It is an acronym for Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access, also referred to as the 3GPP work item on the Long Term Evolution (LTE) also known as the Evolved Universal Terrestrial Radio Access (E–UTRA) in early drafts of the 3GPP LTE specification.
- Radio Access
- Duplexing Techniques
- Modulation and Coding
- Transmission Bandwidth
- LTE FFT Size
1) Radio Access
In 2005, the LTE study group selected two multiplexing techniques for the downlink and uplink air interface.
Downlink – OFDMA (Orthogonal Frequency Division Multiple Access) was selected for its spectral efficiency compared with other technologies such as WCDMA (Wideband Code Division Multiple Access) in channels wider than 5MHz. Furthermore, OFDMA is considered to perform well in hostile radio environments.
Uplink – SC-FDMA (Single Carrier – Frequency Division Multiple Access) was chosen for its low PAPR (Peak to Average Power Ratio) and flexibility which reduced the complexity in the handset and improved the power performance and battery life.
2) Duplexing Techniques
Duplexing TechniquesTo support transmission in paired and unpaired spectrum, the LTE air interface supports both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) modes although much of the initial development has been devoted to FDD operation.
3) Modulation and Coding
LTE uses adaptive modulation and coding to improve data throughput. This technique varies the downlink modulation and coding scheme based on the channel conditions for each user. When the link quality is good, the system can use a higher order modulation scheme (more bits per symbol), which will result in greater system capacity. On the other hand, when link conditions are poor, due to problems such as signal fading, the system can change to a lower modulation scheme to maintain an acceptable radio link margin. The channel coding is turbo coding with a coding rate of 1/3 for Transport channels and a combination of turbo and block coding with varying rates for physical control information.
4) Transmission Bandwidth
In order to address the international wireless market and regional spectrum regulations, LTE includes varying channel bandwidths selectable from 1.4 to 20MHz, with a consistent subcarrier spacing of 15 kHz. Subcarrier spacing isconstant regardless of channel bandwidth. To allow for operation in different sized spectrum allocation, the transmission bandwidth is instead altered by varying the number of subcarriers.
5) LTE FFT Sizes
The LTE specifications define a number of channel bandwidth options; each is assigned a FFT size. The actual subcarrier spacing is always 15kHz. This is calculated by dividing the channel bandwidths sampling rate, e.g. 7.68MHz for a 5MHz channel, by the associated FFT size.
|Channel Bandwidth||FFT Size||Subcarrier Bandwidth (kHz)||Sampling Rate MHz|