Effective Channel Hardening in an Indoor Multiband Scenario

We evaluate channel hardening for a large scale antenna system by means of indoor channel measurements over four frequency bands, \(1.472 \,\hbox {GHz}\), \(2.6 \,\hbox {GHz}\), \(3.82 \,\hbox {GHz}\) and \(4.16 \,\hbox {GHz}\). NTNU’s Reconfigurable Radio Network Platform has been used to record the channel estimates for 40 radio links to a 64 element array with wideband antennas in a rich scattering environment. We examine metrics for channel hardening, namely, the coherence bandwidth, the rms delay spread and the normalized effective subcarrier power, for the effective channel perceived by a single user after precoding and superposition in the downlink. We describe these metrics analytically and demonstrate them with measured data in order to characterize the rate of hardening of the effective channel as the number of antenna elements at the base station increases. The metrics allow for direct insight into the benefits of channel hardening with respect to radio system requirements.