Wireless Future - Telenor

slope of the gain curve corresponds to a diversity
order of 4-6, less than one might expect.
It should be recalled that the gain values shown
above rely on adaptive arrays, which all the time
track variations in the channel and adjust the
weighting factors on each side. This is a somewhat
optimistic situation in practice, where often
a fixed antenna beam will be used. If a fixed
beam is used at one array, then the gain of that
array will vary from M to 1, where the gain of M
will correspond to Fcor small and 1 when Fcor
is large (Andersen [3]). The latter case may also
be understood by the effect of a narrow beam in
a wide scattering situation, the antenna does not
receive (or illuminate) all the scatterers, and the
gain will decrease. There will of course be no
diversity gain for a fixed set of weights.
Gain Impact on Data Rates,
Range and Frequency
The data rate achievable over a link is a function
of many factors like modulation, error distribution,
coding et cetera, but in all cases a certain
energy per bit is required, the well-known
E b /N 0 . When the data rate increases the needed
power increases with the data rate R. This is why
the link antenna gain, the mean value and the
diversity gain, are so important for wideband
services, since the achievable data rate is proportional
to the power gain.
12
10
8
6
4
2
0
-2
-4
Telektronikk 1.2001
The impact on the range depends on the decay of
power with distance, so for a power law like
P = P 0 d –n (13)
the range will vary with gain like
d = d 0 G 1/n (14)
As an example we can choose n = 3.5. If the
required power is increased by a factor of 10 for
a given data rate, the range is reduced by a factor
2, unless the gain is increased correspondingly
by a factor 10. Four elements at each end will
roughly give a mean gain of 10 dB.
As far as the carrier frequency is concerned the
situation is more complicated, since it depends
on how the path loss varies with frequency. The
famous Hata law for urban propagation gives a
frequency dependence of f –2.6 for the received
power, which corresponds approximately to the
free space law and a shadow diffraction. This
is valid for constant gain antennas, but if we
instead consider constant area antennas filled
with adaptive antenna elements, the situation
changes dramatically. For all the situations
discussed above where the gain was MN the
received power now increases with the square of
the frequency. The worst case is case d) above,
where the gain is reduced, and in this situation
the received power is independent of frequency.
Single polarized, Fpin=10
Capacity
Gain 10%
Nchannel
-6
10 -2 10 -1 10 0 10 1
Fcor
Figure 7 Two 4-element
arrays with Fpin = 10 and
SNR = 10dB. Nchannel is the
mean number of active parallel
channels, Gain10% is the gain
in dB at the 10 % level, and
Capacity is the capacity in
b/s/Hz
45