Key LTE Features
The LTE radio interface is based on Orthogonal Frequency Division Multiplexing (OFDM), the same system used in WiFi (802.11) and WiMAX (802.16). OFDM was not sufficiently mature when the 3G radio interface was determined (1998), hence WCDMA was chosen.
OFDM comprises many non-interfering (or orthogonal) sub-carriers, each with a portion of the user data. Increasing (or decreasing) the OFDM radio channel width is relatively simple: just use more (or fewer) subcarriers.
Hence LTE operates over a range of radio channel widths, from 1.4 MHz to 20 MHz. Mobile data capacity depends on radio channel width, hence increasing the radio channel from 5 MHz (the default UMTS channel) to 20 MHz (the maximum LTE channel size) increases data capacity by a factor of 4, other things being equal.
Also, LTE operates over single radio channels. This technique, known as Time Division Duplex (TDD) alternates uplink and downlink transmissions over a single channel. This greatly increases greenfield LTE deployment possibilities, as many potential operators have access to a single frequency band only (e.g. 3.5 MHz). Single channel LTE is known as TD-LTE.
LTE was initially specified to support a minimum 100 Mbps peak rate downlink and 50 Mbps uplink for a 20 MHz radio channel. Actual LTE rates have exceeded this, with 2008 NTT DOCOMO LTE trials achieving peak rates exceeding 250 Mbps for 20 MHz channels.
LTE also introduces a new core network architecture, the combined LTE radio interface and core network known as “System Architecture Evolution” or SAE. A key feature is that LTE is “all IP”, unlike 3G UMTS, which separates IP data traffic and circuit switched voice traffic.
LTE voice calls will eventually operate over this IP interface. However, in the first instance LTE is likely to be used for high speed mobile data services, with existing 3G (and GSM) networks providing voice services.