Inter-symbol interference cancellation for wireless multiple access

• US 7,733,941 B2
• Filed: 06/29/2006
• Issued: 06/08/2010
• Est. Priority Date: 11/15/2005
• Status: Active Grant

• Current Assignee: III Holdings 1, LLC
• Original Assignee: Rambus Inc.
• Inventors: McCloud, Michael L, Guess, Tommy
• Primary Examiner(s): Lugo, David B
• Application Number:
  US11/479,401
• Publication Number:
  US 20070110196A1
• Time in Patent Office: 1,440 Days
• Field of Search: 375/144, 375/148, 375/229, 375/230, 375/232, 375/233, 375/260, 375/267, 375/346
• US Class Current: 375/148
• CPC Class Codes: H04B 2201/70702 Intercell-related aspects
• CPC Class Codes: H04L 2025/03375 Passband transmission
• CPC Class Codes: H04L 2025/0349 as a feedback filter
• CPC Class Codes: H04L 2025/03605 Block algorithms
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence

Abstract

An interference-canceling receiver is configured for cancelling inter-symbol interference due to both inter- and intra-channel interference in coded, multiple-access, spread spectrum transmissions that propagate through frequency-selective communication channels. The receiver mitigates the effects of post-cursor inter-symbol interference using feedback of previously estimated symbols, and mitigates pre-cursor inter-symbol interference using an interference canceller, such as an iterative interference canceller.

Claims

1. An interference canceller for cancelling inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal, comprising:

a post-cursor ISI synthesis module configured for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

a post-cursor ISI cancellation module and a resolving module configured for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation module configured for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback loop configured for coupling the post-cursor symbol estimates to the post-cursor ISI synthesis module,

whereby the post-cursor ISI synthesis module is further configured to receive as input a plurality of symbol estimates in a predetermined number of post-cursor symbol intervals, the post-cursor ISI synthesis module comprising;

a weighting module configured for weighting the plurality of symbol estimates according to symbol-estimate merits to produce weighted symbol estimates;

a code-waveform modulator configured for modulating the weighted symbol estimates onto corresponding code waveforms for producing modulated code waveforms and summing the modulated code waveforms to generate an estimated transmit signal; and

a channel emulator configured for imparting multipath delays and gains to the estimated transmit signal.

2. The interference canceller recited in claim 1, whereby the interference cancellation module is configured to subtract a synthesized post-cursor inter-symbol interference signal from the received signal to cancel post-cursor inter-symbol interference in the received signal.

3. The interference canceller recited in claim 1, whereby each of the symbol-estimated merits comprises a function of average ratio of signal power to interference-plus-noise power of a corresponding symbol estimate.

4. The interference canceller recited in claim 3, whereby the average ratio of signal power to interference-plus-noise power is estimated from a pilot signal.

5. The interference canceller recited in claim 3, whereby the average ratio of signal power to interference-plus-noise power is estimated using techniques of statistical signal processing.

6. The interference canceller recited in claim 1, whereby the function of the average ratio of signal power to interference-plus-noise power is characterized by

$\pi$

= max ⁢

{ C , 1 1 + 1 / SINR } ,

where π

is a symbol weight, max { } is a function for selecting a maximum value from a set of quantities within brackets { }, SINR denotes an averages ratio of signal power to interference-plus-noise power (SINR) of the symbol, and C is a non-negative real constant for enforcing a minimum symbol weight.

7. The interference canceller recited in claim 1, whereby the weighting module is further configured to assign all symbols a common weight.

8. The interference canceller recited in claim 7, whereby the weighting module is further configured to choose the weight to minimize mean square-error between the received signal and a synthesized signal.

9. The interference canceller recited in claim 8, whereby the weighting module is further configured to choose the weight according to

${\pi }_{}$

n = ( θ

n post ) H ⁢

θ

n / 

θ

n post 

2 ,

where π

_n is the weight, θ

_n is a vector having multi-symbol elements produced from Rake processing with combining, θ

_n^post is a vector having multi-symbol outputs after despreading, H denotes a complex-conjugate transpose, and ∥

•

∥

² denotes a sum of an argument'"'"'s elements'"'"' magnitude squares.

10. An interference canceller for cancelling inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal, comprising:

a post-cursor ISI synthesis module configured for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

a post-cursor ISI cancellation module and a resolving module configured for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation module configured for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback loop configured for coupling the post-cursor symbol estimates to the post-cursor ISI synthesis module,

whereby the interference cancellation module comprises,

a symbol-estimation module configured for generating initial symbol estimates;

a sequence of interference cancellation units, the sequence configured to receive the initial symbol estimates, each of the sequence of interference cancellation units configured to weight input symbol estimates with soft weighting, apply stabilizing step sizes to a difference signal, and perform mixed decisions to produce updated, interference-cancelled symbol estimates; and

a partitioning module configured to output final updated, interference-cancelled symbol estimates for an on-cursor symbol interval.

11. The interference canceller recited in claim 10, whereby the partitioning module is further configured to output temporary symbol estimates corresponding to a predetermined number of post-cursor symbol intervals.

12. The interference canceller recited in claim 10, whereby the symbol-estimation module is further configured to employ temporary symbol estimates for a predetermined number of post-cursor symbol intervals for generating the initial symbol estimates.

13. An interference canceller for cancelling inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal, comprising:

a post-cursor ISI synthesis module configured for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

a post-cursor ISI cancellation module and a resolving module configured for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation module configured for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback loop configured for coupling the post-cursor symbol estimates to the post-cursor ISI synthesis module,

whereby the interference cancellation module is further configured for performing iterative pre-cursor ISI cancellation at the symbol level by employing a one-step matrix update equation, comprising an explicit matrix representation of a received-signal correlation matrix over multiple symbol intervals, an implementation matrix over multiple symbol intervals, a soft-weighting matrix, a scalar stabilizing step size, and a mixed decision symbol estimator.

14. The interference canceller recited in claim 13, whereby the matrix update equation is expressed by

${\underset{\_}{\hat{b}}}_n^{\left(i+1\right)}=\Psi$

⁡

( μ

n [ i ] ⁡

( q _ n - R n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) + F n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) ,

wherein {circumflex over (b)}_n^[i] is a column vector containing input symbols decisions of sources of precursor symbols for symbol intervals after an i^th iteration of interference cancellation, q_n is a vector of a resolved received signal over a plurality of post-cursor symbol intervals, μ

_n^[i] is a scalar stabilizing step, Γ

_n^[i] is a weighting matrix used for scaling the input symbol decisions, R_n is the received-signal correlation matrix over the plurality of post-cursor symbol intervals, F_n is an implementation matrix, Ψ

is a mixed-decision operator that returns mixed symbol decisions to produce the updated symbol decisions {circumflex over (b)}_n^[i+1].

15. A method whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the method comprising:

providing for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

providing for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal in which post-cursor ISI has been removed from the received signal;

providing for performing interference cancellation for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

providing for feeding back the post-cursor symbol estimates to the step of providing for synthesizing an estimated post-cursor ISI signal,

whereby providing for synthesizing is further configured to receive as input a plurality of symbol estimates in a predetermined number of post-cursor symbol intervals, providing for synthesizing comprising;

providing for weighting the plurality of symbol estimates according to symbol-estimate merits to produce weighted symbol estimates;

providing for modulating the weighted symbol estimates onto corresponding code waveforms for producing modulated code waveforms and summing the modulated code waveforms to generate an estimated transmit signal;

and providing for performing channel emulation to impart multipath delays and gains to the estimated transmit signal.

16. The method recited in claim 15, whereby providing for performing interference cancellation further comprises subtracting a synthesized post-cursor inter-symbol interference signal from the received signal to cancel post-cursor inter-symbol interference in the received signal.

17. The method recited in claim 15, whereby each of the symbol-estimate merits comprises a function of average ratio of signal power to interference-plus-noise power of a corresponding symbol estimate.

18. The method recited in claim 17, whereby providing for performing interference cancellation further comprises estimating the average ratio of signal power to interference-plus-noise power from a pilot signal.

19. The method recited in claim 17, whereby providing for performing interference cancellation further comprises using techniques of statistical signal processing for estimating the average ratio of signal power to interference-plus-noise power.

20. The method recited in claim 15, whereby the function of the average ratio of signal power to interference-plus-noise power is characterized by

$\pi$

= max ⁢

{ C , 1 1 + 1 / SINR } ,

where π

is a symbol weight, max { }is a function for selecting a maximum value from a set of quantities within brackets { }, SINR denotes an average ratio of signal power to interference-plus-noise power (SINR) of the symbol, and C is a non-negative real constant for enforcing a minimum symbol weight.

21. The method recited in claim 15, whereby providing for weighting further comprises assigning all symbols a common weight.

22. The method recited in claim 21, whereby providing for weighting further comprises choosing the weight to minimize mean square-error between the received signal and at least one synthesized signal.

23. The method recited in claim 22, whereby providing for weighting further comprises choosing the weight as

${\pi }_{}$

n = ( θ

n post ) H ⁢

θ

n / 

θ

n post 

2 ,

where π

_n is the weight, θ

_n is a vector having multi-symbol elements produced from Rake processing with combining, θ

_n^post is a vector having multi-symbol outputs after despreading, H denotes complex-conjugate transpose, and ∥

•

∥

² denotes a sum of an argument'"'"'s elements'"'"' magnitude squares.

24. A method whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the method comprising:

providing for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

providing for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal in which post-cursor ISI has been removed from the received signal;

providing for performing interference cancellation for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

providing for feeding back the post-cursor symbol estimates to the step of providing for synthesizing an estimated post-cursor ISI signal,

whereby providing for performing interference cancellation comprises,

providing for generating initial symbol estimates;

providing for a sequence of interference cancellation steps, the sequence configured to receive the initial symbol estimates, each of the sequence of interference cancellation steps configured to weight input symbol estimates with soft weighting, apply stabilizing step sizes to s difference signal, and perform mixed decisions to produce updated, interference-cancelled symbol estimates; and

providing for outputting final updated, interference-cancelled symbol estimates for an on-cursor symbol interval.

25. The method recited in claim 24, whereby providing for outputting further comprises outputting temporary symbol estimates corresponding to a predetermined number of post-cursor symbol intervals.

26. The method recited in claim 24, whereby providing for generating initial symbol estimates is further configured to employ temporary symbol estimates for a predetermined number of post-cursor symbol intervals for generating the initial symbol estimates.

27. A method whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the method comprising:

providing for synthesizing an estimated post-cursor ISI signal from post-cursor symbol estimates;

providing for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal in which post-cursor ISI has been removed from the received signal;

providing for performing interference cancellation for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

providing for feeding back the post-cursor symbol estimates to the step of providing for synthesizing an estimated post-cursor ISI signal,

whereby providing for performing interference cancellation is further configured for performing iterative pre-cursor ISI cancellation at the symbol level by employing a one-step matrix update equation, comprising an explicit matrix representation of a received-signal correlation matrix over multiple symbol intervals, an implementation matrix over multiple symbol intervals, a soft-weighting matrix, a scalar stabilizing step size, and a mixed decision symbol estimator.

28. The method recited in claim 27, whereby the matrix update equation is expressed by

${\underset{\_}{\hat{b}}}_n^{\left(i+1\right)}=\Psi$

⁡

( μ

n [ i ] ⁡

( q _ n - R n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) + F n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) ,

wherein {circumflex over (b)}_n^[i] is a column vector containing input symbols decisions of sources of precursor symbols for symbol intervals after an i^th iteration of interference cancellation, q_n is a vector of a resolved received signal over a plurality of post-cursor symbol intervals, μ

_n^[i] is a scalar stabilizing step, Γ

_n^[i] is a weighting matrix used for scaling the input symbol decisions, R_n is the received-signal correlation matrix over the plurality of post-cursor symbol intervals, F_n is an implementation matrix, Ψ

is a mixed-decision operator that returns mixed symbol decisions to produce the updated symbol decisions {circumflex over (b)}_n^[i+1].

29. A system whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the system comprising:

a post-cursor ISI synthesis means for synthesizing as estimated post-cursor ISI signal from post-cursor symbol estimates;

a post cursor ISI cancellation means and a resolving means for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation means for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback means for coupling the post-cursor symbols estimates to the post-cursor ISI synthesis means,

whereby the post-cursor ISI synthesis means is configured to receive as input a plurality of symbols estimates in a predetermined number of post-cursor symbol intervals, the post-cursor ISI synthesis means comprising;

a weighting means for weighting the plurality of symbol estimates according to symbol-estimates merits to produce weighted symbol estimates;

a code-waveform modulation means for modulation the weighted symbol estimates onto corresponding code waveforms for producing modulated code waveforms and summing the modulated code waveforms to generate an estimated transmit signal; and

a channel-emulation means for imparting multipath delays and gains to the estimated transmit signal.

30. The system recited in claim 29, whereby the interference cancellation means is further configured to subtract a synthesized post-cursor inter-symbol interference signal from the received signal to cancel post-cursor inter-symbol interference in the received signal.

31. The system recited in claim 29, whereby each of the symbol-estimate merits comprises a function of average ratio of signal power to interference-plus-noise power of a corresponding symbol estimate.

32. The system recited in claim 31, whereby the average ratio of signal power to interference-plus-noise power is estimated from a pilot signal.

33. The system recited in claim 31, whereby the average ratio of signal power to interference-plus-noise power is estimated using techniques of statistical signal processing.

34. The system recited in claim 29, whereby the function of the average ratio of signal power to interference-plus-noise power is characterized by

$\pi$

= max ⁢

{ C , 1 1 + 1 / SINR } ,

where π

is a symbol weight, max { }is a function for selecting a maximum value from a set of quantities within brackets { }, SINR denotes an average ratio of signal power to interference-plus-noise power (SINR) of the symbol, and C is a non-negative real constant for enforcing a minimum symbol weight.

35. The system recited in claim 29, whereby the weighting means is further configured to assign all symbols a common weight.

36. The system recited in claim 35 whereby the weighting means is further configured to choose the weight to minimize mean square-error between the received signal and at least one synthesized signal.

37. The system recited in claim 36, whereby the weighting means is further configured to choose the weight according to

${\pi }_{}$

n = ( θ

n post ) H ⁢

θ

n / 

θ

n post 

2 ,

where π

_n is the weight, θ

_n is a vector having multi-symbol elements produced from Rake processing with combining θ

_n^post is a vector having multi-symbol outputs after dispreading, H denotes complex-conjugate transpose, and ∥

•

∥

² denotes a sum of an argument'"'"'s elements'"'"' magnitude squares.

38. A system whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the system comprising:

a post-cursor ISI synthesis means for synthesizing as estimated post-cursor ISI signal from post-cursor symbol estimates;

a post cursor ISI cancellation means and a resolving means for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation means for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback means for coupling the post-cursor symbols estimates to the post-cursor ISI synthesis means,

whereby the interference cancellation means comprises,

a symbol-estimation means for generating initial symbol estimates;

a sequential interference cancellation means for receiving the initial symbol estimates, the sequential interference cancellation means configured to weight input symbol estimates with soft weighting, apply stabilizing step sizes to a difference signal, and perform mixed decisions to produce updated, interference-cancelled symbol estimates; and

a partitioning means configured for outputting final updated, interference-cancelled symbol estimates for an on-cursor symbol interval.

39. The system recited in claim 38, whereby the partitioning means is further configured to output temporary symbol estimates corresponding to a predetermined number of post-cursor symbol intervals.

40. The system recited in claim 38, whereby the symbol-estimation means is further configured to employ temporary symbol estimates for a predetermined number of post-cursor symbol intervals for generating the initial symbol estimates.

41. A system whereby inter-symbol interference (ISI), intra-cell interference, and inter-cell interference in a multiple-access communication signal are cancelled, the system comprising:

a post-cursor ISI synthesis means for synthesizing as estimated post-cursor ISI signal from post-cursor symbol estimates;

a post cursor ISI cancellation means and a resolving means for processing the estimated post-cursor ISI signal and the received signal to produce a first interference-canceled signal;

an interference cancellation means for cancelling pre-cursor inter-symbol interference, intra-cell interference, and inter-cell interference from the first interference-cancelled signal for producing interference-cancelled symbol estimates and the post-cursor symbol estimates; and

a feedback means for coupling the post-cursor symbols estimates to the post-cursor ISI synthesis means,

whereby the interference cancellation means is further configured for performing iterative pre-cursor ISI cancellation at symbol level by employing a one-step matrix update equation, comprising an explicit matrix representation of a received-signal correlation matrix over multiple symbol intervals, an implementation matrix over multiple symbol intervals, a soft-weighting matrix, a scalar stabilizing step size, and a mixed symbol estimator.

42. The system recited in claim 41, whereby the matrix update equation is expressed by

${\underset{\_}{\hat{b}}}_n^{\left(i+1\right)}=\Psi$

⁡

( μ

n [ i ] ⁡

( q _ n - R n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) + F n ⁢

Γ

n [ i ] ⁢

b ^ _ n [ i ] ) ,

where {circumflex over (b)}_n^[i] is a column vector containing input symbol decisions of sources of pre-cursor symbols for symbol intervals after an i^th iteration of interference cancellation, q_n is a vector of a resolved received signal over a plurality of post-cursor symbol intervals, μ

_n^[i] is a scalar stabilizing step, Γ

_n^[i] is a weighting matrix used for scaling the input symbol decisions, R_n is the received-signal correlation matrix over the plurality of post-cursor symbol intervals, F_n is an implementation matrix, Ψ

is a mixed-decision operator that returns mixed symbol decisions to produce the updated symbol decisions b_n^[i+1].