On Mon, Oct 22, 2012 at 02:16:19AM -0400, johncr...@aol.com wrote:
Dale -
To your question re BPSK and DPSK. In both modes the phase shift is 180
degrees.
Straight PSK has the issue of determining the 1's from the 0's, at the
receiver as there is
no phase reference.
To avoid this DPSK encodes the the serial data stream prior to the
bi-phase modulator.
As I recall (at 1 AM) the method is like this. If the present bit to be
sent is a 1 the phase
of the carrier is inverted. If it is a zero the phase is not inverted.
This is easily sorted out
in the receiver using a flip flop and an XOR.
I might add one note. Non differential PSK has a slight BER
advantage with very weak signals as differential PSK decoding causes TWO
bits to be in error in the recovered data if the phase state of a bit is
incorrectly determined by the receiver and the next and previous bits
were correctly determined.
For this reason most satellite nPSK modulations use absolute
encoding and determine phase in initial lockon by looking for a phase
which causes the inner FEC to work (eg produce valid corrected data).
There have been demodulators for differential nPSK that work
by correlating the last bit with the current bit using some kind of
delay line. Don't typically work as well with weak signals though.
--
Dave Emery N1PRE/AE, d...@dieconsulting.com DIE Consulting, Weston, Mass
02493
An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in
celebration of what could have been, but wasn't and is not to be now either.
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