Abstract
At present, mud pulse transmission systems are widely used in downhole data transmission. But the systems are very low in transmission efficiency, only 5-10 bits/s, with very large anti-inter-symbol-interference (ISI). It cannot meet high requirements for high-speed transmission of modern logging system. The development of communication technology has laid some foundation for this requirement. For this purpose, the Orthogonal Frequency Division Multiplexing (OFDM) Wireless Downhole Transmission Systems are proposed for the first time because of their high transmission rate, anti-inter-symbol-interference (ISI), and high spectral efficiency, etc. Due to non-linear power amplifier (PA) of logging systems with limited dynamic range, the drawbacks of high peak-average power ratio (PAPR) may outweigh all the potential benefits of OFDM wireless downhole transmission systems. Selective mapping (SLM) method can reduce the PAPR of OFDM logging signals without distortion. But at the receiver, the conventional SLM method needs exact bits of side information (SI) to recover the data signal. The probability of erroneous SI detection has a significant influence on the error performance of the system. And individual transmissions of SI result in the reduction of bandwidth efficiency. To restore the exact data signal, our scheme codes the SI bits by linear block codes (LBC), and is easily decoded by syndrome decoding. And then the coding SI bits are superimposed onto the logging signals to omit SI bits transmission. The theory and simulation results show that the proposed method has better performance than the conventional one. Accordingly, the OFDM wireless downhole transmission systems can tackle the high PAPR problem, and highten the transmission rate of logging signals.
Highlights
Nowadays, due to limited data transfer rate of mud pulse transmission [1,2,3], no logging data can be transmitted real-timely to the ground
This paper proposes Orthogonal Frequency Division Multiplexing (OFDM) wireless downhole transmission systems and an improved Selective mapping (SLM) scheme to reduce peak-average power ratio (PAPR)
The simulation of the proposed scheme and the conventional SLM schemes is performed for the OFDM system and OFDMA downlink system specified in IEEE 802.16 standard, which uses 256 and 2048 subcarriers respectively, QPSK, and 16-QAM, and 64-QAM modulations
Summary
Due to limited data transfer rate of mud pulse transmission [1,2,3], no logging data can be transmitted real-timely to the ground. Many schemes have been proposed to reduce PAPR, such as companding technique [17], clipping [18,19], partial transmit sequence (PTS) [20,21], Code Blocking [22,23,24], tone reservation (TR) scheme [25], and selective mapping (SLM) [26,27,28,29] Among these schemes, SLM is more popular than others because it reduces the high PAPR of OFDM signal without distortion. The transmission of side information is one of the main drawbacks of SLM In this case, this paper proposes OFDM wireless downhole transmission systems and an improved SLM scheme to reduce PAPR. The systems will make a good performance on high rate downhole transmission
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