Multi-carrier modulation excitation analysis for acoustic transmission signal in a drillstring

The multiple echoes caused by periodic structure of a drillstring in acoustic telemetry during drilling tend to lead to the serious intersymbol interference and high bit error rate. According to the multipath transmission characteristics of a drillstring, based on the multi-carrier acoustic transmission test analysis along a short drill pipe, a finite difference model for a periodic channel composed of multiple drill pipes and tool joints was established. Considering the ground surface noises and multipath echo interferences in the channel, a least mean square adaptive equalization method was introduced. Then, the multi-carrier modulation performance was simulated by using a drillstring structure with four drill pipes and three tool joints. The simulation results showed that the orthogonal frequency division multiplexing (OFDM)-based multi-carrier modulation can obtain a bit error rate (BER) which is 50% less than that of the phase-shift-keying (PSK) modulation; although its transmission charateristics is sensitive to a non-periodic channel structure, its BER is superior to that of PSK modulation and it can be effectively improved by use of sub-carrier phase compensation.