UWB for Around-the-Body Data Streaming - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 47

Fig. 3 Challenges for existing hearing-aids radios
measurements have been performed with a vector network analyzer on a Phantom
head using single-ended antennas. The reported fading is in excess of 80 dB for
some UWB channels. However, in [ 2 ] novel time domain ear-to-ear measurements
with 7 human subjects using differential antennas are reported with a UWB channel
fading of around 60-65 dB. Moreover, in [ 2 ], it is shown that by using single-ended
antennas it is possible to create deep fades in the measurements due to cable cross
talk. The results in [ 2 ] also indicate that pocket-to-ear has a slightly larger path loss
partially due to the increased distance.
To this point, it seems more reasonable (except the part for the antenna size which
decreases as the frequency increases) to opt for a radio system at lowest possible
frequency, which can still support a wireless link upto multi-meter. One of the good
candidates is the Bluetooth radio, which actually exists in the market. The major
challenges for these existing solutions are coexistence and fading. As shown in
Fig. 3 , the lower GHz band is crowded with different wireless systems, such as GPS,
PCS, 802.11.b/g and Bluetooth, and is also extended to 802.11.a at 5 GHz. Strong
interferences are the first thing to be tackled if the audio streaming device is chosen
to operate at this frequency range. Tighter selectivity, better sensitivity, and larger
transmitted power are the common solutions for interference issues, which means
larger form factor and higher power consumption, thus stringent but solvable. In
contrast, fading is more an inherent issue in the narrowband system. As shown in
Fig. 3 , in a narrow band system, the path loss of the signal varies with time, position,
and frequency. Compensations are usually made in the digital algorithm, again at the
expenses of power consumption and the complexity.
Considering all the above challenges, high-band impulse-radio ultra-wide-band
(IR-UWB) system tackles the spectrum crowdedness and narrow band fading by
nature. The high-band choice offers a frequency range that contains channels for
worldwide coverage. Moreover, the high-band system benefits from a smaller form
factor due to the smaller antenna size. Due to the ability of larger frequency separa-
tion, the high-band IR-UWB is more robust against out-of-band interference, such
as WLAN at 5 GHz, ISM at 2.4 GHz, and GSM at 1.8 GHz.
Except for the frequency range and the fading, the IR-UWB system also benefits
from several other advantages, due to its fundamental structures, i.e., duty-cycling.
In a conventional continuous radio system, the signal is transmitted and received