OFDM Radar Measurements

In the radar measurents, a 5G NR OFDM waveform was used. The channel bandwidth was increased to 40 MHz and subcarrier spacing to 30 kHz. The goal for cancellation is to enable full dynamic range from the receiver similary to calculation show in chapter 2. The receiver used is now PXIe-5840 and the calculations are different for it. Getting the maximum dynamic range is not enough and the noise in the receiver is also mini-mized. Minimum noise and full dynamic range is achieved when the received power is below −50 dBm. The reference level of the receiver can be then set to−50 dBm. This means that the antenna isolation and analog RF SIC should be above65 dBwith transmit power of15 dBm. In the measurements, combination of antenna isolation and analog RF cancellation in open space was75 dBas shown in figure 4.7.

Used measurement setup was similar to the setup in figure 4.1. Dipole antenna was

Fig. 4.3.Spectrum of the residual SI signal after different cancellation stages

switched to more directive horn antenna LB-880 with 10 dBi of antenna gain and the transceiver was National Instruments PXIe-5840 as it has higher bandiwdth. With a cor-rect port combination with the circulator JQL JCC2300T2500S6 the TX and RX isolation is roughly 25 dB. With this configuration the RF canceller provides more than 50 dB of SI cancellation over 40 MHz bandwidth. With the circulator isolation the total RF isolation is 75 dB and as mentioned earlier, it allows use of full ADC dynamic range from the receiver PXIe-5840 with minimal noise.

The first measurement scenario was done in a road called Hervannan valtaväylä and targets were moving cars. Location is perfect as the measurements can be done from a window in Tietotalo at TAU and the horn is pointing the road. Figure of the measurement scenario is shown in 4.4. The radar image can be seen in figure 4.6 and snapshot of the road can be seen in figure 4.5.

The performance was tested in an open space. In this scenario SI is only coming from the circulator and antenna reflection, atleast in theory. Finding such measurement location was difficult as it would require large open space. Suitable location was found in the roof of Tietotalo as the antenna can be pointed to the sky. At the same time the radar detection was tested with a drone carrying a reflector. Figure of this setup is shown in 4.8. The spetrum from open space measurement can be found in 4.7. As the reflections from the enviroment are minimal the cancellation is easier to perform.

Fig. 4.4. Antenna and the webcam pointing the road from window in Tietotalo.

Fig. 4.5.Snapshot of Hervanna valtaväylä in the moment of radar measurement

Fig. 4.6.Radar image of Hervanna valtaväylä at a moment shown in figure 4.5.

Fig. 4.7.Cancellation performance of the radar setup in open space.

Fig. 4.8. Measuring a drone with OFDM radar.

The OFDM radar was tested in the roof for detecting a drone. As a drone is a small device it is more challenging target than a car. First a radar reflector was connected to the drone. Figure about this scenario is shown in 4.8. It was found that the reflector was unnecsesary as the drone could be detected without it. The radar image is shown in 4.9.

The doppler shift seen in the figure is a result of circular movement in drone propellers.

The drone itself is not moving.

If we only plot the radar image in zero velocity, the importance of SI is shown more clearly. In figure 4.10 zero velocity radar image is shown and withouth RF cancellation, the detection of the drone would not be possible.

4.4 Comparison

Over the last decade multiple different full duplex designs have been reported in academia.

To fulfill the premise of full-duplex operation, doubling the frequency effieciency, over 100 dB cancellation is required with a typical transmit output power and receiver noise

Fig. 4.9.Radar image when drone is static in front of the radar

Fig. 4.10. Radar image plotted with zero velocity only. Open space on the left and the drone is present on the right as can be seen on the peak at 40m.

level. A full-duplex techonoly survey has been done in [18]. In the paper, multiple FD can-cellers, this design included, has been compered with each other and multiple designs have been reported to achieve over 100 dBcancellation. Typical FD device is operating at 2.4 GHz ISM-band, transmit power below 30 dBm power and bandwidth of 20 MHz.

Designs upto 120 MHz has been reported.

In the survey, the used channel isolation method is not differentiated. Antenna separation techniques have been reported to achieve high isolation levels and as the main point of this work is the analog RF cancellation, only designs with analog RF cancellation are considered for comparison.

For analog SIC, the general approach is the same. The SI is cancelled by combining it with 180 degree out of phase SI estimation. The SI estimation generation can be divided into two categories. First approach, similar to design from this work, the transmitted signal is splitted for the canceller and transmitter [3], [24] and[33]. Other approach is to have

Table 4.1. Comparing cancellation performance between different solutions in the litera-ture.

Canceller Taps Isolation RF cancellation Total cancellation

This design (rev. 3) 3 32 dB 49 dB 81 dB

revision 2 2 22 dB 34 dB 56 dB

MIT [24] 4 56 dB 22 dB 78 dB

Stanford [23] 16 15 dB 87 dB 72 dB

second transmitter to generate SI estimation [34], [35].

In this design and [33] the SI estimation is based on vector modulators and delayed taps.

The MIT design [24] is also based on delayed taps. Only phase shifters and attenuators are used, the difference between the approaches can be seen in 2.4a.

The other approach for SI estimation is another transmitter [34] and [35]. The SI estima-tion is generated in the digital baseband and is in this way simpler as everything is done in digital domain. The channelge is that the transmitter and SI transmitter are not identical or ideal, so these differences have to be modelled. Another problem is that the transmitter noise is not same for both signals, as in splittet signal approach.

5. CONCLUSIONS

In this thesis, a 3-tap analog RF cancellation device was designed, built, tested and used in multiple measurements. The design is based on previous revisions and the new design has few improvements, mainly third tap and adaptive digital control circuit. Analog RF cancellation is a critical part of the self-interference cancellation as it protects the sensitive receiver parts from SI and at the same time prevents ADC saturation. With high antenna isolation and digital cancellation, IBFD is possible without analog RF canceller, but it requires high antenna isolation and limits the antenna design [20]. On the other hand, for sensing applications, digital cancellation is not always nescesary if antenna isolation and analog RF cancellation is sufficient [8].

The cancellation performance was tested with different transmission powers and band-widths to find out the limits of the cancellation performance. Then the FD canceller was used in two different applications for proof of concept. First, for sensing as an OFDM radar and then for electronic protection as a jamming and detecting device. The device can provide analog cancellation between 40 to 60 dB, support channel bandwidths up to80 MHzand with some extra effor from digital cancellation, the FD canceller supports transmission powers up to 30 dBm. The device is adaptive and it is controlled by an ex-ternal FPGA circuit. This improves the performance not only in in dynamic enviroments, but also in static scenarios with wide bandwidths, as manual tuning is difficult and time consuming with wide bandwidths and three taps.

At the time of bulding the canceller, comparing the results with other cancellers in the literature, shown results are good, achieving over 100 dB of total cancellation, which at the time was the highest recorded in the literature [5]. Multiple applications for canceller shows that the IBFD is not just a new way of increasing the spectral efficiency, but it has also many applications in sensing and defence. Over the years, the designed FD canceller has been usefull in multiple occasions and it has been used in multiple scientific publications: [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15].