This publication contains a general overview of Ultra Massive MIMO (U-M-MIMO) communication technology development over the last decade and comparison of this technology with ultra-dense network communication. We proved that in free space, these two technologies are equivalent. However, taking into account real channel and other practical constrains, each technology has its own advantages and disadvantages. We provide our evaluation of spectral efficiency gain that Massive MIMO (M-MIMO) provides for different channel models (both sub 6GHz and mm Waves). We also outline major challenges faced by M-MIMO technologies, such as channel estimation, cost, and power consumption, and will present most up-to-date solution approaches to resolve these problems. We compare two approaches to reduce M-MIMO cost and power consumption: "hybrid beamforming" vs. "all digital solution" with low resolution ADC's and DAC's. We compare two possible modulations (Multi-Carrier vs. Single-Carrier) for M-MIMO.

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