Chapter 4: Telecommunication systems
1. Name some key features of the GSM, DECT, TETRA, and UMTS systems. Which features do the systems have in common? Why have the three older different systems been specified? In what scenarios could one system replace another? What are the specific advantages of each system?
Key features:
GSM (wide area coverage, bandwidth 9.6-50 kbit/s, voice, SMS, MMS),
DECT (local coverage, voice, data, high density),
TETRA (regional coverage, ad-hoc mode, very fast connection set-up, group call, voice, data, very robust),
UMTS (medium coverage, higher data rates 384 kbit/s, flexible bandwidth assignment).
Common features are:
traditional voice support (circuit switched)
integration into classical fixed telecommunication network
ISDN core network.
The systems have different, unique properties: GSM has wide area coverage, TETRA ad-hoc mode and fast connection setup, DECT can support high user densities.
If allowed from licensing GSM could replace DECT, if modified GSM can replace TETRA (e.g., GSMRail) – under certain conditions (GSM does not offer an ad-hoc mode). But also
UMTS has specific advantages – higher data rates compared to classical GSM (but
lower coverage) and higher coverage than WLANs (but lower data rates).
2. What are the main problems when transmitting data using wireless systems that were made
for voice transmission? What are the possible steps to mitigate the problems and to raise efficiency? How can this be supported by billing?
Systems optimised for voice transmission support certain fixed data rates and operate circuit switched. Data transmission happens quite often spontaneous with varying data rates. Thus either too much bandwidth is reserved to accommodate the maximum expected data rate or data transmission experiences long delays due to connection setup.
One possible step towards the support of data transmission is the introduction of packet switched services as known from the Internet. An example is GPRS in GSM.
Instead of time-based billing providers can now bill based on volume (however, application based billing would make even more sense as customers are not interested in bytes but useful applications).
3. * Which types of different services does GSM offer? Give some examples and reasons why these services have been separated.
The three big categories are bearer, tele, and supplemental services.
Separation of services supports phased introduction of services and separation of concerns:
network providers, service providers, device manufacturers etc. can focus on certain sets of services (e.g., tele services between terminals) and rely on certain interfaces to other services (e.g., to the underlying bearer services).
4. Compared to the TCHs offered, standard GSM could provide a much higher data rate (33.8 kbit/s) when looking at the air interface. What lowers the data rates available to a user?
Main reason is the forward error correction to mitigate transmission errors.
Furthermore, bandwidth is needed for signalling, guard spaces.
5. * Name the main elements of the GSM system architecture and describe their functions. What are the advantages of specifying not only the radio interface but also all internal interfaces of the GSM system?
A GSM system consists of three subsystems, the radio subsystem (RSS), the network and switching subsystem (NSS), and the operation subsystem (OSS). (For info on each subsystem refer to the text book… it’s too big)
Specifying all (or at least many) internal interfaces allows for a larger variety of vendors. As long as vendors stay with the standardised interfaces equipment of different vendors can be combined and network operators are not completely dependent from one manufacturer. However, reality often looks different and network operators often use only equipment from one or two vendor(s).
6. Describe the functions of the MS and SIM. Why does GSM separate the MS and SIM? How and where is user-related data represented/stored in the GSM system? How is user data protected from unauthorized access, especially over the air interface? How could the position of an MS (not only the current BTS) be localized? Think of the MS reports regarding signal quality.
The MS contains all device related functions: device ID, coders/decoders, radio etc.
The SIM contains subscriber related functions and data: authentication, PIN, user id etc.
This separation helps changing phones while keeping personal data: users simply insert their SIM in a new mobile phone and can use, e.g., their personal phone
book, PIN etc. Exceptions are so-called SIM locked phones – in this case a mobile
phone accepts only a certain SIM. However, this is rather a marketing than technical reason. Besides the SIM also the mobile phone itself can store user-related data.
Additional user-related data is stored in the VLR responsible for the location area a user is currently in and the HLR of the network operator the user has a contract with.
User data is protected in several ways: authentication centres are protected parts of the HLR residing at the network operator. Inside the core network only temporary identifiers are used, data is encrypted over the air interface (weak, but still encrypted), and the content of the SIM is protected via a PIN (some cards destroy themselves after being attacked too many times).
Localisation could be terminal assisted: the terminal could gather the current signal strength from all surrounding base stations. Furthermore, using the time of arrival helps calculating the distance. Reflection and attenuation makes the calculation more difficult.
7. Looking at the HLR/VLR database approach used in GSM – how does this architecture limit the scalability in terms of users, especially moving users?
GSM uses only two levels of hierarchy: Network operators store all user related information in the HLR and all information related to visitors within a certain location area in a VLR. Capacities of HLRs is up to some million customers, that of VLRs up to a million. I.e., within the location area a maximum of, e.g., one million users can be active (registered). If many users move between location areas updates have to take place, i.e., the HLR always gets the information about the new VLR. These updates happen independently on the users’ activity (data transmission, calls etc.). For standard scenarios – most users stay most of the time within their location area – the 2-level hierarchy works well. However, if, e.g., many tourists move frequently the updating process puts some load on the network as the HLR in the home network of the tourists always requires update information – probably around the globe. More levels of hierarchy could improve scalability but also raises complexity.
8. Why is a new infrastructure needed for GPRS, but not for HSCSD? Which components are new and what is their purpose?
HSCSD still operates circuit switched as CSD does. It “simply” combines several connections. GPRS introduces a new paradigm in GSM, packet switching.
Basically, the core network needs routers handling the packet stream. These routers (SGSN, GGSN) operate on IP and rely on the traditional GSM network for user localisation. Another new component located at the HLR is a registry for subscribed GPRS services. Furthermore, the system has to set up a context for each active user, account transmitted data, assign IP addresses etc.
9. What are the limitations of a GSM cell in terms of diameter and capacity (voice, data) for the traditional GSM, HSCSD, GPRS? How can the capacity be increased?
Traditional GSM has cell diameters of up to 70 km, i.e., a user may have a maximum distance of 35 km to the base station. This limitation is not because of too strong attenuation, but because of the delay the signals experience. All signals must arrive synchronised at the base station, timing advance adjust the sending point (the further away a terminal is the earlier it has to send its data). With some tricks the diameter can be doubled. The capacity is limited by the number of channels * number of time slots – signalling overhead. The number of channels is operator and regulation dependent. The
capacity is independent of the usage of GSM/CSD, HSCSD or GPRS – all three systems use the same basic frame structure and modulation.
New modulation schemes can offer higher capacity, EDGE is an example. Furthermore, systems like GPRS offer different levels of error protection – this may increase user data rates under good propagation conditions, but does not increase the system
capacity.
10. What multiplexing schemes are used in GSM and for what purpose? Think of other layers apart from the physical layer.
GSM uses SDM, FDM and TDM:
SDM: Operators design the cell layout, place base stations and reuse frequencies according to certain cluster patterns.
FDM: Regulation authorities assign channels to operators, operators assign channels to base stations, and base stations assign a certain channel to a terminal during data transmission.
TDM: Base stations assign a time-slot or several time-slots to a terminal for transmission.
11. How is synchronization achieved in GSM? Who is responsible for synchronization and why is it so important?
The BSS has to create a frame structure. Terminals listen into the medium, receive signals over broadcast channels and synchronise to the frame structure. Within each time-slot during transmission a midample further improves synchronisation.
The terminal itself is responsible for precise synchronisation within the cell. This is very important in TDM systems as otherwise neighbouring data may be destroyed.
12. What are the reasons for the delays in a GSM system for packet data traffic? Distinguish between circuit-switched and packet-oriented transmission.
Examples for delays in packet transmission:
CS: connection setup (some seconds), FEC coding/decoding and interleaving (about 100 ms), propagation delay (some ms).
PS: channel access (depending on the current load), FEC coding/decoding and interleaving (about 100 ms), propagation delay and routing (some ms).
Experiments show that packets in GPRS may experience heavy delays due to channel access Delays: 200-500 ms for 128 byte packets, several seconds for 1-4 Kbyte packets.
13. Where and when can collisions occur while accessing the GSM system? Compare possible collisions caused by data transmission in standard GSM, HSCSD, and GPRS.
Besides problems due to interference, collisions in GSM systems can only occur during connection setup. Terminals have to access the base station suing a slotted Aloha scheme for the layer 2 signalling connection. During this connection attempt several terminals may collide and have to repeat the connection attempt. During data transmission or voice call no collision can occur.
Data transmission in standard GSM (CSD) behaves just as voice calls.
HSCSD has the additional problem of requesting several channels. These may be occupied. However, this does not cause a collision but a simple denial of the connection request for several channels.
Channel assignment and release is handled dynamically in GSM systems.
For GPRS, too, data transmission cannot cause a collision as the terminal wanting to transmit has to request time-slots firs. After the assignment of time-slots the terminal may access these slots without further collisions. Depending on the current load, not too many slots may be available; however, network operators try to offer at least one slot per cell for GPRS traffic to offer a minimum data rate.
14. Why and when are different signaling channels needed? What are the differences?
GSM comprises many different channels for signaling control data. If no traffic channel (TCH) exists, an MS uses an SDCCH for signaling, e.g., authentication and registration data required prior to TCH establishment. TCH and SDCCH use an SACCH for signaling channel quality/signal strength. If a TCH exists and more signaling is required (e.g., during handover), an MS uses a FACCH, which is located in the time-slots otherwise used by the TCH.
15. How is localization, location update, roaming, etc. done in GSM and reflected in the data bases? What are typical roaming scenarios?
The GSM system only stores the current location area for a user in the VLR. Each time a user changes the location area this change is reflected in the VLR. Additionally, periodic updates are possible. Roaming includes changing the network operator. This can happen within the same country (national roaming) or when going to another country (international roaming). The latter is the most common scenario as national roaming typically involves direct competitors. Prerequisite are roaming agreements between the different operators. The HLR always stores the current VLR for the user, no matter if inside the own or inside a foreign network. Precise localisation of users is performed during call setup only (paging within the location area).
16. Why are so many different identifiers/addresses (e.g., MSISDN, TMSI, IMSI) needed in GSM? Give reasons and distinguish between user-related and systemrelated identifiers.
Users of the GSM systems work with telephone numbers. That is all users should see. These phone numbers are completely independent of the current location of the user. The system itself needs some additional information; however, it must not reveal the identity of users. The international identification of users is done with the IMSI (=country code + network code + subscriber ID). During operation within a location area, only a temporary identifier, the TMSI is needed. This hides the identity of a user. The TMSI is not forwarded to the HLR. These are already some examples for identifiers; however, GSM provides some more:
IMEI: MS identification (like a serial number); consists of type approval code (centrally assigned), final assembly code, serial number, and spare (all three manufacturer assigned).
IMSI: Subscriber identification, stored in the SIM. Consists of the mobile country code (3 digits, e.g., 262 for Germany), the mobile network code (2 digits, e.g., 01 for the German T-Mobile), and the mobile subscriber identification number (10 digits). The mobile network code together with the mobile subscriber identification number forms the national mobile subscriber identity.
MSISDN: Mobile subscriber ISDN Number, i.e., the phone number, assigned to a subscriber, not a telephone! The MSISDN is public, not the IMSI nor the mapping MSISDN-IMSI. An MSISDN consists of the country code (up to 3 digits, e.g., 49 for Germany), the national destination code (typically 2 or 3 digits), and the subscriber number (up to 10 digits).
MSRN: The mobile station roaming number is a temporarily assigned, location based ISDN number. The VLR assigns MSRNs and forwards them to the HLR/GMSC for call forwarding.
Assignment happens either upon entering a new LA or upon request from the HLR (call-setup).
LAI: The location area identity describes the LA of a network operator. It consists of a country code (3 digits), a mobile network code (2 digits), and a location area code (16 bit). The LAI is broadcasted on the BCCH for LA identification.
TMSI: The VLR currently responsible for an MS can assign a 32 bit temporary mobile subscriber identity to an MS with a SIM. The tuple (TMSI, LAI) uniquely identifies a subscriber. Thus, for ongoing communication IMSI is replaced by (TMSI, LAI).
LMSI: An additional local mobile subscriber identity (32 bit) can be used by the VLR/HLR for fast subscriber look-up.
CI: Within a LA each cell has a unique cell identifier (16 bit). Thus, the tuple (LAI, CI) uniquely identifies a cell worldwide (global cell identity).
BSIC: The base transceiver station identity code identifies base stations (6 bit) and consists of a 3 bit network colour code and a 3 bit base transceiver station colour code.
All MSCs, VLRs and HLRs have unique ISDN numbers for identification.
17. Give reasons for a handover in GSM and the problems associated with it. What are the typical steps for handover, what types of handover can occur? Which resources need to be allocated during handover for data transmission using HSCSD or GPRS respectively? What about QoS guarantees?
The typical reason for a handover is a weaker signal from the current base station compared with a neighbouring base station. Another reason could be the current load situation: the network could decide to offload some users from a crowded cell. For the typical steps and types of handover see figures 4.11-4.13. For HSCSD to succeed the same resources are needed in the new cell as were available in the old one. I.e., there must be enough time-slots available to handle the same number of simultaneous connections. Otherwise the available bandwidth will decrease. Sure the probability of having several channels available is much lower than having a single channel. For GPRS data rates fluctuate anyway depending on the current load. The same happens during and after handover. Without pre-reservation neither HSCSD nor GPRS can give any QoS guarantees. There is not even a QoS guarantee for a voice call – if the next cell is already completely booked the connection will break upon entering this cell.
18. * What are the functions of authentication and encryption in GSM? How is system security maintained?
The first step is the authentication of the user against the SIM. This is done using a simple PIN. Then, the SIM authenticates itself against the GSM system. This second authentication is much stronger compared to the PIN. This is because the operator is not really interested in who is using the system as long as it is a valid and paying customer. Authentication with the system uses a challenge response scheme with a shared secret on the SIM and in the AuC. Neither the SIM nor the AuC will transmit this secret over the air or reveal it to customers. Encryption only takes places between the MS and the BSS. GSM does not provide strong encryption end-to-end or MS to the gateway into the fixed network. System designers decided for over-the-air encryption only as they thought that the system itself is trustworthy. Thus, authentication of base stations against MSs was neglected, too. This opened ways to fake base stations. UMTS introduces full authentication of all components.
19. How can higher data rates be achieved in standard GSM, how is this possible with the additional schemes HSCSD, GPRS, EDGE? What are the main differences of the approaches, also in terms of complexity? What problems remain even if the data rate is increased?
The classical data rate of GSM is 9.6 kbit/s. Using less FEC 14.4 kbit/s are available, too. These data rates are achievable using a single time-slot per frame in a certain channel. HSCSD combines several time-slots but leaves coding untouched. GPRS can dynamically use several time-slots per frame plus offers 4 different coding schemes that allow for higher data rates per slot. EDGE finally introduces another modulation scheme (PSK) in addition to GMSK, which offers even higher data rates under good propagation conditions. Only EDGE can really increase the capacity of a GSM cell. Independent of the coding and modulation schemes the complexity of handover signalling, handover delay and high
delay due to coding/interleaving remain.
20. * What limits the data rates that can be achieved with GPRS and HSCSD using real devices (compared to the theoretical limit in a GSM system)?
Real devices can (currently) not offer all data rates specified in the standards. While the standards in principle specify devices that use all 8 time-slots in both directions, real devices can often not send and receive at the same time. Furthermore, older devices even need some time to switch from sending into receiving mode, thus wasting another slot or even several slots. Additionally, current GPRS phones often do not offer all coding schemes.
21. Using the best delay class in GPRS and a data rate of 115.2 kbit/s – how many bytes are in transit before a first acknowledgement from the receiver could reach the sender (neglect further delays in the fixed network and receiver system)? Now think of typical web transfer with 10 kbyte average transmission size – how would a standard TCP behave on top of GPRS (see chapters 9 and 10)? Think of congestion avoidance and its relation to the round-trip time. What changes are needed?
The delay is specified between the MS and the exit point of the GPRS core network.The best average delay is 0.5 s. Assuming a data rate of 115.2 kbit/s (a common rate using serial adapters connected to the mobile phone) and a delay of 2*0.5 sec, 115.2 kbit = 14.4 kbyte are in transit. TCP was made for streaming larger amounts of data, i.e., file transfers etc. TCP allows for fair sharing of bandwidth as soon as it is in stable state. This requires the reception of acknowledgements, the adaptation of sending windows and thresholds. However, if the whole transfer is 10 kbyte only, TCP either never gets an acknowledgement back during transmission to adapt sender characteristics (only if the initial sending window is large enough), or TCP wastes bandwidth by using a too small starting sending window (standard case). Real measurements with GPRS exhibit high latencies (examples are round trip times for different packet sizes, class 8 mobile phone): 0.8 s/64 byte, 1.4 s/128 byte, 2.2 s/1024 byte, 2.9 s/2048 byte, and 4.8 s/4096 byte. Additionally, measurements show high jitter. Under these conditions, TCP performs poorly. Chapter 9 lists several proposed changes to TCP (e.g., large initial sending window). (If you want read chapter 9 :-P)
22. How much of the original GSM network does GPRS need? Which elements of the network perform the data transfer?
GPRS still needs the classical CS core for localisation, authentication etc. However, for data transfer the MSCs are not needed any more. The routers in the PS core (SGSN and GGSN) perform data forwarding.
23. What are typical data rates in DECT? How are they achieved considering the TDMA frames? What multiplexing schemes are applied in DECT and for what purposes? Compare the complexity of DECT with that of GSM.
DECT offers 120 full duplex channels, each with a standard rate of 32 kbit/s (unprotected). DECT applies TDM for structuring the frames and multiplexing users (24 slots per frame, typ. 12 up/12 down link). Furthermore, FDM is applied to increase capacity (several DECT cells at the same location, 10 channels). Users can also apply SDM by placing access points further apart. All the multiplexing schemes together result in very high capacities of the system, which is needed, e.g., in office
buildings. Compared to GSM the system is simpler. Although data bases have been defined, too, typical DECT systems consist of a simple base station and several mobile devices. Most scenarios do not require complicated handover (although possible in DECT). Most systems furthermore do not need accounting and billing mechanisms as they are simply connected to the fixed phone network or a PBX.
24. * Who would be the typical users of a trunked radio system? What makes trunked radio systems particularly attractive for these user groups? What are the main differences to existing systems for that purpose? Why are trunked radio systems cheaper compared to, e.g., GSM systems for their main purposes?
Police, fire brigades, ambulances, disaster relief teams, public transportation authorities, taxi drivers, etc. are typical users of trunked radio systems. Trunked radio systems are attractive because of special features like very fast connection setup (sub second), group calls, paging, high robustness, cheap operation, reliable and fast messaging, and ad-hoc capabilities. Existing systems for these special purposes are still often analogue systems operating on special frequencies without strong encryption. This makes it very difficult to cooperate for, e.g., fire brigades, the police and ambulances during disaster relief operations – the teams have to exchange equipment in order to be able to communicate. Trunked radio systems can be cheaper compared to GSM as they can have higher coverage with fewer base stations due to the lower expected load. Furthermore, complex billing and accounting mechanisms are quite often not needed.
25. Summarize the main features of third generation mobile phone systems. How do they achieve higher capacities and higher data rates? How does UMTS implement asymmetrical communication and different data rates?
Main features: higher and more flexible data rates, better voice quality due to new codecs, usage of CDMA (in almost all systems), operation at 2 GHz. Higher cell capacities and higher data rates are mainly achieved by more powerful modulation schemes, better codecs with higher compression rates for voice, CDMA as additional multiplexing scheme, and more powerful devices (more precise power adaptation, utilisation of multipath propagation, …). UMTS implements asymmetrical data rates and different data rates in the same direction via different spreading factors. As the chipping rate of UMTS is always constant, data rates depend on the spreading factor. The more the data is spread the lower the data rate is.
26. Compare the current situation of mobile phone networks in Europe, Japan, China, and North America. What are the main differences, what are efforts to find a common system or at least interoperable systems?
Currently, the situation is not absolutely clear as the different countries are in different stages implementing 3G systems. Right now no one believes in a common worldwide system, not even the same frequencies are available everywhere:
Europe: After a much discussed licensing process (beauty contests and auctions) many operators are currently deploying 3G systems. Some operators already dropped out, some filed bankruptcy. All operators for 3G will use UMTS, in the beginning the UTRA/FDD mode only (no one knows when and if UTRA/TDD will be deployed). Although licensing did not prescribe the usage of UMTS, there were only a few operators thinking of different systems in the beginning. Start of the system was 2002, 50% of the population should have access to UMTS in 2005 (in Germany).
Japan: Two different 3G systems are available in Japan. NTT DoCoMo uses a variation of UMTS in their W-CDMA system marketed as FOMA. KDDI deploys a cdma2000 system, which is 3G from the version 1xEV-DO on. China: While most 2G users today use GSM creating the biggest national market for this system, it may be speculated that UMTS will be a major 3G system in China, too, as this system can easily reuse the existing core network in
its Release 99. The Chinese development TD-SCDMA was incorporated into UMTS (UTRA/TDD, slow chipping option, Release 4). However, it is currently not clear when and if this system will be deployed. There are also some cdma-operators in China which might opt for cdma2000.
North America: The situation in the US and Canada is quite unclear. Already today many systems exist in parallel without a clear winner (compared to GSM in Europe). Furthermore, licensing of 3G spectrum takes a long time and the availability of spectrum is not clear yet. Thus, it could be the case that EDGE enhanced systems (TDMA and GSM) will be deployed offering higher data rates with EGPRS compared to today’s networks. The cdma-operators will go for cdma2000.
27. What disadvantage does OVSF have with respect to flexible data rates? How does UMTS offer different data rates (distinguish between FDD and TDD mode)?
OVSF offers only certain fixed data rates (certain multiples of 15 kbit/s). If users want to send with a data rate in-between the system either drops data (which can be recovered using FEC) or inserts dummy data. In the FDD mode adjusting the spreading factor is the only way for offering different data rates. TDD offers additionally the possibility of requesting more or less slots for up or downlink.
28. How are different DPDCHs from different UEs within one cell distinguished in UTRA FDD?
The spreading codes can always be the same in UTRA FDD to lower the system complexity. However, each UE has an individual scrambling code that is quasiorthogonal to other scrambling codes. In UTRA TDD, scrambling is cell specific.
29. Which components can perform combining/splitting at what handover situation? What is the role of the interface Iur? Why can CDMA systems offer soft handover?
The important characteristic of combining/splitting is that it is never performed inside the (traditional) core network. I.e., the MSCs do not notice anything from the new possibilities offered by CDMA (reception of data via more than one base station). Depending on the location of the handover (between two antennas at the same Node B, between two Node Bs, between two RNCs) the Node Bs or the serving RNC have to perform splitting/combination. The interface Iur is needed to transfer data between RNCs for combination/splitting without any interaction with the CN. For CDMA receiving signals from different base stations looks like multipath propagation. The rake receivers can thus handle both. The handover is then as soft as a change in the strongest signals in a multipath scenario. TDMA/FDMA systems like GSM cannot do this because the currently used time-slot and/or frequency may not be available in the next cell.
30. How does UTRA-FDD counteract the near-far effect? Why is this not a problem in GSM?
The terminals have to measure and adapt their transmission power 1500 times per second in UTRA/FDD to achieve equal signal strength at the base station. In GSM this is no problem as it never happens that two stations send at the same time on the same frequency.
Chapter 5: Satellite Systems
1. * Name basic applications for satellite communication and describe the trends.
The traditional application for satellites is the “big cable in the sky.” i.e., satellites interconnect distant locations. Today, this traditional usage for satellites is not dominant anymore. Thousands of fibres through all oceans connect all continents offering more capacity than currently needed. However, satellites are still required for TV/radio distribution and access to telecommunication networks at remote places, places with destroyed infrastructure, hostile environments etc.
2. Why are GEO systems for telecommunications currently being replaced by fiber optics?
The delay earth-GEO satellite-back to earth is always about 250 ms. This is very high compared to delays in fibre optics. Nothing can change this fact as (currently) the speed of light is the upper limit for the signal propagation speed and the distance of the GEOs is almost the circumference of the earth.
3. * How do inclination and elevation determine the use of a satellite?
The inclination determines the coverage of the satellite. At an inclination of 0° the equator is covered. With a 90° inclination a satellite orbits over the poles. Geostationary satellites are only possible over the Equator, but then reception is poor at higher latitudes. The elevation determines the signal quality. At an elevation of 0° reception is almost impossible. Typically, a signal has a usable quality starting from an elevation of 10°. Optimum signal quality can be achieved at 90°. High elevations are also required in urban or mountainous areas where buildings or mountains block signals from satellites with low elevation.
4. What characteristics do the different orbits have? What are their pros and cons?
Characteristics, pros/cons of different orbits (see chapter 5 for further figures i.e. if u want…):
• GEO: Satellites seem to be pinned to the sky; pros: fixed antennas possible, wide area coverage, simpler system design; cons: long delays, high transmission power, low system capacity (difficult SDM), weak signals at high latitudes, and crowded positions over the equator.
• LEO: low orbiting satellites; pros: low delay, lower transmission power, intersatellite routing; cons: high complexity, high system cost.
• MEO: somewhere in-between GEO and MEO
• HEO: non-circular orbits; pros: higher capacity over certain points; cons: complex systems
5. What are the general problems of satellite signals travelling from a satellite to a receiver?
Attenuation caused by the atmosphere, dust, rain, fog, snow, … Blocking of signals due to obstacles (buildings, mountains). The lower the elevation the longer is the way for the signals through the atmosphere. Without beam forming high output power is needed.
6. * Considered as an interworking unit in a communication network, what function can a satellite have?
Classical satellites were simple amplifiers that amplified the incoming analogue signal and transmitted it again on a different frequency. The next step came with digital signals. Satellite could then work as repeater. This includes regeneration of the digital data and transmission of signals representation the received data without noise (compared to analogue amplifiers that also amplify noise). Many of today’s satellites are repeaters. The next steps are switches/routers. Satellites can
perform data forwarding functions depending on receiver addresses and can even route data through space from satellite to satellite.
7. What special problems do customers of a satellite system with mobile phones face if they are using it in big cities? Think of in-building use and skyscrapers.
Without any additional repeaters on earth, satellite phones only work outdoor (or close to windows). Satellite signals are typically too weak to penetrate roofs. Furthermore, satellite phones often require a line-of-sight even outdoor. Thus, skyscrapers blocking the LOS may block communication, too.
8. Why is there hardly any space in space for GEOs?
In order to stay synchronous with the earth’s rotation, GEOs have to use the common orbit at 35786 km. Furthermore, the inclination must be 0°. This leads to satellites stringed on this orbit like stones on a thread. Additionally, the satellites should orbit above populated regions. Thus, areas above the equator looking towards Europe, America, Asia etc. are crowded. This is also the reason why all satellites must spare some propellant to catapult them out of the orbit after their lifetime. They must not block their position.
Chapter 6: Broadcast Systems
1. * 2G and 3G systems can both transfer data. Compare these approaches with DAB/DVB and list reasons for and against the use of DAB/DVB.
DAB and DVB both offer much higher data rates compared to 2G/3G networks. But they operate only unidirectional and bandwidth is shared (well, the capacity of a 2G/3G cell is shared, too). Thus, broadcast systems are good for distributing mass data relevant to many (in the best case all) users. Good examples are radio and TV, but also system updates, popular web content, news etc. Typically, it is to expensive to broadcast individual data. However, if broadcast bandwidth is available this is feasible, too. DAB/DVB can be complementary to 2G/3G systems. In particular if downloads are needed at higher relative speeds. Mobile phone systems have to lower their bandwidth dramatically at high speeds, while broadcast systems may still work at full bandwidth.
2. Which web pages would be appropriate for distribution via DAB or DVB?
Examples are news, search engines, weather reports, big portals, i.e., web pages that are relevant to many users. But also within individual web pages common parts (commercials, video streams) could use broadcast systems, while the individual parts use mobile telecommunication systems.
3. * How could location-based services and broadcast systems work together?
If the location of a user is known to the system, LBSs may offer individual, location dependent services (next pizzeria, next ATM, cheapest bookstore in close proximity, gaming partners within a certain radius etc.). Depending on the current location, the LBS may program broadcast disks of broadcast providers for individual users or groups of users. If an LBS discovers a group of people standing in front of a museum, it could trigger a video stream on a DVB device showing pictures from the current exhibition.
