The LTE (Long Term Evolution) architecture is part of the Evolved Packet System (EPS), designed as a flat, all-IP based network to provide high-speed data and low-latency communication. It is primarily divided into two main sections: the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and the Evolved Packet Core (EPC).
1. User Equipment (UE) The UE is the mobile device used by the end-user to communicate with the network.
• Protocol Stack: Consists of both a user-plane (for data) and a control-plane (for signaling).
• Key Components: It includes the Non-Access Stratum (NAS) for communicating with the MME and Radio Resource Control (RRC) for managing the wireless connection with the eNodeB.
2. Evolved-UTRAN (E-UTRAN) The E-UTRAN consists solely of base stations known as eNodeBs (eNB).
• eNodeB Functions: It manages radio resource functions, including radio channel ciphering/deciphering, header compression, and radio admission control.
• Mobility Control: It handles handovers between cells and manages paging for UEs in an idle state.
• Inter-eNB Communication: eNodeBs communicate with each other via the X2 interface for seamless handovers and data forwarding.
3. Evolved Packet Core (EPC) The EPC is the central part of the LTE network that manages data traffic and subscribers. It contains several key entities:
• Mobility Management Entity (MME): ◦ Handles signaling between the UE and the EPC.
◦ Responsible for selecting the SAE Gateway (SGW) for a device, handling security keys, and managing mobility in an idle state.
◦ Communicates with the **Home Subscriber Server (HSS)** to retrieve subscriber information and authentication data.
• Serving Gateway (SGW): ◦ Acts as the mobility anchor for the user-plane, routing and forwarding user data packets.
◦ Buffers data for UEs during paging and triggers paging when a device is in idle mode.
◦ Connects to the PDN Gateway (PGW) via the **S5 interface**.
• Packet Data Network Gateway (PDN GW / PGW): ◦ Provides connectivity to external networks, such as the Internet or IMS (for VoLTE).
◦ Performs UE IP address allocation and packet filtering.
◦ Interacts with the **Policy and Charging Rules Function (PCRF)** via the **S7 interface** to manage quality of service (QoS) and charging.
Key Interfaces The components are linked by standardized interfaces:
• S1 Interface: Connects the E-UTRAN (eNB) to the EPC. It is split into S1-MME for control signaling and S1-U for user data.
• X2 Interface: Connects multiple eNodeBs to support terminal mobility and buffered packet forwarding during handovers.
• SGi Interface: The interface between the PDN GW and external packet data networks.
Voice over LTE (VoLTE) is a technology that allows voice calls to be delivered as data streams over the 4G LTE network. Previously, LTE was primarily designed for high-speed data, while voice calls were often shifted back to 2G or 3G networks.
Core Mechanism VoLTE works by converting voice into Voice over IP (VoIP) data packets. To ensure high quality, it uses a simplified version of the IP Multimedia Subsystem (IMS).
The key entities in a reduced VoLTE network include:
• P-CSCF (Proxy Call State Control Function): Acts as the entry point for all signaling to and from the user.
• S-CSCF (Serving Call State Control Function): Manages the subscriber’s call processing within the home network.
• AS (Application Server): Specifically handles the voice service as an application.
• HSS (Home Subscriber Server): The main database containing subscriber details and access status.
Key Benefits
• Efficiency: It uses the wireless spectrum more efficiently than traditional 2G/3G voice services.
• Superior Quality: Provides much higher data throughput and lower latency, resulting in clearer "HD" voice calls.
• Simultaneous Use: Eliminates the need to switch between different networks for voice and data, allowing you to browse the web at 4G speeds while on a call.
• Battery Life: It can increase handset battery life by approximately 40% compared to older methods.
Self-Organizing Network (SON) is an automation technology designed to minimize the human effort involved in network planning, installation, configuration, and optimization. Its primary goal is to provide "Plug-and-Play" functionality for base stations (eNodeBs), allowing them to automatically configure and optimize themselves once powered on.
SON Functionalities The SON lifecycle is divided into three operational states:
• Self-Configuration (Pre-operational): Handles basic setup tasks like IP address configuration, authentication, and software downloads.
• Self-Optimization (Operational): Continuously adjusts parameters to improve coverage, capacity, and neighbor list management.
• Self-Healing (Operational): Automatically detects and localizes failures, applying predefined healing schemes to maintain service.
Architecture of SON The architecture of SON is classified into three types based on where the optimization algorithms are located and executed:
1. Centralized SON
• Location: Optimization algorithms are executed in the OAM (Operation and Maintenance) system.
• Characteristics: SON functionality resides at a high level in the architecture in a small number of locations.
• Merits: Easy to deploy and manage centrally.
• Demerits: It does not support quick, real-time optimization cases and may have limited support for multi-vendor environments.
2. Distributed SON
• Location: Optimization algorithms are executed directly in the eNodeB.
• Characteristics: Functionality resides at a low level in the architecture across many locations.
• Merits: Supports quick optimization responses for cases concerning only one or two eNodeBs.
• Demerits: Requires significant deployment work and is difficult to coordinate for complex schemes involving many base stations.
3. Hybrid SON
• Location: Optimization algorithms are split; some run in the OAM system while others run in the eNodeB.
• Characteristics: Simple, high-speed tasks stay in the eNodeB, while complex, global optimizations are handled by the OAM.
• Merits: Highly flexible and supports multi-vendor optimization via the X2 interface.
• Demerits: Requires more complex deployment effort and interface extension work.
SON in Heterogeneous Networks (HetNet) A HetNet integrates multiple cell sizes and technologies to improve performance, particularly at cell edges where signals are traditionally weak. SON coordinates the following components within a HetNet:
• Varying Cell Sizes: It manages the relationship between Macrocells (large coverage areas controlled by high-power base stations) and Small Cells (Microcells, Picocells, and Femtocells) that provide targeted capacity.
• Specialized Nodes: SON handles the integration of different nodes such as Home eNodeBs (HeNB) for residential coverage, Relay Nodes (RN) to extend cell range, and Remote Radio Heads (RRHs).
• Interference Management: Because these different cells often overlap, SON is essential for managing interference and ensuring that handovers between large and small cells occur smoothly.
