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We evaluate channel hardening for a large scale antenna system by means of indoor channel measurements over four frequency bands, 1.472 GHz , 2.6 GHz , 3.82 GHz and 4.16 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.

A triple-mode dual-polarized dipole antenna topology with a very small frequency separation ratio is proposed. The antenna has a simple structure with a very low manufacturing cost, while maintaining a good performance. A conventional dipole is fed by a feeding network also acting as a radiator to generate a second resonant mode. A shorted conductor loaded to the dipole is used to excite a third resonant mode. It is shown that the three modes can be tuned in order to cover different sets of applications. By fusing the two higher resonances, an antenna with a lower band and a very wide higher band can be obtained. Two orthogonal linear polarizations are achieved by integrating two of the aforementioned topologies. The conceptual structure is prototyped and measured. The realized antenna has a lower band of 2.38-2.54 GHz and a higher band of 3.11-4.15 GHz. Its electrical size is 0.8×0.8 λ 2 , with a profile of 0.27 λ (λ is the wavelength in free space at 2.4 GHz). The mutual coupling between the two ports is below -20 dB, which is quite beneficial for the diversity performance in MIMO systems.

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