助理教授：吳俊興 助教：許力中 國立高雄大學 資訊工程學系 教育部行動寬頻尖端技術人才培育計畫-小細胞基站聯盟中心 示範課程：行動寬頻網路之異質性存取 Week #03 Radio Access Networks - LTE Signal Structure and Physical Channels 助理教授：吳俊興 助教：許力中 國立高雄大學 資訊工程學系

Outline Introduction LTE Signal Structure Introduction to LTE Physical Channels and Reference Signals Resource Block Assignment Downlink Physical Channels - Physical Broadcast Channel (PBCH) - Physical Downlink Shared Channel (PDSCH) - Physical Multicast Channel (PMCH) - Physical Control Format Indicator Channel (PCFICH) - Physical Hybrid ARQ Indicator Channel (PHICH) - Physical Downlink Control Channel (PDCCH) Uplink Physical Channels - Physical Uplink Shared Channel (PUSCH) - Physical Uplink Control Channel (PUCCH) - Physical Random Access Channel (PRACH) Reference: Joydeep Acharya, Long Gao, and Sudhanshu Gaur, Heterogeneous Networks in LTE-Advanced, John Wiley & Sons, Ltd, 2014

Introduction Orthogonal Frequency Division Multiple Access (OFDMA) and Single-Carrier FDMA (SC-FDMA) are employed in the downlink and uplink of LTE, respectively, both of which use OFDM Introduce the OFDM-based LTE signal and frame structure LTE signal structure LTE downlink and uplink operations by RSs Physical channels and how they are mapped

LTE Signal Structure LTE signal structure spans the three dimensions Space Multiple independent data streams can be effectively transmitted by using MIMO at the eNodeB and UE Frequency Transmission over multiple subcarriers is made possible by OFDM Time Consecutive transmitted OFDM symbols are grouped into slots of duration 0.5 ms LTE has two duplexing modes: Frequency Division Duplexing (FDD) A paired spectrum of two separate and equal-sized bands are used, one each for the uplink and the downlink transmission. Time Division Duplexing (TDD). The subframes of each radio frame are divided into two subsets, one each for downlink and uplink transmissions.

LTE Frame Structure in FDD Mode Support multiple spatial MIMO layers Each 0.5 ms slot in a subframe has Seven OFDM symbols if OFDM with normal control plane (CP) length (5 μs) is used, and Six OFDM symbols if extended CP (17 μs) is used PRB spans 12 subcarriers each with spacing 15 kHz, in Frequency One RE is one OFDM subcarrier for the duration of one OFDM symbol and is the smallest indivisible unit in the LTE time frequency grid 84 REs normal CP 72 REs extended CP

Number of PRBs for Different LTE Bandwidths LTE supports a variety of bandwidths up to 20 MHz The minimum LTE bandwidth is 1.4 MHz (6 PRBs) The number of PRBs for different LTE bandwidths is listed below

LTE Frame Structure in TDD Mode ‘D’ and ‘U’ are reserved for downlink and uplink transmissions ‘S’ are special subframes with three fields: Downlink Pilot Time Slot(DwPTS) can be treated as a normal downlink subframe with reduced length, less transmission Guard Period (GP) is used to protect switching between downlink to uplink transmissions Uplink Pilot Time Slot (UpPTS) has a very short duration and is not used for uplink transmission

TDD Downlink–Uplink Configuration Multimedia Broadcast/Multicast Service (MBMS) Multiple eNodeBs transmit the same signal simultaneously so that the received signal to noise ratio increases UE scheduled for reception in a MBSFN subframe will receive multiple versions of the signal coming from multiple eNodeBs with different propagation delays This appears as additional multipaths to the UE and thus the overall delay spread increases. Since the cyclic prefix length has to be greater than the overall delay spread, only the extended CP is used for MBSFN transmission Notw:subframes 0,1, 4, 5, and 9 in each radio frame are not allowed in FDD mode 0, 1, 2, 5, and 6 cannot be configured as MBSFN subframes in TDD mode. Cuz carry Synchronization Signals (SSs) and broadcast system

Physical Channels-Downlink In Downlink, the physical channels can be categorized into two groups Data channel- used to transport information above the physical layer Physical Broadcast Channel (PBCH): carries the master information block (MIB) which has important system information needed by the UEs for initial access and configuration to the network Physical Downlink Shared Channel (PDSCH): carries user data and the system information that are not carried by the PBCH Physical Multicast Channel (PMCH): carries data for Multimedia Broadcast and Multicast Services Control Channels Physical Control Format Indicator Channel (PCFICH): informs the UE of the number of OFDM symbols used for PDCCH which is the control region Physical Hybrid ARQ Indicator Channel (PHICH): carries Hybrid ARQ ACK/NAKs in response to the uplink transmissions Physical Downlink Control Channel (PDCCH): informs the UE of the resource allocation and carries the uplink scheduling grant

Downlink Reference Signals UE for cell synchronization and channel estimation purposes Synchronization Signal (SS): used for initial downlink synchronization to the network Cell-Specific RS (CRS): used for downlink channel estimation and/or demodulation of downlink data signals Channel State Information (CSI) RS: introduced in Release 10 for downlink channel estimation UE-Specific RS associated with PDSCH: introduced in Release 9 for demodulation of downlink data signals

Downlink Logic Channels The RLC layer has different buffers for different types of application data and these are passed to the MAC by logical channels MAC decides the characteristics of the data to be transported Broadcast Control Channel (BCCH): carries the broadcasting system control information Paging Control Channel (PCCH): carries the paging information and system information change notifications Common Control Channel (CCCH): carries the common control information for a UE without a Radio Resource Control (RRC) connection Multicast Control Channel (MCCH): carries the MBMS control information Dedicated Control Channel (DCCH): carries the dedicated control information for a UE with RRC connection Other logical channels Dedicated Traffic Channel (DTCH): carries UE data Multicast Traffic Channel (MTCH): carries data for MBMS

Downlink Physical and Logical Channels • The PDSCH and PMCH transport channels are almost all control-plane and user-plane data (except for the MIB which is transported by the PBCH) • The PCFICH, PHICH, and PUCCH are not mapped to any logical or transport channels, not transport any data from the higher layers • These channels are generated in the physical layer in order to provide necessary support for the transmissions of the PBCH, PDSCH, and PMCH • The downlink RSs are not mapped to any logical or transport channels; thereforealso generated at the physical layer

Uplink Physical and Logical Channels The physical channels are categorized into two groups depending on whether or not they carry the user-plane and data-plane data from the higher layers The single data channel Physical uplink shared channel (PUSCH): carries user data The control channels Physical uplink control channel (PUCCH): carries the scheduling request from the UE,the HARQ acknowledgement for the downlink transmission, and the channel state information(CSI) feedback Physical random access channel (PRACH): is used for initial network access and uplink time synchronization RSs are employed to support the uplink physical channels Demodulation RS: supports the transmissions of the PUCCH and PUSCH Sounding RS: supports the transmissions of the PUSCH

Physical Uplink Mapping The PUCCH and PRACH are not mapped to any logical channels, which means that they do not transport any data from the RLC layer

Resource Block Assignment The most straightforward way for the eNodeB to inform each scheduled UE about the RB assignment is to send a bitmap whose size is equal to the number of RBs in the system For example, a system bandwidth of 20 MHz (i.e. 100 RBs) will require 100 bits for the bitmap, which will incur a significant overhead especially for small packets Virtual resource blocks used to enable distributed transmission in the frequency domain for the downlink Map to PRB-2 VRB module Distributed these blocks need not map to consecutive PRBs Localized pair is directly mapped to a localized PRB pair

Resource Allocation Type 0 In resource allocation type 0, the RBs are grouped (RBGs) according to their locations in the frequency domain, and a bitmap is used to indicate the assigned RBGs to a UE

Resource Allocation Type 1 In resource allocation type 1, the RBGs are formed as in resource allocation type 0. The RBGs are further divided into P RBG subsets Since the eNodeB also needs to send the RBG subset index, the bitmap in resource allocation type 1 is smaller than that in resource allocation type 0 in order to maintain the same total number of bits used for RB assignment As a result, the bitmap does not have enough bits to indicate all RBGs in a RBG subset To resolve this issue, an extra bit is used to indicate whether the bitmap relates to the ‘left’ or ‘right’ part of the RBGs in a RBG subset

Resource Allocation Type 2 (Distributed) RB assignment information indicates a contiguous set of VRBs which could be localized or distributed RB assignment can vary from a single RB up to the maximum available RBs Since different resource allocation types have different parameters, the format of the control signal used to support them should also be different. The format and content of a control signal is known as DCI (Downlink Control Information)

Downlink Physical Channels The first N OFDM symbols are used to carry the physical control channels In the subframes for non-MBMS service2, the value of N can be chosen from {1, 2, 3, 4} This size of the control channel is therefore variable and depends on the amount of control information to be transmitted The design of LTE thus optimizes the Res available for data transmission via the variable length control channel design In the subframes for MBMS transmission, a smaller N is chosen from {1, 2}. Because the resource allocation information is semi-static and common for all UEs in MBMS subframes, which is signalled by higher-layer signaling rather than PDCCH signaling The physical data channels are located in the data channel region, which occupies the remaining part of the subframe

Physical Broadcast Channel (PBCH) The MIB consists Downlink system bandwidth (3 bits) PHICH size (3 bits) System Frame Number (SFN) (8 bits) 10 spare bits The MIB is repeated every 40 ms, which translates to a data rate of just 350 bps on the PBCH Since reading the MIB is vital for initial cell detection and acquisition, reliability is the most important issue while designing the PBCH structure In order to guarantee reliable reception of the PBCH, the following three mechanisms are utilized: Forward Error Correction (FEC) coding, Antenna diversity, and Time diversity

Physical Downlink Shared Channel (PDSCH) One advantage of distributed PRB pairs is frequency diversity Consider the case of semi-static scheduling, where the same PRB pairs are allocated to a specific UE on periodic basis If a UE suffers from deep fading in a particular sub-band, localized PRB allocation increases the chances of both PRBs being in perpetual deep fade Distributed PRB pairing increases the complexity of resource allocation and is not supported in certain transmission modes that use UE-specific RS for demodulation

Physical Multicast Channel (PMCH) The PMCH is used to carry MBMS data; Like PDSCH, the PMCH spans the entire system bandwidth. Config anywhere The resource allocation information for the PMCH is signaled by RRC signaling instead of the PDCCH signaling This makes the control channel region in the subframes containing the PMCH smaller The extended CP is always used for the PMCH due to the large effective delay spread of multi-cell transmission channel Note that the CP length for the control channel region could be different from that for PMCH within the same MBSFN subframe If the non-MBSFN subframes use the normal CP, then the normal CP is also used for the control channel region in the MBSFN subframes This is because the UE has to read the control channel region of each subframe no matter if it is a MBSFN subframe or not, which requires a unique CP length for the control regions in all subframes The RSs embedded in the PMCH are different from those embedded in the PDSCH For example, the CRS is not transmitted on the PMCH

Physical Control Format Indicator Channel CFI is encoded into 32 bits in length to have sufficient error protection These 32 bits are mapped to 16 symbols with QPSK modulation These 16 symbols are mapped into 4 groups of 4 REs each, called a Resource Element Group (REG) These 4 REGs are transmitted in the first OFDM symbol of each subframe and distributed across the entire system bandwidth to exploit the frequency diversity Note that the location of the PCFICH is predefined for a given cell. In order to minimize the probability of collision with the PCFICH from a neighboring cell, a cell-specific frequency shift is applied to the positions of the PCFICH REGs

Physical Hybrid ARQ Indicator Channel (PHICH) The PHICH is used to carry the HARQ indicator, which informs a UE if the eNodeB has correctly received an uplink transmission The HARQ indicator is set to 0 for an Acknowledge (ACK) and 1 for a Negative acknowledge (NACK) The 1-bit HARQ indicator is repeated three times for error protection. Each of them is then mapped to a REG of four REs. Since multiple UEs may be scheduled simultaneously in the PUSCH RBs (see Section 3.6.1) in each subframe, multiple PHICHs may be needed (one for each scheduled UE) To avoid additional signaling to indicate which PHICH carries the HARQ indicator for a given UE, each PHICH is given an index derived from the index of the lowest uplink RB and the DMRS cyclic shift assigned to the corresponding UE Note that if the uplink transmission occurs in subframe N, the corresponding PHICH will always be in subframe N + 4 and thus a UE knows where to look for its PHICH

PHICH

Physical Downlink Control Channel (PDCCH) One PDCCH carries only one DCI for a UE (e.g. the associated PDSCH is used for unicast transmission) or a group of UEs (e.g. the associated PDSCH is used for broadcasting the SIBs) Each DCI message is attached with a CRC, which is then scrambled with a Radio Network Temporary Identifier (RNTI) according to the usage of the PDCCH If the PDCCH is for a specific UE, the CRC will be scrambled with a unique UE identifier, the Cell-RNTI (C-RNTI) If the DCI is associated with the PDSCH which contains the system information, the CRC will be scrambled with the System Information-RNTI (SI-RNTI) In the case where the corresponding PDSCH contains the paging information, the Paging-RNTI (P-RNTI) will be used for the CRC scrambling

Common and UE-specific Search Space Since the UE does not know which DCI format is being carried in the PDCCH, it will perform blind decoding over its search space by checking all possible PDCCH locations and DCI formats This involves multiple blind decodes and is a task of high complexity To reduce the complexity, a separate Common Search Space (CSS) and a UE-specific Search Space (USS) are defined in LTE

Downlink Control Information (DCI) Formats

Uplink Physical Channels Carry uplink data transmission from the UE to the eNodeB Related to uplink synchronization and downlink channel quality feedback Review different types of information that a UE has to transmit in the uplink 1. Data transmission 2. ACK/NACK information about a downlink transmission from the eNodeB 3. Downlink channel quality indicators (CQI/PMI/RI) that the UE has measured and has to report to the eNodeB 4. UE has data to transmit in the uplink, request the eNodeB to schedule it for uplink transmission. Scheduling Request (SR) 5. If the eNodeB responds favorably to the SR, then the UE has to provide further details to the uplink scheduler in the eNodeB These details are the Buffer Status Report (BSR) which indicates how much data the UE has for transmission and the Power Headroom Report (PHR) which indicates status of the UE transmit power The scheduler needs these items of information to decide how many RBs to allocate to the UE 6. UE also has to transmit two reference signals (RSs) Sounding reference signal (SRS) - channel estimation Uplink demodulation reference signal (DMRS) - demodulation The corresponding downlink RSs are CRS/CSI-RS and downlink DMRS, respectively

Physical Uplink Control Channel (PUCCH) Similar to the PDSCH, PUSCH is a shared data channel used by the UEs Differences between the two When the eNodeB schedules a downlink transmission to a UE in a subframe, the PDSCH contains the data and the PDCCH contains the DCI with the associated control information Data and associated control are therefore transmitted in the same subframe For uplink data transmission from the UE, the transmission parameters are still decided by the eNodeB scheduler The associated control information for PUSCH transmission is also carried in the PDCCH

Physical Uplink Control Channel (PUCCH) The PUCCH is used to transmit control information in the uplink similar to PDCCH in the downlink Unlike the PDCCH, the PUCCH transmission is scheduled for two entire slots that lie on the edges of the total system bandwidth The PUCCH is always designed to exploit frequency diversity The two slots are not in the same RB but lie in opposite edges of the total system bandwidth Allocating PUCCH resources at the edge of the system bandwidth also allows the PUSCH to be selected from a contiguous block of RBs from the center of the resource grid The PUCCH region therefore acts as a guard band between PUSCH regions of adjacent carriers

PUCCH Formats Different PUCCH formats are used to carry different types of information. The information bits are Channel coded Scrambled Cyclic shifted by a length of 12 base sequence before DFT precoding Use of the cyclic shift sequence allows up to 12 different UEs to be orthogonally multiplexed in the same PUCCH resource The DMRS signal is multiplexed in the resulting PUCCH symbols before the IFFT operation and this is used for demodulation at the eNodeB PUCCH format 3 was introduced for Rel-10 carrier aggregation (CA) for transmitting multiple ACK/NACKs from multiple component carriers It follows a different generation procedure than the other PUCCH formats

Physical Random Access Channel (PRACH) Time and frequency multiplexed with the PUSCH and PUCCH The location of these subframes are indicated to the UE via SIB-2 which the UE has already decoded during the downlink synchronization process RACH Preamble Transmission The UE chooses a preamble from a set of RACH preambles Means that the eNodeB informs the UE via RRC signaling which preamble it should use Random Access Response Transmits the Random Access Response (RAR) Contains the identity of the detected preamble UEs would receive and be able to decode RRC Connection Request The UE uses the PUSCH grant to transmit an RRC connection request If UE can decode ,then continue next step, or must be reconnect request Contention Resolution Message addressed to the C-RNTI or S-TMSI of the UE whose message was successfully decoded in the previous step

References [1] 3GPP (2012) Evolved Universal Terrestrial Radio access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN), overall description 3GPP TS 36.300 v11.2.0. Third Generation Partnership Project, Technical Report. [2] Hitachi (2011) R1-112587, System Design Considerations for CoMP and eICIC. Technical Report, August 2011. [3] 3GPP (2012) Evolved Universal Terrestrial Radio access (E-UTRA), Multiplexing and channel coding 3GPP TS 36.212 v11.0.0. Third Generation Partnership Project, Technical Report. [4] 3GPP (2012) Evolved Universal Terrestrial Radio access (E-UTRA), Physical layer procedures 3GPP TS 36.213 v11.0.0. Third Generation Partnership Project, Technical Report. [5] Sesia, S., Toufik, I., and Baker, M. (2011) LTE: The UMTS Long Term Evolution: From Theory to Practice. United Kingdom: John Wiley & Sons Ltd.