Cellular Networks PDF

Wireless Mobile & Multimedia Networking (7COM1076) Cellular Wireless Networks 1  Overview Principles of Cellular Networks AMPS (1G) GSM (2G TDMA) and IS-95 (2G CDMA) WiMax and LTE (both 4G OFDM) 2 Evoluti...

Wireless Mobile & Multimedia Networking (7COM1076) Cellular Wireless Networks 1  Overview Principles of Cellular Networks AMPS (1G) GSM (2G TDMA) and IS-95 (2G CDMA) WiMax and LTE (both 4G OFDM) 2 Evolution of Cellular Wireless Systems 3  Cellular Network Organization Use multiple low-power transmitters (100 W or less) Areas divided into cells Each served by its own antenna Served by base station consisting of transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern) 4 Frequency Reuse Patterns 5  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 6  Reuse Factor Definitions D = minimum distance between centers of cells that use the same frequency band (called cochannels) R = radius of a cell d = distance between centers of adjacent cells N = number of cells in a repetitious pattern (each cell in the pattern uses a unique set of frequency bands), termed the reuse factor. Hexagonal cell pattern N = I^2+J^2+(I x J), I, J = 0, 1, 2, 3 D/R = sqrt(3N) D/d = sqrt(N) 7  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 building s hills, and lamp posts , 8 Cellular System Overview 9  Cellular Systems Terms Base Station (BS) – includes an antenna, a controller, and a number of receivers Mobile telecommunications switching office (MTSO) – connects calls between mobile units Two types of channels available between mobile unit and BS Control channels – used to exchange information having to do with setting up and maintaining calls Traffic channels – carry voice or data connection between users 10  Steps in an MTSO Controlled Call between Mobile Users (1) Mobile unit initialization (2) Mobile-originated call 11  Steps in an MTSO Controlled Call between Mobile Users (Cont.) (3) Paging (4) Call accepted 12  Steps in an MTSO Controlled Call between Mobile Users (Cont.) (5) Ongoing call (6) Handoff 13  Additional Functions in an MTSO Controlled Call Call blocking Busy Call termination Hang up Call drop Interference or weak signal Calls to/from fixed and remote mobile subscriber Arranged by MTSO 14  Handoff Performance Metrics Cell blocking probability – probability of a new call being blocked Call dropping probability – probability that a call is terminated due to a handoff Call completion probability – probability that an admitted call is not dropped before it terminates Probability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequate Handoff blocking probability – probability that a handoff cannot be successfully completed Handoff probability – probability that a handoff occurs before call termination Rate of handoff – number of handoffs per unit time Hard handoff and soft handoff 15  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 co-channel 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 16  Overview Principles of Cellular Networks AMPS (1G) GSM (2G TDMA) and IS-95 (2G CDMA) WiMax and LTE (both 4G OFDM) 17  Advanced Mobile Phone Service (AMPS) Two 25-MHz bands allocated to AMPS One for transmission from base to mobile unit (869-894 MHz) One for transmission from mobile unit to base (824- 849MHz) Each band split in two to encourage competition Each 12.5MHz band is divided into 416 channels every 30KHz (21 for control and 395 for traffic) All together 790 full-duplex voice channels and 42 full- duplex control channels Frequency reuse exploited 18  AMPS 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 19  Overview Principles of Cellular Networks AMPS (1G) GSM (2G TDMA) and IS-95 (2G CDMA) WiMax and LTE (both 4G OFDM) 20  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 21  Mobile Station Mobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit Mobile equipment (ME) – physical terminal, such as a telephone or PCS ME includes radio transceiver, digital signal processors and subscriber identity module (SIM) GSM subscriber units are generic until SIM is inserted SIMs roam, not necessarily the subscriber devices 22  Base Station Subsystem (BSS) BSS consists of base station controller and one or more base transceiver stations (BTS) Each BTS defines a single cell Includes radio antenna, radio transceiver and a link to a base station controller (BSC) BSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS, and controls paging 23  Network Subsystem (NS) NS provides link between cellular network and public switched telecommunications networks Controls handoffs between cells in different BSSs Authenticates users and validates accounts Enables worldwide roaming of mobile users Central element of NS is the mobile switching center (MSC) 24  Mobile Switching Center (MSC) Databases Home location register (HLR) database – stores information about each subscriber that belongs to it Visitor location register (VLR) database – maintains information about subscribers currently physically in the region Authentication center database (AuC) – used for authentication activities, holds encryption keys Equipment identity register database (EIR) – keeps track of the type of equipment that exists at the mobile station 25  GSM Radio Link Aspects Two 25-MHz bands allocated to GSM One for transmission from base to mobile unit (935-960 MHz) One for transmission from mobile unit to base (890- 915MHz) Each 25MHz band is divided into 125 radio-frequency carriers every 200kHz, provides for 125 full-duplex channels. Each 200KHz frequency carrier is divided into 8 logical channels defined by the repetitive occurrence of time slots. 26 GSM Frame Format 27  IS-95 Most widely used second-generation CDMA system. Using DSSS on forward and reverse links. All forward channels use the same bandwidth. The chipping code is used to distinguish among the different channels. For the forward channel, chipping codes are the 64 orthogonal 64-bit codes For the reverse channel, chipping codes are still 64 bits, but are not guaranteed orthogonal, but the orthogonarity of signals are guaranteed by an orthogonal modulation scheme. 28  IS-95 Radio Link Aspects Two 25-MHz bands allocated to IS-95 One for transmission from base to mobile unit (869-894 MHz), the forward link One for transmission from mobile unit to base (824- 849MHz), the reverse link Each 25MHz band is divided into 20 radio-frequency carriers every 1250kHz, provides for 20 full-duplex channels. Each 1250KHz frequency carrier is divided into 64 logical channel on the forward link or up to 94 logical channel on the reverse link 29 IS-95 Channel Structure 30  Advantages of CDMA Cellular Frequency diversity – frequency-dependent transmission impairments have less effect on signal Multipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelation Privacy – privacy is inherent since spread spectrum is obtained by use of noise-like signals Graceful degradation – system only gradually degrades as more users access the system 31  Overview Principles of Cellular Networks AMPS (1G) GSM (2G TDMA) and IS-95 (2G CDMA) WiMax and LTE (both 4G OFDM) 32  Third-Generation Capabilities An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic More efficient use of the available spectrum in general Support for a wide variety of mobile equipment Flexibility to allow the introduction of new services and technologies 33  Why 4G ? Current 4G Voice communication VoIP, high quality video conferencing SMS, MMS Video messaging Internet browsing Super-fast internet Downloadable games Online gaming with mobility Downloadable video High quality audio & video streaming No TV service Broadcast TV on-demand Peer-to-peer messaging Wide-scale distribution of video clips Mobile payment File transfer Many other innovative ideas 34  Technical Requirements Increase data rate About 100 Mbps downlink and 50 Mbps uplink Improve wireless performance Better signal reception and better coverage Increase spectrum efficiency More subscribers and more data transfer in the same spectrum High flexibility of allocation Quickly adjust data rate to subscriber according to need 35  4G Enabling Technology Some key technologies made 4G possible Both WiMAX (Worldwide Interoperability for Microwave Access and LTE (Long Term Evolution) use: OFDM, OFDMA and SC-FDMA Channel dependent scheduling Adaptive coding and modulation (ACM) Multiple-In-Multiple-Out (MIMO) antenna processing Turbo coding and decoding Need to fight the fading channel 36 37 38 39 WiMAX architecture 41 42 43 44 45 46  Comparing the End-to-End Network LTE/SAE User Plane & Data Flow Application Multiple layers, Many nodes and proprietary protocols e.g. IP, e.g. IP, PPP PPP Relay Relay PDCP PDCP GTP -U GTP - U GTP -U GTP -U RLC RLC UDP/IP UDP/IP UDP/IP UDP/IP MAC MAC L2 L2 L2 L2 L1 L1 L1 L1 L1 L1 LTE-Uu S1-U SGi UE/MS E-UTRAN Serving GW S5 PDN GW Mobile WiMAX User Plane & Data Flow Based on simple IETF protocols, Fewer nodes & fewer device requirements, Optimized for high speed data 47  Comparison WiMAX and LTE employ similar technologies Both will achieve very high data rates Both will provide new services Both use: OFDMA, MIMO, TURBO LTE has the advantage of large GSM/UMTS customer base WiMAX has the advantage of being already in service in few places in USA 48  Summary Principles of Cellular Networks Frequency reuse Cellular system overview Steps of setting-up a call and maintaining it Handoff & power control First-Generation FDMA AMPS Second-Generation TDMA and CDMA GSM IS-95 Forth-Generation Systems WiMAX & LTE 49  References Wireless Communications & Networks. William Stallings, 2nd edition. Chapter 10 WiMAX vs. LTE, who will lead the broadband mobile Internet? Z. Abichar, J. Chang, and C. Hsu, IEEE IT Professionals Magazine, 2010 WiMAX vs LTE, Slides by P., Gaggar, P. Kalyani, K. Nadkarni and A. Gadodia 50