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

Frequency Domain Independent Component Analysis Applied To Wireless Communications Over Frequency-selective Channels

In wireless communications, frequency-selective fading is a major source of impairment for wireless communications. In this research, a novel Frequency-Domain Independent Component Analysis (ICA-F) approach is proposed to blindly separate and deconvolve signals traveling through frequency-selective, slow fading channels. Compared with existing time-domain approaches, the ICA-F is computationally efficient and possesses fast convergence properties. Simulation results confirm the effectiveness of the proposed ICA-F. Orthogonal Frequency Division Multiplexing (OFDM) systems are widely used in wireless communications nowadays. However, OFDM systems are very sensitive to Carrier Frequency Offset (CFO). Thus, an accurate CFO compensation technique is required in order to achieve acceptable performance. In this dissertation, two novel blind approaches are proposed to estimate and compensate for CFO within the range of half subcarrier spacing: a Maximum Likelihood CFO Correction approach (ML-CFOC), and a high-performance, low-computation Blind CFO Estimator (BCFOE). The Bit Error Rate (BER) improvement of the ML-CFOC is achieved at the expense of a modest increase in the computational requirements without sacrificing the system bandwidth or increasing the hardware complexity. The BCFOE outperforms the existing blind CFO estimator [25, 128], referred to as the YG-CFO estimator, in terms of BER and Mean Square Error (MSE), without increasing the computational complexity, sacrificing the system bandwidth, or increasing the hardware complexity. While both proposed techniques outperform the YG-CFO estimator, the BCFOE is better than the ML-CFOC technique. Extensive simulation results illustrate the performance of the ML-CFOC and BCFOE approaches

Orthogonal Frequency Division Multiplexing modulation and inter-carrier interference cancellation

The Orthogonal Frequency Division Multiplexing (OFDM) technique, wireless channel models, and a pair of new intercarrier interference self-cancellation methods are investigated in this thesis. The first chapter addresses the history of OFDM, along with its principles and applications. Chapter two consists of three parts: the principal, the modern OFDM models, and the Peak to Average Power Ratio (PAPR) problem. Chapter two also summarizes possible PAPR solutions. Chapter three discusses a series of well-known wireless channel models, as well as the general formula for wireless channels. In Chapter four, ICI problem has been discussed, along with its existing solutions. Chapter five focuses on two new ICI self-cancellation schemes, namely the clustering method and the multi-codebook method. These two new methods show promising results through the simulations. A summary of this thesis and the discussion of future research are also provided in Chapter five

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

Statistical characterization of correlation-based time/frequency synchronizers for OFDM

Orthogonal Frequency Division Multiplexing (OFDM) has been widely adopted as a modulation format for reliable digital communication over multipath fading channels, e.g. IEEE 802.11g WiFi networks, as well as broadband wireline channels, e.g. DSL modems. However, its robustness to channel impairments comes at the cost of increased sensitivity to symbol timing and carrier frequency offset errors, and thus requires more complex synchronization methods than conventional single-carrier modulation formats. In this thesis, a class of synchronization methods based upon the intrinsic autocorrelation structure of the OFDM signal is studied from a statistical perspective. In particular, the reasons for the existence of irreducible time and frequency offset estimation errors in the limit of increasing signal-to-noise ratio (SNR) are investigated for correlator-based synchronizers for the non-fading channel case and several fading channel models. It is demonstrated that the primary source of irreducible synchronization errors at high SNR is the natural random distribution of signal energy in the cyclic prefix of the OFDM symbol. Comparisons of the distribution of correlator output magnitude between the non-fading and fading channel cases demonstrates that fading skews the distribution with respect to the non-fading case. A potential mechanism for reducing the effect of innate signal energy variability, correlator output windowed averaging, is studied from the perspective of its influence on the distribution of interpeak intervals in the temporal correlator output signal. While improved performance is realized through averaging for the non-fading channel case, this technique is not as effective for fading channels. In either instance, the windowed averaging method increases the latency of the synchronization process and thus introduces delay in the overall demodulation process

Comparative Analysis of Conventional and ICI-Self-Cancellation Digital Video Broadcasting Transceivers

Digital video broadcasting–terrestrial (DVB-T) is one of the important technologies in the communication area because of its capability of high data-rate multimedia transmission. Orthogonal frequency division multiplexing (OFDM) has been the backbone technique in the current DVB-T systems adopted by Europe and Japan. However, since the OFDM system is very sensitive to the frequency synchronization and phase errors, which will induce the intercarrier interference (ICI), the ongoing research has been dedicated to this ICI problem in the presence of the Doppler frequency drift and the fading channels. A means to deal with the ICI problem is called the ICI self-cancellation or polynomial cancellation coding scheme. In this thesis, we establish the complete simulation environment for the physical layer of the DVB-T system and then evaluate the effectiveness of the ICI self-cancellation coding scheme compared with the existing convolutional coding scheme for different fading channels and different Doppler frequencies. According to our simulation results, we conclude that the ICI self-cancellation scheme significantly outperforms the convolutional coding scheme which is adopted by the existing DVB-T standard in the AWGN and the frequency non-selective fading channels, but both schemes have the similar performance in the frequency selective fading channels