Metas de Uso de Espectro

In the above-discussed schemes (both linear and nonlinear), the precoding is performed in the digital baseband domain. The precoded digital signals are, subsequently, fed to separate RF chains, where they are converted into analog RF signals, and finally trans- mitted through individual antenna elements, as depicted in Figure 2.2. The massive MIMO systems are not expected to be equipped with a dedicated RF chain for each antenna element, as it would be economically expensive and results in large operational power [LLS+_{14]. Therefore, the precoding schemes that are suitable in the conventional}

MIMO systems are inappropriate for the massive MIMO systems. The BSs in massive MIMO systems need novel architectures and corresponding new precoding schemes that take the size of antenna array into account. Below are some solutions considered in the literature for precoding in massive MIMO systems.

One-Bit Precoding: One of the popular techniques proposed in the literature to

reduce the cost and power consumption in a massive MIMO system is to employ in- expensive low-resolution DACs/ADCs instead of costly high-resolution DACs/ADCs [HGR+16] in RF chains. For example, in one-bit precoding architecture [SFS17], each

antenna element is equipped with a dedicated RF chain; however, each RF chain comprises 1-bit resolution DACs/ADCs per complex dimension. The 1-bit resolution DACs/ADCs are relatively inexpensive, and their power consumption is significantly low, which grows exponentially in the number of quantization bits and linearly with an increase in bandwidth and sampling rate. Novel precoding schemes are proposed, and the conventional precoding schemes are extended for the 1-bit precoding architecture, e.g., [JDC+_{16, SFS17, UJMN16, LMLS18].}

26 Chapter 2: Theoretical Background

Antenna Selection: In this scheme, the BS is equipped with a relatively small

number of RF chains, denoted by R, when compared to the number of antenna elements N. From the antenna array only R antenna elements are selected, and each antenna is connected to an RF chain through switches. Appropriate antenna elements are chosen by employing any suitable algorithm to maximize a given utility function [SN04]. Subsequently, the baseband digital precoding is performed, as discussed in Section 2.3.1 for conventional MIMO systems. This approach inherently limits the performance of a massive MIMO system, as a large number of antenna elements are precluded during the transmission.

Hybrid Analog-Digital Precoding: Another effective method to reduce the hard-

ware cost and operational power consumption in massive MIMO systems is hybrid analog-digital precoding. In this scheme, the precoding is performed in two sequential stages: low-dimensional digital precoding in the baseband and high dimensional ana- log precoding in the RF domain, as illustrated in Figure 2.5. Due to low-dimensional digital precoding, this technique requires a much smaller number of RF chains when compared to the number of antenna elements.

s1 sK Digital precoding z = Ds RF chain 1 RF chain 2 RF chain R z1 z2 zR Analog precoding x = Az 1 2 N

Figure 2.5. Hybrid analog-digital precoding architecture.

In the hybrid precoding, instead of having a dedicated RF chain for each antenna element as in the fully-digital precoding, each RF chain is shared by multiple antenna elements. Each antenna element, in this structure, is connected to one or more RF chains through low-cost PSs. A PS is an electronic device. An ideal PS shifts the phase of a narrow-band input signal by a desired phase value, which is typically adjustable, and scales the magnitude of the input signal by a fixed gain value. The analog precoding is implemented using these PSs in the RF domain. As a consequence, the analog

2.3 Precoding 27

precoding offers limited flexibility, where the phase values of the precoder coefficients are adjustable; however, their magnitudes are fixed. In contrast, the digital precoding offers a higher number of degrees of freedom by allowing the adjustment of both phase and magnitude values of precoder coefficients.

Two types of hybrid precoding architecture are typically considered in the lit- erature, namely, fully-connected architecture and partially-connected architecture [SY17, YSZL16]. In a fully-connected architecture, each RF chain is connected to all antenna elements in the antenna array, as shown in Figure 2.6(a). On the other hand, in a partially-connected architecture, as illustrated in Figure 2.6(b), each RF chain is connected to only a subset of antenna elements in order to reduce the number of PSs and hardware complexity.

In the literature, different types of PSs are considered for hybrid precoding archi- tecture, such as tunable full-resolution PSs, finite-resolution PSs, and fixed phase PSs. The full-resolution (high-resolution) PSs can assume any continuous phase value be- tween 0 and 2π. On the contrary, the finite-resolution (low-resolution) PSs can assume only a finite number of discrete values [SY16]. The low-resolution PSs are inexpensive and require lower operational power, at the cost of reduced degrees of freedom, when compared to the full-resolution PSs. The cost and power consumption can be further reduced by employing a bank of inexpensive fixed PSs (both phase and magnitude

s1 sK d11 dR1 d1K dRK RF chain 1 RF chain R a11 aN 1 a1R aN R Tunable PS 1 N Digital precoding Analog precoding (a) s1 sK d11 dR1 d1K dRK RF chain 1 RF chain R a11 aN 1 a1R aN R 1 N Digital precoding Analog precoding (b)

Figure 2.6. Left: Hybrid precoding with fully-connected architecture. Right: Hybrid precoding with partially-connected architecture.

28 Chapter 2: Theoretical Background RF chain 1 RF chain R 1 R 1 2 C c11 cN 1 c12 cN 2 c1C cN C Fixed PS 1 N Codebook of analog precoders Switches

Figure 2.7. Codebook-based fully-connected hybrid precoding architecture. are permanent), to form a codebook of precoding vectors. In this approach, instead of tuning the phase values of individual PSs, selection schemes are applied to choose appropriate precoders from the predefined codebook. Subsequently, the selected pre- coders are connected to the corresponding RF chains using switches [BLHV16, HMP18], as illustrated in Figure 2.7.

In this thesis, we develop precoding schemes for hybrid analog-digital precoding architecture. In particular, the precoding schemes devised in Chapter 4 assume the codebook-based fully-connected hybrid precoding architecture. In Chapter 5, Chapter 6, and Chapter 7, precoding techniques are designed for both fully-connected hybrid precoding architecture with full-resolution PSs and codebook-based fully-connected hybrid precoding architecture. Moreover, as discussed in the corresponding chapters, most of the proposed methods are easily extendable to the partially-connected hybrid precoding architecture.

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Chapter 3

System Model and Problem Statement

3.1

Introduction

This chapter presents a general system model considered in the thesis. Moreover, the general objectives of the thesis under the framework of the introduced system model are highlighted.