Method and apparatus for time tracking in OFDM wireless communication systems

Patent metadata

Field	Value
Publication number	US-9866368-B1
Application number	US-201615059437-A
Country	US
Kind code	B1
Filing date	Mar 3, 2016
Priority date	Mar 3, 2016
Publication date	Jan 9, 2018
Grant date	Jan 9, 2018

Abstract

In a mobile wireless communication system a client terminal needs to continuously maintain its receive and transmit operations in time alignment with the serving base station timing. A client terminal may initially achieve the timing alignment with the serving base station through the cell search procedure. Subsequently, as a client terminal may move toward or away from a serving base station, it may need to continuously track its timing to remain time aligned with the serving base station. In Orthogonal Frequency Division Multiplexing (OFDM) systems, one of the commonly used methods for time tracking may be the Cyclic Prefix (CP) correlation. When the length of the CP is small, the reliability of the CP correlation may be low, especially when the signal quality is poor. A method and apparatus are disclosed that use the initial synchronization signals for time tracking for improved performance and reduced complexity.

First claim

The invention claimed is:

1. A method for time tracking at a client device being served by a base station in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system, the method comprising:
controlling, by a processing device, for each of at least one interval of a received signal,
filtering the received signal to obtain a filtered signal having a bandwidth corresponding to a predetermined central bandwidth,
in which the filtered signal corresponds to a bandwidth of a Primary Synchronization Signal (PSS) and a bandwidth of a Secondary Synchronization Signal (SSS) of the received signal, and
in which a sampling rate of the filtered signal is according to a bandwidth of the received signal, a sampling rate of the received signal and a predetermined estimation accuracy for timing error;
first cross-correlation processing of the filtered signal and a time domain replica of the PSS of the serving base station having a sampling rate of the received signal, to obtain a first cross-correlation output signal;
second cross-correlation processing of the filtered signal and a time domain replica of a SSS of the serving base station having the sampling rate of the received signal, to obtain a second cross-correlation output signal;
combining the first and second cross-correlation output signals to obtain a plurality of combined cross-correlation values; and
when the bandwidth of the received signal is not less than a predetermined bandwidth, estimating a timing error from a position of a maximum of the combined cross-correlation values, and
when the bandwidth of the received signal is less than a predetermined bandwidth, performing interpolation on the combined cross-correlation values to obtain interpolated cross-correlation values and estimating the timing error from a position of a maximum of the combined cross-correlation values and the interpolated cross-correlation values.

2. The method of claim 1 , wherein the time domain replica of the PSS and the time domain replica of the SSS are stored in a memory of the client device.

3. The method of claim 1 , wherein the sampling rate of the filtered signal is less than the sampling rate of the received signal.

4. The method of claim 1 , wherein the time domain replica of the SSS is a time domain replica of a first SSS or a time domain replica of a second SSS of the serving base station.

5. The method of claim 1 , wherein the first cross-correlation processing of the filtered signal and the time domain replica of the PSS of the serving base station is performed only around an expected position of the PSS in the received signal for the serving base station.

6. The method of claim 5 , wherein the first cross-correlation processing of the filtered signal and the time domain replica of the PSS of the serving base station is performed only at a time instant at which the PSS is transmitted by the serving base station.

7. The method of claim 1 ,
wherein the second cross-correlation processing of the filtered signal and the time domain replica of the SSS of the serving base station is performed only around an expected position of the SSS for the serving base station, and wherein the time domain replica of the SSS is a time down replica of a first SSS or a time domain replica of a second SSS of the serving base station.

8. The method of claim 7 ,
wherein the second cross-correlation processing of the filtered signal and the time domain replica of the SSS of the serving base station is performed only at a time instant at which the first SSS or the second SSS as the SSS is transmitted by the serving base station.

9. The method of claim 1 , wherein the combined cross-correlation values include a single set of correlation values for each time offset around (i) a first nominal position for which the first cross-correlation processing of the filtered signal and the time domain replica of the PSS of the serving base station is performed and (ii) a second nominal position for which the second cross-correlation processing of the filtered signal and the time domain replica of the SSS of the serving base station is performed.

10. The method of claim 1 , wherein the at least one interval is a 5 ms interval and the wireless communication system is a 3 rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system.

11. The method of claim 1 , wherein the at least one interval includes a plurality intervals, and
the method further comprising:
estimating a final timing error by determining an average of the timing errors estimated respectively for the plurality of intervals.

12. The method of claim 1 ,
wherein the filtering of the received signal is to retain only sixty-three central subcarriers, and wherein the predetermined central bandwidth is 945 kHz.

13. The method of claim 1 , wherein the sampling rate of the filtered signal is the same as the sampling rate of the received signal.

14. The method of claim 1 ,
wherein the interpolation is performed in accordance with the bandwidth of the received signal and a predetermined desired error granularity for the time tracking.

15. The method of claim 1 , wherein the filtering of the received signal and the interpolation are performed by a single filter.

16. The method of claim 1 , wherein the method is performed for each PSS and SSS occurrence in the received signal or based on a determination of need for speed and accuracy of the time tracking.

17. An apparatus for time tracking at a client device being served by a base station in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system, the apparatus comprising:
circuitry configured to control, for each of at least one interval of a received signal,
filtering the received signal to obtain a filtered signal having a bandwidth corresponding to a predetermined central bandwidth,
in which the filtered signal corresponds to a bandwidth of a Primary Synchronization Signal (PSS) and a bandwidth of a Secondary Synchronization Signal (SSS) of the received signal, and
in which a sampling rate of the filtered signal is according to a bandwidth of the received signal, a sampling rate of the received signal and a predetermined estimation accuracy for timing error;
first cross-correlation processing of the filtered signal and a time domain replica of the PSS of the serving base station having a sampling rate of the received signal, to obtain a first cross-correlation output signal;
second cross-correlation processing of the filtered signal and a time domain replica of a SSS of the serving base station having the sampling rate of the received signal, to obtain a second cross-correlation output signal;
combining the first and second cross-correlation output signals to obtain a plurality of combined cross-correlation values; and
when the bandwidth of the received signal is not less than a predetermined bandwidth, estimating a timing error from a position of a maximum of the combined cross-correlation values, and
when the bandwidth of the received signal is less than a predetermined bandwidth, performing interpolation on the combined cross-correlation values to obtain interpolated cross-correlation values and estimating the timing error from a position of a maximum of the combined cross-correlation values and the interpolated cross-correlation values.

18. The apparatus of claim 17 , wherein the first cross-correlation processing of the filtered signal and the time domain replica of the PSS of the serving base station is performed only around an expecte

Assignees

• Mbit Wireless Inc

Inventors

• Patel Bhaskar

Classifications

• H04L7/0025 Primary

  interpolation of clock signal · CPC title

• H04W72/0446

  Resources in time domain, e.g. slots or frames · CPC title

• H04L5/0007 Primary

  the frequencies being orthogonal, e.g. OFDM(A) or DMT · CPC title

• H04L27/2656 Primary

  Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation · CPC title

• H04L27/2675

  Pilot or known symbols · CPC title