## 2 System Model
### Modulation and Demodulation  
To encode reader-to-tag data for transmission, the reader amplitude modulates the 925 MHz RF carrier wave. The carrier waveform remains at a constant amplitude until the bits are transmitted, making the carrier drop nearly 10% of its normal value. Logical \"one\" and \"zero\" are indicated by the \"break\" duration. The harvester leaves the baseband data signal in the order of 70 kHz and effectively demodulates the 925 MHz carrier signal. Most of this signal is rectified into DC voltage. When the demodulator is enabled, some of the signal travels through the demodulator branch. This signal is also rectified to produce a reference voltage for the bit detection process. The ten (10) diodes in the voltage doubling ladder are the RF Schottky diodes. Instead of using a second mini harvester for demodulation, the demodulator is directly connected to the main harvester; however, it can be disabled to prevent energy leaking from the demodulation's pull-down resistor. In Figure 3, LS is the lever shifter. Figure 2: MIMO UHF RFID tag: (**a**) Architecture; (**b**) photograph. The final extra diode is responsible for an additional rectification step, in which the 70 kHz data signal is removed and leaves a slowly varying average power level that provides a dynamic reference for bit detection. Through a Schmitt trigger inverter and level shifter, the 70 kHz data signal is fed to convert the relative magnitude of the incoming data waveform into a 1.8 V level for the MSP430. The ASK demodulation is used. The NLSVT244 is a 2-bit configurable dual-supply voltage level translator by ON Semiconductor(r). The typical applications are in mobile phones, PDAs, and other portable devices. It has the same function with bit detection. The ultra-low power microcontroller MSP430 has all the general purpose computation abilities of the UHF MIMO passive RFID tags. It is responsible for many core functions such as demodulating reader signals and controlling backscatter signal modulator. These passive tags modulate the impedance of their antenna, which causes a change in the amount of energy reflected back to the reader, rather than actively transmitting radio signals. This modulation is the so-called backscatter modulation. The ADG918 switch, manufactured by ANALOG DEVICES, that connects two different loads is used to change the impedance matching of the corresponding antenna. When the MSP430 is activated via the harvester, the IC will run the algorithm to control the RF switch ADG918. Simultaneously, the IC sends the control signal to the protection components (including the R, C, and diode) to short circuit the two branches of the antenna, changing the antenna impedance state. Digital Section and Power Consumption  
Considering the design of passive tags, power consumption minimization is always required. This minimization is reached through careful component selection as the power available to feed the RFID is extremely limited. Benefiting from the fact that IC manufacturing now allows for the design of discreet components with less than 1 uA current consumption and 1.8 V operation, it is now possible to construct working passively powered RFID tags with discrete components. The 16-bit flash microcontroller used in the MSP430F1232 can run at up to 4 MHz with a 1.8 V supply voltage and consumes approximately 470 uA when active for this choice of frequency and voltage. In the design, the tag readable range has been extended by improving the microcontroller's firmware, allowing the operation at lower voltage and clock frequency. Of particular interest for low power RFID applications is the MSP430 that has various low power modes, and the minimum RAM-retention supply current of only 0.5 uA at 1.5 V. The device provides over 8 Kbytes of flash memory, 256 bytes of RAM, and a 10-bit, 200 kilo samples per second analog-to-digital converter (ADC). The processor uses selective combining techniques to select the signal-to-noise ratio (SNR).