Delta modulation for channel feedback in transmit diversity wireless communication systems

• US 7,376,192 B2
• Filed: 07/22/2004
• Issued: 05/20/2008
• Est. Priority Date: 07/22/2004
• Status: Expired due to Fees

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Koorapaty, Havish, Krasny, Leonid, Ramesh, Rajaram
• Primary Examiner(s): Payne; David C.
• Assistant Examiner(s): Nguyen; Leon-Viet Q
• Application Number:
  US10/897,250
• Publication Number:
  US 20060018389A1
• Time in Patent Office: 1,398 Days
• Field of Search: 375/219, 375/232, 375/296, 375346-348
• US Class Current: 375/259
• CPC Class Codes: H04B 7/0626 Channel coefficients, e.g. channel state information [CSI]
• CPC Class Codes: H04B 7/0641 Differential feedback
• CPC Class Codes: H04L 2025/03802 Signalling on the reverse channel
• CPC Class Codes: H04L 25/0214 of a single coefficient
• CPC Class Codes: H04L 25/0228 with direct estimation from sounding signals
• CPC Class Codes: H04L 25/03343 Arrangements at the transmitter end

Abstract

A method is described herein for reducing the number of feedback bits needed to send channel state information over a feedback channel from a receiving unit (e.g., mobile terminal, base station) to a transmitter unit (e.g., base station, mobile terminal) in a wireless communication system. In the embodiment, the receiver unit is capable of performing the following steps: (1) receiving a pilot signal from a transmit antenna located at a transmitter unit; (2) analyzing a channel tap associated with the transmit antenna and exploiting the temporal correlations of the channel tap to generate the following (i) a delta-modulated feedback bit indicative of a real part of a complex coefficient of the channel tap associated with the transmit antenna and (ii) a delta-modulated feedback bit indicative of an imaginary part of the complex coefficient of the channel tap associated with the transmit antenna; and (3) sending the two feedback bits over a feedback channel to the transmitter unit which analyzes the two feedback bits, and other feedback bits corresponding to other channel taps of this and other transmit antennas, to optimize the subsequent transmission of data to the receiver unit.

Claims

1. A receiver unit comprising:

a receiver for receiving a pilot signal from a transmit antenna located at a transmitter unit; and

a processor for performing the following steps;

computing a channel estimate of a real part of a complex coefficient of a channel tap associated with a channel corresponding to the transmit antenna;

generating a reconstructed channel estimate of the real part of the complex coefficient of the channel tap associated with the channel corresponding

to the transmit antenna using feedback bits that were sent in the past to the transmitter unit;

determining a difference between the channel estimate and the reconstructed channel estimate;

quantizing the difference between the channel estimate and the reconstructed channel estimate to generate a feedback bit to be sent to the transmitter unit;

said processor also performing the computing, generating, determining and quantizing steps to generate a feedback bits for an imaginary part of the complex coefficient of the channel tap associated with the channel of the pilot signal;

a transmitter for sending the two feedback bits corresponding to a channel tap over a feedback channel to the transmitter unit which uses the feedback bits to optimize the subsequent transmission of data to the receiver;

wherein said processor uses more than two feedback bits per channel tap to send a more complete channel estimate to the transmitter unit

     1) during an initial synchronization between the receiver and the transmitter unit;

     2) when said transmitter unit requests a resynchronization between the receiver and the transmitter unit;

     3) when said processor decides to send the more complete channel estimate to said transmitter unit;

or

     4) at periodic intervals.

2. The receiver unit of claim 1, wherein said processor for both the real part and the imaginary part of the channel tap compensates for a feedback delay by performing a step of predicting a future channel estimate based on current and past estimates and uses this predicted channel in the determining and quantizing steps.

3. The receiver unit of claim 1, wherein said processor for both the real part and the imaginary part of the channel tap performs a step of adapting a step size for a current time step based on previous feedback bits wherein the adapted step size indicates a weight to be accorded to the latest feedback bit when generating the reconstructed channel estimate.

4. The receiver unit of claim 3, wherein said transmitter periodically sends the step sizes in addition to the two feedback bits per channel tap to the transmitter unit.

5. The receiver unit of claim 1, wherein said receiver unit is a mobile terminal and said transmitter unit is a base station.

6. The receiver unit of claim 1, wherein said receiver unit is a base station and said transmitter unit is a mobile terminal.

7. A transmitter unit comprising:

a transmit antenna for transmitting a pilot signal to a receiver unit;

a receiver for receiving two feedback bits sent by said receiver unit; and

,

a processor for analyzing the two received feedback bits b(n) for each channel tap corresponding to the transmit antenna to generate reconstructed channel estimates for both the real parts and the imaginary parts of the complex coefficients of the channel taps and for using the reconstructed channel estimates to optimize the subsequent transmission of data to the receiver unit from that transmit antenna;

wherein the receiver for receiving feedback bits receives more than two feedback bits per channel tap representing a more complete channel estimate

     1) during an initial synchronization between said transmitter unit and the receiver unit;

     2) during periodic resynchronization between said transmitter unit and the receiver unit;

or

     3) when said receiver unit decides to send the more complete channel estimate to said transmitter unit.

8. The transmitter unit of claim 7, wherein:

said processor, for both the real part and the imaginary part of a channel tap, performs a step of adapting a step size for a current time step based on previous feedback bits received from the receiver unit wherein the adapted step size indicates a weight to be accorded to the latest feedback bit when reconstructing the channel estimate.

9. The transmitter unit of claim 8, wherein the receiver for receiving feedback bits periodically receives step sizes from the receiver unit in addition to the two feedback bits per channel tap.

10. The transmitter unit of claim 7, wherein said transmitter unit is a mobile terminal and said receiver unit is a base station.

11. The transmitter unit of claim 7, wherein said transmitter unit is a base station and said receiver unit is a mobile terminal.

12. A wireless communication system comprising:

a receiver unit that receives a pilot signal corresponding to a transmit antenna and performs for both a real part and imaginary part of a complex coefficient of a channel tap associated with the transmit antenna the following steps;

computing a channel estimate;

compensating for a feedback delay by computing a predicted channel estimated time steps ahead;

generating a reconstructed channel estimate based on a latest feedback bit sent to the transmitter unit, past reconstructed channel estimates and on the step size;

adapting the step size based on previous feedback bits;

generating a feedback bit by quantizing the difference between the predicted channel estimate and a function of the current and past reconstructed channel estimates into two levels;

transmitting the feedback bit to the transmitter unit;

a transmitter unit for transmitting a pilot signal corresponding to each transmit antenna at the transmitter unit that performs the following steps;

receiving the feedback bits transmitted by said receiver unit;

adapting the step size for a current time step based on previous feedback bits;

reconstructing a channel estimate based on the latest feedback bit received from the receiver unit, past reconstructed channel estimates and on the step size; and

utilizing the reconstructed channel estimate to maximize throughput to said receiver unit;

wherein said receiver unit uses more than two feedback bits per channel tap to send a more complete channel estimate to the transmitter unit

     1) when said transmitter unit requests a resynchronization between said receiver unit and said transmitter unit;

     2) during an initial synchronization between said receiver unit and said transmitter unit;

or

     3) at periodic times to resynchronize said receiver unit and said transmitter unit.

13. The wireless communications system of claim 12, wherein the step size is adapted as follows:

the step size is increased by multiplying with a constant greater than 1 when a fixed number of past feedback bits all have the same value;

the step size is decreased by multiplying with a constant greater less than 1 when the sum of a fixed number of past feedback bits is zero;

the step size is restricted to lie within an upper and a lower limit; and

the upper and lower limits may also be adjusted based on the rate of variation of the channel estimates.

14. The wireless communications system of claim 12, wherein the predicted channel estimate is computed using a Wiener filter.

15. The wireless communications system of claim 12, wherein said receiver unit periodically sends the step size in addition to the feedback bit to the transmitter unit.

16. The wireless communications system of claim 12, wherein said receiver unit uses more than two feedback bits per channel to send a more complete channel estimate to the transmitter unit.

17. The wireless communications system of claim 12, wherein said transmitter unit has multiple transmit antennas.

18. The wireless communications system of claim 12, wherein said transmitter unit is a mobile terminal and said receiver unit is a base station.

19. The wireless communications system of claim 12, wherein said transmitter unit is a base station and said receiver unit is a mobile terminal.

20. A method for feeding back detailed channel information from a receiver unit to a transmitter unit in a wireless communications network, said method comprising the steps of:

transmitting, from a transmit antenna at a transmitter unit, a pilot signal;

receiving, at the receiver unit, the pilot signal corresponding to the transmit antenna and performing for both a real part and imaginary part of a complex coefficient of a channel tap associated with the transmit antenna the following steps;

computing a channel estimate;

compensating for a feedback delay by computing a predicted channel estimate d time steps ahead;

generating a reconstructed channel estimate based on a latest feedback bit sent to the transmitter unit, past reconstructed channel estimates and on the step size;

adapting the step size based on previous feedback bits;

generating a feedback bit by quantizing the difference between the predicted channel estimate and a function of the current and past reconstructed channel estimates into two levels;

transmitting the feedback bit b(n) to the transmitter unit;

receiving at the transmitter unit the feedback bit b(n) and performing the following steps;

adapting step size for a current time step based on previous feedback bits;

reconstructing a channel estimate based on the latest feedback bit received from the receiver unit, past reconstructed channel estimates and on the step size; and

utilizing the reconstructed channel estimate to maximize throughput to said receiver unit;

wherein said receiver unit uses more than two feedback bits per channel tap to send a more complete channel estimate to the transmitter unit

     1) when said transmitter unit requests a resynchronization between said receiver unit and said transmitter unit;

     2) during an initial synchronization between said receiver unit and said transmitter unit;

or

     3) at periodic times to resynchronize said receiver unit and said transmitter unit.

21. The method of claim 20, wherein the step size is adapted as follows:

the step size is increased by multiplying with a constant greater than 1 when a fixed number of past feedback bits all have the same value;

the step size is decreased by multiplying with a constant greater less than 1 when the sum of a fixed number of past feedback bits is zero;

the step size is restricted to lie within an upper and a lower limit; and

the upper and lower limits may also be adjusted based on the rate of variation of the channel estimates.

22. The method of claim 20, wherein the predicted channel estimate is computed using a Wiener filter.

23. The method of claim 20, wherein said receiver unit periodically sends the step size in addition to the feedback bit to the transmitter unit.

24. The method of claim 20, wherein said receiver unit uses more than two feedback bits per channel tap to send a more complete channel estimate to the transmitter unit.

25. The method of claim 20, wherein said transmitter unit has multiple transmit antennas.

26. The method of claim 20, wherein said transmitter unit is a mobile terminal and said receiver unit is a base station.

27. The method of claim 20, wherein said transmitter unit is a base station and said receiver unit is a mobile terminal.