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] why 4, and fsk, psk

[David Rowe]

If implemented correctly, 4FSK gets you 3dB over 2FSK on an AWGN 
channel. The FSKs can use class C PAs.


For DRM/C4FSK they _are_ bandwidth constrained due to the high bit rate 
of AMBE/standards (which is black box with a bit rate so they have no 
choice over).  I presume this is the reason for poorly spaced 4FSK tones 
and a performance hit of 6-ish dB.  Or maybe low SNR performance doesn't 
matter in the use-cases for those standards.


Why is 2FSK more robust than 4FSK (both non-coherent demods) on a 
UHF/VHF mobile channel?


- David


On 06/10/17 10:57, glen english wrote:

Considering we are not bandwidth constrained, why is everyone so
enamoured with 4 level waveforms like 4FSK , When two level/ two state
waveforms like BPSK and 2FSK are far more robust in a mobile environment


Differential BPSK is an easy demod , 2FSK is also an easy demod.

BPSK having the slight edge due to being a antipodal waveform compared
to being an orthogonal waveform.

(though the full 3dB gain (BER = 1e-04) of an antipodal waveform only
occurs with coherent demod) .

(gain at very low SNR closer to 2dB)

complexity of a DBPSK incoherent demod and incoherent 2FSK demod are similar

DBPSK leaves open the option of a highly productive coherent demod, and
2FSK leaves open the option of highly simplified demodulators (slicing
FM demod).


-glen Vk1xx



On 6/10/2017 11:02 AM, 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] why 4, and fsk, psk

[Steve]

There was some neat AM designs a couple of years ago, but I think they
all petered out last year. Narrow-band FM still remains king on VHF
and up.

I play around a little bit with an used FT-857d, but that's a Cadillac
radio, and I use it mostly on HF.

I think that's the problem with the current SSB radios, is that they
throw in the kitchen sink. I think there would be a market for a
single-band SSB rig designed for the whole 2 metre band. No FM
capability.

25 Watts, highly stable, with PSK, and QAM modes and narrow/wideband
codecs. A huge heat sink with attachment to wrap around your
automobiles freon accumulator/dryer (that really cold wet thing under
the hood) :-)

Oops, I'm off on a tangent again...

73/Steve

On Thu, Oct 5, 2017 at 7:27 PM, glen english  wrote:
> Considering we are not bandwidth constrained,

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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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>> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
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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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