CHAPTER 27

Low−Earth−Orbit Satellites (LEOs)

A low Earth orbit (LEO) is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 160–2,000 km (100–1,240 miles) above the Earth's surface. The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.

Satellite systems are employed for telephone and data communications. There are geostationary satellites flying in high orbit (22,000 miles) where they can maintain the same position above the earth's surface at all times. The only problem, with such high-flying satellites is that there is a noticeable delay in real-time communications, and the power requirement to communicate with the satellites is too high for portable devices.

LEOs are more practical for mobile communication devices like mobile phones, PDAs, and automobile communication systems. An LEO satellite orbits in a relatively low earth orbit of a few hundred miles. In this orbit, the round-trip time for transmission is minimal, as are the power requirements for earth-bound communication devices. The downside of LEO satellites is that a fleet of them is required. Because of their low orbit, they move faster relative to a point on the surface, so a fleet of LEO satellites is required to maintain communications over a single point. As one LEO moves out of position, the other moves in. Each satellite covers an area that could be compared to a cell in a cellular system, except that the cell moves as the satellite orbits.

Space debris

The LEO environment is becoming congested with space debris. This has caused growing concern in recent years, since collisions at orbital velocities can be damaging or dangerous, and can produce more space debris in the process (Kessler Syndrome). The Joint Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), currently tracks more than 8,500 objects larger than 10 cm in LEO,[4] however a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters, which are too small to be visible from Earth.

ORBITAL DISTANCES

Any satellite can achieve orbit at any distance from the earth if its velocity is sufficient to keep it from falling to earth. The farter the satellite is from the earth, the longer it takes for a transmission to reach the satellite. The altitudes at which satellites can orbit are split into two categories:

Ø  Low Earth Orbit (LEO)

Ø  Medium Earth Orbit (MEO)

LOW EARTH (LEO)

Satellites in low earth orbit (LEO) satellites complete one orbit roughly every 90 minutes at a height of between 100 and 500 miles above the earth's surface. This means that they are fast moving ( >17,000mph) and sophisticated ground equipment must be used to track the satellite. This makes for expensive antennas that must track the satellite and lock to the signal while moving.

MIDDLE EARTH (MEO)

Most of the satellites in middle earth orbit circle the earth at approximately 6,000 to 12,000 miles above the earth in an elliptical orbit around the poles of the earth. Any orbit that circles around the poles is refered to as a 'polar orbit'. Polar orbits have the advantage of covering a different section of the earth's surface as they circle the earth. As the earth rotates, satellites in polar orbits can cover the entire surface of the earth. Fewer satellites are required to create coverage for the entire earth, as these satellites are higher and have a larger footprint. Spy satellites typically use middle earth, polar orbits to cover as much of the earth's surface as possible from one satellite.

GEOSTATIONARY/GEOSYNCHRONOUS (GEO)

 At 22,240 miles above the earth, craft inserted into orbit over the equator and traveling at approximately 6,880 miles per hour around the equator following the earths rotation. This allows these satellites to maintain their relative position over the earth's surface. Since the satellite follows the earth, and takes 24 hours to complete it's orbit around the earth, geostationary orbits are also called geosynchronous.

http://www.answers.com/topic/low-earth-orbit

http://www.linktionary.com/l/leo.html

http://en.wikipedia.org/wiki/Low_Earth_orbit

CHAPTER 25

THIRD- GENERATION (3G) WIRELESS SYSTEM

The 3G (UMTS and CDMA2000) research and development projects started in 1992. In 1999, ITU approved five radio interfaces for IMT-2000 as a part of the ITU-R M.1457 Recommendation; WiMAX was added in 2007.

There are evolutionary standards (EDGE and CDMA) that are backwards-compatible extensions to pre-existing 2G networks as well as revolutionary standards that require all-new network hardware and frequency allocations. The cell phones used utilise UMTS in combination with 2G GSM standards and bandwidths, but do not support EDGE. The latter group is the UMTS family, which consists of standards developed for IMT-2000, as well as the independently developed standards DECT and WiMAX, which were included because they fit the IMT-2000 definition.

3G systems will provide access, by means of one or more radio links, to a wide range of telecommunication services supported by the fixed telecommunication networks and to other services that are specific to mobile users. A range of mobile terminal types will be encompassed, linking to terrestrial and/or satellite-based networks, and the terminals may be designed for mobile or fixed use.

3G is the next generation of wireless network technology that provides high speed bandwidth (high data transfer rates) to handheld devices. The high data transfer rates will allow 3G networks to offer multimedia services combining voice and data. Specifically, 3G wireless networks support the following maximum data transfer rates:

1. 2.05 Mbits/second to stationary devices.

2. 384 Kbits/second for slowly moving devices, such as a handset carried by a

walking user.

3. 128 Kbits/second for fast moving devices, such as handsets in moving vehicles.

3G networks offer users advantages such as:

1. New radio spectrum to relieve overcrowding in existing systems.

2.More bandwidth, security, and reliability.

3. Interoperability between service providers.

4. Fixed and variable data rates.

5. Asymmetric data rates.

6. Backward compatibility of devices with existing networks.

7. Always-online devices.  3G will use IP connectivity, IP is packet based (not

circuit based).

8. Rich multimedia services.

UMTS offers teleservices (like speech or SMS) and bearer services, which provide the capability for information transfer between access points. It is possible to negotiate and renegotiate the characteristics of a bearer service at session or connection establishment and during ongoing session or connection. Both connection oriented and connectionless services are offered for Point-to-Point and Point-to-Multipoint communication.

Bearer services have different QoS parameters for maximum transfer delay, delay variation and bit error rate. Offered data rate targets are:

1.144 kbits/s satellite and rural outdoor

2.384 kbits/s urban outdoor

3.2048 kbits/s indoor and low range outdoor

UMTS network services have different QoS classes for four types of traffic:

> Conversational class (voice, video telephony, video gaming)

> Streaming class (multimedia, video on demand, webcast)

> Interactive class (web browsing, network gaming, database access)

> Background class (email, SMS, downloading)

UMTS will also have a Virtual Home Environment (VHE). It is a concept for personal service environment portability across network boundaries and between terminals. Personal service environment means that users are consistently presented with the same personalised features, User Interface customisation and services in whatever network or terminal, wherever the user may be located. UMTS also has improved network security and location based services.

http://www.umtsworld.com/technology/overview.htm

http://transition.fcc.gov/3G/

http://www.silicon-press.com/briefs/brief.3g/brief.pdf

CHAPTER 24

 GENERAL PACKET RADIO SERVICE (GPRS)

GPRS is a best-effort service, implying variable throughput and latency that depend on the number of other users sharing the service concurrently, as opposed to circuit switching, where a certain quality of service (QoS) is guaranteed during the connection. In 2G systems, GPRS provides data rates of 56–114 kbit/second.[3] 2G cellular technology combined with GPRS is sometimes described as 2.5G, that is, a technology between the second (2G) and third (3G) generations of mobile telephony.[4] It provides moderate-speed data transfer, by using unused time division multiple access (TDMA) channels in, for example, the GSM system. GPRS is integrated into GSM Release 97 and newer releases.

GPRS usage is typically charged based on volume of data. This contrasts with circuit switching data, which is typically billed per minute of connection time, regardless of whether or not the user transfers data during that period.

GPRS data is typically supplied either as part of a bundle (e.g., 5 GB per month for a fixed fee) or on a pay-as-you-use basis. Usage above the bundle cap is either charged per megabyte or disallowed. The pay-as-you-use charging is typically per megabyte of traffic.

General Packet Radio Services (GPRS) is a packet-based wireless communication service that promises data rates from 56 up to 114 Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computers. GPRS is based on Global System for Mobile (GSM) communication and complements existing services such circuit-switched cellular phone connections and the Short Message Service (SMS).

Protocols supported

GPRS supports the following protocols:

1. Internet protocol (IP). In practice, built-in mobile browsers use IPv4 since IPv6 was not yet popular.

2. Point-to-point protocol (PPP). In this mode PPP is often not supported by the mobile phone operator but if the mobile is used as a modem to the connected computer, PPP is used to tunnel IP to the phone. This allows an IP address to be assigned dynamically to the mobile equipment.

3. X.25 connections. This is typically used for applications like wireless payment terminals, although it has been removed from the standard. X.25 can still be supported over PPP, or even over IP, but doing this requires either a network based router to perform encapsulation or intelligence built in to the end-device/terminal; e.g., user equipment (UE).

When TCP/IP is used, each phone can have one or more IP addresses allocated. GPRS will store and forward the IP packets to the phone even during handover. The TCP handles any packet loss (e.g. due to a radio noise induced pause).

Hardware

Devices supporting GPRS are divided into three classes:

Class A

Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. Such devices are known to be available today.

Class B

Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devices are Class B.

Class C

Are connected to either GPRS service or GSM service (voice, SMS). Must be switched manually between one or the other service.

A true Class A device may be required to transmit on two different frequencies at the same time, and thus will need two radios. To get around this expensive requirement, a GPRS mobile may implement the dual transfer mode (DTM) feature. A DTM-capable mobile may use simultaneous voice and packet data, with the network coordinating to ensure that it is not required to transmit on two different frequencies at the same time.

http://en.wikipedia.org/wiki/General_Packet_Radio_Service

http://searchmobilecomputing.techtarget.com/definition/GPRS