System Level Modeling and Optimization of the LTE Downlink

2. 3GPP Long Term EvolutionNAS procedures IP packet routing network managementRB control broadcast, paging handover, cell selection QoSROHCsecurityCore network (SAE)RRCPDCPsegmentationARQscheduling/priority handlingmultiplexing at UE levelHARQlink adaptation power control cell searchRLCMACPHYRANFigure 2.2: Layer structure for LTE [31, 35].ˆ Signaling between nodes of different core networks for mobility between 3GPPaccess networks including Serving-General packet radio service Support Node(SGSN) selection for handovers to 2G or 3G 3GPP networks.ˆ Security control together with the Home Subscriber Server (HSS), which supportsthe database containing the user subscription information.The Serving Gateway (S-GW) is the termination point towards the RAN. It supportsthe termination of U-Plane packets and its switching when UE mobility requiresit, as well as packet routing and forwarding. For UTRAN (3G) mobility,the U-Plane connection is done directly with the UMTS RAN, while the signalinggoes through the SGSN. For GSM EDGE Radio Access Network (GERAN) (2G)mobility, both C-Plane and U-Plane are routed through the SGSN, as shown inFigure 2.1.Lastly, the PDN Gateway (P-GW) serves as an anchor point for sessions towardsexternal Packet Data Networks (PDNs). It supports:ˆ Packet filtering and/or marking and DHCP functionality (IP address allocation).ˆ Service level charging and rate enforcing, together with the Policy and ChargingRules Function (PCRF).This functional split of the SAE elements allows for a more specialized implementationof the MME, S-GW, and P-GW. Thus, the MME is optimized for C-Planeprocessing, while the S-GW is optimized to process high-throughput U-Plane data.The P-GW performs functions analogous to those of an exit edge router in thecommonly-used IP DiffServ architecture [38] for the LTE core network.13