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.