  Ultrasonic intra-body communication (IBC) is a promising enabling technology
for future healthcare applications, due to low attenuation and medical safety
of ultrasonic waves for the human body. A splitting receiver, referred to as
the splitting-detection separate-decision (SDSD) receiver, is introduced for
ultrasonic pulse-based IBCs, and SDSD can significantly improve bit-error rate
(BER) performance over the traditional coherent-detection (CD) and energy
detection (ED) receivers. To overcome the high complexity and improve the BER
performance of SDSD, a splitting-detection joint-decision (SDJD) receiver is
proposed. The core idea of SDJD is to split the received signal into two steams
that can be separately processed by CD and ED, and then summed up as joint
decision variables to achieve diversity combining. The theoretical channel
capacity and BER of the SDSD and SDJD are derived for M-ary pulse position
modulation (M-PPM) and PPM with spreading codes. The derivation takes into
account the channel noise, intra-body channel fading, and channel estimation
error. Simulation results verify the theoretical analysis and show that both
SDSD and SDJD can achieve higher channel capacity and lower BER than the CD and
ED receivers with perfect channel estimation, while SDJD can achieve the lowest
BER with imperfect channel estimation.
