A transit system in a metro area operates a large rail system and a bus network and it requires a robust land/mobile radio communications system for its operations. In order to serve its patrons, there is a need to have seamless system-wide radio coverage. A network of tunnels comprises a large portion of this system. A communication connection must be established to ensure public safety and routine maintenance and operations, whether a radio user is aboveground or underground in the tunnels.

Customer's needs

Transit officials noticed "nulls" i.e. poor radio coverage spots inside the tunnels. In order to further improve the coverage within its tunnels and metro stations, the customer wanted to determine and analyze the signal strength of a 490MHz simulcast trunked radio system. The analysis would help the customer identify RF coverage "nulls" within the tunnels and stations, and take remedial actions.

GL's Solution

Using an Andrew's advanced test and measuring systems, Invex3G, GL's engineers analyzed RF signal strength (a.k.a. RSSI) along the tunnels. By combining the analysis with the chain markers located in the tunnels, GL identified exact locations of poor signal inside the tunnels that required RF coverage improvements. A threshold for acceptable signal was established and compared to the observed signal strength to identify the "nulls" as indicated in the RSSI plot below for one of the tunnel sections.

Results and Benefits

Our analysis of the RF coverage showed locations inside the tunnels that required improved RF signal strength in order to achieve a reliable coverage of the 490 MHz system within the station platforms and tunnels. Based on our analysis, the customer implemented remedial actions including, but not limited to, adjusting the offset from the wall, replacement of certain bad sections of the cable etc. GL's expertise in RF field testing and analysis was a key to the success of the project.