Modulators and Demodulators

Abstract

Data transmitted between passive wireless microsystems and their base stations must be modulated prior to their transmission. Modulation is a process of converting the information to be transmitted through a medium such as a wire line or a wireless channel to a proper form such that the information can be transferred over the medium with a high degree of fidelity. In digital communications, modulation is to convert a digital bit stream to a sinusoidal carrier with its parameters, such as amplitude, frequency, or phase varied in accordance with the bit value of the digital bit stream. If the amplitude of the analog carrier varies with the digital bit stream, the modulation is termed amplitude–shift–keying (ASK). If the frequency of the carrier is modulated by the digital bit stream, it is called frequency–shift–keying (FSK). If the phase of the carrier varies in accordance with the digital bit stream, it is referred to as phase–shift–keying (PSK). Complex digital modulation schemes, such as quadrature phase–shift–keying (QPSK), offset QPSK (OQPSK), minimum shift–keying (MSK), Gaussian MSK (QMSK), etc, are all built upon generic ASK, FSK, or PSK schemes to improve data rate and biterrorrate (BER). Although a variety of modulation schemes are available for data communications over wireless channels, the limited power resource of passive wireless microsystems and constraints imposed on carrier frequency limit the choice of modulation schemes for these systems to only a few.