[論文レビュー] UAVs with Reconfigurable Intelligent Surfaces: Applications, Challenges, and Opportunities

Aly Sabri Abdalla, Talha Faizur Rahman+1 more|arXiv (Cornell University)|Dec 8, 2020

Advanced Wireless Communication Technologies参考文献 10被引用数 43

ひとこと要約

この論文は、RIS対応UAVをセルラーネットワーク向けに調査し、ユースケース、利点、課題、および将来の研究方向を詳述します。 また、空中RISネットワークを地上および空中のみのシステムと主要指標で比較します。

ABSTRACT

A reconfigurable intelligent surface (RIS) is a metamaterial that can be integrated into walls and influence the propagation of electromagnetic waves. This, typically passive radio frequency (RF) technology is emerging for indoor and outdoor use with the potential of making wireless communications more reliable in increasingly challenging radio environments. This paper goes one step further and introduces mobile RIS, specifically, RIS carried by unmanned aerial vehicles (UAVs) to support cellular communications networks and services of the future. We elaborate on several use cases, challenges, and future research opportunities for designing and optimizing wireless systems at low cost and with low energy footprint.

研究の動機と目的

• 再構成可能な知能表面をUAVと統合して将来のセルラーネットワークを支援する動機付け。
• RIS-UAVがカバレッジ、容量、セキュリティ、大規模接続、スペクトラム共有、SWIPTを強化できるユースケースの要約。
• チャネルモデリング、チャネル推定、RIS/UAV制御オーバーヘッドにおける主要な研究課題の特定。
• RIS-UAVの6G統合に向けた新技術と研究方向の強調。

提案手法

• RISとUAVの既存文献をレビューして統一的なRIS-UAV通信フレームワークを明確化。
• ユースケースと潜在的な性能向上（カバレッジ、容量、PLS、SWIPT）を議論。
• チャネルモデリング、チャネル推定、RIS制御オーバーヘッドなどの課題を概観。
• ML/AI、mmWave/THz、VLC、インデックス変調、6G統合における機会を要約。

実験結果

リサーチクエスチョン

• RQ1 RISを搭載したUAVが都市部およびエッジ環境でセルラーカバレッジを拡張し、スペクトル効率を向上させるにはどうすればよいか？
• RQ2 RIS-UAVシステムの主な課題（チャネルモデリング、推定、制御オーバーヘッド）は何で、それらはどう解決できるか？
• RQ3 カバレッジ、容量、PLS、マス接続、スペクトラム共有、SWIPTの観点から、空中RISと従来の地上・空中ネットワークの比較潜在性はどうか？
• RQ4 効率的なRIS-UAV運用と最適化を実現する6G技術とAI/MLアプローチは何か？

主な発見

Metric	Terrestrial cellular networks	Integration of aerial nodes	Aerial RIS-assisted networks
Coverage	• Coverage extension via relaying, D2D, multi-hop networking, etc. • However, challenges remain and service deteriorates with channel conditions.	• Aerial communications nodes can extend the coverage where and when needed. • For broad coverage, a constellation of UAVs needs to be formed, which is energy consuming as well costly.	• Aerial-RIS increases the coverage by directing the beam towards the UEs in addition to free positioning of the UAV and ability to track and follow target UEs. Additionally, a portion of the RIS elements can be used to charge the on-board battery of the UAV.
Capacity	• Driving technologies include D2D, multi-RAT, and cell-densification, which increases deployment cost and complexity.	• 5G-aerial systems leverage LoS- MIMO to exploit channel capacity. • Such can introduce severe pilot contamination with for ground users and other aerial nodes.	• Aerial-RIS systems leverage the capacity performance of terrestrial and aerial systems and boost them using the flexibility of defining the appropriate number of elements to tackle problems such as pilot contamination. Moreover, the high degrees of freedom enables increasing the spectral efficiency.
PLS	• 5G uses signal processing (precoding) to reduce secrecy outage probability; however, this increases transmitter/receiver complexity.	• 5G aerial systems create safe zones for ground users by transmitting AN. The limited energy at aerial systems may not sustain these prevention schemes.	• The aerial RIS enhances wireless security by directing power to legitimate users. Since passive phase shifting is employed, significant gains in terms of energy efficiency can be achieved.
Massive Access	• Various massive access techniques (orthogonal and non-orthogonal) exist, increasing transceiver design complexity.	• Aerial assisted networks can be dynamically deployed closer to end users to serve a large number of devices. UAV spectrum access and interference compromises scalability.	• An aerial RIS reflects signals to the desired users and mitigates interference. Scalability can be achieved by coordinating multi-UAV-RIS systems with static RISs.
Spectrum Sharing	• Dynamic spectrum sharing is considered for several 5G bands; challenges include coordination, MAC protocols, and interference.	• Aerial assisted networks provide more degrees of freedom for spectrum sharing, e.g. by limiting the interference footprint through strategic UAV positioning.	• Aerial RIS systems enable flexible spectrum sharing with the help of RIS, which allows multiple users to share spectrum without causing harmful interference to each other.
SWIPT	• SWIPT is suggested for terrestrial networks with power splitting or time switching; near–far problem challenges fairness.	• The 3D mobility pattern of UAVs and LoS links can addresses the near-far problem. In addition, aerial nodes are the key enablers to provide SWIPT especially for where terrestrial networks are damaged. • The limited flight time of aerial nodes is a challenge, especially with dispersed users that need to be served.	• Aerial RIS-assisted systems provide beams for EH and ID alongside with the signals from the terrestrial or aerial network so that it can serve multiple users at the same time. The ability to continuous charge the aerial nodes by using a portion of the RIS elements while the user-centered SWIPT tasks are ongoing will extend the lifetime.

• RISを搭載したUAVはカバレッジを拡張できる（IOSは盲点のない360度反射を可能にする）。
• RIS-UAVはパッシブビームフォーミングを通じて容量を向上させ、アクティブリレーと比較してスペクトル効率の向上と電力消費の低減の潜在的利点を持つ。
• RISを搭載したUAVは正当なユーザーへ電力を向けることで物理層セキュリティを強化できる。受動的位相シフトを用いるため、エネルギー効率の顕著な利得が得られる。
• RIS支援UAVは信号条件を形成して干渉を低減し、拡張可能な大規模アクセスとスペクトル共有を実現する。
• RIS-UAVを用いたSWIPTはUAVの軌跡、RIS位相シフト、エネルギー/情報転送の共同最適化を可能にし、エネルギー harvesting・IDデバイスへ同時供給を実現する。
• 空中RIS構成はカバレッジ、容量、セキュリティ、効率の点で地上ネットワークより有利な点を提供するが、チャネルモデリング、チャネル推定、制御オーバーヘッドの課題も伴う。