LOW EARTH ORBIT SATELLITE

LOW EARTH ORBIT SATELLITE

            Traditionally, communications satellites have operated in geo-stationary orbit (GSO), 35,000 km above the equator, and low Earth orbit (LEO) satellites have been used for weather monitoring, resource mapping, and Earth sensing. By the start of the new millennium, in five years, revolution will have occurred in the communication satellite industry with LEO satellite networks becoming key pathways of the information superhighway.

Low Earth Orbit (LEO) satellites are satellites that operate in orbits of around 100 km to 1,000 km above the Earth’s surface – much lower than traditional communications satellites – which bring them into frequent radio contact with ground stations. LEOs are used for a variety of civil, scientific and military roles including Earth observation, radar, optical, telecoms and demonstrator.

LEO satellite constellations now form a vital link in the expanding communications and observation networks that are essential for global economic development, especially in remote areas. Reliability is therefore a key issue for satellite manufacturers and operators.

Low Earth Orbit satellite must travel very quickly to resist the pull of gravity approximately 17,000 mile per hour. Because of this, Low Earth Orbit satellite can orbit the planet in as little as 90 minutes.

Kinds of LEO

Little LEO

            Little LEO are required to offer non-voice services for example vehicle tracking, environmental monitoring and two-way data communication. A little LEO is a constellation of small, low-earth orbiting satellites, use for short, and narrowband communication. Little LEO are small, Low cost, class of satellites.

Big LEO

            Big LEO are used for technology devices such as high speed, high bandwidth data communications, and video conferencing. They can carry voice and high speed data services. They are aimed at data communications and real time voice into hand held devices. Big LEO can also offer global services, which are also subject to regulatory requirements. 

Advantage of LEO

• A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.
• A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

Disadvantage of LEO

• A network of LEO satellites is needed, which can be costly.
• LEO satellites have to compensate for Doppler shifts cause by their relative movement.
• Atmospheric drag affects LEO satellites, causing gradual orbital deterioration.

3G

3G

3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 (IMT-2000) specifications by the International Telecommunication Union.

Time line in using 3G

GPRS

GPRS operates at much higher speeds than cur rent  networks,  providing advantages  from a software perspective. Wireless middleware currently is required to enable slow speed mobile clients to work with fast networks for applications such as e−mail, databases, groupware, or Internet access. With GPRS, wireless middleware will probably be unnecessary, making it easier to deploy wireless solutions.

EDGE

            Beyond GPRS, EDGE takes the cellular community one step closer to UMTS. It provides higher data rates than GPRS and introduces a new modulation scheme called 8−Phase Shift Keying (PSK). The TDMA community also adopted EDGE for their migration to UMTS. The data rates allocated for EDGE are started at 384 Kbps and above as a second stage to GPRS. EDGE uses the same modulation techniques as many of our existing TDMA infrastructures using Gaussian Minimum Shift Keying (GMSK) 8−PSK. Moreover EDGE uses a combination of FDMA and TDMA as the multiple access control methods.

WHY EDGE?

            EDGE is a new modulation scheme that is more bandwidth efficient than the GMSK modulation scheme used in the GSM standard. It provides a promising migration strategy for HSCSD and GPRS. The technology defines a new physical layer: 8−PSK modulation, instead of GMSK. 8−PSK enables each pulse to carry 3 bits of information versus the GMSK 1−bit−per−pulse rate. Therefore, EDGE has the potential to increase the data rate of existing GSM systems by a factor of three.

            EDGE retains other existing GSM parameters, including a frame length, eight time slots per frame, and a 270.833 kHz symbol rate. The GSM 200 kHz channel spacing is also maintained in EDGE, enabling the use of existing spectrum bands. This fact is likely to encourage deployment of EDGE technology on a global scale.

UMTS

            UMTS is a modular concept that takes full advantage of the trend of converging existing and future information networks, devices, and services, and the potential synergies that can be derived from such convergence. UMTS will move mobile communications forward from where we are today into the 3G services and will deliver speech, data, pictures, graphics, video communication, and other wideband information direct to people on the move. UMTS is one of the major new 3G mobile communications systems being developed within the framework, which has been defined by the ITU and is known as IMT−2000.

WCDMA

            WCDMA is an ITU standard derived from CDMA and is officially known as IMT−2000 direct spread. WCDMA is a 3G mobile wireless technology offering much higher data speeds to mobile and portable wireless devices than commonly offered in today's market.

            WCDMA can support mobile/portable voice, images, data, and video communications at up to 2 Mbps (local area access) or 384 Kbps (wide area access). The input signals are digitized and transmitted in coded, spread−spectrum mode over a broad range of frequencies. A 5 MHz wide carrier is used compared with a 200 kHz wide carrier for narrowband CDMA.

Applications of 3G

Online e−mail

Access to the World Wide Web

Enhanced short message services

Wireless imaging with instant photos or graphics

Video services

Document/information sharing

Surveillance

Voice messaging via Internet

Broadcasting

GPRS

GPRS

GPRS is a key milestone f o r GSM data. It offers end users new data services   a n d enables operators to offer radically new pricing options. Using the existing GSM radio infrastructure, up−front investments for operators are relatively low.

What is GPRS

As stated previously, GPRS stands for General (or generic) Packet Radio Service. GPRS extends the packet data capabilities of the GSM networks from Packet Data on Signaling−channel Service (PDSS) to higher data rates and longer messages.

GPRS is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications (GSM). GPRS was originally standardized by European Telecommunication Standard Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3^rd Generation Partnership Project.

GPRS Network View

Why does GPRS was develop

            GPRS was developed to enable GSM operators to meet the growing demands for wireless packet data service that is a result of the explosive growth of the Internet and corporate intranets. Applications using these networks require relatively high throughput and are characterized by bursty traffic patterns and asymmetrical throughput needs. Applications, such as web browsing, typically result in bursts of network traffic while information is being transmitted or received, followed by long idle periods while the data is being viewed.

GPRS Radio Technology

            Packet switching means that the GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to one mobile user for a fixed period of time, the available radio resources can be concurrently shared by several users. This efficient use of the scarce radio resources means that a larger number of GPRS users can share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data each user has to send or receive.

Sample Application of GPRS

Many applications fit into the mode of GPRS and IPs. These applications are merely a means to an end. In other scenarios, the features and applications can be met with other technologies. The issue at hand is that the use of GPRS facilitates these applications and drives the acceptance ratio.

Chat

            Chat can be distinguished from general information services because the source of the information is a person with the chat protocol, whereas it tends to be from an Internet site for information services.

Textual and Visual Information

            A wide range of content can be delivered to mobile phone users, ranging from share prices, sports scores, weather,  flight information, news headlines, prayer reminders, lottery results,   jokes, horoscopes, traffic, location−sensitive services, and so on. This information does not necessarily need to be textual — it may be maps or graphs or other types of visual information.

Still Images

           

            Still images such as photographs, pictures, postcards, greeting cards, presentations, and static web pages can be sent and received over   the mobile network as they are across fixed telephone networks. It will be possible with GPRS to post images from a digital camera connected to a GPRS radio device directly to an Internet site, enabling near real−time desktop publishing.

Moving Images

            Over time, the nature and form of mobile communication is getting less textual and more visual. The wireless industry is moving from text messages to icons, picture messages to photographs, blueprints to video messages, movie previews being downloaded, and on to full−blown movie watching via data streaming on a mobile device.

Web Browsing

           

            Using circuit−switched data for web browsing has never been an enduring application for mobile users. Because of the slow speed of circuit−switched data, it takes a long time for data to arrive from the Internet server to the browser. Alternatively, users switch off the images, just access the text on the Web, and end up with text layouts on screens that are difficult to read. As such, mobile Internet browsing is better suited to GPRS.