Power Amplification Efficient Transmitter Structures for Massive MIMO with SC-FDE Schemes: A Promising Combination for 5G Systems?

RADIOENGINEERING, VOL. 27, NO. 1, APRIL 2018 221 Power Amplification Efficient Transmitter Structures for Massive MIMO with SC-FDE Schemes: A Promising Combination for 5G Systems? Paulo MONTEZUMA1, 2, 3 , Afonso FERREIRA1, 2 , Rui DINIS 1, 2 , Marko BEKO 3, 4 1 Dept. de Engenharia Electrotecnica Faculdade de Ciências e Tecnologia (DEE-FCT), Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal 2 Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal 3 UNINOVA, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal 4 CICANT-CIC.DIGITAL, Universidade Lusófona de Humanidades e Tecnologias, Portugal , Submitted April 13, 2017 / Accepted November 2, 2017 Abstract. It is well-known that massive multiple-input multiple-output (MIMO) systems have high potential for future wireless broadband systems. Requirements such as high spectral and power efficiency are also crucial in 5G. Based on a multi-amplifier structure it is possible to define a double layered structure where each amplification branch is connected to an antenna array to achieve both constellation and power directivies, assuring at same time similar performances to systems using transmitters with 2-dimensional antenna arrays. Thus, a different path can be followed to improve energy efficiency of power amplification where the usage of parallel amplification branches is combined with big arrays of antennas and multi-stream communication systems. These systems can be combined with single-carrier with frequency domain equalization (SC-FDE) schemes to improve the power efficiency in uplink due to the low envelope fluctuations. Keywords Massive MIMO, multi-amplifier, power efficiency, double layered structure 1. Introduction High data rate, spectral and energy efficiencies are key requirements for further 5G systems. High data rates can be supported by multiple-input multiple-output systems (MIMO) which can increase throughput with a reduction of the transmitted power by each antenna. With the advent of millimeter waves large numbers of antennas can be deployed in small areas. This allows the use of multiple antenna systems to obtain very high transmission gains, with beamforming and massive MIMO techniques [1–3]. Beamforming is a versatile technique to achieve high data rates [4] DOI: 10.13164/re.2018.0221 in signal transmission in the presence of noise or interference. In practice, the limited number of transmission antennas cannot guarantee perfect directivity, which means that users are still somewhat interfering. Although the design of a beamforming array to maximize the signal power at the intended user is fairly easy, the minimization of the interference is generally a nondeterministic polynomial-time (NP) hard problem [5]. On the other hand, high spectral efficiency requirements are only attainable with the use of multilevel constellations. However, the use of multilevel comes at the expense of reduced power amplification efficiency, which is undesirable in mobile systems [6], [7]. Energy efficiency in power amplification requires the use of nonlinear (NL) amplifiers, which only work with constant or almost constant envelope signals when it is intended to avoid nonlinear distortion. In this paper the characterization of a new multi-layer transmission system with bi-dimensional antenna arrays is provided. The proposed multi layer-transmitter transmitter combines beamforming with a constellation shaping technique, where multilevel constellations are decomposed into several constant envelope bi-phase shift keying (BPSK) components, that can be amplified separately. The better power amplification efficiency is due to the use of nonlinear amplifiers in such operation [7–9]. Thus, each BPSK component requires a separate radio frequency (RF) chain including a power amplifier. The key difference to MIMO classical implementation relies on the fact that each RF chain is associated to a BPSK component that is combined at channel level to generate the desired multilevel constellation symbol. As we shall see the constellation shaping performed by the transmitter acts as a amplitude and phase distortion of the constellation when the transmitter parameters are unknown. Thus, one might expect that the distortion would affect mutual information (MI) for any user unaware about transmitter configuration. Phase rotations between RF branches lead to SIGNALS 222 P. MONTEZUMA, ET AL., POWER AMPLIFICATION EFFICIENT TRANSMITTER STRUCTURES FOR MASSIVE MIMO . . . an optimization of the transmitted constellation in a desired direction, but do not have any impact on the directivity of the radiated power, which is only assured by a beamforming operation performed at a different transmitter’s layer. The strategy is to blend both techniques in a bi-dimensional antenna array composed by Nv × Nh antennas, where Nv and Nh are the number of antenna elements aligned vertically and horizontally, respectively. The Nv elements are responsible for the constellation shaping (layer 1) and the Nh elements are responsible for azimuthal radiation directivity (layer 2). For a proper reception, any receiver needs to know the constellation parameters and array configuration used by the transmitter. Consequently, robustness against interception and interference becomes enhanced [10]. This two layer transmit structure is particularly suitable for 5G wireless communications as it can take advantage of the large number of antennas of massive MIMO structures, providing azimuthal power directivity and reinforced physical security against interception. To cope with the sensitivity to interference of large or non-uniform constellations, specifically intersymbol interference (ISI) caused by multipath propagation and dispersive channel effects, a single-carrier with frequency-domain equalization (SC-FDE) scheme is considered. SC-FDE schemes have also the additional advantage of a lower peakto-average power ratio (PAPR) when compared with orthogonal frequency-division multiplexing (OFDM). The main motivation for this paper is to present an extended characterization of this layered transmitter structure combining power efficiency with physical layer security. Section 2 characterizes the concepts behind the multi-layered transmission architecture. Layer 1 configuration possibilities are discussed in Sec. 2.1. The inherent security due to layer 1 is discussed in Sec. 3, being the MI and secrecy capacity analyzed in Sec. 3.1. Receiver’s characterization is done in Sec. 4. Methods for transmitter parameters estimation are discussed in Sec. 5, 5.1 and 5.2. Multi-layer implementations are analyzed and evaluated in Sec. 6 and the corresponding simulation results are presented in Sec. 6.1. Finally, Sec. 7 concludes this paper. 2. Multiple Layer Architecture Figure 1 exemplifies the two layered transmission structure, where a massive MIMO scheme is employed with Nv × Nh antenna (...truncated)