Method and apparatus for reducing intermodulation distortion in digital wideband transmission systems

• US 6,108,385 A
• Filed: 07/08/1996
• Issued: 08/22/2000
• Est. Priority Date: 07/08/1996
• Status: Expired due to Term

• Current Assignee: Rateze Remote Mgmt. L.L.C. (Intellectual Ventures LLC)
• Original Assignee: Silicon Wireless Corporation
• Inventors: Worley, William Spencer III
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Liu, Shuwang
• Application Number:
  US08/679,424
• Time in Patent Office: 1,506 Days
• Field of Search: 330/124 R, 330/149, 330/52, 330/151, 330/127, 375/296, 375/297, 375/295, 455/126, 455/127
• US Class Current: 375/296
• CPC Class Codes: H03F 1/3241 using predistortion circuits H03F1/3211, H03F1/3217 take precedence
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

Signal processing to remove unwanted signal components of non-linear elements such as RF amplifiers in broadband wireless communications systems that exhibit unwanted intermodulation products. An information signal is processed to form a processed output signal, V_od. A signal preprocessor includes a base modulator for modulating the information signal to form base modulated signals, a band modulator for modulating the base modulated signals to form an input signal, V_i, a distortion processor for providing a predistortion signal V_pd having selected predistortion terms of the Taylor series form [k₁,(V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ], and a broadband processor for combining the input signal V_i and the predistortion signal V_pd to form a predistorted signal V_id with the selected predistortion terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ] having reduced amplitude. A non-linear element having a non-linear transfer function for processes the predistorted signal V_id to form the output signal with the selected predistortion terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ] having reduced amplitude. Amplifiers achieve substantial improvements in efficiency, an increase of 40-80% of usable power, due to the reduction of intermodulation products. In particular, the third order products are reduced.

Claims

1. In a communications system including,

a modulator for modulating a plurality of information signals to provide a broadband input signal, V_i, having a plurality of frequency terms,

a broadband processor for combining said input signal V_i and a predistortion signal V_pd to form a predistorted input signal V_id,

a non-linear element having a non-linear transfer function for processing the predistortion signal, V_id, to provide an output signal, V_o, where the output signal includes terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ],

a predistortion processor comprising,

term generator means for generating selected predistortion terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ],

phase shifter means for providing phase shifts to the selected predistortion terms to provide phase shifted terms that will reduce corresponding ones of the said terms in the output signal, V_o,

periodic signal generator means for generating periodic signals from said phase shifted terms,

scaling means for scaling the periodic signals in proportion to the transfer function of the non-linear element to form said predistortion signal V_pd.

2. The predistortion processor of claim 1 wherein said periodic signal generator means is a cosine generator.

3. The predistortion processor of claim 1 wherein said term generator means generates third harmonic terms.

4. In the communication system of claim 1 wherein said modulator receives said input information signals as one or more information signals V₁ (t), V₂ (t), . . . , V_j (t), . . . , V_J (t) where J is an integer, receives one or more carriers ω

₁ (t), ω

₂ (t), . . . , ω

_J (t), . . . , ω

_J (t) and receives one or more scalers scalers a₁, a₂, . . . , a_j, . . . , a_J and wherein said modulator includes,

base modulator means for baseband modulation of said information signals to provide one or more baseband signals m₁ (t), m₂ (t), . . . , m_j (t), . . . , m_J (t),

band modulation means for modulation of the carriers by the baseband signals to form said input signals V_i including one or more of the signals V_i(1), . . . , V_i(j), . . . , V.sub.(i)(J),

and wherein said predistortion signal includes V_pd(1), . . . , V_pd(s), . . . , V_pd)(S), where S is an integer.

5. The predistortion processor of claim 4 wherein said carriers ω

₁ (t), ω

₂ (t), . . . , ω

_j (t), . . . , ω

_J (t) are GSM carriers for a cellular communications system.

6. The predistortion processor of claim 1 wherein said term generator means generates said selected predistortion terms as third harmonic terms, H_s(j), using J carriers ω

₁ (t) ω

₂ (t), . . . , ω

_j (t), . . . , ω

_J (t), where J is some integer, as follows;

##EQU12##

7. The predistortion processor of claim 6 wherein J is equal to 2 and said term generator means generates said third harmonic terms of the following forms:

8. The predistortion processor of claim 1 wherein said term generator means generates third and fifth harmonic terms.

9. The signal processor of claim 1 wherein said broadband processor includes a frequency division multiplexer and a digital-to-analog converter.

10. The signal processor of claim 1 wherein said non-linear element is an RF amplifier.

11. A signal processor method for processing an information signal to form a processed output signal comprising:

modulating the information signal to form base modulated signals,

modulating the base modulated signals to form an input signal V_i,

providing a predistortion signal, V_pd, using the steps of,

generating selected distortion terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ] where k₁, k₂, k₃, . . . are amplitudes of the terms (V_i)¹, (V_i)², (V_i)³, . . . , respectively,

phase shifting said selected distortion terms to provide phase shifted terms,

generating periodic signals from the phase shifted terms,

scaling the periodic signals in proportion to a non-linear transfer function to form said predistortion signal, V_pd,

combining said input signal, V_i, and said predistortion signal, V_pd, by broadband processing to form a predistorted signal, V_id, with said selected predistortion terms of the Taylor series form [k₁ (V_i)¹, k₂ (V_i)², k₃ (V_i)³, . . . ] reduced in amplitude,

processing said predistorted signal V_id in a non-linear element having said non-linear transfer function to form said processed output signal.