I. Introduction
Recent years have seen the development of a large number of portable wireless systems and devices. Thanks to electronic miniaturization, portable devices are carried in small pockets or attached to clothes. An important consequence of their proximity to the body is the need to characterize the latter as a radio propagation channel. Research on the on-body channel characterization is being conducted [1]–[3] also including an analysis of the most suitable antenna for each application. Recent new implementations apply to more complicated activities which the body is involved in and they require a larger quantity of data to be transferred with higher speed. Military and sport equipment for communications between wearable transceivers and sensors give an example of this [4]. The need for higher performance systems also suggests the use of multiple antenna systems in an on-body environment. Diversity is a well-known technique used to reduce the fading effect due to a multipath propagation channel, by using a multiple antenna system at the receiver. Two or more uncorrelated signals are received by separate antennas and properly combined. This process results in improved signal-to-noise ratio (SNR) and an increase in the signal reliability, that is, the probability that the signal is greater than a certain threshold. At present, diversity is widely applied in receiving at base stations for mobile cellular systems and recent studies have been conducted on portable devices [5]. In both previous applications, the base station antenna system, whether or not in a diversity configuration, is always fixed and the mobile unit moves in the propagation environment. Therefore, the multipath effect is mostly determined by the environment that closely surrounds the receiving terminal. On the contrary, in an on-body propagation system the performance is mostly determined by the body activity, as both the transmitting and the receiving modules are placed on the same moving holder.