SS/PBCH must be subcarrier aligned to maintain orthogonality?

  • The synchronisation raster defines the set of allowed center frequencies for the Synchronisation Signal / Physical Broadcast Channel (SS/PBCH) Block.
  • The SS/PBCH Block has a bandwidth of 20 Resource Blocks and must be contained within the bandwidth of the downlink channel.
  • The SS/PBCH does not need to be Resource Block aligned with the downlink channel but must be subcarrier aligned to maintain orthogonality.

Can somebody please elaborate the last sentence about orthogonality? I can not fully understand this point.