LTE

Introduction

To support the transmission of downlink and uplink transport channels Downlink Shared Channel(DL-SCH) and Uplink Shared Channel (UL-SCH) control signaling is required. This control signaling enables a UE to successfully receive, demodulate, and decode the DL-SCH. The Downlink Control Information (DCI) is transmitted through the Physical Downlink Control Channel (PDCCH) and includes information about the DL-SCH resource allocation (the set of resource blocks containing the DL-SCH), transport format and information related to the DL-SCH Hybrid Automatic Repeat reQuest (ARQ).

To form the PDCCH payload the DCI undergoes channel coding: addition of a CRC attachment followed by convolutional coding and rate matching according to PDCCH format capacity. The coded DCI bits i.e. PDCCH payload, are then mapped to Control Channel Elements (CCEs) according to the PDCCH format. These coded bits are then converted to complex modulated symbols after performing operations including scrambling, QPSK modulation, layer mapping and precoding. Finally, the modulated symbols are interleaved and mapped to physical Resource Elements (REs).

After performing deinterleaving, deprecoding, symbol combining, symbol demodulation and descrambling at the receiver, the UE is required to perform blind decoding of the PDCCH payload as it is not aware of the detailed control channel structure, including the number of control channels and the number of CCEs to which each control channel is mapped. Multiple PDCCHs can be transmitted in a single subframe which may and may not be all relevant to a particular UE. The UE finds the PDCCH specific to it by monitoring a set of PDCCH candidates (a set of consecutive CCEs on which a PDCCH could be mapped) in every subframe. The UE uses its Radio Network Temporary Identifier (RNTI) to try and decode candidates. The RNTI is used to demask a PDCCH candidate's CRC. If no CRC error is detected the UE determines that PDCCH carries its own control information.

Control Region

Downlink control signaling is located at the start of each downlink subframe (up to the first three OFDM symbols). One of the advantages of transmitting control channel at the start of every subframe is if the UE is not scheduled it may turn off its receiver circuitry for larger part of the subframe which results in reduced power consumption. Downlink control signaling is carried by three physical channels. The Physical Control Format Indicator Channel (PCFICH) to indicate the number of OFDM symbols used for control signaling in this subframe, Physical Hybrid-ARQ Indicator Channel (PHICH) which carries downlink Acknowledgment (ACK)/Negative Acknowledgment (NACK) for uplink data transmission and Physical Downlink Common Control Channel (PDCCH) which carries the downlink scheduling assignment and uplink scheduling grants.

The PDCCH carries scheduling assignments and other control information in the form of DCI messages. A PDCCH is transmitted on one CCE or an aggregation of several consecutive CCEs, where a CCE corresponds to 9 Resource Element Groups (REGs). In PDCCH transmission, only those REGs are used which are not assigned to PCFICH or PHICH. Each REG contains 4 Resource Elements (REs). Thus, REGs are used for defining the mapping of control channels to resource elements.

The control region of a downlink subframe comprises the multiplexing of all PDCCHs bits into a single block of data which is subsequently processed to form complex modulated symbols. These symbols are then divided to form blocks of complex-valued symbols quadruplets. These quadruplets are then interleaved and cyclically shifted prior to PDCCH resource mapping.

The DCI message transmits uplink or downlink scheduling information or an uplink Transmit Power Control (TPC) command. Depending on the purpose of control message, different DCI formats are defined. The information provided contains everything necessary for the UE to be able to identify the resources required to receive the Physical Downlink Data Channel (PDSCH) in that subframe and to decode it. The DCI formats are:

• Format 0 for transmission of Uplink Shared Channel (UL-SCH) allocation

• Format 1 for transmission of DL-SCH allocation for Single Input Multiple Output (SIMO) operation

• Format 1A for compact transmission of DL-SCH allocation for SIMO operation or allocating a dedicated preamble signature to a UE for random access

• Format 1B for transmission control information of Multiple Input Multiple Output (MIMO) rank 1 based compact resource assignment

• Format 1C for very compact transmission of PDSCH assignment

• Format 1D same as format1B with additional information of power offset

• Format 2 and Format 2A for transmission of DL-SCH allocation for closed and open loop MIMO operation, respectively

• Format 2B for the scheduling of dual layer transmission (antenna ports 7 & 8)

• Format 2C for the scheduling of up to 8 layer transmission (antenna ports 7 to 14) using TM9

• Format 2D for the scheduling of up to 8 layer transmission (antenna ports 7 to 14) using TM10

• Format 3 and Format 3A for transmission of TPC command for an uplink channel

• Format 4 for the scheduling of PUSCH with multi-antenna port transmission mode

Search Space and PDCCH Candidates

Once the DCI message is generated and channel coded according to the required DCI and PDCCH format, PDCCH multiplexing, scrambling, modulation, precoding, interleaving and layer mapping are performed to form complex symbols. These complex symbols are now ready to be mapped onto REs. The REs are defined in terms of REGs/CCEs allocated to a transmission. The number of CCEs allocated is given by the PDCCH format. The control region of a subframe is a collection of CCEs and can contain PDCCHs for multiple UEs thus the UE has to monitor a large area to extract its own control information. As the UE is not explicitly informed of the detailed control channel structure it has to blindly attempt to decode the control region. Unfortunately, this may impose a substantial burden on the UE as at large bandwidths the control region may be very large. This may exceed practical hardware limitations and lead to increased cost and/or reduced performance of the UE.

To simplify the decoding task at the UE, the whole control region is sub-divided into common and UE-specific search spaces which the UE should monitor (attempt to decode each of the PDCCHs). Each space comprises 2, 4 or 6 PDCCH candidates whose data length depends on the PDCCH format; each PDCCH must be transmitted on 1, 2, 4 or 8 CCE(s) (1 CCE = 9 REGs = 9*4 REs = 72 bits).

The PDCCH candidates consist of consecutive CCEs. The candidates within the PDCCH candidate set do not have to be unique, especially for smaller bandwidths. The common and UE-specific search spaces can overlap with each other. The size of search space is determined by the number of PDCCH candidates and the size of CCE aggregation level. That is, the size of the search space is integer times the size of CCE aggregation level or the number of PDCCH candidates.

Common Search Space

The common search space carries the common control information and is monitored by all UEs in a cell. The number of CCE aggregation levels supported by the common search space is limited to two i.e. 4 and 8 as compared to the UE-specific search space where four CCE aggregation levels are possible. This reduces the burden on UE for decoding common control information compared to decoding UE-specific control information. The common control space is used to carry important initial information including paging information, system information and random access procedures.

When searching the common control space the decoder always starts decoding from the first CCE. This restriction further simplifies the common search. The decoding is done on every possible PDCCH candidate set for given PDCCH format until it successfully decodes the PDCCH present in common search space.

UE-Specific Search Space

The UE-specific search space carries control information specific to a particular UE and is monitored by at least one UE in a cell. Unlike the common space search, the starting location of the UE-specific search space may be varied for each subframe or UE. The starting location of the UE-specific search space is determined in every subframe using a hash function, as specified in TS36.213, Clause 9.

In the UE-specific search space the UE finds its PDCCH by monitoring a set of PDCCH candidates (a set of consecutive CCEs on which PDCCH could be mapped) in every subframe. If no CRC error is detected when the UE uses its RNTI to demask the CRC (16-bit value also refers as C-RNTI) on a PDCCH, the UE determines that PDCCH carries its own control information. The PDCCH candidate sets correspond to different PDCCH formats. There are 4 PDCCH formats: 0, 1, 2 or 3. If the UE fails to decode any PDCCH candidates for a given PDCCH format it tries to decode candidates for other PDCCH formats. This process is repeated for all possible PDCCH formats until all directed PDCCHs are successfully decoded in UE-specific search space.

Blind Decoding Using DCI Search

The UE is only informed of the number of OFDM symbols within the control region of a subframe and is not provided with the location of its corresponding PDCCH. The UE finds its PDCCH by monitoring a set of PDCCH candidates in every subframe. This is referred to as blind decoding. The UE demasks each control candidate's CRC using its Radio Network Temporary Identifier (RNTI). If no CRC error is detected, the UE considers it as a successful decoding attempt and reads the control information within the successful candidate.

The eNodeB determines a PDCCH format to be transmitted to the UE, creates an appropriate DCI and attaches a CRC. The CRC is then masked with an RNTI according to the owner or usage of PDCCH. If the PDCCH is for a specific UE, the CRC will be masked with a UE unique identifier, for example a Cell-RNTI (C-RNTI). If the PDCCH contains paging information, the CRC will be masked with a paging indication identifier i.e. Paging-RNTI (P-RNTI). If the PDCCH contains system information, a system information identifier i.e. a system information-RNTI (SI-RNTI) will be used to mask the CRC.

With the possibilities of different RNTIs, PDCCH candidates, DCI and PDCCH formats, a significant number of attempts may be required to successfully decode the PDCCH. To overcome this complexity the UE first tries to blindly decode the first CCE in the control channel candidate set of a subframe. If the blind decoding fails, the UE tries to blindly decode the first 2, 4 then 8 CCEs sequentially, where the starting location is fixed for common search case and is given by hash function, as defined in TS36.213, Clause 9, for UE-specific case.

Reference

　　1. TS 36.213

　　2. MathWorks