LTE Standards, Protocols, and Functions

LTE (Long-Term Evolution) of UMTS (Universal Mobile Telecommunications Service) is one of the latest steps in an advancing series of mobile telecommunication systems. The standards body behind the paperwork is the 3rd Generation Partnership Project (3GPP).

Along with the term LTE, the acronyms EPS (Evolved Packet System), EPC (Evolved Packet Core), and SAE (System Architecture Evolution) are often heard. Figure 1 shows how these terms are related to each other: EPS is the umbrella that covers both the LTE of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and the SAE of the EPC network.

 

Figure 1: EPC and LTE under the umbrella of EPS

LTE was and is standardized in parallel to other radio access network technologies like EDGE (Enhanced Data Rates for GSM evolution) and HSPA (High-Speed Packet Access). This means that LTE is not a simple replacement of existing technologies. Rather it is expected that different kinds of radio access will coexist in operator networks.

From this background it emerges that understanding LTE also requires understanding alternative and coexisting technologies. Indeed, one of the major challenges of LTE signaling analysis will concern the analysis of handover procedures. Especially, the options for possible inter-RAT (Radio Access Technology) handovers have multiplied compared to what was possible in UMTS Release 99. However, also intra-system handover and dynamic allocation of radio resources to particular subscribers will play an important role.

The main drivers for LTE development are:

• reduced delay for connection establishment;
• reduced transmission latency for user plane data;
• increased bandwidth and bit rate per cell, also at the cell edge;
• reduced costs per bit for radio transmission;
• greater flexibility of spectrum usage;
• simplified network architecture;
• seamless mobility, including between different radio access technologies;
• reasonable power consumption for the mobile terminal.

It must be said that LTE as a radio access technology is flanked by a couple of significant improvements in the core network known as the EPS. Simplifying things a little, it is not wrong to state that EPS is an all-IP (Internet Protocol) transport network for mobile operators. IP will also become the physical transport layer on the wired interfaces of the E-UTRAN. This all-IP architecture is also one of the facts behind the bullet point on simplified network architecture. However, to assume that to be familiar with the TCP/IP world is enough to understand and measure LTE would be a fatal error. While the network architecture and even the basic signaling procedures (except the handovers) become simpler, the understanding and tracking of radio parameters require more knowledge and deeper investigation than they did before. Conditions on the radio interface will change rapidly and with a time granularity of 1 ms the radio resources assigned to a particular connection can be adjusted accordingly.

For instance, the radio quality that is impacted by the distance between the User Equipment (UE) and base station can determine the modulation scheme and, hence, the maximum bandwidth of a particular connection. Simultaneously, the cell load and neighbor cell interference – mostly depending on the number of active subscribers in that cell – will trigger fast handover procedures due to changing the best serving cell in city center areas, while in rural areas macro cells will ensure the best possibley coverage.

The typical footprint of a LTE cell is expected by 3GPP experts to be in the range from approximately 700 m up to 100 km. Surely, due to the wave propagation laws such macro cells cannot cover all services over their entire footprint. Rather, the service coverage within a single cell will vary, for example, from the inner to the outer areas and the maximum possible bit rates will decline. Thus, service optimization will be another challenge, too.