Low Power Wide Area Networks (LPWAN) LPWAN Specifically - - PowerPoint PPT Presentation

IoTSSC – Low Power Wide Area Networks (LPWAN)

LPWAN

• Specifically targeting IoT applications
• Battery powered devices

LPWAN

• Coverage (lower frequency) vs bandwidth/throughput

(WLAN/ WPAN)?

• Licensed vs unlicensed spectrum?
• Centralised vs decentralized access?
• Compatibility with cellular systems?
• Mobility support?
• Latency constraints?
• Security

LoRaWAN

• One of the main contenders for LPWAN dominance
• Operates in license-free ISM bands: 433, 868, 915 MHz
• Regulated (power, duty-cycle, bandwidth)
• EU: 0.1% or 1% per sub-band duty-cycle limitation
• Protocol stack works on top of a chirp spread spectrum

PHY layer (LoRa) – rates typically between 300bps – 5.5kbps + two ‘high speed’ (FSK) channels (11 and 50kb/s)

• PHY is proprietary (SemTech)
• Can work on 8 different frequencies at a time

LoRaWAN architecture

LoRa modulation

• Chirp spread spectrum - Spread Factors (SF) 7 to 12
• Moving an RF tone through time linearly - breaking

chirps in different places in terms of time and frequency to encode a symbol.

Credits: Thomas Telkamp

LoRa modulation

• Robust to interference
• Symbol rate (SF bits per symbol)

𝑆𝑡 = 𝐶𝑋 2𝑇𝐺

• Bit rate

𝑆𝑐 = 𝑇𝐺 𝐶𝑋 2𝑇𝐺

• Data whitening, interleaving, FEC are then applied
• FEC k/n – for every k bits of information n bits TX

LoRa bit rates vs SF (BW=125kHz)

SF SF Ch Chir irps/symbol Bit Bitrate 7 128 5.469kb/s 8 256 3.125kb/s 9 512 1.758kb/s 10 1024 977b/s 11 2048 537b/s 12 4096 293b/s

LoRa bit rates vs SF (BW=125kHz)

SF SF Ch Chir irps/symbol Bit Bitrate 7 128 5.469kb/s 8 256 3.125kb/s 9 512 1.758kb/s 10 1024 977b/s 11 2048 537b/s 12 4096 293b/s Numbers do not really match the formula. Why?

LoRa bit rates vs SF (BW=125kHz)

SF SF Ch Chir irps/symbol Bit Bitrate 7 128 5.469kb/s 8 256 3.125kb/s 9 512 1.758kb/s 10 1024 977b/s 11 2048 537b/s 12 4096 293b/s Numbers do not really match the formula. Why? FEC is applied here as well. Can you guess the code rate?

LoRa bit rates vs SF (BW=125kHz)

SF SF Ch Chir irps/symbol Bit Bitrate 7 128 5.469kb/s 8 256 3.125kb/s 9 512 1.758kb/s 10 1024 977b/s 11 2048 537b/s 12 4096 293b/s Numbers do not really match the formula. Why? Default code rate used is 4/5

LoRaWAN protocol stack

LoRaWAN device types

Standard defines three classes of devices defined

• Class A: Supported by all devices. Each uplink TX

followed by two short downlink receive windows.

• Class B: Extra receive windows at scheduled times

latency controlled downlink); slotted communication

• Class C: Continuously open receive widow, except when

transmitting (mains powered devices, no latency)

LoRaWAN frame format

LoRaWAN security

Two layers of security

• Network Security Key (nwkSkey) – authenticates node

in the network

• Application Security Key (appSkey) – ensures network
• perator cannot inspect the data, but only service

provider can

• AES 128 used in both cases
• MIC calculated over the ‘network’ part of the message

– works as a signature

Device activation

1) Over The Air Activation (OTAA)

• End-device follows a join procedure.

(+) device can attach any LoRaWAN network, security keys can be updated on a per session basis; enables roaming (-) App server has to answer to join requests each time a device (re)starts, generating more downlink traffic.

Device activation

2) Activation By Personalization (ABP)

• The end-device already pre-registered on the network.

DevAddr and keys are stored in end-device and NS. (+) simpler from application server point of view (-) node tied to a particular network; vulnerable to replay attacks

UoE LoRaWAN infrastructure

• Edge devices: Pycom dev boards (run micropython)
• Gateways deployed @ library, Argyle House, Bush campus
• TTN – The Things Network – open source NS stack

https://www.thethingsnetwork.org

Narrow Band IoT (NB-IoT)

The NB-IoT paradigm

• Standardised by 3GPP (Rel 13) to enable roll out over

existing cellular infrastructure (focus on reliability)

• Target apps: smart metering, smart cities (diagnostics

and control), eHealth

• Three types of deployments:

*Wang et al. “A Primer on 3GPP Narrowband Internet of Things (NB-IoT)”

The NB-IoT paradigm

• Bandwidth: 180kHz (low throughput)
• Data rates: 25kb/s (downlink) and 64kb/s (uplink,

multi-tone)

• Latency <10ms
• Hybrid ARQ scheme (reliability)
• Power saving modes (base station can dictate power

control through signalling)

Cellular IoT (CIoT) architecture

(red) – control plane; (blue) – user plane RAN – Radio Access Network; S/P-GW – Serving/Packet Gateway MME – Mobility Management Entity SCEF - Service Capability Exposure Function (new addition)

Channel access - downlink

• Each NB-IoT subframe spans one physical resource

block (PRB) – 12 subcarriers

• Narrowband Primary Synchronization Signal (NPSS);

Secondary Synchronization Signal (NSSS); Physical Broadcast Channel (NPBCH); Reference Signal (NRS); Physical Downlink Control Channel (NPDCCH); Physical Downlink Shared Channel (NPDSCH)

Channel access - uplink

• Narrowband Physical Random Access Channel

(NPRACH) – similar to LTE, but narrower channel

• Tone frequency index changes from one symbol

group to another - single-tone frequency hopping

Questions?