Polar Modulation and Transmitters Power Efficiency

Introduction

High peak-to-average signals play a key role in the modern wireless communications. More over OFDM technology being applied to the initially constant envelope signal like QPSK, makes it distribution Gaussian with theoretically infinite crest factor. Such signals are very sensitive to the nonlinear distortions.
Quadrature (I/Q) modulation is a most popular technique of baseband signal spectrum translation to the radio frequency (RF) bands. From the mathematical point of view this way provides one of two forms of the complex numbers representation as real and imaginary parts. Practical realization of this method is very convenient and therefore widely used both for BPSK, QPSK GMSK and QAM modulation constellations. But among a lot of advantages it has one serious drawback. In the case of non constant amplitude signal modulation schemes (including OFDM technology) it requires high transmitter linearity. As a result power efficiency suffers. 
Polar modulation is alternative way of complex envelope forming. In this case angle and amplitude but not real and imaginary variations are applied to the carrier signal. It corresponds to the polar representation of complex envelope. Polar modulation realization allows nonlinear operation of RF transmitter. Even high efficiency switching classes of operation of power amplifiers are appropriated. Amplitude component of signal modulation is introduced at the last stage of power amplifier in this case.
Difficulties of practical realization of such approach quite clear – it is necessary to provide sufficient accuracy of amplitude and angle variations during modulation. 
The purpose of the project is practical polar modulation development, hardware prototype elaboration, its experimental research and numerical estimation of the power efficiency improving.

Environment

Experimental set consists of the next blocks:

• bit stream source;
• digital processor (modulator);
• RF transmitter;
• RF receiver;
• digital demodulator;
• BER tester.

Standard measurement equipment is also necessary for a purpose of prototypes tuning and experiments providing:

• vector generator;
• spectrum analyzer;
• digital oscilloscope.

Research Directions

1. Amplitude modulation technique at power amplifiers, operating in the different classes, including switched regimes.
2. Combined angle and amplitude modulation accuracy and its distortions due to hardware imperfection.
3. Development of the polar modulation signals digital processing for a popular communication standards like 802.11, 802.16.

Project Stages

1. Literature analysis. Different classes operation of the power amplifiers simulation. Polar modulation technique simulation. Calculation of achievable modulation parameters and limitations.
2. Choice of practical realization variants. Electronic components analysis. Development of prototypes and its fabrication. Signal processing special algorithms development.
3. Prototypes tuning and parameters measurement. Complete experiments wit different modulation formats. Experimental results summarizing and analysis.

Contacts

Team leader: Vladimir Prikhodko