Differences of LTE Technology to GSM and UMTS

The key difference of 4G to 2G and 3G is the new underlying radio access technology. LTE uses Orthogonal Frequency Division Multiple Access (OFDM) for downlink which enables a high peak data rates achieved in high spectrum bandwidth. On the other hand, Multiple Input Multiple Output (MIMO) systems form an essential part of LTE makes the ambitious requirements for throughput and spectral efficiency viable. MIMO refers to the use of multiple antennas at transmitter and receiver side. Although the use of Wideband Code Division Multiple Access (WCDMA) radio technology on 2G and 3G has efficiency as good as OFDM in delivering peak data rates of about 10 Mbps in 5 MHz of bandwidth. Achieving peak rates in the 100 Mbps range with wider radio channels, however, would result in highly complex terminals and is not practical with current technology. This is where OFDM provides a practical implementation advantage.

A pure OFDMA approach on the uplink results in high Peak to Average Ratio (PAR) of the signal, which compromises power efficiency and, ultimately, battery life. Hence, LTE uses another mean for the uplink transmission, called Single Carrier FDMA (SC-FDMA), SC-FDMA is somewhat similar to OFDMA, but has a 2 to 6 dB PAR advantage over the OFDMA method used by other technologies such as WiMAX.

Another distinctive difference is the characteristic of EUTRA (Evolved UMTS Terrestrial Radio Access Network or Evolved Universal Terrestrial Radio Access Network). EUTRA is unrelated and incompatible with WCDMA. It is categorized as a completely new air interface from the ancestor technologies. All the traffic and signaling is sent over shared channels for both directions of transmission-downlink and uplink because of the total lack of dedicated channels.

The OFDMA approach is also highly flexible in channelization, and LTE will operate in various radio channel sizes ranging from 1.4 to 20 MHz. LTE also boosts spectral efficiency.

Moreover, LTE only support the hard handover. Considering then that handover creates an interruption time in the user plane, the handover performance in terms of success rate and delay of execution is utmost importance.

An important aspect of the design mentioned in Release 8 (Rel-8) LTE is the ability of LTE to use spectrum in a flexible fashion. This allows, for example, an initial LTE deployment with a small amount of spectrum as well as migration of GSM/CDMA frequency bands. Then as the usage of LTE grows, the system can efficiently migrate to increasingly larger bandwidths.

The overall objective for LTE is to provide an extremely high performance radio-access technology that offers higher data rates and truly mobile broadband with full vehicular speed mobility provided and that can readily coexist with HSPA and earlier networks. Because of scalable bandwidth, operators will be able to easily migrate their networks and users from HSPA to LTE over time.

LTE assumes a full Internet Protocol (IP) network architecture and is designed to support voice in the packet domain. It incorporates top-of-the-line radio techniques to achieve performance levels beyond what will be practical with CDMA approaches, particularly in larger channel bandwidths. However, in the same way that 3G coexists with second generation (2G) systems in integrated networks, LTE systems will coexist with 3G and 2G systems. Multimode devices will function across LTE/3G or even LTE/3G/2G, depending on market circumstances.