Discuss-gnuradio mailing list
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On Tue, Dec 2, 2014 at 11:03 AM, Yi Chu [email protected] wrote:
Hello everyone,
I am working on the GNURadio simulation of a 3-hop system using
non-differential QPSK: source generates QPSK-modulated signal and sends
it to relay, relay performs certain operation on the received complex
samples and sends them to the receiver, and the receiver decodes the data.
The flow graph is as below and I attached the .grc file.
same), then the signal can be correctly decoded…
Does anyone have any ideas on this problem? Any advice are appreciated!
Many thanks and best regards
Yi
The ISI is from the transmit pulse shaping filter, a root raised cosine.
The RRC filter introduces ISI, which is what you see there. You need to
pass it through a second RRC filter to remove the ISI.
The problem with non-differential coding is that QPSK has a 4-way phase
ambiguity and nothing in your receiver path is able to line up the
received
symbols in phase to how they were transmitted. It simply locks the
points
to the constellation without regard for which symbol is where. You might
transmit a QPSK with ‘11’ in the first quadrant, but when you receive
it,
where it locks might have that ‘11’ in any one of the four quadrants of
the
complex plane. Differential encoding doesn’t care about that absolute
placement in the plane, just the change in phase, which is invariant
with
the rotation.
Tom