control dynamically attributes resources if specified levels of transmission quality are met. The blocking probability of the two classes of users is determined, demonstrating that the multicode T/CDMA approach performs better than the multislot solution and than a pure CDMA system     

Adaptive multiuser CDMA detector for asynchronous AWGNchannels-steady state and transient analysis

A two-stage adaptive multiuser detector in an additive white Gaussian noise code-division multiple-access channel is proposed and analyzed. Its first stage is an asynchronous one-shot decorrelator which in terms of computational complexity only requires inversion of K symmetric K×K matrices for all K users. In addition, the K inversions can be done in parallel, and the computed results for one user can be reused by all other users as well, resulting in a latency of only one bit, same as its synchronous counterpart. The decorrelated tentative decisions are utilized to estimate and subtract multiple-access interference in the second stage. Another novel feature of the detector is the adaptive manner in which the multiple-access interference estimates are formed, which renders prior estimation of the received signal amplitudes and the use of training sequences unnecessary. Adaptation algorithms considered include steepest descent (as well as its stochastic version), and a recursive least squares-type algorithm that offers a faster transient response and better error performance. Sufficient conditions for the receiver to achieve convergence are derived. The detector is near-far resistant, and is shown to provide substantial steady-state error performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals     

无线传感器网络中基于小波的自适应多模数据压缩算法

基于数据的多模性,设计了一个基于小波的自适应多模数据压缩算法.在给定的相关度阈值的条件下,算法能自适应地对数据调整分类,对相关数据采用最小二乘估计进行拟合,把特征数据抽象成一个矩阵,利用小波变换去除数据的空间和时间相关性.理论分析和仿真实验表明,新算法能够有效地去除数据之间的多模相关性和同种数据的空间和时间相关性,新算法有效地提高了压缩比,降低了网络的能耗.     

Hierarchy schedule sensing protocol for CDMA wireless networks performance study under multipath, multiuser interference, and collision-capture effect

This paper studies a multiple access control protocol, hierarchy schedule sensing protocol, for CDMA wireless data centric networks characterized by high terminal density in a small area. In particular, this paper focuses on the impact of packet collision with capture effect on performance of a network based on the protocol, where a common-coded permission frame beacon is used to schedule-transmission requests to avoid collisions. To further reduce scheduling delay, hierarchical group-based coding is introduced to the scheme. The performance of such a network is analyzed considering packet collisions, capture effect, multipath-interference (Ml), and multiuser interference (MUI) with the help of a Markovian chain model. The average packet collision-rate with and without capture effect is explicitly derived, the performance of the protocol is compared with traditional receiver-based code protocol to show that, despite its low complexity, it is a fairly robust multiple access control protocol featured by uniformly high throughput and low delay.     

Multicarrier CDMA techniques for future wideband wireless networks

In this paper, multicarrier techniques are considered in the context of the future wideband wireless networks. An overview of the different access schemes based on a combination of code division and multicarrier techniques, such as Multi-Carrier Code Division Multiple Access (MC-CDMA), Multi-Carrier Direct Sequence CDMA (MC-DS-CDMA) and Multi-Tone CDMA (MT-CDMA) is presented first. Afterwards, MC-CDMA systems are considered for the downlink of high rate cellular networks. The performance of different mono-user detection techniques are first analytically estimated and then compared with the help of Monte Carlo simulations for a Rayleigh channel. Simulation results on the performance of mono-user and multi-user detection techniques are also provided for Bad Urban (BU) Hilly Terrain (HT) and Vehicular UMTS channels. With regard to the channel coding, convolutional codes and turbo-codes are considered. Among others, a new linear detection technique named Global Minimum Mean Square Error Algorithm (GMMSE) is compared to multi-user Parallel Interference Cancellation detection technique. It is shown that GMMSE offers good performance especially for non-full load systems. Thus, the efficiency of MC-CDMA as a very promising multiple access and robust modulation scheme is successfully demonstrated for the downlink of the future wideband mobile networks.     

Structured multiuser channel estimation for block-synchronousDS/CDMA

Uplink channel estimation for a block-synchronous chip-asynchronous DS/CDMA system as proposed for the time-division duplex option of third-generation cellular systems is considered. Training midambles are employed for joint channel estimation of all users. The standard unstructured approach based on modeling the effective user channels as unknown FIR filters is compared with two structured methods that exploit a priori knowledge about the user channels such as the maximum delay-spread, the transmit chip-shaping pulse and the path delays. Since these are usually unknown, a low-complexity estimator for the path delays of all users is derived from a maximum-likelihood approach. For all channel estimators, optimal sets of training sequences based on perfect root-of-unity sequences are found. For these optimal sets, it is shown that the reduction in channel estimation mean-squared error of the structured estimator versus the unstructured estimator is exactly the ratio of the number of structured parameters to unstructured parameters. Simulation results show that structured channel estimation provide advantages up to 4 dB in terms of output signal-to-interference plus noise ratio with respect to unstructured estimation, for linear RINSE detection. In contrast, for conventional single-user matched filtering, unstructured estimation proves to be sufficiently good     

CDMA/HDR: a bandwidth efficient high speed wireless data servicefor nomadic users

This article presents an approach to providing very high-data-rate downstream Internet access by nomadic users within the current CDMA physical layer architecture. A means for considerably increasing the throughput by optimizing packet data protocols and by other network and coding techniques are presented and supported by simulations and laboratory measurements. The network architecture, based on Internet protocols adapted to the mobile environment, is described, followed by a discussion of economic considerations in comparison to cable and DSL services     

Real-time algorithms and architectures for multiuser channelestimation and detection in wireless base-station receivers

This paper presents algorithms and architecture designs that can meet real-time requirements of multiuser channel estimation and detection in future code-division multiple-access-based wireless base-station receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their real-time implementation difficult on current digital signal processor-based receivers. A maximum-likelihood based multiuser channel estimation scheme requiring matrix inversions is redesigned from an implementation perspective for a reduced complexity, iterative scheme with a simple fixed-point very large scale integration (VLSI) architecture. A reduced-complexity, bit-streaming multiuser detection algorithm that avoids the need for multishot detection is also developed for a simple, pipelined VLSI architecture. Thus, we develop real-time solutions for multiuser channel estimation and detection for third-generation wireless systems by: (1) designing the algorithms from a fixed-point implementation perspective, without significant loss in error rate performance; (2) task partitioning; and (3) designing bit-streaming fixed-point VLSI architectures that explore pipelining, parallelism, and bit-level computations to achieve real-time with minimum area overhead     

Cross‐layer optimization for CDMA/TDD wireless mesh networks

This paper proposes a new cross‐layer optimization algorithm for wireless mesh networks (WMNs). CDMA/TDD (code division multiple access/time division duplex) is utilized and a couple of TDD timeslot scheduling schemes are proposed for the mesh network backbone. Cross‐layer optimization involves simultaneous consideration of the signal to interference‐plus‐noise ratio (SINR) at the physical layer, traffic load estimation and allocation at medium access control (MAC) layer, and routing decision at the network layer. Adaptive antennas are utilized by the wireless mesh routers to take advantage of directional beamforming. The optimization formulation is subject to routing constraints and can be solved by general nonlinear optimization techniques. Comparisons are made with respect to the classic shortest‐path routing algorithm in the network layer. The results reveal that the average end‐to‐end successful packet rate (SPR) can be significantly improved by the cross‐layer approach. The corresponding optimized routing decisions are able to reduce the traffic congestion. Copyright © 2010 John Wiley & Sons, Ltd.