[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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