Tutorials:
Sensors, interfaces and bus systems (SENIN, BUSSY)

Wireless

last updated: 2021-01-17

Quick links to the subchapters

Introduction

Song of this chapter: Roger Waters > Radio K.A.O.S. > Radio Waves

Wireless is quite self explaining: there is no wire, so the transfer of information or power normally passes using radio waves. Depending of the power or technique used short and long distances can be bridged.

The Internet of Things (IoT) or Industrial Internet of Things (IIoT) would not happen without a bunch of performant wireless protocols. Billions of Things have to be connected to the (wired) Internet and doing this wirelessly facilitates the installation a great deal.

But there are also drawbacks using wireless communication. The communication can easily be disturbed, intercepted and the frequency channels are limited.

The magic of radio communication

Radio communication uses radio waves defined as electromagnetic waves of frequency between 30 Hertz (Hz) and 300 Gigahertz (GHz), including microwaves.

They are part of the electromagnetic spectrum seen in ELEFU (alternating current).

Class Denomination Short Frequency Wavelength
------------------------- ------------------------ ----- --------- ---------
  Extremely high frequency EHF 300 GHz

30 GHz
1 mm

1 cm

Microwaves
Super high frequency SHF
3 GHz

1 dm
  Ultra high frequency UHF
300 MHz

1 m
  Very High frequency VHF
30 MHz

10 m
      and ------------------------ ----- --------- ---------
  High frequency HF 30 MHz

3 MHz
10 m

100 m
  Medium frequency MF
300 kHz

1 km
  Low frequency LF
30 kHz

10 km
Radio waves Very low frequency VLF
3 kHz

100 km
  ------------------------ ----- --------- ---------
  Ultra low frequency ULF 3 kHz

300 Hz
100 km

1 Mm
  Super low frequency SLF
30 Hz

10 Mm
  Extremly low frequency ELF
3 Hz

100 Mm

We know current and voltage. Voltage is the cause and the current is the effect. Voltage stands for an electric field. With a current flowing we get mandatory an magnetic field. Is the current changing (alternating voltage and current current) we get electromagnetic fields. These fields have the characteristic to leave a wire as electromagnetic wave.

How much of the electromagnetic energy leaves the wire (or stays as voltage and current) depends on the length of the wire and the frequency respectively the wavelength of the signal.

Let's refresh our knowledge:

Frequency is the number of occurrences of a repeating event per unit of time. The symbol is f and the unit is hertz Hz or s-1 (cycle per second).

The period T is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. It's unit is the second s.

And here a new one:

The wavelength λ is the spatial period of a periodic wave. It is the distance over which the waves shape repeats. It's unit is the meter m.

The wavelength is the distance in space between e.g. two adjacent crests of the wave. It is a characteristic of both travelling waves and standing waves.

radio communication

formula wavelength

for electromagnetic waves in free space:

formula wavelength free space

Radio waves are generated by a device called transmitter connected to an antenna. The antenna gets energy from a current and enables this energy to leave the wire and radiates the electromagnetic wave (radio wave).

An illustrative example of radio waves that propagate around a transmitting antenna are water waves that occur when you throw a stone into a lake

wave water

Photo by Linus Nylund on Unsplash

Another antenna connected to a radio receiver reconverts the received energy of the wave to a current. Antennas work in two directions like speaker for acoustic waves.

Devices integrating a transmitter and a receiver are called a transceiver.

radio communication

Radio waves are very widely used in modern technology, but the bandwidth must be shared by all the users. To prevent interferences, the emission of radio waves is strictly regulated by laws, coordinated by the International Telecommunications Union (ITU), which allocates frequency bands in the radio spectrum for different uses.

"Just do it" Wireless 1:

Fundamentals of Wireless Networks

All communication methods have a maximum maximum error-free information rate for a particular noise level also called channel capacity! Claude Shannon presented in 1948 the noisy-channel coding theorem or Shannon's theorem (from wikipedia):

For any given degree of noise contamination of a communication channel, it is possible to communicate discrete data (digital information) nearly error-free up to a computable maximum rate through the channel.

The channel capacity C can be calculated from the physical properties of a channel; for a band-limited channel with Gaussian noise, using the Shannon–Hartley theorem:

formula shannon

with:

This simplified formula helps us to understand and classify wireless networks. The bit rate is proportional to the bandwidth! Increasing the signal power between the receiver and the sender also increases the bitrate!

Radio communication uses a shared medium: radio waves (electromagnetic waves). A certain standard is allowed to use a certain frequency range. For example WiFi standards 802.11b and 802.11g both use the 2.4–2.5 GHz band, or LoRa uses an ISM band (e.g. 867-869 MHz). A doubling in available frequency range will double the data rate. The WiFi standard 802.11n improved WiFi performance over earlier WiFi standards by going from 20 to 40 MHz of bandwidth!

As we know it also depends were our frequency band is situated in the overall frequency range. Low-frequency signals travel farther and cover large areas called macrocells (with more clients competing for access). Also the antennas are much bigger (wavelength/2). High-frequency signals can transfer more data with a much shorter range (smaller coverage areas called microcells) and a requirement for more receive antennas.

Comparing different wireless protocols:

Technology Range distance Tx Power Bitrate Frequency (EU)
NFC very short <10 cm 200 mW 106-424 kbit/s 2.4 GHz
Bluetooth short 1 m, 10 m, 100 m 1 mW, 2.5 mW, 100 mW 128 kbit/s - 2 Mbit/s 2.4 GHz
ZigBee short 10-100 m 1-100 mW 20-250 kbits/s 868 MHz or 2.4 GHz
Z-Wave short 15-50 m 10 mW 10-40 kbits/s 868 MHz
Wifi medium 50-250 m 80 mW 11-450 Mbit/s 2.4 GHz or  GHz
3G/4G medium 52.4 km 50002.4 mW 1.8-8 GHz
Sigfox long 3-40 km 25 mW (up) 500 mW (down) 100-600 bit/s 868 MHz
NB-IoT long 3-30 km 300 bit/s-200 kbit/s 868 MHz + LTE?
LoRa(WAN) long 2-10 km 20 mW 300 bit/s-50 kbit/s 433 MHz or 868 MHz


wireless standards comparision diagram.svg


Interesting links