5 April 2016

CIS-112 modem 22.22 Bd BPSK stream

other than the π/4 DQPSK burst mode, CIS OFDM 122-tone can be met also as a stream-waveform where all channels use BPSK modulation as part of the basic QPSK in the absolute constellation. The 22.22Bd manipulation speed and 25.6Hz channels separation are the same in both the two waveforms as well as the typical CIS pilot-tone transmitted at 3300 Hz from the suppressed carrier. This sample was recordered at 0940 UTC on 10980.0 KHz/USB (April, 3).

pic. 1 - CIS-112 BPSK stream-waveform

This CIS-112 waveform  has two functionally distinct transmission phases: synchronization preamble phase and data phase. The sync preamble phase is discussed in another post.

During the data phase, the transmit waveform contains both message information and special symbols, say "probes", most likely reserved for equalization and sync by the receive modem. These probes are:

- a single special/service character consisting of the 56 odd tones, transmitted each 72 symbols 

pic. 4 - a special/service character is sent each 72 symbols
pic. 5 - the special/service symbol in the spectrogram
- a four symbol periods (181ms length) pattern transmitted each 144 symbols that causes strong 6500ms ACF spikes. It's worth noting that 144 is just the double of 72.

In my opinion this sequence consists of known-data and should work like the mini-probes of the MS188-110 serial waveforms. Looking at the spectrogram, it seems that only the 3rd symbol is transmitted at higher level.

pic. 6 - fours symbol periods pattern transmitted each 144 symbols
pic. 7 - 6500 ACF spikes

Is quite easy isolate a single channel and measure the manipulation speed but it's very difficult to get the sub-carrier and consequently a clear phase constellation, despite the good quality of this sample (pic. 8). 

It's worth noting that some experts suggest that CIS-112 may use IOTA-OQAM technology and it causes intersymbolic-interference effects when analyzed with classical tools.
pic. 8
IOTA-OQAM technolgy  

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