If you’re considering coverage enhancement in your facility, its important to consider the reasoning behind the decision to separate or combine cellular services with public safety LMR signals. There are pros and cons to either choice, but be sure to consider the following 7 reasons for keeping systems separate when making your decision:
- Cost & Lifecycle
– Public Safety (PS) DAS typically has far fewer antennas, electronic components and cable runs than a cellular DAS.
– PS DAS has additional requirements which would multiply the costs of a large cellular DAS.
– Life cycle difference – cellular DAS may need technology upgrades every 3 to 5 years, PS DAS maybe requires upgrades every 10 years, A cellular DAS owner should expect to re-invest in the DAS for technology upgrades.
- Power levels
– Public Safety LMR signals can be at lower levels than cellular
– LMR User device receivers are more sensitive.
· Operating down to -120 dBm
– Cellular devices typically become unreliable below -100 dBm.
· 20 dB difference is 100 times the power level
– LMR User devices have higher Tx power levels, 2.5W or more compared to <1 W for cellular.
– Conclusion: P.S. antennas can be spaced farther apart.
· This reduces the number of antennas.
· Fewer cable runs and fewer components for the P.S. DAS
- Gain Settings
– LMR systems can have asymmetrical gain between uplink and downlink.
– This allows for more flexibility in system tuning
– In cellular systems the user device auto-adjusts its Tx power to match the Rx power it sees. Asymmetrical gain in the DAS can confuse the user device and base station causing system problems.
– P.S. DAS gain settings might be better to be completely different than in a cellular DAS
- Filtering & Separation
– 700 and 800 MHz bands are intermingled for cellular and LMR bands
– Potential for interference and intermodulation in passive DAS systems
· Separation of cell/LMR signals into separate BDAs
· Additional filtering may be required
· Active DAS has more tools to limit interference
- Greater Reliability Requirements
– PS DAS is recommended to have:
· 12-hours power back-up on electronic equipment,
· 2 hour fire survivability
· Water proofing
· Monitoring – continuous monitoring, alarm reporting to fire panels
· Annual testing and re-certification
– Cellular DAS is typically built for best-effort service levels
- Service Level Agreement (SLA) of DAS Provider
– Cellular DAS systems are typically maintained on a best-effort basis with no uptime commitments
– A Public Safety DAS would typically be installed with a maintenance agreement that obligated a service provider to respond within a specified number of hours of placing a service call.
- Operating Authority
-A cellular DAS is operated under the approval of the carrier networks who hold the frequency licenses.
· DAS owners need carriers’ approval to use the DAS
· Uses the carriers’ frequency licenses
– Owners need to deactivate their DAS elements immediately upon notice by the carrier. If the DAS is integrated, this might mean an obligation to shut down the entire DAS, including Public Safety signal enhancement whenever the carrier’s needs to test their own networks.