一般許可接取使用者頻道配選之機制設計與實作

Abstract

有鑑於無線通信的巨大流量增長，美國聯邦通信委員會（FCC）提出了一個創新的三層式頻譜共享監管框架，以提高頻譜利用率並減輕頻譜缺陷。此三層分別為：具有最高接取權的既有使用者（IA）、優先接取（PAL）及一般許可接取（GAA）。IA 的接取使用要免於干擾，PAL接取使用所受到的干擾則應保持在規範程度以下。GAA不得干擾IA及PAL使用者，而與其它GAA使用者在容許有干擾的情形下和諧共享接取使用。無線創新論壇（WInnF）為3.55-3.7 GHz公民無線電服務頻段提出系統架構，其包含了頻譜存取系統（SAS）、區域代理人（DP）與網路管理系統(NMS)等等以管理基地台(CBSD)的頻譜接取使用權。其中GAA至少能使用80MHz頻寬的一般許可接取最早可開始實現，但細部接取管理機制則尚待設計。 本論文研究探討細部設計GAA營運商的頻道選擇(channel selection) 及SAS的頻譜分配(frequency assignment)機制。頻道選擇為其有需求的CBSD選擇最佳的頻道組合使營運商的總容量(Shannon Capacity)最大化。頻譜分配為調和運營商間的頻道選擇，讓營運商間的總容量(Shannon Capacity)最大化且滿足一定的公平性，同時保障IA跟PAL使用者的接取使用品質。 具體的頻道選擇和頻率分配（CSFA）機制設計研究問題有兩個： 1. 頻道選擇問題:考慮到干擾如何影響總生產容量，單一營運商如何確定頻道選擇以最大化其總容量？ 2. 頻率分配問題: SAS如何通過頻率分配，協調GAA營運商的頻道選擇以最大化營運商間的總生產容量且同時保持營運商之間的公平性？ 前者因干擾和生產容量成高度非線性關係且有諸多頻道組合而使總容量最大化選擇困難。後者挑戰在於如何定義GAA營運商之間的公平性，以及當營運商的總需求大於可用頻道時，SAS應該如何根據公平性和總容量最大化來分配頻道。 本論文研究新設計了單運營商頻道選擇（SOCS）演算法來解決問題一。 SOCS設計是基於非線性背包問題（KP）的數學模型以最大化營運商的總生產容量，並且整合掌握了兩項特色: (一) 輸入使用及輸出結果使用符合WInnF-TS-0016 SAS-DP/CBSD資訊交換協議之資訊項目; (二) 以頻道需求滿足性將GAA不保障品質下頻道滿足CBSD需求，及各CBSD間以平等和諧共享接取使用的精神，導入於頻道選擇決策模型。本論文研究為SAS頻率分配設計了多營運商頻率分配（MOFA）的演算法來解決問題二。 MOFA設計同樣考慮到符合WInnF-TS-0016 SAS-DP/CBSD資料交換協議，基於非線性KP的數學模型以最大化營運商間的總容量，並且有兩項創新巧思: (三) 基於GAA盡量滿足需求但不保證服務品質的本質，定義比例基本容量公平性（PBCF）導入頻率分配決策模型為約束條件。(四)定義協調指標(Coordination Index，CI)，讓SAS的頻率分配盡量依照各營運商所提出之頻道選擇組合，同時提升營運商間可接取使用的總容量。 以數值實驗進行評估SOCS與MOFA演算法的研究發現如下： I.有需求之CBSD總數目大於總可用頻道數: I.1 在均勻分散分布的情境下，最佳選擇配置的特性: 總距離其他CBSD最短的CBSD會獨占單一頻段，其餘CBSD平均分享使用剩餘頻段。 I.2 在分布集中的情境下，最佳選擇配置的特性: 犧牲一頻段給互相干擾很大的CBSDs，其餘可用頻段皆讓某些CBSD獨占單一頻段。 在多營運商頻率分配的情境下，我們的研究結果之發現如下: II. 當營運商間之總頻道需求少於總可用頻段時，若營運商間選擇的頻道有衝突時，則SAS將協調有衝突的頻道到沒有衝突的頻道，亦即調和到每個CBSD都獨占單一頻道。 III.當營運商間之總頻道需求大於總可用頻段時，SAS根據operators所提出之頻道組合計算最佳配置及最佳協調指標，提供對個別operator的配置給operator實施 IV. 營運商間之總生產容量 (total Shannon capacity)在SAS有調和分配下會大於等於Operators間自己頻道選擇之結果。 V. 在均勻分散分布的環境下，雖然多加了基本容量公平性之考慮，最佳配置有時也會像I-1之單一CBSD獨佔某頻道的特性。在分布集中的環境下，營運商間的總生產容量會較公平。 VI. CS和FA的計算時間皆隨CBSD的數目及可用頻段數成指數關係，亦即複雜度為O(NK^N),其中K為可用頻道總數, N為有需求的CBSD總數。以10個有需求的CBSD及3個可用頻道數為例，CS演算法計算時間約為16~19秒、FA演算法計算時間約為19-23秒。 本論文為GAA營運商實做了線上頻道配選平台且實作部分之WInnF-TS-0016 SAS-DP / CBSD-SAS之資料交換規範。我們透過演示兩個情境來概念驗證此系統之可行性及萃取CSFA的特徵。頻道以10MHz為單位，一次配選時間為6小時。情境一:單一營運商，3個GAA的需求、2個可用頻道數。情境二:兩個營運商個別有兩個GAA的需求、4個可用頻道數。本論文結合Python實施頻道選擇及頻道分配演算法並整合至平台上。當SAS回覆頻率分配結果給營運上的DP後，CBSD根據頻率分配結果進行傳輸。 在我們的系統中，我們將正文Gemtek X-Cell做為CBSD在3.55-3.6GHz頻率範圍內傳輸。當頻道配選完成後，頻道配選平台將自動設定X-Cell接取使用所配頻道進行傳輸，整體程序完成時間約28秒，其中演算法部分皆可在0.05秒內完成，傳送及回傳請求之總時間大約2秒，而控制正文X-cell在該配選之頻道上傳輸之時間約為25秒。

In view of tremendous traffic growth of wireless communications, Federal Communications Commission(FCC) announced an innovative, 3-tiered regulatory framework of spectrum sharing to raise spectrum utilization and mitigate spectrum deficiency. The three layers are: Incumbent (IA, tier-1) which has highest access priority, Priority Access License (PAL, tier-2) whose users access can be protected with interference limit, General Authorized Access (GAA, tier-3) whose base station users may access through 3.55-3.7GHz and shall have no expectation of interference protection from other users. Wireless innovation forum(WInnF) proposed management system architecture for Citizens Broadband Radio Service(CBRS) consisting of spectrum access system(SAS), Domain Proxy(DP) and Network Management System(NMS) which manage the base station(CBSD). Although the rulings that overall GAAs can use at least 80 MHz bandwidth have been announced by FCC, detailed mechanism design is an immature stage. In this thesis, we worked on mechanism design and implementation aspects for GAA operators because there is lack of available solutions for fair accesses by GAA operators that increases spectrum efficiency yet. In specific, we shall focus on frequency assignment in SAS and channel selection in DP. The role of channel selection(CS) is to determine channel combinations for GAA CBSDs of operator that enhance its total capacity so that the users’ quality of service(QoS) are raised. The role of frequency assignment(FA) is to coordinate operators’ channel selection by assigning channels that maximizes total capacity of operators as well as protecting IAs and PALs’ QoS. Specific research problems of channel selection and frequency assignment(CSFA) mechanism design have two folds: I. Channel Selection: How does one operator determine channel selection that maximizes total capacity considering how interference affects total capacity? The challenges lie in highly non-linear relationship between interference and Shannon capacity and many combinations of channel selection. II. Frequency Assignment: How does SAS coordinate operators’ channel selections by assigning channels that aims to maximize total capacity of GAA operators while maintaining fairness among operators? The challenges lie in how should fairness among GAA operators be defined and how should SAS assign channel/frequency according to fairness and Shannon capacity maximization when operators’ total demands exceed available channels. To address problem I and its challenges, we designed a novel functionality of channel selection for single operator called single operator channel selection(SOCS). The SOCS design is non-linear knapsack problem(KP)-based formulation that maximizes the total capacity of operator and integrates two innovations. The first innovation is that our design complies with WInnF-TS-0016 DP/CBSD-SAS protocol. The second is we formulated the channel demand satisfaction concept as constraint based on the essences of equal access priority and no expectation of interference protection among GAAs. To address problem II and its challenges, we designed a functionality of SAS frequency assignment called multiple operator frequency assignment(MOFA). The MOFA design is I/O information items complying with WInnF-TS-0016 DP/CBSD-SAS data exchange protocol and non-linear KP-based formulation that maximizes total average capacity of GAA operators and integrates three innovations. The first innovation is defining proportional basic capacity fairness(PBCF) based on the essences of SAS that tries to do best effort to match demands of each operator and the equal quality among GAAs. The second is formulating the PBCF concept as a constraint. The third is defining and formulating coordination index into frequency assignment that try to match operators’ channel selections as much as SAS can. In SOCS scenarios, the findings and insights of our results are as follows: I1) In the spectrum scarcity scenarios with distributed distributions, operator will select an exclusive channel for the CBSD that has minimum total distance with other CBSDs. I2) In the spectrum scarcity scenarios with clustered distributions, operator will select one channel for most of required CBSDs and let remaining CBSDs monopolize other available channels. In MOFA scenarios, the findings and insights of our results are as follows: F1) In the abundant spectrum scenarios, if operators select the same channels to their CBSDs, then SAS coordinates operators’ channel selection until each CBSD monopolizes one channel. I3) In the spectrum scarcity scenarios with distributed distributions, total capacity of operators in SAS frequency assignment(coordination) is always higher than or equal to in operators’ channel selection. I4) In the spectrum scarcity scenarios with clustered distribution, proportional basic capacity fairness will decrease and SAS will assign channels to operators with maximum basic capacity of each CBSD (i.e. Q_max). In other words, the fairer total capacity among GAA operators in clustered distributions. F2) The computation time of SOCS and MOFA grows exponentially with number of required CBSDs and available channels. The time complexity of algorithms are O(NK^N), where K is total number of available channels and N is total number of required CBSDs. In the scenario with 10 required CBSDs and 3 available channels, the computation time of CS is 16-19 seconds and FA is 19-23 seconds. Furthermore, this thesis implements an online platform of channel selection and frequency assignment for operator. We implement a part of WInnF-TS-0016 DP/CBSD-SAS protocol in our platform. The CSFA system proofs of concepts by demonstrating two scenarios to capture the characteristics of CSFA and the feasibility of the system. Each channel has 10 MHz bandwidth and the time period of spectrum assignment is 6 hours. The environment of Scenario I: single operator with 3 required CBSDs and there are 2 available channels. Scenario II: Two operators, each operator has 2 required CBSDs and there are 4 available channels. After SAS replies the response to DP, CBSDs transmit on the channel based on the frequency assignment results. Moreover, in our system, we combine Gemtek X-Cell as CBSD to transmit throughout 3.55-3.6GHz. After channel selection and frequency assignment are done, our platform will control the transmission of X-Cell automatically. The overall processing time is around 28 seconds which algorithms run within 0.05 second, requests and responses of procedures take around 2 seconds and the time from DP control X-Cell’s configuration to X-Cell start transmitting is around 25 seconds.