Telecommunications have grown to be a pillar to a functional society and the urge for reliable and high throughput systems has become the main objective of researchers and engineers. State-of-the-art work considers massive Multiple-Input Multiple-Output (massive MIMO) as the key technology for 5G and beyond. Large spatial multiplexing and diversity gains are some of the major benefits together with an improved energy efficiency. Current works mostly assume the application of well-established techniques in a massive MIMO scenario, although there are still open challenges regarding hardware and computational complexities and energy efficiency. Fully digital, analog, and hybrid structures are analyzed and a multi-layer massive MIMO transmission technique is detailed. The purpose of this article is to describe the most acknowledged transmission techniques for massive MIMO systems and to analyze some of the most promising ones and identify existing problems and limitations.

Nowadays, the demand of high data wireless transfer rates is growing with the big and fast grow of smart phones. As a consequence, the spectrum is getting overloaded with communications, increasing the interference. Therefore a different path of ideas has been followed and the communication process has been taken to the next level by the usage of big arrays of antennas and multi-stream communication (MIMO systems) and wider bandwidths. These systems can be combined with SC-FDE (Single-Carrier Frequency Domain Equalization) schemes to improve the power efficiency in uplink due to the low envelope fluctuations. In this paper we will focus on the equalization field applied in massive MIMO schemes, more specifically in the performance of new low complexity receivers that avoid matrix inversion operation. These ones are based on an IB-DFE (Iterative Block Decision Feedback Equalizer) but are changed to include pre-processed signals from EGC (Equal Gain combiner) and MRC (Maximum Ratio combiner) schemes.

Wireless communication systems are being considered for medical applications to facilitate the doctors' operation and the quality of the medical procedures. A demonstrative example of this is the catheterization laboratory (CathLab), where it is desirable to replace the existent wired connections by wireless alternatives. However, there are some challenging requirements that need to be fulfilled by the wireless link, especially for intra-vascular ultra-sound (IVUS) systems, since the images acquired by the catheter should be transmitted with very high data rate and low latency, together with the highest possible amplification efficiency, to increase the battery life. The communication requirements can be achieved with latest the Wi-Fi standard IEEE 802.11ax (Wi-Fi 6). However, since Wi-Fi is based on orthogonal frequency division multiplexing (OFDM) waveforms, the transmitted signals present high envelope fluctuations, leading to amplification difficulties due to the nonlinear distortion effects and low energy efficiency. In this paper, we present an innovative amplification scheme named quantized digital amplification (QDA). It is shown that the QDA allows a quasi-linear amplification of IEEE 802.11ax signals while maintaining a very high energy efficiency. To demonstrate this, a QDA prototype and a set of performance results, regarding both the linearity of the transmitted signals and the energy efficiency, are presented.