CN105282079A - OFDM null symbol detection method and detection device - Google Patents

Abstract

The invention discloses an OFDM null symbol detection method which can improve digital audio broadcasting. The method is characterized by calculating modulus values (or modulus square) of input signals; carrying out difference on modulus value signals; carrying out moving average on difference signals; carrying out low-pass filtering on the modulus values of the input signals simultaneously to obtain an average modulus value; carrying out amplitude normalization on moving average output signals according to the obtained average modulus value; searching for positive and negative two peaks from the differential signal obtained after normalization, when finding the negative peak, continuing to search for the positive peak with a preset search range, and if finding the positive peak, judging that null symbol detection is successful, or otherwise, restarting to search for the negative peak; and calculating the distance between the positive and negative peaks, the distance being the length of a null symbol, and obtaining a sending mode according to the length. The invention also discloses a null symbol detection device for the method. The method can improve digital audio broadcasting (DAB) OFDM(orthogonal frequency division multiplexing) null symbol detection performance.

Description

The vacant symbol detection method of OFDM and checkout gear thereof

Technical field

The present invention relates to the communications field, particularly relate to OFDM (OFDM) vacant symbol (NullSymbol) detection method of a kind of digital audio broadcasting (DAB).The invention still further relates to a kind of vacant symbol detection apparatus for described method.

Background technology

OFDM technology is widely used in Modern Communication System.Adopt the digital audio standards of OFDM technology, DAB (" RadioBroadcastingSystems; DigitalAudioBroadcasting (DAB) tomobile, portableandfixedreceivers, " ETSIEN300401) multi-frame structure as shown in Figure 1.Wherein, every frame comprises a vacant symbol, a phase reference symbol, and some data symbols.Again, vacant symbol is positioned at the head of frame, and it sends signal power is " 0 ".On the other hand, DAB has four kinds of sending modes, the variable lengths (see table 1) of the vacant symbol of often kind of sending mode.

Sending mode	I	II	III	IV
					Sampling	2656	664	345	1328
Length (us)	1297	324	168	648

Table 1

At the receiving terminal of DAB system, first, the starting point by detecting vacant symbol carries out frame synchronization; Further, also need to detect sending mode.

Traditionally, by detecting the power detection method that changes at vacant symbol place of received signal power, the sync bit of vacant symbol and vacant symbol lengths can be obtained to obtain sending mode.But when snr of received signal is low, the power variation between vacant symbol and proper symbol eases up, the detecting reliability of power detection method reduces, and synchronous error becomes large.More seriously, in fading channel, Received signal strength itself is in fluctuation always, makes the detection perform serious deterioration of power detection method.B.R.Vinod (" ANullSymbolDetectionAlgorithmforDABReceivers; " IEEEICSCN2007, Feb2007) a kind of innovatory algorithm is proposed, by calculating the different signal power value of two kinds of averaging methods, and a set of search control algolithm, improve detection perform.Similar, W.H.He (" Apparatusandmethodfordetectingnullsymbols; " USpatent, US2008/0175338, Jan.2008) improving one's methods of proposing is, by an auxiliary signal power detector, follow the tracks of the signal fluctuation of fading channel, with the power detection of auxiliary vacant symbol.These two kinds of methods, except control method complexity, its accuracy of following the tracks of fading channel and limited reliability, limit its improvement effect to detection perform.How to improve the detection perform of vacant symbol, the performance particularly under serious fading channel, remaining one has problem to be solved.

Summary of the invention

The technical problem to be solved in the present invention is to provide the detection method that one improves OFDM (OFDM) vacant symbol (NullSymbol) detection perform of digital audio broadcasting (DAB), and method of the present invention can reduce false dismissal probability and the probability of false detection of the vacant symbol of OFDM.For this reason, the present invention also will provide a kind of vacant symbol detection apparatus for described method.

For solving the problems of the technologies described above, improving the detection method of vacant symbol detection performance, comprising:

1) modulus value or the mould square of input signal is calculated;

2) modulus value signal is spaced apart to the difference of L;

Difference interval L is the parameter to modulus value signal differential, represents differential delay, and span is 1 to (345-1)/2;

3) carry out to differential signal the rolling average that length is N, rolling average length N: to the length of differential signal rolling average, value is difference interval, i.e. N=L;

4) low-pass filtering is carried out to the modulus value of input signal, obtain average modulus value;

5) utilize the average modulus value obtained, amplitude normalization is carried out to rolling average output signal;

6) to the differential signal after normalization, search for positive negative two peak values, first search for negative peak, if negative peak is less than the first predetermined threshold TH1, enter next step, otherwise continue search negative peak, first predetermined threshold TH1: the negative peak detection threshold of the differential signal after normalization, the first predetermined threshold TH1 scope is-256 ~-1024, representative value-512;

7) K in predetermined hunting zone, continues search positive peak, if positive peak is more than or equal to the second predetermined threshold TH2, shows the success of vacant symbol detection, otherwise, restart to search for negative peak;

Second predetermined threshold TH2: the positive peak detection threshold of the differential signal after normalization, the second predetermined threshold TH2 scope is 512 ~ 2048, representative value 640;

Predetermined hunting zone K: after the negative peak of the differential signal after normalization being detected, the maximum search scope that search positive peak needs, hunting zone is 2656 ~ 3320, representative value 3160;

8) calculate the distance between positive negative peak, this distance is the length of vacant symbol, and obtains out sending mode according to the length of vacant symbol.

For a vacant symbol detection apparatus for said method, comprising:

Magnitude calculation unit, the modulus value calculating input signal according to input signal exports Difference Calculation unit and low-pass filter unit to;

Difference Calculation unit, carries out calculus of differences by the modulus value of the input signal calculated and exports rolling average unit to;

Rolling average unit, carries out rolling average by the differential signal calculated, the impact that noise decrease and signal fluctuation bring, and the differential signal obtained in statistical significance exports as normalization unit;

Low-pass filter unit, carries out low-pass filtering by the modulus value of the input signal calculated, obtains average modulus value, export normalization unit to;

Normalization unit, utilizes the average modulus value calculated, and carries out amplitude normalization to rolling average output signal;

Peak search element, according to obtaining the differential signal after normalization, search one positive negative two peak values, and according to the distance between positive negative peak, obtain the length of vacant symbol to judge sending mode.

First method of the present invention calculates the modulus value (or mould square) of input signal; Difference is carried out to modulus value signal; Rolling average is done to differential signal; Low-pass filtering is carried out to the modulus value of input signal simultaneously, obtain average modulus value; Utilize the average modulus value obtained, amplitude normalization is carried out to rolling average output signal; To the differential signal after normalization, search one positive negative two peak values, after searching negative peak, in predetermined hunting zone, continue search positive peak, if search positive peak, show the success of vacant symbol detection, otherwise, restart to search for negative peak; Calculate the distance between positive negative peak, this distance is the length of vacant symbol, and estimates sending mode accordingly.Method of the present invention can improve the detection perform of OFDM (OFDM) the vacant symbol (NullSymbol) of digital audio broadcasting (DAB).

Accompanying drawing explanation

Below in conjunction with drawings and Examples, the present invention is further detailed explanation:

Fig. 1 is DAB system signal frame structure schematic diagram.

Fig. 2 is vacant symbol detection apparatus structural representation.

Fig. 3 is peak search element process chart in Fig. 2.

Fig. 4 is low-pass filter unit pie graph in Fig. 2.

Fig. 5 is pattern II in sending mode table 1, without differential signal waveform figure after lower rolling average of making an uproar.

Description of reference numerals

Magnitude calculation unit 10

Difference Calculation unit 20

Rolling average unit 30

Low-pass filter unit 40

Normalization unit 50

Peak search element 60

Embodiment

Shown in Fig. 2, be vacant symbol detection apparatus of the present invention, comprise magnitude calculation unit 10, Difference Calculation unit 20, rolling average unit 30, low-pass filter unit 40, normalization unit 50 and peak search element 60.

First, the modulus value (or mould square) of input signal is calculated.Modulus value signal enters Difference Calculation unit 20 again, carries out the calculus of differences being spaced apart L.Usually, interval L is less than the half of the minimum vacant symbol lengths shown in table 1.Next, rolling average is carried out to differential signal, shown in (1):

$z\left(n\right)=\frac{1}{N}\underset{k=1}{\overset{N}{\mathrm{\Σ}}}y(n+k)---\left(1\right)$

In formula, y () is differential output signal, and z () is rolling average output signal, and N is rolling average length, and usual value is difference interval, i.e. N=L.

Near vacant symbol, after rolling average, differential signal as shown in Figure 5 (under the ideal communication channel of noiseless and interference in channel).At starting point place and the destination county of vacant symbol, after signal differential, form respectively trailing edge and rising edge.After further rolling average, obtain two triangles, the initial point position of the corresponding vacant symbol of negative peak, the corresponding final position of positive peak, the spacing of two peak values is the length of vacant symbol.

On the other hand, modulus value signal enters low pass filter as shown in Figure 4, obtains the average amplitude of signal.The relation of its input and output is such as formula shown in (2):

p(n)＝(1-beta)p(n-1)+beta*x(n)(2)

In formula, x () is modulus value signal, and p () is filter output signal, and beta is filter coefficient.

After being mixed into noise and interference in Received signal strength, particularly under serious fading channel, peak value size meeting random fluctuation, therefore, peakvalue's checking thresholding is difficult to determine, thus has influence on detection perform.Therefore, the average modulus value signal utilizing formula (2) to obtain is normalized differential signal.

Signal after normalization sends into peak detector.With reference to Fig. 3 and Fig. 5, first, search negative peak.Input signal D (n) and the first predetermined threshold TH1 compare.When being less than the first predetermined threshold TH1, show successfully to search negative peak; Then, at predetermined search ranges n+1, n+2, maximum D (m) is searched in n+K, n<m<n+K+1, and judge: when D (m) is less than the second predetermined threshold TH2, shows that positive correlation peaks detects unsuccessfully, turn back to the first step; When D (m) is more than or equal to the second predetermined threshold TH2, show that positive correlation peaks detects successfully.At this moment, the starting point of the corresponding vacant symbol of negative peak, the corresponding terminal of positive peak, initial point position is frame synchronization position; Calculating the distance Q=m-n between two peak values, is the vacant symbol lengths estimated.

Hunting zone is, the vacant symbol lengths of pattern I adds the protection gap length of pattern I, i.e. K=2656+504=3160.

According to the vacant symbol lengths Q estimated, according to table 2, obtain sending mode:

When Q is less than 3160 and is more than or equal to 1992, obtaining sending mode is pattern I;

When Q is less than 1992 and is more than or equal to 996, obtaining sending mode is pattern IV;

When Q is less than 996 and is more than or equal to 504, obtaining sending mode is pattern II;

When Q is less than 504, obtaining sending mode is pattern III.

Table 2

Difference interval L: to the parameter of modulus value signal differential, represents differential delay, span [1, (345-1)/2], representative value 128.

The span 0.01 ~ 0.0001 of low-pass filter coefficients beta: low-pass filter coefficients beta, representative value 0.001.

First predetermined threshold TH1: the negative peak detection threshold of the differential signal after normalization, predetermined threshold TH1 representative value-512.

Second predetermined threshold TH2: the positive peak detection threshold of the differential signal after normalization, predetermined threshold TH2 representative value 640.

Predetermined hunting zone K: after the negative peak of the differential signal after normalization being detected, the maximum search scope that search positive peak needs, representative value 3160.

Above by specific embodiment to invention has been detailed description, but these are not construed as limiting the invention.Without departing from the principles of the present invention, those skilled in the art also can make many distortion and improvement, and these also should be considered as protection scope of the present invention.

Claims (2)

1. the vacant symbol detection method of OFDM, is characterized in that, comprising:

1) modulus value or the mould square of input signal is calculated;

2) modulus value signal is spaced apart to the difference of L;

Difference interval L is the parameter to modulus value signal differential, and represent differential delay, span is 1 ~ (345-1)/2;

3) carry out to differential signal the rolling average that length is N, rolling average length N: to the length of differential signal rolling average, value is difference interval, i.e. N=L;

4) low-pass filtering is carried out to the modulus value of input signal, obtain average modulus value;

5) utilize the average modulus value obtained, amplitude normalization is carried out to rolling average output signal;

6) to the differential signal after normalization, search one positive negative two peak values, first search for negative peak, if negative peak is less than the first predetermined threshold (TH1), enter next step, otherwise continue search negative peak;

First predetermined threshold (TH1): the negative peak detection threshold of the differential signal after normalization, the first predetermined threshold (TH1) scope is-256 ~-1024;

7) in predetermined hunting zone (K), continue search positive peak, if positive peak is more than or equal to the second predetermined threshold (TH2), show the success of vacant symbol detection, otherwise, restart to search for negative peak;

Second predetermined threshold (TH2): the positive peak detection threshold of the differential signal after normalization, the second predetermined threshold (TH2) scope is 512 ~ 2048;

Predetermined hunting zone (K): after the negative peak of the differential signal after normalization being detected, the maximum search scope that search positive peak needs, hunting zone is 2656 ~ 3320;

8) calculate the distance between positive negative peak, this distance is the length of vacant symbol, and obtains out sending mode according to the length of vacant symbol.

2., for a vacant symbol detection apparatus for method as claimed in claim 1, it is characterized in that, comprising:

Magnitude calculation unit, the modulus value calculating input signal according to input signal exports Difference Calculation unit and low-pass filter unit to;

Difference Calculation unit, carries out calculus of differences by the modulus value of the input signal calculated and exports rolling average unit to;

Rolling average unit, carries out rolling average by the differential signal calculated, the impact that noise decrease and signal fluctuation bring, and the differential signal obtained in statistical significance exports as normalization unit;

Low-pass filter unit, carries out low-pass filtering by the modulus value of the input signal calculated, obtains average modulus value, export normalization unit to;

Normalization unit, utilizes the average modulus value calculated, and carries out amplitude normalization to rolling average output signal;

Peak search element, according to obtaining the differential signal after normalization, search one positive negative two peak values, and according to the distance between positive negative peak, obtain the length of vacant symbol to judge sending mode.