by Keith Cobler, Industry Marketing Manager – Mobile Wireless, Rohde & Schwarz USA, Inc.
5G is the next-generation mobile communication system aimed to be broadly introduced commercially around 2020. As the 3GPP specification for 5G is yet to be finalized, the majority of work today is to investigate, develop and standardize 5G. The technological framework for 5G calls for ultra-high-speed data transmissions of over 10Gbps with 1,000 times the capacity of 4G LTE. The new system will enable a diverse range of application scenarios as needed for the emerging technologies such as Internet of Things (IoT). The need for higher bandwidth and higher data rates for 5G makes it necessary to adopt significantly higher frequencies compared with today’s cellular network implementations that are below 2 GHz in most cases.
Channel sounding is a process that allows a radio channel to be characterized by decomposing the radio propagation path into its individual multipath components. This information is essential for developing robust modulation schemes to optimize the capacity and quality over a frequency channel. For decades, propagation conditions in cellular networks have been well understood in the frequency bands from 450 MHz to 3 GHz. Starting with the commercialization of GSM systems, channel models were developed based on this knowledge. These are widely used to verify base station and end user device performance in the lab by simulating real-world conditions.
In contrast to these legacy frequency bands, knowledge of the radio channel at frequencies above 6 GHz, and for that matter at millimeter–wave, is still limited. Although a significant amount of channel sounding experiments have been conducted and many results published, new spectrum in the range from 6 GHz to 100 GHz requires much more measurement data. Multiple research projects, as well as the upcoming 3GPP standardization, need comprehensive measurement data in order to derive accurate channel models for efficient testing in the future. Mobile network operators, research institutes, universities and other industry players are conducting extensive channel measurement campaigns in order to define channel models for 5G.
Channel characteristics at millimeter–wave frequencies are expected to be drastically different from today’s frequency channels of a few GHz:
The path loss is significantly higher in the mm-wave domain so that highly directional beamforming will be required.
Oxygen and water absorption (e.g. rain or humidity loss) needs to be taken into account for specific bands below 70 GHz and above 100 GHz and above a distance of 200 m.
The time selectivity of radio channels at mm-wave is much faster so that TDD technologies are preferable.
The attenuation and reflections of most obstacles is stronger, e.g. even foliage loss, etc.
Line-of-sight (LOS) conditions cannot always be ensured, therefore non-line-of-sight (NLOS) communications is essential (and possible).
Channel sounding test solutions support the research activities aimed at exploiting the microwave and millimeter-wave spectrum proposed for 5G networks. More advanced channel sounding test solutions enable direct measurement of the channel impulse response (CIR) in the time domain. Using discrete instruments allows fast CIR measurement over the full bandwidth, which is much faster than a test setup based on a network analyzer.
The entire industry needs to learn how signals in emerging high-frequency bands with very wide signal bandwidths propagate through the radio channel. Microwave and millimeter–wave frequencies will mean a disruptive change for mobile communications, and the industry is far from being able to define channel models at frequencies well above 6 GHz. Millimeter-wave channel sounding contributes to a better understanding of radio propagation, making it possible to derive realistic channel models for the relevant wireless use cases. It will therefore provide the basis for standardizing link and system design within the global 5G effort.