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Cellular Wireless Network

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  • In order to drive the various functions needed to manage connections and mobility, one (or more) slots of the TDMA frame are devoted to carry logical channels for signalling and control purposes. BCCH carries in a point-to-multipoint manner all the system information needed to the mobile terminals roaming in that cell CCCH on the up link carries the random access channel, accessed with contention by all the mobiles roaming in the cell; on the down link, two logical channels are time multiplexed on CCCH, namely: the paging channel, addressing the mobile which is requested by an incoming call the access grant, carrying the acknowledgements to the mobile access. DCCH carries in both up and down links, control channels that are either signalling-dedicated or associated control channels; the former are carrying the call signalling in the phases when the traffic channel has not yet been activated for that call; the latter are active in conjunction with any channel activated (either signalling or traffic channel) and are basically devoted to transport measurement and handover-related information. SDCCH carries the signalling procedure otside the activation of a traffic channel SACCH and FACCH carry the control information associated to traffic (TCH) and signalling channels (SDCCH).
  • The figure summarises the basic concepts driving the possible radio access schemes: FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) e CDMA (Code Division Multiple Access). For each, the related radio resource usage mechanisms are highlighted in the time, frequency and power domains. In the light of the most recent developments, the most innovative access technique in the field of mobile services is the Code Division Multiple Access (CDMA) technique. In January 1998 ETSI took an important decision about the radio access technique to be used for UMTS: for the asymmetrical portion of the spectrum available for UMTS, a hybrid solution based on a TDMA + narrow band CDMA scheme has been chosen for the symmetrical portion of the spectrum, a wide band CDMA scheme has been adopted. The former solution is thought mainly for in-door applications, the latter for public/outdoor/high mobility services. The Code Division Multiple Access (CDMA) belongs to the family of spread spectrum techniques. The following figures are recalling just some of the basic concepts on which the CDMA technique is based; this is done just to supply some background information to facilitate the comprehension of the subsequent network considerations.
  • Transcript

    • 1. CS 218 Fall 2003 October 23, 2003
      • Cellular Wireless Networks
        • AMPS (Analog)
        • D-AMPS (TDMA)
        • GSM
        • CDMA
      • Reference: Tanenbaum Chpt 2 (pg 153-169)
    • 2. Cellular Wireless Network Evolution
      • First Generation : Analog
        • AMPS: Advance Mobile Phone Systems
        • Residential cordless phones
      • Second Generation : Digital
        • IS-54: North American Standard - TDMA
        • IS-95: CDMA (Qualcomm)
        • GSM: Pan-European Digital Cellular
        • DECT: Digital European Cordless Telephone
    • 3. Cellular Evolution (cont)
        • Third Generation: T/CDMA
        • combines the functions of: cellular, cordless, wireless LANs, paging etc.
        • supports multimedia services (data, voice, video, image)
        • a progression of integrated, high performance systems:
        • (a) GPRS (for GSM)
        • (b) EDGE (for GSM)
        • (c) 1xRTT (for CDMA)
        • (d) UMTS
    • 4.  
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    • 7. Digital Cellular Systems World-wide GSM D-AMPS Japan Digital PCS 1900 DCS 1800 CDMA
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    • 13. Cellular Concept
      • Geographical separation
      • Capacity (frequency) reuse
      • Backbone connectivity
      BS BS BS BS BS BS Backbone Network
    • 14. Characteristics of Radio Medium
      • Path Loss
        • Attenuation increases with respect to frequency and distance
          • Free space loss = square of distance
          • Indoor mobile radio = fourth power of distance
      R1 Distance Distance Transmitter 10 100 1000 Power 40 dB per decade 20 dB per decade
    • 15. Characteristics of Radio Medium (cont’d)
      • Fading
        • Multipath fading
        • Shadowing
      • Delay spread
        • Intersymbol interference
      • Interference
        • Across channels
      Object Receiver Power dB Deep fade - 40 dB Transmitter
    • 16. Invented by Bell Labs; installed In US in 1982; in Europe as TACS
    • 17. AMPS (Advance Mobile Phone System):
      • In each cell, 57 channels each for A-side and B -side carrier
      • respectively; about 800 channels total (across the entire AMPS system)
      Frequency Reuse : Frequencies are not reused in a group of 7 adjacent cells FDMA (Frequency Div Multiple Access): one frequency per user channel B A E D C F G B A E D C F G B A E D C F G
    • 18. Advanced Mobile Phone System
      • (a) Frequencies are not reused in adjacent cells.
      • (b) To add more users, smaller cells can be used.
    • 19. Channel Categories
      • The channels are divided into four categories:
      • Control (base to mobile) to manage the system
      • Paging (base to mobile) to alert users to calls for them
      • Access (bidirectional) for call setup and channel assignment
      • Data (bidirectional) for voice, fax, or data
    • 20. Handoff
      • Handoff : Transfer of a mobile from one cell to another
      • Each base station constantly monitors the received power from each mobile.
      • When power drops below given threshold, base station asks neighbor station (with stronger received power) to pick up the mobile, on a new channel.
      • In APMS the handoff process takes about 300 msec.
      • Hard handoff : user must switch from one frequency to another (noticeable disruption)
      • Soft Handoff (available only with CDMA): no change in frequency.
    • 21. To register and make a phone call
      • When phone is switched on , it scans a preprogrammed list of 21 control channels, to find the most powerful signal.
      • It transmits its ID number on it to the MSC – which informs the home MSC (registration is done every 15 min)
      • To make a call, user transmits dest Ph # on random access channel; MSC will assign a data channel
      • At the same time MSC pages the destination cell for the other party (idle phone listens on all page channels)
    • 22. (Freq Division Duplex)
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    • 26. Digital Cellular: IS-54 TDMA System
      • Second generation: digital (as opposed to analog as in AMPS)
      • Same frequency as AMPS
      • Each 30 kHz RF channel is used at a rate of 48.6 kbps
        • 6 TDM slots/RF band (2 slots per user)
        • 8 kbps voice coding
        • 16.2 kbps TDM digital channel (3 channels fit in 30kHz)
      • 4 cell frequency reuse (instead of 7 as in AMPS)
      • Capacity increase per cell per carrier
        • 3 x 416 / 4 = 312 (instead of 57 in AMPS)
        • Additional factor of two with speech activity detection.
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    • 33. IS-54 slot and frame structure BASE TO MOBILE SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6 Frame 1944 bits in 40 ms( 48600 b/s) G6 R6 DATA16 SYNC28 DATA122 SACCH 12 DVCC 12 DATA122 MOBILE TO BASE DATA130 DATA130 DVCC 12 SACCH 12 SYNC28 RSVD 12 G:GUARD TIME R:RAMP TIME DVCC: DIGITAL VERIFFICATION COLOR CODE RSVD: RESERVE FOR FUTURE USE
    • 34.  
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    • 39. GSM (Group Speciale Mobile)
      • Pan European Cellular Standard
      • Second Generation: Digital
      • Frequency Division Duplex (890-915 MHz Upstream; 935-960 MHz Downstream)
      • 125 frequency carriers
      • Carrier spacing : 200 Khz
      • 8 channels per carrier (Narrowband Time Division)
      • Speech coder : linear predictive coding (Source rate = 13 Kbps)
      • Modulation : phase shift keying (Gaussian minimum shift keying)
      • Slow frequency hopping to overcome multipath fading
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    • 44. (To combat multipath fading)
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      • BCCH: Broadcast Control Channel
        • point-to-multipoint unidirectional control channel broadcasting system information to MS
      • CCCH: Common Control Channel
          • up-link: RACH (Random Access CHannel)
          • down-link: PCH (Paging Channel)
              • AGCH (Access Grant CHannel)
      • DCCH: Dedicated Control CHannel
        • point-to-point bidirectional control channel
        • SACCH (Slow Associated Control CHannel)
        • FACCH (Fast Associated Control CHannel)
        • SDCCH (Stand Alone Dedicated Control CHannel)
      GSM Signalling channels
    • 54. CDMA (Code Division Multiple Access): IS-95 QUALCOMM, San Diego
      • Based on DS spread spectrum
      • Two frequency bands (1.23 Mhz), one for forward channel (cell-site to subscriber) and one for reverse channel (sub to cell-site)
      • CDMA allows reuse of same spectrum over all cells. Net capacity improvement:
        • 4 to 6 over digital TDMA (eg. GSM)
        • 20 over analog FM/FDMA (AMPS)
    • 55. Access techniques for mobile communications P - Power T - Time F - Frequency P T P T F P T F FDMA (TACS) TDMA (GSM, DECT) CDMA (UMTS) F ATDMA (UMTS)
    • 56. CDMA (Code Division Multiple Access)
      • unique “code” assigned to each user; i.e., code set partitioning
      • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data
      • Note: chipping rate >> data rate (eg, 64 chips per data bit)
      • encoded signal = (original data bit) X (chipping sequence)
      • decoding: inner-product of encoded signal and chipping sequence
      • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)
    • 57. CDMA Encode/Decode
    • 58. CDMA: two-sender interference
    • 59. CDMA (cont’d)
      • One of 64 PS (Pseudo Random) codes assigned to subscriber at call set up time
      • RAKE receiver (to overcome multi path-fading)
      • Pilot tone inserted in forward link for:
        • power control
        • coherent reference
      • Speech activity detection
      • Voice compression to 8 kbps (16 kbps with FEC)
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