The concept of an experimental test bed for system loss and channel impulse response measurements for off-body and body-to-body radio channels in wireless body area networks (WBANs) is fully described. The possible measurement scenarios that may occur in investigation of off-body and body-to-body channels are classified and described in detail. Additionally, an evaluation is provided of the standard and expanded uncertainties of the presented measurement stand and methodology. Finally, the exemplary results are presented and discussed, in order to point out the need for further investigations of different diversity schemes and their applications in WBANs.
Modern telecommunication technologies and their rapid development cause important changes in the ways humans communicate, as well as changes in communication between humans and machines. In addition, wireless technologies provide independent telecommunication services based on the existence of fixed infrastructure, which results in an improvement of the quality and comfort of human life. Due to the unceasing miniaturization of electronic devices and the simultaneous decrease of their energy consumption, there is an increasing demand for wireless solutions for exchanging information between devices working in close proximity to the human body or even inside it. This kind of short-range radio network, known as the WBAN, was proposed for the first time in 1996 by Zimmermann, who demonstrated the possibility of transmission between two devices using the human body as a transmission medium [
Due to the ability to integrate various portable or wearable devices with a fixed infrastructure, WBANs will play a significant role in the next generation of radio communication systems, including 5G [
Empirical models, except for deterministic ones, are one possible way to model radio channel characteristics such as average system loss with slow and fast fading distributions or channel impulse response. In order to build a reliable empirical channel model, it is necessary to perform a comprehensive measurement campaign considering the different places of the antennas’ installation, user motion scenarios, different environments, etc. It is also well known that by using different diversity schemes, the quality of transmission may be improved, and thus, this kind of technique should be also considered in measurement research of radio channel characteristics. Such research works are carried out within the COST Action CA15104 “Inclusive Radio Communication Networks for 5G and beyond” (IRACON) [
Currently, there are only few studies dealing with experimental investigations of radio channels with space diversity schemes that also describe the measurement methodology and scenarios. Space diversity in an off-body channel at 2.45 GHz has been considered in [
The rest of the article is structured as follows. Section
The block diagram of the versatile measurement stand for radio channel measurements in WBANs with space and polarization diversity schemes is presented in Figure
The block diagram of the versatile measurement stand for radio channel measurements in WBANs with space and polarization diversity schemes.
The ES is divided into two independent parts that can be used depending on the type of radio channel parameters that one needs to measure, i.e., system loss (SL) or channel impulse response (CIR). For the former, one can use the transmitting (Tx) and receiving (Rx) parts. This solution allows for any deployment of Tx and Rx in the environment as well as investigation of longer radio links without significant influence of the RF cables’ attenuation. The Tx part consists of a vector signal generator (VSG) SMU 200A that allows generation of any RF signal at a frequency from 100 kHz up to 6 GHz. It is noteworthy that after hardware extensions, it could be possible to generate two independent RF signals, e.g., for transmitting diversity or MIMO measurements. The Rx part consists of two digital wideband R&S EM550 receivers that can be used for investigations of space diversity or polarization diversity, depending on the antenna set being used. It should be noted that the average time between particular measurements of received signal powers is around 3 ms. Before measurements, both the Tx part and the Rx part should be calibrated.
In the part responsible for CIR measurements, the Agilent E5071C vector network analyzer (VNA) is used. It allows for measurements in the frequency band from 300 kHz up to 20 GHz. While the measurements are always done in the frequency domain, the results provided by the VNA may be given directly in the time domain, or in the frequency domain, and then, CIR can be calculated in postprocessing. The main parameters that have to be set up are the center frequency, the bandwidth, the power transmitted by port 1, and the number of points. The value of the intermediate frequency filter bandwidth (IF bandwidth), which is a key factor in the selectivity of the VNA, should be chosen in order to ensure that the noise level is not larger than a certain value. For example, for a center frequency of 5.8 GHz, bandwidth of 500 MHz, Tx power of 0 dBm, and an IF bandwidth of 70 kHz, the maximum noise level equals -103 dBm, with an average of -111 dBm. Before measurements, the VNA should be calibrated. It should be also stressed that the narrower IF bandwidth causes a longer time requirement for a single measurement.
In order to carry out measurements with different diversity schemes, there is a need to switch RF inputs of the digital receivers (SL measurements) or ports 2 and 3 of the VNA (CIR measurements) between the two types of antenna sets in the AS. The switching mechanism may be manual or automatic. In the first case, the RF connections between ES and AS are configured manually by use of the RF cables. In the case of an automatic mode, remotely controlled electromechanical switches should be used, e.g., Tesoel TS121. In this case, the certain configuration of RF switches is set by the CS via an RS232 interface. The connections between the RF switching section and AS should be realized using a flexible, high-frequency, and high-quality cable, e.g., Sucoflex 126E, that allows for precise measurements with high phase stability combined with low loss and good return loss up to 26.5 GHz.
Depending on the scenario, the Tx antenna may be considered as an off-body or on-body antenna. In the first case, one can use a UWB omnidirectional antenna, e.g., OA2-0.3-10.0 V/1505, designed for operation in the frequency range of 0.3–10.0 GHz. In the case of an on-body antenna, one should remember that during design of this type of antenna, the influence of the human body on the antenna parameters should be considered. An example of such an antenna may be the linearly polarized UWB monopole antenna, described in [
Similarly, the Rx antenna set can also be placed on the user’s body or in some external location. Considering the polarization diversity and the off-body case, one can use a dual-polarized antenna, e.g., the quad-ridged horn antenna LB-OSJ-0760, designed for operation in the frequency range of 0.7–6 GHz [
The CS consists of a portable computer (e.g., a Mecer Toughbook T89 M with Intel Atom Z530) responsible for controlling the work of the whole measurement stand and collecting measurement data and a network switch that allows for control of four devices via a LAN interface at the same time. Communication between the CS and ES is realized using the National Instruments implementation of the Virtual Instrument Software Architecture (NI-VISA) standard and Standard Commands for Programmable Instruments (SCPI).
The principle of measurement stand operation in configuration for SL measurements has been presented as a simplified algorithm for control software (see Figure
Simplified algorithm of control software for performing system loss measurements.
Depending on the “stop mode” set in the application configuration, there are three possible modes of measurements. The first mode is a manual stop mode, in which the measurements are done in a loop until the stop button is pressed. The second mode is a sample size mode, in which a specific number of measurements are performed. The last mode is a sample time mode, in which measurements are completed by a specified time period. Regardless of the mode, a single measurement is done by triggering the power measurement in each receiver and reading the received value. After that, the system loss values are calculated basing on the measured power value, transmitted power, and attenuation values of the Tx and Rx cables. After finishing the measurements, the user can disconnect the receivers and close the file with results, but there is also a possibility of repeating the measurements many times, writing the results to the same file.
A simplified algorithm of control software for performing CIR measurements has been presented in Figure
Simplified algorithm of control software for performing channel impulse response measurements.
A very important part of measurement methodology is establishing a proper measurement scenario. One has to know what kind of information to obtain about the radio channel. The right selection of the environment, the type of user motion, and the location of the antenna are of equal importance. As such, the scenarios that may occur in off-body and body-to-body networks have been classified and are presented in Figure
Classification of the measurement scenarios in off-body and body-to-body radio channels.
The measurement stand presented in the previous section allows for measurements of SL or CIR depending on its configuration. The parameters used in the exemplary result analysis are presented in the following part of the paper.
The system loss (
In addition, while analyzing polarization diversity, based on the SL measurements for orthogonally polarized Rx antennas, it is possible to calculate the cross-polarization discrimination (XPD), which is the most common metric used to characterize depolarization properties of wireless channels. The XPD (in a linear domain) may be calculated as the ratio between the Rx powers received in the copolarized (CP) and cross-polarized (XP) channels [
The CIR,
The next parameter is the number of multipath components, which is defined as the number of peaks in a PDP whose amplitude is not lower than the certain value from the highest peak and above the noise floor. The third statistic is the average delay,
The last statistical parameter is the r.m.s. delay spread,
It is also noteworthy that the r.m.s. delay spread may be used for calculation of the channel coherence bandwidth with different correlation thresholds, as is described in [
Taking the above into account, based on the measurement results obtained with the described measurement stand, one can thoroughly characterize a radio channel with space or polarization diversity.
Reception diversity is a technique in which different independent replicas of the same RF signal propagating through a multipath channel are received and combined in a certain way, in order to minimize the influence of the signal fading. To realize the reception diversity, one has to decorrelate two or more signal replicas and use a well-known combining method, like selection combining, equal-gain combining, or maximal ratio combining [
While analyzing a measurement scenario, one also has to define the mutual visibility of the Tx and Rx antennas (xLoS in Figure
It is considered that the main applications of WBANs are in healthcare and remote patient monitoring, so the main operating environments seem to be indoor, e.g., a hospital or home. However, this kind of network can also operate in outdoor environment, e.g., in a case of soldiers on the battlefield or remote monitoring of vital signs of football players. Thus, both types of environments should be considered in experimental investigations. In both cases, one can consider an empty environment, i.e., without any other individuals except the WBAN users, or a crowded one, with the influence of nonuser persons. Depending on the behavior of people in the environment, one can distinguish between static (people just standing) or dynamic (people are moving, e.g., walking or gesticulating) crowd scenarios.
The best way to determine the antenna placement is human body segmentation, as it has been proposed in [ left or right side of the head, HEL or HER, respectively front or back side of the torso, TOF or TOB, respectively front or back side of the waist, WAF or WAB, respectively left or right side of the arm top, ATL or ATR, respectively left or right side of the arm bottom, ABL or ABR, respectively left or right side of the leg top, LTL or LTR, respectively left or right side of the leg bottom, LBL or LBR, respectively
Segmentation and orientation of the human body proposed in [
The selection of a particular antenna placement depends on the planned application of the WBAN being designed.
As the system loss strongly depends on the mutual orientation of both antennas in the WBAN radio link, as has been proved in [ Co-Directed (CD)—when the maximum gain of one antenna is directed towards the other one Opposite-Directed (OD)—when the maximum gain of one antenna is directed in the opposite direction to the other one Cross-Directed (XD)—when the maximum gain of one antenna is directed in an orthogonal direction to the other one
Taking the above into account and considering the mutual orientation of two antennas, one can distinguish the following cases [
Three different scenarios of user motion can be considered during the measurements. The user may be static in the environment with a lying, sitting, or standing position. The user may be also dynamic (walking or running) with four possible directions of motion: Approaching the external antenna (off-body) or the other user (body-to-body) Departing from the external antenna (off-body) or from the other user (body-to-body) Parallel motion of two users in the same direction (body-to-body) Random motion of user or users in the environment
One can also distinguish a so-called quasi-dynamic scenario, in which the user mimics a particular motion, e.g., walking, running, or rotating in a fixed place.
Regardless of the measurement methodology, it is impossible to absolutely determine the exact value of a physical quantity. The difference between the measurement result and the true value of the measured quantity is called the measurement error. One can distinguish between a reading error, a systematic error, and a random error. A reading error usually arises due to inattentiveness or carelessness of the observer in reading or writing results or as a result of a sudden change in measurement conditions (e.g., shocks). This kind of error is easy to detect and remove, if several series of measurements have been performed. A systematic error results from imperfections of instruments and measurement methods. It can be reduced by using more precise methods and instruments, but it is impossible to completely eliminate this kind of error. Identified systematic error should be taken into consideration by introducing appropriate corrections to the result. The random error is always present in the measurement results. It arises from various accidental and unpredictable factors (e.g., variations of the temperature or reflections of RF signals from a moving object). The existence of random error is demonstrated by the unrepeatability of measurement results of the same physical quantity. This kind of error can be reduced by multiple repetition of the measurements, by which there is a partial compensation for random deviations of the result.
The international standard [
In order to estimate the standard uncertainty of the measurement of a physical quantity
The
If one wants to present the standard uncertainty as an interval around the measurement result, the expanded uncertainty,
The expanded uncertainty allows for the following notation of measured value:
The choice of the coverage factor,
In order to evaluate the standard uncertainty of the versatile measurement stand, the uncertainties of the following factors have been considered: VSG, VNA, Tx and Rx cables, receivers, methodology imperfection, and the human factor. In Table
Evaluation of the standard uncertainty and expanded uncertainty for 95% confidence interval (
Object of measurement |
|
Factor |
|
Pessimistic case | Optimistic case | ||||
---|---|---|---|---|---|---|---|---|---|
|
|
|
|
|
| ||||
System loss | 1 | VSG |
|
0.9 | 2.0 | ±4.0 | 0.9 | 1.5 | ±3.0 |
2 | Tx cable | 0.2 | 0.2 | ||||||
3 | Tx antenna | 1.6 | — | ||||||
4 | RF switch | 0.3 | — | ||||||
5 | Rx antenna | 1.6 | — | ||||||
6 | Rx cable | 0.2 | 0.2 | ||||||
7 | Receiver | 1.5 | 1.5 | ||||||
8 | Methodology imperfection | 1.0 | 1.0 | ||||||
9 | Human factor | 1.5 | 1.5 | ||||||
|
|||||||||
Channel impulse response | 1 | VNA |
|
0.5 | 1.7 | ±3.4 | 0.5 | 1.1 | ±2.2 |
2 | Tx cable | 0.2 | 0.2 | ||||||
3 | Tx antenna | 1.6 | — | ||||||
4 | RF switch | 0.3 | — | ||||||
5 | Rx antenna | 1.6 | — | ||||||
6 | Rx cable | 0.2 | 0.2 | ||||||
7 | Methodology imperfection | 1.0 | 1.0 | ||||||
8 | Human factor | 1.5 | 1.5 |
Because (according to the SL definition [
The versatile measurement stand and the methodology described in the study have been used to performed complex measurements for different diversity schemes and for different scenarios and environments. In this section, some selected results for off-body channels have been presented, i.e., SL measurements for space diversity in the office and ferry environments, SL measurements for polarization diversity in the ferry environment, and CIR measurements for polarization diversity in the office environment.
Measurement of SL in the office environment has been performed for a narrowband off-body channel at 2.45 GHz. The omnidirectional Cobham OA2-0.3-10.0V/1505, with 2 dBi gain and 65° half-power beamwidth in the
The mean (
Rx no. | Rx antenna placement |
|
|
|
---|---|---|---|---|
Rx1 | TOF | 61.6 | 2.3 | 7.2 |
Rx2 | TOB | 59.3 | 7.5 | |
Rx1 | TOF | 61.5 | 0.6 | 6.7 |
Rx2 | ABL | 60.9 | 6.9 | |
Rx1 | TOF | 61.0 | 3.3 | 7.1 |
Rx2 | ABR | 57.7 | 7.0 | |
Rx1 | TOF | 61.5 | 2.3 | 7.3 |
Rx2 | HEL | 59.2 | 7.2 | |
Rx1 | TOF | 61.9 | 7.5 | 6.5 |
Rx2 | HER | 54.4 | 6.8 | |
Rx1 | TOB | 64.0 | 1.8 | 7.0 |
Rx2 | ABL | 62.2 | 7.4 | |
Rx1 | TOB | 64.2 | 7.0 | 7.4 |
Rx2 | ABR | 57.2 | 7.0 | |
Rx1 | TOB | 64.3 | 4.2 | 7.2 |
Rx2 | HEL | 60.1 | 6.9 | |
Rx1 | TOB | 63.8 | 8.7 | 6.7 |
Rx2 | HER | 55.1 | 6.8 |
The absolute difference (
The space diversity scheme may be useful even when the mean SL values are similar, like for the TOF-ABL configuration of the Rx antenna set. The magnitudes of fast fading (
Magnitude of fast fading (
The SL measurements for space diversity in the ferry environment have been performed for a narrowband off-body channel at 2.45 GHz, in a passenger cabin on the M/F Wawel ferry boat running from Poland to Sweden. The cabin’s dimensions are 4.4 m per 2.3 m, with a height of 2.1 m. The walls, floor, and ceiling are made of metal. The patch off-body antenna with linear polarization and hemispherical radiation characteristics has been used as a Tx antenna mounted in the middle of the ceiling. The half-power beamwidth equals 115° in the The user lying on the back with hands along the body The user lying on the front with hands under the head The user lying on the left side with the right hand under the head The user lying on the right side with the right hand under the head
Table
The mean (
Scenario | Rx no. | Rx antenna placement | Upper berth | Lower berth | ||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
| |||
Lying on the back with hands along the body | Rx1 | TOF | 58.7 | 2.9 | 4.7 | 62.5 | 8.2 | 6.6 |
Rx2 | TOB | 61.6 | 4.9 | 54.3 | 5.1 | |||
Rx1 | TOF | 50.8 | 8.3 | 4.0 | 50.4 | 2.5 | 4.2 | |
Rx2 | ABR | 59.1 | 4.5 | 52.9 | 3.9 | |||
|
||||||||
Lying on the front with hands under the head | Rx1 | TOF | 61.4 | 10.0 | 4.7 | 58.5 | 6.1 | 4.3 |
Rx2 | TOB | 51.4 | 5.0 | 52.4 | 5.1 | |||
Rx1 | TOF | 53.4 | 6.7 | 4.1 | 63.4 | 1.5 | 4.2 | |
Rx2 | ABR | 60.1 | 4.0 | 64.9 | 7.7 | |||
|
||||||||
Lying on the left side with the right hand under the head | Rx1 | TOF | 46.8 | 8.4 | 4.3 | 57.2 | 4.3 | 4.8 |
Rx2 | TOB | 55.2 | 4.9 | 61.5 | 5.7 | |||
Rx1 | TOF | 55.7 | 8.4 | 4.1 | 49.3 | 13.6 | 4.1 | |
Rx2 | ABR | 64.1 | 3.8 | 62.9 | 4.3 | |||
|
||||||||
Lying on the right side with the right hand under the head | Rx1 | TOF | 66.5 | 10.8 | 4.7 | 50.1 | 10.2 | 4.3 |
Rx2 | TOB | 55.7 | 5.6 | 60.3 | 5.6 | |||
Rx1 | TOF | 57.2 | 0.2 | 4.4 | 52.0 | 1.6 | 4.1 | |
Rx2 | ABR | 57.4 | 4.4 | 53.6 | 4.0 |
The same ferry environment has been investigated in terms of SL measurements in the off-body narrowband channel at 2.45 GHz, with polarization diversity. In this case, a walking scenario has been considered. The measurements have been carried out in a straight corridor with a length of 20 m, a width of 1 m, and a height of 2 m. Most of the construction elements in this environment (i.e., walls, ceiling, doors, and handrails) are made of steel with different thicknesses. The floor is also made of steel but additionally was covered with a covering. The Tx antenna was a linearly polarized wearable patch antenna with 3 dBi gain and half-power beamwidths of 115° and 40°, respectively, in the
Table
The mean (
Walking scenario | Tx antenna placement | Rx no. | Rx antenna polarization |
|
|
XPD (dB) |
---|---|---|---|---|---|---|
Approaching | HER | Rx1 | H | 45.9 | 7.2 | 2.8 |
Rx2 | V | 43.1 | 6.8 | |||
TOF | Rx1 | H | 45.0 | 7.3 | 9.0 | |
Rx2 | V | 36.0 | 7.1 | |||
ABL | Rx1 | H | 46.2 | 7.5 | 5.5 | |
Rx2 | V | 40.7 | 6.6 | |||
|
||||||
Departing | HER | Rx1 | H | 48.0 | 6.6 | 3.0 |
Rx2 | V | 45.0 | 7.5 | |||
TOF | Rx1 | H | 50.2 | 6.7 | 7.1 | |
Rx2 | V | 43.1 | 7.0 | |||
ABL | Rx1 | H | 48.9 | 6.9 | 2.7 | |
Rx2 | V | 46.2 | 7.5 |
Nevertheless, considering the local values of XPD as a function of the propagation path length, as shown in Figure
XPD vs. length of the propagation path (
Tx antenna placed on ABL
Tx antenna placed on HER
Magnitude of fast fading (
The CIR measurements for a wideband off-body channel at 5.8 GHz with a polarization diversity scheme have been performed in the same office environment as the one described in the section related to SL measurements for space diversity. As for the off-body Tx antenna, the linearly polarized UWB monopole antenna [
The examples of Rx power delay distributions over time for the vertically and horizontally polarized on-body antenna have been shown in Figure
Rx power (
Figures
Total Rx power (
Average delay (
R.m.s. delay spread (
The general aim of the article was to present the versatile measurement stand, as well as the measurement methodology and scenarios being used during research carried out under the umbrella of COST Action CA15104 “Inclusive Radio Communication Networks for 5G and beyond” (IRACON) [
The concept of the experimental test bed for SL and CIR measurements for off-body and body-to-body radio channels in WBANs has been fully described. This test bed consists of the following sections: executive, RF switching, antennas, and control. The possible measurement scenarios that may occur in off-body and body-to-body channel investigation have been classified and described in detail. The considered classification includes the object of measurements, diversity scheme, mutual visibility of Tx and Rx antennas, environments, possible antennas’ placement, mutual antenna orientation, and user motion. Additionally, the evaluation of the standard and expanded uncertainty of the presented measurement stand and methodology has been provided. In the last section, the exemplary results have been presented and discussed. These measurements consider SL measurements in off-body narrowband channels with a space diversity scheme for walking and lying scenarios in office and ferry environments. In addition, the experimental results consider SL and CIR for a polarization diversity scheme, in an off-body narrowband and an ultrawideband channel, respectively. The analysis of mean SL, magnitudes of fast fading, XPD, Rx power delay, average delay, and r.m.s. delay spread allows one to draw the conclusion that there is a need for further investigations of different diversity schemes and their applications in WBANs. The presented measurement stand and methodology are a perfect facility that may be used for this purpose.
The measurement data used to support the findings of this study are available from the corresponding author upon request.
The author declares that there is no conflict of interest regarding the publication of this paper.
The author would like to thank the joint research team of Gdansk University of Technology (Department of Radio Communication Systems and Networks) and University of Lisbon (Instituto Superior Tecnico) for the research work carried out jointly in the field of channel modeling in WBANs.