Ideal, or theoretical, radio frequency (RF) power amplifiers act linearly, faithfully reproducing an amplified RF signal at their output without distortion. Unfortunately, in practice, physical RF power amplifiers are actually non-linear and add a certain amount of unwanted distortion to a signal, e.g., intermodulation distortion (IMD). A portion of such IMD is then realized as adjacent channel power (ACP), and potentially causes adjacent channel interference. Moreover, physical amplifiers have a finite ability to deliver gain and/or power.
RF power amplifiers are often operated and configured in such a manner so as to provide sufficient output power at the lowest possible cost. In general, the cost of an amplifier increases with its maximum output power. Therefore, to maximize the cost effectiveness of a given RF power amplifier, the amplifier is operated close to a saturation point to provide as much output power as possible. Operation of an amplifier close to saturation affords additional output power but is often at the expense of an increase in ACP thereby increasing the potential for adjacent channel interference.
Specifically, operation of an amplifier close to saturation increases the power of unwanted IMD products within the normal operating frequency range of the amplifier. These IMD products may impede the proper transmission and reception of RF signals within the normal operating frequency range of the amplifier. To address this issue, numerous techniques have been developed to reduce IMD products from an amplified signal both within and also outside the operating frequency range of an amplifier. Such techniques include predistortion, feed-forward, and linear amplification with non-linear components (LINC). Predistortion techniques are described further below.
Recent increases in the demand for wireless communications devices have led to new frequency bands to increase capacity, such as, for example, the Universal Mobile Telecommunications System (UMTS) developed by the European Telecommunications Standard Institute for delivering 3G (third generation) services. Modern transmission protocols, such as UMTS, demand high linearity to prevent RF energy in one band from spilling over and interfering with other proximate channels and/or bands. Certain modern transmission protocols also have high Peak-to-Average Power Ratio (PAR) carrier signals that make efficient linear amplifiers difficult to design for such applications.
Another issue to be addressed is that, adjacent channel interference may be compounded as a result of the close proximity of frequency bands. RF power amplifiers therefore must operate at high drive levels in order to achieve the high linearity demanded by broadband applications. Energy leakage resulting from one band spilling over into another can undesirably degrade the signal-to-noise (SNR) ratio or bit-error rate (BER) of the proximate frequency band.
In practice, it is unnecessary to completely eliminate all ACP and/or adjacent channel interference for a selected center frequency. Certain tolerable levels of ACP are acceptable. When the terms “eliminate” or “reduce” are used herein with reference to ACP, it is understood that the ACP should be suppressed below a certain tolerable level, even though it will often not be entirely eliminated.
To address the issues noted above, predistortion circuits, or, more simply, predistorters, have been developed that allow the operation of RF power amplifiers close to saturation but also with improved linearity thereby reducing ACP and the potential for adjacent channel interference. Generally, predistorters multiply an input signal by the multiplicative inverse of the distortion in the response of an amplifier. This develops a predistorted signal, which is then applied to the amplifier. When the amplifier amplifies the predistorted signal, the distortions due to non-linearities in the response of the amplifier are cancelled by the predistortions in the predistorted signal. This results in an improvement in the linearity of the amplifier, particularly when the amplifier is operated close to saturation.
While such predistorters address linearity issues, they also increase the cost of amplification, though typically less so than resorting to a higher power amplifier operated at a reduced output level. Generally, such predistorters may be either analog or digitally based, depending on the type of processing used. Moreover, predistorters will modify an RF input signal as a function of instantaneous input signal power (e.g., amplitude and phase) as well as the rate of change in the input signal power (e.g., memory effects such as self-heating, odd order distortion products, power supply compensation, etc.).
Digitally based predistorters often use look-up tables (LUTs) stored in memory to provide predistortion or correction factors to an input signal. Since some digitally based predistorters compensate based on both the instantaneous input signal envelope (input signal power) and the rate of change in the input signal envelope, a first pair of LUTs are often used for amplitude and phase correction (e.g., one for in-phase (I) and one for quadrature-phase (Q) signals) while a second pair of LUTs are used to compensate for memory effects in I and Q. Moreover, by virtue of the need to modulate an input signal onto a carrier, such predistorters often include a modulator and a third pair of LUTs containing magnitude and phase correction factors for I and Q signals, respectively, for the modulator. The use of these numerous LUTs significantly increases the memory requirements and, consequently, the cost of the predistorter and the amplifier associated therewith. Moreover, the use of multiple LUTs creates numerous other problems as well.
When multiple LUTs are used, some provision must be made for combining the correction factors from the tables. The problem of combining the correction factors is exacerbated by the application of the difference equation to the LUTs that correct for memory effects. The straightforward interpretation of the difference equation is to add to the current sample, or correction factor, the difference between the subsequent sample and the previous sample. Delay circuitry, or clocking, is typically used to apply the desired sequencing to the samples. A practical problem exists in that it is difficult to execute a fast three-operand function to meet requirements imposed by industry on many predistorters. Additionally, the use of delay circuits also increases the cost of a predistorter.
Another problem encountered in many digitally based predistorter implementations deals with the storing of correction factors in the LUTs. In calibrating digitally based predistorters with LUTs for a particular amplifier, coefficients are transmitted or loaded into the LUTs in memory. Often, it is desirable to read back the coefficients from the LUTs to verify whether or not the coefficients were loaded or stored correctly. Such loading or reading back of correction factors requires a substantial amount of processing time and may cause a brief interruption, or glitch, in the output of the amplifier if performed while the amplifier is in use.
Yet another problem associated with LUTs deals with the number and/or size of the LUTs. Since the LUTs stored in memory may become rather large, and memory increases the cost of the predistorter, the use of multiple large LUTs to correct for instantaneous amplitude and phase and memory effects, as well as a modulator, leads to a more costly predistorter.
Existing predistorters operate in an environment where signals are modulated according to a single known modulation format. For example, RF signals may be modulated in accordance with one of any number of modulation formats which are well known in the art, including, TDMA, GSM, CDMA, WCDMA, QAM, and OFDM, to name but a few. For example, the bandwidth for a WCDMA signal is 3.84 MHz (wideband), and the bandwidth for a CDMA signal is 1.25 MHz. By contrast, a GSM signal has a bandwidth of 200 kHz, and a TDMA signal has a bandwidth of only 30 kHz (narrowband). If the signals are located in a PCS frequency band (1920 to 1980 MHz), a 60 MHz bandwidth is used. Thus, the bandwidth of a signal, depending on the modulation format and band used, may vary from 30 kHz to 3.84 MHz. As a result, the number of the LUTs may vary accordingly, since correction may be required over a greater or lesser bandwidth. Moreover, the requirements for ACP may also vary based on the modulation format utilized.
Therefore, a need exists for an IMD reduction technique that corrects for instantaneous amplitude and phase, memory effects, and a modulator having reduced memory requirements and faster processing without exhibiting glitches while at reduced cost.