LTE E-UTRAN and its Access Side Protocols - PDF

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White Paper LTE E-UTRAN and its Access Side Protocols By: Suyash Tripathi, Vinay Kulkarni and Alok Kuma Overview The journey which initially started with UMTS 99 aiming for high peak packet data rates
White Paper LTE E-UTRAN and its Access Side Protocols By: Suyash Tripathi, Vinay Kulkarni and Alok Kuma Overview The journey which initially started with UMTS 99 aiming for high peak packet data rates of 2Mbps with support for both voice and data services is approaching a new destination known as Long Term Evolution. LTE targets to achieve 100Mbps in the downlink (DL) and 50 Mbps in the uplink (UL) directions with user plane latency less than 5ms due to spectrum flexibility and higher spectral efficiency. These exceptional performance requirements are possible due to Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input and Multiple-Output (MIMO) functionality in the radio link at the physical layer. The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), the very first network node in the evolved packet system (EPS), achieves high data rates, lower control & user plane latency, seamless handovers, and greater cell coverage. The purpose of this paper is to highlight the functions, procedures, and importance of the access stratum particularly the radio access side protocols pertaining to E-UTRAN. CONTENTS E-UTRAN, Functions within the Access Stratum pg. 2 Radio Protocol Architecture Access Stratum, User Plane Protocols, Control Plane Protocols pg. 3 Physical Layer for E-UTRAN, Physical Resource in LTE pg. 4 Mapping of Physical, Transport and Logical Channels, Cell Configuration, Link Adaptation, Synchronization Procedures and System Acquisition pg. 5 Cell Search, Slot and Frame Synchronization pg. 6 Random Access Procedure System Access, Contention-Based Random Access Procedure pg. 7 Non-Contention-Based Random Access Procedure, Physical Layer Measurements, Power Control, Layer 2 pg. 8 Transport of NAS Messages pg. 9 E-UTRAN Identities, ARQ and HARQ Processes, ARQ Principles pg. 10 HARQ Principles, Uplink HARQ Operation, Measurement Management pg. 11 QoS Management, QoS Parameters, Bearer Service Architecture pg. 12 Scheduling, CQI Reporting for Scheduling, Downlink Scheduling pg. 13 Uplink Scheduling, Non-Persistent (Dynamic) Scheduling, Persistent Scheduling, Semi-Persistent Scheduling pg. 14 Security, Security Termination Points, Radio Resource Management pg. 15 Summary, References, Authors pg. 17 2 E-UTRAN The E-UTRAN consists of enodebs (enb) which provide E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations toward the user equipment (UE). The enbs are interconnected with each other by means of the X2 interface. The enbs are also connected by means of the S1 interface to the Evolved Packet Core (EPC), more specifically to the Mobility Management Entity (MME) by means of the S1-MME interface and to the Serving Gateway (SGW) by means of the S1-U interface. The S1 interface supports manyto-many relations between MMEs/Serving Gateways and enbs. The E-UTRAN architecture is illustrated in Figure 1. Functions within the Access Stratum The access stratum provides the ability, infrastructure, and accessibility to the UE in acquiring the capabilities and services of the network. The radio access protocols in the E-UTRAN access stratum are comprised of numerous functionalities: Radio Resource Management (RRM) performs radio bearer control, radio admission control, connection mobility control, and dynamic allocation of resources to UEs in both UL and DL (scheduling) Traffic Management, in conjunction with radio resource management, does the following: Supports real- and non-real-time user traffic between the non-access stratum (NAS) of the infrastructure side and the UE side Supports different traffic types, activity levels, throughput rates, transfer delays, and bit error rates Efficiently maps the traffic attributes used by non-lte applications to the attributes of the radio access bearer layer of the access stratum IP header compression and encryption of user data streams Figure 1. E-UTRAN Architecture Selection of an MME at UE attachment when no MME information is provided by the UE Routing of User Plane data toward the SGW Location Management: scheduling and transmission of paging messages (originated from the MME) Scheduling and transmission of broadcast information (originated from the MME or O&M) Measurement and measurement reporting configuration for mobility and scheduling Scheduling and transmission of Earthquake and Tsunami Warning System (ETWS) messages (originated from the MME) Provides initial access to the network, registration, and attach/detach to/from the network Handover Management Intra-eNodeB, IntereNodeB, Inter-eNodeB with change of MME, Inter-eNodeB with same MME but different SGW, and Inter-RAT handovers Macro-diversity & Encryption Radio channel coding 3 Radio Protocol Architecture Access Stratum Logically, LTE network protocols can be divided into control plane (responsible for managing the transport bearer) and user plane (responsible for transporting user traffic). User Plane Protocols Figure 2 shows the protocol stacks for the user plane, where PDCP, RLC, MAC, and PHY sublayers (terminated at the enb on the network side) perform functions like header compression, ciphering, scheduling, ARQ, and HARQ. Control Plane Protocols Figure 3 shows the protocol stacks for the control plane, where: PDCP sublayer performs ciphering and integrity protection RLC, MAC, and PHY sublayers perform the same functions as in the user plane RRC performs functions like System Information Broadcast, Paging, RRC connection management, RB control, Mobility Control, and UE measurement reporting and control Figure 4 depicts the access side protocol suite consisting of RRC, PDCP, RLC, MAC, and PHY layers. RRC configures the lower layers PDCP, RLC, MAC & PHY for respective parameters required at run time for their functionalities. Radio Bearers (RB) exist between RRC & PDCP which are mapped to various logical channels lying between RLC & MAC. There is well-defined mapping between logical channels to transport channels to physical channels as highlighted in Figure 7. Figure 2. User Plane Protocol Stacks Figure 3. Control Plane Protocol Stacks Figure 4. Access Side Protocol Suite at enodeb 4 Physical Layer for E-UTRAN Frame Type in LTE LTE downlink and uplink transmissions are organized into radio frames with 10ms duration. LTE supports two radio frame structures: Type 1, applicable to FDD (paired spectrum) Type 2, applicable to TDD (unpaired spectrum) Figure 5.1. Frame Structure Type 1 Frame structure Type 1 is illustrated in Figure 5.1. Each 10ms radio frame is divided into ten equally sized sub-frames (1ms each). Each sub-frame consists of two equally-sized slots of 0.5ms length. In FDD, uplink and downlink transmissions are separated in the frequency domain. Frame structure Type 2 is illustrated in Figure 5.2. Each 10ms radio frame consists of two half-frames of 5ms each. Each half-frame consists of eight slots of length 0.5ms and three special fields: DwPTS, GP, and UpPTS. The length of DwPTS and UpPTS is configurable subject to the total length of DwPTS, GP, and UpPTS equal to 1ms. Subframe 1 in all configurations, and subframe 6 in the configuration with 5ms switch-point periodicity, consist of DwPTS, GP, and UpPTS. Subframe 6 in the configuration with 10ms switch-point periodicity consists of DwPTS only. All other subframes consist of two equally-sized slots. Figure 5.2. Frame Structure Type 2 For TDD, GP is reserved for downlink-to-uplink transition. Other subframes/fields are assigned for either downlink or uplink transmission. Uplink and downlink transmissions are separated in the time domain. Physical Resource in LTE The LTE physical resource is a time-frequency resource grid where a single resource element corresponds to one OFDM subcarrier during one OFDM symbol interval with carrier spacing (Δf = 15kHz). 12 consecutive subcarriers are grouped to constitute a resource block, the basic unit of resource allocation. In normal CP (cyclic prefix) mode, one time slot contains 7 OFDM symbols and in extended CP there are 6 symbols. Figure 6. Physical Resource in LTE 5 Mapping of Physical, Transport and Logical Channels Figure 7 depicts the mapping between different types of logical channels, transport channels, and physical channels in LTE. Cell Configuration When an enodeb comes up, it involves initialization of hardware and performs hardware tests (memory and peripherals) followed by the bringing up of cells. An enodeb can be associated with more than one cell, of course, and bringing up of a cell involves configuring various common resources. The following configurations happen as part of cell configuration: Physical layer resources (bandwidth, physical channel resources, etc.) Layer 2 MAC resources (logical channel configuration, transport channel configuration, scheduling configuration, etc.) Layer 2 RLC resources (common radio bearers for broadcast, paging, SRB0, etc.) A camped UE on a cell shall be able to do the following: Receive system information from the Public Land Mobile Network (PLMN) If the UE attempts to establish an RRC connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped Listen to paging messages Receive ETWS notifications Some of the cell parameters might be reconfigured and the same is reflected to UEs by means of broadcasted system information. Figure 7. Mapping of Different Channels at the enodeb Link Adaptation LTE link adaptation techniques are adopted to take advantage of dynamic channel conditions. Link adaptation is simply the selection of different modulation and coding schemes (MCS) according to the current channel conditions. This is called adaptive modulation and coding (AMC) applied to the shared data channel. The same coding and modulation is applied to all groups of resource blocks belonging to the same L2 protocol data unit (PDU) scheduled to one user within one transmission time interval (TTI) and within a single stream. The set of modulation schemes supported by LTE are QPSK, 16QAM, 64QAM, and BPSK. The various types of channel coding supported in LTE different for different channels are Turbo coding (Rate 1/3), Convolution coding (Rate 1/3 Tail Biting, Rate 1/2), Repetition Code (Rate 1/3), and Block Code (Rate 1/16 or repetition code). Synchronization Procedures and System Acquisition The enodeb provides all the necessary signals and mechanisms through which the UE synchronizes with the downlink transmission of the enb and acquires the network to receive services. 6 Cell Search Cell search is the procedure by which a UE acquires time and frequency synchronization with a cell and detects the cell ID of that cell. E-UTRA cell search supports a scalable overall transmission bandwidth corresponding to 6 resource blocks (i.e., 72 subcarriers) and upwards. E-UTRA cell search is based on various signals transmitted in the downlink such as primary and secondary synchronization signals, and downlink reference signals. The primary and secondary synchronization signals are transmitted over the center 72 sub-carriers in the first and sixth subframe of each frame. Neighbor-cell search is based on the same downlink signals as the initial cell search. Slot and Frame Synchronization The UE, once powered-up and after performing memory and peripheral hardware tests, initiates downlink synchronization and a physical cell identity acquisition procedure. The UE attempts to acquire the central 1.4MHz bandwidth in order to decode the Primary sync signal (PSCH), Secondary sync signal (SSCH), and the system information block (SIB). The enodeb transmits this information on the subcarriers within the 1.4MHz bandwidth consisting of 72 subcarriers, or 6 radio blocks. In order to perform slot synchronization, the UE attempts to acquire the Primary sync signal which is generated from Zadoff-Chu sequences. There are three possible 62-bit sequences helping the UE to identify the start and the finish of slot transmissions. Next, the UE attempts to perform frame synchronization so as to identify the start and the finish of frame transmission. In order to achieve this, Primary sync signals are used to acquire Secondary sync signals. The Secondary sync signal (a 62-bit sequence) is an interleaved concatenation of two length-31 binary sequences scrambled with the Primary synchronization signal. Once PSCH and SSCH are known, the physical layer cell identity is obtained. Figure 8. Position of Physical Channels in the Time-Frequency Domain in LTE The physical layer cell identities are grouped into 168 unique physical layer cell identity groups, with each group containing three unique identities. The grouping is such that each physical layer cell identity is part of one and only one physical layer cell identity group. There are 168 unique physical-layer cell-identity groups (ranging from 0 to 167), and three unique physical-layer identities (0, 1, 2) within the physical layer cell identity group. Therefore, there are 504 unique physical layer cell identities. Figure 8 depicts the placement of PSCH, SSCH, and PBCH along with other physical channels in the central 1.4MHz (6 RBs, or 72 subcarriers). The UE is now prepared to download the master information block (MIB) that the enodeb broadcasts over the PBCH. The MIB (scrambled with cellid) reception provides the UE with LTE downlink bandwidth (DL BW), number of transmit antennas, system frame number (SFN), PHICH duration, and its gap. After reading the MIB, the UE needs to get system information blocks (SIBs) to know the other system-related information broadcasted by the enodeb. SIBs are carried in the PDSCH, whose information is obtained from the PDCCH indicated by the control format indicator (CFI) field. In order to get CFI information, the UE attempts to read the PCFICH which are broadcasted on the first OFDM symbol of the subframe as shown above in Figure 8. Once bandwidth selection is successful, the UE attempts to decode the DCI (DL control information) to acquaint with SIB Type 1 and 2 to get PLMN id, cell barring status, and various Rx thresholds required in cell selection. 7 Random Access Procedure System Access The UE cannot start utilizing the services of the network immediately after downlink synchronization unless it is synchronized in the uplink direction too. The Random Access Procedure (RAP) over PRACH is performed to accomplish the uplink synchronization. RAP is characterized as one procedure independent of cell size and is common for both FDD & TDD. The purpose of RAP is highlighted in Figures 9, 10, and 11. Figure 9. Purposes of Random Access Procedure The RAP takes two distinct forms: contention-based (applicable to all five events mentioned in Figure 9) and non-contention-based (applicable only to handover and DL data arrival). Normal DL/UL transmission can take place after the RAP. Contention-Based Random Access Procedure Multiple UEs may attempt to access the network at the same time, therefore resulting in collisions which make contention resolution an essential aspect in the RAP. The UE initiates this procedure by transmitting a randomly chosen preamble over PRACH. Figure 10. Contention-Based RACH Procedure Figure 11. Non-Contention-Based RACH Procedure 8 Non-Contention-Based Random Access Procedure The network initiates this procedure, when the UE is already in communication with the enodeb, by transmitting an allocated preamble to the UE. There are no collisions with other UEs because the enodeb controls the procedure and hence has the necessary information to support a non-contention-based RAP. Physical Layer Measurements The LTE physical layer measurements to support mobility are classified as: Within E-UTRAN (intra-frequency, inter-frequency) Between E-UTRAN and GERAN/UTRAN (inter-rat) Between E-UTRAN and non-3gpp RAT (Inter-3GPP access system mobility) For measurements within the E-UTRAN, at least two basic UE measurement quantities shall be supported: Reference symbol received power (RSRP) E-UTRA carrier received signal strength indicator (RSSI) Power Control Apart from providing high data rates and greater spectral efficiency, efficient usage of power is another crucial aspect being considered in LTE. Power control is being supported in both uplink as well as downlink directions. Implementation of intelligent power control schemes is a critical requirement for all enodebs. Power control efficiencies focus on: Limiting power consumption Increasing cell coverage, system capacity, and data rate/voice quality Minimizing interference at the cell edges Uplink power control procedures are relevant in controlling transmit power for the uplink physical channels. Power control procedures on PRACH are slightly different from those on the PUCCH and PUSCH channels. During the RAP the physical layer takes care of the preamble transmission. Since there is no RRC connection established at this point, the actual transmission power must be estimated by the UE. This is done through estimating the downlink path loss with the help of parameters alpha and TPC step size available in the SIB2 broadcasted by the enodeb. While controlling the transmit power on PUCCH and PUSCH, the enodeb continues to measure the uplink power and compares it with the established reference. Based on the comparison, the enodeb issues the power corrections known as transmit power control (TPC) commands through the DCI format to the UE. This TPC command carries the power adjustment information, and upon receiving power adjustments the UE aligns itself to the value assigned by the enodeb. Apart from standard power control procedures, there are a few other features that assist in effective power utilization at the UE. Discontinuous Reception (DRX) is one such feature which is leveraged from previous technologies such as GERAN and UMTS. The enodeb can instruct a UE to control its PDCCH monitoring activity, the UE s C-RNTI, TPC-PUCCH-RNTI, TPC- PUSCH-RNTI, and Semi-Persistent Scheduling C-RNTI (if configured). Layer 2 Layer 2 is divided into the three sublayers: Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Convergence Protocol (PDCP). PDCP: PDCP provides data transfer, header compression using the Robust Header Compression (RoHC) algorithm, ciphering for both user and control planes, and integrity protection for the control plane. 9 Figure 12. Layer 2 Structure in LTE RLC: RLC performs segmentation and reassembly and error correction functions using ARQ (in Acknowledged Mode). MAC: MAC maps logical channels (mapped to radio bearers) to transport channels, multiplexes/demultiplexes MAC SDUs from one or more logical channels onto transport blocks on transport channels, performs scheduling of resources, error correction using HARQ, and transport format selection. Figure 12 highlights various functional modules or entities in the different sublayers of LTE Layer 2. Transport of NAS Messages The access stratum (AS) provides reliable in-sequence delivery of non-access stratum (NAS) messages in a cell. In E-UTRAN, NAS messages are either concatenated with RRC messages or carried in RRC without concatenation. In the downlink direction, when an evolved packet system (EPS) bearer establishment or release procedure is triggered, the NAS message is concatenated with the associated RRC message. When the EPS bearer and/or radio bearer are/is modified, the NAS message and associated RRC message are concatenated. In the uplink direction, concatenation of NAS messages with an RRC message is used only for transferring the initial NAS message during connection setup. 10 Identities Description UE-Id NW-Id C-RNTI Semi-Persistent Scheduling C-RNTI Temporary C-RNTI TPC-PUSCH-RNTI TPC-PUCCH-RNTI RA-RNTI Unique identification at cell level Identifies RRC connection Used for scheduling Unique identification used for semi-persistent scheduling Identification used for the random access procedure Identification used for the power control of PUSCH Identification used for the p
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