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 Copyright © 2016, S.R. Phillips K6JT. All rights reserved. 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 Page i 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. Rev: 0 4 4-Oct-16 White Paper RRI-008 for the DTN Digital System Rev. 1 3 October 2016 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