Study And Application Of Six-port Technology In Microwave Communication Systems

With theexplosive expansion of smart mobile terminals, human beings fully enter the age of mobile information highway, which largely promate the development of wireless communication technologies, while propose higher and higher demand for wireless communication equipment. Radio frequency receiver is on the front end of the wireless communication system, whose performance will tremendously and directly impact the whole wireless communication system. Radio frequency receiver can be classified into two categories by configuration, named superheterodyne and homodyne. Due to the fantastic sensitivity and frequency selectivity, superheterodyne structure is considered to be the most reliable receiver solution, so that holds a central position for a long time in wireless communication system. Compared with the superheterodyne structure, the configuration of the homodyne structure is simpler to be integrated without image frequency disturbing. The homodyne structure seems to be more suitable with the demands of light-weight and low-power consumption of receiver development. However, the traditional homodyne receiver has two drawbacks, which are LO power leakage and DC offset. So the stability and dynamic range of the homodyne structure are worse than the superheterodyne structure. Different characteristics of these two kinds of receiver configurations make them be used in different application scenarios of wireless communication respectively and complement each other.Six-port technology appeared in the 1970 s. The six-port technology was used in microwave power measurement, later expanded to the measurements of voltage, current, impedance and phase; finally it became another implementation scheme of vector network analyzer. In the 1990 s, six-port techonology was introduced into wireless communication field, and it became a whole new implement of homodyne receiver. Six-port is able to demodulate a modulated radio frequency into baseband signal directly, and it has lots of characteristics such as flexible design, simple structure, high linearty, etc. These characteristics give the six-port receiver tremendous technique advantages in several aspects such as high frequency, wide bandwidth, multichannel, multifunction and multistandard, which makes the six-port receiver very applicable for short-range high speed wireless access, software defined radio(SDR) and software defined cognitive radio(SDCR) etc. Thus, the six-port receiver technology has become a research focus recently and it has broad development prospect.In this dissertation, the six-port technology in the wireless communication domain is discussed and studied. The six-port technology is applied in receiving and demodulating of modulated signals with different modulation modes and symbol rates. The content of the dissertation includes following aspects:1. The development of six-port technology is systematically introduced. The basic theory of six-port technology is depicted, and the operational principle of the six-port receiver is also explained.2. Different techniques of wideband six-port junction are studied. Firstly, an ultra-wideband(UWB) power divider which can be used to form a UWB six-port junction is proposed. A transversal filter consisting of two different dual-mode resonators is introduced at input port of the Wilkinson power divider as an impedance transformer to obtain good UWB filtering responses. Sencondly, a 3.1~4.8 GHz planar wideband six-port junction consists of four novel slotted circular patch quadrature hybrid couplers with three-line coupled structure and one 90o Schiffman phase shifter is proposed. Finally, a novel wideband elliptical patch power divider is used to make up of another 5.5~9.5GHz wideband six-port junction along with parallel-coupled based wideband branch-line couplers.3. Miniaturized six-port demodulation(receiver) technique is studied. A newly proposed shunt-stub-based meander line is used to form a compact six-port junction, whose circuit size is just 25% of the conventional one. Based on the proposed six-port junction, a 2.4GHz miniaturized six-port demodulator(receiver) is presented. Using this demodulator, a QPSK modulated signal of which symbol rate reaches up to 50Msymbol/s is successfully demodulated.4. Millimeter-wave six-port demodulation(receiver) technique is studied. A 35 GHz six-port junction is designed based on Al2O3 ceramic substrate. Then a millimeter-wave six-port demodulator(receiver) is fabricated and measured. Measurement results show that the proposed demodulator can demodulate QPSK, 16-QAM and 32-QAM modulated signals with highest symbol rate of 50Msymbol/s.5. To compare with the six-port receiver, a time-division multiplexing duplex(TDD) U-band transceiver is developed. The center frequency of this transceiver is 40.578 GHz, and its opration bandwidth is 120 MHz. Short switching time and high isolation between the transmitter and receiver are two technical difficulties of the proposed transceiver. In order to solve the problem of switching time, a fast automatic gain control(AGC) circuit base on digital-analog mixed technology and a carrier synchronization circuit based on open-loop structure of quadruplicated frequency conversion are used. In order to slove the isolation problem, a TDD millimeter-wave transmission/receiving(T/R) front-end is designed. Finally, dual-IF structure is used to achieve the proposed transceiver. Experimental results show that a QPSK modulated signal with 56Msymbols/s is successfully demodulated, and the measured bit error rate(BER) is maintained on 10-6 level.