## 2 System Overview
A photograph of the prototype AC board is provided in Figure 3.  
Regarding the SI cancellation functionality realized in AC board, it is carried out by a six-stage multi-tap delay compensation circuit shown in Figure 4. In the designed feedthrough circuit, the delay of the reference TX signal can be adjusted by the combination of the six taps (D\({}_{1}\)-D\({}_{6}\)), and the phase can be inverted by the phase shifters in each tap. The amplitude of the reference TX signal can be adjusted by the variable gain amplifier (VGA) of each tap (A\({}_{1}\)-A\({}_{6}\)). The characteristics of the output frequency can be adjusted by the VFE before each delay component and in the output part. In this prototype, the VFE in the output part is for compensating the frequency characteristics of the SI signal, and VFEs before delay components compensate for the frequency characteristics of each delay component. In this prototype, a six-stage multi-tap delay compensation circuit is applied because 6 taps can sufficiently compensate for the possible SI signal delay caused  
Figure 2: The block diagram of the implemented system. by RF front-end and TX/RX antennas. In addition, for adjusting the circuit there is an interface that can receive the command from a computer, so that the users can adjust the frequency response of the AC via computer software. The details about AC design can be found in [13]. It is noted that the AC board adopted here can deal with an analog signal with up to 100 MHz bandwidth, which is a very wide band comparing with other former works with several tens of bandwidth like [12]. The last block of the implemented system is an antenna unit, which consists of a pair of identical antennas set as a Tx- and Rx-antenna, respectively. The antenna, as shown Figure 5, is an in-house developed microstrip patch antenna, designed and fabricated on a thin dielectric substrate with a thickness of 0.5 mm and a relative dielectric constant of 3.3. The microstrip patch antenna is one of the widely used planar antenna in various wireless systems. Figure 6 shows the antenna pair with their parallel (side-by-side) configuration, fabricated on the same dielectric substrate with a fixed distance \(d\) (\(d\) is defined as the distance between the center of each antenna). This form, integrated TX and RX antennas on the same board, is designed and intended to keep our measurement stable and repeatable, which makes the setup easier and saves the time for measurement as well. The antenna board used in this experiment was one with a distance of 150 mm. Measured results  
Figure 4: Block diagram of AC SI canceller. Figure 3: The designed prototype of analog circuit proposed in [13]. including the return loss of the developed antenna, and the SI between two antennas, denoted as S11 and S21, respectively, are shown in Figure 7. From these results, we can confirm that the antenna has a central operating frequency at 4.70 GHz, and the SI is about \(-48\) dB at the frequency (i.e., 48 dB isolation, including the loss of the fed-line for each antenna and coaxial adapter for connection to the measurement equipment). The small rips that occurred in the SI response are considered due to the reflection from experimental environment including the surrounded absorber. The antenna measurement was carried out on a network analyzer (PNA Network Analyzer, Model E8361C, 10 MHz 67 GHz, Agilent Technologies) in an anechoic chamber, and completed prior to the system experiment. More detailed information about the antenna design/dimensions, various antenna configurations, measured results of SI between the TX and RX antennas, and thw measurement system can be found in [33], and our other related works. Figure 5: Planar antenna structure: microstrip patch antenna on a dielectric substrate with thickness of 0.5 mm, and relative dielectric constant of 3.3. The copper foil has thickness of 18 \(\upmu\)m.  
Figure 6: Parallel (side-by-side) configuration of two antennas (set as TX and RX), integrated on the same substrate with fixed distance (\(d\)) between antennas.