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[Discuss-gnuradio] please help - i am stuck - spreading data and bpsk

From: John Andrews
Subject: [Discuss-gnuradio] please help - i am stuck - spreading data and bpsk
Date: Sat, 11 Sep 2010 14:54:56 -0700

Can someone please tell me how to do this?

I want to generate DSSS with BPSK modulation in gnuradio but I have a few doubts that couldn't be cleared even after a lot of looking for an answer.

The DSSS transmitter block diagram from various books shows that i have to XOR the input data with the PN sequence to spread and then BPSK modulate. My doubt is

1. I did the same and then BPSK modulated using the gnuradio DBPSK modulation without the diff encoder block but I am having trouble in acquisition at the receiver. I am using the method described in the book  "Fundamentals of Global Positioning System Receivers" by Tsui. Maybe I am doing something fundamentally wrong at the transmitter.

2. Someone on this list suggested BPSKmod-> RRCFilter ->SPREADING. If samples per symbol were 1 then I can just multiply the symbol with the PN sequence but as it is always greater than 1 I cannot figure out how I can spread. Can someone please help me here?

3. Is it possible to remove the RRC filter completely and instead use (1+0j) and (-1+0j) symbols to modulate the carrier and still get good results at the receiver? If it is possible then life would be easier as I don't have to worry about the problem mentioned above in 2.

3. If anyone can share the transmitter block diagram of a GPS transmitter then I think it can be of great help as it has a DSSS modulation scheme. All i found on the internet was block diagrams but my concern is about pulse shaping and none of these diagrams show nothing about that.

I did put in an effort to find answers by myself for a long time but as I couldn't find an answer I am writing for a little guidance.


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