Bluetooth has been designed to operate in noisy radio frequency environments, and uses a fast acknowledgement and frequency-hopping scheme to make the link robust, communication-wise. Bluetooth radio modules avoid interference from other signals by hopping to a new frequency after transmitting or receiving a packet.
Compared with other systems operating in the same frequency band, the Bluetooth radio typically hops faster and uses shorter packets. This is because short packages and fast hopping limit the impact of microwave ovens and other sources of disturbances. Use of Forward Error Correction (FEC) limits the impact of random noise on long-distance links.
Considering the figure 1, one realizes that hopping out-and-into a continuous range of frequencies that are subject to noise gives the communications link a better chance to remedy transmission errors when out of the disturbed frequencies, than would have been the case if the transmission had stayed for several timeslots within the noisy frequency range. This argument does not hold true if the noise is spread over the whole frequency range in a random fashion, but sources of electro-magnetic noise usually give the pattern illustrated in figure 2.
Whenever a connection (a "piconet") is first established between 2 (or more) units, the Master-unit establishes a frequency-hopping scheme, which is communicated to the other units. This frequency selection scheme consists of two parts:
- selecting a sequence;
- mapping this sequence onto the hop frequencies.
The mapping from the input to a particular hop frequency is performed in the selection box. Basically, the input is the native clock and the current address. In the CONNECTION state, the native clock is modified by an offset to equal the master clock. Only the 27 MSBs of the clock are used. In the page and inquiry substates, all 28 bits of the clock are used. However, in the PAGE substate the native clock will be modified to the master estimate of the paged unit.
The frequency-hopping schemes
10 types of frequency-hopping sequences are defined ?5 for the 79-hop and 5 for the 23-hop system. These sequences are:
- A page hopping sequence with 32 (16) unique wake-up frequencies, distributed equally over the 79 (23) MHz, with a period length of 32 (16).
- A page response sequence covering 32 (16) unique response frequencies that are all in a one-to-one correspondence to the current hopping sequence. Master & Slave use different rules to obtain the same sequence.
- An inquiry sequence with 32 (16) unique wake-up frequencies, distributed equally over the 79 (23) MHz, with a period length of 32 (16).
- A inquiry response sequence covering 32 (16) unique response frequencies that are all in a one-to-one correspondence to the current inquiry hopping sequence.
- A channel hopping sequence with very long period, which does not show repetitive patterns over a short time interval, but which distributes the hop frequencies equally.
