Method and system for performing synchronization in OFDM systems

• US 20070025236A1
• Filed: 09/29/2006
• Published: 02/01/2007
• Est. Priority Date: 10/17/2001
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Tong, Wen, Ma, Jianglei, Zhu, Peiying, Jia, Ming
• Granted Patent: US 8,018,975 B2
• Time in Patent Office: Days
• US Class Current: 370/208
• CPC Class Codes: H04B 7/022 Site diversity; Macro-diversity using two or more spaced independent antennas H04B7/04
• CPC Class Codes: H04B 7/04 using two or more spaced independent antennas
• CPC Class Codes: H04B 7/0413 MIMO systems
• CPC Class Codes: H04B 7/0667 of delayed versions of same signal using space-time coding H04L1/0618
• CPC Class Codes: H04B 7/0697 using spatial multiplexing
• CPC Class Codes: H04J 11/0069 Cell search, i.e. determining cell identity [cell-ID] design of multiplexing codes H04J13/00; processing access restriction or access information H04W48/16; discovery of network devices for network data management H04W8/005; sounding signals for channel estimation H04L25/0226; structure of reference signals in multicarrier modulation systems H04L27/2613; frame, time or carrier synchronisation in multicarrier modulation systems H04L27/2655
• CPC Class Codes: H04L 27/2602 Signal structure
• CPC Class Codes: H04L 27/26025 Numerology, i.e. varying one or more of symbol duration, subcarrier spacing, Fourier transform size, sampling rate or down-clocking allocating sub-channels of the transmission path H04L5/003
• CPC Class Codes: H04L 27/2607 Cyclic extensions
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/2627 Modulators
• CPC Class Codes: H04L 27/2663 Coarse synchronisation, e.g. by correlation
• CPC Class Codes: H04L 27/2665 Fine synchronisation, e.g. by positioning the FFT window
• CPC Class Codes: H04L 27/2672 Frequency domain
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 5/0023 Time-frequency-space
• CPC Class Codes: H04L 5/0037 Inter-user or inter-terminal allocation
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines
• CPC Class Codes: H04W 36/08 Reselecting an access point

Abstract

A method and apparatus are provided for performing acquisition, synchronization and cell selection within an MIMO-OFDM communication system. A coarse synchronization is performed to determine a searching window. A fine synchronization is then performed by measuring correlations between subsets of signal samples, whose first signal sample lies within the searching window, and known values. The correlations are performed in the frequency domain of the received signal. In a multiple-output OFDM system, each antenna of the OFDM transmitter has a unique known value. The known value is transmitted as pairs of consecutive pilot symbols, each pair of pilot symbols being transmitted at the same subset of sub-carrier frequencies within the OFDM frame.

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32. A method of performing synchronization at an OFDM receiver comprising:

at each of at least one receive antenna, sampling a received signal to produce a respective set of time domain samples;

determining at least one course synchronization position;

at each of the at least one receive antenna;

a) for each of a plurality of candidate fine synchronization positions about one of said at least one course synchronization position;

i) for each receive antenna positioning an FFT window to the candidate fine synchronization position and converting by FFT the time domain samples into a respective set of frequency domain components;

ii) for each said at least one transmit antenna, extracting a respective received training sequence corresponding to the transmit antenna from the sets of frequency domain components;

iii) for each transmit antenna, calculating a correlation between each respective received training sequence and a respective known transmit training sequence;

iv) combining the correlations for the at least one transmit antennas to produce an overall correlation result for each candidate synchronization position;

b) determining a fine synchronization position from the plurality of correlation values;

combining the fine synchronization positions from the at least one receive antenna in an overall fine synchronization position.

33. A method according to claim 32 wherein a course synchronization position is determined for each receive antenna and used for determining the respective fine synchronization position.

34. A method according to claim 32 a course synchronization position is determined for each receive antenna and an earliest of the positions is used determining the fine synchronization positions for all receive antennas.

35. A method according to claim 33 wherein the course synchronization position is determined in the time domain for at least one receive antenna by looking for a correlation peak between the time domain samples over two OFDM symbol durations.

36. A method according to claim 32 applied at an OFDM receiver having at least two antennas, combining the fine synchronization positions from the at least one receive antenna in an overall fine synchronization position comprises selecting an earliest of the fine synchronization positions.

37. A method according to claim 32 wherein:

sampling a received signal to produce a set of time domain samples is done for at least three OFDM symbol durations;

determining at least one course synchronization position comprises performing a course synchronization in the time domain by looking for a correlation peak between the time domain samples received over two OFDM symbol durations to identify a course synchronization position by;

a) calculating a plurality of correlation values, each correlation value being a correlation calculated between a first set of time domain samples received during a first period having one OFDM symbol duration and a second set of time domain samples received during a second period immediately following the first period and having OFDM symbol duration, for each of a plurality of starting times for said first period;

b) identifying the course synchronization position to be a maximum in said plurality of correlation values.

38. A method according to claim 32 wherein:

combining the correlations for the at least one transmit antennas to produce an overall correlation result for each candidate synchronization position comprises multiplying together the correlations for the at least one transmit antenna for each candidate synchronization position.

39. A method according to claim 32 applied to a single transmit antenna single receive antenna system.

40. A method according to claim 32 wherein the training sequence is received on common synchronization channel sub-carriers.

41. A method according to claim 32 wherein the training sequence is received during an OFDM frame preamble.

42. A method according to claim 32 wherein the training sequence is received on dedicated pilot channel sub-carriers.

43. A method according to claim 42 wherein the training sequence is received during an OFDM frame preamble.

44. An OFDM receiver comprising:

at least one receive antenna;

for each said at least one receive antenna, receive circuitry adapted to sample a received signal to produce a respective set of time domain samples;

a course synchronizer adapted to determine at least one course synchronization position;

a fine synchronizer comprising at least one FFT, at least one correlator and at least one combiner, adapted to, at each of the at least one receive antenna;

a) for each of a plurality of candidate fine synchronization positions about one of said at least one course synchronization position;

i) for each receive antenna position an FFT window to the candidate fine synchronization position and convert by FFT the time domain samples into a respective set of frequency domain components;

ii) for each said at least one transmit antenna, extract a respective received training sequence corresponding to the transmit antenna from the sets of frequency domain components;

iii) for each transmit antenna, calculate a correlation between each respective received training sequence and a respective known transmit training sequence;

iv) combine the correlations for the at least one transmit antennas to produce an overall correlation result for each candidate synchronization position;

b) determine a fine synchronization position from the plurality of correlation values;

the receiver being further adapted to combine the fine synchronization positions from the at least one receive antenna in an overall fine synchronization position.

45. A receiver according to claim 44 having at least two receive antennas, adapted to combine the fine synchronization positions from the at least one receive antenna in an overall fine synchronization position by selecting an earliest of the fine synchronization positions.

46. A receiver according to claim 44 adapted to combine the correlations for the at least one transmit antennas to produce an overall correlation result for each candidate synchronization position by multiplying together the correlations for the at least one transmit antenna for each candidate synchronization position.

47. A receiver according to claim 44 adapted to receive the training sequence on common synchronization channel sub-carriers.

48. A receiver according to claim 44 adapted to receive the training sequence on dedicated pilot channel sub-carriers.

49. A method of performing fine synchronization comprising:

at each at least one receive antenna receiving OFDM symbols containing a respective received frequency domain training sequence for each of at least one transmit antenna;

performing fine synchronization in the frequency domain by looking for maximum correlations between known frequency domain training sequences and the received frequency domain training sequences.

50. A method of transmitting signals enabling fine synchronization comprising:

from each of at least one transmit antenna, transmitting OFDM symbols containing a respective frequency domain training sequence.

51. A method according to claim 50 wherein a different frequency domain training sequence is transmitted by each transmit antenna, but the same frequency domain training sequence is transmitted by corresponding antenna of other transmitters.

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57. A method comprising:

converting a frequency domain representation of a common sync sequence into a time domain representation;

transmitting the time domain representation twice, preceded by a cyclic prefix of the time domain representation.

58. The method of claim 57 wherein converting a frequency domain representation of a common sync sequence into a time domain representation comprises mapping the common sync sequence to a set of non-adjacent OFDM sub-carriers and performing an IFFT.

59. The method of claim 57 wherein the common sync sequence is common to multiple base stations.

60. A method of performing synchronization at an OFDM receiver comprising:

at each of at least one receive antenna, sampling a received OFDM signal to produce a respective set of time domain samples, the received signal comprising a received training sequence;

performing synchronization based on correlations between the received training sequence and a local copy of the training sequence.

61. The method of claim 60 wherein the received training sequence is a frequency domain training sequence located on selected sub-carriers of the received OFDM signal.

62. The method of claim 60 wherein performing fine synchronization based on correlations between the received training sequence and a local copy of the training sequence is done in the frequency domain.

63. The method of claim 62 wherein performing fine synchronization based on correlations between the received training sequence and a local copy of the training sequence in the frequency domain comprises:

at each of the at least one receive antenna;

a) for each of a plurality of candidate fine synchronization positions;

i) positioning an FFT window to the candidate fine synchronization position and converting by FFT the time domain samples into a respective set of frequency domain components;

ii) for each said at least one transmit antenna, extracting a respective received training sequence corresponding to the transmit antenna from the sets of frequency domain components;

iii) for each transmit antenna, calculating a correlation between each respective received training sequence and a respective known transmit training sequence;

iv) combining the correlations for the at least one transmit antennas to produce an overall correlation result for each candidate synchronization position;

b) determining a fine synchronization position from the plurality of correlation values.

64. The method of claim 63 wherein the at least one antenna comprises a plurality of antennas, the method further comprising:

combining the fine synchronization positions from the at least one receive antenna in an overall fine synchronization position.

65. A method according to claim 64 wherein the training sequence is received on common synchronization channel sub-carriers.

66. A method according to claim 65 wherein another training sequence is received during an OFDM frame preamble.

67. A method according to claim 64 wherein another training sequence is received on dedicated pilot channel sub-carriers.

68. A method according to claim 67 wherein another training sequence is received during an OFDM frame preamble.