Return-Path: X-Spam-DCC: paranoid 1290; Body=2 Fuz1=2 Fuz2=2 X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on lipkowski.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=5.0 tests=BAYES_00,HTML_MESSAGE autolearn=unavailable version=3.1.3 Received: from post.thorcom.com (post.thorcom.com [195.171.43.25]) by paranoid.lipkowski.org (8.13.7/8.13.7) with ESMTP id t04FHLgG005797 for ; Sun, 4 Jan 2015 16:17:21 +0100 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1Y7mq4-000161-6K for rs_out_1@blacksheep.org; Sun, 04 Jan 2015 15:11:32 +0000 Received: from [195.171.43.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1Y7mq3-00015s-NE for rsgb_lf_group@blacksheep.org; Sun, 04 Jan 2015 15:11:31 +0000 Received: from omr-m09.mx.aol.com ([64.12.143.82]) by relay1.thorcom.net with esmtps (TLSv1:DHE-RSA-AES256-SHA:256) (Exim 4.84) (envelope-from ) id 1Y7mq0-0004TT-Ot for rsgb_lf_group@blacksheep.org; Sun, 04 Jan 2015 15:11:30 +0000 Received: from mtaout-mbe01.mx.aol.com (mtaout-mbe01.mx.aol.com [172.26.254.173]) by omr-m09.mx.aol.com (Outbound Mail Relay) with ESMTP id 467DE703E9BF7 for ; Sun, 4 Jan 2015 10:11:26 -0500 (EST) Received: from White (ipb21bee4a.dynamic.kabel-deutschland.de [178.27.238.74]) by mtaout-mbe01.mx.aol.com (MUA/Third Party Client Interface) with ESMTPA id 554E438000091 for ; Sun, 4 Jan 2015 10:11:23 -0500 (EST) Message-ID: <98BD4CA9E5394486A3D53B80EE042D81@White> From: "Markus Vester" To: References: <54A92E4E.2030903@abelian.org> Date: Sun, 4 Jan 2015 16:11:22 +0100 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal Importance: Normal X-Mailer: Microsoft Windows Live Mail 12.0.1606 X-MimeOLE: Produced By Microsoft MimeOLE V12.0.1606 x-aol-global-disposition: G DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mx.aol.com; s=20140625; t=1420384286; bh=mcxuDKkOX86vZDBgtaShf1D7f+LvZMczXoV0eZ4u+sI=; h=From:To:Subject:Message-ID:Date:MIME-Version:Content-Type; b=FPqHVcUqulen9UvNsgPLE0wcto6MhJDGlt/Z1mAdUPRv9HLiC7Ys50kbPln3FYgmf GVw2D/O6YDsbuBImwjRGN43FG+HZxa8jp0bNHyOS0wAMg4OcXOmEsc+07Y8FbSTjXz FiWMsywmMdaXLc31oKzpLo/eC2VKXd67epetMTvY= x-aol-sid: 3039ac1afead54a9581b1f47 X-AOL-IP: 178.27.238.74 X-Scan-Signature: 31e464b680e3427073f16da582bb11f1 Subject: Re: VLF: Transatlantic messages at 8822Hz Content-Type: multipart/alternative; boundary="----=_NextPart_000_004B_01D02839.14D143D0" X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-Scanned-By: MIMEDefang 2.56 on 10.1.3.10 Status: RO X-Status: X-Keywords: X-UID: 1787 Dies ist eine mehrteilige Nachricht im MIME-Format. ------=_NextPart_000_004B_01D02839.14D143D0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Big congratulations again to Paul and Dex!=20 What Paul described in unpretentious and matter-of-fact words should = really be regarded as a major achievement. It has been a well-deserved = fruit of several months of effort, and there were a number of = difficulties to be overcome. As I was lucky to be included in the = preceding email exchange, I had the chance to witness milestones and = setbacks during the process.=20 Most members of the LF group will appreciate the challenges to Dex on = the sub-9kHz transmit side, dealing with large coils and high voltage, = realizing accurate GPS-derived frequencies and sub-second bit timing, = and last not least being able to leave the signal on air reliably for = many hours.=20 The information processing side handled solely by Paul surely presented = an even higher hurdle: In the first place, he searched and found a small = handful of "good" FEC codes. The search involved extensive simulations = on powerful multicore computers hired from the Amazon cloud. Then the = soft Viterby decoding of a potential receive signal is also computer = intensive, especially for longer messages. There need to be numerous = trials while optimising reference phase evolution, bit timing, and = antenna weights. Perhaps the most challenging part was guessing appropriate parameters = before a transmission, ie. how many characters should be sent in which = amount of time. Although some experience had been gained from carrier = measurements during previous nights, ionospheric and atmospheric = variations make it hard to predict SNR accurately enough if you want to = exploit the channel capacity to the last couple of dBs. In the third = round on new year's night, Dex and Paul dared to take a bet, and won. = Allow me to cite from their final email exchange on Dec 31st: Dex: Want to test the limits? Paul:=20 Yes please.=20 Let's go for broke.=20 8 seconds is close to the limit, which is what I'd like to see.=20 The ultimate goal of this work has been to take decoding sensitivity = close to the theroretical limit. An universal metric for this is Eb/N0, = the ratio of the received signal energy per payload bit (Eb in Joules) = and the noise spectral power density (N0 in Watt/Hertz, equivalent to = "noise energy" in Joules). The Shannon limit for long messages spread to = infinite bandwidth is=20 Eb/N0 =3D ln(2) =3D -1.59 dB,=20 which (similar to the speed of light) cannot be surpassed by any = possible encoding scheme. Paul's and Dex' experiments showed that his = codes can come within about a dB of this limit in a real long-distance = propagation experiment.=20 To put that into perspective, let's derive Eb/N0 figures for two popular = digital modes: WSPR-15 transmits 50 information bits in 15 minutes, ie one bit in 18 = seconds. The decoding threshold is -38 dB in 2.5 kHz, or -4 dBHz. This = gives=20 Eb/N0 =3D 10 log(18) - 4 dB =3D +8.5 dB, ie. about 10 dB above the Shannon limit. Note that although different = speed variants (eg WSPR-2) need different power, the minimum energy per = bit has to remain the same. Opera-32 carries 28 information bits in 32.6 minutes, ie. one bit in 70 = seconds. The threshold is about -39.5 dB in 2.5 kHz, (-5.5 dBHz), = referenced to the average power of the 50% dutycycle on-off keying. This = gives Eb/N0 =3D 10 log(70) - 5.5 dB =3D +13 dB or about 14.5 dB above Shannon. Note however that for LF / VLF = transmissions, the limit will often be antenna voltage and peak power = rather than average power, which can result in a further 3 dB = disadvantage for Opera against frequency- or phase-modulated techniques. = =20 The opds correlation decoder can go about 9 dB lower than Opera. But of = course it can only find the best match from an a-priori defined list of = callsigns, and doesn't attempt to decode any message. However we must recognize that the amateur modes spend a significant = part of their energy to provide a reference for synchronisation, so not = all of the Eb/N0 difference is due to less efficient encoding. The = "nude" FEC-PSK mode doesn't contain any such overhead. So it can only = work when the link has a stable phase (like on VLF), and the decoder has = been given accurate information on carrier frequency and symbol timing. All the best, Markus (DF6NM) =20 From: Paul Nicholson=20 Sent: Sunday, January 04, 2015 1:13 PM To: rsgb_lf_group@blacksheep.org=20 Subject: VLF: Transatlantic messages at 8822Hz W4DEX achieved another 'first' recently by sending a series of messages across the Atlantic at 8822 Hz which were successfully copied at Todmorden UK, range 6194km. Transmissions used coherent BPSK signalling with ERP of around 150uW. The modulation encoded the messages using a rate 1/16 terminated convolutional code with constraint length 25, cascaded with an outer error detection code. The first message was received at 2014-12-30 03:00, a 4 character message 'EM95'. Eb/N0 was -0.8dB using 9 second symbols. A second test the following night managed 12 characters 'PAUL HNY DEX' using 14 second symbols giving Eb/N0 of +1.0dB. Conditions were good and we could have used shorter symbols and a longer message. The third test and best result so far was a 25 character message '8822HZ 2015 JAN 1 TA TEST' sent from 2015-01-01 00:00 using 8 second symbols. This was received with Eb/N0 =3D -0.1dB. In the 0.125 Hz bandwidth of a code symbol, the S/N was -13.2dB. That corresponds to -56dB S/N in a 2.5kHz audio bandwidth after sferic blanking. Before the blanker the S/N would be around -76dB. The source encoding uses 6 bits per character to produce a payload of 150 bits. An outer code adds a 16 bit CRC and the convolutional encoder expands the message to 3040 signal bits. The effective code rate is therefore 150/3040 =3D 1/20.27. Of the 3040 signal bits, 1153 were demodulated incorrectly but the FEC was able to fix them all to reveal the message. Received signal was around 0.12 fT and it was necessary to combine H-field and E-field receiver outputs to obtain a sufficient S/N to decode. The decoder is a soft Viterbi list decoder. The signals are too weak to reveal a reference phase by the usual method of summing the squared complex symbol amplitudes. Instead the decoder has to do a brute force trial and error search. The information rate in the 3rd test was 24.6 bits per hour which is 80% of the channel capacity. -- Paul Nicholson http://abelian.org/ -- ------=_NextPart_000_004B_01D02839.14D143D0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Big congratulations again to Paul and = Dex!=20
 
What Paul described in unpretentious = and=20 matter-of-fact words should really be regarded as a major achievement. = It has=20 been a well-deserved fruit of several months of effort, and there were a = number=20 of difficulties to be overcome. As I was lucky to be included in the = preceding=20 email exchange, I had the chance to witness milestones and setbacks = during the=20 process.
 
Most members of the LF group will = appreciate the=20 challenges to Dex on the sub-9kHz transmit side, dealing with large = coils and=20 high voltage, realizing accurate GPS-derived frequencies and sub-second = bit=20 timing, and last not least being able to leave the signal on air = reliably for=20 many hours.
 
The information processing side handled = solely by=20 Paul surely presented an even higher hurdle: In the first place, he = searched and=20 found a small handful of "good" FEC codes. The search involved extensive = simulations on powerful multicore computers hired from the Amazon cloud. = Then=20 the soft Viterby decoding of a potential receive signal is also computer = intensive, especially for longer messages. There need to be numerous = trials=20 while optimising reference phase evolution, bit timing, and antenna=20 weights.
 
Perhaps the most challenging part was = guessing=20 appropriate parameters before a transmission, ie. how many characters = should be=20 sent in which amount of time. Although some experience had been gained = from=20 carrier measurements during previous nights, ionospheric and atmospheric = variations make it hard to predict SNR accurately enough if you want to = exploit=20 the channel capacity to the last couple of dBs. In the third round on = new year's=20 night, Dex and Paul dared to take a bet, and won. Allow me to cite from = their=20 final email exchange on Dec 31st:
 
Dex:
Want to test the = limits?
 
Paul:
Yes please.
Let's go for = broke.
8=20 seconds is close to the limit, which is what I'd like to see. =
 

The ultimate goal of this work has been to take decoding = sensitivity=20 close to the theroretical limit. An universal metric for this is Eb/N0, = the=20 ratio of the received signal energy per payload bit (Eb in Joules) and = the noise=20 spectral power density (N0 in Watt/Hertz, equivalent to "noise energy" = in=20 Joules). The Shannon limit for long messages spread to infinite = bandwidth is=20
 Eb/N0 =3D ln(2) =3D -1.59 dB,
which (similar to the speed = of light)=20 cannot be surpassed by any possible encoding scheme. Paul's and Dex' = experiments=20 showed that his codes can come within about a dB of this limit in a real = long-distance propagation experiment.
 
To put that into perspective, let's derive Eb/N0 figures for two = popular=20 digital modes:
 
WSPR-15 transmits 50 information bits in 15 minutes, ie one bit in = 18=20 seconds. The decoding threshold is -38 dB in 2.5 kHz, or -4 dBHz. This = gives=20
 Eb/N0 =3D 10 log(18) - 4 dB =3D +8.5 dB,
ie. about 10 dB = above the=20 Shannon limit. Note that although different speed variants (eg WSPR-2) = need=20 different power, the minimum energy per bit has to remain the = same.
 
Opera-32 carries 28 information bits in 32.6 minutes, ie. one bit = in 70=20 seconds. The threshold is about -39.5 dB in 2.5 kHz, (-5.5 dBHz), = referenced to=20 the average power of the 50% dutycycle on-off keying. This = gives
 Eb/N0=20 =3D 10 log(70) - 5.5 dB =3D +13 dB
or about 14.5 dB above Shannon. = Note however=20 that for LF / VLF transmissions, the limit will often be antenna voltage = and=20 peak power rather than average power, which can result in a further 3 dB = disadvantage for Opera against frequency- or phase-modulated=20 techniques.  
 
The opds correlation decoder can go about 9 dB lower than Opera. = But of=20 course it can only find the best match from an a-priori defined list of=20 callsigns, and doesn't attempt to decode any message.
 
However we must recognize that the amateur modes spend a = significant part=20 of their energy to provide a reference for synchronisation, so not all = of the=20 Eb/N0 difference is due to less efficient encoding. The "nude" FEC-PSK = mode=20 doesn't contain any such overhead. So it can only work when the link has = a=20 stable phase (like on VLF), and the decoder has been given accurate = information=20 on carrier frequency and symbol timing.
 
All the best,
Markus (DF6NM) 

Sent: Sunday, January 04, 2015 1:13 PM
Subject: VLF: Transatlantic messages at = 8822Hz


W4DEX achieved another 'first' recently by sending a = series=20 of
messages across the Atlantic at 8822 Hz which were = successfully
copied=20 at Todmorden UK, range 6194km.

Transmissions used coherent BPSK=20 signalling with ERP of around
150uW.   The modulation = encoded the=20 messages using a rate
1/16 terminated convolutional code with = constraint=20 length 25,
cascaded with an outer error detection code.

The = first=20 message was received at 2014-12-30 03:00, a 4
character message = 'EM95'. =20 Eb/N0 was -0.8dB using 9 second
symbols.

A second test the = following=20 night managed 12 characters 'PAUL
HNY DEX' using 14 second symbols = giving=20 Eb/N0 of +1.0dB.
Conditions were good and we could have used shorter=20 symbols
and a longer message.

The third test and best result = so far=20 was a 25 character message
'8822HZ 2015 JAN 1 TA TEST' sent from = 2015-01-01=20 00:00 using
8 second symbols.  This was received with Eb/N0 =3D=20 -0.1dB.

In the 0.125 Hz bandwidth of a code symbol, the S/N was=20 -13.2dB.
That corresponds to -56dB S/N in a 2.5kHz audio = bandwidth
after=20 sferic blanking.  Before the blanker the S/N would be
around=20 -76dB.

The source encoding uses 6 bits per character to produce=20 a
payload of 150 bits.  An outer code adds a 16 bit CRC and=20 the
convolutional encoder expands the message to 3040 signal = bits.
The=20 effective code rate is therefore 150/3040 =3D 1/20.27.

Of the = 3040 signal=20 bits, 1153 were demodulated incorrectly
but the FEC was able to fix = them all=20 to reveal the message.

Received signal was around 0.12 fT and it = was=20 necessary to
combine H-field and E-field receiver outputs to obtain=20 a
sufficient S/N to decode.

The decoder is a soft Viterbi list = decoder.  The signals are too
weak to reveal a reference phase = by the=20 usual method of summing
the squared complex symbol amplitudes.  = Instead=20 the decoder has
to do a brute force trial and error = search.

The=20 information rate in the 3rd test was 24.6 bits per hour
which is 80% = of the=20 channel capacity.

--
Paul Nicholson
http://abelian.org/
--

= ------=_NextPart_000_004B_01D02839.14D143D0--