APPLICATION NOTEFrequency Agility Protocol for nRF24XXnAN24-07NORDICFrequency Agility Protocol for nRF24XXL GeneralThis application note describes the low-level Frequency Agility Protocol for nRF24XX This is a protocol that gives protection against disturbing traffic from frequency stationary systems like WLAN and frequency hopping devices like Bluetooth.The protocol is generic and can be used in many different systems that require resistance against disturbance from other systems. By using this protocol, a system can operate in close proximity with systems using different WLAN channels, 2.4GHz cordless phones. 2,4GHz remote controls, microwave ovens, Bluetooth devices and other proprietary 2,4GHz systems. Basing its functionality on recovery by retransmission of lost packets, it will be much more reliable than a uni-directional communication protocol.This document describes the assumptions that the protocol is based on, a description of the functionality, implementation described with state machine diagrams, current consumption calculations and C-code examplesA 2.4GHz-mouse/keyboard application is a typical 2.4GHz application that will be used in proximity with WLAN and Bluetooth nodes This document will for that reason use the wireless mouse/keyboard application when describing the Frequency Agility Protocol for nRF24XX.a aFigure 1: Wireless 2.4GHz mouse/keyboard applicationAs seen in Figure 1, a wireless 2.4GHz mouse/keyboard application consists of a wireless mouse, a wireless keyboard and a receiver unit for the PC side, referred to as a dongle” in the rest of this document.2. AssumptionsThe frequency agility protocol is built on a series of assumptions regarding mouse / keyboard applications and the traffic in the 2.4GHz bandThe traffic in the 2.4GHz band is mainly consistent of frequency stationary systems like WLAN and frequency-hopping systems like Bluetooth. While frequency stationary systems operate in a specific part of the band, frequency-hopping systems will generate traffic in the whole band All traffic generated by systems operating in the 2.4GHz band is packet based.At a given channel in the 2.4GHz band, if a frequency hopping system is present, the likelihood of a collision with traffic from that system is the same in every channel. It is therefore no use in changing the operating channel if disturbed by a frequency hopping system. If the disturbance comes from a frequency stationary system, it is possible to move in such manner that the likelihood for a collision with the same system on the new channel is minimal.A mouse will require a much higher update rate than a keyboard 1( is assumed that when a mouse is used, it should be updated every 8lh millisecond. The mouse will therefore have priority in front of the keyboard regarding updatesThe disturbance from other systems will be strongest close to the PC, and the dongle attached to the PC will suffer the most.3. Frequency agility protocol for a 2.4GHz mouse/keyboard applicationBased on the previous assumptions the definition of the frequency agility protocol emerges: A protocol that will move own traffic to another channel in the 2.4G Hz band if a stationary disturhance occurs at the currently lined frequency. MThe main functionality of the frequency agility protocol will be to: Detect stationary disturbance Move in such manner that new disturbance from the same source will not occur Do not move if disturbed by a frequency hopping source Give priority to mouse traffic.It is important to notice that this protocol will only force a change in operating frequency when a stationary disturbance occurs After it has changed the operating frequency, it will be on the new channel for a relative long time.The frequency agility protocol functionality is based on the communication between the mouse and the dongle. When the mouse is in use, it will send a packet to the dongle every 8lh millisecond and wait for acknowledge The mouse will re-send a packet up lo two times if no acknowledgement has been received. Bluetooth will stay up to 650 microseconds on one channel before hopping This means that if a Bluetooth system is knocking out the mouses first attempt to send a packet, the next two should get trough since each packet 一 acknowledgement cycle takes about one millisecond. Il is therefore not likely that a frequency hopping system will cause a change in frequencyIf all three attempts to send a packet fail, the mouse and dongle will change channel according to a table. The table is built up to take care of the functionality that avoids disturbance from the same source at the new channel. Figure 2 shows a typical table with channels used by the frequency agility protocol. The table is WLAN weighted/* meaning it will find the next channel outside of the assumed WLAN channel that is disturbing the currently