Adaptive pre-equalization method and apparatus

• US 20060056327A1
• Filed: 07/15/2002
• Published: 03/16/2006
• Est. Priority Date: 07/15/2002
• Status: Abandoned Application

• Current Assignee: Nokia Corporation
• Original Assignee: Nokia Corporation
• Inventors: Coersmeier, Edmund
• Application Number:
  US10/520,383
• Publication Number:
  US 20060056327A1
• Time in Patent Office: Days
• US Class Current: 370/315
• CPC Class Codes: H04L 2025/03414 Multicarrier
• CPC Class Codes: H04L 2025/03617 Time recursive algorithms H04L2025/03643 takes precedence
• CPC Class Codes: H04L 25/03343 Arrangements at the transmitter end
• CPC Class Codes: H04L 27/2626 Arrangements specific to the transmitter only
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

An adaptive pre-equalizer is disclosed to compensate amplitude ripples in a low cost transmitter pass-band filter. A filtered-x LMS algorithm is proposed to calculate the equalizer coefficients. To this purpose, the modulated RF signal is demodulated at the transmitter and subtracted from a filtered version of the original base band signal. The impulse response of the low-cost transmit filter is approximated by a delay. The disclosure may be applied to direct conversion or heterodyne transmitters using OFDM.

Claims

1. A method of pre-equalizing a transmission characteristic of a signal processing circuitry , said method comprising the steps of:

a) obtaining a difference between an output signal of said signal processing circuitry and an input signal of an pre-equalizing function;

b) approximating a gradient of said difference based on said obtained difference and an approximation of said transmission characteristic; and

c) updating control values of said equalizing function (15) based on said approximated gradient.

2. A method according to claim 1, wherein said approximating step comprises the step of calculating an approximation of a least mean square gradient vector of said difference.

3. A method according to claim 2, wherein said gradient vector is calculated from a partial differential equation of a system cost function.

4. A method according to claim 1, wherein said difference is obtained by comparing signal envelopes of said output and input signals.

5. A method according to claim 4, wherein said input signal is a digital signal and said output signal is an analog signal.

6. A method according to claim 1, wherein said control values are coefficients of an adaptive digital filter.

7. A method according to claim 1, wherein said transmission characteristic is approximated as a delay function.

8. A method according to claim 7, wherein the delay of said delay function corresponds to the position of the maximum analog filter peak of said transmission characteristic.

9. A method according to claim 8, wherein said gradient vector is calculated using the following equation:

∇

{E}=−

2e[k]·

d[k−

τ

],

wherein

∇

{E} denotes said gradient vector,

e[k] denotes said obtained difference, and

d[k−

τ

] denotes a vector representation of said input signal assessed by said delay approximation of said transmission characteristic.

10. A method according to claim 9, wherein filter coefficients are updated in said updating step based on the following equation:

w[k+1]=w[k]+μ

e[k]·

d[k−

τ

],

wherein

w[k+1] denotes a vector representation of updated filter coefficients,

w[k] denotes a vector representation of current filter coefficients, and

μ

denotes a predetermined proportionality factor.

11. An apparatus for pre-equalizing a transmission characteristic of a signal processing circuitry, said apparatus comprising:

a) comparing means for obtaining a difference between an output signal of said signal processing circuitry and an input signal of an pre-equalizing means

b) approximation means for approximating a gradient of said difference based on said obtained difference and an approximation of said transmission characteristic; and

c) updating means for obtaining control values supplied to said preequalizing means, based on said approximated gradient.

12. An apparatus according to claim 11, wherein said comparing means are arranged to compare said input and output signals based on their envelopes.

13. An apparatus according to claim 11 or 12, wherein said approximation means is arranged to approximate said transmission characteristic as a delay function and to approximate said gradient by using a least mean square approximation function.

14. An apparatus according to claim 11, wherein said signal processing circuitry is a direct conversion or heterodyne transmitter architecture.

15. An apparatus according to claim 11, wherein said apparatus comprises a digital pre-equalizer means.