## 2 Analysis of OFDM-Induced IM2 Distortion Effects  
Let us assume that a desired MedRadio RF signal comes into the receiver along with an OOB OFDM blocker. Since the OFDM signal comprises multiple subcarrier tones that are independently modulated and equally spaced in the frequency domain, any combination of two subcarrier tones within the entire OFDM signal will create multiple intermodulation distortion components. Figure 1a shows the first situation such that two subcarrier tones at \(f_{k}\) and \(f_{m}\) create the third-order intermodulation (IM3) distortion at \(2f_{k}-f_{m}\). The resulting IM3 falls inside the desired RF channel at \(f_{RF}\), directly leading to SNR degradation at \(f_{RF}\). Considering that the typical subcarrier spacing is in the range of a few 100 Hz to a few kHz for the communication and broadcasting signals, this effect is only possible when the two subcarrier tones are very close to \(f_{RF}\). Such a close blocker signal will be likely to exist inside the desired band. Then, the in-band interference mitigation techniques such as the channel-scan-and-search-based LBT will be effective enough for its mitigation. On the other hand, when the OFDM blocker signal is far away from \(f_{RF}\) as shown in Figure 1b and is, thus, likely to be out of the MedRadio band, then its IM3 tone cannot directly affect the desired band at \(f_{RF}\). Yet, the IM2 distortion at \(f_{m}-f_{k}\) will fall in the down-converted IF or baseband channel at \(f_{IF}\) and lead to the SNR degradation. The LBT technique that works for the close blocker signal will not work for this situation because the blocker signal is located far away out of the band and is, thus, likely out of the channel scan range. It should be noted that a variety of communication and broadcasting services that are being offered worldwide in the UHF and VHF bands employs the OFDM signaling. Hence, the IM2 distortions induced by the OOB OFDM blocker signal need to be analyzed rigorously to accurately assess the SNR degradation in the MedRadio receiver. Figure 2 illustrates that the OOB OFDM blocker signal at \(f_{BL}\) appears beside the desired MedRadio RF signal at \(f_{RF}\). Let the number of subcarriers and subcarrier spacing of the OFDM blocker are \(N_{sub}\) and\(f_{sub}\), respectively. For example, in the ATSC 3.0 standard [21], given the channel bandwidth of 6 MHz, \(f_{sub}\) is 843.7, 421.8, and 210.9 Hz depending on the OFDM FFT modes of 8K (\(N_{sub}=6913\)), 16K (\(N_{sub}=13826\)), and 32K (\(N_{sub}=27,649\)), respectively. Similarly, we can find that \(f_{sub}\) is 279-8929 Hz for DVB-T2 [22], and 992-3968 Hz for ISDB-T [23]. Figure 1: Two-tone blocker effects. (**a**) Third-order intermodulation effect for the close-in blocker, (**b**) second-order intermodulation effects for the far-out blocker. As shown in Figure 2, any combination of two subcarrier tones out of the total \(N_{sub}\) subcarrier tones will create (\(N_{sub}-1\)) of IM2 tones at the baseband, starting from the first at \(f_{sub}\) to the last at (\(N_{sub}-1\))\(f_{sub}\). If the desired IF signal band overlaps with the IM2 tones, the total IM2 distortion power can be computed by integrating its power spectral density (PSD) over the IF channel bandwidth. Then, the SNR degradation caused by the OFDM-induced IM2 distortions can be accurately evaluated. For example, the IF signal bandwidth of Figure 2 encompasses the 3 rd IM2 tone at 3\(f_{sub}\) through the 10 th IM2 tone at 10\(f_{sub}\). Thus, the total IM2 distortion power should be computed by integrating from the 3 rd through the 10 th IM2 distortion power. The well-known input-referred second-order intercept point power (\(II\!P_{2}\)) of an RF receiver is expressed as  
\[II\!P_{2}=P_{in}+(P_{out}-P_{OIM2}) \tag{1}\]  
where \(P_{in}\) is the input power, \(P_{out}\) is the output power, and \(P_{OIM2}\) is the output-referred IM2 distortion power, all expressed in dB or dBm.