# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### Communication Function Requirements
The dynamic range requirements for the communication services are largely driven by the variation in signal input levels corresponding to advanced high-order Quadrature Amplitude Modulation (QAM) schemes that support increasingly higher bit rates. The shipboard EIRP requirements in Table 1 for the communication services reflect values that are necessary to compensate for rain attenuation of the signal at these frequencies, and for the one-way geometric propagation loss over the signal path, such that the SNR at the off-board receiver is high enough to support the desired channel capacity. The EIRP requirement for the TDCL function is noticeably lower than that for the SATCOM services primarily because TDCL is specified to operate over ranges of only up to 200 km, whereas the one-way signal propagation distances involved in SATCOM are approximately 36,000 km for geostationary satellites. The one-way beamwidth values that appear in Table 1 are not independent beamwidth requirements. Rather, as the antenna gain can be computed from the beamwidth, the beamwidth values serve to indicate in this case the associated antenna gains required to ensure that the SNR value of the signal received at the shipboard receiver is consistent with the desired channel capacity. The transmission duty cycle of a communication function may be up to 100% when the function is activated. The polarization requirements listed in Table 1 are somewhat different depending upon the service. A common characteristic of the SATCOM services is that Rx and Tx polarizations are orthogonal, where orthogonality is achieved for linear polarizations by using horizontal and vertical polarizations, and for circular polarization by using right-hand circular polarization and left-hand circular polarization. The use of polarization orthogonality provides added isolation between Rx and Tx channels beyond that afforded only by duplex operation. More importantly however, this capability allows a SATCOM satellite to use orthogonal polarizations within the same frequency channel. This frequency re-use doubles the satellite's communication capacity within a given bandwidth constraint, which is especially important for the crowded Ku spectrum. Orthogonal polarizations are not required for TDCL. Circular polarization is used for X-band SATCOM because at X-band frequencies and lower, Faraday rotation of linear polarizations in the ionosphere [17] causes unacceptable leakage between orthogonal polarization states, which adversely affects frequency re-use. In contrast, horizontal/vertical polarizations are employed for K\({}_{\text{u}}\)-band SATCOM because Faraday rotation is not an issue for linear polarization at K\({}_{\text{u}}\) frequencies and above, and although rain causes leakage between all orthogonal polarization states at these frequencies due to the interaction of the SATCOM signal with large flattened raindrops along the propagation path, the leakage is less pronounced for orthogonal linear polarizations [18]. Note that the use of linear polarization requires precise stabilization of the shipboard antenna in the presence of ship motion to prevent misalignments between the shipboard antenna and satellite antenna that can contribute to leakage between polarization states. Notwithstanding the depolarization effects of rain described above, circular polarization is used for K\({}_{\text{a}}\)-band SATCOM to address a greater concern. Signals at K\({}_{\text{a}}\) frequencies are generally more severely attenuated by rain than K\({}_{\text{u}}\) and lower frequencies, and in particular, there is a more significant difference in attenuation between orthogonal linear polarizations at K\({}_{\text{a}}\)-band for large raindrops, due to the fact that they have dimensions comparable to K\({}_{\text{a}}\)-band wavelengths [18]. Specifically, the polarization along the axis of the widest dimension of the flattened raindrop is more attenuated than the orthogonal polarization.