PHYSICAL LAYER

6.2 Transport Channels and their Mapping to the Physical Channels

6.2 TRANSPORT CHANNELS AND THEIR MAPPING TO THE PHYSICAL CHANNELS In UTRA the data generated at higher layers is carried over with transport channels which are mapped in the physical layer to different physical channels Physical layer support variable bit rate transport channels to offer bandwidth-on-demand services multiplex several services to one connection

Transport channels may have a different number of blocks at any moment not all the transport channels are necessarily active Each transport channel accompanied by Transport Format Indicator (TFI) TFI indicates the local UMTS air interface transport format to be used for the transmission time interval

Physical layer combines TFI information from different transport channels to Transport Format Combination Indicator (TFCI) TFCI transmitted in physical control channel to inform the receiver which transport channels are active for the current frame decoded 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 6.1 two transport channels are mapped to a single physical channel error indication is provided for each transport block

Coded Composite Transport Channel (CCTrCh) a technology used in the UMTS physical layer the 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 channel one or more physical data channels

Two types of transport channel dedicated channels common channels Dedicated channel a resource, identified by a certain code on a certain frequency, is reserved for a single user only Common channel a resource, divided between all or a group of users in a cell

6.2.1 Dedicated Transport Channel Common Transport Channels Mapping of Transport Channels onto the Physical Channels Frame Structure of Transport Channels

6.2.1 DEDICATED TRANSPORT CHANNEL The only dedicated transport channel ： dedicated channel (DCH) Dedicated transport channel carries all the information intended for the given user coming from layers above the physical layer

Dedicated transport channel carries both service data such as speech frames higher layer control information such as handover commands or measurement reports from the terminal Dedicated transport channel is characterized by fast power control fast data rate change on a frame-by-frame basis the possibility of transmission to a certain part of the cell or sector Dedicated channel supports soft handover

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

Common channels do not have soft handover some of them can have fast power control The new transport channel in Release 5 High-speed Downlink Shared Channel (HS-DSCH)

Broadcast Channel Forward Access Channel Paging Channel Random Access Channel Uplink Common Packet Channel Downlink Shared Channel Required Transport Channels

BROADCAST CHANNEL Broadcast Channel (BCH) used to transmit information specific to the UTRA network or for a given cell needed for transmission with relatively high power to reach all the users within the intended coverage area The information rate on the broadcast channel is limited by the ability of low-end terminals to decode the data resulting in a low and fixed data rate for the UTRA broadcast channel

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

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

PAGING CHANNEL Paging Channel (PCH) a downlink transport channel that carries data relevant to the paging procedure i.e., when the network wants to initiate communication with the terminal e.g., a speech call to the terminal the 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 area Design of the paging channel affects the terminal’s power consumption in standby mode the less often the terminal has to tune the receiver in to listen for a possible paging message, the longer the terminal’s battery will last in standby mode

RANDOM ACCESS CHANNEL Random Access Channel (RACH) an uplink transport channel to be used to carry control information from the terminal, such as requests to set up a connection can also be used to send small amounts of packet data from terminal to network The random access channel must be heard from the whole cell coverage area which means that practical data rates have to be rather low

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

In physical layer, the main differences to RACH use of fast power control a physical layer-based collision detection mechanism a CPCH status monitoring procedure Uplink transmission CPCH data ： last several frames RACH message ： one or two frames

DOWNLINK SHARED CHANNEL Downlink shared channel (DSCH) used to carry dedicated user data and/or control information can be shared by several users It is similar to the forward access channel, but shared channel supports fast power control variable bit rate on a frame-by-frame basis DSCH does not need to be heard in the whole cell area always associated with a downlink DCH

REQUIRED TRANSPORT CHANNELS Mandatory common transport channels RACH, FACH and PCH Optional common transport channels DSCH and CPCH

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

In addition to the transport channels there exist physical channels to carry only information relevant to physical layer procedures Those 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 station Synchronization Channel (SCH) Common Pilot Channel (CPICH) Acquisition Indication Channel (AICH)

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

Dedicated channel (DCH) is mapped onto two physical channels Dedicated Physical Data Channel (DPDCH) carries higher layer information, including user data Dedicated 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 layer the 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 CHANNELS UTRA channels use a 10ms radio frame structure The frame structure also employs a longer period, called the system frame period System Frame Number (SFN) a 12-bit number and is used by procedures that span more than a single frame Physical layer procedures, such as the paging procedure or random access procedure, are examples of procedures that need a longer period than 10ms