- The paper demonstrates that 28 GHz channels exhibit Rician fading with K-factors of 9–15 dB in LOS and 5–8 dB in NLOS vertical co-polarized scenarios.
- It employs half-wavelength spatial increments over a 33-wavelength trajectory to reveal decorrelation distances of 2 wavelengths in LOS and 5 in NLOS.
- The research advises revisiting traditional Rayleigh models to enhance MIMO and beamforming strategies in next-generation 5G systems.
Small-Scale Fading Models at 28 GHz Millimeter-Wave Frequencies
The paper by Samimi, MacCartney, Sun, and Rappaport presents a comprehensive investigation into the small-scale fading characteristics of 28 GHz millimeter-wave ultrawideband wireless channels. This research is a pivotal piece contributing to our understanding of the fading statistics and spatial autocorrelation behaviors crucial for the deployment and improvement of millimeter-wave systems, particularly for next-generation mobile networks like 5G.
The authors conducted empirical measurements of small-scale fading in outdoor environments using directional horn antennas. These investigations were set in a typical base-to-mobile communication scenario, involving various line of sight (LOS) and non-line of sight (NLOS) configurations. Measurements were performed using a channel sounder on the NYU Brooklyn campus, highlighting realistic urban communication conditions. The key focus was on capturing the power delay profiles (PDPs) at half-wavelength spatial increments over a 33-wavelength trajectory. This approach enabled an in-depth analysis of fading characteristics over local distances.
Methodology and Findings
The research leveraged high-gain directional antennas to acquire extensive data on multipath configurations and their fades, analyzing these through the lens of statistical models. One of the significant findings is the distribution characteristics of voltage path amplitudes. In both LOS and NLOS scenarios, fading followed a Rician distribution rather than the typically assumed Rayleigh distribution, with K-factors ranging as follows:
- LOS vertical-to-vertical co-polarized scenarios: $9 - 15$ dB
- NLOS vertical-to-vertical co-polarized scenarios: $5 - 8$ dB
- Both LOS and NLOS vertical-to-horizontal cross-polarized scenarios: $3 - 7$ dB
These results suggest that even in NLOS conditions, the potential for resolved multipath components or coherent sums manifests in non-Rayleigh fading for wideband channels.
Further, spatial autocorrelation functions were studied, showing that in LOS and NLOS environments, it typically takes 2 and 5 wavelengths, respectively, for signal amplitudes to decorrelate entirely. This spatial behavior is important for practical channel modeling and the development of multi-element antenna systems, impacting techniques such as beamforming and spatial diversity schemes.
Implications for Millimeter-Wave Systems
The models and findings presented in this paper have a direct impact on how millimeter-wave systems can be designed and optimized for real-world usage. The Rician distribution of amplitudes across LOS and NLOS environments suggests that existing channel models, often based on Rayleigh assumptions, need reconsideration for millimeter-wave frequencies. Additionally, the spatial autocorrelation results are critical for implementing effective MIMO (multiple-input multiple-output) systems and beamforming strategies, essential components in achieving high data rates in 5G and beyond.
Future Directions
This paper lays the groundwork for subsequent studies to refine antenna element designs and deploy more efficient beamforming algorithms. Future research could extend these findings across a variety of urban environments and different antenna configurations to further understand the nuanced behaviors of millimeter-wave propagation. Beyond empirical expansion, integrating these insights into the development of comprehensive simulation tools could significantly aid in the design and deployment of robust wireless communication systems.
Through diligent empirical measurement and insightful analysis, this research contributes meaningfully to the understanding of millimeter-wave small-scale fading characteristics, offering practical paths forward in the development of next-generation wireless communication technologies.