CN113225284B - 8FSK incoherent soft decision demodulation method for high-performance channel coding and decoding - Google Patents
8FSK incoherent soft decision demodulation method for high-performance channel coding and decoding Download PDFInfo
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Abstract
The invention discloses an 8FSK incoherent soft decision demodulation method for high-performance channel coding and decoding, belonging to the technical field of wireless communication. The method is realized in a digital domain, and firstly, a baseband signal at a receiving end is sampled; then, on the basis of strict receiving and transmitting synchronization, carrying out fast Fourier transform on sampling points of a single symbol period; then calculating the fast Fourier output amplitude corresponding to 8 modulation frequencies according to the frequency interval and the fast Fourier point number set by the 8FSK modulation; and finally, carrying out joint operation on the amplitudes corresponding to the 8 frequencies to obtain soft information corresponding to each bit after symbol demodulation. The method can solve the problem of soft decision demodulation at the receiving end of the 8FSK modulation communication system, and obviously improves the decoding performance of error correction coding at the receiving end.
Description
Technical Field
The invention belongs to the technical field of wireless communication, in particular to an 8FSK incoherent soft decision demodulation method for high-performance channel coding and decoding, which can be used for various wireless communication transmission systems adopting 8FSK modulation and demodulation.
Background
The wireless communication system has various modulation modes such as amplitude modulation, phase modulation and Frequency modulation, wherein FSK (Frequency-Shift Keying) is relatively insensitive to the multipath effect of a wireless transmission channel and channel fading, and is relatively widely applied to the wireless communication system. With the development of coding and decoding technology, high-performance channel coding and decoding put new requirements on a demodulation mode of FSK modulation, and conventional hard decision demodulation cannot be applied to channel coding modes such as convolutional codes, Turbo codes and LDPC codes.
And Modulation modes such as MPSK (Multiple Phase Shift Keying) and MQAM (Multiple quadrature amplitude Modulation) can calculate the soft information bit by bit corresponding to each symbol by using a maximum likelihood posterior probability formula on the basis of quadrature demodulation by calculating Euclidean distance of constellation point distribution. However, the euclidean distance cannot be calculated by using the constellation point distribution during FSK demodulation, thereby increasing the difficulty of acquiring soft information of each bit.
How to effectively acquire bit soft information corresponding to the FSK modulation symbol at a receiving end of a communication system becomes a key for the combined application of the FSK modulation mode and the high-performance channel coding and decoding mode. For 2FSK modulation, the Euclidean distance of the two frequency points is calculated through the energy difference value of the two frequency points, and then soft-decision demodulation information is obtained. For 4FSK modulation, a single symbol corresponds to 2 bits, energy values of 4 frequency points are calculated at a receiving end, and then the 4 energy values are combined according to a modulation mapping relationship, so as to obtain 2-bit soft-decision demodulation information in the single symbol (see "a 4FSK soft-demodulation method" in all-the-go, all-the-west, and patent CN 1852279 a). In addition, for FSK modulation transmission system adopting fixed Viterbi decoding mode, M frequency energy values corresponding to FSK modulation symbol can be used as branch metric value of decoder to directly make Viterbi decoding so as to avoid calculation of no bit soft information of FSK modulation symbol (refer to ' energy measurement Viterbi soft decoding algorithm performance analysis of M-FSK signal ' in Dongbixian, Tangpeng, Duyang, etc. ' electronic and informatics declaration, volume 37, period 8, page number: 1920-. With the advancement of signal digitization processing technology, the frequency estimation of the received symbols by FFT can realize hard decision demodulation of FSK modulation scheme, which can estimate the energy of FSK signal frequency points, while avoiding the complex carrier recovery problem of coherent demodulation (see "FFT demodulation method of FSK signal under short wave fading channel" in queawa, radio communication technology, volume 41, phase 6, page number: 27-31).
For the FSK modulation mode, as the modulation order increases, the bandwidth required by transmission increases to a certain extent, but the signal-to-noise ratio E required by system demodulation b /N 0 The threshold is reduced, so that in the transmission system with strong channel noise and not strict limitation to transmission bandwidth, the high-order modulation such as 8FSK is more suitable to be adopted, and the E required by effective demodulation of the transmission system is further reduced b /N 0 It is necessary to ensure that 8FSK can implement soft decision demodulation to meet the application requirements of high performance decoding modes of Turbo codes and LDPC codes.
Disclosure of Invention
In view of this, the present invention provides an 8FSK incoherent soft decision demodulation method for high performance channel coding and decoding, which is capable of implementing the combination of the FSK modulation mode and the high performance channel coding and decoding for a communication system using the FSK modulation mode, and improving the transmission performance of the transmission system under a low bit signal-to-noise ratio.
In order to achieve the purpose, the invention adopts the technical scheme that:
an 8FSK incoherent soft-decision demodulation method for high-performance channel coding and decoding comprises the following steps:
(1) adopting full digital domain processing to convert the received signal into baseband in quadrature and then carrying out ADC sampling with the sampling frequency f s MR, where R is the symbol rate and M is a positive integer;
(2) setting the number of points of FFT to be N, N > M, and setting M sampling points r in the received 8FSK symbol period to be (r) 1 r 2 … r M ) 1×M Zero-filling to N to obtain r ═ r 1 r 2 … r M 0 … 0) 1×N ；
(3) Performing N-point FFT operation on the sequence r' after zero padding to obtain a sequence X ═ X 1 ,…,x N ) 1×N And calculating 3-bit soft information corresponding to a single 8FSK symbol by using X to obtain 8FSK soft-decision demodulation output.
Further, the specific way of calculating the 3-bit soft information corresponding to the single 8FSK symbol by using X in step (3) is as follows:
(3-1) frequency spacing f in accordance with 8FSK modulation parameters Δ Calculating the frequencies f corresponding to different symbols in the 8FSK modulated baseband signal 1 To f 8 8 modulation symbols and frequency values f 1 To f 8 Between them, using Gray coding, f 1 To f 8 Respectively as follows:
(3-2) from the sampling frequency f s Calculating the position sequence (K) of the sequence X corresponding to the 8 frequency values according to the FFT point number N 1 ,K 2 ,K 3 ,K 4 ,K 5 ,K 6 ,K 7 ,K 8 ) The calculation method is as follows:
wherein round (, denotes an integer, i ═ 1, 2., 8;
(3-3) calculating position K i The amplitude of the corresponding 8 samples of the FFT transformed sequence, i.e.
And calculating 3-bit soft decision information of the modulation symbol according to the amplitudes of the 8 sampling points.
Further, the specific way of calculating the 3-bit soft decision information of the modulation symbol according to the amplitudes of the 8 samples in step (3-3) is as follows:
(3-3-1) the 8 modulation symbols of the 8FSK are represented as (b) 1 ,b 2 ,b 3 )，b 1 ,b 2 ,b 3 All values of (A) are 0 or 1;
b is to 1 The sample amplitudes of the 4 modulation symbols corresponding to 0 are recorded as
b 1 The sample amplitudes of the 4 modulation symbols corresponding to 1 are recorded as
B is to 2 The sample amplitudes of the 4 modulation symbols corresponding to 0 are recorded as
B is to 2 The sample amplitudes of the 4 modulation symbols corresponding to 1 are recorded as
B is to 3 Sampling point of 4 modulation symbols corresponding to 0The amplitude is recorded as
B is to 3 The sample amplitudes of the 4 modulation symbols corresponding to 1 are recorded as
(3-3-2) calculating bits b, respectively j Vectors corresponding to 0 and 1
And
the element mean values of (1) are respectively recorded as
And
j＝1,2,3；
(3-3-3) is prepared from
And
determining 3-bit soft decision information for 8FSK modulation symbols
Namely, it is
The invention has the beneficial effects that:
1. the method does not need to carry out complex carrier recovery at a receiving end, and is suitable for the 8FSK communication transmission system adopting high-performance channel coding modes such as convolutional codes, Turbo codes, LDPC codes and the like.
2. The invention adopts a guiding auxiliary synchronization mode based on multi-path differential weighting correlation, and can solve the problem of 8FSK demodulation adaptive to channel coding soft information decoding input in a communication transmission system.
3. The invention has the same performance as the conventional incoherent demodulation method, can provide soft decision information for a decoder at a receiving end, further remarkably improves the error correction performance of codes such as convolutional codes and LDPC (low density parity check) and is suitable for an 8FSK modulation-demodulation transmission system which needs the soft decision information for decoding.
Drawings
Fig. 1 is a schematic diagram of a soft-decision demodulation method according to an embodiment of the present invention.
Fig. 2 is a graph comparing the performance of the demodulation method according to the embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the detailed description.
As shown in fig. 1, an 8FSK incoherent soft decision demodulation method for high performance channel coding and decoding is implemented by performing analog domain processing on a received signal at a receiving end of an 8FSK communication transmission system through low-noise amplification, filtering and down-conversion, digitizing the signal, and simultaneously ensuring that the center frequency thereof is shifted to 0Hz by analog or digital frequency conversion to be converted into a baseband signal.
For processing, before processing the digital domain received signal, setting the ADC sampling clock or performing subsequent interpolation, decimation and other processes, and setting the final digital domain signal sampling rate f s Becoming an integer multiple of the 8FSK modulation symbol rate R, i.e. f s MR, M is a positive integer.
And setting FFT parameters, taking the number of FFT points as N, wherein the number of the FFT points is required to be an integer power of 2 in order to ensure the realization of the fast Fourier transform algorithm. When FFT processing is performed on a baseband reception signal, the FFT processing is performed in units of 8FSK symbols.
Due to 8FSK baseband signal sampling rate f s The symbol rate R is not much greater than that of the sample rate R, and the number of FFT points N is generally greater than the number of sampling points M in a single 8FSK symbol period, so zero padding is required before FFT transformation of M sampling points, and a sequence with length N after zero padding is denoted as R' ═ R (R ═ R) 1 r 2 …r M 0…0) 1×N . After r' is subjected to FFT, an output sequence X with the length of N is obtained (X) 1 ,…,x N ) 1×N 。
Determining corresponding parameters required for 8FSK soft decision demodulation according to frequency interval f in 8FSK modulation parameters Δ Calculating the frequencies corresponding to different symbols in the 8FSK modulated baseband signal, wherein the 8FSK symbols are used as (b) 1 ,b 2 ,b 3 ) Is represented by b i Is 0 or 1, 8 modulation symbols and frequency value f 1 To f 8 The gray coding mode is usually adopted, namely:
calculating 8 frequencies, f, of an 8FSK modulation symbol 1 To f 8 At the position (K) corresponding to the FFT transformed output sequence X 1 ,K 2 ,K 3 ,K 4 ,K 5 ,K 6 ,K 7 ,K 8 ) The specific process is as follows:
where round (×) denotes an integer.
For 8FSK modulation, the symbol may be represented as (b) 1 ,b 2 ,b 3 )，b 1 ,b 2 ,b 3 All the values of (1) are 0 or 1, each symbol corresponds to 3 bits, and 8 positions K are set according to the value of each bit as 0 or 1 i Corresponding to the elemental grouping in X.
Bit 1 b 1 The 4 sampling point amplitudes corresponding to 0 are recorded as 1 group
B is to 1 The amplitudes of 4 samples corresponding to 1 are recorded as 1 group
2 nd bitb 2 The 4 sampling point amplitudes corresponding to 0 are recorded as 1 group
B is to be 2 The amplitudes of 4 samples corresponding to 1 are recorded as 1 group
Bit 3 b 3 The 4 sampling point amplitudes corresponding to 0 are recorded as 1 group
B is to 3 The sample amplitudes of 4 modulation symbols corresponding to 1 are recorded as 1 group
Separately calculating vectors
And
mean value of the elements, note
And
mean value of
And
respectively calculating 3-bit soft decision information of 8FSK modulation symbols
The relationship is calculated as follows:
the following is a more specific example:
compared with 4FSK and 2FSK, the 8FSK modulation mode needs lower bit signal to noise ratio for effective demodulation under the condition of the same error rate, and simultaneously adopts LDPC channel coding with performance approaching to Shannon limit to ensure the reliability of information transmission.
The LDPC coding rate adopted by an 8FSK modulation wireless communication system is set to be 1/2, the code length is set to be 1792, the transmission information rate is 9.6Mbps, and the coded modulation symbol rate is 9.6 Mbps/(1/2)/(log) 2 8) The modulation frequency interval of different symbols is 7.04Mbps, 6.4 MSps. At the receiving end of the system, the sampling rate is set to be 16 times of the symbol rate, namely 102.4MHz, and the number of FFT points is 1024. On the basis of the parameters, 8 frequency points for receiving baseband 8FSK signals are respectively as follows: -24.64MHz, -17.6MHz, -10.56MHz, -3.52MHz, 10.56MHz, 17.6MHz and 24.64 MHz. And further calculating the positions in the FFT conversion output sequences corresponding to the 8 frequency points, wherein the positions are respectively as follows: 778. 848, 918, 989, 36, 107, 177, and 247.
And under the parameter conditions, performing transmission performance simulation by adopting a Gaussian white noise channel, and counting the average 1000 Monte Carlo simulation results. Fig. 2 shows an 8FSK hard-decision demodulation performance curve, an 8FSK soft-decision demodulation performance curve of the method of the present invention, and a 4FSK soft-decision demodulation performance curve. As can be seen from comparison of three performance curves in the figure, the method can effectively improve the transmission performance of the 8FSK modulation communication system, and the bit signal-to-noise ratio E b /N 0 And when the bit error rate is 5.7dB, the bit error rate is reduced by more than 3 orders of magnitude compared with hard decision demodulation, and the bit error rate is reduced by more than 4 orders of magnitude compared with 4FSK soft decision demodulation.
In summary, the method estimates the energy values of the received symbols at different frequency points through FFT conversion, and simultaneously calculates the corresponding distribution distances of different bits in the symbols by using different combinations of the energy values, thereby obtaining the soft decision information of the bits as the input of the high-efficiency channel coder-decoder, and ensuring that the performance of Viterbi decoding and iterative decoding can be optimized as much as possible. The invention can well solve the problem of combination with a high-performance channel coding mode, reduce the threshold of the signal-to-noise ratio required by the demodulation of a receiving end, effectively improve the overall transmission performance of a link, and is suitable for various communication systems adopting 8FSK modulation modes.
Claims (1)
1. An 8FSK noncoherent soft-decision demodulation method for channel coding and decoding, characterized by comprising the following steps:
(1) adopting full digital domain processing to convert the received signal into baseband in quadrature and then carrying out ADC sampling with the sampling frequency f s MR, where R is the symbol rate and M is a positive integer;
(2) setting the number of points of FFT to be N, N > M, and setting M sampling points r in the received 8FSK symbol period to be (r) 1 r 2 …r M ) 1×M Zero-filling to N to obtain r ═ r 1 r 2 …r M 0…0) 1×N ；
(3) Performing N-point FFT operation on the sequence r' after zero padding to obtain a sequence X ═ X 1 ,…,x N ) 1×N Calculating 3 bit soft information corresponding to a single 8FSK symbol by using X to obtain 8FSK soft decision demodulation output; the specific way of calculating 3-bit soft information corresponding to a single 8FSK symbol by using X is as follows:
(3-1) frequency spacing f in accordance with 8FSK modulation parameters Δ Calculating the frequencies f corresponding to different symbols in the 8FSK modulated baseband signal 1 To f 8 8 modulation symbols and frequency values f 1 To f 8 Between them, using Gray coding, f 1 To f 8 Respectively as follows:
(3-2) from the sampling frequency f s Calculating the position sequence (K) of the sequence X corresponding to the 8 frequency values according to the FFT point number N 1 ,K 2 ,K 3 ,K 4 ,K 5 ,K 6 ,K 7 ,K 8 ) The calculation method is as follows:
wherein round (, denotes an integer, i ═ 1, 2., 8;
(3-3) calculating position K i Amplitude of 8 samples of the corresponding FFT transformed sequence, i.e.
Calculating 3-bit soft decision information of the modulation symbol according to the amplitudes of the 8 sampling points, wherein the specific mode is as follows:
(3-3-1) the 8 modulation symbols of the 8FSK are represented as (b) 1 ,b 2 ,b 3 )，b 1 ,b 2 ,b 3 All values of (A) are 0 or 1;
b is to 1 The sample amplitudes of the 4 modulation symbols corresponding to 0 are recorded as
b 1 The sample amplitudes of the 4 modulation symbols corresponding to 1 are recorded as
B is to 2 The sample amplitudes of the 4 modulation symbols corresponding to 0 are recorded as
B is to 2 The sample amplitudes of the 4 modulation symbols corresponding to 1 are recorded as
B is to be 3 The sample amplitudes of the 4 modulation symbols corresponding to 0 are recorded as
B is to 3 4 tones corresponding to 1The amplitude of the sample point for making the symbol is recorded as
(3-3-2) calculating bits b, respectively j Vectors corresponding to 0 and 1
And
the element mean values of (1) are respectively recorded as
And
(3-3-3) is prepared from
And
determining 3-bit soft decision information for 8FSK modulation symbols
Namely, it is
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| CN110912847A (en) * | 2019-11-26 | 2020-03-24 | 中国电子科技集团公司第四十一研究所 | GMSK signal demodulation method |
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| CN101335729A (en) * | 2008-05-19 | 2008-12-31 | 重庆重邮信科通信技术有限公司 | Fixed-point soft decision demodulation method of quadrature amplitude modulation technique |
| CN110971344A (en) * | 2019-11-20 | 2020-04-07 | 中国地质大学（武汉） | Soft demodulation method of linear frequency modulation spread spectrum modulation technology |
| CN110912847A (en) * | 2019-11-26 | 2020-03-24 | 中国电子科技集团公司第四十一研究所 | GMSK signal demodulation method |
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