1 Lecture #11: Cellular Radio. Satellite Communications. C o n t e n t s l Cellular Radio –Evolution of wireless and mobile communications –Cellular mobile systems l Communication Satellites –Geosynchronous satellites –Low-orbit satellites

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

3 Paging Systems l 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 l Addressing is based on unique number like the phone system l Calls are made to the service company and the messages are transmitted locally by antenna or to remote client via satellite. l Possibility for broad- and multi-casting l Narrow bandwidth (because of the small buffer of the receiving device); transmission band is in the diapason MHz. 2/53

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

5 Mobil Phones evolution l Single channel push-to-talk systems l IMTS (Improved Mobile Telephone System): –bi-directional analog transmission on two-frequencies- channel; –up to 23 channels in system in the MHz 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. l 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  Not to be mixed with the ATM cell! 2/54a

6 Analog Cellular Telephone Systems l Small retransmission cells with low-power transceiver (radio transmitter+receiver); low-power telephones (05.-3W depends on usage: hand-set, car-set). l Channel adjacency scheme that allocates two equal sets of channels distanced by two cell’s dimensions. l 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 l 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) l MTSO is connected to one or more end offices of the telephone system and also to other MTSO l Hierarchy of MTSO - packet switching network l Intercell transfer of the mobile telephone based on the detection of the phone location by its signal strength

8 Mobile Phone Channels l In AMPS 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 l Signal distortions: absorption and reflection l 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 l 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 l 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  ) –control information is transmitted in digital form although the voice channels are analog l 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 l Call to another phone #: the request to the base station is send via one of the access channels l 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 l Digital coding makes cellular transmission more secure against direct taping and gives more possibilities for encryption, computer and data services, etc. l 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 kHz- band channels - FDM. For each of those channels multiple user processes are multiplexed -TDM

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

12 Satellite Communications l 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 l 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 l Types: –geosynchronous satellites –low-orbit satellites

13 Geosynchronous Satellites l Satellite rotates around the Earth with a 24-hour period i.e. synchronously to given surface point on a constant height of km l Interference limitations: up to 2 0 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 l 3 bind available for public communications: l 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 l 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. l Frequency band is split between the transponders statically or dynamically (FDM) and the channels are congested by TDM

15 Surface Equipment l 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) l 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 l 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 l 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 l 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 moving cells serve mobile clients by 1,6 GHz full-duplex channels ; support by intersatellite communications l #SATspots(cells)/totalchannels/total SATcellscellchannels 6624(av.) /57a 2/57b

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