On the Road to 4G
by Tom Lecklider, Senior Technical Editor
The long term evolution (LTE) of the 3rd Generation Partnership Project (3GPP) is the most recent step toward an IP-based 4G network technology that promises to overcome current 3G problems. LTE also is called evolved universal terrestrial radio access network (e-UTRAN) or sometimes just e-UTRA, although technically this isn’t a complete description.
LTE has two parts: the e-UTRAN access network and the all-IP evolved packet core (EPC). The EPC supports efficient packet-oriented multimedia services while e-UTRAN provides access to them. LTE was defined in 3GPP release 8 and further enhanced in release 9. Release 10 specifies LTE Advanced, the first true 4G network.
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Courtesy of Agilent Technologies |
As Joe Zeto, senior manager of product development at Ixia, commented, “For the most part, 3G is a best-effort data network. The sharp rise in data subscribers and the explosion of data are causing vendors and operators to rethink how mobile data networks are designed. In contrast, LTE networks are specified to have much more intelligence.
“Operators have realized they cannot build their way out of capacity issues by just adding more spectrum and cell sites,” Mr. Zeto continued. “The network needs to become more intelligent to properly manage the different subscriber and service types. LTE is much more concerned with policy control and QoS management. These capabilities will support new business models such as tiered service levels, on-demand performance boosts, and preferred third-party applications to maximize revenue for the operator. They also will allow operators to have granular control of the finite network resources to best manage the different service types to provide the expected service quality.”
Mr. Zeto’s emphasis on increased data traffic is at the heart of the move to LTE. However, as stated in an Agilent Technologies application note, existing 2G and 3G networks still will be needed for voice traffic until operators implement IP-based voice services.
And this need for LTE to coexist with legacy networks is one reason that handover between networks is such an important part of LTE deployment. Of course, LTE coverage will not be available everywhere nor immediately, and handover to or from a legacy network also will be required to sustain data transfer. As the user equipment (UE) moves from one cell to another, the necessary handover may be within the same network or between different networks.1
LTE uses orthogonal frequency division multiple access (OFDMA) for the downlink and single carrier FDMA (SC-FDMA) for uplink. SC-FDMA requires additional signal processing but provides a lower peak-to-average ratio (PAR) and consequently uses lower power in the mobile device.
Both the uplink and downlink schemes are based on orthogonal frequency division multiplexing (OFDM), which distributes the information to be transmitted across a large number of narrowband channels. This approach can rapidly adapt to local conditions that may create high loss for certain frequencies.
The physical signals are generated by inverse Fourier transforms performed on the group of frequencies actually being used within a 10-ms radio frame. The specification supports frequency division duplex (FDD) as well as time division duplex (TDD) variants. Both divide the 10-ms frame into 10 subframes and each of these into two 0.5-ms slots.
FDD uses frame structure type 1, which allocates all of the 20 time slots within a frame for either uplink or downlink. TDD uses frame structure type 2, which groups the first five subframes and the second five. Switching from uplink to downlink or vice versa can occur at either the 10-ms frame boundary or the 5-ms half-frame point.
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