Seamless call handover

Seamless Call Handover is a Capacity Max feature that allows a radio terminal to continue call activity (transmitting, receiving or in a hangtime state) while roaming onto an adjacent site. When enabled, the terminal will continue the same call while roaming, with almost no noticeable audio hole.

Limitations

The following limitations apply:

• It works on all voice call types but not data calls.
• It works on both Capacity Max Open System Mode and Capacity Max Advantage Mode but not Capacity Max Open Radio Mode.
• It will work only if the current and roamed sites which have IP connectivity to the Trunking Controller.
• The home site and all adjacent sites must have different frequencies.
• The home site and all adjacent sites must have the same color code.

Operation

While in a call, the terminal periodically measures the signal strength of its current payload channel. If the signal strength drops below the Roaming RSSI Threshold of the home site, the terminal samples the signal strength of all adjacent site payload channels that carry the same call.

The terminal will conduct a call handover if the signal of the home payload channel is lower than the Handover RSSI Threshold value for the home site and there is at least one adjacent site payload channel carrying the group call that has a signal strength of more than 6dB greater than the current (home) payload channel.

If the conditions are not fulfilled, the radio continues without any handover.

A failure in the current payload channel for a receiving radio can also initiate a call handover, as long as the signal conditions are fulfilled. If not, the radio drops the call and initiates rejoining the call at the (same) home site.

When a call handover is successful, the radio acquires a new site, which becomes its new home site. The radio then rebuilds a new set of adjacent payload channels based on the announcement in the new home payload channel. The new set is sampled and used for subsequent call handover. A successful call handover enables the radio to join the call at the new site with a reduced audio hole. The radio joins the call without registration, which occurs when the call ends, or when the subsequent handover fails.

When a transmitting or receiving terminal performs a call handover to an adjacent site with no existing payload channel for the ongoing call, one of the following things happen:

• If the radio is entitled to new channel resources in this roaming site, the system allocates available channel resources.
• If the radio is not entitled to new channel resources (due to affiliation, talkgroup site restrictions or Subscriber Access Control restrictions), the system does not allocate new channel resources. If the system has been configured to use Fast Start as the service setup method, the probability of not entitling the radio to a new channel increases.

If a payload channel was allocated for the ongoing call before the radio roams into the site, for example due to another radio or a static talkgroup/site affiliation, the radio continues using that channel and completes the call.

Configuration

Seamless Call Handover feature is configured in the Radio Management (RM) application. The tick-box can be found in the Capacity Max System Set. Software release M2020.01 is required in the terminals; CMSS and repeaters in order for this feature to work.