4.1. Frequency Bands and Channel Arrangement

The Bluetooth system operates in the license-free 2.4 GHz ISM (Industrial Science Medial) band using frequency hopping spread spectrum (FHSS). In the vast majority of countries around the world this frequency band is 2400 - 2483.5 MHz. Some countries have, however, national limitations on the frequency range. In order to comply with these national limitations, special frequency hopping algorithms have been specified for these countries. It should be noted that products implementing the reduced frequency band will not work with products implementing the full band. Products implementing the reduced frequency band must therefore be considered local versions.

The Bluetooth frequency band is divided into distinct channels with 1 MHz channel spacing. In order to comply with out-of-band regulations in each country, a guard band is used at the lower and upper band edge. The frequency range is 2.400 - 2483.5 MHz, and the corresponding channels are f = 2402 + k MHz; k = 0 - 78. Transmission utilizes channel hopping over the specified range at 1600 kHz hop frequency. When operating in countries that permit the use of only a subset of the overall spectrum, transmission utilizes only the approved portions of the spectrum. The Bluetooth system utilizes Gaussian frequency shift keying (GFSK). The signaling rate is 1 Mbit/s.

4.2. Power Considerations

The Bluetooth system transceivers are classified into three power classes to support different link ranges.

• Power Class 1. Output power is 1 - 100 mW (0 - 20 dBm) with mandatory power control ranging from 4 to 20 dBm.

• Power Class 2. Output power is 0.25 - 2.5 mW (-6 - +4 dBm) with optional power control.

• Power Class 3. Output power is less than 1 mW (0 dBm) with optional power control.

Bluegiga's WRAP products support a 100 meter link range with Option 1 (Power Class 1).

4.3. Radio Frequency Propagation

The radio frequency signal propagates in free space as a spherical wave, from a point source to all directions equally. In reality, the actual signal source always differs from a theoretic isotropic signal source. The power distribution of wireless telecommunication equipment in space is determined by the antenna radiation pattern. In free space the signal propagates with the speed of light and attenuates with 1/r2 relation. In reality, the environment always differs from free space. The propagation environment of wireless telecommunication equipment is restricted by all obstacles.

The basic mechanism of radio propagation is attributed to reflection, diffraction, and scattering depending on existing obstacles. Since the radio frequency signal propagates omnidirectionally, the transmitted signal arrives at the receiver following multiple paths deformed by the aforementioned propagation mechanisms. The received signal is the superposition of attenuated and delayed replicas of the transmitted signal, leading to fading of the transmitted signal and broadening of the duration of the transmitted pulse. The transmitted pulse delay spread leads to inter-symbol interference (ISI) because the subsequent symbols interfere with each other. The ISI leads to a bit error probability (BIT) floor that is independent of the signal to noise ratio (SNR). Depending on the time delay spread of the transmitted pulse or the amount of widening that the transmitted pulse experiences across the radio channel, the multipath interference differs. When the time delay spread of the transmitted signal is very small with respect to the signaling time, the multipath interference essentially leads to the signal fading phenomena of the received signal. When the time delay spread of the transmitted signal is high with respect to the signaling time, the multipath interference leads to the symbol interference phenomena of the received signal as well.

A major difference between indoor and outdoor environments is that the former is considerably more sensitive to changes in the geometry of the environment than the latter. This is because of the differences in distance between obstacles. For example, a door being shut rather than open may have a major impact on an indoor environment whereas a comparable event in an outdoor environment may have a minor impact.

The Bluetooth standard has been designed to operate in noisy radio frequency environments. Transmission utilizes fast frequency hopping and short packages to make the link efficient and robust. Fast hopping and short packages limit the impact of interfering devices on the same frequency band.