3.3 Satellite Communication

 Satellites have been traditionally utilized in various areas, each serving distinct purposes:

1. Weather Forecasting: Satellites provide essential data for weather forecasting by capturing images of the Earth using infrared or visible light. This data is crucial for predicting natural phenomena like hurricanes.

2. Radio and TV Broadcast: Satellite technology enables the transmission of hundreds of radio and TV programs globally. This medium competes with cable due to its cost-effectiveness and widespread availability.

3. Military Applications: Satellites have been utilized for military purposes, including espionage and secure communication links due to their reliability and resistance to enemy attacks.

4. Navigation Systems: The Global Positioning System (GPS), initially developed for military use, has become indispensable for precise worldwide localization in various sectors, including maritime, aviation, and terrestrial navigation.

5. Global Telephone Backbones: Satellites have historically served as international telephone backbones, offering communication channels that complement terrestrial networks, especially in remote or inaccessible areas.

6. Connections for Remote Areas: Satellites provide connectivity to regions lacking direct wired access to telecommunication networks, such as remote research stations in Antarctica or underdeveloped regions.

7. Global Mobile Communication: Satellites extend the coverage of mobile communication networks, particularly in areas where terrestrial infrastructure is lacking or too costly to install. They enable worldwide connectivity and supplement existing cellular networks.

8. Modern Satellite Capabilities: Contemporary satellites function as sophisticated communication hubs, resembling flying routers. They offer digital signal processing capabilities, error correction, and inter-satellite routing, enhancing communication quality and efficiency.

Geostationary Earth Orbit (GEO) satellites orbit the Earth at a distance of approximately 36,000 kilometers. They play vital roles in various applications, including TV and radio broadcasting, weather monitoring, and backbone support for the telephone network. Here are the key characteristics, advantages, and disadvantages of GEO satellites:

Advantages:

1. Complete Coverage: Only three GEO satellites are needed to cover almost any location on Earth. This extensive coverage makes them ideal for global communication and broadcasting.
2. Fixed Antenna Positions: GEO satellites appear stationary in the sky from the perspective of ground-based antennas. This stationary position simplifies antenna setup and alignment since there is no need for constant adjustment.
3. Ideal for Broadcasting: The fixed position of GEO satellites makes them well-suited for TV and radio broadcasting, as receivers can maintain a constant connection without tracking the satellite's movement.
4. Long Lifetime: GEO satellites typically have a long operational lifespan, lasting up to 15 years or more, which reduces the need for frequent replacements.
5. No Doppler Shift: The relative movement between GEO satellites and ground stations is negligible, resulting in minimal Doppler shift, which simplifies signal processing.

Disadvantages:

1. Limited Coverage in Polar Regions: Regions near the North and South Poles experience challenges in receiving signals from GEO satellites due to their low elevation angles. Larger antennas are often required in these areas, increasing infrastructure costs.
2. Signal Shading in Urban Areas: Tall buildings and low satellite elevation angles in urban environments can obstruct or degrade signal quality, particularly for receivers located farther from the equator.
3. High Power Requirements: Transmitting signals to GEO satellites requires relatively high power levels, which can be impractical for battery-powered devices or small mobile phones.
4. High Latency: The round-trip time for signals to travel to and from GEO satellites results in significant latency, often exceeding 0.25 seconds. This latency can negatively impact real-time communication applications and may require specialized protocols to mitigate delays.
5. Limited Frequency Reuse: Due to their large coverage areas, GEO satellites face challenges in reusing frequencies efficiently. This limitation can lead to spectrum congestion or the need for specialized antenna designs to focus signals on smaller footprints.
6. Costly Deployment: Launching and deploying GEO satellites into orbit is a costly endeavor, requiring substantial investment in spacecraft design, manufacturing, and launch services.

Medium Earth Orbit (MEO) satellites operate at distances ranging from approximately 5,000 to 12,000 kilometers from the Earth's surface. Although there have been relatively few satellites in this class historically, upcoming systems like ICO are exploring the use of MEOs for various purposes. Here are the key characteristics, advantages, and disadvantages of MEO satellites:

Advantages:

1. Optimal Balance: MEO satellites offer a balance between the coverage provided by Geostationary Earth Orbit (GEO) and the orbital dynamics of Low Earth Orbit (LEO) satellites. They occupy a middle ground in terms of orbit altitude, providing a compromise between coverage area and system complexity.
2. Reduced System Complexity: Compared to GEO and LEO systems, MEO satellite constellations require fewer satellites to achieve global coverage. Typically, only a dozen satellites are needed for MEO systems, making them more manageable and cost-effective to deploy and maintain.
3. Extended Coverage: MEO satellites, positioned at orbits around 10,000 kilometers, can cover larger populations due to their broader footprint. This extended coverage reduces the need for frequent handovers between satellites, enhancing the continuity of communication services.

Disadvantages:

1. Increased Signal Delay: Despite being closer to Earth than GEO satellites, MEO satellites still experience significant signal delay, averaging around 70-80 milliseconds. This delay can impact real-time communication applications and may require latency mitigation strategies.
2. Higher Power Requirements: Transmitting signals to MEO satellites requires higher power levels compared to LEO systems due to the greater distance. Additionally, specialized antennas may be needed to focus signals on smaller footprints, especially for applications requiring high data rates or precise coverage.

Low Earth Orbit (LEO) satellites operate at altitudes ranging from 500 to 1,500 kilometers above the Earth's surface. Initially used primarily for espionage, many modern satellite systems now utilize LEO satellites due to their unique advantages. Here are the key characteristics, advantages, and disadvantages of LEO satellites:

Advantages:

1. High Transmission Rates: Despite their lower orbit, LEO satellites can achieve high transmission rates, reaching up to 2,400 bits per second (bps) with advanced compression schemes. This bandwidth is sufficient for voice communication and even supports mobile terminals with omni-directional antennas using low transmit power.
2. Low Signal Delay: LEO satellites offer relatively low signal delay, typically around 10 milliseconds (ms). This delay is comparable to long-distance wired connections, making LEO systems suitable for real-time communication applications.
3. Improved Frequency Reuse: The smaller footprints of LEO satellites enable better frequency reuse, similar to the concepts used in cellular networks. This optimization enhances spectral efficiency and overall system capacity.
4. Enhanced Global Coverage: LEO satellites provide better coverage in polar regions and ensure high-quality communication links by maintaining a high elevation for every spot on Earth.

Disadvantages:

1. Requirement for Numerous Satellites: Achieving global coverage with LEO satellites requires a large number of satellites in orbit, often ranging from 50 to 200 or more. The need for multiple satellites increases the complexity and cost of the satellite system.
2. Complex Handover Mechanisms: Due to their short visibility windows and high elevation angles, LEO satellites require additional mechanisms for seamless handover between different satellites as users move between coverage areas. Managing handovers adds complexity to the system.
3. Limited Satellite Lifespan: LEO satellites have a relatively short lifespan of about five to eight years due to atmospheric drag and radiation exposure. This short lifespan necessitates frequent satellite replacements, adding to operational costs and logistical challenges.
4. Routing Complexity: Data packets transmitted via LEO satellites may require routing between multiple satellites or between satellites and ground stations to reach their destination. This routing complexity increases with the number of satellites and can impact network performance.

Aspect	LEO	MEO	GEO
Altitude	500 - 1,500 km	5,000 - 12,000 km	Approximately 36,000 km
Period	Short (typically 95 - 120 minutes)	Longer (about 6 hours)	24 hours (synchronous with Earth)
Coverage	Limited, multiple satellites needed	Medium, fewer satellites required	Global, fewer satellites required
Signal Delay	Low (around 10 milliseconds)	Moderate (around 70 - 80 milliseconds)	High (around 250 milliseconds)
Number of Satellites	Many (50 - 200 or more)	Fewer (a dozen or less)	Typically three for global coverage
Lifespan	Short (about 5 - 8 years)	Moderate (typically 10 - 15 years)	Long (about 15 years)
Handover Complexity	High due to short visibility windows	Moderate	Minimal due to fixed position
Frequency Reuse	Limited due to small footprints	Moderate	High due to large coverage area
Application Examples	Global mobile communication, broadband internet	Global navigation, regional communication	TV and radio broadcasting, backbone for telephone network

The routing of data transmissions in a satellite system, together with gateways and fixed terrestrial networks, depends on whether the satellites offer Inter-Satellite Links (ISLs) or not. Here's a breakdown of the two scenarios:

1. With ISLs:

   • When the satellite system supports ISLs, data transmission between users occurs primarily within the satellite network in space.
   • One user sends data up to a satellite, which then forwards it to the satellite responsible for the receiver via other satellites.
   • The final satellite sends the data down to the earth for delivery to the receiver.
   • This approach requires only one uplink and one downlink per direction, reducing the reliance on gateways on earth.
   • Routing within the satellite network decreases the latency of data transmission.
   • However, implementing ISLs increases system complexity due to additional antennas and routing hardware and software on the satellites.

2. Without ISLs:

• If the satellite system does not offer ISLs, data transmission follows a route through fixed terrestrial networks.
• The user sends data up to a satellite, which then relays it to a gateway on earth.
• Routing proceeds through the fixed terrestrial networks until another gateway responsible for the satellite above the receiver is reached.
• The data is then sent up to the satellite again, which forwards it down to the receiver.
• This approach requires two uplinks and two downlinks, increasing the latency compared to ISL-enabled routing.
• The reliance on terrestrial networks for routing may introduce additional delays.
• However, this approach may be simpler to implement in terms of satellite hardware and software.

In satellite networks, the localization of users is similar to that in terrestrial cellular networks, but with some additional considerations due to the movement of satellites. Here's an overview of how user localization works in satellite networks:

1. Registers in Satellite Networks:

   • Home Location Register (HLR): Stores static information about users and their current location.
   • Visitor Location Register (VLR): Stores the last known location of a mobile user.
   • Satellite User Mapping Register (SUMR): Stores the current positions of satellites and maps each user to the satellite through which communication is possible.

2. Registration Process:

   • When a mobile station initiates communication, it sends a signal that one or more satellites can receive.
   • Satellites detecting this signal report the event to a gateway.
   • The gateway uses the information from the satellites to determine the location of the user.
   • User data is then requested from the user's HLR, and the VLR and SUMR are updated accordingly.

3. Calling Process:

• When someone calls a mobile station, the call is forwarded to a gateway.
• The gateway localizes the mobile station using the HLR and VLR.
• With the assistance of the SUMR, the gateway determines the appropriate satellite for communication.
• The connection is established through the identified satellite.

Handover is a critical aspect of satellite systems, especially those using Medium Earth Orbits (MEOs) and Low Earth Orbits (LEOs), due to the movement of satellites. Here's an explanation of the different types of handover in satellite systems:

1. Intra-satellite handover:

   • Occurs when a user moves from one spot beam of a satellite to another within the same satellite's footprint.
   • Special antennas on the satellite can create multiple spot beams within its coverage area.
   • Movement of the satellite itself can also trigger intra-satellite handover.

2. Inter-satellite handover:

   • Happens when a user moves out of the footprint of one satellite or when the satellite itself moves away.
   • Can be a hard handover, where the connection switches abruptly to the next satellite, or a soft handover, where the connection is maintained with both satellites simultaneously (possible with CDMA systems).
   • Inter-satellite handover can also occur between satellites if they support Inter-Satellite Links (ISLs), allowing for smoother transitions.
   • The satellite system may adjust transmission quality in exchange for handover frequency, with higher quality requiring more frequent handovers.

3. Gateway handover:

   • Takes place when a satellite moves away from its current gateway, requiring connection to a different gateway for communication.

4. Inter-system handover:

   • Involves switching between satellite-based communication systems and terrestrial cellular networks.
   • Users may switch to terrestrial networks when available due to cost-effectiveness or lower latency.
   • Seamless handover between satellite and terrestrial networks has been challenging, despite the availability of dual-mode mobile phones.