HBC channel defined by IEEE 802.15.6

IEEE 802.15.6 channel modeling subcommittee has approved measurements for frequency range 13.550 to 13.571 MHz and the surprising result is that human body exhibits almost similar path loss as that of free space for a narrow band of 21 kHz. According to these measurements, signal amplitude reduction through the regions of hand, wrist, torso (front to back), thigh, ankle, left to right ear is 3.3 %, 2.8 %, 3.4 %, 1.9 %, 2.8 % and 2.0 % respectively [1]. 

 

It has also been specicified that human body can be used as a communication channel from 5 to 50 MHz without need of any modulation. It has been shown that for a transmission distance of 150 cm from the fingertips of one to the fingertips of the other hand with receiver load impedance of 10 M-ohm and electrode sizes of 2x2 cm² the amplitude and phase mean value of response is as follows at different frequencies [1].

	5 MHz	10 MHz	20 MHz	30 MHz	40 Mhz	50 Mhz
Amplitude (dB)	-47.2 dB	-48 dB	-48.65 dB	-50.7 dB	-52 dB	-54.8 dB
Phase (degrees)	-29.2 º	-47.4 º	-87.7 º	-117.6 º	-150 º	- 172.7 º

The channel also exhibits Gaussian noise with zero mean and 2.55×10^-5 variance.

References

[1] K.-Y. Yazdandoost and K. Sayrafian, “Channel model for body area network (BAN),” 15-08-0033-04-0006-draft-of-channel-model-for- body-area-network.doc https://mentor.ieee.org/802.15/file/08/15-08-0780-09-0006-tg6-channel-model.pdf

Thursday discussion

Today there were intense discussions on amplifiers and rejection ratios.

Health and monitoring circuits

Texas Instruments discusses the use of their circuitry in so called bio-patches. A way to monitor the health and cognitive behavior of a person.

• See more in on-line ECN magazine

Quite a lot of speculation though at the moment, but yet. There are a few other interesting points on sensor hubs, etc., in the Jan 2014 issue of the magazine.

Effect of electrode configuration on signal transmission through human body communication channel

 There could be two simple types of electrode configurations with respect to dominant electric field direction on the surface of human body channel:

One  type could be the coplanar capacitor formed by placing signal electrode adjacent to  ground  electrode or inter-digitated signal- and ground-fingers in the horizontal plane with the assumption of horizontal direction of electric field vector.

 The  second type could be the overlapping capacitive electrode formed by placing the signal  and  ground electrodes in the vertical plane. The direction of electric field intensity vector is  assumed vertical in this configuration.

 For  both configurations of electrodes, we are forming some kind of electrical dipole on the  surface  of human body. The electric field intensity has been calculated on the surface of  human body with finite permitivity and conductivity for the vertical infinitesimal electrical  dipole in [1]. However, it's also important to calculate electric field intensity for horizontal  dipole in order to compare the two coupling schemes and decide which one is better.

If  the human body acts as a waveguide medium as claimed in a european patent [2] then it's important  to see, for which component of electric field vector (horizontal or vertical) human body serves as a better medium. For example, it could be that human body attenuates one component of electric field say horizontal more than the other component of electric field say vertical. This would then define the better configuration of electrode for signal transmission on human body if we assume that the vertical component of the electric field is produced by the overlapping electrode configuration and the horizontal component of electric field is produced by the coplanar electrode configuration.

References

[1] J.Bae, H. Cho, K. Song, H. Lee, and H.-J.Yoo, “The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication,” IEEE Trans. Microw. Theory Tech., vol. 60, No. 3, pp. 582-593, March 2012

[2] R. Bedini, A. Buratto, G. Casadio, G. Palagi and A. Ripoli, “Transmission system using the human body as waveguide,” European Patent EP0824889 A1, Feb 25, 1998.

A qualitative overview of modulation schemes for body channel communication

Analog Modulation AM, FM/PM

Human body suffers from amplitude fading for different environmental and surrounding conditions. Moreover it is subjected to different electromagnetic interference patterns which also deforms the amplitude. Therefore analog amplitude modulation (AM) is not favorable for such a channel. Moreover, AM has lower noise immunity than frequency (FM) or phase (PM) modulation which are harder to distingish from each other when modulation is in analog domain and also requires PLL or VCO (more complex circuitry) for generating accurate frequency. SNR improves quadratically at the expense of transmitting BW for FM but there is no such tradeoff in case of AM. 

Pulse Modulation PAM, PWM, PPM

Pulse modulation schemes offer higher SNR at the expense of larger BW than analog modulation schemes by allowing analog signal to be represented as a periodic pulse train and varying either pulse amplitude, pulse duty cycle or triggering pulse modulated signal at rising or falling edge etc proportional to the sampled value. 

Digital Modulation

Pulse code modulation (PCM) involves both time sampling and digital encoding or quantization of analog signal before transmission. The resolution of ADC ensures higher SNR. Different line codes like unipolar/ bipolar NRZ, unipolar/ bipolar RZ, Manchester and differential encoding could be used with this scheme. Other digital modulation schemes involve amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK) which are produced by modulating AM, FM and PM with pulse modulated signal.

Conclusion

A digital signal when transmitted as a bit stream should have theoretically highest noise immunity as compared to analog or pulse modulation schemes at the cost of higher bandwidth and increased circuit complexity. Increased circuit complexity is however addressed by decreasing transistor sizes in newer technologies giving rise to lesser power consumption and smaller form factor. Moreover digital transmission allows infinite possibilities to regenrate the bit sequence provided that they are not corrupted by noise or distortion. BAN transceivers could therefore take advantage of digital modulation schemes for increased data rates as well as higher noise immunity.