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Hi,
What exactly does it mean to "look at change in
phase over 1 SYMBOL"? Is it just measuring the phase difference of two
samples (separated by the number of samples in one symbol) and then subtracting
the phase rotation of the center frequency across one symbol? Or does your
algorithm look at all the samples in-between too?
Can you please give a brief
explanation?
Thanks,
Hans
----- Original Message -----
Sent: Monday, October 15, 2007 1:28
PM
Subject: [Discuss-gnuradio] Improvements
made to GMSK demod
All-
I've made some improvements to the built-in GMSK
demodulator, based mostly on Simon & Wang's "Differential Detection of
Gaussian MSK in a Mobile Radio Environment". Also see "Differential Detection
of GMSK Using Decision Feedback", Yongacoglu, A. Makrakis,
D. Feher, K.
The intermediate steps I
took were as follows:
-Place a low-pass filter in front of the clock
recovery block. Optimal bandwidth is a function of both Eb/N0 and xmitter BT
product.
-Remove the quadrature demod (which was looking at change in phase
over 1 sample). Replace with blocks to look at change in phase over 1 SYMBOL.
(this is a true 1 bit differential detector)
-Instead of 1 bit
differential detector, look at change in phase over 2 symbols (2 bit
differential detector). (requires diff encode on xmit side). This produces an
asymmetric eye diagram, so you have to bias it a little bit prior to clock
recovery.
Initial Bit Error Rate results (just to get an idea of
relative performance):
(xmit BT=0.25, 8 samp/sym, same input to all demods,
at a certain Eb/N0)
Original, unchanged GMSK
demod:
BER=0.38
Original + pre CR filter:
BER=0.0090
True
1-bit diff detect + pre CR filter:
BER=0.0056
True 2-bit diff detect
+ pre CR filter:
BER=0.00076
Is this something that people would
like to see checked in? If so, how's the best way to do so...via a developer
branch?
Tom Tsou- thanks for pointing the way to some of this
stuff.
-Steven
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