28 GHz and 3.5 GHz Wireless Channels: Fading, Delay and Angular Dispersion

We employ a ray tracing framework to extract the three dimensional (3D) channel parameters characterizing outdoor small cell deployments providing services to indoor users close to the exterior wall and windows at 28 GHz and 3.5 GHz. The wireless channel is highly dependent on 3D antenna patterns, the environment specific characteristics such as 3D geometry of the buildings, materials used for building construction and their specific propagation properties. These dependencies are particularly important at higher frequencies, where the range of radio signals may be significantly limited due to the path loss and shadowing by obstacles. A good understanding of the channel propagation characteristics at these frequencies, and their correlation to propagation at lower bands, is thus critical for designing and deploying reliable radio systems. The ray tracing approach has substantial merits in absence of relevant field measurements, facilitating extraction of the 3D site-specific channel parameters pertaining to 28 GHz small cell deployments and giving useful insights to foster innovation of new wireless technologies at 28 GHz. Furthermore, we derive channel statistics at both 28 GHz and 3.5 GHz for the same environment drawing conclusions on path loss, delay spread, and angle spread in azimuth and vertical directions. Our results indicate that for the environment considered the angle spread of azimuth angle of arrival exceeds 20° in more than 45% of the terminal locations even at 28 GHz, suggesting that very high beamforming gain at the terminal is not feasible. Furthermore, with a 15° grid of beams based transmitted signal, the best beam at 28 GHz is different from that 3.5 GHz for almost 60% of the locations.