Wireless communication

Learning goals

• know typical wireless communication protocols

Overview

Network Type	Frequency (Hz)	Bit rate (bit/s)	Band	Max. range (m)
Bluetooth	2.4 G	3 M	Unlicensed ISM	300 m
IR	100–200 T	16 M	IR-B	5 m
Wi-Fi	2.4–5 G	108 M	ISM to U-NII	100 m
ZigBee	900 M	256 K	-	10 m
Cellular Networks	850–1900 M	20 M	-	5 km
WiMAX	10–66 G	75 M	-	40 km
LMDS	10–40 G	512 M	-	5 km

Exercise 34

You are a consulting engineer. A hospital is deploying a telemedicine system to improve patient care. The system includes:

1. Wearable Health Monitors: Patients wear sensors that track heart rate and oxygen levels, sending small data packets every 10 seconds to nearby nurse stations (max range 20 m).

2. Remote Patient Consultations: Doctors conduct live video calls with patients at home, requiring high-speed internet with a max range of 5 km.

3. Emergency Drones: Drones deliver medical supplies to accident sites and transmit real-time video to hospitals (max range 3 km).

4. Hospital Equipment Monitoring: Wireless sensors monitor critical hospital devices and send status updates to a central server every minute (max range 100 m).

For each case, select the most suitable wireless technology using the table above and explain the hospital management why.

Bluetooth

• A controller device can hold an active connection to up to seven devices

  • up to 255 further slave devices can be inactive

• Uses adaptive frequency hopping between 79 frequencies to reduce EMI

• low-cost and integrated in microcontrollers

  • e.g., TI CC2651R3

• Three classes

  • 3, 30, 300 m

Infrared (IR)

• Usually associated with heat

  • The sun emits both visible and IR

• Our body emits IR

  • useful for search and rescue

• Typically used in TV remotes

• Three types:

Abbreviation	Wavelength	Freq. (THz)
IR-A	780–1400 nm	215–384
IR-B	1400–3000 nm	100–215
IR-C	3–1000 µm	0.3–100

• IR-A

  • night vision

• IR-B

  • long range optical communication

• transmitter must be aligned close

  • dependent on the used LEDs

• LED or laser diode goes on and off

• mature tech

• requires LOS, but no interference in return

• standardized by the Infrared Data Association (IrDA)

  • bit rate from 8.6 kbit/s to 1 Gbit/s

• I believe IR used to be popular, but Bluetooth replaces it on medical devices

  • Could not find significant amount of devices using IR

  • A ECG device mentions wireless options based on Bluetooth and IR

Wireless local area network (WLAN) and Wi-Fi

• More complex setup compared to IR

• IEEE 802.11 protocol family

• Nowadays popular Wi-Fi 6

  • probably you see its logo labeled with 6 on your smartphone’s screen

  • up to ~10 Gbit/s

• Based on

  • access point (AP)

  • station

• Supports roaming

  • station selects the AP with the strongest signal from the APs with the same name (SSID)

• More APs or wireless relay (repeater) for extension

• Availability and thus safety is an issue

  • uses unlicensed ISM radio band

    • makes jamming easier, because many easy to access

• Wi-Fi is a WLAN tech, but WLAN is not necessarily Wi-Fi. WLAN can be any kind of wireless networking

Example details from my laptop connected to eduroam:

$ iw dev wlp0s20f3 station dump
...
	signal avg:	-49 dBm
    ...
	tx bitrate:	400.0 MBit/s VHT-MCS 9 40MHz short GI VHT-NSS 2
    ...
...

• 400 Mbit/s

  • data rate

• VHT-MCS 9:

  • MCS index 9, which corresponds to 256-QAM. Very High Throughput (VHT), used in 802.11ac (Wi-Fi 5).

• 40 MHz

  • channel bandwidth

• short GI (Guard Interval)

  • GI is the time period between transmitted symbols to prevent inter-symbol interference (ISI) caused by multipath propagation

  • Using a short guard interval (400 ns), which improves performance slightly over the default 800 ns.

• VHT-NSS 2

  • number of spatial streams (NSS) 2

  • two independent antennas are used

ZigBee

• low-rate wireless personal area network (PAN)

• used in home automation

• supports star, tree, and mesh topologies

Three kinds of devices

• full-function devices (FFD)

  • can do all the required functions

• reduced-function devices (RFD)

  • less capabilities

  • cheaper

  • low energy

Fig. 11 Different kinds of nodes available in a ZigBee network. The violet coordinator maintains a table of all available nodes in a network and is an FFD. Yellow Router routes packets between nodes, but can have also additional sensor/actuator functionality. End devices are usually RFDs are sensor/actuators. (CC BY-NC-SA http://www.swappa.it/wiki/Uni/RW-13Aprile)

Fig. 12 Star topology. Yellow routers do not have to route in this case. The coordinator can be a bottleneck, if more participants are added to the network. (CC BY-NC-SA http://www.swappa.it/wiki/Uni/RW-13Aprile)

Fig. 13 Tree topology. Here the coordinator can become a bottleneck if sub-trees want to exchange data often. (CC BY-NC-SA http://www.swappa.it/wiki/Uni/RW-13Aprile)

Fig. 14 Mesh topology. Often found in ad-hoc network scenarios, where nodes can come and go. (CC BY-NC-SA http://www.swappa.it/wiki/Uni/RW-13Aprile)

wireless ad-hoc network

a decentralized type of wireless network. Does not require pre-existing infrastructure like dedicated routers

ad hoc

Latin phrase meaning for this. Signifies a solution designed for a specific purpose, which is typically provisional and temporary.

Exercise 35

Given above definitions, why do you think is ZigBee an ad-hoc network?

• low-cost and integrated in microcontrollers

  • e.g., TI CC2651R3

• also useful in body area networks

• requires a coordinator and router

• Comparison of radio modules

  • Receive sensitivity (lowest expected power received by the antenna): -90 dBm — -122 dBm.

Li-Fi

• optical wireless

• alternative to Wi-Fi

• compared to IR, additionally uses ultraviolet and visible light

• IEEE 802.11bb introduced in 2023 motivates broader industry adoption

• 10 Mbit/s - 9.6 Gbit/s

• less interference, because

  • most of the devices and communication protocols operate at lower frequencies

  • requires LoS, no penetration of walls

• less interference means a higher communication density for a group of devices

• can also penetrate the Faraday cage of an MRI room

• security

  • data cannot be intercepted outside the room

Fig. 15 Working principle of Li-Fi
CC BY-SA 4.0. By Бумбаяр. Source: Wikimedia Commons

Exercise 36

A website advertises that Li-Fi 100x faster than Wi-Fi. Can this be true? Explain.

Cellular networks

• current standard 5G with three application areas as separate networks

  • enhanced mobile broadband (eMBB)

    • higher bandwidth, more capacity, e.g., for stadiums

  • ultra-reliable low-latency communications (URLLC)

    • short packets for mission critical applications, e.g., telesurgery

  • massive machine type communications (mMTC)

    • low bandwidth with higher coverage, e.g., IoT devices in rural areas with bad connectivity

• 5G as a wireless power grid

  • 6 μW at 180 m

  • interesting for energy-harvesting by low power sensors

• Illustration of 4G and 5G device types Source:Ericsson RedCap whitepaper

• Narrowband IoT

  • based on LTE but limits bandwidth to 180 kHz

  • for example LTE Cat NB2: ⬇️ 127 kbit/s, ⬆️ 159 kbit/s, ↔️ 180 kHz

• 5G NR frequency ranges

  • 5G can use up to 71 GHz

More details

• 5G in telemedicine

WISP

Wireless Internet service provider (WISP)

an internet service provider with a network based on wireless networking

Can use:

• Wi-Fi mesh networking

• proprietary (non-standardized) protocols over unlicensed bands

• licensed bands in ultra high frequency (UHF), 300 MHz to 3 GHz (according to ITU)

• LMDS

You can find a list of all bands here.

Fig. 16 (2015) A CableFree point to multipoint radio base station installed for a WISP in Rotterdam, The Netherlands. The radio base station has 4 radio interfaces each connected to a separate sector antenna, each providing 90 degrees coverage of the city for a full 360 degrees coverage. Within 5-20km of this base station, Subscriber Units (CPEs) with high gain directional antennas are installed on sites which can then can connect to the Base Station to receive broadband data connections of typically 10-200 Mbps capacity.
CC BY-SA 4.0. By Millsbi. Source: Wikimedia Commons

Fig. 17 A microwave radio relay dish for wireless networking
Public domain. By Unknown author. Source: Wikimedia Commons

Wireless broadband

also called broadband wireless access. An alternative to cellular networks. We only cover here:

• LMDS

• WiMAX

• Local multipoint distribution service (LMDS)

  • proprietary (non-standardized)

  • originates from 90s, for TV broadcasting, but failed adoption. Nowadays used in Europe for backhaul (intermediate connections to the backbone network used to interconnect different local area networks (LAN)) between base stations.

• Worldwide Interoperability for Microwave Access (WiMAX) additionally specifies physical and media access layer options

  • based on the IEEE 802.16 called WirelessMAN (metropolitan area network) standard.

  • conceived as a last-mile broadband access and as an alternative to cable broadband

  • used in many countries including Denmark), however the last one was launched in Bangladesh in 2014 according to Wikipedia and it was not successful also in Danmark.

  • lost against LTE in 4G according to this article.

Satellite networks

• point-to-point (PTP) link from a computer to a satellite

  • e.g., Starlink

• used in underdeveloped areas for teleconsulting in medicine

Licensed and unlicensed frequency bands

• unlicensed: everyone can use it without getting permission from an authority

• licensed: the opposite

  • guarantees more reliability, because it is more known who will use it compared to an unlicensed frequency band :

M-health and telemedicine applications

• mobile health

• nowadays, chips allow integration of many components to a single chip

  • makes mobile compact applications possible

• challenges of m-health, e.g., in context of an oxygen saturation meter

  • movement artifacts

    • sensor’s position may shift, which can affect readings

  • ambient light changes

    • may affect readings

The outdoor operating environment

attenuation

when a signal gets weaker as it travels through a material or space. Reduction in power density.

• the more attenuation, the more the chances are for interference

path loss

attenuation of an electromagnetic wave as it propagates.

Path loss causes a decrease in the usable signal energy by the receiver

diffraction

deviation of waves from straight-line propagation through an obstacle or aperture

Fig. 18 Waves coming from the sea are diffracted and they become waves in form of an arc. additionally, reflection can be seen on the top left corner of the quarry.
CC BY-SA 4.0. By Verbcatcher. Source: Wikimedia Commons

radio wave diffraction example

reflection

change of direction so that the signal returns back to the origin. reflection happens:

• when the wavelength is much smaller than the hit object

• or if the hit object has holes much smaller than the wavelength

  • e.g., mesh filter on the door of a microwave oven. (~10cm wavelength)

scattering

signal is forced to deviate from a straight trajectory scattering:

• includes waves that do not follow the law of reflection

• the hit object is much smaller than the wavelength (compared to diffraction)

  • e.g., rough surfaces, dust

  • may lead to more energy received by the receiver

diffraction, reflection, scattering

• all the above phenomena cause multipath propagation

• waves can interfere constructively or destructively

fading

the variation of signal attenuation over variables like time, position, and frequency.

signal loss vs fading

• signal loss is a baseline reduction in power, while fading refers to fluctuations around that baseline

• both signal loss and fading affect signal quality

• antenna diversity can help

  • even one antenna picks up a faded signal due to diffraction, other one can still receive a usable signal

LOS (line of sight) may not enough for an acceptable signal quality

• Fresnel zone defines a space between two antennas which should be clear

• if the signal direction is changed by an object outside the Fresnel zone, its impact is insignificant

• rule of thumb: Fresnel zone must be at least 60% clear

Fig. 19 Fresnel zone
CC BY-SA 3.0. By Jcmcclurg. Source: Wikimedia Commons

• weather conditions can affect signal quality

  • rain-induced attenuation insignificant for systems operating under 10 GHz or rainfall rate below 20 mm/h

  • a higher frequency signal is attenuated more than a lower frequency signal

  • the attenuation increases linearly for increasing rainfall

• polarization diversity

  • an antenna diversity technique that uses antennas with orthogonal polarizations, e.g., vertical and horizontal

  • horizontal polarization

    • gets attenuated more by the rainfall

    • the attenuation difference increases linearly with rainfall rate

Exercise 37

You are designing a communication link from a remote medical facility to an ambulance for a tropical region. You know that usually more accidents happen during heavy rain. How would you cope with the problem of rain attenuation?

• cross-polarization diversity

  • utilizes different polarizations (e.g., horizontal and vertical)

  • it may feel logical get rid of horizontally polarized antennas, but using diversity is better. We will see why in the next chapter

• multipath fading

  • a signal interferes with the required signal in amplitude as well as phase due to indirect paths

  • generally an issue with signals below 10 GHz

    • issue for >10 GHz: attenuation by rain

Fig. 20 Multipath fading
Public domain. By wikipedia:en:user:Cadmium. Source: Wikimedia Commons

Fig. 21 Ghost signal due to multipath propagation
Public domain. By Original image: Lithium57 / English translation: MichaelBillington. Source: Wikimedia Commons

• Doppler spread

  • moving signals have a frequency shift called Doppler shift

  • the difference in Doppler shifts between different signal components contributing to a signal fading

  • issue in vehicular communication

RFID in telemedicine

RFID (radio-frequency identification)

tech for identifying something using radio frequency signals

• like an invisible barcode

tag

• object that is being identified

reader

• object that identifies a tag

• typically stationary

Fig. 22 An RFID tag
CC BY-SA 3.0. By derivative work: Sakurambo (talk) EPC-RFID-TAG.jpg: SMARTCODE Corporation. Source: Wikimedia Commons

A tag can be passive or active

• if passive: needs a reader to be powered

• if active: has a battery and can be read from a longer distance

Examples:

• Student id

• Bank card

RFID demo

Install the app NFSee and read your student id

Exercise 38

Give three examples for RFID applications that you encountered.

Exercise 39

How can we utilize RFID in telemedicine? Provide at least two ideas.

• frequency ranges

  • low: ~130 kHz

  • high: ~13 MHz

  • ultra-high: ~900 MHz

• security

  • ultra-high can be read from a larger distance

  • for telemedicine, low and high frequency may be better suitable

• fading problems for body sensors

  • for low and high frequencies

    • multipath fading

  • for ultra-high

    • energy absorption by water in tissues

• example: glucose meter for diabetes monitoring (Christiansen et al 2018)

  • not as critical as a pacemaker

  • glucose data read into the RFID tag and the tag transmits the data

  • data must be sent immediately due to low storage capacity

• example: patient tracking (Cao et al. 2014)

  • real-time location information

  • integrated in wristband or clothing

  • problem: tag-collision (Xiao et al. 2018)

• solutions for tag-collision

  • slotted Aloha

    • broadcasts an initialization command and a parameter that the tags individually use to pseudo-randomly delay their responses.

  • adaptive binary tree

    • the reader sends one bit at a time, until only a single tag responds

    • may not be suitable for clinic due to the need of simultaneous tagging of patients which may incur too much time

Fig. 23 How a device is searched for using adaptive binary tree
CC BY-SA 2.5 es. By Rob Blanco. Source: Wikimedia Commons