# Energy Efficiency Analysis of MIMO Wideband RF Front-End Receivers
## 1 Introduction
Common to the previously mentioned low-power wideband LNAs is their focus on providing low noise figure at the expense of reduced gain and linearity. Alternatively, Figure 1 presents the low-noise transconductance amplifier (LNTA) architecture. The multiple off-chip bulky SAW-filters are replaced by on-chip reconfigurable filters, denoted as N-path filters [14]. Then, the combination  
Figure 1: LNA and LNTA front-end receiver architectures for wideband operation. of the high output impedance of LNTA with the low impedance of the N-path circuitry enables significant attenuation of frequencies outside the desired band, thus providing high RF selectivity on-chip. Therefore, the association of the LNTA with the multiple reject-band filters (RBF) in parallel allow the reception and amplification of the desired signal in wideband, through multiple smaller bands, allowing reduced noise figure at the front-end. In addition, to reduce power-consumption, a passive mixer is placed right-after the LNTA. The position of the RBF could change around the LNTA, depending on the desired behavior. LNTAs have recently drawn considerable interest due to their greater linearity performance [15]. For instance, inductorless designs implementing noise and distortion cancellation techniques are presented in [16; 17]. Moreover, a high-linearity RF receiver architecture is shown in [14], adopting Miller band-pass filters for channel selection. To reduce the front-end receiver power consumption, in [18] the LNTA is followed by a current mode passive mixer that provides sufficient linearity to permit coexistence with large out-of-band interference arising from other transceivers. Nevertheless, despite the enhanced linearity of LNTAs, these circuits face severe requirements for impedance matching networks and bandpass filters. As a result, occupied area enlarges significantly. In addition, to improve receiver sensitivity, LNTAs are designed to provide a sufficient gain to the weak incoming RF signals, increasing the power consumption [19]. In summary, the above mentioned LNTA-based designs aim to improve linearity and reduce the noise figure, at the expense of a significant increase in the power consumption. Given all the challenges imposed by energy-constrained applications, in this paper we present an energy efficiency analysis that merges aspects of front-end receiver architectures with the spatial diversity improvements of using multiple antennas. Then, we take into account the power consumption of the required circuit blocks, for both LNA and LNTA architectures, in two MIMO scenarios: one based on AS scheme, in which only one pair of antennas remains active in each transmission; and other based on SVD beamforming, which uses all transmitting and receiving antennas to increase robustness against channel fading. Moreover, as a reference, we also consider the SISO case. Results show that low power consumption is not enough to guarantee the best performance in terms of energy efficiency. There is a tradeoff between link distance, noise figure and power consumption that can be optimized through the proper combination of receiver architectures and spatial diversity techniques. The remainder of this paper is organized as follows. Section 2 details the RF receiver designs and the communication model. The considered transmission schemes are mathematically detailed in Section 3. Then, some numerical results are discussed in Section 4, while Section 5 concludes the paper.