Amir ahmadian tlt-6507-seminar report final version-

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Amir ahmadian tlt-6507-seminar report final version-

  1. 1. An Overview of the LTE Radio-Interface Architecture Amir Mehdi Ahmadian Electronics and Communications Department, Tampere University of Technology Tampere,Finland amir.ahmadiantehrani@tut.fi Abstract— In this report we consider the advent of the future cellular networks known as Long-Term Evolution (LTE) radio interface architecture. Taking into account the variety of system architectures and access networks, the functionality of radio protocols in order to provide reliable, efficient data flow along with deploying comprehensive channel protocol model, scheduling mechanism and QoS plan must be accounted as well. Keywords— E-UTRAN, LTE, EPC, SAE, eNodeB I. INTRODUCTION Developments behind radio interface in Wideband Code Division Multiple access (WCDMA) successor, known as the Long-Term Evolution (LTE) soon became clear that the system architecture would also need to be evolved. Since the LTE is the first cellular communication system optimized to support packet-switched services on the one hand, and it had been shown the co-located efficient functionality performed in NodeB on the other hand, the System Evolution Architecture (SAE) followed the radio interface development and it was agreed to schedule the completion of work in Release 8. According to [1], the following lists some of aims shaped the outcomes:  Optimization for packet switched services in general, when there is longer a need to support the circuit switched mode of operation  Optimized support for higher throughput required for higher end user bit rate  Improvement in the response times for activation and bearer set-up  Improvement in the packet delivery delays  Optimized inter-networking with other 3GPP access networks  Optimized inter-networking with other wireless access networks In this work the overall architecture for both Radio Access Network (RAN) and the Core Network (CN) were considered separately from functionality issues point of view led to flatter RAN architecture and the Evolved Packet Core (EPC). All the radio related functionality of the overall network like scheduling, radio resource handling and retransmission protocols are controlled and planned in RAN. Whereas, all issues regarded to providing a complete mobile-broadband network like authentication or end-to-end setup connections are handled separately by the EPC having several radio-access technologies. Driving different architecture developments has indicated the need to introduce a set of new functions and maybe new interfaces to support specific protocols for each one of them. Considering the target of keeping architecture simple and supporting all potential inter-networking scenarios, the 3GPP architecture specifications were split into three deployment scenarios:  Basic system architecture with only E-UTRAN (Evolved Universal Terrestrial Radio Access Network).  Legacy 3GPP basic system architecture with existing E-UTRAN.  E-UTRAN basic system architecture with non-3GPP access networks. II. SYSTEM ARCHITECTURE A. Basic Architecture with Only E-UTRAN An overall overview of simplest architecture and network elements proposed by 3GPP TS 23.401[6] is depicts in Figure1. The new architectural development comparing to those in existing 3GPP is limited to EPC and E-UTRAN(or RAN) As it can be seen UE,E-UTRAN and EPC represent the main function provides the IP connectivity turns out this part to be called Evolved Packet System (EPS). Apart from EPC and E-UTRAN as main new network elements, it is seen an element called SAE GW which the combination of the two gateways ,Serving Gateway (S-GW) and Packet Data Network (P-GW) defined for unit protocol handling in EPC. Following we have a brief explanation of each network element is provided: User Equipment (UE): typically it is a hand held device such as a smart phone or a data card such as those used currently in 2G and 3G. Functionally the UE is a platform for communication applications, which signal with the network
  2. 2. for setting up, maintaining and removing the communication links the end user needs.[3]    Fig. 1 Basic System Architecture with Only E-UTRAN  E-UTRAN NodeB (eNodeB): the main developments in E UTRAN is concentrated on this complex logical node which Is the termination point for all radio related protocols. As Figure2 represents the eNodeB is connected to the EPC by means of the S1 interface, more specifically to the S-GW by means of the S1 user-plane part, S1-u, and to the MME by means of the S1 control-plane part, S1-c to play important role in Mobility Management specially includes exchanging handover signalling between other eNodeBs and MME.. One eNodeB can be connected to multiple MMEs/S-GWs for the purpose of load sharing and redundancy.[3] As a network, EUTRAN is simply a mesh of eNodeBs connected to neighboring eNodeBs with the X2 interface. Moreover, the X2 interface is used for active-mode support mobility and multicell radio Radio Recourse Management (RRM) functions like Inter-cell interference Coordination (ICIC). Figure3 summarizes all eNodeB responsibilities in a nutshell.[2] Apart from user management or ensuring QoS like latency and minimum bandwidth requirements for real-time bearers, eNodeB serves a set of both MMEs and S-GW being pooled to a single eNodeB and it has to keep track of this association. Evolved Packet Core (EPC): as a dedicatedly packetswitched support core network structured represented in Figure 1 & 4 , contains 5 following main elements:  Mobility Management Entity (MME) : as the Main control element in the EPC is responsible for connection/release of bearers to a terminal and has a logically direct CP connection to the UE to provide functions like authentication, handover support, establishment of bearers, Internetworking with other radio networks and the mobility of functionality operating between the EPC and the terminal known as Non-Access Stratum (NAS). Serving Gateway(S-GW) : as a high level operator of unit protocol management and switching functionality, it mainly acts as mobility anchor when terminals move between eNodeBs as well as mobility anchor between other technologies (GSM/GPRS and HSPA ) made by MME. Moreover, it handles the collection information and statistics necessary for charging[]. Along with configuring in a one-to-many fashion , One S-GW may be serving only a particular geographical area with a limited set of eNodeBs and likewise there may be a limited set of MMEs that control that area. Packet Data Network Gateway (P-GW) or (PDNGW): acting as the highest level mobility anchor in the system, it is the edge router between the EPS and external packet data networks. Simply put, it provides IP address allocation to the UE. Policy and Charging Resource Function (PCRF): all the operations regarding policies and procedures is charged by a server usually located in operator switching centres in RNC. Home Subscription Server (HSS) : records the location of the user in the level of visited network control node such as MME along with subscribing data repository for all permanent user. Briefly, in all signaling related integrity protection or authentication HSS interacts with MMS. Fig. 2 Evolved UTRAN Architecture B. Basic Architecture with E-UTRAN and Legacy 3GPPP Access Networks As one of the common 3GPP Inter-working system architecture configurations, provides the similar connectivity services from the end user point of view via different functionalities (Figure5).As it can be seen, all defined 3GPP access networks are connected to EPC. Main issues concerned in this system are to how the bearers are managed in the EPS compared to the existing networks with UTRAN or GERAN access along with optimized connectivity between GERAN (GSM/EDGE Radio Access Network) and UTRAN as before
  3. 3. by assuming the S-GW as role of GGSN (Gateway GPRS Support Node). Therefore, a few new interfaces are needed as it is shown in EPC, UTRAN and GERAN. Moreover, optimized inter-working is achieved when the network is in control of mobility events, such handovers and minimizing interruptions in services.[1] optimizations, and the same procedures are applicable in both connected and idle mode.[1] Whereas, the second category includes a specific solution for cdma 2000 HRPD (High Rate Packet Data). Considering the only the first category (Figure6) in this report, it relies only on loose coupling with generic interfacing means, and without AN level interfaces, there are so many different kinds of ANs, they have been categorized to two groups, the trusted and un-trusted non-3GPP ANs, depending on whether it can be safely assumed that 3GPP defined authentication can be run by the network, which makes it trusted, or if authentication has to be done in overlay fashion and the AN is un-trusted.[4] depending on which access network is un-trusted or trusted, the non-3GPP access networks are connected to it either via the S2a or the S2b interface. In addition, the UE may register in any non-3GPP AN, receive an IP address from there, and register that to the Home Agent in P-GW. Fig. 3. eNodeB connections and main functions Fig. 5. System architecture for 3GPP access networks Fig. 4. EPC Overall Architecture This solution addresses the mobility as an overlay function. While the UE is served by one of the 3GPP ANs, the UE is considered to bein home link, and thus the overhead caused by additional MIP headers is avoided. Thus, this system architecture requires additional interfaces and updated logical elements supporting different scenarios described in 3GPP Release 8. C. Basic Architecture with E-UTRAN and Non-3GPPP Access Networks According to [4], the Inter-working with non-3GPP access networks was one of the key design goals for SAE, and to support it a completely different architecture designed in 3GPP.The non-3GPP Inter-working System Architecture includes a set of solutions in two categories. The first category contains a set of generic and loose inter-working solutions that can be used with any other non-3GPP AN. Mobility solutions defined in this category are also called handovers wi thout Fig. 6. System architecture for 3GPP and non-3GPP access networks
  4. 4. III. RADIO PROTOCOL ARCHITECTURE Generally speaking, protocols entities are common to the user and control planes. However, the protocol stack split into two parts.[2]&[5] as Figure7 illustrates the control plane protocols are shown in the left side starts with NAS layer used for mobility management and other purposes between the mobile device and the MME. NAS messages are tunneled through the radio network, and the eNodeB just forwards them transparently. Radio Resource Control (RRC) managing the air interface connection used, for example, for handover along with encapsulating NAS messages. In fact, The main difference on the user data plane shown on the right of Figure 7 is that RRC message does not necessarily have to include a NAS message. Therefore, IP packets are always transporting user data and are sent only if an application wants to transfer data. attachment by physical layer to the transport block would be the final for the relevant operations. IV. LTE DOWNLINK & UPLINK CHANNEL MODEL All higher layer signaling and user data traffic are organized in channels [7]. All protocols regarding transport, physical and logical layers for downlink and uplink channels are demonstrated in Figure10. This layered structure is to separate the logical data flows from the properties of physical channel. Briefly speaking, downlink channel model elements have depicted bellow: Downlink Logical Channles:     Paging Control Channel (PCCH) : used for paging of terminals whose location on a cell level is not known to the network .The paging message therefore needs to be transmitted in multiple cells Broadcast Control Channel (BCCH): used for transmission of system information from the network to all terminals in a cell . Dedicated Traffic Channel (DTCH): used for transmission of user data to/from a terminal and for transmission of all uplink user data. Dedicated Control Channel (DCCH): used for individual configuration of terminals such as different handover messages. Fig. 7. Radio interface protocol stack and main functions Packet Data Convergence Protocol (PDCP) is in charge of encapsulating as the first unifying transport layer IP packets for ciphering to reduce the number of bits to transmit over the radio interface. One layer below is RLC (Radio Link Control) which is responsible for is responsible for segmentation /concatenation , retransmission handling of higher layer packets to adapt them to a packet size that can be sent over the air interface. MAC layer simply speaking, multiplexes data fromdifferent radio bearers and ensures QoS and also responsible for HARQ packet retransmission functionality and address field ,power management and time advance control. Keep all radio protocol layers in mind, Figure 8 illustrates the architecture for downlink in LTE . remember that not all functionalities pointed out can be applicable in all scenarios. As example, hybrid ARQ with soft combining is not used for broadcast of the basic system information [3]. Simply put, the LTE data flow represents in Figure 9 in order to distinguish each radio protocol layer functionality precisely. After adding RLC header the RLC PDU (Packet Data Unit) is forwarded to MAC layer. After multiplexing the MAC layer would be added to transport block e formed. CRC Fig.8. Downlink Radio interface protocols in LTE
  5. 5. Downlink Transport Channles:     Transport format (TF) : Specifying how the transport block is to be transmitted over the radio interface includes information about transport-block size , modulation and coding scheme and antenna mapping . Broadcast Channel (BCH): has a fixed transport format used for transmission of parts of the BCCH system information, more specifically Master Information Block (MIB). Paging channel (PCH) : is used for transmission of paging information from the PCCH logical channel . Downlink Shared Channel (DL-SCH) : the main transport channel used for transmission of downlink data in LTE supporting features like dynamic rate adaption and channel dependent scheduling , hybrid ARQ with soft combining ,possibility of transmitting of BCCH system not mapped to the BCH. Downlink Physical Channels:  Physical Downlink Shared Channel (PDSCH) : The main physical channel used for unicast data transmission and transmission of paging information.  Physical Broadcast Channel (PBCH) : Carries part of system information , required by terminal in order to access the network.  Physical Downlink Control Protocol (PDCCH): Used for downlink scheduling information required for reception of PDSCH and aslo enabling transmission on the PUSCH. Fig.10. LTE Downlink Channels by how and with what characteristics the information is transmitted over the radio interface. In uplink channel a similar channel model is used as in the downlink direction as we would have transport, logical and physical channel (Figure11) . The most important channel here is Physical Uplink Shared Channel (PUSCH) which carries the user data in addition to signaling information and signal quality feedback. Other channels have been explained briefly by the followings:  Common Control Channel (CCCH) : transports  signaling messages establishment. during connection Physical Random Access Channel (PRACH) : synchronizing and requesting initial uplink resources Ensures the contention based procedure is performed when the connection establishment is repeated Fig.11. LTE Uplink Channels V. SCHEDULING Fig.9. LTE Data Flow Example As it is shown in Figure10, a transport channel is defined Data transmissions in LTE in both the uplink and the downlink directions are controlled by the network [3]
  6. 6. Which is eNodeB as high level radio network control in LTE. Therefore, in LTE we would have network-based scheduling being advantageous in network reaction to changing radio conditions of each user and ensuring the QoS for each user. As it is shown in Figure12, the eNode-B’s scheduler is responsible for forwarding the data that it receives from the network, for all users it serves in both downlink and uplink over the air interface. Since the transmission buffer is not always quiet, the scheduler has to decide which users and bearers are given an assignment grant for the next sub frame and how much capacity is allocated to each. Therefore, a dynamic scheduling should is taken into account [2]&[4]: For each eNodeB decides the number of users wants to schedule and the number of resource blocks that are assigned that are assigned to each user.  Required number of symbols on the time axis in each sub frame for the control region.  Depending on the system configuration and the number of users to schedule, one to four symbols are used across the complete bandwidth for control region. The downlink scheduler is responsible for (dynamically) controlling which terminal(s) to transmit to and, for each of these terminals, the set of resource blocks upon which the terminal’s DL-SCH should be transmitted. Whereas, The uplink scheduler serves per terminal a similar purpose, namely to (dynamically) control which terminals are to transmit on their respective UL-SCH and on which uplink time–frequency resources (component carrier). Thus,the eNodeB scheduler controls the transport format and the terminal controls the logical-channel multiplexing.  Fig. 12. Transport format selection in downlink and uplink VI. QOS The development of the SAE bearer model and the QoS concept started with the assumption that improvements compared to the existing 3GPP systems with, e.g. UTRAN access, should be made, and the existing model should not be taken for granted.[8] There has been many QoS parameters introduced [1], however the most common ones are: Priority : used to define the priority for packet scheduling of the radio interface Delay Budget: Helps the packet scheduler to maintain sufficient scheduling rate to meet the delay requirements for the bearer Loss Rate :Helps to use appropriate RLC settings number of re-transmission Considering accuracy and integrity in QoS plan following problems might be encountered[8]:  It had not seen easy for operators to use Qos in legacy 3GPP systems  It may Only reduced set of parameters for SAE  Network resource management is the solely network controller  Network decides how the parameters are set and main bearer set-up logic consists of only one signaling transaction from the network to the UE. VII. CONCLUSIONS To summarize the discussion, we could list up the followings :  the overall system architecture of LTE including EUTRAN and EPC were revisited in 3GPP but separately unlike previous systems even though it is mainly focused on E-UTRAN as the most complex logical node.  Radio interface architecture in LTE adds different features to facilitate configuration with different access networks although not all architectures are applicable in all scenarios.  Radio access network functionalities in LTE has been efficiently compressed and colocated in order to compensate redundancy and increase integrity and accuracy .  Scheduling plays an important role in both uplink and downlink channel to dedicate shared resources through sub frames in MAC layer.  The overall goal for network orientation in bearer set-up is to minimize the need for QoS knowledge and configuration in the UE.  Apart from different effective parameters , the QoS plan in LTE must be comprehensive enough to cover all legacy 3GPP scenarios as before
  7. 7. References [1] [2] [3] [4] [5] [6] [7] [8] H. Holma and A. Toskala , LTE for UMTS OFDMA and SC-FDMA based radio access, Willey & Sons, 2009. E. Dahlman, E. Parkvall, J. Skold 4G LTE/LTE Advanced for Mobile Broadband, Elsevier, 1989, vol. 61. M. Sauter, LTE From GSM to LTE, Willey & Sons, 2011. M. Baker, . LTE the UMTS Long term Evolution from theory to practice, Willey & Sons, 2009. 3GPP TS 36.413, ‘ Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network (EUTRAN) overall descripts 3GPP TS 23.401, ‘General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (Release 8)’. H. Holma and A. Toskala , WCDM for UMTS –HSPA Evolution and LTE. L.Li,, “End-to-End QoS performance management across LTE networks,”Network operations and management symposium(APNOMS., Sep. 2011.

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