Studies On The Key Techniques For Frequency Hopping OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is an effective technique for high speed transmission which is commendable to be applied in the region of military communication. Frequency Hopping OFDM (FH-OFDM) system improves its anti-jamming ability by switching frequencies in succession, which makes it bearing with the features of both continuation and burst communication mode. It comes up with higher requirement on the carrier frequency offset (CFO) estimation and channel estimation, which requires to guarantee the estimated performance and to improve the transmission efficiency as much as possible at the same time. Moreover the transient state during the communication would become a significant factor impacting the system performance.In conventional OFDM systems, the pilots are interpolated for the tracking of residual CFO and channel, which expend a lot of system resources. In order to improve the utilization ratio of bandwidth, we propose two new joint CFO and channel estimation algorithms which need no pilots. The phase distortion suppression technique for RF FHOFDM systems is also studies. The main research findings are shown as follows.1. By choosing Cramer-Rao Bound (CRB) as the unified reference point and Maximum Likelihood (ML) criteria as the unified estimation criteria for CFO and channel estimation, the impaction between CFO and channel estimation and the performance upper limit of joint estimation are analyzed systemically. The conclusion is drawn that the JCRB (Joint CRB) is beeter than the CRB of dependent estimation where the statistical characteristicr is known for CFO and is closed to the CRB in ideal conditions for the channel.2. Propose a joint Detected-Directed (DD) CFO and channel estimation technique, and obtain the expressions of Mean Square Error (MSE) of the CFO and channel estimation results with a certain Symbol Error Rate (SER) by theoretical derivation. In order to improve the estimation performance furtherly, open loop and closed loop schemes are suggested for different SNR ( Signal Noise Ratio) and channel environment. The rules that how to set the values of the loop factors are also suggested for both continuous transmission and burst transmission modes. In consideration of the problems of error spreading and error platform phenomenon, this algorithm should be assisted with the channel preference algorithm. We use the normalized Vector Signal Errors (NVSE) as the metric for the algorithm which is more robust than conventional ones.3 . Propose two CFO and channel joint estimation algorithm based on Expectation-Maximization (EM) algorithm, which are named as time domain EM (TEM) and frequency domain (FEM) algorithms according to the disposal domain respectively. The refinement algorithms to improve the performance or decrease the complexity are also proposed. The former is constructed on Bayesian criteria for both two schemes and the latter is disposed subchannel by subchannel for FEM. According to the methods of data reconstruction, the algorithm can be combined with the A Posteriori Probabilities (APP) detector or Soft-In Soft-Out (SISO) decoder to implement the joint detection of CFO, channel and data, which can improve the estimated performance greatly. The estimation performance, complexity and application settings of the two EM algorithms are analyzed and compared. The algorithm disposed in the frequency domain has lower complexity and better performance for deep fade channels and high order modulation systems. They all conquer the shortcoming of error spreading.4. Build the new state transition model for CFO and channel parameters in time varying channels. The joint estimation algorithm based on EM criteria is proposed which can be realized with EM-Kalman algorithm and tracking the time varying channel effectively. As compared with the conventional EM-Kalman algorithm based on pilots, it can improve the utilization ratio of bandwidth more than 10% and make the performance loss less than 0.5dB at the same time.5. The phase distortion of RF FH -OFDM system is made up of phase jitter and spurs. We build the corresponding statistical model for them and analyze the impact one system performance. The unified Bit Error Rate (BER) expression is made for MPSK modulation systems. Based on the analysis above, the corresponding phase jitter suppression and spurs suppression algorithm are proposed which are based on Least Square (LS) and Minimum Mean Square Error (MMSE) criteria respectively. We estimate the equalizer directly and reduce the effects of phase distortion greatly with a few pilots.