Recently, due to the proliferation of wearable devices and the reduction of their size, there is a large number of applications including wearable communication systems. Due to this development, new trends have emerged in communications that led to a new and interesting research area, body-centric wireless communications. Body-centric wireless communications use the human body as the propagation environment between two or more devices that operate in proximity to the body and communicate with each other through wireless technologies. Wearable systems operate in a constantly varying environment, thus causing fading. More specifically, fading occurs because of the presence and the movement of the user and the surrounding environment. Moreover, in contrast to mobile communications, in on-body channels both the transmitter and the receiver are moving and changing positions in the propagation environment as well as relative to each other. Besides fading, another important issue that concerns the devices on-body is the transmitted power, which should be kept as low as possible in order to increase the life of the battery and at the same time reduce the value of the specific absorption rate (SAR).Apart from the above mentioned issues, the ever increasing use of wireless devices in health, recreation, safety, personal identification etc. drives research to implement. More efficient and reliable links between devices located on-body. For this reason, a significant increase in applications that include multiple antennas in body-centric systems is observed lately. This increase is followed by significant scientific interest in this research area.
As results from the previous analysis, it is very important for on-body channels to mitigate fading as much as possible since they affect the reliability and the quality of services and to increase the signal-to-noise ratio (SNR) without increasing the transmitting power. Spatial diversity is a very popular technique to overcome fading and achieve an efficient link in mobile communications. On the other side, MIMO systems could be used to increase the data rate. For these reasons, the study of wireless body-centric networks and the modeling of the wireless channel when the antennas of both the transmitter and the receiver are on the user’s body, presents a huge research interest.
This doctoral thesis, having as a main subject the wireless body-centric communication systems including multiple antennas, addresses two relating issues, the evaluation and modeling of these systems.
At first, studies the performance evaluation of MIMO antenna systems focusing on the impact of the human body on the channel performance. This evaluation is very important from the antenna design and system modeling point of view. Particularly, we perform a performance study of a MIMO system which is equipped with a wireless terminal at the user’s side, for two typical cases of showing the impact of the human body on the system performance. The first case regards the impact of the user’s hand in “multimedia viewing position” while the user holds the MIMO terminal, whereas in the second case the impact of the full human body is studied, while the terminal is in the user’s pocket in operating mode. The analysis of the results shows that the human body can cause degradation of the system performance that ranges between 10% and 60%. Moreover the performance of a MIMO system that includes multiple wearable antennas at the user’s side is investigated. Finally a comparative study is performed between the previously mentioned systems.
Secondly, the modeling of the on-body channel of a wireless body-centric communication system is studied, in terms of the single branch channel modeling and the performance analysis and modeling of the on-body diversity channel. In this context, an extensive measurement campaign has been conducted at 2.45 GHz, for various scenarios using wearable antennas. In order to perform measurements, a textile substrate wearable antenna was designed and manufactured, that operates in the ISM frequency band of 2.45 GHz. The measurement environments were a realistic propagation environment that includes the multipath effect (office) and a reflectionless environment where fading are caused alone by the body movements (anechoic chamber). Through extensive statistical analysis of the measurements data the impact of the antenna position and the user movement on the propagation characteristics of the on-body channel and the on-body diversity channel was investigated. Moreover the on-body channel diversity performance was evaluated. As resulted, the theoretical model that best fits, both the single branch and the diversity channels, in a reflectionless environment is lognormal distribution. On the other side, in a multipath environment, best fit is provided by Weibull distribution for single branch channel and α-μ for the diversity channel.