Re: [Freetel-codec2] 2400A VHF modem performance
(cc-ed to list) Sure, SM1000 purchase details here: http://www.rowetel.com/?page_id=3902 The LDPC codes have much better performance (coding gain) than Hamming codes. I've found Codec 2 (especially the lower rates like 700) fall over at a few % BER. Cheers, David On 08/10/17 23:58, Adrian Musceac wrote: Hi David, I just tried 700B today on VHF (2 km path) and I have to say I'm very impressed. I got a completely readable and clear signal at only 3 dB SNR, that's in the context where SSB required 2 dB SNR for a very noisy copy. This without any channel coding. Do you have any idea if I can still buy the SM1000 and from where, as I'd like to test my SSB transceiver with the 700C mode against the full SDR mode I use currently. Regarding channel coding, I was wondering if you considered that LDPC and similar codes have very abrupt slopes at high BER numbers, compared to for example Hamming. I was actually able to understand voice at 25% BER, but at these numbers there's not much channel coding can do. I'm very interested in your next steps :) Thanks, Adrian On 10/6/17, David Rowewrote: Hi Adrian, There is Octave and C code for the mFSK modem we have developed in codec2-dev, that we have tested against the ideal FSK performance curves. Not sure I'd call it matched filter, but take a look at the code. It's important to check your modem implementation against theory at a couple of points on the BER versus Eb/No curves. Very easy for bugs to creep in. I'm currently working on improved quality Codec 2 at 700 bit/s, but it's a slow process with no release date in mind. - David On 06/10/17 10:32, Adrian Musceac wrote: Hi David, Thanks very much for the tips! Would you suggest that the matched filter approach is better for 4FSK as well? I am using it for 2FSK and it works well, it's just that being lazy I wanted to avoid too much complexity in the code for the 4FSK variant. Regarding PSK: I have several things to try and real world tests will show which one is more practical. Right now for PSK I am using Codec2 at 1400 bits as what I find a good compromise between quality and bitrate. This gives me just enough space for synchronization bits and other protocol data (which may span on multiple frames). I am considering moving down to 700 bits per second and I wanted to ask you if you think you will be making major changes to it's quality in the near future. This would give me 3 additional dB to play with, but at this point I don't think we can afford to have more than 1% errors per frame, as each bit carries a lot of information. I tried rate 1/2 convolutional encoding with real world tests and it seems to give an additional 2 dB of space. The advantage is that frame sizes are short, so we don't have large gaps when errors occur. On top of that, Viterbi soft symbol decoding and trellis to 8PSK add to the computational cost, which I have a low budget for. Best regards, Adrian On 10/6/17, David Rowe wrote: Hi Adrian, It's very important to avoid using an analog FM demodulator with FSK - it's the reason C4FM/DMR are such a poor performers: http://www.rowetel.com/?p=3799 http://www.rowetel.com/?p=4279 At 1% BER, Eb/Nos reqd are roughly: 2FSK 9dB 4FSK 6dB PSK 4dB The PSK results are for coherent demodulation, which is hard to do without overhead (e.g. pilot symbols or a unique word). I suspect non-coherent PSK is worse than FSK, so not worth doing unless you are really concerned about bandwidth. The FSK results are for non-coherent demodulators which are really simple to implement and get real-world results right on ideal. Convolutional codes are a bit old hat - we're getting gd results on HF with short-ish LDPC codes. But best to sort out your uncoded demodulator performance first. Cheers, David On 05/10/17 20:05, Adrian Musceac wrote: Hi David, Thanks for the answer! I have just simulated a 2FSK modem on AWGN channel, but this time without using FM demodulation. It performs just like you said, ~2 dB worse than QPSK (at 5% frames dropped). This means that the FM demodulator I used for 2400A must be introducing some symbol errors. What I can't figure out is the 10 dB difference to analog FM. My experimental results (test in urban environment, with distances between 500 meters and 1 km between sender and receiver) show ~6 dB between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4 dB between 4FSK and FM. Could the non-coherent demodulation explain this? I know I can obtain up to 6 dB SNR improvement by going to Codec2 700 bits/sec and using Viterbi soft symbol decoding, but I'd like to get the optimal results before. Would convolutional encoding in 2400A be worth considering? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot
Re: [Freetel-codec2] 2400A VHF modem performance
Hi David, Thanks for pointing me at the Octave code. I'll have to do some reading as I'm pretty sure my implementation is buggy. Regarding, the 700 bit/s mode, I do hope there will be no major change in the library version, as we have to use the distribution packaging, on stable. Thanks, Adrian On 10/6/17, David Rowewrote: > Hi Adrian, > > There is Octave and C code for the mFSK modem we have developed in > codec2-dev, that we have tested against the ideal FSK performance > curves. Not sure I'd call it matched filter, but take a look at the code. > > It's important to check your modem implementation against theory at a > couple of points on the BER versus Eb/No curves. Very easy for bugs to > creep in. > > I'm currently working on improved quality Codec 2 at 700 bit/s, but it's > a slow process with no release date in mind. > > - David > > > On 06/10/17 10:32, Adrian Musceac wrote: >> Hi David, >> >> Thanks very much for the tips! Would you suggest that the matched >> filter approach is better for 4FSK as well? I am using it for 2FSK and >> it works well, it's just that being lazy I wanted to avoid too much >> complexity in the code for the 4FSK variant. >> >> Regarding PSK: I have several things to try and real world tests will >> show which one is more practical. Right now for PSK I am using Codec2 >> at 1400 bits as what I find a good compromise between quality and >> bitrate. This gives me just enough space for synchronization bits and >> other protocol data (which may span on multiple frames). I am >> considering moving down to 700 bits per second and I wanted to ask you >> if you think you will be making major changes to it's quality in the >> near future. This would give me 3 additional dB to play with, but at >> this point I don't think we can afford to have more than 1% errors per >> frame, as each bit carries a lot of information. >> >> I tried rate 1/2 convolutional encoding with real world tests and it >> seems to give an additional 2 dB of space. The advantage is that frame >> sizes are short, so we don't have large gaps when errors occur. On top >> of that, Viterbi soft symbol decoding and trellis to 8PSK add to the >> computational cost, which I have a low budget for. >> >> Best regards, >> Adrian >> >> On 10/6/17, David Rowe wrote: >>> Hi Adrian, >>> >>> It's very important to avoid using an analog FM demodulator with FSK - >>> it's the reason C4FM/DMR are such a poor performers: >>> >>> http://www.rowetel.com/?p=3799 >>> http://www.rowetel.com/?p=4279 >>> >>> At 1% BER, Eb/Nos reqd are roughly: >>> >>> 2FSK 9dB >>> 4FSK 6dB >>> PSK 4dB >>> >>> The PSK results are for coherent demodulation, which is hard to do >>> without overhead (e.g. pilot symbols or a unique word). I suspect >>> non-coherent PSK is worse than FSK, so not worth doing unless you are >>> really concerned about bandwidth. >>> >>> The FSK results are for non-coherent demodulators which are really >>> simple to implement and get real-world results right on ideal. >>> >>> Convolutional codes are a bit old hat - we're getting gd results on HF >>> with short-ish LDPC codes. >>> >>> But best to sort out your uncoded demodulator performance first. >>> >>> Cheers, >>> >>> David >>> >>> On 05/10/17 20:05, Adrian Musceac wrote: Hi David, Thanks for the answer! I have just simulated a 2FSK modem on AWGN channel, but this time without using FM demodulation. It performs just like you said, ~2 dB worse than QPSK (at 5% frames dropped). This means that the FM demodulator I used for 2400A must be introducing some symbol errors. What I can't figure out is the 10 dB difference to analog FM. My experimental results (test in urban environment, with distances between 500 meters and 1 km between sender and receiver) show ~6 dB between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4 dB between 4FSK and FM. Could the non-coherent demodulation explain this? I know I can obtain up to 6 dB SNR improvement by going to Codec2 700 bits/sec and using Viterbi soft symbol decoding, but I'd like to get the optimal results before. Would convolutional encoding in 2400A be worth considering? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >>> >>> -- >>> Check out the vibrant tech community on one of the world's most >>> engaging tech sites, Slashdot.org! http://sdm.link/slashdot >>>
Re: [Freetel-codec2] 2400A VHF modem performance
Hi Adrian, There is Octave and C code for the mFSK modem we have developed in codec2-dev, that we have tested against the ideal FSK performance curves. Not sure I'd call it matched filter, but take a look at the code. It's important to check your modem implementation against theory at a couple of points on the BER versus Eb/No curves. Very easy for bugs to creep in. I'm currently working on improved quality Codec 2 at 700 bit/s, but it's a slow process with no release date in mind. - David On 06/10/17 10:32, Adrian Musceac wrote: Hi David, Thanks very much for the tips! Would you suggest that the matched filter approach is better for 4FSK as well? I am using it for 2FSK and it works well, it's just that being lazy I wanted to avoid too much complexity in the code for the 4FSK variant. Regarding PSK: I have several things to try and real world tests will show which one is more practical. Right now for PSK I am using Codec2 at 1400 bits as what I find a good compromise between quality and bitrate. This gives me just enough space for synchronization bits and other protocol data (which may span on multiple frames). I am considering moving down to 700 bits per second and I wanted to ask you if you think you will be making major changes to it's quality in the near future. This would give me 3 additional dB to play with, but at this point I don't think we can afford to have more than 1% errors per frame, as each bit carries a lot of information. I tried rate 1/2 convolutional encoding with real world tests and it seems to give an additional 2 dB of space. The advantage is that frame sizes are short, so we don't have large gaps when errors occur. On top of that, Viterbi soft symbol decoding and trellis to 8PSK add to the computational cost, which I have a low budget for. Best regards, Adrian On 10/6/17, David Rowewrote: Hi Adrian, It's very important to avoid using an analog FM demodulator with FSK - it's the reason C4FM/DMR are such a poor performers: http://www.rowetel.com/?p=3799 http://www.rowetel.com/?p=4279 At 1% BER, Eb/Nos reqd are roughly: 2FSK 9dB 4FSK 6dB PSK 4dB The PSK results are for coherent demodulation, which is hard to do without overhead (e.g. pilot symbols or a unique word). I suspect non-coherent PSK is worse than FSK, so not worth doing unless you are really concerned about bandwidth. The FSK results are for non-coherent demodulators which are really simple to implement and get real-world results right on ideal. Convolutional codes are a bit old hat - we're getting gd results on HF with short-ish LDPC codes. But best to sort out your uncoded demodulator performance first. Cheers, David On 05/10/17 20:05, Adrian Musceac wrote: Hi David, Thanks for the answer! I have just simulated a 2FSK modem on AWGN channel, but this time without using FM demodulation. It performs just like you said, ~2 dB worse than QPSK (at 5% frames dropped). This means that the FM demodulator I used for 2400A must be introducing some symbol errors. What I can't figure out is the 10 dB difference to analog FM. My experimental results (test in urban environment, with distances between 500 meters and 1 km between sender and receiver) show ~6 dB between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4 dB between 4FSK and FM. Could the non-coherent demodulation explain this? I know I can obtain up to 6 dB SNR improvement by going to Codec2 700 bits/sec and using Viterbi soft symbol decoding, but I'd like to get the optimal results before. Would convolutional encoding in 2400A be worth considering? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2 -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2 -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2 -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot
Re: [Freetel-codec2] 2400A VHF modem performance
Hi David, Thanks very much for the tips! Would you suggest that the matched filter approach is better for 4FSK as well? I am using it for 2FSK and it works well, it's just that being lazy I wanted to avoid too much complexity in the code for the 4FSK variant. Regarding PSK: I have several things to try and real world tests will show which one is more practical. Right now for PSK I am using Codec2 at 1400 bits as what I find a good compromise between quality and bitrate. This gives me just enough space for synchronization bits and other protocol data (which may span on multiple frames). I am considering moving down to 700 bits per second and I wanted to ask you if you think you will be making major changes to it's quality in the near future. This would give me 3 additional dB to play with, but at this point I don't think we can afford to have more than 1% errors per frame, as each bit carries a lot of information. I tried rate 1/2 convolutional encoding with real world tests and it seems to give an additional 2 dB of space. The advantage is that frame sizes are short, so we don't have large gaps when errors occur. On top of that, Viterbi soft symbol decoding and trellis to 8PSK add to the computational cost, which I have a low budget for. Best regards, Adrian On 10/6/17, David Rowewrote: > Hi Adrian, > > It's very important to avoid using an analog FM demodulator with FSK - > it's the reason C4FM/DMR are such a poor performers: > >http://www.rowetel.com/?p=3799 >http://www.rowetel.com/?p=4279 > > At 1% BER, Eb/Nos reqd are roughly: > >2FSK 9dB >4FSK 6dB >PSK 4dB > > The PSK results are for coherent demodulation, which is hard to do > without overhead (e.g. pilot symbols or a unique word). I suspect > non-coherent PSK is worse than FSK, so not worth doing unless you are > really concerned about bandwidth. > > The FSK results are for non-coherent demodulators which are really > simple to implement and get real-world results right on ideal. > > Convolutional codes are a bit old hat - we're getting gd results on HF > with short-ish LDPC codes. > > But best to sort out your uncoded demodulator performance first. > > Cheers, > > David > > On 05/10/17 20:05, Adrian Musceac wrote: >> Hi David, >> >> Thanks for the answer! I have just simulated a 2FSK modem on AWGN >> channel, but this time without using FM demodulation. It performs just >> like you said, ~2 dB worse than QPSK (at 5% frames dropped). This >> means that the FM demodulator I used for 2400A must be introducing >> some symbol errors. >> >> What I can't figure out is the 10 dB difference to analog FM. My >> experimental results (test in urban environment, with distances >> between 500 meters and 1 km between sender and receiver) show ~6 dB >> between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4 >> dB between 4FSK and FM. Could the non-coherent demodulation explain >> this? >> I know I can obtain up to 6 dB SNR improvement by going to Codec2 700 >> bits/sec and using Viterbi soft symbol decoding, but I'd like to get >> the optimal results before. Would convolutional encoding in 2400A be >> worth considering? >> >> Thanks, >> Adrian >> >> -- >> Check out the vibrant tech community on one of the world's most >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot >> ___ >> Freetel-codec2 mailing list >> Freetel-codec2@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > > -- > Check out the vibrant tech community on one of the world's most > engaging tech sites, Slashdot.org! http://sdm.link/slashdot > ___ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] 2400A VHF modem performance
On 10/5/17, Stevewrote: > You might also do your testing in a more remote spot. The use of ISM > in the city is probably pretty high, and interference would be an > issue. It might not break the squelch (spread spectrum, etc), but the > bits would be corrupted. > > I see a lot of users on that band, with just a junkbox antenna and > SDR, and I'm in a residential area. > > 73/Steve > Hi Steve, Actually, there was not much noise on those particular frequencies at that time. I know because I could monitor the signals on the spectrum scope in a 1 MHz bandwidth around my frequencies of interest (doing this on a cellphone with limited screen estate is quite challenging). I plan to repeat the test in a more organized fashion on 5 GHz and 2.4 GHz. I expect better performance and reliable 1 square km coverage with the following setup: 1 milliwatt transmitter into 16 dBi sector antennas and 6 dBi RX antennas (with and w/o diversity). I will process the data and hopefully make it available before the SDRA conference in July next year for interested parties. I recommend reading ETSI ETR 300-1 for details on the test methodology. Resistance to wideband interference from 802.11x and avoidance of other narrow band signals like 802.15 is one of the goals. SDR allows us to monitor several MHz of spectrum and choose the channel with the best SNR. One simple way to do this is a multichannel modem with OFDMA, where active carrier allocation is done based on noise sensing. Cheers, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] 2400A VHF modem performance
Hi David, Thanks for the answer! I have just simulated a 2FSK modem on AWGN channel, but this time without using FM demodulation. It performs just like you said, ~2 dB worse than QPSK (at 5% frames dropped). This means that the FM demodulator I used for 2400A must be introducing some symbol errors. What I can't figure out is the 10 dB difference to analog FM. My experimental results (test in urban environment, with distances between 500 meters and 1 km between sender and receiver) show ~6 dB between QPSK and analog FM (with 2.5 kHz deviation) and no more than 4 dB between 4FSK and FM. Could the non-coherent demodulation explain this? I know I can obtain up to 6 dB SNR improvement by going to Codec2 700 bits/sec and using Viterbi soft symbol decoding, but I'd like to get the optimal results before. Would convolutional encoding in 2400A be worth considering? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] 2400A VHF modem performance
You might also do your testing in a more remote spot. The use of ISM in the city is probably pretty high, and interference would be an issue. It might not break the squelch (spread spectrum, etc), but the bits would be corrupted. I see a lot of users on that band, with just a junkbox antenna and SDR, and I'm in a residential area. 73/Steve -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] 2400A VHF modem performance
Hi Adrian, This blog post compares a bunch of different modems at the 2% BER operating point: http://www.rowetel.com/?p=4663 Given all things being equal (e.g. sync overheads, bit rate etc) PSK is about 3dB better than 4FSK at the 2% BER point. In practice real world PSK modems tend to have a bit more overhead so 2dB would be more likely. It also depends on your operating point, see reference curves for PSK and 4FSK. Our work with FreeDV 2400A, e.g. http://www.rowetel.com/?p=5119 indicates FreeDV 2400A is about 10dB better than analog FM. I'd suggesting testing your various modems using software simulations of the channel first, rand try to reconcile that with theory, then real world/off-air results. Cheers, David On 05/10/17 00:42, Adrian Musceac wrote: Hi, I was able to perform some field measurements on Sunday to compare the performance of the 4FSK modem and the QPSK modem in the 433 MHz ISM band. My transmitter was set to 1 mW output power into a 2 dBi gain antenna located at approx. 21 meters above ground on one of the buildings in my city. I walked around a perimeter of about 600 meters in diameter, recording the RSSI of the data channel and the percentage of the data frames lost. For the QPSK modem, the performance was as expected: 5% lost frames at ~7 dB SNR. However the 4FSK modem as specified in 2400A performed about 3 dB worse, with the same percentage of dropped frames at ~11 dB SNR. For comparison, the analog FM signal was readable at 14 dB SNR. My question is: is this performance expected of the 2400A modem, or should I go back to the drawing board? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2 -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
[Freetel-codec2] 2400A VHF modem performance
Hi, I was able to perform some field measurements on Sunday to compare the performance of the 4FSK modem and the QPSK modem in the 433 MHz ISM band. My transmitter was set to 1 mW output power into a 2 dBi gain antenna located at approx. 21 meters above ground on one of the buildings in my city. I walked around a perimeter of about 600 meters in diameter, recording the RSSI of the data channel and the percentage of the data frames lost. For the QPSK modem, the performance was as expected: 5% lost frames at ~7 dB SNR. However the 4FSK modem as specified in 2400A performed about 3 dB worse, with the same percentage of dropped frames at ~11 dB SNR. For comparison, the analog FM signal was readable at 14 dB SNR. My question is: is this performance expected of the 2400A modem, or should I go back to the drawing board? Thanks, Adrian -- Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2