與IEEE 802.11ac公平共存之LTE MAC協定設計

Abstract

長期演進技術(LTE)蜂巢式網路中的流量快速增長，預期將造成需執照頻譜的供給短缺。為了滿足大量的流量需求，在市區及人口擁擠的地方將需要更多的頻譜及小型基地台的布建。有些組織提出LTE-U的概念，使得LTE能夠使用免執照頻譜區段，藉以將部分需執照頻譜區段的流量分流到免執照頻譜區段。然而，這產生了一個問題：如何讓LTE-U小型基地台與擁有一半網路流量，且為免執照頻譜區段的大宗─WiFi公平的共存？ 針對這個問題，目前主流的兩種解法是：工作週期(Duty Cycle)及先聽後說(Listen-Before-Talk)機制。前者依照基地台數量等分頻道的使用時間，但卻無法及時的利用空閒的頻譜，而後者雖然可以有效的運用頻譜，但很難達到公平的共享頻譜。 因此，基於先聽後說機制，設計一個可以公平共享頻譜的LTE-U協定成為一個重要的問題。在本論文研究中，我們主要處理四個問題： 1. 如何在LTE-U和WiFi之間定義可量測的公平性指標？ 2. 如何以先聽後說機制為基礎設計LTE-U協定以達成我們所定義的公平？ 3. 如何盡可能的使用保有現存的LTE協定，並加入我們的先聽後說LTE-U協定設計？ 4. 設計後的公平性及資料傳輸率如何？是否能使個別WiFi使用者的資料傳輸率比僅WiFi的網路環境還好？ 該問題的挑戰主要是 1. 如何客觀的定義容易量測的公平性指標 2. 如何讓我們的設計可達成與WiFi公平共存 3. 如何在現存LTE架構中實現我們的設計 4. 如何達成公平，同時使WiFi的資料傳輸率保持，甚至更好？ 我們的主要貢獻有： C1. 定義資料傳輸率(throughput)對傳送速率(bit rate)的比值作為衡量公平性的標準，並以珍的公平性指標(Jain’s Fairness Index)作為量測基準。此公平性定義的概念是基於機會均等，也就是每個人所付出的與所得到的比值相等。 C2. 我們證明此公平性定義達成的是相同的成功傳輸時間。並且讓媒體存取層排程器動態的調整最大傳輸時間限制(TXOP limit)。此最大傳輸時間限制可以查表，因此其計算複雜度為O(1)。 C3. 我們設計了一個新的LTE-U協定，稱為FaM-CSMA/CA。此設計在媒體存取層排程器中加入頻譜資源尋覓者，此尋覓者的行為類似於CSMA/CA，並且傳送類似RTS及ACK的控制訊號。 C4. 以數值分析效能結果顯示，此協定可達我們所設計的公平性，且在共存環境下與僅WiFi環境下，WiFi的資料傳送率(throughput)可提升約0.7%。與現有的其他LTE-U協定設計相比，FaM-CSMA/CA雖未大幅度提升throughput，但卻達成了完全的時間公平。

The rapid traffic growth of LTE cellular network is expected to result in the shortage of licensed spectrum. To satisfy the tremendous traffic demand, more spectrum and widely spreading LTE small cells are required, especially in metropolitan area with high population density. A proposed concept of LTE-U is that enabling LTE to operate in unlicensed band in order to offloads data traffic from licensed band to shared access of unlicensed band. However, it raises the coexistence issue with WiFi, which is the major user of unlicensed band and holds almost a half of all internet traffic. Two main approaches are duty cycle and listen before talk. The former shares the channel time equally without flexibly utilize the shared channel when it is idle, while the latter makes better use of shared channel, yet, it is hard to achieve fair sharing. Therefore, to design a listen-before-talk based LTE-U protocol and enable fair coexistence with WiFi in unlicensed band becomes a significant issue. In this research, we consider following questions: P1. How to define fairness in LTE-U & WiFi coexistence wireless network, and can be quantified? P2. How to design the LBT-based LTE-U protocol to satisfy our fairness definition? P3. How to fully exploit existing LTE protocol to realize out LBT-based LTE-U protocol design? P4. How is the performance of the design in fairness and total throughput compared with other LBT scheme? Challenges are: 1. How to define an objective and easy-to-measure fairness definition between LTE-U and WiFi. 2. How to achieve fairness in LBT-based LTE-U to coexist with WiFi 3. How to realize the protocol in the framework of existing LTE protocol? 4. How to achieve fairness while the throughput of WiFi can be no worse than WiFi-only environment? Our contributions are: C1) Give a fairness definition in term of the ratio of throughput to available bit rate of the technology, which is a measurable instance of Jain’s Fairness Index [JCH84]. The concept of fairness definition is based on equal opportunity that the ratio of outcome to input is equal among each user. C2) Prove that the fairness definition is equivalent to equal time of successful packet transmission, which enables listen before talk mechanism to achieve fairness by adjusting TXOP limit. The TXOP limit is a look-up table stored in HeNB, which has O(1) time complexity. C3) FaM-CSMA/CA is designed to enable listen-before-talk by adding a new module ‘Spectrum Resource Seeker’ in MAC scheduler. Spectrum Resource Seeker performs multiple CSMA/CA processes with RTS-like signal and designed HARQ ACK. C4) Numerical performance analysis shows that fair coexistence LTE-U protocol achieves almost extreme fairness between LTE-U and WiFi, while throughput of coexisting WiFi users slightly increase 0.7% from that of WiFi only network. Comparing to the existing LTE-U designs, FaM-CSMA/CA reaches totally time fairness between LTE-U and WiFi while compromising on high throughput increase.