Lecture #11: Cellular Radio. Satellite Communications.

1. Lecture #11: Cellular Radio. Satellite Communications. C o n t e n t s • Cellular Radio • Evolution of wireless and mobile communications • Cellular mobile systems • Communication Satellites • Geosynchronous satellites • Low-orbit satellites 2 6 12 13 17

2. Mobile Telecommunication Services • Extension of the traditional telephone and data services, including • satellite paging • cordless phones • cellular phones • mobile data applications

3. Paging Systems • Non-interactive unidirectional transmission of short messages from [conventional] telephone to one or more subscribers based on • small receiving device portable by the subscriber or • computer plug-in device for receiving of arbitrary long messages • Addressing is based on unique number like the phone system • Calls are made to the service company and the messages are transmitted locally by antenna or to remote client via satellite. • Possibility for broad- and multi-casting • Narrow bandwidth (because of the small buffer of the receiving device); transmission band is in the diapason 930-932 MHz. 2/53

4. Cordless Phones • Two-part cordless extension of standard telephone, including base station and portable telephone connected by bi-directional radio line • Preset frequency or user-selectable frequency or frequency hops on predefined channels to avoid interference with other devices and for privacy • Analog or digital (newer standard) coding • Range under 1km (usually wider range is under different regulation) • Some new versions support roaming between the base stations

5. Mobil Phonesevolution • Single channel push-to-talk systems • IMTS (Improved Mobile Telephone System): • bi-directional analog transmission on two-frequencies-channel; • up to 23 channels in system in the 150-450MHz band and up to 23 simultaneous conversations; • powerful 200W transmitter covering up to 100km area; • long distance between adjacent transmitters, separation by uncovered zone to avoid interference. • Cellular telephone systems: • small areas of the region (“cells”) use subset of frequency channels so that the same channels can be reused in one or several near-by but not adjacent cells • Analog systems: AMPS (Advanced Mobile phone System) • Digital systems: GSM 1946 1962 Not to be mixed with the ATM cell! 2/54a 1982 1987

6. Analog Cellular Telephone Systems • Small retransmission cells with low-power transceiver (radio transmitter+receiver); low-power telephones (05.-3W depends on usage: hand-set, car-set). • Channel adjacency scheme that allocates two equal sets of channels distanced by two cell’s dimensions. • Cell size can be reduced by reducing the transmitting power and congesting the transceivers in the area. The result is higher frequency reuse i.e. higher number of service users. 2/54b

7. Mobile Phone Cell • Cell consists of a base station that includes computer controlled transceiver (with antenna) and connection to MTSO (Mobile Telephone Switching Office) or MSC (Mobile Switching Center) • MTSO is connected to one or more end offices of the telephone system and also to other MTSO • Hierarchy of MTSO - packet switching network • Intercell transfer of the mobile telephone based on the detection of the phone location by its signal strength

8. Mobile Phone Channels • In AMPS - 832 full duplex channels separated by their frequency (i.e. FDM) as follows: • 832 * 30kHz simplex transmission channels (total bandwidth 25MHz) in the range 824849MHz • 832 * 30kHz simplex receiving channels (total bandwidth 25MHz) in the range 869894MHz • Signal distortions: absorption and reflection • Those 832 channels are separated among the cells - usually ~50 channels/cell  ~17 (but  21) different kind of cells are to be allocated according some pattern for maximal distance between the similar cells • Channel types • Control (station to mobile) system management (AMPS: 21 reserved control channels hardwired in each telephone PROM) • Paging (station to mobile) incoming calls • Access (bidirectional) outcoming calls and channel assignment • Data (bidirectional) conversation, fax, data exchange

9. Mobile Phone Calls • Mobile phone numbers are 34b addresses written in PROM: • 10b - 3 decimal digits for area code (: not ASCII but binary coding! 10b binary code ranges 0  (1K-1), 3 positions of decimal code range 0  999) • 24b - 7 decimal digits for subscriber # (24b code 0  16M-1; 7 decimal digits code 0  9 999 999 ) • control information is transmitted in digital form although the voice channels are analog • Switching the phone on proceeds as follows: • scanning of the 24 control channels for nearest base station • accepting the numbers of paging and access channels • MTSO records the new customer and informs the originating (home) MTSo for his current location • periodical reregistration • Call to another phone #: the request to the base station is send via one of the access channels • Call from another phone is received on paging channel which is scanned periodically by the telephone for message addressed to its number

10. Digital Cellular Telephony • Digital coding makes cellular transmission more secure against direct taping and gives more possibilities for encryption, computer and data services, etc. • Standards: • AMPS digital upgrade are IS-54/135. The 30kHz AMPS channel packs 48.6 kb/S shared between 3 active users - i.e. 13 kb/S for each user • GSM (Global System for Mobile communications) uses the 10 MHz frequency bandwidth round 1.8 GHz. It is divided into 50 200kHz-band channels - FDM. For each of those channels multiple user processes are multiplexed -TDM

11. Personal Communication Services • PCS/PCN (Personal Communication {Services|Network}) world-wide mobile telephone with data-transfer capabilities and global addressing system • Technology: dense set of mcells, that allows low-powered portable and autonomous user devices (phones, etc.)

12. Satellite Communications • Communication satellites act as signal reflectors or repeaters (reflection + amplification). They contain several transponders which: • scan signals in given frequency band • amplifies and retransmit the signals in at another frequency that not interfere the incoming signal • Functions: • overcome the terrestrial curve for straight transmitted unguided media (e.g. microwave) • broadcasting a signal over wide area • focusing the signal in a narrow surface spot • Types: • geosynchronous satellites • low-orbit satellites

13. Geosynchronous Satellites • Satellite rotates around the Earth with a 24-hour period i.e. synchronously to given surface point on a constant height of 36000 km • Interference limitations: up to 20 angle between adjacent satellites and the Earth center - up to 180 satellite orbit slots. Number of geosynchronous satellite may be bigger because of the frequency separation • 3 bind available for public communications: • One or more spot beams of the satellite cover the entire visible area or smaller elliptic areas that correspond to the national territories - depending on the number of transponders and equipment features 2/55

14. The Geosynchronous Satellite • Number of transponders vary between 10 and 20 and their single bandwidth is around 50MHz i.e. • more than 50Mb/S data path (depending on modulation) or • 800*64kb/S voice channels. • Frequency band is split between the transponders statically or dynamically (FDM) and the channels are congested by TDM

15. Surface Equipment • System of low-power VSAT (Very Small Aperture Terminals) - low uplink rate (19.2 kb/S) because of the weak transmitter; high down-link rate (512 kb/S) • Hub - powerful transceiver with huge antenna retransmits inter-terminal communications. Thus all communication passes through the hub like in the networks of star-topology.

16. Geosynchronous Satellites Communication • Features: • relatively big delays for signal traveling to and from the remote satellite - 270mS and 540mS for VSAT-via-Hub communication • effective broadcasting • security measures • surface distance insensitive

17. Low-Orbit Satellite • Communication use of chains similar satellites that are ordered in spinning necklace. The data transfer function in regard to given surface object is switched between the consecutively appearing satellites • Motorola Iridium project - 66 satellites chain in 6 necklaces (~ 33° latitude angle) ; the satellites have a mean of 24 beam spots (i.e. transponders) separated by the frequency; frequency reuse in 2-3 cells distance. 1628 moving cells serve mobile clients by 1,6 GHz full-duplex channels ; support by intersatellite communications • # SAT spots(cells)/ total channels/ total SAT cells cell channels 66 24(av.) 1628 174 283 272 2/57a 2/57b