Antenna selection in LTE: from motivation to specification

Transmit antenna selection technology has been adopted for the uplink by the next generation Long Term Evolution wireless standard in order to harness the spatial diversity offered by multiple antennas at the mobile transmitter, while keeping the hardware complexity and cost of a mobile low. In TAS, the number of RF chains for processing/up-conversion is smaller than the number of available antenna elements, so at any time the signals can only be transmitted from a (dynamically optimized) subset of antenna elements. As a result, the training procedure for AS needs to be carefully engineered. In LTE, this is accomplished by reusing the wideband sounding reference signal for the purpose of AS training. Furthermore, new mechanisms are required to facilitate feedback from the receiver (the base station) to the transmitter about which subset is optimal and should thus be used by the mobile. In LTE, this is accomplished by employing a unique masking technique on the downlink control channel that eliminates the feedback overhead at the expense of a minor increase in complexity at the mobile. This article provides an in-depth and systematic overview of all physical and higher layer features in the LTE standard that enable TAS. Also highlighted are the variety of technical and standardization challenges that drove the specification of AS in LTE, and the aspects of the LTE standard that are impacted by AS.