Cellular Networks Principles of Cellular Networks First Generation - PowerPoint PPT Presentation

CMPE 477 – Wireless and Mobile Networks Cellular Networks  Principles of Cellular Networks  First Generation Analog, AMPS  Second Generation TDMA, GSM  System Architecture  Radio Interface  Localization and Calling  Handover CMPE 477

Cellular Concept and Organization Motivation: Instead of using high-power transmitter/receiver (mobile radio telephone), lower-power systems with  shorter radius  Numerous transmitters/receivers Idea: Space division multiple access Areas divided into cells  Each served by its own antenna  Served by base station consisting of transmitter, receiver, and control unit  Mobile stations communicate only via the base station

Cell Structure A structure where adjacent antennas are equidistant?  A hexagonal pattern  Simplifies the task of determining when to switch to the adjacent antenna and which one to choose  Cell radius R (radius of the circle that circumscribes the hexagon)  d 3 R  Distance between cells In practice hexagonal pattern cannot be used due to  topographical limitations  signal propagation conditions  limitations on the antennas

Cellular Network Organization Advantages of cell structures:  higher capacity, higher number of users  less transmission power needed  more robust, decentralized  base station deals with interference, transmission area etc. locally Challenges:  fixed network needed for the base stations  handover (changing from one cell to another) necessary  interference with other cells

Frequency Reuse Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells  10 to 50 frequencies assigned to each cell  Transmission power controlled to limit power at that frequency escaping to adjacent cells  The issue is to determine how many cells must intervene between two cells using the same frequency, reuse factor (N) D      2 2 N I J I J I J ( * ), , 0 , 1 , 2 , 3 ,... N d D: distance between centers of cells that use the same frequency

Example Reuse Patterns

Approaches to Cope with Increasing Capacity Adding new channels Frequency borrowing – frequencies are taken from adjacent cells by congested cells Cell splitting – cells in areas of high usage can be split into smaller cells Cell sectoring – cells are divided into a number of wedge- shaped sectors, each with their own set of channels Microcells – antennas move to buildings, hills, and lamp posts

Power Control Design issues making it desirable to include dynamic power control in a cellular system  Received power must be sufficiently above the background noise for effective communication  Desirable to minimize power in the transmitted signal from the mobile  Reduce cochannel interference, alleviate health concerns, save battery power  In SS systems using CDMA, it’s desirable to equalize the received power level from all mobile units at the BS

Types of Power Control Open-loop power control  Depends solely on mobile unit  No feedback from BS  Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength Closed-loop power control  Adjusts signal strength in reverse channel based on metric of performance  BS makes power adjustment decision and communicates to mobile on control channel

Development of mobile telecommunication systems CT0/1 FDMA AMPS CT2 NMT IMT-FT DECT IS-136 TDMA TDMA EDGE IMT-SC D-AMPS IS-136HS GSM GPRS UWC-136 PDC IMT-DS UTRA FDD / W-CDMA IMT-TC UTRA TDD / TD-CDMA CDMA IMT-TC TD-SCDMA IS-95 IMT-MC cdma2000 1X cdmaOne cdma2000 1X EV-DO 1X EV-DV (3X) 1G 2G 2.5G 3G

First-Generation Analog Advanced Mobile Phone Service (AMPS)  In North America, two 25-MHz bands allocated to AMPS  One for transmission from base to mobile unit  One for transmission from mobile unit to base  Each band split in two to encourage competition  Frequency reuse exploited Operation: Subscriber initiates call by keying in phone number and presses send key MTSO verifies number and authorizes user MTSO issues message to user’s cell phone indicating send and receive traffic channels MTSO sends ringing signal to called party Party answers; MTSO establishes circuit and initiates billing information Either party hangs up; MTSO releases circuit, frees channels, completes billing

Differences Between First and Second Generation Systems  Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital  Encryption – all second generation systems provide encryption to prevent eavesdropping  Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception  Channel access – second-generation systems allow channels to be dynamically shared by a number of users

2 nd Generation TDMA: GSM formerly: Groupe Spéciale Mobile (founded 1982)  now: Global System for Mobile Communication Pan-European standard (ETSI, European Telecommunications Standardisation Institute) simultaneous introduction of essential services in three phases (1991, 1994, 1996) by the European telecommunication administrations  seamless roaming within Europe possible  today many providers all over the world use GSM (more than 184 countries in Asia, Africa, Europe, Australia, America)  around 800 million subscribers  GSM900, GSM1800, GSM1900

Performance characteristics of GSM (wrt. analog systems) Total mobility  international access, chip-card enables use of access points of different providers Worldwide connectivity  one number, the network handles localization High capacity  better frequency efficiency, smaller cells, more customers per cell High transmission quality  high audio quality and reliability for wireless, uninterrupted phone calls at higher speeds (e.g., from cars, trains) Security functions  access control, authentication via chip-card and PIN

GSM: Mobile Services GSM offers  several types of connections  voice connections, data connections, short message service  multi-service options (combination of basic services) Three service domains  Bearer Services  Telematic Services  Supplementary Services

Bearer Services  Services to transfer data between access points  Transparent Bearer Services use the physical layer to transmit data  Forward error correction is used  Non-Transparent Bearer Services use layer 2 and 3 protocols for error correction and flow control Specification of services up to the terminal interface (OSI layers 1-3)  Different data rates for voice and data (original standard)  data service (packet switched)  synchronous: 2.4, 4.8 or 9.6 kbit/s  asynchronous: 300 - 9600 bit/s

Tele Services  Telecommunication services that enable voice communication via mobile phones  Offered services  mobile telephony primary goal of GSM was to enable mobile telephony offering the traditional bandwidth of 3.1 kHz  Emergency number common number throughout Europe (112); mandatory for all service providers; free of charge; connection with the highest priority (preemption of other connections possible)  Short Message Service (SMS) alphanumeric data transmission to/from the mobile terminal using the signaling channel, thus allowing simultaneous use of basic services and SMS  Group-3 fax: Fax data is transmitted as digital data over the analog phone network

Supplementary services  Services in addition to the basic services, cannot be offered stand-alone  May differ between different service providers, countries and protocol versions  Important services  identification: forwarding of caller number  suppression of number forwarding  automatic call-back  conferencing with up to 7 participants  locking of the mobile terminal (incoming or outgoing calls)

Architecture of the GSM system several providers setup mobile networks following the GSM standard within each country  components  MS (mobile station)  BS (base station)  MSC (mobile switching center)  LR (location register)  subsystems  RSS (radio subsystem): covers all radio aspects  NSS (network and switching subsystem): call forwarding, handover, switching  OSS (operation subsystem): management of the network

GSM: overview OMC, EIR, AUC HLR GMSC fixed network NSS with OSS VLR MSC MSC VLR BSC BSC RSS

GSM: elements and interfaces radio cell BSS MS MS U m radio cell MS RSS BTS BTS A bis BSC BSC A MSC MSC signaling NSS VLR VLR ISDN, PSTN HLR GMSC PDN IWF O OSS EIR AUC OMC

System architecture: radio subsystem radio network and switching subsystem subsystem MS MS Components  MS (Mobile Station) U m  BSS (Base Station Subsystem): consisting of A bis BTS  BTS (Base Transceiver Station): BSC MSC sender and receiver BTS  BSC (Base Station Controller): controlling several transceivers Interfaces  U m : radio interface A  A bis : standardized, open interface with BTS MSC 16 kbit/s user channels BSC BTS  A : standardized, open interface with BSS 64 kbit/s user channels