3 research outputs found
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
Cancelamento de interferência em sistemas celulares distribuÃdos
Doutoramento em Engenharia ElectrotécnicaO tema principal desta tese é o problema de cancelamento de interferência
para sistemas multi-utilizador, com antenas distribuÃdas. Como tal, ao iniciar,
uma visão geral das principais propriedades de um sistema de antenas
distribuÃdas é apresentada. Esta descrição inclui o estudo analÃtico do impacto
da ligação, dos utilizadores do sistema, a mais antenas distribuÃdas.
Durante essa análise é demonstrado que a propriedade mais importante do
sistema para obtenção do ganho máximo, através da ligação de mais antenas
de transmissão, é a simetria espacial e que os utilizadores nas fronteiras das
células são os mais bene ciados. Tais resultados são comprovados através
de simulação. O problema de cancelamento de interferência multi-utilizador
é considerado tanto para o caso unidimensional (i.e. sem codi cação) como
para o multidimensional (i.e. com codi cação). Para o caso unidimensional
um algoritmo de pré-codi cação não-linear é proposto e avaliado, tendo
como objectivo a minimização da taxa de erro de bit. Tanto o caso de
portadora única como o de multipla-portadora são abordados, bem como o
cenário de antenas colocadas e distribuidas. É demonstrado que o esquema
proposto pode ser visto como uma extensão do bem conhecido esquema
de zeros forçados, cuja desempenho é provado ser um limite inferior para
o esquema generalizado. O algoritmo é avaliado, para diferentes cenários,
através de simulação, a qual indica desempenho perto do óptimo, com baixa
complexidade. Para o caso multi-dimensional um esquema para efectuar
"dirty paper coding" binário, tendo como base códigos de dupla camada é
proposto. No desenvolvimento deste esquema, a compressão com perdas de
informação, é considerada como um subproblema. Resultados de simulação
indicam transmissão dedigna proxima do limite de Shannon.This thesis focus on the interference cancellation problem for multiuser distributed
antenna systems. As such it starts by giving an overview of the
main properties of a distributed antenna system. This overview includes, an
analytical investigation of the impact of the connection of additional distributed
antennas, to the system users. That analysis shows that the most
important system property to reach the maximum gain, with the connection
of additional transmit antennas, is spatial symmetry and that the users at
the cell borders are the most bene ted. The multiuser interference problem
has been considered for both the one dimensional (i.e. without coding) and
multidimensional (i.e. with coding) cases. In the unidimensional case, we
propose and evaluate a nonlinear precoding algorithm for the minimization
of the bit-error-rate, of a multiuser MIMO system. Both the single-carrier
and multi-carrier cases are tackled as well as the co-located and distributed
scenarios. It is demonstrated that the proposed scheme can be viewed as an
extension of the well-known zero-forcing, whose performance is proven to be
a lower bound for the generalized scheme. The algorithm was validated extensively
through numerical simulations, which indicate a performance close
to the optimal, with reduced complexity. For the multi-dimensional case, a
binary dirty paper coding scheme, base on bilayer codes, is proposed. In the
development of this scheme, we consider the lossy compression of a binary
source as a sub-problem. Simulation results indicate reliable transmission
close to the Shannon limit
Adaptive Resource Allocation for Statistical QoS Provisioning in Mobile Wireless Communications and Networks
Due to the highly-varying wireless channels over time, frequency, and space
domains, statistical QoS provisioning, instead of deterministic QoS guarantees, has
become a recognized feature in the next-generation wireless networks. In this dissertation,
we study the adaptive wireless resource allocation problems for statistical QoS
provisioning, such as guaranteeing the specified delay-bound violation probability,
upper-bounding the average loss-rate, optimizing the average goodput/throughput,
etc., in several typical types of mobile wireless networks.
In the first part of this dissertation, we study the statistical QoS provisioning for
mobile multicast through the adaptive resource allocations, where different multicast
receivers attempt to receive the common messages from a single base-station sender
over broadcast fading channels. Because of the heterogeneous fading across different
multicast receivers, both instantaneously and statistically, how to design the efficient
adaptive rate control and resource allocation for wireless multicast is a widely cited
open problem. We first study the time-sharing based goodput-optimization problem
for non-realtime multicast services. Then, to more comprehensively characterize the
QoS provisioning problems for mobile multicast with diverse QoS requirements, we
further integrate the statistical delay-QoS control techniques — effective capacity
theory, statistical loss-rate control, and information theory to propose a QoS-driven
optimization framework. Applying this framework and solving for the corresponding optimization problem, we identify the optimal tradeoff among statistical delay-QoS
requirements, sustainable traffic load, and the average loss rate through the adaptive
resource allocations and queue management. Furthermore, we study the adaptive
resource allocation problems for multi-layer video multicast to satisfy diverse statistical
delay and loss QoS requirements over different video layers. In addition,
we derive the efficient adaptive erasure-correction coding scheme for the packet-level
multicast, where the erasure-correction code is dynamically constructed based on multicast
receivers’ packet-loss statuses, to achieve high error-control efficiency in mobile
multicast networks.
In the second part of this dissertation, we design the adaptive resource allocation
schemes for QoS provisioning in unicast based wireless networks, with emphasis
on statistical delay-QoS guarantees. First, we develop the QoS-driven time-slot and
power allocation schemes for multi-user downlink transmissions (with independent
messages) in cellular networks to maximize the delay-QoS-constrained sum system
throughput. Second, we propose the delay-QoS-aware base-station selection schemes
in distributed multiple-input-multiple-output systems. Third, we study the queueaware
spectrum sensing in cognitive radio networks for statistical delay-QoS provisioning.
Analyses and simulations are presented to show the advantages of our proposed
schemes and the impact of delay-QoS requirements on adaptive resource allocations
in various environments