A Modeling Method of the IBC System Based on the Composite Fading Channel

Maoyuan Li,Xu Zhang,Yong Song,Yu Chen,Wu Ren

Abstract:Intra-body communication (IBC) is a novel short-range non-RF(radio frequency) wireless communication technique specified by the IEEE 802.15.6 using the human body as a transmission medium.In this work,a new modeling method of the IBC system based on the composite fading channel is proposed,where the cascaded filter is used to express the composite fading channel and the modulation method in transmitter and receiver.The composite fading channel combines with the average attenuation,group delay,multipath effect,and shadowing effect.The modulation is adopted orthogonal frequency division multiplexing (OFDM),and thereby the constellation,cyclic prefix,bit error rate,and pilot mode are determined.As a result,the whole process and multiparameter simulation of IBC system can be achieved.It provides a theoretical foundation for the system design of the intra-body communication and will promote its application to the wireless body area network (WBAN).

Keywords:intra-body communication (IBC);modeling;orthogonal frequency division multiplexing (OFDM);fading channel

1 Introduction

The ever-advancing miniaturization and lowpower consumption of electronic devices,combined with recent developments in wireless communication,is leading to a rapidly increasing demand for wireless communications in the human body area.In a scenario of human body area communications,various communication devices may locate on,in,or near the human body to form a wireless link or a small-scaled network to share data,reduce functional redundancies,and allow for new services.As an emerging communication technique,it is especially expected to be useful in medical,healthcare,and consumer electronics applications.It may provide new possibilities in high-quality medical and healthcare services by linking various on/in-body vital sensors to establish a body area network (BAN) of personal health information [1].

One of the alternative technologies for BAN is intra-body communication (IBC),which is referred to as human body communication [2],body channel communication [3],body coupled communication [4],conductive communication [5],and in-body radio [6],etc.Human body communication (HBC) uses the conductive properties of the human body to transmit the signal [7].Compared with other IEEE 802.15.6 technology,such as NB,UWB [8],and IEEE 802.15.04 technology,such as Bluetooth,ZigBee [9],IBC has a lower operating frequency of 0.1 MHz–100 MHz for research [10].The lower frequency yields relatively smaller power absorption in the human tissue,resulting in deeper penetration into the tissue,and smaller diffracted field attenuation,leading to the shadow effect not so obvious [11].Moreover,generally,the received field component of IBC with a larger wavelength is mainly contributed by the electrostatic field,denoting the radiation field components is uneasily resonant within the human body [12].In a word,IBC is advantageous for low consumption,high efficiency,security,and anti-interference.

IBC models contribute to the design process as well as to the understanding of transmission mechanisms through biological tissues [13].Some modeling methods such as circuit model,numerical model,analytical model are widely used to investigate the characteristics of IBC [14].In previous works,the transmitter is simply abstracted as voltage/current source and internal resistance in series,while the receiver is simplified as a load impedance,which lacks the description regarding modulation/demodulation,coding/decoding,and so on.The whole simulation process has not been achieved to provide an accurate basis for system design.On the other hand,there is another way that can achieve the whole process simulation,i.e.,filter method [14].Such method mainly abstracts the transmitter,receiver,and channel as the cascaded filter to express linear time-varying systems with randomness [15].The foremost prerequisite of filter simulation for optimizing the system performance is acquiring the accurate channel parameter such as averaged attenuation,group delay,and fading characteristics.

In this paper,a new modeling method of the IBC system based on the composite fading channel is proposed,where the composite fading channel combines with the average attenuation,group delay,multipath effect,and shadowing effect.The modulation is adopted orthogonal frequency division multiplexing (OFDM),and thereby the constellation,cyclic prefix,bit error rate,pilot mode are determined.As a result,the whole process and multi-parameter simulation of IBC system can be achieved.It provides a theoretical foundation for the system design of the intrabody communication and will promote its application to the wireless body area network(WBAN).

2 The Overall Model

The overall model of the IBC system based on the composite fading channel is plotted in Fig.1,where the modulation is adopted OFDM.The model includes a transmitter (TX),human channel,and receiver (RX).In the transmitter,the encoded data stream d is segmented into the partitioned data Xk.Sequentially,the pilot is inserted into each partitioned data,where the pilot insertion is to estimate the time-varying and frequency-varying channels. The partitioned data can be regarded as Fourier spectrum coefficients(X0,X1,...,XN–1),and the PSK (phase shift keying) or QAM (quadrature amplitude modulation) is employed to conduct constellation mapping operation.The signal is converted into the time domain signal by IFFT (inverse fast Fourier transform).

Next,the cyclic prefix is added to the modulated signal.The signal,s(n),is processed by upconversion in turn and s'(n) is injected into the human body.The human body is modeling as filter,which will state in Section 3 in detail.In the receiver,signal r'(n) is detected by electrode,and down-conversion is operated to baseband.Fast Fourier transform (FFT) is conducted after cyclic prefix deduction. The signal Y' is recovered by pilot signal and received signal d' is restored by constellation inverse mapping and data registration.

2.1 OFDM

In the sub-section,OFDM is mathematically introduced.The core idea of OFDM is to use parallel transmission technology to reduce the communication rate on the sub-channel,so as to increase the symbol length and reduce the influence of inter-symbol interference (ISI) caused by the multipath effect.

In Fig.1,the allocated channel bandwidth is divided into N sub-channels.Given the time period T,the complex signal s(n) can be express as

Fig.1 Block diagram of the IBC system based on the composite fading channel

where Xkis the kth Fourier spectrum coefficients,tsis the initial time,and Tsis the sampling time.

Change the sign bit from serial transmission to parallel transmission,so that Ts=0,thus,

where a normalization factoris used to maintain the total signal power unchanged,and ψ represents the sub-carrier to achieve IFFT.

To reduce the consumption,the up-conversion process should be mixed by a mixer with the center frequency,f0,after IFFT operation.

Correspondingly,the down conversion operating in a receiver is the inverse process of the transmitter.

Moreover,the receiver signal over the time domain can be express as

where h(n'Ts) is the unit impulse response,n0is the noise of the channel,mTsis the delay spread.The demodulated signal can be expressed as

where N0is the discrete Fourier transform spectrum of n0,and H(k') is the frequency response of a channel,i.e.,

According to (5),ICI (inter-carrier interference) caused by multipath effect will occur at the first M–1 sampling point in the signal after demodulation,thus cyclic prefix with the length of Tg>mTsshould be added to eliminate ICI.

2.2 Pilot Estimation

As shown in Fig.1,the IBC system based on the composite fading channel includes a pilot signal module in the transmitter,while the pilot extraction and equalization module are in the receiver.Under the premise of strictly satisfying the synchronization,the receiver signal over the frequency domain can be derived from (4),and

where 0≤k≤N–1,Yk(k),Xk(k),H(k),N0'(k) is the frequency spectrum express of r(n),s(n),h(n),n0'(n),respectively. Given the number of the sub-carrier is N,and known the pilot information in the receiver,the inverse channel 1/H*(k)can be obtained.

where Θ is the nonuniform error.

In general,since the body channel slowly varies with time,it is necessary to have pilot symbols for equalizing the channel response.The basic 2D arrangement of the pilot can be categorized into two types:block-type pilot and combtype pilot,which respectively represent inserting pilot signal with interval Dtand Dfat time and frequency domain.Such pilot modes should respectively meet the conditions as follows

where fmaxand tmaxare the maximum Doppler shift and maximum delay spread of the channel,respectively.Moreover,the least-square (LS) algorithm is used to estimate the pilot.

3 Composite Fading Channel

Different from the transmission line,coaxial cable,optical fiber,and other constant parameter channels,the intra-body communication experiences random fluctuations over time by multipath effect [16],shadowing effect [17],and even group delay [18].Such randomness leads to amplitude change,inter-symbol interference,frequency shift.As a result,the human channel can be regarded as a composite fading channel.

Since the human body is a non-free space and lossy medium,the reflection,scattering,and diffraction would happen during electromagnetic wave propagation in the human body.The signal component along different paths has a time delay,namely,the multipath effect.It will make the amplitude and phase of the signal fluctuate,when the channel has frequency selectivity.

Moreover,due to the non-linear phase-frequency characteristic,the EM wave with different frequencies has different propagation speeds causing by the group delay,which means the envelope of the wideband signal will distort while propagating in the human body.

On the other hand,the multipath signal components have different incident angles between signal transmission direction and motion directions of the receiver. Different frequency shift occurs at different path component due to Doppler effect,when the channel has time selectivity.

The dielectric properties is a biological tissue can be considered as a composition of water and organic material.The aging and the fluid intake will make the amount of water inside the biological tissue change,thus changing the dielectric property correspondingly. As a signal propagates through a human channel,it experiences random fluctuations in time and can be regarded as a large-scale fading characteristic.

Therefore,the composite fading channel model is proposed by establishing a filter. As shown in Fig.2,where the average attenuation,group delay,multipath effect,and shadowing effect has been added in the model.

Fig.2 Composite fading channel of intra-body communication

As shown in Fig.2,at first,the injected signal S'(f),is filtered by averaged attenuation and phase characteristics module,and expressed as

where the l th path component with the power of aland time delay of lTs,and Pmis the power expression in dB,and corresponding to am.

Regarding the Doppler effect module,given that the relative velocity between transmitter and receiver is v,the maximum Doppler frequency shift is express as fd=v/λ,where λ is the wavelength.The power density of the dispersion spectrum can be express as follows,assumed that the angle of incidence at receiver performs even distribution [19].

where fcis the center frequency of the subcarrier.After that,the convolution is operated at the Doppler filter module,

Known the received signal will typically experience random variation,a model for the random attenuation due to these effects is also needed. Since the location,size,and dielectric properties of the blocking objects as well as the changes in reflecting surfaces and scattering objects that cause the random attenuation is generally unknown,statistical models must be used to characterize this attenuation.Until now,log-normal distribution p(ψ) is seemly accepted as an additional attenuation [14].

where ψ=Pt/Pris the ratio of transmit-to-receive power,ξ=10/ln10,μψdBis the mean of ψdB=10log10ψ in dB and σΨdBis the standard deviation of ψdB,also in dB.Moreover,the shadowing effect module is used to filte as follows

4 Simulation Paramters and Results

4.1 System Parameters Design

The parameters of IBC transmitter/receiver should follow some conditions.Firstly,according to IEEE 802.15.6 protocol standard,the center frequency is set as 21 MHz,while the bandwidth is less than 5.25 MHz [8].Secondly,the delay spread is less than 200 ns [20].Finally,the transmission rate is up to 6 MHz.

Therefore,the parameters of IBC transmitter/receiver are set as Tab.1.Basically,the interval Tgis the 2–5 times of the delay spread,and thereby the interval is set as 1 μs.Moreover,the symbol period without the interval is generally the 4–5 times of the interval,thus setting symbol period T as 6 μs.In another word,the bandwidth of the sub-carrier is 1/(T–Tg)=200 kHz.To meet the requirement of the transmission rate bigger than 6 Mbit/s,the transmission rate is initially set as 8 Mbit/s,while every symbol period could carry 8 Mbit/s×6 μs=48 bit.Therefore the 16QAM modulation with 12 sub-carriers can be used to cover the bandwidth of 2×(12+1)×200 kHz=5.2 MHz，which is less than the required bandwidth of 5.25 MHz.

Tab.1 Parameters of IBC transmitter/ receiver for simulation

4.2 Composite Fading Channel Parameters

Here listed are composite fading channel parameters.The averaged attenuation has used the results in [21],where the fitting equation is

We extracted the impulse response of IBC channel from [22],and then got the corresponding delay power density spectrum.At the 20 dB threshold,the delay spread of the multipath effect module of the IBC channel is 200 ns.Thus the path components can be divided into

Tab.2 Multipath effect module parameters

Due to the low operating spectrum for HBC and slow speed caused by physiological motion,the Doppler effect can be negligible for HBC(Doppler spread is smaller than 2 Hz at 400 MHz) [23].As a result,the frequency shift is set as 0 Hz in this work.Moreover,in [14],the large-scale fading is investigated,and thereby the typical value σΨdB=0.42 is used.

4.3 Simulation Results

Based on the established model,the bit error rate(BER) performance is emulated by the Monte Carlo method with the 107 amount of pseudorandom code.The noise is regarded as AWGN(additive white Gaussian noise),and the signal to noise ratio (SNR) of 10,20,30,and 40 dB is simulated.Meanwhile,in order to facilitate comparison,the ideal channel (transfer function is a constant without any fading characteristics) is also obtained.According to the BER,the system performance is optimized.

As shown in Tab.3,the mapping constellation comparison of the simulation results with different SNR is listed.It can be seen that the mapping constellation of 16QAM received signal under different SNR conditions is filtered by composite fading channel of intra-body communication.When the signal sequence is without a pilot,the mapping of the signal with cyclic prefix and without cyclic prefix is more divergent with the decrease of SNR.Despite that the cyclic prefix can restrain divergency at high SNR(30,40 dB),the subcarriers no longer keep orthogonal and the phase rotation emerges due to non-flat characteristics of the channel.

Tab.3 Mapping constellation comparison of the simulation results

Moreover,the signal sequences with block and comb pilot have different performances.According to the mapping constellation,the comb and block pilot can restore the in-phase component and quadrature component positions to some extent.However,the overall performance of the block pilot is lower than that of the comb pilot.As shown in Fig.3,under the same SNR,the BER of the simulation with cyclic prefix and without pilot is greater than that with the comb and column pilot.Moreover,under the condition of high SNR (SNR>20 dB),the BER of comb pilot is lower than 3 orders of that of block pilot.

Fig.3 Simulation results of signal-to-noise ratio (SNR) vs.bit error rate (BER) on the condition of without pilot,with block pilot and with comb pilot

5 Conclusions

Intra-body communication (IBC) is a novel short-range non-RF wireless communication technique specified by the system based on the composite fading channel is proposed,where the cascaded filter is used to express the composite fading channel and the modulation method in transmitter and receiver.The composite fading channel combines with the average attenuation,group delay,multipath effect,and shadowing effect.The modulation is adopted OFDM,and thereby the constellation,cyclic prefix,bit error rate,pilot mode are determined.It can be seen that,under the same SNR,the BER of the simulation with cyclic prefix and without pilot is greater than that with the comb and column pilot.Moreover,under the condition of high SNR(SNR>20 dB),the BER of comb pilot is lower than 3 orders of that of block pilot.

As a result,the whole process and multiparameter simulation of IBC system can be realized.It provides a theoretical foundation for the system design of the IBC and will promote its application to the WBAN.