CN101814932B - Variable bandwidth CDMA digital filtering method based on digital intermediate frequency - Google Patents

Abstract

The invention belongs to the technical field of digital filters and in particular relates to a variable bandwidth CDMA digital filtering method based on digital intermediate frequency. The following steps are completed in sequence by adopting the filter coefficient symmetry mode: (1) storing the input signals into a storage in sequence; (2) numbering the input value in sequence and adding the input value symmetrically; (3) multiplying the result obtained in the step (2) by the corresponding filter coefficient; and (4) accumulating all the results obtained in the step (3), thus obtaining the output value. The capacity of the storage is consistent with the number of the filter coefficient. By the method, the hardware resources are saved, filter switching in two different frequency ranges can be realized, the filter is shared by frequency selection and wideband signals, and the method still has amplitude-frequency responses in different shapes under the condition of data at different rates.

Description

CDMA digital filtering method based on the bandwidth varying of digital intermediate frequency

Technical field

The invention belongs to the digital filter techniques field, relate in particular to a kind of CDMA digital filtering method of the bandwidth varying based on digital intermediate frequency.

Background technology

Along with the development of chip technology, can finish increasing function by digital technology.Increasing product design makes the transition from simulation-to-digital, and former simulation CDMA frequency-selector also can be transformed into the frequency-selector of numeral.The digital CDMA frequency-selector has a lot of advantages with respect to the CDMA frequency-selector of simulation: 1, be convenient to integrated more function; 2, be convenient to device upgrade, in a single day the simulation frequency-selector is finished, and is difficult to change design, device upgrade is very difficult, and the major function of digital frequency-selecting device is finished by write-in program in the chip, as long as the change program, again download and both can finish new function, it is extremely convenient to upgrade; 3, the better control signal of digital frequency-selecting device, for example noise control, single-channel power control etc.Corresponding intermediate-frequency filter also can be finished by digital FIR filter.

The main effect of digital filter is to provide enough big attenuation outside a channel at intermediate frequency.For the signal in broadband and the signal of frequency-selecting, because the bandwidth difference need the different digital filter of design, but under limited ground resource, finishing two filters and designing corresponding communication link often needs more hardware resource.In general design, often have only the signal in broadband to handle, perhaps have only the signal of frequency-selecting to handle.Therefore generally, CDMA frequency-selector and choosing band device can not be general, can not realize that the filtering of two kinds of different frequency ranges is switched, and can waste a large amount of hardware resources when equipment uses.In addition, it is worthy of note that domestic existing C DMA frequency-selector all adopts analog form, what its intermediate frequency filtering adopted is that the intermediate frequency SAW (Surface Acoustic Wave) filter is finished.

Summary of the invention

The present invention is intended to overcome the deficiencies in the prior art part and a kind of saving hardware resource is provided, the filtering that can realize two kinds of different frequency ranges is switched, frequency-selecting and broadband signal share, under the data of different rates, still has the CDMA digital filtering method based on the bandwidth varying of digital intermediate frequency of identical shaped amplitude-frequency response.

For achieving the above object, the present invention is achieved in that

Based on the CDMA digital filtering method of the bandwidth varying of digital intermediate frequency, adopt the filter coefficient symmetric pattern, finish following steps successively:

(1) deposits input signal in memory in order;

(2) according to sequencing input value is numbered, will import symmetrical addition;

(3) step (2) gained result is multiplied each other with corresponding filter coefficient;

(4) all results of step (3) are added up, namely get output valve.

What the present invention was primarily aimed at is CDMA frequency-selecting and choosing band device.It is integrated to frequency-selecting and select the intermediate frequency filtering function of being with.Can realize the filtering switching of two kinds of different frequency ranges by software.And with respect to using two other filters of branch to save hardware resource more.

The invention solves the frequency-selecting function and select sharing of band function, only design a filter, share for frequency-selecting and broadband signal.Utilization under the data of different rates, still has identical shaped amplitude-frequency response with the wave digital lowpass filter of spline coefficient, is corresponding band connection frequency, the stop-band frequency difference.If suitably regulate handled data signaling rate, just can realize the expansion of passband and dwindle.

Description of drawings

The invention will be further described below in conjunction with the drawings and specific embodiments.Protection scope of the present invention not only is confined to the statement of following content.

Fig. 1 is linear pattern digital filter configuration figure;

Fig. 2 is digital filter configuration figure of the present invention;

Fig. 3 is digital filtering method FB(flow block) of the present invention;

Fig. 4 is one embodiment of the present invention FB(flow block);

Fig. 5 is one embodiment of the present invention amplitude-frequency response schematic diagram;

Fig. 6 is impulse response figure of the present invention;

Fig. 7 is step response diagram of the present invention;

Fig. 8 is zero pole plot of the present invention;

Fig. 9 is another kind of execution mode amplitude-frequency response schematic diagram.

Embodiment

As shown in the figure, based on the CDMA digital filtering method of the bandwidth varying of digital intermediate frequency, utilize the filter coefficient symmetric pattern, finish following steps successively:

(1) deposits input signal in memory in order;

(2) according to sequencing input value is numbered, will import symmetrical addition;

(3) step (2) gained result is multiplied each other with corresponding filter coefficient;

(4) all results of step (3) are added up, namely get output valve.

The capacity of above-mentioned memory is consistent with number of filter coefficients.

1, the operation principle of CDMA digital filter

The difference equation expression formula of FIR filter is:

$y\[n\]=\underset{m=0}{\overset{N-1}{\mathrm{\Σ}}}h\[m\]x\[n-m\]---\left(1\right)$

Wherein, N is the exponent number of filter, h[m] be the coefficient of filter.Directly the digital filtering structure of type is utilized the characteristics of filter coefficient symmetry as shown in Figure 1, is optimized to the form as Fig. 2.

From Fig. 2, can draw the implementation method of digital filter:

At first, deposit input signal in memory in order, the capacity of memory is consistent with number of filter coefficients.According to sequencing input value is numbered, will import symmetrical addition.

a[m]=x[m]+x[N-m] (2)

Multiply each other with corresponding filter coefficient again,

b[m]=a[m]*h[m] (3)

Then all results are added up, remember output valve

y[n]=b[1]+…+b[m] (4)

The FB(flow block) of designing filter is as Fig. 3.

2, process route design of the present invention

The present invention can select that the CDMA broadband signal of the CDMA frequency-selecting signal of one-segment 1.23MHz and 10MHz bandwidth is done filtering and handle.If the AD sampling rate of design is 100Mbps, when the broadband signal of 10MHz is carried out the filtering processing, sampling rate need be reduced to 25Mbps, when 1.23MHz is carried out the filtering processing, sampling rate need be reduced to 3.125Mbps.Thereby the data for low sampling rate can also reach the saving hardware resource by multiplexing multiplier, realize the function of maximum 8 frequency-selectings, as Fig. 4.

Because with respect to the frequency-selecting signal, broadband signal needs littler transition band, therefore comes the designing filter coefficient according to broadband signal.The semaphore request that obtains the broadband is: bandwidth 10MHz, and stop-band frequency is 10.75MHz, stopband attenuation is 35dbc.The minimum exponent number of process emulation is 120 rank.

3, the principle of filter and design

Understand the needed amplitude-frequency response of CDMA Channel, just can design the needed filter of CDMA broadband signal according to this:

Sample frequency f _s: 25MHz

Passband fluctuation:＜1db

Bandwidth: 9.8MHz(semibelt width 4.9MHz)

Stopband requires :-40dbc, 5.5MHz;

Utilize the filter design tool of MATLAB as requested: designed 94 rank filters, the 1.5dB bandwidth is 10MHz, and the 3db bandwidth is 10.2MHz, and the 30db bandwidth is 11MHz.Amplitude-frequency response as shown in Figure 5.The impulse response figure of filter is Fig. 6.The step response diagram of filter is Fig. 7.The zero pole plot of filter is Fig. 8.

The designed filter coefficient of the present invention is:

To the binary number that is quantized into 12bit of filter coefficient, after normalized coefficient changed into the coefficient of integer, the filter amplitude-frequency response figure that obtains saw Fig. 9 when considering design.

The filter of contrast front normalization coefficient, the amplitude-frequency response error of the filter of integer very I to accept.If corresponding sampling rate is 3.125Mbps.Through emulation, for the data-signal of 3.125Mbps, the 1.5db bandwidth of same filter is 1.25MHz, and the 3db bandwidth is 1.275Mhz, and the 30db bandwidth is 1.375Mhz, satisfies the requirement of CDMA frequency-selecting signal far away.

Claims (1)

1. based on the CDMA digital filtering method of the bandwidth varying of digital intermediate frequency, it is characterized in that, utilize the filter coefficient symmetric pattern, finish following steps successively:

(1) deposits input signal in memory in order;

(2) according to sequencing input value is numbered, with the addition of input value symmetry; A[m]=x[m]+x[N-m]; Wherein, N is the exponent number of filter;

(3) step (2) gained result is multiplied each other with corresponding filter coefficient; B[m]=a[m] * h[m]; Wherein, h[m] be the coefficient of filter;

(4) all results of step (3) are added up, namely get output valve; Y[n]=b[1]+... + b[m].

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