Industrial, Scientific and Medical Band (ISM)
Bluetooth
Bluetooth Air Interface
Bluetooth Application

Development of the Bluetooth Standard

Wireless Personal Area Network (WPAN)

Driven by the feature phone, the mobile phone was connected to a number of other devices via cable:

• PC: For data backup and to provide modem functionality
• Headphones/Microphone: For conducting conversations without holding the phone or for listening to music
• Hands-free system: For making hands-free phone calls in the car

Wireless transmission already existed in the PC sector. Computer mice that were connected to the PC via radio had been available since 1991. However, this was done using a simple analog radio standard.

Wireless transmission in the immediate vicinity of the end device is generally called Wireless Personal Area Networks (WPAN).

Industrial, Scientific and Medical band (ISM)

Like all other wireless systems also a WPAN system with low power transmission requires a frequency band that is approved by the authorities for radio application. Not every user of a WPAN system should apply for a license. Therefore the authorities provide license-free bands. Of course within this band the systems require to follow certain requirements e.g. low transmission power. Such a license free band is the ISM band (Industrial, Scientific and Medical).

One license free ISM band is for example the range around 27 MHz is known, which can be used for walking talkies. This was very popular as CB radio in the eighties. The same frequency could also be used for baby monitors. Another license free band at 433 MHz is still in use. It has long been used for analog wireless headphones, for outdoor thermometers or for car keys.

The most important ISM band is around 2.4 GHz. The fact that it exists is mainly due to the invention of the microwave oven. When radar was developed in the 1940s, it was discovered that food heated up in the radar’s microwave radiation. The reason is that at a frequency of around 2.4 GHz, water molecules absorb the electrical energy through torsional vibration. This in turn causes the water to warm up accordingly. At this frequency it was also particularly cheap to generate microwaves using so-called magnetrons. Therefore the 2.4 GHz band was unlicensed to be used for microwave ovens.

Bluetooth

In the early 1990s, Nils Rydbeck was technical manager of Ericsson Mobile, the division of Ericsson that manufactured mobile communications devices. He wanted a wireless connection to a headset specifically for cell phones. Of course, this connection should/had to be very small and very energy efficient. Two engineers who worked for Ericsson then created a new digital radio system which was introduced in 1997.

In the development of the IBM Labtops (ThinkPad), driven by the progress of the GSM standard, people worked on ways to integrate a radio modem into a labtop. They worked with Ericsson, but studies concluded that the power consumption was still too high for integration, at least at that time. Finally, the idea arose to establish a wireless digital connection between the laptop and the cell phone. Ericsson would introduce its new radio system and IBM would supply the corresponding protocol SW. Intel was also included as the leading CPU supplier. An engineer from Intel provided a name for the joint project: Bluetooth. This name comes from a legendary Danish king named Bluetooth who united Denmark and Norway in the tenth century. It was assumed that the new standard would eventually get a new name or abbreviation, but in the end the project name became the name of the standard. A consortium was soon formed to take care of the further development of this standard. It was called the Bluetooth Special Interest Group (SIG).

In 1999, the first headset with Bluetooth was introduced. The first mobile phone with integrated Bluetooth was already available in 2001, of course an Ericsson: T36. At the same time, IBM also brought the first PC with a Bluetooth interface onto the market.

Bluetooth technology and air interface

Bluetooth is a digital radio standard. Like GSM it uses a carrier frequency to modulate digital signals. In its first versions of Bluetooth, Gaussian minimum shift keying (GMSK) was used, the same modulation as used in GSM. With 1 MHz, the channel bandwidth is significantly higher than with GSM and therefore allows bit rates of 780 kbit/s. However, there is only one channel per Bluetooth connection or network (see below). There is also no uplink and a downlink channel like with GSM. All connections must share the same channel. For this purpose it is divided into time slots with a length of 625 us.

The channels are in the ISM band between 2400 MHz and 2483.5 MHz. This results in 79 available channels. But it must kept in mind, that other systems also use this band and interfere with Bluetooth, especially the microwave oven and later the WLAN standard. Even worse, other Bluetooth connections must transmit in the same band and there is no central element that monitors different Bluetooth connections. For this reason, frequency hopping was used in Bluetooth. This means, after each time slot, a new frequency is selected. This way the Bluetooth signal is distributed across the entire frequency band. This technology is called frequency hopping spread spectrum (FHSS). The frequency jumps are apparently random and prevent strong interference with other systems. In later versions of Bluetooth, frequency hopping was refined. If there was a permanent interference with some channels or channel ranges, for example from a competing 2.4 GHz device, these could be detected and bypassed. This was then called „adaptive frequency hopping“.

Bluetooth can establish connections between two devices (e.g. between a cell phone and a headset). But it is also possible to set up a small network, e.g. PC with a connection to a mouse, keyboard and headset. In any case, a device (usually a cell phone or PC) takes control. These devices are called „masters“. All devices that are connected are then called „slaves“. With Bluetooth, a piconet can consist of a master and up to 7 slaves.

Two types of data transfer have been defined. A synchronous mode (like the normal traffic channel in GSM) in which (voice) data is sent continuously or an asynchronous mode in which data packets are sent.

In synchronous mode, SCO (Synchronous Connection Oriented), slots (bursts) are sent and received at regular intervals in such a way that a data rate of exactly 64 kbit/s are established. This corresponds to standard speech sampling at 8 kSample/s. Depending on the transmission quality, error correction can be used, which of course increases the number of raw data. It is also possible to transmit control data, for example to control the volume in a headset.

In asynchronous mode, ASC (Asynchrounous Connection-Less), the master sends 1 – 5 slots to a slave. This responds in the following slot or after 5 slots at the latest. The answer can also be 1 – 5 slots long. The master can then send further data slots. A slave must respond in any case, even with an empty slot, which provides an acknowledgment of reception. If the master stops sending because its data buffer is full, it can pause up to 800 slots (0.5 s) and thus save power.

The figure above shows the structure of a Bluetooth slot. The first 72 bits serve as the address or identification of the associated piconet. This is followed by a header consisting of 18 bits. The first three bits of the header indicate the number of the slave to be addressed (7 possible). The following type identifier is important. 4 bits indicate whether it is an SCO or an ASC slot and how this error protection is handled. There are 4 ways to protect or define an SCO and 5 ways to protect a data packet. With the SCO’s it is also determined how many slots are used for transmission (1, 3 or 5). A flow bit signals to the remote station whether a data buffer is full. If the flow bit is set, the other side is asked not to send any more data. The following ARQN bit is used to signal that the last packet was received correctly (error-free). If necessary, the other party must repeat the transmission. The SEQN signals whether entire packets were transmitted correctly. Finally, an error check of the header is carried out with 8 bits to guarantee correct transmission.

In order to set up a pico network, the end device must know all participants so that it can address them via the access code. The Bluetooth standard uses a so-called pairing for this purpose. In this procedure, a terminal device looks for possible slaves and can establish a connection with them. A peripheral device can then be connected to the terminal through interaction with the user.

Bluetooth Applications

In 1999, the first headset with Bluetooth was introduced. The first mobile phone with integrated Bluetooth was already available in 2001, of course an Ericsson: T36. A headset, the HBH-10, was also released along with the mobile phone. At the same time, IBM also brought the first PC with a Bluetooth interface onto the market.

Bluetooth was not an immediate success. It took a few years for Bluetooth to become established as an application. However, it soon became a necessary feature of a high-quality cell phone. More and more Bluetooth enabled phones were sold. There were more cell phones with Bluetooth interface than end devices like headsets. Bluetooth became an “expensive” feature. In general, Bluetooth headsets were not very popular. Years later, Bluetooth became more important for connecting a cell phone to the car and a built-in hands-free system. In the mid 2000, young people also began to discover Bluetooth, but more as a tool for exchanging data with each other.