A-Level Physics: Topic 7.

5 - Communication Systems by Kanyai TT

1. Communication Channels
Information can be transmitted through various physical pathways, each with its
own advantages and limitations.
• Wire-pairs: Simple copper wires. Prone to high signal attenuation and
interference (noise). Low cost, used for short-distance telephone lines.
• Coaxial Cables: A central conductor surrounded by a shield. Better than
wire-pairs as the shield reduces interference and cross-talk. Used for
cable TV and internet. Higher bandwidth than wire-pairs.
• Radio and Microwave Links: Electromagnetic waves transmitted
through the atmosphere.
– Radio waves can travel long distances and diffract around
obstacles but have lower bandwidth.
– Microwaves have higher bandwidth but require a direct line of
sight (cannot diffract well). Used for satellite communication and
mobile phone networks.
• Optic Fibres: Thin strands of glass that transmit information as pulses of
light.
– Advantages: Extremely high bandwidth, very low signal
attenuation, immune to electromagnetic interference, secure.
– Disadvantages: Higher installation cost, fragile.

2. Modulation
Modulation is the process of superimposing a low-frequency information signal
(e.g., audio from a microphone) onto a high-frequency carrier wave for
transmission.
Why is it necessary?
1. Antenna Size: For efficient transmission and reception, the antenna should be
on the order of the wavelength of the signal. Audio signals have very long
wavelengths (km), requiring impossibly large antennas. Carrier waves have
short wavelengths (m), allowing for practical antenna sizes.
2. Preventing Signal Mixing: Multiple signals can be transmitted
simultaneously by using different carrier frequencies, preventing them from
interfering with each other.
Types of Modulation:
• Amplitude Modulation (AM):
– The amplitude of the carrier wave is varied in proportion to the
amplitude of the information signal.
– The frequency of the carrier wave remains constant.
– Bandwidth: An AM signal has a bandwidth equal to twice the
highest frequency in the information signal (BW = 2f_max).
– Disadvantages: More susceptible to noise (static), as noise
primarily affects amplitude.
• Frequency Modulation (FM):
– The frequency of the carrier wave is varied in proportion to the
amplitude of the information signal.
– The amplitude of the carrier wave remains constant.
– Bandwidth: An FM signal uses a wider bandwidth than an AM
signal for the same information signal.
– Advantages: Less susceptible to amplitude noise, resulting in
higher quality audio reception.
Mathematical Representation: A carrier wave modulated by a single audio
frequency produces three frequencies:
1. The original carrier wave frequency, f_c
2. An upper sideband frequency, f_c + f_a
3. A lower sideband frequency, f_c - f_a
3. Digital Communication
Digital signals represent information as a series of discrete binary values (0s
and 1s).
Advantages over Analogue Transmission: * Noise Immunity: Digital signals
can be regenerated perfectly by repeaters, removing any noise picked up during
transmission. Analogue amplifiers amplify the signal and the noise. * Error
Detection and Correction: Digital systems can use codes to detect and correct
errors that occur during transmission. * Security: Digital data can be easily
encrypted. * Multiplexing: It is easier to combine multiple digital signals into
one transmission.
The Process: ADC and DAC To transmit analogue data (e.g., voice, music)
digitally, it must be converted.
1. Analogue-to-Digital Conversion (ADC) at the transmitter: * Sampling: The
analogue signal’s amplitude is measured at regular time intervals (sampling
rate). * Quantisation: Each sampled amplitude value is rounded to the nearest
discrete level. The number of available levels is determined by the bit depth
(number of bits per sample). * Encoding: Each quantised level is assigned a
unique binary code.
2. Digital-to-Analogue Conversion (DAC) at the receiver: The binary codes are
converted back into an approximate analogue signal, which is then smoothed to
reconstruct the original wave.
Factors Affecting Quality: * Sampling Rate: Must be at least twice the highest
frequency in the input signal (Nyquist Theorem). A higher sampling rate
produces a more accurate reconstruction. * Bit Depth: A higher number of bits
per sample allows for more amplitude levels, reducing quantisation error (the
difference between the original and quantised value) and improving dynamic
range and fidelity.
4. Noise, Attenuation, and Signal Degradation
• Noise: Unwanted energy added to a signal from external sources (e.g.,
lightning, machinery) or internal sources (e.g., thermal agitation in
components). It degrades the signal-to-noise ratio.
• Signal Attenuation: The gradual loss of signal strength (power or
amplitude) as it travels through a medium due to absorption and
scattering.
– It is expressed in decibels (dB).
– For a cable, attenuation is often given as dB per unit length (e.g.,
dB km⁻¹).
• Cross-talk: When a signal from one channel unintentionally interferes
with another, adjacent channel.

Maintaining Signal Integrity: * Repeaters / Regenerators: Devices placed at

intervals along a transmission path. * For analogue signals, they amplify the
weakened signal (and the noise). * For digital signals, they regenerate the
signal. They detect the incoming 0s and 1s and produce a new, clean, noise-free
output signal.

5. The Decibel (dB) Scale

The decibel is a logarithmic unit used to compare two power levels.
Formula: Number of dB = 10 log₁₀(P₁ / P₂)
Where: * P₁ and P₂ are the two power levels being compared. * A positive dB
value means P₁ > P₂ (gain). * A negative dB value means P₁ < P₂
(loss/attenuation).
Example: If a signal’s power drops from 20 W to 5 W, the attenuation is:
Attenuation = 10 log₁₀(5 / 20) = 10 log₁₀(0.25) = 10 × (-0.60)
= -6.0 dB

6. Satellite Communication
Satellites act as microwave repeater stations in space, receiving signals from a
ground station and retransmitting them back to a different location on Earth.
• Geostationary Satellites:
– Orbit: ~36,000 km above the equator. Orbital period is 24 hours,
so they remain fixed over one point on Earth.
– Merits: Constant connection; ground antennas don’t need to track
the satellite.
– Demerits: High latency (signal delay of ~0.24 s each way) due to
large distance. Requires powerful transmitters and large receivers.
Coverage is limited to about one-third of the Earth’s surface.
 • Polar Orbiting Satellites:
– Orbit: Much lower altitude (e.g., 500-1500 km), passing over or
near the North and South Poles.
– Merits: Lower latency, requires less power for transmission. Can
provide coverage of the entire Earth’s surface over time.
– Demerits: Not always in sight of a given ground station;
connection is intermittent. Requires a network (constellation) of
satellites for continuous coverage. Ground antennas must track the
moving satellite.

Summary Table
Concept Key Points
Channels Optic fibres > Coaxial > Wire-pairs
(for bandwidth, noise immunity).
Radio/Microwaves for wireless.
Modulation AM: Varies amplitude, narrower BW,
prone to noise. FM: Varies frequency,
wider BW, better quality.
Digital vs Analogue Digital allows regeneration, error
correction, encryption. Requires
ADC/DAC conversion.
ADC Quality Higher sampling rate and higher bit
depth = better quality reproduction
of original signal.
Attenuation Signal power loss. Measured in dB. dB
= 10 log₁₀(P₁/P₂).
Repeaters Amplify analogue signals.
Regenerate digital signals (remove
noise).
Satellites Geostationary: High latency, fixed
position. Polar Orbit: Low latency,
moving, needs a constellation.
Channel Typical Uses Advantages Disadvantages
Wire-pair Traditional Inexpensive, easy Low bandwidth,
telephone lines, to install. high attenuation,
DSL internet. very susceptible
to noise and
cross-talk.
Coaxial Cable Cable television Higher More expensive
(CATV), bandwidth than and rigid than
broadband wire-pairs, better wire-pairs, higher
internet. shielding reduces attenuation than
noise and cross- fibre optics.
talk.
Radio Waves AM/FM radio, Can travel long Limited
television distances, can bandwidth,
broadcasting, diffract around susceptible to
mobile phones obstacles (e.g., atmospheric
(lower bands). hills), no physical interference and
connection fading, low
needed. security (anyone
with a receiver
can listen).
Microwave Satellite Very high Requires line-of-
Links communication, bandwidth, no sight, affected by
terrestrial physical cabling atmospheric
microwave links needed over conditions (e.g.,
(between difficult terrain. rain fade),
towers), radar. requires
licensing.
Optical Fibres Internet Extremely high High installation
backbone, high- bandwidth, very cost, fragile
speed LANs, low attenuation, (requires careful
telephone and immune to handling and
cable TV trunk electromagnetic splicing), requires
lines. interference, specialized
highly secure equipment for
(taps are easy to transmission and
detect), small size reception.
and weight.
8. Key Concepts: Bandwidth and Noise
• Bandwidth: The range of frequencies a communication channel can
transmit effectively. It is a key measure of the information-carrying
capacity of the channel.
– A wider bandwidth allows more data to be sent per second (a
higher bit rate).
– Example: An optical fibre has a bandwidth of terahertz (THz),
while a coaxial cable is in the megahertz (MHz) range, and a
twisted pair is in the kilohertz (kHz) range.
• Signal-to-Noise Ratio (SNR): A measure of the level of a desired signal
compared to the level of background noise. It is crucial for signal clarity
and is also expressed in decibels (dB). A higher SNR means a clearer,
better-quality signal.

9. Practical and Investigative Activities (As per Syllabus)

• Visiting a Broadcasting Station: This provides first-hand insight into the
practical application of modulation (AM/FM transmitters), the scale of
antennas, and the use of satellite links for national broadcasts.
• Comparing Digital and Analog Transmission:
– Experiment: Transmit a simple tone or music signal over a short
distance using a wire. First, transmit it as an analogue signal and
intentionally add noise (e.g., by placing a cell phone near the wire).
Observe the degraded output.
– Then, convert the signal to digital (using a simple ADC in a
microcontroller or software) and transmit the digital data. After
reception, convert it back to analogue (DAC). The output will be
clear and free from the added noise, dramatically demonstrating
the advantage of digital regeneration.
• Inviting a Resource Person: An engineer working in
telecommunications or satellite ground operations can provide real-
world context about the merits and challenges of different systems,
linking theory to industry practice.
Final Summary and Checklist
To master this topic, ensure you can:

Learning Objective Checkpoint

Channels List the main channels and state one
key advantage and disadvantage for
each.
Modulation Explain why modulation is necessary.
Describe the difference between AM
and FM in terms of what property of
the carrier wave is varied.
Digital Communication State three advantages of digital over
analogue transmission. Explain the
roles of ADC and DAC.
Quality & Fidelity Explain how sampling rate and bit
depth affect the quality of a digitized
signal.
Attenuation & Noise Define attenuation and noise.
Explain the crucial difference between
a repeater (for analogue) and a
regenerator (for digital).
The Decibel Recall and use the formula number
of dB = 10 log₁₀(P₁ / P₂) to
calculate a power ratio gain or loss.
Satellites Compare geostationary and polar
orbiting satellites in terms of orbit,
latency, and coverage.