Tuesday, June 25, 2019

4800 symb/sec QPSK signal on 10160 kHz (2)

This in an update related to the last blog post. As before, all analysis is done in GNU octave.

In order to extract the bit stream from the signal on 10160 kHz we assume that it (D)QPSK modulated and use the gray code mapping [0,1,2,3] -> [0,1,3,2].

Assuming QPSK modulation


The extracted bits are shown below in terms of 894-bit long frames.

bit stream in terms of 24×41 frames (QPSK)


The rank-deficiency analysis of this bit stream indicates the presence of a convolutional code:

rank-deficiency (QPSK)

For obtaining the rank-deficiencies above, the bit stream has been aligned by using an offset for which the R(k)/k is minimal, i.e., at 10+m×24:

alignment of bit stream for the rank-deficiency analysis (QPSK)

As a result, the code rate is n/k=1/4, and the length of the dual code is 41. The latter is quite large and therefore hints at the use of some form of Turbo Code, or something similar to it.

Assuming DQPSK modulation


The extracted bits are shown below in terms of 894-bit long frames. Unlike for QPSK modulation the bits in positions [1,18,19,24]+m×24 are the same in each frame.

zoom of bit stream in terms of 24×41 frames (DQPSK)


Also assuming DQPSK modulation, the presence of a convolutional code is detected; it has code rate 7/12 and the length of the dual code is 41. (This might be a Turbo Code with 14 systematic bits and 5+5 parity bits)

rank deficiency (DQPSK)

alignment of bit stream for the rank-deficiency analysis (DQPSK)

Summary


It is unlikely by coincidence that the frame length is 24×41, the length of the dual code is 41, and the distance between the rank deficiencies is 24.

The fixed bits in each frame when assuming DQPSK modulation are likely being used for frame synchronization: in this case, 4 of the 14 systematic bits are used for frame alignment and there are 5+5 parity bits.

Obviously it would be interesting to figure out the full parameters of the convolutional coding.

Friday, June 14, 2019

4800 symb/sec QPSK signal on 10160 kHz


Currently there is a signal active on 10160 kHz. It is QPSK modulated with 4800 symb/sec. The autocorrelation function for the QPSK symbols has peaks at multiples of 492=12×41 symbols:

symbol autocorrelation

Arranging the QPSK symbols in frames of 492 symbols, some interesting structures can be seen

492 symbols / frame

When the symbols are arranged in terms of 12-symbol long frames, some columns of the resulting matrix have a periodicity of 41 in the squared symbols.

TDoA geo-location indicates a location close to Luxembourg.

TDoA