Research On Optimal Pilot Sequences Design For MIMO-OFDM System Channel Estimation

As one of the most important technologies in next wireless communication systems, multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems have gained considerable interest in recent years. OFDM that is a multicarrier modulation technique has attracted a lot of attention, due to its simple implementation and robustness against frequency-selective fading channels. OFDM can avoid inter symbol interference (ISI) through converting the frequency selective fading channel into flat fading subchannels. OFDM has been standardized for a variety of applications, such as digital audio broadcasting (DBA), digital television broadcasting wireless local area networks. MIMO takes advantage of the spatial diversity that is obtained by separated antennas in multi-path scattering environment and can increase channel capacity without the bandwidth increasing. Combining OFDM with MIMO has been shown to provide a significant increase in system capacity and transmission rates.Channel estimation information is used for data detection and channel equalization. Channel estimation information can be obtained in different ways; one is based on block pilot sequeces and comb pilot sequences that are a priori known at the receiver, whereas the other is blind, i.e., exploiting statistical information and/or transmitted symbol properties (like constant modulus, etc.). However, compared with block pilot sequences and comb pilot sequences, blind channel estimation generally requires a long data record. Hence, it is limited to slowly time-varying channels and entails high complexity. We restrict our attention to block pilot sequences and comb pilot sequences-bansed channel estimation in this thesis.In this thesis, we analyze the factors that may affect the performance of the channel estimation in practical mobile communication systems, and introduce the summary of research status, pivotal problems and main algorithms. Having the applications of the channel estimation algorithms in mind, we propose the time-domain least square (LS) algorithms for channel estimation and optimal pilot sequences in MIMO-OFDM systems, and study the channel estimation analysis in the channel environment with Doppler frequency shift.In OFDM systems, to avoid the transmitted data being distorted by the low-pass filter on the transmitter side, the subcarriers that fall in the roll-off region of the filter transfer function are not used for transmission. These subcarriers that are on both edges of allocated bandwidth, are ofen referred to as guard band. In the conventional optimal pilot sequences (OPS) scheme, the first subcarrier of the OFDM band is generally selected as initial pilot tone of the OPS, the channel estimation must be seriously degraded, because the initial pilot tone i.e. first subcarrier fall in the guard band. For the single-input single-output (SISO)-OFDM systems, a new OPS scheme is proposed in this thesis. It is shown that the OPS are equipowered, equispaced. The OPS can avoid the guard band through adjusting the postion of initial pilot tone of the OPS, if and only if the bandwidth of OFDM band is less than or equal to the maximum guard bandwidth (MGB), the area of the initial potision of the OPS is also computed.There is a different Doppler frequency shift for each subcarrier in the wireless mobile OFDM systems due to the relative movement between the mobile users and the base station. The Doppler frequency shift that varies directly as the subcarrier frequency and the relative speed between the transmitter and the receiver can degrade the performance of the channel estimation. To tackle the problem of Doppler frequency shift for channel estimation in OFDM systems, we evaluate the effect of Doppler frequency shift on the channel estimation performance by setting up OPS. We also derive the mathematical expression of Doppler frequency shift and MSE of channel estimation. It is shown in this expression that the MSE of channel estimation is proportional to the square of Doppler frequency spread (The Doppler frequency spread is the Doppler frequency shift of the subcarrier which frequency is biggest in OFDM band).The time-domain LS channel estimation scheme is proposed for MIMO-OFDM systems based on the comb pilot sequences in this thesis. The basic idea is that the time-domain samples that are generated by the pilot sequences of the transmitter and the receiver are used to estimate the time-domain channel responses. The OPS with regard to (w.r.t) the mean square error (MSE) of LS channel estimation are also presented. It is shown that the OPS are equipowered, equispaced, and position orthogonal. In order to obtain the minimum MSE (MMSE) of the LS channel estimation, the OPS mus be able to avoid the guard band. The OPS can avoid the guard band, if and only if the guard bandwidth of the MIMO-OFDM system is less than or equal to the MGB. On the transmitter side, the more the guard bandwidth is bigger, the more the transmitted data being distorted by the low-pass filter can be avoided effectively. The bigger guard bandwidth can be used to avoid the transmitted data being distorted in this thesis, because the MGB of the proposed OPS is bigger than that of the conventional OPS. To reduce the training overhead, the time-domain LS channel estimation scheme over multiple OFDM symbols is also discussed. Through simulations, it is shown that the proposed OPS scheme outperforms the conventional OPS shcme in terms of MSE and bit error rate (BER).The block pilot sequences are generally used for the slowly time-varying channel estimation. The block pilot sequences are position orthogonal in the conventional pilot sequences design. The more data resources are taken up by pilot sequences in OFDM band as the transmit antennas are increasing in number, thus making the OFDM systems transmission ineffective. In order to increase the transmission efficiency, we propose a OPS scheme in which the pilot sequences are phase shift orthogonal to each other in different antennas. The proposed scheme can avoid the transmission efficiency being degraded by the increasing of transmit antennas efficiently. The proposed scheme is shown, to provide a significant increase in system cpacity and transmission efficiency.