Personal tools

5G and Satellite Evolution

LEO_and_GEO_Satellites_Unidata_072120A
[LEO and GEO Satellites - Unidata]

 

- Overview

To fully realize the promise of fifth-generation mobile networks (5G) -- the near-ubiquitous, instant connectivity of vast numbers of devices around the world -- terrestrial telecommunications systems that rely heavily on buried fiber-optic cables are not enough.

Instead, we will need to move from (a) largely separate satellite and terrestrial communications systems to one where satellites are primarily used to solve "last mile" problems (areas where laying fiber is a physical or economic challenge) or for discrete use cases (Such as processing credit card payments at gas stations) to (b) an integrated 5G “network of networks” where satellites play an increasing role alongside terrestrial networks.

 

- Three Types of Communication Satellites: LEO, MEO, and GEO

Satellite systems provide voice, data, and broadcast services with widespread, often global, coverage to high-mobility users as well as to fixed sites. They have the same basic architecture as cellular systems, except that the base stations are satellites orbiting the earth. 

Satellites are characterized by their orbit distance from the earth: low-earth orbit (LEO) at 500 to 2,000 km, medium-earth orbit (MEO) at 10,000 km, and geosynchronous orbit (GEO) at 35,800 km. A geosynchronous satellite has a large coverage area that is stationary over time, since the earth and satellite orbits are synchronous. Satellites with lower orbits have smaller coverage areas, and these coverage areas change over time so that satellite handoff is needed for stationary users or fixed-point service.

LEO and GEO satellites are big players in this skyrocketing small satellite industry. They can provide Internet access to the ground. With suitable protocols and sophisticated algorithms, it will be possible to modify the other TCP layers like transport or network layers to satisfy the optical network requirement.

 

- Geosynchronous (GEO) Satellites

GEO satellites are earth-orbiting about 22,300 miles (35,800 Kilometers) directly above the equator. They travel in the same direction as the rotation of the Earth. This gives the satellites the ability to stay in one stationary position relative to the Earth. One advantage of GEO satellites is that it is easier to implement new technologies as they arise and additional capacity one satellite at a time, unlike LEO satellites. The disadvantage is that making these improvements is more time-consuming and costly.

Since GEO satellites have such large coverage areas, just a handful of satellites are needed for global coverage. However, GEO systems have several disadvantages for two-way communication. It takes a great deal of power to reach these satellites, so handsets are typically large and bulky. The large round-trip propagation delay is quite noticeable in two-way voice communication. Recall that high-capacity cellular systems require small cell sizes. Since geosynchronous satellites have very large cells, these systems have small capacity, high cost, and low data rates, less than 10 Kbps. The main geosynchronous systems in operation today are the global Inmarsat system, MSAT in North America, Mobilesat in Australia, and EMS and LLM in Europe.

 

NASA Rocket_010322A
[America’s Rocket for Deep Space Exploration (Concept View) - NASA’s Space Launch System, or SLS, is a super-heavy-lift launch vehicle that provides the foundation for human exploration beyond Earth’s orbit. With its unprecedented power and capabilities, SLS is the only rocket that can send Orion, astronauts, and cargo to the Moon on a single mission. Offering more payload mass, volume capability, and energy, SLS is designed to be flexible and evolvable and will open new possibilities for payloads, including robotic scientific missions to places like the Moon, Mars, Saturn, and Jupiter.]

- Low-Earth Orbit (LEO) Satellites  

LEO satellites, like their name implies, orbit much closer to earth. LEOs tend to be smaller in size compared to GEO satellites, but require more LEO satellites to orbit together at one time to be effective. To deliver full communication services over selected locations, more than 1,000 LEO satellites may be needed to achieve sufficient performance. GEO satellites should handle this job for now, but LEO satellites can play a significant role in testing out the new technologies and concepts for GEOs.

The trend in current satellite systems is to use the lower LEO orbits so that lightweight handheld devices can communicate with the satellites and propagation delay does not degrade voice quality. The best known of these new LEO systems are Globalstar, Iridium, and Teledesic. Both Globalstar and Iridium provide voice and data services to globally roaming mobile users at data rates under 10 Kbps. Teledesic uses 288 satellites to provide global coverage to fixed-point users at data rates up to 2 Mbps. The cell size for each satellite in a LEO system is much larger than terrestrial cells, with the corresponding decrease in capacity associated with large cells. The cost to build, launch, and maintain these satellites is much higher than that of terrestrial base stations, so these new LEO systems are unlikely to be cost-competitive with terrestrial cellular and wireless data services. LEO systems can complement terrestrial systems in low-population areas and may appeal to travelers desiring just one handset and phone number for global roaming.

Rather than competing with GEOs, LEOs should be used as more of an accomplice, at least for now (with current technology). It is much cheaper and faster to test new technologies and concepts in LEO devices compared to GEOs.  

 

- Comparison between GEO and LEO Satellites

GEO and LEO satellites both have advantages. To choose which is best for a given need, application, or market, it’s important to understand the top factors that play a role in determining how a LEO or GEO will perform for various communication applications. 

Here are the top seven criteria you should review when trying to understand LEO and GEO satellite performance: Latency, Coverage, Efficiency, Cost, Complexity, Frequency spectrum. Time to market and adaptability.

  • Latency: Lower orbits tend to have lower latency for time-critical services because of the closer distance to earth. Latency can many times vary depending on the specific application. Latency is mostly important when looking at online and very interactive applications, such as gaming and electronic services trading. GEO satellite latency typically has limited impact on user experiences. However, with the increase of 4G/LTE and even 5G technology, latency is becoming less of a concern. 
  • Coverage: It’s important to reiterate that many LEO satellites must work together to offer sufficient coverage to a given location. Although many LEOs are required, they require less power to operate because they are closer to earth. Choosing to go with more satellites in the LEO orbit on less power, or using fewer larger satellites in GEO, is the biggest decision to make here.
  • Efficiency: Because LEO satellites are constantly moving relative to earth at a given moment, they tend to spend a lot of time over oceans and other unpopulated areas, making them less efficient in that sense. However, this would be the best option when trying to cover a larger geographical area as opposed to GEO satellites. Remember, GEOs stay in one location relative to a specific spot relative to earth. This makes GEOs more efficient for smaller, more specific regions. 
  • Cost: Although smaller LEO satellites are less expensive to manufacture, more are typically needed at one time to have effective communication operations. GEO satellites on the other hand are larger with more capability, but are only needed to work successfully in small amounts. In the future, it is expected that new innovations in GEO technology will help to significantly reduce their cost per unit. This not only applies to GEO satellites but the entire satellite industry.  LEO satellites get very complex and expensive when looking at the number of gateways required on the ground to operate. This drives the total cost of the overall system up. However, large-scale production of identical gateways can help reduce these costs. 
  • Complexity: As mentioned above, low earth orbiting satellites are more complex when it comes to their expensive ground antennas. The need for many ground satellites comes from the many traveling LEOs in orbit at one time. New phased array antennas can help reduce complexity here, but will need to be used in a wider variety of elevations, which can be difficult to install. 
  • Frequency spectrum: LEO satellites are constantly overlapping each other geographically. This makes managing their frequency synchronization complex between systems, because there are many different LEOs traveling all around the world. Managing all these chaotic two-way systems in LEO orbits can become extremely difficult for engineers and operators to allow all the LEO satellites to function without traffic deprivations. And that’s just the beginning! Things get even more complex with LEOs when they must also coordinate with other GEO satellites at certain points along their orbital journey. 
  • Time to market and adaptability: Since a single GEO satellite can cover a specific region, it is much easier to progressively add new satellites to the system or replace old ones. However, when it comes to the many LEO satellites (a LEO constellation) in orbit, a large portion of the constellation will have to be built before adding on to or replacing the current constellation to provide service to a region. Sometimes, the entire LEO constellation will have to be replaced by a new constellation when upgrading performance or implementing alternative services. This adds time, costs, and complexity as opposed to GEOs.



[More to come ...]



Document Actions