[MLB-WIRELESS] Need details on 802.11a outdoors...

[John Dalton]

There is also the question of whether 802.11a will work reliably
outdoors.

By itself, Orthogonal Frequency Division Multiplexing (OFDM)
as used in 802.11a does not provide much protection against
intersymbol interference due to delay spread in the channel.

In plain english, this means your receiver may see radio waves
bouncing off two (or more) different objects.  Typically each radio
wave will be shifted in time, as each has taken a different
length path from the transmitter to the receiver.  When
the receiver adds these components up, the edges of each data symbol
becomes 'blurred'.  At high data rates, these 'blurred' edges meet and
the entire symbol becomes 'blurred'.  As the difference in length of
each radio path increases (ie. the delay spread of the channel increases)
the effect cuts in at lower data rates.

To combat this, 802.11a extends each symbol, in time, by a small amount.
During receiver processing, the 'blurred' edges of each symbol are thrown
away, leaving the clean part of the symbol for further processing.

The length of this 'cyclic extension' or 'guard inverval' has been chosen
by the designers of 802.11a to deal with the longest expected delay
spread 802.11a will encounter.  There is a trade off here
as increasing cyclic extension reduces the data rate.  Typically
an outdoor channel has a longer delay spread than an indoor channel.
To picture this, think that the walls in an average room may be
of the order of 10 metres away.  When radio waves bounce of these walls
you might get a delay spread of 70 nanoseconds (2x10m/speed of light).
Outdoors, you will probably get reflections from that building 150 metres
away, corresponding to a maximum delay spread of 1000 nanoseconds.  Compare
these numbers with the guard interval of 802.11a, which is 800 nanoseconds.
Result: outdoors, the 802.11a receiver might not be able to remove
all the 'blurring' and performance drops off.

In summary, 802.11a just might not work well outdoors.

Regards
John

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