Low Earth Orbit

LEO (Low Earth Orbit) satellite communication systems is a recent development of systems for mobile satellite communications that now exist, such as Inmarsat, AMSC. Mobile Satellite systems (satellites for mobile communications) are in operation today use satellite traveling 36,000 kilometers on the surface of the Earth and have STP 24-hour time. With a career that coincides with the equatorial zone, from a point on the Earth, the satellite appears as if the movement Geostationary Earth Orbit (GEO).

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

The concept of the LEO arrangement is shown. In this particular case, the satellites

are traversing the earth's surface at a height of 400+ nautical miles above the earth, in a polar orbit.

In the polar orbit, the satellite moves around the earth's poles and passes over any specific point

along its path very quickly. The satellites move at approximately 7,400 meters per second in

different orbits. Therefore, as one target site moves out of view, a new one comes into view at

approximately the same time. A handoff will take place between the individual satellites (using the

Ka band).

A variety of different types of satellite use the LEO orbit levels. These include different types and applications including:

-Communications satellites - some communications satellites including the Iridium phone system use LEO.

-Earth monitoring satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth.

-The International Space Station is in an LEO that varies between 320 km (199 miles) and 400 km (249 miles) above the Earth's surface. It can often be seen from the Earth's surface with the naked eye.

Space debris in LEO

Apart from the general congestion experienced in Low Earth Orbit, the situation is made much worse by the general level of space debris that exists.

There is a real and growing risk of collision and major damage - any collisions themselves are likely to create further space debris.

The US Joint Space Operations Center currently tracks over 8 500 objects that have dimensions larger than 10 centimetres. However debris with smaller dimensions can also cause significant damage and could render a satellite unserviceable after a collision.

Third Generation Wireless System

EDGE introduced a new modulation scheme for high speed data rate which is the 8-Phase Shift Keying (PSK). 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. The channel separations are 45 MHz, and the carrier spacing is a 200 kHz channel capacity, the same as GSM and GPRS. The number of TDMA slots on each carrier is the same (eight) as the GSM and GPRS architecture.

Preparing for the

revolution, existing Time Division Multiple Access (TDMA) operators must evolve their networks to

take advantage of Mobile Multimedia applications and the eventual shift to an all−IP architecture.

One way to do that is through the evolution of General Packet Radio Services (GPRS). However,

soon after we see the installation of GPRS, some operators will begin the next step in the evolution

process to Enhanced Data for Global Environment (EDGE). With EDGE, existing TDMA networks

can host a variety of new applications, including

· 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 operates at much higher speeds than current 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.

Although current wireless applications are text oriented, GPRS' high throughput finally makes

multimedia content, including graphics, voice, and video, practical. Imagine participating in a

videoconference while waiting for your flight at the airport, something that is completely out of the

question with today's data networks.

General Packet Radio Service

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.

General Packet Radio Services may also be called 2.5G technology because it is an enchancement of GSM (Global System for Mobile Communication) or TDMA (IS-136) network. It exist in our cellular network infrastructure and enhances the technology by software upgrades at the base station to create a GPRS gateway that connects to the internet.

Because GPRS is a packet switched network, a GPRS user station doesn't occupy a dedicated path during an Internet connection. However, each end user station (e.g. mobile phone) is allocated several time slots out of 8 GSM/TDMA available time-slots for GPRS service. Each time slot has a maximum capacity of 14.4 kbps. Depending on how many time slots are allocated for the downlink (from a base station to a user station) and the uplink (from a user station to a base station), GPRS devices are divided into multi-slot classes. A multi-slot class is often represented by the number of downlink and uplink slots. For example, Class 10 is also known as Class 4+2. While active slots indicate the maximum number of slots that can be allocated for both downlink and uplink in a specific class. The following table lists available multi-slot classes.

GPRS is a key milestone for GSM data. It offers end users new data services and enables

operators to offer radically new pricing options. Using the existing GSM radio infrastructure, up−front

investments for operators are relatively low. GPRS solutions began appearing initially in 1999

through 2000 using the infrastructures that are already in place. Pricing for use of the voice side of

the network has become commoditized, whereas pricing models for the new data access will cause

a revolution. One such threshold looks at an all−you−can−eat model whereby users of wireless

phones add a data subscription at $29.95 per month for unlimited use. Another such model is the

one used in Japan by DoCoMo, charging a rate of the U.S. equivalent to $.0025 per packet. Others

will emerge that will shake the industry mode and create new dynamics in the use of data

anywhere.

GPRS services were targeted at the business user. However, the services will soon be available

networkwide, targeting both the business and the residential consumer. The widespread adoption

and acceptance of GPRS will create a critical mass of users, driving down costs while offering better

services. These components will form the basis of a healthy mobile data market with growth figures

comparable to GSM voice−only services today. Research by Infonetics indicates that the movement

of the user community will also be to a more mobile community. In fact, the study indicates that by

2005, more wireless devices will be used for the Internet than PCs on the Net, as shown in Figure

24−3. This form of growth is again a driver that will force the rapid deployment by carriers and

manufacturers alike.

MMDS and LMDS

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.

Advantage of MMDS

• It has chunks of under-utilized spectrum that will, once completely digital, become increasingly valuable and flexible.
• System Implementation, which is little more than putting an installed transmitter on a high tower and a small receiving antenna on the customer’s balcony or roof, is quick and inexpensive.
• Moreover, since MMDS services have been around for 20 years, there is a wealth of experience--at least in respect to the one-way distribution technology.

Disadvantage of MMDS:

• Large upstream bandwidth in MMDS band requires careful planning, filtering etc.
• Limited capacity without sectorization, cellularization which adds complexity and cost

MMDS is a broadcasting and communications service that operates in the ultra−high−frequency

(UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless

cable. It was conceived as a substitute for conventional cable television (TV). However, it also has

applications in telephone/fax and data communications.

In MMDS, a medium−power transmitter is located with an omni−directional broadcast antenna at or

near the highest topographical point in the intended coverage area. The workable radius can reach

up to 70 miles in flat terrain (significantly less in hilly or mountainous areas). There is a monthly fee,

similar to that for satellite TV service.

MMDS frequencies provide precise, clear, and wide−ranging signal coverage. Customers are

protected from interference from other users when the provider uses the licensed frequencies. Rain,

snow, and fog do not interfere with signal performance as we saw in the microwave radio chapter

(see Chapter 17, "Microwave− and Radio−Based Systems"). Many of the carriers use a super−cell

concept with a service area spanning a 35−mile radius from each of its MMDS transmitters.

Whenever the concept of the competitive environment enters a discussion, two other discussions

ensue: the WLL and the use of LMDS. This chapter will look at some of the movement in this area

to understand how and why the last mile has become so critical in meeting the demands for

higher−speed broadband communications. Moreover, when looking at the incumbent local

exchange carriers' (ILEC) copper−based plant, one can only marvel at the lack of foresight in

fending off the competition.