| 摘要: | 3GPP 於2017年發表第五代行動通訊標準 IMT-2020 的準備時程，其將使用於 Release-15、Release-16 以及後續相關 5G 標準規格。IMT-2020 確立了三大應用領域，分別為加強行動寬頻 (enhanced Mobile BroadBand，eMBB)，大規模機器通訊 (massive Machine Type Communication，mMTC) 以及高可靠低延遲通訊 (Ultra Reliable Low Latency Communication，URLLC)。鑒於授權頻譜昂貴，且頻寬需求日益增加，使用非授權頻譜有望成為未來通訊之趨勢。3GPP LTE-A 標準中，已採用 LAA (Licensed Assisted Access) 作為存取非授權頻譜 (Unlicensed Band，UB) 之核心技術，目前 3GPP 亦討論 5G New Radio 於非授權頻譜技術之可行性。未來發展趨勢是盡可能充分利用 UB 提供多樣化的服務。而考量到 URLLC 對於低延遲以及高可靠度之門檻，欲將其實現於 UB 上極其困難，故如何於 UB 上支援 URLLC 之技術目前僅有少數的相關研究。
由於 IEEE 802.11 標準已廣泛利用，LAA 欲使用 UB 皆需與 WiFi 共同競爭通道，因此需系統共存機制。LAA採用 Listen-Before-Talk (LBT) 機制，其運作原理為通訊前先監聽，使 LAA 與 WiFi 共存並在頻譜方面取得較好效率，然而執行 LBT 機制所需之時間應無法達成 URLLC 之低延遲門檻。考慮 URLLC 低時間延遲容忍度與高可靠度要求，若於 UB 上支援 URLLC 應用，設備須持續確保任何時間擁有通道使用權，隨時可進行送收 URLLC 資料，且需符合佔用通道限制之相關法規。本論文設計通道預約機制，藉由全雙工無線電 (Full Duplex Radio，FDR) 因包含特殊自我干擾消除模組 (Interference Cancellation System，ICS) 故允許設備間同時同頻雙向傳輸之特性；於不違反通道使用標準規範之下，當URLLC 設備偵測一通道為空閒狀態時，設備於該通道上發送 Busy Tone 以達成完整佔據通道，並藉由ICS消除自身發出之 Busy Tone，使其他設備之傳送訊號或 Busy Tone 皆可被順利接收。因受限通道最長存取時間 (Maximum Channel Occupancy Time，MCOT) 限制，通道預約機制將以跳頻方式達成。;3GPP had released the preparation schedule for the fifth-generation mobile communication standard IMT-2020 in 2017, which includes the Release-15, Release-16 and subsequent 5G standard specifications. The IMT-2020 focuses on three application scenarios, Enhanced Mobile BroadBand (eMBB), Massive Machine Type Communication (mMTC) and Ultra Reliable Low Latency Communication (URLLC). Since the licensed band is expensive and the increasing requirement for bandwidth, the use of unlicensed spectrum is expected to become a trend of future mobile communication industry. In 3GPP LTE-A standard, the Licensed Assisted Access (LAA) is used as a technology for accessing Unlicensed Band (UB). Currently 3GPP also discusses the feasibility of implementing 5G New Radio over unlicensed spectrum. How to utilize UB is a main trend of developing new technologies to support various applications. Considering the low latency and high reliability of URLLC, it is extremely difficult and challenge to realize URLLC service on UB. Therefore, there are only few researches related to the design of supporting URLLC over UB.
As the IEEE 802.11 (WiFi) system is matured and wildly deployed, the LAA shall consider the coexistence with WiFi products. LAA applying the Listen-Before-Talk (LBT) mechanism to coexist with WiFi users, meanwhile promoting the overall system throughput. However, the time required to perform the LBT mechanism may not fulfill the latency requirement of URLLC. Taking the features of URLLC into consideration, all URLLC devices need to have the access right on at least one channel on UB for transmitting URLLC data at any time. This research will propose two channel reservation mechanisms in order to accomplish the goal. To do this, whenever a URLLC device detects an idle channel, it will send a special busy tone on that channel in order to occupy it. To avoid the self-interference, the URLLC device equips with the Interference Cancellation System (ICS) in the Full Duplex Radio (FDR) to allow the concurrently bidirectional communications between devices. Thanks to the ICS technology, the busy tone issued from a device can be well eliminated by itself. Consequently, the signals transmitted from others URLLC devices can be received successfully. Due to the restriction of Maximum Channel Occupancy Time (MCOT), the channel reservation mechanism incorporates the frequency hopping technique. |
