CN1992697A - Method of coarse symbol timing estimation employed in a radio communication system - Google Patents

Abstract

According to the invention, a coarse symbol timing estimation method in the radio communication system is provided, the said method including: using the relative window whose length less than the protection interval length to form the relative functions needed by the coarse symbol timing estimation; using the said relative functions to gain the relative patterns of input sampling signal after relative operations; and then, using the falling-edge of relative peak fluctuation platform followed closely to the relative patterns, and the arbitrary value below the relative peak fluctuation platform as threshold value to obtain the coarse symbol timing position.

Description

The coarse symbol timing estimation method that adopts in the radio communications system

Technical field

The present invention relates to the coarse symbol timing estimation method that adopts in a kind of radio communications system, described radio communications system is the DRM system particularly.The method according to this invention can alleviate the influence of various channel unfavorable factors to thick symbol timing estimation.

Background technology

DRM (digital radio mondiale broadcasting, Digital Radio Mondiale) is a kind of new Digital wireless broadcast system, and it is unique universal non-patent digital radio broadcasting system of shortwave, medium wave and long wave amplitude modulation broadcasting frequency range.Under same coverage condition, the DRM transmitter power is than the low 6-9dB of traditional analog transmissions acc power, and digital broadcasting is lower than the same adjacent frequency protective rate of analog broadcasting, and the anti-multipath interference performance is strong, is convenient to move receive; Tonequality can reach the quality of CD or FM multiplex; Additional data and multimedia messages can be provided; Compare with DAB, its receiver price is easier to be accepted by the mass audience.Its appearance is the sign that the following frequency range broadcasting of 30MHz is revived, and has become international standard at present.

The DRM system has adopted OFDM (OFDM) technology.OFDM can be regarded as a kind of special multi-transceiver technology.Compare multicarrier system more complicated synchronously with single-carrier system.Compare with other multicarrier system, the short wave channel transmission environment of DRM system is then more abominable, shows: the expansion of bigger time delay, stronger delay time signal power, bigger relative Doppler's expansion and have Doppler frequency shift etc.Therefore, the design of the synchronized algorithm of DRM system will be more complicated and challenging.

In the DRM receiver, can be divided into five tasks synchronously, i.e. robustness mode detection, timing synchronization, carrier frequency synchronization, sampling clock synchronously and frame regularly synchronously, as shown in Figure 1.In the DRM system, timing synchronization can be divided into three main portions: thick symbol timing estimation (acquisition phase), thick symbol timing deviation are estimated (acquisition phase) and smart symbol timing deviation estimation (tracking phase).This specification will be absorbed in the thick symbol timing estimation technology of DRM system.

The DRM system has four kinds of robustness mode modes, i.e. Mode A, B, C and D.Table 1 has been listed the typical use of these four kinds of robustness mode modes.

The typical use of table 1 robustness mode

Robustness mode	Typical case's communication environments
		A	Gaussian channel has slight decline
B	Time and frequency-selective channel have bigger time delay expansion
		C	With robustness mode B, but bigger Doppler's expansion is arranged
D	With robustness mode B, but serious time delay expansion and Doppler's expansion are arranged

Corresponding to different robustness modes, defined different OFDM parameter sets, as shown in table 2.

Table 2OFDM signal parameter

In table 2, T _sThe duration of expression OFDM symbol, T _gThe expression protection duration at interval, T _uThe duration of expression OFDM symbol useful part, T represents the basic time cycle, equals 83 (1/3) μ s.If with T, as chronomere, then corresponding to four kinds of Mode As, B, C and D, its protection length at interval equals 32,64,64 and 88 successively.In like manner, the length of useful part equals 288,256,176 and 112 successively.Based on above-mentioned parameter, can determine the length of IFFT and FFT.Usually the length of IFFT and FFT is the integral multiple of useful part length, and 4 times be a parameter commonly used.Under this condition, the length of IFF and FFT equals 288 * 4=1152,256 * 4=1024,176 * 4=704 and 112 * 4=448 successively.

Table 3 has been listed the parameter value of the DRM receiver employing of a reality.

The OFDM parameter value of one group of reality of table 3

Parameter	Mode A	Mode B	Pattern C	Pattern D
					Protection length (N at interval g)	128	256	256	352
Length (the N of useful part u)	1152	1024	704	448
					IFFT length	1152	1024	704	448

After the robustness mode detection was finished, the synchronous next task of DRM was carried out thick timing synchronization exactly.As everyone knows, the location of mistake of FFT demodulation window will cause intersymbol interference (ISI), even inter-carrier interference (ICI), and follow-up synchronous and channel estimating is produced very big influence.Two main targets of thick symbol timing estimation are: guarantee that 1) position of estimating is positioned at the ISI free zone, as shown in Figure 2; 2) fluctuation of estimated position is little.As can be seen from Figure 2, the symbol timing position of estimation may drop on four zones: the data area of the data area of previous symbol, ISI zone, current sign and do not have the ISI zone.In these four zones, have only last zone, promptly there is not the ISI zone, be the symbol timing position zone of wishing.

The coarse symbol timing estimation method relevant at interval based on protection is widely used.In the content below, we will at first introduce a kind of traditional coarse symbol timing estimation method.

Coarse symbol timing estimation method is usually based on the relevant at interval thought of protection.Its correlation function is defined as

λ(θ)＝|γ(θ)|-ρΦ(θ) (1)

Here,

$\mathrm{\γ}\left(\mathrm{\θ}\right)=\underset{k=\mathrm{\θ}}{\overset{\mathrm{\θ}+L-1}{\mathrm{\Σ}}}r\left(k\right)\·r^{*}(k-N_u)---\left(2\right)$

$\mathrm{\Φ}\left(\mathrm{\θ}\right)=\frac{1}{2}\underset{k=\mathrm{\θ}}{\overset{k+L-1}{\mathrm{\Σ}}}{\left|r\left(k\right)\right|}^2+{\left|r(k-N_u)\right|}^2---\left(3\right)$

Here the OFDM sampled signal that r (k) expression receives, k represents signal sampling point sequence number.N _uThe length of the useful part of expression OFDM symbol is as above shown in the table 3.L represents the length of associated window.Certainly, correlation function of the present invention is not limited thereto, and it can be other correlation functions that adopted in the range radio communication system.

Traditional coarse symbol timing estimation method has utilized the information of whole protecting interval, and promptly the length L of associated window equals the length N of whole protecting interval _gUnder the different robustness mode situations, parameter N _gAnd N _uValue as above shown in the table 3.Under high s/n ratio (SNR:Signal-to-Noise Ratio) situation, the value that parameter ρ is set is 1.

The estimation criterion of tradition coarse symbol timing estimation method can be expressed as

$\widehat{\mathrm{\θ}}=\arg \underset{\mathrm{\θ}}{\max }\mathrm{\λ}\left(\mathrm{\θ}\right)---\left(4\right)$

Above-mentioned estimation criterion can be described as making the maximum timing position θ of correlation function λ (θ) to be thick symbol timing estimation value

As everyone knows, traditional coarse symbol timing estimation method has good estimation performance under the additive white Gaussian channel situation.But at multidiameter fading channel, particularly under the long time delay extended channel situation, the estimated performance of conventional method significantly descends.This is because associated window has comprised the information of other OFDM symbol, the information of for example previous OFDM symbol, as shown in Figure 3.The information of other symbol can be counted as a kind of interference signal, and it will influence correlated results, thereby causes the decline of estimated performance.And under the Gaussian channel condition, owing to have only a transmission path, so will not have interference signal in the associated window, as shown in Figure 4.

Yet we know that in a mobile radio communications system, multipath transmisstion is its transmission channel most important character.AM broadcast wireless transmissions channel also is like this.But compare with other most mobile radio system, the channel model of DRM system is more complicated, and this makes its synchronous also become more complicated.The channel model of DRM system has following outstanding feature: 1) bigger time delay is expanded; 2) bigger inhibit signal average power; 3) bigger Doppler expands; 4) there is Doppler frequency shift etc.In the DRM system specifications, listed six kinds of AM broadcast wireless transmissions channel models altogether, as shown in table 4.We are example with channel model 5, explain the distinguishing feature of the AM of DRM system broadcast transmission channel.As shown in table 4, the time delay expansion of channel model 5 is 4ms nearly, has almost accounted for 3/4 of the Mode B whole protecting time; The signal power in second path equals the power of main footpath signal, and the power of inhibit signal all is far smaller than main footpath signal power usually.In addition, the Doppler in 5 two paths of channel model expansion even greater than 4% of Mode B subcarrier spacing.For other channel model, for example channel 3, and it also exists the Doppler frequency shift factor.

The channel parameter of table 4DRM system

Channel number 1:AWGN		Typical good/middle equal difference		LF，MF，HF LF，var.SNR
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0
Path gain, rms (ρ k)	1
				Doppler frequency shift (D sh)	0
Doppler spread (D sp)	0

Channel number 2: Rice channel with delay		Typical good/middle equal difference		MF，HF
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0	1ms
Path gain, rms (ρ k)	1	0.5
				Doppler frequency shift (D sh)	0	0
Doppler spread (D sp)	0	0.1Hz

Channel number 3:US association		Typical good/middle equal difference		HF MF
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0	0.7ms	1.5ms	2.2ms
Path gain, rms (ρ k)	1	0.7	0.5						0.25
				Doppler frequency shift (D sh)	0.1Hz	0.2Hz	0.5Hz	1.0Hz
Doppler spread (D sp)	0.1Hz	0.5Hz	1.0Hz						2.0Hz

The bad channel of channel number 4:CCIR		Typical good/middle equal difference		HF
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0	2ms
Path gain, rms (ρ k)	1	1
				Doppler frequency shift (D sh)	0	0
Doppler spread (D sp)	1Hz	1Hz

Channel number 5		Typical good/middle equal difference		HF
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0	4ms
Path gain, rms (ρ k)	1	1
				Doppler frequency shift (D sh)	0	0
Doppler spread	2Hz	2Hz

(D sp)

Channel number 6		Typical good/middle equal difference		HF
						Path
1	Path 2	Path 3	Path 4
				Postpone (Δ k)	0	2ms	4ms	6ms
Path gain, rms (ρ k)	0.5	1	0.25						0.0625
				Doppler frequency shift (D sh)	0	1.2Hz	2.4Hz	3.6Hz
Doppler spread (D sp)	0.1Hz	2.4Hz	4.8Hz						7.2Hz

In the emulation of DRM system link, we find that traditional coarse symbol timing estimation method has very high estimate variance.More bad is that the symbol timing position of estimation falls into ISI zone or data area through regular meeting, as shown in Figure 2.In order to overcome these shortcomings of conventional method, we have proposed a kind of according to coarse symbol timing estimation method of the present invention.

Summary of the invention

Proposed the present invention in order to overcome defective of the prior art, therefore, the objective of the invention is the coarse symbol timing estimation method that adopts in a kind of radio communications system, described radio communications system is the DRM system particularly.The method according to this invention can alleviate the influence of various channel unfavorable factors to thick symbol timing estimation.

To achieve these goals, according to the present invention, proposed the coarse symbol timing estimation method that adopts in a kind of radio communications system, described method comprises: utilize length to form the required correlation function of thick symbol timing estimation less than the associated window of protection length at interval; Utilize described correlation function to obtain the sampled signal of input is carried out relational pattern behind the related operation; And utilize on the trailing edge of the relevant peaks fluctuation platform follow relational pattern closely, the arbitrary value that is lower than relevant peaks fluctuation platform obtains thick symbol timing position as threshold value.

Preferably, the parameter of reflection signal to noise ratio situation is set to less than 1 in the described correlation function.

Preferably, the time span of the length representative of described associated window deducts the maximum multipath time delay of multipath in disturbing less than the time span of protection length representative at interval.

Preferably, described threshold value equals zero.

Preferably, described radio communications system is the DRM system.

Preferably, described coarse symbol timing estimation method based on protection at interval.

Description of drawings

Below in conjunction with the detailed description of preferred embodiment of accompanying drawing to being adopted, above-mentioned purpose of the present invention, advantage and feature will become apparent by reference, wherein:

Fig. 1 shows the block diagram of DRM synchronous task;

Fig. 2 shows the schematic diagram in four zones that the thick symbol timing position of estimation may fall into;

Fig. 3 shows the schematic diagram of the associated window under the multidiameter fading channel condition;

Fig. 4 shows the schematic diagram of the associated window under the Gaussian channel condition;

Fig. 5 shows the schematic diagram according to coarse symbol timing estimation method of the present invention;

Fig. 6 shows the curve chart of the probability distribution (Mode A, channel 1) that method of the present invention compares with conventional method;

Fig. 7 shows the curve chart of the probability distribution (Mode B, channel 3) that method of the present invention compares with conventional method; And

Fig. 8 shows the performance curve chart relatively of method of the present invention and traditional method of estimation.

Embodiment

In order to overcome these shortcomings of conventional method, we have proposed a kind of according to coarse symbol timing estimation method of the present invention.Although according to coarse symbol timing estimation method of the present invention equally based on the at interval relevant thought of protection; and its correlation function is identical with conventional method; but in order to alleviate the influence of various channel unfavorable factors to thick symbol timing estimation, thick signal-timing method according to the present invention has been taked the estimation strategy of three kinds of keys.These three kinds of influences that strategy has effectively alleviated various channel unfavorable factors, thus guarantee that the symbol timing position of estimating falls into no ISI zone and has littler estimated position fluctuation.

According to coarse symbol timing estimation method of the present invention equally based on the at interval relevant thought of protection.Its correlation function is identical with conventional method, shown in the formula (1).But in order to alleviate the influence of various channel unfavorable factors to thick symbol timing estimation, we have taked three kinds of strategies:

(1) in correlation function formula (1), the length L of associated window is less than protection length N at interval _g, preferably, the time span (L multiply by the sampling period) of the length L representative of described associated window is less than protection interval N _gThe time span (N of length representative _gMultiply by the sampling period) deduct the maximum multipath time delay of multipath in disturbing.

(2) in correlation function formula (1), the value of parameter ρ is less than 1; This parameter ρ is generally the parameter of reflection system signal noise ratio situation.

According to above (1) and (2) two conditions, can form according to the required correlation function of thick symbol timing estimation of the present invention.But, it should be noted that in the present invention condition (2) is optional condition.

(3) thus,, revised the timing estimation criterion, promptly obtained the rule of symbol timing position utilizing after described correlation function obtained the relational pattern of sampled signal after carrying out related operation to input.

Just, it satisfies two conditions: 1) be positioned on the trailing edge of relevant peaks fluctuation platform; 2) arbitrary value that will be lower than relevant peaks fluctuation platform is selected thick symbol timing position as threshold value.

As example, in Fig. 5, the threshold value of selection equals zero.Certainly, can select to follow closely on the trailing edge of relevant peaks fluctuation platform of relational pattern, be lower than the arbitrary value of relevant peaks fluctuation platform as threshold value.

Particularly, above-mentioned the 3rd strategy as shown in Figure 5.In Fig. 5, parameter L is set equals N _g/ 4, ρ equals 0.85.Other simulation parameter is: Mode B, and channel 3, the 10KHz bandwidth, SNR is 25.4dB.

Assessed traditional and according to the performance of coarse symbol timing estimation method of the present invention by Computer Simulation.Select following two performance index: the 1) probability distribution of the symbol timing position of Gu Jiing; 2) mean square error of the symbol timing position of Gu Jiing.

Common simulation parameter: 10KHz signal bandwidth, carrier frequency shift 2.45F (F represents subcarrier spacing), sampling clock frequency offset 50ppm, baseband signal sampling point speed 48KHz.For coarse symbol timing estimation method according to the present invention, L=N _g/ 4, ρ=0.85.

Can understand effect of the present invention thus.As shown below.

(1) probability distribution

The DRM system has 6 kinds of channel models.We select two typical channels: channel 1 and 3, its parameter is as shown in table 4.Channel 1 is Gaussian channel.With respect to channel 1, robustness mode is A and SNR 15.6dB.With respect to channel 3, robustness mode is B and SNR 25.4dB.Simulation result as shown in Figure 6 and Figure 7.As a comparison, we have also provided the simulation result that reduces the conventional method of correlation length, and correlation length equals N _g/ 4.

It is emphasized that if use

Represent thick symbol estimated value regularly, then when the estimated position falls into the data area of previous symbol, $\widehat{\mathrm{\&theta;}}<0;$ When the estimated position falls into ISI or ISI free zone, $0\&le;\widehat{\mathrm{\&theta;}}\&le;N_g;$ When the estimated position falls into the data area of current sign, $\widehat{\mathrm{\&theta;}}>N_g.$

As can be seen from Figure 6, under the Gaussian channel condition, the estimated position of conventional estimated method is usually located near the timing position 0.Though conventional method has good focus characteristics, almost half estimated position has all fallen into the data area of previous symbol.And not only have good focus characteristics according to coarse symbol timing estimation method of the present invention, and its estimated position can fall into no ISI zone usually.

As can be seen from Figure 7, the focus characteristics of conventional estimated method begins variation under the multipath channel condition.Worse, its estimated position can fall into the ISI zone usually.This is for being unacceptable synchronously.We know, suitably reduce the influence that correlation window length can alleviate the channel unfavorable factor that causes owing to multipath.As shown in Figure 7, when adopting the correlation window length that reduces, the estimated position of conventional estimated method has mainly fallen into no ISI zone, and this is a very big raising.But the regional extent that its estimated position falls into is very big, for example from 64 to 192.The fluctuation that this means the estimated position is very big.Can effectively overcome this shortcoming according to method of estimation of the present invention.As can be seen from Figure 7, fall into a scope littler zone relatively, for example from 192 to 256 according to the estimated position of method of estimation of the present invention.This hint coarse symbol timing estimation method according to the present invention has littler estimated position fluctuation.

(2) mean square error

Mean square error is defined as

$\mathrm{MSE}=\frac{1}{N}\mathrm{\&Sigma;}{\left(\frac{\widehat{\mathrm{\&theta;}}-{\mathrm{\&theta;}}_{\mathrm{opt}}}{N_g}\right)}^2$

Here, θ _OptThe symbol timing position that expression is best equals N _g, as shown in Figure 2.N represents the OFDM symbolic number.In emulation, each simulating scenes uses 100000 symbols.For each simulating scenes, its simulation parameter is as shown in table 5.

Simulation result as shown in Figure 8.As can be seen from Figure 8, compare with conventional method, coarse symbol timing estimation method according to the present invention has littler estimation mean square error.

The setting of table 5 simulation parameter

Scene	Channel

1	Channel 2	Channel 3	Channel 4	Channel 5	Channel 6
						Parameter is provided with	Mode A channel 1 15.6dB	Mode A channel 2 16.6dB	Mode B channel 3 25.4dB	Mode B channel 4 23.5dB	Mode B channel 5 22.7dB	Pattern D channel 6 25.2dB

Compare with the transmission channel of other most of mobile radio systems, the channel condition of DRM system is more abominable.This cause traditional coarse symbol timing estimation method can not be in the DRM system operate as normal.For this reason, we have proposed above-mentioned effective coarse symbol timing estimation method.

Be traditional or all be based on the relevant at interval thought of protection according to coarse symbol timing estimation method of the present invention.These two kinds of main differences of method are:

1) they have adopted different correlation window length.Traditional coarse symbol timing estimation method, its correlation window length L equals the length N of whole protecting interval _gAnd according to coarse symbol timing estimation method of the present invention, the length L of its associated window is less than protection protection length N at interval _g

2) they have adopted different estimation criterions.The estimation criterion of conventional method can be described as: make the maximum timing position of correlation function λ (θ) be selected as the estimated value of thick symbol timing position.And the estimation criterion of new method is more complicated.Timing position need satisfy two conditions: 1) be positioned at after the relevant peaks; 2) its corresponding correlation function value is first less than 0 value; Just can be selected as thick symbol timing position estimated value.

Compare with traditional coarse symbol timing estimation method, reduce the influence that associated window can effectively alleviate various channel unfavorable factors, thereby guarantee that the symbol timing position of estimating falls into no ISI zone.And can guarantee that according to estimation criterion of the present invention the symbol timing position of estimating has littler fluctuation.Therefore, coarse symbol timing estimation method according to the present invention has three significant advantages:

1) guarantees that the thick symbol timing position of estimating can not fall into ISI or data area;

2) fluctuate in littler estimated position;

3) take still less hardware resource.

Simulation result also shows.Compare with traditional method of estimation, coarse symbol timing estimation method according to the present invention has better estimated performance.

Although below show the present invention in conjunction with the preferred embodiments of the present invention, one skilled in the art will appreciate that under the situation that does not break away from the spirit and scope of the present invention, can carry out various modifications, replacement and change to the present invention.Therefore, the present invention should not limited by the foregoing description, and should be limited by claims and equivalent thereof.

Claims (6)

1, the coarse symbol timing estimation method that adopts in a kind of radio communications system, described method comprises:

Utilize length to form the required correlation function of thick symbol timing estimation less than the associated window of protection length at interval;

Utilize described correlation function to obtain the sampled signal of input is carried out relational pattern behind the related operation; And

The arbitrary value that utilization follows closely on the trailing edge of relevant peaks fluctuation platform of relational pattern, be lower than relevant peaks fluctuation platform obtains thick symbol timing position as threshold value.

2, method according to claim 1 is characterized in that the parameter of reflection signal to noise ratio situation in the described correlation function is set to less than 1.

3, method according to claim 1 is characterized in that the time span of the length representative of described associated window deducts the maximum multipath time delay of multipath in disturbing less than the time span of protection length representative at interval.

4, method according to claim 1 is characterized in that described threshold value equals zero.

5, method according to claim 1 is characterized in that described radio communications system is the digital radio mondiale broadcast system.

6, method according to claim 1 is characterized in that described coarse symbol timing estimation method based on protection at interval.

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