Abstract
The usage of wireless capsule endoscopes to gain insights into the small intestine is a proven method for the diagnosis of certain gastrointestinal diseases. A capsule endoscope is a pill-sized device equipped with a camera and a radio transmitter that is used to transmit pictures from the inside of the digestive tract to an on-body receiver. With this, the range of classical endoscopes is extended to otherwise unreachable areas of the digestive tract. Current devices are capable of transmitting images with a rather limited video quality. Hence, it is desired to improve the video quality for future capsule endoscopes. To increase the data rates of the wireless communication link ultra wideband transmission is considered. In this Ph.D. thesis a simplified channel modeling technique is introduced and used to determine fundamental theoretic limits of the communication by computing the channel capacity and investigate the bit error rate performance of an exemplary transmission scheme.
The main contribution of this thesis is a simplified channel model for ultra wideband communication which can easily be adapted to different setups and extended to different human body models. Similar as for well-established wireless communication channel models, it is not the target to model every detail of the physical channel. Instead, a channel model is built which captures the essential characteristics but is simple enough to allow for the design and simulation of the in-body communication system. The general idea is to model the propagation path of an electromagnetic wave by a multi-layered dielectric resembling the tissue structure on the path between transmitter and receiver. Hence, it is called the layer modeling approach.
It is shown by a numerical simulation of the wave propagation that the layer modeling approach predicts the path loss, the frequency dependency and the power delay profile very well. The resulting path loss models also fit well to other results from literature.
Based on the layer model, the channel capacity is then computed for twelve digital human phantoms, which are all based on medical imaging data of real human patients. The differences in channel capacity between the digital human phantoms, due to differences in their physiology, are significant. For five optimally placed receive antennas on the abdomen the 10%-outage channel capacity varies between 217.5 Mbit/s and 1.5 Gbit/s. It is shown that the exact placement of the receive antennas on the abdominal surface plays a crucial role for the channel capacity. As soon as the optimum placement is not met exactly, the channel capacity deteriorates drastically by around one order of magnitude. Hence, a suboptimal placement scheme is investigated which needs much more receive antennas to achieve the same performance as for the optimized antenna positions. Moreover, it is found that the values for the dielectric properties of human tissue have a drastic influence on the resulting channel capacity and may lead to a reduction by more than one order of magnitude.
Finally, an exemplary ultra wideband communication system using pulse position modulation is designed and its bit error probability is analyzed. It is shown that the effective discrete time equivalent baseband transfer function is frequency flat. Moreover, it is analyzed how the different receive antenna placements affect the transmitter coverage in the digestive tract. A coverage of 99.8 % of the digestive tract can be achieved with an uncoded bit error probability of less than at a data rate of 3 Mbit/s for a specific antenna setup.
