Re: [Freetel-codec2] 2400A VHF modem performance

[David Rowe]

(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 Rowe  wrote:

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

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Re: [Freetel-codec2] 2400A VHF modem performance

[Adrian Musceac]

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 Rowe  wrote:
> 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

 --
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>>> 

Re: [Freetel-codec2] 2400A VHF modem performance

[David Rowe]

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

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Re: [Freetel-codec2] 2400A VHF modem performance

[Adrian Musceac]

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
>>
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Re: [Freetel-codec2] 2400A VHF modem performance

[Adrian Musceac]

On 10/5/17, Steve  wrote:
> 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

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Re: [Freetel-codec2] 2400A VHF modem performance

[Adrian Musceac]

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

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Re: [Freetel-codec2] 2400A VHF modem performance

[Steve]

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

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Re: [Freetel-codec2] 2400A VHF modem performance

[David Rowe]

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

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[Freetel-codec2] 2400A VHF modem performance

[Adrian Musceac]

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

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