In this thesis, channel estimation technique in Multiple Input Multiple Output - Orthogonal Frequency Division Multiplexing (MIMO-OFDM) systems is investigated. Channel estimation plays an important role in the receiver performance of the MIMO-OFDM systems. In this thesis, the Space-Frequency Block Codes - Orthogonal Frequency Division Multiplexing (SFBC-OFDM) structure with the superimposed training scheme are adopted to increase bandwidth efficiency. In addition to using the first-order statistics of the received signal to reduce the interference induced by the unknown signals, the modified least square (MLS) estimator and the decision-feedback scheme are combined together to improve the channel estimation. In contrast to the conventional interference cancellation scheme for channel estimations adopting the decision feedback mechanism, the structure of the SFBC is utilized to decrease the interference. Furthermore, the effects of the decision-feedback scheme with erroneous decisions upon channel estimation are also examined. Analytical results show that the proposed method has better performance and computation complexity is not increased.

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Orthogonal frequency-division multiplexing (OFDM) has been employed in several current and future 4-th generation (4G) wireless standards. Frequency offsets in OFDM introduce intercarrier interference (ICI). Channel estimations are also required. This thesis focuses on the channel and frequency offset estimation for OFDM-based systems. For cooperative-relay OFDM with frequency offsets, where inter-relay interference (IRI) exists, channel estimation is developed. Optimal pilot designs are proposed by minimizing the IRI in the mean square error (MSE) of the least square (LS) channel estimation. The impact of frequency offset on the channel estimation accuracy is derived. The pairwise error probability (PEP) with both the frequency offset and channel estimation errors is evaluated. The power allocation is discussed. For multiple-input multiple-output (MIMO) OFDM systems, channel and frequency offset estimation errors are investigated. The signal-to-interference-and-noise ratio (SINR) is first analyzed given channel and frequency offset estimation errors. The bit error rate (BER) is then approximated for multiple-antenna reception with maximal ratio combing (MRC) and equal gain combining (EGC). For orthogonal frequency-division multiplexing access (OFDMA) systems, the variance of the frequency offset estimation is derived as a function of SINR and signal-to- noise ratio (SNR). This variance information is exploited to improve the accuracy of frequency offset estimators. A successive interference cancellation (SIC)-based frequency offset estimator is also developed. The accuracy of frequency offset estimation of the OFDMA uplink can also be improved by using the cooperative relaying. Both conventional amplify-and-forward (AF) relays and new decode-and-compensate-and-forward (DcF) relays are studied. The frequency offset estimate is derived from combining different link estimates. In addition, when CSI is available, a scheme is proposed to adaptively switch between the cooperative and conventional (no relaying) transmissions to optimize the frequency offset estimation. / Wireless Communications

In this thesis, we investigate and implement the channel estimation schemes for digital video broadcasting - handheld (DVB-H) and IEEE 802.16d world interoperability for micro-wave access (WiMAX). Both systems are based on the orthogonal frequency division multiplexing (OFDM) technique. The performance of the channel estimation schemes is first verified by using simulation experiments. Then, the channel estimation algorithms are realized by hardware implementation. For the DVB-H systems, since the mobile device may have a relatively high speed, the channel condition is time-varying, leading to serious degradation in channel estimation. In this thesis, the decision feedback mechanism is adopted to improve the performance of the channel estimation. The adopted structure for channel estimation is realized by using Verilog hardware description language (HDL). Then, all the baseband signal processing related algorithms, which include timing synchronization, frequency offset estimation/compensation, and scattered pilot detection, are integrated together using 0.18 ASIC process. For the IEEE 802.16d WiMAX, since it is dedicated for fixed wireless applications, the channel condition is basically stationary and easier to obtain. Therefore, the decision feedback mechanism is not adopted to save hardware complexity. The system performance is verified by conducting simulation experiments. The adopted channel estimation algorithm is implemented by using Verilog HDL. Finally, the whole baseband receiver is realized by using the field programmable gate array (FPGA) and verified by using the Agilent logic analyzer.

The fourth generation cellular mobile communication system in the future needs to provide high data rate transmissions. Multi-Tone CDMA (MT-CDMA) system combines the advantages of CDMA and OFDM, and it is one of the promising choices for the fourth cellular mobile communication system. In order to raise the performance, we amend MT-CDMA named Modified MT-CDMA. We use different pilot arrangement in different channel to enhance the channel estimation of system. The receiver architecture includes a baseband signal detection and a channel estimation. We use a pilot symbol to estimation the channel. In this thesis, we make a study of pilot arrangement on different channel estimation performance of system. We use Least Square estimation(LS)¡Blinear interpolation¡Bspline interpolation¡BFFT/IFFT interpolation and spline interpolation combine FFT/IFFT algorithm to estimate the system performance.

Multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) technology has been used widely in many wireless communication systems. Signals will be distorted when they are transmitted in wireless channels. For the reason that wireless channel is time or location variant, we have to estimate the channel impulse response and use the channel state information to compensate the channel distortion. Pilot signals can be spaced separated in the transmitted symbols. In the receiver, the channel impulse response can be estimated at the positions of pilot signals. The other channel information at the data signals can be obtained by interpolating the estimated channel impulse response. However, error caused by channel interpolation can not be avoided. In this paper we propose a pilot-aided iterative maximum likelihood (ML) channel estimation algorithm for MIMO OFDM systems. At first, an ML channel estimate is obtained by using pilot tones. The receiver uses the estimated channel to help the detection/decision of data signals. And then the channel estimation treats the detected signals as known data to perform a next stage channel estimation iteratively. By utilizing the iterative channel estimation and signal detection process we can reduce the estimation error caused by channel interpolation between pilots. The accuracy of the channel estimation can be improved by increasing the number of iteration process. Simulation results demonstrate the iterative ML channel estimation algorithm can provide better mean-square-error and bit-error-rate performance than conventional methods. By changing the system parameters we can see that the improvement provided by this algorithm is different. Systems with fewer pilots have more improvement from the iterative ML algorithm. On the other hand, systems with more pilots have less improvement from the iterative algorithm. Finally, several channel environments are also considered in this thesis to compare the performance improvement introduced by the iterative algorithm.

The direct sequence ultra-wideband (DS-UWB) system has the characteristics of low transmission power constraints and wide transmitted bandwidth. The wide bandwidth will introduce fine timing resolution and severe inter-symbol interference (ISI), which will is cause larger delay spread. Therefore, the traditional acquisition methods are no longer suitable here. A modified acquisition method and a coarse channel estimation algorithm are proposed in this thesis. Three different strategies with composition of acquisition and channel estimation are compared. Based on three strategies, the tasks of coarse acquisition and channel estimation can be achieved with limited length of acquisition sequence. From simulation results, the parallel strategy is robust enough to operate under the UWB channel in severe ISI conditions and the time duration for completing the tasks of coarse acquisition and channel estimation meets the specification of IEEE 802.15.3a.

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In this thesis, a precoding scheme is proposed for channel estimation in orthogonal frequency-division multiplexing (OFDM) systems. The precoding scheme utilizes a special code matrix before the inverse fast Fourier transform (IFFT) at the transmitter. The row vectors of the matrix have constant amplitudes in both time domain and frequency domain. With the prcoding scheme, a semi-blind channel estimation method is proposed by using the characteristics of the row code sequences. In the proposed scheme, the channel frequency responses of all sub-carriers can be obtained by using only one pilot sub-carrier, and the proposed architecture can not only increase the data rate, but also avoid interpolation error in channel estimation. In addition, the normalized mean square error (NMSE) function is derived and parameters are optimized to improve system performance. The proposed precoding architecture and channel estimation scheme are shown to have better performances in bit error rate by conducting computer simulation experiments.