Analysis and design of MIMO indoor communication system using terahertz patch antenna based on photonic crystal with graphene

Highlights

• Electrical properties of MIMO antennae based on photonic crystals and graphene.

• Enhanced return loss, bandwidth, gain, and radiation efficiency.

• Enhanced capacity for an indoor communication scenario using photonic crystals.

• Further enhancement of communication scenario by using graphene load.

• Photonic crystals effectiveness against fading in terahertz communication system.

Abstract

In this study, a multiple input/multiple output (MIMO) indoor communication system was developed using microstrip antennae based on a photonic crystal substrate, where the graphene load operated in the terahertz band. First, the characteristics of graphene were analyzed by determining the operating modes related to the chemical potential of graphene. Next, three MIMO antennae were designed and analyzed using homogenous, photonic crystals, or optimized photonic crystal substrates, and by utilizing a planted graphene load. The results obtained in CST simulations indicated enhancement of the bandwidth as the frequency reached 356 GHz and an improvement in the gain was also achieved. An indoor communication environment based on the terahertz band was then studied and compared with previously proposed methods. The path loss and reflection loss were analyzed for single input/single output, single input/multiple output, multiple input/single output, and MIMO systems. The geometrical parameters of the indoor environment operating in the terahertz band were investigated. Finally, an interesting enhancement of the channel capacity was achieved using the aforementioned design for MIMO antennae. Moreover, the extra channel capacity could be modified in the proposed system by manipulating the spacing between the transmission antennae or receiver antennae, or the distance between the transmission and receiver antennae.

Introduction

For the next generation of wireless communication systems, transitioning the carrier frequencies to the terahertz band is a viable solution to meet the long-term demand for extra-high data transmission rates [1], thereby allowing important real-life technologies to emerge. Increasing the available bandwidth is important for establishing high data rate communication links. However, the terahertz channel has distinct characteristics compared with lower band systems, such as a higher transmission path loss and extra molecular loss due to absorption [2]. These problems have been addressed in many studies [3], where highly directional antennae were proposed to overcome the losses and to increase the link capacity for practical indoor environment applications in order to implement extra-high speed communication systems. Moreover, the reflection and scattering of waves differ at the lower bands [4], [5], [6]. For terahertz rays, the surfaces of indoor objects may be perceived as rough surfaces rather than smooth surfaces because the roughness of indoor surfaces such as plaster or wallpaper is comparable to the traveling wavelength.

The motions of moving objects are quite slow in indoor environments compared with terahertz signals through a certain time window. Thus, a moving object can be considered as a static object between the transmission antenna and receiver antenna [7]. Therefore, the channel may be influenced by fading in most situations and this will impact the bit error rate. Hence, diversity is required for the receiver antenna to capture several redundant waves, where the probability of instantaneously affected received waves is excessively diminished. Several strategies can be employed for increasing the channel capacity, such as using the multiple input/multiple output (MIMO) technique in order to introduce a diversity scheme into communication systems [8]. However, MIMO systems are affected by limitations in terms of size, isolation of antennae, and spacing, so it is necessary to develop other techniques to address these constraints, where using a photonic crystal substrate for antennae and making them directional [9] is a promising strategy.

Microstrip patch antennae are widely used in various applications because of their low profile, low cost, simple deployment on molded surfaces, and compatibility with integrated circuit technology [8]. However, the substrate used for the antenna exhibits two main types of loss, which comprise the conduction loss from the substrate and the loss of surface waves due to the substrate's high permittivity and comparatively large thickness [10]. In previous studies, high-dielectric or thick substrates were used to boost the electrical characteristics of microstrip antennae [11], [12], [13], [14]. However, adding high dielectric materials with considerable thickness contributes to the excitation of shock waves in the millimeter and terahertz frequency ranges [15]. Indeed, the use of a thick substrate contributes to surface wave excitation due to the absorption of energy within the substrate [16], but decreasing the thickness of the substrate degrades the antenna's radiation pattern.

Photonic crystals are widely employed as substrates in the design of microstrip antennae for operation at high frequencies to reduce the aforementioned surface wave loss [17], [18], [19], [20], [21], [22]. In addition, two-dimensional photonic crystals are attractive because they are much easier to manufacture than three-dimensional photonic crystals [23], [24], [25], and they have a wide range of uses in addition to antennae and planar waveguides [26], [27]. However, characterizing photonic crystals using strictly analytical methods is generally difficult due to their complexity. Alternatively, full-wave simulators such as the finite integral technique in CST Microwave Studio have been used to analyze antennae implemented with a photonic crystal substrate. The application of photonic crystals in integrated silicon-based on-chip antennae is also appealing in the area of terahertz technology, such as MIMO communication systems, because they are more cost effective in terms of packaging and compactness compared with the traditional packaging with separate antennae and transceivers. In general, the system-on-chip technology is of interest for many terahertz applications, including extreme high analog interfaces, and digital logic for processing and low-cost mass production [28].

Graphene was the first two-dimensional structure made from graphite to be discovered [29]. Many devices such as optical sensors [30] and filters [31] have been constructed based on graphene due to its special properties. The most interesting feature of graphene is that the surface conductivity can be controlled by changing the electrostatic voltage bias. Studies have investigated the possible application of graphene in antennae to improve their radiation characteristics [32], [33], [34], [35], [36], [37].

Manufacturing techniques are constantly developing and improving, and novel techniques were recently employed to fabricate structures at the nanometer scale, which can be employed in the construction of terahertz microstrip antennae [38], [39], [40]. In addition, measurement is a challenging issue that can be overcome by developing measuring techniques for use in the near field [41], [42], [43], thereby allowing the manufacturing and testing of terahertz microstrip antennae with various applications.

The remainder of this paper is organized as follows. In Section 2, we describe our analysis and investigation of the interesting characteristics of graphene load. In Section 3, we explain our MIMO antenna design based on homogenous, photonic crystals and the optimized photonic band gap structures with the graphene load. In Section 4, we describe the application of the proposed antenna in a normal indoor communication environment, as well as various analyses of the capacity enhancement and comparisons with previously reported methods. In Section 5, we give our conclusions.