Telecommunications Systems: Signals, Modulation, and Microwave Links
Defining Telecommunications
Telecommunications is responsible for the transport of information over long distances via a communication medium or channel by means of signals.
The Birth of Modern Telecommunications
Modern telecommunications was born in 1844 with the birth of the telegraph. The telegraph, the first form of electrical communication, was invented by Samuel Morse.
Elements of a Telecommunications System
- Transmitter: Shapes the message to be sent into a signal suitable for the channel.
- Transmission Channel: The electrical link (medium) between the transmitter and receiver, bridging the source and destination.
- Receiver: Extracts the desired signal from the channel and delivers it to the output transducer.
What is Modulation?
Modulation is the systematic alteration of a carrier wave according to the message (modulating signal). It can also involve coding.
Benefits of Modulation
- Facilitates the propagation of the information signal via cable or air.
- Organizes the radio spectrum, distributing different information channels.
- Optimizes antenna dimensions.
- Avoids co-channel interference.
- Protects information from noise degradation.
- Sets the quality of the conveyed information.
Advantages of Digital Communication
- Noise Immunity: Received pulses are evaluated during a sampling interval, and a determination is made whether the pulse is above (1) or below (0) a specific threshold.
- Storage and Processing: Digital signals can be easily stored and processed, unlike analog signal processing.
- They produce a noise-resistant analog counterpart.
Terrestrial Microwave Radio Links
A terrestrial microwave radio link, or microwave link, provides connectivity between two locations (earth stations) in line of sight (Line-of-Sight, LOS) using radio equipment with carrier frequencies above 1 GHz. The emitted waveform may be analog (conventional FM) or digital.
Microwave System Applications
- Long-distance communications.
- Alternative for basic telephony (telephone channels).
- Video transmission.
- Data transmission.
- Telegraph/Telex/Facsimile.
- TV channels.
- Mobile phone communication (between trunks).
Main Frequencies Used in Microwave Systems
- 12 and 18 GHz: Connect two locations between 1 and 15 miles apart.
- 2 and 6 GHz: The equipment can transmit at distances between 20 and 30 miles.
- 23 GHz.
Microwave Antennas
Dish antennas: Typically 3 meters in diameter, they transmit a narrow beam focused directly towards the receiving antenna. They must be at a considerable height to achieve greater separation. The maximum distance is approximately 7.14 km.
Advantages and Disadvantages of Microwave Links
Advantages:
- Lower cost.
- Simple installation.
- Quicker and generally more economical maintenance.
- Can overcome uneven terrain.
Disadvantages:
- Operation restricted to sections with line of sight.
- Requires adequate access to repeater stations.
Characteristics of HF Radio Waves
HF radio waves broadcast from antennas on Earth follow two paths:
- The ground wave follows the Earth’s surface.
- The sky wave bounces back and forth between the Earth’s surface and several layers of the Earth’s ionosphere.
Direct Infrared Laser Transmissions
Direct infrared laser transmissions use the same techniques as optical fiber transmission, except the medium is open air. The laser has a range of up to 10 miles, although most applications today are at distances less than a mile. Typically, infrared transmissions are used where cable installation is not feasible to connect two sites.
Low Earth Orbit (LEO) Satellites
Low Earth Orbit (LEO) satellite orbits are less than 36,000 km, with a rotation period less than the Earth’s. Therefore, their relative position in the sky changes constantly.