Data Communication · Lecture 14

Wireless Communication Basics

Radio propagation, path loss and fading, Wi-Fi, Bluetooth, cellular networks and satellites

Md Mahbub E Noor · Assistant Professor, Department of CSE, University of Barishal

Course: Data Communication · 2nd Year, 2nd Semester

Going wireless: today's goals

Explain why wireless links lose power and suffer from fading
Compute free-space path loss and a simple link budget in dB
Describe 802.11 architecture (BSS, ESS) and its four addresses
Explain Bluetooth piconets and cellular frequency reuse
Compare GEO, MEO and LEO satellites using Kepler's law

Lecture 13 gave us

CSMA/CA, RTS/CTS and channelization (FDMA, TDMA, CDMA).

Today

the radio channel itself, and the wireless networks built on it: Wi-Fi, Bluetooth, cellular and satellite.

Reading: Forouzan (5th ed.), Chapters 15 and 16

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Lecture 14: Wireless Communication Basics

University of Barishal

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Wired versus wireless

	Wired LAN	Wireless LAN
Medium	cable, point-to-point (switched)	air, shared broadcast
Attenuation	small, predictable	large: power spreads in all directions
Interference	rare	other senders on the same band
Multipath	none	echoes arrive with different delays and phases
Errors	rare (low BER)	frequent: low and changing SNR
Access	full-duplex switch; CSMA/CD in old LANs	CSMA/CA (cannot detect collisions)
Hosts	fixed point of attachment	mobile; may move between cells

A wireless LAN changes only the two lowest layers: swap the network cards and the switch for an access point; IP addresses stay the same.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Radio frequencies and wavelength

Wireless systems use radio waves and microwaves (Lecture 4)
Higher frequency → shorter wavelength → smaller antennas
Higher frequency also → more loss and worse penetration of walls
ISM bands (902–928 MHz, 2.400–2.4835 GHz, 5.725–5.850 GHz) need no licence
Cellular bands are licensed to operators

λ = c / f

c = 3 × 10⁸ m/s · antenna size scales with λ (a quarter-wave antenna is λ/4 long)

System	f	λ	λ / 4
Mobile (GSM 900)	900 MHz	33 cm	8 cm
Wi-Fi / Bluetooth	2.4 GHz	12.5 cm	3.1 cm
Wi-Fi (5 GHz)	5 GHz	6 cm	1.5 cm
GEO TV (Ku band)	12 GHz	2.5 cm	dish ≈ 60 cm

Textbook typo: Forouzan gives the ISM band as 2.400–4.835 GHz; it is 2.400–2.4835 GHz.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Free-space path loss

Power spreads over a sphere: received power falls as 1/d²
In dB, loss grows by 6 dB for every doubling of distance
Higher frequency: smaller receiving antenna, so more loss (+6 dB per doubling of f)
Worked: Wi-Fi at 2.4 GHz over 100 m loses 80 dB (a factor of 10⁸)
Real buildings lose more: walls, bodies, furniture

L_FS = 20 log d_km + 20 log f_MHz + 32.44 dB

10 m

100 m

1 km

100

loss, dB

80 dB at 100 m

86 dB at 200 m

2.4 GHz

5 GHz

2.4 GHz, 100 m

−20 + 67.6 + 32.44

80.0 dB

Double d or f

20 log₁₀ 2

+6 dB

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Lecture 14: Wireless Communication Basics

University of Barishal

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A link budget

Add up gains and losses along the link, all in dB
Received power must stay above the receiver's sensitivity
The surplus is the fade margin: room for walls, rain, multipath
−56 dBm is about 2.5 × 10⁻⁹ W: tiny, but plenty
Free space would reach ~2 km; walls cut indoor Wi-Fi to tens of metres

P_r = P_t + G_t + G_r − L

all in dB / dBm; dBm = dB relative to 1 mW (Lecture 3)

Item	Value
Transmit power P_t (100 mW)	+20 dBm
Antenna gains G_t + G_r	+2 + 2 dBi
Path loss (2.4 GHz, 100 m)	−80.0 dB
Received P_r	−56.0 dBm
Receiver sensitivity	−82 dBm
Margin	+26 dB

Free-space range at 0 dB margin

L = 106 dB → 20 log d = 5.96

≈ 2 km (indoors: tens of m)

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it: will the signal arrive?

Interactive

Md Mahbub E Noor

Lecture 14: Wireless Communication Basics

University of Barishal

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Multipath and fading

Reflections give the receiver several copies of the signal
Copies in phase add; half a cycle apart they cancel: a fade
Moving a few centimetres (λ/2) can change strong to weak
Long echo delays smear symbols into each other: inter-symbol interference
Fading is why wireless links need margin, error control and clever modulation

direct

echo

wall / ground

Echo delay τ

30 m ÷ c

100 ns

Fade spacing

1 / τ

10 MHz

Echo in symbols

100 ns ÷ 50 ns

Remedies: diversity (two antennas), OFDM (many slow subcarriers: Wi-Fi, 4G), spread spectrum and error correction.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it: two paths, one receiver

Interactive

Md Mahbub E Noor

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University of Barishal

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How fast can a wireless link go?

Bandwidth sets the symbol rate; SNR sets the bits per symbol
Shannon: C = B log₂(1 + SNR), with SNR as a ratio, not dB
Worked: 20 MHz at 25 dB SNR → at most about 166 Mbps
Radios adapt: dense QAM near the AP, robust BPSK/QPSK at the edge
So the same Wi-Fi shows different speeds in different rooms

C = B log₂(1 + SNR)

Shannon capacity (Lecture 3): the upper limit for any modulation

Wi-Fi channel

B = 20 MHz, SNR = 25 dB = 316

log₂ 317 = 8.31

Capacity

20 × 10⁶ × 8.31

≈ 166 Mbps

Distance from AP	SNR	Typical choice
close	high (30+ dB)	64/256-QAM, high code rate
middle	~20 dB	16-QAM
edge	low (< 10 dB)	BPSK/QPSK, strong coding

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Lecture 14: Wireless Communication Basics

University of Barishal

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Checkpoint 1 · 3 minutes

Check your understanding

Q1

Free-space loss at 2.4 GHz over 1 km? (Use the formula; 20 log₁₀ 2400 ≈ 67.6.)

Q2

P_t = 20 dBm, both antennas 3 dBi, path loss 90 dB, sensitivity −75 dBm. Received power and margin?

Q3

An echo travels 60 m farther than the direct path. What is its delay, and how often do fades repeat across frequency?

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Lecture 14: Wireless Communication Basics

University of Barishal

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IEEE 802.11 architecture

BSS: the basic building block; ad hoc (no AP) or infrastructure (with an AP)
Access point: joins the wireless and wired worlds
ESS: several BSSs linked by a distribution system
Stations may roam between BSSs of one ESS
Like cellular: BSS ≈ cell, AP ≈ base station

distribution system

BSS 1

BSS 2

= one ESS

Term	Meaning
BSS	basic service set: stations (+ an AP for infrastructure; none for ad hoc)
ESS	two or more BSSs whose APs are joined by a distribution system (usually Ethernet)
Mobility	no-transition · BSS-transition (within an ESS) · ESS-transition

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Lecture 14: Wireless Communication Basics

University of Barishal

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802.11 MAC: DCF, PCF and the frame

DCF: CSMA/CA with RTS/CTS and ACKs (Lecture 13)
PCF: optional polling by the AP for time-sensitive data
When both run, a repetition interval mixes a contention-free and a contention period
The Duration field sets other stations' NAV
Up to four addresses; FCS is a CRC-32 (Lecture 11)

DCF

Distributed coordination: CSMA/CA, DIFS, RTS/CTS, NAV, ACK (Lecture 13). Mandatory; contention service.

PCF

Point coordination: the AP polls stations; optional, for time-sensitive traffic; contention-free service.

FC 2

D 2

Addr 1

Addr 2

Addr 3

SC 2

Addr 4

Body ≤ 2312

FCS 4

Frame fields in bytes: frame control, duration (for NAV), four 6-byte addresses, sequence control, body, CRC-32

Frame type	Examples
Management	beacons, association, authentication
Control	RTS, CTS, ACK
Data	user data

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Lecture 14: Wireless Communication Basics

University of Barishal

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802.11 addressing: four cases set by To DS and From DS

To DS	From DS	Address 1	Address 2	Address 3	Address 4	Case
0	0	Destination	Source	BSS ID	–	station → station, same BSS
0	1	Destination	Sending AP	Source	–	AP → station
1	0	Receiving AP	Source	Destination	–	station → AP
1	1	Receiving AP	Sending AP	Destination	Source	AP → AP (wireless DS)

Worked: laptop A (BSS 1) → B (BSS 2) over a wired distribution system

Hop	To / From DS	Addr 1	Addr 2	Addr 3
A → AP1	1 0	AP1	A	B
AP1 → AP2	Ethernet frame
AP2 → B	0 1	B	AP2	A

Address 1 = next receiver

Address 2 = previous sender

Address 3 = the end station not named yet

Address 4 = original source (wireless DS only)

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it: the four 802.11 address cases

Interactive

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Lecture 14: Wireless Communication Basics

University of Barishal

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802.11 physical layers

Standard	Technique	Band	Rate (Mbps)
802.11 (1997)	FHSS / DSSS / infrared	2.4 GHz	1, 2
802.11b	DSSS (CCK)	2.4 GHz	5.5, 11
802.11a	OFDM	5 GHz	6 – 54
802.11g	OFDM	2.4 GHz	up to 54
802.11n (Wi-Fi 4)	OFDM + MIMO	2.4 and 5 GHz	up to 600
802.11ac / ax (Wi-Fi 5 / 6)	OFDM / OFDMA, MU-MIMO	5 (and 6) GHz	Gbps range

Corrections to Forouzan's Table 15.4: the 2.4 GHz band is 2.400–2.4835 GHz (not 4.835); 802.11n works in both 2.4 and 5 GHz; 802.11g's standard rate is 54 Mbps. The last row is beyond the 5th edition.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Checkpoint 2 · 3 minutes

Check your understanding

Q1

A campus has 40 access points on one wired network, all with the same network name. In 802.11 terms, what is this?

Q2

A station sends a frame to its AP for a host in another BSS. Give To DS, From DS and Address 1–3.

Q3

Why does 802.11 need an ACK for every data frame when classic Ethernet did not?

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Lecture 14: Wireless Communication Basics

University of Barishal

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Bluetooth: piconets and scatternets

A short-range (~10 m) ad hoc network of gadgets (IEEE 802.15.1)
Piconet: one primary, up to 7 active secondaries (more may be parked)
Secondaries follow the primary's clock and hopping sequence
Scatternet: a station in two piconets links them
Radio: FHSS over 79 channels, 1600 hops/s; time slots of 625 µs (TDD)

S in one, member of both

piconet 1

piconet 2

Radio

2.4 GHz ISM, 79 × 1 MHz

FHSS

Slot (one hop)

1 / 1600 hops/s

625 µs

Piconet

1 primary + up to

7 active

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Lecture 14: Wireless Communication Basics

University of Barishal

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Cellular networks: frequency reuse

The area is divided into cells, each with a base station (BS)
Base stations connect to a mobile switching centre (MSC) and the phone network
Neighbouring cells use different frequency groups
A reuse pattern of N cells repeats; red cells share group 1
Low transmit power keeps each cell's signal local

Reuse factor

N = i² + ij + j², i = 2, j = 1

N = 7

Reuse distance

D = R √(3N) = R √21

D ≈ 4.58 R

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Lecture 14: Wireless Communication Basics

University of Barishal

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How many calls fit in a cell?

Channels per cell = channels available ÷ N
AMPS: 832 channels, two operators, control channels, N = 7 → 56 calls per cell
GSM: 124 carriers × 8 slots = 992; N can be as low as 3
GSM: digital voice, error correction → tolerates more interference
Smaller cells (micro/pico) reuse frequencies more often → more capacity

Step	AMPS (1G, FDMA)	GSM (2G, FDMA + TDMA)
One-way band	25 MHz (824–849 / 869–894)	25 MHz (890–915 / 935–960)
Channel width	30 kHz → 832 channels	200 kHz → 124 carriers
Users per carrier	1	8 time slots → 992
Per operator / control	416, minus 21 control = 395	–
Reuse factor N	7	as low as 3
Channels per cell	395 / 7 ≈ 56	992 / 3 ≈ 330

GSM channel bit rate

(1/120 ms) × 26 × 8 × 156.25 bits

270.8 kbps

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it: reuse the frequencies

Interactive

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Lecture 14: Wireless Communication Basics

University of Barishal

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Calls, handoff and roaming

The MSC sets up calls and assigns channels
Incoming call: the MSC pages cells to find the phone
Moving between cells: the call is handed off to a new BS
Hard handoff: break before make; soft: make before break
Roaming: service from another provider's network

Event	What happens
Making a call	phone scans for the strongest setup channel → BS → MSC → called party; MSC assigns a voice channel
Receiving	MSC pages all cells to find the phone, then rings it and assigns a channel
Hard handoff	break with the old BS, then connect to the new one (1G, 2G)
Soft handoff	talk to old and new BS at once, then drop the old (CDMA, 3G)
Roaming	another operator serves you under an agreement

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Lecture 14: Wireless Communication Basics

University of Barishal

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Cellular generations

Generation	Systems	Service	Multiple access (Lecture 13)
1G (1980s)	AMPS	analog FM voice	FDMA
2G (1990s)	GSM, D-AMPS, IS-95	digital voice, SMS	TDMA + FDMA; CDMA (IS-95)
3G (2000s)	UMTS (W-CDMA), CDMA2000	voice + mobile data	CDMA
4G (2010s)	LTE, WiMAX	all-IP data, VoIP	OFDMA (MIMO)
5G (2020s)*	5G NR	fast data, IoT, low latency	OFDMA, wide bands, mmWave

Every generation reuses ideas from this course: modulation (L7), multiplexing and spread spectrum (L9), switching (L10), error control (L11–12) and multiple access (L13). *5G is beyond the 5th edition.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Satellites and Kepler's law

A satellite relays signals between distant points on Earth
Kepler: period = C × distance^1.5, C ≈ 1/100 (s, km from Earth's centre)
At 35,786 km the period is 24 h: geostationary (GEO)
Orbits: equatorial, inclined or polar
Footprint: the area a satellite's signal covers

LEO 550 km (hugs the Earth)

MEO / GPS 20,200 km

GEO 35,786 km (outer ring)

Earth radius 6,378 km

Ex. 16.2: GEO

(1/100) × 42,164^1.5 = 86,579 s

≈ 24 h

Ex. 16.1: the Moon

(1/100) × 390,378^1.5 = 2.44 × 10⁶ s

≈ 1 month

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Lecture 14: Wireless Communication Basics

University of Barishal

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GEO, MEO and LEO compared

Orbit	Altitude	Period	Up + down delay (overhead)	Examples
LEO	500 – 2,000 km	~90 – 120 min	≈ 3.7 ms (550 km)	Iridium, Starlink, Earth imaging
MEO	5,000 – 20,000 km	3 – 12 h	≈ 135 ms (GPS)	GPS (24+ satellites, 6 orbits)
GEO	35,786 km	24 h	≈ 239 ms	TV broadcast, weather, VSAT

GEO up + down

2 × 35,786 km / 300,000 km/s

≈ 239 ms

Question + answer via GEO

two up-and-down trips

≈ 0.48 s

Three GEO satellites

120° apart

cover nearly all the Earth

Textbook inconsistency: Forouzan puts MEO at 5,000–15,000 km (6–8 h) but GPS at “about 18,000 km”. GPS actually orbits at about 20,200 km with a 12 h period (Kepler: 43,329 s).

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it: higher orbit, longer period and delay

Interactive

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University of Barishal

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Wireless systems compared

System	Range	Rate	Access (Lecture 13)	Use
Bluetooth	~10 m	1–3 Mbps	polling in a piconet, FHSS	earbuds, keyboards
Wi-Fi (802.11)	~50 m indoors	11 Mbps – Gbps	CSMA/CA (DCF)	home, campus LAN
Cellular	1–10+ km per cell	kbps (2G) – Gbps (5G)	FDMA, TDMA, CDMA, OFDMA	phones, mobile data
Satellite	continent	Mbps – 100s of Mbps	TDMA / FDMA; ALOHA requests	TV, GPS, remote Internet

One trade-off everywhere: range ↔ rate ↔ power. Longer range costs path loss; higher rate needs more SNR or bandwidth.

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Lecture 14: Wireless Communication Basics

University of Barishal

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Try it yourself: Wireless Lab

1 · Link budget: path loss, received power and range
2 · Multipath: move the echo and watch fades appear
3 · Cell reuse: colour N = 3, 4, 7, 12; channels per cell
4 · Satellites: Kepler's law and delay for any orbit

Scan to open the lab on your phone

Open the lab ↗

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Lecture 14: Wireless Communication Basics

University of Barishal

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See it in action: going wireless

Watch signal strength fall as distance doubles
See an echo cancel the direct wave
Follow a laptop from one BSS to another
See frequency reuse in a cell pattern, and two orbits
When a “Pause & predict” card appears, answer before the video moves on

Click the video to play or pause · Press → for full screen

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Lecture 14: Wireless Communication Basics

University of Barishal

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Checkpoint 3 · 3 minutes

Check your understanding

Q1

An operator has 840 two-way channels and uses N = 7. How many channels per cell, and what is D for R = 1.5 km?

Q2

Using Forouzan's Kepler formula, what is the period of a satellite 1,200 km above the Earth?

Q3

Why is a GEO satellite link poorly suited to Stop-and-Wait ARQ (Lecture 12)?

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Lecture 14: Wireless Communication Basics

University of Barishal

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Key takeaways

Radio loses power with distance (≈ 6 dB per doubling) and fades with multipath
Link budget: P_r = P_t + G_t + G_r − L; keep a margin above sensitivity
802.11: BSS/ESS, DCF (CSMA/CA) and PCF, four addresses; Bluetooth piconets
Cellular: frequency reuse N, D = R√(3N), handoff; 1G → 5G
Satellites: Kepler's law; GEO 24 h and ~0.24 s delay; LEO low delay, many satellites

Course complete

From signals to wireless networks

Signals and bits (Lectures 1–9), switching and the data link layer (10–13), wireless networks (14). Revise with the course map, the labs and each deck's checkpoints.

Reading: Forouzan, Chapters 15 and 16 · Practise in the Wireless Lab

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Lecture 14: Wireless Communication Basics

University of Barishal

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The course in one map

Introduction to Data Communications

Data and Signals I

Data and Signals II

Transmission Media

Analog-to-Digital Conversion

Digital-to-Digital Conversion

Digital-to-Analog Conversion

Analog-to-Analog Conversion

Multiplexing and Spread Spectrum

Switching

Data Link Layer I

Data Link Layer II

Multiple Access

Wireless Communication Basics

Physical layer (1–10): signals, media, conversion, multiplexing, switching · Data link layer (11–13): framing, error and flow control, MAC · Wireless (14): everything together

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University of Barishal

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