# A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz-2.4 GHz Range
## 2 Materials and Methods
### Methodology and Scope  
The goal of the reference measurement is to compare narrowband path loss as a function of distance in a fixed scenario using various wireless technologies for on-body communication at different frequencies. The IEEE 802.15.6 standard for on-body communication considers frequencies from 400 MHz-3.5 GHz. In this frequency span, we have performed measurements from 420 MHz-3 GHz, with different wireless technologies per studied frequency. Path loss (or equivalently path gain) is a commonly studied quantity in (on-body) RF propagation and is the ratio of the received power and the input power, usually as a function of distance or frequency [34]. Note that in the definition used in this paper, path loss expressed in decibels is negative. Generally, a power loss should be subtracted in a power balance and consequently path loss should be positive in decibels. However, to keep formatting in line with literature and S-parameter measurements, path loss is reported in this paper as negative values in decibels. It should be noted that other authors also refer to the same quantity as path gain. Additionally, it should be noted that the International Telecommunication Union (ITU) recommends the use of the term system loss instead of path loss [35], referring to the loss between antenna or electrode terminals instead of the loss purely caused by propagation.All the studied wireless, on-body technologies are studied in the same reference scenario shown in Figure 1. A transmitting node (TX) is placed on the left wrist of a male subject (30 y old, height 191 cm, mass 83 kg) in upright anatomical position. Consequently, the TX position was 93 \(\pm\) 5 cm above the floor. A receiving node (RX) is placed on the left arm at different separation distances (_d_) from the TX. This path loss is measured over distance, where distance was determined using tape measure from aperture to aperture along the arm and is varied from 10 cm to 50 cm in steps of 5 cm. All measurements are executed in an indoor environment. The parameters and (additional) configurations used for every studied technology are listed in the next sections. This reference scenario is chosen because it represents a commonly targeted on-body application where a wearable device or a prosthetic is worn on the wrist and needs to communicate along the arm with another body-worn RF node. As a note, three-dimensional radiation characteristics of the used technologies are not reported, given the variable influence of the body on these parameters. Figure 1: Frontal view of the on-body setup of the reference path-loss measurement scenario. The blue marker indicates the TX location, while the red markers indicate different RX positions. Each of the wireless technologies has a different method of (indirectly) registering received powers on the RX nodes. Eventually, these are all converted to values in relative powers, i.e. ratios of the received power over the input power (\(P_{\text{dB}}\)) in decibel. These are fed into a log-linear least-square fit for each technology using the following two path loss models ([PERSON], 1996):  
\[P_{\text{dB}}(d)=P_{0}-10n\cdot\log_{10}\left(\frac{d}{d_{0}}\right)+N\left(0, \sigma_{p}^{2}\right), \tag{1}\]  
with \(P_{0}\) the path loss at the reference distance \(d_{0}\) of 10 cm, \(d\) the separation between TX and RX, \(n\) the path loss exponent, and \(N\left(0,\sigma_{p}^{2}\right)\) is a term quantifying the (zero-mean) lognormal variance on the path loss. \(\sigma_{p}\) is the standard deviation on the path loss model. This path loss model is used frequently when quantifying (on-body) path loss measurements in indoor environments ([PERSON], 1996). While this model is simplistic, it uses only three parameters, and is therefore easier to interpret physically and allow for an intuitive comparison over different frequencies and technologies.