Hi Sean,
Don't really know much about this but I played around with the FSM code
a little. Outputs would read the same as the state transitions:
if in state 0 and a 0 is received, output A, else output B
Using the fsms generation routines, I got a different shape FSM than
yours (higher output cardinality). For instance (and I might be using
this routine totally wrong),
t=trellis.fsm(3, 2, 1)
Parameters:
P ???? h=K/P (relatively prime)
M alphabet size
L pulse duration
generates:
2 3 6
0 1
1 2
2 0
0 3
1 4
2 5
Since the FSM is used internally by the decoder, it doesn't have output
states that map to output data bits. If L=2, then it's:
2 6 12
0 3
1 4
2 5
1 4
2 5
0 3
0 6
1 7
2 8
3 9
4 10
5 11
Judging by the FSM examples, definition of useful FSMs is black magic.
- Jeff
On 09/09/2015 09:17 PM, Nowlan, Sean wrote:
I’m trying to demodulate a CPFSK signal using the Viterbi algorithm. The
gr-trellis module seems to have most of the components I’ll need, but
I’m having trouble figuring out how to wrangle my problem into the FSM
format. Basically, I have a binary CPFSK signal with a modulation index,
h=25/3. Following the guidance of [1] section 3.3-2, I know that for CPM
with rational h=m/p, (here, m=25 and p=3), with m odd, I have 2p=6 phase
states. I worked out the phase trajectories and took them modulo 2*pi to
get this set of terminal phase states: {0, pi/3, 2*pi/3, pi, 4*pi/3,
5*pi/3}.
I’m trying to understand the FSM format in gr-trellis, and how to build
my FSM. If you consider the set of terminal phase states above as being
zero-indexed, the phase transitions should go according to the “Next
state” mapping of the FSM file format below. I’ve added my comments as
to my understanding of the format. Please correct me if I’m wrong.
FSM file:
2 6 2 # 2 possible input symbols // 6 possible FSM states // 2
possible output symbols
# Next state mapping
5 1 # if in state 0 and a 0 is received, go to state 5, else (1
is received, and) go to state 1
0 2 # if in state 1 and a 0 is received, go to state 0, else go
to state 2
1 3 # ...
2 4
3 5
4 0
# Output symbol mapping
[…]
I’m having trouble understanding how to define the output symbol
mapping. Is there a plain-language interpretation like the one I spelled
out for Next state mapping in my comments above?
Thanks!
Sean
[1] Proakis & Salehi, Digital Communications, 5ed.
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