CN110784233B - Physical layer scrambling code sequence recovery method in DVB-S2 standard - Google Patents
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- H—ELECTRICITY
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- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M13/00—Coding, decoding or code conversion, for error detection or error correction; Coding theory basic assumptions; Coding bounds; Error probability evaluation methods; Channel models; Simulation or testing of codes
- H03M13/03—Error detection or forward error correction by redundancy in data representation, i.e. code words containing more digits than the source words
- H03M13/05—Error detection or forward error correction by redundancy in data representation, i.e. code words containing more digits than the source words using block codes, i.e. a predetermined number of check bits joined to a predetermined number of information bits
- H03M13/11—Error detection or forward error correction by redundancy in data representation, i.e. code words containing more digits than the source words using block codes, i.e. a predetermined number of check bits joined to a predetermined number of information bits using multiple parity bits
- H03M13/1102—Codes on graphs and decoding on graphs, e.g. low-density parity check [LDPC] codes
- H03M13/1148—Structural properties of the code parity-check or generator matrix
- H03M13/1154—Low-density parity-check convolutional codes [LDPC-CC]
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- H—ELECTRICITY
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- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
- H04N21/40—Client devices specifically adapted for the reception of or interaction with content, e.g. set-top-box [STB]; Operations thereof
- H04N21/43—Processing of content or additional data, e.g. demultiplexing additional data from a digital video stream; Elementary client operations, e.g. monitoring of home network or synchronising decoder's clock; Client middleware
- H04N21/438—Interfacing the downstream path of the transmission network originating from a server, e.g. retrieving encoded video stream packets from an IP network
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Abstract
The invention provides a physical layer scrambling code sequence recovery method in DVB-S2 standard, which uses the relation between the check matrix of low density parity check code and code word, and uses the minimum of check sum as judgment criterion, the method traverses all possible scrambling code sequences, after descrambling and constellation map inverse mapping under each sequence, calculates the check sum of LDPC code word, the scrambling code sequence corresponding to the minimum check sum is the sequence used for scrambling; the method is irrelevant to the characteristics of the information source, can realize single-frame scrambling code recovery, and is particularly suitable for application of a burst co-communication system.
Description
Technical Field
The invention relates to the field of scrambling code analysis, in particular to a physical layer scrambling code sequence recovery method in DVB-S2 standard.
Background
The second generation digital video broadcasting (Digital Video Broadcasting Second Generation, DVB-S2) standard is one of the most successful digital communication standards and is therefore widely used in various communication systems. In the DVB-S2 standard, physical layer scrambling is performed on symbols in addition to bit scrambling on the source to reduce mutual interference between different traffic, different beams, and the scrambling code can also be used as an address code for multiple access communication. However, there is no explicit instruction in the standard to transmit the physical layer scrambling sequence parameters, and in applications such as adaptive communications, the receiving party does not know the scrambling sequence parameters, and the receiving party needs to recover the scrambling sequence first and then descramble.
In terms of DVB-S2 physical layer scrambling sequence recovery, the main methods currently adopted are: firstly, descrambling the information source, and then, utilizing the 0 and 1 unbalance of the information source, recovering the scrambling sequence bit by bit through multi-frame accumulation. However, this approach has significant problems:
firstly, multi-frame accumulation is needed, and the number of times of accumulation is increased exponentially along with the decrease of the unbalance degree of the source 0 and 1, if the unbalance of the source 0 and 1 is very small due to the source self-body characteristic or the adoption of source coding, the number of times of accumulation is too large to practically apply the algorithm;
secondly, in the burst communication system, the receiving party receives the data frame very limited and does not have to have the condition of multi-frame accumulation;
third, when the error rate is large, the existence of the error bit can destroy the 0 and 1 imbalance of the information source, and the algorithm is invalid.
Therefore, there is no better means for recovering the physical layer scrambling sequence in the DVB-S2 standard to break through the above method.
Disclosure of Invention
The invention provides a physical layer scrambling code sequence recovery method in DVB-S2 standard, which uses the relation between the check matrix of low density parity check code (Low Density Parity Check, LDPC) and code word, and uses the minimum of check sum as judgment criterion, the method traverses all possible scrambling code sequences, after descrambling under each sequence and constellation diagram inverse mapping, calculates the check sum of LDPC code word, the scrambling code sequence corresponding to the minimum check sum is the sequence used for scrambling; the method is irrelevant to the characteristics of the information source, can realize single-frame scrambling code recovery, and is particularly suitable for application of a burst co-communication system.
In the art, the DVB-S2 physical layer scrambling principle is as follows:
the general transmission procedure of the DVB-S2 communication link is: after the information source is subjected to bit scrambling, binary BCH coding is firstly carried out, and then LDPC coding is carried out; the coded bit stream is subjected to constellation mapping according to the selected modulation mode, and a symbol sequence corresponding to the modulation mode is generated; after constellation mapping, physical layer scrambling is performed; the symbol after physical layer scrambling is shaped by baseband and quadrature modulated to generate intermediate frequency modulation signal for output. According to different modulation modes and channel conditions, links such as symbol interleaving, pilot frequency insertion and the like can exist.
The physical layer scrambling procedure in the DVB-S2 communication link is as follows:
construction of pseudo-random sequences x and y
x(0)＝1,x(1)＝x(2)＝…＝x(17)＝0 (1)
y(0)＝y(1)＝y(2)＝…＝y(17)＝1 (2)
Wherein the method comprises the steps of
Representing the modulo-2 addition.
(a) Constructing a pseudo-random sequence z n
Wherein mod (i+d, 2 18 -1) represents i+d vs 2 18 -1 modulo the remainder; d is the offset.
(b) Constructing scrambling code sequence R d
R d (i)＝2z d (mod(i+131072,2 18 -1))+z d (i)，i＝0,1,...,2 18 -2 (6)
(c) Physical layer scrambling
S c (i)＝S(i)exp(jR d (i)π/2)，i＝0,1,…,L-1 (7)
Wherein S (i), S c (i) The i-th symbol before and after scrambling, L is the frame length in the unit of symbol;
thus, as can be seen from the above-mentioned physical layer scrambling procedure, the recovery physical layer scrambling sequence in the present invention is the recovery sequence R d So long as R is obtained d The scrambling sequence may be descrambled,
S(i)＝S c (i)exp(-jR d (i)π/2)，i＝0,1,…,L-1 (8)
from formulas (5) and (6), R is reconstructed d The key to (a) is to estimate the offset d.
The DVB-S2 inner code adopts LDPC coding, and LDPC coding parameters can be obtained according to the physical frame header of the DVB-S2 frame; in addition, modulation mode information can be obtained through a physical frame header, and the physical frame header does not participate in physical layer scrambling, and LDPC parameters are set, modulation modes are known, and operations such as carrier removal, timing extraction and the like are completed.
LDPC coding is essentially a linear block code, and assuming a message packet length of k and a codeword length of n, the message packet m= [ m 1 ,m 2 ,…,m k ]And the corresponding codeword c= [ c ] 1 ,c 2 ,…,c n ]There is a relationship that,
c＝mG (9)
where G is a kXn-dimensional matrix, called a generator matrix. There is a (n-k) x n-dimensional matrix H, satisfying,
GH T ＝0 k×(n-k) (10)
h is called a check matrix corresponding to G, wherein 0 k×(n-k) Representing a k x (n-k) dimensional all-zero matrix. Can be obtained by the formulas (9) and (10),
cH T ＝mGH T ＝0 1×(n-k) (11)
wherein 0 is 1×(n-k) For a 1× (n-k) all zero matrix, i.e., the codeword is multiplied by the corresponding check matrix transpose, an n-k dimensional all zero row vector is obtained. The generation method of the check matrix H when different values of n and k are given in the DVB-S2 standard, so that H can be obtained only by knowing n and k, and the values of n and k can be determined according to the information in the physical frame header.
Suppose that scrambling sequence R is generated by a certain offset l l Descrambling according to the formula (8) to obtain a sequence S l ，
S l (i)＝S c (i)exp(-jR l (i)π/2)，i＝0,1,…,L-1 (12)
Then according to the modulation mode to S l Performing constellation diagram inverse mapping to obtain an n-dimensional row vector c l . Definition of check word ch l Checksum chs l In order to achieve this, the first and second,
ch l ＝c l H T ，l＝0,1,...,2 18 -2 (13)
from this, it can be seen that: if there is no error, c when l=d l ＝c d Is an LDPC codeword, its checksum chs l ＝chs d =0; chs when l=d, if there is an error code l ＝chs d Minimum should be taken. Thus, the estimated expression of the offset d is,
wherein the method comprises the steps of
Represents the estimated value of d +.>
Representing the get chs l Minimum value of l. Obtain->
After that, the +.5 can be obtained according to the formula>
And then the physical layer scrambling sequence is restored according to (6)>
In summary, the symbol sequence S is received from the received symbol sequence S at known coding parameters n, k c Medium recovery scrambling code sequence
The steps of (a) are as follows:
1) Constructing pseudo-random sequences x and y
x(0)＝1,x(1)＝x(2)＝…＝x(17)＝0
y(0)＝y(1)＝y(2)＝…＝y(17)＝1
2) Obtaining a check matrix H according to n and k;
3) Taking l=0;
4) Calculating z l R is R l
R l (i)＝2z l (mod(i+131072,2 18 -1))+z l (i)，i＝0,1,...,L-1
5) By R l For symbol sequence S c The descrambling is performed such that the data is not scrambled,
S l (i)＝S c (i)exp(-jR l (i)π/2)，i＝0,1,…,L-1
6) According to modulation mode pair S l Performing constellation diagram inverse mapping to obtain c l ；
7) A check word and a check sum are calculated,
ch l ＝c l H T ，
8) If l < 2 18 -2, then take l=l+1, go to 4), otherwise go to 9);
9) The amount of offset is estimated and,
10 Recovering scrambling code sequences
The beneficial effects of the invention are as follows:
the invention uses the relation between the low density parity check code check matrix and the code word to traverse all possible sequences to check the minimum judgment criterion, thus realizing the recovery of the physical layer scrambling code sequence; meanwhile, the invention is irrelevant to the information source characteristic, can realize single frame recovery without multi-frame accumulation, has stronger adaptability, and is particularly suitable for burst communication application environments; in addition, due to the sparsity of the LDPC check matrix, the error resistance is also strong.
Drawings
Fig. 1 is a flow chart of physical layer scrambling code sequence recovery in accordance with the present invention.
Fig. 2 is a schematic diagram of the calibration and the variation of the offset value l according to the embodiment.
Detailed Description
The invention provides a physical layer scrambling code sequence recovery method in DVB-S2 standard, which uses the relation between the check matrix of low density parity check code and code word, and uses the minimum of the check sum as the judgment criterion, the method traverses all possible scrambling code sequences, after descrambling under each sequence and inverse mapping of constellation diagram, calculates the check sum of LDPC code word, the scrambling code sequence corresponding to the minimum check sum is the sequence used for scrambling, namely the recovery sequence R d So long as R is obtained d The scrambling sequence may be descrambled and the recovery flow is as shown in figure 1.
Examples of restoration according to the above flow chart are as follows:
in the DVB-S2 communication link, the modulation mode is QPSK, the frame length l=8100, the corresponding LDPC coding parameter is n=16200, the offset d=11 when k=7200 physical layer scrambling sequences, and the signal to noise ratio is 0dB. The process of reconstructing the physical layer scrambling code sequence by the receiving end is as follows:
a) Constructing pseudo-random sequences x and y;
b) Obtaining a check matrix H according to n=16200 and k=7200;
c) Let l=0, 1, & 2 18 -2, and at each value of l, calculating R l Descrambling to obtain S l Performing constellation diagram inverse mapping according to the modulation mode to obtain c l Thereby obtaining the checksum chs l ，chs l The variation with l is shown in figure 2, and only 0-21 is taken for the convenience of observing l;
d) Thereby can be obtained
e) According to
Structure->
From the above application examples, it can be seen that the physical layer scrambling code sequence is correctly recovered according to the method of the present invention, and has a strong error resistance.
Claims (3)
1. A physical layer scrambling code sequence recovery method in DVB-S2 standard is characterized in that: traversing all possible scrambling sequences by using the relation between the check matrix of the low density parity check code and the code word and taking the minimum check sum as a judgment criterion, performing inverse mapping of the constellation diagram under each scrambling sequence, and calculating the check sum of the LDPC code word to obtain a scrambling sequence R corresponding to the check sum d Then the scrambling sequence R is recovered d ；
When the coding parameters n, k are known, the sequence S of symbols is received from c Medium recovery scrambling code sequence
The specific steps of (a) are as follows:
(1) Constructing pseudo-random sequences x and y;
(2) Obtaining a check matrix H according to the coding parameters n and k; where n is the codeword length and k is the message packet length;
from the message packet length k and codeword length n, a message packet m= [ m ] is derived 1 ,m 2 ,…,m k ]And the corresponding codeword c= [ c ] 1 ,c 2 ,…,c n ]Relationship between:
c＝mG；
g is a kXn-dimensional matrix, called a generator matrix;
there is an (n-k) ×n-dimensional matrix H satisfying:
GH T ＝0 k×(n-k) ；
the matrices H and G are corresponding check matrices, wherein 0 k×(n-k) Representing a k× (n-k) dimensional all-zero matrix;
(3) Taking a set offset l, and generating a corresponding set scrambling code sequence R by making the set offset l=0 l Setting the scrambling code sequence R l Descrambling is performed according to the following formula to obtain a sequence S l ：
S l (i)＝S c (i)exp(-jR l (i)π/2)
Wherein: i=0, 1, …, L-1, L is the frame length in symbols;
(4) The pseudo-random sequence z is calculated according to the following formula l Setting scrambling code sequence R l ：
R l (i)＝2z l (mod(i+131072,2 18 -1))+z l (i)，i＝0,1,...,L-1；
(5) By using the set scrambling code sequence R l For symbol sequence S c Descrambling was performed according to the following formula:
S l (i)＝S c (i)exp(-jR l (i)π/2)，i＝0,1,…,L-1；
(6) According to modulation mode pair S l Performing constellation diagram inverse mapping to obtain an n-dimensional row vector c l ；
(7) Calculate check word ch l Checksum chs l ：
ch l ＝c l H T ，
(8) If l < 2 18 -2, taking l=l+1, going to step (4), otherwise going to step (9);
(9) Estimating an offset:
wherein the method comprises the steps of
Estimated value representing offset, +.>
Representing the get chs l A minimum value of l;
(10) Recovering scrambling sequences
2. The method for recovering the physical layer scrambling code sequence in the DVB-S2 standard according to claim 1, wherein: according to DVB-S2 standard, when obtaining different values of n and k, the generation method of check matrix H can obtain H according to n and k, and the values of n and k are determined according to the information in the physical frame head.
3. The method for recovering physical layer scrambling code sequences in DVB-S2 standard according to claim 1 wherein step (7) calculates check word ch l Checksum chs l And when the checksum is minimum, the judgment criterion is that:
if there is no error, c when l=d l ＝c d Is an LDPC codeword, its checksum chs l ＝chs d ＝0；
Chs when l=d, if there is an error code l ＝chs d Minimum should be taken.
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