communication

COMMUNICATION

Introduction

The process of transmitting and receiving meaningful information or intelligence is termed communication. Electronic communication involves converting speech or intelligence into electrical signals using transducers. The signals are processed and transmitted. A receiver located kilometers away from the transmitter receives these signals. The receiver processes the received signals and finally drives the transducer, which converts the processed signals back into speech or intelligence.

Transducers

Transducers are devices that convert energy from mechanical to electrical forms and vice versa. Radio communication (transmission and reception) stands as one of the most effective communication systems.

Definition of Some Terminology

• Signal: A changing electric current or waveform that carries information.

• Encoding: The process of transforming information into a suitable form for transmission.

• Decoding: The process of extracting the original information from a received signal.

Modulation and Demodulation

The effective transmission and reception of information is accomplished through two main processes: Modulation and Demodulation.

Modulation

Modulation involves combining low-frequency audio waves with very high-frequency radio waves. The low-frequency wave is termed the Modulating Wave, whereas the very high-frequency radio wave that carries the low-frequency audio information is known as the Carrier Wave. The resultant wave is referred to as the Modulated Carrier.

Demodulation

Demodulation is the process of recovering the low-frequency audio wave from the Modulated Carrier Wave and is performed at the receiving end. This process serves as the reverse of modulation.

Need for Modulation

Audio frequency signals are inherently low-frequency signals, which present some disadvantages for unmodulated transmission:

1. Low-frequency signals cannot propagate over long distances, resulting in short-range transmission.

2. Direct transmission of low-frequency signals can lead to interference, causing unclear information at the reception point.

3. The antenna length required for transmitting audio frequency signals is approximately 75 meters, which is practically very large.
   Thus, low-frequency signals cannot be effectively transmitted without modulation, and therefore, radio frequency carrier waves are modulated with low-frequency signals to reach longer distances. The antenna sizes required for radio frequency wave transmission are more reasonable, making modulation essential.

Types of Modulation

Based on the variation in characteristic properties of the carrier wave when combined with low-frequency audio signals, modulation can be classified into two types:

1. Amplitude Modulation (AM)

In amplitude modulation, the amplitude of the radio-frequency (RF) carrier wave is varied by low-frequency audio signals (AF) without altering the frequency and phase of the carrier wave.

2. Frequency Modulation (FM)

In frequency modulation, the frequency of the RF carrier wave is varied by low-frequency audio signals, while the amplitude and phase of the carrier wave remain unchanged.

Comparison between AM and FM

Advantages of FM:

1. FM offers better quality and audio fidelity compared to AM.

2. FM signals utilize a larger bandwidth, allowing for greater information capacity than AM signals.

3. FM signals experience less interference from adjacent bands.

4. FM receivers are less affected by noise and distortion due to the constant amplitude of the carrier wave.

Disadvantages of FM:

1. The circuits necessary for FM transmission are more complex than those for AM.

2. FM transmission is more costly and cannot travel as far, necessitating multiple repeater stations for national radio reach.

Carrier Wave

A Carrier Wave is a high-frequency radio wave generated using radio-frequency oscillators, with radio frequencies ranging from 3 kHz to 300 GHz. In radio transmission, carrier waves are preferred within the hundreds of kHz to a few MHz range.

Sidebands and Bandwidth

If $f_c$ is the carrier RF and $f_m$ is the smaller modulating frequency, the three resultant waves have frequencies of:

1. Carrier frequency: $f_c$

2. Upper sideband frequency: $f_c + f_m$

3. Lower sideband frequency: $f_c - f_m$

Example

If $f_c = 1$ MHz and the highest modulating frequency $f_m = 0.01$ MHz, then:

• Upper sideband frequency = $f_c + f_m = 1 + 0.01 = 1.01$ MHz

• Lower sideband frequency = $f_c - f_m = 1 - 0.01 = 0.99$ MHz

Bandwidth Definition

Bandwidth is defined as the range of frequencies carried within a signal, measured as the difference between the upper and lower frequencies in a continuous frequency set.

Radio Communication Systems

A typical radio communication system from a broadcasting station comprises a transmitter that transmits a modulated carrier into space via an antenna. This wave propagates through space. In a remote location, a receiver receives the modulated carrier through a receiving antenna aided by a tuning circuit. The receiver demodulates the modulated carrier and converts it back to speech or intelligence.

Radio Transmitter Components

• Microphone: Converts sound waves into electrical signals (within the range of 20 Hz to 20 kHz).

• Audio Frequency Amplifier: Amplifies the electrical signals.

• Radio Frequency Generator: Produces the carrier frequency.

• Modulator: Superimposes audio onto the carrier signal.

• Radio Frequency Power Amplifier: Boosts the low power modulated carrier amplitude.

• Aerial: Produces electromagnetic waves radiated into space.

Radio Receiver Components

• Antenna: Picks up the signal frequency and converts it to an alternating current (AC) voltage when tuned to the RF carrier wave.

• Amplification: The received modulated RF carrier is amplified before passing through a demodulator that extracts AF signals.

• Amplification (Continued): The AF signal is further amplified in an AF amplifier, then passed to an AF power amplifier, producing maximum sound energy from the loudspeaker, which converts it into speech or intelligence.

Tuning Circuit

The tuning circuit includes an inductor coil and a capacitor (often variable), arranged in parallel, featuring high impedance at its resonant frequency (ideally infinity). At other frequencies, impedance is lower.

Tuning Process

• At the resonant frequency, the voltage across the circuit reaches its maximum value.

• Capacitors and inductors exhibit equal effects in this circuit, showing equal reactance.

• Phase Relationship: The current through the capacitor is equal to that through the inductor.

Operating Characteristics

Below the Resonant Frequency:

• Lower voltage across the circuit

• Inductor carries most of the current

• Capacitor has reduced effect; current lags behind voltage

Above the Resonant Frequency:

• Lower voltage across the circuit

• Capacitor carries most of the current

• Inductor has diminished effect; current leads the voltage

At Resonant Frequency Equations

The resonant frequency $f$ is expressed as:
$f = rac{1}{2 ext{π} imes ext{sqrt}(LC)}$
where:

• $C$ is the capacitance (Farads, F)

• $L$ is the inductance (Henrys, H)

Further Relationships

1. $L = rac{1}{4 ext{π}^2f^2C}$

2. $C = rac{1}{4 ext{π}^2f^2L}$

3. The capacitor is generally variable for radio tuning.

4. Inductors may be ferrite or iron core types.

Examples

1. At what resonant frequency will a tuning circuit operate if $C = 0.4$ µF and $L = 0.4$ H?

2. In a radio tuning circuit, the capacitor and inductor are always connected in parallel. Illustrate graphs showing the following variations:
      - i) Resonant frequency and capacitance
      - ii) Resonant frequency and inductance

3. At resonance, the frequency of a tuning circuit is 50 Hz. Determine the inductance if the capacitance is 12 µF.

4. Calculate the lowest and highest capacitor values needed with a 0.1 H inductor to tune through the commercial broadcast band from 88 MHz to 108 MHz.

Wave Guides

A wave guide is a medium (material) through which electromagnetic (EM) waves can be transmitted from one point to another without allowing the waves to spread. Wave guides are efficient for energy transmission, facilitating minimal energy loss. They perform optimally when the wavelength is comparable to the guide width. Rectangular wave guides with metal walls transmit microwaves, efficiently carrying large power quantities with minimal heating effects.

Types of Wave Guides

1. Co-axial Cable:
      - Consists of a central copper wire surrounded by an insulating roll.
      - A copper sheath surrounds the roll with an outer insulating layer.
      - Shields inner wire from unwanted signals and supports energy transmission without loss.

2. Optical Fibre:
      - A thin transparent thread made from glass or plastic that permits light travel.
      - Encapsulated by a lower refractive index material called cladding.
      - Utilizes total internal reflection for rapid light signal transmission.
      - Light pulses converted back into electrical signals via a photodiode upon reaching the receiving end.
      - Can transmit hundreds of photo signals simultaneously due to the speed of LASER and photodiode.

Advantages of Optical Fibre over Copper Cables

1. Greater bandwidth permits larger transmission capacity.

2. Less signal power losses lead to reduced necessity for regeneration amplifiers (boosters) and allow higher speed data transmission over long distances.

3. Generally more cost-effective compared to copper wires.

4. More secure since they do not radiate energy, resulting in negligible cross-talk between fibres.

5. Ideal for digital transmission.

6. Extremely reliable due to immunity against many environmental factors impacting copper cables.

Data Transmission

Transmission of information relies on two signal types, analog and digital:

a) Analog Transmission

This system involves continuous varying voltage or current for conveying signals. Telephone and radio systems deliver voice as analog signals, permitting any value within the designated range.

Disadvantages:

• Excessive noise generation from the modulator, transmitter, communication link, receiver, and demodulator.

• Analog systems are unable to identify corrupt signals.

b) Digital Transmission

This system utilizes digits or numbers (digital signals). A digital signal possesses two possible values for current or voltage at any given time. Data is defined as any information transformed into a digital form for transmission via digital systems.

Conversion Process

Digital data transmission necessitates Analog to Digital Converters (ADC) at the transmitting end and Digital to Analog Converters (DAC) at the receiving end. Analog signals derived from speech, which vary continuously, are converted into digital signals by ADC. The original signal is recovered when the DAC reverts the digital signal back into an analog format.

Modems

Within telephone lines, a modem facilitates the conversion of digital signals to analog and demodulates the analog signals at both communication endpoints.

Advantages of Digital Transmission over Analog

1. High quality transmission across various distances.

2. Easier signal amplification and boosting without noise production.

3. Digital signals can be encrypted for enhanced security and privacy.

4. Digital signals are readily stored.

5. More information can be encoded into a smaller bandwidth.

6. Enhanced error detection and maintenance.

Disadvantages:

1. Higher associated costs.

2. Challenges in the disposal of outdated analog technologies.

Mobile Telephone

A mobile phone is a portable telephone that sends and receives calls through a cell site (transmitting load). It functions as a transceiver, transmitting one radio signal while receiving a different signal from a base station, as the two radio signals operate at different frequencies.

Functionality

When a user speaks into the telephone, a microphone converts sound waves (analog) into electrical signals (current or voltage) which generate radio waves, now digital. These signals are then transmitted to a microwave tower positioned at a base station.

Block Diagram of a Mobile Phone

As this document might be illustrative, it should represent the basic layout and functional components of a mobile phone system.

The Cellular Network

A cellular or mobile network represents a radio network distributed across land areas termed cells, each served by at least one fixed-location transceiver known as a cell site or base station. Base stations are strategically placed several kilometers apart, with an automated central station facilitating signal transfer between cells as users move.

Frequency Utilization

In a cellular network, each cell employs distinct frequency sets compared to neighboring cells to minimize interference and ensure guaranteed bandwidth. The linkage of these cells ensures coverage over extensive geographical areas, enabling multiple portable transceivers to communicate seamlessly with each other and with the entire network.

Satellite Telephone System

For calling, a telephone transmits radio waves directly to a satellite, which relays the signal to a ground station, subsequently connecting to the telephone network. The GPS represents a satellite system with ground monitoring stations and receivers detailing exact locations on or above the Earth’s surface, requiring signals from a minimum of four (4) different satellites for accurate positioning.

The Mobile Telephone Network

A cellular network is employed by mobile phone operators, including CAMTEL, ORANGE, NEXTEL, and MTN. Modern mobile phone networks utilize cells as radio frequencies are a limited shared resource; cells switch frequencies under computer control to maximize simultaneous usage with minimal interference. This division into smaller cells prevents signal loss and accommodates a high number of active phones in a given area. All cell sites connect to a telephone switch (or exchanger), interfacing with the Public Switch Telephone Network (PSTN).

Communication Technologies

All mobile phones utilize the GSM (Global System for Mobile Communication), but various digital cellular technologies are used, including:

• CDMA (Code Division Multiple Access)

• GPRS (General Packet Radio Service)

• SDMA (Space Division Multiple Access)

• Ev-DO (Evolution Data Optimized)

• FDMA (Frequency Division Multiple Access)

• TDMA (Time Division Multiple Access)

• AMPS (Advanced Mobile Phone System)

Advantages of Communication Channels

Telecommunication involves concurrent information transmission and reception through the same path or channel. The most efficient method is simultaneous signal transmission, achievable through multiplexing (electric switching). Multiplexing subdivides or splits channel information while retaining distinct separation to prevent confusion or interference at the receiving end.

Methods of Multiplexing

There are two primary multiplexing methods:

1. Frequency Division Multiplexing (FDM): Each telephone signal demands a signal bandwidth of 3.1 kHz, which shifts frequencies at the modulating and demodulating ends, accommodating multiple signals within distinct 3.1 kHz channels, depending on cabling capacity.

2. Time Division Multiplexing (TDM): Signals are segmented into bursts of information lasting 8 milliseconds, comprised of digitally encoded information. TDM achieves a bandwidth advantage over FDM by maintaining a lower system bandwidth while transmitting increased information volumes in shorter intervals.

Degradation of Information

Every communication channel is susceptible to noise, distortion, and attenuation.

Attenuation

Attenuation represents the weakening of signals, leading to strength or form loss. In telephone systems, this weakening is mainly due to wire resistance, while optical fibers experience attenuation stemming from scattering from total internal reflections. Amplifiers can compensate for attenuation along communication channels. Regenerators (repeaters) of digital signals are also utilized to clean up or reshape the pulse, effectively producing a new pulse replicating the original.

Noise

Noise is an unwanted, random signal added to the information signal being transmitted, originating from electronic system interactions due to thermal electron motion and external conditions like thunder or other artificial sources (e.g. electric motors, switches for extensive currents). This noise can be easily captured by radio and television receivers.

Superheterodyne System

The superheterodyne system, invented to tackle the limitations of standard radio receivers, is integrated into all modern receivers. The functional block diagram outlines an AM receiver system of the superheterodyne type.

Operating Mechanism

Radio waves from various broadcasting stations are intercepted by the receiving antenna, which couples to the RF amplifier to select desired radio waves and enhance their strength. The amplified output is then sent to a mixer, combining it with the local oscillator's output, generating an intermediate frequency (IF). This IF frequently stabilizes and enhances the receiver's overall sensitivity and selectivity. The IF output is subsequently processed through a tuned intermediate amplifier and a diode detector circuit, extracting the audio signal. The audio frequency signal, initially weak, is amplified further by the audio frequency amplifier, leading to the loudspeaker converting the audio signals back into sound waves akin to the original audio from the broadcasting station.

Superheterodyne Principle

The superheterodyne principle involves mixing incoming RF signals with local oscillator frequencies to yield the intermediate frequency (IF).

Advantages

1. High RF amplification, yielding a stable intermediate frequency.

2. Enhanced selectivity owing to reduced losses within tuned circuits at lower IFs.

3. Cost-effectiveness compared to other receiver types maintaining a consistent IF regardless of scattered radio waves.

4. Exceptional audio fidelity.

Necessity of Heterodyning

The necessity for heterodyning practice stems from the following reasons:

1. Difficulty in designing high gain, high bandwidth RF amplifiers.

2. The relative simplicity of designing a high gain IF amplifier with uniform gain across narrower bands of lower intermediate frequencies.

3. Conversion of RFs to IFs is essential for optimized processing.

Additional Definitions

• SIM: Subscriber Identification Module

• SMS: Short Message Service

• WIFI: Wireless Fidelity

• GPRS: General Packet Radio Service

• GPS: Global Positioning System

• GSM: Global System for Mobile Communications.