Low-Earth Orbit satellites

Chapter 27: Low-Earth Orbits satellites

                                                                                     

Low Earth Orbit

In December 1990, Motorola filed an application with the FCC for the purposes of constructing,

launching, and operating a LEO global mobile satellite system known as Iridium. This was the hot

button that sparked the world into a frenzy. Iridium was a concept of launching a series of 66

satellites

[1] Originally the Iridium proposal was for 77 satellites, but Motorola amended this number after the

World Administrative Radio Council meeting in the spring of 1992. around the world to provide

global coverage for a mobile communications service operating in the 1.610 to 1.6265 GHz

frequency bands. The concept was to use a portable or mobile transceiver with low profile antennas

to reach a constellation of 66 satellites. Each of the satellites would be interconnected to one

another through a radio communications system as they traversed the globe at 413 nautical miles

above the earth in multiple polar orbits

[2] The original concept was to use 7 polar orbits with 11 satellites in each. This would provide

worldwide coverage, much similar to an orange slice concept . This would provide a continuous

line−of−sight coverage area from any point on the globe to virtually any other point on the globe,

using a spot beam from the radio communications services on−board each of the satellites. The use

of this spot beam concept, which had been discussed for years in the satellite industry, allowed for

high frequency reuse capacities that had not been achieved before. Iridium wanted to provide the

services outlined in Table 27−2. Motorola also suggested that an interconnection arrangement

would be set up with all providers around the world through an arrangement with the local Post

Telephone and Telegraph (PTT) organizations. The concept was sound, and the approach would

have provided for the coverage that was lacking in the past to remote areas. In the table, the two

columns are used as exclusive of each other. The services can be provided in any of the coverage

areas regardless of which service is selected.

A communications satellite in orbit 400 to 1600 miles above the earth. Being much closer than 22,282 mile-high geosynchronous satellites (GEOs), LEO signals make the round trip from earth much faster. Thus, low-powered "pizza dishes" and handheld devices can be used. LEOs are also better suited to interactive conferencing. Unlike GEOs, which travel at earth speed, LEOs revolve around the globe every couple of hours, and any single LEO is in view for only a few minutes. In order to maintain continuous communications, multiple LEOs must be used. From 48 to 66 LEOs are needed to cover the earth.

Advantages to LEO Satellite Telemetry

• Global applications in developing a remote environmental monitoring communication system.
• Two-way communications
• Easy to set up and low maintenance costs
• Low profile, non-directional whip helix antenna
• Easy access to data
• No FCC or other governmental agency requirements for data transmission
• Coverage in very remote areas
• Data is proprietary
• Stevens is able to quickly detect any problems with transmission of data
• Systems verifies that data has been transmitted which minimizes risk of missing data.
• Event notification on line, by pager, etc.
• Lower power transceivers compared to GEO transmitters.
• LEO Transceivers hardware system is lower in cost than a GEO transmitter system.

Disadvantages to LEO Satellite Telemetry

• Monthly service fee which could be expensive with frequent transmissions of data.
• Power outage at GCC would shut down the communication server, which could delay transmission of data to end user until power is restores. However, no data is lost.
• LEO satellites have a much shorter life span (five to eight years) than GEO satellites.

Third-Generation(3G) Wireless system

Chapter 25: Thrird-Generation(3G) Wireless system

First Generation Phones
The very first mobile phones are now referred to as first generation (1G) mobile phones. The first handheld phone to be released on the us market was the Motorola DynaTAC 8000X in 1983. Mobile phones started to gain popularity in the eighties with the introduction of cellphones that were based on cellular networks. 
Mobile phones were originally much larger than current ones, & they were initially designed for installation in vehicles only, which is why they were called the car phone. These chunky units were later converted for use as transportable phones the size of a small suitcase & the rest is history.

Second Generation Phones

Second generation, also called 2-G, mobile telephones were introduced in the nineties. 2-G telephone systems were different because of their use of digital circuit switched transmission & the introduction of advanced & quick telephone to network signals. The introduction of 2-G systems saw telephones move from historic 1G telephones to small hand held items, which were much more portable. This change was made possible through improvements in technology such as more advanced batteries &Second Generation Phones

Second generation, also called 2-G, mobile telephones were introduced in the nineties. 2-G telephone systems were different because of their use of digital circuit switched transmission & the introduction of advanced & quick telephone to network signals. The introduction of 2-G systems saw telephones move from historic 1G telephones to small hand held items, which were much more portable. This change was made possible through improvements in technology such as more advanced batteries & energy efficient electronics. 

The second generation mobile telephones had several advantages over 1G items. These included SMS messaging, which initially became possible on GSM networks & eventually on all digital networks. The very first machine-sent message was sent in Great Britain in 1991. The very first person-to-person SMS text message was sent in 1993 in Finland. SMS text messaging soon became the communication method of choice & the general public now prefer sending messages to placing voice calls.

The first pre-commercial 3-G mobile phone network was launched in Japan in May of 2001. 3G was later commercially released in Japan on October, 1 2001.

3-G is the third generation of mobile phone standards & technology, which succeeded 2-G mobile technology. The birth of 3-G technologies enabled network operators including orange to give their users a wider range of advanced services. This includes broadband internet, in addition to high-tech video calls.

Despite the success of 3-G, there has also been many complaints about it. These include criticism over the cost of 3G phones, a lack of network coverage because it's still a new service & the high power usage of 3G phones.

Ever since the launch of 3G mobile telephone technology, people have been discussing 4-G. 4-G technology will signify the future of mobile telephones, producing the most advanced handsets & best services to date. In actual fact, one of the next services to be developed is thought to be the live streaming of radio and television shows to 3G handsets is & businesses including Disney & Real recently announced that they'll be offering services like these. electronics. 

The second generation mobile telephones had several advantages over 1G items. These included SMS messaging, which initially became possible on GSM networks & eventually on all digital networks. The very first machine-sent message was sent in Great Britain in 1991. The very first person-to-person SMS text message was sent in 1993 in Finland. SMS text messaging soon became the communication method of choice & the general public now prefer sending messages to placing voice calls.


The first pre-commercial 3-G mobile phone network was launched in Japan in May of 2001. 3G was later commercially released in Japan on October, 1 2001.

3-G is the third generation of mobile phone standards & technology, which succeeded 2-G mobile technology. The birth of 3-G technologies enabled network operators including orange to give their users a wider range of advanced services. This includes broadband internet, in addition to high-tech video calls.

Despite the success of 3-G, there has also been many complaints about it. These include criticism over the cost of 3G phones, a lack of network coverage because it's still a new service & the high power usage of 3G phones.

Ever since the launch of 3G mobile telephone technology, people have been discussing 4-G. 4-G technology will signify the future of mobile telephones, producing the most advanced handsets & best services to date. In actual fact, one of the next services to be developed is thought to be the live streaming of radio and TV shows to 3G handsets is & businesses including Disney & Real recently announced that they'll be offering services like these.

What is 3G?



3G refers to the third generation of mobile telephony (that is, cellular) technology. The third generation, as the name suggests, follows two earlier generations.

The first generation (1G) began in the early 80's with commercial deployment of Advanced Mobile Phone Service (AMPS) cellular networks. Early AMPS networks used Frequency Division Multiplexing Access (FDMA) to carry analog voice over channels in the 800 MHz frequency band.

The second generation (2G) emerged in the 90's when mobile operators deployed two competing digital voice standards. In North America, some operators adopted IS-95, which used Code Division Multiple Access (CDMA) to multiplex up to 64 calls per channel in the 800 MHz band. Across the world, many operators adopted the Global System for Mobile communication (GSM) standard, which used Time Division Multiple Access (TDMA) to multiplex up to 8 calls per channel in the 900 and 1800 MHz bands.

The International Telecommunications Union (ITU) defined the third generation (3G) of mobile telephony standards IMT-2000 to facilitate growth, increase bandwidth, and support more diverse applications. For example, GSM could deliver not only voice, but also circuit-switched data at speeds up to 14.4 Kbps. But to support mobile multimedia applications,

3G had to deliver packet-switched data with better spectral efficiency, at far greater speeds.

However, to get from 2G to 3G, mobile operators had make "evolutionary" upgrades to existing networks while simultaneously planning their "revolutionary" new mobile broadband networks. This lead to the establishment of two distinct 3G families: 3GPP and 3GPP2.

The 3rd Generation Partnership Project (3GPP) was formed in 1998 to foster deployment of 3G networks that descended from GSM. 3GPP technologies evolved as follows.

• General Packet Radio Service (GPRS) offered speeds up to 114 Kbps.

• Enhanced Data Rates for Global Evolution (EDGE) reached up to 384 Kbps.

• UMTS Wideband CDMA (WCDMA) offered downlink speeds up to 1.92 Mbps.

• High Speed Downlink Packet Access (HSDPA) boosted the downlink to 14Mbps.

• LTE Evolved UMTS Terrestrial Radio Access (E-UTRA) is aiming for 100 Mbps.

GPRS deployments began in 2000, followed by EDGE in 2003. While these technologies are defined by IMT-2000, they are sometimes called "2.5G" because they did not offer multi-megabit data rates. EDGE has now been superceded by HSDPA (and its uplink partner HSUPA). According to the 3GPP, there were 166 HSDPA networks in 75 countries at the end of 2007. The next step for GSM operators: LTE E-UTRA, based on specifications completed in late 2008.

A second organization, the 3rd Generation Partnership Project 2 (3GPP2) -- was formed to help North American and Asian operators using CDMA2000 transition to 3G. 3GPP2 technologies evolved as follows.

• One Times Radio Transmission Technology (1xRTT) offered speeds up to 144 Kbps.

• Evolution Data Optimized (EV-DO) increased downlink speeds up to 2.4 Mbps.

• EV-DO Rev. A boosted downlink peak speed to 3.1 Mbps and reduced latency.

• EV-DO Rev. B can use 2 to 15 channels, with each downlink peaking at 4.9 Mbps.

• Ultra Mobile Broadband (UMB) was slated to reach 288 Mbps on the downlink.

1xRTT became available in 2002, followed by commercial EV-DO Rev. 0 in 2004. Here again, 1xRTT is referred to as "2.5G" because it served as a transitional step to EV-DO. EV-DO standards were extended twice – Revision A services emerged in 2006 and are now being succeeded by products that use Revision B to increase data rates by transmitting over multiple channels. The 3GPP2's next-generation technology, UMB, may not catch on, as many CDMA operators are now planning to evolve to LTE instead.

In fact, LTE and UMB are often called 4G (fourth generation) technologies because they increase downlink speeds an order of magnitude. This label is a bit premature because what constitutes "4G" has not yet been standardized. The ITU is currently considering candidate technologies for inclusion in the 4G IMT-Advanced standard, including LTE, UMB, and WiMAXII. Goals for 4G include data rates of least 100 Mbps, use of OFDMA transmission, and packet-switched delivery of IP-based voice, data, and streaming multimedia.

General Packet Radio System (GPRS)

Chapter 24: General Packet Radio System (GPRS)



Introduction

Wireless communications lets people live and work in ways never before possible. With over two hundred million cellular subscribers worldwide, users have overwhelmingly embraced the concept of having a telephone that is always with them. And now business users also want a data connection with the office wherever they go, so that they can have access to e-mail, the Internet, their files, faxes and other data wherever and whenever it is needed, giving them a competitive advantage and more flexible lifestyles. A number of wireless data services are available today, but none are as exciting as a forthcoming data service for GSM networks called General Packet Radio Service (GPRS).

GPRS refers to a high-speed packet data technology, which is expected to be deployed in the next two years. It is expected to profoundly alter and improve the end-user experience of mobile data computing, by making it possible and cost-effective to remain constantly connected, as well as to send and receive data at much higher speeds than today. Its main innovations are that it is packet based, that it will increase data transmission speeds from the current 9.6 Kbps to over 100 Kbps, and that it will extend the Internet connection all the way to the mobile PC -- the user will no longer need to dial up a separate ISP. GPRS will complement rather than replace the current data services available through today’s GSM digital cellular networks, such as circuit-switched data and Short Message Service. It will also provide the type of data capabilities planned for "third generation" cellular networks, but years ahead of them.

Why is GPRS important?

The most important aspects of GPRS are that it allows data transmission speeds to over 100 Kbps, that it is packet based, and that it supports the world's leading Internet communications protocols, Internet Protocol (IP) and X. 25. 

The fact that GPRS will operate at much higher speeds than current networks should provide a huge advantage from a software perspective. Today, wireless middleware is often required to allow 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 often be unnecessary, and thus it should be easier to deploy wireless solutions than ever before.
Whereas today’s wireless applications tend to be text oriented, the high throughput offered by GPRS will finally make multimedia content, including graphics, voice and video practical. Imagine participating in a video conference while waiting for your flight at the airport, something completely out of the question with today’s data networks.
Why is packet data technology important? Because packet provides a seamless and immediate connection from a mobile PC to the Internet or corporate intranet allowing all existing Internet applications such as e-mail and Web browsing to operate smoothly without even needing to dial into an Internet service provider. The advantage of a packet-based approach is that GPRS only uses the medium, in this case the precious radio link, for the duration of time that data is being sent or received. This means that multiple users can share the same radio channel very efficiently. In contrast, with current circuit-switched connections, users have dedicated connections during their entire call, whether or not they are sending data. Many applications have idle periods during a session. With packet data, users will only pay for the amount of data they actually communicate, and not the idle time. In fact, with GPRS, users could be "virtually" connected for hours at a time and only incur modest connect charges. For detailed information about how GPRS works, see "For Network Managers" below.
While packet-based communications works well with all types of communications applications, it is especially well suited for frequent transmission of small amounts of data, what some call short and bursty, such as "real time" e-mail and dispatch. But packet is equally well suited for large batch operations, and other applications involving large file transfers.
GPRS will support the widely used Internet Protocol (IP) as well as the X.25 protocol. IP support is becoming increasingly important as companies are now looking to the Internet as a way for their remote workers to access corporate intranets. For further discussion about remote access, see "GPRS and Remote Access" below.
The IP protocol is ubiquitous and familiar, but what is X.25, and why is support for it important? X.25 defines a set of communications protocols that prior to the Internet constituted the basis of the world’s largest packet data networks. These X.25 networks are still widely used, especially in Europe, and so wireless access to these networks will benefit many organizations. But what does this really mean? Quite simply it means that any existing IP or X.25 application will now be able to operate over a GSM cellular connection. You can think of cellular networks with GPRS service as wireless extensions of the Internet and existing X.25 networks, as shown in figure one.