Ultra-wideband channel characterisation for body-centric communication in indoor environments

• Main Author: Catherwood, P. A.
• Published: Queen's University Belfast 2012
• Subjects: 621.38415
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.554357

The release of the Ultrawideband (UWB) regulations by the Federal Communications Commission (FCC) in 2002, and the complimentary IEEE standard 802.15.4-2006 P802.15.4a amendment in 2007 for UWB Personal Area Networks (PANs), have promised high-speed wireless personal area network (PAN) communications, and have led to an increasing interest in using the 3.1 - 10 GHz radio band for transmitting large amounts of data from a body-centric antenna over distances of less than lOm. For operation in indoor environments the proximity of the human body can cause shadowing and perturbations of the transmitted signal. Radio frequency sounding is often employed to characterise the UWB radio channel in an effort to investigate such phenomena. This thesis explores statistical characterisation of the off-body S1S0 and MISO UWB indoor radio channel between 3.1 GHz and 6.0 GHz However, the sounding equipment may itself alter the radio channel parameters and to this end, a comparison of an optical and radiofrequency (RP) cable feeding the body-mounted antenna for an off-body UWB link was performed. This revealed that the RP cables add measurement error due to their reflective construction, increasing launch scattering and post-launch reflections. This was found to be particularly evident for low reflectivity environments and for non line of sight (NLOS) configurations. Indeed, the use of cables was found to increase received power, increase multipath delay and had the potential to alter the channel's statistical distribution model. This was found to be true for 89% ofNLOS and 44% of LOS experiments. The same novel optical cable fed UWB sounder was implemented to investigate off-body links in a 49 m2 hospital environment for both stationary and mobile UWB radio transmitters mounted on the waist and chest. The results showed that received signal strength values were dependent on whether transmit and receive antennas had line of sight (LOS) and were also affected by body- shadowing and antenna-body position. For mobile conditions, received signal strength tended to be Lognormally distributed with NLOS links having significantly lower mean values. Excess time delay results for the mobile user tests were best described by the Weibull distribution. Overall, the results favoured the chest mounted antenna position, with higher mean signal levels, reduced mean excess delay and less difference between LOS and NLOS channels. Additionally, it was found that the Rician k-factor values for the anechoic and hospital environment were significantly affected by environmental conditions, highlighting that the human body acts primarily as part of the radiating structure. Nonetheless, variations in the k-factor values for the differing antenna mounting positions also indicated that this was also a factor affecting off-body radiation. Finally, the performance of a multiple antenna body-centric transmitter was investigated in an office and corridor environment. It was found that when using such a multi-antenna array, all the distributions for each of the channels were Lognormally distributed in the office and Rician in the corridor. It was also found that using selection combining techniques, all the distributions for two-channel diversity and three-channel diversity for both LOS and NLOS journeys were mostly Lognormally distributed in the office and the corridor. Thus, use of simple channel diversity techniques changed the channel's statistical distribution from Rician for a single channel to Lognormal for combined channels in the corridor. A general correlation between mutual coupling between the antennas in the array and the received power values on each channel was found. It was also found that when the antenna array was placed on the body with such a configuration as presented here, diversity gains were poor, in the region of only 1 dB. Overall, the results in this thesis indicate that off-body UWB channel characterisation should be conducted with optical fed antennas and more research into antenna positioning to attain diversity gain is required.