| Popis: |
Because of a progress in marine development for seabed natural resource, deep sea exploring becomes essential. Then, wireless underwater communication has become important to communicate with distanced terminals. In case that the signal propagation channel contains reflection objects such as sea surface and structures in the sea, plural delayed signals are generated and cause the Inter Symbol Interference (ISI) between transmitting symbols. To avoid the ISI degradation, acoustic OFDM communication system inserts gap interval between successive OFDM symbols such as Cyclic Prefix (CP). However, the insertion of CP lowers data transmission bandwidth. This study evaluates an ISI suppression performance for underwater communication system using Space Time Block Code (STBC) with MIMO OFDM system [1]–[2] with no CP insertion. Previously we have reported computer simulation results as [3], and this paper describes improved signal processing method and Ocean experiment results. Simulated Bit Error Rate (BER) performance vs maximum Multipath Delay L. Multipath condition is one direct wave and additional L delayed waves with linearly degraded amplitude and random phase. In case of SNR (dB) more than 20 dB, 70 points delay (corresponding to 11ms) for 128 points OFDM symbol (20ms length) is successfully suppressed, which corresponds to 55% ISI condition. We uused two transmitting and 4 receiving transducers. The distance between TX and RX is roughly 6.0m. 3.3 ms multipath delay is observed with 20ms OFDM symbol. Which corresponds to 16.5% Inter-Symbol-Interference (ISI) condition. Measured constellations for simple OFDM with no CP and 2×4 STBC-MIMO OFDM with no CP. And corresponding measured BERs are 0.0401 and 0.0023, respectively. Because of ISI, the OFDM with no CP suffers the ISI and the BER is degraded, while CP-free 2×4 STBC- MIMO OFDM has shown improved BER because of ISI suppression capability. In this simulation, time series distortion was observed. For time series distortion, BER was improved when AIF canceled. And corresponding measured BER is 0.0017. the BER of CP-free STBC-MIMO OFDM (AIF can cell), which we used to deal with distortion, was the smallest. |
