Estimating frequency-offsets and multi-antenna channels in MIMO OFDM systems

• US 8,718,185 B2
• Filed: 02/26/2013
• Issued: 05/06/2014
• Est. Priority Date: 05/21/2003
• Status: Active Grant

• Current Assignee: Regents of The University of Minnesota (University of Minnesota)
• Original Assignee: Regents of The University of Minnesota (University of Minnesota)
• Inventors: Giannakis, Georgios B., Ma, Xiaoli
• Primary Examiner(s): Ahn, Sam K
• Assistant Examiner(s): Ali, Shawkat M
• Application Number:
  US13/777,993
• Publication Number:
  US 20130163693A1
• Time in Patent Office: 434 Days
• Field of Search: 370/203, 370/295, 370/213, 370/329, 370/442, 370/486, 370/509, 375/130, 375/146, 375/147, 375/259, 375/260, 375/267, 375/299, 375/308, 375/316, 375/346, 375/347, 375/365, 380/274, 455/59, 455/69, 455/435.2, 455/456.1, 455/464, 455/515, 455/522
• US Class Current: 375/295
• CPC Class Codes: H04B 7/0413 MIMO systems
• CPC Class Codes: H04B 7/046 taking physical layer constraints into account
• CPC Class Codes: H04B 7/063 Parameters other than those covered in groups H04B7/0623 - H04B7/0634, e.g. channel matrix rank or transmit mode selection
• CPC Class Codes: H04B 7/066 Combined feedback for a number of channels, e.g. over several subcarriers like in orthogonal frequency division multiplexing [OFDM]
• CPC Class Codes: H04J 11/0063 of multipath interference, e.g. Rake receivers
• CPC Class Codes: H04L 25/0202 Channel estimation
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0228 with direct estimation from sounding signals
• CPC Class Codes: H04L 25/0242 using matrix methods
• CPC Class Codes: H04L 27/2607 Cyclic extensions
• CPC Class Codes: H04L 27/2613 Structure of the reference signals
• CPC Class Codes: H04L 27/26134 Pilot insertion in the transmitter chain, e.g. pilot overlapping with data, insertion in time or frequency domain
• CPC Class Codes: H04L 27/2626 Arrangements specific to the transmitter only
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2671 Time domain
• CPC Class Codes: H04L 27/2672 Frequency domain
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 27/2692 with preamble design, i.e. with negotiation of the synchronisation sequence with transmitter or sequence linked to the algorithm used at the receiver
• CPC Class Codes: H04L 27/2695 with channel estimation, e.g. determination of delay spread, derivative or peak tracking channel estimation H04L25/0202
• CPC Class Codes: H04L 5/0016 Time-frequency-code

Abstract

Techniques are described for carrier frequency offset (CFO) and channel estimation of orthogonal frequency division multiplexing (OFDM) transmissions over multiple-input multiple-output (MIMO) frequency-selective fading channels. A wireless transmitter forms blocks of symbols by inserting training symbols within two or more blocks of information-bearing symbols. The transmitter applies a hopping code to each of the blocks of symbols to insert a null subcarrier at a different position within each of the blocks of symbols, and a modulator outputs a wireless signal in accordance with the blocks of symbols. A receiver receives the wireless signal and estimates the CFO, and outputs a stream of estimated symbols based on the estimated CFO.

Claims

1. A method comprising:

in a base station;

forming two or more blocks of output symbols for orthogonal frequency division multiplexing (OFDM) transmissions over a multiple-input multiple-output (MIMO) channel, wherein the forming comprises (i) identifying different positions within the two or more blocks of output symbols based at least on (a) one or more block index values, (b) one or more subcarrier index values, (c) a cyclic prefix parameter selected to compensate for intersymbol interference (ISI) associated with the MIMO channel, and (d) a hopping code that is based, at least in part, on the cyclic prefix parameter, and (ii) inserting, based at least on the hopping code, training symbols and null subcarriers within two or more blocks of information-bearing symbols, the hopping code directing at least a portion of the null subcarriers to be inserted at the different positions within the two or more blocks of output symbols; and

transmitting, via two or more antennas, transmission signals in accordance with the two or more blocks of output symbols, wherein the two or more blocks of output symbols include a first block of output symbols and a second block of output symbols, and wherein inserting the training symbols and null subcarriers comprises;

inserting a first null subcarrier at a first subcarrier position within the first block of output symbols; and

inserting a second null subcarrier at a second subcarrier position within the second block of output symbols, wherein the first subcarrier position is different from the second subcarrier position.

2. The method of claim 1, wherein transmitting the transmission signals comprises:

transmitting, via a first antenna of the two or more antennas, a first transmission signal in accordance with the first block of output symbols; and

transmitting, via a second antenna of the two or more antennas, a second transmission signal in accordance with the second block of output symbols.

3. The method of claim 1, wherein transmitting the transmission signals comprises inserting a cyclic prefix within each of the blocks of output symbols, and wherein the transmission signals provide information for estimating a carrier frequency offset associated with received versions of the transmission signals.

4. The method of claim 3, wherein the transmission signals provide information for estimating a phase noise of the received versions of the transmission signals based on the estimated carrier frequency offset.

5. The method of claim 1, wherein the training symbols within the two or more blocks collectively provide information for estimating the MIMO channel.

6. The method of claim 1, wherein inserting the training symbols and the null subcarriers comprises inserting at least one training symbol adjacent to at least one null subcarrier.

7. The method of claim 1, wherein transmitting the transmission signals comprises transmitting transmission signals for a multi-user wireless communications system.

8. The method of claim 1, further comprising:

encoding the information-bearing symbols in space and time within the two or more blocks of output symbols.

9. A system, comprising:

two or more antennas; and

a base station configured to (i) form two or more blocks of output symbols for orthogonal frequency division multiplexing (OFDM) transmissions over a multiple-input multiple-output (MIMO) channel by identifying different positions within the two or more blocks of output symbols based at least on (a) one or more block index values, (b) one or more subcarrier index values, (c) a cyclic prefix parameter selected to compensate for intersymbol interference (ISI) associated with the MIMO channel, and (d) a hopping code that is based, at least in part, on the cyclic prefix parameter, and inserting, based at least on the hopping code, training symbols and null subcarriers within two or more blocks of information-bearing symbols, the hopping code directing at least a portion of the null subcarriers to be inserted at the different positions within the two or more blocks of output symbols, and (ii) transmit, via the two or more antennas, transmission signals in accordance with the two or more blocks of output symbols,

wherein the two or more blocks of output symbols include a first block of output symbols and a second block of output symbols, and wherein the base station is further configured to;

insert a first null subcarrier at a first subcarrier position within the first block of output symbols; and

insert a second null subcarrier at a second subcarrier position within the second block of output symbols, wherein the first subcarrier position is different from the second subcarrier position.

10. The system of claim 9, further comprising:

a wireless communication device configured to receive the transmission signals as received signals, wherein the wireless communication device is configured to (i) estimate a carrier frequency offset based on the received signals and (ii) perform channel estimation of the MIMO channel based on the received signals.

11. The system of claim 10, wherein the wireless communication device is configured to estimate a phase noise of the received signals based on the carrier frequency offset.

12. The system of claim 9, wherein the base station is configured to:

transmit, via a first antenna of the two or more antennas, a first transmission signal in accordance with the first block of output symbols; and

transmit, via a second antenna of the two or more antennas, a second transmission signal in accordance with the second block of output symbols.

13. The system of claim 9, wherein the base station is configured to insert a cyclic prefix within each of the blocks of output symbols, and wherein the transmission signals provide information for estimating a carrier frequency offset associated with received versions of the transmission signals.

14. The system of claim 9, wherein the training symbols within the two or more blocks collectively provide information for estimating the MIMO channel.

15. The system of claim 9, wherein the base station is configured to insert at least one training symbol adjacent to at least one null subcarrier.

16. The system of claim 9, wherein the base station is configured to transmit transmission signals for a multi-user wireless communications system.

17. The system of claim 9, wherein the base station is configured to encode the information-bearing symbols in space and time within the two or more blocks of output symbols.

18. A method comprising:

in a base station;

forming two or more blocks of output symbols for orthogonal frequency division multiplexing (OFDM) transmissions over a multiple-input multiple-output (MIMO) channel;

identifying, via a hopping code and based at least on a cyclic prefix parameter, different positions within the two or more blocks of output symbols, the different positions comprising a block index value, a subcarrier index value, and the cyclic prefix parameter selected to compensate for intersymbol interference (ISI) associated with the MIMO channel;

inserting, using the different positions identified via the hopping code, a first set of two or more training symbols and two or more null subcarriers (i) into a first block of two or more blocks of information-bearing symbols and (ii) at a first position within the first block of information-bearing symbols;

inserting, using the different positions identified via the hopping code, a second set of two or more training symbols and two or more null subcarriers (i) into a second block of the two or more blocks of information-bearing symbols and (ii) at a second position within the second block of information-bearing symbols, wherein the hopping code directs the first position to be different from the second position; and

transmitting over the MIMO channel, via two or more antennas, transmission signals in accordance with the two or more blocks of output symbols.

19. The method of claim 18, wherein the block index value is a positive integer.

20. The method of claim 19, wherein the subcarrier index value is a positive integer.

21. The method of claim 20, wherein the cyclic prefix parameter is a positive integer.

22. The method of claim 18, wherein the first block of the two or more blocks of output symbols and the second block of the two or more blocks of output symbols are consecutive.

23. The method of claim 18, wherein the first block of the two or more blocks of output symbols and the second block of the two or more blocks of output symbols each comprises a vector of size 1×

N, where N is a total number of subcarriers in a block of output symbols.

24. The method of claim 18, wherein inserting the training symbols into the first block of the two or more blocks of information-bearing symbols further comprises inserting the training symbols into the first block of the two or more blocks of information-bearing symbols such that there is at least one information-bearing symbol between each of the training symbols in the first block of the two or more blocks of information-bearing symbols.

25. The method of claim 18, wherein inserting the training symbols into the first and second blocks of the two or more blocks of information-bearing symbols further comprises inserting the training symbols into the first and second blocks of the two or more blocks of information-bearing symbols such that there is at least one information-bearing symbol between each of the training symbols in the first and second blocks of the two or more blocks of information-bearing symbols.

26. The method of claim 18, wherein each block of output symbols contains a number of training symbols equal to a number of transmit antennas.

27. The method of claim 18, wherein inserting the training symbols into the first and second blocks of the two or more blocks of information-bearing symbols further comprises inserting a number of training symbols, wherein the number of training symbols inserted per block of information-bearing symbols is based at least on a coherence time of the MIMO channel.

28. The method of claim 27, further comprising increasing the number of training symbols inserted per block of information-bearing symbols based at least on a reduction in the coherence time of the MIMO channel.

29. The method of claim 18, further comprising space-time encoding the information-bearing symbols in the first and second blocks of information-bearing symbols.

30. The method of claim 18, wherein forming the two or more blocks of output symbols, identifying the different positions, and inserting the training symbols and the null subcarriers are repeated for each of two or more transmit antennas.