This paper proposes a demodulation scheme based on channel prediction for differential multiple-input multipleoutput (MIMO) spatial multiplexing employing per-transmit antenna differential mapping (PADM). As PADM is one of the differential space-time trellis codings (DSTTCs), it requires per-survivor processing (PSP) based on the Viterbi algorithm (VA) for its demodulation. Therefore, PADM has excellent tracking performance on fast time-varying channels. However, aiming at higher frequency efficiency, PADM for a higher modulation index suffers from the following two disadvantages: (1) poor receiver sensitivity; (2) high computational complexity. In order to solve the items (1) and (2), this paper employs a spatial multiplexing structure and a state reduction algorithm for the VA, i.e., decision-feedback sequence estimation (DFSE), respectively. Finally, for a frequency efficiency of 4bps/Hz with two receive antennas, computer simulation results confirm that the proposed scheme has excellent bit error rate (BER) performance on fast time-varying channels in the case of the maximum Doppler frequency normalized by a symbol rate of 4%.
In order to improve channel sounding performance, this paper discusses channel sounders employing modified maximal length (M) sequences for acoustic communications in severe time and frequency selective environment, i.e., doubly selective environment. There still exist the following issues in the conventional channel sounders employing M sequences: 1) M sequences have side-lobe for their auto-correlations; 2) they output acoustic signals in undesired frequency band; 3) estimated channel impulse responses are time-varying due to sampling frequency deviation between Tx/Rx sides. This paper employs the modified M sequences, band pass filters in Tx/Rx sides and over-sampling signal processing employing interpolation for the items 1), 2) and 3), respectively. Next, computer simulation results and experimental results confirm that the proposed schemes improve channel sounding performance. Finally, this paper evaluates actual acoustic channel environments using the proposed channel sounders.
This paper discusses channel sounders for underwater acoustic communications (UWAC), which estimate channel impulse response (CIR) using commercial electronic acoustic devices and off-line signal processing. Although transmission channel models are inevitable in performance evaluation of wireless communications, there are a few reports on channel models for UWAC. In order to obtain channel models for UWAC, channel sounders play important roles. Thus, if channel sounders can be realized by commercial electronic acoustic devices, it is easy to sound CIR in UWAC channel environment. The proposed simple acoustic channel sounders can suppress frequency distortion and frequency deviation due to mismatch between a transmitter and a receiver of commercial electronic acoustic devices. Furthermore, the channel sounder can precisely detect time-varying CIR by means of cyclic correlation. Finally, this paper evaluates performance of the proposed channel sounders in actual shallow UWAC environment in Lake Biwa.
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