Biomedical Engineering Reference
In-Depth Information
Mutual Coupling Between Diversity Branch Antennas
A spacing of
λ
0
/2 between diversity branch antennas is considered as sufficient for
minimising the effect of mutual coupling [
2
]. For the five on-body links (i.e. chest-to-
waist, back-to-waist, wrist-to-waist, head-to-waist and ankle-to-waist), the spacing
between the two branches was varied from 5 mm (0.11
λ
0
) to 32 mm (0.73
λ
0
) and the
mutual coupling measured;
λ
0
is the free-space wavelength at the centre frequency of
6.85 GHz. It is noted from that, for a minimum spacing of 15 mm (0.34
λ
0
) between
diversity branch antennas, the mutual coupling remained below
15 dB across
the whole UWB band. This indicates that the antennas are suitably decoupled. The
differences observed in the mutual coupling for the five on-body channels are mainly
attributed to variation in the effective permittivity surrounding the antenna elements,
due to changes in the tissue properties in the chosen on-body positions. However,
for all measured on-body channels, the mutual coupling remained below
−
15 dB.
Therefore, for the following measurement campaign, 0.34
λ
0
spacing between the
branches was applied.
−
Diversity Combining and Diversity Gain Calculation
Three commonly-used diversity combining techniques are used in this study: selec-
tion combining (SC), equal-gain combining (EGC) and maximum-ratio combining
(MRC). The channel responses for the two diversity branches are captured by the
PNA in the frequency domain and converted to the time domain using an inverse fast
Fourier transform (IFFT). Diversity combining is achieved by using the expressions
givenin[
3
] for combining the time domain signal:
SC(
t
)
=
max(
r
1
(
t
),
r
2
(
t
))
(3)
r
1
(
t
)
+
r
2
(
t
)
EGC(
t
)
=
(4)
2
r
1
(
t
)
r
2
(
t
)
MRC(
t
)
=
+
(5)
where
r
1
(
t
) and
r
2
(
t
) are the two received branch signal envelopes. The DG was cal-
culated by plotting the cumulative distribution functions (CDFs) of the two branch
signals and the diversity-combined signal. The DG is the difference between the
strongest of the two branch signals and the diversity-combined signal at some spec-
ified outage probability [
3
,
4
]. In this thesis, the outage probability is assumed to
be 10 % for the DG calculation. For MRC and EGC, co-phasing of the two branch
signals was achieved by shifting the phase of one signal with respect to other signal
using the simple procedure taken from [
2
]. Figure
4
shows the CDF plot for the
head-to-waist channel for the indoor environment (at location 1).
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