Biomedical Engineering Reference
In-Depth Information
Diversity Technique Analysis
Doppler Shift
In mobile communications, a Doppler shift in the signal frequency occurs due to the
relative speed of motion between the transmitter and receiver; this is also true for the
on-body case.
In order to capture all movements made by the human body during measurements,
the sampling frequency of the measurement should be greater than twice the maxi-
mum expected Doppler shift. This shift is calculated for all channels using Eq. 1, as
givenin[
28
]:
v
λ
0
f
m
=
(1)
where
v
is the velocity of motion of the human body in metres per second and
λ
0
is the
wavelength in metres. For the UWB band (3.1-10.6 GHz), with a centre frequency
of 6.85 GHz,
f
m
=
68.5 Hz for
a speed of 3 m/s. The maximum Doppler shift is observed for the waist-to-wrist
channel, compared to the other channels. During measurements, the sampling time
was set to 6.6 ms on the PNA, to capture even the fastest movement of 3 m/s by
keeping the sampling frequency equal to 150 Hz, which is greater than twice the
maximum expected Doppler shift (i.e. 68.5 Hz).
22.83 Hz for an average speed of 1 m/s and
f
m
=
Envelope Correlation Coefficients
For a diversity scheme to be effective, each antenna element should receive statis-
tically independent versions of the transmitted signal reducing the likelihood that
all branches are experiencing correlated fading. Two signals are said to be suitably
uncorrelated if their correlation is less than 0.7 [
2
]. Signal decorrelation is usu-
ally introduced by multi-path components. The envelope correlation coefficient (
ρ
e
)
between the two diversity branches is calculated by the following expression [
29
]:
i
=
i
N
(
r
1
(
i
)
−
r
1
)(
r
2
(
i
)
−
r
2
)
ρ
e
=
N
r
1
)
2
N
(2)
i
=
i
i
=
1
r
2
)
2
(
r
1
(
i
)
−
(
r
2
(
i
)
−
where N is total number of samples and
ri
is the mean value of fast fading envelope
ri
of received diversity branch signal
i
.
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