[e2e] narrow vs. wide bandwidth (hz)
D. Ridgely Bolgiano
bolgiano at ieee.org
Tue Oct 7 17:44:44 PDT 2003
A brief word from the "Physical Layer" by a "wet blanket" physicist. ;-)
47CFR2.202(a) "Occupied bandwidth. The frequency bandwidth such that,
below its lower and above its upper frequency limits, the mean powers
radiated are each equal to 0.5 percent of the total mean power radiated
by a given emission. ..."
While intended for wireless transmission, this definition does apply
equally well to wireline and is a good approximation of the "bandwidth"
used by Claude Shannon in defining information rate (H).
The best transmission occurs when the receive acceptance bandwidth is
equal to that transmitted. Indeed, in many modern implementations (such
as Cellular, D-AMPS or T1, OC3, etc.) typically the receive filter
characteristic is made identical to the transmit filter characteristic.
Shannon assumed matched filters for maximum H.
Per se, whether a bandwidth is particularly narrow or wide has nothing
to do with information rate. If the receive filter is wider than the
transmit bandwidth, an excess of noise is received, thus S/N is
decreased when compared to the matched case because N is larger.
Conversely, if the receive filter is narrower than the transmit
bandwidth, then the signal is attenuated and S/N is decreased when
compared to the matched case because S is smaller. It doesn't matter
whether we are discussing 5 MHz bw spread-spectrum or 5 kHz bw SSB.
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