Communication baseband signal generator with continuously variable symbol rate

Abstract

The invention belongs to the technical field of spacecraft measurement and control communication, and particularly relates to a communication baseband signal generator with a continuously variable symbol rate, which comprises a forming filter used for performing baseband forming on an IQ symbol sequence and converting the IQ symbol sequence into a baseband waveform meeting requirements; the interpolation controller is used for determining interpolation points and fractional intervals through input symbol rate control words; and the fractional interpolator is used for generating the baseband waveform under the DA sampling frequency by adopting a fractional interpolation technology at an interpolation point for the baseband waveform. The method can adapt to any baseband forming filtering, baseband waveforms with different roll-off coefficients can generate baseband waveforms with any symbol rate, the stepping precision of the symbol rate generated by the method can be easily controlled below 1Hz, and even the stepping precision superior to 10 <-6 > Hz can be realized; and the requirements on symbol rate fine adjustment in various wireless communication scenes such as spacecraft measurement and control communication, satellite communication and the like can be well met.

Description

Technical Field

[0001] The present invention belongs to the technical fields related to satellite communication, spacecraft measurement and control communication, and wireless communication, and in particular relates to a communication baseband signal generator with continuously variable symbol rate. Background Art

[0002] In wireless communication fields such as satellite communication and spacecraft tracking and control communication, the baseband waveform of the physical layer signal is the basic unit of information. Its style directly determines the bandwidth occupied by the communication link and the physical channel rate. It is subject to ITU radio regulations and frequency allocation. It usually needs to be reasonably designed to ensure that it occupies as little frequency bandwidth as possible without generating inter-code interference. It is an important design link in the field of wireless communication. For example, the CCSDS protocol widely used in the field of spacecraft tracking and control communication, and the DVB-S2 / S2X protocol used in the field of satellite communication both have detailed definitions and descriptions of the baseband waveform.

[0003] The transmission rate of the baseband waveform is the symbol rate. The symbol rate determines the bandwidth occupied by the baseband signal. In the communication system, the symbol rate of the baseband signal is often required to be flexibly and freely adjusted. Here are the following three scenarios: 1) In the multiple access protocol, the symbol rate needs to be adjusted according to the frequency allocation. For example, in the MF-TDMA multiple access mode, the appropriate symbol rate needs to be set according to the bandwidth specified by the random access time-frequency resources; 2) Different communication scenarios have different requirements for symbol rates. For example, at different satellite elevation angles, different symbol rates need to be switched according to different signal-to-noise ratios to ensure reliable communication; 3) Changes in idle frequencies in cognitive radio also require corresponding adjustments to the symbol rate to adapt to changes in bandwidth. However, since baseband shaping filtering is required to suppress the out-of-band power of the baseband signal and reduce the frequency band occupancy, the existing symbol rate is generally adjusted between several fixed rates. For example, a set of symbol rates with a two-fold relationship is a common symbol rate combination. Such a coarse-grained step-by-step adjustment method makes it difficult to achieve optimal communication power efficiency and bandwidth efficiency, and it is also difficult to fully utilize frequency resources in changing frequency band resources. Summary of the invention

[0004] In view of the problem in the prior art that coarse-grained step-type symbol rate adjustment is difficult to achieve optimal power efficiency and bandwidth efficiency, the purpose of the present invention is to overcome the above-mentioned defects of the prior art and propose a communication baseband signal generator with continuously variable symbol rate.

[0005] In order to achieve the above object, the present invention discloses a communication baseband signal generator with continuously variable symbol rate, comprising:

[0006] A shaping filter is used to perform baseband shaping on the IQ symbol sequence and convert the IQ symbol sequence into a baseband waveform that meets the requirements;

[0007] an interpolation controller for determining interpolation points and fractional intervals by inputting a symbol rate control word; and

[0008] The fractional interpolator is used for generating a baseband waveform at a DA sampling frequency by applying a fractional interpolation technique to the baseband waveform at an interpolation point.

[0009] Preferably, the input of the shaping filter is: an IQ symbol sequence x obtained by upsampling the modulated IQ symbol twice or four times IQ (m), the output is: the formed baseband waveform x s (m), the sampling frequency is f i , the interval period is T i , and satisfies f i ≤f s ,T i ≥T s , where f s is the DA sampling frequency, T s is the sampling interval.

[0010] Preferably, when the shaping filter adopts a root raised cosine filter SRRC, its frequency domain response H(f) is:

[0011]

[0012] Where, f represents the frequency, f N =R s / 2 is the Nyquist frequency, R s is the symbol rate; α is the roll-off factor, which is set according to the specific communication standard adopted.

[0013] Preferably, the interpolation controller is implemented by a digitally controlled oscillator NCO, and the input is a symbol rate control word SR_ctrl, which satisfies the following formula:

[0014]

[0015] Digitally controlled oscillator NCO at f s The following accumulation operation is performed at the working frequency:

[0016] ph(k)=[ph(k-1)+SR_ctrl]mod1

[0017] Wherein, ph(k) and ph(k-1) are the accumulated values ​​of the register at the kth and k-1th times, respectively, and ph(0) = 0;

[0018] The output is the fractional interval μ used for each interpolation of the fractional interpolator. k , satisfying the following formula:

[0019] μ k = ph(k);

[0020] The interpolation point is for each f s The sampling point.

[0021] Preferably, update x once when ph(k-1)+SR_ctrl≥1 s (m), that is, x s (m) walks an interval period T i .

[0022] Preferably, the fractional interpolator adopts a third-order polynomial interpolation of a FARROW structure, and has four parallel finite impulse response filter FIR branches, namely FIR1, FIR2, FIR3 and FIR4, each FIR branch has the same structure and has 4 tap coefficients.

[0023] Preferably, the input of the fractional interpolator is: the shaped baseband waveform x s (m) and fractional interval μ k ; Output: At DA sampling frequency f s Baseband waveform y(k) under the symbol rate R s , satisfying the following formula:

[0024]

[0025] Among them, l is 0 to 3, corresponding to FIR1, FIR2, FIR3 and FIR4 respectively; v1 to v4 correspond to the output of FIR1, FIR2, FIR3 and FIR4 respectively, and complete the following operations respectively:

[0026]

[0027]

[0028]

[0029]

[0030] Among them, i represents the i-th tap, and b1(i)~b4(i) correspond to the i-th tap coefficients of FIR1, FIR2, FIR3 and FIR4 respectively.

[0031] Compared with the prior art, the advantages of the present invention are:

[0032] The present invention can adapt to any baseband shaping filter, and baseband waveforms with different roll-off coefficients can generate baseband waveforms with any symbol rate. The symbol rate step accuracy generated by the present invention can be easily controlled below 1 Hz, and can even achieve better than 10 -6 The step accuracy of Hz can well meet the needs of fine adjustment of symbol rate in various wireless communication scenarios such as spacecraft measurement and control communication, satellite communication, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a block diagram of a communication baseband signal generator with continuously variable symbol rate;

[0034] Figure 2 This is the time domain waveform of the root raised cosine shaping filter with a roll-off factor of 0.25;

[0035] Figure 3 It is a fractional interval relationship diagram;

[0036] FIG4 is a baseband waveform and spectrum diagram before and after interpolation at a symbol rate of 11Msps, wherein FIG4(a) is a waveform scatter plot before and after interpolation, and FIG4(b) is a baseband waveform diagram after interpolation; FIG4(c) is a baseband spectrum diagram before interpolation, and FIG4(d) is a baseband spectrum diagram after interpolation;

[0037] FIG5 is a baseband waveform and spectrum diagram before and after interpolation at a 7.5Msps symbol rate, wherein FIG5(a) is a waveform scatter plot before and after interpolation, and FIG5(b) is a baseband waveform diagram after interpolation; FIG5(c) is a baseband spectrum diagram before interpolation, and FIG5(d) is a baseband spectrum diagram after interpolation;

[0038] FIG6 is a baseband waveform and spectrum diagram before and after interpolation of a 7.500001Msp symbol rate, wherein FIG6(a) is a waveform scatter plot before and after interpolation, and FIG6(b) is a baseband waveform diagram after interpolation; FIG6(c) is a baseband spectrum diagram before interpolation, and FIG6(d) is a baseband spectrum diagram after interpolation;

[0039] Figure 7 is a diagram of the baseband waveform and spectrum before and after interpolation at a symbol rate of 7.50000001Msps, where Figure 7(a) is a scatter plot of the waveform before and after interpolation, and Figure 7(b) is a diagram of the baseband waveform after interpolation; Figure 7(c) is a diagram of the baseband spectrum before interpolation, and Figure 7(d) is a diagram of the baseband spectrum after interpolation. DETAILED DESCRIPTION

[0040] We have invented a communication baseband signal generator that can achieve continuously variable symbol rate. It uses fractional interpolation technology to shape baseband waveforms with arbitrary symbol rate and roll-off factor. The symbol rate step accuracy can be easily controlled below 1Hz, truly achieving continuously variable symbol rate.

[0041] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0042] Example

[0043] An embodiment of the present invention provides a communication baseband signal generator with continuously variable symbol rate, such as Figure 1 The figure shows a structural block diagram of a communication baseband signal generator with continuously variable symbol rate. It should be emphasized that the present application emphasizes the design concept of the structural block diagram and does not limit the implementation method using software or hardware.

[0044] It mainly includes three parts: 1) The shaping filter SRRC in the upper left corner is responsible for baseband shaping of the IQ symbol sequence and converting the IQ symbols into a baseband waveform that meets the requirements; 2) The interpolation controller NCO in the lower left corner is responsible for controlling the update of the interpolator input data and generating the fractional intervals required for interpolation; 3) Except for 1) and 2), the remaining part is the fractional interpolator, namely the FARROW interpolator, which generates the baseband waveform at the DA (digital-to-analog converter) sampling rate through interpolation.

[0045] (2) The shaping filter uses a root raised cosine filter (SRRC), which is a finite impulse response filter. Its specific response in the frequency domain is as follows:

[0046]

[0047] Among them, f N =R s / 2 is the Nyquist frequency. α is the roll-off factor, which can be set according to the specific communication standard adopted. For example, 0.35, 0.25, 0.20, 0.15, 0.10 and 0.05 can be selected in DVB-S2 / S2X. The smaller the roll-off factor, the smaller the bandwidth occupied by the signal. Different roll-off factors only affect the style of the symbol waveform and have no effect on the subsequent waveform generation of any symbol rate, and can be applied. Figure 2 The time domain waveform of SRRC when α=0.25 is shown, and its span is 8 symbol periods. The shaping filter here can also be other types of shaping filters, such as Gaussian shaping filter, etc. The type of shaping filter has no effect on subsequent processing.

[0048] (3) The specific shaping and filtering process is as follows:

[0049] First, the modulated IQ symbols are upsampled twice or four times to obtain x IQ (m), the upsampling process is to directly fill zero after the IQ symbol, 1 zero is added for 2 times upsampling, and 3 zeros are added for 4 times upsampling, and then sent to the shaping filter with the same oversampling multiple for shaping filtering to obtain the shaped baseband waveform x s (m).

[0050] (4) The interpolation controller is implemented using a digitally controlled oscillator (NCO). The interpolation point and fractional interval are determined by the input symbol rate control word SR_ctrl. This is a key module in the entire framework. The specific implementation process is as follows:

[0051] (5) Let the sampling frequency of DA be f s , the sampling interval is T s . Let the formed baseband waveform x s The sampling frequency of (m) is f i , the interval period is T i , if 2 times oversampling is used, then f i =2R s , if 4 times oversampling is used, then f i =4R s , R s is the symbol rate, and satisfies f i ≤f s ,T i ≥T s . Therefore, the symbol rate control word can be expressed as:

[0052]

[0053] SR_ctrl is a constant less than 1. When the DA sampling rate is determined, its accuracy directly determines the step accuracy of the symbol rate: Taking a 2x oversampling rate as an example, R s =SR_ctrl·f s / 2. In hardware implementation, SR_ctrl is usually represented by a register with a bit width. Let the bit width be N, then the step accuracy of the symbol rate is f s / 2 N+1 .

[0054] NCO in f s The following accumulation operation is performed at the working frequency:

[0055] ph(k)=[ph(k-1)+SR_ctrl]mod1

[0056] Wherein ph(k) and ph(k-1) are the accumulated values ​​of the register at the kth and k-1th times respectively, and ph(0)=0.

[0057] (6)x s (m) Update:

[0058] when

[0059] ph(k-1)+SR_ctrl≥1

[0060] Update once xs (m), that is, x s (m) One clock cycle T has passed i .

[0061] (7) Interpolation points and fractional intervals μ k Determination of:

[0062] Because f i ≤f s , the entire interpolation belongs to the upsampling process, so each f s The sampling points of are all interpolation points, and the fractional interval of each interpolation is:

[0063] μ k =ph(k)

[0064] Interpolation interval μ k With T s and T i See the relationship Figure 3 .like Figure 3 As shown, the horizontal axis is the time axis, and the interval between every two real columns is T i , the interval between every two virtual columns is T s , the real column and the imaginary column at time zero coincide, and the vertical axis represents μ k The relationship curve presents a periodic triangular tooth shape as a whole, and the slope is SR_ctrl / T s .

[0065] (8) Figure 1 As shown, the fractional interpolator adopts a third-order polynomial interpolation of a FARROW structure. There are four FIR (finite impulse response) filter branches, namely FIR1, FIR2, FIR3 and FIR4. The FIR filter of each branch has 4 tap coefficients, and the structure of each FIR branch is consistent, as shown in the expanded structure of FIR1 in the figure.

[0066] (9) The fractional interpolator uses the x obtained in the previous step s (m) and μ k The final interpolated y(k) is obtained through the following process:

[0067] FIR1, FIR2, FIR3, and FIR4 complete the following operations respectively:

[0068]

[0069]

[0070]

[0071]

[0072] The final output of the fractional interpolator is obtained by the following operations:

[0073]

[0074] Among them, l is 0 to 3, corresponding to FIR1, FIR2, FIR3 and FIR4 respectively.

[0075] The tap coefficients of the third-order polynomial interpolator are shown in the following table:

[0076] Table 1. Tap coefficients of third-order polynomial interpolator

[0077] i <![CDATA[b1]]> <![CDATA[b2]]> <![CDATA[b3]]> <![CDATA[b4]]> 0 1 / 6 0 -1 / 6 0 1 -1 / 2 1 / 2 1 0 2 1 / 2 -1 -1 / 2 1 3 -1 / 6 1 / 2 -1 / 3 0

[0078] (10) The final y(k) is the symbol rate R s , the sampling rate is converted to the DA sampling rate f s The communication baseband signal can be directly sent to the DA chip for digital-to-analog conversion and subsequent RF module processing. The symbol rate is controlled by SR_ctrl. If a double oversampling rate is used in the shaping filter process, R s =SR_ctrl·f s / 2, if the shaping filter process uses a four-fold oversampling rate, then R s =SR_ctrl·f s / 4. When f s After the symbol rate is determined, the step accuracy of the symbol rate is completely determined by SR_ctrl. By selecting the bit width of the control word, it is easy to achieve a symbol rate step accuracy much better than 1 Hz: for example, if the control word width is 32 bits, the double oversampling rate is formed, and the DA sampling rate is 100Msps, the symbol rate step accuracy is about 0.012Hz. If a 48-bit control word is used, the symbol rate step accuracy can reach about 1.78×10 -7 Hz!

[0079] (11) The following is a specific example of using this method to generate a communication baseband waveform with an arbitrary symbol rate: the DA sampling rate is 100Msps, the SRRC shaping filter roll-off coefficient is 0.25, the span is 8, and the oversampling rate is 4. This method is used to generate communication baseband signals with symbol rates of 11Msps, 7.5Msps, 7.5Msps+1sps, and 7.5Msps+0.01sps, respectively. The baseband waveforms and corresponding spectrum diagrams before and after interpolation are given, as shown in Figures 4 to 7, respectively. Figure 4 is a baseband waveform and spectrum diagram before and after interpolation of a symbol rate of 11Msps, where Figure 4(a) is a waveform scatter plot before and after interpolation, and Figure 4(b) is a baseband waveform diagram after interpolation; Figure 4(c) is a baseband spectrum diagram before interpolation, and Figure 4(d) is a baseband spectrum diagram after interpolation;

[0080] FIG5 is a baseband waveform and spectrum diagram before and after interpolation at a 7.5Msps symbol rate, wherein FIG5(a) is a waveform scatter plot before and after interpolation, and FIG5(b) is a baseband waveform diagram after interpolation; FIG5(c) is a baseband spectrum diagram before interpolation, and FIG5(d) is a baseband spectrum diagram after interpolation;

[0081] FIG6 is a baseband waveform and spectrum diagram before and after interpolation of a 7.500001Msp symbol rate, wherein FIG6(a) is a waveform scatter plot before and after interpolation, and FIG6(b) is a baseband waveform diagram after interpolation; FIG6(c) is a baseband spectrum diagram before interpolation, and FIG6(d) is a baseband spectrum diagram after interpolation;

[0082] Figure 7 is a diagram of the baseband waveform and spectrum before and after interpolation at a symbol rate of 7.50000001Msps, where Figure 7(a) is a scatter plot of the waveform before and after interpolation, and Figure 7(b) is a diagram of the baseband waveform after interpolation; Figure 7(c) is a diagram of the baseband spectrum before interpolation, and Figure 7(d) is a diagram of the baseband spectrum after interpolation.

[0083] The baseband waveforms of these four rates all involve non-integer multiple interpolation, and the baseband waveforms before and after interpolation remain consistent, which verifies the effectiveness of the method. It can be seen from the spectrum diagram that the image suppression of the baseband waveform generated by the present invention is better than 45dB.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A communication baseband signal generator with continuously variable symbol rate, characterized in that: include: A shaping filter is used to perform baseband shaping on the IQ symbol sequence and convert the IQ symbol sequence into a baseband waveform that meets the requirements; An interpolation controller, used for determining interpolation points and fractional intervals by inputting a symbol rate control word; and The fractional interpolator is used for generating a baseband waveform at a DA sampling frequency by applying a fractional interpolation technique to the baseband waveform at an interpolation point.

2. The communication baseband signal generator with continuously variable symbol rate according to claim 1, characterized in that: The input of the shaping filter is: the IQ symbol sequence x obtained by upsampling the modulated IQ symbol twice or four times IQ (m), the output is: the formed baseband waveform x s (m), the sampling frequency is f i , the interval period is T i , and satisfies f i ≤f s ,T i ≥T s , where f s is the DA sampling frequency, T s is the sampling interval.

3. The communication baseband signal generator with continuously variable symbol rate according to claim 2, characterized in that: When the shaping filter adopts a root raised cosine filter SRRC, its frequency domain response H(f) is: Where, f represents the frequency, f N =R s / 2 is the Nyquist frequency, R s is the symbol rate; α is the roll-off factor, which is set according to the specific communication standard adopted.

4. The communication baseband signal generator with continuously variable symbol rate according to claim 2, characterized in that: The interpolation controller is implemented by a digitally controlled oscillator NCO, and the input is a symbol rate control word SR_ctrl, which satisfies the following formula: Digitally controlled oscillator NCO at f s The following accumulation operation is performed at the working frequency: ph(k)=[ph(k-1)+SR_ctrl]mod1 Wherein, ph(k) and ph(k-1) are the accumulated values ​​of the register at the kth and k-1th times, respectively, and ph(0) = 0; The output is the fractional interval μ used for each interpolation of the fractional interpolator. k , satisfying the following formula: μ k =ph(k); The interpolation point is for each f s The sampling point.

5. The communication baseband signal generator with continuously variable symbol rate according to claim 4, characterized in that: Update x once when ph(k-1)+SR_ctrl≥1 s (m), that is, x s (m) walks an interval period T i .

6. The communication baseband signal generator with continuously variable symbol rate according to claim 4, characterized in that: The fractional interpolator adopts a third-order polynomial interpolation of a FARROW structure, and has four parallel finite impulse response filter FIR branches, namely FIR1, FIR2, FIR3 and FIR4. Each FIR branch has the same structure and has 4 tap coefficients.

7. The communication baseband signal generator with continuously variable symbol rate according to claim 6, characterized in that: The input of the fractional interpolator is: the shaped baseband waveform x s (m) and fractional interval μ k ; Output: At DA sampling frequency f s Baseband waveform y(k) under the symbol rate R s , satisfying the following formula: Among them, l is 0 to 3, corresponding to FIR1, FIR2, FIR3 and FIR4 respectively; v1 to v4 correspond to the output of FIR1, FIR2, FIR3 and FIR4 respectively, and complete the following operations respectively: Among them, i represents the i-th tap, and b1(i)~b4(i) correspond to the i-th tap coefficients of FIR1, FIR2, FIR3 and FIR4 respectively.

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