Chapter 15 — Communication Systems

Fig 15.1 — Educational Diagram: Introduction to Communication Systems

What is a communication system? A communication system transfers information (voice, data, image) from a source to a destination using a suitable physical medium. Information is carried by signals which are often translated to electromagnetic waves for transmission.

Basic blocks. A simple communication system has three main blocks: Transmitter (encodes and conditions the message; may modulate it onto a carrier), Channel (medium — free space, cable, optical fiber — that may add attenuation and noise), and Receiver (demodulates and decodes to recover the message). Typical transmitter/receiver sub-blocks include sources, modulators/demodulators, amplifiers, filters and antennas.

Why modulation? Most information signals (audio, sensor outputs) are low-frequency (baseband). Direct transmission of baseband over long distances is inefficient or impossible because of antenna size and channel properties. Modulation maps a baseband signal onto a high-frequency carrier so it can be efficiently radiated, separated in frequency from other users, and delivered with smaller antennas. Demodulation at the receiver retrieves the original baseband signal.

Types of signals & systems. Signals can be analog (continuous amplitude/time) or digital (discrete levels/time). Communication systems can be analog (AM/FM radio) or digital (mobile phones, Wi‑Fi). Digital systems are robust to noise and allow efficient multiplexing and error correction.

Noise and performance. Noise (thermal, atmospheric, interference) degrades received signal. Important performance measures are bandwidth (frequency extent of the signal or channel) and signal-to-noise ratio (SNR). Trade-offs exist: higher data rates require more bandwidth or higher SNR.

Baseband vs passband. Baseband: signal content centered around 0 Hz (e.g., raw audio). Passband: signal shifted to a carrier frequency fc (e.g., radio transmission). Modulation converts baseband into passband and demodulation converts back.

Common modulation methods (brief). Amplitude Modulation (AM): message varies amplitude of carrier. Frequency Modulation (FM): message varies instantaneous frequency of carrier. Phase Modulation (PM): message varies carrier phase. Digital schemes include ASK, FSK, PSK, QAM.

Examples in everyday life. AM/FM radio, television broadcast, mobile phones, Wi‑Fi, Bluetooth, satellite TV, GPS, optical fiber internet. Each uses modulation, a transmitter, a channel, and a receiver, with design choices suited to required range, data rate and environment.

Design considerations. Choose carrier frequency (higher gives smaller antennas but different propagation), bandwidth (determines data rate), power (determines range), and coding/modulation scheme (trade-off of data rate vs noise tolerance). Regulatory aspects (frequency allocation) are also critical.

Fig 15.2 — Educational Diagram: Electromagnetic Spectrum and Frequency Bands

What is the electromagnetic spectrum?
The electromagnetic (EM) spectrum is the complete range of all possible frequencies (or wavelengths) of electromagnetic radiation. Different frequency ranges (bands) have different physical properties and practical uses in communication, imaging and industry.

Key idea (wave relation)
Every EM wave satisfies c = f · λ, where c is the speed of light in vacuum (≈ 3.00 × 10⁸ m/s), f is frequency (Hz) and λ is wavelength (m). Higher frequency means shorter wavelength and higher photon energy.

Typical bands (approximate ranges and notes)

• Radio waves (used for wireless communications) — subdivided commonly as:
  • LF / LF-like long wave: tens of kHz (large wavelengths) — maritime navigation, lighthouse beacons.
  • MF (Medium wave): ~300 kHz – 3 MHz — AM broadcast (≈0.5–1.7 MHz).
  • HF (Short wave): ~3 MHz – 30 MHz — international shortwave broadcasting, sky-wave (ionospheric) propagation.
  • VHF: ~30 MHz – 300 MHz — FM radio (88–108 MHz), TV channels, line-of-sight communication.
  • UHF: ~300 MHz – 3 GHz — TV, mobile phones, Bluetooth, some Wi‑Fi bands.
• Microwaves (~3 GHz – 300 GHz) — satellite links, radar, Wi‑Fi (2.4 GHz, 5 GHz), microwave ovens (2.45 GHz). Short wavelengths allow compact antennas and high data rates, but more atmospheric attenuation at higher microwave frequencies.
• Infrared (IR) (~3 × 10¹¹ – 4 × 10¹⁴ Hz) — remote controls, thermal imaging, short-range optical communication.
• Visible light (~4 × 10¹⁴ – 7.5 × 10¹⁴ Hz; wavelength ~700–400 nm) — optical communications (fiber optics uses near-infrared/visible), imaging and human vision.
• Ultraviolet (UV) (~7.5 × 10¹⁴ – 3 × 10¹⁶ Hz) — sterilization, fluorescence.
• X‑rays (~3 × 10¹⁶ – 3 × 10¹⁹ Hz) — medical imaging, material inspection.
• Gamma rays (> 3 × 10¹⁹ Hz) — nuclear processes, high-energy astrophysics, cancer therapy.

Propagation and practical consequences

• Low-frequency radio (long wavelength) diffracts around obstacles and follows ground; used for long-range communication (e.g., AM broadcasting, submarine communications at ELF/VLF).
• HF (short waves) can be reflected by the ionosphere (sky-wave), enabling long-distance communication without satellites.
• VHF/UHF and microwaves are largely line-of-sight; antenna height and direct path matter. Higher frequencies allow smaller antennas (antenna size ~ λ/2) and larger available bandwidth → higher data rates.
• Atmospheric absorption increases at some bands (water vapour and oxygen absorption bands in microwave and mm-wave), producing windows and opaque regions important for satellite and radar design.

Class-12 level summary points

• EM spectrum orders bands by increasing frequency (and decreasing wavelength and increasing photon energy).
• Communication systems choose bands based on propagation, antenna size, available bandwidth and regulation.
• Key relations: c = f·λ and E = h f relate wave and quantum properties.

Worked numerical examples (quick)

• Wavelength of FM radio carrier at 100 MHz: λ = c/f = 3.00 × 10⁸ / 1.00 × 10⁸ = 3.0 m.
• Wavelength of Wi‑Fi at 2.4 GHz: λ = 3.00 × 10⁸ / 2.4 × 10⁹ ≈ 0.125 m (12.5 cm).
• Photon energy of visible light at f = 6.0 × 10¹⁴ Hz: E = h f ≈ 6.626 × 10⁻³⁴ × 6.0 × 10¹⁴ ≈ 3.98 × 10⁻¹⁹ J (~2.5 eV).

Fig 15.3 — Educational Diagram: Modes of Communication

Definition: Modes of communication are the physical paths or media through which information (signals) is transmitted from a transmitter to a receiver. In telecommunication these are broadly classified into guided (wired) and unguided (wireless) modes.

1. Guided (Wired) Communication: Signals are confined to a physical medium that guides the waves. Main types:

• Twisted pair (wire): Two insulated copper wires twisted to reduce interference. Used in telephony and Ethernet (LAN).
• Coaxial cable: Central conductor, dielectric, metallic shield and jacket. Higher bandwidth and lower loss than twisted pair; used for cable TV and broadband.
• Optical fibre: Glass or plastic core with higher refractive index cladding. Carries light pulses with very high bandwidth and low loss over long distances. Used for long‑haul links and high‑speed internet.

Key characteristics of guided media: well‑defined path, controlled attenuation and dispersion, higher security, but installation and repair costs can be high.

2. Unguided (Wireless) Communication: Signals propagate through air or space without a guiding physical medium. Main types:

• Radio waves: Long to short wavelength radio frequency bands used for AM/FM radio, broadcast, mobile phones, Wi‑Fi.
• Microwaves: Shorter wavelengths used for point‑to‑point links, satellite uplink/downlink, radar and microwave ovens.
• Infrared (IR): Short range line‑of‑sight links such as TV remotes and some indoor communication systems.
• Satellite communication: Earth–space links using microwave frequencies; enable global coverage for TV, telephony and data.

Key characteristics of unguided media: mobility, easy deployment, wide coverage; but subject to attenuation, interference, multipath fading and security concerns.

Other classifications (by direction): Simplex (one‑way), Half‑duplex (two‑way but one at a time), Full‑duplex (simultaneous two‑way).

Important physical points: Signal behaviour depends on frequency, medium properties (permittivity, permeability, conductivity), and geometry. For optical fibres, light is guided by total internal reflection; numerical aperture (NA) determines how much light is accepted. For wireless links, free‑space path loss and antenna gains determine received power.

Practical considerations: Choose mode by required range, bandwidth, cost, security and environment. E.g., optical fibre for backbone internet links; coax/twisted pair for last‑mile copper; radio/microwave/satellite for mobility and wide area coverage.

Fig 15.4 — Educational Diagram: Bandwidth and Information Capacity

What is bandwidth?
Bandwidth (in Hz) is the width of the frequency band that a signal or channel occupies or can pass. If a channel passes frequencies from f1 to f2, its bandwidth B = f2 − f1. In communications, bandwidth limits how fast the signal can change and therefore limits how much information can be sent per second.

Types of bandwidth

• Baseband bandwidth: frequencies from 0 up to B (e.g., digital pulses on a wire).
• Bandpass bandwidth: a band centered at some carrier frequency (e.g., radio channels).

Information capacity (channel capacity)
Information capacity is the maximum error-free data rate (bits per second) that a channel can support under given conditions. Two key formulae give theoretical limits:

1) Nyquist (noiseless channel)
For an ideal noise-free channel of bandwidth B, using M discrete signal levels per symbol, the maximum symbol rate is 2B symbols/s (for baseband signals). So the maximum bit rate is
C = 2 B log2(M) bits/s. For binary signaling (M = 2): C = 2 B bits/s.

2) Shannon–Hartley (noisy channel)
For a channel of bandwidth B (Hz) with additive white Gaussian noise and a signal-to-noise power ratio S/N, the theoretical maximum reliable data rate (capacity) is
C = B · log2(1 + S/N) bits/s. S/N must be linear (not in dB). This formula shows capacity grows with both bandwidth and SNR but with diminishing returns for SNR.

Key ideas and intuition

• Bandwidth limits how quickly you can change the waveform; shorter symbols require more spectral width.
• Noise limits distinguishable signal levels; more noise requires fewer levels or more error-correction, reducing net rate.
• Nyquist gives an upper bound when noise is negligible and signals are ideal; Shannon gives the ultimate limit when noise is present.

Units and conversions
Bandwidth in Hz, capacity in bits per second (bps). SNR in dB: SNR(dB) = 10 log10(S/N). To use Shannon, convert dB back: S/N = 10^(SNR(dB)/10).

Practical considerations
Actual rates are lower than these ideal limits due to imperfect filters, inter-symbol interference, non-ideal modulation/demodulation, coding overhead, multipath fading, and regulatory channel allocation. Engineers choose modulation (M-ary), coding, equalization and error-correction to approach these limits.

Fig 15.5 — Educational Diagram: Need for Modulation

What is modulation? Modulation is the process of varying a high-frequency carrier wave (sinusoid) in accordance with a low-frequency information (message or baseband) signal so that the information can be transmitted efficiently over long distances.

Why modulation is needed

• Efficient radiation and practical antenna size: Antennas radiate efficiently when their size is comparable to the carrier wavelength (λ). For low-frequency baseband signals (audio, e.g. a few kHz) λ would be extremely large, so direct transmission is impractical. By shifting the information to a much higher carrier frequency f_c (smaller λ), antennas become physically manageable (e.g. mobile handset, radio tower).
• Frequency-division multiplexing (FDM): Modulation moves different signals to different carrier frequencies so many channels can share the same medium simultaneously without overlapping (e.g. many radio stations, TV channels, cellular channels).
• Reduced propagation loss and long-distance transmission: Some frequency bands propagate farther or are more suitable for a channel (satellite, terrestrial microwave). Carrier frequencies are chosen to reduce attenuation and exploit favorable propagation characteristics.
• Avoiding interference and selective reception: Tuning a receiver to a carrier frequency allows selective reception of one transmitter among many. Without modulation, baseband signals would overlap and interfere.
• Improved signal-to-noise performance and demodulation techniques: Certain modulation schemes (e.g. FM) convert the desired signal into a parameter less affected by typical channel noise (FM reduces sensitivity to amplitude noise). Also, modulation enables use of filters, amplifiers, and frequency-selective components optimized for the carrier band.
• Bandwidth management and regulatory allocation: By allocating slices of spectrum to different services, authorities can manage interference and ensure orderly use of the electromagnetic spectrum.

How modulation works (conceptual): A baseband signal m(t) (low-frequency) is used to vary a carrier c(t) = A_c cos(2πf_c t + φ). For amplitude modulation (AM) the carrier amplitude is varied; for frequency modulation (FM) the instantaneous frequency is varied; for phase modulation (PM) the carrier phase is varied. After propagation, the receiver tunes to f_c and applies demodulation to recover m(t).

Practical considerations: Choice of modulation depends on required bandwidth, power efficiency, noise immunity, hardware complexity and regulatory constraints. For example, AM is simple and used in long-distance broadcasting (AM radio), FM provides better noise immunity (FM radio), and complex digital modulation (QAM, PSK, OFDM) is used in modern digital communications (Wi‑Fi, LTE) for high data rates.

Summary: Modulation is essential to make electromagnetic transmission practical (antennas of reasonable size), allow many simultaneous channels, control interference, exploit propagation properties, and improve overall system performance through appropriate modulation choices.

Fig 15.6 — Educational Diagram: Amplitude Modulation (AM)

Definition: Amplitude Modulation (AM) is a linear modulation technique in which the amplitude of a high-frequency carrier wave is varied in proportion to the instantaneous amplitude of a lower-frequency message (modulating) signal, while the carrier frequency and phase remain essentially constant.

Basic idea:

• Carrier: vc(t) = Ac cos(ωc t) — a high-frequency sinusoid (Ac = carrier peak amplitude).
• Message (modulating) signal: vm(t) — a low-frequency baseband signal that contains the information.
• AM signal (DSB-FC, single-tone modulating): s(t) = Ac[1 + m cos(ωm t)] cos(ωc t), where m is the modulation index and ωm, ωc are angular frequencies of message and carrier.

Single-tone analysis (useful for understanding spectrum):

For a single-tone message vm(t) = Am cos(ωm t) and amplitude sensitivity ka, the AM wave can be written as:

s(t) = Ac[1 + k_a Am cos(ωm t)] cos(ωc t) = Ac[1 + m cos(ωm t)] cos(ωc t)

Expanding using trigonometric identities gives:

s(t) = Ac cos(ωc t) + (m Ac/2) cos((ωc + ωm)t) + (m Ac/2) cos((ωc - ωm)t)

This shows three spectral components: the carrier at ωc and two sidebands at ωc ± ωm. AM with a general message produces sidebands around the carrier over a bandwidth equal to twice the highest modulating frequency.

Key features:

• Bandwidth: BW = 2 f_m(max) (for baseband highest frequency f_m(max)).
• Modulation index m (depth of modulation) measures how strongly carrier amplitude is varied. For single-tone m = k_a Am.
• Percent modulation = m × 100%.
• Detectability: If 0 ≤ m ≤ 1, envelope of the AM wave follows the message and can be recovered with a simple envelope detector. If m > 1 (overmodulation), envelope crosses zero and causes distortion (envelope detector fails).
• Power distribution: carrier carries no information but consumes most power; sidebands carry the information.

Demodulation: Simple envelope (diode) detectors recover the message when m ≤ 1. Coherent (synchronous) detectors may be used for better performance/noise rejection.

Advantages & limitations:

• Simple transmitter and receiver designs (envelope detector).
• Inefficient in power (carrier wastes power) and spectrum (each message requires two sidebands).
• Susceptible to noise (amplitude noise directly affects AM).

Fig 15.7 — Educational Diagram: Amplitude Demodulation

What is Amplitude Demodulation?

Amplitude demodulation (also called AM demodulation or detection) is the process of extracting the original baseband message signal m(t) from an amplitude-modulated carrier wave s(t). It is the inverse operation of amplitude modulation.

Types of AM signals and required demodulators

• DSB-FC (Standard AM with carrier, a.k.a. commercial AM): s(t)=A_c[1 + k m(t)] cos(ω_c t). Simple envelope (diode) detectors can recover m(t) when the carrier is present and modulation is not excessive.
• DSB-SC (Double-sideband suppressed-carrier): s(t)=A_c m(t) cos(ω_c t). Carrier is suppressed, so coherent (synchronous) detection is required.

Envelope detection (non-coherent)

For DSB-FC, if the modulation index is ≤ 1 (no overmodulation) the instantaneous envelope of s(t) equals A_c[1 + k m(t)]. An envelope detector (diode + RC) follows this envelope and, after low-pass filtering, outputs a signal proportional to m(t).

Important design condition for the detector time constant RC:

1/ω_c << RC << 1/ω_m

Meaning: RC must be large enough that the capacitor does not discharge appreciably during each carrier cycle (so the diode conducts only near peaks), but small enough that the capacitor follows changes in the envelope (message) without excessive smoothing.

Coherent (synchronous) detection

For DSB-SC or when higher fidelity is needed, multiply the received signal by a locally generated carrier of the same frequency and phase, then low-pass filter the product to recover the baseband. This requires carrier synchronization (phase and frequency lock), typically implemented with a PLL or carrier-recovery circuit.

Effects of phase/frequency error

If the local carrier used in coherent detection has a phase error φ, the recovered amplitude is scaled by cos φ (and can be zero if φ = 90°). Thus accurate carrier recovery is essential.

Practical notes

• Envelope detectors are cheap and widely used in commercial AM radio where the carrier is transmitted with enough amplitude.
• Coherent detectors give better SNR and are necessary when the carrier is absent or suppressed (DSB-SC) or in digital communications where phase matters.

Summary of steps in a typical AM receiver

1. RF amplification and bandpass filtering around ω_c
2. Downconversion to IF (optional) and filtering
3. Detection: envelope detector for AM (DSB-FC) or synchronous detector for DSB-SC
4. Low-pass filtering and audio amplification

Fig 15.8 — Educational Diagram: Modulation Index and Power Relations in AM

Introduction. Angle modulation is a class of modulation where the information is carried in the instantaneous angle (phase) of the carrier, rather than its amplitude. The carrier has the form s(t) = A_c cos[θ(t)], where θ(t) is the instantaneous phase. Two important types are Phase Modulation (PM) and Frequency Modulation (FM).

General expressions.

• Carrier (unmodulated): s_c(t) = A_c cos(ω_c t).
• Angle-modulated signal: s(t) = A_c cos[θ(t)], where θ(t) = ω_c t + φ(t) and φ(t) is the message-dependent phase term.

Phase Modulation (PM).

• Definition: In PM the instantaneous phase is directly varied by the message m(t).
• Time domain: s_PM(t) = A_c cos[ω_c t + k_p m(t)], where k_p is the phase sensitivity (radians per unit of m).
• Instantaneous phase: θ_PM(t) = ω_c t + k_p m(t).
• Instantaneous angular frequency: ω_i(t) = dθ_PM/dt = ω_c + k_p dm(t)/dt. Thus frequency deviation in PM depends on the derivative of m(t).
• Modulation index for PM: β_PM = k_p A_m for a sinusoidal message m(t)=A_m cos ω_m t.

Frequency Modulation (FM).

• Definition: In FM the instantaneous frequency of the carrier is varied according to the message m(t).
• Time domain: s_FM(t) = A_c cos[ω_c t + k_f ∫_0^t m(τ) dτ], where k_f is the frequency sensitivity (radians per second per unit of m).
• Instantaneous angular frequency: ω_i(t) = dθ_FM/dt = ω_c + k_f m(t). (In Hz form: f_i(t) = f_c + k_f' m(t), where k_f' is Hz per unit of m.)
• For a sinusoidal message m(t)=A_m cos ω_m t, the peak angular frequency deviation Δω = k_f A_m and modulation index β_FM = Δω/ω_m = (k_f A_m)/ω_m. In Hz terms, β = Δf / f_m.

Relationship between FM and PM. FM and PM are both angle modulation. PM depends directly on m(t) (instantaneous phase ∝ m(t)). FM depends on the integral of m(t) (instantaneous phase ∝ ∫ m). Therefore integrating a message before PM produces FM-like behavior, and differentiating before FM gives PM-like behavior.

Spectrum and bandwidth.

• Angle-modulated signals have infinite number of sidebands in theory. The amplitude of the nth sideband is given by Bessel functions J_n(β), where β is the modulation index. So s(t) = A_c Σ_{n=-∞}^{∞} J_n(β) cos(ω_c + n ω_m)t for a single-tone modulating signal.
• Carson's rule (practical bandwidth estimate): BW ≈ 2(Δf + f_m) = 2 f_m (β + 1), where Δf is peak frequency deviation and f_m is the maximum modulating frequency.
• Narrowband FM (β << 1) approximates to a carrier plus two first-order sidebands (similar to PM narrowband). Wideband FM (β >> 1) requires much larger bandwidth and has many significant sidebands.

Advantages of angle modulation.

• Greater noise immunity: FM/PM are less affected by amplitude noise because information is in angle, not amplitude.
• Constant envelope: Allows use of non-linear (efficient) RF amplifiers since amplitude remains constant.

Disadvantages.

• Requires wider bandwidth than AM for comparable audio quality.
• Demodulators and transmitters are more complex (require PLLs, discriminators, etc.).

Demodulation methods. FM: frequency discriminator, phase-locked loop (PLL). PM: phase detector, PLL (or convert PM to FM by differentiation and use FM demodulator).

CBSE/Practical notes. For single-tone analysis use m(t)=A_m cos ω_m t. Use the FM formula to find Δω and β, then apply Bessel function results for amplitude of carrier and sidebands if required. For bandwidth questions, Carson's rule gives a good exam-appropriate estimate.

Fig 15.9 — Educational Diagram: Frequency Modulation (FM)

What is demodulation?
Demodulation is the process of extracting the original message signal m(t) from a modulated carrier. For angle modulation, the carrier carries information in its phase (PM) or frequency (FM). Demodulators convert variations of phase or frequency back into the baseband message.

Signals and basic relations

General angle-modulated carrier: s(t) = A_c cos[ω_c t + φ(t)], where φ(t) is the instantaneous phase. For PM: φ(t) = k_p m(t). For FM: φ(t) = 2π k_f ∫ m(τ) dτ (i.e. frequency deviation is proportional to m(t)).

Instantaneous frequency
f_i(t) = f_c + (1 / 2π) dφ(t)/dt. This shows the close relation between FM and PM: frequency is the time derivative of phase, so PM and FM can be converted into one another by differentiation or integration of the message.

FM demodulation methods

• Slope detector (frequency-to-amplitude conversion): a tuned circuit (sloped region of resonance) converts small frequency shifts into amplitude changes. The output is then detected (envelope detected) and filtered to recover m(t). Simple but sensitive to amplitude changes and not very linear.
• Foster–Seeley discriminator: uses two tuned circuits and a phase/diode bridge to produce an output voltage proportional to frequency deviation (more linear and widely used in analog FM receivers).
• Ratio detector: like Foster–Seeley but provides amplitude limiting and better AM rejection (commonly used in FM receivers for improved noise/AM immunity).
• Phase-Locked Loop (PLL) FM demodulator: the most common modern method. A PLL locks a VCO to the incoming carrier. The control (error) voltage that keeps the VCO locked is proportional to the instantaneous frequency deviation and thus is the recovered message (after filtering). PLLs give good linearity, capture range and noise performance.

PM demodulation methods

• Direct phase detector (coherent demodulation): multiply incoming signal by a locally generated carrier of same frequency and phase reference, then low-pass filter. This requires carrier phase synchronization. The output is proportional to the instantaneous phase (hence message for PM).
• PLL for PM: a PLL can directly track the phase of the incoming signal. The PLL phase detector output (error signal) is proportional to the phase difference and can be processed to recover m(t).
• Differentiate then FM demodulate: since FM is the derivative of phase, differentiate the PM signal to convert phase modulation into frequency modulation and then use an FM discriminator. This is useful when implementing PM demodulation with FM demodulators.

Practical notes
FM demodulators are designed to be insensitive to amplitude noise (they often include amplitude limiting). PLLs are widely used in modern receivers for both FM and PM because they provide good linearity, stability and can implement capture and lock functions.

Block diagram (conceptual)
[RF front end / limiter] → [discriminator (Foster–Seeley / ratio / PLL)] → [de-emphasis / low-pass] → [audio output].

Common issues: carrier frequency offset, amplitude modulation interference (use limiters), noise (FM offers better noise performance for large modulation index), nonlinear discriminator response (requires linearization or PLL).

Fig 15.10 — Educational Diagram: FM vs AM Comparison

What is Pulse Modulation?
Pulse modulation is a class of modulation techniques in which the information (message) signal is conveyed by varying one or more characteristics of a train of pulses (amplitude, width/duration, position or by converting samples to digital codes). It converts a continuous-time message into a sequence of pulses suitable for transmission, switching, multiplexing and digital processing.

Why use pulse modulation?
It allows efficient use of bandwidth, easier multiplexing, better noise immunity (especially when digitised), and convenient interfacing with digital systems.

Sampling (basic step)
All pulse-modulation methods that start from an analog signal first sample the message signal. According to the sampling theorem (Nyquist–Shannon), a band-limited signal whose maximum frequency is f_m is completely represented by samples taken at sampling frequency f_s ≥ 2f_m. Sampling produces discrete-time values which are then used to form pulses.

Main types of pulse modulation

• Pulse Amplitude Modulation (PAM): The amplitude of each pulse is made proportional to the instantaneous value of the message at sampling instants. A simple mathematical form: x_p(t) = \sum_n x(nT) p(t - nT), where p(t) is the pulse shape and T = 1/f_s. PAM is an intermediate step before PCM and is used in some analog multiplexing systems.
• Pulse Width Modulation (PWM) / Pulse Duration Modulation (PDM): The width (duration) of each pulse varies in proportion to the message amplitude. Larger message amplitude means wider pulse. PWM is widely used in power control, motor drives, and LED dimming.
• Pulse Position Modulation (PPM): The time position of each pulse (within a fixed time slot) is shifted in proportion to the message amplitude. PPM is used in some optical and radio control systems because it can be more robust to amplitude noise.
• Pulse Code Modulation (PCM): Analog samples are quantized to finite levels and encoded into binary numbers. PCM is a digital pulse-modulation technique used in telephony, audio CDs and digital audio systems. PCM involves three steps: sampling, quantization and encoding.

Basic block (for PCM)
A typical PCM transmitter: Anti-aliasing filter → Sampler → Quantizer → Encoder. At the receiver: Decoder → Reconstruct using hold/filter → Anti-imaging filter.

Advantages and disadvantages (summary)
Advantages: Good noise immunity (especially PCM), easy multiplexing and digital processing, precise recovery (if sampling and quantization adequate). Disadvantages: Requires higher bandwidth (often), quantization introduces distortion (in PCM) and more complex circuitry.

Practical considerations
Choice of pulse shape p(t), sampling frequency f_s, number of quantization levels (for PCM) and synchronization (for PPM/PPM and PCM bit framing) are important design decisions. Pulse shaping and filtering affect bandwidth and intersymbol interference.

Fig 15.11 — Educational Diagram: Propagation of Electromagnetic Waves

Overview
Pulse Code Modulation (PCM) is a method to convert an analog message signal into a digital bit stream so it can be transmitted or stored reliably. PCM is fundamental to digital communication systems and consists of three main steps: sampling, quantization, and encoding.

Steps in PCM

• Sampling: Measure the amplitude of the continuous-time signal at discrete time instants. According to the sampling theorem (Nyquist), sampling frequency fs must satisfy fs > 2 fm, where fm is the highest frequency present in the analog signal, to avoid aliasing.
• Quantization: Each sample amplitude is rounded to the nearest value from a finite set of levels. This introduces quantization error (noise). For a uniform quantizer with L levels and range Vmin to Vmax, the step size is Delta = (Vmax - Vmin)/L.
• Encoding: Each quantized level is represented by an n-bit binary code, where L = 2^n. The sequence of bits forms the PCM output bit stream.

Key performance aspects
Bit rate: Rb = fs * n (bits/s). Increasing fs or n increases the bit rate. Quantization introduces noise; increasing n reduces quantization noise and improves signal-to-quantization-noise ratio (SNR). For a full-scale sinusoid and uniform quantizer, SNR (in dB) approximates SNR_dB ≈ 6.02 n + 1.76 dB.

Companding and practical telephony
To improve perceived quality for speech at low bit rates, nonuniform quantizers (companders) such as u-law and A-law compress small amplitudes more finely and expand at the receiver. Standard telephony uses 8-bit companded PCM sampled at 8 kHz -> 64 kbps per voice channel (G.711).

Digital communication basics related to PCM

• Source coding: PCM is a form of pulse code source coding (uniform or nonuniform).
• Channel considerations: After encoding, the bit stream is transmitted over a channel using digital modulation schemes such as ASK, FSK, or PSK, and requires pulse shaping to limit bandwidth and reduce intersymbol interference.
• Capacity limits: For a noiseless channel of bandwidth B and M discrete signal levels, Nyquist maximum bit rate is R_max = 2 B log2 M. For a noisy channel, Shannon capacity is C = B log2(1 + SNR_linear).
• Tradeoffs: Higher n improves fidelity but increases bit rate and required channel bandwidth or more complex modulation.

Advantages and disadvantages

• Advantages: Noise immunity, ease of multiplexing, simple regeneration with digital repeaters, good for storage (CDs) and digital networks.
• Disadvantages: Requires higher bandwidth and bit rate than analog alternatives; quantization causes distortion if n is small; needs anti-aliasing filter before sampling.

Typical practical numbers
Audio CD: fs = 44.1 kHz, n = 16 -> Rb = 705.6 kbps per channel. Telephony: fs = 8 kHz, n = 8 -> Rb = 64 kbps per voice channel (companded).

Block diagram suggestion
Typical PCM transmitter: Analog input -> Anti-aliasing low-pass filter -> Sampler (sample-and-hold) -> Quantizer -> Encoder -> Channel encoder / Modulator -> Channel. Receiver does the inverse operations.

Fig 15.12 — Educational Diagram: Ground Wave Propagation

Definition and purpose
Multiplexing is a technique that allows two or more independent signals to share a single transmission medium (wire, optical fibre or radio channel) simultaneously. It increases the utilisation of the channel and reduces cost by sending multiple information streams over the same physical link.

Why it is needed
Most communication channels (cables, fibres, radio links) have far greater capacity than a single signal needs. Multiplexing combines several signals so the medium is used efficiently, reducing the number of parallel links required and simplifying network design.

Main types

• Frequency Division Multiplexing (FDM): Several baseband signals modulate different carrier frequencies. Each modulated signal occupies a frequency band; bands are separated by guard bands to avoid overlap. FDM is used for analog signals and also for subcarrier channels in cable TV and radio.
• Time Division Multiplexing (TDM): Multiple digital signals share the same channel by assigning each a different time slot in a repeating frame. Each user transmits only during its slot. Two common forms are synchronous (fixed slots per user) and statistical (slots assigned dynamically to active users).
• Wavelength Division Multiplexing (WDM): Optical equivalent of FDM. Different data channels are carried on different light wavelengths (colours) in the same optical fibre. Dense WDM (DWDM) packs many closely spaced wavelengths for very high capacity.
• Code Division Multiplexing (CDM/CDMA): All users transmit simultaneously over the same bandwidth but use distinct orthogonal or pseudo-random codes to separate their signals at the receiver (used in some mobile systems).
• Space Division Multiplexing (SDM): Uses separate physical paths (separate fibres, antenna elements or spatial channels) to carry different signals simultaneously.

How it works (basic blocks)
A typical multiplexing system has: modulators/encoders for each input channel → multiplexer that combines channels → transmission medium → demultiplexer/receiver that separates channels → demodulators/decoders. For FDM each input modulates a different carrier; for TDM inputs are sampled/serialized into time slots; for WDM lasers of different wavelengths are combined with an optical multiplexer.

Advantages

• Efficient use of resources (fewer links for many users).
• Lower installation and maintenance cost per user.
• Flexible: different multiplexing schemes suit analog, digital and optical systems.

Limitations and practical issues

• Inter-channel interference: requires guard bands (FDM) or precise timing (TDM).
• Complexity of multiplexers/demultiplexers and synchronization (TDM).
• Non-ideal filters and crosstalk reduce capacity and require signal processing.

Applications (short)
Telephone trunking (digital TDM), broadcast radio and cable TV (FDM), optical backbone networks (WDM/DWDM), cellular multiple access (FDMA/TDMA/CDMA), satellite communications, DSL (multitone FDM), sensor networks and many embedded buses.

Fig 15.13 — Educational Diagram: Sky Wave Propagation

Noise in Communication Systems

What is noise? Noise is any unwanted random electrical signal that corrupts the desired information in a communication system. It is stochastic (random) and usually treated statistically. Noise reduces the clarity of received signals and increases the probability of error.

Sources and types of noise

• Thermal (Johnson–Nyquist) noise: Generated by the random thermal motion of charge carriers in resistors and electronic components. It is present in all circuits at non‑zero temperature and is often modelled as additive white Gaussian noise (AWGN).
• Shot noise: Occurs in devices where current results from discrete charge carriers (e.g., diodes, photodiodes). It is proportional to the average current and has a white spectrum.
• Flicker (1/f) noise: Dominates at low frequencies, its power spectral density varies approximately as 1/f.
• Impulse / burst noise: Short, high‑amplitude spikes caused by switching transients, lightning, or faulty components.
• Interference and cross‑talk: External signals from other transmitters, power lines (hum), or nearby channels that couple into the signal path.
• Quantization noise: Introduced when an analog signal is digitized (ADC); depends on quantizer resolution.

Key characteristics

• Randomness: Described statistically (mean, variance). Thermal and shot noise often have zero mean and Gaussian distribution.
• Power spectral density (PSD): Noise distribution over frequency. White noise has a flat PSD; 1/f noise increases at low f.
• Additive noise model: Received signal r(t) = s(t) + n(t) where n(t) is noise.

Effects on communication

Noise reduces the signal‑to‑noise ratio (SNR), leading to poorer detection and higher bit error rate (BER) in digital systems. In analog systems it produces hiss, static, and distortion. Designers aim to maximize SNR at the detector and minimize noise contribution from receivers.

Ways to reduce noise

• Increase signal power (within limits).
• Limit bandwidth with filters (reduces integrated noise power).
• Use low‑noise amplifiers and good shielding/grounding to reduce external interference.
• Use error detection/correction coding and modulation schemes robust to noise.

CBSE‑level summary

At Class 12 level, treat most fundamental noise as thermal (AWGN). Understand how noise power depends on temperature and bandwidth, how SNR is defined and expressed in decibels, and how stronger noise increases communication errors.

Fig 15.14 — Educational Diagram: Line of Sight (LOS) Propagation

Overview
A receiver extracts information (audio, data, video) from a received modulated electromagnetic wave. Two main classical types discussed in Class 12 are the Tuned Radio Frequency (TRF) receiver and the Superheterodyne (superhet) receiver.

1. Tuned Radio Frequency (TRF) Receiver
A TRF receiver uses one or more cascaded tuned amplifier stages at the incoming radio frequency (RF) followed by a detector and audio amplifier. Each RF stage must be tuned to the desired carrier frequency.

• Advantages: simple in concept, fewer stages.
• Disadvantages: difficult to maintain constant bandwidth and gain across tuning range, poor selectivity and stability, complicated multi‑stage tuning.

2. Superheterodyne Receiver (Superhet)
The superhet converts the incoming RF to a fixed intermediate frequency (IF) using mixing with a local oscillator (LO). Most amplification and filtering are done at this fixed IF where high selectivity and stable gain are easy to achieve.

Block diagram (logical order):

• Antenna / RF input → RF amplifier / preselector (tuned) → Mixer (frequency converter) + Local Oscillator (LO) → IF amplifier & IF filters → Detector / Demodulator → AF (audio) amplifier → Speaker / output

How mixing works (concept):
The mixer multiplies the incoming RF signal (frequency f_RF) with the LO (frequency f_LO). Multiplication produces sum and difference frequencies: f_RF + f_LO and |f_RF - f_LO|. The latter (difference) is selected as the intermediate frequency f_IF by IF filters.

Key operational relations
f_IF = |f_RF - f_LO|. The LO is chosen so that the desired f_RF is converted to a fixed f_IF (e.g., 455 kHz for many AM receivers, 10.7 MHz for FM).

Image frequency and preselection
Because |f_candidate - f_LO| = f_IF has two solutions (the desired f_RF and another unwanted frequency), an unwanted signal called the image frequency can also be converted to the same IF and interfere. The image frequency f_img can be shown as the other solution of |f - f_LO| = f_IF, which leads to f_img = f_LO ± f_IF (the solution not equal to f_RF), equivalently f_img = f_RF ± 2 f_IF depending on LO placement. An RF input filter (preselector) ahead of the mixer suppresses image signals.

Advantages of Superheterodyne

• High and stable selectivity and sensitivity because IF amplifiers use fixed-frequency high‑Q filters.
• Single tuning control (or controlled tuning) — easier to tune across bands with consistent response.
• Better overall gain and noise performance with fewer tuned stages.

Disadvantages

• More complex circuitry (mixer and LO), larger cost and power consumption.
• Image-frequency issue and possible spurious responses; need good preselection and filtering.

Practical notes and typical IF values
Common IF choices: AM radios ≈ 455 kHz, FM receivers ≈ 10.7 MHz. Modern receivers (TV, mobile) use multiple conversion stages (double or triple conversion superhets) to improve image rejection and selectivity.

Summary
TRF receivers amplify at the incoming frequency (simple but limited). Superheterodyne receivers convert to a fixed intermediate frequency for the bulk of amplification and filtering, providing superior selectivity and sensitivity and are the dominant architecture in practical radio/TV/mobile receivers.

Fig 15.15 — Educational Diagram: Satellite Communication

What is propagation of electromagnetic waves?
Propagation of electromagnetic (EM) waves is the way radio waves travel through space and atmosphere from a transmitter to a receiver. Different mechanisms (reflection, refraction, diffraction, scattering, absorption) and different atmospheric layers determine how far and how reliably a wave can travel.

Main modes of propagation

• Ground (Surface) wave: Wave travels along the Earth's surface and is guided by the ground. Important at low frequencies (kHz to a few MHz). Range decreases with frequency because of ground absorption.
• Sky wave (Ionospheric propagation): Waves are refracted or reflected back to Earth by the ionosphere (layers D, E, F). Useful for medium and short wave broadcasting and long-distance HF communication. Characteristic features: skip distance (zone of no reception near transmitter) and maximum usable frequency (MUF) which depends on ionospheric electron density and angle of incidence.
• Space wave (Line-of-sight): Direct (LOS) wave and ground-reflected wave travel in straight lines. Dominant at VHF, UHF and microwaves (used for FM radio, TV, mobile, satellite uplink/downlink). Range limited by radio horizon (slightly beyond geometric horizon due to refraction).

Physical processes that affect propagation

• Reflection: Occurs at surfaces or ionospheric layers; causes multipath and fading.
• Refraction: Gradual bending of wave direction in a medium with varying refractive index (e.g., ionosphere or troposphere).
• Diffraction: Bending of waves around obstacles; enables reception in shadow zones — more prominent for long wavelengths.
• Scattering: Caused by irregularities (raindrops, buildings, rough ground) and leads to signal spread and fading.
• Absorption (attenuation): Energy loss in medium (e.g., ground losses, atmospheric gases, rain), increases with frequency and distance.

Ionospheric concepts (brief)
The ionosphere contains free electrons; its electron density controls whether a transmitted HF wave will be reflected. Critical frequency (f_c) is the highest frequency reflected for vertical incidence; for oblique incidence the maximum usable frequency (MUF) is larger and related by MUF = f_c / cos(θ), where θ is the angle between the wave direction and the vertical. Night and solar activity change ionospheric density, so HF propagation varies with time.

Attenuation and path loss
Free-space spreading causes power density to fall approximately as 1/R^2 (inverse square law). In practical channels additional losses from absorption, reflection and diffraction apply. For link design, the free-space path loss (FSPL) formula is commonly used.

Practical consequences and design points

• Low-frequency (LF/VLF) signals follow ground waves and are used for long-range navigation and submarine communications.
• Medium frequency (MF) AM broadcasting uses both ground and sky waves; at night sky-wave reach increases due to reduced D-layer absorption.
• VHF/UHF (FM, TV, mobile) rely on line-of-sight; antenna height increases range (radio horizon), and obstacles cause multipath fading.
• Satellites use space-wave propagation with very high frequencies and need clear line-of-sight to ground stations.

Summary
Propagation depends on frequency, antenna heights, Earth and atmospheric properties. Understanding the dominant mode (ground, sky or space) and the governing effects (refraction, diffraction, absorption) is essential for designing communication links and predicting coverage.

Fig 15.16 — Educational Diagram: Fiber Optic Communication

Satellite communication is the use of artificial satellites to relay radio signals between widely separated points on Earth. A satellite communication system typically has three segments: the space segment (satellite with transponders and antennas), the ground segment (earth stations, user terminals) and the control segment (mission/control stations).

Types of communication satellites: GEO (geostationary) — appears fixed above the equator (period = Earth rotation), MEO (medium earth orbit) — e.g., navigation constellations, LEO (low earth orbit) — used by many broadband constellations and mobile-phone satellites. Choice depends on latency, coverage, and launch cost.

Basic orbital mechanics (circular orbit): For a satellite of mass m orbiting at radius r around Earth (mass M), gravity provides centripetal force:

GMm/r^2 = m v^2 / r = m ω^2 r

From this we get orbital speed and period:

v = sqrt(GM / r)

T = 2π sqrt(r^3 / GM)

Geostationary orbit (GEO): A GEO satellite must have orbital period equal to Earth’s rotation (sidereal day T ≈ 86 164 s). Solving for r gives r ≈ 4.2164 × 10^7 m (distance from Earth center). Subtracting Earth's mean radius (≈ 6.371 × 10^6 m) gives altitude h ≈ 3.5786 × 10^7 m ≈ 35 786 km above Earth’s surface. GEO satellites provide continuous coverage of the same Earth area but have one-way signal delay ≈ 119 ms (round-trip ≈ 238 ms), which affects two-way applications like voice.

Link principles and key parameters:

• Friis transmission (free-space) — received power depends on transmit power, antenna gains and path loss.
• Free-space path loss (FSPL) increases with frequency and distance; calculated in linear or dB form.
• EIRP (effective isotropic radiated power) = Pt + Gt (in dBW/dBm and dBi) determines transmitted power into space.
• G/T (ground antenna gain / system noise temperature) and C/N (carrier-to-noise) determine link quality.
• Transponders convert uplink frequencies to downlink frequencies, amplify and bandpass signals; polarization (linear or circular) reduces interference.

Frequency bands and real effects: Typical satellite bands: L-band (~1–2 GHz), S-band (~2–4 GHz), C-band (~4–8 GHz), X-band (~8–12 GHz), Ku (~12–18 GHz), Ka (~26–40 GHz). Higher frequencies give larger bandwidth but suffer more free-space loss and rain attenuation (especially Ku/Ka).

Multiple access: Satellites support FDMA, TDMA, CDMA and combinations for sharing bandwidth among users.

Typical applications: TV broadcasting (DTH), satellite internet (VSAT, modern LEO constellations), GPS/GNSS (MEO), weather monitoring, remote sensing, maritime/aircraft communications and emergency links.

Advantages & disadvantages (summary):

• Advantages: wide-area coverage (especially GEO), rapid deployment, connectivity in remote areas.
• Disadvantages: propagation delay (GEO), high launch and space-segment cost, spectrum congestion, weather attenuation at high bands.

Fig 15.17 — Educational Diagram: Digital Communication Basics

Definition and basic idea
Optical fiber communication is a method of transmitting information from one place to another by sending light pulses through thin flexible fibers of glass or plastic. It uses total internal reflection (TIR) to confine light in a high‑index core surrounded by a lower‑index cladding.

Structure of an optical fiber

• Core: central region (glass/plastic) that carries light.
• Cladding: lower refractive index layer that causes TIR at the core–cladding interface.
• Coating/jacket/buffer: mechanical protection and moisture barrier.

Working principle
Light entering the fiber within an acceptance cone undergoes repeated total internal reflection at the core–cladding boundary and propagates along the fiber. The acceptance cone is defined by the numerical aperture (NA) of the fiber.

Types of fibres

• Step‑index multimode: core has uniform refractive index; many propagation modes; larger core diameters (≈50–100 μm); inexpensive sources (LED).
• Graded‑index multimode: core refractive index decreases gradually from center (typically parabolic), reducing modal dispersion and increasing bandwidth.
• Single‑mode (step‑index): very small core (≈8–10 μm); only the fundamental mode propagates; used for long‑distance and high‑bit‑rate links (lasers as sources).

Sources and detectors
Light sources: LEDs (multimode, short distance), semiconductor lasers / DFB lasers (single‑mode, long distance). Detectors: PIN photodiodes, avalanche photodiodes (APD).

Losses and dispersion (limitations)

• Attenuation: loss of signal power (absorption, Rayleigh scattering, bending losses). Attenuation depends on wavelength; low‑loss windows near 850 nm, 1310 nm and 1550 nm (1550 nm often best for long distance).
• Modal dispersion: in multimode fibers different modes take different paths and arrive at different times, broadening pulses and limiting bit rate.
• Chromatic (material) dispersion: different wavelengths travel at different speeds in the medium; important for sources with finite spectral width.

Important performance concepts

• Bandwidth‑distance product: a measure of how much data rate the fiber supports over a distance (e.g., MHz·km).
• Acceptance cone & NA: only rays within the acceptance angle at the input are guided.

Advantages: very high bandwidth, low attenuation (especially at 1550 nm), immunity to electromagnetic interference, light weight, secure (difficult to tap).

Disadvantages: higher initial installation cost, need for precise connectors and alignment, specialized splicing equipment.

Summary
Optical fibers form the backbone of modern high‑speed communication networks (internet backbone, undersea cables, metropolitan networks) because of their huge capacity and low loss. Choosing the right fiber type (single‑mode vs multimode) and wavelength/window depends on distance, cost, and bandwidth requirements.

Fig 15.18 — Educational Diagram: Pulse Code Modulation (PCM)

What is an antenna? An antenna is a transducer that converts guided electromagnetic waves on a transmission line into free-space electromagnetic waves (radiation) and vice versa. In communication systems an antenna is used to transmit radio waves into space and to receive them.

Basic functions: radiate power efficiently into desired directions, receive incoming waves, provide correct impedance matching to the transmitter/receiver, control polarization and bandwidth.

Key concepts

• Wavelength and size: Antenna physical size is usually expressed relative to the operating wavelength λ = c/f (c ≈ 3×10^8 m/s). Typical simple antennas: short dipole (length ≪ λ), half-wave dipole (length ≈ λ/2).
• Radiation pattern: Spatial variation of radiated power (or radiation intensity). Patterns are often shown as polar plots (2D) or 3D plots. Examples: isotropic (ideal spherical), half-wave dipole (doughnut-shaped), highly directional (narrow beam) antennas like parabolic dishes.
• Directivity (D): How concentrated the radiation is in a particular direction compared to an isotropic radiator. Higher directivity → narrower beam.
• Gain (G): Directivity scaled by antenna efficiency η: G = η·D. Gain includes losses and is the practical measure used for transmit/receive power calculations.
• Polarization: Orientation of the electric field (vertical, horizontal, circular). For best reception the antenna polarization should match the incoming wave's polarization.
• Input impedance and matching: Antenna must be impedance matched to the transmission line (typical 50 Ω or 75 Ω) to avoid reflections. Reflection coefficient Γ and VSWR quantify mismatch.
• Bandwidth and efficiency: Bandwidth is the frequency range over which antenna performance (impedance, gain) is acceptable. Efficiency accounts for ohmic and mismatch losses.
• Reciprocity: Antennas are reciprocal: the radiation characteristics when transmitting are the same as the reception characteristics.

Simple antenna examples: the short (small) dipole acts as a basic radiator for understanding current distribution; the half-wave dipole is a common resonant element used in many antennas; arrays (multiple elements) form beams and increase directivity; parabolic reflectors give high gain and narrow beams for satellite links and radar.

Practical notes: Antenna choice depends on frequency (hence size), desired coverage (omni-directional vs directional), polarization, available space and required gain/bandwidth. Real antenna design also considers ground effects, mounting, and environmental factors.

Fig 15.19 — Educational Diagram: Cellular Mobile Communication Basics

Overview: Modulation transfers an information signal onto a carrier (sinusoid) by changing its amplitude, frequency or phase. Major analog schemes: AM (Amplitude Modulation), FM (Frequency Modulation), PM (Phase Modulation). Major digital schemes: ASK, FSK, PSK (including BPSK, QPSK), QAM and their M-ary forms. Choosing a modulation involves trade-offs among bandwidth, power efficiency, noise immunity, complexity and spectral efficiency.

Key performance metrics:

• Bandwidth – how much spectrum the modulated signal occupies.
• Power efficiency – transmitted power needed to achieve a target SNR/BER.
• Noise immunity – resistance to channel noise and interference.
• Spectral efficiency – bits/second per Hz (important for digital systems).
• Complexity – transmitter/receiver hardware and signal processing required (e.g., coherent detection, synchronization).

Analog modulation comparison:

• AM: Simple transmitter/receiver, uses narrowband around carrier (carrier + two sidebands). Poor noise immunity because noise adds to amplitude. Power wasted in carrier (no information). Used in medium-wave radio broadcasting and amplitude-shift TV video (older systems).
• FM: Information in frequency variations. Much better noise immunity for amplitude noise (capture effect), higher fidelity for audio. Requires larger bandwidth than AM (many sidebands). More complex receiver (freq. discriminator or PLL). Used for VHF FM radio, two-way radio.
• PM: Information in instantaneous phase. Behavior and noise immunity similar to FM; often used where phase control is convenient. Useful in coherent systems and as a building block for digital modulation.

Digital modulation comparison:

• ASK (Amplitude Shift Keying): Simple but poor noise immunity (amplitude affected by noise/fading). Used in simple RF links and optical ON–OFF keying.
• FSK (Frequency Shift Keying): Better noise performance than ASK; used in modem, Bluetooth (GFSK). Non-coherent detection possible (simpler RX).
• PSK (Phase Shift Keying) (BPSK, QPSK): Good power efficiency and noise performance (especially with coherent detection). QPSK transmits 2 bits/symbol; used in cellular, satellite links.
• QAM (Quadrature Amplitude Modulation): Combines amplitude and phase to increase spectral efficiency (e.g., 16-QAM, 64-QAM). More bits/s/Hz but requires higher SNR and linear amplifiers; used in Wi‑Fi, LTE, cable modems.

Typical trade-offs:

• Bandwidth vs. Power: FM (or wideband PM) uses more bandwidth but can give better SNR (noise immunity) for a given power. Narrowband schemes (AM, ASK) use less bandwidth but need higher power or have poorer noise performance.
• Spectral Efficiency vs. Noise Margin: Higher-order digital constellations (e.g., 64-QAM) carry more bits per Hz but require higher SNR (smaller noise margin) and more linear transmitters.
• Complexity vs. Performance: Coherent detection (phase-locked loops, carrier recovery) yields better BER for PSK/QAM but increases receiver complexity compared with non-coherent schemes (some FSK variants).
• Latency and Channel Constraints: Some high-efficiency schemes require precise timing and channel equalization (e.g., OFDM + QAM) which adds processing delay and complexity.

How to choose a modulation: Match to requirements — for long-range broadcast audio, resilience and simplicity (FM) matter; for spectrum-limited high-data-rate links (4G/5G, Wi‑Fi), spectral efficiency (QAM/OFDM) is critical; for very low-power IoT links, low complexity and power efficiency (BPSK, FSK) may be preferable.

Fig 15.20 — Educational Diagram: Summary, Formulas, and Practice Problems

Overview: Communication systems solve the problem of transferring information (voice, data, video, control signals) from a source to a destination reliably and efficiently. A basic system comprises transmitter, channel (medium), and receiver, with important blocks such as modulators/demodulators, encoders/decoders, multiplexers/demultiplexers and amplifiers/repeaters.

Where theory meets practice: Practical systems apply principles learned in the chapter: modulation (AM, FM, PM; digital modulation), sampling and analogue-to-digital conversion (ADC/PCM), multiplexing (FDM, TDM), channel characteristics (bandwidth, attenuation, noise), link design (antennas, repeaters, towers, satellites), and information limits (Nyquist sampling, Shannon capacity). Engineering choices depend on bandwidth availability, power, distance, propagation medium and required quality (S/N, bit-error-rate).

Key practical points:

• Modulation transfers baseband information into a band suitable for transmission: AM used for medium-wave broadcast; FM for high-fidelity VHF radio; digital modulation used in mobile/Wi‑Fi.
• Multiplexing increases channel utilisation: FDM used in analog broadcast and cable, TDM in digital telephony and packet networks.
• Sampling and PCM convert voice/video into digital form for error control, compression and multiplexing in modern networks.
• Propagation and medium matter: free-space/microwave for satellites and line-of-sight links, guided media (optical fibre) for very high bandwidth and low loss, and wireless for mobile access.
• Noise, attenuation and interference require filtering, amplification, error-control coding and link budgeting to ensure reliable reception.

Typical system examples and how they use chapter concepts:

• Broadcast radio (AM/FM): uses modulation (AM for amplitude-modulated medium-wave, FM for VHF frequency-modulated high-fidelity audio) and antennas; spectrum shows carrier plus sidebands (AM) or wideband FM spectrum.
• Television: analogue TV used AM for video and FM for audio with large bandwidth; modern TV is digital (MPEG compression + QAM/OFDM modulation) and transmitted by terrestrial transmitters or satellites.
• Mobile cellular networks: use digital modulation, cell splitting, frequency reuse, handoff between base stations, multiplexing and error-control coding to provide voice/data across many users.
• Satellite communication: uses microwave uplink/downlink, large antennas, repeaters (transponders); geostationary satellites provide TV/VSAT links while LEO constellations provide low-latency broadband.
• Optical fibre networks: use guided-wave propagation with lasers/LEDs, very high bandwidth and low attenuation for long-haul internet backbone and local access (FTTx).
• Radar and navigation (e.g., GPS): use pulsed or continuous-wave transmissions, time-of-flight and Doppler principles to measure distance/velocity and provide positioning.
• Telemetry and remote sensing: transmit sensor data from remote platforms (satellites, rockets) using robust modulation and error-control coding.