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.