Characteristics of the polarised off-body channel in indoor environments
Turbic et al. EURASIP Journal on Wireless Communications and Networking
Characteristics of the polarised off-body channel in indoor environments
Kenan Turbic 0 2
Slawomir J. Ambroziak 1
Luis M. Correia 0 2
0 Instituto Superior Técnico, INESC-ID, University of Lisbon , Lisbon , Portugal
1 Faculty of Electronics, Telecommunications and Informatics, Gdansk University of Technology , Gdansk , Poland
2 Instituto Superior Técnico, INESC-ID, University of Lisbon , Lisbon , Portugal
This paper addresses the depolarisation effect in off-body body area networks channels, based on measurements performed at 2.45 GHz in an indoor environment. Seven different scenarios, involving both static and dynamic users, were considered, taking a statistical perspective. The analysis of the cross-polarisation discrimination is performed, as well as the analysis of path loss in co- and cross-polarised channels. Results show a strong dependence of the cross-polarisation discrimination and of channel characteristics on the polarisation and propagation condition, i.e. line-of-sight (LoS), non-LoS or quasi-LoS. Distance, varied between 1 and 6 m in the considered scenarios, is observed to have very little impact on the cross-polarisation discrimination. In the considered dynamic scenario, the channel is characterised by lognormal-distributed shadowing and Nakagami-distributed multipath fading. Parameters of the Nakagami distribution have essentially different values in the co- and cross-polarised channels, showing a trend towards Rice in the former and Rayleigh in the latter. Based on results, a model is proposed for a dynamic off-body channel.
1 Introduction
While the depolarisation of an electromagnetic wave
transmitted over a wireless channel is a well-known
phenomenon, the interest in describing it arose from the
fact that orthogonal polarisations can be exploited as
additional degrees of freedom in a channel, in order to
improve communication quality by means of polarisation
diversity [
1
], or to increase the available data rates by means
of polarisation multiplexing [
2
]. Recently, dual-polarised
antennas are being considered for using high data rates in
multiple input multiple output (MIMO) systems, when the
channel matrix is rank-deficient due to the presence of
strong LoS (line-of-sight) [
3
].
The depolarisation effect in wireless channels yields
mismatched polarisations in between the Rx antenna
and the impinging E-field, arising from several factors,
addressed in what follows. Depolarisation of the LoS
component is due to the physical misalignment of the
transmitter (Tx) and receiver (Rx) antennas, and also to
imperfect antenna cross-polarisation isolation (XPI),
where practical antennas inevitably radiate some power
in the undesired polarisation other than the one it was
designed for (co-polarisation). While this can be avoided
in fixed radio links, if antennas’ orientation is carefully
chosen, somewhat random antenna rotations in mobile
and off-body communications will unavoidably yield
variable LoS depolarisation during user’s motion. In
addition, interaction with the environment causes
additional depolarisation of multipath components (MPCs).
According to the geometrical theory of depolarisation [
4
],
the extent of this depolarisation depends on the relative
geometry between the antennas and the scattering object,
i.e. orientation of the plane of incidence, as well as on the
electromagnetic properties of scattering objects, yielding
different attenuation and phase changes associated with
the orthogonal components of reflected, diffracted, and
scattered waves. The channel’s depolarisation
characteristics depend on the environment (i.e. its geometry and
electromagnetic properties), radiation/polarisation
patterns of antennas, propagation conditions (due to the
dominance of different depolarisation factors), as well as
user’s dynamics.
Several researchers have addressed the depolarisation
effect, providing statistical models for the channel
depolarisation effects based on measurements, while
only few have provided physical models explaining the
actual source of depolarisation [
4, 5
]. An important step
in understanding the depolarisation of MPCs was made
in [4], where channel coefficients corresponding to
orthogonal polarisation components of MPCs at the Rx
are obtained from a three-dimensional geometry
environment, accounting for Tx and Rx’s relative positions.
The model assumes ideally conducting reflection
surfaces, therefore, neglecting the depolarisation due to
different attenuation of the perpendicular and parallel
components. On the other hand, depolarisation due to
realistic scattering is modelled in [
5, 6
]. In [7], the
depolarisation effect due to antennas’ mismatch is
analysed, where the derivation of the polarisation
rotation angle for the LoS component is based on a
three-dimensional geometry, for arbitrary orientations of
Tx and Rx antennas. The depolarisation of MPCs is
modelled by i (...truncated)