Communications system employing non-polluting pilot codes

• US 7,224,942 B2
• Filed: 07/26/2001
• Issued: 05/29/2007
• Est. Priority Date: 07/26/2001
• Status: Expired due to Fees

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Dent, Paul W.
• Primary Examiner(s): Appiah; Charles
• Assistant Examiner(s): Daniel, Jr.; Willie J.
• Application Number:
  US09/915,895
• Publication Number:
  US 20030054828A1
• Time in Patent Office: 2,133 Days
• Field of Search: 455/63.1, 455/67.11, 455/67.13, 455/67.14, 455/67.15, 455/67.16, 455/43, 370/203, 370/208, 370/241, 370/249, 370/341, 370/324, 370/329, 375/267, 375/247
• US Class Current: 455/69
• CPC Class Codes: H04B 7/005 Control of transmission; Equalising
• CPC Class Codes: H04B 7/022 Site diversity; Macro-diversity using two or more spaced independent antennas H04B7/04
• CPC Class Codes: H04B 7/0619 using feedback from receiving side feedback signaling for adaptive modulation/coding H04L1/0001
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/0226 sounding signals per se

Abstract

Transmitting non-polluting dummy pilot signals in a wireless communication network permits estimation of the downlink propagation channels between a number of joint transmitters and a lesser number of receivers. With fewer receivers than transmitters, loop back information from the receivers is not sufficient to determine the downlink propagation channels between the transmitters and receivers. For N transmitters and M receivers, (N-M) dummy pilot symbols are transmitted to facilitate downlink channel estimation. Each dummy pilot signal is transmitted to an imagined or dummy receiver that is virtually located such that its downlink channel coefficient vector is orthogonal to those of the real receivers. Transmit pre-filtering based on estimated propagation channels is applied to the information signals for the real receivers and to the dummy pilot signals. The extent to which the dummy pilot signals interfere at each receiver is an indication of mismatch between estimated and actual propagation channels.

Claims

1. A method of facilitating estimation of propagation channels between a first number of transmitters and a lesser number of receivers, the method comprising:

transmitting information signals for said receivers jointly from said transmitters based on propagation channel estimates, such that interference between information signals is reduced at each said receiver; and

transmitting a number of dummy pilot signals equal to a difference between the number of transmitters and receivers, such that said dummy pilot signals cause substantially no interference in reception of said information signals by said receivers when said propagation channel estimates substantially match said propagation channels.

2. The method of claim 1, further comprising, for each said dummy pilot signal transmitted, calculating propagation channel estimates for a virtual receiver location corresponding to said dummy pilot signal.

3. The method of claim 1, further comprising choosing said virtual receiver location to be a location relative to said transmitters such that reception of said dummy pilot signal jointly transmitted by said transmitters would be strongest.

4. The method of claim 3, wherein propagation channel estimates for each said receiver relative to said transmitters comprise a channel estimate vector, and wherein choosing a virtual receiver location comprises determining a supplemental channel estimate vector orthogonal to said channel estimate vectors of said receivers.

5. The method of claim 1, further comprising forming said propagation channel estimates into a channel estimate matrix comprising a matrix column for each said transmitter and a matrix row for each said receiver, such that a matrix element represents propagation channel estimates between a given transmitter and a given receiver, and wherein each said matrix row comprises a channel estimate vector.

6. The method of claim 5, further comprising making said channel estimate matrix square for use in pre-filtering said information signals and said dummy pilot signals before transmission by supplementing said channel estimate matrix based on adding supplemental channel estimate vectors as additional matrix rows, wherein each supplemental channel estimate vector corresponds to one of said dummy pilot signals being transmitted by said transmitters.

7. The method of claim 6, further comprising computing said supplemental matrix rows to be orthogonal to said channel estimate vectors for said receivers.

8. The method of claim 6, wherein transmitting information signals for said receivers jointly from said transmitters based on propagation channel estimates comprises pre-filtering said information signals based on said channel estimate matrix such that each transmitter transmits a weighted combination of said information signals, and wherein said weighted combinations of said information signals combine at each said receiver to substantially cancel the information signals for the other receivers.

9. The method of claim 8, wherein transmitting a number of dummy pilot signals equal to a difference between the number of transmitters and receivers comprises pre-filtering said dummy pilot signals based on said channel estimate matrix, such that said dummy pilot signals substantially cancel at each said receiver, and wherein an amount of interference caused by said dummy pilot signals at each said receiver is a function of mismatch between said propagation channel estimates for that receiver and the actual propagation channels to that receiver.

10. The method of claim 8, further comprising:

determining dummy pilot signal interference at said receivers; and

adjusting said propagation channel estimates based on said determined dummy pilot signal interference.

11. A method of estimating propagation channels between a number transmitters and a lesser number of receivers, the method comprising:

initializing a set of propagation channel estimates for said propagation channels;

generating a number of dummy pilot signals equal in number to a difference between said number of transmitters and receivers;

determining supplemental propagation channel coefficients for each said dummy pilot signal based on said propagation channel estimates intended to cause said dummy pilot signals to substantially cancel at each said receiver, thus causing no interference at said receivers;

pre-filtering said dummy pilot signals and an information signal for each said receiver using said propagation channel estimates and said supplemental propagation channel estimates;

transmitting said information signals and said dummy pilot signals jointly from said transmitters to said receivers after said pre-filtering; and

adjusting said propagation channel estimates based on observed dummy pilot signal interference at said receivers.