83 research outputs found
Linearisation, error correction coding and equalisation for multi-level modulation schemes
University of Technology, Sydney. Faculty of Engineering.Orthogonal frequency division multiplexing (OFDM) has been standardised for digital audio broadcasting (DAB), digital video broadcasting (DVB) and wireless local area networks (WLAN). OFDM systems are capable of effectively coping with frequency- selective fading without using complex equalisation structures. The modulation and demodulation processes using fast fourier transform (FFT) and its inverse (IFFT) can be implemented very efficiently. More recently, multicarrier code division multiple access (MC-CDMA) based on the combination of OFDM and conventional CDMA has received growing attention in the field of wireless personal communication and digital multimedia broadcasting. It can cope with channel frequency selectivity due to its own capabilities of overcoming the asynchronous nature of multimedia data traffic and higher capacity over conventional multiple access techniques.
On the other hand, multicarrier modulation schemes are based on the transmission of a given set of signals on large numbers of orthogonal subcarriers. Due to the fact that the multicarrier modulated (MCM) signal is a superposition of many amplitude modulated sinusoids, its probability density function is nearly Gaussian. Therefore, the MCM signal is characterised by a very high peak-to-average power ratio (PAPR). As a result of the high PAPR, the MCM signal is severely distorted when a nonlinear high power amplifier (HPA) is employed to obtain sufficient transmitting power. This is very common in most communication systems, and decreases the performance significantly. The simplest way to avoid the nonlinear distortion is substantial output backoff (OBO) operating in the linear region of the HPA. However, because of the high OBO, the peak transmit power has to be decreased. For this reason, many linearisation techniques have been proposed to compensate for the nonlinearity without applying high OBO. The predistortion techniques have been known and studied as one of the most promising means to solve the problem. In this thesis, an improved memory mapping predistortion technique devised to reduce the large computational complexity of a fixed point iterative (FPI) predistorter is proposed, suitable especially for multicarrier modulation schemes. The proposed memory mapping predistortion technique is further extended to compensate for nonlinear distortion with memory caused by a shaping linear filter. The case of varying HPA characteristics is also considered by using an adaptive memory mapping predistorter which updates the lookup table (LUT) and counteracts these variations. Finally, an amplitude memory mapping predistorter is presented to reduce the LUT size.
Channel coding techniques have been widely used as an effective solution against channel fading in wireless environments. Amongst these, particular attention has been paid to turbo codes due to their performance being close to the Shannon limit. In-depth study and evaluation of turbo coding has been carried out for constant envelope signaling systems such as BPSK, QPSK and M-ary PSK. In this thesis, the performance of TTCM-OFDM systems with high-order modulation schemes, e.g. 16-QAM and 64-QAM, is investigated and compared with conventional channel coding schemes such as Reed-Solomon and convolutional coding. The analysis is performed in terms of spectral efficiency over a multipath fading channel and in presence of an HPA. Maximum a-priori probability (MAP), soft output Viterbi algorithm (SOVA) and pragmatic algorithms are compared for non-binary turbo decoding with these systems. For this setup, iterative multiuser detection in TTCM/MC-CDMA systems with M-QAM is introduced and investigated, adopting a set of random codes to decrease the PAPR. As another application of TTCM, the performance of multicode CDMA systems with TTCM for outer coding over multipath fading channels is investigated
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
Performance Analysis of BER in CDMA using Various Coding & Simulation Techniques
Wireless Communication is the most important part of our life in today’s time. CDMA system has made it more secure system to communicate within the system. CDMA system has been developed enough to improve various problems like multipath fading, interference, cross talk etc. This paper inculcated various approaches to improve BER in CDMA system with different Coding & Simulation Techniques. This also represents various advantages and limitations of different evaluation/analysis methodology used to evaluate BER
Effective radio resource management for multimedia broadcast/multicast services in UMTS networks
WOS:000242097300001 (Nº de Acesso Web of Science)Broadcast and multicast offer a significant improvement of spectrum utilization, and become particularly important where information channels are shared among several users. Mobile cellular environments are expected to evolve with the technological approaches necessary to facilitate the deployment of multimedia services, such as streaming, file download, and carousel services. The perspective that video streaming in wireless networks services is an attractive service to end-users has spurred the research in this area. To provide for a video delivery platform in UMTS, the third generation partnership project (3GPP) addressed this problem with the introduction of the multimedia broadcast and multicast services (MBMS) in 3GPP Release 6. In this document we analyse several effective radio resource management techniques to provide MBMS, namely, use of nonuniform QAM constellations, multicode, and macrodiversity to guarantee the optimal distribution of QoS depending on the location of mobiles.info:eu-repo/semantics/publishedVersio
Coded Parity Packet Transmission Method for Two Group Resource Allocation
Gap value control is investigated when the number of source and parity packets
is adjusted in a concatenated coding scheme whilst keeping the overall coding
rate fixed. Packet-based outer codes which are generated from bit-wise XOR
combinations of the source packets are used to adjust the number of both source
packets. Having the source packets, the number of parity packets, which are the
bit-wise XOR combinations of the source packets can be adjusted such that the
gap value, which measures the gap between the theoretical and the required
signal-to-noise ratio (SNR), is controlled without changing the actual coding
rate. Consequently, the required SNR reduces, yielding a lower required energy
to realize the transmission data rate. Integrating this coding technique with
a two-group resource allocation scheme renders efficient utilization of the total
energy to further improve the data rates. With a relatively small-sized set of
discrete data rates, the system throughput achieved by the proposed two-group
loading scheme is observed to be approximately equal to that of the existing
loading scheme, which is operated with a much larger set of discrete data rates.
The gain obtained by the proposed scheme over the existing equal rate and
equal energy loading scheme is approximately 5 dB. Furthermore, a successive
interference cancellation scheme is also integrated with this coding technique,
which can be used to decode and provide consecutive symbols for inter-symbol
interference (ISI) and multiple access interference (MAI) mitigation. With this
integrated scheme, the computational complexity is signi cantly reduced by
eliminating matrix inversions. In the same manner, the proposed coding scheme
is also incorporated into a novel fixed energy loading, which distributes packets
over parallel channels, to control the gap value of the data rates although the
SNR of each code channel varies from each other
Coded Parity Packet Transmission Method for Two Group Resource Allocation
Gap value control is investigated when the number of source and parity packets
is adjusted in a concatenated coding scheme whilst keeping the overall coding
rate fixed. Packet-based outer codes which are generated from bit-wise XOR
combinations of the source packets are used to adjust the number of both source
packets. Having the source packets, the number of parity packets, which are the
bit-wise XOR combinations of the source packets can be adjusted such that the
gap value, which measures the gap between the theoretical and the required
signal-to-noise ratio (SNR), is controlled without changing the actual coding
rate. Consequently, the required SNR reduces, yielding a lower required energy
to realize the transmission data rate. Integrating this coding technique with
a two-group resource allocation scheme renders efficient utilization of the total
energy to further improve the data rates. With a relatively small-sized set of
discrete data rates, the system throughput achieved by the proposed two-group
loading scheme is observed to be approximately equal to that of the existing
loading scheme, which is operated with a much larger set of discrete data rates.
The gain obtained by the proposed scheme over the existing equal rate and
equal energy loading scheme is approximately 5 dB. Furthermore, a successive
interference cancellation scheme is also integrated with this coding technique,
which can be used to decode and provide consecutive symbols for inter-symbol
interference (ISI) and multiple access interference (MAI) mitigation. With this
integrated scheme, the computational complexity is signi cantly reduced by
eliminating matrix inversions. In the same manner, the proposed coding scheme
is also incorporated into a novel fixed energy loading, which distributes packets
over parallel channels, to control the gap value of the data rates although the
SNR of each code channel varies from each other
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