Physical interface

Frequency band

Bluetooth operates in the unlicensed 2.4 GHz ISM (Industrial Scientific Medicine) band, which uses the range 2400 MHz to 2483.5 MHz.

It uses 79 frequency channels, spaced 1 MHz apart. Channel n (where n is 0...78) uses a carrier frequency of 2402+n MHz.

Modulation

1 MBit/sec (GFSK)

The original, or Basic Rate, modulation scheme. Uses GFSK (Gaussian Frequency Shift Keying) modulation:

Logic '1' represented by a positive frequency deviation (of at least 115 kHz, nominally 160 kHz).
Logic '0' represented by a negative frequency deviation (of at least 115 kHz, nominally 160 kHz).

Symbols are transmitted at 1 Mbaud, with one bit per symbol, giving a raw data rate of 1 MBit/sec.

2 MBit/sec (π/4-DQPSK)

One of two EDR (Enhanced Data Rate) modulation schemes that may be used for the packet payload (the header always uses GFSK modulation). Uses square root raised cosine filtered π/4-DQPSK (π/4 rotated Differential encoded Quaternary Phase Shift Keying):

Each symbol is encoded by the phase shift relative to the last symbol (the Differential part of the modulation scheme). There are four possible phase shifts, allowing two bits of data to be encoded per symbol.

Symbols are transmitted at 1 Mbaud, with two bits per symbol, giving a raw data rate of 2 MBit/sec.

3 MBit/sec (8DPSK)

One of two EDR (Enhanced Data Rate) modulation schemes that may be used for the packet payload (the header always uses GFSK modulation). Uses square root raised cosine filtered 8DPSK (Differential encoded 8-ary Phase Shift Keying):

Each symbol is encoded by the phase shift relative to the last symbol (the Differential part of the modulation scheme). There are eight possible phase shifts, allowing three bits of data to be encoded per symbol.

Symbols are transmitted at 1 Mbaud, with three bits per symbol, giving a raw data rate of 3 MBit/sec.

Time slots

Each frequency channel is divided into 1600 time slots per second; each slot is 625 µs long. Each data packet may use between 1 and 5 slots.

Each data packet is transmitted on a different frequency channel, following a pseudo-random hopping sequence determined by the device address of the master device. The position in the sequence is determined by a 27-bit counter in the master device.

The master device may only start transmitting on an even numbered slot. A slave device may only start transmitting on an odd numbered slot, following a message from the master which was directed specifically to that slave.