Massive multiple-input multiple-output systems (mMIMO) are the most prevalent candidates for the next generation of wireless communication. Yet even with mMIMO systems the joint optimization of spectral and energy efficiencies can be only attained by combining high order signal constellations and efficient power amplification. In order to push this limitation, the transmitter can spread the information into several amplification branches, which are the result of the decomposition of multilevel constellation symbols into quasi constant envelope signals. Nevertheless, the high number of antennas involved in this type of communication leads to an increase of the channel matrix's size and therefore the complexity of the equalization process can create drawbacks for the power consumption and latency. In this paper we will study the combination of a multi-layer transmitter with a low complexity receivers based on an iterative block decision feedback equalizer (IB-DFE). These receivers avoid the matrix inversion operation in the equalizer the feed-forward by replacing it with an equal gain combiner (EGC) or a maximum ratio combiner (MRC) module. Results show that can be used without penalties on performance provided that the number of antennas involved is high.

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 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 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, we present a QDA prototype and a set of performance results regarding both the linearity of the transmitted signals and the energy efficiency.

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Massive multiple-input multiple-output (mMIMO) with perfect channel state information (CSI) can lead array power gain increments proportional to the number of antennas. Despite this fact, constrains on power amplification still exist due to envelope variations of high order constellation signals. Although, power amplification constrains of multilevel constellations can be overpassed by a transmitter with several amplification branches where each one is associated to a component signal that results from the decomposition of a multilevel constellation as a sum of several quasi constant envelope signals that are sent independently. Under these conditions, both physical security and higher power amplification efficiency could be attainable by a transmission structure based on multiple amplification branches transmitting signal components of the constellation symbol. When combined with antenna arrays at the end of each amplification branch, the security due constellation shaping becomes increased due to the energy separation achieved by beamforming. In a mMIMO scenario several structures could be used to achieve energy separation as well as to increase the security among different users. However, for each user, all the beams related with the signal components should be directed in same direction to avoid distortion on the signal that results from the combination of all components at channel level. In such conditions it is crucial to assess the security level achieved by these structures, as well as the impact of impairments in the alignment of the several beams related with the signal components associated to each user. This solution for this problem is the purpose of this work. The set of results presented here show that physical security comes with good tolerance against misalignment of the beam components, which can be also used to increase the physical security achieved by these transmission structures.