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2G ::3G

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These multipath signals can be received and combined using a RAKE receiver. It consists of correlators, also known as RAKE fingers, each receiving a multipath signal. ... – PowerPoint PPT presentation

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Title: 2G ::3G


1
W-CDMA and RNC
2
W-CDMA
  • W-CDMA stands for Wideband Code Division
    Multiple Access.
  • It is based on IMT-2000 specification and this
    specification is developed by 3GPP.
  • It is the main third generation air interface in
    the world.
  • 3G mobile communication systems are referred as
    UMTS (Universal Mobile Telecommunication System).
  • W-CDMA is targeted at 2GHz.
  • Within 3GPP, W-CDMA is called UTRA (Universal
    Terrestrial Radio Access) FDD(Frequency Division
    Duplex) and TDD(Time Division Duplex).
  • W-CDMA is completely a new air interface without
    any constraints from the past.
  • Data Rate
  • Circuit Switched 384 kbps at hot spots and 144
    kbps in wide coverage.
  • Packet Switched 2Mbps

3
W-CDMA cont
  • WCDMA uses a digital wideband spread spectrum
    technology to transmit multiple independent
    conversations across single or multiple 5MHz
    segments of radio spectrum.
  • All of signals power are spread simultaneously
    over a wide frequency band.
  • The spread signal would then appear in a noise
    like signal scattered over the same wide
    transmission band with a much lower power per
    bandwidth.
  • Each user is tagged with a distinctive
    Pseudo-random Noise (PN) code that only the
    users receiver can recognize.
  • Receiver knows in advance how the transmitter
    will spread the spectrum. Based on this info, it
    compresses and reconstitutes the desired signal.
  • In nutshell,
  • In WCDMA, multiple users share the same
    frequency band at the same time. Each
    conversation is specially encoded and decoded for
    each particular user.
  • It uses spread-spectrum. The core principle of
    spread spectrum is the use of noise-like carrier
    waves.

4
W-CDMA cont
  • Each carrier has a bandwidth 5MHz. Four times
    than that of CDMA.
  • The large bandwidth is needed to support higher
    bit rates. Concept of Bandwidth on demand (BoD)
    is well supported.
  • Chip-rate is 3.84 Mcps. A chip is a bit in the
    code word, which is used to modulate the
    information signal.
  • Spreading factor ratio b/w chip rate and data
    rate.

5
W-CDMA cont
  • UTRA FDD mode
  • the uplink and downlink use separate freq bands.
  • in Europe and most of Asia, bands of 2 x 60 MHz
    ( 1920-1980MHz 2110-2170 MHz) are used
  • each carrier is divided into 10ms radio frames
  • UTRA TDD mode
  • both uplink and downlink use the same frequency
    carrier but different time slots.
  • in Europe and most of Asia, 25 MHz is available
    for TDD ( 1900-1920MHz 2020-2025 MHz) are used
  • each carrier is divided into 10ms radio frames
    and each frame further into 15 time slots.

6
High level 2G and 3G Architectures
7
Power Control
  • Near-far problem during uplink.
  • Optimum strategy is to equalize the received
    power per bit of all the MS at all the times.
  • Solution Fast closed-loop power control.
  • BS performs frequent estimates of received
    Signal-to-interference Ratio (SIR) and compares
    it to a target SIR.
  • If measured SIR is higher than the target SIR,
    base station commands the MS to lower the power
    and vice versa.
  • This measure-command-react cycle is executed at
    1.5KHz for each MS.
  • Same technique is also used on downlink.
  • Additional power to MS at the cell edge, as they
    suffer from increased other-cell interference.
  • Outer loop power control
  • RNC adjusts the target SIR setpoint in the base
    station according to the needs of the individual
    radio link and aims at a constant quality.
  • Target SIR depends on MS speed and multipath
    profile.

 
8
Rake Receiver
  • In a multipath channel, the original transmitted
    signal reflects off obstacles in its way, and the
    receiver receives several copies of the original
    signal with different delays.
  • These multipath signals can be received and
    combined using a RAKE receiver.
  • It consists of correlators, also known as RAKE
    fingers, each receiving a multipath signal. After
    despreading by correlators with a local copy of
    the appropriately delayed version of the
    transmitter's spreading code, the signals are
    combined.
  • Since the received multipath signals are fading
    independently, combined composite signals in a
    rake receiver improves the overall combined
    signal quality and performance.
  • Both MS and BS have rake receiver.

9
Handoffs
  • Handoff The process whereby a mobile station
    moves to a new traffic channel is called handoff
  • Soft hand-off Soft Handoff is a process in
    which the mobile is directed to hand off to the
    same frequency assigned to an adjacent cell
    without dropping the original RF link.The mobile
    keeps two RF links during the soft-handoff
    process.
  • Softer hand-off During softer handover, a MS is
    in the overlapping cell coverage area of two
    adjacent sectors of a base station.
  • Hard handoff occurs when two base stations are
    not synchronized or are not on the same frequency
    resulting in interruption in voice or data.
  • Inter-frequency hard hand-off
  • Inter-system hard hand-off

10
RNC
  • RNC stands for Radio Network Controller.
  • In 2G it corresponds to called Base Station
    Controller.
  • RNC is a key network element in emerging 3G
    wireless networks.
  • Provides the control functions and physical
    links
  • Between the SGSN/MSC and Node B
  • RNC interfaces the core network via lu interface
    and uses lub interface to control Node B.
  • Provides functions such as
  • call set-up and tear down
  • voice and data traffic processing
  • call handover
  • cell configuration data and control of RF power
    levels in base transceiver stations
  • Resource management of radio network and
    optimization of radio resources

11
Diff between RNC and BSC
sasharma
  • In RNC, lur --a new interface is added for
    performance in WCDMA networks and to make
    mobility transparent to the core network
  • RNCs has more radio functions as WCDMA requires
    more soft handovers between cells, base stations
    and RNCs.
  • RNC provides bridging functions for connecting to
  • IP packet-switched networks,
  • traditional ATM AAL2(voice) and AAL5 (data)
    functions,
  • IP over ATM (IPoATM) and
  • packet over SONET(POS).
  • Support for both IPv4 and IPv6.
  • RNC can be viewed as a heterogeneous networking
    device.
  • RNC handles more signaling per call.
  • More processing is required in RNC and thereby
    more processing power.

12
Diff between BSC and RNC cont
  • Most significant difference lies in the nature of
    processing
  • BSC transcoders with signal processors account
    for the majority of processing resources.
  • RNC the brunt of processing entails shuffling
    data using general-purpose processors.
  • Internal data paths
  • BSC works with strict multipexing architecture
  • RNC requires full switching capabilities.

13
Interface Reference Points of RNC
  • lub
  • Connects Node B transceivers and the RNC usually
    via an OC-3 link
  • lur
  • RNC to RNC connection for call handover, usually
    via an OC-3 link
  • lu-cs
  • Core network interface between the RNC and the
    circuit switched networks. Usually implemented as
    an OC-12 rate link.
  • lu-ps
  • Core network interface between the RNC and
    packet-switched data networks. Usually
    implemented as an OC-12 rate link.

14
Radio Access Network
  • Two roles of RNC
  • Serving RNC
  • It has the overall control of the MS that is
    connected to RAN. It controls the connection on
    the lu interface for the handset
  • Drift RNC
  • When a MS must use the resources in a cell not
    controlled by its Serving RNC, the Serving RNC
    must ask the controlling RNC for those resources.
  • This request is made via lur interface.
  • In this case, Controlling RNC is called Drift
    RNC for this particular handset.

15
Radio Access Bearer
  • The main service offered by WCDMA RAN is the
    Radio Access Bearer (RAB).
  • RAB carriers the subscriber data between the MS
    and the core network.
  • 3GPP has defined four different classes of RABs.
  • Conversational (used for e.g. voice telephony)
  • low delay, strict ordering
  • Streaming (used for e.g. watching a video clip)
  • moderate delay, strict ordering
  • Interactive (used for e.g. web surfing)
  • moderate delay
  • Background (used for e.g. file transfer)
  • -no delay requirement
  • Conversational and Streaming RABs require a
    reservation of resources in network and are
    primarily meant for real-time services.
  • Interactive and Background RABs are called best
    effort as they require no reservation of
    resources .
  • Core network asks the RNC to provide a RAB based
    on the service request.

16
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