Pilot patterns for OFDM systems with multiple antennas
View Patent ↗The present invention relates to orthogonal frequency-division multiplexing (OFDM) communication systems with multiple transmit antennas receive antennas, and in particular to methods for inserting scattered pilots (SPs) into the transmit signals of such OFDM systems, for estimating channel properties on the basis of the scattered pilots, a multi-antenna OFDM transmitter, and an OFDM receiver. In this context, it is the particular approach of the present invention to keep the same SP pattern like in the single-transmitter case, to partition the pilots into as many subsets as there are transmitters (transmit antennas), and to interleave these subsets both in time and in frequency. In this manner, the granularity of pilots of the same subset is reduced. This offers increased flexibility in designing the scattered pilot patterns and greater accuracy of the estimated channel properties.
1. A multi-antenna OFDM transmitter having N antennas, N being an integer greater than or equal to two, the multi-antenna OFDM transmitter comprising:
a multi-antenna encoder for generating a plurality of data streams, one for each of the N transmit antennas, each data stream consisting of a succession of OFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality of subcarriers;
a pilot generation unit for generating, for each of the plurality of data streams, a plurality of scattered pilots, said plurality of scattered pilots being partitioned into M subsets, each of the scattered pilots being encoded on the basis of the subset to which the scattered pilot belongs and the data stream into which the scattered pilot is to be inserted, and
a plurality of pilot inserting units, each pilot inserting unit for inserting one of the plurality of scattered pilots into a corresponding one of the plurality of data streams in accordance with a predefined periodic pattern in which a temporal spacing between two OFDM symbols having scattered pilots in OFDM cells associated with the same subcarrier is equal to D s and a frequency spacing of two subcarriers bearing scattered pilots in any of the OFDM symbols is equal to D k , each of D s and D k being an integer greater than or equal to two, wherein
M is greater than or equal to N and satisfies a relationship M=M s M k , each of M s and M k being an integer greater than or equal to two, and
each of the plurality of pilot inserting units inserts the scattered pilots in such a manner that a temporal spacing between two OFDM symbols having scattered pilots of the same subset in OFDM cells associated with the same subcarrier is equal to D s M s and a frequency spacing of two subcarriers bearing scattered pilots of the same subset in any of the OFDM symbols is equal to D k M k .
2. The multi-antenna OFDM transmitter according to claim 1 , wherein
each of M and N is equal to four, and
each of M s and M k is equal to two.
3. The multi-antenna OFDM transmitter according to claim 2 , wherein
D k is equal to two, three, or four.
4. The multi-antenna OFDM transmitter according to claim 3 , wherein
the pilot generation unit encodes the scattered pilots by, for each subset, multiplying all scattered pilots of the subset with a constant coefficient that depends on the subset and the data stream into which said all scattered pilots of the subset are to be inserted.
5. The multi-antenna OFDM transmitter according to claim 2 , wherein
the pilot generation unit encodes the scattered pilots by, for each subset, multiplying all scattered pilots of the subset with a constant coefficient that depends on the subset and the data stream into which said all scattered pilots of the subset are to be inserted.
6. The multi-antenna OFDM transmitter according to claim 5 , wherein
a matrix formed by the constant coefficients used for multiplying the scattered pilots is invertible, in particular a unitary diagonal matrix or a Hadamard matrix.
7. The multi-antenna OFDM transmitter according to claim 1 , wherein
with respect to one of the OFDM cells, each of the plurality of pilot inserting units inserts, in a subcarrier direction, not all but at least two of scattered pilots that belong to respective subsets and correspond to the pilot inserting unit.
8. An OFDM receiver comprising:
an OFDM demodulator for obtaining a data stream consisting of a succession of OFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality of subcarriers;
a pilot extraction unit for (i) extracting scattered pilots from the data stream in accordance with a predefined periodic pattern in which a temporal spacing between two OFDM symbols having scattered pilots in OFDM cells associated with the same subcarrier is equal to D s and a frequency spacing of two subcarriers bearing scattered pilots in any of the OFDM symbols is equal to D k , each of D s and D k being an integer greater than or equal to two, and (ii) partitioning the extracted scattered pilots into M subsets; and
a channel estimation unit for estimating a plurality of channel components from the M subsets of scattered pilots, each channel component representing a channel condition between one of a plurality of transmitters and the OFDM receiver, wherein
M satisfies a relationship M=M s M k , each of M s and M k being an integer greater than or equal to two, and
a temporal spacing between two OFDM symbols having scattered pilots of the same subset in OFDM cells associated with the same subcarrier is equal to M s D s and a frequency spacing of two subcarriers bearing scattered pilots of the same subset in any of the OFDM symbols is equal to M k D k .
9. The OFDM receiver according to claim 8 , wherein
M is equal to four, and
each of M s and M k is equal to two.
10. The OFDM receiver according to claim 9 , wherein
D k is equal to two, three, or four.
11. The OFDM receiver according to claim 10 , wherein
the pilot extraction unit extracts, for each OFDM symbol, at least one continual pilot from OFDM symbols associated with predefined subcarriers and partitions the extracted continual pilots into the M subsets; and
the channel estimation unit estimates the plurality of channel components from the M subsets of scattered pilots and continual pilots.
12. The OFDM receiver according to claim 9 , wherein
the pilot extraction unit extracts, for each OFDM symbol, at least one continual pilot from OFDM symbols associated with predefined subcarriers and partitions the extracted continual pilots into the M subsets; and
the channel estimation unit estimates the plurality of channel components from the M subsets of scattered pilots and continual pilots.
13. The OFDM receiver according to claim 12 , wherein
the predefined subcarriers are the subcarriers bearing scattered pilots.
14. The OFDM receiver according to claim 13 , wherein
continual pilots extracted from the same subcarrier are partitioned into at least two different subsets.
15. The OFDM receiver according to claim 12 , wherein
the predefined subcarriers are distinct from the subcarriers bearing scattered pilots.
16. The OFDM receiver according to claim 15 , wherein
continual pilots extracted from the same subcarrier are partitioned into the same subset.
17. The OFDM receiver according to claim 15 , wherein
continual pilots extracted from the same subcarrier are partitioned into at least two different subsets.
18. The OFDM receiver according to claim 8 , wherein
with respect to one of the OFDM cells, the pilot extraction unit extracts, in a subcarrier direction, not all but at least two of scattered pilots that belong to respective subsets.
19. A method used by a multi-antenna transmitter with N transmit antennas for inserting scattered pilots into transmit signals, said method comprising the steps of:
generating a plurality of data streams, one for each of the N transmit antennas, each data stream consisting of a succession of OFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality of subcarriers;
generating, for each of the plurality of data streams, a plurality of scattered pilots, said plurality of scattered pilots being partitioned into M subsets, each of the scattered pilots being encoded on the basis of the subset to which the scattered pilot belongs and the data stream into which the scattered pilot is to be inserted, and
inserting one of the plurality of scattered pilots into a corresponding one of the plurality of data streams in accordance with a predefined periodic pattern in which a temporal spacing between two OFDM symbols having scattered pilots in OFDM cells associated with the same subcarrier is equal to D s and a frequency spacing of two subcarriers bearing scattered pilots in any of the OFDM symbols is equal to D k , each of D s and D k being an integer greater than or equal to two, wherein
M is greater than or equal to N and satisfies a relationship M=M s M k , each of M s and M k being an integer greater than or equal to two, and
in the inserting step, the scattered pilots are inserted in such a manner that a temporal spacing between two OFDM symbols having scattered pilots of the same subset in OFDM cells associated with the same subcarrier is equal to D s M s and a frequency spacing of two subcarriers bearing scattered pilots of the same subset in any of the OFDM symbols is equal to D k M k .
20. The method according to claim 19 , wherein
each of M and N is equal to four, and
each of M s and M k is equal to two.
21. The method according to claim 19 , wherein
with respect to one of the OFDM cells, not all but at least two of scattered pilots that belong to respective subsets are inserted in a subcarrier direction in the inserting step.
22. A method for estimating, at an OFDM receiver, channel properties between the OFDM receiver and each of N transmit antennas, said method comprising the steps of:
obtaining a data stream consisting of a succession of OFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality of subcarriers;
extracting scattered pilots from the data stream in accordance with a predefined periodic pattern in which a temporal spacing between two OFDM symbols having scattered pilots in OFDM cells associated with the same subcarrier is equal to D s and a frequency spacing of two subcarriers bearing scattered pilots in any of the OFDM symbols is equal to D k , each of D s and D k being an integer greater than or equal to two, and partitioning the extracted scattered pilots into M subsets; and
estimating a plurality of channel components from the M subsets of scattered pilots, each channel component representing a channel condition between one of a plurality of transmitters and the OFDM receiver, wherein
M is greater than or equal to N and satisfies a relationship M=M s M k , each of M s and M k being an integer greater than or equal to two, and
a temporal spacing between two OFDM symbols having scattered pilots of the same subset in OFDM cells associated with the same subcarrier is equal to M s D s and a frequency spacing of two subcarriers bearing scattered pilots of the same subset in any of the OFDM symbols is equal to M k D k .
23. The method according to claim 22 , wherein
each of M and N is equal to four, and
each of M s and M k is equal to two.
24. The method according to claim 22 , wherein
with respect to one of the OFDM cells, not all but at least two of scattered pilots that belong to respective subsets are extracted in a subcarrier direction in the extracting step.