CN101764775A - Method and system for transmitting OFDM synchronizing pilot - Google Patents

Abstract

The invention relates to a system for transmitting an OFDM synchronizing pilot, which comprises a time synchronization module (103), a relevant operation module (104), a decimal-multiple frequency-deviation estimation module (105), an integral-multiple frequency-deviation estimation module (107), a repeating type conversion module (108) and a data demodulation module (109). The time synchronization module (103) carries out time deviation estimation at a receiving terminal; the relevant operation module (104) carries out relevant operation on the two parts of a sequence used for synchronization according to a corresponding order; the decimal-multiple frequency-deviation estimation module (105) estimates frequency deviation which is smaller than half of a subcarrier; the integral-multiple frequency-deviation estimation module (107) estimates frequency deviation which is times of the subcarrier; the repeating type conversion module (108) converts a signal from a time domain to a frequency domain; and the data demodulation module (109) demodulates the transmitted data. By adopting the invention, the integral-multiple frequency deviation can be estimated without carrying out repeating type conversion, thereby the realization complexity is reduced greatly.

Description

OFDM synchronizing pilot sending method and system

Technical field

The present invention relates to the field of information transmission in the wireless cellular system, particularly the pilot tone setting of wireless cellular system and the method and system of frequency offset estimating.

Background technology

OFDM (OFDM) technology is actually a kind of of MCM (Multi-CarrierModulation, multi-carrier modulation).Its main thought is, channel is divided into some orthogonal sub-channels, converts high-speed data signal to parallel low speed sub data flow, is modulated on each subchannel and transmits.Orthogonal signalling can separately can reduce the phase mutual interference (ICI) between the subchannel like this by adopt correlation technique at receiving terminal.Signal bandwidth on each subchannel is less than the correlation bandwidth of channel, so can regard the flatness decline as on each subchannel, thereby can eliminate intersymbol interference.And because the bandwidth of each subchannel only is the sub-fraction of former channel width, it is relatively easy that channel equalization becomes.But ofdm system is again that less frequency shift (FS) will cause more serious intersymbol interference to frequency shift (FS) sensitivity very, thereby influences systematic function, so the design of frequency deviation estimating method is a key job of ofdm system.

OFDM (OFDM) system is the hot issue that current industrial circle and academia pay close attention to, there have been many wireless communication systems to be based on MIMO-OFDM's, as IEEE802.16e, IEEE802.11n etc., CMMB, and MIMO-OFDM dominate in next generation mobile communication system very likely.

Adopt the main cause of MIMO-OFDM to be that the MIMO-OFDM system can provide the ability of broadband wireless access, and in broadband access, design effectively and the few targeting sequencing of resource requirement be very challenging problem.

Here the system of Cai Yonging is the system of OFDM, and its symbolic construction comprises 601 Cyclic Prefix and OFDM symbol as shown in Figure 6, and the part 602 of OFDM symbol back is the same with circulation 601 prefixes of front.

Time-frequency synchronization is summarized as follows in the classical ofdm system: use targeting sequencing, with the relevant of OFDM symbol can estimated time and frequency shift (FS).

After frame arrives, at first pass through the targeting sequencing on the time domain, the relevant synchronous frequency offset estimating that reaches in 1/2 subcarrier of symbol time that obtains with the OFDM symbol, then by fourier-transform, mate with the pilot frequency sequence on the frequency domain and to obtain integral multiple subcarrier spacing frequency deviation (abbreviation integer frequency bias) and estimate.

At present, frequency offset estimating is at first utilized the relevant of Cyclic Prefix and OFDM, estimates fractional part of frequency offset, and then by fourier-transform, estimates integer frequency offset at frequency domain by being correlated with, as shown in Figure 5.

The symbol lengths that the pilot tone that is used for frequency offset estimating is placed each OFDM is the sampling value that N is ordered, and then the L of back point is carried out repetition in the front of OFDM symbol, L≤N, as shown in Figure 4.Wherein, N is the number of sampling of each OFDM symbol, and L is the number of sampling of Cyclic Prefix.

Frequency offset estimation

Be by the correlation computations between OFDM symbol and the Cyclic Prefix thereof is obtained.Its function calculation method is as follows:

$\mathrm{\Δ}\widehat{\mathrm{\ϵ}}=\frac{1}{2\mathrm{\π}}\∠\mathrm{\γ}$

Wherein, γ be by the sampling value among the cp of baseband receiving signals and in the OFDM symbol correlation between the corresponding sampling value obtain

$\mathrm{\γ}=\underset{k=0}{\overset{L-1}{\mathrm{\Σ}}}r\left(k\right)\·r^{*}(k-N)$

Wherein, r (k) is a k sampling value of baseband receiving signals, and N is the time interval number that carries out between the sample of signal of correlation computations, just in time equals the OFDM symbol and carries out counting of FFT computing, L is the length of related operation window, just in time equals the length of OFDM symbol cyclic prefix CP here.Its principle as shown in Figure 3.

In the existing frequency offset estimation methods; be to carry out according to the correlation computations between OFDM symbol and the Cyclic Prefix thereof; being limited in scope of Frequency offset estimation is the width of 1/2 subcarrier, and the frequency shift (FS) of ofdm system through regular meeting the width greater than 1/2 subcarrier.This estimates with regard to the method for estimation that needs extra employing integer frequency offset.

What need improve is exactly the characteristics of placing by the change pilot tone, the scope of increasing Frequency offset estimation, and under the very little situation of implementation complexity increase, what the scope of Frequency Estimation became meets the demands.

Summary of the invention

The purpose of this invention is to provide a kind of frequency offset estimation methods and system.

According to an aspect of of the present present invention, a kind of OFDM synchronizing pilot transmitting system comprises: at transmitting terminal,

The pilot frequency sequence generation module is used for producing pilot frequency sequence signal, and wherein pilot frequency sequence satisfies following condition:

M (n)=m (N+n) or m (n)=m (2N-n);

The fourier-transform module, with signal from the frequency domain transform to the time domain.

According to another aspect of the present invention, a kind of OFDM synchronizing pilot receiving system comprises: at receiving terminal,

The time synchronized module is carried out time deviation and is estimated;

The related operation module, two parts that will be used for synchronous sequence are carried out related operation according to corresponding order;

The fractional part of frequency offset estimation module is estimated the frequency deviation less than half subcarrier;

The integer frequency offset estimation module is estimated the frequency shift (FS) of subcarrier multiple;

Anti-fourier-transform module transforms from the time domain to frequency domain with signal;

Data demodulation module is come out the data demodulates of transmission.

The present invention can carry out anti-fourier-transform, just can estimate integer-times frequency offset, and implementation complexity is reduced greatly.

Description of drawings

Fig. 1 is the structure chart that improves the communication system of the frequency offset estimating that pilot tone is provided with;

Fig. 2 is the structure chart that improves pilot frequency sequence;

Fig. 3 is fractional part of frequency offset estimation principles figure;

Fig. 4 is the structure chart of present improvement pilot frequency sequence;

Fig. 5 is the structure chart of the communication system of the frequency offset estimating that is provided with of present pilot tone;

Fig. 6 is that the symbolic construction of ofdm system is formed.

Embodiment

Frequency offset estimation methods of the present invention at transmitting terminal, comprises the steps: as shown in Figure 1

Two continuous synchronizing symbols, length is identical, transmits 2N data symbol, or a synchronizing symbol, is divided into two parts;

The pilot frequency sequence that sends satisfies following conditions:

M (n)=m (N+n) or

m(n)＝m(2N-n)

1≤n≤N-1

Wherein, m (n) is the value of symbol of n point sampling, and N is a data symbol length.

The pilot frequency sequence design of transmitting terminal as shown in Figure 2.

Frequency offset estimation methods of the present invention as shown in Figure 1, at receiving terminal, received signal is

r(n)＝m(n)*e ^j2πεn/2N

Wherein, r (n) is a n sampling value of baseband receiving signals, and m (n) is the signal that sends, and ε is a frequency deviation value, and N is the time interval number that carries out between the sample of signal of correlation computations, and Frequency offset estimation obtains by frequency pilot sign is carried out following calculating.Its function calculation method is as follows, and promptly selecting suitable frequency deviation value to make the frequency values of the value maximum of following this function promptly is the frequency offseting value that estimates:

$f=\underset{f}{\arg \max \left({\left|\mathrm{\φ}\left(f\right)\right|}^2\right)}$

Wherein, $\mathrm{\&phi;}\left(f\right)=\underset{n=1}{\overset{N-1}{\mathrm{\&Sigma;}}}\mathrm{\&alpha;}\left(n\right)*e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="H2008101849316E0000012.png,H2008101849316E0000022.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;fn}}$

And α (n)=r (n) * r ^*(2N-n)

Wherein, α (n) multiplies each other by two-part sampling value before and after the baseband receiving signals and obtains.

Top formula is simplified, obtained

We know, when the estimated value of frequency deviation

The time,

The value maximum, promptly when the scope of f was 1/2 subcarrier, the estimation range of frequency deviation was a N/2 subcarrier.Wherein, r (k) is a k sampling value of baseband receiving signals, and N is the time interval number that carries out between the sample of signal of correlation computations, just in time equals the OFDM symbol and carries out counting of FFT computing.

In actual implementation process, we can make

Multiple increase progressively, till the scope that reaches frequency offset estimating.

The invention provides a kind of under the very little situation of implementation complexity, a kind of pilot setting system that channel estimated accuracy is improved.The structure of system as shown in Figure 1.Comprise:

Pilot frequency sequence generation module (101) is used for producing pilot frequency sequence signal;

Fourier-transform module (102), with signal from the frequency domain transform to the time domain;

At receiving terminal,

Time synchronized module (103) is carried out time deviation and is estimated;

Related operation module (104), two parts that will be used for synchronous sequence are carried out related operation according to corresponding order.

Fractional part of frequency offset is estimated (105), and the frequency deviation less than half subcarrier is estimated;

Integer frequency offset is estimated (107), and the frequency shift (FS) of subcarrier multiple is estimated;

Anti-fourier-transform module (108) transforms from the time domain to frequency domain with signal;

Data demodulates (109) is come out the data demodulates of transmission.

Embodiment

Present embodiment has adopted two continuous synchronizing symbols, and length is identical, transmits 4096 data symbols, is divided into two parts.The pilot frequency sequence that sends satisfies following conditions:

M (n)=m (N+n) or

m(n)＝m(2N-n)

1≤n≤N-1

At receiving terminal, comprise the steps that wherein, received signal is:

r(n)＝m(n)*e ^j2πεn/2N

Wherein, r (n) is a n sampling value of baseband receiving signals, and m (n) is the signal that sends, and ε is a frequency deviation value, and N is the time interval number that carries out between the sample of signal of correlation computations, and Frequency offset estimation obtains by frequency pilot sign is carried out following calculating.Its function calculation method is as follows, and promptly selecting suitable frequency deviation value to make the frequency values of the value maximum of following this function promptly is the frequency offseting value that estimates:

$f=\underset{f}{\arg \max \left({\left|\mathrm{\&phi;}\left(f\right)\right|}^2\right)}$

Wherein, $\mathrm{\&phi;}\left(f\right)=\underset{n=1}{\overset{N-1}{\mathrm{\&Sigma;}}}\mathrm{\&alpha;}\left(n\right)*e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="H2008101849316E0000012.png,H2008101849316E0000022.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;fn}}$

And α (n)=r (n) * r ^*(2N-n)

Wherein, α (n) multiplies each other by two-part sampling value before and after the baseband receiving signals and obtains.

Top formula is simplified, obtained

Then when f=ε/N,

The value maximum, promptly when the scope of f was 1/2 subcarrier, the estimation range of frequency deviation was a N/2 subcarrier.The width of each subcarrier is 39.0625kHz, and total bandwidth is that the maximum of the such frequency offset estimating of 8M. is 1MHz.

Wherein, r (k) is a k sampling value of baseband receiving signals, and N is the time interval number that carries out between the sample of signal of correlation computations, just in time equals the OFDM symbol and carries out counting of FFT computing.

Improved pilot tone setting, and improved frequency deviation estimating method makes the scope of the frequency offset estimating of ofdm system increase N/4 doubly.

Claims (10)

1. OFDM synchronizing pilot transmitting system comprises: at transmitting terminal,

Pilot frequency sequence generation module (101) is used for producing pilot frequency sequence signal, and wherein pilot frequency sequence satisfies following condition:

M (n)=m (N+n) or m (n)=m (2N-n);

Fourier-transform module (102), with signal from the frequency domain transform to the time domain.

2. OFDM synchronizing pilot transmitting system comprises: at receiving terminal,

Time synchronized module (103) is carried out time deviation and is estimated;

Related operation module (104), two parts that will be used for synchronous sequence are carried out related operation according to corresponding order;

Fractional part of frequency offset estimation module (105) is estimated the frequency deviation less than half subcarrier;

Integer frequency offset estimation module (107) is estimated the frequency shift (FS) of subcarrier multiple;

Anti-fourier-transform module (108) transforms from the time domain to frequency domain with signal;

Data demodulation module (109) is come out the data demodulates of transmission.

3. by claim 1 or 2 described systems, it is characterized in that described system is the OFDM wireless communication system.

4. by claim 1 or 2 described systems, it is characterized in that described system is for comprising the Cyclic Prefix in OFDM System system.

5. OFDM synchronizing pilot sending method comprises: at transmitting terminal,

Produce pilot frequency sequence signal by pilot frequency sequence generation module (101), wherein pilot frequency sequence satisfies following condition:

M (n)=m (N+n) or m (n)=m (2N-n);

By fourier-transform module (102) with signal from the frequency domain transform to the time domain.

6. OFDM synchronizing pilot sending method comprises: at receiving terminal,

Carrying out time deviation by time synchronized module (103) estimates;

Two parts that will be used for synchronous sequence by related operation module (104) are carried out related operation according to corresponding order;

By fractional part of frequency offset estimation module (105) frequency deviation less than half subcarrier is estimated;

By integer frequency offset estimation module (107) frequency shift (FS) of subcarrier multiple is estimated;

By anti-fourier-transform module (108) signal is transformed from the time domain to frequency domain;

By data demodulation module (109) data demodulates of transmission is come out.

7. by the described method of claim 5, it is characterized in that described frequency pilot sign is two continuous synchronizing symbols.

8. by the described method of claim 7, it is characterized in that the length of described two continuous synchronizing symbols is identical.

9. by the described method of claim 5, it is characterized in that described frequency pilot sign is a continuous synchronizing symbol.

10. by the described method of claim 9, it is characterized in that a described continuous synchronizing symbol is divided into two parts.

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