OFDM carrier frequency domain frequency offset compensation system and method based on FPGA

Abstract

The present invention provides an OFDM carrier frequency domain frequency offset compensation system and method based on FPGA. The implementation steps of the compensation method are: the pilot extraction module extracts the pilot of the frequency domain received signal; the data buffer module buffers the frequency domain received signal; the frequency offset estimation module obtains the phase value of the carrier frequency offset; the phase value of the carrier frequency offset is normalized to construct the corresponding simplified matrix required for compensation; the matrix composed of compensation factors is used to compensate the signal buffered by the data buffer module. The OFDM carrier frequency domain frequency offset compensation system in the present invention is implemented based on FPGA, which avoids the problem that a large amount of data in the prior art for calculating the carrier frequency offset compensation value is completed through the transmission between the host computer and the hardware, and the use of the carrier frequency offset compensation value to compensate for the frequency offset of each signal still requires the consumption of more hardware resources due to the large amount of data. Under the premise of ensuring a low bit error rate, the compensation process is simple and the resource consumption is low.

Description

Technical Field

[0001] The present invention belongs to the technical field of satellite communications, and relates to a frequency domain frequency offset compensation system and method, and specifically to an OFDM carrier frequency domain frequency offset compensation system and method based on FPGA, which is suitable for carrier recovery in the field of satellite OFDM communications. Background Art

[0002] The integration of satellite and terrestrial cellular networks is a promising technology for providing global communications. With the adoption of orthogonal frequency division multiplexing (OFDM) in 3GPP standards, such as 4G LTE and the ongoing 5G NR, achieving high interoperability with terrestrial cellular networks LTE or 5G will become an important feature of satellites. The use of OFDM technology similar to terrestrial wireless communications in satellite-to-ground transmission links is one of the current optional technologies. The main advantage of OFDM technology is that it can resist a certain degree of frequency selective fading or narrowband interference due to the orthogonality between subcarriers. In a multi-carrier system, frequency selective fading only damages a small number of carriers, and the receiver can also perform error correction on the damaged subcarriers. Frequency selective fading caused by multipath will not have a serious impact on the bit error rate of the entire system because it only affects the information carried by the subcarriers in the trough of the frequency band. From the above characteristics, it can be seen that OFDM is suitable for transmitting data in a satellite environment.

[0003] Since a large Doppler frequency offset exists in the satellite communication channel, it poses a more difficult challenge to the carrier frequency offset recovery of the OFDM system. The carrier frequency offset recovery performance directly affects the overall performance of the receiver. The phase noise caused by the error caused by the instability of the clock oscillators of the transmitter and receiver and the Doppler frequency offset caused by the relative motion of the transmission and reception will cause the carrier frequency offset. On the one hand, the carrier frequency offset will cause the phase offset of the symbols received by the receiver, and on the other hand, it will cause the overall spectrum of the received signal to shift, resulting in the spectrum of the baseband signal not being zero frequency. Therefore, it is necessary to recover the carrier frequency offset. The carrier frequency offset recovery can generally be completed in the time domain or in the frequency domain. The carrier frequency offset needs to be estimated first and then the frequency offset compensation is completed. However, the calculation amount of completing carrier compensation in the frequency domain by hardware is large and requires a lot of resources.

[0004] In order to reduce the resource consumption of hardware-implemented carrier frequency offset compensation, researchers have proposed many methods. For example, the patent application with application publication number CN 107171993 A and titled "Hardware Implementation Method for Uplink Joint Carrier Synchronization Based on Large-Scale MIMO" discloses a hardware implementation method for frequency domain frequency offset compensation of joint OFDM. The specific implementation steps are: the base station intercepts the sequence signal and calculates the carrier frequency offset through the sequence signal; the host computer uses the calculated expected value of the carrier frequency offset to update the carrier frequency offset compensation value of each channel of the base station; the base station uses the updated carrier frequency offset compensation value to compensate for the frequency offset of each received signal. However, in this method, a large amount of data for calculating the carrier frequency offset compensation value is completed through the transmission between the host computer and the hardware, and the use of the carrier frequency offset compensation value to compensate for the frequency offset of each signal still consumes a lot of hardware resources due to the large amount of data. Summary of the invention

[0005] The purpose of the present invention is to provide an OFDM carrier frequency domain frequency offset compensation system and method based on FPGA to address the defects of the above-mentioned prior art, so as to solve the technical problem of high hardware resource consumption in the prior art.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] An OFDM carrier frequency domain frequency offset compensation system and method based on FPGA, comprising a pilot extraction module, a data buffer module and a frequency offset compensation module implemented based on FPGA and connected in sequence, wherein a frequency offset estimation module is loaded between the output end of the pilot extraction module and the input end of the frequency offset compensation module, wherein:

[0008] The pilot extraction module is used to extract the pilot of the frequency domain received signal after orthogonal frequency division multiplexing OFDM demodulation;

[0009] The data buffer module is used to receive and buffer the frequency domain received signal;

[0010] The frequency offset estimation module is used to calculate the phase value of the frequency offset estimation according to the pilot signal;

[0011] The frequency offset compensation module is used to compensate the cached frequency domain received signal through the frequency offset estimation value.

[0012] The above-mentioned OFDM carrier frequency domain frequency offset compensation system and method based on FPGA, the frequency offset compensation module includes multiple shift registers arranged in parallel and a read-only memory, and a complex multiplier connected to the output ends of the multiple shift registers and the read-only memory.

[0013] A compensation method comprises the following steps:

[0014] (1) The pilot extraction module extracts the pilot of the frequency domain received signal:

[0015] The pilot extraction module obtains L OFDM demodulated frequency domain received signals in a time slot and each frequency domain received signal Y containing K subcarriers l The pilot enable signal is used to extract M pilots of L frequency domain received signals according to the pilot enable signal, where L≥3, K≥1, and Y l The kth subcarrier in is Y l (k), the pilot enable signal is e l (k), the mth pilot Y m The kth subcarrier is Y m (k);

[0016] (2) The data buffer module buffers the received signal in the frequency domain:

[0017] The data buffer module buffers L OFDM demodulated frequency domain received signals;

[0018] (3) The frequency deviation estimation module obtains the phase value of the carrier frequency deviation:

[0019] The frequency offset estimation module uses each pilot Y m and its corresponding local pilot X m The phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals Estimation, where X m ={X m (k)|1≤m≤M,1≤k≤K}, the mth local pilot X m The kth subcarrier is X m (k);

[0020] (4) The frequency offset compensation module obtains the compensation result of the carrier frequency domain frequency offset:

[0021] The frequency deviation compensation module uses the phase value of the carrier frequency deviation Each frequency domain received signal Y cached by the data cache module l Each subcarrier Y in l (k) Perform frequency offset compensation to obtain L OFDM demodulated frequency domain received signals after frequency offset compensation.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The OFDM carrier frequency domain frequency offset compensation system in the present invention is implemented based on FPGA, which avoids the defect that a large amount of data in the prior art for calculating the carrier frequency offset compensation value is completed through the transmission between the host computer and the hardware, and the frequency offset compensation of each signal using the carrier frequency offset compensation value still requires more hardware resources due to the large amount of data. Under the premise of ensuring a low bit error rate, the compensation calculation process is simplified, thereby reducing resource consumption.

[0024] 2. The present invention simplifies the complex compensation calculation of frequency domain frequency deviation and can select the number of compensation factors. When implemented in hardware such as FPGA, specific design and configuration can be performed according to current performance requirements, supporting application flexibility and multi-channel parallel processing, and can maximize the use of FPGA resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the compensation system of the present invention.

[0026] Figure 2 It is a structural schematic diagram of a frequency deviation compensation module in the compensation system of the present invention.

[0027] Figure 3 It is a flow chart of the implementation of the compensation method of the present invention. DETAILED DESCRIPTION

[0028] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0029] Reference Figure 1 The frequency offset compensation system of the present invention comprises a pilot extraction module, a data buffer module and a frequency offset compensation module which are implemented based on FPGA and connected in sequence, and a frequency offset estimation module is loaded between the output end of the pilot extraction module and the input end of the frequency offset compensation module, wherein:

[0030] The pilot extraction module is used to extract the pilot of the frequency domain received signal after orthogonal frequency division multiplexing OFDM demodulation;

[0031] The data buffer module is used to receive and buffer the frequency domain received signal;

[0032] The frequency offset estimation module is used to calculate the phase value of the frequency offset estimation according to the pilot signal;

[0033] Reference Figure 2 The frequency offset compensation module is used to compensate the cached frequency domain received signal through the frequency offset estimation value, and includes multiple shift registers arranged in parallel and a read-only memory, and a complex multiplier connected to the output ends of the multiple shift registers and the read-only memory.

[0034] The above-mentioned OFDM carrier frequency domain frequency offset compensation system and method based on FPGA, the frequency offset compensation module,

[0035] Reference Figure 3 The frequency offset compensation method of the present invention comprises the following steps:

[0036] Step 1) Extracting the pilot signal The pilot signal extraction module extracts the pilot signal of the frequency domain received signal:

[0037] The pilot extraction module obtains L OFDM demodulated frequency domain received signals in a time slot and each frequency domain received signal Y containing K subcarriers l The pilot enable signal is used to extract M pilots of L frequency domain received signals according to the pilot enable signal, where L≥3, K≥1, and Y l The kth subcarrier in is Y l (k), the pilot enable signal is e l (k), the mth pilot Y m The kth subcarrier is Y m (k);

[0038] Y l The kth subcarrier Y in l (k), pilot enable signal e l (k) and the mth pilot Y m The kth subcarrier Y m (k), their interrelationships are:

[0039] Y m (k) = Y l (k) e l (k)

[0040] in,

[0041] Step 2) The data cache module caches the frequency domain received signal:

[0042] The data buffer module buffers L OFDM demodulated frequency domain received signals;

[0043] Step 3) The frequency deviation estimation module obtains the phase value of the carrier frequency deviation:

[0044] The frequency offset estimation module uses each pilot Y m and its corresponding local pilot X m The phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals Estimation, where X m ={X m (k)|1≤m≤M,1≤k≤K}, the mth local pilot X m The kth subcarrier is Xm (k);

[0045] The phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals Make an estimate, the estimation formula is:

[0046]

[0047] in,() * represents the conjugate operation, and D represents the distance between two adjacent pilots.

[0048] Step 4) The frequency offset compensation module obtains the compensation result of the carrier frequency domain frequency offset:

[0049] The frequency deviation compensation module uses the phase value of the carrier frequency deviation Each frequency domain received signal Y cached by the data cache module l Each subcarrier Y in l (k) Perform frequency offset compensation to obtain L OFDM demodulated frequency domain received signals after frequency offset compensation.

[0050] (4a) The frequency offset compensation module calculates the phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals. Normalize and calculate the normalized frequency deviation value in the range of [-C, C] Perform N-point equal-interval quantization and calculate each quantization point The corresponding A compensation factors ψ n (i) Later cached into read-only memory:

[0051]

[0052]

[0053] Among them, N FFT Indicates the number of FFT points for OFDM demodulation, N CP represents the length of the cyclic prefix CP, A∈{3,5,…,2K+1}, e represents the base of natural logarithm;

[0054] (4b) The frequency deviation compensation module calculates the phase value of the carrier frequency deviation. Calculate the address n of the corresponding compensation factor in the read-only memory c , and according to n c Find the compensation factor corresponding to the current estimate in read-only memory

[0055]

[0056] in, Indicates rounding down;

[0057] (4c) Using the compensation factor found Construct a K×K main diagonal Parallel to the main diagonal, the top is Parallel to the main diagonal, below is Toeplitz matrix Ψ with 0 in other positions;

[0058] (4d) The frequency offset compensation module stores the L OFDM demodulated frequency domain received signals cached by the data cache module into a plurality of serial shift registers one by one, and compares the frequency domain received signals stored in the shift registers with the compensation factors. Complex multiplication operations are performed respectively to complete the frequency offset compensation process of multiplying the Toeplitz matrix Ψ by the L OFDM demodulated frequency domain receiving signals cached by the data cache module, and the sum of the complex multiplication operations obtained is the L OFDM demodulated frequency domain receiving signals after frequency offset compensation.

[0059] Based on the above matrix characteristics, this matrix is ​​simplified into a matrix consisting of several main diagonals, and the values ​​on the same diagonal are the same. The number of required compensation factors is determined according to the performance requirements and the simulation diagram. It should be noted that the number of compensation factors A taken satisfies A∈{3,5,…,2K-1}. The required compensation factor is determined to be ψ(i), where,

[0060] Table 1. Comparison of complexity before and after simplification

[0061]

[0062] It is worth noting that the range of the quantized normalized frequency offset [-C, C] needs to be determined in combination with the range of the frequency offset estimation algorithm and the range of the frequency offset to be compensated. The larger the normalized frequency offset, the more compensation factors are required, and the bit error rate performance is better. The number of compensation factors A needs to be selected in combination with performance requirements.

[0063] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for compensating frequency offset of OFDM carrier frequency domain based on FPGA, characterized in that This is achieved through a compensation system, which includes the following steps: (1) The pilot extraction module extracts the pilot of the frequency domain received signal: The pilot extraction module obtains L OFDM demodulated frequency domain received signals in a time slot and each frequency domain received signal Y containing K subcarriers l The pilot enable signal is used to extract M pilots of L frequency domain received signals according to the pilot enable signal, where L≥3, K≥1, and Y l The kth subcarrier in is Y l (k), the pilot enable signal is e l (k), the mth pilot Y m The kth subcarrier is Y m (k); (2) The data buffer module buffers the received signal in the frequency domain: The data buffer module buffers L OFDM demodulated frequency domain received signals; (3) The frequency deviation estimation module obtains the phase value of the carrier frequency deviation: The frequency offset estimation module uses each pilot Y m and its corresponding local pilot X m The phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals Estimation is performed, where Xm = {Xm(k)|1≤m≤M,1≤k≤K}, the mth local pilot X m The kth subcarrier is X m (k); (4) The frequency offset compensation module obtains the compensation result of the carrier frequency domain frequency offset: The frequency deviation compensation module uses the phase value of the carrier frequency deviation Each frequency domain received signal Y cached by the data cache module l Each subcarrier Y in l (k) Perform frequency offset compensation, the implementation steps are as follows: (4a) The frequency offset compensation module calculates the phase value of the carrier frequency offset of the L OFDM demodulated frequency domain received signals. Normalize and calculate the normalized frequency deviation value in the range of [-C, C] Perform N-point equal-interval quantization and calculate each quantization point The corresponding A compensation factors ψ n (i) Later cached into read-only memory: Among them, N FFT Indicates the number of FFT points for OFDM demodulation, N CP represents the length of the cyclic prefix CP, A∈{3,5,…,2K-1}, e represents the base of natural logarithm; (4b) The frequency deviation compensation module calculates the phase value of the carrier frequency deviation. Calculate the address n of the corresponding compensation factor in the read-only memory c , and according to n c Find the compensation factor corresponding to the current estimate in read-only memory in, Indicates rounding down; (4c) The frequency offset compensation module uses the compensation factor found The main diagonal of the construction size is K×K. Parallel to the main diagonal, the top is Parallel to the main diagonal, the following are Toeplitz matrix Ψ with 0 in other positions; (4d) The frequency offset compensation module stores the L OFDM demodulated frequency domain received signals cached by the data cache module into a plurality of serial shift registers one by one, and compares the frequency domain received signals stored in the shift registers with the compensation factors. Complex multiplication operations are performed respectively to complete the frequency offset compensation process of multiplying the Toeplitz matrix Ψ by the L OFDM demodulated frequency domain receiving signals cached by the data cache module, and the sum of the complex multiplication operations obtained is the L OFDM demodulated frequency domain receiving signals after frequency offset compensation.

2. The method according to claim 1, characterized in that: The compensation system comprises a pilot extraction module, a data buffer module and a frequency offset compensation module which are implemented based on FPGA and connected in sequence, and a frequency offset estimation module is loaded between the output end of the pilot extraction module and the input end of the frequency offset compensation module, wherein: The pilot extraction module is used to extract the pilot of the frequency domain received signal after orthogonal frequency division multiplexing OFDM demodulation; The data buffer module is used to receive and buffer the frequency domain received signal; The frequency offset estimation module is used to calculate the phase value of the frequency offset estimation according to the pilot signal; The frequency offset compensation module is used to compensate the cached frequency domain received signal through the frequency offset estimation value.

3. The method according to claim 2, characterized in that The frequency offset compensation module includes a plurality of shift registers arranged in parallel and a read-only memory, and a complex multiplier connected to the output ends of the plurality of shift registers and the read-only memory.

4. The method according to claim 1, characterized in that: Y in step (1) l The kth subcarrier Y in l (k), pilot enable signal e l (k) and the mth pilot Y m The kth subcarrier Y m (k), their interrelationships are: AND m (k)=Y l (k)·e l (k) in, 5. The method according to claim 1, characterized in that The phase value of the carrier frequency deviation of the L OFDM demodulated frequency domain received signals described in step (3) Make an estimate, the estimation formula is: in,(·) * represents the conjugate operation, and D represents the distance between two adjacent pilots.

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