White Paper RRI-008 Pactor-3 Characteristics and Implications for Digital Traffic Network Hub and Digital Traffic Station Operations in the Radio Relay International

White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016

White Paper RRI-008 Pactor-3 Characteristics and Implications for Digital Traffic Network Hub and Digital Traffic Station Operations In the Radio Relay International

Prepared By: Steve Phillips K6JT Plano, TX 75025 [email protected] Interim Coordinator Central US Radio Relay International

White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016

Table of Contents 1. SCOPE ...... 2

1.1. GENERAL ...... 2 1.2. INTRODUCTION ...... 2 2. APPLICABLE DOCUMENTS ...... 3 3. PACTOR-3 DETAILED CHARACTERISTICS ...... 4

3.1. CHANNEL DATA RATES AND THEIR PROPERTIES AND PERFORMANCE ...... 4 3.2. CREST FACTOR AND TRANSMITTER POWER ...... 6 3.3. AUTOMATIC-CONTROL SUB-BAND OPERATION ...... 6 3.3.1. 40 Meters ...... 8 3.3.2. Operation Outside the Auto-Control Segments ...... 8 4. TRANSMITTER AND RECEIVER IMPLICATIONS ...... 10 5. NOTES ...... 11

5.1. ACRONYMS AND ABBREVIATIONS ...... 11

List of Tables

TABLE 1, PACTOR-3 TONES FOR EACH CHANNEL DATA RATE ...... 5 TABLE 2, PACTOR-3 MODULATION AND CODING CHARACTERISTICS ...... 5 TABLE 3, DIAL AND CENTER FREQUENCY LIMITS INCLUDING SIDELOBE (3600 BPS) ...... 7 TABLE 4, DIAL AND CENTER FREQUENCY LIMITS EXCLUDING SIDELOBES (3600 BPS) ...... 7 TABLE 5, 40 METER CENTER FREQUENCY ALTERNATIVES ...... 8

List of Figures

FIGURE 1. PACTOR-3 SPECTRUM AT 3600 BPS CHANNEL DATA RATE ...... 4 FIGURE 2. PACTOR-3 THROUGHPUT WITH CHANNEL NOISE (WITH RESPECT TO 200 BPS CHANNEL RATE) ...... 5 FIGURE 3. PACTOR-3 BIT ERROR RATE PERFORMANCE ...... 6

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White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 1. Scope

1.1. General This paper describes the technical characteristics of the Pactor-3 waveform, as defined in document ITU-R-Rec-M.1798 by Hans-Peter Helfert and Thomas Rink, SCS GmbH & Co. KG, Hanau, Germany. That document was obtained thanks to the efforts of Peter Dintelmann, DL4FN, who is the European hub for DTN Digital. The paper also describes the implications for DTN Digital operations in the United States, considering the limitation to operation of automatically controlled stations (DTN Digital Hubs) to specific, limited portions of the HF spectrum, and the use of Pactor-3 by Digital Traffic Stations for both DTN and Winlink operation.

1.2. Introduction The information herein was derived primarily from the ITU document referenced above but also from empirical observations made by the author while using an SCS DR-7400 modem for HF DTN and Winlink operation. The Pactor-3 waveform is a multi-tone system that operates at one of 6 specific raw channel data rates, 200, 600, 1400, 2800, 3200, and 3600 bits per second. Each of these occupies a different bandwidth and has a specific modulation and channel coding. The maximum data bandwidth required by a Pactor-3 signal is 2.2 KHz, but that is only when using 18 tones at the highest (3600 bps) channel data rate. Lower data rates have fewer tones, thus less bandwidth. The lowest rate (200 bps) occupies about 960 Hz and uses only two tones. Because the tones are centered around 1500 Hz, which is the default frequency offset for an SCS modem, the spectrum runs from about 400 Hz to a maximum of 2600 Hz. But the signal shaping and filtering done by the SCS modem results in some side-lobes only 20 dB down such that the effective bandwidth, at the -40 dB level, is 2440 Hz, or about 2.4 kHz. As a result, the DTN Digital hubs must assure that their selected center frequencies do not result in emissions outside the automatic control sub-bands for inter-hub automatic communication. Furthermore, because the composite signal is applied to an Upper Sideband modulator in an amateur radio transceiver, the transmit and receive filter bandwidths should be set to accommodate the highest usable frequency of 2600 Hz if operation at 3600 bps is to be supported.

Rev: 0 2 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 2. Applicable Documents

RECOMMENDATION Characteristics of HF radio equipment for the exchange of digital data and ITU-R M.1798 electronic mail in the maritime mobile service DTN Digital Operations DTN Digital Operations White Paper, Via the link on http://www.k6jt.com/ DTN Digital Website http://www.nts-digital.net

Rev: 0 3 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 3. Pactor-3 Detailed Characteristics

Figure 1. Pactor-3 Spectrum at 3600 bps Channel Data Rate The above figure shows the spectrum of the 18 tone 3600 bps signal. The 0 Frequency is the SCS Modem center frequency of 1500 Hz. Notice the sidelobes that extend beyond the actual signal spectrum. They are down 40 dB from the peak at the 2.4 KHz overall bandwidth.

3.1. Channel Data Rates and Their Properties and Performance The Pactor-3 signal operates at a 100 symbol per second (baud) rate, well within the FCC’s 300 baud limit. It does this by having multiple tones, or sub-carriers, within the composite signal. Note that the actual data throughput is less than the channel data rate because for most channel rates, rate ½ convolutional encoding (with Viterbi decoding) is used with the data, with the loss somewhat offset by built-in data compression algorithms. The following figure shows the actual data throughput in the presence of channel noise. Note that the peak throughput (at the 3600 bps channel rate) is 2720 bps. Note that it takes a 20 dB signal to noise ratio to achieve the highest throughput. Note also that it is possible to achieve a throughput of approximately 100 bps even when the signal is 5 dB below the channel noise level (in a 3 KHz bandwidth). This is borne out by empirical observations when the signal could not be heard above the noise but a connection and data transfer were still able to be made, albeit at a low rate with numerous retransmission attempts. Noise is reduced with a lower bandwidth, but the full Pactor-3 signal cannot be contained in less than about a 2 KHz USB radio bandwidth setting (when operating with the 200 bps channel rate). See the table following the figure for the tone frequencies.

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Figure 2. Pactor-3 Throughput with Channel Noise (with respect to 200 bps channel rate) The following table shows the tones used and their positions for the different channel data rates.

Table 1, Pactor-3 Tones for Each Channel Data Rate

200 x x 600 x x x x x x 1400 x x x x x x x x x x x x x x 2800 x x x x x x x x x x x x x x 3200 x x x x x x x x x x x x x x x x 3600 x x x x x x x x x x x x x x x x x x

Tone 480 600 720 840 960 1080 1200 1320 1440 1560 1680 1800 1920 2040 2160 2280 2400 2520 The tones are separated by 120 Hz at the highest rates, which is the assumed bandwidth of each. Differential Phase Shift Keying (DPSK) is used for all tones but binary (DBPSK) is used for the lower 3 and quadrature (DQPSK) is used for the higher 3 data rates. This is shown in the following table along with Constraint Length (convolutional coding, CL), Coding Rate (CR), Channel Rate CHR, Information Rate (IR), and Crest Factor (CF) of the signal. Crest Factor is important because it is essentially “audio compression” to maintain a high average power output in the transmitter (more about that later). Constraint length varies for the higher throughputs as does coding rate, but code puncturing (bits are thrown away and recovered in the Viterbi decoder) is used to achieve an acceptable channel data rate. Table 2, Pactor-3 Modulation and Coding Characteristics

Modulation CL CR CHR IR CF (dB) DBPSK 9 ½ 200 76.8 1.9 DBPSK 7 ½ 600 247.5 2.6 DBPSK 7 ½ 1400 588.8 3.1

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Modulation CL CR CHR IR CF (dB) DQPSK 7 ½ 2800 1186.1 3.8 DQPSK 7 ¾ 3200 2039.5 5.2 DQPSK 7 8/9 3600 2722.1 5.7 As a result of the encoding, the following bit error rate performance is achieved.

Figure 3. Pactor-3 Bit Error Rate Performance The different lines in the figure correspond, left to right, with the 200, 600, 1400, 2800, 3200, and 3600 channel data rates. Note that the 200 bps rate is capable of operating in a negative S/N environment.

3.2. Crest Factor and Transmitter Power One of the most important characteristics of the Pactor-3 signal is the low CF, especially with the lower channel rates. As most HF power amplifiers are peak-power limited and use a peak-power automatic level control (ALC), Pactor-3 provides considerably more transmitter output power than comparable multicarrier modes like, for example, WINMOR, when using the same power amplifier, thereby increasing the S/N at the receiver. Up to 2800 bps, the CF fairly compares to the CF of single-carrier modes. Even with 3200 and 3600, the CF is about 3 dB lower than the CF of typical Frequency Division Multiplex modes, thereby doubling the transmitted RMS power. This is one reason that a DTN station should be operating the transmitter at lower power settings, typically around 60 watts peak, to avoid overheating a 100 watt transmitter.

3.3. Automatic-Control Sub-band Operation An analysis of the foregoing results in the highest USB dial and center frequencies that should be used when 3600 bps channel rate operation is desired in order to contain the entire 2.4 kHz spectrum completely within the automatic control sub-bands. A dial frequency slightly lower than the lower edge of the automatic control sub-band may actually be used since the lowest tone is

Rev: 0 6 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 400 Hz above the suppressed carrier (dial) frequency. However, because of the sidelobe that extends about 150 Hz lower than that, it may be best to limit the lowest dial frequency to exactly the lower limit of the auto-control sub-band. Note that other DTN frequencies OUTSIDE the auto-control sub-bands may be provided for the exclusive use of the Digital Traffic Stations, who manually connect with the DTN hub. Similarly, Winlink frequencies may be outside these bands. However, ALL transmissions outside these bands MUST use 500 Hz or less bandwidth modes and are thus limited to Pactor-1 or Pactor-2. Adhering to this limitation will completely satisfy all known FCC requirements. Table 3, Dial and Center Frequency Limits Including Sidelobe (3600 bps)

Band Lowest Dial Lowest Center Highest Dial Highest Center 3585-3600 3585 3586.5 3597.28 3598.78 7100-7105* 7100 7101.5 7102.28 7103.78 10140-10150 10140 10141.5 10147.28 10148.78 14095-140995** 14095 14096.5 14096.78 14098.28 141005-14112** 14100.5 14102 14109.28 14110.78 18105-18110 18105 18106.5 18107.28 18108.78 21090-21100 21090 21091.5 21097.28 21098.78 24925-24930 24925 24926.5 24927.28 24928.78 28120-28189 28120 28121.5 28186.28 28187.78 * See section 3.3.1 for 40 meter alternatives ** The 20 meter frequencies are divided into two segments to protect the 14.100 beacon frequency The above table takes into account the sidelobe occurring at the upper band limit (see Figure 1). Since the sidelobes are down 20 dB from the peak, it should be permissible to operate with only the signal width. Sidelobes end 1.22 kHz from center while the signal ends 1.1 kHz from center. These revised numbers are shown in the following table. Table 4, Dial and Center Frequency Limits Excluding Sidelobes (3600 bps)

Band Lowest Dial Lowest Center Highest Dial Highest Center 3585-3600 3585 3586.5 3597.4 3598.9 7100-7105 7100 7101.5 7102.4 7103.9 10140-10150 10140 10141.5 10147.4 10148.9 14095-140995 14095 14096.5 14096.9 14098.4 141005-14112 14100.5 14102 14109.4 14110.9 18105-18110 18105 18106.5 18107.4 18108.9 21090-21100 21090 21091.5 21097.4 21098.9 24925-24930 24925 24926.5 24927.4 24928.9 28120-28189 28120 28121.5 28186.4 28187.9

Rev: 0 7 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 Which table we use is open to discussion and a consensus opinion will be added here once obtained (TBD). Observation of current center frequencies in use appears to support the use of the signal width rather than sidelobe width. The lower band edge dial frequency is set to the same frequency as the edge, but in actuality, it may be set lower since signal energy does not start until 280 Hz (sidelobe) or 400 Hz (ignoring sidelobe) above the suppressed carrier (dial) frequency.

3.3.1. 40 Meters It is highly desirable to be able to fit two Pactor-3 signals into the 5 KHz band allocation on 40 meters. If one inspects the above tables, particularly Table 3, it may be seen that the two center frequencies are too close to each other to avoid some mutual interference when operating at the 3600 bps channel rate. Since we need not be concerned about interference between the sidelobes, the 1.1 KHz offset from center (higher for the low frequency center and lower for the high frequency center) may be used to calculate the frequencies. Because of the offset of 400 Hz from carrier, lowering the first dial frequency below 7100 would not put energy outside the band, but the lower sidelobe should be taken into account to assure any radiated energy is 40 dB below the signal peak level. Thus the following shows two alternative sets of 40 meter frequencies. Table 5, 40 Meter Center Frequency Alternatives

Lowest Dial Lowest Upper Energy Upper Dial Upper Lower Energy Separation Center Center 7100 7101.5 7102.6 7102.3 7103.8 7102.7 0.1 7099.8 7101.3 7102.4 7102.3 7103.8 7102.7 0.3

For this table, the upper dial and center is set so that the sidelobe still fits within the allocated band for both alternatives. The first 7101.5 and 7103.8 center frequencies result in a 100 Hz separation between the energy envelopes of two 3600 bps Pactor-3 signals. The sidelobes will “collide”, however, possibly causing some intermodulation distortion of one signal if it is much weaker than the other. A better alternative is the second set of center frequencies – 7101.3 and 7103.8. This still keeps energy within the band since even the sidelobe does not start for 280 Hz from carrier. An 80 Hz lower guard band is thus maintained, even if the two station frequencies are not exactly on center. At the upper end, all energy, including all but 20 Hz of the sidelobe, is within the band. Both sidelobes (upper for the lower frequency and lower for the upper frequency) similarly have a small guard band of 100 Hz between them. As with the last section, a consensus among all parties will be reached and the result published in an update here (TBD).

3.3.2. Operation Outside the Auto-Control Segments Section 3 stated that frequencies outside the auto-control sub-bands could be provided, exclusively for use of DTS stations (i.e., not hub to hub transfers). While this is true, and often quite useful during congested operating hours, some restrictions do apply. Thanks to Steve, KB1TCE for sending them, the following regulations apply to this operation, both for DTS access to a DTN hub and also for Winlink RMS access. Dave, WB2FTX, has responded to complaints about this on the digitalradio Yahoo group as well. From KB1TCE: 97.221(c) says:

Rev: 0 8 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 (c) Except for channels specified in §97.303(h) (the auto-control sub-bands), a station may be automatically controlled while transmitting a RTTY or data emission on any other frequency authorized for such emission types provided that: (1) The station is responding to interrogation by a station under local or remote control; (valid for a DTS connecting to a DTN hub or Winlink RMS) and (2) No transmission from the automatically controlled station occupies a bandwidth of more than 500 Hz. This clearly states that the signal from the DTN hub (or RMS) be restricted to 500 Hz bandwidth. That eliminates Pactor-3 outside of the auto-control sub-band. Pactor-2 uses 450 Hz bandwidth and Pactor-1 even less. It also eliminates the use of WINMOR 1600 for RMS access outside the auto-control sub-bands. Since all the hubs are using SCS modems, which automatically respond to the incoming waveform type negotiations, it is the responsibility of the DTS stations to assure that they do not force the hubs to operate contrary to the regulations. BPQ-32 also allows setting the mode limits for each frequency scanned. For this reason, I am recommending that the system.DTNxx.ini files for all areas be modified to remove the Pactor-3 notation for all frequencies established for use outside the auto-control sub- bands. That will cause Airmail to configure the SCS modem to use Pactor-2. I have modified my own copy of the file. Once we get all frequency changes ironed out pursuant to hubs changing to BPQ32, then the DTNxx files can be modified and distributed globally. Note that Dave, WB2FTX, is the “keeper” of these files and copies sent to the other DTN Digital Coordinators for distribution in their area.

Rev: 0 9 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 4. Transmitter and Receiver Implications Section 3.2 described the significance of the crest factor and suggested that a maximum of 60 watts be used for transmitters operating with Pactor-3 signals. The spectrum shown in Figure 1 and the tones shown in Table 1 dictate that a radio bandwidth of 2600 Hz be used in order to pass all significant energy of the 3600 bps channel rate signal. Some transceivers allow the configuration of SSB transmit bandwidth. For those rigs, assure that the bandwidth is set to at least 2600 Hz. Setting it a bit higher will not hurt, but setting it lower will result in attenuation of the highest tone(s), making copy at the receive end less than ideal and possibly even preventing operation at the higher channel rates. The receive bandwidth of the transceiver should likewise be set to 2600 Hz if higher channel rate operation is anticipated. But if distances and conditions typically limit speeds to 1400 bps or even 2800 bps, then a lower bandwidth, such as 2400 Hz, may be used. It depends on the shape factor of the filtering, of course. For example, my Ten-Tec Omni VII has a 2.5 KHz hardware filter with relatively sharp rolloff. Digital filters typically are not as sharp, but some implementations may be, so use your judgment and manufacturer information. The point is that the wider the receive bandwidth, the more noise will also be received. So a tradeoff must be made and for manually controlled DTS operation, the operator can start with a lower bandwidth (e.g., 2400 Hz) and if a connection has the modems ratchet up speed to greater than 1400 bps, increase the bandwidth to see if it helps hold that speed.

Rev: 0 10 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 5. Notes

5.1. Acronyms and Abbreviations

CF Crest Factor – the ratio (often expressed in dB) of the peak value to RMS (average) value of a waveform. Lower values represent better “compression”. CHR and IR Channel Rate – the number of symbols (or bits) transmitted. Contrast to IR, information rate, which is the recovered data following decoding of the convolutional coded information CL Constraint Length – the depth of the convolutional encoder where the output is a function of the previous CL symbols CR Coding Rate – convolutional encoder rate often referred to as n/k where n is the input data rate and k is the output code rate DBPSK and DQPSK Differential Binary and Quadrature Phase Shift Keying DTN Digital Traffic Network DTS Digital Traffic Station (in the DTN) Hub An area or regional node in the Digital Traffic Network running the BPQ32 or other store-and-forward control software Puncturing A method whereby a base CR is increased, for example from ½ to 7/8 by not transmitting a portion of the code symbols RRI Radio Relay International – A corporation that includes all digital operations and all voice/CW networks at region level and above. May also include other affiliated nets and operators. Not affiliated with the American Radio Relay League or the National Traffic System at Section level. SCS Specialty Communications Systems (company in Germany) that manufactures all Pactor-2/3/4 modems Viterbi An algorithm named after Andrew Viterbi that is used for decoding convolutional codes over noisy links Winlink Winlink communications system (http://www.winlink.org ) WINMOR Sound-card hosted waveform developed by the Winlink Development Team that is used for Winlink system access by those who do not have a Pactor-2 or -3 modem

Rev: 0 11 4-Oct-16