Wireless 101 · Module 1

How wireless communication works

By the end you will be able to say how far a signal goes, how fast it can carry data, and how many users can share the air.

[Presenter name]Wireless 101[Date]

Every wireless system answers three questions

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

From Maxwell's equations to 5G took more than a century

1. 1865

   Maxwell

   Equations predict electromagnetic waves

2. 1886–88

   Hertz

   Generates and detects radio waves in the lab

3. 1901

   Marconi

   Receives a transatlantic radio signal

4. 1979–83

   1G

   First analog cellular networks

5. 1997

   Wi-Fi

   First IEEE 802.11 standard

6. 2019

   5G

   First commercial 5G networks

Wavelength is the speed of light divided by frequency

λ

wavelength, in metres

c

speed of light, ≈ 3 × 10⁸ m/s

f

frequency, in hertz (Hz)

100 MHz → 3 m · 2.4 GHz → 12.5 cm · 28 GHz → about 1.07 cm

A radio wave is an oscillating electric and magnetic field

The radio spectrum runs from 3 kHz to 300 GHz

Decibels turn multiplying gains into adding them

dB

ratio between two powers

dBm

power relative to 1 mW: 10 · log₁₀(P / 1 mW)

dBi

antenna gain relative to an ideal isotropic antenna

Three rules of thumb cover most link arithmetic

Antenna gain focuses power; it does not create it

• An antenna converts current into a radio wave, and back again.
• Isotropic antenna: the 0 dBi reference. Half-wave dipole: 2.15 dBi.
• Fence: gain does not add energy. A directional antenna sends the same power into a narrower beam.
• Phones and home routers use omnidirectional antennas; point-to-point links use directional ones.

Free-space path loss grows with distance and frequency

d

distance, in km

f

frequency, in MHz

Each doubling of distance or frequency: +6 dB

Wi-Fi at 2.4 GHz over 100 m loses about 80 dB

Free-space path loss, worked: 20·log₁₀(0.1) = −20.0; 20·log₁₀(2400) = 67.6; −20.0 + 67.6 + 32.44 ≈ 80 dB.

Module 1, section 7 (worked example)

Real environments reflect, bend, scatter and absorb signals

• Reflected by large flat surfaces: buildings, the ground, walls.
• Diffracted around edges, reaching shadowed areas.
• Scattered by objects smaller than a wavelength: foliage, street signs.
• Absorbed by concrete, water and human bodies.
• Path loss grows as dⁿ: n = 2 in free space, 2.7–3.5 urban cellular, 4–6 obstructed indoors.

Multipath makes the signal fade over half a wavelength

• Reflections give the receiver several delayed copies of the signal.
• Copies in phase add up; copies out of phase cancel.
• At 2.4 GHz, half a wavelength is about 6 cm: move that far and the signal can drop sharply.
• Delay spread: echoes that arrive spread out in time overlap symbols.
• Doppler: f_d = v / λ. A car at 30 m/s receiving 2 GHz sees up to 200 Hz shift.

A link budget adds every gain and loss in decibels

P_t

transmit power, dBm

G_t, G_r

antenna gains, dBi

L

losses, dB (path loss and others)

P_r

received power, dBm

Wi-Fi, 100 m: 20 + 2 + 2 − 80 = −56 dBm

Wi-Fi at 100 m arrives at −56 dBm, 34 dB above sensitivity

Part 2

How fast can we send data?

Modulation, bandwidth and noise set the data rate.

Modulation changes a wave's amplitude, frequency or phase

• Amplitude: AM (analog), ASK (digital).
• Frequency: FM (analog), FSK (digital).
• Phase: PSK (digital).
• Amplitude and phase together: QAM (digital).
• Bits per symbol = log₂(number of points in the constellation).

Each extra bit doubles the points and needs a cleaner signal

More points per symbol means more bits, but the points sit closer together.

Shannon: bandwidth and SNR set the maximum data rate

C

maximum error-free rate, bit/s

B

bandwidth, in Hz

SNR

as a linear ratio, not in dB

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

Bandwidth is the strongest lever; SNR helps only logarithmically

• More bandwidth: the rate grows linearly with it.
• More SNR: the rate grows only logarithmically.
• New standards add wider channels and new bands, and MIMO adds parallel streams.
• OFDM splits a wide channel into many narrow subcarriers, robust to multipath. Used by Wi-Fi (802.11a/g onward), 4G LTE and 5G NR.

Four multiple-access schemes separate users

1. Separated by frequency: 1G AMPS

2. Separated by time slot: 2G GSM

3. Separated by code: 3G UMTS, IS-95

4. Separated by subcarriers and time: 4G LTE, 5G NR, Wi-Fi 6

Two-way links use FDD (separate frequencies) or TDD (taking turns)

• FDD: separate frequencies for uplink and downlink, at the same time.
• TDD: the same frequency, taking turns in time.
• TDD makes it easy to shift capacity between uplink and downlink.
• Wi-Fi uses CSMA/CA: devices listen before talking and back off if the channel is busy.

Small cells reuse frequencies, so capacity multiplies

Regulators split spectrum into licensed and unlicensed bands

Same physics, different trade-offs across five systems

1. 2.4 GHz · about 10 m (typical) · unlicensed

2. 2.4, 5, 6 GHz · tens of metres indoors · unlicensed

3. 600 MHz – 2.6 GHz (varies by country) · hundreds of metres to km · licensed

4. about 3.5 GHz · hundreds of metres · licensed

5. 24–40 GHz · about 100–300 m, line of sight · licensed

Quiz

Check your understanding (1 of 2)

1. What is the wavelength of a 5 GHz signal?

2. Convert 100 mW to dBm.

3. A link has 70 dB of path loss at 50 m. About how much at 100 m in free space?

Quiz

Check your understanding (2 of 2)

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

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

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

Answers

1. 6 cm
2. 20 dBm
3. About 76 dB
4. 64-QAM: 6 bits vs 2; needs more SNR
5. Doubling bandwidth
6. OFDMA

Summary: nine ideas to take away

1. λ = c / f · 0 dBm = 1 mW · FSPL ≈ 80 dB at 100 m, +6 dB per doubling · keep a link margin

2. Denser modulation needs more SNR · C = B · log₂(1 + SNR): bandwidth is the strongest lever

3. FDMA → TDMA → CDMA → OFDMA · FDD vs TDD · cells reuse frequencies · licensed and unlicensed bands