Chapter 8

Summary and Future Challenges

Body area communications is a short range wireless communication technique in the vicinity of, or inside, a human body. The different operating environments mean that the body area communications is classified into on-body, in-body or in-body/on-body to off-body communication scenarios. It can provide a wide range of applications such as medical and healthcare services, assistance to people with disabilities, and consumer electronic connectivity and user identification.

As an introduction to body area communications, we have started with basic electromagnetic characterization and modeling methodology for the human body in various possible frequency bands. Based on the basic knowledge, we have focused on three major areas: channel modeling, modulation/demodulation performance, and EMC consideration in body area communications.

Body area communications is being considered to operate in UWB, MICS, ISM and HBC bands. Since electrical properties of human tissue are frequency-dependent, a Debye-type approximation expression is a useful means for modeling them. The basis for modeling the frequency dependence uses the dispersion phenomena in the dielectric spectrum of tissue. The dielectric spectrum is characterized by several dispersion regions. Each of them can be characterized by a single relaxation time constant in the Debye expression. The dielectric properties of the human body are then expressed as a summation of terms corresponding to the main dispersion mechanisms. For a frequency range from several Hz to 10 GHz, four Debye-type dispersion region expressions provide a good model for most human body tissues.

The frequency-dependent electrical properties mean that propagation along the human body is via different mechanisms at different frequency bands. For on-body communication:

• At HBC band such as 10 MHz, almost 80% of the received electric field component is contributed by an electrostatic field term at a unit communication distance. The signal transmission is actually realized by electrostatic coupling.
• At MICS or WMTS band around 400 MHz, almost 80% of the received electric field component is contributed by the surface propagation term, which acts as a main on-body propagation mechanism.
• In UWB band, more than 95% of the received electric field component is contributed by the surface propagation term. Actually it completely dominates the on-body propagation.

On the other hand, on-body communication mainly suffers from a path loss fluctuation or shadowing due to the body shape and structure and a multipath fading due to the body movement, while in-body communication mainly undergoes severe signal decay during the transmission through the lossy human tissue. These features mean that care has to be taken in choosing a frequency band for the on-body or in-body communication. In general, UWB and HBC bands provide some advantages for on-body communication. The former's advantages are based on its very low PSD and robustness to multipath fading, and the latter's are based on its small on-body path loss compared with other frequency bands. On the other hand, the MICS band and UWB low band may be more adequate for in-body communication because of the relatively small penetration depth in human tissues or the possibility of high data rates for real time transmission.

Based on the basic propagation mechanisms, we have generalized the channel model into a path loss model and an impulse response multipath model. The path loss model mainly describes the channel loss, including propagation loss, absorption loss as well as diffraction loss. It has been shown that the empirical power decay law can give a reasonable expression for all of the considered frequency bands. The path loss exponent may range from 2 to 5 for on-body UWB full band propagation, and is about 10 for in-body UWB low band and 6 for in-body MICS band propagation. The HBC propagation exhibits a smaller path loss around 30 dB per meter after 10 cm away from the transmitter. Moreover, in any frequency bands, the path loss fluctuation from the average path loss, that is, the shadowing, is found to always follow a log-normal distribution.

The multiple paths between transmitter and receiver, produced by body movement or transmitter movement, introduce complexity into the channel model. To take into account the time-varying channel properties, we have established a discrete time impulse response channel model based on the classical Saleh–Valenzuela model. First, the time delays of the multipaths are generated as follows: the first path is generated at a fixed arrival time, after which a temporal delay between two successive paths is generated according to the inverse Gaussian distribution and added to the arrival time of the previous path. Next, the gain coefficient for each path is determined acording to the log-normal distribution, whose mean follows an exponential power decay formula. In this discrete time impulse response model, four paths are usually necessary for representing an on-body UWB multipath channel, while two paths are sufficient for representing an in-body to on-body multipath channel in a capsule endoscope application.

The communication performance of body area communication systems is directly influenced by the channel it operates in. Based on the generalized body area channels in the forms of a static shadow fading channel and a dynamic multipath fading channel, we have evaluated the BER performance in terms of frequency bands and communication channels, that is, according to the following four types: on-body UWB communication, in-body UWB communication, in-body MICS band communication and human body communication, respectively. Link budget analysis and RAKE or diversity reception improvement are also carried out. The results can be summarized as follows:

• For on-body UWB communication, it is sufficient to realize a data rate not exceeding 10 Mbps within a 1 m communication distance in the multipath fading environment. If the RAKE reception is adopted, a system margin larger than 0 dB can be obtained almost on the whole body.
• For in-body UWB communication in the capsule endoscope scenario, diversity reception provides effective improvement on BER performance. In almost all of the transmitter locations inside the digestive organs, a data rate of 0.1 Mbps can always yield a system margin larger than 0 dB. When the data rate is increased to 1 Mbps or 10 Mbps, however, the corresponding communication distance will be reduced to about 10 cm. In order to make the communication possible in all the digestive organs, more than two-branch diversity reception is necessary. On the other hand, for in-body UWB communication in the cardiac pacemaker scenario, even without RAKE reception, a system margin can still be obtained larger than 0 dB at a data rate as high as 10 Mbps.
• For in-body MICS band communication in the capsule endoscope scenario, the permissible transmitting power is only −16 dBm. The conventional correlation receiver only supports a communication distance of 8 cm at a data rate of 1 Mbps. With the aid of diversity reception, the communication distance can be extended to basically cover all the digestive organs. However, transmission at a data rate of 10 Mbps seems still difficult.
• For HBC, the path loss is relative small and a low transmitting power can therefore be expected. It is basically available to communicate over the entire body area at a data rate up to 1 Mbps.

Finally, we have addressed the EMC issues in body area communications. One is SAR analysis for human safety evaluation, and the other is the EMI with a cardiac pacemaker. The SAR is basically low enough due to the low transmitting power in most of the body area transmitters. On the other hand, the EMI with a cardiac pacemaker is mainly due to the nonlinearity of the internal analog sensing circuit. Based on this mechanism of malfunction, we have presented a two-step approach for EMI evaluation with a pacemaker. In the first step, the input voltage of the sensing circuit of the pacemaker is calculated using a numerical electromagnetic analysis method by considering the pacemaker as a receiving antenna. In the second step, a Volterra series for a nonlinear system is employed to analyze the output voltage of the nonlinear sensing circuit for EMI evaluation. This two-step approach has provided an effective means of EMC consideration in the design of body area communication systems.

Although body area communications has shown obvious potential and rapid progress in medical, healthcare and consumer electronics areas, there are still many topics to be studied as well as many problems to be solved. Some topics for consideration are summarized as follows:

• Body area communications processes vital signals for patients and elderly people in situations that may be a matter of life and death. This feature requires that the communication link must be established at a highly reliable level, which does not allow a failure of communication and a loss of information. How to realize a body area communication at an error-free level should be a big challenge.
• Because of the short range and near-field coupling feature, body area channel models usually include the effects of the transmitting and receiving antennas used in measurement. This makes the derived channel models lack generality. How to develop an antenna-independent channel model is important for a new application of body area communications.
• On-body and in-body antenna design plays an important role in realizing high-quality body area communications. These antennas are obviously different from conventional ones, and a new design methodology is required. In view of the near-field feature, the antenna radiation pattern in far-field is meaningless in body area communications. In on-body communication, special attention should be paid to make the propagation along the body surface and the energy radiation not towards the human body. For in-body communication, how to make the radios penetrate into/through the lossy tissue is a challenge to antenna designers. Moreover, the transmitting antennas, the human body and the receiving antenna should be considered to be linked by an impedance matrix. Its optimization for energy transmission over the human body is the basic methodology for body area antenna design.
• Unifying the sensor and transceiver into a size as small as possible is a continuous challenge. Low consumption power and long battery time is also a key factor for the spread of applications. A wireless power transmission for in-body transceivers is especially expected.
• EMC issues will always rise in body area communications. The issues not only include the interference from other communication systems but also the interference to medical equipments. Due to the miniaturization and low-voltage operation of body area devices, how to solve the mutual coupling and interference problems inside the devices will also be a challenge to realize a highly reliable body area communication system.