I have been working on a DSSS system for some time now.
A few comments:
this project grew out of the DARPA spectrum challenge:
our team eventually dropped out of the race because of other time commitments but
I decided to finish up the design and make it publicly available
since DARPA was generous enough to send us USRPs for testing etc.
Now about the gr-cdma project:
The idea is to build a fully parametrized DS system.
Users can set spreading , modulation, framing parameters etc.
The structure is as follows:
There is a frame structure and in each frame we superimpose 2 orthogonal
channels: 1) training/synchronization and 2) data.
At the receiver, there is a time/frequency searching engine
(consisting of a user defined number of correlators at different freqiuencies)
that looks at the sync channel and can move nicely between
Acquisition mode and Tracking mode
This part is really cool, since at Ack we spend
a lot of computational resources (N chip based matched filters over the entire
frame, each of length chips_per_frame),
while at tracking very few (1 despreading MF of length chips_per_symbol).
Once ack/tracking is done the rest of the Rx is trivial:
The frames are handed to the rest of the Rx which correlates with the data pn-sequence, despreads, phase-tracks and demodulates data.
That said the project is not yet ready for prime time, but I took the opportunity to
announce it due to this request on the list.
The basic idea of the automatic Acquisition/Tracking is ready and tested and
can be found in the application
apps/timing_test.grc
as well as the individual apps (in case you want to try to to different machines)
apps/timing_test_tx.grc and apps/timing_test_rx.grc
The cool thing about those is that they are build exclusively on grc using
"probe" and "function probe" blocks to detect SNR level and switch between
acquisition and tracking (there is an option to do it manually or auto).
I anticipate that in a few weeks the system can be fully functional...but cannot promise that..I work on it on my spare time...
best,
Achilleas