- Research Article
- Open Access
Experimental Characterization of a UWB Channel for Body Area Networks
© Lingli Xia et al. 2011
Received: 28 October 2010
Accepted: 14 January 2011
Published: 20 January 2011
Ultrawideband (UWB) communication is a promising technology for wireless body area networks (BANs), especially for applications that require transmission of both low and high data rates with excellent energy efficiency. Therefore, understanding the unique UWB channel propagation characteristics around the human body is critical for a successful wireless system, especially for insuring the reliability of important vital sign data. Previous work has focused only on on-body channels, where both TX and RX antennas are located on the human body. In this paper, a 3–5 GHz UWB channel is measured and analyzed for human body wireless communications. Beyond the conventional on-body channel model, line-of-sight (LOS) and non-line-of-sight (NLOS) channel models are obtained using a TX antenna placed at various locations of the human body while the RX antenna is placed away from the human body. Measurement results indicate that the human body does not significantly degrade the impedance of a monopole omnidirectional antenna. The measured path loss and multipath analysis suggest that a LOS UWB channel is excellent for low-power, high-data-rate transmission, while NLOS and on-body channels need to be reconfigured to operate at a lower data rate due to high path loss.
A high data rate is not typically an important concern for body area networks, as sampling frequencies of front-end sensors is typically less than 1 kHz. For example, a heart reading using ECG requires at most 12 kbps or 12 b at 1 kHz. However, for body sensor applications that require tens or hundreds of sensing channels , a large bandwidth is necessary. One example is a handheld, wireless ultrasound module with hundreds of ADC channels, which need to send several megabits of data. Another example is in next-generation brain implants, which will require hundreds of cortical implant channels streamed wirelessly to a stationary receiver . This large communication bandwidth will also be needed for an application where BAN data may firstly be stored locally on the sensor node, such as in a local data storage memory. Then when the patient goes to the hospital, the doctor can read these data through high-data-rate transmission and make a thorough diagnosis, as shown in Figure 1(b).
LOS measurement results comparison.
48 mW at 0 dBm
50.4 mW at 0 dBm
63.6 mW at 0 dBm
4.44 mW at −41.3 dBm/MHz
BER at RSSI
0.2% at −51 dBm
0.1% at −63 dBm
<0.1% at −64 dBm
1% at −50 dBm
Knowledge of the channel model for UWB transmission is critical for any robust transceiver system. Moreover, body area networks exhibit unique radio propagation characteristics combining line of sight, creeping wave, multiple reflections from surrounding environments, and diffraction around the human body. Ever since the FCC released unlicensed spectrum for UWB, several previous works on UWB channel modeling have been published. Molisch et al.  developed an IEEE 802.15.4a channel model for various low-rate UWB applications, where the body area network channel model is analyzed using a finite difference time-domain (FDTD) simulator with antennas moving around the human body. Wang et al.  also used FDTD method to simulate various body postures based on a realistic human body model. Unfortunately, these numerical approaches neglect considerations of the surrounding environments, which are the main sources of multipath.
Furthermore, the previous investigations only considered data transmission with both TX and RX antennas on the human body, which is not the dominant usage model. In this paper, we present a complete UWB channel model that not only considers on-body UWB propagation but also extends to include LOS and NLOS channel measurement, using a TX antenna placed on the human body and a separate RX antenna located externally. Section 2 introduces the measurement setup of this work, Section 3 discusses the measurement results and provides a thorough analysis on different channels, and Section 4 draws a conclusion.
2. Measurement Setup
In this work, the UWB radio channel measurement is performed in an EM-shielded lab with a height of 3.5 m. The lab resembles an ordinary room with concrete walls, ceiling, desks, and chairs. When the door is closed, the lab is protected from EM interferences by metallic panels behind the walls and ceiling. This enables accurate estimation of local multipath propagation, with sufficient interference rejection.
There is no object obstructing the TX and RX antennas. The TX antenna is placed on the head, chest, and left thigh of the human body while the RX antenna is placed at the same height off the human body.
The transmission between the TX and RX antennas is interrupted by the human body.
Both TX and RX antennas are placed on the human body. The RX antenna is worn on the left wrist while the TX antenna is able to freely move around.
3. UWB Radio Channel Measurement Results
3.1. Propagation Path Loss
3.1.1. Frequency Dependence
3.1.2. Distance Dependence
Comparison of parameter values for distance-dependent path loss model.
Path loss of on-body channels.
Left wrist to
path loss (dB)
3.2. RMS Delay Spread
Ultrawideband communication is a promising technology for next generation body sensor networks due to its potential for both low power and large bandwidth, currently unavailable using conventional narrowband systems. In this paper, both line-of-sight (LOS) and non-line-of-sight (NLOS) channels with various TX and RX antennas placed near the human are characterized. The frequency- and distance-dependent characteristics of a UWB channel are analyzed in this paper, where an NLOS channel is shown to have larger path loss than a LOS channel due to the physical interruption of the human body. Moreover, the path loss of an on-body channel is comparable with an NLOS channel. RMS delay spread is presented which provides an intuitive inspection of the multipath richness of a variety of channels. According to the experimental and analytical results, UWB systems with high data rate will require LOS channel characteristics. For sensor network application where only low-data-rate transmission is needed, NLOS and on-body channels can exhibit good performance using UWB.
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