Method for implementing common pilot-based CPM state zeroing and carrier synchronization

Abstract

The application discloses a CPM state zeroing and carrier synchronization implementation method based on common pilot, and mainly solves the problem of system performance reduction caused by not using pilot as zeroing symbol in the prior art.The scheme is as follows: a pseudo-random sequence is selected to modulate to obtain a synchronization head signal; a pilot symbol with zero pilot is selected to be placed after an information sequence to form an information pilot sequence, and the information pilot sequence is modulated to obtain a state zeroing signal; the zeroing signal is combined with the synchronization head signal and transmitted; at a receiving end, frequency offset estimation and compensation are performed on a received signal to obtain a frequency offset compensated signal; a coarse estimation value of phase offset is calculated according to the frequency offset compensated signal, a phase offset search interval is determined, and an accurate estimation value of the phase offset is found; the accurate estimation value is used to perform phase offset compensation on the frequency offset compensated signal again to obtain a carrier synchronized signal and demodulate the carrier synchronized signal.The application can reduce the error probability of demodulation, improve the error code performance of the system, and can be used in a high-speed transmission short burst digital communication system.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology, and specifically relates to a method for implementing CPM state zeroing and carrier synchronization, which can be used in high-speed short burst digital communication systems. Background Technology

[0002] Short burst signals, with their short duration, difficulty in acquisition and detection, and strong anti-interference capabilities, have made short burst broadband communication systems widely used in high-speed mobile communications, satellite communications, and military communications. However, the Doppler effect caused by the relative movement of the communicating parties results in a significant frequency offset in the received signal, while the available synchronization overhead is quite limited. Therefore, a key problem to be solved is to achieve a burst communication system with high power utilization, high bandwidth utilization, and high reliability through carrier synchronization using efficient modulation techniques and low-overhead pilot sequences.

[0003] Continuous phase modulation (CPM) is a highly efficient modulation technique characterized by constant envelope and continuous phase. The instantaneous carrier frequency or phase of CPM contains all the information of the data signal. Due to the memory characteristics of the CPM phase, its carrier phase is continuous in time, thus avoiding spectral jumps and resulting in a relatively concentrated power spectral density and very low out-of-band power. Furthermore, nonlinear amplifiers can be used to process CPM signals. CPM's constant envelope characteristic can reduce the impact of fading channels.

[0004] In order to make the CPM signal periodically enter a known phase state, since the CPM signal is a modulation with memory, a return-to-zero symbol can be added after the unknown data symbol so that the CPM signal reaches a fixed phase state at the end of the data, thereby reducing the demodulation error probability and improving the system's bit error rate performance.

[0005] In his paper “Research on Carrier Synchronization of CPM Signal [D]. Xi’an University of Electronic Science and Technology, 2008,” Liu Yi mentioned that an additional return-to-zero symbol is added to the data frame structure of the CPM signal to make the CPM signal return to a fixed phase state at the end of the data.

[0006] In his paper “Research on Key Technologies of Continuous Phase Modulation Single Carrier Frequency Domain Equalization [D]. Zhejiang University, 2010,” Fu Tao proposed a method to return the phase state of the CPM signal to zero by inserting a cyclic prefix in the data block. This method is similar to the method mentioned by Liu Yi in his paper, both of which use additional symbol symbols to ensure that the phase of the CPM signal can return to the zero state.

[0007] Both of the above methods add an extra return-to-zero symbol in communication systems with extremely limited synchronization overhead. Since the total number of symbols available for synchronization and return-to-zero is limited, although this method can improve the system's bit error rate performance, it also reduces the number of symbols used for synchronization, further increasing the difficulty of system carrier synchronization and reducing the system's bit error rate performance. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of the prior art by proposing a method for achieving CPM state zeroing and carrier synchronization based on shared pilots. This method uses shared pilots and zeroing symbols to reduce the probability of demodulation errors and improve the system's bit error rate performance.

[0009] The technical idea of ​​this invention is to adjust the position of the pilot at the beginning of the data to the end of the data, so that the pilot can be used as a return-to-zero symbol for the CPM signal and also for carrier synchronization of the signal. That is, with extremely limited pilot overhead, the pilot is used for carrier synchronization and also as a return-to-zero symbol, thereby improving the bit error rate performance of the system.

[0010] Based on the above approach, the present invention provides a method for achieving CPM state zeroing and carrier synchronization based on a shared pilot, characterized by the following steps:

[0011] (1) Select the pilot sequence o with zero pilot symbol and place it after the information sequence c to form the information pilot sequence [co];

[0012] (2) Select a sequence with good pseudo-random characteristics and modulate it to obtain the synchronized head modulated signal u;

[0013] (3) Map and modulate the information pilot sequence [co] onto f c At a frequency of f, the modulated signal [dp] of the information pilot sequence is obtained, which is the signal whose state has been returned to zero. Then, it is combined with the modulated signal u of the synchronization head to obtain the modulated signal s = [u dp]; where f c d is the carrier frequency, d is the modulated data, and p is the modulated pilot signal;

[0014] (4) The modulated signal s is sent into the Gaussian white noise channel for transmission;

[0015] (5) Extract the receiver synchronization header u' from the received signal s' = [u' d' p'] at the receiving end;

[0016] (6) The frequency offset of the synchronization header u' is estimated using the RPA algorithm to obtain the estimated frequency offset of the received signal s'.

[0017] (7) The frequency offset of the received signal s' is calculated as follows: Frequency offset compensation is performed to obtain the frequency offset compensated signal s”=[u” d” p”], so as to eliminate the frequency offset of the received signal;

[0018] (8) Calculate a coarse estimate of the phase offset based on the frequency offset compensated signal s”.

[0019] (9) Based on the coarse estimate of the phase deviation Determine the phase bias search interval In this interval, a phase-biased search is performed using the mean square value of the demodulated soft information of the data as the objective function to find the mean square soft output function of the data. Maximum value This is the precise estimate of the phase bias, where Λ d (n) represents the soft value of the data bits after demodulation. It is half the length of the search interval;

[0020] (10) Using phase bias precise estimate The frequency offset compensated signal s” is then subjected to phase offset compensation to obtain the carrier synchronized signal r, which is then demodulated by CPM.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] First, compared to the pilot signal being located at the beginning of the data, this invention sets the pilot symbol to zero and places it at the end of the data. Therefore, without increasing the symbol overhead, the pilot signal at the end can be used for both carrier synchronization and as the return-to-zero symbol of the CPM signal. This allows the CPM signal to reach a fixed phase state at the end of the data, thereby reducing the probability of demodulation errors and improving the system's bit error rate performance.

[0023] Secondly, compared to adding a return-to-zero symbol at the end of the data, this invention places the pilot at the end and does not require adding a return-to-zero symbol. This can save unnecessary symbol overhead under the condition that the synchronization overhead is extremely limited. These saved symbols can be used for data transmission, thereby improving the efficiency of information transmission.

[0024] Third, the zero-return symbol of this invention uses tilted symbols. Adding an all-zero pilot sequence can bring the CPM signal state to zero, eliminating the need to calculate the pilot sequence based on the data symbols. Compared with existing zero-return symbol methods, this simplifies the computational complexity of the pilot and improves the efficiency of pilot selection. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the short burst system used in this invention;

[0026] Figure 2 This is a flowchart illustrating the implementation of the present invention;

[0027] Figure 3 This is a schematic diagram showing the pilot positions of the present invention and the prior art;

[0028] Figure 4 The figures show a comparison of the bit error rate performance simulations of the present invention and existing technologies. Detailed implementation method:

[0029] The embodiments and effects of the present invention will be further described below with reference to the accompanying drawings.

[0030] Reference Figure 1 This embodiment is applied to a high-speed, short-burst communication system. The system includes a transmitter and a receiver, and uses an additive white Gaussian noise channel as the channel model. The basic working principle of this system is as follows:

[0031] At the transmitting end, the pseudo-random sequence is modulated by MSK, and the information sequence and pilot sequence are modulated by CPM. Then, they are combined to obtain the modulated signal and sent into the channel for transmission.

[0032] At the receiving end, the received signal is carrier synchronized using the local synchronization header and the received synchronization header, as well as the local pilot and the received pilot. The carrier-synchronized received signal is then demodulated to obtain the transmitted binary information sequence.

[0033] Reference Figure 2 This invention is based on Figure 1 The system uses a shared pilot signal for CPM state zeroing and carrier synchronization, and its implementation steps include the following:

[0034] Step 1: The transmitting end selects a sequence with good pseudo-random characteristics and modulates it to obtain the synchronized head modulated signal u. It is then placed before the information sequence c and pilot sequence o that have been modulated by CPM to obtain the modulated signal s, which is then sent into the Gaussian white noise channel for transmission.

[0035] (1.1) At the transmitting end, a sequence with good pseudo-random characteristics is selected and MSK modulated to obtain the synchronized header modulated signal u:

[0036] u=[u(0),u(1),…,u(i),…,u(N u -1)]

[0037] Where u(i) represents the i-th sampling point in the local synchronization header u, i = 0, 1, ..., N u -1, N u The length of the modulated signal u of the synchronization head;

[0038] (1.2) The transmitting end uses the information sequence c and the pilot sequence o to construct the information pilot sequence [co]. The information sequence c in the information pilot sequence [co] is used to obtain the M-ary symbol sequence according to the Gray mapping. Each all-zero symbol in the pilot sequence o is mapped to -(M-1), and it is modulated onto f. c At the frequency, the information pilot modulated signal [dp] is obtained, and it is combined with the synchronization head modulated signal u to obtain the modulated signal s = [udp];

[0039] (1.3) The transmitting end sends the modulated signal s into the Gaussian white noise channel for transmission.

[0040] Step 2: The receiving end extracts the synchronization header u' from the received signal s'.

[0041] (2.1) The receiving end receives the signal sent by the transmitting end and obtains the received signal s':

[0042] s'=[u' d' p']

[0043] Where u', d', and p' represent the received synchronization header, information, and pilot signal, respectively;

[0044] (2.2) The m-th sample point in the received signal s' is represented as:

[0045]

[0046] Where m = 0, 1, ..., N-1, and Δf is the carrier frequency offset. For carrier phase offset, f s Where is the signal sampling rate, N is the length of the received signal s', w(m) represents complex Gaussian random noise with zero mean and variance N0 / 2, N0 is the one-sided power spectral density of the noise, and j is the complex unit;

[0047] (2.3) The receiving end extracts the synchronization header u' from the received signal s'.

[0048] Step 3: The receiving end uses the received synchronization header u' and the local synchronization header u to perform frequency offset estimation on the received signal s' using the RPA algorithm.

[0049] (3.1) Multiply the local synchronization header u and the received synchronization header u' by pointwise conjugate to obtain the demodulation sequence Z:

[0050] z(n)=u'(n)u * (n)

[0051] Where z(n) represents the nth bit of the demodulation sequence Z, u'(n) represents the nth sampling point in the received synchronization header u', u *(n) represents the conjugate of the nth sampling point in the local synchronization header u, where n = 0, 1, ..., N u -1;

[0052] (3.2) Perform L frequency offset rotations on the modulation sequence Z, with each rotation having a corresponding frequency offset of f. l L frequency shift sequences Z are obtained. l :

[0053]

[0054] in l = 0, ..., L-1, T is the symbol period, N fft The number of points in the FFT;

[0055] (3.3) For the frequency shift sequence Z l By plotting the average periodicity chart, we obtain the periodicity chart matrix C. k,l (f l ) and average period chart

[0056] C k,l (f l ) = FFT(Z l N fft )

[0057]

[0058] Where k = 0,...,N fft -1, l=0,...,L-1, N fft The number of points in the FFT;

[0059] (3.4) Search Periodic Graph Matrix C k,l (f l Find the two variables k corresponding to the maximum value in the matrix. m and l m ;

[0060]

[0061]

[0062] (3.5) Calculate the frequency offset estimate based on the above parameters.

[0063]

[0064] (3.6) The frequency offset of the received signal s' is... Frequency offset compensation yields the compensated received signal s":

[0065] s”=[u” d” p”]

[0066] Where u”, d”, and p” represent the synchronization header, information, and pilot after frequency offset compensation, respectively;

[0067] (3.7) The m-th sample of the compensated received signal s” is represented as:

[0068]

[0069] Where s'(m) represents the m-th sampling point in the received signal s', m = 0, 1, ..., N-1, f s denoted as the signal sampling rate, N as the length of the received signal s', and j as the complex unit.

[0070] Step 4: Based on the received signal after frequency offset compensation, the MSSO search algorithm is used to perform coarse and fine phase estimation.

[0071] (4.1) Extract pilot p” from the received signal s” after frequency offset compensation, multiply it by the local pilot p point by point conjugate, and sum the results. Take the angle value to obtain a coarse estimate of the phase.

[0072]

[0073] Where p”” is the nth sampling point of the pilot signal p” after frequency offset compensation. * (n) is the conjugate of the nth sampling point of the local pilot p, and K is the number of sampling points of the pilot.

[0074] (4.2) Based on the coarse estimate of the phase bias Determine the phase bias search interval In this interval, a phase-biased search is performed using the mean square value of the demodulated soft information of the data as the objective function to find the mean square soft output function of the data. Maximum value This is the precise estimate of the phase bias, where Λ d (n) represents the soft value of the data bits after demodulation. It is half the length of the search interval.

[0075] Step 5: Perform phase offset compensation on the frequency offset compensated signal s”.

[0076] (5.1) Using accurate phase deviation estimates Phase offset compensation is performed on the received signal s” after frequency offset compensation, that is, s” is corrected by complex phase rotation to obtain the carrier-synchronized signal r:

[0077]

[0078] Where r(m) represents the m-th sampling point of signal r after carrier synchronization, and s"(m) represents the m-th sampling point of received signal s" after frequency offset compensation.

[0079] (5.2) Demodulate the carrier-synchronized signal r to complete the CPM state zeroing and carrier synchronization based on the shared pilot.

[0080] The numbers used in the above steps are only for the purpose of clearly describing the solution of the present invention, and their order is not limited.

[0081] The effects of this invention can be further illustrated by the following simulations:

[0082] 1. Simulation conditions:

[0083] The simulation software is MATLAB R2017a.

[0084] The simulation data structure consists of 400 blocks in total, including a synchronization header and data blocks. The first 20 blocks are the synchronization header, and the remaining 380 blocks are data and pilot blocks. Each block contains 32 symbol symbols, with 30 data symbols and 2 pilot symbols in the data and pilot blocks. The data and pilot blocks use a 4CPM modulation scheme with M=4, modulation index h=1 / 4, memory length L=2, and a 2RC raised cosine pulse.

[0085] Channel-added carrier frequency offset Δf = 8kHz, random phase offset The symbol rate is 5 Msps.

[0086] Set the pilot position as follows: Figure 3 ,in:

[0087] Figure 3 (a) is the pilot position where the pilot is used as a return-to-zero symbol in this invention.

[0088] Figure 3 (b) Pilot locations where existing methods do not use pilots as return-to-zero symbols.

[0089] 2. Simulation content:

[0090] Under the above conditions, the present invention and existing methods were respectively used to... Figure 1 The system performs transmit and receive simulations to obtain a comparison of the system's bit error rate performance, such as... Figure 4 As shown.

[0091] from Figure 4 As can be seen, the system using pilots as return-to-zero symbols in this invention has a lower bit error rate, more accurate estimation, and lower signal-to-noise ratio threshold requirements. Its performance is significantly better than existing methods that do not use pilots as return-to-zero symbols. Furthermore, the performance graph shows that this invention achieves a bit error rate of 10% at a signal-to-noise ratio of 10dB.-3 Existing technologies only achieve a bit error rate of 10 when the signal-to-noise ratio is 13dB. -3 This invention achieves a signal-to-noise ratio gain of 3dB. This is because the pilot sequence is placed as a return-to-zero symbol at the end of the information sequence, causing the CPM signal to periodically enter a known phase state. The CPM signal is a modulation with memory; adding a return-to-zero symbol after the unknown data symbol ensures that the CPM signal reaches a fixed phase state at the end of the data, thereby reducing the demodulation error probability and improving the system's bit error rate performance.

[0092] In summary, this invention achieves higher error rate performance while saving return-to-zero symbols, greatly improving the transmission reliability of the system. It also saves the return-to-zero symbols following the CPM data symbols, thus improving the information transmission efficiency of the system.

[0093] The above description is merely a specific example of the present invention and does not constitute any limitation on the present invention. Obviously, those skilled in the art, after understanding the content and principles of the present invention, may make various modifications and changes in form and detail without departing from the principles and structure of the present invention. However, these modifications and changes based on the ideas of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A method for achieving CPM state zeroing and carrier synchronization based on a shared pilot, characterized in that, The following steps are required: (1) Select the pilot sequence o with zero pilot symbol and place it after the information sequence c to form the information pilot sequence [co]; (2) Select a sequence with good pseudo-random characteristics and modulate it to obtain the synchronized head modulated signal u; (3) Map and modulate the information pilot sequence [co] onto f c At a frequency of f, the modulated signal [dp] of the information pilot sequence is obtained, which is the signal whose state has been returned to zero. Then, it is combined with the modulated signal u of the synchronization head to obtain the modulated signal s = [udp]; where f c d is the carrier frequency, d is the modulated data, and p is the modulated pilot signal; (4) The modulated signal s is sent into the Gaussian white noise channel for transmission; (5) Extract the receiver synchronization header u' from the received signal s' = [u' d' p'] at the receiving end; (6) The frequency offset of the synchronization header u' is estimated using the RPA algorithm to obtain the estimated frequency offset of the received signal s'. (7) The frequency offset of the received signal s' is calculated as follows: Frequency offset compensation is performed to obtain the frequency offset compensated signal s”=[u” d”p”], so as to eliminate the frequency offset of the received signal; (8) Calculate a coarse estimate of the phase offset based on the frequency offset compensated signal s”. (9) Based on the coarse estimate of the phase deviation Determine the phase bias search interval In this interval, a phase-biased search is performed using the mean square value of the demodulated soft information of the data as the objective function to find the mean square soft output function of the data. Maximum value This is the precise estimate of the phase bias, where Λ d (n) represents the soft value after demodulation of the data bits. It is half the length of the search interval; (10) Using phase bias precise estimate The frequency offset compensated signal s” is then subjected to phase offset compensation to obtain the carrier synchronized signal r, which is then demodulated by CPM.

2. The method according to claim 1, characterized in that, Step (2) selects a sequence with good pseudo-random characteristics and performs MSK modulation to obtain the synchronized head modulated signal u, as shown below: u=[u(0),u(1),…,u(i),…,u(N u -1)] Where u(i) represents the i-th sampling point in the modulated signal u of the synchronization head, i = 0, 1, ..., N u -1, N u The length of the synchronized signal u is the length of the synchronized head.

3. The method according to claim 1, characterized in that, In step (3), the information sequence c in the information pilot sequence [co] is mapped to an M-ary symbol sequence according to the Gray mapping, and each all-zero symbol in the pilot sequence o is mapped to -(M-1), and then modulated onto f. c At the frequency, modulation is achieved through continuous phase modulation (CPM), and the formula is as follows: Where α is the M-ary symbol sequence mapped from the information pilot sequence, α = (α0, α1, ..., α) i ,...), α i ∈{±1,±3,±(M-1)}, where T and E represent the period and energy of the symbol α, respectively, and f c Let t be the carrier frequency, h be the modulation index, M be the symbol base, and q(t) be the phase pulse function.

4. The method according to claim 1, characterized in that, The received synchronization header u' extracted in step (5) is represented as follows: u'=[u'(0),u'(1),…,u'(i),…,u'(N u -1)] Where u'(i) represents the i-th sampling point, N u The length of the receive synchronization header u'.

5. The method according to claim 1, characterized in that, Step (6) uses the RPA algorithm to estimate the frequency offset of the synchronization header u', and obtains the estimated frequency offset value of the received signal s'. The implementation steps include the following: (6a) Multiply the local synchronization header u and the received synchronization header u' by pointwise conjugate to obtain the demodulation sequence Z: z(n)=u'(n)u * (n) Where z(n) represents the nth bit of the demodulation sequence Z, u'(n) represents the nth sampling point in the received synchronization header u', u * (n) represents the conjugate of the nth sampling point in the local synchronization header u, where n = 0, 1, ..., N u -1; (6b) Perform L frequency offset rotations on the demodulation sequence Z, with each rotation having a corresponding frequency offset of f. l L frequency shift sequences Z are obtained. l ; in, T is the symbol period, N fft The number of points in the FFT; (6c) For the frequency shift sequence Z l By plotting the average periodicity chart, we obtain the periodicity chart matrix C. k,l (f l ) and average period chart C k,l (f l )=FFT(Z l ,N fft ) Where k = 0,...,N fft -1, l=0,...,L-1, N fft The number of points in the FFT; (6d) Search periodic graph matrix C k,l (f l Find the two variables k corresponding to the maximum value in the matrix. m and l m ; (6e) Calculate the frequency offset estimate based on the above parameters.

6. The method according to claim 1, characterized in that, In step (7), the frequency offset of the received signal s' is... The frequency offset compensation yields s", as shown in the following formula: Where s(m) represents the m-th sampling point of the received signal s” after frequency offset compensation, and s'(m) represents the m-th sampling point of the received signal s', m = 0, 1, ..., N-1, f s denoted as the signal sampling rate, N as the length of the received signal s', and j as the complex unit.

7. The method according to claim 1, characterized in that, Step (8) Calculate a coarse estimate of the phase offset based on the frequency offset compensated signal s”. The formula is as follows: Where p”” is the nth sampling point of the pilot signal p” after frequency offset compensation. * (n) is the conjugate of the nth sampling point of the local pilot p, and K is the number of sampling points of the pilot.

8. The method according to claim 1, characterized in that, Step (10) Use the phase offset precise estimate The frequency offset compensated signal s” is then subjected to phase offset compensation to obtain the carrier synchronized signal r, as shown in the following formula: Where r(m) represents the m-th sampling point of signal r after carrier synchronization, and s"(m) represents the m-th sampling point of received signal s" after frequency offset compensation.

Images