Broad-band direct QPSK modulator/demodulator for wireless gigabit communication

Abstract

The design and measured results of a broad-band direct quadrature phase shift keying (QPSK) modulator and demodulator are described in this paper. The circuits are fabricated using 1-m GaAs HBT technology. To suppress the local oscillator (LO) leakage, the double-balanced mixer is selected as the core unit in the modulator/demodulator. An embedded four-way quadrature divider which includes a Lange coupler and two Baluns is utilized in the system to generate quadrature-phase LO signals. As results of a back-to-back test, the system can operate at data rates in excess of 2 Gb/s (1 Gb/s per I and Q channels) at 30 GHz. The growing data communication speed, bandwidth limitation and congestions at lower frequencies have resulted in the move toward high frequency system. Wireless systems at Ka-band frequencies have various applications such as in point-to-point, point-to-multipoint, and very small aperture terminal (VSAT) systems. One of the most promising wireless communications service is the Local Multipoint Communication Service (LMCS). Additionally, there is a great demand on the Ka-band phased-array radar for military and commercial applications, which includes communications, navigation, weather forecast and so on. To meet the market demands, many researchers have developed highly monolithic microwave integrated circuits (MMICs) design and compact chip size for the high speed system applications [1,2]. Direct conversion architecture (DCA) greatly reduces front-end hardware complexity, lower cost and no image frequency comparing to heterodyne technique [3][4][5]. Therefore, passive reflection-type demodulator and modulator have been reported in various process [6][7][8] for millimeterwave (MMW) direct conversion systems, even gigabit data rate. However, the local oscillator (LO) leakage and image suppression in these transceivers need be carefully considered. Because the LO signal and RF signal are connected in the same device. In addition, the baseband signals of the refection-type modulators need be calibrated, which will limit its applications.The Ka-band has several features that render it ideal for high-data-rate radio links. The wide bandwidth available permits high speed transmission by utilizing a simple modulation scheme such as quadrature phase-shift keying (QPSK). Most importantly, antennas with high gain and directivity are easily realizable in this frequency range with apertures less than 2.3 f t [9]. However, in the compact chip die the passive components need be carefully considered to ensure broad-band phase and amplitude balance of the IQ signals.This paper presents a broad-band modulator and demodulator using 1-m GaAs HBT technology for direct conversion