Digital Signal Processing Reference
In-Depth Information
Selection diversity combining (SDC)
Equal gain combining (EGC)
antenna
measure
RF/baseband
independent
several
f
o
r fading
λ
RF/baseband
several λ
RF/baseband
monitor incoming
power
measure
Phased-array antenna
Maximum ratio combining (MRC)
measure
RF/baseband
RF/baseband
several
λ
RF/baseband
= gain tuning
= phase equalization
measure
Figure 4.9.
Spatial diversity improves the link reliability over fading channels.
Diversity combining should not be confused with phased-array
antennas with a directive sensitivity pattern.
originates from the lack of rms delay spread rather than from a malfunctioning
of the subchannel loading mechanism.
It is for this reason that spatial diversity has recently regained considerable in-
terest as a reception enhancement technique for ofdm-based wlan receivers.
The importance of this form of diversity was already recognized in the early
1970s [Dav71], but only recently found widespread use thanks to the availabil-
ity of digital signal processing. In order to combat the dreadful misery caused
by destructive intra-symbol interference, a spatial-rake receiver combines the
energy from different signals collected by two or even more antennas.
As with any other problem, your mileage may vary depending on the complex-
ity of the solution. The most widespread implementation of an multi-antenna
receiver employs a
selection diversity combining
(sdc) scheme. In this sce-
nario, the signal quality at each antenna is monitored while only the antenna
which offers the highest snr is retained for reception (Figure 4.9). Because
of its simplicity, selection diversity is used in many 802
.
11a/b/g devices. The
performance of sdc is somewhat limited, of course, since this kind of receiver
will not be able to use all the signal power that is available from both antenna
terminals. The only reason for the success of sdc is that it does not require a
duplication of (part of) the receiver front-end hardware, apart from the physi-
cally separated antennas.
