15 August 2014

The (other) RSSI Threshold

To facilitate the use of a shared physical channel, a repeater in an IP Site Connect; Capacity Plus  or Linked Capacity Plus system, will monitor its RX frequency and will not transmit if the RSSI from signals from another radio system is greater than a configurable threshold. This ensures that the system will not use a channel if another nearby system is currently using that channel.

This RSSI threshold is CPS configurable anywhere between -40dBm and -130dBm. The threshold should be chosen wisely as:
  • Setting it too low may result in interference from background noise inhibiting a repeater from transmitting. 
  • Setting it too high may result in a channel being used while interference is present. In Capacity Plus and Linked Capacity Plus systems, this would result in data messages and call setup requests being missed.

RSSI Threshold in a Capacity Plus or Linked Capacity Plus Channel.

While setting up the system, or performing a FAT, the RSSI Threshold can be set to -40dBm. Once deployed on site, the RSSI Threshold should be set to a reasonable value, based on the average noise level on site. Setting the the RSSI Threshold to this high value, while setting up and testing in the laboratory, will eliminate RSSI Threshold  as a root cause for the system failing to operate as expected.

RSSI Threshold in a Conventional Channel.


Using RDAC, one can measure the average signal level, under no traffic conditions, on the repeater channel. It is important to only consider the RSSI Threshold while there is no traffic on the channel. It is also important to observe the RSSI Threshold over several minutes to ensure a reliable "noise floor" value is obtained. Once this is done, the RSSI Threshold can be set  to 10dB above this "noise floor". It can also be tweaked at a later stage if anything changes.

8 August 2014

Roaming RSSI Threshold

The Roaming RSSI Threshold is a CPS configurable parameter, that controls the signal strength a radio needs to see before searching for another site. If the RSSI measurement of the site, which the radio is on now, is above the specified Roaming RSSI Threshold, then the radio will remain on that site and not roam. Once the RSSI measurement drops below this threshold, the radio will begin a Passive Site Search process to find a site with higher signal strength. Essentially, this setting controls the distance away from a site, at which a radio will begin looking for another site.

It is important to note that while passively roaming, the radio temporarily leaves the current home site to determine if a stronger site is available. Since the radio is temporarily away from the home channel, it is possible that it will miss the beginning of a transmission (i.e. enter the call late).

The adjustment of the Roaming RSSI Threshold is a balance between when a radio will leave one site and look for the next, versus how often the radio will go into roaming. If the Roaming RSSI Threshold is too low, the radio will remain on a low signal strength site, even though there might be a stronger site available. If the Roaming RSSI Threshold is too high, the radio will be roaming in strong signal conditions hence resulting in excessive late entries. The above diagram shows the impact of the Roaming RSSI Threshold value, in relation to the good coverage line (dotted) which most system coverage is designed to meet. The shaded area is where the radio would roam.

The default value of the Roaming RSSI Threshold is -108 dBm. It can be programmed for anything between -80 dBm and -120 dBm. A value of -108 dBm is roughly 80% of the good coverage. Therefore roaming will occur in the outer 20% of coverage. This default value is okay for most configurations but may not be optimal in certain configurations where some tweaking. Before setting the Roaming RSSI Threshold, one must consider the customer’s site configuration.

Consider the following four basic site configurations:
  • Dense Overlapping Coverage (Urban) – This type of coverage consists of dense sites with generous overlap. This coverage type is often found in large cities or highly populated areas. Overlapping sites utilize different frequencies. Non-overlapping sites may share frequencies, but those that do share frequencies need to have different color codes if they need to be distinguished while roaming. This type of coverage is highly likely to encountered shared use on one or all of its sites. A radio user may be within coverage of three to four sites at a time. The time it takes a radio user to move from the coverage of one site to another is in the range of 10 minutes.
  • Isolated No Overlapping Coverage (Rural) – This type of coverage consists of isolated sites with little to no overlap. This coverage type is often used for isolated sites in rural areas, although could be used to cover a single part of a small city. Non-overlapping sites may share frequencies, but those that do share frequencies need to have different color codes if they need to be distinguished while roaming. This type of coverage is less likely to encountered shared use although possible. A radio user will only be within coverage of one site at any time. The time it takes a radio user to move from the coverage of one site to another is in the range of multiple hours.
  • Corridor Coverage – This type of coverage consists of in-series slightly overlapping sites. This coverage type is often used for covering highways, train tracks, shore lines, or rivers. Frequency re-use is common in this configuration since one site only overlaps with its two adjacent sites. Non-overlapping sites may share frequencies, but those that do share frequencies need to have different color codes if they need to be distinguished while roaming. A radio will only be within coverage of one to two sites at a time. The time it takes a radio user to move from the coverage of one site to another is in the range of an hour.
  • Multi-Floor Coverage – This type of coverage consists of dense extremely close sites with short range coverage and generous overlap. This coverage type is often used for covering tall buildings, or deep tunnels. Frequency re-use is not common due to the small coverage footprint usually implemented with in-building radiax antenna systems. This coverage type also often encounters quick signal strength drop offs due to the nature of in building coverage. Non-overlapping sites may share frequencies, but those that do share frequencies need to have different color codes if they need to be distinguished while roaming. A radio will only be within coverage of one to two sites at a time. The time it takes a radio user to move from the coverage of one site to another is in the range of one minute.

The site configuration should be taken into consideration when setting the Roaming RSSI Threshold. For example, if the customer has Isolated No Overlapping Coverage, the threshold can be set to its lowest value of -120dBm. Because there is no overlap, there is no reason for the radio to start roaming until well outside of the coverage range of the repeater. For extremely close sites with large overlaps and quick signal drop off like in Multi-Floor Coverage, it might be better to set to it to a higher value so that the radios search for stronger sites.

It is important to note that since the roaming algorithm in the radio uses the outbound (repeater to radio) transmission to determine when to roam, having an unbalanced system can cause radios not to roam even though they can no longer reach the repeater. This can lead to radio transmissions that do not reach the repeater and are therefore not repeated.

One method to rectify this problem is to lower the output power of the repeater. This decreases the outbound coverage area, but ensures that if a radio can hear the repeater well, it can respond successfully. If lowering the output power is not desirable, the Roaming RSSI Threshold needs to be raised higher (less negative) than the recommended values. This forces the radios to roam to another site within very good RF coverage of another. This value may be different for portables and mobiles since they have different output power and therefore different inbound coverage. Portables may need a higher (less negative) Roaming RSSI Threshold than mobiles.

Also note that there is one Roaming RSSI Threshold per roam list in the CPS. This means that if one site has an inbound outbound imbalance and another does not, it may be difficult to find the correct Roaming RSSI Threshold to exactly accommodate both sites. In other words if you set the threshold to roam correctly on the imbalanced site, it may end up roaming too early on a balanced site.

31 July 2014

Ten things you probably didn't know about RF?

Here's a short quiz (10 questions) about a few obvious things we tend to forget about. Take the quiz by clicking here.

8 July 2014

New MOTOTRBO firmware

On 10 June, Motorola released new firmware for the current generation of MOTOTRBO radios. Included in this release are a number of enhancements which make MOTOTRBO an even more compelling choice for customers on the move.

Some of the highlights from firmware release R02.30.10 include:

  • Enhanced audio and data routing to the option board interface. This provides application developers with a generic interface for routing audio and data to a radio option board.
  • Hospitality  focused Job Ticket enhancements which make the radio even more useful for customers in this market. This includes the ability to sign in or out capability in the Hotel Management System from the radio menu. 
  • Radio users can now generate new job tickets from the radio and send them to the Hotel Management System. The ticket template and codes can be configured in CPS, rather than being fixed in radio firmware. 
  • When searching for a contact in the contact list, results are now displayed by matching the first character instead of any character in the name. The user can also delete user defined entries from Contact List via radio keypad (note: the radio user is still not able to delete CPS defined entries from the Contacts List). 
  • Data added to the radio by the user via Front Panel Programming or radio operations will be stored in a separate partition from the rest of CPS programmable data when writing or updating the radio. This will be limited to Call Log, Contacts, Job Tickets and Sent/Received Text messages, and does not include programmable button, power level, scan list states etc. 
  • NAI based applications (such as SmartPTT Plus and TRBONET Plus) are now able to support phone calls from a phone to an individual radio user, from radio to phone and from phone to talk groups. Before this was only possible via an analogue phone patch or via a control station radio (and an individual call setup). 
  • NAI based applications can now also receive RSSI values (i.e. the RSSI seen by the radio unit) with location data and this information can be recorded and mapped on a map.
  • The new firmware also provides better mobility and presence data in NAI by checking if the subscriber is online at any time. The update also supports ARS query based radio presence check through DDMS. 
  • There is now a base station option in the CPS which allows DM1000/2000 series 25W mobiles, using compatible accessories, to be deployed as part of a fixed installations in R&TTE countries. 
  • R02.03.10 allows non-display radios to receive (and optionally acknowledge) Emergency Alarms. Previously, this was not possible.  
  • The ring tone for incoming telephone calls has been improved allowing it to be heard in noisier environments. 
  • Satellite Receivers in Capacity Plus / Linked Capacity Plus systems employing Digital Voting no-longer require Capacity Plus / Linked Capacity Plus licenses. 
  • CSBK features have been implemented equally across the DP/DM2000 series radios. This means that a DM2600 can now encode Radio Check, Remote Monitor, Radio Disable and Radio Enable as well as encode capability supported for MDC Radio Check and MDC Remote Monitor. Digital Telephone Patch supported as a CfS feature. 
  • Hungarian has been added as a user interface language.
  • R02.30.10 and R01.12.10 addresses the security issue that allowed the codeplug password to be easily discovered and provides three configurable codeplug password policies. A read-write codeplug password protection option is now possible - it is not possible to packet grab this password and there is no backdoor password for EMEA. It is therefore highly recommended that the read-only password option is chosen because if read-write password protection is selected and the password forgotten, then the radio will have to be returned to the Motorola ERSC for recovery.
This firmware also requires the latest CPS: Version 10.5 (Build 551) or Radio Management Version 1.5. A new version of Tuner and RDAC are also available. All of these*, together with the latest System Planner, can be found on Motorola Online (or ask your local Motorola representative).

* Except RDAC