14-CellularWirelessNetworks - Data and Computer...

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Unformatted text preview: Data and Computer Communications Communications Chapter 14 – Cellular Wireless Networks Cellular Chapter Eighth Edition by William Stallings Lecture slides by Lawrie Brown Cellular Wireless Networks Cellular After the fire of 1805, Judge Woodward was the central After figure involved in reestablishing the town. Influenced by Major Pierre L’Enfant’s plans for Washington, DC, Judge Woodward envisioned a modern series of hexagons with major diagonal avenues centered on circular parks, or circuses, in the center of the hexagons. Frederick Law Olmstead said, "nearly all of the most serious mistakes of Detroit's past have arisen from a disregard of the spirit of Woodward's plan." from —Endangered Detroit, Friends of the Book-Cadillac Hotel Cellular Wireless Networks Cellular key technology for mobiles, wireless nets etc developed to increase mobile phone capacity based on multiple low power transmitters area divided into cells in a tiling pattern to provide full coverage each with own antenna each with own range of frequencies served by base station adjacent cells use different frequencies to avoid adjacent crosstalk crosstalk Cellular Geometries Cellular Frequency Reuse Frequency must manage reuse of frequencies power of base transceiver controlled allow communications within cell on given frequency limit escaping power to adjacent cells allow re-use of frequencies in nearby cells typically 10 – 50 frequencies per cell example for Advanced Mobile Phone Service (AMPS) • • • • N cells all using same number of frequencies cells K total number of frequencies used in systems total each cell has K/N frequencies each K/N K=395, N=7 giving 57 frequencies per cell on average =395, Frequency Frequency Reuse Patterns Increasing Capacity Increasing add new channels not all channels used to start with taken from adjacent cells by congested cells or assign frequencies dynamically non-uniform topography and traffic distribution non-uniform use smaller cells in high use areas frequency borrowing cell splitting Cell Splitting Cell Increasing Capacity Increasing cell sectoring cell divided into wedge shaped sectors (3–6 per cell) each with own channel set directional antennas move antennas from tops of hills and large buildings move to tops of small buildings and sides of large buildings to use reduced power to cover a much smaller area good for city streets, roads, inside large buildings microcells Frequency Reuse Example Frequency Overview of Cellular System Overview Cellular System Channels Cellular system is fully automated see two types of channels between mobile see and base station (BS) and control channels set up and maintain calls establish relationship between mobile unit and establish nearest BS nearest carry voice and data traffic channels Call Stages Stages Other Functions Other call blocking if all traffic channels busy when user hangs up when BS cannot maintain required signal strength MTSO connects mobile user and fixed line via PSTN MTSO connects to remote MTSO via PSTN or MTSO dedicated lines dedicated call termination call drop calls to/from fixed and remote mobile subscriber Mobile Radio Propagation Effects Propagation signal strength strength of signal between BS and mobile unit strong strength enough to maintain signal quality at the receiver enough not too strong to create cochannel interference not must handle variations in noise time variation of received signal caused by changes in transmission path(s) even if signal strength in effective range, signal even propagation effects may disrupt the signal propagation fading Design Factors Design propagation effects max transmit power level at BS and mobile units typical height of mobile unit antenna available height of the BS antenna these factors determine size of individual cell use model based on empirical data eg. model by Okumura et al & refined by Hata detailed analysis of tokyo area produced path loss info for an urban environment Hata's model is an empirical formulation Multipath Propagation Multipath Effects of Multipath Propagation Propagation Types of Fading Types fast fading rapid changes in strength over half wavelength distances rapid • eg. 900MHz wavelength is 0.33m see 20-30dB slow fading slower changes due to user passing different height slower buildings, gaps in buildings etc. buildings, over longer distances than fast fading affects all frequencies in same proportion different frequency components affected differently flat fading selective fading Error Compensation Mechanisms Mechanisms forward error correction applicable in digital transmission applications typically, ratio of total bits to data bits is 2-3 has a big overhead applied to transmissions that carry analog or digital applied information information used to combat intersymbol interference gathering the dispersed symbol energy back together gathering into its original time interval into techniques include so-called lumped analog circuits techniques and sophisticated digital signal processing algorithms and adaptive equalization Error Compensation Mechanisms Mechanisms diversity based on fact that individual channels experience based independent fading events independent use multiple logical channels between transmitter and use receiver receiver send part of signal over each channel doesn’t eliminate errors reduce error rate equalization, forward error correction then cope with equalization, reduced error rate reduced space diversity involves physical transmission paths more commonly refers to frequency or time diversity First Generation Analog First original cellular telephone networks analog traffic channels early 1980s in North America Advanced Mobile Phone Service (AMPS) Advanced also common in South America, Australia, also and China and replaced by later generation systems AMPS Spectral Allocation In North America North two 25-MHz bands are allocated to AMPS from BS to mobile unit (869–894 MHz) from mobile to base station (824–849 MHz) bands is split in two to encourage competition operator is allocated only 12.5 MHz in each direction operator channels spaced 30 kHz apart (416 channels / operator) control channels are 10 kbps data channels control voice channels carry analog using frequency modulation control info also sent on voice channels in bursts as data number of channels inadequate for most major markets for AMPS, frequency reuse is exploited Operation Operation AMPS-capable phone has numeric assignment AMPS-capable module (NAM) in read-only memory module NAM contains number of phone serial number of phone when phone turned on, transmits serial number and when phone number to MTSO MTSO has database of mobile units reported stolen MTSO uses phone number for billing iif phone is used in remote city, service is still billed to f user's local service provider user's AMPS Call Sequence AMPS 1. 2. 2. 3. 4. 5. 6. subscriber initiates call keying in number subscriber MTSO validates telephone number and checks MTSO user authorized to place call user MTSO issues message to user's phone MTSO indicating traffic channels to use indicating MTSO sends ringing signal to called party when called party answers, MTSO establishes when circuit and initiates billing information circuit when one party hangs up MTSO releases when circuit, frees radio channels, and completes billing information billing AMPS Control Channels AMPS 21 full-duplex 30-kHz control channels transmit digital data using FSK data transmitted in frames control information can be transmitted over voice control channel during conversation channel Mmobile unit or the base station inserts burst of data Mmobile • turn off voice FM transmission for about 100 ms • replacing it with an FSK-encoded message used to exchange urgent messages • change power level • handoff Second Generation CDMA Second provide higher quality signals, higher data rates, provide support digital services, with overall greater capacity capacity key differences include digital traffic channels encryption error detection and correction channel access • time division multiple access (TDMA) • code division multiple access (CDMA) Code Division Multiple Access (CDMA) Access have a number of 2nd gen systems for example IS-95 using CDMA each cell allocated frequency bandwidth is split in two half for reverse, half for forward uses direct-sequence spread spectrum (DSSS) Code Division Multiple Access (CDMA) Advantages Access frequency diversity noise bursts & fading have less effect chipping codes have low cross & auto correlation inherent in use of spread-spectrum more users means more noise leads to slow signal degradation until unacceptable multipath resistance privacy graceful degradation Code Division Multiple Access Code (CDMA) Disadvantages (CDMA) self-jamming some cross correlation between users signals closer to receiver are received with signals less attenuation than signals farther away less near-far problem RAKE Receiver RAKE IS-95 IS-95 second generation CDMA scheme primarily deployed in North America transmission structures different on transmission forward and reverse links forward IS-95 Channel Structure IS-95 IS-95 Forward Link IS-95 four types of channels Pilot (channel 0) • allows mobile unit to acquire timing information Synchronization (channel 32) • 1200-bps channel used by mobile station to obtain 1200-bps identification information about the cellular system identification Paging (channels 1 to 7) • Contain messages for one or more mobile stations Traffic (channels 8 to 31 and 33 to 63) • 55 traffic channels all channels use same bandwidth Forward Link Processing Processing Forward Link - Scrambling Forward after interleaver, data scrambled privacy mask prevent sending of repetitive patterns reduces probability of users sending at peak power at reduces same time same pseudorandom number from 42-bit shift register initialized with user's electronic serial number output at a rate of 1.2288 Mbps scrambling done by long code scrambling Forward Link - Power Control Forward inserts power control info in traffic channel to control the power output of antenna robs traffic channel of bits at rate of 800 bps robs by stealing code bits by 800-bps channel carries information directing 800-bps mobile unit to change output level mobile power control stream multiplexed to 19.2 kbps Forward Link - DSSS Forward spreads 19.2 kbps to 1.2288 Mbps using one row of Walsh matrix assigned to mobile station during call setup if 0 presented to XOR, 64 bits of assigned row sent if 1 presented, bitwise XOR of row sent final bit rate 1.2288 Mbps bit stream modulated onto carrier using QPSK data split into I and Q (in-phase and quadrature) data channels data in each channel XORed with unique short code IS-95 Reverse Link IS-95 up to 94 logical CDMA channels each occupying same 1228-kHz bandwidth supports up to 32 access and 62 traffic channels each station has unique long code mask based on each serial number serial • 42-bit number, 242 – 1 different masks • access channel used by mobile to initiate call, respond to access paging channel message, and for location update paging traffic channels are mobile unique Reverse Reverse Link Processing Reverse Link - DSSS Reverse llong code unique to mobile XORed with output ong of randomizer of 1.2288-Mbps final data stream modulated using orthogonal QPSK modulation differs from forward channel in use of delay differs element in modulator to produce orthogonality element forward channel, spreading codes orthogonal reverse channel orthogonality of spreading codes not reverse guaranteed guaranteed Third Generation Systems Third high-speed wireless communications to support high-speed multimedia, data, and video in addition to voice multimedia, 3G capabilities: • • • • • • • • • • voice quality comparable to PSTN 144 kbps available to users over large areas 384 kbps available to pedestrians over small areas support for 2.048 Mbps for office use symmetrical and asymmetrical data rates packet-switched and circuit-switched services adaptive interface to Internet more efficient use of available spectrum support for variety of mobile equipment allow introduction of new services and technologies Driving Forces Driving trend toward universal personal telecommunications trend universal communications access GSM cellular telephony with subscriber identity module, GSM is step towards goals is personal communications services (PCSs) and personal personal communication networks (PCNs) also form objectives for third-generation wireless third-generation technology is digital using time division multiple access technology or code-division multiple access or PCS handsets low power, small and light IMT-2000 Terrestrial Radio Alternative Interfaces Alternative CDMA Design Considerations – Bandwidth and Chip Rate Bandwidth dominant technology for 3G systems is CDMA 3 CDMA schemes, share some design issues CDMA 5 MHz reasonable upper limit on what can be MHz allocated for 3G allocated 5 MHz is enough for data rates of 144 and 384 kHz given bandwidth, chip rate depends on desired data given rate, need for error control, and bandwidth limitations rate, chip rate of 3 Mbps or more reasonable bandwidth (limit channel to 5 MHz) chip rate CDMA Design Considerations – Multirate Multirate provision of multiple fixed-data-rate channels to user different data rates provided on different logical channels llogical channel traffic can be switched independently ogical through wireless fixed networks to different destinations through flexibly support multiple simultaneous applications flexibly efficiently use available capacity by only providing the efficiently capacity required for each service capacity use TDMA within single CDMA channel or use multiple CDMA codes CDMA Multirate CDMA Time and Code Multiplexing Summary Summary principles of wireless cellular networks operation of wireless cellular networks first-generation analog second-generation CDMA third-generation systems ...
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