Elaborate on forward W-CDMA channel.

Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

1 Answer

WCDMA uses Direct Sequence Spread Spectrum multiple access technique. The multiple communication channels can be shared by multiple users using different orthogonal codes. There are Logical channels, Transport channels and physical channels in WCDMA.

I. Logical WCDMA channels: The MAC layer provides data transfer services on logical channels. A set of logicalchannel types is defined for different kinds of data transfer services as offered by MAC. Each logical channel type is defined by the type of information that is transferred. There are of two types- signaling and control logical channels and traffic logical channels. The forward Logical channels are as follows:

A. Forward Logical control channels in WCDMA:

  • Broadcast control channel (BCCH): Downlink channel for broadcasting system controlinformation.

  • Paging control channel (PCCH): Downlink channel that transfers paging information and isused when:

    • Network does not know the location cell of the mobilestation.

    • The mobile station is in the cell connected state (utilizing sleep mode procedures)

  • Common control channel (CCCH): Bidirectional channel that transferscontrol informationbetween network and mobile stations. This channel is used:

    • By the mobile stations having no RRC connection with the network

    • By the mobile stations using common transport channels when accessing a new cell after cell reselection

  • Dedicated control channel (DCCH): Point-to-point bidirectional channel that transmits dedicatedcontrol information between a mobile station and the network.This channel is established through RRC connection setupprocedure.

  • ODMA common control channel (OCCCH):Bidirectional channel for transmitting control informationbetween mobile stations.

  • ODMA dedicated control channel (ODCCH):Point-to-point bidirectional channel that transmits dedicatedcontrol information between mobile stations. This channel isestablished through RRC connection setup procedure.

B. Forward Logical traffic channels in WCDMA:

  • Dedicated traffic channel (DTCH): Point-to-point channel, dedicated to one mobilestation, for the transfer of user information. ADTCH can exist in both uplink and downlink.

  • ODMA dedicated traffic channel (ODTCH): Point-to-point channel,dedicated to one mobilestation, for the transfer of user informationbetween mobile stations. An ODTCH exists inrelay link. A point-to-multipoint unidirectionalchannel for transfer of dedicated user informationfor all or a group of specified mobile stations.

II. Transport WCDMAchannels: A transport channel is defined by how and with what characteristics data is transferred over the air interface. There exist two types of transport channels- Dedicated Transport channels and Common Transport channels.

A. Dedicated Transport channels:

There is one dedicated transport channel, the dedicated channel (DCH), which isa downlink or uplink transport channel. The DCH is transmitted over the entire cell orover only a part of the cell using beam-forming antennas. The DCH is characterized bythe possibility of fast rate change (every 10 ms), fast power control, and inherentaddressing of mobile stations.

B. Forward Common Transport channels:

  • Broadcast channel (BCH): Downlink transport channel that is used tobroadcast system- and cell-specific information.The BCH is always transmitted over the entirecell with a low fixed bit rate.

  • Forward access channel (FACH): Downlink transport channel. The FACH istransmitted over the entire cell or over only a partof the cell using beam-forming antennas. TheFACH uses slow power control.

  • Paging channel (PCH): Downlink transport channel. The PCH is alwaystransmitted over the entire cell. The transmissionof the PCH is associated with the transmission ofa physical layer signal, the paging indicator, tosupport efficient sleep mode procedures.

  • Downlink shared channel (DSCH): Downlink transport channel shared by several mobile stations. The DSCH is associated with aDCH.

III. Physical WCDMAchannels: Physical channels consist of radio frames and time slots. The length of a radio frame is 10 ms and one frame consists of 15 time slots. A time slot is a unit, which consists of fields containing bits. The number of bits per time slot depends on the physical channel. Depending on the symbol rate of the physical channel, the configuration of radio frames or time slots varies. The basic physical resource is the code/frequency plane. There are three types of forward Physical channels- Dedicated physical channel, Shared Physical channel, Common control Physical channel.

A. Dedicated physical channel:

On the DPCH, thededicated transport channel is transmitted time multiplexed with control informationgenerated at layer 1 (known pilot bits, power-control commands, and an optionaltransport-format combination indicator). DPCH can contain several simultaneousservices when TFCI is transmitted or a fixed rate service when TFCI is not transmitted.

B. Physical downlink shared channel (PDSCH):

Itcarries DSCH shared by users based on code multiplexing associated with DPCH.

C. Primary common control physical channel (PCCPCH):

It carries BCH, rate30 kbps, SF =256. It supports continuous transmission. There is no power control.

D. Secondary common control physical channel (SCCPCH):

It carries FACH and PCH. It is transmitted when data is available. SF range is from 256 to 4.

E. Synchronization channel (SCH):

It is used for cell search. It consists of two sub-channels- primary SCH transmits a modulated code of 256 chips once every slot and secondary SCH transmits repeatedly 15 codes of 256 chips.

F. Acquisition indicator channel (AICH):

It carries acquisition indicators.Page indicator channel (PICH) carries a page for UE at fixed rate, SF =256.

G. Primary common pilot channel (PCPICH):

It is used as phase reference forSCH, PCCPCH, AICH, PICH, and default phase reference for all other DL physical channels. It is one per cell and broadcast over entire cell.

H. Secondary common pilot channel (SCPICH):

It is a continuous channel withthe same spreading and scrambling codes, transmitted on different antennasin case of DL transmit diversity SF =256. It may be transmitted over onlypart of the cell.

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