22 September 2016

PACTOR-IV ("Dragon" modem) BPSK/QPSK

PacTOR-IV transmission using BPSK and QPSK modulation and speed of 1800 symbols/sec (in both the cases), without the characteristic PacTOR-IV spreading factor used at lower data rates. The presence of the spreading can be detected running the 'AM detector' module of SA and looking at the phase vector of the signal: in fig. 2 the AM detector exhibits a single baudrate line (so no spread factor) and the phase vector has a normal trend.

fig. 2
Instead, the AM detector in fig.3 exhibits multiple baudrate lines and the phase vector assumes the characteristic sawtooth ramp. Note that the base speed is about 112.5 Baud (the lower baudrate line and the frequency in the phase vector) while the speed detected by tha analyzer is 1800 Baud: this is a spread-16 since 112.5Bd * 16 just results 1800Bd.

fig. 3
Modulation vary according to the channel condition (PACTOR-IV is an adaptive mode) and the resulting constellations are visible in fig. 4, the related phase vectors are shown in fig. 5

fig. 4 - constellations
fig.5 - phase vectors for BPSK and QPSK blocks
A distinguishing characteristic, other than the 3300ms block length, is the 239 symbols lenght of the BPSK and QPSK frame structures:
239 symbols = 239 bit for BPSK (two modulation symbols, 1 binary digit)
239 symbols = 478 bits for QPSK (four modulation symbols, 2 binary digits)
Bitstreams after demodulations and frame structures are shown in figs 6,7. It may happen that the bistreams have the period length which  is the double of the expected one: most likely this is due to the  un-initialized state of the (generic) psk demodulator of SA.

fig. 6 - PACTOR-IV BPSK frame structure
fig. 7 - PACTOR-IV QPSK frame structure


  1. What software is that you are using?

  2. Hi Aaron, software used is "SA - Signals Analyzer":