ITU-R - International Telecommunication Union/ITU Radiocommunication Sector
Scope
Introduction Mobile-satellite communication services are now being offered all over the world. In order to share the limited frequency bandwidth among many MSS systems@ a perpetual requirement is to improve the spectrum utilization efficiency. For this purpose@ it is important to consider how to not only share the same frequency bandwidth between different systems but also improve spectrum utilization efficiency within an MSS system. Another MSS system utilizes V/H dual linear polarization. Figure 2 shows an example of spectrum utilization in a V/H dual linear polarization system. Each user earth station communicates using either V or H polarization. In Fig. 2@ V polarization is assigned to user earth station A and H polarization is assigned to user earth station B. Since user earth station A and user earth station B share the same frequency bandwidth with different polarization@ polarization tracking is required at each user earth station so as to eliminate cross-polarization interference on the other user earth station. In Fig. 2@ if each user earth station sends at the bit rate of R with bandwidth of W@ the total bit rate@ 2R@ is achieved with the bandwidth of W. Compared to dual linear polarization@ circular polarization excels in terms of its polarization-tracking-free nature which yields simple user earth stations. However@ linear polarization offers double the spectrum utilization efficiency; 2R/W (bit/s/Hz) compared to 2R/2W (bit/s/Hz). This means that dual linear polarization makes better use of spectrum resources than circular polarization. It is true that dual linear polarization is attractive from the viewpoint of spectrum utilization efficiency. However@ in practice@ accurate polarization tracking is difficult to realize@ especially for mobile user earth stations with low-profile antennas. Figure 3 shows the degradation in spectrum utilization efficiency that occurs when the mobile user earth station experiences polarization misalignment. As shown@ to handle this misalignment@ the spectrum utilization efficiency of both user earth station A and user earth station B should be reduced to hold communication quality steady in the face of cross-polarization interference between user earth stations. Figure 3 also shows one solution to the problem of mutual interference: single linear polarization. This approach does not employ polarization multiplexing and so avoids the mutual interference between user earth stations. Its weakness is that it fails to increase the spectrum utilization efficiency.