Practical Considerations for Reconfigurable Intelligent Surface aided Systems

Introduction

Reconfigurable intelligent surfaces (RISs) have become a very hot topic in academic as well as industrial research. Numerous theoretical analyses have been conducted and recently experimental studies have also been initiated. Large companies have even constructed RIS prototypes and presented some benefits RIS technology brings forth. However, since RIS may still be considered by some to be in its early stages of research, there are many assumptions made and these reduce the practicality of the system. Recently, RIS has been considered a study item which is a key step in its advancement of being considered for wireless communication standards. Hence, research has to shift towards more practical scenarios when including a RIS aided set up to show the true benefits and expectations RIS technology has to put forth. This script will explain key practical points that must be taken into consideration when designing RIS aided wireless communication systems.

RIS Placement

The strategic placement of RIS plays a pivotal role in enhancing wireless communication systems. Where you place the RIS can detrimentally impact the performance gain it brings to the system. As it can be seen in most RIS literature, one major factor to consider is the double fading path loss the signal reflecting off the RIS experiences. The signal impinging off of the RIS is generally not the shortest path to the destination, hence the signal experiences significant path loss. Therefore, ideally the RIS should be situated near either the transmitter (Tx) or receiver (Rx), though sometimes an intermediate location is beneficial, particularly in cases of significant path loss or to circumvent obstacles. Optimal positioning is instrumental in maximizing RIS efficiency, as it allows for more direct manipulation of electromagnetic waves, thereby enhancing signal strength and coverage. It should be noted that both the transmitter to RIS and RIS to reciever path should be jointly considered in this optimal placement. In densely built urban environments, positioning an RIS near the receiver can be advantageous to effectively navigate around physical barriers, whereas in open rural settings, a placement closer to the transmitter might be preferable to extend the signal reach.

Figure 1: Example of a RIS near the transmitter. (https://ieeexplore.ieee.org/document/10338963)

Figure 2: Example of a RIS near the recievers. (https://www.semanticscholar.org/paper/Reconfigurable-Intelligent-Surface-Enabled-Joint-Al-Hilo-Samir/f7e194ebcad02287068629cf928fcbcdf37087e7/figure/0)

When we consider indoor scenarios, the RIS may be placed in the corner of a hall way.

Figure 3: Example of RIS aided indoor scenario for an L-shaped hallway. (https://arxiv.org/pdf/2211.07188.pdf)

The decision on RIS placement thus requires a careful consideration of the specific environmental context and the intended network configuration.

Antenna Characteristics

The characteristics of antennas in RIS-aided systems are crucial for maximizing their effectiveness. For the transmitter, directional antennas such as horn antennas might be suitable without affecting the practicality of the system, as they can focus the signal in a specific direction (generally towards the RIS to maximize reflection signal). However, for receivers, omnidirectional antennas are more appropriate since it is not practical for all receiving users to have horn antennas and assume that they are always facing the required direction of the incoming signal or towards the RIS. These omnidirectional antennas capture signals from all directions, accommodating the realistic dynamic nature of wireless environments where the optimal signal path can frequently change. These signals from all directions can even cause destructive and constructive interference which should be modeled and taken into consideration. This is especially important in RIS applications, as the technology often seeks to enhance coverage and signal quality in various directions. The use of omnidirectional antennas at the receiver’s end ensures a broader reception range, enabling the RIS to effectively interact with signals regardless of their originating direction. This setup enhances network flexibility and overall system efficiency, making it adaptable to a wide range of scenarios.

Figure 4: a) omni directional antennas transmit and receive from all directions. b) directional antennas transmit and receive from a certain and limited angle/direction. (https://link.springer.com/referenceworkentry/10.1007/978-981-4560-44-3_52)

Passive nature of an RIS

In the realm of RIS-aided communication, the passive nature of these systems is a defining feature, particularly in scenarios involving larger cells. Initially in the literature, RISs have been introduced as passive devices however, as research went on, researchers starting proposing RIS configurations, hardware designs, complex algorithms, and many other aspects that would drift from the passive concept. At some point in the literature, researchers began to include amplifiers in RIS elements and introduce a concept of active RIS. Whether or not a RIS should be active or passive may be debated however it is a serious consideration and must be thought through carefully. It may vary from scenario to scenario and possibly a hybrid architecture may even be proposed.

Let us focus on the most common passive RIS case which is generally desired in future wireless networks. The preference for passivity stems from a desire to minimize energy consumption and simplify operational complexity. Passive RIS units are capable of manipulating electromagnetic waves without the need for active power sources, which is a significant advantage in terms of energy efficiency and system design simplicity. For larger coverage areas, the emphasis should be on employing materials and structural designs that inherently reflect and modify the signal path. This approach avoids the need for active electronic components, ensuring the RIS remains as passive as possible, thereby reducing power requirements and simplifying the overall system architecture.

Some considerations that may aid in a passive RIS system are:

• Optimal material selection for a cost-effective RIS operation.
• Circuits with minimal power to operate and possibly incorporate energy-saving modes during times of inactivity.
• Simple RIS designs for ease of deployment, use and maintenance.
• Keeping signal control and manipulations to a minimum and execute them with minimal computational resources.
• Keep the RIS from communicating and connecting/synchronizing/signaling with the network to a minimum.
• Any algorithms (codebook generation, ML/AI, optimization, etc.) at the RIS should be quick and have low complexity.

Figure 5: Comparison between an a) passive RIS structure and b) active RIS structure. (https://www.researchgate.net/publication/350484632_Active_RIS_vs_Passive_RIS_Which_Will_Prevail_in_6G/figures?lo=1)

Size and number of RIS elements

The scalability and adaptability of RIS units are crucial for their effective application across different environments and use-cases. In indoor settings, smaller RIS panels are often more suitable due to space constraints and the need for discretion. Conversely, outdoor or larger-scale applications might benefit from larger RIS panels to achieve broader signal coverage. However, the size of a RIS and its elements also varies significantly based on the frequency it intends to operate on. Higher frequencies with shorter wavelengths can be effectively manipulated with smaller RIS units. However, they will most probably require a larger number of elements for precise operation. On the other hand, lower frequencies with longer wavelengths might require larger panels to interact effectively with the signals but these models may not require as many elements on the RIS. Thus, the design of RIS panels must be versatile, allowing for various sizes and element counts to suit specific deployment scenarios, from compact indoor environments to expansive outdoor areas. These parameters and considerations will greatly affect the cost of the RIS production as well so it is important to design your use case and scenario accordingly.

Frequency Band Considerations

Selecting the appropriate frequency bands is critical for RIS systems to function efficiently and harmoniously within existing wireless frameworks. The choice of frequency bands should be based on the specific use case and environment. This careful selection helps avoid interference with other services and takes advantage of the unique propagation characteristics of different frequency bands. It is essential for RIS systems to be versatile in their frequency band usage to cater to a wide range of network environments and requirements.

In the context of RIS-aided systems, frequency band selection must consider several key factors:

1. Propagation Characteristics: Different frequency bands have distinct propagation characteristics. Lower frequency bands (such as sub-6 GHz) are known for their ability to provide wide coverage and penetrate through obstacles more effectively, making them suitable for rural or suburban environments where broader coverage is essential. On the other hand, higher frequency bands (like mmWave frequencies) offer higher data rates and capacity but have a shorter range and are more susceptible to attenuation and blockage. These high-frequency bands are ideal for dense urban areas or indoor environments where large amounts of data need to be transmitted over shorter distances.
2. Interference Management: Avoiding interference with existing wireless services is crucial. The frequency band chosen for RIS should not overlap with bands heavily used by other services unless it is specifically designed to coexist without causing interference. This requires a thorough understanding of the spectrum usage in the intended area of deployment to identify the most appropriate and least congested frequency bands for RIS operations.
3. Environmental Impact: The environment in which the RIS is deployed plays a significant role in frequency band selection. Urban environments with numerous buildings and obstacles might benefit more from higher frequency bands, despite their limitations in range, due to their ability to support higher data rates and capacity. In contrast, more open and rural environments, where obstacles are fewer, could leverage lower frequency bands for their superior coverage and penetration capabilities.
4. Regulatory Considerations: Frequency band allocation is subject to regulatory policies and varies from region to region. It’s important to consider the legal and regulatory framework governing spectrum usage in the area where the RIS is to be deployed. Compliance with these regulations is essential to avoid legal issues and ensure the successful deployment and operation of RIS systems.
5. Future-Proofing: As wireless technology evolves, considering future trends in frequency band usage is crucial. For instance, as 5G networks expand, the utilization of mmWave frequencies is becoming more common. An RIS system designed with future developments in mind can adapt more easily to upcoming changes in spectrum usage and technology standards.
6. Cost and Resource Optimization: Different frequency bands may require varying levels of resource investment in terms of both hardware and software. Lower frequency bands might necessitate larger RIS panels to effectively manipulate longer wavelengths, potentially increasing material costs. Conversely, higher frequency bands might require more sophisticated and precise manufacturing processes. Balancing these factors is key to ensuring cost-effective and resource-efficient RIS solutions.
7. Application-Specific Requirements: The intended application of the RIS system also dictates the choice of frequency band. For instance, applications requiring high throughput, like video streaming in an indoor setting, might favor higher frequency bands. In contrast, applications prioritizing wide-area coverage, such as IoT networks in rural areas, would be better served by lower frequency bands.

Figure 6: An illustration showing different use cases for different frequencies. (https://terasense.com/terahertz-technology/radio-frequency-bands/)

Integration with Existing Infrastructure

For RIS technology to achieve widespread adoption, it must seamlessly integrate with existing wireless networks, including 4G, 5G, and Wi-Fi systems. This integration involves designing RIS units to operate within the frequency bands utilized by current technologies and ensuring compatibility with existing base stations and network protocols. Effective integration is crucial for the coexistence of RIS with traditional network infrastructure, enabling a smoother transition to more advanced wireless communication technologies and enhancing the overall capabilities of existing networks.

User-Controlled vs Network-Controlled Operation

The operational control of RIS systems should be tailored to the specific environment and use case. In indoor settings, where user requirements can be dynamic and varied, a user-controlled approach might be more effective, allowing individuals to adjust settings as needed. Conversely, in outdoor or broader network environments, a network-controlled RIS could be advantageous for more comprehensive management and optimization. This determination in control mechanisms necessitates flexibility in RIS design, enabling either user-centric adjustments through simple interfaces or more complex network integrations for dynamic network-wide adjustments based on overall network conditions.

Reduced hardware

Current RIS literature mostly relies on excessive hardware such as backhaul links connecting the RIS to the basestation and as mentioned previously, horn antennas at each node and these all introduce impracticality to the system. Designing RIS systems to function with minimal reliance on traditional backhaul networks and hardware is a significant step toward reducing deployment costs and complexity. This is particularly beneficial in remote or underserved areas. Such a design would involve RIS units that can be controlled and managed locally, potentially incorporating edge computing capabilities to reduce dependence on centralized network infrastructure. Localized control and management of RIS systems offer a more cost-effective and efficient approach to deploying advanced wireless communication technologies in various environments.

Acknowledgement:

• Turkcell 6Gen Lab is supported by The Scientific and Technological Research Council of Turkey (TUBITAK) through the 1515 Frontier Research and Development Laboratories Support Program under Project 5229901–6GEN. Lab: 6G and Artificial Intelligence Laboratory