Hi,
If I want to transmit a signal which is DSSS spread using BPSK. Is it
correct for me to first digitally spread the incoming data bits and then
apply BPSK before sending it to the USRP or should I first convert the
signal to BPSK and then multiply the output of the BPSK modulator with
the
PN sequence although I don’t know how to do it.
- If ‘1’ is the data bit then convert it to ‘1010100111’ (lets say this
is
the PN sequence) and then input it into a BPSK modulator.
- If ‘1’ is the data bit first input it into a BPSK modulator and then
mutiply the interpolated output with the PN sequence.
Theoretically, these two methods will lead to the same result but I am
not
sure how to do this in gnuradio.
Any ideas?
Thanks
John
this would be similar to 802.11b’s PHY at 1Mbps, except it uses the
barker
sequence as opposed to the sequence you mention. It spreads a BPSK
signal
using the barker sequence.
You can take a look at the BBN 802.11b code in CGRAN to see how they
implement this:
https://www.cgran.org/browser/projects/bbn_80211/branches/usrp2_version/gr-bbn/src/examples/bbn_80211b_pkt.py#L126
It modulates to BPSK, and then spreads it out via the interpolation
filter.
On Tue, Apr 27, 2010 at 21:19, John A. [email protected] wrote:
If I want to transmit a signal which is DSSS spread using BPSK. Is it
correct for me to first digitally spread the incoming data bits and then
apply BPSK before sending it to the USRP or should I first convert the
signal to BPSK and then multiply the output of the BPSK modulator with the
PN sequence although I don’t know how to do it.
Technically, these are equivalent. However, it is easier and uses
less CPU to XOR your data sequence with your PN code, then send it to
the modulator.
Johnathan
On Wed, 2010-04-28 at 11:23 -0700, Johnathan C. wrote:
the modulator.
I think there is a difference, if you use a root raised cosine filter,
set for your datarate and roll-off factor.
I think the correct version should then be:
BPSKmod -> RRCfilter -> DSSS
Martin
Ok.
The block diagram in most of the textbook says the modulated signal is
multiplied with the PN sequence.
- If I decide to do the same then my setup will look like this
byte -> bytes_to_chunks -> chunks_to_symbols(BPSK) -> rrc_filter(X
samples per bit) -> DSSS
I am confused about the DSSS part. I am using a 63-bit PN sequence and
spreading the BPSK symbols with this sequence means I have to pass each
of
the BPSK symbol through an interpolation filter with PN sequence as the
taps(like in the barker example). Am I right? If this is not how it is
done
then can someone please suggest me a way.
Thanks,
John