# Energy Efficiency Analysis of MIMO Wideband RF Front-End Receivers
## 1 Introduction  
A very relevant use case in wireless communication systems presently is the Internet of Things (IoT) [1]. Moreover, with the fifth-generation (5G) technology, the market is expanding towards massive IoT deployments connected to complex smart sensor networks, which require low-cost devices with low power consumption [2]. Since IoT devices typically consist of battery-powered nodes, energy efficiency (EE) is a critical issue, so that long-term operation without battery replacement is feasible. This demand makes EE a key design goal for 5G IoT applications [3]. Multiple antenna (MIMO) systems have been used to mitigate the effects of fading, which allows improving the link reliability so that wireless nodes transmit with reduced power. However, each antenna should be connected to a radio frequency (RF) chain and, therefore, multiple antennas may increase the power consumption at the circuit level. In that sense, antenna selection (AS) is an important technique [4], yielding the same diversity gains as MIMO, but with lower power consumption due to the reduced number of active RF chains. For instance, the EE optimization of some MIMO techniques, when considering the effect of reconfigurable RF transceivers, is discussed in [5]. Along with AS, singular value decomposition (SVD) beamforming and single antenna (SISO) are considered. Results show that EE can be improved considerably with the AS technique for short and moderate distances. On the other hand, the spatial diversity gains provided by SVD are important to extend the range. In addition, the combination of antenna selection and low-resolution phase shifters for device-to-device communications is investigated by [6]. The proposed antenna selection algorithm is shown to increase the energy efficiency by suppressing interference from other devices in the network, as well as improving the received signal power. Another important venue to improve the EE is to reduce the power consumption at the circuit level, e.g., through battery management techniques [7] or receiver architecture redesigns [8]. CMOS technology downscaling imposes huge challenges for RF designers, which need to meet severe system requirements while keeping power-consumption as low as possible. Seeking for low-power front-end receivers compliant with the required specifications of 5G-based IoT scenarios, both receiver architecture and circuits have to be optimized. To support a huge number of applications in different communication standards, usually multiple RF front-ends are designed to operate in a single band/standard each, thus potentially consuming considerable power overall [9]. Hence, a low-power wideband RF front-end receiver is essential for multi-standard communications. Figure 1 illustrates two different wideband receiver architectures. The conventional receiver chain in Figure 1 has a band-pass filter (BPF), followed by a low-noise amplifier (LNA) and an active mixer. In wideband applications, BPF are generally surface acoustic wave (SAW)-filters organized in banks, which are switched on/off depending on the band or channel required. These SAW-filters make the receiver bulky and costly. Several strategies on the LNA designs can be found in the literature for wideband receivers, depending on the metric to be optimized. To save die area, wideband inductorless LNAs with tunable active shunt-feedback architecture have been proposed in [10]. Also employing an active shunt-feedback, but focusing on reducing noise figure, a wideband noise-canceling CMOS LNA with enhanced linearity was proposed by [11]. In addition, aiming at reducing the power-consumption and improving the linearity, the circuit designed in [12] is a dual-path noise and nonlinearity canceling LNA. Furthermore, instead of designing a single wideband LNA, a multi-standard beamforming RF front-end is presented in [13], which consists of four independent receiving paths. This circuit covers a wideband operation, yielding better noise figure and linearity at the expense of increasing the overall power-consumption.