I. Introduction
Wireless communication is an attractive option for connecting various electronic devices carried by a person on their body. Many applications that could benefit from such technology have been suggested, including personal healthcare, the smart home, personal entertainment and the future infantry soldier [1], [2]. In spite of much work reported to date on the characterization of the received signal variation for different on-body channels in terms of slow and fast fading [3]–[8] and path gain measurement [8]–[10], few publications are available on the polarization behavior of channels in body area networks, including on-body, off-body and in-body. However, depolarization of the electromagnetic wave is an important factor that has to be considered when describing the channel behavior of most wireless communication channels [11], [12]. Understanding depolarization in the channel is useful as it allows for optimized wearable antenna design. It also provides information about the performance of the channel when optimally polarized antennas cannot be used due to the higher importance of other factors, such as cost or available space for the antenna. In addition, the information about channel depolarization is valuable for evaluating the performance of either polarization diversity used to decrease the effect of multipath fading or multiple input multiple output (MIMO) systems used to increase the capacity by employing dual-polarized antennas [13]–[15]. In on-body channels, the depolarization phenomena can be very significant due to the body movement, as well as the scattering of the wave from the body and surrounding environment. In most of the published works related to the polarization investigation for on-body applications, identical antennas with polarizations either normal or parallel to the body surface have been used in on-body channels and different parameters, such as path gain, root mean square (RMS) delay spread, and electric field distribution within the channel, have been investigated both in the narrowband scenario and in ultrawideband (UWB) channels [16]–[19]. In [20], the authors described the importance of the polarization for on- and in-body applications and highlights the adverse effect of polarization mismatch of transmit and receive antennas on the path gain, but no experimental evidence was reported. A general full-wave solution for a point source near a human trunk has been derived in [21], [22]. In all the papers, it is indicated that the vertically polarized antenna provides a better path gain in different channels than a horizontally polarized antenna. However, very few studies of mixed antenna types has been published and, furthermore, no data on the depolarization in such channels in dynamic cases has been reported.