為降低高頻通訊鏈結建立延遲之軸幅式網路架構及 ALE 流程設計

Abstract

高頻(High Frequency, HF)通訊為頻率範圍介於3MHz~30MHz之無線電通訊，由於高頻電波可透過電離層反射傳播，具有通訊距離長、佈署成本低、靈活性高等優點，卻也受限於電離層的狀態變化，使得穩定性與傳輸速率普遍不高。近年來隨著自動鏈結建立機制(Automatic Link Establishment, ALE)以及機器學習技術發展，提升對高頻電離層通道的預測性及掌握度，進而提高頻段的使用率，讓高頻通訊重新被受到重視。第四代的高頻通訊(4G-HF)，又稱寬頻高頻(Wideband HF, WBHF)，透過增加各頻段傳輸頻寬以及同時接取多個頻道等方式，提高傳輸速度，在此發展趨勢下，鏈結建立延遲對傳輸效率的影響變得更明顯，以傳輸固定大小(139kB)檔案為例，當鏈結頻寬由3KHz提升至48KHz，鏈結建立所造成之額外時間佔比約由8%提升至55%，故我們希望設計一網路架構及ALE流程來改善鏈結建立延遲。 本論文針對特定高頻通訊應用情境—小範圍(每組傳輸距離小於500公里)非軍用高頻通訊系統進行研究，提出通道協調式側鏈通訊之軸福式高頻網路系統(Channel Allocation Sidelink Communication of Hub-and-Spoke HF System, CASC-HaS HF)，並希望滿足以下兩點技術目標：(T1)可行性，資訊傳遞及無線電功能設計皆符合現有高頻通訊標準；(T2)低延遲，針對ALE機制修改以降低鏈結建立延遲。CASC-HaS HF包含一個新的高頻網路架構設計—協調式軸幅高頻網路架構(Hub-and-spoke Architecture with Coordination, HaSAC)，以及兩階段的ALE流程設計—角色分層通道探測(Role-tiered Channel Sounding, RCS)與中央協調通道分配側鏈鏈結建立(Central Coordinated Channel Allocation Sidelink LSU , CCCAS LSU)。 CASC-HaS HF設計發想的動機如下。首先，HaSAC網路架構由一個基地台(Base Station, BS)，以及多個使用者裝置(User Eqipment, UE)組成，BS中搭載通道篩選(Channel Filter)及通道分配(Channel Allocation)功能。接著，BS採用RCS收集各UE在各通道上的信噪比並縮小系統的可用通道數量，以降低UE-UE間鏈結建立瓶頸項目—捕捉探針持續時間。最後，當UE間通訊需求產生時，採用CCCAS鏈結建立，透過BS分配閒置通道給UE，且捕捉探針時間下降量會大於BS-UE協調時間成本，可有效降低鏈結建立延遲。 基於上述基礎，本研究之主要研究問題(P)、挑戰(C)、及解決方案(M)為: P1. 新世代高頻通訊系統採用更高頻寬進行傳輸，卻也使鏈結建立對資料傳輸造成之額外負擔更為明顯，如何針對商用高頻通訊系統設計具降低鏈結建立延遲潛力之架構? C1. ALE鏈結建立延遲瓶頸為捕捉探針持續時間，該時間是為了確保通訊接收端能掃描到發送端選定之通道，並且該延遲項目時間與系統可用通道數量線性相關。 M1. 在軸幅式網路架構的基礎上設計HaSAC架構，透過BS統整降低系統內各UE通道掃描數量，以降低UE間通訊所需之捕捉探針持續時間。若BS-UE通訊所增加的時間小於減少的捕捉探針時間，就可以達到降低鏈結建立延遲的效果。 P2. 在ALE鏈結建立流程中，捕捉探針持續時間為一正比於通道掃描數量之延遲項目，目的為確保接收端可在其掃描程序中接收到呼叫訊號。在新世代高頻通訊系統中，為滿足寬頻通道接取的需求，提升掃描通道數量為一趨勢，並會使捕捉探針持續時間上升，成為鏈結建立延遲的瓶頸。如何解決捕捉探針持續時間隨著可用通道數量增加而線性上升的問題? C2. 高頻通道預測模型可將可用通道進行篩選，減少掃描之通道數量，然而HaSAC架構中各通訊裝置若各自進行通道篩選，會有系統內各裝置掃描通道不一致的情況，要適當運用通道篩選並統一系統掃描通道是一挑戰。 M2. 我們提出角色分層通道探測(RCS)，將HaSAC架構中的BS與現有的電離層通道狀態預測模型結合，在通道探測前BS可以事先篩選出狀態較佳的通道，並依照BS與UE層級不同進行ALE通道探測流程設計，使探測訊號只由BS發出，UE則執行通道掃描並在接收到探測訊號時(1)更新掃描通道集，以降低所需的捕捉探針時間長度；(2)回傳該通道上測得之信噪比，作為後續執行通道分配的依據。 P3. 在高頻通訊系統中，可能會有多組通訊同時進行，在封包傳輸的過程中產生相互干擾，導致封包丟失和錯誤率增加，拉長鏈結建立延遲。基地台如何管理通道使用以降低壅塞對ALE時間的影響? C3. 在HaSAC系統中加入通道篩選機制後，由於系統通道掃描集減小，將使封包碰撞發生機率更高。要使各組通訊在鏈結建立時能快速找到未被占用的通道是一挑戰。 M3.¬ 我們提出中央協調通道分配側鏈鏈結建立(CCCAS LSU)，當一UE對另一UE之通訊需求產生時，先傳送通道分配請求至BS，BS會及時將狀態最佳之閒置通道分配給該UE，使其在所分配的通道上進行ALE鏈結建立。 P4. 如何設計支持4G-ALE通訊協定的高頻通訊模擬環境，用來模擬高頻通訊的傳輸並實驗本論文提出的HaSAC架構、RCS以及CCCAS LSU流程，以評估降低鏈結建立延遲與減輕干擾發生的效果? C4. 為模擬HaSAC架構下的通訊，支持本論文設計的通道探測及鏈結建立機制，同時也必須滿足4G-ALE通訊協定的規範，尋找適合的開發套件並實作程式為一挑戰。 M4. 創新設計通道協調式側鏈通訊之軸福式高頻網路系統(CASC-HaS HF)實驗平台。主要採用Python語言開發並整合Simpy離散事件系統模擬套件。此平台包括以下模組：（一）HF通訊裝置4G-ALE控制器模組，模擬BS及UE的ALE流程控制；（二）高頻通道狀態模組，簡化假設各篩選通道狀態皆為可用，並模擬各通道之占用狀態；（三）中央協調器模組，模擬通道分配流程；（四）資料分析模組，分析鏈結建立延遲。 本論文研究的創新與貢獻包含: (1) 創新設計HaSAC架構，為小範圍商用高頻通訊系統設計，結合了支援4G-ALE之商用基地台及通道協調器作為通道集中管理及分配的角色，通過基地台統一管理和減少UE的通道掃描需求，可以降低UE間通訊鏈結建立瓶頸項目所需時間，且在符合MIL-STD-188-141D通訊標準的情況下，搭配ALE流程設計以控制UE-BS通訊所增加的延遲小於減少的時間，進而降低鏈結建立延遲。 (2) 設計HaSAC上的RCS機制，透過BS進行通道篩選及探測，統一縮小系統中各UE的通道掃描集，以降低鏈結建立延遲瓶頸項目—捕捉探針時間長度。以有40個可用通道，20個UE的系統為例，透過RCS機制將通道掃描集篩選為10個後，各UE捕捉探針時間長度由8.4秒降低為2.4秒，減少約71.4%。 (3) 設計HaSAC上的CCCAS LSU，當UE之間通訊需求產生時，向BS請求通道分配，BS及時將狀態最佳之閒置通道分配給該UE以建立鏈結。其中UE-BS互動所增加的延遲會小於被捕捉探針減少的延遲，且BS分配可避免碰撞。採用CCCAS LSU可將鏈結建立時間由點對點ALE的6.5~10.5秒降低為約5.5秒，對於一每次皆傳送139kB之一48KHz寬頻通道，此1~5秒之延遲減少可使該通道使用率及總傳輸量增加約8~17%，提升高頻通道使用效率及靈活性。 (4) 設計與實作CASC-HaS HF實驗平台，透過修改HaSAC架構中的參數，可以觀察在不同可用通道數，UE數量、篩選通道數等調整下，鏈結建立延遲分析結果的變化，有助於後續相關領域的研究。

High-frequency (HF) communication, with a frequency range between 3MHz to 30MHz, is known for its long communication distance, low deployment cost, and high flexibility, facilitated by ionospheric reflection. However, its stability and transmission rate are generally low due to the variability of the ionosphere. With the advancement of Automatic Link Establishment (ALE) mechanisms and machine learning technologies, there has been an improved predictability and control over HF ionospheric channels, leading to an increased interest in HF communication. The fourth generation of HF communication (4G-HF), also known as Wideband HF (WBHF), enhances transmission speed by increasing bandwidth and accessing multiple channels simultaneously. In this context, the impact of link establishment delay on transmission efficiency becomes more pronounced. For instance, the delay caused by link establishment increases from 8% to 55% when the link bandwidth is enhanced from 3KHz to 48KHz for transmitting a fixed-size (139kB) file. This study aims to design a network architecture and ALE process to mitigate link establishment delay. This thesis focuses on a specific application scenario of non-military HF communication systems within a small range (less than 500 km per transmission group) and proposes the Channel Allocation Sidelink Communication of Hub-and-Spoke HF System (CASC-HaS HF). The technical goals are two-fold: (T1) Feasibility - ensuring that information transmission and radio function designs comply with existing HF communication standards, and (T2) Low Delay - modifying the ALE mechanism to reduce link establishment delay. The CASC-HaS HF includes a new network architecture design - Hub-and-Spoke Architecture with Coordination (HaSAC), and a two-phase ALE process design - Role-tiered Channel Sounding (RCS) and Central Coordinated Channel Allocation Sidelink LSU (CCCAS LSU). The motivation for the CASC-HaS HF design is as follows: First, the HaSAC network architecture consists of a base station (BS) and multiple user equipment (UE), with the BS incorporating channel filter and channel allocation functions. Then, the BS uses RCS to collect the signal-to-noise ratio of each UE on each channel, reducing the number of available channels in the system to alleviate the bottleneck in UE-UE link establishment - the duration of probe capture. Finally, when communication needs arise between UEs, CCCAS LSU is used for link establishment, with the BS allocating idle channels to UEs. The reduction in probe capture time exceeds the coordination time cost between BS-UE, effectively reducing link establishment delay. The main research questions (P), challenges (C), and solutions (M) of this study are: P1. How to design a framework with the potential to reduce link establishment delay for commercial HF communication systems, especially when the new generation of HF communication systems employs higher bandwidth, making the extra burden of link establishment on data transmission more apparent? C1. The bottleneck in ALE link establishment delay is the duration of capturing probes, which is essential to ensure the communication receiver can scan the channel selected by the sender. This delay factor is linearly related to the number of available channels in the system. M1. The HaSAC architecture is designed on the basis of a hub-and-spoke network structure, utilizing the Base Station (BS) to consolidate and reduce the number of channel scans required by each User Equipment (UE), thereby shortening the duration for capturing probes necessary for UE-UE communication. If the time added by BS-UE communication is less than the time saved from reduced capturing probe duration, this approach can effectively decrease the delay in establishing links. P2. In the ALE link establishment process, the duration of probe capture is directly proportional to the number of channels scanned, ensuring that the receiver can capture the call signal during its scanning process. In the new generation of HF communication systems, increasing the number of scanned channels to meet the demand for wideband channel access is a trend, which elevates the duration of probe capture and becomes a bottleneck in link establishment delay. How to address the issue of linear increase in probe capture duration with the increase in available channel numbers? C2. HF channel prediction models can filter available channels, reducing the number of channels to be scanned. However, if each communication device in the HaSAC architecture conducts channel filtering independently, there will be inconsistencies in the scanned channels within the system. Appropriately utilizing channel filtering and unifying system scanning channels is a challenge. M2. We propose Role-tiered Channel Sounding (RCS), combining the BS in the HaSAC architecture with existing ionospheric channel state prediction models. Before channel sounding, the BS can pre-filter channels with better conditions, and design the ALE channel sounding process differently for BS and UE tiers. The sounding signal is only emitted by the BS, and the UEs perform channel scanning and update the scanning channel set upon receiving the sounding signal, thus reducing the required duration of probe capture. They also send back the signal-to-noise ratio measured on that channel, serving as a basis for subsequent channel allocation. P3. In HF communication systems, multiple communication groups might be operating simultaneously, causing mutual interference during packet transmission, leading to packet loss and increased error rates. This in turn prolonging link establishment delay. How can the base station manage channel usage to reduce the impact of ALE by jamming? C3. After incorporating channel filtering mechanisms into the HaSAC system, the reduction in the system's channel scanning set increases the likelihood of packet collisions. Quickly finding unoccupied channels for communication groups during link establishment is a challenge. M3. We propose the Central Coordinated Channel Allocation Sidelink LSU (CCCAS LSU). When communication demand arises between UEs, a channel allocation request is sent to the BS. The BS promptly allocates the best idle channel to the UE for ALE link establishment. The delay added by UE-BS interaction is offset by the reduction in probe capture duration. Moreover, BS allocation can avoid collisions and reduce the probability of ALE timeouts. With CCCAS LSU, link establishment time can be reduced from 6.5~10.5 seconds in point-to-point ALE to about 5.5 seconds. P4. How to design a high-frequency communication simulation environment that supports the 4G-ALE communication protocol, to simulate HF transmission and experiment with the proposed HaSAC architecture, RCS, and CCCAS LSU process in this thesis, thereby assessing the effects of reducing link establishment delay and minimizing interference? C4. Simulating communication under the HaSAC architecture, supporting the channel sounding and link establishment mechanisms designed in this thesis, while also meeting the specifications of the 4G-ALE communication protocol, is a challenge. Finding suitable development kits and implementing the program are essential. M4. The innovative design of the Channel Allocation Sidelink Communication Hub-and-Spoke HF Network System (CASC-HaS HF) experimental platform is developed using Python and integrates the Simpy discrete event simulation package. This platform includes modules for: (1) a 4G-ALE controller module for HF communication devices, simulating ALE process control for BS and UEs; (2) an HF channel status module, assuming all filtered channels are available and simulating the occupancy status of each channel; (3) a central coordinator module, simulating the channel allocation process; and (4) a data analysis module for analyzing link establishment delays. The innovations and contributions of this thesis include: 1)The HaSAC architecture innovatively designed for small-scale commercial HF communication systems integrates commercial base stations supporting 4G-ALE with channel coordinators for centralized channel management and allocation. By managing and reducing the channel scanning requirements of UEs through the base station, it reduces the time needed to establish communication links between UEs. This approach not only complies with the MIL-STD-188-141D communication standard but also ensures that the delay added by UE-BS communication is less than the time saved, thereby reducing link establishment delays. 2)Designing the RCS mechanism on HaSAC, where the BS performs channel filtering and sounding, unifying and reducing the channel scanning set for all UEs in the system, thereby decreasing the bottleneck in link establishment delay - the duration of probe capture. For a system with 40 available channels and 20 UEs, filtering the channel scanning set to 10 channels through the RCS mechanism reduces the probe capture duration from 8.4 seconds to 2.4 seconds, a reduction of approximately 71.4%. 3)Designing CCCAS LSU on HaSAC, where UEs request channel allocation from the BS when communication needs arise. The BS promptly allocates the best idle channel to the UE for establishing the link. The delay added by UE-BS interaction is offset by the reduction in probe capture duration, and BS allocation can avoid collisions, reducing the probability of ALE timeouts. Using CCCAS LSU can reduce the link establishment time from 6.5~10.5 seconds in point-to-point ALE to about 5.5 seconds. For a system with 40 available channels and 20 UEs, if the filtered channel number is 10, using CCCAS LSU reduces the link establishment delay to about 5 seconds. For a 48KHz wideband channel transmitting a 139kB file each time, this reduction in delay of 25 seconds can increase the channel usage rate and total transmission volume by approximately 8~17%, enhancing the efficiency and flexibility of HF channel use. 4)Designing and implementing the CASC-HaS HF experimental platform allows for observing changes in link establishment delay analysis results by adjusting parameters within the HaSAC architecture, such as the number of available channels, the number of UEs, and the number of filtered channels. This can contribute to research in related fields by providing insights into how different configurations affect communication efficiency and delay.