BT fundamental and FDD/TDD Concept

1. BT Radio Interface

With many other users on the ISM band from microwave ovens to Wi-Fi, the hopping carrier enables interference to be avoided by BT devices.
A BT transmission only remains on a given frequency for a short time, and if any interference is present the data will be re-sent later when the signal has changed to a different channel which is likely to be clear of other interfering signals. The standard uses a hopping rate of 1600 hops /second, and the system hops over all the available frequencies using a pre-determined pseudo-random hop sequence based upon the BT address of the master node in the network.

}During the development of the BT standard it was decided to adopt the use of frequency hopping system rather than a direct sequence spread spectrum approach because it is able to operate over a greater dynamic range.

}If direct sequence spread spectrum techniques were used then other transmitters nearer to the receiver would block the required transmission if it is further away and weaker.

2. BT Channel allocation

}BT frequencies are all located within the 2.4 GHz ISM band. The ISM band typically extends from 2400 MHz to 2483.5 MHz (i.e. 2.4 ~ 2.4835 GHz).The BT channels are spaced 1 MHz apart, in the range 2402 ~ 2480 MHz. This can be calculated as 2401 + n, where n = 1 ~ 79.

}In France, Japan and Spain, the hop sequence has to be restricted to only 23 frequencies because of the ISM band allocation is smaller.

}There are also some BT frequency accuracy requirements for BT transmissions. The transmitted initial centre frequency must be within ±75 kHz from the receiver centre frequency. The initial frequency accuracy is defined as being the frequency accuracy before any information is transmitted and as such any frequency drift requirement is not included.

3. BT FDD/TDD Method comparison

FDD : Frequency Division Duplexing
TDD : Time Division Duplexing

4. BT Power Level

The transmitter powers for BT are quite low, although there are three different classes of output dependent upon the anticipated use and the range required : 

	Range	Max Pwr	Power Control
Power Class 1	Long	20dBm	Must
Power Class 2	Normal	6dBm	Optional
Power Class 3	Short	0dBm	Optional