Long Term Evolution (Advanced) - LTE(-A)
LTE/LTE-A Physical layer

The physical layer communication in LTE/LTE-A is different to the one used in UMTS. It considers OFDMA and SC-FDMA techniques for access in downlink and uplink respectively. Also the larger spectrum of frequency bands is defined for LTE. At this time, 25 frequency bands are defined for 4G (17 paired bands for FDD and 8 unpaired for TDD). For example, bands at 2 or 2.6 GHz or bands around 3.5 GHz and under 1 GHz (700 - 900 MHz) are considered.

ODFMA is multiple access method exploiting OFDM. The OFDM is a multi-carrier transmission combining TDMA and FDMA (see Figure 23). The whole bandwidth is split into relatively densely spaced subcarriers. To enable dense spacing of subcarriers, orthogonal spectrum of the subcarriers must be ensured. It means, spectral maximum of one subcarrier overlaps with minimum of other subcarriers. The TDMA is considered by sharing each subcarrier by multiple users in time division manner. As a result, the whole available radio resources are split in time and frequency and can be shared by multiple users. A time interval at a subcarrier represents an OFDM symbol. Each symbol contains modulated data and the modulation can be different for each symbol. To avoid ISI (InterSymbol Interference), a cyclic prefix (CP) is introduced. The CP is composed by copying the last samples of the symbol and its purpose is to avoid of overlapping of individual symbols.

Note that contrary to UMTS, radio resources are understand rather as OFDMA symbols at physical layer in LTE/LTE-A.

OFDMA and resource allocation in LTE/LTE-A

A weakness of OFDMA consists in significant difference in power allocated at each subcarrier as user data are modulated independently over individual subcarriers. It means a subcarrier can be allocated with high power while other subcarrier power can be very low. Consequently, data at each subcarrier are modulated without considering information modulated on other subcarriers. This leads to high PAPR (Peak to Average Power Ratio), which negatively influences energy consumption. The SC-FDMA is used in uplink instead of OFDMA to reduce the PARP. In SC-FDMA, all data transmitted in the same time interval are modulated as a linear combination of this data symbols. Therefore, a symbol at subcarriers contains components related to information mapped onto other subcarriers. This way, a PAPR (Peak to Average Power Ratio) is lowered and consequently, it leads to minimizing interference and UE’s battery consumption.

Like UMTS, LTE supports both FDD and TDD modes for data transmission as well. Two types of physical layer frames (labelled as Type 1 and Type 2) are defined as shown in Figure 24. Type 1 is applicable to FDD transmission in either full duplex or half duplex while Type 2 is intended for TDD. In both cases, the transmission is organized into frames with duration of 10 ms. Each frame is divided into ten subframes. In both types of frame, each subframe consists two slots with equal duration of 0.5 ms. The slots are composed of so-called resource blocks, which further comprise resource elements. The number of resource elements in a resource block is defined as a product of multiplication of a number of subcarriers per resource block and a number of symbols. Depending on the size of CP, the amount of symbols in one resource block is either six (if extended CP is applied) or seven (if normal CP is applied). The subcarriers in LTE(-A) systems are spaced equally with distance of 15 kHz and one resource block is composed of 12 subcarriers. Like in HSDPA, LTE/LTE-A uses adaptive modulation and coding. Hence, the actual modulation and coding rate are selected according to the quality of signal level. Therefore, the amount of bits carried in one resource element depends on selected Modulation and Coding Scheme (MCS). Three modulations are available in LTE/LTE-A: QPSK, 16-QAM, 64-QAM.

LTE-A TDD and FDD frame structures

In FDD frame, the different frequencies are considered for each direction. Therefore, ten subframes can be used for DL (downlink) as well as for UL (uplink) transmission simultaneously. This approach leads to equal distribution of radio resources in both directions if the same bandwidth is used.

In TDD frame, both transmission directions occupy the same frequency. To support various ratios of DL and UL, the LTE-A defines several different configurations for assignment of subframe to either DL or UL. The ratio of resources associated to DL and to UL can vary from 2:3 to 9:1. Generally, each subframe can be dedicated to DL transmission (in Table denoted as D subframe), to UL transmission (denoted as U subframe), or to combination of both. In the last case, the subframe is called as a Special (S) subframe. The first and sixth subframes are always assigned to DL direction regardless the selected configuration. Moreover, the second subframe is always dedicated to the S subframe and the third one is assigned to UL transmission. Based on the length of switch period between DL and UL, the seventh subframe can be associated to either D (10 ms switch period) or S subframe (5 ms switch period). The content of the other subframes depends on DL-UL configuration as defined in table.

Assignment of subframe according to DL-UL configurations for TDD
DL-UL configuration | DL-UL switch period [ms] | Subframe #
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
0 | 5 | D | S | U | U | U | D | S | U | U | U
1 | 5 | D | S | U | U | D | D | S | U | U | D
2 | 5 | D | S | U | D | D | D | S | U | D | D
3 | 10 | D | S | U | U | U | D | D | D | D | D
4 | 10 | D | S | U | U | D | D | D | D | D | D
5 | 10 | D | S | U | D | D | D | D | D | D | D
6 | 5 | D | S | U | U | U | D | S | U | U | D

The S subframe contains three parts: DL transmission part, known as Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and UL transmission part denoted as Uplink Pilot Time Slot (UpPTS). The DwPTS part is usually occupied by DL data such as in conventional D subframe, only with reduced length. Its length varies between three and twelve symbols according to S subframe configuration. The UpPTS can consume either one or two SC-FDMA symbols and it is utilized for transmission of control channels only (i.e., no data transmission). The GP is scheduled right after the DwPTS and it is used for switching antennas from transmitting to receiving mode and vice versa. Thus no user’s data can be transmitted during the GP. Its length determines the maximum supportable cell size as it is proportional to the signal propagation delay.