RF power amplifiers used for wireless communication transmitters, with spectrally efficient modulation formats, require high linearity to preserve modulation accuracy and to limit spectral regrowth. Typically, a linear amplifier, Class-A type, Class-AB type or Class-B is employed to faithfully reproduce input signals and to limit the amplifier output within a strict emissions mask. Linear amplifiers are capable of electrical (DC power in to RF power out or DC-RF) efficiencies 50% or greater when operated at saturation. However, they are generally not operated at high efficiency due to the need to provide high linearity. For constant envelope waveforms, linear amplifiers are often operated below saturation to provide for operation in their linear regime. Time varying envelopes present an additional challenge. The general solution is to amplify the peaks of the waveform near saturation, resulting in the average power of the waveform being amplified at a level well backed-off from saturation. The back-off level, also referred to as output power back-off (OPBO), determines the electrical efficiency of a linear amplifier.
For example, the efficiency of a Class-A type amplifier decreases with output power relative to its peak value (EFF=POUT/PPEAK). The efficiency of Class-B type amplifiers also decreases with output power relative to its peak value (EFF=(POUT/PPEAK)1/2). Class-AB type amplifiers have output power variations intermediate between these values. Thus, there is customarily an inherent tradeoff between linearity and efficiency in amplifier designs.
Modern transmitters for applications such as cellular, personal, and satellite communications employ digital modulation techniques such as quadrature phase-shift keying (QPSK) in combination with code division multiple access (CDMA) communication. Shaping of the data pulses mitigates out-of-band emissions from occurring into adjacent channels but produces time-varying envelopes. In addition to amplifying individual waveforms with time varying envelopes, many transmitters (especially in base stations) are being configured to amplify multiple carriers. Multi-carrier signals have a wide distribution of power levels resulting in a large peak-to-average ratio (PAR). Therefore, the operation of the linear amplifiers in these types of signals is very inefficient, since the amplifiers must have their supply voltage sized to handle the large peak voltages even though the signals are much smaller a substantial portion of the time. Additionally, the size and cost of the power amplifier is generally proportional to the required peak output power of the amplifier. Techniques that limit out-of-band (OOB) emissions while the amplifier operates at or near saturation are highly desirable.
Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Division Multiplexing (OFDM), and multi-carrier versions of Global Standard for Mobile Communication (GSM) and Code Division Multiple Access 2000 (CDMA 2000) are wireless standards and application growing in use. Each requires amplification of a waveform with high PAR levels, above 10 dB in some cases. The sparse amount of spectrum allocated to terrestrial wireless communication requires that transmissions minimize out-of-band (OOB) emissions to minimize the interference environment. A linear amplifier used to amplify a waveform with a PAR of 10 dB or more provides only 5-10% DC-RF efficiency. The peak output power for the amplifier is sized by the peak waveform. The cost of the amplifier scales with its, peak power. Several other circuit costs including heat sinks and DC-DC power supplies scale inversely to peak power and dissipated heat (which results from the electrical inefficiency). Related base station costs of AC-DC power supplies, back-up batteries, cooling, and circuit breakers also scale inversely with efficiency as does the electrical operating costs. Clearly, improving DC-RF efficiency is a major cost saver both for manufacture and operation.
Many modern digital communications systems transmit complex waveforms consisting of multiple carriers, multiple code channels, or other signals that give rise to large, infrequent peaks in signal power. These signals, while rich in information content, are costly to transmit in terms of hardware and electrical consumption. Any scheme that reduces the size of the peaks without introducing substantial levels of error is desirable. Most modern day communication standards strictly limit the amount of signal distortion and OOB emissions that can occur in a signal transmission. There are a variety of schemes to clip a signal which result in substantial amounts of signal distortion and/or OOB emissions. The strict regulation of OOB emissions is often the limiting factor in the degree to which peak signals can be limited by clipping.
Linearization techniques generally improve wanted signal distortion and reduce OOB emissions. Some linearization techniques, such as digital pre-distortion and versions of digital cross cancellation require a priori information on the mechanisms that cause distortion and OOB emissions. Intentional clipping of a signal results in OOB emissions that can be readily predicted. Most linearization techniques operate over a limited bandwidth but these can be adapted to work in several parallel channels with greater efficacy.