LTE has been designed as a completely packet-oriented multi service system, without the reliance on circuit-switched connection-oriented protocols prevalent in its predecessors. In LTE, this philosophy is applied across all the layers of the protocol stack.
The route towards fast packet scheduling over the radio interface was already opened by HSDPA, which allowed the transmission of short packets having a duration of the same order of magnitude as the coherence time of the fast fading channel, as shown in Below Figure .
This calls for a joint optimization of the physical layer configuration and the resource management carried out by the link layer protocols according to the prevailing propagation conditions.
This aspect of HSDPA involves tight coupling between the lower two layers of the protocol stack – the MAC (Medium Access Control layer ) and the physical layer.
In HSDPA, this coupling already included features such as fast channel state feedback, dynamic link adaptation, scheduling exploiting multi-user diversity, and fast retransmission protocols. In LTE, in order to improve the system latency, the packet duration was further reduced from the 2 ms used in HSDPA down to just 1 ms.
This short transmission interval,together with the new dimensions of frequency and space, has further extended the field of cross-layer techniques between the MAC and physical layers to include the following techniques in LTE:
- Adaptive scheduling in both the frequency and spatial dimensions;
- Adaptation of the MIMO configuration including the selection of the number of spatial layers transmitted simultaneously;
- Link adaptation of modulation and code-rate, including the number of transmitted
- codewords;
- Several modes of fast channel state reporting.
These different levels of optimization are combined with very sophisticated control signalling.
What is the Packet-Switched Radio Interface in LTE?
The Packet-Switched Radio Interface in LTE refers to the interface used for transmitting packet-based data (such as internet traffic) between the User Equipment (UE) and the Evolved NodeB (eNB). Unlike traditional circuit-switched systems, LTE relies entirely on a packet-switched architecture to deliver data efficiently, supporting high-speed internet and multimedia services.
Components of the Packet-Switched Radio Interface
- UE (User Equipment): The mobile device (e.g., smartphone, tablet) that communicates with the network to access services like browsing, video streaming, and VoIP.
- eNB (Evolved NodeB): The base station that connects the UE to the LTE network. The eNB is responsible for transmitting data to and from the UE over the packet-switched interface.
- Serving Gateway (SGW): The gateway that handles packet data routing and forwarding between the eNB and the core network (EPC). It ensures the data is delivered to the appropriate destination.
- Evolved Packet Core (EPC): The core network that controls packet data routing, mobility management, and service delivery to users in LTE. It includes several nodes like the Mobility Management Entity (MME), SGW, and PGW (Packet Gateway).
How the Packet-Switched Radio Interface Works
- Data Transmission: The UE sends packet data (such as internet requests) to the eNB. The eNB forwards this data to the SGW, which routes it to the appropriate destination within the EPC or to external services.
- IP-Based Communication: LTE uses Internet Protocol (IP) for data communication, which allows for flexible and efficient routing of packets over the network. This enables a variety of services, such as video, voice, and browsing, to run over the same data connection.
- Quality of Service (QoS): The packet-switched interface in LTE supports QoS, which ensures that different types of data (e.g., voice, video) are prioritized and delivered according to their specific requirements (e.g., low latency for voice calls).
Why Packet-Switched Radio Interface Matters in LTE
The Packet-Switched Radio Interface is a fundamental part of LTE because it enables the efficient and high-speed transmission of data, supporting the internet, multimedia services, and applications on mobile devices. By using IP-based packet switching, LTE networks can handle a large variety of services simultaneously while maintaining high performance and flexibility. This approach is crucial for the modern mobile experience, where users demand fast, reliable internet and multimedia connectivity.