Abstract
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.
Highlights
Massive multiple input multiple output (MIMO) systems are envisioned as a key feature of the generation of communication systems which provide large sum capacity as well as spectral and energy efficiency, while simultaneously serving multiple users
We show that coherence bandwidth ceases to be a practical measure for effective channel evaluation
The frequency correlation is calculated as the Fourier transform of the effective power delay profile (PDP), coherence bandwidth can be obtained as a metric for evaluation of the effective channel
Summary
Massive multiple input multiple output (MIMO) systems are envisioned as a key feature of the generation of communication systems which provide large sum capacity as well as spectral and energy efficiency, while simultaneously serving multiple users. Channel hardening is considered from two points of view: firstly, as a property that causes the effective channel transfer function (CTF) between the UE and the BS to become more deterministic, secondly as a property to focus the received signal in the delay domain as the number of BS antennas increases. We illustrate these properties in the effective channel in order to determine how many antennas are sufficient to achieve a certain level of channel hardening.
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