Bluetooth Physical layer | Header and data payload processing
This article describes Bluetooth physical layer modules or blocks. The processing of Bluetooth packet header through bluetooth physical layer and data payload through bluetooth physical layer have been described.
Specifications of Bluetooth technology are developed and maintained by Bluetooth SIG (Special Interest Group). There are various versions of bluetooth starting from V1.2 till V4.1, V4.2, V5.0 and V5.1 (recently published). The bluetooth devices use 2.4 GHz unlicensed frequency band. It uses total 40 channels out of which 3 are used for advertising and 37 are used for data transmission/reception. Bluetooth compliant devices communicate within distance of about 50 meters (indoor) and 150 meters (outdoor).
There are two types of network topologies in bluetooth network viz. piconet and scatternet. Piconet is formed by 1 master and 1 slave or 1 master and multiple slaves. Combinations of multiple piconets is called scatternet. A device can participate in multiple piconets and will timeshare and need to be synchronized with master of current piconet.
The figure depicts Bluetooth protocol stack and different layers. The layer-1 is known as physical layer or baseband layer. Refer Bluetooth protocol stack for functions of each of these layers.
Bluetooth packet format
The Bluetooth packet structure is shown below. It consists of preamble (1 byte), access address (4 bytes), PDU ( max. 257 bytes) and CRC (3 bytes). PDU part carry either advertising packet or data packet. Both of these packet types will have header part and payload part.
Both the header (2 bytes) and payload parts (max. 37 bytes in advertising packet and max. 255 bytes in data packet) are processed separately using bluetooth physical layer modules/blocks.
The packet structure for BR and EDR is shown in the figure.
Header processing through Bluetooth Physical Layer
Following section describes header processing through Bluetooth Physical Layer.
➤(Transmitter Part :)
• As shown in the figure, HEC is generated and added to the packet header part.
• Header bits are scrambled using whitening word.
• FEC encoding is applied to the above scrambled data bits.
• FEC encoded data bits are modulated.
• The pulse shaping is applied before RF up conversion.
• The pulse shaped data is upconverted at 2.4 GHz frequency and transmitted over the air.
➤(Receiver Part :)
• The reverse operations are performed at the bluetooth receiver part viz. RF down conversion, pulse de-shaping, demodulation, FEC decoding, de-whitening, HEC checking before the header is decoded.
Data payload processing through Bluetooth Physical Layer
Following section describes data payload processing through Bluetooth Physical Layer.
➤The payload processing use all the modules as applied for the header processing. In addition, it uses encryption module either E0 or AES-CCM. When E0 encryption is applied, the entire payload should be encrypted. When AES-CCM encryption is applied, only payload body and MIC shall be encrypted; where as payload header and CRC shall not be encrypted.
➤Transmitter part of bluetooth physical layer processing and receiver part of processing are self explanatory as shown in the figure.
Bluetooth Physical Layer Modules or blocks
Following section describes brief of the bluetooth physical layer modules or blocks. For detailed specifications refer
bluetooth specification vol-2, part-B published by Bluetooth SIG.
Error Checking: HEC generation and checking, CRC generation is defined in the standard with their respective generator polynomials.
Data Whitening:The data whitening shall not be applied to synchronization train packets. Both header and payload is scrambled with data whitening word in order to randomize the data from highly redundant patterns. This helps in minimizing DC bias in the bluetooth packet before being applied to FEC encoder module. Whitening word is generated with polynomial g(D) = D7 + D4 + 1 which 221 in octal representation.
Error Correction: The three error correction schemes used in bluetooth are 1/3 rate FEC, 2/3 rate FEC and ARQ scheme. Using ARQ scheme different packet types are transmitted until ack of successful reception is returned by the destination (or timeout is exceeded).
Modulation: There are two main modulation modes viz. basic rate and enhanced data rate. Binary FM modulation is applied to basic rate where as PSK modulation is applied to enhanced data rate. There are two variants of PSK modulation viz. π/4-DQPSK, 8-DPSK. Binary FM modulation is also referred as GFSK (Gaussian Frequency Shift Keying). The symbol rate for all the three modulation types are same i.e. 1 MSym/Sec. The different gross data rates viz. 1 Mbps (for basic rate), 2 Mbps (for EDR using π/4-DQPSK) and 3 Mbps (for EDR using 8-DPSK) can be achieved. For transmission of access code and packet header, GFSK is used. For transmission of sync. sequence, payload and trailer sequence, PSK modulation with data rate of 2 Mbps or 3 Mbps is used.
Pulse Shaping:The square-root raised cosine pulse shaping is applied to generate the equivalent low pass information-bearing signal v(t).
RF frequency conversion: The pulse shaped data signal is up converted as per ISM band carrier frequency (2.4 GHz) out of the 40 assigned bluetooth frequency channels >> as per requirement (i.e. advertising channel or data channel). Adaptive frequency hopping is applied as defined in Vol-2, Part-C, section 4.1.4.
Bluetooth physical layer (i.e. Bluetooth baseband layer, Layer-1) specifications are defined in bluetooth core document Vol-2,Part-B,section 7.
Bluetooth RELATED LINKS
BLE (Bluetooth Low Energy) Links
➤Bluetooth v4.2 versus v5.0 >>
➤Bluetooth v5.0 versus v5.1 >>
➤BLE states and state diagram >>
➤BLE advertising and data channels >>
➤BLE protocol stack architecture >>
➤BLE connection process >>
➤BLE advertising and data packet formats >>
Other Standard Physical Layers
• LTE Physical layer • WiMAX OFDM Physical layer • Wireless physical layer overview • 11b physical layer • 11a physical layer • fixed wimax physical layer-OFDM • mobile wimax physical layer-OFDMA • 11n physical layer • GSM Physical layer • TD-SCDMA Physical layer • GPRS physical layer • LDACS1 Physical layer • 10,40,100 Gigabit Ethernet Physical layer • Zigbee Physical layer • WCDMA Physical layer • Bluetooth Physical layer • WLAN 802.11ac Physical layer • WLAN 802.11ad Physical layer • 5G Physical layer