1,261 research outputs found
SYNCHRONIZATION AND RESOURCE ALLOCATION IN DOWNLINK OFDM SYSTEMS
The next generation (4G) wireless systems are expected to provide
universal personal and multimedia communications with seamless connection
and very high rate transmissions and without regard to the users’ mobility and
location. OFDM technique is recognized as one of the leading candidates to
provide the wireless signalling for 4G systems. The major challenges in
downlink multiuser OFDM based 4G systems include the wireless channel, the
synchronization and radio resource management. Thus algorithms are required
to achieve accurate timing and frequency offset estimation and the efficient
utilization of radio resources such as subcarrier, bit and power allocation.
The objectives of the thesis are of two fields. Firstly, we presented the
frequency offset estimation algorithms for OFDM systems. Building our work
upon the classic single user OFDM architecture, we proposed two FFT-based
frequency offset estimation algorithms with low computational complexity.
The computer simulation results and comparisons show that the proposed
algorithms provide smaller error variance than previous well-known algorithm.
Secondly, we presented the resource allocation algorithms for OFDM
systems. Building our work upon the downlink multiuser OFDM architecture,
we aimed to minimize the total transmit power by exploiting the system
diversity through the management of subcarrier allocation, adaptive
modulation and power allocation. Particularly, we focused on the dynamic
resource allocation algorithms for multiuser OFDM system and multiuser
MIMO-OFDM system. For the multiuser OFDM system, we proposed a lowiv
complexity channel gain difference based subcarrier allocation algorithm. For
the multiuser MIMO-OFDM system, we proposed a unit-power based
subcarrier allocation algorithm. These proposed algorithms are all combined
with the optimal bit allocation algorithm to achieve the minimal total transmit
power. The numerical results and comparisons with various conventional nonadaptive
and adaptive algorithmic approaches are provided to show that the
proposed resource allocation algorithms improve the system efficiencies and
performance given that the Quality of Service (QoS) for each user is
guaranteed.
The simulation work of this project is based on hand written codes in the
platform of the MATLAB R2007b
A study of BER Performance of OFDM Modulation in Multi-fading Channel
The diversity and complexity of Multipath fading can influence performance of the Orthogonal Frequency Division Multiplexing (OFDM) modulation. The system performance analysis is based on correct design of a channel model. According to system characters of OFDM, a frequency selective slow fading channel model is built up, by combining the Trapped delay line model and the slow fading characters, such as the Rayleigh, Rician or Nakagami distribution. The theoretical Bit Error Rate (BER) of OFDM system under this channel model is deduced based on the BER or Symbol Error Rate of MQAM under Additive White Gauss Noise (AWGN) channel and the Probability Density (PDF) Function of different slow fading channel. The applicability of this channel model and the System BER performance under different slow fading channel is verified by simulation. The results indicate that the simulation result is consistent with the theoretical analysis under MQAM modulation method, which illustrates that the frequency selective slow fading channel model is suitable for the performance analyzing of OFDM system
Error Rate Analysis for Coded Multicarrier Systems over Quasi-Static Fading Channels
This paper presents two methods for approximating the performance of coded
multicarrier systems operating over frequency-selective, quasi-static fading
channels with non-ideal interleaving. The first method is based on
approximating the performance of the system over each realization of the
channel, and is suitable for obtaining the outage performance of this type of
system. The second method is based on knowledge of the correlation matrix of
the frequency-domain channel gains and can be used to directly obtain the
average performance. Both of the methods are applicable for
convolutionally-coded interleaved systems employing Quadrature Amplitude
Modulation (QAM). As examples, both methods are used to study the performance
of the Multiband Orthogonal Frequency Division Multiplexing (OFDM) proposal for
high data-rate Ultra-Wideband (UWB) communication.Comment: 5 pages, 3 figures, 2 tables. Submitted to Globecom 200
Compressive sensing based Bayesian sparse channel estimation for OFDM communication systems: high performance and low complexity
In orthogonal frequency division modulation (OFDM) communication systems,
channel state information (CSI) is required at receiver due to the fact that
frequency-selective fading channel leads to disgusting inter-symbol
interference (ISI) over data transmission. Broadband channel model is often
described by very few dominant channel taps and they can be probed by
compressive sensing based sparse channel estimation (SCE) methods, e.g.,
orthogonal matching pursuit algorithm, which can take the advantage of sparse
structure effectively in the channel as for prior information. However, these
developed methods are vulnerable to both noise interference and column
coherence of training signal matrix. In other words, the primary objective of
these conventional methods is to catch the dominant channel taps without a
report of posterior channel uncertainty. To improve the estimation performance,
we proposed a compressive sensing based Bayesian sparse channel estimation
(BSCE) method which can not only exploit the channel sparsity but also mitigate
the unexpected channel uncertainty without scarifying any computational
complexity. The propose method can reveal potential ambiguity among multiple
channel estimators that are ambiguous due to observation noise or correlation
interference among columns in the training matrix. Computer simulations show
that propose method can improve the estimation performance when comparing with
conventional SCE methods.Comment: 24 pages,16 figures, submitted for a journa
Frequency-domain receiver design for doubly-selective channels
This work is devoted to the broadband wireless transmission techniques, which are serious candidates to be implemented in future broadband wireless and cellular systems, aiming at providing high and reliable data transmission and concomitantly high mobility.
In order to cope with doubly-selective channels, receiver structures based on OFDM
and SC-FDE block transmission techniques, are proposed, which allow cost-effective implementations, using FFT-based signal processing.
The first subject to be addressed is the impact of the number of multipath components,
and the diversity order, on the asymptotic performance of OFDM and SC-FDE, in
uncoded and for different channel coding schemes. The obtained results show that the
number of relevant separable multipath components is a key element that influences the performance of OFDM and SC-FDE schemes.
Then, the improved estimation and detection performance of OFDM-based broadcasting systems, is introduced employing SFN (Single Frequency Network) operation.
An initial coarse channel is obtained with resort to low-power training sequences estimation, and an iterative receiver with joint detection and channel estimation is presented.
The achieved results have shown very good performance, close to that with perfect channel estimation.
The next topic is related to SFN systems, devoting special attention to time-distortion
effects inherent to these networks. Typically, the SFN broadcast wireless systems employ OFDM schemes to cope with severely time-dispersive channels. However, frequency errors, due to CFO, compromises the orthogonality between subcarriers. As an alternative approach, the possibility of using SC-FDE schemes (characterized by reduced envelope fluctuations and higher robustness to carrier frequency errors) is evaluated, and a technique, employing joint CFO estimation and compensation over the severe time-distortion effects, is proposed.
Finally, broadband mobile wireless systems, in which the relative motion between
the transmitter and receiver induces Doppler shift which is different or each propagation path, is considered, depending on the angle of incidence of that path in relation to the direction of travel. This represents a severe impairment in wireless digital communications systems, since that multipath propagation combined with the Doppler effects, lead to drastic and unpredictable fluctuations of the envelope of the received signal, severely affecting the detection performance. The channel variations due this effect are very difficult to estimate and compensate. In this work we propose a set of SC-FDE iterative receivers implementing efficient estimation and tracking techniques. The performance results show that the proposed receivers have very good performance, even in the presence of significant Doppler spread between the different groups of multipath components
Bit error rate estimation in WiMAX communications at vehicular speeds using Nakagami-m fading model
The wireless communication industry has experienced a rapid technological evolution from its basic first generation (1G) wireless systems to the latest fourth generation (4G) wireless broadband systems. Wireless broadband systems are becoming increasingly popular with consumers and the technological strength of 4G has played a major role behind the success of wireless broadband systems. The IEEE 802.16m standard of the Worldwide Interoperability for Microwave Access (WiMAX) has been accepted as a 4G standard by the Institute of Electrical and Electronics Engineers in 2011. The IEEE 802.16m is fully optimised for wireless communications in fixed environments and can deliver very high throughput and excellent quality of service. In mobile communication environments however, WiMAX consumers experience a graceful degradation of service as a direct function of vehicular speeds. At high vehicular speeds, the throughput drops in WiMAX systems and unless proactive measures such as forward error control and packet size optimisation are adopted and properly adjusted, many applications cannot be facilitated at high vehicular speeds in WiMAX communications. For any proactive measure, bit error rate estimation as a function of vehicular speed, serves as a useful tool. In this thesis, we present an analytical model for bit error rate estimation in WiMAX communications using the Nakagami-m fading model. We also show, through an analysis of the data collected from a practical WiMAX system, that the Nakagami-m model can be made adaptive as a function of speed, to represent fading in fixed environments as well as mobile environments
MAXIMIZING AVAILABLE DIVERSITY IN A MIMO FADING CHANNEL
The effects of Rayleigh fading on a wireless communications link and the diversity techniques that are available to
combat fading are explored in this thesis. The thesis consists of two main portions; an exploration on the use of spatial
diversity and an exploration on the use of space-frequency diversity.
In harnessing spatial diversity, the use of space-time coding is proposed to combat fading. Several codes of
diversity two and four are explored, including Quasi-Orthogonal (QO) codes that are able to achieve a unity rate. The
concept of constellation rotation for QO codes to maximize the available diversity is studied. QO codes are shown by
simulation to be able to achieve full diversity, despite the fact that the codes are non-orthogonal.
The use of space-time-frequency (STF) code to harness both space and frequency diversity is also explored.
Simulations were performed for a variety of channel correlations to simulate varying degrees of frequency-selectivity.
Results show that STF codes are very effective when the correlations in frequency response are less than 0.5. STF
codes also have a power advantage over space-time codes with comparable diversity and bandwidth efficiency.Military Expert 5, Republic of Singapore Air ForceApproved for public release; distribution is unlimited
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