11.6 Tunnel Solutions, T-Systems
One of the most commonly used designs is to use a ‘T-feed’ system. This system has several advantages over the cascaded system in Section 11.5. Signal distribution to the RUs (Remote Units) is done in parallel with the radiating cable system. Each RU (Remote Unit) is fed an input, and provides output in both directions of the radiating cable. In the case of RU malfunction, or a broken radiating cable, most of the system will actually still work. Centralized control and monitoring are done at the ‘optical fiber unit’ co-located with the base station. This main unit of the tunnel system also feeds the base station with external alarms in case of malfunction. Remote configuration can be done via an RF modem or IP for easy trouble-shooting and maintenance.
11.6.1 T-systems, Principle
The principle for the ‘T-feed’ system (as illustrated in Figure 11.11) is that the base station connects to an optical converter that converts the RF into optical signals. Using optical transmission the signal can be transmitted over relatively long distances with only limited degradation compared with RF transmission over coaxial cable. Each RU (Remote Unit) has an optical interface and converts the signal back to RF, and then a power amplifier that transmits the radio power in both directions on the radiating cable. Naturally the RU has a reverse amplifier and RF-to-optical converter for the uplink signal.