From the beginning, LTE was designed so that the base station and mobile could both use multiple antennas for radio transmission and reception. This chapter covers the three main multiple antenna techniques, which have different objectives and which are implemented in different ways.
The most familiar is diversity processing, which increases the received signal power and reduces the amount of fading by using multiple antennas at the transmitter, the receiver or both. Diversity processing has been used since the early days of mobile communications, so we will only review it briefly.
In spatial multiplexing, the transmitter and receiver both use multiple antennas so as to increase the data rate. Spatial multiplexing is a relatively new technique that has only recently been introduced into mobile communications, so we will cover it in more detail than the others. Finally, beamforming uses multiple antennas at the base station in order to increase the coverage of the cell.
Spatial multiplexing is often described as the use of multiple input multiple output (MIMO) antennas. This name is derived from the inputs and outputs to the air interface, so that ‘multiple input’ refers to the transmitter and ‘multiple output’ to the receiver. Unfortunately, the name is a little ambiguous, as it can either refer to spatial multiplexing alone or include the use of transmit and receive diversity as well. For this reason, we will generally use the term ‘spatial ...