Research On The Pilot Design For NC-OFDM Systems

With the development of communication, Internet and multimedia, more and more users hope that wireless communication systems can provide richer mobile services, such as image transmission, audio and video on demand, multimedia services. Telecom operators also expect that the wireless networks in the future can easily expand some new services without frequent replacement of network infrastructure and equipment. Drived by this demand, the research on the 4G mobile communication systems is currently being developed around the world.As an effective spectrum utilization technology, Non-Contiguous OFDM (NC-OFDM) can be used in the environment of discrete spectrum. It has not only the traditional OFDM advantages of against multipath fading and high speed transmission, but also the flexibility in spectrum aggregation, power control and spectrum allocation. Therefore, it is considered as one of the most competitive candidate transmission technology in 4G mobile communication systems.NC-OFDM looses the restriction that traditional communication system should work within a continuous spectrum, and in the meanwhile, some technical challenges are brought. Especially, the pilot design is one of the key technologies to be resolved. In traditional OFDM systems, equispaced and equipowered pilot sequence is generally used. However, this pilot sequence cannot be applied to NC-OFDM systems due to the non-continuous spectrum. Therefore, the study on the pilot design for NC-OFDM system has the theoretical and practical significance.In this thesis, the existing research is reviewed, and the technical features and new challenges in the NC-OFDM systems are also described. Specially, the pilot design is fully explored. By the theoretical analysis and simulation experiments, several pilot designs with low complexity for NC-OFDM systems are proposed. The main contributions of this thesis are as follows.1. The pilot design for SISO-NC-OFDM systems.By minimizing the normalized mean square error of symbol estimate (SENMSE) with least squares (LS) channel estimation, firstly analyze the constraint relations among the optimal data power, the optimal pilot power and the optimal pilot placement, and then present a suboptimal pilot design, where the number of pilot tones for each frequency band is firstly determined, and then the position of each pilot tone can be obtained through a simple search method.2. The pilot design for MIMO-NC-OFDM systems with independent channels.By minimizing the MSE of the channel frequency response over the data subcarriers (DSCFR MSE) with LS channel estimation, firstly derive the constraint relations between the optimal pilot power and the optimal pilot placement of each transmitted antenna by disjoint pilots placement, then give two suboptimal pilot designs which can approximate the optimum theoretical solution through sequentially solving the pilot position of each transmitted antenna.3. The pilot design for MIMO-NC-OFDM systems in correlated channel.By the minimizing the DSCFR MSE with linear minimum mean square error (LMMSE) channel estimation, firstly establish a new system model which is equivalent to the original MIMO-NC-OFDM system, then present a suboptimal pilot design by disjoint pilots placement. In the proposed pilot design, firstly allocate the total pilot power for each transmitted antenna, and then find out the pilot position of each transmitted antenna sequentially.