In this thesis work, we study the performance of 2400 bps serial type high frequency (HF) modem algorithms via computer simulation. To obtain the performance of the HF modem algorithms, the whole HF modem building blocks are implemented in software and simulated using a software HF channel simulator.
At the HF modem transmitter, eight-phase DPSK encoding is done and then modulates and transmits it to the HF channel. The HF modem receiver receives the modulated signal from the HF channel and demodulates the received signal to recover the original input data stream. However, the input signal at the HF-modem receiver experiences distortions due to Doppler shift, Doppler spread, multipath delay, and multipath spread when it passes through the HF channel.
To compensate these time-varying channel distortion, we use an adaptive decision feedback equalizer (ADFE) in the modem receiver. For ADFE tap coefficient adaptation, three kinds of adaptation algorithms are used and their performances are compared. They are the least mean square algorithm (LMS), the least square lattice (LSL) algorithm, and the square root Kalman (SRK) algorithm. The error signal in the ADFE is used to update ADFE tap coefficients as well as to adjust the phase of carrier phase tracking loop. So the carrier tracking loop operates simultaneously with ADFE as a first-order or second-order digital phase-locked loop (DPLL) according to the order of a loop filter.
Computer simulation of the HF modem algorithm is done by varying the HF channel parameters. Simulation result shows that, in order to compensate the time-varying characteristics of the HF channel, fast converging adaptation algorithm such as Kalman filtering and LSL algorithms should be used to update ADFE tap coefficients. Among the three adaptation algorithms considered here, SRK tap adaptation algorithms yields the best performance in both the symbol error rate and the final mean square error (MSE).
본 논문에서는 2400 bps seriel-type HF modem algorithm의 성능을 computer simulation을 통하여 연구하였다. HF modem의 성능을 분석하기 위하여 전체 modem을 software로 구성하였고 software HF channel simulator를 통하여 simulation 하였다.
HF modem 송신기에서는 입력된 데이타를 8-phase DPSK encoding을 한 다음 변조하여 HF channel을 통하여 전송한다. HF modem 수신기는 HF channel을 통과한 신호를 수신하고 복조하여 송신된 데이타 신호를 얻는다. 그러나 HF modem 수신기의 입력은 Doppler shift, Doppler spread, multipath delay, multipath spread 등에 의해 distortion을 받게된다. 이와같은 channel distortion을 보상하기 위해 DFE를 사용한다. 본 논문에서는 DFE의 tap 계수 adaptation을 위해 LMS, LSL 및 square root Kalman을 사용하였고, 이에 대한 성능을 분석하였다. DFE의 error signal은 DFE의 tap 계수를 update 하고 carrier tracking loop의 phase를 estimate 하는데 사용된다. 그래서 carrier tracking loop은 DFE와 동시에 simulation 하였다. 사용된 carrier tracking loop은 first-order 또는 second-order DPLL과 data-aided loop을 결합한 형태를 사용하였다.
HF modem algorithm의 computer simulation을 통하여 HF channel의 여러가지 parameter들에 대해서 simulation 하였으며, 그 결과 time-varying HF channel을 보상하기 위해서는 수렴속도가 빠른 Kalman algorithm이나 LSL algorithm이 사용되어야 함을 알 수 있었다. 본 연구에서는 위의 세가지 algorithm 중 square root Kalman algorithm이 가장 우수한 특성을 보였다.
