Electromagnetic Waves: The Physical Layer Begins
Wireless communication starts with electromagnetic (EM) waves. These waves are oscillating electric and magnetic fields traveling through space at the speed of light.
Electric Field (E)
↑
| ~~~~~~~~
| ~~
| ~~
-----+--------------------> Propagation Direction
| ~~
| ~~
| ~~~~~~~~
Magnetic Field (H) perpendicular to E
Key characteristics:
- Frequency (Hz)
- Wavelength (λ)
- Amplitude
- Phase
The relationship: λ = c / f
Where:
- λ = wavelength
- c = speed of light (~3 × 10^8 m/s)
- f = frequency
Higher frequency → shorter wavelength → smaller antenna possible
Lower frequency → longer wavelength → better penetration
And this leads us naturally to the concept of spectrum allocation.
Frequency Spectrum and Bands: Organizing the Air
The electromagnetic spectrum is huge, but we use specific bands for communication.
|----LF----|----HF----|----VHF----|----UHF----|----SHF----|
kHz MHz MHz GHz GHz
Common embedded wireless bands:
- 433 MHz (Sub-GHz IoT)
- 868 MHz (Europe ISM)
- 915 MHz (US ISM)
- 2.4 GHz (Wi-Fi, BLE, Zigbee)
- 5 GHz (Wi-Fi)
Examples:
- Bluetooth Low Energy → 2.4 GHz
- Wi‑Fi → 2.4 GHz / 5 GHz
- Zigbee → 2.4 GHz / Sub-GHz
Why ISM bands matter:
- No license required
- Limited power
- Shared medium → interference expected
Modulation: Turning Bits into Waves
Digital systems generate 0s and 1s. But antennas radiate analog waves.
So we modulate a high-frequency carrier signal.
Carrier: Carrier: ~~~~~~~~~~~~~~~~~~~~~~~
Amplitude Shift Keying (ASK)
1 → High amplitude
0 → Low amplitude
1: ~~~~~~~~
0: ~~
Frequency Shift Keying (FSK)
1 → High frequency
0 → Low frequency
1: ~~~~~~ ~~~~~~
0: ~~ ~~ ~~ ~~
Phase Shift Keying (PSK)
1 → 0°
0 → 180° shift
Modern protocols use combinations:
- QPSK
- OFDM (Wi-Fi)
- GFSK (BLE)
And now once we transmit modulated waves, they must travel through a non-ideal environment.
Propagation and Path Loss: The Real World Problem
In real environments, signals:
- Attenuate
- Reflect
- Diffract
- Scatter
Free-space path loss: Received Power ∝ 1 / d²
TX ))))))))))))) ))))))))) ))))))) RX
Strong Medium Weak
Multipath:
Wall
|
TX ----->|----> RX
\ ^
\____Reflect__|
Consequences:
- Fading
- Inter-symbol interference
- Packet loss
This is why link budget calculation is essential.
Link Budget: Can My Signal Survive?
The link budget determines whether communication will succeed.
TX Power
- Cable Loss
+ Antenna Gain
- Path Loss
- Environmental Loss
= RX Power
If: RX Power > Receiver Sensitivity → Communication OK
Example typical BLE:
- TX Power: 0 dBm
- Sensitivity: -90 dBm
- Margin: ~90 dB
Engineers must calculate this before PCB layout or enclosure design.
And once the physical layer works, we must share the medium.
Which brings us to a critical concept: how do we encode bits onto a carrier?
In wireless systems, two major design questions always exist:
1) Who talks when? → Medium Access (CSMA, FDMA, TDMA…)
2) Who talks to whom? → Network Topology (Star, Mesh…)
Medium Access: Who Talks When?
_(Who is allowed to transmit?)_
Wireless is a shared medium. If two devices transmit at the same time on the same frequency → collision.
Device A ---> )))))
Device B ---> )))))
X (Collision)
So we need rules.
Common access mechanisms:
- CSMA/CA (Wi-Fi)
- Time slots
- Frequency hopping (BLE)
- TDMA
Example CSMA/CA:
Node A: Listen → Free → Transmit
Node B: Listen → Busy → Wait
BLE frequency hopping: Channel 1 → Channel 18 → Channel 5 → Channel 32 ...
This reduces interference and improves robustness.
Now we move from radio theory to real embedded protocols.
CSMA – Carrier Sense Multiple Access
Used in:
- Wi-Fi
- Ethernet (wired version)
Idea:
Before transmitting → listen first.
Step 1: Listen
Step 2: If channel free → transmit
Step 3: If busy → wait random time
---
Channel: ____________BUSY____________
Node A: Listen → WAIT
Node B: Listen → WAIT
Node C: Listen → FREE → TRANSMIT
In Wi-Fi (CSMA/CA – Collision Avoidance):
- Device listens
- Waits random backoff time
- Then transmits
Why “avoidance”?
Because wireless cannot detect collision while transmitting.
Pros:
- Simple
- Flexible
- Good for burst traffic
Cons:
- Collisions still happen
- Performance degrades under heavy load
- Hidden node problem
Hidden node example:
A ----> Router <---- B
A and B can't hear each other
But both collide at Router
This leads us to a different strategy.
FDMA – Frequency Division Multiple Access
Instead of sharing time…
We share frequency.
Total Spectrum
|----F1----|----F2----|----F3----|
User A User B User C
Each device gets its own frequency band.
No listening required.
No collision (if well separated).
Used in:
- Cellular systems
- Some industrial RF systems
Embedded example:
Sub-GHz IoT with 3 channels:
Channel 1 → Sensor A
Channel 2 → Sensor B
Channel 3 → Sensor C
Pros:
- Deterministic
- No collisions
- Low latency
Cons:
- Wastes spectrum if idle
- Limited number of users
- Requires filtering hardware
Common Embedded Wireless Technologies
Bluetooth Low Energy
- 2.4 GHz
- Low power
- Star topology
- Advertising + connection model
- Uses GFSK
Ideal for:
- Wearables
- Sensors
- Mobile integration
Wi‑Fi
- High throughput
- Higher power consumption
- Uses OFDM
- Infrastructure-based
Ideal for:
- OTA updates
- High data transfer
- Cloud connectivity
Zigbee
- Mesh topology
- Low data rate
- Ultra-low power
- Self-healing networks
Ideal for:
- Smart home
- Industrial sensor networks
And once devices connect, security becomes the most critical layer.
Wireless Security Fundamentals
Wireless = broadcast medium → anyone can listen.
Threats:
- Eavesdropping
- Replay attacks
- Man-in-the-middle
- Jamming
- Rogue access points
Security mechanisms:
- AES encryption
- Secure pairing
- Rolling keys
- Frequency hopping
- TLS (for Wi-Fi/IP devices)
BLE example security flow:
Device A ---- Pair Request ----> Device B
Device B ---- Public Key ------> Device A
Shared Secret Generated
Session Key Derived
Encrypted Link Established
As an embedded security engineer, you must always assume:
The attacker is listening.