Chapter 27: Low Earth Orbit Satellites (LEOs)

     Satellite systems are employed for telephone and data communications. there are geostationary satellites flying in high orbit where they can maintain the same position above the earth's surface ate 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 system. 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 on. Each satellites covers an area that could be compared to a cell in a cellular system, except that the cell moves as the satellite orbits. 

Advantages and Disadvantages of LEO and GEO

TYPE	GEO	LEO
Description	Geostationary Earth Orbit	Low Earth Orbit
Height	36,000 km	200-3000 km
Time in LOS	24 hrs	15 min.
Advantages	1. Covers 42.2% of the earth's surface 2. Constant view 3. No problems due to Doppler 24 hrs 36,000 km	1. Lower launch costs 2. Very short round trip delays 3. Small path loss
Disadvantages	1. Larger round trip delays – 250 ms latency uıp/down link 2. Poor look angle elevations at higher latitudes 3. Must be pointed to acquire a satellite	1.Short life 2.Short LOS 3.Short call interruptions depending to the position

Benefits of LEO

Ubiquitous services 

With continuous and global coverage, any−to−any connections can occur. As users travel either domestically or abroad, the service travels with them. It will eliminate the need for special access arrangements and special numbers that must be dialed. Users should never be out of range from their network. Remote areas with limited demand and finances now have the capability to connect anywhere in the world.

Spectral efficiency 

As already mentioned, the frequency reuse patterns for the bandwidth allocation will be significant. No other satellite system has achieved these reuse ratios. Iridium was first to claim this capability of efficiency. RDSS portion of the Iridium network is contained in the same spectral arrangement, freeing up 16.5 MHz of spectrum. This is a quantum leap in the efficient use of the spectrum.

Public benefits due to flexible design

The digital technology deployed allows the total connection for all voice and data services on a seven day by twenty−four hour basis. This allows the flexibility of service provisioning. The LEO overcomes some of the limitations of the higher transport systems, such as the delay in the round trip transmission. Because the satellites are low, the user set needs a lower power output device. This orbit has been selected to be the most flexible.

The potential to save lives 

It's common for the news media to publish stories of people stranded in remote areas with no life support systems who died because of their inability to communicate. The press today is filled with stories of cellular and PCS users notifying authorities of casualties. If only people in remote areas had a means of notifying authorities and/or rescue parties, their lives could be spared in the event they get into a life−threatening situation.

Capabilities of the vendor 

Motorola states that they are uniquely qualified to provide these types of services due to their background in the development and sales of other ancillary equipment that works in the wireless world. Specifically, they have been one of the major developers in the production, research, and development of private mobile−radio services.

LEO deployment promotes international communications 

The LEO networks deliver modern digital−transmission services to remote areas of the world. The FCC and the U.S. government are attempting to use telecommunications as a strategic and economic tool to foster development in these areas.

Chapter 25: 3rd Generation (3G) Wireless System

     3G or 3rd Generation Mobile Telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunication-2000 (IMT-2000) specifications by the International Telecommunication Union. Application services include wide-area wireless voice telephone , mobile Internet Access, Video Calls and Mobile TV, All in a mobile environment.

     Several Telecommunication companies market wireless mobile Internet as 3G, indicating that the advertised service is provided over a 3G wireless network. Services advertised as 3G are required to meet IMY-2000 technical standards, including standards for reliability and speed. To meet IMT-2000 standards , a system is required to provide peak data rates of at least 200 Kbit/s. However, many services advertised as 3G provide higher speed than the minimum technical requirements for a 3G service.

The following Standards are typically branded 3G:

• The UMTS system, first offered in 2001, standardized bu 3GPP, used widely and other regions by GSM 2G system infrastructure. The cell phones are typically UMTS and GSM hybrids. Several radio interfaces are offered, sharing the same infrastructure:

1. The Original and most widespread radio interface is called W-CDMA.
2. The TD-SCDMA radio interface was commercialized in 2009.
3. The latest UMTS release, HSPA+, can provide peak data rates up to 56MBit/s in the downlink and 22MBit/s in uplink.

• The CDMA2000 system, first offered in 2002, standardized by 3GPP2, sharing infrastructure with the IS-95 2G standard. The cell phones are typically CDMA2000 and IS-95 hybrids. 

     The above systems and radio interfaces are based on kindred spread spectrum radio transmission technology. while the GSM EDGE standard (2.9G), DECT cordless phones and mobile WiMAX standard formally also fulfill the IMT-2000 requirements and are approved as 3G standards by ITU.

  
Enhanced Data GSM Environment(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. If we look at this from an OSI stack model, EDGE uses FDMA and TDMA at the MAC layer (bottom half of layer 2 OSI).

Specialty of 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.

Universal Mobile Telecommunication Service(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.

Wideband Code Division Multiple Access(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.

Chapter 24: General Packet Radio Services

     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 the Global System for Mobile (GSM) communication and complements.

     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 system are no longer be needed. As GPRS has become more widely available, along with other 2.6G and 3G services, mobile users of Virtual Private Networks (VPN) 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. It is an evolutionary step toward Enhanced Data GSM Environment (EDGE) and Universal Mobile Telephone Service (UMTS).

Services Offered


GPRS extends the GSM Packet circuit switched data capabilities and makes following services possible:

• SMS messaging and broadcasting
• Always on internet access
• Multimedia Messaging Services (MMS)
• Push to talk over Cellular Phone (PoC)
• Instant Messaging and presence wireless village
• Internet Applications for smart devices through Wireless Application Protocol (WAP)
• Point-to-Point (P2P) Service   

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, but using only one or the other at a given time. During GSM service, GPRS service is suspended. and then resumed automatically after the GSM service has concluded. Most GPRS mobile devices are Class B.

Class C

   Are connected to either GPRS service or GSM service. Must be switched manually between one or the other service.

The following table summarises some possible configurations of GPRS and Circuit  switched data services.

Technology	Download (kbit/s)	Upload (Kbit/s)	TDMA Timeslots allocated (DL+UL)
CSD	9.6	9.6	1+1
HSCSD	28.8	14.4	2+1
HSCSD	43.2	14.4	3+1
GPRS	80.0	20.0 (Class 8 & 10 and CS-4)	4+1
GPRS	60.0	40.0 (Class 10 and CS-4)	3+2
EGPRS (EDGE)	236.8	59.2 (Class 8, 10 and MCS-9)	4+1
EGPRS (EDGE)	177.6	118.4 (Class 10 and MCS-9)	3+2

Cells and Routing Areas


     The geographic coverage area of a GPRS network is divided into smaller areas known as cells and routing areas, as shown in Figure 24−14. A cell is the area that is served by a set of radio base stations (BSs). When a GPRS MS wants to send data or prepare to receive data, it searches for the strongest radio signal that it can find. Once the mobile scans for the strongest signal and locates the strongest BS, it then notifies the network of the cell it is receiving the strongest and selects it. At this point, the mobile listens to the BS for news of incoming data packets.


Data Transfer


    Once the MS has attached to a SGSN and activated a PDP address, it is now ready to begin communicating with other devices. For example, a GPRS mobile is communicating with a computer system connected to an X.25 or IP network. The other computer may be unaware that the MS is, in fact, mobile. 


     From this point onward, they are routed based on their original (internal) header. Using tunneling within GPRS solves the mobility problem for the packet networks and helps to eliminate the complex task of protocol interworking. Mobile IP also makes use of tunneling to route packets to mobile nodes. In mobile IP, packets are only tunneled from the fixed network to the MS. Packets flowing from the mobile to fixed nodes use normal routing. GPRS, by contrast, uses tunneling in both directions.

Some Applications for GPRS



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. The information intensity, the amount of information transferred per message, tends to be lower with chat, where people are more likely to state opinions than factual data. In the same way as Internet chat groups have proven to be a very popular application of the Internet, groups of like−minded people, so−called communities of interest, have begun to use non-voice mobile services as a means to chat and discuss.


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.


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.


Audio. Despite many improvements in the quality of voice calls on mobile networks, such as Enhanced Full Rate (EFR), they are still not broadcast quality. In some scenarios, journalists or undercover police officers with portable professional broadcast−quality microphones and amplifiers capture interviews with people or radio reports that they have dictated and need to send this information back to their radio or police station. Leaving a mobile phone on or dictating to a mobile phone would not give
sufficient voice quality to enable that transmission to be broadcast or analyzed for the purposes of background noise analysis or voice printing, where the speech autograph is taken and matched against those in police storage.


Internet E−mail. Services come in the form of a gateway service where the messages are not stored or mailbox services in which messages are stored. In the case of gateway services, the wireless e−mail platform translates the message from SMTP, the Internet e−mail protocol, into SMS and sends it to the SMS Center. In the case of mailbox e−mail services, the e−mails are actually stored, and the user receives a notification on his or her mobile phone and can then retrieve the full e−mail by dialing in to collect it, forward it, and so on.