Dithering scheme using multiple antennas for OFDM systems

• US 8,331,217 B2
• Filed: 03/27/2009
• Issued: 12/11/2012
• Est. Priority Date: 08/28/2002
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agere Systems LLC (Broadcom, Inc.)
• Inventors: Celebi, Samel, Song, Jie, Yun, Paul M.
• Primary Examiner(s): Sheikh, Ayaz
• Assistant Examiner(s): HALIYUR, VENKATESH N
• Application Number:
  US12/412,607
• Publication Number:
  US 20090180564A1
• Time in Patent Office: 1,355 Days
• Field of Search: 455/101, 455/129, 455/503, 455/561, 455/574, 370/208, 370/316, 370/319, 370/331, 370/334, 370/342, 370/344, 370/503
• US Class Current: 370/208
• CPC Class Codes: H04B 7/0671 using different delays between antennas
• CPC Class Codes: H04B 7/0682 using phase diversity (e.g. phase sweeping)
• CPC Class Codes: H04B 7/18517 Transmission equipment in earth stations
• CPC Class Codes: H04W 52/34 TPC management, i.e. sharing limited amount of power among users or channels or data types, e.g. cell loading

Abstract

A ground- or roof-top-based repeater in an OFDM system uses multiple transmission antennas to transmit multiple identical OFDM signals. Dithering is performed by introducing a slight variable-frequency phase offset to all but one of the multiple identical transmitted OFDM signals. The effective overall channel is more dynamic and provides spatial diversity to minimize long periods of fading in fading subchannels of the OFDM signals when the receiver is in a slow moving or stationary situation. To overcome the additional cancellation problem that can occur when two or more of the transmitting antennas are (i) in a line-of-site position with the receiver and (ii) approximately the same distance from the receiver, a delay is deliberately introduced to make the delayed signals appear to be reflected signals. This delay will not negatively impact the OFDM receiver performance as long as the delay is within the guard interval used in the OFDM process.

Claims

1. A method for transmitting an information stream over a wireless communication medium to one or more receivers, the method comprising:

(a) applying both orthogonal-frequency division multiplexed (OFDM) modulation and splitting to the information stream to generate a first copy and M−

1 other copies of an OFDM signal;

(b) applying M−

1 variable-frequency phase offsets to the M−

1 other copies to generate M−

1 phase-offset copies; and

(c) transmitting the first copy and the M−

1 phase-offset copies via M respective antennas over the wireless communication medium, where M is an integer value greater than 1.

2. The invention of claim 1, wherein the M−

1 variable-frequency phase offsets are applied in step (b) by multiplying each of the M−

1 other copies by a phase generated by a Rayleigh process.

3. The invention of claim 1, wherein the OFDM modulation is applied to the information stream before the splitting.

4. The invention of claim 3, wherein the OFDM signal is multiplied by 1/M after the OFDM modulation and prior to the splitting.

5. The invention of claim 1, wherein:

step (b) comprises applying M−

1 delays and the M−

1 variable-frequency phase offsets to the M−

1 other copies of the OFDM signal to generate M−

1 delayed, phase-offset copies, wherein the M−

1 delays are relative to the first copy; and

the M−

1 phase-offset copies transmitted in step (c) are the M−

1 delayed, phase-offset copies.

6. The invention of claim 5, wherein the M−

1 delays are applied prior to the M−

1 variable-frequency phase offsets.

7. The invention of claim 1, wherein the application of the M−

1 variable-frequency phase offsets increases effective channel response between the M antennas and the one or more receivers.

8. An apparatus for transmitting an information stream over a wireless communication medium to one or more receivers, the apparatus comprising:

both an orthogonal-frequency division multiplexing (OFDM) modulator and a splitter adapted to apply OFDM modulation and splitting, respectively, to the information stream to generate a first copy and M−

1 other copies of an OFDM signal;

a first multiplier adapted to apply M−

1 variable-frequency phase offsets to the M−

1 other copies to generate M−

1 phase-offset copies; and

a transmission device adapted to transmit the first copy and the M−

1 phase-offset copies via M respective antennas over the wireless communication medium.

9. The invention of claim 8, wherein the first multiplier applies the M−

1 variable-frequency phase offsets step (b) by multiplying each of the M−

1 copies by a phase generated by a Rayleigh process.

10. The invention of claim 8, wherein OFDM modulation is applied by the OFDM modulator before splitting is applied by the splitter.

11. The invention of claim 10, further comprising a second multiplier adapted to multiply the OFDM signal by 1/M before the OFDM signal is split.

12. The invention of claim 8, wherein:

the apparatus comprises a delay buffer and the first multiplier adapted to apply M−

1 delays and the M−

1 variable-frequency phase offsets, respectively, to the M−

1 other copies of the OFDM signal to generate M−

1 delayed, phase-offset copies, wherein the M−

1 delays are relative to the first copy; and

the M−

1 phase-offset copies transmitted by the transmission device are the M−

1 delayed, phase-offset copies.

13. The invention of claim 12, wherein the M−

1 delays are applied by the delay buffer before the variable-frequency phase offsets are applied by the first multiplier.

14. The invention of claim 8, wherein the application of the M−

1 variable-frequency phase offsets increases effective channel response between the M antennas and the one or more receivers.