Method for frame and frequency synchronization of an OFDM signal and method for transmitting an OFDM signal

• US 7,339,882 B2
• Filed: 03/15/2002
• Issued: 03/04/2008
• Est. Priority Date: 03/28/2001
• Status: Active Grant

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Schaefer, Wolfgang, Hansen, Christian
• Primary Examiner(s): Yao; Kwang Bin
• Assistant Examiner(s): Lai; Andrew
• Application Number:
  US10/473,631
• Publication Number:
  US 20040146003A1
• Time in Patent Office: 2,181 Days
• Field of Search: 370/350, 370/503, 370/516, 370/343, 370/208, 370/203, 370/615
• US Class Current: 370/203
• CPC Class Codes: H04L 1/0056 Systems characterized by the type of code used H04L1/08 takes precedence
• CPC Class Codes: H04L 1/0071 Use of interleaving interleaving per se H03M13/27
• CPC Class Codes: H04L 27/2656 Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

Abstract

A method for frame and frequency synchronization of an OFDM signal and a signal for transmitting an OFDM signal is described, the purpose of which is to impress a pilot phase profile that is then used at the receiving end for frame and frequency synchronization on pilots which are already contained in the OFDM signal for channel estimation. This has the advantage that no additional transmission capacity has to be used for the synchronization. The method according to the present invention is initiated by a rough time synchronization unit connected upline, which searches for the beginning of the guard interval in the OFDM signal. The comparison between a stored pilot phase profile and the received subcarrier symbol is performed using a cross-correlation, whose result is then evaluated to determine the frame and frequency synchronization.

Claims

1. A method for a frame and frequency synchronization of an OFDM (orthogonal frequency division multiplexing) signal, the OFDM signal being used to receive an OFDM symbol, each of which has a pilot symbol as a subcarrier symbol, the method comprising:

adding an additional phase rotation to the pilot symbol;

transmitting the pilot symbol including the additional phase rotation;

performing a comparison of the subcarrier symbol with at least one pilot phase profile that is stored; and

performing the frame and frequency synchronization of the OFDM signal as a function of the comparison;

wherein the pilot phase profile is unique to each different OFDM symbol.

3. The method as recited in claim 1, wherein:

a unique pilot phase profile is produced by adding the additional phase rotation to the pilot symbol in a transmission frame.

4. The method as recited in claim 1, further comprising:

prior to the comparison of the subcarrier symbol with the at least one pilot phase profile, performing a rough time synchronization by searching for a guard interval in the OFDM signal.

5. The method as recited in claim 1, wherein:

the comparison is performed using a cross-correlation, and

the cross-correlation is then evaluated to determine the frame and frequency synchronization.

11. The method as recited in claim 5, wherein:

a main to secondary peak ratio is used to evaluate the cross-correlation.

12. The method as recited in claim 5, wherein:

a merit factor is used to evaluate the cross-correlation.

2. A method for transmitting an OFDM signal, the OFDM signal being used to transmit an OFDM symbol having a subcarrier symbol, comprising:

appending a guard interval to the OFDM symbol;

transmitting the subcarrier symbol as at least one pilot; and

impressing an additional phase rotation phase on the at least one pilot prior to transmitting, so that at least one pilot phase profile results, the transmitted at least one pilot including the impressed additional phase rotation phase;

wherein the pilot phase profile is unique to each different OFDM symbol.

7. The method as recited in claim 2, wherein:

the at least one pilot phase profile is determined by one of an equation and a pseudo-random series.

10. The method as recited in claim 2, wherein:

the at least one pilot is distributed uniformly in an OFDM symbol.

6. A method for a frame and frequency synchronization of an OFDM (orthogonal frequency division multiplexing) signal, the OFDM signal being used to receive an OFDM symbol, each of which has a pilot symbol as a subcarrier symbol, the method comprising:

adding an additional phase rotation to the pilot symbol;

performing a comparison of the subcarrier symbol with at least one pilot phase profile that is stored; and

performing the frame and frequency synchronization of the OFDM signal as a function of the comparison;

wherein;

the comparison is performed according to the following equation;

$\bigwedge <br><br>\left(l,p<br><br>\left(l,k\right),s,i\right)=\mathrm{ABS}\left[\underset{k}{\sum <br><br>}<br><br>W^{*}<br><br>\left(l,p<br><br>\left(l,k\right)\right)·<br><br>W<br><br>\left(l,p<br><br>\left(l,k+1\right)\right)·<br><br>R<br><br>\left(s,p<br><br>\left(l,k\right)+i\right)·<br><br>R^{*}<br><br>\left(s,p<br><br>\left(l,k+1\right)+i\right)\right].$

8. A method for transmitting an OFDM signal, the OFDM signal being used to transmit an OFDM symbol having a subcarrier symbol, comprising:

appending a guard interval to the OFDM symbol;

transmitting the subcarrier symbol as at least one pilot; and

impressing an additional phase rotation phase on the at least one pilot prior to transmitting, so that at least one pilot phase profile results;

wherein;

the at least one pilot phase profile is determined by the following equation;

$P_{l,p<br><br>\left(l,k\right)}=\sqrt{2}·<br><br>W_{p<br><br>\left(l,k\right)}=\sqrt{2}·<br><br>{\mathrm{e<br><br>}}^{j<br><br>\frac{\mathrm{\pi <br><br>}·<br><br>p<br><br>{\left(l,k\right)}^2}{N_0}}·<br><br>{\mathrm{e<br><br>}}^{{\mathrm{j\phi <br><br>}}_{\mathrm{RND}}<br><br>\left(l,k\right)<br><br>}.$

9. A method for transmitting an OFDM signal, the OFDM signal being used to transmit an OFDM symbol having a subcarrier symbol, comprising:

appending a guard interval to the OFDM symbol;

transmitting the subcarrier symbol as at least one pilot; and

impressing an additional phase rotation phase on the at least one pilot prior to transmitting, so that at least one pilot phase profile results;

wherein;

the at least one pilot phase profile is determined by the following equation;

${\mathrm{\phi <br><br>}}_{\mathrm{RND}}<br><br>\left(l,p<br><br>\left(l,k\right)\right)={\mathrm{\phi <br><br>}}_{\mathrm{RND}}<br><br>\left(l,k_l+i<br><br><br><br>x<br><br><br><br>y\right)=\arg <br><br>\left\{Z<br><br>\left(l\right)\right\}+2<br><br>\mathrm{\pi <br><br>}·<br><br>x<br><br><br><br>y·<br><br>\frac{T_G+l·<br><br>T_S}{T_U}·<br><br>i+2<br><br>\mathrm{\pi <br><br>}·<br><br>\frac{i^2<br><br>\left(1+l\right)}{P_0}.$

13. A transmitter, comprising:

a memory containing a pilot phase profile;

an OFDM modulator;

an antenna that radiates an OFDM signal; and

a device for feeding at least one pilot with the pilot phase profile, an additional phase rotation being added to the pilot by the pilot phase profile;

wherein the pilot phase profile adding the additional phase rotation;

includes a set of at least one element that results after (a) discarding data carriers of an OFDM symbol of the OFDM signal and (b) taking arguments of remaining pilot subcarriers; and

is unique to each different OFDM symbol.

14. A receiver for receiving an OFDM symbol of an OFDM signal, the OFDM symbol including a pilot symbol as a subcarrier symbol, comprising:

a first time synchronization unit for performing a rough time synchronization;

an OFDM demodulator; and

a processor including a memory for;

performing a comparison between the received subcarrier symbol and a stored pilot phase profile, the received subcarrier symbol including an additional phase rotation added prior to transmission; and

performing a frame and frequency synchronization of the OFDM signal as a function of the comparison;

wherein the pilot phase profile is unique to each different OFDM symbol.