Controller Algorithms for Single Feed Switched Parasitic Antennas

Abstract

Controller Algorithms for Single Feed Switched Parasitic AntennasElyas Palantei * (1), and David V. Thiel (1)(1) Centre for Wireless Monitoring & Applications (CWMA)Griffith University NathanQueensland, Australia Q4111E-mail: Elyas.Palantei@student.griffith.edu.au, d.thiel@griffith.edu.auAn ideal smart antenna should provide a continuous high quality RF signal tothe transmitter/receiver circuit independent of multipath fading andinterference. A switched parasitic antenna with a central feed element encircledby a number of parasitic elements with DC activated switches can serve thispurpose. In receive mode, a power splitter provides RF output to both thereceiver and the controller circuits so they can operate independently. The mainbeam direction must be updated should the signal level fall below a predefinedthreshold. In this implementation, a simple, battery powered microcontrollerprovides the intelligence to determine the RF signal condition and to generatethe bit sequence for beam steering. Experimental results of one smart antennaconfiguration will be presented.Threshold crossing algorithm: The RF level is monitored and converted toanalog signal Va_RF(t). This is compared electronically with a preset fadingthreshold level Vfad_th(t). If Va_RF(t) is less than Vfad_th(t), and an update sequenceis run. This involves checking the RF level in all possible beam switch positions(5 parasitic elements require 5 switch positions) and setting the parasiticconfiguration for maximum signal. The output of the comparator is 1 or 0.If logic-1 is detected the controller generates the bit sequences for the parasiticelement switches of 01111, 10111, 11011, 11101, and 11110 every 64.5 ¼Sec(Delay 1). Note that a logic 0 opens the switch and de-activates the parasiticelement. This algorithm provides the smart antenna with the best beamdirection to achieve the maximum power. However, while Va_RF(t) is greaterthan Vfad_th(t) the logic-0 of comparator output will appear meaning that thereceived RF-signal is sufficient for communications. At this time, the parasiticelements are set in one of the five possible combination of bit sequence for alonger time (Delay 2) unless the RF signal level drops below the threshold.When the antenna is in transmit mode, the antenna beam is directed tomaximum reception. With appropriate component selection, the systemfunctions adequately for Vfad_th(t) less than -60 dBm with a dynamic range of 80dB.BER algorithm: A controller computation algorithm suitable for wirelessTDMA application can include a check of BER rather than simple RF signallevel. The antenna must be more closely integrated into the receiver output toobtain this information.