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5G in the sub-6 Ghz spectrum bands

Straight to mmWave? Not so fast

It is becoming increasingly accepted that mmWave (extremely high frequency), the frequencies in the spectrum between 30 Ghz and 300 Ghz, will define 5G networks. But there are still problems to be solved with mmWave before it is ready for prime time.

Nutaq provides a list of mmWave weaknesses:

  • Atmospheric absorption– The atmosphere absorbs millimeter waves, thus restricting their transmission range. Rain, fog, and moisture in the air make the signal attenuation very high. Oxygen (O2) absorption is especially high at 60 GHz.
  • Mechanical resonance– The mechanical resonance frequencies of gaseous molecules also coincide with the millimeter wave signal. For current technology, the important absorption peaks occur at 24 and 60 GHz.
  • Scattering– Millimeter wave propagation is also affected by rain. Raindrops are roughly the same size as the radio wavelengths and therefore cause scattering of the signal.
  • Non-line of sight issues –When a line-of-sight path between transmitter and receiver isn’t present, the travelling signal still has alternative ways to reach the receiver, be it through diffraction, reflection or bending. Diffraction in millimeter waves is scarce due to the short wavelengths.
  • Brightness temperature –When millimeter waves are subjected to absorption by water vapor, oxygen and rain, these molecules absorb high frequency electromagnetic radiation. This energy emission, when received by a receiver antenna, is called brightness temperature and it degrades system performance
source: The University of Texas at Austin
source: The University of Texas at Austin In a report titled Comparing Massive MIMO at Sub-6 GHz and Millimeter Wave Using Stochastic Geometry by Dr. Robert W. Heath Jr

The difficulties associated with mmWave means standards organizations and mobile operators will continue to rely heavily on sub-6 Ghz spectrum. Especially since it likely take three years or more for mmWave technology to be developed and to harmonize the availability of the new spectrum bands.

Sub-6Ghz, a more immediate solution

Some of the advantages of sub-6 Ghz spectrum include, according to the Pacific Telecommunications Council (PTC):

  • The existing investment by satellite industry (169 commercial satellites, $50-60b and growing, user base = several hundred millions)
  • Unique technical properties (rain fade, coverage) & often preferred solution for broadcasting, telemetry, disaster relief, meteorological, aeronautical, etc.

The International Telecommunications Union’s (ITU) WRC-15, held in Geneva, reached agreements and identified new spectrum for mobile communications for International Mobile Telecommunications (IMT), the collective term for 3G, 4G and 5G.

  • A decision was reached to make the L-Band (1427 – 1518MHz) and part of the C-Band (3.4 – 3.6GHz) available for mobile broadband on a global basis.
  • The 700MHz band (694 – 790MHz) is now also globally harmonized following the initial decisions made at WRC-12 and the follow-up action at WRC-15 for its use in Europe, Middle East and Africa.
  • Additional spectrum is identified in some countries in the frequency bands 470 – 694/698MHz, 3.3-3.4GHz, 3.6-3.7GHz and 4.8 – 4.99GHz.
  • Spectrum at higher frequencies in the range from 24.25GHz up to 86GHz will be subject to study work for 5G (IMT-2020) usage in ITU, providing one of the cornerstones for future 5G services.

WRC-19, held in 2019, will decide mmWave spectrums.

Companies already on the way to sub-6Ghz 5G.

source: Huawei
source: Huawei

Last year Huawei proposed bands blow 6 GHz as the primary working frequency of 5G. At the Mobile World Congress Shanghai 2015, Huawei demonstrated the world’s first 5G testbed working on sub-6 GHz frequency band.

Qualcomm announced a 5G New Radio (NR) prototype system and trial platform. The 5G NR prototype system operates in the sub-6 GHz spectrum bands and is being utilized to showcase the company’s 5G designs to efficiently achieve multi-gigabit per second data rates and low latency.

“We are happy to be working with Qualcomm to showcase the sub-6 GHz 5G prototype system at Mobile World Congress Shanghai,” said Madam Huang Yuhong, the DGM of China Mobile Research Institute in a statement. “This is a great example of the 5G technology collaboration we set out to accomplish when we announced the 5G Joint Innovation Center earlier this year.”

5G will make the best use of a wide range of spectrum bands, and utilizing spectrum bands below 6 GHz is a critical part of allowing for flexible deployments with ubiquitous network coverage and a wide range of use cases, according to Qualcomm. Designs implemented on the company’s prototype system are being utilized to drive 3GPP standardization for a new, OFDM-based 5G NR air interface. The prototype system will closely track 3GPP progress to help achieve 5G NR trials with mobile operators, infrastructure vendors, and other industry players, as well as future 5G NR commercial network launches.

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