LTE Introduction
LTE (Long Term Evolution) or the E-UTRAN (Evolved Universal Terrestrial Access Network), introduced in 3GPP R8, is the access part of the Evolved Packet System (EPS). The main requirements for the new access network are high spectral efficiency, high peak data rates, short round trip time as well as flexibility in frequency and bandwidth.
LTE is a standard for wireless data communications technology and an evolution of the GSM/UMTS standards. Next development of LTE Network is to redesign and simplify network architecture to an IP-Based system with significant reduction of transfer latency compared to previous networks. The LTE wireless interface is incompatible with 2G and 3G networks, so that it must be operated on a separate wireless spectrum.
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LTE Networks are based on OFDMA
LTE: The Evolved Packet System (EPS) is purely IP based. Both real time services and data communications services will be carried by the IP protocol. The IP address is allocated when the mobile is switched on and released when switched off.
The new access solution, LTE, is based on OFDMA (Orthogonal Frequency Division Multiple Access) and in combination with higher order modulation (up to 64QAM), large bandwidths (up to 20 MHz) and spatial multiplexing in the downlink (up to 4×4) high data rates can be achieved. The highest theoretical peak data rate on the transport channel is 75 Mbps in the uplink, and in the downlink, using spatial multiplexing, the rate can be as high as 300 Mbps.
Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of the popular Orthogonal Frequency-Division Multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of sub-carriers to individual users as shown in the illustration below. This allows simultaneous low data rate transmission from several users.
LTE: OFDM Frequency-Time Representation
LTE: Comparison od OFDMA & SC-FDMA transmitting a series of QPSK data symbols
In OFDMA these sub-carriers can be shared between multiple users. The OFDMA solution leads to high Peak-to-Average Power Ratio (PAPR) requiring expensive power amplifiers with high requirements on linearity, increasing the power consumption for the sender. This is not a problem in the e-NB, but would lead to very expensive handsets. Hence a different solution was selected for the UL. As illustrated, the SC-FDMA solution generates a signal with single carrier characteristics, hence with a low PAPR.
In LTE technology band guard between sub-carriers is of 15 kHz and it is constant valued, independent from channel bandwidth. Number of sub-carriers varies between 72, in a 1,4 MHz channel, and 1200 in a 20 MHz channel.
OFDMA Advantage:
- Flexibility of deployment across various frequency bands with little needed modification to the air interface
- Averaging interference from neighboring cells, by using different basic carrier permutations between users in different cells.
- Interference within the cell are averaged by using allocation with cyclic permutations.
- Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage.
- Offers Frequency diversity by spreading the carriers all over the used spectrum.
- Allows per channel or per sub-channel power OFDM is a multi-carrier technology subdividing the available bandwidth into a multitude of mutual orthogonal narrow-band sub-carriers.
To enable possible deployment around the world, supporting as many regulatory requirements as possible, LTE is developed for a number of frequency bands – E-UTRA operating bands – currently ranging from 700 MHz up to 2.7 GHz.
The available bandwidths are also flexible starting with 1.4 MHz up to 20 MHz.
LTE is developed to support both the time division duplex technology (TDD) as well as frequency division duplex (FDD).
RB UL (SC-FDMA) to generate signal in the frequency domain use same parameters of DL (OFDMA) . Is is based in 15 kHz sub-carriers.
Any sub-carriers has 15 kHz band allocation so that any RB has 180 kHz band allocation.
Resource Block (RB) include 12 sub-carriers in a slot of 0,5 ms.
LTE
Minimum resource allocation is 12 sub-carriers and than it takes 180 kHz.
LTE
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