Pre-empted channel selection algorithm
Original Publication Date: 2010-Sep-21
Included in the Prior Art Database: 2010-Sep-21
When a high priority call pre-empts an active call in a radio system, there is no guarantee that the high priority radio will be able to transmit over the pre-empted radio. The high priority radio may not have a strong enough radio signal to overcome the pre-empted radio. This invention uses Received Signal Strength Indication (RSSI) to maximize the probability of success for a pre-empting high priority call.
Pre-empted Channel Selection Algorithm
By James E. Streed
Global ASTRO Product Solutions
Emergency communications in mission critical radio systems has to be serviced immediately – lives are at stake. If a radio system is busy, it has to determine which active call to pre-empt in order to complete a high priority call. Since the pre-empted radio is not aware it is being pre-empted, the high priority radio has to have a strong enough signal to transmit over the pre-empted radio. This invention uses Received Signal Strength Indication (RSSI) to identify the channel on which the high priority radio is most likely to be heard.
Public safety two-way radio systems are typically designed to service the largest voice call load the public safety organization can envision. When first responder safety is at risk, voice communication availability is critical.
Even so, during busy times or catastrophic events, all of the channel resources at a radio site may be occupied when a new critical voice call must be serviced. To ensure that a high priority voice call, such as an emergency call, can be completed when no unassigned radio channels are available, the radio system pre-empts a call in progress. This pre-emption ensures that the scarce radio channel resources are used for the most important calls.
Pre-emption decisions can be based on any combination of factors. The radio system may pre-empt the oldest call or the lowest priority call, or the radio system may merely use the next channel in its call assignment list.
When a high priority call pre-empts an active call, there is no guarantee that the high priority radio will be able to be heard over the pre-empted radio. The pre-empted radio is not aware that it is being pre-empted, so it does not stop transmitting. If the high priority radio has a lower signal strength than the pre-empted radio, the high priority radio will not be heard by its talkgroup members.
The current methods for choosing a call to pre-empt do not account for the probability that the high priority radio has enough signal strength to overpower the radio it is pre-empting.
To maximize the probability that a pre-empting radio will be successful in using the channel to which it is assigned, the call controller assigns the pre-empting radio to the channel with the lowest Receive Signal Strength Indication value.
All channels that are involved in a call send periodic Receive Signal Strength Indication (RSSI) messages to the call controller. The RSSI value indicates the relative strength of the mobile radio’s signal received by the channel. When a high priority call request requires a pre-emption of an active call, the call controller chooses the channel with the lowest RSSI reading.
In multi-site radio systems, the radio sites involved in a call are designated as either source sites or destination sites for a call. The source site is where the transmitting mobile radio is located. A destina...