Bluetooth Low Energy

Network Topology

There are two devices, Peripheral and Central/Scanner. There are also advertisers, but when connected they become peripherals.

graph
	P1((P1)) <--> C((C))
	P2((P2)) <--> C
	A((A)) --> C

This is a simplification. A single device can have multiple roles simultaneously.

• Scan + Advertise
• Peripheral + Scanner + Advertise
• Peripheral + Scan + Central + Advertise

One peripheral can also be connected to multiple centrals.

BLE Layers

There are three main layers:

• Host: Configuration and Server
  • GAP: Generic Access Profile. This is where we configure advertising.
  • GATT: Generic ATTribute Profile. This is where we configure connections between devices.
• HCI: Host Controller Interface
  • In the chip there, is a CPU, BLE chip, microcontroller, etc. The HCI is so then the chip can talk to other components inside itself.
• Controller: Communication
  • Link Layer: sending packets
  • RF and PHY: sending bits Finally, the application sits on top of this stack. This layout is ordered form high level to low level.

BLE runs of 2.4 GHz with forty 2-MHz channels at up to 1 Mbps data rate.

• 37, 38, 39 are for advertising
• 0-36 are for connections. Although BLE runs on the same frequency as WiFi, there are techniques to avoid interference.

BLE Modulation

Gaussian Frequency-Shit Keyring (GFSK)

• improvement on FSK
• smoother transitions between bits $\to$ reduce nearby interference
• lessens spectral leakage at the expense of some loss in intersymbol discriminability
  • i.e. reduce bandwidth at the cost of bit errors

Packet Structure

Preamble                  1 Byte
AccessAddress             4 Bytes
Protocol Data Unit (PDU)  2-257 Bytes
CRC                       3 Bytes

• The same packet structure is used for both advertisements and connections.
• Fields are little endian.
• Advertisement packets use fixed Access Address: 0x8E89BED6
• Established connections use a (randomly chosen) unique Access Address
  • not actually random, just has a bit pattern to not look too much like noise.

Data Whitening

This is a technique to avoid long series of repetitive bits (all zeroes or all ones). In particular, you want a signal to have an equal distribution of $1$s and $0$s. Otherwise, it would imply something has gone wrong. RF noise should not be more focused in one direction.

Thus, radio hardware outputs with zero bias (or close to it).