Satellite vs. Terrestrial Communication: A Comprehensive Comparison

This article delves into the differences between satellite and terrestrial communication systems, exploring their respective strengths and weaknesses. Both systems facilitate the transmission and reception of information, voice, data, and video between geographically dispersed users and offer global connectivity.

In satellite communication, satellites serve as the transmission medium between the transmitter and receiver. Conversely, terrestrial communication relies on ground-based infrastructure, including copper cables, fiber optic cables, point-to-point microwave links, and wireless cellular networks like 3G, 4G LTE, and 5G.

Microwave frequencies are commonly used for wireless communication due to their ability to penetrate the ionosphere. However, they experience attenuation when used as ground or surface waves. Consequently, microwave communication is primarily line-of-sight (LOS) based.

Both satellite and terrestrial systems consist of transmit and receive components. The transmit system converts the baseband signal to a microwave signal, while the receive system performs the reverse process. The baseband signal is a multiplexed signal carrying multiple individual low-bandwidth signals, such as voice, data, and video, using either Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM).

Terrestrial Communication Systems

Terrestrial systems facilitate the transmission of voice, data, and video signals over land-based physical infrastructure. These systems employ various communication technologies and mediums operating on the Earth’s surface, including cables, fiber optics, Ethernet, and microwave wireless transmission systems.

Unlike satellite communication, which relies on space-based satellites, terrestrial communication depends on fixed and mobile networks located on the ground. It is widely used for applications such as internet connectivity, television broadcasting, telephone services, and data transmission, forming the backbone of modern communication infrastructure for seamless connectivity in residential and commercial settings.

A terrestrial system comprises a transmitter, a transmission medium, and a receiver. The transmission medium can be wired or wireless. Wired communication utilizes copper cables, fiber optic cables, Ethernet, and the like. Wireless communication employs radio frequency waves or microwave signals to transmit data without physical cables.

The following block diagram illustrates a terrestrial microwave communication system:

These microwave terrestrial links enable long-distance communication between fixed locations, connecting remote offices or transmitting data across point-to-point (LOS) locations. The range of terrestrial communication is limited because microwave frequencies are susceptible to attenuation from buildings, trees, and geographical features.

To extend the range, multi-section relays or repeaters are used. Repeaters receive a signal from one end, amplify it, and retransmit it to the other end, compensating for RF losses. Repeaters are typically spaced 32 to 80 km apart.

Terrestrial systems use both analog and digital modulation techniques. In analog systems, data information signals are frequency multiplexed (FDM), then frequency modulated (FM) and up-converted for transmission using an RF antenna. In digital systems, data information signals are time multiplexed (TDM) to form a baseband signal, which is then modulated (using either Phase Modulation (PM) or Phase-Shift Keying (PSK)) and up-converted for transmission using an RF antenna.

Satellite Communication Systems

Satellite-based networks operate in two primary modes: mesh and star. In this system, baseband information is transmitted at a microwave carrier frequency from a ground station (VSAT) to the satellite using a directional antenna.

The satellite receives the signal using an on-board antenna, shifts the received frequency to another band, and amplifies the translated signal before relaying it over a large area of the earth. For example, a 6 GHz uplink frequency and a 4 GHz downlink frequency are common, with a 2 GHz difference achieved using a Local Oscillator (LO) frequency of 2225 MHz. Here, 6 GHz refers to the frequency range from 5.925 to 6.425 GHz, and 4 GHz refers to the range from 3.7 to 4.2 GHz.

Satellite frequencies are chosen to mitigate the effects of the ionosphere and absorption by gases and water vapor. Geostationary satellites are positioned at an altitude of 35,800 km and travel at approximately 11,000 km/hour. This allows for fixed antenna placement at the ground station because satellite tracking is not required.

In mesh mode, VSAT1 and VSAT2 communicate directly via the satellite. In star mode, VSAT1 and VSAT2 communicate through the satellite/hub. Both configurations operate at 6 GHz (uplink) / 4 GHz (downlink) bands. Other satellite system bands include Ku bands, operating at 14 GHz (uplink) / 11 GHz (downlink), and 17/12 GHz frequency bands.

Similar to terrestrial systems using repeaters, satellite systems employ transponders to provide connectivity between source and destination stations. A typical satellite may have 12 transponders, each with a 36 MHz bandwidth, covering a total satellite band of approximately 500 MHz.

Key Differences Between Satellite and Terrestrial Systems

Several differences between satellite and terrestrial wireless communications impact system design:

• Coverage Area: Satellite systems offer a significantly larger coverage area than terrestrial systems. A GEO satellite with a single antenna can cover about one-fourth of the Earth.
• Signal Quality: Satellite communication links may experience more signal degradation compared to terrestrial links, but the overall transmission quality is usually quite good.
• Latency: The round-trip delay in a satellite link (Earth-to-satellite-to-Earth) is approximately 240 ms, whereas the delay in a terrestrial link is much lower.
• Cost: The transmission cost in a satellite system is independent of distance within the satellite antenna’s coverage area. In contrast, terrestrial system costs vary with distance.
• EIRP and Bandwidth: Satellite Effective Isotropic Radiated Power (EIRP) and bandwidth are crucial design parameters for both the satellite and earth station.
• Bandwidth and Data Rates: Satellite-based communication systems can achieve very high bandwidths and data rates.
• Signal Reception: In satellite systems, all Earth stations/VSATs can receive their own transmissions. Therefore, transmit power must be carefully determined based on the RF link budget. However, the use of different transmitting and receiving frequencies minimizes potential interference. Transmit reject filters should be adequate to mitigate any remaining interference.

The following table summarizes the differences between satellite and terrestrial communication systems:

Conclusion

Satellite and terrestrial communication systems are often used together to leverage the benefits of both. This hybrid approach creates robust and versatile communication networks that meet diverse needs and provide reliable connectivity across various scenarios globally.