Research On The Channel Estimation In OFDM Systems Over HF Channel

In modern shortwave communication systems, the communication capability and data transmission rate are limited by the channel, which is mostly frequency-selective and band-limited. Orthogonal Frequency Division Multiplexing (OFDM) has recently been applied widely in wireless communication systems due to its high data rate transmission capability with high bandwidth efficiency and its robustness to multi-path delay. It has been used in shortwave communication systems. In order to achieve good performance, channel estimation is necessary before the demodulation of OFDM signals since the radio channel is frequency selective and time-varying. The channel estimation techniques for OFDM systems over HF channels based on pilot symbol aided modulation are investigated.First, the features of shortwave channel are analyzed. And then the time delay spread and inter-symbol interference caused by multi-path effect are discussed. In order to measure performance, the classic Watterson channel model for HF channel is proposed. Next, the design of OFDM systems over HF channel is discussed; also, the principles of OFDM systems including digital modulation, cyclic prefix et al are introduced. And the design of OFDM systems over HF channel is organized. At last, the channel estimation techniques for HF channel based on pilot arrangement are deeply investigated. The channel estimation based on comb type pilot arrangement is studied through different interpolating algorithms including linear interpolation, second order interpolation and low-pass interpolation et al. Furthermore, the channel estimation based on block type pilot arrangement is performed by sending pilots at every sub-channel and implementing decision equalizer for all sub-channels followed by periodic block-type pilots. Also the wiener filter algorithms based on two dimension pilot is discussed. This paper compared the performances of all schemes by measuring bit error rate with 16QAM as modulation schemes and multi-path Rayleigh fading channel as channel models. The results demonstrate the algorithms use more pilots in order to achieve definite BER and are not frequency-efficient.Different pilot schemes can induce different estimation performance. In this paper a new pilot scheme for the estimation of HF channel is proposed, and a corresponding algorithm for the specific scheme is discussed. The algorithm makes use of the pilot clusters, which clusters two neighboring pilot together so as to increase the channel estimate accuracy by averaging over the two pilot symbols. Also it makes use of the channel response of pilot clusters and the sub-channels which have the same time and frequency as pilot clusters. This new scheme could give a better performance in terms of BER with lower pilot density.