Research On Key Technologies For Pulse Shaping OFDM/OQAM Systems

The traditional Orthogonal Frequency Division Multiplexing (OFDM) system, which is regarded as a most popular multicarrier modulation system in the current communication field, is widely used in the design of the physical layer communication under different scenarios. But OFDM also has shortcomings. For example, the use of cyclic prefix reduces spectrum efficiency, the rectangular shaping pulse leads to high out-of-band radiation, and the system is sensitive to Carrier Frequency Offset (CFO). Therefore, in the future5generation (5G) mobile communication field, exploring a better multicarrier modulation scheme becomes the focus of many researchers.Using Pulse Shaping Orthogonal Frequency Division Multiplexing Modulation with Offset Quadrature Amplitude Modulation (Pulse Shaping OFDM/OQAM) technology is considered to be an appropriate scheme. Firstly OFDM/OQAM system uses a waveform of good time-frequency localization as the prototype pulse. That is, the pulse’s energy can be well focused in time-frequency domain, which leads to the system with satisfactory ability of resisting Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). What is more, OFDM/OQAM no longer needs a Cyclic Prefix (CP) as the protection interval, which improves spectrum efficiency. Certainly, these nice charactors come at the expense of relaxation in the subcarrier orthogonality, namely the orthogonality is strictly true only in the real domain. This means that there is an inherent imaginary interference between subcarriers and symbols. Although the receiver can remove the interference very easily by taking real operation, but when the signal comes through a complex channel, it is difficult to eliminate the inherent interference. It makes the classical channel estimation and equalization, cannot be directly applied in a pulse shaping OFDM/OQAM system.Channel estimations in pulse shaping OFDM/OQAM systems, can be grouped into two categories, including the scatterd-based and the preamble-based channel estimations. In view of the scatterd-based channel estimation, the existing method called Auxiliary Pilot, has some problems such as decimal pilots of high energy. This paper makes some improvements, by introducing iteration to eliminate the interference on the pilot. In view of the preamble-based channel estimation, the existing Interference Approximation Method (1AM) has the problem of large pilot overhead. The paper simplifies the pilot structure, and proposes three methods to solve the interference between pilots and data symbols.For channel equalization in pulse shaping OFDM/OQAM systems, the paper finds that the classical Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE) equalization can be easily transplanted in OFDM/OQAM systems. While, the Maximum Likelihood (ML) equalization, which is optimal in additive gaussian noise environment, cannot be applied directly. Therefore, the paper proposes the ML channel equalization algorithm based on interference offset, and improves the accuracy of inherent interference estimation by multiple iterations, thus enhances the performance of equalization and reduces the symbol error rate accordingly. Moreover, the derivation of channel equalization algorithms under MIMO-OFDM/OQAM systems is obtained.Finally, this paper simply reviews the research ideas. In view of the shortcomings, the paper also discusses the direction of the future research.