25 October 2014

CODAN-9001/3012: MPSK-16 QPSK

The CODAN 9001 modem uses the 16 QPSK carriers for the transport of data (payload), each carrier is independently modulated with data so it carries a distinct channel-packet. All the 16 concurrent channel-packets constitute a frame and a number of frames constitute a multi-frame.

This "data" waveform is an asynchronous adaptive ARQ system. The modulation rate of each of the 16 tones is 75 Baud; the modulation type is quaternary phase-shift keying (QPSK). The QPSK scheme uses from 656.25 Hz to 2343.75 Hz, these center frequencies are derived from a 600 Hz to 2400 Hz frequency spread and 112.5 Hz per QPSK channel (http://signals.radioscanner.ru/base/signal85/)

While the CODAN-9001 transports data, the CODAN Chirp provides the ALE/selcall (Automatic Link Establishment) part between the peers.

Each payload data packet has a constant length and a sequence number. However, the numbering only serves as an example, and due to the use of ARQ-based retransmissions the numbering may not be sequential.

CODAN-9001 16 tones schema
Independent of the payload data field, the sequence number field has its own error detecting and correcting code. Payload data in each channel packet is protected by a cyclic redundancy code (CRC). This feature is included in order to allow the ARQ protocol to request retransmission of packets received in error.
A session consists of one or more multi-frames. Depending on the amount of data queued for transfer the length of a multi-frame may vary. The receiving modem will extract the frames from the multi-frame determining the number of channel packets and checking whether payload data was received without errors. If a channel packet was received in error a re-transmission is requested. It should be clear from this that a multi-frame may consist of a mixture of new data and re-transmitted data. Re-transmitted data may appear on any channel and in any position within a multi-frame.
Additionally the transmitting modem may opt to send ALE-like parity bit packets in a separate frame and even on another channel within the same multi-frame as the payload data packet to which it belongs. This is indicated by the two packets belonging together carrying the same sequence number. This mechanism is predominantly seen when the link quality deteriorates and consequently the number of re-transmissions increases.

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