103 research outputs found
Frequency- domain multiuser detection for highly overloaded DS-CDMA systems
A DS-CDMA (Direct Sequence-Coded Division Multiple
Access) system has maximum spectral efficiency if the
system is fully loaded (i.e., the number of users is equal to the
spreading factor) and we employ signals with bandwidth equal
to the chip rate. However, due to implementation constraints we
need to employ signals with higher bandwidth, decreasing the
systemâs spectral efficiency. In this paper we consider prefixassisted
DS-CDMA systems with bandwidth that can be significantly
above the chip rate. To allow high spectral efficiency we
consider highly overloaded systems where the number of users
can be twice the spreading factor or even more. To cope with
the strong interference levels we present an iterative frequencydomain
receiver that takes full advantage of the total bandwidth
of the transmitted signals. Our performance results show that the
proposed receiver can have excellent performance, even for highly
overloaded systems. Moreover, the overall system performance
can be close to the maximum theoretical spectral efficiency, even
with transmitted signals that have bandwidth significantly above
the chip rate
Hybrid Iterative Multiuser Detection for Channel Coded Space Division Multiple Access OFDM Systems
Space division multiple access (SDMA) aided orthogonal frequency division multiplexing (OFDM) systems assisted by efficient multiuser detection (MUD) techniques have recently attracted intensive research interests. The maximum likelihood detection (MLD) arrangement was found to attain the best performance, although this was achieved at the cost of a computational complexity, which increases exponentially both with the number of users and with the number of bits per symbol transmitted by higher order modulation schemes. By contrast, the minimum mean-square error (MMSE) SDMA-MUD exhibits a lower complexity at the cost of a performance loss. Forward error correction (FEC) schemes such as, for example, turbo trellis coded modulation (TTCM), may be efficiently combined with SDMA-OFDM systems for the sake of improving the achievable performance. Genetic algorithm (GA) based multiuser detection techniques have been shown to provide a good performance in MUD-aided code division multiple access (CDMA) systems. In this contribution, a GA-aided MMSE MUD is proposed for employment in a TTCM assisted SDMA-OFDM system, which is capable of achieving a similar performance to that attained by its optimum MLD-aided counterpart at a significantly lower complexity, especially at high user loads. Moreover, when the proposed biased Q-function based mutation (BQM) assisted iterative GA (IGA) MUD is employed, the GA-aided systemâs performance can be further improved, for example, by reducing the bit error ratio (BER) measured at 3 dB by about five orders of magnitude in comparison to the TTCM assisted MMSE-SDMA-OFDM benchmarker system, while still maintaining modest complexity
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 âCMA-SICâ and âBA-PICâ, 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 sourceâs 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 usersâ 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 âUser Collaborationâ 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 userâs receiver, decoding of composite
codeword is carried out to extract the userâs 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 âUser Collaborationâ approach, referred to as CS-CDMA-UL is presented next. When
usersâ 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
Multiuser Detection for the Uplink of Prefix-Assisted DS-CDMA Systems Employing Multiple Transmit and Receive Antennas
In this paper we consider the uplink transmission within a DS-CDMA system employing CP-assisted (Cylic Prefix) block transmission techniques combined with spatial multiplexing techniques that require multiple antennas at both the transmitter and the receiver. We present an efficient frequency-domain receiver structure with iterative MUD (MultiUser Detection). The performance of the proposed receiver can be close to the single user matched filter bound, even for fully loaded systems and/or severely time-dispersive channels.POSI/CPS/46701/2002 - MC-CDMA and the FCT/POCI 2010 research
grant SFRH / BD / 24520 / 200
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 stationâs or radio portâs 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
Reduced-Complexity Maximum-Likelihood Detection in Downlink SDMA Systems
The literature of up-link SDMA systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the down-link. Hence, in this paper a Space Division Multiple Access (SDMA) down-link (DL) multi-user communication system invoking a novel low-complexity Maximum Likelihood (ML) space-time detection technique is proposed, which can be regarded as an advanced extension of the Complex Sphere Decoder (CSD). We demonstrate that as opposed to the previously published variants of the CSD, the proposed technique may be employed for obtaining a high effective throughput in the so-called âover-loadedâ scenario, where the number of transmit antennas exceeds that of the receive antennas. The proposed method achieves the optimum performance of the ML detector even in heavily over-loaded scenarios, while the associated computational complexity is only moderately increased. As an illustrative example, the required Eb/N0 increased from 2 dB to 9 dB, when increasing the normalized system load from unity, representing the fully loaded system, to a normalized load of 1.556
Avaliação da probabilidade de erro de bit e da eficiĂȘncia espectral de sistemas celulares MC-CDMA que utilizam detecção multiusuĂĄrio
Orientador: Celso de AlmeidaTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia ElĂ©trica e de ComputaçãoResumo: Uma tĂ©cnica que combina mĂșltiplo acesso por divisĂŁo de cĂłdigo (CDMA) e multiplexação por divisĂŁo de frequĂȘncias ortogonais (OFDM) foi proposta como uma opção para futuros padrĂ”es de comunicaçÔes mĂłveis. Esta tĂ©cnica Ă© conhecida como mĂșltiplo acesso por divisĂŁo de cĂłdigo com multiportadoras (MC-CDMA), a qual alĂ©m de herdar as vantagens das tĂ©cnicas CDMA e OFDM, tambĂ©m possui uma inerente diversidade em frequĂȘncia. Apesar de ser uma tĂ©cnica de mĂșltiplo acesso, MC-CDMA foi tipicamente estudada usando detectores de um Ășnico usuĂĄrio no receptor. AlĂ©m disso, alguns trabalhos que tĂȘm estudado seu desempenho com detectores multi-usuĂĄrio usam apenas simulaçÔes. Ademais, cenĂĄrios de uma cĂ©lula sĂŁo tipicamente considerados, embora sistemas mĂłveis operem em ambientes celulares. Esta dissertação visa complementar parcialmente as pesquisas prĂ©vias sobre MC-CDMA. Em geral, este trabalho aborda o desempenho do enlace reverso de sistemas MC-CDMA em termos da taxa de erro de bit (BER) e da eficiĂȘncia espectral celular. Para isto, um sistema celular que usa os esquemas de reuso de frequĂȘncias fracionĂĄrio (FFR) e suave (SFR) Ă© suposto. Entrelaçamento no domĂnio da frequĂȘncia Ă© usado no transmissor dos equipamentos dos usuĂĄrios e, detecção multiusuĂĄrio e um arranjo de antenas sĂŁo considerados nos receptores das estaçÔes radio base. O transmissor dos equipamentos dos usuĂĄrios tambĂ©m realiza controle de potĂȘncia perfeito. AlĂ©m disso, ruĂdo aditivo Gaussiano branco, perda de percurso e desvanecimento lento e seletivo que segue a distribuição de Rayleigh sĂŁo considerados no modelo do canal. As contribuiçÔes desta dissertação sĂŁo resumidas a seguir. ExpressĂ”es fechadas sĂŁo obtidas para avaliar a BER mĂ©dia de um sistema celular que usa os detectores multiusuĂĄrio: zero-forcing (ZF), minimum mean square error (MMSE) e maximum likelihood detector (MU-MLD). Adicionalmente, a tĂ©cnica signal space diversity (SSD) Ă© usada no sistema celular MC-CDMA. Para isto, o MU-MLD precisa ser empregado no receptor da estação radio base. Uma expressĂŁo precisa para avaliar a BER mĂ©dia neste cenĂĄrio Ă© tambĂ©m derivada. Ademais, uma anĂĄlise assintĂłtica das expressĂ”es da BER Ă© feita para se obter mais informaçÔes sobre a ordem da diversidade e o comportamento do sistema no regime de alta relação sinal-ruĂdo mais interferĂȘncia. A complexidade computacional dos detectores multi-usuĂĄrio tambĂ©m Ă© obtida em termos do nĂșmero de operaçÔes complexas realizadas durante o processo de detecção. Em particular, o MU-MLD Ă© implementado atravĂ©s de um algoritmo de decodificação esfĂ©rica (SD), a fim de reduzir sua complexidade. Algumas tĂ©cnicas sĂŁo fornecidas para reduzir ainda mais a complexidade da SD. Finalmente, uma expressĂŁo para avaliar a eficiĂȘncia espectral celular mĂ©dia do sistema MC-CDMA nos cenĂĄrios FFR e SFR Ă© obtida. Esta anĂĄlise Ă© baseada em um algoritmo que calcula os raios de cobertura da cĂ©lula para cada modulação usada no sistema, assumindo que modulação adaptativa Ă© empregada. Para todos os cenĂĄrios, modulaçÔes BPSK e M-QAM sĂŁo consideradas. SimulaçÔes de Monte Carlo corroboram a precisĂŁo da anĂĄlise matemĂĄtica apresentadaAbstract: A hybrid technique combining code division multiple access (CDMA) and orthogonal frequency division multiplexing (OFDM) has been proposed as an option for future mobile communication standards. This technique is known as multicarrier code division multiple access (MC-CDMA), which, besides inheriting the advantages of CDMA and OFDM techniques, also possesses an inherent frequency diversity. Despite being a multiple access technique, MC-CDMA has been typically studied employing single-user detectors in the receiver. Moreover, some works that have studied their performance with multiuser detectors have done so far using only simulations. Furthermore, single cell scenarios are typically considered although mobile systems operate in cellular environments. This dissertation aims to partially complement previous research on MC-CDMA. In general, this work addresses the uplink performance of MC-CDMA systems in terms of the bit error rate (BER) and the cellular spectral efficiency. For this, a cellular system employing fractional frequency reuse (FFR) and soft frequency reuse (SFR) schemes is assumed. Frequency domain interleaving is performed in the transmitter into the user equipments and, multiuser detection and an antenna array are considered in the receivers at the base stations. The transmitter into the user equipments also performs perfect power control. Furthermore, additive white Gaussian noise, path-loss and slow frequency-selective Rayleigh fading are considered in the channel model. The contributions of this dissertation are summarized in the following. Closed-form expressions are derived to evaluate the mean BER of MC-CDMA cellular systems using the multiuser detectors: zero-forcing (ZF), minimum mean square error (MMSE) and maximum likelihood detector (MU-MLD). In addition, signal space diversity (SSD) is used in the MC-CDMA cellular system. For this, MU-MLD must be employed in the receiver at the base station. An accurate expression to evaluate the mean BER in this scenario is also derived. Moreover, an asymptotic analysis of the BER expressions is performed to obtain further insights of the diversity order and system behavior at the high signal-to-noise-plus-interference ratio regime. The computational complexity of the multiuser detectors is also obtained in terms of the number of complex operations performed during the detection process. In particular, MU-MLD is implemented via a sphere decoder (SD) algorithm in order to reduce its complexity. Some techniques are provided in order to further reduce the SD complexity. Finally, an expression to evaluate the mean cellular spectral efficiency of the MC-CDMA system in FFR and SFR scenarios is obtained. This analysis is based on an algorithm that calculates the cell coverage radius for each modulation used in the system, assuming that adaptive modulation is employed. For all analyzed scenarios, BPSK and M-QAM modulations are considered. Monte Carlo simulations corroborate the accuracy of the presented mathematical analysisDoutoradoTelecomunicaçÔes e TelemĂĄticaDoutor em Engenharia ElĂ©tricaCAPE
Efficient Radio Resource Allocation Schemes and Code Optimizations for High Speed Downlink Packet Access Transmission
An important enhancement on the Wideband Code Division Multiple Access
(WCDMA) air interface of the 3G mobile communications, High Speed Downlink
Packet Access (HSDPA) standard has been launched to realize higher spectral
utilization efficiency. It introduces the features of multicode CDMA transmission
and Adaptive Modulation and Coding (AMC) technique, which makes radio resource
allocation feasible and essential. This thesis studies channel-aware resource
allocation schemes, coupled with fast power adjustment and spreading code optimization
techniques, for the HSDPA standard operating over frequency selective
channel.
A two-group resource allocation scheme is developed in order to achieve a
promising balance between performance enhancement and time efficiency. It only
requires calculating two parameters to specify the allocations of discrete bit rates
and transmitted symbol energies in all channels. The thesis develops the calculation
methods of the two parameters for interference-free and interference-present
channels, respectively. For the interference-present channels, the performance of
two-group allocation can be further enhanced by applying a clustering-based channel
removal scheme.
In order to make the two-group approach more time-efficient, reduction in
matrix inversions in optimum energy calculation is then discussed. When the
Minimum Mean Square Error (MMSE) equalizer is applied, optimum energy allocation
can be calculated by iterating a set of eigenvalues and eigenvectors. By
using the MMSE Successive Interference Cancellation (SIC) receiver, the optimum
energies are calculated recursively combined with an optimum channel ordering
scheme for enhancement in both system performance and time efficiency.
This thesis then studies the signature optimization methods with multipath
channel and examines their system performances when combined with different
resource allocation methods. Two multipath-aware signature optimization methods
are developed by applying iterative optimization techniques, for the system
using MMSE equalizer and MMSE precoder respectively. A PAM system using
complex signature sequences is also examined for improving resource utilization
efficiency, where two receiving schemes are proposed to fully take advantage of
PAM features. In addition by applying a short chip sampling window, a Singular
Value Decomposition (SVD) based interference-free signature design method is
presented
Low density spreading multiple access
The need for ubiquitous coverage and the increasing demand for high data rate services, keeps constant pressure on the cellular network infrastructure. There has been intense research to improve the system spectral efficiency and coverage, and a significant part of this effort focused on developing and optimizing the multiple access techniques. One such technique that has been recently proposed is the low density spreading (LDS), which manages the multiple access interference to offer efficient and low complexity multiuser detection. The LDS technique has shown a promising performance as a multiple access technique for cellular systems. This chapter will give an overview on the LDS multiple access technique. The motivations for the LDS design will be highlighted by comparing it to conventional spreading techniques, including brief history of the early work on LDS. Furthermore, a background on the design of LDS in multicarrier communications, such as signatures design, a belief propagation multiuser detection, etc., will be presented along with the challenges and opportunities associated with the multicarrier LDS multiple access
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