## 2 Indoor Positioning System in 433 MHz ISM Band
### GPS Down-Converting Repeater  
The GPS down-converting repeater subsystem comprises RF blocks and supporting blocks, which are presented in Table 1. The RF blocks refer to the components that together form the RF path, through which the GPS signal propagates, while the supporting blocks sustain the regular operation of RF blocks. The GPS down-converting repeater subsystem is implemented using the listed components and blocks in Table 1.  
The implemented GPS down-converting repeater is depicted in Figure 4. Fabricated and modelled 1575.42 MHz outdoor directional GPS antenna and 433 MHz dipole antenna are demonstrated in Figure 5. The RF blocks of the GPS down-converting repeater subsystem are as follows; a directional outdoor GPS antenna (proposed in [32]) and its bias-tee, a 1575.42 to 433 MHz down-converter, a signal power conditioner and filter block (where an LNA, a variable attenuator, a 433 MHz band pass filter, and a second LNA are cascaded), and an indoor 433 MHz dipole antenna. The supporting blocks are a controller over Wi-Fi that interfaces between the RF blocks and the user, and a voltage regulator that provides the required DC to RF blocks. The active directional GPS antennas pick-up the GPS signals from satellites in the 3 dB beam width. The conic reflector reduces the beam width by 30 degrees. Therefore, the 3 dB beam width of the fabricated antenna is measured as 60 degrees. A bias-tee is placed before the down-converter to provide the DC voltage that is required by the active directional GPS antenna. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Blocks** & **Components** & **Block Properties** \\ \hline Directional GPS Antenna & Tango 20 off-the-shelf active GPS antenna & 34.45 dBi gain \\  & with conic reflector & 60 degree 3 dB beam width \\ Directional GPS Antenna & TB-JEBT-4R2 GW + bias tee & Provides DC to GPS antenna and transmits RF to \\  & & down converter \\ \hline Down-Converter & ADRF6820 quadrature demodulator, & 8.7 dB block loss \\  & & Programmable over SPI from controller block \\ Down-Converter & ZX10Q-2-5-S 90 degree power combiner & Combines down-converter I/Q outputs \\ Signal Power Conditioner & LHA-13 LN + low noise amplifier & 22.43 dB gain \\  & & 0.9 dB noise figure \\ Signal Power Conditioner & DAT-31R5A-SP + digital step attenuator & 0 to 31.5 dB adjustable attenuation by 0.5 dB step \\  & (variable attenuator) & size and can be adjusted from controller \\ Signal Power Conditioner & DBP.433.T.A.30 band pass filter at 433 MHz & 1.7 dB insertion loss at 433 MHz, 19 MHz 3 dB \\  & & bandwidth \\ Signal Power Conditioner & LHA-13 LN + as the second low noise & 22.43 dB gain \\  & amplifier & 0.9 dB noise figure \\  & & 2.1 dB gain \\
433 MHz Dipole & 433 MHz dipole on FR4 & 80 degree 3 dB beam width on both sides \\  & Voltage Regulator & Provides DC to system components and blocks \\  & Controller with Wi-Fi for programming & Wi-Fi connection to remote PC, SPI connection to \\  & attenuator and down-converter & ADRF6820, and attenuator \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Repeater subsystem and its components. Figure 3: RF blocks in down-converting repeater (**left**) and up-converting receiver (**right**). When the GPS signal at 1575.42 MHz is received by the antenna, the signal passes to the down-converter. The down-converter converts the signal from 1575.42 to 433 MHz. The amplifiers in the signal power conditioner and filter block then amplify the signal. The amplification level can be adjusted by changing the attenuation of the digital step attenuator, which is able to attenuate the signal from 0 to 31.5 dB. By adjusting the attenuation value of each attenuator, the gain of the overall down-converter subsystem may be set to different values. The bandpass filter at 433 MHz is also deployed to eliminate signals and harmonics out of the band. Furthermore, the signal power conditioner block has another amplifier following the band pass filter.