Magnetic Induction Based Through-the-Earth Emergency Communications

When geological disasters such as earthquakes and mudslides as well as production safety accidents such as gas explosions and coal mine collapses occur,the burial of underground spaces such as underground shopping malls and mines is very likely to cause people to be trapped.Achieving efficient and stable through-the-earth information delivery is the key to ensuring underground emergency rescue.However,using electromagnetic waves as the propagation medium to achieve through-the-earth transmission has problems such as serious signal attenuation,channel instability,and large antenna size.Thus,it is desired to develop a new technique to improve the emergency rescue capability for the underground post-disaster space.As compared with electromagnetic waves based communication techniques,magnetic induction(MI)communications,which are based on Faraday’s laws of electromagnetic induction,has been widely considered as an effective technique to achieve through-the-earth communication due to the high penetration efficiency,stable channel,and small antenna size.However,the conventional coil antenna has only one resonant frequency,which results in a small number of available frequencies.In addition,the rapid attenuation of magnetic field leads to the low energy efficiency and spectrum efficiency.To solve above problems,we focus on the following researches:1.To solve the problems of limited frequency resources,low spectrum efficiency and energy efficiency in existing through-the-earth MI communications,we propose a multifrequency resonating compensation(MuReC)based multi-band,multiple input,and multiple output(MbMIMO)simultaneous wireless information and power transfer(SWIPT)system,where an access point(AP)is equipped with a MuReC coil array to simultaneously deliver data and power to multiple receivers(RXs)with different resonant frequencies.As compared with the traditional single-resonance MI transmission,each coil of the MuReC coil array expands the available frequency-band and can resonate at multiple frequencies.Thus,MuReC coil array can comprehensively utilize frequency resources and antenna resources to support multi-band MI transmission.We propose the magnetic beamforming scheme for MuReC-MbMIMO-SWIPT,which can minimize the transmit power and satisfy the total throughput,total receive power,and individual constraints of each frequencydomain channel.Then,for two special cases of MuReC-MbMIMO-SWIPT,that is,MuReCMbMIMO communications and MuReC-MbMIMO wireless power transfer(WPT),the optimal magnetic beamforming schemes are given.The optimal beamforming scheme in closed-form for MuReC-MbMIMO-communications is derived.Finally,the parameter design scheme for MuReC coil is given,which can make the MuReC coil resonate at arbitrary pre-scheduled frequencies.Numerical results verify our theoretical analyses and show that MuReC-MbMIMO transmission with the proposed beamforming schemes can fully utilize antenna resources and effectively support multi-frequency access.2.To solve the problem of low energy efficiency and spectrum efficiency in existing works about bidirectional MI through-the-earth communications,we propose a backscatter based bidirectional full-duplex MI(BFM)communication technique,where the additional information can be sent with the inherent backscatter property of the MI channel and the backscatter device(BD)is free of self-interference(SI).For BFM communication and time division multiple access(TDMA)based multi-user networks,we formulate the joint magnetic beamforming and time allocation optimization problem,which aims to minimize the average energy consumption and guarantee the downlink signal to noise ratio(SNR),uplink signal to interference plus noise ratio(SINR),and energy harvesting requirements.The formulated problem is non-convex and transformed into magnetic beamforming and time allocation sub-problems.Based on block coordinate descent(BCD)method,the subproblems are solved iteratively.For BFM communication and frequency division multiple access(FDMA)based multi-user network,the magnetic beamforming scheme is also given.Numerical results show that the BFM communication and the proposed beamforming and time allocation scheme can effectively reduce the average energy consumption and enhance the spectrum efficiency and energy efficiency.3.To solve the problem of low spectrum efficiency in existing works about pointto-point MI through-the-earth communications,we employ multiple input multiple output orthogonal frequency division modulation(MIMO-OFDM)technique to improve the spectrum efficiency.For multiplexing MIMO-OFDM-MI communications,the closed-form solution to the optimal current control scheme is proposed,which can maximize the spectrum efficiency under the constraint of transmit power.For diversity MIMO-OFDM-MI communications,a magnetic beamforming schemes is proposed.In addition,for two special cases of MIMO-OFDM-MI communications:single input multiple output(SIMO)based OFDM-MI communications and single input single output(SISO)based OFDM-MI communications,the closed-form solutions of optimal power control schemes are given.For multiple input single output(MISO)based OFDM-MI communications,the optimal magnetic beamforming scheme is derived.Numerical results verify our theoretical analyses and show that MIMO-OFDM-MI communication system and the proposed current control as well as magnetic beamforming schemes can significantly enhance the spectrum efficiency.