Automatic Frequency Selection
The operator may configure up to 20 different frequencies at the BSU and SUs. If the BSU detects a loss in performance on the frequency being used currently, and if this problem cannot be bought under control with ARQ, Adaptive Coding and Modulation or the other mechanisms in place, then it initiates a frequency change process. Once the BSU changes frequency, the SU automatically initiates a frequency search process until it locks on to the new frequency that the BSU is using. This mechanism is useful in the following situations:
- When deployed in the License Exempt bands such as the 5 GHz bands, AFS provides a way for the system to automatically change frequencies if the current frequency becomes un-usable due to interference.
- AFS can be used to provide automatic backup in case the primary BSU fails and there is a secondary BSU present. In this case, the SUs will automatically lock to the signal coming from the secondary BSU if they detect a loss of signal from the primary BSU.
In the license-exempt wireless spectrum, Aperto developed AFS to allow delivery of business quality services. Using AFS, a service provider can adjust the level of interference that triggers a switch to another frequency, and can determine the time allowed to mitigate the problem before the frequency switch is made. In addition, AFS may be used to move from one licensed or unlicensed frequency to another, to balance the load within a cell. Subscriber units do not have to be reset, and inactive subscriber units will automatically start using the new frequency upon activation.