How to LTE Radio Air Interface?
The state-of-the-art design of the LTE air interface is characterised by OFDMA (DL) and SC-FDMA (UL) together with MIMO.
The downlink modulation is based on OFDMA (Orthogonal Frequency Division Multiple Access). OFDMA is a variant of OFDM which has the advantage that receiver complexity is at a reasonable level, it can handle scalable bandwidth requirements and it supports various modulation schemes from BPSK, QPSK,
16QAM to 64QAM. This allows adaptive modulation on a per user base.
In uplink direction a variant of OFDMA called SC-FDMA (Single Carrier Frequency Division Multiple Access) is used. It has the advantage against OFDMA to have a lower PAPR (Peak-to-Average Power Ratio), which leads to lower power consumption and less expensive RF amplifiers in the terminal.
LTE will support MIMO. It describes the possibility to have multiple transmitter and receiver antennas in a system. Other names are beam-forming or smart antennas.
Up to four antennas can be used by a single LTE cell. This allows having spatial
multiplexing and beam-forming. MIMO is considered to be the core technology to increase spectral efficiency. Currently the performance of MIMO for high mobility cases is still under investigation.
HARQ implements a protocol on layer 1/layer 2 that allows for fast retransmission. Furthermore blocks can be retransmitted with increased coding.
In contrast to UMTS where physical resources are either shared or dedicated, the Evolved Node B in EUTRAN handles all physical resource via a scheduler and assigns them dynamically to users and channels. This provides greater flexibility than the older system.