Channel Estimation Methods Over Time-varying Channels Based On IEEE 802.16e Ofdm/ofdma Systems

Broadband wireless access (BWA) based on the IEEE 802.16 standard is a promising technique for last-mile access. IEEE Std 802.16e is an enhancement to IEEE Std 802.16d to support subscriber stations moving at vehicular speeds and the operation is limited to licensed bands suitable for mobility between 2 to 6 GHz. Thereby, it specifies a system for combined fixed and mobile broadband wireless access.Orthogonal frequency division multiplexing (OFDM) is one of the most important physical layer transmission techniques in future wireless communications due to its robustness to multi-path channel and high spectrum utilization. OFDMA is a strong candidate multi-access scheme for high-data-rate mobile wireless communications, due to its flexibility on sub-carrier allocation and multi-user diversity utilization over multi-path fading channels. Hence, OFDM and OFDMA techniques are defined as the main physical layer transmission techniques in IEEE Std 802.16e.In mobile communications, the high speed motion of subscribers give rise to Doppler effects that destroy orthogonality among sub-carriers, leading to Inter-Carrier Interference (ICI) which degrades performance of the OFDM/OFDMA receiver. Conventional OFDM time varying channel estimation methods have provided promising results, but are unavailable in practical OFMD/OFDMA systems. Because the time variations of channel impulse response (CIR) are tracked in the time-domain with an ideal assumption that pilots are evenly spaced through the whole frequency band and the channel is an ideal sample-spaced channel. But concerning a practical OFDM/OFDMA system where not noly the non-sample spaced multipath delay but also the unevenly distributed pilots will lead to CIR leakage, the perforamance of conventional algorithms will decrease rapidly.Based on analysis above, this dissertation investigate the frequency domain time varying channel estimation techniques for practical IEEE 802.16e OFDM/OFDMA systems over time varying channels. The main contributions are listed as follows:( 1 ) According to practical IEEE 802.16e OFDM systems, a piece wise linear model based frequency domain time varying channel estimation method for practical OFDM system is proposed. The time variations of channel frequency transmission function are tracked by linear model using pilots in adjacent symbols. Then the channel matrix can be obtained and ICI is well migigated in equalization.( 2 ) In high mobility applications, the ICI energy increased greatly. We find that ICI on pilots make the estimated channel frequency responses at pilot sub-carriers not accurate and inevitably lead to estimation errors. To further improve the estimation precision, this paper considers the use of frequency-domain correlative coding to compress the ICI from data to pilots. The proposed method makes pilots less sensitive to ICI from data sub-carriers, especially in high Doppler environments, more favorable benefits will be received.( 3 ) With the user mobility increase, the channel varies non-linearly during one OFDM symbol period. We futher investigate a BEM based frequency domain time varying channl estimation method for practical OFDM systems. The non-linearly time variations of the channel frequency transmission function are model by BEM. According to the degree of channel variations and the pilot costs for channel estimation, two kinds of channel estimation methods are proposed respectively, which are the adjacent symbols assisted channel estimation method and the channel estimation method by using pilot clusters in the current symbol. Additionaly, their different application ranges are given.( 4 ) Conventional time varying channel estimation methods for OFDMA systems assumed that the channel is constant in a symbol period and the ICI can be ignored. It is inappropriate for high mobile environments. In our dissertation, frequency domain time varying channel estimations methods for practical IEEE802.16e OFDMA systems are proposed, which are performed in each OFDMA time-frequency blocks and the channel variariations during one OFDMA symbol period are tracked well. Hence, ICI caused by channel variations can be mitigated after equalization.( 5 ) Finally, the applicability of all above achievements in MIMO systems are discussed. Conclusions are as follows: If the pilots on different transmit antennas are orthogonal, channel estimation can be performed independently between a pair of transmit and receive antennas. Then, the above achievements will continue to apply in MIMO-OFDM/OFDMA systems. But for MIMO systems, the parameters to be estimated increase manyfold. On the premise of limited pilot overhead costs, compared to single transmit antenna systems, the eatimation performance will drop more or less. In practical applications, comprehensive considerations are necessary.