Research On Channel Estimation And Multiple Access Techniques For OFDM Communication Systems

The rapid development of terrestrial wireless commmunications promote the satellite commmunications, and the intergrated infrastructure of terrestrial and satellite becomes the trend of global wireless mobile communication systems. Orthogonal Frequency Division Multiplexing(OFDM) technology is the key technology of the fourth generation(4G) wireless mobile communication as it has the characteristics, strong robust to multipath distortion and the high frequency, and it has been widely used in terrestrial wireless network. However, there are still some practical problems need to be solved when OFDM is applied to the intergrated terrestrial and satellite communication system. Moreover, there are still no mature techniques in satellite communication system yet.The intergrated satellite and terrestrial system is the application background of this dissertation, and the problems caused by the 4G key techniques based on OFDM applied to the scenarioes of satellite and terrestrial segments are discussed systematically. The characteristics of OFDM technology and the principles of conventional channel estimation are analyzed firstly. The key problems such as the disadvantages of conventional pilot aided channel estimation algrithms, joint the Peak-to-Average Power(PAPR) recuction and channel estimation and the problem about the appropriate multiple access scheme which have affections on the performance of transmitter and receiver are investigated to make the further improvements in the intergrated satellite and terrestrial system. The main contents are described in detail as the following:1. Channel estimation and equalization. Channel estimation is a necessary part in OFDM systems, the improvement of channel estimation and the length estimation of channel impulse response are focused on in the part. (1) The proposed effective channel impulse response length estimation method is based on the principle of centralized energy in time domain to detect the tap by setting of threshold for the estimated channel impulse response length. The channel impulse response length can be estimated accurately by the above operation. Meanwhile, it has low complexity. The improvement of performance can also be proved by the theoretical analysis. The proposed time-domain channel estimation method is based on the channel impulse response length and correlation operation, and the received signal with cyclic prefix is adopted for the cyclic correlation to obtain the coarse channel impulse response. And the coarse channel impulse responses are aslo used to obtain the threshold of detecting the channel impulse response length. Moreover, the principle which is similar to the sliding correlation and the guard interval are adopted for obtaining the different channel impulse responses. Then the accurate channel impulse responses are obtained by noise reduction. The improvement of estimation accuracy of the proposed method is proved by simulation and theoretical analysis. Moreover, it is more proper to the OFDM system which has long cycic prefix. And the complexity of the proposed method is low. Based on the proposed algorithm, a channel estimator which can estimate the channel length and the channel impulse response accurately is proposed. (2) An improved channel estimation method based on time-domain Lest Squares is proposed for the large delay spread channel. It can improve the aliasing effect which is caused by channel impulse responseoverlapping of time-domain DFT channel estimation method. The channel impulse response length is obtained and optimized by the principle of centralized power and the threshold detection technique. For vehicular channel model, it is proved by simulation and theoretical analysis. This method is simple and practical. Base on the above methods, a time-domain channel estimator is proposed. (3) The application of OFDM and Single Carrier-Frequency Domain Equalization to satellites based on the idea of the intergrated satellite and terrestrial are discussed. Frequency domain equalization technology is the approprite candidate for equalization as the conventional time-domain equalization has high complexity in this scenario, especially for the severe frequency selective fading satellite channel. The differences between Single Carrier-Frequency Domain Equalization and OFDM which have both low complexity are introduced first, and then the performance of Single Carrier-Frequency Domain Equalization when it is applied to the satellite downlink with high power amplier is analyzed. It is proved that Single Carrier-Frequency Domain Equalization can overcome the shortcomings of OFDM system under the high signal to noise condition, and it is robust to frequency selective fading.2. Joint PAPR and channel estimation. A linear PAPR reduction method is proposed to solve the inherent PAPR problem in OFDM systems, by selecting the appropriate pilot pattern and the information of chanel state as side information which is obtained by the pilot channel estimation at receiver. And it is also a method which belongs to joint channel coding and precoding. It is shown that the performance is greatly improved without increasing the complexity. The non-linearity of satellites based on OFDM is analyzed, and it is also proved that OFDM technology can be adopted in the satellite communication system. The minimum of total degradation is chosen for the optimum value when the problem is analyzed. A method is proposed to estimate the non-linear degradation introduced by the on-board high power amplier. Moreover, some typical satellite transponder characteristics and PAPR performance of on-board signal are verified for different sub-carrier modulation modes. 3. Multiple access techniques of intergrated satellite and terrestrial communication system based on OFDM. Multiple access and on board processing problems for satellite communication systems based on OFDM for the intergrated infrustructure are studied. Firstly, the development of next generation communication technology. Moreover, the future objective and further evolution of satellite communication systems based on all-IP are discussed according to the research on the key techniques and requirements of Beyond 3G/4G wireless communication. Satellite communication network as a necessary complement and extension of terrestrial network will be integrated with the various terrestrial networks, and a seamless integrated network is combined with the two parts including satellite and terrestrial network. Furthermore, it is a trend of satellite communication to integrate the industry of satellite communication with the traditional or Beyond 3G/4G technologies. To satisfy the requirements for future communication systems, the infrastructure of next generation broadband satellite system with inter-satellit link for our country is proposed for the first time. Then, the problem that the alternative of the two multiple access based on OFDM, Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) for the up/down link is discussed. Moreover, time-division pilot structure is proposed for SC-FDMA. The comparisons between OFDMA and SC-FDMA for satellite links are are discussed and analyzed, and some improvements are applied to the on board processing model. Under the Golay coded scheme, the performance of satellite link adopted OFDMA and SC-FDMA schemes is analyzed, and Bit Error Rate and Block Error Rate performance of OFDMA scheme is better than that of SC-FDMA, respectively.