26 December 2014

PACTOR-II (FEC, ARQ)

PacTOR-II is a modern, narrow-band and reliable two-channel phase-shift keyed mode which covers the needs of both amateur and professional users for a secure and fast data transmission system. The good performance during disturbances and a small bandwidth are achieved by the use of a two channel DPSK modulation. PACTOR-II is suited to the transfer of large files rather than hand-typed text. The data may be transmitted as ASCII without compression or can be compressed with HUFFMAN or PSEUDO-MARKOV compression. In addition, run length encoding (RLE) is included.

As propagation conditions change, PacTOR-II ARQ (as well as the FEC option) is able to select the most suitable modulation (DBPSK, DQPSK, DPSK.8, DPSK-16 and PSK-16) and the length of the frame (SHORT or LONG): under very poor conditions PACTOR-II changes to DBPSK short block, which has a frame duration of 1.25 seconds and carries only a small amount of data; for very good conditions, DPSK-16 with a long frame is used, which transfers a large amount of data and needs 3.75 seconds.
PacTOR-II ARQ sonagram
To achieve a data transmission which is robust against all kind of disturbances, a decision must be made, whether a short or a long frame-length should be used. The resulting input bit stream is then terminated with a convolutional encoded and interleaved CRC-sum. After that, the symbol mapping for the two channels is done, and then this signal is applied to the DPSK or PSK demodulator.

PacTOR-II FEC DQPSK

PacTOR-II FEC DPSK-8
Where in PacTOR-II ARQ an acknowledge message from the receiving station is required, PacTOR-II-FEC has no acknowledge return channel. Thus, the receiving station must be able to do all the error correction. This is achieved by using a Viterbi decoder with k = 9 and code rate = ½. Interleaving protects the transmission against fading. Using a DQPSK modulation is a good compromise when looking at the amount of bits transmitted and the necessary signal-to-noise-ratio.

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