80 research outputs found
Optimum power allocation and bit loading for BICM systems
This paper introduces a joint bit loading and power allocation algorithm for systems combining bit-interleaved coded modulation (BICM) with multicarrier transmission. The proposed algorithm maximizes the mutual information, so it can be regarded as a generalization of mercury/waterfilling policy
that incorporates bit loading.
The followed approach relies on irregular modulation and power to cast the problem in the framework of convex optimization.
This allows to derive the optimum solution without resorting to greedy algorithms, embedding the bit loading in the definition of an equivalent constellation such that the complexity increase with respect to mercury/waterfilling is negligible. While irregular modulation plays a key role in algorithm definition, it is proved that only a few subcarriers employ it and it is shown that a practical low complexity algorithm can
be obtained with minimal losses that does not use irregular modulation.Peer ReviewedPostprint (published version
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Optimum power allocation and bit loading with code rate constraints
In this paper, a new power allocation and bit
loading policy is defined for those systems working with a preselected binary channel code and specific bit error rate (BER) requirements. It consists on the maximization of the spectral efficiency with a constraint on the average mutual information per coded bit (bit MI), exploiting the relationship of the bit MI with the BER and the code rate.
An irregular modulation approach is employed in order to express the policy as a convex optimization problem, solved without the need of greedy algorithms. Results are compared with those obtained with other algorithms in the literature.Postprint (published version
LDPC code-based bandwidth efficient coding schemes for wireless communications
This dissertation deals with the design of bandwidth-efficient coding schemes
with Low-Density Parity-Check (LDPC) for reliable wireless communications. Code
design for wireless channels roughly falls into three categories: (1) when channel state
information (CSI) is known only to the receiver (2) more practical case of partial CSI
at the receiver when the channel has to be estimated (3) when CSI is known to the
receiver as well as the transmitter. We consider coding schemes for all the above
categories.
For the first scenario, we describe a bandwidth efficient scheme which uses highorder
constellations such as QAM over both AWGN as well as fading channels. We
propose a simple design with LDPC codes which combines the good properties of
Multi-level Coding (MLC) and bit-interleaved coded-modulation (BICM) schemes.
Through simulations, we show that the proposed scheme performs better than MLC
for short-medium lengths on AWGN and block-fading channels. For the first case,
we also characterize the rate-diversity tradeoff of MIMO-OFDM and SISO-OFDM
systems. We design optimal coding schemes which achieve this tradeoff when transmission
is from a constrained constellation. Through simulations, we show that with
a sub-optimal iterative decoder, the performance of this coding scheme is very close
to the optimal limit for MIMO (flat quasi-static fading), MIMO-OFDM and SISO OFDM systems.
For the second case, we design non-systematic Irregular Repeat Accumulate
(IRA) codes, which are a special class of LDPC codes, for Inter-Symbol Interference
(ISI) fading channels when CSI is estimated at the receiver. We use Orthogonal Frequency
Division Multiplexing (OFDM) to convert the ISI fading channel into parallel
flat fading subchannels. We use a simple receiver structure that performs iterative
channel estimation and decoding and use non-systematic IRA codes that are optimized
for this receiver. This combination is shown to perform very close to a receiver
with perfect CSI and is also shown to be robust to change in the number of channel
taps and Doppler.
For the third case, we look at bandwidth efficient schemes for fading channels
that perform close to capacity when the channel state information is known at the
transmitter as well as the receiver. Schemes that achieve capacity with a Gaussian
codebook for the above system are already known but not for constrained constellations.
We derive the near-optimum scheme to achieve capacity with constrained constellations
and then propose coding schemes which perform close to capacity. Through
linear transformations, a MIMO system can be converted into non-interfering parallel
subchannels and we further extend the proposed coding schemes to the MIMO case
too
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
Low-Density Hybrid-Check Coded Superposition Mapping and its Application in OFDM and MIMO
Since Shannon’s landmark paper, many approaches have been proposed to achieve the channel capacity. In the low SNR regime, the problem has almost been solved by capacity achieving channel codes. The research on coded modulation in the high SNR regime is still under development. Among many methods in accomplishing this goal, superposition mapping is an elegant way as it does not require extra shaping to generate a Gaussian-like distributed signal. Superposition mapping has been shown to offer very close to capacity performance for the AWGN channel by combining with an irregular channel code. The aim of this thesis is to search for a code which provides stable performance for moderate sequence length and sufficient number of iterations, which is more suitable for implementation.
Concerning channel coding for superposition mapping, a generalized code design has recently been proposed. The so-called low-density hybrid-check (LDHC) coding intends to contrive coding and modulation in a joint way. The LDHC coding is constructed by integrating modulation into the Tanner graph. Thus, the complete code can be obtained by taking the effects of all the components into account. In this thesis, the LDHC code design is extended to OFDM and MIMO. For OFDM, the bit loading can be realized in the graph. In case of MIMO with spatial multiplexing, the code is extended to the spatial domain. In both cases, a suitable system structure will be proposed in this thesis. It will also be shown how this novel code design improves the system performance
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
MIMO Systems
In recent years, it was realized that the MIMO communication systems seems to be inevitable in accelerated evolution of high data rates applications due to their potential to dramatically increase the spectral efficiency and simultaneously sending individual information to the corresponding users in wireless systems. This book, intends to provide highlights of the current research topics in the field of MIMO system, to offer a snapshot of the recent advances and major issues faced today by the researchers in the MIMO related areas. The book is written by specialists working in universities and research centers all over the world to cover the fundamental principles and main advanced topics on high data rates wireless communications systems over MIMO channels. Moreover, the book has the advantage of providing a collection of applications that are completely independent and self-contained; thus, the interested reader can choose any chapter and skip to another without losing continuity
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