Chapter 5. Multiple Antenna Transmission and Reception

The expanded and more advanced use of multiple antennas at both the transmitter and receiver promises to be among LTE’s largest advantages over incumbent technologies. Multicarrier modulation enables richer, more efficient use of multiple antennas and receivers in wideband channels. Multiple antenna techniques can be grouped into roughly three different categories: diversity, interference suppression, and spatial multiplexing. Spatial diversity allows a number of different versions of the signal to be transmitted and/or received, and provides considerable resilience against fading. Interference suppression uses the spatial dimensions to reject interference from other users, either through the physical antenna gain pattern or through other forms of array processing such as linear precoding, postcoding, or interference cancellation. Spatial multiplexing allows two or more independent streams of data to be sent simultaneously in the same bandwidth, and hence is useful primarily for increasing the data rate. And, of course, combinations of techniques from these broad categories can be used simultaneously, to achieve some of the benefits of each. LTE’s current options include techniques from each of the three categories.

All three of these different approaches are often collectively referred to as multiple input-multiple output (MIMO) communication, although this is inaccurate unless there are at least two antennas at both the transmitter and the receiver.^[1] This chapter will provide a tutorial on multiantenna technologies organized around the above three categories, and attempt to provide intuition on the strengths and weaknesses of each technique. We will particularly highlight the approaches that are specified or likely to be used in LTE, with precise implementation details left mostly to Chapter 7 (downlink) and Chapter 8 (uplink). As LTE matures in the future, more advanced techniques will be offered, but this chapter provides a technical foundation broad enough to cover nearly any possible multiantenna technique. For example, many possible multiuser MIMO techniques^[2] can be viewed as a hybrid of all three categories.

^[1] Another common, and stricter, convention is for MIMO to specifically refer to only the third type, spatial multiplexing, since only this technique actually transmits multiple independent data streams and hence has multiple inputs and outputs.

^[2] Also known as space division multiple access, as introduced in Chapter 4 and discussed in more detail here in Section 5.9.1.