Diversity Properties of Multiantenna Small Handheld Terminals
EURASIP Journal on Applied Signal Processing
Diversity Properties of Multiantenna Small Handheld Terminals
Wim A. Th. Kotterman 0
Gert F. Pedersen 0
0 Department of Communication Technology (KOM), Aalborg University , 9220 Aalborg Ø , Denmark
Experimental data are presented on the viability of multiple antennas on small mobile handsets, based on extensive measurement campaigns at 2.14 GHz with multiple base stations, indoors, from outdoor to indoor, and outdoors. The results show medium to low correlation values between antenna branch signals despite small antenna separations down to 0.16λ. Amplitude distributions are mainly Rayleigh-like, but for early and late components steeper than Rayleigh. Test users handling the measurement handset caused larger delay spread, increased the variability of the channel, and induced rather large mean branch power differences of up to 10 dB. Positioning of multiple antennas on small terminals should therefore be done with care. The indoor channels were essentially flat fading within 7 MHz bandwidth (−6 dB); the outdoor-to-indoor and outdoor channels, measured with 10 MHz bandwidth, were not. For outdoor-to-indoor and outdoor channels, we found that different taps in the same impulse response are uncorrelated.
and phrases; mobile radio channel; small multiantenna devices; measurement analysis; branch correlation; Doppler spectrum; user influence
1. INTRODUCTION
Research on smart antennas or smart algorithms seem to
have focused on base stations (BSs) and fixed terminals with
relatively little research being devoted to the benefits of
multiple antennas on small mobile terminals. A reason for this
surely must be the still frequently expressed opinion that a
separation between antennas of at least half a wavelength is
needed to get branch correlation coefficients under a
threshold of 0.7 needed for exploiting the diversity potential. In this
context, one often quotes Jakes [
1
], but he considered
amplitude correlation coefficients for early narrowband mobile
systems, whereas for GSM-like systems, it was shown that
at least for some forms of diversity, such a threshold does
not exist. Diversity gain then increases continuously with
decreasing correlation [
2
]. Moreover, Vaughan and Andersen
[
3
] showed that in the ideal case, the antenna patterns are
orthogonal with respect to the incoming wave field, which
theoretically can be achieved even at zero separation for
particular environments. This of course implies that the
achievable diversity gain depends on both the antenna design and
the specific propagation environment. In this respect,
spatial separation is merely a factor in decorrelation between
antenna signals as are polarisation properties. Experimental
confirmation has been documented from the early 1990’s
onwards [
4, 5, 6, 7
]. Please note that the overriding importance
of handset antennas being small, while efficient and
wideband, leaves little room for engineering radiation patterns.
In the framework of a project on smart antennas for
small handsets at Aalborg University (AAU), three
measurement campaigns were organised in different
propagation environments with and without users, as users have
a strong influence on the reception by handheld terminals
[
8, 9
]. During these campaigns, we used our proprietary
measurement system [10] with our “optical” handset
without conducting cables, but using signal transport by
optic fibre instead [
11
]. This paper reports on the findings,
with some emphasis placed on the three classical quantities
BS2
BS3
BS1
Measurement routes
BS1
Measurement routes
BS1
BS2
50 metres
BS3
BS3
50 metres
determining diversity gain: branch correlation coefficients,
amplitude distributions, and (mean) branch power
differences [
12
]. The structure of this paper is as follows: first, the
measurement setup is discussed with the chosen scenarios,
the use of test users, and the equipment. Next, the processing
of the data is described, followed by results and discussion.
Conclusions form the last section.
MEASUREMENT SETUP
The measurement campaigns should provide realistic data
for channel models to be used for research into smart
antennas for small handsets. Therefore, the data should be
gathered in a way that reflects typical use of handheld devices and
typical handheld devices themselves, including size, antenna
types, and locations of major components like display,
keypad, and antennas. This means measuring in different
cellular scenarios, with users handling the terminal in different
ways. Some aspects of the choices made for the campaigns
will be treated in the next sections.
2.1. Cellular scenarios
Three cellular scenarios were chosen: indoor,
outdoor-toindoor, and outdoor.
For the indoor campaigns, we selected two different
buildings as the type of construction determines the
propagation regime. One is the university building in downtown
Aalborg as example of the early twentieth-century building
style: heavy (...truncated)