5.2 Layer 1 Services and Functions

25.3013GPPRadio interface protocol architectureTS

This subclause shall provide an overview on services and functions provided by the physical layer. A detailed description of Layer 1 general requirements can be found in [4].

5.2.1 L1 Services

The physical layer offers information transfer services to MAC and higher layers. The physical layer transport services are described by how and with what characteristics data are transferred over the radio interface. An adequate term for this is ‘Transport Channel’.

NOTE: This should be clearly separated from the classification of what is transported, which relates to the concept of logical channels. Thus DCH is used to denote that the physical layer offers the same type of service for both control and traffic.

5.2.1.1 Transport channels

A general classification of transport channels is into two groups:

– common transport channels (where there is a need for inband identification of the UEs when particular UEs are addressed); and

– dedicated transport channels (where the UEs are identified by the physical channel, i.e. code and frequency for FDD and code, time slot and frequency for TDD).

Common transport channel types are (a more detailed description can be found in [4]):

– Random Access Channel (RACH)

A contention based uplink channel used for transmission of relatively small amounts of data, e.g. for initial access or non-real-time dedicated control or traffic data.

– Forward Access Channel (FACH)

Common downlink channel without closed-loop power control used for transmission of relatively small amount of data. In addition FACH is used to carry broadcast and multicast data.

– Downlink Shared Channel (DSCH)

A downlink channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

– Uplink Shared Channel (USCH)

An uplink channel shared by several UEs carrying dedicated control or traffic data, used in TDD mode only.

– Broadcast Channel (BCH)

A downlink channel used for broadcast of system information into an entire cell.

– Paging Channel (PCH)

A downlink channel used for broadcast of control information into an entire cell allowing efficient UE sleep mode procedures. Currently identified information types are paging and notification. Another use could be UTRAN notification of change of BCCH information.

– High Speed Downlink Shared Channel (HS-DSCH)

A downlink channel shared between UEs by allocation of individual codes, from a common pool of codes assigned for the channel.

– Enhanced Dedicated Channel (E-DCH) (FDD and 1.28 Mcps TDD only)

For FDD, in CELL_FACH state and IDLE mode, this channel is used for uplink transmission and is shared between UEs by allocation of individual codes, from a common pool of codes assigned for the channel. For 1.28 Mcps TDD, in CELL_FACH state and IDLE mode, this channel is used for uplink transmission and is shared by UEs in CELL_FACH, CELL_DCH and Idle mode. The common E-DCH is subject to Node-B controlled scheduling and HARQ.

Dedicated transport channel types are:

– Dedicated Channel (DCH)

A channel dedicated to one UE used in uplink or downlink.

– Enhanced Dedicated Channel (E-DCH)

In CELL_DCH, a channel dedicated to one UE used in uplink only. The E-DCH is subject to Node-B controlled scheduling and HARQ.

To each transport channel, there is an associated Transport Format (for transport channels with a fixed or slow changing rate) or an associated Transport Format Set (for transport channels with fast changing rate). A Transport Format is defined as a combination of encodings, interleaving, bit rate and mapping onto physical channels (see [4] for details). A Transport Format Set is a set of Transport Formats. E.g., a variable rate DCH has a Transport Format Set (one Transport Format for each rate), whereas a fixed rate DCH has a single Transport Format.

5.2.2 L1 Functions

The physical layer performs the following main functions:

– Macrodiversity distribution/combining and soft handover execution;

– Error detection on transport channels and indication to higher layers;

– FEC encoding/decoding and interleaving/deinterleaving of transport channels;

– Multiplexing of transport channels and demultiplexing of coded composite transport channels;

– Rate matching;

– Mapping of coded composite transport channels on physical channels;

– Power weighting and combining of physical channels;

– Modulation and spreading/demodulation and despreading of physical channels;

– Frequency and time (chip, bit, slot, frame) synchronisation;

– Measurements and indication to higher layers (e.g. FER, SIR, interference power, transmit power, etc.);

– Closed-loop power control;

– RF processing;

– Support of timing advance on uplink channels (TDD only);

– Support of Uplink Synchronisation as defined in [12] (TDD only).