Shirish Nagaraj

Set-membership adaptive equalization and an updator-shared implementation for multiple channel communications systems

This paper considers the problems of channel estimation and adaptive equalization in the novel framework of set-membership parameter estimation. Channel estimation using a class of set-membership identification algorithms known as optimal bounding ellipsoid (OBE) algorithms and their extension to tracking time-varying channels are described. Simulation results show that the OBE channel estimators outperform the least-mean-square (LMS) algorithm and perform comparably with the RLS and the Kalman filter. The concept of set-membership equalization is introduced along with the notion of a feasible equalizer. Necessary and sufficient conditions are derived for the existence of feasible equalizers in the case of linear equalization for a linear FIR additive noise channel. An adaptive OBE algorithm is shown to provide a set of estimated feasible equalizers. The selective update feature of the OBE algorithms is exploited to devise an updator-shared scheme in a multiple channel environment, referred to as updator-shared parallel adaptive equalization (USHAPE). U-SHAPE is shown to reduce the hardware complexity significantly. Procedures to compute the minimum number of updating processors required for a specified quality of service are presented.

Adaptive multiuser detection and beamforming for interference suppression in CDMA mobile radio systems

This paper considers the problem of interference suppression in direct-sequence code-division multiple-access (DS-CDMA) systems over fading channels. An adaptive array receiver is presented which integrates multiuser detection, beamforming, and RAKE reception to mitigate cochannel interference and fading. The adaptive multiuser detector is formulated using a blind constrained energy minimization criterion and adaptation is carried out using a novel algorithm based on set-membership parameter estimation theory. The proposed detector overcomes the shortcomings of conventional LMS- and RLS-type algorithms, namely, that of slow convergence and large computational load, respectively. This is especially the case when strong interferers are present or when the number of adaptive weights is relatively large. DS-CDMA systems can have a relatively large number of spatially distributed interferers. Thus beamforming is based on direction-of-arrival (DOA) estimates provided by an approximate maximum-likelihood estimator (DOA-MLE). Unlike previous approaches, the DOA-MLE exploits the structure of the DS-CDMA signaling scheme resulting in robust performance and simple implementation in the presence of angle spreading. The overall method is suitable for real-time implementation and can substantially improve the interference suppression capabilities of a CDMA system.

Intelligent antenna solutions for UMTS: algorithms and simulation results

This article outlines the development of intelligent antenna (IA) solutions for UMTS, a third-generation W-CDMA system. Since the selection of an antenna configuration paired with realizable uplink/downlink algorithms that can satisfy all operating environments is a broad task, this article focus is on cost-effective antenna arrays for macrocells. Algorithms that exploit the antenna configurations and act at both the physical and MAC layers are highlighted and supported by simulation results. Two solutions stand out for UMTS: a universal beamforming algorithm that unifies user-specific and fixed beamforming under one framework, and multibeam scheduling (MBS) that significantly increases downlink packet data throughput using the concept of code reuse in conjunction with beamforming. The article summarizes the critical issues that were faced in the development of an IA solution capable of delivering the theoretically promised benefits to end users.

A rate control algorithm for uplink high-speed packet data transmission in UMTS

High speed data services are becoming increasingly important in the uplink of wireless communication systems in order to support applications such as FTP, E-Mail, and gaming. In Release 6 of the universal mobile telecommunications system (UMTS) standard, an enhanced dedicated channel (E-DCH) has been introduced to provide high-speed packet data transmission in the uplink. This channel has the capability to provide high data rate services, both for best-effort and delay-sensitive applications, by introducing scheduling of users with fast resource allocation at the base station. In this paper, we propose an efficient uplink resource allocation algorithm comprising an overload control algorithm, a rate control algorithm, and a soft handoff (SHO) load control algorithm. System-level simulation results are presented that demonstrate the performance of the proposed algorithm in a multi-cell, multi-user environment. Our simulation result shows that scheduling many users with similar rates leads to a desirable short rise-over-thermal (RoT) tail behavior and better user fairness than scheduling a few high data rate users. The performance analysis of the proposed SHO load control algorithm shows that the algorithm is effective in controlling the tradeoff between cell throughput and user fairness.

Multiple antenna transmission with channel state information: a low-rate feedback approach

This paper presents a novel multiple antenna transmission scheme that depends on channel state information. The proposed approach uses low-rate feedback to adaptively estimate the downlink fading channel at the base-station. The optimal transmission scheme given channel state information can in turn be determined from these channel estimates. It is shown that the channel estimation requires selective feedback of the adaptive filter prediction errors, that can be computed at the mobile by receiving a beamformed pilot signal. To this end, a randomized channel inversion pilot beamforming scheme is developed that makes possible this feedback of prediction errors possible. Further, this method is shown to be relatively insensitive to feedback quantization effects for a given average feedback rate. Simulation results indicate that the feedback scheme provides significant closed-loop beamforming gains with low bit rate feedback even for large number of transmit antennas.

A minimax approach to open-loop downlink beamforming

A new method for downlink beamforming is analyzed in this paper. In this open-loop solution, a minimax algorithm based on optimal set membership filtering is utilized to synthesize a wide and power efficient beam. It is shown that the robustness of this method against angle spread and error of DOA estimation is greater than that of the classical methods.

Low complexity adaptive receiver for CDMA with multipath fading

A new receiver scheme for a DS-CDMA system similar to the IS-95 uplink is proposed in this paper that is of lower complexity and superior bit error rate performance in comparison to the conventional non-coherent RAKE receiver. The proposed receiver consists of a coherent RAKE receiver in the form of a transversal filter, and uses channel estimates obtained by an adaptive channel identification algorithm. Adaptation follows the set-membership filtering methodology, that features fast convergence and lower computational complexity compared to least-squares techniques.

Optimal Set-Membership Filtering Using an Adaptive Minimax Algorithm with Automatic Bound Tuning

Abstract This paper shows that optimal set-membership filtering (SMF) is identical to minimax (l∞) filtering, if the optimal SMF corresponds to the smallest error bound specification that leads to a non-empty feasible solution. An adaptive minimax filtering algorithm that uses optimal quadratic approximations to the minimax cost function is proposed for adaptive implementation of optimal SMF. The proposed algorithm automatically tunes the error bound in an optimal manner, and can also be used for set-membership identification (SMI) when the noise bounds are not known. Simulation results verify the convergence of the proposed algorithm to the minimax solution.