Baltimore County (Project 25)

System Information

• FCC Information
• The database entry for this system is here

Linear Simulcast Modulation

Baltimore County's new Motorola P25 7.x system employs an RF simulcast method known as Linear Simulcast Modulation. This technology supposedly enhances a subscriber radio's ability to overcome digital simulcast multipath distortion by shaping the digital packet waveforms using CQPSK (Compatible Quadrature Phase Shifting Key) modulation. Because of this, if the digital packets arrive at a subscriber radio from multiple transmitters out of sync, the radio is able to organize them and decode them without distortion.

Because scanners are not readily prepared to capture digital packets with these modified shapings, scanners have trouble decoding the packets. This can hold true not only for voice signals, but also control channel signals. This is why you may be in a solid coverage footprint area, but not receiving the system signals in a clear manner.

Multi-Zone

The UPGRADED Baltimore County system is a two-zone system. The reason for this is because you can only effectively simulcast from a certain number of towers before timing sync is degraded.

Your scanner (and the RRDB) refer to these zones as "sites." While typically a "site" is referring to a single tower site/transceiver location, in this case, a "site" is a group of tower sites all transmitting the same EXACT signal (known as simulcasting). With the old system, all 9 tower sites always transmitted the same exact signal -- it was a total simulcast --- therefore, scanners and radios only see the system as one "site."

With the upgraded system, there will be two "sites" or "zones" to cover the county, because, with the addition of other tower site locations, there are too many towers to have a total simulcast. The reason is very technical and beyond the scope of this post.

The main "zone" or "site" covers the majority of the county. The tower sites simulcasting the signal of this zone are as follows:

1. Towson (Fire Station #1)
2. Woodlawn (county highways shop on Windsor Mill Rd)
3. Hunt Valley (on Warren Rd east of I83)
4. Cub Hill (on the grounds of Hickey School for Boys)
5. Kingsville (Belair Rd at Harford County line)
6. Allender (Eastern Sanitary Landfill on Days Cove Rd)
7. Essex (rooftop site at CCBC Essex mid-rise building)
8. Northpoint (Precinct 12)
9. Sparrows Point (Fire Station #57)
10. Halethorpe (county highways shop on Brady Ave near Washington Bl and I695)
11. Catonsville (northwest corner of Rt. 40 & Rolling Rd)
12. Red Run (New Towne High School in Owings Mills)
13. Arcadia (Arcadia Volunteer Fire Company carnival grounds)
14. MEMA (Camp Fretterd military reservation in Reisterstown)
15. Jacksonville (Jacksonville Elementary School)

The following sites will cover the extreme northern end of the county, comprising the second "site" or zone:

1. Maryland Line (Rt. 439 just east of I83)
2. Hereford (state highways yard on Mt. Carmel Rd just east of I83)
3. Spookhill (at the city water facility on Prettyboy Reservoir)

It seems clear at this point that, because of the large footprint covered by the south site, it will carry most traffic on the system. It is unclear exactly how much of the traffic will be carried on the north site. It is likely that there will be a few talk groups "forced" onto the north site at all times, while most talk groups will only be carried on the north site based on registration/affiliation. What this means is that, as a radio enters the footprint of the north site, it will register with the site's controller, identifying the talk group with which is it affiliated, causing the site to begin carrying that talk group. If no radio is registered/affiliated with the north site for a specific talk group, that talk group will not be carried by the site. This is significant for scanner users because a scanner cannot register, therefore, some talk groups may be silent on this zone.

To add to the fun, there are a handful of talk groups that are ONLY for the north site. These talk groups appear to be reserved for a condition called "site trunking." Site trunking is a fallback function that occurs when a remote site (such as the north site) loses contact with the system controller, causing wide area calls (calls over multiple sites) to fail. If this were to occur, users on the "detached" site would revert to talk groups specific to that site, allowing users to continue to communicate within the footprint of the site. Dispatchers may also continue to connect with the site via some sort of RF link or back-up microwave path.

Object Oriented Programming

If you own the Pro-106, Pro-197, PSR500, or PSR600 scanner, you have some options as to how you want to monitor the system:

1. If you will only ever monitor from within the footprint of the main (south) site, you may chose to ONLY enter the south system frequencies (remember, you only ever have to enter the control channel frequencies). You may miss calls that only occur on the north site, but these calls should be rare.
2. If you move between the coverage footprints of the two sites, you may chose to set your scanner to "ROAM." Is this mode, your scanner will evaluate the strengths of the signals of the control channels from both sites and chose the strongest signal to lock on to. If the signal strength drops below a set threshold, your scanner will automatically evaluate the signal strength of the other site's control channel, and, if it is stronger, will switch to that site. Remember, if your scanner locks onto the north site, calls specific to the south site will be missed.
3. If you are within reception of both sites, and want to ensure that you hear all calls, you can set your scanner to "STAT" or stationary mode. In this mode, your scanner will sample the control channels of both sites for brief amounts of time, almost like scanning two different systems, but referring to the same list of talk groups IDs.

If you chose option 2 or 3 above, you need to enter in all control channel frequencies (8 total) and select the appropriate multi-site mode.

Thanks to ocguard in this thread for this information

Software Examples

• These screens should be used as a starting point. You may need to play around with other settings to make it work in your situation. This is particularly true of utilizing the attenuator to cut signal overload issues. Both the North and South site frequencies are shown.

• 

  PSREdit500 screen

• 

  Win500 screen

Unidentified Talkgroups

• as of Nov 25 2011
• 9433
• 9618
• 9622
• 9629
• 9630
• 9647
• 9648
• 9652
• 9656
• 9657
• 9665
• 9668
• 9670
• 9866
• 9869
• 9870
• 9871
• 9872
• 9873
• 9874
• 9876
• 9877
• 9880
• 9884

Unit Ids

About 115 radio IDs seen so far (using Pro96Com)