An input signal having amplitude information is pre-distorted and converted into two pre-distorted signals without amplitude information. The two pre-distorted signals are separately amplified and then recombined to generate a linearized amplified output signal having amplitude information. The pre-distortion and conversion may be implemented using a pre-distorter and a LINC modulator. Alternatively, the pre-distortion and conversion may be implemented in circuitry that combines the functions of a pre-distorter and a LINC modulator. The amplified, pre-distorted signals are preferably combined using circuitry that provides at least some impedance matching, such as a transformer or a transmission line tee with transmission line stubs.

FIELD OF THE INVENTION

The present invention relates to signal processing, and, in particular, to techniques for linearizing amplifiers used in communications systems.

BACKGROUND OF THE INVENTION

Both high efficiencies and high linearities can be achieved in RF amplifiers using a set of techniques known as amplitude reconstruction. In amplitude reconstruction, the amplitude information of a signal is removed, and the remaining phase-modulated signal is amplified using a highly efficient nonlinear amplifier. After amplification, the amplitude information is somehow returned to the signal.

One such technique for amplitude reconstruction is LINC (LInear amplification with Nonlinear Components), also referred to in older literature as outphasing. In this technique, the amplitude information in the signal is converted into phase modulation for two different signals. The phase modulation is performed in such a manner that when the two signals are amplified and then recombined, the resulting signal has the desired output amplitude. If the input signal has zero amplitude, then the two amplified signals will be 180 degrees out-of-phase and will cancel when recombined. If the input signal is at maximum amplitude, then the two amplified signals will be in-phase and will combine perfectly.

FIG. 1is a block diagram of a LINC system100of the prior art. LINC system100comprises LINC modulator102, power amplifiers PA1and PA2, and combiner104. The input signal to LINC system100is an amplitude-modulated carrier represented as A sin(ωt+φ). LINC modulator102generates two signals with phases φ+cos−1(A) and φ−cos−1(A). These two signals are then amplified by amplifiers PA1and PA2, respectively, and combined by combiner104to produce an amplified replica γ of the input signal. Peak output is obtained when the two amplifiers add in-phase; zero output is obtained when they add out-of-phase. Intermediate phase values produce intermediate amplitudes.

In phasor notation, the input signal may be written as in Equation (1) as follows:
u=Aejφ.  (1)
The outputs of amplifiers PA1and PA2may be written as in Equations (2) and (3) as follows:
V1=Ge+j(φ−cos−1A)(2)
and
V2=Ge+j(φ+cos−1A),  (3)
where G is the gain of both power amplifier PA1and power amplifier PA2. The output γ of combiner104may be written as in Equation (4) as follows:
γ=2GAejφ.  (4)
There are two common methods for combining the two amplified signals generated by amplifiers PA1and PA2. These two methods are described below in the context ofFIGS. 2 and 3.

FIG. 2shows a block diagram of a LINC system200of the prior art that employs a first method for combining the amplified signals generated by two power amplifiers. LINC system200has a LINC modulator and two power amplifiers that are analogous to those in LINC system100of FIG.1. In LINC system200, combiner104ofFIG. 1is implemented using a four-port hybrid combiner204, also known as a power combiner. Combiner204receives the amplified signals from amplifiers PA1and PA2as two inputs and generates the sum and difference of the two signals as its two outputs. The sum is an amplified version of the input signal to the LINC system, while the difference signal is wasted in a dummy load. The advantage to such a technique is that each amplifier sees a perfectly matched load. However, some power is wasted in the dummy load, resulting in a loss of efficiency. (Note that, at zero input amplitude, all power is wasted in the difference port.)

FIG. 3shows a block diagram of a LINC system300of the prior art that employs a second method for combining the amplified signals generated by two power amplifiers. Like LINC system200, LINC system300has a LINC modulator and two power amplifiers that are analogous to those in LINC system100of FIG.1. In LINC system300, combiner104ofFIG. 1is implemented using a three-port, lossless combiner304. Combiner304is implemented using a transmission line tee306with transmission line stubs (e.g., shunt reactances)308and310for impedance matching. Alternatively, combiner304may be implemented using a transformer. In either case, this LINC system has the advantage of efficiency over the four-port hybrid technique of LINC system200, since no power is lost in the combiner. Unfortunately, the amplifiers no longer see perfectly matched loads at all output amplitudes. As a result, while the combiner itself is extremely efficient, most amplifiers that are used in such systems lose efficiency when connected to mismatched loads. In addition, their power outputs and phases may vary with the output amplitude of the system.

LINC system300uses shunt reactances (jBSand −jBSinFIG. 3) to improve the efficiency of a basic three-port system to improve amplifier matching at output amplitudes other than the maximum amplitude. In particular, shunts308and310are preferably placed at the electrical equivalent of one-quarter wavelength away (e.g., via quarter-wave delays312and314) from combiner tee306, where the shunts improve the load matching at a variety of output amplitudes. This greatly increases efficiency and linearity at some output amplitudes at the expense of some efficiency and linearity in other amplitudes. The optimum compensation depends heavily on the peak-to-average ratio of the signal to be amplified.

DETAILED DESCRIPTION

Despite the efficiency benefits of LINC systems using four-port hybrid combiners or three-port lossless combiners, LINC systems are typically not linear enough for use with many modern signals. In the case of the four-port hybrid combiner ofFIG. 2, a small difference in the gain or phase offsets of each amplifier will result in different powers going into the combiner. As a result, the phase component of one signal will dominate over the other, which will result in distortion at the output signal.

In the LINC system ofFIG. 3, load matching is significantly improved, but is still not perfect. The changing gains of the amplifiers at various load conditions will cause both gain distortion (amplitude modulation to amplitude modulation or AM-AM distortion) and phase distortion (amplitude modulation to phase modulation or AM-PM distortion). According to embodiments of the present invention, the LINC system ofFIG. 3is linearized by pre-distorting the input signal in a manner that compensates for the distortion of the LINC system, which is usually measurable and consistent.

FIG. 4shows a block diagram of a LINC system400, according to one embodiment of the present invention. LINC system400has pre-distorter401, LINC modulator402, power amplifiers PA1and PA2, and combiner404. In preferred implementations, power amplifiers PA1and PA2and combiner404are analogous to the power amplifiers and combiner ofFIG. 3, where combiner404may be implemented as a transformer (as represented inFIG. 4) or as a transmission line tee with transmission line stubs for impedance matching (as represented in FIG.3).

In one possible implementation, pre-distorter401and LINC modulator402are implemented using distinct circuitry, where LINC modulator402is analogous to the LINC modulator of FIG.3and pre-distorter401may be any suitable type of pre-distorter as is known in the art. In this case, LINC modulator402receives and processes the pre-distorted signal from pre-distorter401as its input.

In phasor notation, for the input signal of Equation (1), the output of pre-distorter401may be represented as in Equation (5) as follows:
u=g(A)ej(φ+p(A)),  (5)
where g(A) is the amplitude-dependent gain adjustment and p(A) is the amplitude-dependent phase adjustment of the of the pre-distortion processing. When this pre-distorted signal is applied to LINC modulator402, the two LINC-modulated, pre-distorted outputs x1and x2may be represented as in Equations (6) and (7) as follows:
x1=e+j(φ+p(A)−cos−1g(A))(6)
and
x2=e+j(φ+p(A)+cos−1g(A)).  (7)

FIG. 5shows a block diagram of a combined implementation of pre-distorter401and LINC modulator402of LINC system400ofFIG. 4, according to an alternative embodiment of the present invention. According to this embodiment, the pre-distortion and LINC modulation functions are performed by a single set of combined circuitry500.

In particular, combined pre-distortion/LINC modulation circuitry500comprises phase detector502, envelope detector504, look-up tables (LUTs)506and508, difference node510, summation nodes512-516, and modulators518-520. Phase detector502detects the phase of the input signal and applies the detected phase φ to summation nodes514and516. Envelope detector504detects the amplitude of the input signal and applies the detected amplitude A to LUTs506and508, which use the detected amplitude as an index into their respective stored data.

Each LUT is loaded with information based on the known distortion properties of the system. Such tables can be used to automatically correct for the gain and phase distortions of the amplifier. In particular, LUT506maps amplitude A to the amplitude-dependent, pre-distortion phase adjustment term p(A), while LUT508maps amplitude A to the amplitude-dependent, LINC modulation phase offset term cos−1(g(A)). Both of these LUT values are applied to difference node510and summation node512.

Difference node510generates the difference between the two LUT values and applies the resulting difference to summation node514. Summation node512generates the sum of the two LUT values and applies the resulting summation to summation node516. The outputs of summation nodes514and516are applied to modulators518and520, respectively.

Each modulator modulates its received summation signal at the carrier frequency w. As such, modulator518generates the LINC-modulated, pre-distorted output signal x1whose phasor representation is given by Equation (6), while modulator520generates the LINC-modulated, pre-distorted output signal x2whose phasor representation is given by Equation (7). The output signals x1and x2from modulators518and520are applied to power amplifiers PA1and PA2, respectively, of LINC system400of FIG.4.

If appropriate, the output signals can be monitored and the lookup tables adjusted for changes in the distortion properties of the system during operation.

Although combined circuitry500ofFIG. 5adds and subtracts different signals in a particular sequence, it will be understood that the present invention can alternatively be implemented using other sequences of addition and subtraction nodes that generate equivalent results.

The present invention may be implemented in the context of wireless signals transmitted from a base station to one or more mobile units of a wireless communication network. In theory, embodiments of the present invention could be implemented for wireless signals transmitted from a mobile unit to one or more base stations. The present invention can also be implemented in the context of other wireless and even wired communication networks to reduce spurious emissions.

Embodiments of the present invention may be implemented as circuit-based processes, including possible implementation on a single integrated circuit. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program. Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer.