Wireless 101 · Module 1

How wireless communication works

By the end you can say how far a signal goes, how fast it can carry data and how many users can share the air, and work out the numbers.

Wireless 101Module 150 minutes

Image: a collage of everyday wireless devices: a phone, a Wi-Fi router, Bluetooth earbuds, a GPS map, an FM radio, an RFID tag and a satellite dish.

Why wireless matters

A wireless link swaps the cable for a wave

An electromagnetic wave travelling through space does the job of the wire.

Wi-Fi, 4G and 5G, Bluetooth, GPS, broadcast radio and TV, RFID tags, satellite links.

Every wireless system answers three questions

How far can the signal go? propagation · path loss · link budget
How fast can we send data? modulation · bandwidth · noise
How many users can share the air? multiple access · cells · regulation

A short history

About 120 years from equations to a phone in every pocket

Not to scale. Each generation mainly changed how efficiently the same spectrum is used.

Part 1 · How far

Wavelength and frequency are locked together: λ = c / f

Higher frequency, shorter wave: 2.4 GHz is 12.5 cm

Frequency	Use	Wavelength
100 MHz	FM radio	3 m
900 MHz	cellular	33 cm
2.4 GHz	Wi-Fi, Bluetooth, microwaves	12.5 cm
5 GHz	Wi-Fi	6 cm
28 GHz	5G millimetre wave	about 1.07 cm

Part 1 · How far

Low bands travel far; high bands carry the bandwidth

Part 1 · How far

Decibels shrink a huge range of powers to small numbers

dBm = 10 · log₁₀(P / 1 mW)

dB: a ratio: 10 · log₁₀(P₂ / P₁)
dBm: absolute power, relative to 1 milliwatt
dBi: antenna gain, relative to an ideal isotropic antenna
P: the power you are converting

100 mW → 10 · log₁₀(100) = 20 dBm

Part 1 · How far

On the dB scale, ×10 is just +10 dB

Image: real antennas side by side: a half-wave dipole, a Yagi, a flat patch and a dish.

Part 1 · How far

Antenna gain focuses power; it never creates it

Isotropic: equal in all directions, 0 dBi, a theoretical reference
Half-wave dipole: 2.15 dBi
Yagi, patch, dish: more gain, narrower beam
Arrays: MIMO and beamforming steer beams electronically

Part 1 · How far

Even empty space costs about 80 dB over 100 m of Wi-Fi

Part 1 · How far

Real environments bend, bounce, scatter and swallow the signal

Part 1 · How far

Indoors and in cities, loss grows faster than in free space

Path loss grows with distance as dⁿ.

Part 1 · How far

Echoes can add up or cancel out: that is fading

Multipath is the main enemy

Slow fading: buildings and hills, over tens of metres
Delay spread: late echoes overlap symbols
Doppler, f_d = v / λ: 30 m/s at 2 GHz gives up to 200 Hz
Defences: diversity, MIMO, error-correcting codes, OFDM

Part 1 · How far

A link budget is just addition

P_r = P_t + G_t + G_r − L

P_r: received power (dBm)
P_t: transmit power (dBm)
G_t, G_r: antenna gains, transmit and receive (dBi)
L: losses, such as path loss (dB)

Part 1 · How far

Our Wi-Fi link arrives at −56 dBm, with 34 dB to spare

Part 2 · How fast

To carry bits, change the wave's amplitude, frequency or phase

The bits 1 0 1 1 0, sent three ways. QAM changes amplitude and phase together.

Part 2 · How fast

More points per symbol means more bits, and less room for noise

Part 2 · How fast

Shannon sets the speed limit of every channel

C = B · log₂(1 + SNR)

C: maximum error-free data rate (bit/s)
B: bandwidth (Hz)
SNR: signal-to-noise ratio, as a linear ratio

20 MHz, SNR 30 dB (×1000): 20 × 10⁶ · log₂(1001) ≈ 199 Mbit/s

Part 2 · How fast

Bandwidth is the strongest lever for speed

More bandwidth

Rate grows linearly, so standards add wider channels and new bands.

More SNR

Rate grows only logarithmically, so MIMO adds antennas for parallel streams.

OFDM

Many narrow, slow subcarriers resist multipath. Used by Wi-Fi, 4G LTE and 5G NR.

Part 3 · How many

Users can be split by frequency, time, code, or all of them

In each box time runs left to right and frequency bottom to top; each shade is one user.

Part 3 · How many

A radio must both talk and listen

FDD

Separate frequencies for uplink and downlink, at the same time.

TDD

One frequency, taking turns in time; capacity shifts easily between up and down.

Wi-Fi

Listen before talking (CSMA/CA) and back off if the channel is busy.

Part 3 · How many

Many small cells reuse the same frequencies

Image: a home Wi-Fi router next to a microwave oven, both working in the 2.4 GHz ISM band.

Part 3 · How many

Spectrum is shared, so regulators decide who may use it

ITU coordinates globally; the FCC (US) and Ofcom (UK) allocate locally
Licensed: exclusive, often auctioned; cellular
Unlicensed: anyone within power rules; Wi-Fi, Bluetooth
2020: FCC opened 1,200 MHz at 6 GHz for Wi-Fi 6E and 7

(1 min) Spectrum is a limited, shared public resource, so it is regulated. The ITU, the International Telecommunication Union, coordinates globally; national regulators allocate locally, for example the FCC in the United States and Ofcom in the United Kingdom.

Licensed spectrum: an operator buys exclusive rights, often at auction. That is cellular: predictable and interference-managed. Unlicensed spectrum: anyone may transmit if they follow power rules. That is Wi-Fi, Bluetooth and many IoT devices: free, but shared and congested.

The 2.4 GHz ISM band, 2.400 to 2.4835 GHz, is used by Wi-Fi, Bluetooth and microwave ovens, which is why the picture shows a router next to a microwave. In 2020 the FCC opened 1,200 MHz of the 6 GHz band, 5.925 to 7.125 GHz, for unlicensed use, used by Wi-Fi 6E and Wi-Fi 7.

Putting it together

Every system is the same physics, tuned differently

System	Typical bands	Typical range	Spectrum
Bluetooth	2.4 GHz	about 10 m	unlicensed
Wi-Fi	2.4, 5, 6 GHz	tens of metres indoors	unlicensed
4G LTE	600 MHz – 2.6 GHz	hundreds of m to km	licensed
5G mid-band	about 3.5 GHz	hundreds of metres	licensed
5G mmWave	24–40 GHz	about 100–300 m, line of sight	licensed

Check your understanding · 1 of 3

Three quick ones on how far

What is the wavelength of a 5 GHz signal?
6 cm
Convert 100 mW to dBm.
20 dBm
70 dB of path loss at 50 m: roughly how much at 100 m in free space?
About 76 dB

Check your understanding · 2 of 3

Two on how fast

QPSK or 64-QAM: which carries more bits per symbol, and at what cost?
64-QAM, 6 bits against 2; it needs a higher SNR
At high SNR, which raises capacity more: doubling bandwidth or doubling SNR?
Doubling bandwidth

Check your understanding · 3 of 3

Which multiple-access technique do 4G and 5G use?

FDMA
TDMA
CDMA
OFDMA

Summary

You can now answer how far, how fast and how many

How far

λ = c / f
+3 dB ≈ ×2, +10 dB = ×10
P_r = P_t + G_t + G_r − L; keep a margin

How fast

Denser constellations need more SNR
C = B · log₂(1 + SNR)
Bandwidth is the strongest lever

How many

FDMA → TDMA → CDMA → OFDMA
FDD or TDD; cells reuse frequencies
Licensed and unlicensed bands

(1 min) The main idea for the third time: every wireless system answers three questions, and you now have the tools for each.

How far: lambda equals c over f, so higher frequency means smaller antennas, more bandwidth and shorter range. Decibels turn multiplication into addition: 0 dBm is 1 mW, plus 3 dB about doubles, plus 10 dB is ten times. Free-space loss is about 80 dB for 100 m at 2.4 GHz, plus 6 dB per doubling of distance. Real channels reflect, diffract, scatter and fade; multipath is the main enemy. And the link budget adds it all up; keep a margin.

How fast: modulation packs bits per symbol, denser constellations need more SNR, and Shannon says bandwidth is the strongest lever. How many: FDMA to TDMA to CDMA to OFDMA, FDD versus TDD, cells reuse frequencies, and regulators split spectrum into licensed and unlicensed bands.

Next module, Module 2: antennas and propagation in depth, radiation patterns, MIMO and beamforming. You walked in with no radio background; you are walking out able to size a real link on paper. Well done, and see you next week.

How wireless communication works

A wireless link swaps the cable for a wave

Every wireless system answers three questions

About 120 years from equations to a phone in every pocket

Wavelength and frequency are locked together: λ = c / f

Higher frequency, shorter wave: 2.4 GHz is 12.5 cm

Low bands travel far; high bands carry the bandwidth

Decibels shrink a huge range of powers to small numbers

On the dB scale, ×10 is just +10 dB

Antenna gain focuses power; it never creates it

Even empty space costs about 80 dB over 100 m of Wi-Fi

Real environments bend, bounce, scatter and swallow the signal

Indoors and in cities, loss grows faster than in free space

Echoes can add up or cancel out: that is fading

Multipath is the main enemy

A link budget is just addition

Our Wi-Fi link arrives at −56 dBm, with 34 dB to spare

To carry bits, change the wave's amplitude, frequency or phase

More points per symbol means more bits, and less room for noise

Shannon sets the speed limit of every channel

Bandwidth is the strongest lever for speed

More bandwidth

More SNR

OFDM

Users can be split by frequency, time, code, or all of them

A radio must both talk and listen

FDD

TDD

Wi-Fi

Many small cells reuse the same frequencies

Spectrum is shared, so regulators decide who may use it

Every system is the same physics, tuned differently

Three quick ones on how far

What is the wavelength of a 5 GHz signal?

Convert 100 mW to dBm.

70 dB of path loss at 50 m: roughly how much at 100 m in free space?

Two on how fast

QPSK or 64-QAM: which carries more bits per symbol, and at what cost?

At high SNR, which raises capacity more: doubling bandwidth or doubling SNR?

Which multiple-access technique do 4G and 5G use?

You can now answer how far, how fast and how many

How far

How fast

How many