MIMO decoding

• US 20050265465A1
• Filed: 05/27/2004
• Published: 12/01/2005
• Est. Priority Date: 05/27/2004
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Ikram, Muhammed Z., Goel, Manish, Balakrishnan, Jaiganesh, Hosur, Srinath, Polley, Michael O.
• Granted Patent: US 7,724,832 B2
• Time in Patent Office: Days
• US Class Current: 375/260
• CPC Class Codes: H04L 25/0204 of multiple channels
• CPC Class Codes: H04L 25/067 providing soft decisions, i.e. decisions together with an estimate of reliability H04L25/068 and H04L25/069 take precedence; sequence estimation techniques H04L25/03178
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01

Abstract

In MIMO wireless communications employing LMMSE receiver, the symbols transmitted through a transmit antenna are estimated at the receiver in the presence of interference consisting of two main components: one due to the additive noise and the other due to (interfering) symbols transmitted via the remaining antennas. This has been shown to hamper the performance of a communication system resulting in incorrect symbol decisions, particularly at low SNR. IMMSE has been devised as a solution to cope with this problem; In IMMSE processing, the symbols sent via each transmit antenna are decoded iteratively. In each stage of processing, the received signal is updated by removing the contribution of symbols detected in the previous iterations. In principle, this reduces the additive interference in which the desired symbols are embedded in. Therefore, the interference level should reduce monotonically as one goes down in processing order. In a noisy environment, however, any incorrect decision made on a symbol in an iteration leaves its contribution in the updated received signal available for processing in the following iterations. Fortunately, if the level of interference is estimated and the soft bits are scaled appropriately by the estimated interference power, the performance of IMMSE receiver can be greatly improved. Preferred embodiments estimate the interference by computing the probability of error in decoding the symbols of the previous stage(s). The computation of decision error probability depends on the constellation size of transmitted symbols and introduces very little processing overhead.

Claims

1. A method of detecting, comprising:

(a) receiving a Q-vector signal representing a set of P symbols where P and Q are both integers greater than 1;

(b) estimating a first symbol of said set of P symbols;

(c) subtracting an estimate of the portion of said received signal due to said first symbol estimate from step (b) to give a first interference-cancelled received signal;

(d) soft-estimating a second symbol of said set of P symbols from said first interference-cancelled received signal; and

(e) providing a scaling for said soft-estimated second symbol from step (d) as a combination of (i) a detection scaling for said second symbol using said first interference-cancelled received signal and (ii) a first symbol estimate error term wherein said first symbol estimate error term includes a statistic relating to error in said first symbol estimate.

2. The method of claim 1, wherein:

(a) each of said P symbol is a vector of components with each component an element of a constellation where the number of components is greater than 1.

3. The method of claim 2, wherein:

(a) P=Q=2; and

(b) said components of each of said symbols correspond to tones of an orthogonal frequency division multiplex system with K tones where K is an integer greater than 1.

4. The method of claim 1, further comprising:

(a) estimating said second symbol of said set of P symbols;

(b) subtracting an estimate of the portion of said first interference-cancelled received signal due to said second symbol estimate from step (a) to give a second interference-cancelled received signal;

(c) soft-estimating a third symbol of said set of P symbols from said second interference-cancelled received signal; and

(d) providing a scaling for said soft-estimated third symbol from step (c) as a combination of (i) a detection scaling for said third symbol using said second interference-cancelled received signal and (ii) a first symbol estimate error and/or second symbol estimate error term wherein said first symbol estimate error and/or second symbol estimate error term includes a statistic relating to error in said first symbol estimate and/or said second symbol estimate.

5. The method of claim 1, wherein:

(a) said first symbol error term is α

|w^Hh₁|² where w is a Q-vector of detection weights for soft-estimating said second symbol from said said first interference-cancelled received signal, h₁s₁ is an estimate of the portion of said received signal due to said first symbol estimate when s₁ is said first symbol estimate, and α

relates to error in said first symbol estimate.

6. The method of claim 5, wherein:

(a) α

equals the expectation of |s₁−

ŝ

₁|² where s₁ is said first symbol and ŝ

₁ is said first symbol estimate.

7. The method of claim 5, wherein:

(a) α

equals 1/(β

+SINR₁) where β

is a positive number and SINR₁) is the estimated signal to interference plus noise ratio of said first symbol.

8. The method of claim 5, wherein:

(a) α

equals 1/(β

+SINR₁) where β

is a positive number and SINR₁ is the estimated decorrelator signal to interference plus noise ratio.

9. The method of claim 1, wherein:

(a) said soft-estimated second symbol from step (d) of claim 1 is minimum mean squared error estimation.

10. The method of claim 1, wherein:

(a) said soft-estimated second symbol from step (d) of claim 1 is zero-forcing estimation.

11. An iterative detector, comprising:

(a) a first detector for estimating a first symbol from a received Q-vector signal representing a set of P symbols where P and Q are both integers greater than 1;

(b) a regenerator-subtractor coupled to said first detector and for (i) regenerating an estimate of the portion of said received signal due to a first symbol estimate from said first detector and (ii) subtracting this regenerated estimate from said received signal to give a first interference-cancelled received signal;

(c) a second detector coupled to said regenerator-subtractor and for soft-estimating a second symbol of said set of P symbols from said first interference-cancelled received signal; and

(d) a scaling circuit coupled to said second detector and for scaling a soft-estimated second symbol from said second detector, wherein said scaling circuit combines (i) a detection scaling for said second detector and (ii) a first symbol estimate error term wherein said first symbol estimate error term includes a statistic relating to error in said first symbol estimate.

12. The detector of claim 11, wherein:

(a) said first detector, said regenerator-subtractor, said second detector, and said scaling circuit are implemented as a programmable processor plus program memory plus peripheral circuitry.

13. The detector of claim 11, wherein:

(a) said first detector outputs a soft estimate of said first symbol to said scaling circuit; and

(b) said scaling circuit outputs soft estimates and scalings for both said first symbol and said second symbol to a decoder.

14. The detector of claim 11, wherein:

(a) P=Q=2; and

(b) said components of each of said symbols correspond to tones of an orthogonal frequency division multiplex system with K tones.

15. The detector of claim 14, further comprising:

(a) a discrete Fourier transform circuit coupled to said first detector and said regenerator-subtractor, said discrete Fourier transform circuit for K-point Fourier transforms.

16. The detector of claim 11, wherein:

(a) said first symbol error term of said scaling circuit is α

|w^Hh₁|² where w is a Q-vector of detection weights for soft-estimating said second symbol from said said first interference-cancelled received signal, h₁s₁ is an estimate of the portion of said received signal due to said first symbol estimate when s₁ is said first symbol estimate, and a relates to error in said first symbol estimate.

17. The detector of claim 16, wherein:

(a) α

equals the expectation of |s₁−

ŝ

₁|² where s₁ is said first symbol and ŝ

₁ is said first symbol estimate.

18. The detector of claim 16, wherein:

(a) α

equals 1/(β

+SINR,) where β

is a positive number and SINR₁) is the estimated signal to interference plus noise ratio of said first symbol.

19. The detector of claim 16, wherein:

(a) α

equals 1/(β

+SINR₁) where β

is a positive number and SINR₁ is the estimated decorrelator signal to interference plus noise ratio.

20. The detector of claim 11, wherein:

(a) said soft-estimated second symbol of said second detector is a minimum mean squared error estimator.