Using antenna arrays in multipath environment

• US 20040156443A1
• Filed: 02/10/2003
• Published: 08/12/2004
• Est. Priority Date: 02/10/2003
• Status: Active Grant

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Dent, Paul W.
• Granted Patent: US 7,184,492 B2
• Time in Patent Office: Days
• US Class Current: 375/267
• CPC Class Codes: H04B 7/0408 using two or more beams, i.e. beam diversity
• CPC Class Codes: H04B 7/0617 for beam forming
• CPC Class Codes: H04B 7/0671 using different delays between antennas
• CPC Class Codes: H04B 7/0695 using beam selection

Abstract

The invention comprises using beamforming antenna to coherently transmit an information signal to a receiver using two or more directional beams. In one embodiment, the phase and timing of the information signals carried by each directional beams are adjusted such that the signals arrive synchronously at the mobile terminal. Time synchronization may be obtained by delaying signals transmitted on selected directional beams to compensate for different propagation delays, or by preconditioning and filtering the signals using a channel coefficient matrix.

Claims

1. A method of transmitting an information signal to a first mobile terminal comprising:

forming a plurality of directional beams, each directional beam oriented in a different direction and having a propagation delay;

selecting two or more directional beams to use for transmitting the information signal to the first mobile terminal;

for each selected directional beam, generating a transmit signal derived from the information signal for transmission to the first mobile terminal;

transmitting the transmit signals to the first mobile terminal over respective ones of the selected directional beams; and

compensating for differences in propagation delay between the selected directional beams such that the transmit signals arrive time aligned at the first mobile terminal.

2. The method of claim 1 wherein compensating for differences in propagation delay between the selected directional beams comprises delaying transmit signals transmitted on at least one of the selected directional beams.

3. The method of claim 2 wherein forming a transmit signal for a selected directional beam comprises:

determining a channel coefficient polynomial describing the estimated amplitude and phase attenuation associated with the selected directional beam; and

prefiltering the information signal using the channel coefficient polynomial.

4. The method of claim 3 wherein prefiltering the information signal using the channel coefficient polynomial comprises prefiltering the information signal with a time reverse conjugate of the channel coefficient polynomial.

5. The method of claim 4 wherein the channel coefficient polynomial includes complex coefficients encoding phase and amplitude attenuation associated with the directional beam and propagation delay values.

6. The method of claim 5 wherein compensating for differences in propagation delay between the selected directional beams such that the transmit signals arrive time aligned at the first mobile terminal comprises adjusting the propagation delay values in the channel coefficient polynomial to compensate for the differences in propagation delay between selected directional beams.

7. The method of claim 4 wherein forming a transmit signal for a selected directional beam further comprises filtering the information signals using a pulse shaping filter.

8. The method of claim 1 wherein forming a transmit signal for a selected directional beam comprises pre-filtering the information signal in a transmit signal generator to compensate for inter-symbol interference in the selected directional beams.

9. The method of claim 1 wherein forming a transmit signal for a selected directional beam comprises pre-filtering the information signal in a transmit signal generator to compensate for interference between the at least two directional beams.

10. The method of claim 1 wherein forming a transmit signal for a selected directional beam comprises filtering the information signal in an interference cancellation filter such that interfering signals for other mobile terminals cancel at the first mobile terminal.

11. The method of claim 10 wherein said interference cancellation filter comprises an infinite impulse response filter.

12. The method of claim 1 wherein forming a transmit signal for a selected directional beam comprises weighting the transmit signal in each directional beam according to an estimated amplitude attenuation.

13. The method of claim 1 wherein selecting at two or more directional beams to use for transmitting an information signal to the first mobile terminal comprises estimating path loss associated with the plurality of directional beams and selecting directional beams from the plurality of directional beams based on the estimated path loss.

14. The method of claim 1 wherein selecting at two or more directional beams to use for transmitting an information signal to the mobile terminal comprises measuring the strength of signals received from the first mobile terminal on the plurality of directional beams and selecting directional beams from the plurality of directional beams based on the strength of the received signals.

15. The method of claim 1 wherein selecting at two or more directional beams to use for transmitting an information signal to the first mobile terminal comprises:

estimating multi-path propagation channel characteristics between the transmitter and the first mobile terminal for the plurality of directional beams; and

choosing the selected directional beams based on the estimated multipath propagation channel characteristics.

16. The method of claim 15 wherein estimating multipath propagation channel characteristics comprises determining the multipath channel characteristics based on a loopback signal from the first mobile terminal.

17. The method of claim 1 further comprising reselecting at least one directional beam responsive to path loss variations.

18. The method of claim 1 further comprising changing a transmitted power level associated with at least one directional beam responsive to path loss variations.

19. The method of claim 1 wherein forming a plurality of directional beams comprises forming the plurality of directional beams with one or more beamforming antennas, where each beamforming antenna comprises a plurality of antenna elements.

20. The method of claim 19 wherein forming the plurality of directional beams with the at least one beamforming antenna comprises forming a number of directional beams less than the number of antenna elements in each beamforming antenna.

21. The method of claim 19 wherein selecting two or more directional beams comprises selecting at least two directional beams from the same beamforming antenna.

22. The method of claim 19 wherein the azimuth of each directional beam is fixed.

23. A communication system to transmit an information signal to a first mobile terminal comprising:

one or more beamforming antennas to form a plurality of directional beams, each directional beam oriented in a different direction and having a propagation delay, where each beamforming antenna comprises a plurality of antenna elements;

a controller to select two or more directional beams from at least one beamforming antenna for transmitting the information signal to the first mobile terminal;

a transmit signal generator coupled to each of said beamforming antenna and said controller to derive a transmit signal from the information signal for each selected directional beam; and

a delay element to compensate for differences in propagation delay between the selected directional beams such that the transmit signals arrive time aligned at the first mobile terminal.

24. The communication system of claim 23 wherein the delay element compensates for differences in the propagation delay between the selected directional beams by delaying the transmit signal on at least one of the selected directional beams.

25. The communication system of claim 24 wherein the transmit signal generator determines a channel coefficient polynomial describing the estimated amplitude and phase attenuation associated with the selected directional beams.

26. The communication system of claim 25 wherein the delay element adjusts the propagation delay in the channel coefficient polynomial to compensate for the differences in propagation delay between selected directional beams.

27. The communication system of claim 24 wherein the transmit signal generator includes a filter to pre-filter the information signal using the channel coefficient polynomial.

28. The communication system of claim 27 wherein the filter comprises a finite impulse response filter to pre-filter the information signal with a time reverse conjugate of the channel coefficient polynomial.

29. The communication system of claim 28 wherein the channel coefficient polynomial determined by the transmit signal generator further includes complex coefficients encoding phase and amplitude attenuation associated with the directional beams and propagation delay.

30. The communication system of claim 23 wherein the transmit signal generator includes a pulse shaping filter to pre-filter the information signals.

31. The communication system of claim 23 wherein the transmit signal generator includes a filter to compensate for inter-symbol interference in the selected directional beams.

32. The communication system of claim 23 wherein the transmit signal generator includes a filter to compensate for interference between the at least two directional beams.

33. The communication system of claim 23 wherein the transmit signal generator includes an interference cancellation filter to filter the information signal such that interfering signals for other mobile terminals cancel at the first mobile terminal.

34. The communication system of claim 33 wherein said interference cancellation filter comprises an infinite impulse response filter.

35. The communication system of claim 23 wherein the transmit signal generator further weights the transmit signal in each directional beam according to an amplitude attenuation.

36. The communication system of claim 23 wherein the controller estimates path loss associated with the plurality of directional beams and selects directional beams from the plurality of directional beams based on the estimated path loss.

37. The communication system of claim 23 wherein the controller measures the strength of signals received from the first mobile terminal on the plurality of directional beams and selects directional beams from the plurality of directional beams based on the strength of the received signals.

38. The communication system of claim 23 wherein the controller estimates multi-path propagation channel characteristics between the transmitter and the first mobile terminal for the plurality of directional beams and chooses the selected directional beams based on the estimated multi-path propagation channel characteristics.

39. The communication system of claim 38 wherein the controller estimates the multipath propagation channel characteristics by determining the multi-path channel characteristics based on a loopback signal from the first mobile terminal.

40. The communication system of claim 23 wherein the controller further reselects at least one directional beam responsive to path loss variations.

41. The communication system of claim 23 wherein the transmit signal generator further changes a transmitter power level associated with at least one directional beam responsive to path loss variations.

42. The communication system of claim 23 wherein the transmit signal generator includes a beamformer.

43. The communication system of claim 23 wherein each beamforming antenna forms a number of directional beams less than the number of antenna elements in the beamforming antenna.