Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 1919

e) header FEC. Note.— Immediately after these segments follows an AVLC frame with the format as contained in the data link service description in the Manual on VDL Mode 2 Technical Specifications. 6.4.3.1.1.1 Transmitter ramp-up and power stabilization. The purpose of the first segment of the training sequence, called the ramp-up, is to provide for transmitter power stabilization and receiver AGC settling, and it shall immediately precede the first symbol of the unique word. The duration of the ramp-up shall be five symbol periods. The time reference point (t), for the following specification is the centre of the first unique word symbol, a point that occurs half a symbol period after the end of the ramp-up. Conversely stated, the beginning of the ramp-up starts at t = –5.5 symbol periods. The transmitted power shall be less than –40 dBc prior to time t = –5.5 symbol periods. The ramp-up shall provide that at time t = –3.0 symbol periods the transmitted power is 90 per cent of the manufacturer’s stated output power or greater (see Figure 6-1*). Regardless of the method used to implement (or truncate) the raised cosine filter, the output of the transmitter between times t = –3.0 and t = –0.5 will appear as if ‘000’ symbols were transmitted during the ramp-up period. Note. 1.— For Mode 3, the timing reference point is the same as the “power reference point”. Note 2.— It is desirable to maximize the time allowed for the AGC settling time. Efforts should be made to have power above 90 per cent of nominal output power at t – 3.5 symbol periods. 6.4.3.1.1.2 Synchronization and ambiguity resolution. The second segment of the training sequence shall consist of the unique word: 000 010 011 110 000 001 101 110 001 100 011 111 101 111 100 010 and shall be transmitted from left to right. * All figures are located at the end of this chapter. Annex 10 — Aeronautical Communications Volume III 22/11/07 I-6-10 6.4.3.1.1.3 Reserved symbol. The third segment of the training sequence shall consist of the single symbol representing 000. Note.— This field is reserved for future definition. 6.4.3.1.1.4 Transmission length. To allow the receiver to determine the length of the final Reed-Solomon block, the transmitter shall send a 17-bit word, from least significant bit (lsb) to most significant bit (msb), indicating the total number of data bits that follow the header FEC. Note.— The length does not include those bits transmitted for: the Reed Solomon FEC, extra bits padded to ensure that the interleaver generates an integral number of 8-bit words, or the extra bits padded to ensure that the data encoder generates an integral number of 3-bit symbols. 6.4.3.1.1.5 Header FEC. To correct bit errors in the header, a (25, 20) block code shall be computed over the reserved symbol and the transmission length segments. The block code shall be transmitted as the fifth segment. The encoder shall accept the header in the bit sequence that is being transmitted. The five parity bits to be transmitted shall be generated using the following equation: [P1 , ... , P5] = [R1 , ... , R3 , TL1 , ... , TL17] HT where: P is the parity symbol (P1 shall be transmitted first); R is the reserved symbol; TL is the transmission Length symbol; T is the matrix transpose function; and H is the parity matrix defined below: 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0 0 0 0 1 1 1 1 1 1 1 1 H = 1 1 0 0 0 1 1 1 0 0 1 1 0 0 0 0 1 1 1 1 1 1 0 1 1 0 1 1 0 1 0 1 0 0 1 1 0 0 1 1 0 1 1 0 1 0 0 1 1 1 1 0 0 1 0 1 0 1 0 1 6.4.3.1.1.6 Bit transmission order. The five parity bits of the resultant vector product shall be transmitted from the left bit first. 6.4.3.1.2 Forward error correction. In order to improve the effective channel throughput by reducing the number of required retransmissions, FEC shall be applied after the training sequence, regardless of frame boundaries. 6.4.3.1.2.1 FEC calculation. The FEC coding shall be accomplished by means of a systematic fixed-length Reed- Solomon (RS)(255,249) 28-ary code. Note 1.— This code is capable of correcting up to three octets for data blocks of 249 octets (1992 bits). Longer transmissions must be divided up into 1992 bit transmissions and shorter transmissions must be extended by virtual fill with trailing zeros. Six RS-check octets are appended for a total block of 255 octets. The field defining the primitive polynomial of the code shall be as follows: Part I Annex 10 — Aeronautical Communications I-6-11 22/11/07 p(x) = (x8 + x7 + x2 + x + 1) The generator polynomial shall be as follows: i 120 ( ) = −α ∏ i x where: α is a primitive element of GF(256); GF(256) is a Galois field (GF) of size 256. Note 2.— The Reed-Solomon codes are described in the Recommendation for Space Data System Standards Telemetry Channel Coding, by the Consultative Committee for Space Data Systems (see the Appendix to this chapter). 6.4.3.1.2.2 Block lengths. The six RS-check octets shall be calculated on blocks of 249 octets. Longer transmissions shall be split into blocks of 249 octets, per 6.4.3.1.3. Blocks of shorter length shall be extended to 249 octets by a virtual fill of trailing zeros. The virtual fill shall not be transmitted. Blocks shall be coded according to 6.4.3.1.2.3 through 6.4.3.1.2.3.3. 6.4.3.1.2.3 No error correction. For blocks with 2 or fewer non-fill octets, no error correction shall be used. 6.4.3.1.2.3.1 Single-byte error correction. For blocks with 3 to 30 non-fill octets, all six RS-check octets shall be generated, but only the first two shall be transmitted. The last four RS-check octets shall be treated as erasures at the decoder. 6.4.3.1.2.3.2 Two-byte error correction. For blocks with 31 to 67 non-fill octets, all six RS-check octets shall be generated, but only the first four shall be transmitted. The last two RS-check octets shall be treated as erasures at the decoder. 6.4.3.1.2.3.3 Three-byte error correction. For blocks with 68 or more non-fill octets, all six RS-check octets shall be generated and transmitted. 6.4.3.1.3 Interleaving. To improve the performance of the FEC, an octet-based table-driven interleaver shall be used. The interleaver shall create a table having 255 octets per row and c rows, where transmission length (bits) c = 1992 (bits) where:

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