Adaptive DS-CDMA multiuser detection for time variant frequency selective Rayleigh fading channel

The current digital wireless mobile system such as IS-95, which is based on direct sequence Code Division Multiple Access (DS-CDMA) technology, will not be able to meet the growing demands for multimedia service due to low information exchanging rate. Its capacity is also limited by multiple accessed interference (MAI) signals. This work focuses on the development of adaptive algorithms for multiuser detection (MUD) and interference suppression for wideband direct sequence code division multiple access (DS-CDMA) systems over time-variant frequency selective fading channels. In addition, channel acquisition and delay estimation techniques are developed to combat the uncertainty introduced by the wireless propagation channel. This work emphasizes fast and simple techniques that can meet practical needs for high data rate signal detection. Most existing literature is not suitable for the large delay spread in wideband systems due to high computational/ hardware complexity. A de-biasing decorrelator is developed whose computational complexity is greatly reduced without sacrificing performance. An adaptive bootstrap symbolbased signal separator is also proposed for a time-variant channel. These detectors achieve MUD for asynchronous, large delay spread, fading channels without training sequences. To achieve high data rate communication, a finite impulse response (FIR) filter based detector is presented for M-ary QAM modulated signals in a multipath Rayleigh fading channel. It is shown that the proposed detector provides a stable performance for QAM signal detection with unknown fading and phase shift. It is also shown that this detector can be easily extended to the reception of any M-ary quadrature modulated signal. A minimum variance decorrelating (MVD) receiver with adaptive channel estimator is presented in this dissertation. It provides comparable performance to a linear MMSE receiver even in a deep fading environment and can be implemented blindly. Using the MVD receiver as a building-block, an adaptive multistage parallel interference cancellation (PIC) scheme and a successive interference cancellation (SIC) scheme were developed. The total number of stages is kept at a minimum as a result of the accurate estimating of the interfering users at the earliest stages, which reduces the implementation complexity, as well as the processing delay. Jointly with the MVD receiver, a new transmit diversity (TD) scheme, called TD-MVD, is proposed. This scheme improves the performance without increasing the bandwidth. Unlike other TD techniques, this TDMVD scheme has the inherent advantage to overcome asynchronous multipath transmission. It brings flexibility in the design of TD antenna systems without restrict signal coordination among those multiple transmissions, and applicable for both existing and next generation of CDMA systems. A maximum likelihood based delay and channel estimation algorithm with reduced computational complexity is proposed. This algorithm uses a diagonal simplicity technique as well as the asymptotically uncorrelated property of the received signal in the frequency domain. In combination with oversampling, this scheme does not suffer from a singularity problem and the performance quickly approaches the Cramer-Rao lower bound (CRLB) while maintaining a computational complexity that is as low as the order of the signal dimension

Interference suppression and diversity for CDMA systems

In code-division multiple-access (CDMA) systems, due to non-orthogonality of the spreading codes and multipath channels, the desired signal suffers interference from other users. Signal fading due to multipath propagation is another source of impairment in wireless CDMA systems, often severely impacting performance. In this dissertation, reduced-rank minimum mean square error (MMSE) receiver and reduced-rank minimum variance receiver are investigated to suppress interference; transmit diversity is applied to multicarrier CDMA (MC-CDMA) systems to combat fading; packet combing is studied to provide both interference suppression and diversity for CDMA random access systems. The reduced-rank MMSE receiver that uses a reduced-rank estimated covariance matrix is studied to improve the performance of MMSE receiver in CDMA systems. It is shown that the reduced-rank MMSE receiver has much better performance than the full-rank MMSE receiver when the covariance matrix is estimated by using a finite number of data samples and the desired signal is in a low dimensional subspace. It is also demonstrated that the reduced-rank minimum variance receiver outperforms the full-rank minimum variance receiver. The probability density function of the output SNR of the full-rank and reduced-rank linear MMSE estimators is derived for a general linear signal model under the assumption that the signals and noise are Gaussian distributed. Space-time coding that is originally proposed for narrow band systems is applied to an MC-CDMA system in order to get transmit diversity for such a wideband system. Some techniques to jointly decode the space-time code and suppress interference are developed. The channel estimation using either pilot channels or pilot symbols is studied for MC-CDMA systems with space-time coding. Performance of CDMA random access systems with packet combining in fading channels is analyzed. By combining the current retransmitted packet with all its previous transmitted copies, the receiver obtains a diversity gain plus an increased interference and noise suppression gain. Therefore, the bit error rate dramatically decreases with the number of transmissions increasing, which in turn improves the system throughput and reduces the average delay

Interference characterization and suppression for multiuser direct-sequence spread-spectrum system

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 175-184).In this thesis we investigate efficient modulation and detection techniques for the uplink (i.e. transmission from mobile to base station) of a DS-CDMA network. Specifically, the thesis contains three parts. In the first part, we focus on the mobile transmitter. In particular, we evaluate and compare the spectral efficiency of two promising variable rate DS-CDMA transmission techniques, multicode (MCD) and variable-spreading-gain (VSG), under the presence of multiple-access (user-to-user) interferences (MAI) and multipath interferences. The uniqueness of our study is that in bit-error-rate evaluation, instead of approximating the interference as Gaussian noise (which has been done in most of the previous studies), we incorporate both power and distribution of interferences into consideration. We show where the Gaussian assumption may give misleading answers and how our results in these cases are different from those obtained in the past. In part two and three of the thesis, we focus on the base station receiver. Specifically, we present effective joint detection techniques that have good performance-complexity tradeoff. Part two of the thesis introduces a class of novel multistage parallel interference cancellation algorithms based on stage-by-stage minimum mean-squared error (MMSE) optimization. We show that this scheme is capable of achieving significantly better performance than other algorithms with similar complexity. Part three of the thesis presents a low-complexity dual-mode multiuser detector that dynamically switches its detection mode between the matched-filter receiver and the decorrelator. We show that this detector is capable of achieving the performance of a decorrelator but with significant savings in processing power and complexity.by Mingxi Fan.Ph.D

Advanced receivers for high data rate mobile communications

Improving the spectral efficiency is a key issue in the future wireless communication systems since the spectrum is a scarce resource. Both the number of users as well the demanded data rates are increasing all the time. Furthermore, in mobile communications the wireless link is required to be reliable even when the mobile is in a fast moving vehicle. Using Multiple-Input Multiple-Output (MIMO) antennas is a well known technique to provide higher spectral efficiency as well as better link reliability. Additionally, higher order modulation methods can be used to provide higher data rates. In order to benefit from these enhancements in practise, sophisticated signal processing methods as well as accurate estimates of time-varying wireless channel parameters are needed. This thesis addresses the problem of designing multi-antenna receivers in high data rate systems. The case of multiple transmit antennas is also considered. System specific features of High Speed Downlink Packet Access (HSDPA) which is part of 3rd generation (3G) Wideband Code Division Multiple Access (WCDMA) evolution are exploited in channel estimation methods and in MIMO receiver design. Additionally, complexity reduction methods for Minimum Mean Square Error (MMSE) equalization are addressed. Blind channel estimation methods are spectrally efficient, since no extra resources are needed for pilot signals. However, in mobile communications accurate estimates are needed also in fast fading channels. Consequently, semi-blind channel estimation methods where the receiver combines blind and pilot based channel estimation are an appealing alternative. In this thesis blind and semi-blind channel estimation methods based on knowledge of multiple spreading codes are derived. A novel semi-blind combining scheme for code multiplexed pilot signal and blind estimation is proposed. Another important factor in receiver design criteria is the structure of interference in the received signals. Interference mitigation techniques in MIMO systems have been shown to be potential methods for providing improved performance. A chip level inter-antenna interference cancellation method has been developed in this thesis for HSDPA. Furthermore, this multi-stage ordered interference canceler is combined with the semi-blind channel estimation scheme to enhance the system performance further.Langattomassa tiedonsiirrossa radiospektrin tehokas käyttö on tulevaisuuden suuria haasteita. Taajuuksia on käytössä vain rajoitetusti, kun taas käyttäjien määrä sekä vaaditut siirtonopeudet kasvavat jatkuvasti. Lisäksi langattomien yhteyksien on toimittava luotettavasti myös nopeasti liikkuvissa kulkuneuvoissa. Moniantennijärjestelmät, joissa on useita antenneita sekä tukiasemissa että päätelaitteissa mahdollistavat radiospektrin tehokkaamman käytön sekä parantavat yhteyksien laatua. Tiedonsiirtonopeutta voidaan myös kasvattaa erilaisilla modulaatiotekniikoilla. Hyötyjen saavutamiseksi käytännössä tarvitaan sekä kehittyneitä vastaanotinrakenteita että tarkkoja estimaatteja aikamuuttuvasta radiokanavasta. Tässä työssä on kehitetty vastaanotinrakenteita ja kanavan estimointimenetelmiä kolmannen sukupolven (3G) nopeiden datayhteyksien (HSPA) järjestelmissä. Työssä on johdettu menetelmiä, jotka hyödyntävät HSPA järjestelmien erikoispiirteitä tehokkaasti. Lisäksi on kehitetty laskennallisesti tehokkaita menetelmiä vastaanottimien signaalinkäsittelyyn. Ns. sokeat menetelmät mahdollistavat taajuuskaistan tehokkaan käytön, koska ne eivät vaadi tunnettuja harjoitussignaaleja. Mobiileissa tietolikennejärjestelmissä radiokanava saattaa kuitenkin muuttua hyvin nopeasti, jonka vuoksi kanavan estimoinnissa on tyypillisesti hyödynnetty tunnettua pilottisignaalia. Yhdistämällä pilottipohjainen ja sokea kanavaestimointimenetelmä, voidaan saavuttaa molempien menetelmien edut. Tässä työssä kehitettiin sokeita kanavaestimointimenetelmiä, jotka hyödyntävät useita tunnettuja hajoituskoodeja. Sokean ja koodijakoiseen pilottisignaaliin pohjautuvien kanavan estimaattien yhdistämiseksi kehitettiin uusi menetelmä. Signaalin laatua ja siten vastaanottimen suorituskykyä voidaan langattomissa järjestelmissä parantaa vaimentamalla interferenssiä eli häiriöitä. Vastaanottimen toimintaa voidaan tehostaa oleellisesti, jos häiriösignaalin rakenne tunnetaan. Käytettäessä useampaa lähetysantennia HSPA järjestelmissä vastaanotetussa signaalissa olevia häiriötä voidaan kumota usealla eri tasolla. Tässä työssä on kehitetty chippitasolla häiriöitä kumoava vastaanotinrakenne, joka hyödyntää HSPA järjestelmän ominaisuuksia. Vastaanottimen suorituskykyä on edelleen parannettu yhdistämällä se aiemmin esitettyyn puolisokeaan kanavan estimointimenetelmään.reviewe

High Capacity CDMA and Collaborative Techniques

The thesis investigates new approaches to increase the user capacity and improve the error performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation and collaborative spreading and space diversity techniques. Collaborative Coding Multiple Access (CCMA) is also investigated as a separate technique and combined with CDMA. The advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the uplink and downlink are proposed and evaluated. Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM) property of digital signals to blindly suppress interference during the despreading process and obtain amplitude estimation with minimum mean squared error for use in cancellation stages. Two new blind adaptive receiver designs employing successive and parallel interference cancellation architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respectively, are presented. These techniques have shown to offer near single user performance for large number of users. It is shown to increase the user capacity by approximately two fold compared with conventional IC receivers. The spectral efficiency analysis of the techniques based on output signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore, an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI without any knowledge of channel amplitudes and allows large number of users compared with equal gain and maximum ratio combining techniques normally used in practice. New user collaborative schemes are proposed and analysed theoretically and by simulations in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First, a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA are designed and analysed. Next, a new user collaborative scheme with successive interference cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce MAI and achieve improved diversity. To further improve the performance of C-SIC under high system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed. It is shown to minimize the residual MAI, leading to improved user capacity and a more robust system. It is known that collaborative diversity schemes incur loss in throughput due to the need of orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time encoding methods. At the receiver collaborative joint detection is performed to separate each paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth as inter-user channel SNR becomes high. A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA for both the downlink and uplink. First, collaborative group spreading technique for the downlink of overloaded CDMA system is introduced. It allows the sharing of the same single spreading sequence for more than one user belonging to the same group. This technique is referred to as Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative coding is used for each group to form a composite codeword signal of the users and then a single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite codeword is carried out to extract the user’s own information while maintaining a high SINR performance. To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions, Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative coding multiple access and space-time coding principles. A new scheme for uplink of CDMA using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved by grouping multiple users to share the same spreading sequence and performing MUD on per group basis followed by a low complexity ML decoding at the receiver. This approach has shown to support much higher number of users than the available sequences while also maintaining the low receiver complexity. For improved performance under highly correlated channel conditions, T-user collaborative coding is also investigated within the CS-CDMA-UL system

Trickle-based interference cancellation schemes for CDMA systems

In this thesis, we introduce a novel approach to interference cancellation for code division multiple access uplink transmission. Several models combining principles of serial and parallel interference cancellation are discussed. The proposed scheme is derived from the analysis of these hybrid models and applies a user configuration algorithm (termed "trickle") in order to provide an improved bit-error-rate performance. The algorithm utilizes an adaptive matrix to compute the required configuration to be used for the subsequent interference cancellation stage. Bit-streaming, pipelined multiuser detection is employed and channel estimates are obtained using sample pilot data known at the receiver. We demonstrate that significant performance improvements can by achieved over various hybrid schemes. A reduced-complexity version of the trickle algorithm is also introduced where the processing delay is greatly reduced while maintaining similar performance. We present several numerical examples through which we demonstrate the efficacy of the proposed algorithms relative to existing interference cancellation algorithms

Near far resistant detection for CDMA personal communication systems.

The growth of Personal Communications, the keyword of the 90s, has already the signs of a technological revolution. The foundations of this revolution are currently set through the standardization of the Universal Mobile Telecommunication System (UMTS), a communication system with synergistic terrestrial and satellite segments. The main characteristic of the UMTS radio interface, is the provision of ISDN services. Services with higher than voice data rates require more spectrum, thus techniques that utilize spectrum as efficiently as possible are currently at the forefront of the research community interests. Two of the most spectrally efficient multiple access technologies, namely. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) concentrate the efforts of the European telecommunity.This thesis addresses problems and. proposes solutions for CDMA systems that must comply with the UMTS requirements. Prompted by Viterbi's call for further extending the potential of CDMA through signal processing at the receiving end, we propose new Minimum Mean Square Error receiver architectures. MMSE detection schemes offer significant advantages compared to the conventional correlation based receivers as they are NEar FAr Resistant (NEFAR) over a wide range of interfering power levels. The NEFAR characteristic of these detectors reduces considerably the requirements of the power control loops currently found in commercial CDMA systems. MMSE detectors are also found, to have significant performance gains over other well established interference cancellation techniques like the decorrelating detector, especially in heavily loaded system conditions. The implementation architecture of MMSE receivers can be either Multiple-Input Multiple Output (MIMO) or Single-Input Single-Output. The later offers not only complexity that is comparable to the conventional detector, but also has the inherent advantage of employing adaptive algorithms which can be used to provide both the dispreading and the interference cancellation function, without the knowledge of the codes of interfering users. Furthermore, in multipath fading channels, adaptive MMSE detectors can exploit the multipath diversity acting as RAKE combiners. The later ability is distinctive to MMSE based receivers, and it is achieved in an autonomous fashion, without the knowledge of the multipath intensity profile. The communicator achieves its performance objectives by the synergy of the signal processor and the channel decoder. According to the propositions of this thesis, the form of the signal processor needs to be changed, in order to exploit the horizons of spread spectrum signaling. However, maximum likelihood channel decoding algorithms need not change. It is the way that these algorithms are utilized that needs to be revis ed. In this respect, we identify three major utilization scenarios and an attempt is made to quantify which of the three best matches the requirements of a UMTS oriented CDMA radio interface. Based on our findings, channel coding can be used as a mapping technique from the information bit to a more ''intelligent" chip, matching the ''intelligence" of the signal processor

Multiuser MIMO-OFDM for Next-Generation Wireless Systems

This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station’s or radio port’s coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems