Bluetooth
Bluetooth wireless technology was created by the Ericsson mobile phone company as an alternative to using wired data cables to connect devices. Today the Bluetooth standard is overseen by the Bluetooth Special Interest Group (SIG). The IEEE previously standardized Bluetooth as IEEE 802.15.1.
Bluetooth operates at frequencies between 2402 and 2480 MHz, or 2400 and 2483.5 MHz including guard bands 2 MHz wide at the bottom end and 3.5 MHz wide at the top.
Frequency Spread:
- 2.4GHZ to 2.4835GHz
Bluetooth is a packet-based protocol with a master/slave architecture. A master BR/EDR Bluetooth device can communicate with a maximum of seven devices in a piconet. The devices can switch roles, by agreement, and the slave can become the master.
Bluetooth is short range. Because the devices use a radio broadcast communications system, they do not have to be in visual line of sight of each other.
- Class 1 Bluetooth range 300ft allocated for industrial use
- Class 2 Bluetooth range 30ft allocated for consumer mobile devices
- Class 3 Bluetooth range 3ft allocated for consumer mobile devices
Most Bluetooth applications are for indoor conditions, where attenuation of walls and signal fading due to signal reflections make the range far lower. Most Bluetooth applications are battery powered Class 2 devices, with little difference in range whether the other end of the link is a Class 1 or Class 2 device as the lower powered device tends to set the range limit.
Bluetooth and Wi-Fi have some similar applications: setting up networks, printing, or transferring files. Wi-Fi is intended as a replacement for high speed cabling for general local area network access in work areas or home. Bluetooth was intended for portable equipment and its applications. Wi-Fi is usually access point-centered. Bluetooth is usually symmetrical, between two Bluetooth devices. Bluetooth serves well in simple applications where two devices need to connect with minimal configuration.
Bluetooth Radio
For Bluetooth transmission 79 RF channels spaced 1 MHz apart are defined. The transmitting frequencies can be calculated by:
f = (2402 + k) MHz k= 0,...,78 (channel number)
In the standard the transmitting power level is classified into three classes: 0 dBm = 1mW, 4 dBm = 2,5 mW and 20 dBm = 100 mW. The requirement for a Bluetooth receiver is an actual sensitivity level of -70 dBm or better.
Bluetooth FCC Regulation
Bluetooth devices must receive the Federal Communications Commission (FCC) CFR47 Telecommunications, Part 15 Subpart C “Intentional Radiators” modular approval in accordance with Part 15.212 Modular Transmitter approval.
Bluetooth and Wifi Coexistence
The problem is that both Bluetooth and Wifi use 2.4GHz. For the radio transmission Bluetooth and WLAN (IEEE 802.11b) both uses the unprotected ISM-band (Industrial, Scientific, Medical) at 2.4 GHz. In the US and in most countries of Europe, a band of 83.5 MHz width is available.
In 802.11b systems the channel bandwidth is 22 MHz. The impact of Bluetooth personal area networks on a WLAN system is a problem. The degree to which an IEEE 802.11b terminal is susceptible to interference from nearby Bluetooth transmitters is clearly dependent upon the strength of the desired DSSS signal from the access point.
- An 11 Mbps DSSS radio can provide reliable service with a narrow band interferer (such as a Bluetooth transmitter) falling within its pass band as long as the Signal-to-Interference Ratio (SIR) is greater than roughly 10 dB.
- There is only about a 25% probability that an active Bluetooth transmitter will be in the DSSS passband on any given hop period.
- Collision is also reduced because a Bluetooth transmitter is only active for 366 msec in each 625 msec Bluetooth hopping period.
- An active Bluetooth device present can drop down of the WLAN throughput rate from about 6 Mbit/s without interferences to 3.5 Mbit/s at a typical packet size of 750 bytes.
The impact of a 20dBm 802.11 Direct-Sequence WLAN system on a 0dBm Bluetooth link is pretty much minimal. When transmitting in its 22 MHz channel, the WLAN system effectively occupies about 17MHz of the 2.45GHz ISM band (20dBm bandwidth). The total amount of power transmitted amounts to 20dBm. When the Bluetooth receiver hops in the WLAN band, it filters out the Bluetooth hop bandwidth. For the Bluetooth receiver, the WLAN signal is regarded as white noise. Assuming a 0.85MHz noise bandwidth in the Bluetooth receiver, a filter suppression of 13dB is achieved.
Under normal traffic conditions in the WLAN (40 emails, 20 file transfers and 1000 Internet accesses per WLAN terminal in 8 hours, resulting in a total transfer of approximately 11 Mbyte per terminal), the Bluetooth voice user is not affected as long as his operating distance remains below 6½ feet. At five times that distance the probability that there is a noticeable interference on the link increases to 8%. A throughput reduction of more than 10% occurs with 24% probability at an operating distance of 32 feet.
| Usage | Wireless Service | Frequency Band MHz |
|---|---|---|
| Wide Area Network (WAN) | WiMAX |
|
| Local Area Network (LAN) | WiFi |
|
| Personal Area Network (PAN) | Bluetooth |
|
