Research On Some Key Technologies For Ultrasonic Through-Metal Communication Based On OFDM

Ultrasonic through-metal communication is a kind of communication method which uses ultrasonic waves as the carrier to transmit data through the metal without drilling a hole.Electromagnetic waves cannot penetrate metal effectively due to the electromagnetic shielding caused by the skin effect of the metal.Drilling a hole in the metal will destroy the structural integrity of the metal,bringing potential dangers.Since ultrasonic waves can effectively transmit through metal material without drilling any holes in the metal,many researchers utilize ultrasonic to achieve through-metal communication without drilling holes.After entering the metal plate,ultrasonic waves generated by the piezoelectric transducer will reflect back and forth in the metal plate,resulting in multiple echo signals received by the receiving end.This echo effect is similar to the multipath effect,resulting in certain particularities in ultrasonic through-metal communication.Orthogonal frequency division multiplexing(OFDM)technique can effectively resist multipath fading in the ultrasonic through-metal communication,but it also brings new problems,for example,how to estimate the state information of the ultrasonic throughmetal channel that suffers severe frequency selective fading in the OFDM system and how to improve the reliability of encoding transmission for the ultrasonic through-metal communication system.This dissertation treats the ultrasonic through-metal communication system based on OFDM as the research object and mainly focuses on the five issues below: ultrasonic through-metal channel model,ultrasonic through-metal channel estimation method,dynamic subcarrier distribution technology,bit interleaver of encoding transmission,and the cascaded integrator comb(CIC)filter compensation in digital up-down conversion.The main work and innovations of this dissertation are as follows:(1)Ultrasonic through-metal channel has a multi-layer structure,which involves many material parameters and complex coupling relationships.A simplified frequency domain model of the ultrasonic through-metal channel is proposed to reduce the complexity of modeling the ultrasonic through-metal channel.By combining the equaltime interval characteristic of echoes and the exponential decay characteristic of echoes during ultrasonic transmission in the metal with the tap delay line model,the frequency response of ultrasonic propagation in the metal is derived and then multiplied by the approximate amplitude-frequency response of the piezoelectric transducer to obtain the proposed simplified frequency domain channel model.Simulation results show that the proposed model can effectively approximate the frequency response of the ultrasonic through-metal channel while retaining the characteristics of the ultrasonic through-metal channel.(2)Because the ultrasonic through-metal channel suffers a severe frequency selective fading,it is difficult to estimate subcarriers’ state in the frequency domain of ultrasonic through-metal communication based on OFDM.Therefore,by taking advantage of the equal-time interval characteristic of echoes and the exponential decay characteristic of echoes during ultrasonic transmission in the metal,a piecewise suboptimal threshold selection algorithm based on the transform domain technique is proposed.The proposed algorithm first segments the mean square error(MSE)between channel estimate values and real values,resulting that each segment contains only one echo signal.The derived piecewise sub-optimal thresholds corresponding to each segment are utilized to select the effective channel impulse response signals.And then,a further selection is adopted to get rid of the possible noise samples for the subsequent segments according to the selection result in the first segment.Finally,the selected impulse signals are transformed into the frequency response of the ultrasonic through-metal channel.Simulation results show that,compared with the existing channel estimation algorithms based on the transform domain technology,the proposed algorithm can achieve a lower MSE of channel estimate values and has strong robustness,which means it is independent of the initial ultrasonic velocity,and applicable to different thickness metals,and not affected by the number of estimated echoes at a low signal-to-noise ratio.(3)Dynamic subcarrier allocation technology can improve the bit throughput of the OFDM system for ultrasonic through-metal communication.Under the condition of limited power,the bit loading algorithm based on greedy strategy can achieve the optimal bit allocation,but need to calculate subcarriers’ two Bit Error Rates(BER)before and after allocation,resulting in a large computation.Therefore,a bit loading algorithm with the fast selection of subcarriers for ultrasonic through-metal communication is proposed.The proposed algorithm adopts the bit error rate(BER)approximation formula to derive the BER increment approximation expression of loading extra bits on the subcarrier.Taking into account the influence of different bit increments on the average BER,the BER increment approximation expression is further modified.And then,the modified BER increment approximation expression is expanded by the Taylor formula to select subcarriers by the first several terms.Simulation results show that the bit throughput by using the proposed algorithm is close to that of the greedy bit loading algorithm,but about3/4 of the BER calculation for subcarriers is saved,which greatly reduces the computation amount of the greedy bit loading algorithm.(4)The frequency response of the ultrasonic through-metal channel has a certain vibration period,resulting that the OFDM subcarriers in each cycle may suffer deep fading and then generate multi-segment consecutive error bits.Aiming at the problem of multi-segment consecutive error bits in the ultrasonic through-metal communication based on OFDM,a multiple-input bit interleaver of ultrasonic through-metal communication is proposed.The proposed bit interleaver utilizes the periodicity of the frequency response of the ultrasonic through-metal channel to group the coded bits,and then the coded bits in each group are shifted and interleaved by the rectangular interleaver to maximize the minimum span of grouped bits,which not only reduces the influence of unequal error protection on BER in the higher-order modulation but also effectively disperses the continuous error bits in a cycle into different positions of different cycles.The approximate BER performance of the proposed interleaver is analyzed and verified by simulation.Simulation results show that the proposed interleaver achieves a lower system BER than random interleaver,and effectively improves the reliability of the ultrasonic through-metal communication.(5)In the development of the OFDM-based ultrasonic through-metal communication system,digital up-down conversion is the key to the conversion between the baseband signal and the band signal,and the sampling rate conversion is a key in digital up-down conversion.Although multistage CIC filters can realize the sampling rate transformation with low complexity,it has a serious passband attenuation problem.Therefore,aiming at the passband attenuation problem of multistage CIC filters in the development of an ultrasonic through-metal communication system,an integer programming model for designing the compensating filter is proposed,which transforms the optimization model whose optimization variables are fractional coefficients of compensating filter into an integer programming.By using the relationship between filter coefficients,the number of optimization variables in the integer programming is reduced.Finally,the particle swarm optimization algorithm is used to improve the solving efficiency for the integer programming.Simulation results show that when the number of the compensator’s coefficients is seven,the passband deviation can be less than 0.03 d B,which means the passband attenuation of the CIC filter is greatly reduced.