4 General description of Layer 1

25.2013GPPPhysical layer - general descriptionRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.1 Relation to other layers

4.1.1 General Protocol Architecture

Radio interface which is prescribed by this specification means the Uu point between User Equipment (UE) and network. The radio interface is composed of Layers 1, 2 and 3. Layer 1 is based on WCDMA/TD-SCDMA technology and the TS 25.200 series describes the Layer-1 specification. Layers 2 and 3 of the radio interface are described in the TS 25.300 series.

Figure 1: Radio interface protocol architecture around the physical layer

Figure 1 shows the UTRA radio interface protocol architecture around the physical layer (Layer 1). The physical layer interfaces the Medium Access Control (MAC) sub-layer of Layer 2 and the Radio Resource Control (RRC) Layer of Layer 3. The circles between different layer/sub-layers indicate Service Access Points (SAPs). The physical layer offers different Transport channels to MAC. A transport channel is characterized by how the information is transferred over the radio interface. MAC offers different Logical channels to the Radio Link Control (RLC) sub-layer of Layer 2. A logical channel is characterized by the type of information transferred. Physical channels are defined in the physical layer. There are two duplex modes: Frequency Division Duplex (FDD) and Time Division Duplex (TDD). In the FDD mode a physical channel is characterized by the code, frequency and in the uplink the relative phase (I/Q); in addition E-HICH and E-RGCH are also defined by a specific orthogonal signature sequence. In the TDD mode the physical channels are also characterized by the timeslot and additionally, the E-HICH is further defined by a specific orthogonal signature sequence. For 1.28Mcps TDD, the E-HICH can be also further defined by a group of orthogonal signature sequences. The physical layer is controlled by RRC.

4.1.2 Service provided to higher layers

The physical layer offers data transport services to higher layers. The access to these services is through the use of transport channels via the MAC sub-layer. The physical layer is expected to perform the following functions in order to provide the data transport service. See also TS 25.302:

– Macrodiversity distribution/combining and soft handover execution.

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

– FEC encoding/decoding of transport channels.

– Multiplexing of transport channels and demultiplexing of coded composite transport channels (CCTrCHs).

– Rate matching of coded transport channels to physical channels.

– 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.

– Radio characteristics measurements including FER, SIR, Interference Power, etc., and indication to higher layers.

– Inner – loop power control.

– RF processing. (Note: RF processing is defined in TS 25.100 series).

– synchronization shift control

– Beamforming

– MIMO transmission

– Hybrid ARQ soft-combining for HS-DSCH and E-DCH

When network elements (UEs and network) provide compatible service bearers (for example support a speech bearer) they should be assured of successful interworking. Moreover, different implementation options of the same (optional) feature would lead to incompatibility between UE and network. Therefore, this shall be avoided.

4.2 General description of Layer 1

4.2.1 Multiple Access

The access scheme is Direct-Sequence Code Division Multiple Access (DS-CDMA) with information either spread over approximately 5 MHz (FDD and 3.84 Mcps TDD) bandwidth, thus also often denoted as Wideband CDMA (WCDMA) due that nature, 10MHz (7.68 Mcps TDD) bandwidth, or 1.6MHz (1.28Mcps TDD), thus also often denoted as Narrowband CDMA. UTRA has two modes, FDD (Frequency Division Duplex) & TDD (Time Division Duplex), for operating with paired and unpaired bands respectively. The possibility to operate in either FDD or TDD mode allows for efficient utilisation of the available spectrum according to the frequency allocation in different regions. FDD and TDD modes are defined as follows:

FDD: A duplex method whereby uplink and downlink transmissions use two separated radio frequencies. In the FDD, each uplink and downlink uses the different frequency band. A pair of frequency bands which have specified separation shall be assigned for the system.

TDD: A duplex method whereby uplink and downlink transmissions are carried over same radio frequency by using synchronised time intervals. In the TDD, time slots in a physical channel are divided into transmission and reception part. Information on uplink and downlink are transmitted reciprocally.

UTRA TDD has three options, the 3.84Mcps option and the 1.28Mcps option, the 7.68Mcps TDD option. In UTRA TDD there is TDMA component in the multiple access in addition to DS-CDMA. Thus the multiple access has been also often denoted as TDMA/CDMA due to the added TDMA nature.

A 10 ms radio frame is divided into 15 slots (2560 chip/slot at the chip rate 3.84 Mcps). A physical channel is therefore defined as a code (or number of codes) and additionally in TDD mode the sequence of time slots completes the definition of a physical channel. In FDD, for HS-DSCH, E-DCH and associated signalling channels, 2ms sub-frames consisting of 3 slots are defined.

The information rate of the channel varies with the symbol rate being derived from the 3.84 Mcps chip rate and the spreading factor. Spreading factors are from 256 to 2 with FDD uplink, from 512 to 4 with FDD downlink, and from 16 to 1 for TDD uplink and downlink. Thus the respective modulation symbol rates vary from 1920 k symbols/s to 15 k symbols/s (7.5 k symbols/s) for FDD uplink (downlink), and for TDD the momentary modulation symbol rates shall vary from 3.84 M symbols/s to 240 k symbols/s.

For the 7.68Mcps TDD option, a 10 ms radio frame is divided into 15 slots (5120 chip/slot). A physical channel is therefore defined as a code (or number of codes) and the sequence of time slots.

The information rate of the channel varies with the symbol rate being derived from the 7.68 Mcps chip rate and the spreading factor. Spreading factors are from 32 to 1 for both uplink and downlink. Thus the respective modulation symbol rates vary from 7.68 M symbols/s to 240 k symbols/s.

For 1.28Mcps TDD option, a 10 ms radio frame is divided into two 5ms sub-frames. In each sub-frame, there are 7 normal time slots and 3 special time slots. Note that in case of entire carrier dedicated to MBSFN there are 7 normal MBSFN Traffic time slots and 1 short MBSFN Special time slot in each sub-frame. A basic physical channel is therefore characterised by the frequency, code and time slot.

The information rate of the channel varies with the symbol rate being derived from the 1.28 Mcps chiprate and the spreading factor. Spreading factors is from 16 to 1 for both uplink and downlink. Thus the respective modulation symbol rates shall vary from 80.0K symbols/s to 1.28M symbols/s.

4.2.2 Channel coding and interleaving

For the channel coding in UTRA two options are supported for FDD and three options are supported for TDD:

– Convolutional coding.

– Turbo coding.

– No coding (only TDD).

Channel coding selection is indicated by higher layers. In order to randomise transmission errors, bit interleaving is performed further.

4.2.3 Modulation and spreading

The UTRA modulation scheme is QPSK (8PSK is also used for 1.28Mcps TDD option). For HS-DSCH transmission, 16QAM and 64QAM can also be used. 16QAM and 64QAM are further supported for E-DCH transmission, and 16QAM is supported for MBSFN^[1] FACH transmissions. Pulse shaping is specified in the TS 25.100 series.

With CDMA nature the spreading (& scrambling) process is closely associated with modulation. In UTRA different families of spreading codes are used to spread the signal:

– For separating channels from same source, channelisation codes derived with the code tree structure as given in TS 25.213 and 25.223 are used.

– For separating different cells the following solutions are supported.

– FDD mode: Gold codes with 10 ms period (38400 chips at 3.84 Mcps) used, with the actual code itself length 218-1 chips, as defined in TS 25.213.

– TDD mode: for the 3.84Mcps and 1.28Mcps TDD options, scrambling codes with the length 16 are used as defined in TS 25.223; for the 7.68Mcps TDD option, scrambling codes with length 32 are used.

– For separating different UEs the following code families are defined.

– FDD mode: Gold codes with 10 ms period, or alternatively S(2) codes 256 chip period.

– TDD mode: for the 3.84Mcps and 1.28Mcps TDD options, codes with period of 16 chips and midamble sequences of different length depending on the environment; for the 7.68Mcps TDD option, codes with period of 32 chips and midamble sequences of different length depending on the environment.

4.2.4 Physical layer procedures

There are several physical layer procedures involved with UTRA operation. Such procedures covered by physical layer description are:

1) The power control, inner loop for FDD mode, and for the 3.84Mcps TDD and 7.68Mcps TDD options open loop in uplink and inner loop in downlink, for 1.28Mcps TDD option, open loop in uplink and inner loop in both uplink and downlink.

2) Cell search operation.

3) Uplink synchronization control with open and closed loop.

4) Random access

5) Procedures related to HS-DSCH transmission, including HS-SCCH less operation and MIMO transmission.

6) Procedures related to E-DCH transmission, including MIMO transmission.

7) Procedures related to discontinuous transmission and reception.

4.2.5 Physical layer measurements

Radio characteristics including FER, SIR, Interference power, etc., are measured and reported to higher layers and network. Such measurements are:

1) Handover measurements for handover within UTRA. Specific features being determined in addition to the relative strength of the cell, for the FDD mode the timing relation between for cells for support of asynchronous soft handover.

2) The measurement procedures for preparation for handover to GSM900/GSM1800.

3) The measurement procedures for UE before random access process.

4) The measurement procedures for Dynamic Channel Allocation (DCA) of TDD mode.

5) UTRAN measurements.

4.2.6 Relationship of the physical layer functions

The functionality of the layer 1 is split over several specifications each for FDD and TDD. The following figures, although not categorical, show as an introduction the relationship of layer 1 functions by specification in terms of users plane information flow.

Figure 2 – FDD layer 1 functions relationships by specification

Figure 3 – TDD layer 1 functions relationships by specification