Modulation and Demodulation Technologies

Modulation is the process of varying one property of a high-frequency carrier signal (amplitude, frequency, or phase) to carry lower-frequency information over long distances. Demodulation is the reverse — extracting the original information from the received carrier at the destination.

Overview of analog modulation (AM, FM, PM) and digital modulation (ASK, PSK, QAM) techniques.

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1. Why Is Modulation Needed?

Without modulation, a voice signal (300–3400 Hz) cannot be transmitted efficiently through air because:

• Antenna size problem: An antenna must be roughly ¼ the wavelength of the signal. A 3 kHz signal has a wavelength of ~100 km — an impractically large antenna. A 900 MHz carrier reduces antenna size to a few centimeters.
• Frequency sharing (Multiplexing): Multiple signals can be shifted to different frequency bands and transmitted simultaneously over the same medium.
• Noise resistance: Higher-frequency carriers with certain modulation types (especially FM) are much more resilient to noise than low-frequency baseband transmission.

Exam point: Modulation allows simultaneous transmission of multiple conversations over the same medium by placing each at a different carrier frequency — this is FDM (Frequency Division Multiplexing).

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2. Analog Modulation

In analog modulation, the information (message) signal is a continuous analog waveform. The carrier is a high-frequency sine wave; modulation changes one of its three properties.

Property Changed	Modulation Type	Abbreviation
Amplitude	Amplitude Modulation	AM
Frequency	Frequency Modulation	FM
Phase	Phase Modulation	PM

2.1 Amplitude Modulation (AM)

The amplitude of the carrier varies in step with the message signal. The frequency and phase of the carrier stay constant.

In AM, the carrier's amplitude envelope follows the shape of the message signal while the carrier frequency stays fixed.

Key facts:

• Simple to implement and demodulate
• Highly susceptible to noise (noise affects amplitude)
• Used in: MW/SW radio broadcasting, aviation communication (VHF AM)
• Bandwidth = $2 \times f_m$ (where $f_m$ is the highest message frequency)
• Sidebands: Two sidebands are produced — Upper Sideband (USB) at $f_c + f_m$ and Lower Sideband (LSB) at $f_c - f_m$
• Modulation index μ must be ≤ 1 to avoid overmodulation and distortion

Exam point: AM bandwidth = $2 \times f_m$. For a 5 kHz audio signal on AM, occupied bandwidth = 10 kHz.

Variants of AM:

Variant	Full name	What it transmits	Bandwidth	Advantage
DSB-FC	Double Sideband Full Carrier	Both sidebands + carrier	2fm	Simple receiver; standard AM broadcast
DSB-SC	Double Sideband Suppressed Carrier	Both sidebands, no carrier	2fm	Power efficient
SSB	Single Sideband	One sideband only	fm	Half the bandwidth; used in HF/military
VSB	Vestigial Sideband	One full + vestige of other	~fm	Used in analog TV

Exam point: SSB uses half the bandwidth of DSB and is power-efficient — commonly tested comparison.

A modulated carrier creates a Lower Sideband (LSB) at fc − fm and an Upper Sideband (USB) at fc + fm. Together they determine the occupied bandwidth.

2.2 Frequency Modulation (FM)

The frequency of the carrier varies in proportion to the message signal amplitude. Amplitude stays constant.

Key facts:

• Much better noise immunity than AM (noise mainly affects amplitude, not frequency)
• Bandwidth is wider than AM
• Used in: FM radio (88–108 MHz), VHF voice links, stereo audio
• Modulation index β = Δf / fm (ratio of peak frequency deviation to message frequency)
• Bandwidth (Carson's Rule): BW = 2(Δf + fm)

Exam point: FM is preferred over AM when noise immunity is critical. FM is used for high-fidelity audio (commercial FM radio).

In FM, the spacing between carrier cycles (instantaneous frequency) changes with the message amplitude while the carrier amplitude stays constant.

2.3 Phase Modulation (PM)

The phase of the carrier changes with the message signal. PM and FM are mathematically related — a changing phase also instantaneously changes frequency.

Key facts:

• PM is the basis of many digital modulation schemes (PSK is derived from PM)
• Used in some satellite and microwave links
• Less common as a standalone scheme compared to AM and FM

In PM, the phase of the carrier shifts in proportion to the message signal value, producing visible leading and lagging relative to a reference carrier.

AM vs FM — Key Comparison

Property	AM	FM
What changes	Amplitude	Frequency
Amplitude constant?	No	Yes
Noise immunity	Low (noise hits amplitude)	High (noise ignores frequency)
Bandwidth	2fm (narrow)	2(Δf + fm) (wider)
Circuit complexity	Simple	More complex
Power efficiency	Low (power wasted in carrier)	Better
Typical use	MW/SW radio, aviation	FM radio, VHF links

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3. Digital Modulation

In digital modulation, the message is a digital bitstream (0s and 1s). The carrier's amplitude, frequency, or phase is switched between discrete states, one per symbol.

Symbol vs Bit:

• A symbol is one transmitted unit. A symbol can carry more than one bit.
• Bits per symbol = log₂(M), where M is the number of distinct states (modulation order)

3.1 ASK — Amplitude Shift Keying

The carrier amplitude switches between values to represent bits. Simple but highly susceptible to noise. Used in optical fiber (on/off keying is a form of ASK) and RFID.

3.2 FSK — Frequency Shift Keying

The carrier frequency switches between values. More noise-resistant than ASK. Used in older modems, caller-ID, paging systems. BFSK (Binary FSK) uses two frequencies for 0 and 1.

3.3 PSK — Phase Shift Keying

The carrier phase switches between values. Better spectral efficiency than FSK.

PSK Type	States (M)	Bits/Symbol	Use
BPSK	2	1	Satellite links, deep-space; most robust
QPSK	4	2	3G (WCDMA), satellite; good balance
8-PSK	8	3	DVB-S2, some microwave links

Exam point: BPSK carries 1 bit/symbol; QPSK carries 2 bits/symbol. QPSK is twice as spectrally efficient as BPSK at the same symbol rate.

3.4 QAM — Quadrature Amplitude Modulation

QAM combines both amplitude and phase changes, producing a large number of distinct symbols. Each symbol represents multiple bits, enabling very high data rates.

QAM Order	States (M)	Bits/Symbol	Typical use
16-QAM	16	4	LTE, Wi-Fi (802.11)
64-QAM	64	6	LTE, DOCSIS cable
256-QAM	256	8	LTE-Advanced, 5G, cable broadband
1024-QAM	1024	10	5G NR (good channel conditions)

$\text{Bit Rate} = \text{Symbol Rate} \times \log_2(M)$

Exam point: Higher-order QAM carries more bits per symbol but needs a cleaner (higher SNR) channel. 64-QAM needs a better signal than 16-QAM to work reliably.

A QAM constellation maps each symbol to an (in-phase, quadrature) point. 16-QAM has 16 points — each carrying 4 bits. More points = more bits per symbol = higher SNR required.

3.5 GMSK — Gaussian Minimum Shift Keying

A special form of FSK where transitions are smoothed using a Gaussian filter. Produces a compact spectrum. Used in GSM (2G) networks.

Exam point: GSM uses GMSK modulation. GMSK is efficient and compact, ideal for mobile channels.

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4. OFDM — Orthogonal Frequency Division Multiplexing

OFDM is not a single modulation scheme but a multi-carrier transmission method. A high-speed data stream is split into many parallel slower streams, each modulated onto a closely spaced subcarrier.

How it works:

• Data is split into many parallel streams
• Each stream modulates its own narrow subcarrier (using QPSK or QAM)
• Subcarriers are mathematically orthogonal — they do not interfere with each other even though they overlap
• All subcarriers are combined and transmitted together

Why OFDM is powerful:

• Multipath resistance: Urban environments cause signals to reflect off buildings, arriving at the receiver at slightly different times. OFDM handles this efficiently using a guard interval (Cyclic Prefix).
• Spectral efficiency: Closely packed subcarriers use spectrum efficiently.
• Easy equalization: Each narrow subcarrier sees a flat frequency response, simplifying the receiver.

Standard	OFDM variant
LTE (4G) downlink	OFDMA (OFDM + multiple access)
LTE (4G) uplink	SC-FDMA (single-carrier variant)
5G NR	OFDM with numerology (variable subcarrier spacing)
Wi-Fi (802.11a/g/n/ac/ax)	OFDM / OFDMA
DVB-T (digital TV)	OFDM

Exam point: LTE uses OFDMA for downlink (base station to phone) and SC-FDMA for uplink (phone to base station). 5G NR also uses OFDM.

OFDM packs many narrow orthogonal subcarriers tightly together. Each subcarrier carries a low-speed stream (QPSK or QAM), but combined they deliver very high aggregate throughput.

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5. Demodulation

Demodulation is the receiver-side process of recovering the original message from the received modulated signal.

Modulation	Demodulation method
AM	Envelope detector (non-coherent) or synchronous detector (coherent)
FM	Discriminator / limiter-discriminator, PLL demodulator
PM	Phase detector / Costas loop
PSK / QAM	Coherent detector (requires carrier recovery and synchronization)

Coherent vs Non-Coherent Detection

Feature	Coherent	Non-Coherent
Phase synchronization required?	Yes	No
Performance (BER)	Better (lower error rate)	Worse (higher error rate)
Circuit complexity	High	Low
Examples	BPSK, QPSK, QAM	BFSK, ASK envelope detector

Exam point: Coherent detection gives better performance but is more complex. Non-coherent detection is simpler but less accurate.

A typical receiver: RF front-end selects the channel → amplifies → down-converts to baseband → demodulates → recovers the original message signal.

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6. Modulation in Real Telecom Systems

System	Modulation used
AM radio (MW/SW)	AM (DSB-FC)
FM radio	FM
GSM (2G)	GMSK
CDMA (3G) / WCDMA	QPSK (data), BPSK (control)
LTE (4G) downlink	OFDMA (with QPSK, 16-QAM, 64-QAM)
LTE (4G) uplink	SC-FDMA
5G NR	OFDM (with up to 256-QAM or 1024-QAM)
Wi-Fi (802.11ac/ax)	OFDM (up to 256-QAM / 1024-QAM)
Satellite (broadband)	BPSK, QPSK, 8-PSK, 16-APSK
Optical fiber (coherent)	DP-QPSK, 16-QAM

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7. Key Formulas (Exam Quick Reference)

Formula	What it gives
$BW_{AM} = 2 f_m$	Bandwidth of an AM signal
$BW_{FM} = 2(\Delta f + f_m)$	Bandwidth of an FM signal (Carson's Rule)
$\beta = \Delta f / f_m$	FM modulation index
$\text{Bits/symbol} = \log_2(M)$	Bits carried per symbol for M-ary modulation
$R_b = R_s \times \log_2(M)$	Bit rate from symbol rate

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8. Quick Revision — Key Facts for the Exam

Topic	Remember this
AM changes	Amplitude of carrier
FM changes	Frequency of carrier
PM changes	Phase of carrier
AM noise immunity	Low (noise affects amplitude)
FM noise immunity	High (noise does not affect frequency)
AM bandwidth	2fm
FM bandwidth (Carson's)	2(Δf + fm)
SSB advantage	Half the bandwidth of DSB
BPSK bits/symbol	1
QPSK bits/symbol	2
16-QAM bits/symbol	4
64-QAM bits/symbol	6
256-QAM bits/symbol	8
GSM modulation	GMSK
LTE downlink multiple access	OFDMA
LTE uplink multiple access	SC-FDMA
OFDM strength	Multipath resistance, spectral efficiency
Coherent detection	Better BER; needs phase sync
Non-coherent detection	Simpler; higher BER

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9. Practice Questions

1. Which property of the carrier signal is changed in FM?
2. Why is FM preferred over AM for high-fidelity audio?
3. What is the bandwidth of an AM signal carrying a 5 kHz audio message?
4. What is the bandwidth of an FM signal with a frequency deviation of 75 kHz and a message frequency of 15 kHz?
5. How many bits per symbol does 64-QAM carry?
6. What modulation scheme is used in GSM (2G) networks?
7. What multiple access scheme does LTE use for the downlink?
8. What is the advantage of SSB over DSB-AM?
9. Explain the difference between coherent and non-coherent detection.
10. What is OFDM and why is it used in 4G/5G systems?
11. A QAM system uses 256 symbols. How many bits does each symbol carry?
12. What does Carson's Rule calculate?

1. Frequency — the carrier frequency varies with the message signal amplitude.
2. FM has better noise immunity; noise mainly affects amplitude, which FM ignores. FM also has higher audio fidelity.
3. BW = 2 × 5 = 10 kHz.
4. BW = 2(75 + 15) = 2 × 90 = 180 kHz.
5. log₂(64) = 6 bits per symbol.
6. GMSK (Gaussian Minimum Shift Keying).
7. OFDMA (Orthogonal Frequency Division Multiple Access).
8. SSB uses half the bandwidth (only one sideband is transmitted), making it more spectrum-efficient.
9. Coherent detection requires the receiver to synchronize its local carrier in phase and frequency with the transmitter — better performance but more complex. Non-coherent detection recovers the signal without phase synchronization — simpler, but less accurate (higher bit error rate).
10. OFDM splits a high-speed data stream into many parallel low-speed streams on closely spaced orthogonal subcarriers. It is used because it handles multipath distortion well (common in cities) and uses spectrum efficiently.
11. log₂(256) = 8 bits per symbol.
12. Carson's Rule estimates the required bandwidth for an FM signal: BW = 2(Δf + fm).