MMSE receivers for multirate DS-CDMA systems

Minimum-mean squared error (MMSE) receivers are designed and analyzed for multiple data rate direct-sequence code-division multiple-access (DS-CDMA) systems. The inherent cyclostationarity of the DS-CDMA signal is exploited to construct receivers for asynchronous multipath channels. Multiple- and single-bandwidth access are treated for both single and multicarrier scenarios. In general, the optimal receiver is periodically time-varying. When the period of the optimal receiver is large, suboptimal receivers are proposed to achieve a lower complexity implementation; the receivers are designed as a function of the cyclic statistics of the signals. In multiple chipping rate systems, the complexity of receivers for smaller bandwidth users can also be controlled by changing their front-end filter bandwidth. The effect of front-end filter bandwidth on receiver performance and system capacity is quantified for a variable chipping rate system. Analysis and simulation show that significant performance gains are realized by the periodically time-varying MMSE receivers over their time-invariant counterparts     

Analysis of decorrelator-based receivers for multirate DS/CDMAcommunications

Decorrelator-based receivers have been investigated for demodulating data in a dual-rate synchronous direct-sequence code-division multiple-access (DS/CDMA) system. The proposed receivers were a high-rate decorrelator (HRD) matched to the data rate of the high-rate users and a low-rate decorrelator (LRD) matched to the data rate of the low-rate users. In this paper, the probabilities of error attainable for high-rate and low-rate users with the use of the HRD and the LRD are analyzed. It is proven that the LRD offers superior performance to that of the HRD for all users. Asymptotic analysis provides bounds for the performance difference for high-rate users. The results are generalized to multirate systems. To increase fidelity, a sliding-window decorrelator (SWD) is proposed which demodulates a high-rate user's data by a soft-decoding rule from the outputs of several decorrelators sliding along the received signal sequence. The results show that it performs better than the HRD while maintaining smaller demodulation delay and computational complexity than the LRD. To further exploit the characteristics of multirate systems, a decorrelating decision-feedback detector is proposed and its asymptotic multiuser efficiency is analyzed. It is shown that this detector incurs little demodulation delay for high-rate users and provides better performance for low-rate users than that of the LRD when the energies of the interfering users are comparable to that of the desired user     

The impact of timing errors on the performance of linear DS-CDMAreceivers

In this paper, an asynchronous direct-sequence code-division multiple-access (DS-CDMA) communication system operating over an additive white Gaussian noise (AWGN) channel is considered. In many applications, the near-far problem can be the limiting factor for the capacity of a DS-CDMA system. Several near-far resistant receivers have, therefore, been proposed (e.g., the decorrelating receiver). These receivers assume perfect knowledge of the propagation delay from all users to the receiver. In practice, the delays are estimated and therefore subject to errors. The performance degradation these errors impose on linear detectors, especially the decorrelating detector, is the topic of this paper     

码分多址系统中多速率方案的分析和比较

本文讨论了直接序列－码分多址（ＤＳ－ＣＤＭＡ）系统中的几种多速率方案,分别分析了它们在加性白高斯噪声和多径瑞利衰落条件下的性能。对这几种方案在瑞利衰落信道下的性能做了仿真,结果表明,多扩频增益方案与多信道方案性能接近,多调制方案在信息速率较高时,性能不如其它方案。     

Distributed resource allocation for DS-CDMA-based multimedia ad hocwireless LANs

Power control in direct sequence code division multiple access (DS-CDMA) systems and power/rate allocation in multirate DS-CDMA based networks is an open and interesting research area which has attracted much attention. However, with a few exceptions, most researchers have emphasized centralized resource allocation algorithms for cellular systems where the base station keeps track of the requirements of the various users and is thus responsible for the management of network resources. Ad hoc wireless local area networks (WLANs), on the other hand, are generally configured as peer-to-peer networks with no centralized hub or controller. Thus resource allocation has to be conducted in a distributed fashion. We address the issue of distributed resource management for multirate DS-CDMA based multimedia WLANs by (1) presenting a distributed resource allocation protocol, known as distributed resource negotiation protocol (DRNP) that builds on the RTS/CTS bandwidth reservation mechanism provided by IEEE 802.111, and provides quality of service (QoS) guarantees through distributed control of resources in DS-CDMA based multimedia WLANs and (2) investigating the performance of various resource allocation schemes within the context of DRNP, in terms of network wide metrics such as overall throughput and blocking rates     

Comparison of maximum-likelihood-based detection for two multirateaccess schemes for CDMA signals

Future wireless systems will need to accommodate information sources with different data rates. Direct-sequence code-division multiple-access (DS/CDMA) is a multiple access technique that is well suited to provide multirate access. Thus, in this paper, multirate communication systems are considered for the transmission of DS/CDMA wireless signals. Performance for maximum-likelihood-based detection is studied in the context of two multirate access methodologies: multicode access, where high data rate users multiplex their information streams onto multiple codes; and variable spreading length access where signature sequences of different lengths are assigned to users with different data rates. Various maximum-likelihood-based detection schemes for the variable spreading length system are considered as they can achieve near-optimal performance and thus provide reference points for comparison with suboptimal schemes. In addition, asymptotic multiuser performance measures are calculated and bounded to compare performance of the two systems     

Multistage linear receivers for DS-CDMA systems

A new family of multistage low-complexity linear receivers for direct sequence code division multiple access (DS-CDMA) communications is introduced. The objective of the proposed design is to mitigate the effect of multiple access interference (MAI), the most significant limiting factor of user capacity in the conventional DS-CDMA channel. The receivers presented here employ joint detection of multiple users and therefore require knowledge of all the signature codes and their timing. In addition, for a multipath environment, reliable estimates of the received powers and phases are assumed available for maximal ratio RAKE combining. Each stage of the underlying design recreates the overall modulation, noiseless channel, and demodulation process. The outputs of these stages are then linearly combined. The combining weights can be chosen to implement different linear detectors, including the decorrelating and minimum mean square error (MMSE) detectors. In this paper, we focus on implementing the MMSE detector. Simulation results illustrate that significant performance gains can be achieved in both synchronous and asynchronous systems.This work was presented in part at IEEE Communication Theory Workshop, April 23–26, 1995, and at IEEE MILCOM '95, November 5–8, 1995.This work was submitted in partial fulfillment of Ph.D. requirements at The City University of New York.     

Rate-Distortion Optimized Video Transmission Over DS-CDMA Channels with Auxiliary Vector Filter Single-User Multirate Detection

In this paper, we consider the rate-distortion optimized resource allocation for video transmission over multi-rate wireless direct-sequence code-division-multiple-access (DS-CDMA) channels. We consider the performance of transmitting scalable video over a multipath Rayleigh fading channel via a combination of multi-code multirate CDMA and variable sequence length multirate CDMA channel system. At the receiver, despreading is done using adaptive space-time auxiliary-vector (AV) filters. We propose a new interference cancelling design that uses just a single AV filter for single-user mutirate despreading. Our experimental results show that the proposed interference cancelling design has excellent performance in scalable video transmission over DS-CDMA systems that use a combination of multicode multirate and variable processing gain multirate CDMA. The proposed design takes advantage of the fact that single user's video data is transmitted using two spreading codes, one for the base layer and one for the enhancement layers, and of the fact that these spreading codes can have different processing gains. The proposed interference cancelling design is compared with two conventional single-user multirate CDMA receiver configurations, however now we use an AV filter rather than a simple matched filter. We also propose a resource allocation algorithm for the optimal determination of source coding rate, channel coding rate and processing gain for each scalable layer, in order to minimize the expected distortion at the receiver.     

BER Performance of Walsh Hadamard Like Codes Based on Complementary Sequence Sets in a CDMA and Cognitive Underlay System

In this paper, a method of constructing a sequence set similar to Walsh Hadamard (WH) codes is described. In general, two complementary sequence set of length 2 is used as a basic kernel to construct WH codes. In this work, four complementary sequence set of length 3 is used as a basic kernel. The proposed code set have a nearly non linear phase sequences with a more evenly spread frequency spectrum when compared with WH codes. It is shown that the proposed spreading codes perform comparable to WH in a Direct Sequence code division multiple access (DS-CDMA) system in an Additive White Gaussian Noise and Rayleigh channel at a reduced code length. In this work, bit error rate performance of the Underlay Cognitive Radio system implemented using DS-CDMA system is simulated. Primary and Secondary Users are distinguished by assigning spreading codes from different families and it is assumed that the channel information is known by both the Primary and Secondary User receivers. This new code set offers good performance in a Cognitive Radio Underlay system.     

Space-Time Adaptive MMSE Multiuser Decision Feedback Detectors With Multiple-Feedback Interference Cancellation for CDMA Systems

In this paper, we propose a novel space-time minimum mean square error (MMSE) decision feedback (DF) detection scheme for direct-sequence code-division multiple access (DS-CDMA) systems with multiple receive antennas, which employs multiple-parallel-feedback (MPF) branches for interference cancellation. The proposed space-time receiver is then further combined with cascaded DF stages to mitigate the deleterious effects of error propagation for uncoded schemes. To adjust the parameters of the receiver, we also present modified adaptive stochastic gradient (SG) and recursive least squares (RLS) algorithms that automatically switch to the best-available interference cancellation feedback branch and jointly estimate the feedforward and feedback filters. The performance of the system with beamforming and diversity configurations is also considered. Simulation results for an uplink scenario with uncoded systems show that the proposed space-time MPF-DF detector outperforms existing schemes such as linear, parallel DF (P-DF), and successive DF (S-DF) receivers in terms of bit error rate (BER) and achieves a substantial capacity increase in terms of the number of users, compared with the existing schemes. We also derive the expressions for MMSE achieved by the analyzed DF structures, including the novel scheme, with imperfect and perfect feedback and expressions of signal-to-interference-plus-noise ratio (SINR) for the beamforming and diversity configurations with linear receivers.     

A noncooperative power control game for multirate CDMA data networks

The authors consider a multirate code-division multiple acess system, in which all users have the same chip rate and vary their data rate by adjusting the processing gain. The receivers are assumed to be implemented using conventional matched filters, whose performance is sensitive to the received power levels. The authors' goal is to maximize the total system throughput by means of power control. A game theoretic approach is adopted. It is shown that for a certain type of pricing function, a unique Nash equilibrium solution exists and it possesses nice global properties. For example, it can be shown that for the optimal solution a high-rate connection should maintain a higher energy per bit than low-rate ones. The asymptotic spectral efficiency is also derived.     

Optimal decorrelating receivers for DS-CDMA systems: a signalprocessing framework

Code division multiple access (CDMA) schemes allow a number of asynchronous users to share a transmission medium with minimum cooperation among them. However, sophisticated signal processing algorithms are needed at the receiver to combat interference from other users and multipath effects. A discrete-time multirate formulation is introduced for asynchronous CDMA systems, which can incorporate multipath effects. This formulation reveals interesting links between CDMA receivers and array processing problems. In this framework, linear receivers are derived that can completely suppress multiuser interference (decorrelating receivers). A criterion is introduced, which guarantees the decorrelating property, while providing optimal solutions in the presence of noise. Parametric FIR designs as well as nonparametric solutions are delineated, and their performance is analyzed. The proposed receivers are resistant to near-far effects and do not require the estimation of the users' and noise powers     

Slow frequency-hopping multicarrier DS-CDMA for transmission overNakagami multipath fading channels

A novel multiple access scheme based on slow frequency hopping multicarrier direct-sequence code division multiple access (SFH/MC DS-CDMA) is proposed and investigated, which can be rendered compatible with the existing second-generation narrowband CDMA and third-generation wideband CDMA systems. The frequency hopping patterns are controlled by a set of constant-weight codes. Consequently, multirate communications can be implemented by selecting the corresponding sets of constant-weight codes having the required weights controlling the SFH patterns invoked. Two FH schemes, namely random and uniform FH, are considered and their advantages as well as disadvantages are investigated. We assume that the system operates in a multipath fading environment and a RAKE receiver structure with maximum ratio combining (MRC) is used for demodulation. The system's performance is evaluated over the range of multipath Nakagami (1960) fading channels, under the assumption that the receiver has all explicit knowledge of the associated frequency-hopping (FH) patterns invoked. Furthermore, the performance of the SFH/MC DS-CDMA system is compared to that of the conventional single-carrier (SC) DS-CDMA system and that of the conventional MC DS-CDMA system, under the assumptions of constant system bandwidth and of constant transmitted signal power     

A new power control function for multirate DS-CDMA systems

This paper proposes a closed-form power control function for the reverse link of a multirate single chip-rate variable processing gain DS-CDMA system in a mobile radio environment that assumes a Rayleigh fading channel with log-normal shadowing and path loss. A closed-form open-loop power control function based on a newly defined traffic exponent is proposed, and nonlinear programming is used to perform the optimization. In addition, a user model that allows users to dynamically switch traffic rates for different connection applications is implemented. Results obtained using random chip sequences demonstrate improvement in the system capacity with the new power control function compared to the conventional power control function. Furthermore, the proposed function also simplifies the power control processing     

A Low Complexity Downlink Beamforming Scheme for DS-CDMA System

The downlink capacity of frequency division duplex (FDD) based DS-CDMA system can be improved if multielement antenna array is equipped at the base station. In this paper, a novel two-path downlink beamforming (TPDB) scheme is proposed to reduce multiple access interference as well as providing two-order path diversity in downlink. An analysis model is presented for the capacity evaluation of multirate DS-CDMA system with base station antenna array, and the capacity performance of the TPDB is compared with other schemes based on this model. The comparison results show that the TPDB would be a promising candidate for the downlink transmission if both performance and implementation complexity are considered. Moreover, a simple algorithm is proposed for the steering vector estimation in the TPDB, and the robustness of this estimation algorithm in the presence of fading and interference is also confirmed by computer simulations.     

Design and Evaluation of Multichannel Multirate Wireless Networks

In a multirate wireless network, low data rate nodes consume proportionately more channel resources than high data rate nodes, resulting in low overall network performance. The use of multiple non-overlapping frequency channels in multirate wireless networks can overcome the performance degradation by having nodes communicate on different channels based on their data rates. However, no effort has been invested to utilize the multiple channels for a multirate wireless network. In this paper, we introduce the Data Rate Adaptive Channel Assignment (DR-CA) algorithm for a multichannel multirate single-hop wireless network to provide higher network throughput and network efficiency. The main idea is to assign links having same or comparable data rates on the same channel to minimize the wastage of channel resources due to interference between high data links and low data rate links. We also design a new Intermediary Multichannel Layer (IML) which resides between network layer and link layer, at which we implement the DR-CA algorithm. The IML design requires no modifications to the underlying MAC layer and upper layers of the network stack. To evaluate the proposed algorithm we define new performance metrics—channel efficiency and network efficiency for a multichannel multirate wireless network. Using OPNET simulations, we show that the multichannel enhancement using our proposed algorithm provides significant performance improvement in terms of network throughput, channel efficiency, and network efficiency over existing approaches in multirate wireless networks. Under heavy load condition, the network efficiency using DR-CA algorithm reaches 90% of the maximum limit. To the best of our knowledge, this is the first work to utilize the benefits of multiple channels in the multirate wireless network environment.     

OMAC: A new access control architecture for overlay multicast communications

Multicast communications concern the transfer of data among multiple users. Multicast communications can be provided at the network layer—an example is IP multicast—or at the application layer, also called overlay multicast. An important issue in multicast communications is to control how different users—senders, receivers, and delivery nodes—access the transmitted data as well as the network resources. Many researchers have proposed solutions addressing access control in IP multicast. However, little attention has been paid to overlay multicast. In this paper, we investigate the access control issues in overlay multicast and present OMAC: a new solution to address these issues. OMAC provides access control for senders, receivers, and delivery nodes in overlay multicast. The proposed architecture, which is based on symmetric key cryptosystem, centralizes the authentication process in one server whereas it distributes the authorization process among the delivery nodes. Moreover, delivery nodes are utilized as a buffer zone between end systems and the authentication server, making it less exposed to malicious end systems. To evaluate our work, we have used simulation to compare the performance of OMAC against previous solutions. Results of the simulation show that OMAC outperforms previous multicast access control schemes. Copyright © 2010 John Wiley & Sons, Ltd.     

A practical transmit beamforming strategy for closed‐loop MIMO communication

A new beamforming strategy is proposed for multiuser systems with N transmit antennas at the transmitter and M ? N single antenna receivers. The proposed scheme remarkably improves on the classical spatial division multiple access, and achieves the same data rates as spatial multiplexing for all users but with significantly superior performance/diversity gain. When compared with the Bell labs layered space–time system, the symbol rate is the same and the performance is much superior because of the presence of diversity gain. In addition, unlike the Bell labs layered space–time system, the receivers do not need to know each other's vector channels. Finally, the proposed algorithm is based on dirty‐paper coding, but does not require much complexity and is implementable. Copyright © 2011 John Wiley & Sons, Ltd.     

Power Control for Link Quality Protection in Cellular DS-CDMA Networks with Integrated (Packet and Circuit) Services

The problem of admission control in a DS-CDMA network carrying a heterogeneous mix of traffic is addressed. In an interference limited system such as DS-CDMA, admission of a new user impacts the performance of all other users, as well as the system capacity. The admission process is concerned with two factors: (1) maintaining the QoS of active users, (2) allocating bandwidth to new users. We propose a simple power control algorithm and prove that it is optimal in the sense of maintaining active link quality while maximizing free capacity for new admissions. The algorithm scales the powers of active links appropriately to achieve link protection and improved tolerance of the link to new interference from bursty sources. This algorithm can be used to overlay bursty packet data services without compromising QoS of active circuits.     

Signal Processing by Generalized Receiver in?DS-CDMA Wireless Communication Systems with?Frequency-Selective Channels

The generalized receiver (GR) based on a generalized approach to signal processing (GASP) in noise is investigated in a direct-sequence code-division multiple access (DS-CDMA) wireless communication system with frequency-selective channels. We consider four avenues: linear equalization with finite impulse response (FIR) beamforming filters; channel estimation and spatially correlation; optimal combining; and partial cancellation. We investigate the GR with simple linear equalization and FIR beamforming filters. Numerical results and simulation show that the GR with FIR beamforming filters surpasses in performance the optimum infinite impulse response beamforming filters with conventional receivers, and can closely approach the performance of GR with infinite impulse response beamforming filters. Channel estimation errors are taken into consideration so that DS-CDMA wireless communication system performance will not be degraded under practical channel estimation. GR takes an estimation error of a maximum likelihood (ML) multiple-input multiple-output (MIMO) channel estimation and GR spatially correlation into account in computation of minimum mean square error (MMSE) and log-likelihood ratio (LLR) of each coded bit. The symbol error rate (SER) performance of DS-CDMA employing GR with a quadrature sub-branch hybrid selection/maximal-ratio combining (HS/MRC) scheme for 1-D modulations in Rayleigh fading is obtained and compared with that of conventional HS/MRC receivers. Procedure of selecting a partial cancelation factor (PCF) for the first stage of a hard-decision partial parallel interference cancellation (PPIC) of the GR employed in DS-CDMA wireless communication system is proposed. A range of optimal PCFs is derived based on the Price’s theorem. Computer simulation results show superiority in bit error rate (BER) performance that is very close to that potentially achieved and surpasses the BER performance of the real PCF for DS-CDMA systems discussed in literature.