Bluetooth baseband structure

General description

The Bluetooth® baseband is a part of the Bluetooth system that describes or implements media access and physical layer procedures between Bluetooth devices.

Two or more devices share the same physical channel of the piconet. A Bluetooth device will act as the master device of the piconet, and the other devices will act as slave devices. There can be up to seven active slave devices in a piconet. In addition, other connected slave devices in the network can maintain the sleep state.

A piconet with a single slave (a), multiple slaves (b), and scatternet (c).

data pack

Data is transmitted wirelessly through data packets. The symbol rate of all modulation procedures is 1 Ms/s. The total air data rate for the basic rate is 1 Mbps.

Standard basic rate packet format.

The enhanced data rate has a primary modulation mode with a total air data rate of 2 Mbps and a secondary modulation mode with a total air data rate of 3 Mbps.

Standard enhanced data rate packet format

Bluetooth clock

The local clock of each Bluetooth device is derived from the self-running system clock. Synchronize with other devices and use offset when adding to the local clock to provide a temporary Bluetooth clock synchronized with each other.

Bluetooth device addressing

Each Bluetooth device is assigned a unique 48-bit Bluetooth device address (BD_ADDR), which comes from the IEEE Registration Authority.

Access code

In the Bluetooth system, all transmissions through the physical channel begin with an access code. The defined access codes are divided into three different categories:

1. Device access code (DAC)

2. Channel Access Code (CAC)

3. Ask for access code (IAC)

Physical channel

Physical channel definition

The physical channel is defined by the pseudo-random radio frequency channel frequency hopping sequence, data packet (gap) timing and access code. The frequency hopping sequence is determined by the Bluetooth device address and the selected frequency hopping sequence. The phase of the frequency hopping sequence is determined by the Bluetooth clock. All physical channels can be subdivided into gaps of unequal length, depending on the physical channel.

Basic Piconet Physical Channel

The basic piconet physical channel is defined by the main device of the piconet. The master device controls the flow of the physical channel of the piconet through a polling mechanism. By definition, the device that establishes a connection through a call is the master device. After the piconet is established, the roles of the master and slave devices can be exchanged. The basic piconet physical channel can be divided into gaps each with a length of 625 μs.

Adapt to piconet physical channel

The adaptive piconet physical channel can be used for connected devices that have enabled adaptive frequency hopping (AFH). There are two differences between basic and adapted piconet physical channels. The first point is to use the same channel mechanism to keep the frequency of the slave device consistent with the previous master device transmission. The second point is that the frequency used to adapt the physical channel of the piconet is less than the full 79 frequencies of the physical channel of the basic piconet.

Call scan physical channel

Although the master device and the slave device are not established before the connection is established, the master device is used to refer to the calling device (the master device in the connected state), and the slave device is used to refer to the call scanning device (the slave device in the connected state). The frequency hopping pattern that the call scanning physical channel follows is slower than that of the basic piconet physical channel, and is a shorter pseudo-random frequency hopping sequence generated through the radio frequency channel.

Ask to scan the physical channel

Although the master device and the slave device are not established before the connection is established, the master device is used to refer to the interrogating device, and the slave device is used to refer to the interrogating scanning device. The frequency hopping pattern followed by the inquiry scan channel is slower than that of the basic piconet physical channel, and is a shorter pseudo-random frequency hopping sequence generated through the radio frequency channel.

Frequency hopping frequency decision

A total of six frequency hopping sequences are defined, five are used for basic frequency hopping systems, and one is used for adaptive frequency hopping position sets used for adaptive frequency hopping (AFH). The six types of frequency hopping are:

1. Call frequency hopping sequence, 32 wake-up frequencies, evenly distributed in 79 MHz, cycle length is 32;

2. The call response frequency hopping sequence, including 32 response frequencies, corresponds to the existing call frequency hopping sequence one-to-one; the master device and the slave device use different rules to obtain the same sequence;

3. Query the frequency hopping sequence, 32 wake-up frequencies, evenly distributed in 79 MHz, and the period length is 32;

4. Query response frequency hopping sequence, including 32 response frequencies, corresponding to the existing query frequency hopping sequence;

5. The basic channel frequency hopping sequence has a very long period, and there will be no repetitive pattern in a short period of time. The frequency hopping is evenly distributed in 79 MHz in a short period of time;

6. The adaptive channel frequency hopping sequence is derived from the basic channel frequency hopping sequence, and the same channel mechanism is used, and the frequency used may be less than 79; the adaptive channel frequency hopping sequence is only used to replace the basic channel frequency hopping sequence. All other frequency hopping sequences are not affected by frequency hopping sequence adaptation.

Physical link

The physical link refers to the baseband connection between devices. A physical link is always associated with a physical channel. The general attributes of the physical link apply to all logical transmissions on the physical link. The general attributes of physical links include:

1. Power Control

2. Link Supervision

3. Encryption

4. Channel quality drives data rate changes

5. Multi-slot packet control

Logical transmission

Different types of logical links can be established between the master device and the slave device. The five logical transmissions defined are:

1. Synchronous connection (SCO) logic transmission

2. Extended synchronous connection (eSCO) logical transmission

3. Asynchronous connection (ACL) logical transmission

4. Active Slave Broadcast (ASB) logical transmission

5. Sleep Slave Broadcast (PSB) logical transmission

Logical link

The five logical links defined are:

1. Link Control (LC)

2.ACL control (ACL-C)

3. User asynchronous/isochronous (ACL-U)

4. User synchronization (SCO-S)

5. User extended synchronization (eSCO-S)

The control logic link LC and ACL-C are used on the link control and link manager respectively. The ACL-U logical link is used to carry 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, and all other logical links are carried in the packet payload. ACL-C and ACL-U logical links are displayed in the payload header ID and LLID fields in the logical link. SCO-S and eSCO-S logical links are only carried by synchronous logical transmission; ACL-U links are generally carried by ACL logical transmission; however, it can also be carried by data in DV packets on SCO logical transmission. The ACL-C link can be carried by SCO or ACL logical transmission.

data pack

A general basic rate packet consists of three parts: access code, header, and payload.

The general enhanced data rate data packet contains 6 parts: access code, header, guard period, synchronization sequence, enhanced data rate payload and tail. The access code and header use the same modulation method as the basic rate data packet, while the synchronization sequence, enhanced data rate payload and tail use enhanced data rate modulation. The guard time allows the transition between the two modulation methods.

Stream processing

Before the payload is sent out through the radio interface, multiple bit operations are performed in the transmitter to increase reliability and security. HEC is added to the data packet header, the header bits are scrambled and encrypted with whitened words, and FEC decoding is adopted. The reverse process takes place in the receiver.

Link controller operation

The figure shows the state diagram of the link controller in different states. There are three main states: standby, connected and hibernation. In addition, it also includes seven sub-states: call, call scan, inquiry, inquiry scan, master device response, slave device response and inquiry response. The sub-state is a temporary state used to establish a connection and enable device discovery. To switch from one state or sub-state to another state or sub-state, you can use the instructions of the link manager or the internal signals of the link controller (such as the trigger signal and timeout signal of the correlator)

Audio

64 kb/s logarithmic pulse code modulation (PCM) format (A law or μ law) or 64 kb/s continuously variable slope delta modulation (CVSD) can be used in the air interface. The latter format uses adaptive delta modulation algorithm and performs syllable compression and extension. The voice coding of the line interface is designed to have a quality equal to or higher than that of 64 kb/s logarithmic PCM. The following table summarizes the voice encoding methods supported by the air interface.
Voice codec