20 research outputs found
Adaptive channel estimation in WCDMA STTD systems
The receiver performance with the use of a space
time transmit diversity (STTD) scheme is more susceptible to
the accuracy of channel estimate than that without the use of
the STTD scheme since the despreading signals suffer from the
effect of crosstalk and the transmit power is equally divided
into multiple transmit antennas. As a result, the efficiency of
channel estimation in the WCDMA STTD system becomes an
important issue more than that in the non-STTD system. In this
paper, an adaptive channel estimator (ACE) is designed to
mitigate the performance degradation due to inaccurate
channel estimation. Numerical results show that the
performance improvement significantly increases with the use
of the proposed ACE, particularly when the channel condition
becomes worse
Transmit diversity and linear and decision-feedback equalizations for frequency-selective fading channels
Abstract-Rapid growth and increasing demands for near-ubiquitous high-quality high-data-rate services present the most challenges for wireless system design. As an effective method to provide such services, space-time (ST) coding is gaining more and more attention. This paper extends ST coding, originally designed for known frequency-nonselective fading channels, to unknown frequency-selective channels. A novel scheme is presented to suppress intersymbol interference, coherently demodulate the information symbols with estimated channel state information in an ST transmit diversity wireless time-division multiple-access system that is equipped with multiple antennas at both transmit and receive sides. The proposed algorithm is powerful and computationally efficient. In addition to the discussion of system identifiability, both theoretical analysis and numerical simulation are presented to illustrate the performance of the proposed estimator and receiver in multipath fading channels
Soft-demodulation of QPSK and 16-QAM for turbo coded WCDMA mobile communication systems
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Linear space-time modulation in multiple-antenna channels
This thesis develops linear spaceātime modulation techniques for (multi-antenna) multi-input multi-output (MIMO) and multiple-input single-output (MISO) wireless channels. Transmission methods tailored for such channels have recently emerged in a number of current and upcoming standards, in particular in 3G and "beyond 3G" wireless systems. Here, these transmission concepts are approached primarily from a signal processing perspective.
The introduction part of the thesis describes the transmit diversity concepts included in the WCDMA and cdma2000 standards or standard discussions, as well as promising new transmission methods for MIMO and MISO channels, crucial for future high data-rate systems. A number of techniques developed herein have been adopted in the 3G standards, or are currently being proposed for such standards, with the target of improving data rates, signal quality, capacity or system flexibility.
The thesis adopts a model involving matrix-valued modulation alphabets, with different dimensions usually defined over space and time. The symbol matrix is formed as a linear combination of symbols, and the space-dimension is realized by using multiple transmit and receive antennas. Many of the transceiver concepts and modulation methods developed herein provide both spatial multiplexing gain and diversity gain. For example, full-diversity full-rate schemes are proposed where the symbol rate equals the number of transmit antennas. The modulation methods are developed for open-loop transmission. Moreover, the thesis proposes related closed-loop transmission methods, where spaceātime modulation is combined either with automatic retransmission or multiuser scheduling.reviewe
Space time transceiver design over multipath fading channels
Imperial Users onl
Iterative multiuser receivers for coded DS-CDMA systems
The introduction of cellular wireless systems in the 1980s has resulted in a continuous and
growing demand for personal communication services. This demand has made larger capacity
systems necessary. With the interest from both the research community and industry in wireless
code-division multiple-access (CDMA) systems, the application of multiuser detection (MUD)
techniques to wireless systems is becoming increasingly important. MUD is an important area
of interest to help obtain the significant increase in capacity needed for future wireless services.
The standardisation of direct-sequence CDMA (DS-CDMA) systems in the third generation of
mobile communication systems has raised even more interest in exploiting the capabilities and
capacity of this type of technology. However, the conventional DS-CDMA system has the major
problem of multiple-access interference (MAI). The MAI is unavoidable because receivers
deal with information which is transmitted not by a single source but by several uncoordinated
and geographically separated sources. As a result, the capacity of these systems is inherently interference
limited by other users. To overcome these limitations, MUD emerges as a promising
approach to increase the system capacity.
This thesis addresses the problem of improving the downlink capacity of a coded DS-CDMA
system with the use of MUD techniques at the mobile terminal receiver. The optimum multiuser
receiver scheme is far too computational intensive for practical use. Therefore, the aim of this
thesis is to investigate sub-optimal multiuser receiver schemes that can exploit the advantages
of MUD but also simplify its implementation. The attention is primarily focused on iterative
MUD receiver schemes which apply the turbo multiuser detection principle. Essentially this
principle consists of an iterative exchange of extrinsic information among the receiver modules
to achieve improved performance.
In this thesis, the implementation of an iterative receiver based on a linear MUD technique and
a cancellation scheme over an additive white Gaussian noise (AWGN) channel is first proposed
and analysed. The interference analysis shows that good performance is achieved using a lowcomplexity
receiver structure. In more realistic mobile channels, however, this type of receiver
suffers from the presence of higher levels of interference resulting in poor receiver performance.
The reason for this is that in such scenarios the desired signals are no longer linearly separable.
Therefore, non-linear detection techniques are required to provide better performance. With
this purpose, a hybrid iterative multiuser receiver is investigated for the case of a stationary
multipath channel. The incorporation of antenna arrays is an effective and practical technique to
provide a significant capacity gain over conventional single-antenna systems. In this context, a
novel space-time iterative multiuser receiver is proposed which achieves a large improvement in
spectral efficiency and performance over multipath fading channels. In addition, it is shown that
this architecture can be implemented without a prohibitive complexity cost. The exploitation
of the iterative principle can be used to approach the capacity bounds of a coded DS-CDMA
system. Using the Shannonās sphere packing bound, a comparison is presented to illustrate how
closely a practical system can approach the theoretical limits of system performance