Fronthaul Size: Calculation of maximum distance between RRH (at cell site) and BBU (at CO)

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Fronthaul Size: Calculation of maximum distance between RRH (at cell site) and BBU (at CO)

  1. 1. 1 NETMANIAS TECH-BLOG Please visit www.netmanias.com to view more posts Fronthaul Size: Calculation of maximum distance between RRH (at cell site) and BBU (at CO) April 1, 2014 | By Dr. Harrison J. Son (tech@netmanias.com), S.M. Shin (smshin@hfrnet.com) In C-RAN (Centralized/Cloud RAN), the more RRHs are covered by one CO (BBU hostelling site), i) the fewer COs are required, and ii) the more BBUs can be centralized at one CO. This allows for the most efficient utilization of BBU resources, thereby significantly reducing CAPEX/OPEX. For this reason, the maximum separation distance between RRH and BBU should be secured. This distance is constrained by the timing requirement of Hybrid Automatic Retransmit reQuest (HARQ) protocol used as a retransmission mechanism between UE and eNB in an LTE network. Figure 1. Latency requirement in C-RAN Tp: Propagation Delay TA: Timing Advance (a) Timing requirement of uplink synchronous HARQ processing RxTx RxTx Subframe (1ms) Round trip latency budget for HARQ =3ms+TA PUSCH RxTx RxTx 2. eNB processing ≤ 3ms Tprop. (b) Impact of fronthaul delay (expedited BBU processing needed) in C-RAN 1. Data 3. ACK/NACK TA=2Tp Data 3. ACK/NACK 1. Data 2.3 Modem (CO) to Rx antenna (cell site) Baseband Processing (L1, L2, L3) eNB Fronthaul (Fiber, Active equipments) RRH BBU Antenna (TxRx) eNB RRH BBU Antenna (TxRx) UE Cell Site UE Cell Site Central Office (CO) ACK/NACK 2.1 Rx antenna (cell site) to Modem (CO) 2.2 BBU processing 2. eNB processing ≤ 3ms To secure UL/DL time alignment at a cell site (TX, Rx antenna), BBU allocates a TA value to UE after subtracting the fiber latency from the TA value. LTE preamble format: 0 Antenna Antenna
  2. 2. Netmanias Tech-Blog: Fronthaul Size: Calculation of maximum distance between RRH and BBU 2 As seen in Figure 1(a) above, according to this requirement, UE should receive ACK/NACK from eNB in three subframes after sending uplink data, i.e. in the fourth subframe. Otherwise, the UE retransmits the data. In LTE, uplink and downlink subsframes are typically time-aligned at eNB antenna port as shown in Figure 1(a). So, eNB should complete eNB processing (UL CPRI processing, UL frame decoding, ACK/NACK creation, DL frame creation, DL CPRI processing) within 3 msecs after receiving uplink data from UE in subframe n, and then send downlink ACK/NACK in subframe n+4 back to the UE. Now, let's consider C-RAN where HARQ is processed between UE and BBU at the CO. In usual C-RAN, BBU and RRH are located several kms, or even tens of kms, away from each other. So, additional delays like transmission delay via optical fiber, processing time at active equipment in a fronthaul network (e.g. Active WDM, PON, etc.) are caused while data are delivered from the antenna at a cell site to BBU at CO. The sum of these delays and baseband processing time at BBU must be less than 3 msecs. In order to maintain the timing presented in Figure 1(a), the additional delay caused in the fronthaul network must be compensated somewhere, somehow, for example, by expediting the BBU processing as seen in Figure 1(b). Base station vendors design BBU to complete the processing and send ACK/NACK usually within 2.75 msecs, intead of 3 msecs, in order to compensate the delay additionally caused in the fronthaul network in C-RAN. Therefore, about 250 μsecs can be allowed in the fronthaul network. Based on this delay budget, the maximum separation distance between BBU and RRH can be calculated (round-trip transmission latency of 10μs/Km). Table 1. Delay component at fronthaul (Uplink latency calculation) 2.1 Rx antenna (cell site) to Modem (CO) 2.2 BBU processing (CO) 2.3 Modem (CO) to Rx antenna (cell site) a. RRH/RF processing (UL) b. RRH/CPRI processing (UL) c. Fiber latency (RRH to BBU) d. Active equipment processing e. BBU/CPRI processing f. PHY: UL frame decoding g. MAC: ACK/NACK creation h. PHY: DL frame creation i. BBU/CPRI processing j. Fiber latency (BBU to RRH) k. Active equipment processing l. RRH/CPRI processing (DL) m. RRH/RF processing (DL) In a fronthaul network built with Active WDM, delay components involved in the data transmission after RRH receives data from UE and before it sends ACK/NACK to UE are listed in Table 2 below. In the table, delay components No. 1 through 3, which are caused at RRH and BBU, must be kept minimized by base station vendors while delay component no. 4 must be kept minimized by fronthaul vendors.
  3. 3. Netmanias Tech-Blog: Fronthaul Size: Calculation of maximum distance between RRH and BBU 3 Table 2. Maximum fiber distance between RRH (cell site) and BBU (CO) Delay Components Related NEs Description Typical values 1. Round trip RF processing time RRH a+m ~ 25-40sec 2. Round trip CPRI processing time RRH, BBU b+e+i+l ~ 10 sec 3. BBU round trip baseband processing time BBU f+g+h ~ 2,700 sec 4. Fronthaul equipments round trip processing delay Fronthaul equipments d+k ~40 sec (OTN encapsulation) ~ few sec (Non-OTN encapsulation) Maximum Fiber RTT = 3msec - (1. round-trip RF processing delay + 2. round-trip CPRI processing delay + 3. BBU round-trip processing time + 4. Fronthaul equipments round trip processing delay) Maximum Fiber distance [Km] = Maximum Fiber RTT / 10sec/Km Example) Maximum Fiber RTT (Non-OTN encapsulation) = 3msec - (40sec + 10 sec + 2,700 sec + 4 sec) = 246 sec Maximum Fiber distance = 246 sec / (10sec/Km) = 24.6 Km
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