||Study on Channel Estimation Methods for OFDM Based Next Generation Wireless Communication Systems in Higher Time-Varying Fading Channel
The Orthogonal Frequency Division Multiplexing (OFDM) technique has been received a lot of attentions especially in the wireless communication systems because of its efficient usage of frequency bandwidth and robustness to the multi-path fading. From these advantages, OFDM technique has been already adopted in many wireless communication systems as the standard transmission technique such as the wireless LAN (WLAN) system, broadband wireless communication system, the 4th generation mobile telecommunication (4G) system and the terrestrial digital broadcasting systems including the Digital Audio Broadcasting (DAB) and the Digital Video Broadcasting (DVB). Recently, many researchers proposed the various new systems based on OFDM technique such as Multi-Input and Multi-Output (MIMO)-OFDM system which can achieve higher data transmission rate or higher signal quality, uplink OFDM Access (OFDMA) system which can accommodate multiple users flexibly and efficiently, two-way relay communication system with broadband analogue network coding (ANC) which can increase the channel capacity and transmission efficiency. To achieve the potential capability for the proposed OFDM based wireless communication systems, there are still remaining many unsolved problems. In this thesis, several practical solutions are proposed to realize the above proposed OFDM based systems which can achieve higher signal quality and higher transmission efficiency in higher time-varying fading channel. One of the limitations of using OFDM technique is the larger peak to average power ratio (PAPR) of its time domain signal which causes the severe degradations of bit error rate (BER) performance and undesirable spectrum re-growth due to the non-linear distortion occurring at the non-linear amplifier. In the wireless communication systems especially for the user terminal, a non-linear amplifier is usually employed at the transmitter which is required for the efficient usage of transmission power because of its battery operation. To solve this problem, the Partial Transmit Sequence (PTS) method was proposed as one of promising PAPR reduction methods which can achieve better PAPR performance with reasonable computation complexity. However the PTS method is required to inform the side information (SI) to the receiver for the correct demodulation of data information by using the separate channel which leads the complexity of transceiver and inefficient usage of frequency bandwidth. To solve this problem, this thesis proposes a new PAPR reduction method based on the packet-switched transmission systems in which all the clusters within the certain number of OFDM symbols have the sequential cluster ID numbers embedded in the header of each cluster. The proposed method enables the reduction of PAPR performance by re-ordering of clusters (ROC) in the frequency domain at the transmitter and reconstructs the original ordering of clusters by using the cluster ID numbers demodulated from each cluster at the receiver which requests no side information. Intelligent Transport Systems (ITS) have been expanding with the popularization of Electrical Toll Collection System (ETC) and Vehicle Information and Communication System (VICS). The OFDM based Road to Vehicle Communication (RVC) system has been considered as one of the promising ITS technologies which can provide the drivers for safety and comfortable driving and collection of variable information from network in the real-time. This thesis proposes a RVC system of using the STBC (Space Time Block Coding) MIMO-OFDM technique which can achieve better BER performance even in higher time-varying fading channel. To achieve the potential capability in the proposed RVC system, it is the essential to realize the accurate channel estimation method. This thesis proposes a novel channel frequency response (CFR) estimation method by using the scattered pilot subcarriers in the frequency domain. From computer simulation results, this thesis demonstrates that the STBC MIMO-OFDM based RVC system of using the proposed CFR estimation method can achieve higher channel estimation accuracy and better BER performance even in the time varying fading channel. To achieve a potential capability of MIMO-OFDM system, it is the essential to realize an efficient and accurate channel estimation method. The conventional Discrete Fourier Transform Interpolation-based channel estimation (DFTI-CE) method of using the scattered pilot symbol can achieve higher estimation accuracy only when the transmission OFDM signal is sampled by the Nyquist rate. However the estimation accuracy of using the conventional DFTI-CE method would be degraded relatively when null subcarriers (zero padding) are inserted at both ends of data subcarrier to reject the aliasing occurring at the output of Digital to Analogue (D/A) converter which corresponds to the non-Nyquist sampling rate. To solve this problem, the Maximum Likelihood (ML) channel estimation method was proposed for MIMO-OFDM system which can achieve better estimation accuracy than the conventional DFTI-CE method. However its estimation accuracy would be degraded at around the both ends of data subcarrier at the non-Nyquist rate especially when increasing the number of transmit antennas and zero padding. To solve the above problem, this thesis proposes the ML based time domain channel estimation (TD-CE) method for MIMO-OFDM system which can achieve higher estimation accuracy even when the non-Nyquist rate and increasing the number of transmit antennas. From computer simulation results, this thesis demonstrates the effectiveness of propose ML based TD-CE method for MIMO-OFDM system. OFDMA technique is considered as one of promising wireless access techniques which can accommodate multiple users flexibly and efficiently. In the uplink OFDMA system, since all users transmit their information data symbols to the base station (BS) simultaneously, it is required to estimate all user’s channel frequency responses all together at the BS before receiving the information data symbols. The estimation of multiple users’ channels in the uplink OFDMA system is similar to that the estimation of multiple channels between the transmit antenna and the receive antenna in the MIMO-OFDM system. From computer simulation results, this thesis also demonstrates the effectiveness of above proposed ML based TD-CE method when employing in the uplink OFDMA system. Broadband ANC of using the OFDM technique has been widely investigated to increase the capacity of two-way relay communication. In the two-way relay communication, it can be simply modeled by two user terminals (UTs) and one relay station (RS) in which each UT sends his information data to the other UT through the RS by using two timeslots. Each UT can demodulate the other user’s information data by removing its self-information data with frequency domain equalization (FDE). To conduct the removal of self-information data and the FDE precisely, it is the essential to realize the accurate channel estimation method for the combined CFR in the 1st and 2nd timeslots from the superimposed received signal sent from both UTs at each UT. To solve this problem, this thesis proposes the ML based combined CFR estimation method for two-way relay communication system by using a novel scattered pilot assignment method including the null subcarriers to avoid the collision of pilot subcarriers sent from both UTs. From the various computer simulation results, this thesis demonstrates the effectiveness of proposed ML based combined CFR estimation method which can achieve higher estimation accuracy and better BER performance even in higher time-varying fading channel. This thesis also demonstrates the effectiveness of proposed ML based combined CFR estimation method when employing the two-way communication system of using the SFBC (Space-Frequency Block Coding) MIMO-OFDM technique to improve the BER performance. The salient feature of proposed CFR estimation method is to employ the pilot subcarriers with Walsh code to differentiate the pilot subcarriers sent from both UTs. From computer simulation results, this thesis confirms that the proposed SFBC MIMO-OFDM system of using the ML based combined CFR estimation method can achieve higher CFR estimation accuracy and better BER performance with higher transmission efficiency even in higher time-varying fading channel. From the numerous computer simulation results, it is confirmed that the proposed methods presented in this thesis can be employed as the practical solutions to realize the next generation OFDM based wireless communication systems.
本文 / Division of Systems Engineering, Graduate School of Engineering, Mie University