*5.3. Speed Measurement*

Figure 10 shows the measurement results for two different speeds. The speed of a single target (0.03 × 0.03 m2) is set to 2.34 m/s in Figure 10a, while Figure 10b shows the result for a single target (0.015 × 0.02 m2) moving at 5 m/s. The assembler, which contains the target mounted on a moving belt, is 0.26 m long and is placed 0.67 m from the receiving antenna. We can notice that both speed and distance can be identified, but due to the smaller size of the moving target, the received signal strength in Figure 10b is weaker than in Figure 10a.

The result in Figure 10c are obtained by simultaneously placing a static target at 0.6 m and a moving target (0.015 × 0.02 m2) at 2.34 m/s located between 0.67 m to 0.9 m. It is observed that the setup can accurately measure the speed in different arrangements within the speed resolution of 0.79 m/s and the range of the static target within the range resolution of 0.042 m. Measurements for the speed values in our experiment required an interval of 1.96 ms to capture sinusoidal variations caused by the Doppler frequency. Multiple reflections from the moving target cause echoes with higher speeds, as indicated in Figure 10a. The ± speed in the results is due to the use of a double-sideband W-band mixer for up/down conversion in the experiment. The ± speed ambiguity can be removed by using single sideband devices such as IQ modulators.

#### *5.4. Data Communication*

To demonstrate the convergence of communication and sensing by using the same 16-QAM OFDM waveforms, we also measured data communication performance in terms of BER. A subframe wise processing is used, where 12 OFDM symbols form a subframe, and each OFDM symbol has 300 active subcarriers. In one subframe, four OFDM symbols are multiplexed with pilot subcarriers (4 × 50 pilot subcarrier in a subframe), and a code

rate of 0.76 is used. In the experiment, we placed the receiver and transmitter in the LOS link distance of 0.6 m, with a bit rate of 8.08 Gbps. Due to non-ideal environment for the measurement such as limited dimensions of the lab and surrounding objects, multiple reflections of the transmitted signal arrive at the receiving antenna. Changes in the spectrum of the received signal in Figure 11 (red color) confirm the presence of the multiple reflections. Therefore, channel equalization is necessary to recover the transmitted data, which increases BER compared to AWGN channel [38]. We performed frequency domain equalization (zeroforcing) by using the transmitted pilot subcarriers for the channel estimation. As a result, a BER of 0.01 is recorded by comparing the transmitted and received data bits, when the average received SNR is around 15 dB, which is below the soft-decision forward error correction (SD-FEC) threshold of 1.5 × <sup>10</sup>−<sup>2</sup> [39]. As a comparison, a BER of 3.6 × <sup>10</sup>−<sup>4</sup> is obtained in the simulation due to the absence of the background reflections. A comparison of the received spectrum and the corresponding constellations is provided in Figure 11, for illustration purposes.

**Figure 11.** Data transmission results. The baseband spectrum and 16–QAM constellation, and red color for a LOS link of 0.6 m with background reflections. Blue color is the corresponding simulation result.
