Architecture - Baseband

Bluetooth Baseband

General Description

The Bluetooth Baseband is the part of the Bluetooth system that specifies or implements the medium access and physical layer procedures between Bluetooth devices

Two or more devices sharing the same physical channel form a piconet. One Bluetooth device acts as the master of the piconet, whereas the other device(s) act as slave(s). Up to seven slaves can be active in the piconet. Additionally, many more slaves can remain connected in a parked state.



Piconets with a single slave operation (a), a multi-slave operation (b) and a scatternet operation (c).

Packets

Data is transmitted over the air in packets. The symbol rate for all modulation schemes is 1 Ms/s. The gross air data rate is 1 Mbps for Basic Rate.


Standard Basic Rate packet format.

Enhanced Data Rate has a primary modulation mode that provides a gross air data rate of 2 Mbps, and a secondary modulation mode that provides a gross air data rate of 3 Mbps.


Standard Enhanced Data Rate packet format

BluetoothClock

Every Bluetooth device has a native clock that shall be derived from a free running system clock. For synchronization with other devices, offsets are used that, when added to the native clock, provide temporary Bluetooth clocks that are mutually synchronized.

Bluetooth Device Addressing

Each Bluetooth device is allocated a unique 48-bit Bluetooth device address (BD_ADDR) obtained from the IEEE Registration Authority.

Access Codes

In the Bluetooth system all transmissions over the physical channel begin with an access code. Three different access codes are defined:

• device access code (DAC)
• channel access code (CAC)
• inquiry access code (IAC)

Physical Channels

Physical Channel Definition

Physical channels are defined by a pseudo-random RF channel hopping sequence, the packet (slot) timing and an access code. The hopping sequence is determined from the Bluetooth device address and the selected hopping sequence. The phase in the hopping sequence is determined by the Bluetooth clock. All physical channels are subdivided into time slots whose length is different depending on the physical channel.

Basic Piconet Physical Channel

The basic piconet physical channel is defined by the master of the piconet. The master controls the traffic on the piconet physical channel by a polling scheme.

By definition, the device that initiates a connection by paging is the master. Once a piconet has been established, master-slave roles may be exchanged.

The basic piconet physical channel is divided into time slots, each 625 μs in length.

Adapted Piconet Physical Channel

Adapted piconet physical channels can be used for connected devices that have adaptive frequency hopping (AFH) enabled. There are two distinctions between basic and adapted piconet physical channels. The first is the same channel mechanism that makes the slave frequency the same as the preceding master transmission. The second aspect is that the adapted piconet physical channel may be based on less than the full 79 frequencies of the basic piconet physical channel.

Page Scan Physical Channel

Although master and slave roles are not defined prior to a connection, the term master is used for the paging device (that becomes a master in the CONNECTION state) and slave is used for the page scanning device (that becomes a slave in the CONNECTION state).

The page scan physical channel follows a slower hopping pattern than the basic piconet physical channel and is a short pseudo-random hopping sequence through the RF channels.

Inquiry Scan Physical Channel

Although master and slave roles are not defined prior to a connection, the term master is used for the inquiring device and slave is used for the inquiry scanning device.

The inquiry scan channel follows a slower hopping pattern than the piconet physical channel and is a short pseudo-random hopping sequence through the RF channels.

Hop Selection

In total, six types of hopping sequence are defined − five for the basic hop system and one for an adapted set of hop locations used by adaptive frequency hopping (AFH). These sequences are:

• A page hopping sequence with 32 wake-up frequencies distributed equally over the 79 MHz, with a period length of 32;
• A page response hopping sequence covering 32 response frequencies that are in a one-to-one correspondence to the current page hopping sequence. The master and slave use different rules to obtain the same sequence;
• An inquiry hopping sequence with 32 wake-up frequencies distributed equally over the 79 MHz, with a period length of 32;
• An inquiry response hopping sequence covering 32 response frequencies that are in a one-to-one correspondence to the current inquiry hopping sequence.
• A basic channel hopping sequence which has a very long period length, which does not show repetitive patterns over a short time interval, and which distributes the hop frequencies equally over the 79 MHz during a short time interval.
• An adapted channel hopping sequence derived from the basic channel hopping sequence which uses the same channel mechanism and may use fewer than 79 frequencies. The adapted channel hopping sequence is only used in place of the basic channel hopping sequence. All other hopping sequences are not affected by hop sequence adaptation.

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Physical Links

A physical link represents a baseband connection between devices. A physical link is always associated with exactly one physical channel. Physical links have common properties that apply to all logical transports on the physical link. The common properties of physical links are:

• Power control
• Link supervision
• Encryption
• Channel quality-driven data rate change
• Multi-slot packet control

Logical Transports

Between master and slave(s), different types of logical transports may be established. Five logical transports have been defined:

• Synchronous Connection-Oriented (SCO) logical transport
• Extended Synchronous Connection-Oriented (eSCO) logical transport
• Asynchronous Connection-Oriented (ACL) logical transport
• Active Slave Broadcast (ASB) logical transport
• Parked Slave Broadcast (PSB) logical transport

Logical Links

Five logical links are defined:

• Link Control (LC)
• ACL Control (ACL-C)
• User Asynchronous/Isochronous (ACL-U)
• User Synchronous (SCO-S)
• User Extended Synchronous (eSCO-S)

The control logical links LC and ACL-C are used at the link control level and link manager level, respectively. The ACL-U logical link is used to carry either asynchronous or isochronous user information. The SCO-S, and eSCO-S logical links are used to carry synchronous user information. The LC logical link is carried in the packet header, all other logical links are carried in the packet payload. The ACL-C and ACL-U logical links are indicated in the logical link ID, LLID, field in the payload header. The SCO-S and eSCO-S logical links are carried by the synchronous logical transports only; the ACL-U link is normally carried by the ACL logical transport; however, it may also be carried by the data in the DV packet on the SCO logical transport. The ACL-C link may be carried either by the SCO or the ACL logical transport.

Packets

The general Basic Rate packet consists of 3 entities: the access code, the header, and the payload.

The general Enhanced Data Rate packet consists of 6 entities: the access code, the header, the guard period, the synchronization sequence, the Enhanced Data Rate payload and the trailer. The access code and header use the same modulation scheme as for Basic Rate packets while the synchronization sequence, the Enhanced Data Rate payload and the trailer use the Enhanced Data Rate modulation scheme. The guard time allows for the transition between the modulation schemes.

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Bitstream Processing

Before the payload is sent over the air interface, several bit manipulations are performed in the transmitter to increase reliability and security. An HEC is added to the packet header, the header bits are scrambled with a whitening word, and FEC coding is applied. In the receiver, the inverse processes are carried out.

Link Controller Operation

The figure to the left shows a state diagram illustrating the different states used in the link controller. There are three major states: STANDBY, CONNECTION, and PARK; in addition, there are seven substates, page, page scan, inquiry, inquiry scan, master response, slave response, and inquiry response. The substates are interim states that are used to establish connections and enable device discovery. To move from one state or substate to another, either commands from the link manager are used, or internal signals in the link controller are used (such as the trigger signal from the correlator and the timeout signals).

Audio

On the air-interface, either a 64 kb/s log PCM (Pulse Code Modulation) format (A-law or μ-law) may be used, or a 64 kb/s CVSD (Continuous Variable Slope Delta Modulation) may be used. The latter format applies an adaptive delta modulation algorithm with syllabic companding. The voice coding on the line interface is designed to have a quality equal to or better than the quality of 64 kb/s log PCM. The table below summarizes the voice coding schemes supported on the air interface.

Voice Codecs

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