1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modern wireless RF transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes such as frequency shift keying (FSK) and phase shift keying (PSK), and variants thereof, often in combination with code division multiple access (CDMA) communication. Independent of the particular communications scheme employed, the RF transmitter output signal, sRF(t), can be represented mathematically assRF(t)=r(t)cos(2πfct+θ(t))  (1)where fc denotes the RF carrier frequency, and the signal components r(t) and θ(t) are referred to as the envelope and phase of sRF(t), respectively.
Some of the above mentioned communication schemes have constant envelope, i.e.,r(t)=R, and these are thus referred to as constant-envelope communications schemes. In these communications schemes, θ(t) constitutes all of the information bearing part of the transmitted signal. Other communications schemes have envelopes (amplitudes) that vary with time and these are thus referred to as variable-envelope communications schemes. In these communications schemes, both r(t) and θ(t) constitute information bearing parts of the transmitted signal.
The most widespread standard in cellular wireless communications is currently the Global System for Mobile Communications (GSM). The GSM standard employs Gaussian Minimum Shift Keying (GMSK), which is a constant-envelope binary modulation scheme allowing raw transmission at a maximum rate of 270.83 kilobits per second (kbps). Even higher data rates are achieved in the specification of the Enhanced Data rates for GSM Evolution (EDGE) cellular telephony standard by selectively applying a 3π/8 offset, 8-level PSK (8-PSK) modulation scheme. With this variable-envelope communication scheme, the maximum bit rate is tripled compared to GSM, while the chosen pulse shaping ensures that the RF carrier bandwidth is the same as that of GSM, allowing for the reuse of the GSM signal bandwidths.
As mentioned above, the 8-PSK modulation scheme of EDGE is an example of a variable envelope communications scheme. A common transmitter used in such variable-envelope modulation communications schemes is the polar transmitter. In a typical polar transmitter architecture, digital baseband data enters a digital processor that performs the necessary pulse shaping and modulation to some intermediate frequency (IF) carrier fIF to generate digital amplitude-modulated and digital phase-modulated signals. The digital amplitude-modulated signal is input to a digital-to-analog converter (DAC), followed by a low pass filter (LPF), along an amplitude path, and the digital phase-modulated signal is input to a phase-locked loop (PLL) along a phase path. The output of the LPF on the amplitude path is an analog amplitude signal, while the output of the PLL on the phase path is an analog RF output signal. The RF output signal is modulated in a non-linear power amplifier (PA) by the analog amplitude signal. Thus, in polar transmitter architectures, the phase component of the RF signal is amplified through the non-linear PA while the amplitude modulation is performed at the output of the PA.
In practice, the power spectrum emitted from an EDGE polar transmitter will not be ideal due to various imperfections in the RF transmitter circuitry. Thus, quality measures of the transmitter performance have been established as part of the EDGE standard and minimum requirements have been set. One RF transmitter quality measure of the EDGE standard is the modulation accuracy, which relates the RF transmitter modulation performance to an ideal reference signal. Modulation accuracy is related to the so-called error vector magnitude (EVM), which is the magnitude of the difference between the actual transmitter output and the ideal reference signal. The error vector is, in general, a complex quantity and hence can be viewed as a vector in the complex plane. Modulation accuracy is stated in root-mean-square (RMS), 95th percentile, and peak values of the EVM and is specified as a percentage. For a given transmitter to qualify for EDGE certification, the RMS EVM must be less than 9%, the 95th percentile of EVM values must be less than 15%, and the peak EVM value must be less than 30%.
One component of the RF circuitry that significantly affects the performance of a polar transmitter is the PLL in the phase path. A significant source of nonlinearities in the PLL that contributes to the degradation of the EVM is the charge pump (CP). The transistor operating points in the CP fluctuate based on the magnitude of the phase signal due to control voltage variation. This causes the charge pump output current to be slightly signal dependent, hence causing a phase modulation error in the RF phase modulated signal. With prior art PLLs, the phase modulation error is normally about 5%, which is acceptable in many GSM applications. However, recent GSM and EDGE applications are requiring a much lower phase modulation error, on the order of less than 2%.
Therefore, what is needed is a PLL architecture for use in a polar transmitter that is capable of compensating for nonlinearities in the charge pump.