Power Scaling Laws and Near-Field Behaviors of Massive MIMO and Intelligent Reflecting Surfaces
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Large arrays might be the solution to the capacity problems in wireless communications. The signal-to-noise ratio (SNR) grows linearly with the number of array elements N when using Massive MIMO receivers/relays. Moreover, intelligent reflecting surfaces (IRSs) have recently attracted attention since their SNR grows as N^2, which seems like a major benefit. In this paper, we use a deterministic propagation model for a planar array of arbitrary size, to demonstrate that the mentioned SNR behaviors, and associated power scaling laws, only apply in the far-field. They cannot be used to study the regime where N→∞. We derive an exact channel gain expression that captures the near-field behavior and use it to revisit the power scaling laws. We derive new finite asymptotic SNR limits but also conclude that these are unlikely to be approached in practice. We further prove that an IRS setup cannot achieve a higher SNR than the corresponding Massive MIMO setups, despite its faster SNR growth. The IRS typically must have a much larger array size to achieve the same SNR. Finally, we show that an optimized IRS can be interpreted as a reconfigurable lens and that it is generally suboptimal to operate it as an "anomalous" mirror.
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