Wednesday, October 31, 2012

Latest FCC News on VHF Digital Use

Most of our readers know that FCC Coordinators recently placed a 10 watt Effective Power Limitation (ERP) on TDMA base/repeater station digital radios (i.e. MotoTRBO and Hytera plus a couple of lesser contenders).  

Within the last week new policies have been submitted by the frequency coordinators to the FCC to apply restrictions on all VHF radios coordinated in the Public Safety Frequency Pool operating in the TDMA digital mode. These new policies  apparently could include P25 systems operating on TDMA! We should be hearing more about this in the near future. 

Or, in engineering bureaucratic double speak…….

For Base/Repeater operations:

1) You must have 250 km (155 miles) protection from your proposed base station antenna to existing co-channel and 7.5 kHz adjacent channel incumbent mobile licensees;

2) Assume an incumbent mobile receiver antenna height equal to the transmitter antenna height as licensed on incumbents granted license or pending application

3)   If a receiver antenna height cannot be inferred from step 2, then a receive antenna height of 60 meters (196 feet) will be assumed;

4)   Perform a LongleyRice propagation study at 50%, 50%, 50% (time location, probability) for proposed digital system; and,

5) A signal level of -110 dBm or greater at the incumbent site will cause the proposed sttion to fail the protection standard.

Or, in layman’s language as understood by this writer…….

If you are a public safety user contemplating a move to digital – ANY digital TDMA (not FDMA which is offered by ICOM and Kenwood) system, conventional or trunked involving a base or repeater station, you must obtain coordination for the proposed frequency(s) to be used based on that frequency being unused  within a radius of 155 miles in all directions as well as all mobile users on adjacent channels within 7.5 kHz.

This is based on the assumption that the incumbent (the guy who already has a license for analog operation) is licensed for an antenna height equal to the height of your base or repeater station.  If you can’t confirm what the incumbents station antenna height is, then a height of 196 feet will be assumed. 

You have the option of performing a propagation study AND demonstrating that you will not produce a signal level of -110 dBm at the incumbent (the guy who has an analog OR digital license) receiver site.  

The typical receiver sensitivity for 12 db sinad is -119dbm. This is 9 db below what the new coordination standards will allow. Thus the FCC is saying that at the antenna of the existing user that you might interfere with, the maximum level from your new digital radio must be no stronger than roughly 8 times the minimum signal a typical receiver can receive.

Bottom line translation for any public safety user planning to go VHF digital……. 

Talk to the frequency coordinator of your choice before signing up to purchase any VHF wide area TDMA radio system or upgrade.  Or better yet, talk to us.  You can call us at 205.854.2611 for the straight story, always!