## 2 System Architecture
### Analog Component
#### 2.2.1 RF Frontend Modules  
USRP channels can operate with frequencies up to 6 GHz (max), so to allow the system to operate at 28 GHz, frequency conversion units are required to be attached to the USRP channels, connected to  
Figure 4: Flow control between the receiver and transmitter during Python and Matlab interfaces. Figure 3: Block diagram of software interface between Host-PC and Universal Software Radio Peripheral (USRP) N310. the antennas. Therefore, two RF frontend units were developed, allowing this conversion from 3.84 to 28 GHz and vice versa. The chosen topology uses the heterodyne configuration. The upconverter is composed, in each channel, by an active mixer, a bandpass filter and an amplifier. To distribute the signal from the local oscillator (LO) to each channel mixer, discrete power splitters were also used. The key element for the up/down conversion is the mixer. The chosen mixer was the HMC264 LC3B from Analog Devices. It is a subharmonic mixer that operates from 21 to 31 GHz at the RF port, using a signal from Direct Current (DC) to 6 GHz at the IF port, and from 10.5 to 15.5 GHz at the LO port. The used amplifier is the MAAL-011129 from MACOM, which is a low noise broadband amplifier (18 to 31.5 GHz) with a gain of 18 dB @ 28 GHz [19]. The local oscillator signal is generated by the phased locked loop (PLL) LMX2595 evaluation board which operates over a wideband (from 10 MHz up to 19 GHz) with low noise characteristics. The power splitter that divides the LO into two channels is the Mini-Circuits EP2k1+, which works from 1.8 to 28 GHz. The selected LO was 12.08 GHz. Finally, a fifth order parallel coupled microstrip band pass filter was designed with 5 GHz of bandwidth, an attenuation of approximately 40 dB at the image frequency (20 GHz) and 0.9 dB of insertion loss at 28 GHz. The upconverter module comprises four IF inputs, four RF outputs and two LO inputs. It was designed for a maximum power of about 10 dBm at the RF outputs, and for a maximum IF input power of 7 dBm, resulting in an expected conversion gain of 3 dB. The purpose of the designed filter is to mitigate the image frequency signal and other spurious signals generated by the mixer. It is assumed that the IF signals at the inputs are bandpass at 3.84 GHz with the proper filtering done in the USRP. A photograph of the fabricated prototype can be seen in Figure 5b). The upconverter was tested using a 12 GHz signal in the LO port and with an IF of 4 GHz. Figure 6a shows the output spectrum at one of the four RF outputs of the upconverter. It is possible to find out the image signal (\(24-4\) GHz = 20 GHz), the LO signal (\(2\times 12\) GHz) and the desired RF signal (\(24+4\) GHz = 28 GHz). The behaviour of the filter can also be observed, since both the image and LO signals are highly attenuated compared to the RF signal, 28.8 and 26.4 dBm, respectively. Figure 6b presents the measured conversion gain of the upconverter at different RF frequencies. It should be noted that the RF variation was made through the LO. The measured gain was \(-1\) dB at 28 GHz, slightly below the expected 3 dB. Regarding the 1 dB compression point (P1 dB) at the output, 8.6 dBm was obtained, a value relatively close to the expected (10 dBm, as can be seen on the link power budget in Figure 5a). Figure 5: Upconverter designed module: (**a**) block diagram, (**b**) fabricated prototype board. The downconverter module was designed similarly as the upconverter. The purpose of the filter is to reject any image signal that can be captured at the RF inputs. In this structure, the mixer is the element that imposes the maximum power on the RF inputs since it has a P1 dB compression point of 3 dBm. Thus, the maximum RF input power will be \(-13\) dBm and the maximum power at the IF output will be \(-10\) dBm, resulting in an expected conversion gain of 3 dB. The photograph of the fabricated prototype is shown in Figure 7b. In Figure 8a is shown the output spectrum at one IF port of the downconverter module.