Measurements of interference between microwave and personal communications systems for spectrum sharing

Future Personal Communications Services (PCS) may be able to share frequency spectrum in bands currently occupied by microwave point-to-point communications systems without causing harmful interference. Telesis Technologies Laboratory (TTL) has undertaken experimental spectrum sharing feasibility measurements in the San Francisco Bay area to quantify the extent to which microwave link spectrum is available for use on a shared basis, and to identify constraints that need to be placed on PCS systems in order to share with fixed link microwave users. The measurements indicate that exclusion zones for potential PCS transmission exist along the main beam of the fixed microwave link and mainly near the microwave terminals. PCS users located in high-rise buildings and hilltops produce significantly higher interference to the fixed microwave link. The results of measurements made in PacTel building show, for worst case, a 45 dB increase in interference level from the ground floor to the roof. In dense urban areas, higher interference also results from specular reflections off building. In downtown San Francisco, a 40 to 50dB higher interference level was observed due to building reflections in a wide area outside the microwave main beam. Because the microwave links are fixed and of known location with directional energy, the exclusion areas are usually local. It is suggested that a system with a channel bandwidth less than 5 MHz is more feasible for spectrum sharing through deliberate frequency planning