Example 5.8
A radio receiver is limited in its performance by
thermal noise generated in the receiver front-end. The receiver is capable of decoding
both coherent binary ASK and binary PSK signals. If the average power rating of the
transmitter is constant when it is sending both ASK and PSK data, which modulation format
will be able to send data at the fastest rate for a given bit error probability in the
receiver, and how much faster can it be?
Solution
The BER equations for coherent ASK and coherent PSK
are as follows:
Pe ASK = 0.5erfc[Eb/2N0]1/2
Pe PSK = 0.5erfc[Eb/N0]1/2
It can be seen that the PSK system can
tolerate a noise power density that is twice that for ASK or an Eb value that
is half that for ASK.
In this example, the noise power density at the
input to the receiver is fixed for both ASK and PSK, and therefore it is possible to halve
the Eb value for PSK and still match the ASK performance, allowing the
PSK system to send data at twice the rate for the same average transmitter power.