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Increasing handset performance using True polarization diversity
Valenzuela-Valds, Juan F.
EMITE Ing
Edificio CEEIM. Campus Espinardo
E-30100 Murcia , SPAIN
Snchez-Hernndez, David A.
Technical University of Cartagena
Plaza del Hospital, 1
E-30202 Cartagena, SPAIN
AbstractWhile many different simulation tools are available to
compute MIMO capacity and diversity gain system performance,
few works tackle the problem from the measurements
standpoint. In this paper, several measurements of MIMO
capacity and diversity gain performance of dipole antenna arrays
are performed through the use of a reverberation chamber,
which includes many effects present in real MIMO channels but
generally avoided in simulations. Results show that polarization
diversity can be effectively combined to spatial diversity even for
Rayleig-fading MIMO scenarios to achieve increased diversity
gain and MIMO capacity.
Index Terms Diversity gain, MIMO, spatial diversity,polarization diversity, reverberation chamber.
I. INTRODUCTIONThe improvement granted by polarization diversity in wireless
systems is typically obtained by an additional de-correlated
channel provided by a polarization state made orthogonal tothe existing one, usually at the transmitting end. A randomly
oriented linearly-polarized antenna is also typically used at the
receiver for evaluating polarization diversity. In this scheme
the cross-polarization discrimination (XPD) factor is the usual
evaluation parameter, with low correlation coefficients being
achieved in LOS and non-LOS situations [1]. Somecombinations of two-branch orthogonal polarization and
spatial diversity have been reported [2]. In mobile
communications scenarios, however, multiple scattering may
not be sufficient for a given polarization to decouple half its
power into the orthogonal polarization [3]. A recent letter [4]
has proposed a novel true polarization diversity (TPD)technique by rotating one antenna by a certain angle with
respect to the contiguous element in the MIMO array. In this
way an arbitrary angular separation between contiguous
dipoles is employed in an equivalent way that an arbitrary
spatial separation is employed for spatial diversity. Inaddition, an accurate prediction of the correlation coefficient
between two dipoles separated by both a spatial distance and
an arbitrary angular position has not been available until very
recently [5]. This has made possible the evaluation of TPDperformance for handset MIMO. Results to be presented in
this contribution demonstrate that under Rayleigh-fadingscenarios TPD can be effectively combined with spatial
diversity to nearly double the diversity gain and MIMO
capacity for the same available volume.
II. MEASUREMENT TECHNIQUE AND SET-UPThe capacity for fading channels can be defined in a
number of ways, very much depending upon the amount of
channel knowledge made available to the transmitter,
including delay and signaling constraints and the statistical
nature of the channel. The instantaneous channel capacity for
MIMO systems is well defined by,
zbits/s/H'**detlog2
+= HH
T
SNRIC RMIMO (1)
The system has Tantennas at the transmitter andR antennas
at the receiver andIR is the identity matrix with dimension R.
In an independent identically distributed (i.i.d.) Rayleigh
environment this capacity can be approximated for high
signal-to-noise ratio (SNR) to Cmin(T,R)log2(SNR) bits/s/Hz.
This growth potential is extraordinary since each 3-dB
increase in transmit SNR will result in roughly min(T,R)
b/s/Hz capacity increase in comparison to 1 b/s/Hz capacity
gain in single-antenna systems. For correlated Rayleigh-fading
environments, however, which represent real scenarios due to
lack of scattering at different antenna elements at reception
and/or transmission ends, the capacity slope changes as a
function of the number of antennas in reception and
transmission, among other parameters, and more detailed
simulation studies are required and available in the literature
[4].
The multipath environment can be generated artificially in a
reverberation chamber fed by wall-mounted antennas, which
thereby provides a statistically repeatable laboratory-produced
environment for characterizing mobile terminals and their
antennas. A reverberation chamber is a metal cavity
sufficiently large to support many resonant modes (multimode
cavity), which are perturbed with stirrers in order to create a
fading environment similar to the ones found in indoor and
urban environments but with a uniform elevation distribution
of the incoming waves. The reverberation chamber can be
efficiently used to measure radiation efficiency, which
characterizes the performance of single antennas in an
isotropic multipath environment according to the above. The
received signal becomes then normally (Gaussian) distributed,
and from this their associated magnitudes get a Rayleigh
distribution, and the phases get a uniform distribution over 2.
The system is completed with an elevation distribution factor
to account for the larger probability of having waves coming
from close to vertical than from close to horizontal directions
in real multipath environments and with the use of the
embedded element pattern as a way to emulate the fact that for
real multipath environments the received signal on each port is
transmitted or detected independent of other ports. The
chamber can also be used to measure the diversity gain and
channel capacities of MIMO antennas with useful and
978-1-4244-2517-4/09/$20.00 2009 IEEE
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accurate repeatability. Since all excitations and parameters can
be weighted, different fading scenarios can also be obtained.
With the use of a reference antenna and the MIMO antenna
array several MIMO parameters can be evaluated through
adequate processing of the measured S-parameters, which are
gathered between the measured port and the wall-mounted
antennas for all positions of the platform and mechanical
stirrers and for all frequency points. The measurement
procedure is then repeated for every antenna port, with theuncorrected ports terminated in 50 , for exactly the same
stirrer positions and position of the array inside the chamber,
and a similar procedure is employed with the reference
antenna. Thus, the field environment is exactly the same when
measuring every port.
S-parameters are first pre-processed on the wall-mounted
exciting antennas (Smn) and of the MIMO array antennas (Snn)
with a complex averaging over stirrer positions by,
== stirrerpos nnnnstirrerpos mnmn SNSandSS
11(2)
Where N is the total number of stirrer positions. In fact,
averaging is also performed for platform positions andpolarization stirring at the users perusal. At low frequencies it
is also advantageous to perform averaging over a small
frequency band (frequency stirring) to get more independent
field samples representing a richer multipath environment.
Since some commercial systems like GSM use frequency
hoping, this is yet another more realistic emulating
characteristic of the reverberation chamber. S-parameters are
then normalized to,
22 )(1)(1 nnmn
mnmn
SS
SS
= (3)
to obtain a better accuracy for the radiation efficiency. Theaverage net transfer function of the chamber becomes,
= stirrerpos mnST21
(4)
S-parameters can also be weighted with mismatch factors
before the frequency stirring, and corrected to a reference
level corresponding to 100 % radiation efficiency. The
processed S-parameters represent estimates of the channel
matrix H of multipath communication channels set up between
the wall antennas and the MIMO array inside the chamber.
Apparent diversity gain for the selection- or maximal ratio
combining techniques is obtained from the processed S-
parameters by evaluating the cumulative probability
distributions of the measured channel samples received at each
MIMO array antenna by,
ref
nnrefmn
mnT
SeSh
)1(2
= (5)
where Tref is the net chamber transfer function for the
reference antenna and eref is its known radiation efficiency.
The effective diversity gain is the increase from the reference
level to the combined signal that is observed at the 1%
probability level. Channel capacity, also known as spectral
efficiency is calculated using the channel estimates hmn in eqn.
(5) between each of the MIMO array antenna and each one of
the wall-mounted exciting antennas, i.e., a m x n radio
channels represent a m x n MIMO system, with n being the
number of MIMO array antennas under test and m the number
of wall-mounted antennas. With only one wall-mounted
antenna the normalized channel estimates form the channel
vector becomes,
[ ]nnx hhhH 112111 ...= (6)
For the maximal ratio combining, for instance, the channel
capacity C1 x n for the system can easily be calculated as,
)1(log2
1 121 =+=n
i inxhSNRC (7)
For each channel matrix estimates H1 x n, the channel capacity
is calculated for a specific SNR range, and all channel capacity
estimates are averaged to produce a maximum average
channel capacity as a function of the SNR. Maximal ratio
combining represents the maximum theoretical capacity and
the upper boundary. Since coupling is inherently accounted for
in the measurements, mean capacity is then derived by,
))(det(log*
2 nxmnxmRnxm HHm
SNRIC += (8)
Similarly, other MIMO parameters such as the correlation
coefficients can also be evaluated in the chamber.
III. MEASUREMENTSCENARIOSReverberation chambers have already demonstrated their
ability to reproduce multipath propagation environments
typically found in indoor and urban wireless environments [6].
In order to evaluate the full potential of TPD techniques,
combined-diversity systems with both spatial and TPD
techniques have been tested. Measurements were performed fordifferent 3x6 linear MIMO systems. The diverse scenarios are
illustrated in figure 1. The correlation coefficients and MIMO
capacity performance were measured for the MIMO array
formed by the 3 wall-mounted transmission antennas and the
combination of 6 receiving dipole antennas. The only
possibility of using conventional orthogonal polarizationdiversity (OPD) in this scheme is to alternate the polarization
orthogonal sate between contiguous elements (VHVHVH).
Such a system is really a particular case of TPD with d=90.In fact, the spatial-only diversity scheme can also be
considered a particular case of TPD with d=0.
Figure 1. Tested TPD array schemes
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IV. MEASUREMENTRESULTSMeasured correlation coefficients for the linear arrays are
illustrated in figure 2 for small element spacing. The
correlation coefficients depicted in figure 2 are measured with
respect to the first dipole in the array. The results confirmedthe enormous potential of TPD for combined-diversity
schemes. It is easily observed from figure 2 that the alternating
orthogonal polarization scheme (TPD with d
=90) has ajigsaw correlation behaviour, which could diminish MIMO
capacity. It is also interesting to observe from figure 2 that
more general TPD schemes (different from d=90) depict adifferent correlation pattern with respect to more conventional
OPD. Diversity gain measurements were performed in the
reverberation chamber for two receiving dipoles separated byboth an angular and a spatial separation. Figure 3 shows how
the measured diversity gain depend on angular separation d() and wavelength-normalized dipole spatial separation
D=(d/). As expected, the combination of both spatial
diversity and TPD provided increased diversity gain with only
two elements in the array. The combination has a stronger
effect when both separations are not large, i.e. when the spatial
separation is large (D 0.24), the angular separation canhardly improve the diversity gain, and vice versa when the
angular separation is large (d 54), the spatial separation
can barely improve the diversity gain. This suggests that agood combination of the two techniques represents the most
efficient technique for optimum diversity performance within
the same reduced volume made available to the complete
array. This is also expected to have an effect on MIMO
capacity. Figure 4 depicts the measured MIMO capacities for
different combined-diversity systems at SNR=15 dB. Allcombined-diversity tested systems provide increased capacity
with respect to the spatial-diversity-only linear MIMO system.
In addition, when the spatial-diversity antenna spacing D is
small enough (D
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IEEE Transactions on Antennas and Propagation, Vol. 6, No. 6, pp. 776-781, June 1998.
[3] M. Kang and M-S. Alouni, Capacity of correlated MIMO RayleighChannels, IEEE Transactions on Wireless Communications, Vol. 5, No.
1, pp. 143-155, Jan. 2006.[4] J.F. Valenzuela-Valds, M.A. Garca-Fernndez,. A.M. Martnez-
Gonzlez, and D. Snchez-Hernndez,, The role of polarization
diversity for MIMO systems under Rayleigh-fading scenarios, IEEEAntennas and Wireless Propagation Letters, Vol. 5, pp. 534-536, 2006.
[5] J.F. Valenzuela-Valds, A.M. Martnez-Gonzlez, and D. Snchez-Hernndez, Estimating combined correlation functions for dipoles inRayleigh-fading scenarios, IEEE Antennas and Wireless Propagation
Letters, Vol. 6, pp. 349-352, 2007.
[6] K. Rosengren and P.S Kildal, Study of distributions of modes and planewaves in reverberation chamber for characterizacion of antenas in
multipath environment, Microwave and Optical Technology Letters,Vol. 30, pp. 386-391, 2001.