Low Earth Orbit Satellite (LEOs)

CHAPTER 27 - LEOs (Low Earth Orbit Satellite)

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 requirements 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

• Low Earth Orbit (LEO) refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles.
• Low Earth Orbit satellites must travel very quickly to resist the pull of gravity — approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.
• Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
• Low Earth Orbit satellites typically operate in polar orbits.
• Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS).
• Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

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.[1][2] The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.

The delta-v needed to achieve low earth orbit starts around 9.4km/s. With the exception of the lunar flights of the Apollo program, all human spaceflights have either been orbital in LEO or sub-orbital. The altitude record for a human spaceflight in LEO was Gemini 11 with an apogee of 1,374.1 km.

Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit height. LEO is an orbit around Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km because that would be impractical due to the larger atmospheric drag.

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement of any orbit. Orbits with a high inclination angle are usually called polar orbits.

Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation.

The International Space Station is in a LEO that varies from 320 km (199 mi) to 400 km (249 mi) above the Earth's surface.

While a majority of artificial satellites are placed in LEO, where they travel at about 7.8 km/s (28,080 km/h), making one complete revolution around the Earth in about 90 minutes, many communication satellites require geostationary orbits, and move at the same angular velocity as the Earth. Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. ENVISAT is one example of an Earth observation satellite that makes use of this particular type of LEO.

Although the Earth's pull due to gravity in LEO is not much less than on the surface of the Earth, people and objects in orbit experience weightlessness due to the effects of freefall.

Atmospheric and gravity drag associated with launch typically adds 1.5–2.0 km/s to the delta-v launch vehicle required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h).

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

              http://www.ssec.wisc.edu/sose/pirs/pirs_m1_leo.html

Chapter 25: Third Generation Wireless System

3RD GENERATION WIRELESS SYSTEM

Wireless continues to develop around the world. Several different standards committees are working on integrating wireless architecture into the overall fold of the network. 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 WiMAX II. 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.

SOURCES: http://searchtelecom.techtarget.com/definition/3G

                    http://home.wlv.ac.uk/~a9814003/3g_wireless_system.htm

Chapter 24: GPRS

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)

In theory, GPRS packet-based services cost users less than circuit-switched services since communication channels are being used on a shared-use, as-packets-are-needed basis rather than dedicated to only one user at a time. It is also easier to make applications available to mobile users because the faster data rate means that middleware currently needed to adapt applications to the slower speed of wireless systems are no longer be needed. As GPRS has become more widely available, along with other 2.5G and 3G services, mobile users of virtual private networks (VPNs) have been able to access the private network continuously over wireless rather than through a rooted dial-up connection.
GPRS also complements Bluetooth, a standard for replacing wired connections between devices with wireless radio connections. In addition to the Internet Protocol (IP), GPRS supports X.25, a packet-based protocol that is used mainly in Europe. GPRS is an evolutionary step toward Enhanced Data GSM Environment (EDGE) and Universal Mobile Telephone Service (UMTS).


Advantages of GPRS

GPRS brought mobile phone users out from the world of WAP, and into a world where Internet was finally available on mobiles. This in itself was a monumental feat, and hence GPRS took off with quite a bang. With GPRS, large amounts of data can be transferred to and from the mobile device over the Internet

Drawbacks

Since GPRS uses the cellular network’s GSM band to transmit data, more often than not, when a connection is active, calls and other network-related functions cannot be used. The data session will go on standby. This is a characteristic typical of the Class B GPRS device. There are Class A devices as well, where there are two radios incorporated into the device, allowing both features to run simultaneously. However, Class A devices tend to be more expensive, and by extension, less popular. Most mobile phones fall in the Class B category.
GPRS is usually billed per megabyte or kilobyte, depending on the individual service provider. However, this has changed in many places, where GPRS downloads are no longer charged as per usage, but are unlimited, and there is merely a flat fee to be paid every month.


EDGE

Enhanced GPRS goes by many monikers, but is essentially the next generation GPRS. It employs much the same technology from the user’s end; however it requires some basic modification at the transmitter’s base stations. Theoretically, EDGE can transmit and receive data three times as fast as a normal GPRS connection, subject of course to ideal conditions.
EDGE and GPRS are still used in lesser developed countries across the globe, mainly because hotspots are not as prolific as other more developed nations. Additionally, 3G technology has not spread across the globe, making GPRS a very viable option as of now

Sources: http://www.brighthub.com/mobile/symbian-platform/articles/16995.aspx
              http://ntrg.cs.tcd.ie/undergrad/4ba2.01/group1/gprs.htm

Microwaves (the actual radio waves) are between 1 mm and 30 cm long, and operate in a frequency range from 300 MHz to 300 GHz. Microwaves were first used in the 1930s, when British scientists discovered the application in a new technology called radar.

 In the 1950s, microwave radio was used extensively for long−distance telephone transmission. With the need to communicate over thousands of miles, the cost of stringing wires across the country was prohibitive. However, the equipment was both heavy and expensive. The radio equipment used vacuum tubes that were bulky as well as highly sensitive to heat. All of that changed dramatically when integrated circuits and transistors were used in the equipment. Now the equipment is not only lightweight, but also far more economical and easy to operate. In 1950, the typical microwave radio used 2,100 watts to generate three groups of radio channels (each group consists of 12 channels), yielding 36−voice−grade−channel capacity.

 Each voice grade channel operated at the standard 4 kHz. Today, equipment from many manufacturers (and Harris/Farinon, specifically) requires only 22 watts of output to generate 2,016 voice channels. Although there have been two orders of magnitude improvements in the quality of the voice transmission, the per−channel cost has plummeted from just over $1,000 to just under $37. This makes the transmission systems very attractive from a carrier's perspective. However, the use of private microwave radio has also blossomed over the years because of the cost and performance improvements.

What About Bandwidth?

Bandwidth is always a touchy subject. It can become a "never satisfied drain" on the corporate bottom line if due diligence is not practiced. There is a direct relationship to cost and total bandwidth. The more bandwidth needed, the greater the cost. Everyone would like as much bandwidth as possible, and at the same time wants it to be affordable. Many people make the mistake of buying more than they need, anticipating future growth. In this industry, prices keep falling as competition increases. If an organization needs an OC−3 (155 Mbps) today, then laying fiber is probably the most affordable solution. However, 155 Mbps microwave systems are available and the prices are constantly dropping, giving short−haul fiber a run for the money.
Conversely, if 10 Mbps Ethernet is the current rate of transmission, then this demand can be
immediately met. Additional bandwidth can be bought later. In two to three years, the costs will
plummet so that the new requirements can be met with incremental or marginal costs.
It's wiser to buy bandwidth as you need it and not before (there will be a small amount of
incremental add−on, but limited). In the future, there will be the following:

• More choices
• Increased providers
• Greater availability
• Lower costs




What should be done in the interim to satisfy the need? The answer is the following:

• Lease (dark) fiber instead of paying the cost of installation

• Lease services from the Incumbent Local Exchange Carrier (ILEC) or CLEC if sufficient
bandwidth is available
• Buy a wireless connection such as point−to−point microwave

Having too much bandwidth is possible. Having too much reliability is just the opposite.
Organizations lose significant amounts of money when the network connection is too slow, but far more when the link is down completely. One hour of network downtime can cost more than the profits and productivity achieved from a year of uptime. In this scenario, automatic backup is an absolute must. Buy the appropriate amount of bandwidth and make sure that the reliability is built in. Plan for the worst−case scenario! Consider an alternate backup plan. Use circuit−switched or packet−switched (frame−switched) alternative connections in case of an outage.

Prior to the 1970s, microwave was the most widely used wireless communications medium in the world. Microwave usage is making a comeback now with end users. Many user organizations were reluctant to experiment with microwave radio transmission due to misconceptions surrounding the technology as well as confusion between the "wireless" products. It is important to recognize that
the difference between one wireless device and another can be as different as fiber and copper wire. Both fiber and copper are "wired," but that is where the commonality ceases. The same is true between microwave and laser, spread spectrum, or cellular service. There are even differences between one type of microwave and another. The differences are due primarily to their respective operating frequencies. Some frequencies are good for distances of 30 or 40 miles and others can barely get you across an office park. Some can only support a couple of T1s or a single video channel and others go to 10 to 45 Mb.

MMDS and LMDS

The local multipoint distribution service (LMDS) and multichannel multipoint distribution service (MMDS) have their historical roots in television. MMDS's pre-cursor, the multipoint distribution service (MDS), was established by the Federal Communications Commission (FCC) in 1972. The Commission originally thought MDS would be used primarily to transmit business data. However, the service became increasingly popular for transmitting entertainment programming. Unlike conventional broadcast stations, whose transmissions are received universally, MDS programming is designed to reach only a subscriber-based audience.

LMDS is a fixed broadband line-of-sight, point-to-multipoint, microwave system, which operates at a high frequency (typically within specified bands in the 24-40GHz range) and can deliver at a very high capacity, depending on the associated technologies. Given the complexity of the equipment required (and the power needed to deliver signals) both of these technologies are regarded as prohibitively expensive for the consumer market. Therefore, LMDS operators will initially be targeting enterprises and network operators, although the consumer market is likely to emerge over time as the cost of CPE comes down (partly driven by the take-up of IP). It should be noted that CPE costs $5,000 for LMDS in the 26GHz range.

MMDS allows two-way voice, data and video streaming. It operates at a lower frequency than LMDS (typically within specified bands in the 2-10GHz range) and therefore has a greater range and requires a less powerful signal than LMDS. MMDS is a less complicated, cheaper system to implement. As a consequence, the CPE is cheaper, thus it has a wider potential addressable market. It is also less vulnerable to rain fade - the interference caused by adverse weather conditions that can undermine the quality of the microwave signal. However, the bandwidth offered by LMDS makes this the more viable option.

BENEFITS

Wireless systems are being deployed to fulfil a number of functions. On a network level they are suitable for both access and backbone infrastructure. It is generally agreed, however, that it is in the access market where the key advantages are held over wireline alternatives. The principal strengths of LMDS/MMDS are:

• Speed of network deployment is much quicker with wireless systems enabling rapid, early market entry
• Entry, deployment and upgrading costs are much lower than for wireline alternatives, for which engineering (cabling and trenching) costs are significantly higher
• The maintenance, management and operation expenditure is lower. Wireless systems can be rolled out much faster, enabling an earlier return on investment
• Scalable architectures enable expanded coverage and services in direct relation to the level of demand
• Only one network architecture is required to provide a full suite of interactive voice, video and data services that can be expanded as and when desired

SOURCE: http://www.mobilecomms-technology.com/projects/mmds/

xDSL or Digital Subscriber Lines

The term DSL is an acronym for Digital Subscriber Lines and this term is the name of the family that provide technology for digital data transmission through the network over the wires of the local telephone network, which are usually made up of copper wires. The best example of this technology is broadband, where the internet ability is provided to the user over these telephone lines.
The two main categories of this xDSL are ADSL or Asymmetric Digital Subscriber Line and SDSL or Symmetric digital subscriber line. Apart from these two major categories, there are two other types of categories present in the market as well, named HDSL or High Data Rate DSL and VDSL or Very High DSL. These two types of the DSL are quite expensive and are installed only in the large sized enterprise, as they installation and the operational costs are both high. The most common type of the DSL that is used by many people throughout the world is ADSL, as it is cheaper and the operational costs are also low.
This DSL technology basically pack the data on to the copper wire by using a sophisticated modulation technique and schemes and this technology is only used for connection from the telephone lines to homes and offices but not for providing connections between the switching stations. This xDSL technology is quite similar to the ISDN technology as they both operate on the copper wires and both require the short run to the telephone office. However, the major difference between these two lies in the matter of speed.
The xDSL technology is capable of giving speed up to 32 Mbps for upstream traffic and max speed of 1 Mbps for the downstream traffic. This technology is currently undergoing revision and it is quite possible that the speed that this technology has to offer to the users might even increase drastically, after all the main thing is loading the cables with the data packets with the help of modulation schemes.

- A xDSL modem is the device found at the customer’s premise which is used to transmit & receive xDSL signals


Benefits

• High-speed data service–DSL typically >10x faster than 56-kbps analog modem
• Always on connection–No need to “dial-up”
• Uses existing copper wires–Co-exists w/ POTS service
• Reasonably priced today and getting cheaper.
•  

Applications

• High speed Internet access
• SOHO
• Multimedia, Long distance learning, gaming
• Video on Demand
• VPN
• VoDSL 

Asymmetric” => faster downstream rate vs. upstream

• Suitable for applications such as web-browsing, MP3 downloading, Video on demand (VoD)
• Types of asymmetric DSL
• Asymmetric DSL (ADSL)
• The original and most popular
• Other asymmetric DSL technologies derived from ADSL
• Universal ADSL (UDSL), a.k.a. G.Lite or DSL Lite
• Expedites and reduces cost of deployment process by moving the splitting process from the CP to the CO
• Splitter-less nature slows the bit rate considerably