2.2. Narrowband On-Body Path Loss Characterisation
The path loss for the different receiver locations was calculated directly from the measurement data of S21 (10 sweeps) averaging at 2.45 GHz. It is well known that the average received signal decreases logarithmically with distance (for both indoor and outdoor environments). The path loss can be modelled as a linear function of the logarithmic distance between the transmitter and receiver, as explained in [
32],
where
d is the distance between the transmitter and receiver,
d0 is a reference distance set to a measurement (in this study, it is set to 10 cm),
PLdB(
d0) is the path loss value at the reference distance and
Xσ is the shadowing fading. The parameter,
γ, is the path loss exponent that indicates the rate at which the path loss increases with distance.
In order to model the path loss as a linear function of the logarithmic distance, a least-squares fit was performed on the measured path loss data for 34 different receiver locations, as shown in
Figure 4.
Table 2 lists the value of path loss exponents and path loss at the reference distance
d0 obtained for the eight different test subjects. Due to the different body sizes, shapes and heights, the path loss exponent,
γ, varies for the different human bodies. In this study for the narrowband case, a maximum variation in the path loss exponent of 0.85 is noticed (Male 1 and Male 8). The results show that the path loss exponent generally increases with body size.
In the case of subjects with a low value of body chest and waist circumferences, such as Male 1, Male 2 and Male 3, the path loss exponent is lower, whereas with large values of chest and waist circumferences (Male 6, Male 7 and Male 8), the path loss exponent is higher. In this case, for thinner subjects (Male 1, Male 2 and Male 3), the propagation between the Tx (transmitter) and Rx (receiver) has more line of sight (LOS) than the test subjects with a higher volume of chest and waist circumferences, resulting in lower path loss exponents (γ = 3.2, γ = 3.25 and γ = 3.31 for Male 1, Male 2 and Male 3, respectively). For the subjects with a higher radius of the curvature of the trunk, such as Male 6, Male 7 and Male 8, the wave reaches the receiver through creeping wave propagation, which has higher signal attenuation, thus leading to a higher value of the exponent (γ = 3.71, γ = 3.85 and γ = 4.05 for Male 6, Male 7 and Male 8, respectively). For the subjects with a higher volume of chest and waist circumferences, the communication for some of the receiver locations is heavily blocked by the different body parts, compared to the subject with a lower value of chest and waist circumferences. In addition, the body tissues, reflection, diffraction and scattering from the body parts are also different for various subjects, which contribute to the variation of path loss. For the on-body radio channel, the propagation is mainly through creeping wave, free space and guided wave. Different shapes and sizes of the test subjects affect the propagation mode for on-body propagation links.
Table 2.
Narrowband on-body path loss parameters for the eight different test subjects. The parameter, γ, is the path loss exponent, PLdB(d0) is the path loss at the reference distance and σ is the standard deviation of the normally distributed shadowing factor.
Table 2.
Narrowband on-body path loss parameters for the eight different test subjects. The parameter, γ, is the path loss exponent, PLdB(d0) is the path loss at the reference distance and σ is the standard deviation of the normally distributed shadowing factor.
Path Loss Parameters | Male 1 | Male 2 | Male 3 | Male 4 | Male 5 | Male 6 | Male 7 | Male 8 |
---|
γ | 3.20 | 3.25 | 3.31 | 3.39 | 3.48 | 3.71 | 3.85 | 4.05 |
PLdB(d0) (dB) | 41.0 | 40.8 | 40.7 | 43.8 | 42.0 | 42.8 | 44.2 | 41.7 |
σ (dB) | 7.62 | 6.80 | 7.12 | 6.31 | 8.01 | 7.09 | 7.17 | 8.12 |
Figure 4.
Measured and modelled path loss for narrowband on-body channels versus the logarithmic Tx-Rx separation distance of different human test subjects (Male 1–Male 8).
Figure 4.
Measured and modelled path loss for narrowband on-body channels versus the logarithmic Tx-Rx separation distance of different human test subjects (Male 1–Male 8).
Xσ is a zero mean, normal distributed statistical variable and is introduced to consider the deviation of the measurements from the calculated average path loss.
Figure 5 shows the deviation of measurements from the average path loss fitted to a normal distribution for the eight different test subjects for the narrowband case.
Table 2 lists the values of standard deviation of the shadowing factor obtained for eight different test subjects. The standard deviation,
σ, of the normal distribution was found to be the highest for Male 5 and Male 8, whereas the lowest is noticed for Male 2 and Male 4. The results indicate that the standard deviation value,
σ, varies for different test subjects.
Figure 5.
Deviation of the measurements from the average path loss for different test subjects (Male 1–Male 8) fitted to a normal distribution at 2.45 GHz.
Figure 5.
Deviation of the measurements from the average path loss for different test subjects (Male 1–Male 8) fitted to a normal distribution at 2.45 GHz.
In order to compare the path loss for eight different human body types, eight different on-body channels (shown in
Figure 6) have been chosen.
Figure 7 shows the variation in path loss for the eight different on-body links for the eight test subjects.
Figure 6.
The considered eight different on-body links chosen for the path loss comparison of different test subjects.
Figure 6.
The considered eight different on-body links chosen for the path loss comparison of different test subjects.
Figure 7.
The variation of path loss for eight different narrowband on-body radio propagation channels for different human test subjects.
Figure 7.
The variation of path loss for eight different narrowband on-body radio propagation channels for different human test subjects.
For the eight different links considered, due to the different shapes and sizes of the human body, a maximum of 13.02 dB of variation of the path loss of an on-body channel was observed. This was noted for the transmitter for the right wrist link (Rx 19) of Male 1 and Male 8, where the variation of the path loss for this link of eight different subjects is mainly due to the different trunk sizes of the different subjects. In the case of Male 1, the trunk size is much smaller than the trunk of Male 8, which creates less non-line-of-sight propagation (NLOS) and less blocked communication, resulting in a lower path loss value for this link for Male 1. For the receivers on the wrists and on the ankles, the variation of the path loss between the different subjects is found to be higher. The maximum variation of the path loss of different subjects for the left and right ankle links is 11.89 dB and 11.69 dB, respectively. For the ankle channels, the variation of the path loss is due to the different height and size of the legs of different subjects. The lowest path loss for both ankle links is noticed for the shorter subject with a medium body size (Male 5), whereas the highest is noticed for the taller subjects with a fatty body (Male 7 and Male 8). In the case of Male 5, the sizes of the legs are smaller than those of Male 7 and Male 8; hence, the communication distance between Tx and Rx is less, leading to a lower path loss value for this channel for Male 5.
For the receiver placed on the ear, it is possible to note that the path loss is higher for the taller subjects with a larger curvature radius at the trunk, such as Male 7 and Male 8, and lower for the thinner and shorter subjects with a smaller curvature radius (Male 5, Male 1, Male 2 and Male 3). The maximum variation of the path loss of different test subjects for the left and right ear links is 8.64 to 9.25 dB, respectively. In this study, for different test subjects, the lowest path loss variation is noticed for the ear and chest links.
Table 3 summarizes the maximum path loss variation of each channel for different test subjects.
Table 3.
The maximum path loss variation of each on-body link obtained among different test subjects (narrowband).
Table 3.
The maximum path loss variation of each on-body link obtained among different test subjects (narrowband).
Channel | Maximum Path Loss (dB) Variation |
---|
Left chest | 7.52 |
Right chest | 8.95 |
Left ear | 9.25 |
Right ear | 8.64 |
Left wrist | 12.10 |
Right wrist | 13.02 |
Left ankle | 11.79 |
Right ankle | 11.69 |
For the narrowband case, the lowest path loss is noticed for the left wrist and left chest links for all eight test subjects. The path loss for each different location is averaged over the eight test subjects. The subject-averaged path loss values for the left the chest, the right chest, left ear, right ear, left wrist, right wrist, left ankle and right ankle are 52.64, 70.77, 68.32, 72.71, 44.14, 70.72, 66.41 and 67.47 dB, respectively. Results show that, for the narrowband case, the subject-averaged highest path loss is noticed for the right ear, right wrist and right chest channels, while the lowest is at the left wrist and left chest channels.