Wireless 101 · Module 1

How wireless communication works

50 minutes, no radio background needed

By the end you can answer three questions about any wireless system

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

Image: a phone, a Wi-Fi router, a GPS receiver and an RFID tag shown together

1 · Why wireless matters

A wireless link swaps a cable for a wave in space

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

2 · A short history

From equations to a phone in every pocket in about 120 years

1865
Maxwell
Equations predict electromagnetic waves
1886–88
Hertz
Generates and detects radio waves in the lab
1901
Marconi
Transatlantic radio signal
1991
GSM
First 2G digital call, in Finland
2019
5G
First commercial networks

3 · Radio waves

A radio wave is an oscillating field with wavelength λ = c / f

3 · Radio waves

Higher frequency means shorter wavelength: 100 MHz is 3 m, 28 GHz about 1 cm

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

4 · The radio spectrum

The ITU splits 3 kHz–300 GHz into bands; the low ones reach far

Band	Range	Example uses
VLF	3–30 kHz	Submarine communication
LF	30–300 kHz	Navigation beacons
MF	300 kHz–3 MHz	AM radio
HF	3–30 MHz	Shortwave, amateur radio

4 · The radio spectrum

The upper bands carry FM, TV, cellular, Wi-Fi, GPS, satellite and radar

Band	Range	Example uses
VHF	30–300 MHz	FM radio, TV, air-band
UHF	300 MHz–3 GHz	Cellular, Wi-Fi 2.4 GHz, Bluetooth, GPS
SHF	3–30 GHz	Wi-Fi 5/6 GHz, satellite, radar
EHF	30–300 GHz	Millimetre-wave 5G, radar

4 · The radio spectrum

Low frequencies travel far; high frequencies carry more bandwidth

5 · Decibels

Decibels turn multiplying into adding

dB = 10 · log₁₀(P₂ / P₁)

+3 dB: about double the power (−3 dB: half)
+10 dB: 10 times the power (−10 dB: one tenth)
Add: gains and losses add and subtract instead of multiplying
Why: powers span many orders of magnitude

5 · Decibels

dBm anchors decibels to 1 mW: 0 dBm = 1 mW

Power	dBm
1 W	30 dBm
100 mW	20 dBm
1 mW	0 dBm
1 µW	−30 dBm
1 pW	−90 dBm

6 · Antennas

Gain does not create energy: it focuses it into a narrower beam

Omnidirectional suits phones and routers; directional suits point-to-point links and base-station sectors. Arrays (MIMO, beamforming) steer beams electronically.

7 · Propagation

Even in empty space, signal power falls with distance

FSPL = 20·log₁₀(d) + 20·log₁₀(f) + 32.44

d, f: distance in km, frequency in MHz
Worked: 2.4 GHz, 100 m: −20.0 + 67.6 + 32.44 ≈ 80 dB
×2 d: doubling distance adds about 6 dB
×2 f: doubling frequency adds about 6 dB

7 · Propagation

The same power spreads over a bigger sphere, so each doubling costs 6 dB

8 · Propagation in the real world

Real signals are reflected, diffracted, scattered and absorbed

8 · Propagation in the real world

Path loss grows as dⁿ, and the exponent n depends on where you are

Environment	Path-loss exponent n
Free space	2
Urban cellular	2.7–3.5
Shadowed urban cellular	3–5
In building, line of sight	1.6–1.8
In building, obstructed	4–6

9 · Multipath and fading

Reflections deliver several copies of the signal, and they add up or cancel

Defences: diversity (several antennas), MIMO, error-correcting codes, OFDM.

9 · Multipath and fading

Moving a few centimetres can sharply change the signal

6 cmFast fading: half a wavelength at 2.4 GHz

10s of mSlow fading (shadowing): buildings, hills

ISIDelay spread: echoes spread in time, symbols overlap

200 HzDoppler: car at 30 m/s, 2 GHz signal

10 · Link budget

A link budget just adds up gains and losses: −56 dBm arrives with 34 dB to spare

11 · Modulation

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

11 · Modulation

Denser constellations carry more bits per symbol, but need a cleaner signal

Bits per symbol = log₂(points). Radios adapt: 256-QAM near the router, BPSK or QPSK at the edge.

Source: Wireless 101 Module 1. 1024-QAM is Wi-Fi 6, 4096-QAM is Wi-Fi 7.

12 · Bandwidth, noise and the Shannon limit

Shannon: bandwidth is the strongest lever for data rate

C = B · log₂(1 + SNR)

Worked: 20 MHz, SNR 30 dB (ratio 1000): ≈ 199 Mbit/s per stream
More B: rate grows linearly
More SNR: rate grows only logarithmically
OFDM: many narrow subcarriers: robust to multipath

SNR, the signal-to-noise ratio, is how much stronger the signal is than the background noise, usually in dB. The Shannon–Hartley theorem gives the maximum error-free data rate: C equals B times log2 of 1 plus SNR, C in bit/s, B in Hz, SNR as a linear ratio.

Worked example: a 20 MHz Wi-Fi channel with SNR 30 dB, a ratio of 1000. C is 20 times 10 to the 6 times log2 of 1001, about 9.97, so about 199 Mbit/s per spatial stream. This is a theoretical ceiling.

Two levers: more bandwidth, where rate grows linearly, or more SNR, where it grows only logarithmically. That is why new standards keep adding spectrum and antennas; MIMO adds parallel streams. Quiz question 5.

OFDM splits a wide channel into many narrow subcarriers, each modulated slowly, which makes it robust to multipath. Wi-Fi since 802.11a/g, 4G LTE and 5G NR all use OFDM. This is the answer to how fast.

13 · Sharing the air

Multiple access separates users by frequency, time, code or subcarriers

Technique	Users separated by	Example system
FDMA	Frequency	1G AMPS
TDMA	Time slot	2G GSM (8 slots per 200 kHz carrier)
CDMA	Code	3G UMTS (WCDMA), IS-95
OFDMA	Subcarrier groups and time	4G LTE, 5G NR, Wi-Fi 6

13 · Sharing the air

Duplexing: send and receive on two frequencies, or on one by taking turns

Wi-Fi shares differently: devices listen before talking (CSMA/CA) and back off if the channel is busy.

14 · The cellular concept

Many small cells reuse frequencies, so capacity multiplies

Same letter = same frequencies, reused by cells far enough apart (illustrative labels).

15 · Who controls the spectrum

Spectrum is a limited, shared public resource, so it is regulated

Licensed

Operator buys exclusive rights, often at auction
Cellular networks
Predictable, interference-managed

Unlicensed

Anyone may transmit within power rules
Wi-Fi, Bluetooth, many IoT devices
Free, but shared and congested

16 · Putting it together

Each system is the same physics: frequency sets range, bandwidth and SNR set speed

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 2

Quiz: wavelength, dBm and path loss

Questions

What is the wavelength of a 5 GHz signal?
Convert 100 mW to dBm.
A link has 70 dB of path loss at 50 m. Roughly how much at 100 m in free space?

Answers

6 cm
20 dBm
About 76 dB

Check your understanding · 2 of 2

Quiz: modulation, Shannon and access

Questions

QPSK or 64-QAM: more bits per symbol, and the cost?
Shannon: double bandwidth or double SNR?
Multiple access in 4G and 5G?

Answers

64-QAM: 6 vs 2 bits; needs a higher SNR
Doubling bandwidth
OFDMA

Summary

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

λ = c / f; FSPL ≈ 80 dB at 100 m, 2.4 GHz; each doubling adds 6 dB. P_r = P_t + G_t + G_r − L: keep a margin.
C = B · log₂(1 + SNR)
Bandwidth is the strongest lever; denser constellations need more SNR.
FDMA → TDMA → CDMA → OFDMA; FDD vs TDD; cells reuse frequencies; licensed vs unlicensed.

How wireless communication works

By the end you can answer three questions about any wireless system

A wireless link swaps a cable for a wave in space

From equations to a phone in every pocket in about 120 years

Maxwell

Hertz

Marconi

GSM

5G

A radio wave is an oscillating field with wavelength λ = c / f

Higher frequency means shorter wavelength: 100 MHz is 3 m, 28 GHz about 1 cm

The ITU splits 3 kHz–300 GHz into bands; the low ones reach far

The upper bands carry FM, TV, cellular, Wi-Fi, GPS, satellite and radar

Low frequencies travel far; high frequencies carry more bandwidth

Decibels turn multiplying into adding

dBm anchors decibels to 1 mW: 0 dBm = 1 mW

Gain does not create energy: it focuses it into a narrower beam

Even in empty space, signal power falls with distance

The same power spreads over a bigger sphere, so each doubling costs 6 dB

Real signals are reflected, diffracted, scattered and absorbed

Path loss grows as dⁿ, and the exponent n depends on where you are

Reflections deliver several copies of the signal, and they add up or cancel

Moving a few centimetres can sharply change the signal

A link budget just adds up gains and losses: −56 dBm arrives with 34 dB to spare

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

Denser constellations carry more bits per symbol, but need a cleaner signal

Shannon: bandwidth is the strongest lever for data rate

Multiple access separates users by frequency, time, code or subcarriers

Duplexing: send and receive on two frequencies, or on one by taking turns

Many small cells reuse frequencies, so capacity multiplies

Spectrum is a limited, shared public resource, so it is regulated

Licensed

Unlicensed

Each system is the same physics: frequency sets range, bandwidth and SNR set speed

Quiz: wavelength, dBm and path loss

Questions

Answers

Quiz: modulation, Shannon and access

Questions

Answers

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

C = B · log₂(1 + SNR)