**4. Simulation Results**

To verify the processing of the proposed OFDM-based converged system, a baseband equivalent model is implemented and simulated in MATLAB. An impulse response, having taps at the round-trip delay of the targets, and each tap varying over OFDM symbols according to the complex exponential of the Doppler frequency, is used to represent the sensing channel, whereas SNR = 15 dB is set for simulation. Equal signal strength is used for different targets, while other parameters, listed in Table 2, are selected to match parameters used in our experiment.

**Table 2.** Simulation parameters.


It is important to note that for scenarios where we use only static targets, *M* = 36 is used, which provides sufficient gain for range detection but results in a high value of Δ*v* in the range/speed plot, although this is irrelevant for static targets. For the range/speed plot, the absolute of the delay/Doppler matrix is first normalized to unit (by dividing with the maximum absolute value) and then converted to dB scale. Figure 4a–c presents the range plots for static targets at distances 0.6 m, 1.3 m 1.5 m, and 10 m, respectively, using the conventional OFDM-based sensing. Results show that the targets are identified correctly at the distances in Figure 4a,b.

**Figure 4.** Simulation results of the conventional OFDM–based sensing vs. the proposed range extension method. (**a**–**c**) Range plots for a target at 0.6 m, two targets at 1.3 m and 1.5 m, and a target at 10 m, respectively, by using conventional OFDM–based sensing. (**d**–**f**) Range plots obtained by using the proposed range extension method.

However, Figure 4c clearly shows that the sensing performance has been compromised, in terms of SNR, for the target located at a 10 m distance. This reduction of SNR happens because the CPI covers a range up to 3.2 m. Beyond this range limit, ISI occurs, and the processing gain is also reduced for that target located at 10 m. In comparison, the scenario mentioned above is also processed for sensing using our proposed method by finding the zero-delay for each target, as shown in Figure 4d–f. Our proposed method offers better performance for the target at 10 m by avoiding the loss in processing gain.

Figure 5 represents the results when multiple targets exist and one of them is moving. In Figure 5a, the conventional approach provides accurate results for two targets, one static target located at 0.6 m and the other target moving with a speed of 5 m/s and situated at 0.85 m. In comparison to Figure 5a, Figure 5b represents the results obtained by using our proposed method where peak height is similar to the peak height in Figure 5a, but ISI effect is used for locations away from the targets.

In order to verify the proposed method for the range beyond the OFDM symbol, we also simulated the case that three targets are placed at 0.85 m (moving with 2.34 m/s), 12.65 m, and 17.65 m, respectively, and the sensing results are shown in Figure 5c,d. The farthest target at 17.65 m is beyond the range of an OFDM symbol duration (12.77 m) and it does not appear in Figure 5c using the conventional approach, but it is detectable with our proposed method, as shown in Figure 5d. There is another peak with height ≈ −20 dB at 4.88 m (12.77 m behind the target at 17.65 m) in Figure 5d, which appears due to the CP effect.

**Figure 5.** (**a**) One static target at 0.6 m and one moving with 5 m/s using the conventional OFDM– based sensing. (**b**) Results by using the proposed range extension method for the scenario in (**a**). (**c**) Range–speed plot for targets at 0.85 m and moving with 2.34 m/s, with second and third static targets at 12.65 m and 17.65 m, respectively. (**d**) Range–speed plots obtained by using the proposed range extension method for targets mentioned in (**c**).

#### **5. Experimental Setup and Results**

In this work, we also implement an experimental demonstration. Figure 6 shows the configuration of our system in the experiment, which is composed of several blocks in the digital and analog domain for transmission and reception. In our experiment, the baseband 16-QAM OFDM waveform is digitally generated for communication and sensing using parameters listed in Table 2. An oversampling factor of 20 is used before the signal is digitally up-converted to an intermediate frequency (IF) at 3 GHz through the IQ mixing. The IF signal is then fed to an arbitrary waveform generator (AWG) operating at 120 GS/s. Before the free-space transmission, the signal is up-converted to the W-band with a carrier frequency of 97 GHz using a commercially available W-band mixer at 94 GHz. Subsequently, a W-band amplifier with 10 dB gain is used to boost the signal to around 0 dBm, and a pair of conical horn antennas with a gain of 30 dBi is used for transmission and reception. At the receiver, the signal is first down-converted into the IF domain using a similar W-band mixer, sampled using a digital sampling oscilloscope (DSO) (KEYSIGHT MXR608A, sampling rate of 16 GS/s, bandwidth of 6 GHz) and then processed digitally to retrieve the baseband signal for further processing. In the digital domain, typical Fourier sidelobes are suppressed by using the Hamming window.

The photos of the experimental setup are shown in Figure 7a–c. Figure 7a shows the arrangement for realizing the reflective sensing to measure the range of two static targets with flat reflective surfaces. Figure 7b shows the setup for speed and range measurement with a static target and a moving target. The speed is measured via a reflective target mounted on a belt, which moves the target along the LOS and away from the receiver with adjustable speed. In this case, the Doppler frequency shift is induced in the signal reflected from the moving target, and the down-conversion from W-band carrier frequency to an IF yields a sinusoidal of the Doppler frequency, which can be observed at the DSO. Observation of the sinusoidal-like variations in the received signals at the DSO indicates the correct measurement arrangement for speed measurement.

**Figure 6.** Block diagram of the measurement setup, showing different stages of processes in the digital and analog domains at the transmitter *TX* and the receiver *RX*.

**Figure 7.** Experimental setup for the measurements. (**a**) Photo of the setup to measure range of two static targets. (**b**) Range-speed measurement setup of a static target and a target mounted on a belt capable to move with adjustable speed up to 5 m/s. (**c**) Data link arrangement.

The setup is calibrated during range measurement, and we measure different cases for illustrating the range and speed measurements, as listed in Table 3, where *d*<sup>1</sup> and *d*<sup>2</sup> are the actual distances of targets from the receiving antenna and ˆ*d*1, ˆ*d*<sup>2</sup> represent measurement results. Similarly, *v* and *v*ˆ in Table 3 represent actual and measured speeds, respectively. For the data transmission part, a LOS link is established by placing the receiver at the location of the static target, as shown in Figure 7c. In the following subsections, we discuss experimental results obtained from the measurements.
