## 6 Discussion and Recommendations
### Discussion  
In the following, we discuss the results presented in Table 4.  
First, considering OFDM1-MCS1, we observe that a reduction in the bandwidth of the interference signal (i.e., going from OFDM2-MCS2 to OFDM4-MCS5 interference) requires a 2 dB SIR increase to provide a \(\mathrm{PDR}>80\%\). This can be attributed to the fact that the reduction in the bandwidth causes an increase in the PSD (Power Spectral Density) of the interference (i.e., a 3 dB increase each time the bandwidth is reduced by half) meaning that the probability of a symbol being corrupted increases. Second, the SIR required for OFDM2-MCS2 and OFDM3-MCS3 is equivalent to OFDM1-MCS1 and OFDM2-MCS2 regardless of packet size (2-4 dB and 5-6 dB, resp.). However, the effect of OFDM3-MCS3 interference causes a 3 dB SIR increase to the OFDM3-MCS3 signal (i.e., 7-8 dB) to maintain \(\mathrm{PDR}>80\%\), whereas the OFDM2-MCS2 signal remains unaffected (5-6 dB). Such results can be attributed to the fact that, despite both OFDM2-MCS2 and OFDM3-MCS3 using the same modulation (QQPSK with coding rate \(R=1/2\)), the former takes advantage of the 2x frequency repetition providing an additional protection of 3 dB. The result is independent of packet length, which confirms the importance of using such protection mechanisms at the physical layer. Third, the SIR required for OFDM4-MCS5 to maintain a \(\mathrm{PDR}>80\%\) is higher compared to the other SUN-OFDM modulations and increases as the bandwidth of the interferer is reduced (going from OFDM1-MCS1 to OFDM4-MCS5 interference). This is an expected result, as the PSD of the interference increases by 3 dB every time the bandwidth is reduced by two, and taking into account that OFDM4-MCS5 uses an aggressive modulation (16-QAM) in combination with a coding rate \(R=1/2\), but does not employ time or frequency symbol repetition, leading to a higher BER (Bit Error Rate) in the presence of interference. Fourth, the SIR required for the OQPSK-DSSS modulation remains constant (8-12 dB depending on packet length) regardless of the type of the SUN-OFDM interference and, hence, the occupied bandwidth. In contrast, OQPSK-DSSS modulation requires a smaller SIR (6-8 dB depending on packet length) when the interference is OQPSK-DSSS. This can be attributed to a combination of facts including that SUN-OFDM interference is composed of multiple pilots spread over the channel and has a higher PSD, as well as the fact that OQPSK-DSSS does not provide any additional mechanism to enhance the robustness of the transmission against interference except for the processing gain of DSSS (\(10\cdot\log_{10}(2\) M/250 k) = 9 dB). Fifth, SUN-OFDM modulations are robust against OQPSK-DSSS interference, requiring negative SIR values (\(\mathrm{SIR}<0\) dB) to maintain a \(\mathrm{PDR}>80\%\). This is an expected result that can be attributed to the fact that SUN-OFDM modulation is a combination of narrowband modulation, whereas the OQPSK-DSSS is inherently wideband, i.e., it has a very low PSD. In fact, the results show that OFDM2-MCS2 and OFDM3-MCS3 require the same SIR, which is lower than OFDM1-MCS1. This can be explained given that OFDM2-MCS2 employs \(2\times\) frequency repetition, whereas OFDM3-MCS3 occupies half the bandwidth (400 kHz vs. 800 kHz, resulting in a 3 dB PSD increase). As a numeric example of the above, for OFDM2-MCS2, the PSD for a transmit power of 0 dBm and a bandwidth of 800 kHz is \(-29.03\) dBm/Hz. In contrast, for OFDM3-MCS3, the PSD for a transmit power of 0 dBm and a bandwidth of 400 kHz is \(-26.02\) dBm/Hz. Finally, OQPSK-DSSS interference with a transmit power of 0 dBm and a bandwidth of 2 MHz has a PSD of \(-33.01\) dBm/Hz. Notice that, for an SIR = 0 dB, OFDM2-MCS2 has \(\mathrm{SIR}=4\) dB (\(-29.03--33.01=3.98\) dB), whereas OFDM3-MCS3 has \(\mathrm{SIR}=7\) dB (\(-26.02--33.01=6.99\) dB).