I did some experiments of Ping, between two USRP N210 nodes, over the
OFDM
link. I need to know if any other has the same performance as I did.
This OFDM link is using two frequencies, 900MHz and 1800MHz, as the SBX
daughter board is used. The antenna is VERT900 which is working in 824
to
960 MHz, 1710 to 1990 MHz Quad-band .
Two computers of intel i7 (4 cores @3.4GHz) are used to run gnuradio.
My performance is that as below:
Static ARP entries are used for both the computer. Tunnel devices are
setup receptively to run ping on it.
Bandwidth = 500K, occ=400/512, average RTT time is 14ms, and packet loss
of
ping is 17%
Bandwidth = 1M, occ = 400/512, average RTT time is 8ms, and packet loss
of
ping 33%
Bandwidth = 1.2M, occ = 400/512, average RTT time is 7ms, but packte
loss
of ping is degraded to >90%, means the PC can not support such kind
bandwidth.
If I use two intel core 2 PC, the supported bandwidth with acceptable
paccket loss rate (35%) is a little higher than 500K (RTT = 20ms). With
more bandwidth, the computer would be slow down significantly and the
link
quality is near crash.
So I need to know if any other friend have tested such experiment and
what
the corresponding performance is.
It seems that the GMSK modulation is doing well in supporting higher
datarate over GNURadio, than OFDM modulation.
Today I did test using the GMSK between two USRP N210 nodes, the maximum
data rate with acceptable packet loss rate of ping (2%) is 3Mbps. It can
support video transmission.
This result is much better than the existing OFDM implementation in
GNURadio. I beleive that OFDM signal processing is much more complex than
GMSK and cost much more resource than GMSK, but I did not expect that
GNURadio OFDM linkcan only support less than 200kbps data rate.
What’s the EVM of your OFDM signal coming out of the RF daughterboard?
Are you backing off enough to allow for OFDM’s high PAPR to remain in
the linear region?
GMSK has the nice advantage of being constant envelope, so compression
doesn’t matter. OFDM, on the other hand, doesn’t get that luxury and,
in fact, requires a significant backoff.
Have you tried running the code without a USRP and just simulating
AWGN and possibly a slight frequency offset to see if the OFDM code is
adequately robust? What are your subcarrier modulations? Are you
running with any FEC?
It seems that the GMSK modulation is doing well in supporting higher
datarate over GNURadio, than OFDM modulation.
Today I did test using the GMSK between two USRP N210 nodes, the maximum
data rate with acceptable packet loss rate of ping (2%) is 3Mbps. It can
support video transmission.
This result is much better than the existing OFDM implementation in
GNURadio. I beleive that OFDM signal processing is much more complex
than
GMSK and cost much more resource than GMSK, but I did not expect that
GNURadio OFDM link can only support less than 200kbps data rate.
Thanks, I understand the backoff now. And I searched the archive, Tom
did
mentioned that the --tx-amplitude can not be set too big due to the
large
PAPR of OFMD.
I tried to set the tx-amplitude as 0.2 ～ 0.3, and the observed supported
bandwidth is still about 1~1.2MHz on my i7 4core computer.
The ping packet loss is 20% ~ 40%.
Actually, I just want to know any performance on such OFDM link has been
reported or not, as my benchmark.
Alex,
I’m not entirely sure you correctly understood Brian’s point…he’s
referring to “backing off” the Tx gain to ensure that the RF power amp
remains entirely linear throughout the power envelope of the OFDM
signal. The PAPR is directly proportional to the number of carriers
employed and is one of the problematic aspects of OFDM. Most USRP
daughter cards (or any radios) tend to loose linearity as they approach
the top end of there gain range. The GMSK signal however being FSK based
has a constellation that remains on the unit circle and hence has
constant amplitude allowing it to operate well in the non-linear region
of an RF amplifier (hence it’s popularity in low cost, high efficiency
single chip radios). It’s easy therefore to see why empirically it might
appear that GMSK is “out performing” OFDM here, given that this is a
simple PHY demonstration, rather than an application providing a robust
link with appropriate channel coding. Tom has addressed similar
questions as yours in the past here, I suggest you search the archive, I
think you’ll find more useful information.
-Ian
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