Wireless 101
M05 · L02
Module 5 · Lesson 2

Keying a Carrier

The message is now bits. A sine wave has only three things you can change — amplitude, frequency, phase. Switch one of them per symbol, and you have digital modulation.

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Wireless 101
M05 · L02
Three knobs

Amplitude, Frequency, Phase

Switch the amplitude → ASK. Switch the frequency → FSK. Switch the phase → PSK. Keying means holding one discrete state per symbol — two states for binary, so one bit each.

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Wireless 101
M05 · L02
Keying the amplitude

ASK & On-Off Keying

Carrier on = 1, carrier off = 0. The cheapest scheme to build and to receive — an envelope detector (Lesson 3.3) is enough. But noise is amplitude, and so is the message, so it is the least robust, and its varying envelope needs a linear amplifier.

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Wireless 101
M05 · L02
Keying the frequency

FSK — Two Tones

A high tone for 1, a low tone for 0. The amplitude never moves, so FSK is constant-envelope: efficient class-C amplifiers, immune to amplitude noise and fading. The cost is bandwidth. Gaussian-smoothed GFSK is what Bluetooth uses.

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Wireless 101
M05 · L02
Keying the phase

BPSK — a 180° Flip

Binary Phase-Shift Keying
s(t) = \pm A\cos(2\pi f_c t),\quad \phi \in \{0,\ \pi\}

A 1 is the cosine, a 0 its exact negative — the two symbols point in opposite directions, as far apart as equal-power signals can be. Constant-envelope and the best binary noise performance.

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Wireless 101
M05 · L02
Two bits at once

QPSK — the Workhorse

Four phases, 90° apart, carry 2 bits per symbol. With Gray coding it keeps BPSK’s error rate at the same energy per bit — but needs half the bandwidth for the same data rate. A free doubling of efficiency.

Phases
4
Bits / symbol
2
Bandwidth vs BPSK
½×
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Wireless 101
M05 · L02
Try it

One byte, three ways

Slide to change the 8-bit message. The same bits become bursts (ASK), tone shifts (FSK), and phase flips (PSK). Watch the phase reversals at every bit change — 0 → 1 as well as 1 → 0.

Byte 10110101
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Wireless 101
M05 · L02
How reliable?

The 3 dB Gap

Bit-Error Rate · coherent
Q\!\left(\sqrt{\tfrac{2E_b}{N_0}}\right)_{\text{PSK}} \;<\; Q\!\left(\sqrt{\tfrac{E_b}{N_0}}\right)_{\text{ASK, FSK}}

The factor of 2 inside the square root means BPSK/QPSK hit any target error rate at half the Eb/N0 of ASK/FSK — a 3 dB saving in power (Lesson 2.4).

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Wireless 101
M05 · L02
Where each lives

Out in the Wild

  • ASK / OOK — RFID tags, IR remotes, garage openers, on-off optical fiber
  • FSK / GFSK — Bluetooth, pagers, caller-ID, ISM sensors, old modems
  • BPSK — GPS, deep-space probes, low-rate control channels
  • QPSK — satellite links, WiFi, LTE and 5G
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Wireless 101
Knowledge Check

Check what stuck

Four questions from this lesson. Answer to see why — the explanation appears whether you were right or wrong. Nothing is scored or saved.

Question 1 of 0
Score 0/0

10 / 11
Wireless 101
M05 · L02
Recap

What you learned

  • Amplitude → ASK, frequency → FSK, phase → PSK; one discrete state per symbol
  • ASK is cheapest but least robust; FSK is robust but widest; PSK is thrifty and strongest
  • BPSK beats ASK/FSK by 3 dB — the 2 lives inside the Q-function’s root
  • QPSK sends 2 bits/symbol at BPSK’s error rate: half the bandwidth, so it is everywhere
Up next in Module 5
Constellation Diagrams →
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