隨著5G的到來，基地台需要確保三種使用場景不同的需求，這三種場景分別為增強型行動寬頻通訊 (Enhanced Mobile Broadband, eMBB)，大規模機器型通訊 (Massive Machine Type Communications, mMTC) 和超可靠度和低延遲通訊 (Ultra-Reliable and Low Latency Communications, URLLC)。在5G中，UE分配到專屬資源傳送資料前須先完成隨機接入(Random Access, RA)，若隨機接入失敗則會造成延遲上升。隨機接入包含前導碼傳送(Preamble Transmission)、隨機接入響應(Random Access Response)、訊息三(Message 3)和解決競爭訊息(Contention Resolution)等四條訊息。當有多個UE在前導碼傳送時選擇相同的前導碼(Preamble)，這些UE會被分配到相同的時頻空間傳送訊息三，由於這些在相同時頻空間傳送的訊息未經過處理，使得這些訊息發生碰撞，造成基地台無法解出這些UE的資料，導致這些UE這次的隨機接入失敗且必須從前導碼傳送開始重做隨機接入，這類問題叫做前導碼碰撞(Preamble Collision)。在5G中，一個基地台要服務更多的UE，參與隨機接入的UE數量越多，發生前導碼碰撞的機率會急遽的上升造成延遲上升，使得5G的要求無法被滿足。為了解決這個問題，5G透過擴增隨機接入資源的方式減低碰撞發生，然而隨機接入保留越多的資源將導致使用者資料 (User-plane data) 的頻寬越少。為此，本篇提出一個動態調配隨機接入資源的方法，一方面透過擴增隨機接入資源的方式減低碰撞發生機率減少延遲，另一方面盡可能少占用頻寬保留資源給使用者資料傳輸。

In 5G networks, the cell needs to ensure the requirement of three deployment scenarios which are Enhanced Mobile Broadband (eMBB), Massive Machine Type Communication (mMTC) and Ultra-Reliable and Low Latency Communication (URLLC). Before UE is allowed to transmit User-plane data, UE needs to perform Random Access (RA) to get its own dedicate resource. The procedure of RA contains four messages which are preamble transmission, random access response, message 3 and contention resolution. When more than one UEs select the same preamble in preamble transmission, it will lead to a $preamble\ collision$ problem which causes RA failed. The preamble collided UEs will be assigned with the same resource to transmit message 3 and this result in the cell cannot decode their data.
With more User Equipments (UE) in a cell, satisfying low latency of RA is much more challenging since the probability of $preamble\ collision$ during RA will be higher. As a result, the latency will significantly increased when there are too many of UEs join RA procedure together. To reduce the preamble collision rate, 5G comes with RA resource expanding technique. However, with more resources allocated to RA, there will be less resources for U-plane (User-plane) data transmission. As a result, in this paper, we proposed a dynamic RA resource allocation scheme for 5G network. Our proposed method not only reduces the preamble collision rate when RA loading is high, but also reserves the resource for U-plane transmission as much as possible.

[1] “Final Report of 3GPP TSG RAN WG1 #89 v1.0.0,” 3GPP, Tech. Rep., Aug. 2017, R1-1712031.
[2] “Final Report of 3GPP TSG RAN WG1 #AH_NR2 v1.0.0,” 3GPP, Tech. Rep., Aug. 2017, R1-1712032.
[3] “Remaining issues on RACH procedure,” NTT DOCOMO, INC., Tech. Rep., Jan. 2018, R1-1800654.
[4] Study on RAN Improvements for Machine-Type Communications, 3GPP, Aug. 2011, TR 37.868 V0.8.1.
[5] 5G; Study on New Radio (NR) access technology, 3GPP, Sep. 2018, TR 38.912
V15.0.0.
[6] NR; Medium Access Control(MAC) protocol specification, 3GPP, Sep. 2018, TS 38.321 V15.3.0.
[7] NR; Physical channels and modulation, 3GPP, Oct. 2018, TS 38.211 V15.3.0.
[8] NR; Physical layer procedures for control, 3GPP, Oct. 2018, TS 38.213 V15.3.0.
[9] NR; Radio Resource Control(RRC); Protocol specification, 3GPP, Oct. 2018, TS 38.331 V15.3.0.
[10] Study on scenarios and requirements for next generation access technologies, 3GPP, Sep. 2018, TR 38.913 V15.0.0.
[11] 5G; NR; User Equipment (UE) radio transmission and reception; Part 1: Range 1 Standalone, 3GPP, Apr. 2019, TS 38.101-1 V15.4.0.
[12] 5G; NR; User Equipment (UE) radio transmission and reception; Part 2: Range 2 Standalone, 3GPP, Apr. 2019, TS 38.101-2 V15.4.0.
[13] Y.-J. Chen, L.-Y. Cheng, and L.-C. Wang, Prioritized Resource Reservation for Reducing Random Access Delay in 5G URLLC,” IEEE 28th Annual International Symposium on Personal, Indoor, and Mobile Radio Communication, Oct. 2017.
[14] S. Duan, V. Shah-Mansouri, and V. W. S. Wong, “Dynamic Access Class Barring for M2M Communication in LTE Networks,” Globecom, pp. 4747–4752, Dec. 2013.
[15] Minimum requirements related to technical performance for IMT-2020 radio interface(s), International Telecommunication Union Radiocommunication Sector, Nov. 2017, ITU-R M.2410-0.
[16] M. Koseoglu, “Lower Bounds on the LTE-A Average Random Access Delay Under Massive M2M Arrivals,” IEEE Transaction on Communications, no. 5, pp. 2104–2115, May. 2016.
[17] Y. Liang, X. Li, J. Zhang, and Z. Ding, “Non-Orthogonal Random Access for 5G Networks,” IEEE Transaction on Wireless Communications, no. 7, pp. 4817–4831, Jul. 2017.
[18] T.-M. Lin, C. han Lee, J.-P. Cheng, and W.-T. Chen, “PRADA: Prioritized Random Access With Dynamic Access Barring for MTC in 3GPP LTE-A Networks,” IEEE Transaction on Vehicular Technology, no. 5, pp. 2467–2472, Jun. 2014.
[19] N. K. Pratas, H. Thomsen, Čedomir Stefanović, and P. Popovski, “Code-Expanded Random Access for Machine-Type Communications,” IEEE Globecom Workshops, pp. 1681–1686, Dec. 2012.
[20] Y. Saito, Y. Kishiyama, A. Benjebbour, T. Nakamura, A. Li, and K. Higuchi, “Non-Orthogonal Multiple Access(NOMA) for Cellular Future Radio Access,” IEEE Vehicular Technology Conference, Jun. 2013.
[21] S. Vural, N. Wang, P. Bucknell, G. Foster, R. Tafazolli, and J. Muller, “Dynamic Preamble Subset Allocation for RAN Slicing in 5G Networks,” IEEE Access, pp.13 015–13 032, Mar. 2018.
[22] Z. Wang and V. W.S.Wong, “Optimal Access Class Barring for Stationary Machine Type Communication Devices With Timing Advance Information,” IEEE Transactions
on Wireless Communications, no. 10, pp. 5374–5387, Oct. 2015.
[23] D. T. Wiriaatmadja and K. W. Choi, “Hybrid Random Access and Data Transmission Protocol for Machine-to-Machine Communications in Cellular Networks,” IEEE Transactions on Wireless Communications, no. 1, pp. 33–46, Jan. 2015.
49