SYSTEM ACCESS AND SYNCHRONIZATION METHODS FOR MIMO OFDM COMMUNICATIONS SYSTEMS AND PHYSICAL LAYER PACKET AND PREAMBLE

• US 20090060076A1
• Filed: 11/12/2008
• Published: 03/05/2009
• Est. Priority Date: 10/17/2001
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Tong, Wen, JIA, MING, MA, JIANGLEI, ZHU, PEIYING
• Granted Patent: US 8,213,292 B2
• Time in Patent Office: Days
• US Class Current: 375/260
• 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.

Claims

1. A transmitter adapted to transmit a packet data frame structure comprising:

a superframe having a length corresponding to a synchronization period of a network;

the superframe containing a plurality of radio frames;

each radio frame containing a plurality of TPS (transmission parameter signalling) frames corresponding to an adaptive coding and modulation period;

each TPS frame containing a plurality of slots corresponding to an air interface slot size;

each slot containing a plurality of OFDM symbols, with the first two symbols of the first slot of the first TPS frame of each OFDM frame being used as header OFDM symbols.

2. A transmitter according to claim 1 wherein the header OFDM symbols have a header OFDM symbol format in which sub-carriers of a header OFDM symbol are divided into a non-contiguous set of sub-carriers for each of a plurality of antennas, with each antenna transmitting the header OFDM symbol only on the respective set of sub-carriers.

3. A transmitter according to claim 1 wherein the header OFDM symbols contain multiplexed pilot channel sub-carriers and common synchronization channel sub-carriers for each of the plurality of antennas.

4. A transmitter according to claim 1 wherein the header OFDM symbols further contain multiplexed broadcasting channel sub-carriers for each of the plurality of antennas.

5. A transmitter according to claim 1 adapted to transmit in a plurality of different modes by transmitting a different number of OFDM symbols per slot with an unchanged slot duration and with no change to the frame structure above the slot.

6. A transmitter according to claim 5 wherein modes with an increased number of OFDM symbols per slot are realized by shortening OFDM symbol duration, and shortening FFT size, but not changing sampling frequency.

7. A transmitter according to claim 1 adapted to transmit to a respective set of users for each TPS frame and to signal for each TPS frame which users should demodulate the entire TPS frame.

8. A receiver adapted to receive and process OFDM frames transmitted by the transmitter of claim 1.

9. 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.

10. A method according to claim 9 wherein a course synchronization position is determined for each receive antenna and used for determining the respective fine synchronization position.

11. A method according to claim 9 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.

12. A method according to claim 10 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.

13. A method according to claim 9 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.

14. A method according to claim 9 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.

15. A method according to claim 9 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.

16. A method according to claim 9 applied to a single transmit antenna single receive antenna system.

17. A method according to claim 9 wherein the training sequence is received on common synchronization channel sub-carriers.

18. A method according to claim 9 wherein the training sequence is received during an OFDM frame preamble.

19. A method according to claim 9 wherein the training sequence is received on dedicated pilot channel sub-carriers.

20. A method according to claim 19 wherein the training sequence is received during an OFDM frame preamble.

21. 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.

22. A receiver according to claim 21 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.

23. A receiver according to claim 21 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.

24. A receiver according to claim 21 adapted to receive the training sequence on common synchronization channel sub-carriers.

25. A receiver according to claim 21 adapted to receive the training sequence on dedicated pilot channel sub-carriers.

26. 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.

27. 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.

28. A method according to claim 27 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.

29. A method of performing cell selection 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) 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) selecting the M strongest correlation peaks for further processing;

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 C/I or similar value for each transmitter thus identified;

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

30. A method according to claim 29 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 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 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.

31. A method according to claim 30 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.

32. A method according to claim 31 applied to a 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.

33. A method according to claim 29 further comprising:

for transmitter switching, averaging the C/I or similar 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 or similar value if different from a currently selected transmitted.

34. A method comprising:

transmitting an OFDM preamble comprising a prefix followed by a plurality of correlated header symbols.

35. The method of claim 34 wherein the prefix is a cyclic repetition of a portion of one of the header symbols.

36. The method of claim 34 wherein the plurality of correlated header symbols comprises two header symbols.

37. The method of claim 34 wherein the plurality of correlated header symbols comprises two identical symbols.

38. The method of claim 34 wherein the plurality of correlated header symbols comprises two identical symbols, and wherein the prefix is a cyclic repetition of one of the header symbols.