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mmWave Change Everything

 Is it better to have a high-gain or low-gain antenna?

The plan to introduce cellular services in frequency bands >6 GHz is driving an abrupt and unprecedented change in how devices and systems have to be designed, operated and tested.

The Friis propagation equation predicts losses at mmWave frequencies:

mmWave Change Everything

  1. To overcome these losses and provide a realistic link budget, it is necessary to use high-gain antennas comprised of multiple elements at both ends of the link
  2. High-gain antennas create narrow beamwidth signals
  3. Radio propagation at mmWave is very different: 
    • Very sparse and spatially dynamic, unlike rich multipath with Rayleigh fading
    • The simplest use of large antenna arrays at the base station is beam steering – create narrow beams within the cell to direct signals to specific locations, possibly with reflections involved
    • The steering only needs to know the direction of the user. Beamforming requires precise real-time channel state information (CSI)
    • Beamforming requires full digital control of the amplitude and phase of every antenna element while beam steering can be done using simple analog phase shifters
    • In a predominantly line of sight channel with several users in different locations, beamforming would simultaneously generate a beam towards each user much like beam steering
    • The benefits of beamforming become more apparent as the channel becomes more scattered, which is when simpler beam steering is less effective

5G Operation at mmWave Frequecies

  1. mmWave has great potential (spectrum!)
  2. mmWave signals do not bend around corners (diffract) and are easily blocked orattenuated
  3. mmWave signals do bounce (reflect) readily giving rise to local scattering(multipath)
  4. mmWave signals act more like light rays, so can be directed using special antennas
  5. Path loss through the air is much greater at mmWave than at LTE bands
  6. Changing from 1 GHz to 28 GHz path loss increases by 28 dB over 1 m
  7. Cables are lossy and expensive, so most testing will be done over the air

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