Method and system for performing cell selection for OFDM communications

• US 7,912,012 B2
• Filed: 10/02/2006
• Issued: 03/22/2011
• Est. Priority Date: 10/17/2001
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Ma, Jianglei, Jia, Ming, Zhu, Peiying, Tong, Wen
• Primary Examiner(s): Mattis, Jason E
• Application Number:
  US11/529,222
• Publication Number:
  US 20070064586A1
• Time in Patent Office: 1,632 Days
• Field of Search: 370/208, 370/329, 370/332
• US Class Current: 370/332
• 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 27/2607 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.

Claims

1. A method of performing cell selection at an orthogonal frequency division multiplexing (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 coarse synchronization position;

at each of the at least one receive antenna;

a) performing a frequency domain correlation between at least one received common synchronization sequence extracted from common synchronization channel sub-carriers in the received signal and a corresponding common synchronization sequence of a respective plurality of transmit antennas to identify a plurality of candidate correlation peaks;

b) from the plurality of candidate correlation peaks, selecting the M strongest correlation peaks for further processing, where M is an integer equal to or greater than 1;

c) at each correlation peak, reconverting time domain samples into frequency domain components and processing pilot channel sub-carriers, these containing transmitter specific information, to identify a transmitter associated with each correlation peak;

d) determining a carrier-to-interference (C/I) value for each transmitter thus identified;

selecting the transmitter having the largest C/I value determined for any of the at least one receive antenna.

2. A method according to claim 1 wherein performing a frequency domain correlation between at least one received common synchronization sequence extracted from common synchronization channel sub-carriers in the received signal and a corresponding common synchronization sequence of a respective plurality of transmit antennas to identify a plurality of candidate correlation peaks comprises:

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

i) for each receive antenna positioning a fast Fourier transform (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 of at least one common synchronization sequence, each common synchronization sequence having been transmitted by a transmit antenna of each of at least one transmitter, extracting a respective received training sequence corresponding to the transmit antennas from the sets of frequency domain components;

iii) for each of the at least one common synchronization sequence, calculating a correlation between each respective received common synchronization sequence and a respective known common synchronization sequence;

iv) combining the correlations to produce an overall correlation result for each candidate synchronization position;

b) determining at least one peak in the correlations, each said at least one peak being local maxima in the correlations.

3. A method according to claim 2 further comprising:

reconverting time domain samples into frequency domain components based on the fine synchronization position of the selected transmitter and performing a further fine synchronization based on a dedicated pilot channel for that transmitter.

4. A method according to claim 3 applied to a multiple in multiple out orthogonal frequency division multiplexing (MIMO-OFDM) frame format having a header symbol format in which subcarriers of a header symbol are divided into a non-contiguous set of subcarriers for each of a plurality of antennas, with each antenna transmitting header symbols only on the respective set of sub-carriers, and wherein the header symbols contain multiplexed pilot channel sub-carriers and common synchronization channel sub-carriers for each of the plurality of antennas, the frame beginning with two identical header OFDM symbols during which contents of the pilot channel sub-carriers are repeated and contents of the synchronization channel sub-carriers are repeated, the common synchronization channel sub-carriers carrying a complex sequence which is different for respective antenna of one base station and being common across multiple base stations, and contents of the dedicated pilot channel sub-carriers being at least locally unique to a particular base station.

5. A method according to claim 1 further comprising:

averaging the C/I value over a time interval for each transmitter thus identified, and at the end of the time interval instigating a transmitter switch to the transmitter with the largest average C/I value if different from a currently selected transmitter.

6. A method comprising:

receiving a respective orthogonal frequency division multiplexing (OFDM) signal at each of at least one receive antenna;

for each of the at least one receive antenna, determining candidate correlation peaks in the frequency domain by performing frequency domain correlation of synchronization sequences to identify a plurality of candidate correlation peaks;

determining a carrier-to-interference (C/I) value for each candidate correlation peak;

performing cell selection or transmitter switching based on the C/I value of the candidate correlation peaks.

7. The method of claim 6 wherein at each of the at least one receive antenna, determining candidate correlation peaks in the frequency domain to identify a plurality of candidate correlation peaks comprises:

at each of the at least one receive antenna;

a) performing a frequency domain correlation between at least one received common synchronization sequence extracted from common synchronization channel sub-carriers in the received signal and a corresponding common synchronization sequence of a respective plurality of transmit antennas to identify a plurality of candidate correlation peaks; and

b) from the plurality of candidate correlation peaks, selecting the M strongest correlation peaks for further processing, where M is an integer equal to or greater than 1.

8. The method of claim 6 further comprising:

at each candidate correlation peak, reconverting time domain samples into frequency domain components and processing pilot channel sub-carriers, these containing transmitter specific information, to identify a transmitter associated with each candidate correlation peak.

9. The method of claim 6 wherein performing a frequency domain correlation between at least one received common synchronization sequence extracted from common synchronization channel sub-carriers in the received signal and a corresponding common synchronization sequence of a respective plurality of transmit antennas to identify a plurality of candidate correlation peaks comprises:

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

i) for each receive antenna positioning a fast Fourier transform (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 of at least one common synchronization sequence, each common synchronization sequence having been transmitted by a transmit antenna of each of at least one transmitter, extracting a respective received training sequence corresponding to the transmit antennas from the sets of frequency domain components;

iii) for each of the at least one common synchronization sequence, calculating a correlation between each respective received common synchronization sequence and a respective known common synchronization sequence;

iv) combining the correlations to produce an overall correlation result for each candidate fine synchronization position;

b) determining at least one peak in the correlations, each said at least one peak being local maxima in the correlations.

10. A method according to claim 6 further comprising:

averaging the C/I value over a time interval for each transmitter thus identified, and at the end of the time interval instigating a transmitter switch to the transmitter with the largest average C/I value if different from a currently selected transmitter.