Architecture Overview NCHU CSE LTE - 1

Transcription

1 Architecture Overview NCHU CSE LTE - 1

2 System Architecture Evolution (SAE) Packet core networks are also evolving to the flat System Architecture Evolution (SAE) architecture. This new architecture optimizes network performance, improve cost-efficiency and facilitate the uptake of massmarket IP based services. There are only two nodes in the SAE architecture user plane: LTE base station (enodeb) SAE Gateway. Home Subscriber Server Mobility Management Entity Policy and Charging Rules Function Serving GPRS Support Node Packet Data Serving Node NCHU CSE LTE - 2

3 High Level Architecture Serving GPRS Support Node Home Subscriber Server Mobility Management Entity Policy and Charging Rules Function NCHU CSE LTE - 3

4 High Level Architecture Functional Elements Evolved Radio Access Network (RAN) Serving Gateway (SGW) Mobility Management Entity (MME) Packet Data Network Gateway (PDN GW) Key Features EPC to EPS» Evolved Packet Core -> Evolved Packet System» Separation of the network entity that performs control-plane functionality (MME) from the network entity that performs bearer-plane functionality (SGW) with a well defined open interface between them (S11). S1-flex Mechanism» provides support for network redundancy and load sharing of traffic across network elements in the CN (core network), the MME and the SGW, NCHU CSE LTE - 4

5 Radio-Interface Architecture In 3GPP the overall system architecture of both the Radio-Access Network (RAN) and the Core Network (CN) was revisited, including the split of functionality between the two network parts. For LTE» The RAN is responsible scheduling, radio-resource handling, retransmission protocols, coding and various multi-antenna schemes. it was known as the System Architecture Evolution (SAE) and resulted in a flat RAN architecture, as well as a new core network architecture referred to as the Evolved Packet Core (EPC). The RAN and the EPC can be referred to as the Evolved Packet System (EPS). NCHU CSE LTE - 5

6 Radio-Interface Architecture The EPC is responsible for providing a complete mobile-broadband network. This includes authentication, charging functionality, and setup of end-to-end connections. Handling these functions separately, instead of integrating them into the RAN, is beneficial as it allows for several radio-access technologies to be served by the same core network. NCHU CSE LTE - 6

7 Core Network The EPC is a radical evolution from the GSM/GPRS core network used for GSM and WCDMA/HSPA. EPC supports access to the packet-switched domain only, with no access to the circuit switched domain. NCHU CSE LTE - 7

8 Core Network The Mobility Management Entity (MME) is the controlplane node of the EPC. include connection/release of bearers to a terminal, handling of IDLE to ACTIVE transitions, and handling of security keys. The Serving Gateway (S-GW) is the user-plane node connecting the EPC to the LTE RAN. The S-GW acts as a mobility anchor when terminals move between enodebs, as well as a mobility anchor for other 3GPP technologies (GSM/GPRS and HSPA). The Packet Data Network Gateway (PDN Gateway, P-GW) connects the EPC to the internet. allocates the IP address for a specific terminal, as well as quality-of service enforcement according to the policy controlled by the Policy and Charging Rules Function (PCRF). NCHU CSE LTE - 8

9 Core Network The EPC also contains other types of nodes such as Policy and Charging Rules Function (PCRF) responsible for quality-of-service (QoS) handling and charging. Home Subscriber Service (HSS) node, a database containing subscriber information. In an actual physical implementation, several of them may very well be combined. For example, the MME, P-GW, and S-GW could very well be combined into a single physical node. NCHU CSE LTE - 9

10 Radio-Access Network LTE radio-access network uses a flat architecture with a single type of node enodeb (enb). The enodeb is responsible for all radio-related functions. enb is connected to the EPC by means of the S1 interface to the S-GW by S1 user-plane part, S1-u. to the MME by S1 control-plane part, S1-c. NCHU CSE LTE - 10

11 Radio-Access Network One enb can be connected to multiple MMEs/S-GWs for the purpose of load sharing and redundancy. The X2 interface, connecting enodebs to each other, is mainly used to support active-mode mobility. is used to support lossless mobility between neighboring cells by packet forwarding. is used for multi-cell Radio Resource Management (RRM) functions such as Inter-Cell Interference Coordination (ICIC). NCHU CSE LTE - 11

12 Summary The E-UTRAN (Evolved Universal Terrestrial Radio Access) consists of enodebs, providing the user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations. The enodebs are interconnected with each other by means of the X2 interface. The enodebs are also connected to the EPC (Evolved Packet Core) by the S1 interface, to the MME (Mobility Management Entity) by the S1-MME to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and enodebs. NCHU CSE LTE - 12

13 Overall Architecture NCHU CSE LTE - 13

14 Functional Split between E-UTRAN and EPC enb Inter Cell RRM RB Control Connection Mobility Cont. Radio Admission Control enb Measurement Configuration & Provision Dynamic Resource Allocation (Scheduler) RRC Control Plane MME NAS Security Idle State Mobility Handling EPS Bearer Control Non Access Stratum PDCP RLC MAC PHY S1 S-GW Mobility Anchoring P-GW UE IP address allocation Packet Filtering internet E-UTRAN User Plane EPC NCHU CSE LTE - 14

15 User Plane UE enb PDCP RLC MAC PHY PDCP RLC MAC PHY NCHU CSE LTE - 15

16 Control Plane UE enb MME NAS NAS RRC PDCP RLC MAC PHY RRC PDCP RLC MAC PHY NCHU CSE LTE - 16

17 Non Access Stratum The non-access stratum (NAS) is highest stratum of the control plane between UE and MME at the radio interface. Main functions of the protocols that are part of the NAS are the support of mobility of the user equipment (UE) and the support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW). NAS control protocol performs followings: Evolved Packet System (EPS) bearer management; Authentication; ECM-IDLE mobility handling; Paging origination in ECM-IDLE; Security control. 3GPP TS Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 EPS Connection Management (ECM) NCHU CSE LTE - 17

18 Control Plane PDCP sublayer (terminated in enb on the network side) performs the functions ciphering and integrity protection; RLC and MAC sublayers (terminated in enb on the network side) perform the same functions as for the user plane; RRC (terminated in enb on the network side) performs the functions listed in subclause 7, e.g.: Broadcast; Paging; RRC connection management; RB control; Mobility functions; UE measurement reporting and control. NAS control protocol (terminated in MME on the network side) performs among other things: EPS bearer management; Authentication; ECM-IDLE mobility handling; Paging origination in ECM-IDLE; Security control. NCHU CSE LTE - 18

19 enb Functions enb hosts the following functions Radio Resource Management:» Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling); IP header compression and encryption of user data stream; Selection of an MME at UE attachment when no routing to an MME can be determined from the information provided by the UE; Routing of User Plane data towards S-GW; Scheduling and transmission of paging messages (originated from the MME); Scheduling and transmission of broadcast information (originated from the MME or operation&maintain (O&M)); Measurement and measurement reporting configuration for mobility and scheduling. NCHU CSE LTE - 19

20 MME Functions MME hosts the following functions NAS (Non Access Stratum) signalling; NAS signalling security; Access Stratum (AS) security control; Inter CN node signalling for mobility between 3GPP access networks; Idle mode UE reachability (including control and execution of paging retransmission); Tracking area list management (for UE in idle and active mode); P-GW and S-GW selection; MME selection for handovers with MME change; SGSN (Serving GPRS Support Node) selection for handovers to 2G or 3G 3GPP access networks; Roaming; Authentication; Bearer management functions including dedicated bearer establishment. NCHU CSE LTE - 20

21 Serving Gateway (S-GW) Functions Serving Gateway (S-GW) hosts the following functions The local mobility anchor point for inter-enb handover; Mobility anchoring for inter-3gpp mobility; E-UTRAN idle mode downlink packet buffering and initiation of network triggered service request procedure; Lawful Interception; Packet routing and forwarding; Transport level packet marking in the uplink and the downlink; Accounting on user and QCI (QoS class identifier) granularity for inter-operator charging; UL and DL charging per UE, PDN (Packet Data Network), and QCI NCHU CSE LTE - 21

22 PDN Gateway (P-GW) Functions PDN Gateway (P-GW) hosts the following functions Per-user based packet filtering (by e.g. deep packet inspection); Lawful Interception; UE IP address allocation; Transport level packet marking in the downlink; UL and DL service level charging, gating and rate enforcement; DL rate enforcement based on aggregate maximum bit rate (AMBR); NCHU CSE LTE - 22

23 Radio Protocol Architecture RAN protocol architecture is designed for the user as well as the control planes. NCHU CSE LTE - 23

24 NCHU CSE LTE - 24

25 Radio Protocol Architecture 1. Packet Data Convergence Protocol (PDCP) performs IP header compression. Robust Header Compression (ROHC). Ciphering, the control plane, integrity protection of the transmitted data, as well as in-sequence delivery and duplicate removal for handover. 2. Radio-Link Control (RLC) is responsible for segmentation/concatenation, retransmission handling, duplicate detection, and in-sequence delivery to higher layers. provides services to the PDCP in the form of radio bearers. NCHU CSE LTE - 25

26 Radio Protocol Architecture 3. Medium-Access Control (MAC) handles multiplexing of logical channels, hybrid-arq retransmissions, and uplink and downlink scheduling. The scheduling functionality is located in the enodeb for both uplink and downlink. provides services to the RLC in the form of logical channels. 4. Physical Layer (PHY) handles coding/decoding, modulation/demodulation, multi-antenna mapping, and other typical physical-layer functions. offers services to the MAC layer in the form of transport channels. NCHU CSE LTE - 26

27 LTE Data Flow 1. The PDCP performs (optional) IP-header compression, followed by ciphering. A PDCP header is added, carrying information required for deciphering in the terminal. 2. The RLC protocol performs concatenation and/or segmentation of the PDCP SDUs and adds an RLC header. The RLC PDUs are forwarded to the MAC layer, which multiplexes a number of RLC PDUs and attaches a MAC header to form a transport block. 3. The physical layer attaches a CRC to the transport block, performs coding and modulation, and transmits the resulting signal, possibly using multiple transmit antennas. NCHU CSE LTE - 27

28 LTE Data Flow NCHU CSE LTE - 28

29 Radio-Link Control It is responsible for segmentation/concatenation of (header-compressed) IP packets, and RLC SDUs are from the PDCP into suitably sized RLC PDUs. retransmission of erroneously received PDUs, as well as removal of duplicated PDUs. ensures in-sequence delivery of SDUs to upper layers. For LTE, the RLC PDU size varies dynamically. NCHU CSE LTE - 29

30 Radio-Link Control Since the RLC, scheduler, and rate adaptation mechanisms are all located in the enodeb, dynamic PDU sizes are easily supported for LTE. By monitoring the sequence numbers of the incoming PDUs, the receiving RLC can identify missing PDUs. Status reports are then fed back to the transmitting RLC entity, requesting retransmission of missing PDUs. MAC-based hybrid-arq protocol. NCHU CSE LTE - 30

31 Medium-Access Control The MAC layer handles logical-channel multiplexing, hybrid-arq retransmissions, and uplink and downlink scheduling. Is also responsible for multiplexing/demultiplexing data across multiple component carriers when carrier aggregation is used. The MAC provides services to the RLC in the form of logical channels. A logical channel is defined by the type of information it carries and is generally classified as a control channel, used for transmission of control and configuration information necessary for operating. as a traffic channel, used for the user data. NCHU CSE LTE - 31

32 Logical Channels The Broadcast Control Channel (BCCH), used for transmission of system information from the network to all terminals in a cell. A terminal uses the system information to find out how the system is configured. The Paging Control Channel (PCCH), used for paging of terminals whose location on a cell level is not known to the network. The Common Control Channel (CCCH), used for transmission of control information in conjunction with random access. The Dedicated Control Channel (DCCH), used for transmission of control information to/from a terminal. NCHU CSE LTE - 32

33 Logical Channels The Multicast Control Channel (MCCH), used for transmission of control information required for reception of the MTCH (see below). The Dedicated Traffic Channel (DTCH), used for transmission of user data to/from a terminal. This is the logical channel type used for transmission of all uplink and non-mbsfn (Multicast-Broadcast Single Frequency Network) downlink user data. The Multicast Traffic Channel (MTCH), used for downlink transmission of MBMS services. NCHU CSE LTE - 33

34 RLC Logical Channels Part of the MAC functionality is multiplexing of different logical channels and mapping of the logical channels to the appropriate transport channels. MAC PHY NCHU CSE LTE - 34

35 Mapping between logical channels and transport channels PCCH BCCH DCCH DTCH MCCH MTCH Logical channels PCH BCH RACH SCH MCH Transport channels NCHU CSE LTE - 35

36 DL Transport Channels Broadcast Channel (BCH) Fixed format Must be broadcast over entire coverage area of cell Downlink Shared Channel (DL-SCH) Supports Hybrid ARQ (HARQ) Supports dynamic link adaption by varying modulation, coding and transmit power Suitable for transmission over entire cell coverage area Suitable for use with beamforming Support for dynamic and semi-static resource allocation Support for discontinuous receive (DRX) for power save support for MBMS transmission. Paging Channel (PCH) Support for UE DRX Requirement for broadcast over entire cell coverage area Mapped to dynamically allocated physical resources Multicast Channel (MCH) Requirement for broadcast over entire cell coverage area Support for MB-SFN (Single-Frequency-Network) Support for semi-static resource allocation NCHU CSE LTE - 36

37 Transport Channels For the physical layer, the MAC layer uses services in the form of transport channels. A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface. The following transport-channel types are defined for LTE: The Broadcast Channel (BCH) has a fixed transport format, provided by the specifications. for transmission of parts of the BCCH system information, more specifically the so-called Master Information Block (MIB) NCHU CSE LTE - 37

38 Transport Channels The Paging Channel (PCH) is used for transmission of paging information from the PCCH logical channel. Supports discontinuous reception (DRX) to allow the terminal to save battery power by waking up to receive the PCH only at predefined time instants The Downlink Shared Channel (DL-SCH) is the main transport channel used for transmission of downlink data in LTE. Supports dynamic rate adaptation and channel dependent scheduling in the time and frequency domains, hybrid ARQ with soft combining, and spatial multiplexing. Also uses for transmission of the parts of the BCCH system information not mapped to the BCH. NCHU CSE LTE - 38

39 Transport Channels The Multicast Channel (MCH) is used to support MBMS. the scheduling and transport format configuration are coordinated among the transmission points involved in the MBSFN transmission. The Uplink Shared Channel (UL-SCH) is the uplink counterpart to the DL-SCH. Uplink transport channel used for transmission of uplink data. The Random-Access Channel (RACH) is also defined as a transport channel, although it does not carry transport blocks. NCHU CSE LTE - 39

40 Logical Channels DL-SCH and UL-SCH are the main downlink and uplink transport. NCHU CSE LTE - 40

41 Uplink Transport Channels Uplink Shared Channel (UL-SCH) Support possible use of beamforming Support dynamic link adaptation (varying modulation, coding and/or Tx power) Support for HARQ Support for dynamic and semi-static resource allocation Random Access Channel (RACH) Supports transmission of limited control information Possible risk of collision NCHU CSE LTE - 41

42 Uplink Transport Channels Mapping Uplink Physical Channels to Transport Channels UL-SCH RACH Uplink Transport channels PUSCH PRACH PUCCH Uplink Physical channels NCHU CSE LTE - 42

43 RLC Format To each RLC PDU, there is an associated subheader in the MAC header. contains the identity of the logical channel (LCID) from which the RLC PDU originated and the length of the PDU in bytes. There is also a flag indicating whether this is the last sub-header or not. NCHU CSE LTE - 43

44 MAC Format The MAC layer inserts the MAC control elements into the transport blocks to be transmitted over the transport channels. A MAC control element is used for inband control signaling timing-advance commands and random access response. Control elements are identified with reserved values in the LCID field, where the LCID value indicates the type of control information. NCHU CSE LTE - 44

45 Scheduling The basic operation of the scheduler is dynamic scheduling, each 1 ms interval takes a scheduling decision and sends scheduling information to the selected set of terminals. across multiple cells residing in different enodebs is supported using signaling over the X2 interface. The downlink scheduler is responsible which terminal(s) to transmit and uses the set of resource blocks in the terminal s DL-SCH Similarly, terminals are to transmit on their respective UL-SCH and on which uplink time frequency resources NCHU CSE LTE - 45

46 Scheduling The uplink scheduling decision is taken per terminal and not per radio bearer. Although the enodeb scheduler controls the payload of a scheduled terminal, the terminal is still responsible for selecting from which radio bearer(s) the data is taken. TF: Transport Format NCHU CSE LTE - 46

47 Scheduling Downlink channel-dependent scheduling is supported through channel-state reports, reflecting the instantaneous channel quality, as well as information necessary to determine the appropriate antenna processing in the case of spatial multiplexing. In the uplink, the channel-state information necessary for uplink channel-dependent scheduling can be based on a sounding reference signal transmitted from each terminal for which the enodeb wants to estimate the uplink channel quality. NCHU CSE LTE - 47

48 Physical Layer The physical layer is responsible for coding physical-layer hybrid-arq processing modulation multiantenna processing mapping of the signal to the appropriate physical time frequency resources The physical layer provides services to the MAC layer in the form of transport channels. Data transmission in downlink and uplink use the DL- SCH and UL-SCH transport-channel types respectively. NCHU CSE LTE - 48

49 Physical Layer These channels, known as L1/L2 control channels, are used for downlink control information (DCI), providing the terminal with the necessary information for proper reception and decoding of the downlink data transmission, uplink control information (UCI) used for providing the scheduler and the hybrid-arq protocol with information about the situation at the terminal. NCHU CSE LTE - 49

50 Mapping Downlink Physical Channels to Transport Channels BCH MCH PCH DL-SCH Downlink Transport channels PBCH PMCH PDSCH PDCCH Downlink Physical channels NCHU CSE LTE - 51

51 Physical Channels LTE DL physical channels are: Physical Downlink Shared Channel (PDSCH) Physical Downlink Control Channel (PDCCH) Common Control Physical Channel (CCPCH)» PBCH Physical Broadcast Channel» PCFICH Physical Control Format Indicator Channel» PHICH Physical Hybrid ARQ Indicator Channel» PMCH Physical Multicast Channel UL PRACH Physical Random Access Channel PUCCH Physical Uplink Control Channel PUSCH Physical Uplink Shared Channel NCHU CSE LTE - 52

52 General Structure for DL Physical Channels The baseband signal representing a DL physical channel is defined in terms of the following steps: scrambling of coded bits in each of the code words to be transmitted on a physical channel modulation of scrambled bits to generate complex-valued modulation symbols mapping of the complex-valued modulation symbols onto one or several transmission layers precoding of the complex-valued modulation symbols on each layer for transmission on the antenna ports mapping of complex-valued modulation symbols for each antenna port to resource elements generation of complex-valued time-domain OFDM signal for each antenna port NCHU CSE LTE - 53

53 General Structure for DL Physical Channels code words layers antenna ports Scrambling Modulation mapper Resource element mapper OFDM signal generation Layer mapper Precoding Scrambling Modulation mapper Resource element mapper OFDM signal generation NCHU CSE LTE - 54

54 Physical Channels (DL) The PDSCH is the main physical channel used for unicast data transmission, also for transmission of paging information. The PBCH carries part of the system information, required by the terminal in order to access the network. The PMCH is used for MBSFN operation. The PDCCH is used for downlink control information, mainly scheduling decisions, required for reception of PDSCH, and for scheduling grants enabling transmission on the PUSCH. NCHU CSE LTE - 55

55 Physical Channels (DL) The PHICH carries the hybrid-arq acknowledgement to indicate to the terminal whether a transport block should be retransmitted or not. The PCFICH is a channel providing the terminals with information necessary to decode the set of PDCCHs. NCHU CSE LTE - 56

56 Physical Channels (UL) The PUSCH is the uplink counterpart to the PDSCH. at most one PUSCH per uplink component carrier per terminal. The PUCCH is used by the terminal to send hybrid-arq acknowledgements, indicating to the enodeb whether the downlink transport block(s) was successfully received or not. to send channel-state reports aiding downlink channeldependent scheduling, and for requesting resources to transmit uplink data upon. at most one PUCCH per terminal. The PRACH is used for random access. NCHU CSE LTE - 57

57 Physical Downlink Shared Channel (PDSCH) The PDSCH is utilized basically for data and multimedia transport. Therefore it is designed for very high data rates. Modulation options therefore include QPSK, 16QAM and 64QAM. Spatial multiplexing is exclusive to the PDSCH. It is not used on either the PDCCH or the CCPCH. CRC insertion: 24 bit CRC is the baseline for PDSCH NCHU CSE LTE - 58

58 Physical Downlink Control Channel (PDCCH) The PDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. 1.Scheduling decisions for Downlink, including PDSCH resource indication, transport format, hybrid- ARQ information, and control information related to spatial multiplexing (if applicable). for Uplink, including PUSCH resource indication, transport format, and hybrid-arq-related information. 2.Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants. NCHU CSE LTE - 59

59 PDCCH A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs) a control channel element corresponds to a set of resource elements. PDCCH format Number of CCEs Number of PDCCH bits NCHU CSE LTE - 60

60 PDCCH The PDCCH conveys UE-specific control information. Robustness rather than maximum data rate is the consideration. QPSK is the only available modulation format. The PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first slot of a subframe. sync sync NCHU CSE LTE - 61

61 PDCCH Subframe Number of OFDM symbols for PDCCH Subframe 1 and 6 for frame structure type 2 1, 2 MBSFN subframes on a carrier supporting both PMCH and PDSCH 1, 2 MBSFN subframes on a carrier not supporting PDSCH 0 All other subframes 1, 2, 3 Maximum number of OFDM symbols used for PDCCH NCHU CSE LTE - 62

62 PDCCH Processing NCHU CSE LTE - 63

63 PDCCH Processing A CRC is attached to each DCI message payload. The identity of the terminal (or terminals) addressed the Radio-Network Temporary Identifier (RNTI) is included in the CRC calculation. After CRC attachment, the bits are coded with a rate- 1/3 tail-biting convolutional code and rate matched to fit the amount of resources used for PDCCH transmission. Tail-biting convolutional coding is similar to conventional convolutional coding with the exception that no tail bits are used. NCHU CSE LTE - 64

64 PDCCH Processing The sequence of bits corresponding to all the PDCCH resource elements to be transmitted in the subframe including the unused resource elements, is scrambled by a cell- and subframe-specific scrambling sequence to randomize inter-cell interference, followed by QPSK modulation and mapping to resource elements. This structure is based on Control-Channel Elements (CCEs), which in essence is a convenient name for a set of 36 useful resource elements (nine resource-element groups). The number of CCEs, one, two, four, or eight, required depends on the payload size of the control information (DCI payload) and the channel-coding rate. NCHU CSE LTE - 65

65 PDCCH Processing The number of CCEs depends on the size of the control region the cell bandwidth the number of downlink antenna ports the amount of resources occupied by PHICH The size of the control region can vary dynamically from subframe to subframe as indicated by the PCFICH. A specific PDCCH can be identified by the numbers of the corresponding CCEs in the control region. NCHU CSE LTE - 66

66 PDCCH Processing As the number of CCEs may vary and is not signaled, the terminal has to blindly determine the number of CCEs used for the PDCCH it is addressed upon. The sequence of CCEs should match the amount of resources available in a given subframe - the number of CCEs varies according to the value transmitted on the PCFICH NCHU CSE LTE - 67

67 Blind Decoding of PDCCHs Each PDCCH supports multiple formats and the format used is a priori unknown to the terminal. The terminal needs to blindly detect the format of the PDCCHs. In each subframe, the terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces. NCHU CSE LTE - 68

68 Blind Decoding of PDCCHs It is required to have mechanisms to limit the CCE aggregations that the terminal is supposed to monitor. From a scheduling point of view: CCE aggregations are undesirable as they may influence the scheduling flexibility and require additional processing at the transmitter side. From a terminal-complexity point of view: the terminal to monitor all possible CCE aggregations, also for the larger cell bandwidths, is not attractive. NCHU CSE LTE - 69

69 Blind Decoding of PDCCHs To impose as few restrictions as possible on the scheduler while at the same time limit the maximum number of blind decoding attempts in the terminal. LTE defines search spaces, which describe the set of CCEs the terminal is supposed to monitor for scheduling assignments/ grants relating to a certain component carrier. A search space is a set of candidate control channels formed by CCEs on a given aggregation level, which the terminal is supposed to attempt to decode. For multiple aggregation levels, corresponding to one, two, four, and eight CCEs, a terminal has multiple search spaces. NCHU CSE LTE - 70

70 Blind Decoding of PDCCHs The terminals will attempt to decode all the PDCCHs that can be formed from the CCEs in each of its search spaces. NCHU CSE LTE - 71

71 Example 1. Terminal A cannot be addressed on a PDCCH starting at CCE number 20, whereas terminal B can. 2. If terminal A is using CCEs 16 23, terminal B cannot be addressed on aggregation level 4 as all CCEs in its level-4 search space are blocked by the use for the other terminals. each terminal in the system has a terminal-specific search space at each aggregation level. 3. The terminal-specific search spaces partially overlap between the two terminals in this subframe (CCEs on aggregation level 8) but, as the terminalspecific search space varies between subframes, the overlap in the next subframe is most likely different. NCHU CSE LTE - 72

72 Blind Decoding of PDCCHs LTE has also defined common search spaces. all terminals in the cell monitor the CCEs for control information. It is primarily transmission of various system messages, it can be used to schedule individual terminals as well. Also, it can be used to resolve situations where scheduling of one terminal is blocked due to lack of available resources in the terminal-specific search space. NCHU CSE LTE - 73

73 Blind Decoding of PDCCHs As the main function of the common search space is to handle scheduling of system information intended for multiple terminals, and such information must be receivable by all terminals in the cell. scheduling is used the common search space. The common search spaces are only defined for aggregation levels of four and eight CCEs and only for the smallest DCI formats, 0/1A/3/3A and 1C. (Table 10-4) Without support for DCI formats with spatial multiplexing in the common search space. NCHU CSE LTE - 74

74 Blind Decoding of PDCCHs The downlink DCI formats to decode in the terminalspecific search space depend on the transmission mode configured for the terminal. The DCI monitoring is described for the case of a single component carrier. NCHU CSE LTE - 75

75 Blind Decoding of PDCCHs For multiple component carriers, the above procedures are applied in principle to each of the activated downlink component carriers. (Ch 12) There is one UE-specific search space per aggregation level and per (activated) component carrier upon which PDSCH can be received (or PUSCH transmitted), although there are some carrier-aggregation-specific modifications. UE-specific search space per aggregation level and component carrier used for the PDSCH/PUSCH. NCHU CSE LTE - 76

76 Blind Decoding of PDCCHs Where PDSCH/PUSCH transmissions on component carrier 1 are scheduled using PDCCHs transmitted on component carrier 1. For component carrier 2, a carrier indicator is assumed in the UE-specific search space as component carrier 2 is cross-carrier scheduled from PDCCHs transmitted on component carrier 1. NCHU CSE LTE - 77

77 Blind Decoding of PDCCHs For component carriers 3 and 4, the terminal will handle the two search spaces independently, assuming (in this example) a carrier indicator for component carrier 4 but not for component carrier 3. If the UE-specific and common search spaces relating to different component carriers happen to overlap for some aggregation level when crosscarrier scheduling is configured, the terminal only needs to monitor the common search space. NCHU CSE LTE - 78

78 Physical Hybrid ARQ Indicator Channel (PHICH) The PHICH is used to transmit the hybrid-arq acknowledgements in response to UL-SCH transmission. The hybrid-arq acknowledgement (one single bit of information per transport block) is repeated three times, followed by BPSK modulation on either the I or the Q branch and spreading with a length-four orthogonal sequence. NCHU CSE LTE - 79

79 PHICH Multiple PHICHs mapped to the same set of resource elements constitute a PHICH group, where PHICHs within the same PHICH group are separated through different orthogonal sequences. Sequence index seq n PHICH Normal cyclic prefix PHICH N SF = 4 Orthogonal sequence Extended cyclic prefix PHICH N SF = [ ] [ ] 1 [ ] [ + 1 1] 2 [ ] [ + j + j] 3 [ ] [ + j j] 4 [ + j + j + j + j] - 5 [ + j j + j j] - 6 [ + j + j j j] - 7 [ + j j j + j] - 2 NCHU CSE LTE - 80

80 PHICH PHICH duration in MBSFN and non-mbsfn subframes Non-MBSFN subframes MBSFN subframes PHICH duration Subframes 1 and 6 in case of All other cases On a carrier supporting frame structure type 2 both PDSCH and PMCH Normal Extended NCHU CSE LTE - 81

81 PHICH After forming the composite signal representing the PHICHs in a group, cell-specific scrambling is applied and the 12 scrambled symbols are mapped to three resource-element groups. separated by approximately one-third of the downlink cell bandwidth. In the first OFDM symbol in the control region, resources are first allocated to the PCFICH, the PHICHs are mapped to resource elements not used by the PCFICH NCHU CSE LTE - 82

82 PHICH Each downlink subframe is normally divided into a control region, and a data region. control region consisting of the first few OFDM symbols, data region consisting of the remaining part of the subframe. The control region carries L1/L2 signaling necessary to control uplink and downlink data transmissions. NCHU CSE LTE - 83

83 PCFICH The PCFICH indicates the size of the control region in terms of the number of OFDM symbols that is, indirectly where in the subframe the data region starts. If PCFICH is incorrectly decoded, neither know how to process the control channels nor where the data region starts for the corresponding subframe. The terminal could blindly try to decode all possible control channel formats and, from which format that was correctly decoded, deduce the starting position of the data region, but this can be a very complex procedure. Two bits of information (32-bit codeword), corresponds to the three control-region sizes of one, two, or three OFDM symbols. NCHU CSE LTE - 84

84 PCFICH The coded bits are scrambled with a cell- and subframespecific scrambling code to randomize inter-cell interference, QPSK modulated, and mapped to 16 resource elements. As the size of the control region is unknown until the PCFICH is decoded, the PCFICH is always mapped to the first OFDM symbol of each subframe. NCHU CSE LTE - 85

85 PCFICH The transmission power of the PCFICH is under control of the enodeb. For coverage in a certain cell, the power of the PCFICH can be set higher than for other channels by borrowing power from, for example, simultaneously transmitted PDCCHs. increasing the power of the PCFICH to improve the performance in an interference-limited system. As the PCFICH-to-resource-element mapping depends on the cell identity, the probability of (partial) collisions with PCFICH in neighboring cells in synchronized networks is reduced, thereby improving the performance of PCFICH power boosting as a tool to control the error rate. NCHU CSE LTE - 86

86 PCFICH The mapping of resource elements in the first OFDM symbol in the subframe is done in groups of four resource elements (resource-element groups) and separated in different frequencies. a symbol quadruplet consisting of four (QPSK) symbols is mapped. instead of simply mapping the symbols one by one, is the support of transmit diversity. transmit diversity with up to four antenna ports is specified for L1/L2 Transmit diversity for four antenna ports is specified in terms of groups of four symbols (resource elements). To avoid collisions in neighboring cells, the location of the four groups in the frequency domain depends on the physical-layer cell identity. NCHU CSE LTE - 87

87 PCFICH In first OFDM symbol; there are two resource element groups per resource block, as every third resource element is reserved for reference signals (or non-used resource elements corresponding to reference symbols on the other antenna port). NCHU CSE LTE - 88

88 PCFICH In second OFDM symbol; (if part of the control region) there are two or three resource-element groups depending on the number of antenna ports configured. In third OFDM symbol (if part of the control region) there are always three resource-element groups per resource block. NCHU CSE LTE - 89

89 Summary for PCFICH The four resource-element groups are separated by one-fourth of the downlink cell bandwidth in the frequency domain, with the starting position given by physical-layer cell identity. 16 QPSK symbols are used for the transmission of four resource-element groups NCHU CSE LTE - 90

90 Common Control Physical Channel (CCPCH) CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than maximum data rate is the chief consideration. QPSK is therefore the only available modulation format. CCPCH is transmitted as close to the center frequency as possible. CCPCH is transmitted exclusively on the 72 active subcarriers (=6 PRBs) centered on the DC subcarrier. Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol (0..5/6) within the slot and l refers to the subcarrier. NCHU CSE LTE - 91

91 Downlink L1/L2 Control Signaling Downlink control signaling supports the transmission of downlink and uplink transport channels. the corresponding information partly originates from the physical layer (Layer 1) and partly from Layer 2 MAC. Downlink L1/L2 control signaling consists of downlink scheduling assignments, including information required for the terminal to be able to properly receive, demodulate, decode the DL-SCH on a component carrier, uplink scheduling grants informing the terminal about the resources transport format to use for uplink (UL-SCH) transmission hybrid-arq acknowledgements in response to UL-SCH transmissions. NCHU CSE LTE - 92

92 Downlink L1/L2 Control Signaling The downlink L1/L2 control signaling is transmitted within the first part of each subframe. Each subframe can be divided into a control region followed by a data region, where the control region corresponds to the part of the subframe in which the L1/L2control signaling is transmitted. NCHU CSE LTE - 93

93 Downlink L1/L2 Control Signaling The control signaling can be dynamically adjusted radio resources to match the instantaneous traffic situation. For example, a small number of users being scheduled in a subframe, the required amount of control signaling is small and a larger part of the subframe can be used for data transmission. The downlink L1/L2 control signaling consists of four different physical-channel types: PCFICH, informing the terminal about the size of the control region (one, two, or three OFDM symbols). There is one and only one PCFICH on each component carrier or, equivalently, in each cell. NCHU CSE LTE - 94

94 Downlink L1/L2 Control Signaling PDCCH carries signal downlink scheduling assignments and uplink scheduling grants. Each PDCCH carries signaling for a single terminal, but can also be used to address a group of terminals. Multiple PDCCHs can exist in each cell. PHICH, used to signal hybrid-arq acknowledgements in response to uplink UL-SCH transmissions. Multiple PHICHs can exist in each cell. R-PDCCH (Relay Physical Downlink Control Channel), used for relaying. the R-PDCCH is not transmitted in the control region. NCHU CSE LTE - 95

95 Downlink Scheduling Assignment Downlink scheduling assignments are valid for the same subframe in which they are transmitted. The scheduling assignments use one of the DCI formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, or 2C and the DCI formats used depend on the transmission mode configured NCHU CSE LTE - 96

96 Uplink The principle advantage of SC-FDMA over conventional OFDM is a lower PAPR (by approximately 2 db) than would otherwise be possible using OFDM. Data is mapped onto a signal constellation that can be QPSK, 16QAM, or 64QAM depending on channel quality. NCHU CSE LTE - 97

97 Uplink Physical Channels Physical Uplink Shared Channel (PUSCH) Resources for the PUSCH are allocated on a sub-frame basis by the UL scheduler. Subcarriers are allocated in multiples of 12 (PRBs) and may be hopped from sub-frame to sub-frame. The PUSCH may employ QPSK, 16QAM or 64QAM modulation. CRC insertion: 24 bit CRC Physical Uplink Control Channel (PUCCH) It is never transmitted simultaneously with PUSCH data. For frame structure type 2, the PUCCH is not transmitted in the UpPTS field. PUCCH conveys control information including channel quality indication (CQI), ACK/NACK, HARQ and uplink scheduling requests (SR). The PUCCH transmission is frequency hopped at the slot boundary NCHU CSE LTE - 98

98 PUSCH The baseband signal representing the physical uplink shared channel is defined in terms of the following steps: scrambling modulation of scrambled bits to generate complex-valued symbols transform precoding to generate complex-valued symbols mapping of complex-valued symbols to resource elements generation of complex-valued time-domain SC-FDMA signal for each antenna port Scrambling Modulation mapper Transform precoder Resource element mapper SC-FDMA signal gen. NCHU CSE LTE - 100

99 PUCCH The physical uplink control channel supports multiple formats. Formats 2a and 2b are supported for normal cyclic prefix only. PUCCH Modulation Number of bits per format scheme subframe, M bit 1 N/A N/A 1a BPSK 1 1b QPSK 2 2 QPSK 20 2a QPSK+BPSK 21 2b QPSK+QPSK 22 NCHU CSE LTE - 101

100 Summary for Physical Channels Physical broadcast channel (PBCH) The coded BCH transport block is mapped to four subframes within a 40 ms interval; 40 ms timing is blindly detected, i.e. no explicit signalling indicating 40 ms timing; Each subframe is assumed to be self-decodable Physical control format indicator channel (PCFICH) Informs the UE about the number of OFDM symbols used for the PDCCHs; Transmitted in every subframe. Physical downlink control channel (PDCCH) Informs the UE about the resource allocation of PCH and DL-SCH, and Hybrid ARQ information related to DL-SCH; Carries the uplink scheduling grant. (rf. MAPs) Physical Hybrid ARQ Indicator Channel (PHICH) Carries Hybrid ARQ ACK/NAKs in response to uplink transmissions. Physical downlink shared channel (PDSCH) Carries the DL-SCH and PCH. Physical multicast channel (PMCH) Carries the MCH. NCHU CSE LTE - 102

101 Summary for Physical Channels Physical uplink control channel (PUCCH) Carries Hybrid ARQ ACK/NAKs in response to downlink transmission; Carries Scheduling Request (SR); (rf. BW-REQ) Carries CQI reports. Physical uplink shared channel (PUSCH) Carries the UL-SCH. Physical random access channel (PRACH) Carries the random access preamble. NCHU CSE LTE - 103

102 Uplink Physical Signals Uplink physical signals are used within the PHY and do not convey information from higher layers. Two types of UL physical signals are defined: Demodulation reference signal, associated with transmission of PUSCH or PUCCH Sounding reference signal, not associated with transmission of PUSCH or PUCCH NCHU CSE LTE - 104

103 Uplink Reference Signals The demodulation signal facilitates coherent demodulation. It is transmitted in the fourth SC-FDMA symbol of the slot and is the same size as the assigned resource. There is also a sounding reference signal used to facilitate frequency dependent scheduling. Both variants of the UL reference signal are based on Zadhoff-Chu sequences. NCHU CSE LTE - 105

104 Downlink Channel Coding Different coding algorithms are employed for the DL physical channels. The PDSCH uses up to 64-QAM modulation. Rate 1/3 turbo coding has been selected for the PDSCH. For the common control channel (CCPCH), modulation is restricted to QPSK. For control channels, coverage is the paramount requirement. Convolutional coding has been selected for use with the CCPCH, though a final determination regarding code rate has not yet been made. NCHU CSE LTE - 106

105 Channel Coding and Interleaving Channel coding scheme for transport blocks Turbo Coding with a coding rate of R=1/3, Two 8-state constituent encoders and A contention-free quadratic permutation polynomial (QPP) turbo code internal interleaver. Maximum information block size of 6144 bits Error detection is supported by the use of 24- bit CRC NCHU CSE LTE - 107

106 Channel Coding Usage of channel coding scheme and coding rate for transport CHs TrCH Coding scheme Coding rate UL-SCH DL-SCH PCH Turbo coding 1/3 MCH BCH Tail biting convolutional coding 1/3 Usage of channel coding scheme and coding rate for control information Control Information Coding scheme Coding rate Tail biting DCI convolutional coding 1/3 CFI Block code 1/16 HI Repetition code 1/3 UCI Block code Tail biting convolutional variable 1/3 coding NCHU CSE LTE - 108

107 Channel Coding Channel coding for DL-SCH (as well as for PCH and MCH) is based on Turbo coding. The encoding consists of two rate-1/2, eight-state constituent encoders, implying an overall code rate of 1/3, in combination with QPP-based interleaving. QPP : Quadrature Permutation Polynomial. NCHU CSE LTE - 109

108 Channel Coding The QPP interleaver provides a mapping from the input (noninterleaved) bits to the output (interleaved) bits according to the function: i is the index of the bit at the output of the interleaver, c(i) is the index of the same bit at the input of the interleaver, K is the code-block/interleaver size. The values of the parameters f 1 and f 2 depend on the code-block size K. The range of code-block sizes is from 40 bits to 6144 bits, together with the associated values for the parameters f 1 and f 2. NCHU CSE LTE - 110

109 Downlink Physical-Layer Processing (10) LTE downlink, there are four different types of transport channels defined, the Downlink Shared Channel (DL-SCH), the Multicast Channel (MCH), the Paging Channel (PCH), the Broadcast Channel (BCH). DL-SCH is mapping to the resource elements of the OFDM time frequency grid. To transmit user data and dedicated control information, as well as part of the downlink system information. The DL-SCH physical-layer processing is to a large extent applicable. also to MCH and PCH transport channels, although with some additional constraints. NCHU CSE LTE - 111

110 Processing Steps Within each transmission time interval (TTI), corresponding to one subframe of length 1 ms, up to two transport blocks of dynamic size are delivered to the physical layer and transmitted over the radio interface for each component carrier. NCHU CSE LTE - 112

111 Processing Steps In the case of no spatial multiplexing there is at most a single transport block in a TTI. In the case of spatial multiplexing, with transmission on multiple layers in parallel to the same terminal, there are two transport blocks within a TTI. CRC Insertion Per Transport Block a 24-bit CRC is calculated for and appended to each transport block. NCHU CSE LTE - 113

112 Code-Block Segmentation and Per-Code- Block CRC Insertion The LTE Turbo-coder internal interleaver is only defined a maximum block size of 6144 bits. including the transport-block CRC, exceeds this maximum code-block size, code-block segmentation, is applied before the Turbo coding. Code-block segmentation implies that the transport block is segmented into smaller code blocks. matching the set of code-block sizes supported by the Turbo coder. the specification includes the possibility to insert dummy filler bits at the head of the first code block. NCHU CSE LTE - 114

113 Code-Block Segmentation and Per-Code- Block CRC Insertion Transport block implies that an CRC is calculated for and appended to rear. CRC is small for the transport block. The code block also adds additional error-detection capabilities (using CRC) and thus further reduces the risk for undetected errors in the code block. NCHU CSE LTE - 115