LTE Performance Overview
LTE (Long Term Evolution) was initially developed to provide IP connectivity to a set of services (including Internet). LTE network consists of a number of elements. These elements are divided into two categories: Radio Access elements (usually referred as Radio Access Network, RAN) and Core Network elements. A key part impacting the entire system efficiency and performance is a set of algorithms and techniques used on radio interface between eNodeB (eNB) and User Equipment (UE). There are key LTE radio aspects considered below.
Cell radius. According to LTE requirements spectral efficiency and peak bit rate targets should be meet up to 5 km cell radius. For cell radius up to 30 km there can be some degradation.
LTE supports both FDD (Frequency Division Duplex) and TDD (Time Division Duplex). There are 29 paired bands (from 450 MHz till 3.5 GHz) defined by 3GPP for FDD and 12 unpaired bands for TDD (from 700 MHz till 3.8 GHz). LTE channel bandwidth can be 1.4, 3, 5, 10, 15 or 20 MHz (also notice there some 3GPP activities to add a support of another bandwidth like 4.4, 4.6, 4.8, 7 MHz and etc. However it's not standardized yet). To provide multiple access LTE uses OFDMA (Orthogonal Frequency-Division Multiple Access) in downlink (DL) channel and SC-FDMA (Single-Carrier Frequency Division Multiple Access) in uplink (UL) channel.
In case of OFDMA all available spectrum is divided into orthogonal subcarriers. Number of subcarriers depends on bandwidth and can be 72, 180, 300, 600, 900 or 1200. Each carrier can use different modulation. LTE supports QPSK, 16QAM, 64QAM and 256QAM modulations. Multiple access is realized via allocation of a part of subcarriers to one user while another part can be allocated to a next user.
One of OFDMA advantages is tolerance to multipath propagation. While OFDMA has its disadvantages as well, e.g. high sensitivity to frequency synchronization (OFDMA requires accurate frequency synchronization), OFDMA signal has high PAPR (Peak to Average Ratio). The last item increases enegry consumption in mobile terminals. Because of that it was decided to use SC-FDMA in UL channel instead of OFDMA to optimize battery usage. To achieve that there is one more step in signal processing chain (in other words there is one more Fourier transformation) in case of SC-FDMA.
LTE supports multiple antenna technology as well (MIMO - Multiple Input Multiple Output). MIMO usage allows significantly increase spectral efficiency and peak data rates. MIMO utilize multiple antennas on eNB and UE to transmit and receive data. There are several MIMO schemes. For example, different antennas can transmit the same data to increase transmission reliability and decrease error rate. Another example, different antennas can transmit different data streams to increase spectral efficiency and data rates (i.e. at the same radio resources simultaneously eNB can transmit different data). LTE supports MIMO 2x2 (2 transmission antennas and 2 antennas to receive), 4x4 and 8x8 in DL channel and MIMO 2x2 and 4x4 in UL channel. In case of MIMO 4x4 data rate will be in almost 4 times higher (in fact a bit less than 4 times due to additional overhead to carry pilot signals).
Using MIMO and 20 MHz channel maximal data rate is up to 300 Mbps in DL channel and 170 Mbps in UL channel. For additional details see "How to calculate LTE bit rate". LTE-Advanced allows to increase it further using Carrier Aggregation, MIMO with higher order and another technologies.
According to LTE requirements spectral efficiency should be 5 bps/Hz in DL channel and 2.5 bps/Hz in UL channel (it leads to 100 Mbps in DL and 50 Mbps in UL correspondingly). These targets should be met even in case of high mobile users (speed up to 120 kph).
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