STA400A datasheet(22/117 Pages) STMICROELECTRONICS

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STA400A

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1.3 TERRESTRIAL DEMODULATION (To Be Completed)

The MultiCarrier Modulated (MCM) terrestrial signal is sampled at 23.92 MHz and converted to 10-bit. The ter-

restrial demodulator processes these samples at four time the MCM symbol rate (4*2.99 MHz = 11.96 MHz) and

includes the final down-convertion and I/Q symbol generation, low-pass filtering and down-sampling to MCM

symbol rate (2.99 MHz). The MCM demodulation is performed by an FFT over 768 samples. The MultiCarrier

Demodulator also includes frequency and symbol synchronization, Amplitude-Modulated Synchronisation Sym-

bol (AMSS) detection for frame synchronisation, guard interval removal, differential decoding, demapping and

metric generation.

All the MCM demodulator functionality are programmable by the microcontroller; the Register Map is listed in

sections 2.3 and 2.9.

1.4 TDM DECODING (To Be Completed)

There are two different TDM structures within the DARS system, the satellite TDM and the Terrestrial TDM, car-

rying the same Payload Channels (PC), which are included in a 432 msec framed packet consisting of one or

more Prime Rate Channel (PRC) with Reed-Solomon protection (outer encoding).

The combined satellite transmission includes a punctured rate 3/8 convolutional inner encoder (from a mother

code of rate 1/3 and "1 out of 9" punctured scheme) and a convolutional interleaved with 4.698 sec. delay.

Each satellite transports one half of the punctured and interleaved PC resulting in an effective inner encoder

rate of 3/4. Up to 256 PRCs are multiplexed together into a Time Division Multiplex (TDM) structure. The Time

Slot Control Channel (TSCC), containing information of the TDM structure, is added at the begin of the fully

FEC protected PCs. After the insertion of 1 Master Frame Preamble (MFP), 205 Fast Synchronization Preamble

(FSP) and the padding field (PAD) the complete satellite Master Frame (MF) contains 1416960 bits over 432

msec, resulting in a satellite TDM bit-rate of 3.28 Mbits/sec.

The terrestrial bit-stream is a repeater signal from the satellite transmission. The inner encoder has a convolu-

tional code rate of 3/5 (from a mother code of rate 1/3 and "4 out of 9" punctured scheme) and does not include

convolutional interleaving. The terrestrial MF structure contains only 3 fields: MFP, Data Field (TSCC/PCs) and

PAD resulting in a 1755360 bits over 432 msec. for a bit-rate of 4.063333 Mbits/sec.

The TDM decoder receives the demodulated symbol streams from the two satellite and the terrestrial demodu-

lators.

The TDM processing includes Sat-Sat combining, frame synchronization, external memory management and

PRC demultiplexing.

The frame synchronization is based on the MFP detection. The MFP and FSP are also used for the phase am-

biguity resolution of the satellite demodulated data and for fast synchronization after a short dropout and cycle-

slip.

The PRC demultiplexer processes the TSCW (Time Slot Control Word) from the TSCC field to extract all the

information needed for the allocation of the PRCs to the selected PCs. Up to 50 PRCs (48 PRCs plus 2 TSCC)

can be demultiplexed resulting in a maximum PC bit stream rate of 414.8 Kbit/s (384Kbit/s of useful data).

The programmable register for the TDM Decoding are explained in sections 2.4 and 2.8.

1.5 FEC (To Be Completed)

The PRC and the TSCC data from the external memory are decoded by the FEC (Viterbi Decoder, Reed-So-

lomon Decoder and Block Deinterleaver) and sent to the PC-BitStream Interface.

The PRC-based packet structure of the Service Layer and the external memory allow the use of one Viterbi De-

coder and one RS Decoder for both the combined-satellite and terrestrial TDM frame.

The first operation of the FEC block is a data pre-processing for buffering, reordering and demultiplexing of the

data streams coming from the external memory and containing both the satellite and terrestrial TDM. In this

phase the depuncturing process is also performed.

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