PHYSICAL LAYER

6.2 Transport Channels and their Mapping to the Physical Channels

6.2 TRANSPORT CHANNELS AND THEIR MAPPING TO THE PHYSICAL CHANNELSIn UTRAthe data generated at higher layers is carried over with transport channelswhich are mapped in the physical layer to different physical channelsPhysical layersupport variable bit rate transport channels to offer bandwidth-on-demand servicesmultiplex several services to one connection

Transport channelsmay have a different number of blocksat any moment not all the transport channels are necessarily activeEach transport channelaccompanied by Transport Format Indicator (TFI)TFIindicates the local UMTS air interface transport format to be used for the transmission time interval

Physical layercombines TFI information from different transport channels to Transport Format Combination Indicator (TFCI)TFCItransmitted in physical control channel to inform the receiver which transport channels are active for the current framedecoded in the receiver and the resulting TFI is given to higher layers for each of the transport channels that can be active for the connection

Figure 6.1two transport channels are mapped to a single physical channelerror indication is provided for each transport block

Coded Composite Transport Channel (CCTrCh) a technology used in the UMTS physical layerthe connection between Transport Channel and Physical Channel which results a data stream from encoding and multiplexing of one or several transport channels CCTrCh consists of one physical control channelone or more physical data channels

Two types of transport channeldedicated channelscommon channelsDedicated channela resource, identified by a certain code on a certain frequency, is reserved for a single user onlyCommon channela resource, divided between all or a group of users in a cell

6.2.1 Dedicated Transport Channel6.2.2 Common Transport Channels6.2.3 Mapping of Transport Channels onto the Physical Channels6.2.4 Frame Structure of Transport Channels

6.2.1 DEDICATED TRANSPORT CHANNELThe only dedicated transport channeldedicated channel (DCH)Dedicated transport channelcarries all the information intended for the given user coming from layers above the physical layer

Dedicated transport channel carries bothservice datasuch as speech frameshigher layer control informationsuch as handover commands or measurement reports from the terminalDedicated transport channel is characterized by fast power controlfast data rate change on a frame-by-frame basisthe possibility of transmission to a certain part of the cell or sectorDedicated channel supports soft handover

6.2.2 COMMON TRANSPORT CHANNELSSix different common transport channel types defined for UTRA in Release 99Broadcast Channel (BCH)Forward Access Channel (FACH)Paging Channel (PCH)Random Access Channel (RACH)Uplink Common Packet Channel (CPCH)Downlink Shared Channel (DSCH)

Common channelsdo not have soft handoversome of them can have fast power controlThe new transport channel in Release 5High-speed Downlink Shared Channel (HS-DSCH)

6.2.2.1 Broadcast Channel6.2.2.2 Forward Access Channel6.2.2.3 Paging Channel6.2.2.4 Random Access Channel6.2.2.5 Uplink Common Packet Channel6.2.2.6 Downlink Shared Channel6.2.2.7 Required Transport Channels

6.2.2.1 BROADCAST CHANNELBroadcast Channel (BCH)used to transmit information specific to the UTRA network or for a given cellneeded for transmission with relatively high power to reach all the users within the intended coverage areaThe information rate on the broadcast channel is limited by the ability of low-end terminals to decode the dataresulting in a low and fixed data rate for the UTRA broadcast channel

6.2.2.2 FORWARD ACCESS CHANNELForward Access Channel (FACH)a downlink transport channel that carries control information to terminals known to be located in the given cellthis is used, for example, after a random access message has been received by the base stationIt is possible to transmit packet data on FACH

There can be more than one FACH in a cellone of the forward access channels must have such a low bit rate that it can be received by all the terminals in the cell areawith more than one FACH, the additional channels can have a higher data rateFACH does not use fast power control

6.2.2.3 PAGING CHANNELPaging Channel (PCH)a downlink transport channel that carries data relevant to the paging procedurei.e., when the network wants to initiate communication with the terminale.g., a speech call to the terminalthe network transmits paging message to the terminal on the paging channel of the cells

The terminals must be able to receive the paging information in the whole cell areaDesign of the paging channel affects the terminals power consumption in standby modethe less often the terminal has to tune the receiver in to listen for a possible paging message, the longer the terminals battery will last in standby mode

6.2.2.4 RANDOM ACCESS CHANNELRandom Access Channel (RACH)an uplink transport channel to be used to carry control information from the terminal, such as requests to set up a connectioncan also be used to send small amounts of packet data from terminal to networkThe random access channel must be heard from the whole cell coverage areawhich means that practical data rates have to be rather low

6.2.2.5 UPLINK COMMON PACKET CHANNELUplink common packet channel (CPCH)an extension to the RACH channel that is intended to carry packet-based user data in the uplink directionThe reciprocal channel providing the data in the downlink direction is the FACH

In physical layer, the main differences to RACHuse of fast power controla physical layer-based collision detection mechanism a CPCH status monitoring procedureUplink transmissionCPCH datalast several framesRACH messageone or two frames

6.2.2.6 DOWNLINK SHARED CHANNELDownlink shared channel (DSCH)used to carry dedicated user data and/or control informationcan be shared by several usersIt is similar to the forward access channel, but shared channel supportsfast power controlvariable bit rate on a frame-by-frame basisDSCHdoes not need to be heard in the whole cell areaalways associated with a downlink DCH

6.2.2.7 REQUIRED TRANSPORT CHANNELSMandatory common transport channelsRACH, FACH and PCHOptional common transport channelsDSCH and CPCH

6.2.3 MAPPING OF TRANSPORT CHANNELS ONTO THE PHYSICAL CHANNELSDifferent transport channels are mapped to different physical channelsthough some of the transport channels are carried by identical (or even the same) physical channel

In addition to the transport channelsthere exist physical channels to carry only information relevant to physical layer proceduresThose channels that are not directly visible to higher layers and are mandatory from the system function point of view, to be transmitted from every base stationSynchronization Channel (SCH)Common Pilot Channel (CPICH)Acquisition Indication Channel (AICH)

Those channels are needed if CPCH is usedCPCH Status Indication Channel (CSICH)Collision Detection/Channel Assignment Indication Channel (CD/CA-ICH)

Dedicated channel (DCH) is mapped onto two physical channelsDedicated Physical Data Channel (DPDCH)carries higher layer information, including user dataDedicated Physical Control Channel (DPCCH) carries the necessary physical layer control information

these two dedicated physical channels are needed to support efficiently the variable bit rate in the physical layerthe bit rate of the DPCCH is constant, while the bit rate of DPDCH can change from frame to frame

6.2.4 FRAME STRUCTURE OF TRANSPORT CHANNELSUTRA channels use a 10ms radio frame structureThe frame structure also employs a longer period, called the system frame periodSystem Frame Number (SFN)a 12-bit number and is used by procedures that span more than a single framePhysical layer procedures, such as the paging procedure or random access procedure, are examples of procedures that need a longer period than 10ms

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