Generalized reference signaling scheme for multi-user, multiple input, multiple output (MU-MIMO) using arbitrarily precoded reference signals

• US 7,809,074 B2
• Filed: 03/16/2007
• Issued: 10/05/2010
• Est. Priority Date: 03/16/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Mundarath, Jayakrishnan C., Kotecha, Jayesh H.
• Primary Examiner(s): Torres; Juan A
• Application Number:
  US11/687,508
• Publication Number:
  US 20080225960A1
• Time in Patent Office: 1,299 Days
• Field of Search: 375/259, 375/260, 375/267, 455/59
• US Class Current: 375/267
• CPC Class Codes: H04B 7/0413 MIMO systems
• CPC Class Codes: H04B 7/0456 Selection of precoding matrices or codebooks, e.g. using matrices antenna weighting
• CPC Class Codes: H04B 7/0665 Feed forward of transmit weights to the receiver
• CPC Class Codes: H04L 2025/03426 transmission using multiple-input and multiple-output channels
• CPC Class Codes: H04L 25/03343 Arrangements at the transmitter end
• CPC Class Codes: H04W 48/16 Discovering, processing access restriction or access information

Abstract

A multi-user multiple input, multiple output (MIMO) downlink beamforming system (200) is provided to enable transmit beamforming vectors to be efficiently provided to a subset of user equipment devices (201.i), where spatial separation or zero-forcing transmit beamformers (w_i) are computed at the base station (210) and used to generate precoded reference signals (216). The precoded reference signals (216) are fed forward to the user equipment devices (201.i) which apply one or more hypothesis tests (207.i, 208.i) to the precoded reference signals to extract the precoding matrix (W), including the specific transmit beamforming vector (w_UE) designed for the user equipment, and this extracted information is used to generate receive beamformers (v_i).

Claims

1. A forward reference signaling method for a multiple input, multiple output (MIMO) space division multiple access (SDMA) system, comprising:

receiving at a transmitting device effective channel information from a plurality of receiving devices;

generating a plurality of transmit beamforming vectors based on the effective channel information;

generating precoded reference signals by using each transmit beamforming vector to encode a reference signal; and

feeding forward the precoded reference signals to a plurality of receiving devices for use in generating receive beamforming vectors at each receiving device, where each receiving device extracts the plurality of transmit beamforming vectors from the precoded reference signals and identifies which transmit beamforming vector is designed for said receiving device without requiring additional information to be fed forward that identifies the transmit beamforming vector or precoded reference signal that is designed for said receiving device.

2. The method of claim 1, where generating a plurality of transmit beamforming vectors comprises using a spatial separation scheme to generate a plurality of transmit beamforming vectors.

3. The method of claim 1, where generating a plurality of transmit beamforming vectors comprises selecting transmit beamforming vectors from a defined set of transmit beamforming vectors.

4. The method of claim 1, where receiving effective channel information comprises receiving a preferred precoding vector from each of the plurality of receiving devices.

5. The method of claim 1, where generating precoded reference signals comprises:

encoding a first reference signal with a first transmit beamforming vector to generate a first precoded reference signal that is designed for a first receiving device; and

encoding a second reference signal with a second transmit beamforming vector to generate a second precoded reference signal that is designed for a second receiving device.

6. The method of claim 1, further comprising applying a first test to the precoded reference signals received at a receiving device to identify which transmit beamforming vector is designed for said receiving device, where the first test comprises selecting a reference signal from a finite set of transmit reference signals y_j which maximizes a first objective measure |v_opt^Hy_j|², where v_opt is an optimal blind receive beamforming vector initially designed by said receiving device and y_j are the precoded reference signals.

7. The method of claim 1, further comprising applying a test to the precoded reference signals received at a receiving device to identify each of the plurality of transmit beamforming vectors generated by the transmitting device, where the test comprises selecting, for each precoded reference signal, a transmit beamforming vector from a finite set of transmit beamforming vectors w_i which minimizes a distance measure ∥

y−

H^Hw_i∥

², where y is a precoded reference signal and H is a channel matrix to said receiving device.

8. A forward reference signaling method for a multiple input, multiple output (MIMO) space division multiple access (SDMA) system, comprising:

feeding back to a transmitting device effective channel information from a receiving device;

receiving over a feed forward channel a plurality of precoded reference signals from the transmitting device which are generated by using a plurality of transmit beamforming vectors which are generated based at least in part on the effective channel information;

extracting at the receiving device the plurality of transmit beamforming vectors from the plurality of precoded reference signals;

identifying which transmit beamforming vector is designed for said receiving device without requiring additional information to be fed forward that identifies the transmit beamforming vector or precoded reference signal that is designed for said receiving device; and

using the transmit beamforming vectors to generate a receive beamforming vector at the receiving device.

9. The method of claim 8, where the plurality of transmit beamforming vectors are generated at the transmitting device using a spatial separation algorithm to generate the plurality of transmit beamforming vectors.

10. The method of claim 8, where extracting the plurality of transmit beamforming vectors comprises selecting transmit beamforming vectors from a defined set of transmit beamforming vectors.

11. The method of claim 8, where feeding back effective channel information comprises feeding back a preferred precoding vector from the receiving device.

12. The method of claim 8, where the plurality of precoded reference signals comprises:

a first reference signal encoded with a first transmit beamforming vector to generate a first precoded reference signal that is designed for a the receiving device; and

a second reference signal encoded with a second transmit beamforming vector to generate a second precoded reference signal that is designed for a different receiving device.

13. The method of claim 8, where identifying which transmit beamforming vector is designed for said receiving device comprises applying a first test to the plurality of precoded reference signals to identify which transmit beamforming vector is designed for said receiving device, where the first test comprises selecting a reference signal from a finite set of transmit reference signals y_j which maximizes a first objective measure |v_opt^Hy_j|², where v_opt is an optimal blind receive beamforming vector initially designed by said receiving device, y_j are the precoded reference signals, and H is a channel matrix to said receiving device.

14. The method of claim 8, where extracting at the receiving device the plurality of transmit beamforming vectors comprises applying a test to the plurality of precoded reference signals to identify the plurality of transmit beamforming vectors generated by the transmitting device, where the test comprises selecting, for each precoded reference signal, a transmit beamforming vector from a finite set of transmit beamforming vectors w_i which minimizes a distance measure ∥

y−

H^Hw_i∥

², where y is a precoded reference signal and H is a channel matrix to said receiving device.

15. A forward reference signaling method for a multiple input, multiple output (MIMO) space division multiple access (SDMA) system, comprising:

receiving at a transmitting device effective channel information from a plurality of receiving devices;

using a spatial separation algorithm to design a transmit beamforming matrix based on the effective channel information;

generating one or more precoded reference signals by using all or part of the designed transmit beamforming matrix to encode said one or more reference signals; and

feeding forward said one or more precoded reference signals to a plurality of receiving devices for use in generating receive beamforming vectors at each receiving device, where each receiving device extracts the transmit beamforming matrix from at least one of said one or more precoded reference signal and identifies a transmit beamforming vector that is designed for said receiving device without requiring additional information to be fed forward that identifies the transmit beamforming vector that is designed for said receiving device.

16. The method of claim 15, where using a spatial separation algorithm to design a transmit beamforming matrix comprises selecting a transmit beamforming matrix from a defined set of transmit beamforming matrices.

17. The method of claim 15, where receiving effective channel information comprises receiving a preferred precoding vector or information representative thereof from each of the plurality of receiving devices.

18. The method of claim 15, where generating one or more precoded reference signals comprises encoding a first reference pilot with a predetermined column of the transmit beamforming matrix.

19. The method of claim 15, where using a spatial separation algorithm to design a transmit beamforming matrix comprises designing a beamforming matrix W using (X=[c₁ c₂ . . . c_m]), W=X[X^HX+α

I]<sup>−

1</sup>, where c₁, c₂, . . . c_m are candidate transmit beamforming vectors received from ‘

m’

receiving devices, α

is a smoothing function constant and I is an identity matrix.

20. The method of claim 15, further comprising applying a hypothesis test to said one or more precoded reference signals received at a receiving device to extract the transmit beamforming matrix, where the hypothesis test comprises selecting a transmit beamforming matrix from a finite set of candidate transmit beamforming matrices, where the selected transmit beamforming matrix has a predetermined column w_t which minimizes a distance measure z_test=∥

y−

H^Hw_t∥

², where y is a received precoded reference signal, and H is a MIMO downlink channel matrix to said receiving device.

21. The method of claim 15, where generating one or more precoded reference signals comprises encoding a first reference pilot with a first vector b selected from a predetermined set of vectors, where each vector in the predetermined set of vectors uniquely represents a candidate transmit beamforming matrix.

22. The method of claim 21, further comprising applying a hypothesis test to said one or more precoded reference signals received at a receiving device to extract the transmit beamforming matrix, where the hypothesis test comprises selecting a transmit beamforming matrix from a finite set of candidate transmit beamforming matrices which uniquely corresponds to a vector b which is selected by testing said one or more precoded reference signals to identify which vector b from the predetermined set of vectors minimizes a distance measure z_test=∥

y−

H^Hb_i∥

², where y is a received precoded reference signal, H is a MIMO downlink channel matrix to said receiving device, and b_i is the predetermined set of vectors.

23. The method of claim 15, where each transmit beamforming matrix comprises a plurality of columns which are arranged in a predetermined order that is known to the transmitting device and the plurality of receiving devices.

24. The method of claim 15, where each transmit beamforming matrix is associated with one or more vectors from a feedforward vector codebook which identify said transmit beamforming matrix uniquely.

25. The method of claim 24, where said one or more vectors is generated with a Grassmanian-type codebook.

26. The method of claim 24, where said one or more vectors comprises a predetermined column from said transmission beamforming matrix.

27. The method of claim 24, where generating one or more precoded reference signals comprises encoding a first reference pilot with said one or more vectors that are associated with the designed transmit beamforming matrix.

28. The method of claim 15, where using spatial separation algorithm to design a transmit beamforming matrix comprises designing a beamforming matrix W=[w₁ w₂ . . . w_m] such that w_i={tilde over (w)}_i/∥

{tilde over (w)}_i∥ and

{tilde over (w)}_i^Hu_j≧

γ

₁, if i=j

{tilde over (w)}_i^Hu_j≦

γ

₂, if i≠

j

where γ

₁>

0 and γ

₂ are constants such that γ

₂<

<

γ

_i and u₁, u₂, . . . u_m are candidate transmit beamforming vectors received from ‘

m’

receiving devices.

29. A forward reference signaling method for a multiple input, multiple output (MIMO) space division multiple access (SDMA) system, comprising:

feeding back to a transmitting device effective channel information from a receiving device;

receiving over a feed forward channel a precoded reference signal from the transmitting device, where the precoded reference signal is generated using all or part of a transmit beamforming matrix which is generated using a spatial separation algorithm based at least in part on the effective channel information;

extracting at the receiving device the generated transmit beamforming matrix from at least the precoded reference signal; and

using the transmit beamforming matrix to generate a receive beamforming vector at the receiving device without requiring additional information to be fed forward that identifies a transmit beamforming vector that is designed for said receiving device.

30. The method of claim 29, where extracting the generated transmit beamforming matrix comprises selecting transmit beamforming matrix from a defined set of transmit beamforming matrices.

31. The method of claim 29, where the precoded reference signal comprises a first reference pilot that is encoded with a predetermined column of the transmit beamforming matrix.

32. The method of claim 29, where the transmit beamforming matrix is designed at the transmitting device using regularized zero-forcing beamforming W using (X=[c₁ c₂ . . . c_m]), W=X[X^HX+α

I]<sup>−

1</sup>, where c₁, c₂, . . . c_m are candidate transmit beamforming vectors received from ‘

m’

receiving devices, α

is a smoothing function constant and I is an identity matrix.

33. The method of claim 29, where the precoded reference signal is generated by encoding a first reference pilot with a first vector b selected from a predetermined set of vectors, where each vector in the predetermined set of vectors uniquely represents a candidate transmit beamforming matrix.

34. The method of claim 33, where extracting the generated transmit beamforming matrix from at least the precoded reference signal comprises hypothesis testing the precoded reference signal received at the receiving device against a predetermined set of vectors, where each vector in the predetermined set of vectors uniquely represents a candidate transmit beamforming matrix.

35. The method of claim 29, further comprising identifying at the receiving device a column from the generated transmit beamforming matrix that is designed for the receiving device by testing all columns of the generated transmit beamforming matrix.

36. The method of claim 29, where extracting the generated transmit beamforming matrix comprises applying a hypothesis test to the precoded reference signal received at a receiving device to extract the transmit beamforming matrix, where the hypothesis test comprises selecting a transmit beamforming matrix from a finite set of candidate transmit beamforming matrices, where the selected transmit beamforming matrix has a predetermined column w_t which minimizes a distance measure z_test=∥

y−

H^Hw_t∥

², where y is the received precoded reference signal, and H is a MIMO downlink channel matrix to said receiving device.

37. The method of claim 29, further comprising applying a first test to the precoded reference signal received at a receiving device to identify which transmit beamforming vector is designed for said receiving device, where the first test comprises selecting a reference signal from a finite set of transmit reference signals y_j which maximizes a first objective measure |v_opt^Hy_j|², where v_opt is an optimal blind receive beamforming vector initially designed by said receiving device and y_j are the precoded reference signals.

38. The method of claim 29, where extracting the generated transmit beamforming matrix from at least the precoded reference signal comprises hypothesis testing the precoded reference signal received at a receiving device against all possible candidate transmit beamforming matrices.

39. The method of claim 29, where extracting the generated transmit beamforming matrix from at least the precoded reference signal comprises hypothesis testing the precoded reference signal received at a receiving device against a subset of all possible candidate transmit beamforming matrices when a transmit beamforming vector for the receiving device is determined by the effective channel information fed back to the transmitting device.

40. A forward reference signaling method for a multiple input, multiple output (MIMO) space division multiple access (SDMA) system, comprising:

feeding back to a transmitting device a preferred precoding vector or information representative thereof from a receiving device;

receiving over a feed forward channel one or more index bits representative of a transmit beamforming matrix which is generated using a spatial separation algorithm based at least in part on the preferred precoding vector or information representative thereof;

using the index bits at the receiving device to retrieve the generated transmit beamforming matrix from a finite set of candidate transmit beamforming matrices; and

using the transmit beamforming matrix to generate a receive beamforming vector at the receiving device without requiring additional information to be fed forward that identifies which transmit beamforming vector in the transmit beamforming matrix is designed for said receiving device.