Adaptive filter

1. A filter apparatus comprising:

a channel estimation filter circuit having a fixed Finite Impulse Response (FIR) portion for providing a predetermined selectivity and having an adaptive portion that uses a filter parameter for determining the frequency response of said filter circuit; and

a controller for controlling said parameter to change said frequency response as a function of an output of the fixed portion, a feedback from the adaptive portion and conditions imposed to maintain stability of the filter.

2. An apparatus according to claim 1 , wherein the conditions include a range of acceptable values for the filter parameter and wherein said apparatus is configured and arranged for channel estimation in a receiver of a communication system.

3. An apparatus according to claim 1 , wherein said filter circuit is adapted to apply a single pole filter operation to a common pilot channel of a wireless communication system, the filter operation including a first pole that is controlled by the modification of the filter parameter and wherein the conditions include a predefined limit on the value of the filter parameter and, in response to the filter parameter reaching the predefined limit, the filter is configured to provide a two pole filter operation in which the second pole is changed using the filter parameter and without changing the first pole.

4. An apparatus according to claim 3 , wherein said controller is adapted to change said frequency response by changing at least one pole of said filter circuit while keeping the zeros constant and wherein the controller is configured and arranged to iteratively introduce additional poles and to remove the additional poles as a function of the feedback.

5. An apparatus according to claim 1 , wherein said controller is adapted to apply a filter coefficient adjustment procedure based on a second order statistic of an error function.

6. An apparatus according to claim 1 , further comprising frequency offset estimation means for estimating a frequency offset in the signal supplied to said filter circuit.

7. An apparatus according to claim 6 , further comprising frequency offset compensation means for compensating said frequency offset in said signal based on an output signal of said frequency offset estimation means.

8. A circuit-implemented filtering method comprising the steps of:

setting predetermined zero values of a filter characteristic to provide a predetermined filter selectivity;

filtering a received signal using the predetermined zero values;

generating a feedback signal from the filtered signal;

controlling, in response to the feedback signal, filter parameters to change a first pole value of said filter characteristic so as to provide frequency response adaptation; and

changing, in response to the filter parameter reaching a limit, a second pole value of the filter characteristic while maintaining the first pole value.

9. A method according to claim 8 , further comprising an estimation step for estimating a signal to noise ratio based on a statistic extracted by a filter input and output.

10. A method according to claim 9 , wherein a signal bandwidth estimation is derived from a value reached by a filter coefficient at an equilibrium, and said estimated signal-to-noise ratio.

11. A method according to claim 9 , wherein said estimation step comprises an iterative refinement procedure based on a determination of filter shaping factors.

12. A method according to claim 8 , further comprising an adaptation step for increasing the filter order based on a determined value of a filter coefficient.