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Intended Learning Outcomes for this Topic
2
➌ Identify the primary architectures for RF interfaces used in embedded systems
By the end of this topic, you should be able to:
➋ Describe the key roles of a medium access control (MAC) in RF communications
➍ Describe the physical and link layer of Bluetooth Low Energy and its design tradeoffs
➊ Describe the primary components of RF signal chains used in embedded systems
➎ Identify choices for short- and long-range communications in embedded systems
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Typical RF Transceiver Signal Path
3
LNA .
PA
ADC
DAC
RF Switch
Transmit Path
Receive Path
Digital data
(from MAC software)
Digital data
(to MAC software)
Antenna
Analog/RF Digital
Often SPI or UART
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Medium Access Control (MAC) Protocols
4
The primary objective of MAC protocols is to prevent collisions
Where do collisions occur (conceptually): At sender, or at the receiver, or both?
▶ Reasons: ➊ Hidden terminal problem; ➋ single antenna with RF switch
Unlike wired interfaces (e.g., I2C), RF systems typically cannot detect if another device is transmitting
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To Understand Collisions: RF Radiation Patterns
5
Source: Panasonic PAN1326 Bluetooth LE Radio Module Datasheet
Transmitter
Disk of
radius r
centered at
transmitter
Example radiation pattern
resulting from transceiver,
matching network, and
antenna properties
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Hidden Terminal Problem
6
BA
?
Ideal depiction of
radio range locus
28
Collisions: Occur At the Receiver
7
BA
C
?
Ideal depiction of
radio range locus
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Typical Transceiver Architecture Dictates Protocol Possibilities
8
LNA .
PA
ADC
DAC
RF Switch
Transmit Path
Receive Path
Digital data
(from MAC software)
Digital data
(to MAC software)
Antenna
Analog/RF Digital
Because of ➊ this and ➋ hidden terminal problem, cannot detect collision while transmitting
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Families of Medium Access Control (MAC) Protocols
9
Slotted Unslotted
Globally scheduled
(TDMA)
Unscheduled
(per-timeslot CSMA)
Contention-based
(unslotted/unsynchronized CSMA)
CSMA
(Carrier-Sense Multiple Access)
Long preambles/
preamble sampling
Low duty-cycle facilitation
Indirect communication
(Random Access)
TDMA: Time Division Multiple Access
CSMA: Carrier Sense Multiple Access
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Common Architectures for RF Interfaces in Embedded Systems
10
Communication Stack /
Network Stack /
Protocol Stack
Terminology
UART
The software and state implementing a collection of communication
protocols at different layers of the OSI hierarchy, from physical layer (e.g.,
RF) to application (e.g., HTTP, SSH)
Radio Module
Typically a circuit board with a microcontroller (“μC”) implementing
the protocol stack, an RF transceiver, an antenna, and other components
Transceiver
Transmitter / receiver: takes digital data as input and modulates data
onto RF communication medium (physical layer)
Universal asynchronous receiver/transmitter. A serial communication
interface comprising a transmit (TX) signal, a receive (RX) signal, and with a
pre-defined number of bits (e.g., 7 or 8) per transmitted value/“packet”.
There is no clock signal. Instead, the boundaries of packets are defined
by start/stop bits and an assumed bit rate. A parity bit enables error detection
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Two Common Architectures for RF Interfaces in Embedded Systems
11
SPI
GPIO
Microcontroller
Application 1
Application 2
MAC
RF Transceiver
TX FIFOs
RX FIFOs
Alternative 1: Protocol stack in μC + simple transceiver
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Two Common Architectures for RF Interfaces in Embedded Systems
12
RF Module or SoC
UART
TX FIFOs
RX FIFOs
MAC
Dedicated
Microcontroller Core
Microcontroller
Application 1
Application 2
Alternative 2: Protocol stack in RF module
SoC=“System-on-Chip”
9.5 mm
PAN1326 Module