CN105635026B - A kind of position modulation in spread spectrum communication and soft demodulating method - Google Patents

Abstract

The invention discloses a position modulation and soft demodulation method in spread spectrum communication, which belongs to the technical field of wireless communication. The present invention includes two parts: the algorithm of the transmitting end and the algorithm of the receiving end; at the transmitting end, multiple copies of different cyclic shifts obtained after a certain original spreading sequence is shifted are used as the spreading sequence; at the receiving end, the present invention adopts The soft demodulation method uses the cyclic shift of the received sequence to correlate with the local sequence, and then uses the demodulation algorithm to calculate the soft demodulation value of the amplitude modulation data bit and the soft demodulation value of the position bit through the correlation value at different positions. Finally, it cooperates with the soft decision of the decoder to obtain the decoded data. The present invention combines the traditional modulation mode with the C-ary sequence position modulation, so that each modulation symbol can carry more data of log ₂ C bits, and improve the spectrum utilization rate; in the position demodulation, the method of soft demodulation is used to obtain the position bits, which can be combined with the soft demodulation method of encoding and reduce the bit error rate of the system.

Description

A Position Modulation and Soft Demodulation Method in Spread Spectrum Communication

technical field

The invention relates to a position modulation and soft demodulation method in spread spectrum communication, belonging to the technical field of wireless communication.

Background technique

In the spread spectrum communication system, the transmitter uses a high-speed pseudo-random code whose bandwidth is much larger than the signal bandwidth to modulate, so that the bandwidth of the original data signal is expanded, and then modulated by the carrier and sent out. The receiving end uses exactly the same pseudo-random code for correlation processing with the received bandwidth signal, and replaces the broadband signal with the narrowband signal of the original information data, that is, despreading, to realize information communication.

Common sequences in spreading codes include m-sequence and Gold sequence. The m-sequence is a code sequence with a period of 2 ⁿ -1 generated by an n-stage linear shift register, and is an abbreviation for the longest linear shift register sequence. This sequence has excellent characteristics such as long period, easy generation, and good randomness. The Gold sequence is obtained by adding a pair of m-sequences with the same period and rate but different codewords by modulo 2. A preferred pair refers to two m-sequences in the set of m-sequences whose absolute value of the maximum cross-correlation function is smaller than certain two values. Gold sequences consist of preferred pairs of m sequences. The preferred pair of m-sequences means that the cross-correlation function of two m-sequences of the same length has ternary value, that is, only three values will appear in the correlation value:

The spectrum efficiency of the traditional spread spectrum system is low, if the data rate is to be increased, a higher order modulation method must be adopted. If the data rate is increased without changing the modulation and coding method, some information can be carried in the sequence position, that is, the sequence after the spread spectrum is cyclically shifted, and the number of bits shifted is determined by the position bit, so that it can be used in different bandwidths. Increase the data rate under changing conditions. Different shifting methods will cause the correlation value between the shifted spreading code and the local spreading code to be different, thus resulting in different demodulation performance of the receiver.

In position demodulation, the traditional demodulation method is to correlate the received sequence with the local sequence after different shifts in turn, and find the peak value and the shifted digits of the corresponding local sequence by comparing the magnitude of the correlation value several times. In this way, all position bits can be solved at one time. However, this hard demodulation method cannot be used in conjunction with the encoded soft decision. If the correlation peak position is affected by noise and changes, the position data obtained by hard demodulation will inevitably be wrong, and the position bits that control the shift number may be all wrong. .

Contents of the invention

The purpose of the present invention is to propose a position modulation method in spread spectrum communication and its soft demodulation method, which includes two parts, the transmitter algorithm and the receiver algorithm.

At the transmitting end, the present invention uses a plurality of copies of different cyclic shifts obtained after an original spreading sequence is shifted as the spreading sequence, and transmits data by shifting information. Since the cross-correlation characteristic of the Gold sequence has three values, that is, except for the correlation value at which the cyclic shift is 0, other correlation values will only have the same three values as the preferred pair of m-sequences of the same length: Therefore, when the spread spectrum sequence is cyclically shifted to a position with a correlation value of -1, that is, the number of shifted steps corresponds to the position of -1 in the autocorrelation function of the spread spectrum sequence. It enables the system to obtain the lowest bit error rate, so that a higher transmission data rate can be achieved.

The specific steps of position modulation at the transmitting end are: assuming that the received signal sequence is r(n), the local spreading code sequence is s(n), the length is N, and the number of position modulation is C, then there can be 2 ^C Different shift values, the sequence referenced during shifting is the position sequence p(n), and the elements of the position sequence p(n) are different shift values. It can be seen that the spreading sequence is shifted according to the elements of p(n) The correlation value with the original spreading sequence should be -1, and the length of the position sequence p(n) should be C. The number of shifting steps required for each symbol constitutes a shifting sequence p _m (n). Then the modulation steps are as follows:

(1) grouping the data to be modulated, each group comprising M bits for data modulation and log ₂ C bits for position modulation;

(2) Map M bits used for data modulation to data on the constellation diagram, and use a spreading code to spread;

(3) The log ₂ C bits used for position modulation in each group are converted into decimal shift numbers to form the nth element of the shift sequence p _m (n). If the log ₂ C bits used for position modulation are all 0, no shifting is performed;

(4) Substituting the displacement sequence p _m (n) into the position sequence _p ( _n ) to form a displacement step sequence p _step (n), wherein p _step (n)=p(p _m (n));

(5) Carry out cyclic right shift (or left shift) to the spread spectrum symbol that each grouping forms, and the number of shifting steps is the corresponding symbol in p _step (n), namely the nth symbol cyclically shifts to the right (or Move left) p _step (n) steps.

At the receiving end, the present invention adopts the soft demodulation method, first uses the received sequence cyclic shift to correlate with the local sequence, and then uses the demodulation algorithm to calculate the amplitude modulation data bit soft demodulation value and The soft demodulation value of the position bit, and finally cooperate with the soft decision of the decoder to obtain the decoded data.

The specific steps of demodulation are as follows:

(1) Perform cyclic left shift (or right shift) on the received spread spectrum sequence according to the elements of the position sequence p(n), and correlate with the local spread spectrum code sequence s(n) after each shift. The cyclic shift of the received spreading sequence is equivalent to the cyclic shift of the local spreading code sequence, i.e.,

Among them, %N represents the remainder of N, s ^* [n] represents the conjugation of the local spreading code sequence, and uses a sequence d(k) to store C correlation values after performing C shift correlations;

(2) Select the largest modulus among the C correlation values, directly read the correlation value and demodulate it to obtain M bits for data modulation;

(3) use Calculate the soft demodulation value of the kth bit of the position data, |□| represents modulo operation, k=1,2,...log ₂ C. Obtain soft demodulation values of all log ₂ C bits used for position modulation;

(4) The soft demodulation values of M bits used for modulating data and the soft demodulation values of log ₂ C bits used for position modulation are combined into one group. All packets are sent to the decoder in order to complete the demodulation and decoding process.

The advantages of the present invention are:

(1) The traditional modulation method is combined with the C-ary sequence position modulation, so that each modulation symbol can carry more log ₂ C bits of data, and the spectrum utilization rate is improved.

(2) Find the modulation position with the best performance based on the position modulation of the C-ary sequence.

(3) In the position demodulation, the soft demodulation method is used to obtain the position bits, which can be combined with the coded soft demodulation method to reduce the system bit error rate.

Description of drawings

Figure 1 is a schematic diagram of the 63-point Gold sequence autocorrelation function.

Fig. 2 is a schematic diagram of sequence position modulation to a correlation value of -1.

Fig. 3 is a performance comparison of sequences modulated to different correlation value positions.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

An embodiment of position modulation and soft demodulation method in a kind of spread spectrum communication of the present invention is given below, wherein the generation of spread spectrum code, the position modulation of sequence, comprise the following steps:

(1) As shown in Figure 1, a Gold sequence with a length of 63 points is used. The initial state is [010110], the tap position is [16] m-sequence and another initial state is [000101], and the tap position is [1256] The m-sequence is done modulo 2 and generated.

(2) The modulation mode is selected as QPSK+binary position modulation, the shift mode is circular right shift, and the channel coding is Turbo code with coding efficiency R=1/2. Here we simulate the performance of the position modulation position at -1 of the correlation value, and at the same time simulate the modulation at the larger value and smaller value of the correlation value except the peak value (for the 63-point long sequence, the corresponding correlation value is 15 and -17) for comparison, as shown in Figure 2, where the position bit is 0 to indicate that the sequence is not shifted.

(3) The received position-modulated spread spectrum sequence is cyclically shifted to the left according to the number of shift steps of the position modulation, and correlated with the local spread spectrum code sequence after each shift. Use a sequence d _QPSK (n) to store the correlation value twice after performing two shift correlations;

(4) select the group with the larger modulus value in the two correlation values, and directly read the real part and the imaginary part of the correlation value to obtain the QPSK data of 2 bits;

(5) Utilize |d _QPSK (1)|-|d _QPSK (2)| to obtain the soft demodulation value of the position bit, and |□| represents a modulo operation;

(6) Send the QPSK data and position demodulation data to the turbo decoder, and the decoding method adopts Viterbi decoding.

(7) Compare the change curves of the bit error rate with the signal-to-noise ratio when modulated to different positions, and find the modulation position with the best performance.

The simulation results are shown in Figure 3. It can be seen that the performance of the position modulated to the smaller value -17 is significantly worse than that of the other two positions. The performance of modulating to the -1 position and modulating to the larger value of 15 is relatively close. When the signal-to-noise ratio is low, the performance of modulation at position 15 is slightly better than position -1, but when the ratio of signal-to-noise is high, the performance of modulation with autocorrelation value -1 is significantly better than position 15, and the signal-to-noise ratio The larger the value is, the more obvious the effect is (BER=0 when the curve is modulated to the autocorrelation value -1 in the figure when SNR=-6).

Claims (1)

1. position modulation and soft demodulation method in a kind of spread spectrum communication, comprise transmitting end algorithm and receiving end algorithm two parts, it is characterized in that: At the transmitting end, multiple copies of different cyclic shifts obtained after an original spreading sequence is shifted are used as spreading sequences, and data is transmitted by shifting information, and the spreading sequence is cyclically shifted to a correlation value of - When at the position of 1, that is, the number of steps of the shift corresponds to the position of -1 in the autocorrelation function of the spread spectrum sequence; if the received signal sequence is r(n), the local spread spectrum code sequence is s(n), The length is N, the position modulation base number is C, then there are 2 ^C different shift values, the reference sequence when shifting is the position sequence p(n), and the elements of the position sequence p(n) are different shift values It can be seen that the correlation value between the spreading sequence and the original spreading sequence should be -1 after being shifted according to the elements of p(n), and the length of the position sequence p(n) should be C; each symbol needs to be shifted The number of bit steps constitutes the shift sequence p _m (n), and the modulation steps are as follows: (1) grouping the data to be modulated, each group comprising M bits for data modulation and log ₂ C bits for position modulation; (2) Map M bits used for data modulation to data on the constellation diagram, and use a spreading code to spread; (3) Convert the log ₂ C bits used for position modulation in each group into decimal shift numbers to form the nth element of the shift sequence p _m (n) of each group; if log ₂ C bits are used If the bits modulated at the position are all 0, no shifting is performed; (4) Substituting the displacement sequence p _m (n) into the position sequence p (n) to form a displacement step sequence p _step (n), wherein, p _step (n)=p(p _m (n)); (5) The spread spectrum symbols formed by each group are cyclically shifted to the right or left, and the number of shift steps is the corresponding symbol in p _step (n), that is, the nth symbol is cyclically shifted to the right or to the left by p _step (n) steps; At the receiving end, the soft demodulation method is adopted, and the received sequence cyclic shift is used to correlate with the local sequence first, and then the correlation values at different positions are used to calculate the soft demodulation value of the amplitude modulation data bit and the position bit by using the demodulation algorithm. The soft demodulation value, and finally cooperate with the soft decision of the decoder to obtain the decoded data; The specific steps of soft demodulation at the receiving end are as follows: (1) Carry out cyclic left shift or right shift to the received spread spectrum sequence according to the elements of the position sequence p(n), and correlate with the local spread spectrum code sequence s(n) after each shift; the received The cyclic shift of the spreading sequence is equivalent to the cyclic shift of the local spreading code sequence, that is, Among them, %N represents the remainder of N, s ^* [n] represents the conjugation of the local spreading code sequence, and uses a sequence d(k) to store C correlation values after performing C shift correlations; (2) Select the largest modulus among the C correlation values, directly read the correlation value and demodulate it to obtain M bits for data modulation; (3) use Find the soft demodulation value of the kth bit of the position data, |·| represents the modulo operation, k=1,2,...log ₂ C; get all the soft demodulation values of log ₂ C bits used for position modulation demodulation value; (4) the soft demodulation values of every M bits used for modulating data and the soft demodulation values of log ₂ C bits used for position modulation form 1 packet; all packets are sent into the decoder in sequence, That is, the demodulation and decoding process is completed.