# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### Communication Function Requirements  
In general, communication function requirements are fundamentally determined by the Shannon-Hartley Theorem, which states that:  
\[C=B_{c}\log_{2}(1+\text{SNR}),\]  
where: \(C\) is the capacity of the communication channel, defined as the maximum achievable error-free data rate, in bits/second; \(B_{c}\) is the channel bandwidth in Hz; and the signal-to-noise ratio SNR is defined as the ratio of the average received signal power to the average channel noise-plus-interference power, where it is assumed that the noise and interference can be characterized by Gaussian white noise. The received signal power is a function of the EIRP of the transmitting terminal, signal losses over the propagation path, and gain of the receive antenna. In lower frequency bands, where communication services have historically been located due to good signal propagation characteristics at these frequencies, data rates tend to be less because channel bandwidths are constrained by spectrum crowding. In higher bands, namely X-band and above, larger channel capacity is possible because greater spectrum availability allows wider channel bandwidths to be used. However, if the benefit of a wider channel bandwidth is to be realized, the EIRP and receive antenna gain of a shipboard communication terminal must typically be larger than those used in lower bands, in order to overcome the higher rain attenuation suffered at these higher frequencies along the signal propagation path. A naval vessel employs different types of communication services that span frequencies up to \(\text{K}_{a}\) band. However, some of these services are not attractive candidates for inclusion in an AESA-based MFRF system. For example, there are a number of terrestrial-based services with low data rates, including voice and tactical data links (such as Link 16 and Link 22), which are operated at UHF frequencies or lower. These services typically employ omnidirectional monopole or dipole antennas, which are relatively simple and inexpensive, but must be large in length to provide efficient radiation at the low frequencies involved. These necessary antenna characteristics preclude such low-frequency services from consideration for an AESA implementation. There are also requirements for a naval vessel to utilize communication services involving military satellite systems operating in UHF, X and \(\text{K}_{a}\) bands, as well as commercial satellite systems in L, \(\text{K}_{u}\) and \(\text{K}_{a}\) bands. Some examples include the military Wideband Global SATCOM system operating at X and \(\text{K}_{a}\) bands, and the commercial Telstar 12 VANTAGE system at \(\text{K}_{u}\) band. Compatibility with future V-band satellite systems may also be a requirement, but the characteristics of ground terminals for such systems are not yet standardized. Consequently, V-band systems will not be discussed here. A common characteristic of all existing communication satellite systems is the use of geostationary orbits. This results in very slow changes of the ship-to-satellite pointing angle, since the only contributor to such changes is the ship translational motion. UHF and L-band _satellite communications_ (SATCOM) terminals for shipboard application commonly employ simple low-gain helical and/or conical antennas to provide a hemispherical beam pattern, which removes the need for any pointing stabilization. Examples of such systems are the Thales QHASS UHF SATCOM terminal [11], which is currently on several of the Royal Canadian Navy Iroquois-class and Halifax-class vessels, and the L-band JRCJUE-87 Inmarsat C terminal [12]. These antenna packages are also compact, being less than 0.5 m in both diameter and height. Such low-gain antennas lead to EIRP values in the order of only 15 dBW, which are nonetheless sufficient because both communication data rates and rain attenuation of the RF signal are very low at these frequencies.