# A Software-Defined Radio Platform for Teaching Beamforming Principles
## 2 System Architecture
### Hardware Architecture  
The central element of the beamforming system is the USRP N310 device [18], a multichannel SDR device featuring four transmit (TX) and four receive channels, operating in the frequency range of 10 MHz-6 GHz. The hardware architecture of the USRP N310 is based on two RF daughterboards, each with a dedicated local oscillator (LO). An external LO followed by an RF splitter was used to achieve phase synchronization on all four transmit channels, as N310 does not support LO sharing between the two daughterboards (a feature that was fixed in the USRP X series [19]). The Anapico APSIN20G RF frequency generator [20] was used to generate an LO frequency twice the frequency of the desired RF output (f\({}_{\text{LO}}\) = 2 \(\times\) f\({}_{\text{TX}}\)) with an amplitude of +5 dBm, ensuring that the amplitude after the RF splitter remains between 0 and 6 dBm [18]. The antenna under test (AUT) can be composed of individual monopole antenna elements like those used in Wi-Fi routers or a 1 \(\times\) 4 array of patch antenna. Each of the USRP TX channels can be physically connected to the AUT ports either by direct SMA RF connection or equal-length RF cables to ensure phase stability among the antenna elements.  
A small dimension Aaronia PBS1 electric field probe [21] with a sensor diameter of 3 mm was connected to one RX channel of the USRP device. The E field probe is used to measure the amplitude received over the air (OTA) at different azimuthal points in a semicircle. The distance between the AUT and the receiving E field probe could be varied between 20 cm and 6 m, depending on the length of the RF cable used for connecting the E field probe to the USRP RX channel. The measurement distance should be chosen to meet the far-field criteria (2D\({}^{2}\)/\(\lambda\)), calculated according to the operating frequency/wavelength (\(\lambda\)) and the device under test dimension (D). Thus, given the minimum operating frequency of 300 MHz, 6 m should be considered sufficient to meet the far-field criteria for a device under test with a maximum dimension of 3 m. A computer running Ubuntu with GNU Radio 3.10.7 open-source software [22] was used to control the USRP. Figure 2 presents the diagram of the proposed system architecture.  
The designed platform supports experimental flexibility, as it can be used on any beamforming device with up to four radiating elements operating in the frequency range of 300 MHz to 4000 MHz. The platform enables variation of the antenna element type, size, or spacing to include intuitive experiments on array beam direction and sidelobe variations. The frequency range limitation for the USRP N310 device is given by the acceptable LO inputs [18]. More advanced SDR hardware can be used to increase the frequency range or the number of TX channels. To this extent, USRP X410 supports direct LO sharing between the RF motherboards, resulting in an increased frequency range (1 MHz up to 7.2 GHz), and USRP X440 can be used to increase the number of channels up to eight. The Cyan SDR manufactured by Per Vices [17] can support up to 16 combinations of RX and TX channels in the near DC to 18 GHz frequency range. As SDR hardware platforms continue to evolve, they are expected to support even higher frequency ranges, including millimeter-wave applications in the 5G/6G FR2 bands, which extend up to 40 GHz.