Channel Characterization and Robust Tracking for Diversity Reception over Time-Variant Off-Body Wireless Communication Channels
EURASIP Journal on Advances in Signal Processing
Hindawi Publishing Corporation
Channel Characterization and Robust Tracking for Diversity Reception over Time-Variant Off-Body Wireless Communication Channels
Patrick Van Torre 1 2
Luigi Vallozzi 2
Hendrik Rogier 2
Marc Moeneclaey 0
Jo Verhaevert 1
Markus Rupp
0 Department of Telecommunications and Information Processing (TELIN), Ghent University , St. Pietersnieuwstraat 41, 9000 Ghent , Belgium
1 Hogeschool Gent, INWE Department , Schoonmeersstraat 52, 9000 Gent , Belgium
2 Information Technology Department (INTEC), Ghent University , St. Pietersnieuwstraat 41, 9000 Ghent , Belgium
In the 2.45 GHz band, indoor wireless off-body data communication by a moving person can be problematic due to time-variant signal fading and the consequent variation in channel parameters. Off-body communication specifically suffers from the combined effects of fading, shadowing, and path loss due to time-variant multipath propagation in combination with shadowing by the human body. Measurements are performed to analyze the autocorrelation, coherence time, and power spectral density for a person equipped with a wearable receive system moving at different speeds for different configurations and antenna positions. Diversity reception with multiple textile antennas integrated in the clothing provides a means of improving the reliability of the link. For the dynamic channel estimation, a scheme using hard decision feedback after MRC with adaptive low-pass filtering is demonstrated to be successful in providing robust data detection for long data bursts, in the presence of dramatic channel variation.
1. Introduction
The safety of rescue workers can be improved by smart
textiles that allow a data communication system to be
integrated into their garment. Textile antenna systems provide a
convenient means for unobtrusive integration that does not
disturb the movements of the rescue worker. The structure of
the patch antennas used, with a ground plane near the body,
limits the radiation exposure of the body and the influence of
the body on the radiation pattern of the antenna. Operations
are often performed indoors, where dramatic signal fading
occurs due to multipath propagation. Given the rapid
movements by rescue workers in combination with the multipath
fading of the indoor environment and the shadowing by the
human body, the channel seen by the multiantenna system
varies rapidly in time. Often short bursts are used to counter
this problem. However, this introduces a significant overhead
caused by the retransmission of the necessary preambles for
each burst.
The measurements presented in this paper are performed
in the same environment where earlier measurements for
off-body communication, documented in [
1, 2
], indicated
Rayleigh-distributed small-scale fading. The rapid
fluctuations over larger distance scales are caused by both
smallscale fading and shadowing. The cumulative distribution
function and level crossing rate in these cases fitted to
the Nakagami distribution. Measurements were performed
deploying two textile antenna patches integrated in a
garment on the human body, using short data bursts and
treating the channel as time-invariant.
In this paper, a continuous transmission, consisting
of a data stream that lasts for much longer than the
channel’s coherence time, will be performed. Measurements
are presented deploying four wearable antenna patches at
the rescue worker and capturing longer time frames, subject
to very significant variations of the channel behavior in
time. In [
1, 2
], no such drastic channel variation occurred
during the same received burst. Important parameters of
the time-varying off-body communication channel, such as
autocorrelation, coherence time, and power spectral density,
are determined. The values are shown to be influenced by
walking speed as well as antenna location on the body.
Measurements are performed for line-of-sight (LoS) and
Non line-of-sight (NLoS) situations.
Diversity techniques, using multiple antennas, are widely
known to combat the effects of fading. At the receiving
side, maximal ratio combining (MRC) is used to combine
signals from different antennas. To use MRC effectively,
an accurate channel estimation is necessary at all times.
Channel estimation can be performed using a series of pilot
symbols. In previous publications [
1, 2
], the transmitted
bursts are kept short, allowing the channel to be treated as
invariant for the duration of the burst. However, as each
separate burst needs to contain a number of pilot symbols, a
significant overhead is introduced, limiting the effective data
throughput.
For the measurements presented, long data bursts
containing one million data symbols and lasting over one
second are transmitted. During this transmission time, the
channel is definitely not invariant when communicating with
a walking person; therefore, a robust system of dynamic
channel tracking is needed. As we p (...truncated)