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cheapest WiFi router has an internal second antenna.
Diversity may be
spacial
, the two antennas suffi-
ciently apart to ensure that they are not destructively
interfered with at one time.
They may occupy the same space, but be differ-
ently
polarised
. (It is unlikely that both vertical and
horizontal polarisation will be in destructive mode at
the same time.)
The diversity may be in
frequency
, either within
or across the 2,45 and 5,8 GHz bands.
They can also cleverly use
time domain
repeti-
tion, as a form of time diversity in a rapidly changing
environment.
Naturally, there is all the above, essentially what
IEEE802.11n, MIMO (Multiple Input Multiple Out-
put) uses, but noting that it takes computing power
either to select or combine the outputs from the an-
tennas, which also consume electrical input power.
Hence intelligent MIMO devices turn off MIMO
unless absolutely required in order to conserve pow-
er, especially if battery driven.
Another major ‘indoor’, i.e. not ‘outdoor’ problem
is non-metallic attenuation. The problem here is the
wall, cabinet, chair, passage way, tool chest, etc that
gets between the transmitter and receiver. Sadly, a
human looks like a lump of water at these frequen-
cies, and unfortunately, tends to move about, chang-
ing the electromagnetic environment.
This challenge is only met by more power, greater
antenna gain, repositioning, or adaptive mesh net-
working (getting around the obstacle/s).
3.4 Wireless coexistence
Radio systems do not exist in isolation.
We all share the same ‘ether’. Even systems that
operate at different frequencies can still interact by
RF swamping of sensitive receiver stages, etc.
Of particular interest is narrow-band interference
killing wide-band systems. A strong Bluetooth signal
often kills WiFi. They operate in the same frequen-
cy band, but Bluetooth divides it into 95 channels
through which it hops in time, whereas WiFi has only
11 channels (three non-overlapping), much wider,
but static in time.
Although both spreading and hopping strategies
were developed to reduce the possibility of intercept