Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-05-11
2003-01-28
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000
Reexamination Certificate
active
06512417
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to high power linear amplifiers and in particular relates to the same using digital pre-distortion.
BACKGROUND OF THE DRAWINGS
First and second generation cellular systems have historically used forms of modulation which are either constant envelope (e.g. GMSK in GSM) or which result in relatively low levels of amplitude modulation. The linearity of the high power amplifiers used for such systems has therefore not been an important technical issue; indeed, for the constant envelope systems it is standard practice to operate the amplifiers either close to or actually in compression in order to maximise power efficiency.
Third generation cellular systems however typically use linear spread-spectrum modulation schemes with a large amount of amplitude modulation of the signal envelope. When passed through a high power amplifier, the output is typically distorted in amplitude and phase by the non-linearity of the amplifier: the amplitude and phase distortion effects are commonly referred to as AM-AM conversion and AM-PM conversion respectively. Both distortion effects are a function only of the amplitude envelope of the input signal and are insensitive to the input phase envelope.
In systems such as Code Division Multiple Access (CDMA) modulation schemes, a plurality of signals are transmitted in a communication system and are amplified simultaneously. When a plurality of signals are applied to a linear amplifier, its non-linear characteristics will tend to produce interaction between the signals being amplified and the amplifier output will contain intermodulation products. Such intermodulation products reduce signal quality by allowing cross-talk to occur and such spillage often falls outside a particular licensed spectrum and must be controlled. Such intermodulation distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components with the amplifier output and feeding forward the distortion component to cancel the distortion of the amplifier output signal.
There are many ways of linearising a high power amplifier: direct RF feedback, envelope feedback, feed-forward and pre-distortion. For cellular power amplifiers, feed-forward amplifiers are commonly used. Feed forward amplifiers are more complicated in that they require the modification of the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis and require an error amplifier which is typically similar in power handling to the main amplifier which incurs a heavy penalty in RF device cost and power efficiency.
Envelope feedback methods (polar and Cartesian) perform much better than feed-forward amplifiers in terms of device cost and efficiency since the RF signal linearistaion processing is done before the power amplifier on a small signal. However, envelope feedback is fundamentally limited in the correction bandwidth obtainable by the delay of the feedback loop. As systems migrate to wider band modulation (e.g. CDMA2000 and WCDMA) a linearisation technology is required which is fundamentally a wideband technique.
Most implementations of pre-distortion are inherently wideband, however the performance achievable has been limited by the difficulty of matching the complex distortion characteristics of typical power amplifier devices with simple analogue pre-distortion networks.
U.S. Pat. No. 4,700,151 (Nagata) provides a baseband (analogue or digital ) modulation system and technique which employs a look-up table for adaptation. U.S. Pat. No. 5,049,832 (Cavers) provides a digital pre-distortion arrangement which reduces memory requirements to under 100 complex pairs, with a resultant reduction in convergence time and removes the need for a phase shifter or PLL in a feedback path.
The applicant's U.S. patent application Ser. No. 09/209,386 filed on Dec. 10, 1998 provides a method of linearising a power amplifier by pre-distorting the input to correct for the instantaneous non-linear transfer function of a power amplifier. The input is split into two paths. The main path goes through a delay line into a modulator and then into an amplifier. The second signal is sent to an amplitude detector whose output is converted into a digital number and used to address a look-up table. The look-up table output is converted into an analogue signal which is used to modulate the main signal such that the power amplifier is linearised. Although this does reduce unwanted emissions out of the licensed bandwidth, further reductions without incurring much extra cost are desirable.
OBJECT OF THE INVENTION
The present invention seeks to provide an improved linear amplifier arrangement which achieves correction over a wide bandwidth with low system cost and high efficiency. More particularly the present inventIon seeks to provide a linear amplifier arrangement capable of amplifying and combining a number of frequency carriers or bearers.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided an amplifier arrangement comprising:
an amplifier for amplifying an input radio frequency (RF) signal to generate an output RF signal;
a linear function processor for implementing a linear function of previous samples of the input signal to generate a linear function correction signal; and
a pre-distortion processor for distorting the current input signal in accordance with the linear function correction signal in order to compensate for distortion in the output signal.
As well has exhibiting instantaneous amplitude and/or phase distortion effects a power amplifier will generally exhibit distortion effects based on the past history of the input signal. For example amplitude and/or phase distortion in the amplifier may be different when the input sample has just undergone a rapid reduction in amplitude than when the input signal has just undergone a rapid increase in amplitude. These historical distortion effects can be greatly reduced while adding little extra cost to the amplifier arrangement by applying a pre-distortion to the input signal based on a linear function of past samples of the input signal.
It should be noted that the pre-distortion processor may take into account other inputs than the linear function when distorting the input signal.
The linear function processor may by a finite impulse response filter and the linear function may have the form:
g
(
t
)
=
∑
K
=
0
N
h
(
k
)
&LeftBracketingBar;
x
(
t
-
k
)
&RightBracketingBar;
where
g(t) is the linear function,
h(k) is a scaling factor,
|x(t−k)| are the previous samples of the input signal, and
N is the number of previous samples
The fact that g(t) is a linear function of the past samples of the input signal |x(t−k)| makes the pre-distortion easy to implement in a cost effective manner. A particularly simple and yet effective implementation is provided when the linear function has the form:
h
(
t
)=
K
(|
x
(
t−
2)|−|
x
(
t−
3)|)
where
h(t) is the linear function,
K is a scaling factor,
|x(t−2)| is the previous sample of the input signal two samples before the current input signal, and
|x(t−3)| is the previous sample of the input signal three samples before the current input signal.
A linear function processor for implementing this linear function can be formed using three delay lines, a subtractor and a multiplier. This can enable the linear function processor to be implemented in spare space in existing processing hardward already used in other parts of the amplifier arrangement.
Where the memory dependent distortion effects in the amplifier predominantly effect the amplitude of the output signal the linear function processor may be an amplitude linear function processor for generating an amplitude linear function correction s
Booth Andrew
Lewsey David
Smith Howard
Widdowson Scott
Lee Mann Smith McWilliams Sweeney & Ohlson
Nguyen Khanh Van
Nortel Networks Limited
Pascal Robert
LandOfFree
Linear amplifier arrangement does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Linear amplifier arrangement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Linear amplifier arrangement will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3063960