bbu cloud wdm

Demonstration of digital fronthaul over self-seeded WDM-PON in commercial LTE

environment Yiran Ma,1,* Zhiguang Xu,2 Chengliang Zhang,1 Huafeng Lin,2 Qing Wang,3 Min Zhou,2

Heng Wang,2 Jingwen Yu,1 and Xiaomu Wang1 1China Telecom Co. Ltd. Beijing Research Institute, 118 Xizhimenneidajie, Xicheng District, Beijing, 100035, China

2 Advanced Technology Department, Huawei Technologies, Bantian, Longgang District, Shenzhen, 518129, China 3 Hubei P&T plan-design Co. LTD., 2 Changqing Third Road, Jianghan District, Wuhan, 430023, China

*[email protected]

Abstract: CPRI between BBU and RRU equipment is carried by self-seeded WDM-PON prototype system within commercial LTE end-to-end environment. Delay and jitter meets CPRI requirements while services demonstrated show the same performance as bare fiber.

©2015 Optical Society of America

OCIS codes: (060.4250) Networks; (060.2330) Fiber optics communications.

References and links

1. F. Saliou, P. Chanclou, B. Charbonnier, B. Le Guyader, Q. Deniel, A. Pizzinat, N. Genay, Z. Xu, and H. Lin, “Up to 15km cavity self seeded WDM-PON system with 90km maximum reach and up to 4.9Gbit/s CPRI links,” presented at European Conference and Exhibition on Optical Communication, paper We.1.B.6, Amsterdam Netherlands, September 2012.

2. F. Saliou, G. Simon, P. Chanclou, M. Brunero, L. Marazzi, P. Parolari, M. Martinelli, R. Brenot, A. Maho, S. Barbet, G. Gavioli, G. Parladori, S. Gebrewold, and J. Leuthold, “Self-Seeded RSOAs WDM PON Field Trial for Business and Mobile Fronthaul Applications,” presented at Optical Fiber Communication Conference, paper M2A.2, Los Angeles, USA, March 2015.

3. Y. Ma, D. Liu, J. Yu, and X. Wang, “System evaluation of economic 16/32chs 1.25Gbps WDM-PON with self-seeded RSOA,” Opt. Express 20(20), 22523–22530 (2012).

4. A. Chiuchiarelli, M. Presi, and E. Ciaramella, “Effective architecture for 10 Gb/s upstream WDM-PONs exploiting self-seeding and external modulation,” presented at Optical Fiber Communication Conference, paper JTh2A, Los Angeles, USA, March 2012.

5. F. Xiong, W. Zhong, M. Zhu, H. Kim, Z. Xu, and D. Liu, “Characterization of Directly Modulated Self-Seeded Reflective Semiconductor Optical Amplifiers Utilized as Colorless Transmitters in WDM-PONs,” J. Lightwave Technol. 31(11), 1727–1733 (2013).

6. U. R. Duarte, R. S. Penze, F. R. Pereira, F. F. Padela, J. B. Rosolem, and M. A. Romero, “Combined Self-Seeding and Carrier Remodulation Scheme for WDM-PON,” J. Lightwave Technol. 31(8), 1323–1330 (2013).

7. T. Komljenovic, D. Babic, and Z. Sipus, “C and L band Self-seeded WDM-PON Links using Injection-locked Fabry-Pérot Lasers and Modulation Averaging,” presented at Optical Fiber Communication Conference, paper W3G.1, San Francisco, USA, March 2014.

8. P. Parolari, L. Marazzi, M. Brunero, M. Martinelli, R. Brenot, A. Maho, S. Barbet, G. Gavioli, G. Simon, S. Le, F. Saliou, and P. Chanclou, “C- and O-Band Operation of RSOA WDM PON Self-Seeded Transmitters up to 10 Gb/s,” J. Opt. Commun. Netw. 7(2), A249–A255 (2015).

9. J. Zhu, S. Pachnicke, M. Lawin, S. Mayne, A. Wonfor, R. V. Penty, R. Cush, R. Turner, P. Firth, M. Wale, I. H. White, and J. Elbers, “First Demonstration of a WDM-PON System Using Full C-band Tunable SFP+ Transceiver Modules,” J. Opt. Commun. Netw. 7(1), A28–A36 (2015).

10. S. Le, A. Lebreton, F. Saliou, Q. Deniel, B. Charbonnier, and P. Chanclou, “Up to 60 km Bidirectional Transmission of a 16 Channels × 10 Gb/s FDM-WDM PON Based on Self-Seeded Reflective Semiconductor Optical Amplifiers,” presented at Optical Fiber Communication Conference, paper Th3G.8, San Francisco, USA, March 2014.

11. Y. Ma, Z. Xu, H. Lin, M. Zhou, H. Wang, C. Zhang, J. Yu, and X. Wang, “Demonstration of CPRI over Self-seeded WDM-PON in Commercial LTE Environment,” presented at Optical Fiber Communication Conference, paper M2J.6, Los Angeles, USA, March 2015.

12. P. Chanclou, F. Effenberger, R. Heron, D. Hood, D. Khotimsky, and A. Rafel, “Next-generation 2 access network technology,” Full-service access network white paper, 2012.

#233852 - $15.00 USD Received 4 Feb 2015; revised 18 Apr 2015; accepted 19 Apr 2015; published 28 Apr 2015 © 2015 OSA 4 May 2015 | Vol. 23, No. 9 | DOI:10.1364/OE.23.011927 | OPTICS EXPRESS 11927

1. Introduction

Passive optical network (PON) such as Ethernet PON (EPON) and Gigabit PON (GPON) has been widely used in fiber-to-the-home (FTTH) deployment nowadays. Many advantages have been shown for PON system, such as passive infrastructure, no line interference, high bandwidth, and etc. Currently, PON is more and more used to carry multiple services such as fronthaul of Common Public Radio Interface (CPRI) protocol data of wireless distributed sites because it can take advantage of the rich installed fiber resource. EPON and GPON will be not sufficient to provide enough bandwidth and satisfied performance for fronthaul applications. Though EPON and GPON can be upgraded to 10G EPON and 10G GPON respectively, the nature problems of time division multiplexed PON (TDM-PON) such as security problem and bandwidth sharing problem still prevent it from being used for fronthaul. Recently, wavelength division multiplexed PON (WDM-PON) is frequently proposed to carry CPRI data between building base band unit (BBU) and remote radio unit (RRU) in long term evolution (LTE) scenarios due to its advantages on high and dedicated bandwidth, low jitter and latency [1,2]. However, only a small portion of CPRI performance parameters such as latency, bit error rate (BER) and sensitivity have been analyzed. Compared with the current solution where bare fiber is used to carry CPRI, WDM-PON could save trunk fiber, extend the transmission reach, provide protection schemes. Moreover, if BBUs are concentrated as a pool, many BBUs require tremendous connections to RRUs if using bare fiber. Many optical solutions have been raised to meet strict CPRI requirements using one trunk fiber with WDM technology, such as optical transport network (OTN) and dense WDM (DWDM). However, these technologies are not able to perform real colorless operation as manual adjustment of wavelengths is still required. There have been discussions in Full Service Access Network (FSAN) that one of the main scenarios of next generation PON (NG-PON2) especially WDM-PON would be fronthaul of CPRI data. The key technology for WDM-PON is colorless transceivers to achieve convenience of installation and low inventory. Self-seeded reflective semiconductor optical amplifier (RSOA) is proposed as a promising technique to achieve colorless transceivers [3–10].

In this paper, a WDM-PON with self-seeded RSOAs both in downstream and upstream is demonstrated. Different from the previous reports by us [3], a novel WDM-POM integrated optical module is demonstrated for the first time and applied in our self-seeded system. The optical module has a high density integrated structure and the whole size is perfectly fabricated in Quad Small Form-factor Pluggable (QSFP) type. Due to an innovative design with both optical and electrical interfaces at the same side, the optical module can be flexibly pluggable and there are no complex connected fibers between modules except for only one main optical output port for launching to the feeder fiber, so as to make the whole WDM-PON mainframe look simple and efficient. More detailed descriptions and performance evaluation of the WDM-PON prototype system are provided based on our previous work [11]. Then a trial that uses the WDM-PON system to carry CPRI is conducted for the first time with commercial LTE equipments including LTE core network system architecture evolution (SAE), BBU, RRU, LTE antenna working in 1800 MHz band, LTE data card and LTE cell phone. It is shown WDM-PON could meet stringent CPRI requirements including latency, frequency error, error vector magnitude (EVM) and latency accuracy. Additional tests are performed in this paper including phase noise compared with our previous work [11]. Then real network services such as file transfer protocol (FTP) and high definition video are carried to show that WDM-PON will not introduce any performance degradation compared with bare fiber.

2. Setup of self-seeded WDM-PON system

The principle of self-seeded WDM-PON is to put a faraday rotator mirror (FRM) with partially reflectivity next to the arrayed waveguide grating (AWG) [12]. Figure 1 shows the schematic diagram of self-seeded WDM-PON system with the newly designed integrated optical module. The left square can be considered as an optical line terminal (OLT) board


with four integrated optical modules inserted into the board cages. Each module is connected with AWG 1 and AWG 2 through two 4 × ch fiber arrays, respectively. One is for upstream and the other is for downstream. All the downstream fiber arrays are connected with the drop ports of AWG 1, while upstream fiber arrays are connected with drop ports of AWG 2. The common ports of AWG 1and AWG 2 are combined by Circulator 1 to the feeder fiber. FRM 1 with 70% light reflection and 30% pass is located between AWG 1 and Circulator 1. In detail, Port 1of Circulator 1 is connected with the FRM output, Port 2 is connected to the feeder fiber, and Port 3 is connected to AWG 2.

Fig. 1. Schematic diagram of self-seeded WDM-PON system.

The other integrated optical module at the right side of Fig. 1 can be considered as the optical network unit (ONU). FRM 2, Circulator 2, AWG 3 and AWG 4 form a remote node (RN) in WDM-PON ODN with 0 to 5 km drop fiber reaching the ONU modules. AS for symmetry, FRM 2 is also designed with 70% light reflection and 30% pass. The whole architecture of WDM PON is symmetrical for upstream and downstream. In this trial, 20km Standard Single Mode Fibers (SSMF) are used as the feeder fiber and 0 to 5 km SSMF are used as the drop fiber.

Since it is complicated for us to prepare two types of optical modules with different bands for up/downstream, only C-band modules are fabricated in this trial. Therefore, circulators are applied for up/downstream separation instead of WDM filters. As for FRM, it not only acts as a partial reflector but also controls the polarization state of reflected light to effectively alleviate the impact of polarization dependent gain of the RSOA.

The dotted line square at the top of Fig. 1 illustrates the inner structure and component arrangement of colorless optical module. Taking the OLT module for example, a 4 × ch C-band RSOA array is used as gain medium and located on a thermo-electric-cooler (TEC) substrate. Herein the TEC is used to control the temperature of RSOA chips at 25°C for their stable performance. Downstream fiber array and upstream fiber array are arranged in parallel on a same plane. A square planar lightwave circuit (PLC) substrate with four waveguide channels is set between the downstream fiber array and RSOA array. The light input port array and output port array of waveguide are coupled to the RSOA array and downstream fiber array, respectively. Because the RSOA array and the downstream fiber array are put perpendicular to each other, four waveguide channels are designed bent to assure a low-loss coupling among RSOA, waveguide and fiber. The coupling loss of waveguide to fiber and waveguide to RSOA are 0.6dB and 1.5dB, respectively.

The RSOA array, the fiber array, FRM and the corresponding AWG ports form a four-channel light oscillating cavity, both at OLT side and ONU side. A 4 × ch avalanche photo


diode (APD) array is used as the receiver and located where its photosensitive surface is opposite to the upstream fiber array output. So the upstream fiber array can directly pass forward parallel to the PLC edge and reach the receiver APD array.

As shown at top right of Fig. 1, all the optical and electrical components are integrated in a QSFP module, which means the system is commercially mature and ready for real installation. Besides, this module has a hybrid port with optical and electrical interfaces at the same side. Looking into the hybrid port interface, there is a gold electrical array at the left side and an optical transceiver port array at the right side. Therefore, if the module is inserted into the cages on board, the electrical link and optical link will be connected synchronously. And as the module’s exposed surface opposite to the hybrid port interface, it looks very neat without any complex fiber connections outside but only one main optical output coming from the built-in AWG common port, which is launched into the feeder fiber.

3. Performance of WDM-PON system

Fig. 2. Experimental optical spectra: (a) ASE spectrum with different bias current; (b) Lasing wavelength at different AWG channels; (c) Modulated spectrum with different drop fiber length.

Before this WDM-PON prototype system can be applied to carry CPRI, general performance of the system and the colorless optical module are demonstrated. The value of 3dB electro-optical bandwidth of the RSOAs becomes wider as the injected current increasing. When the injected current is set at 10mA, 20mA, 30mA, 40mA, 50mA, 60mA and 70mA, the 3 dB bandwidths are obtained as 0.3GHz, 0.4GHz, 0.8GHz, 1.4GHz, 2GHz, 2.1GHz, 2.2GHz, respectively. For an optical module at ONU side, the experimental optical spectra has been recorded which is shown in Fig. 2. The broad amplified spontaneous emission (ASE) light originally emits from RSOA and is injected into one drop port of AWG. Figure 2(a) has shown the ASE emitting spectrum of the optical module with the bias current increasing from 15 mA to 70 mA. It is obviously observed that not only the emitting power increases but also the gain peak presents a blue-shift as the bias current turned up. The gain peak changes from 1570 nm to 1546 nm when the bias current increases from 15 mA to 70 mA. In this trial, the injected current is usually set to be 60~70 mA with the corresponding ASE output power as 2.5 to 3 mW. Different RSOA components in the QSFP module have tiny differences regarding ASE figure and output power which will not affect the trial results.

Due to the wavelength selection effect of AWG, only the light with wavelength corresponding to the injected port will go through the AWG channel and reach FRM 2. Figure 2(b) shows three different lasing wavelengths coming out of FRM 2, which are respectively Channel 1, 8 and 16 of AWG4 with 5 km drop fiber. Due to more insertion loss at short wavelength of AWG, the peak power of channel 1 is about 3 dB lower than the other two channels.

Figure 2(c) illustrates the continuous wave (CW) spectrum outputs of Channel 8 with different lengths of drop fibers. For self-seeded WDM-PON, longer drop fiber means a longer oscillation cavity, which leads to broader spectra and more significant ripple. This is because the optical feedback is less coherent in longer cavity. The mode competition and multiple


reflections in the cavity become more intense. Therefore, the wider spectra and spectrum ripple will be observed for longer drop fibers. The laser linewidth becomes narrower as the injected current increasing. For Channel 8 with 3km drop fiber, when the current is set to 20mA, 30mA, 40mA, 50mA, 60mA, 70mA and 80mA, the corresponding linewidth are 1.18nm, 0.59nm, 0.23nm, 0.13nm, 0.11nm, 0.08nm, 0.07nm.

Figure 3 presents 2.5 Gb/s transmitted eye diagrams of Channel 1, 8 and 16 with different drop fiber of 0 km and 5 km. In details, Fig. 3(a) shows the eye diagram of ASE modulation. The extinction ratio (ER) is 5 dB without any signal noise. Figures 3(b)-3(g) show various eye diagrams on conditions of 0 km drop fiber of Channel 1, 0 km of Channel 8, 0 km of Channel 16, 5 km of Channel 1, 5 km of Channel 8 and 5 km of Channel 16, respectively. And the ERs of Figs. 3(b)-3(g) are 4.79 dB, 4.5 dB, 4.2 dB, 4.56 dB, 4.27 dB and 3.98 dB, respectively. At the same cavity length, ER becomes lower with the channel number growing bigger. As shown in Fig. 2(b), bigger number channels have higher average power, which leads to less openness of modulation eye. Meanwhile, for a same channel, eye diagrams become noisier and ERs become lower with the cavity length getting longer, which is also resulted from the more intense mode competition and multiple reflections in cavity.

Fig. 3. Measurement of self-seeded emitting eye diagram: (a) Eye diagram of ASE modulation; (b) Eye diagram at channel 1 with 0km drop fiber; (c) Eye diagram at channel 8 with 0km drop fiber; (d) Eye diagram at channel 16 with 0km drop fiber; (e) Eye diagram at channel 1 with 5km drop fiber; (f) Eye diagram at channel 8 with 5km drop fiber; (g) Eye diagram at channel 16 with 5km drop fiber.


A real-time WDM-PON system using this RSOA-based self-seeded optical module under 2.5 Gb/s modulation is established and shown in Fig. 4. 4 QSFP modules counting to 16 channels are tested. The measurement results of receiver sensitivity, laser linewidth, bit error ratio, lasing power and loss budget at selected channels are summarized in Table 1 for different cavity lengths. When the FEC function is open, all the channels achieve error free performance without data package loss by associating the drop fiber length of 0~3 km. With 5 km drop fiber, only some channels could achieve error free performance. It is obtained from Table 1 that the transmission performance gets worse as the length of drop fiber increases. The receiver sensitivity degrades from around −23 dBm at 0 km drop fiber to around −16 dBm at 5 km drop fiber. When the drop fiber length is fixed, the transmission performance depends on the channel characteristic according to the recorded value from Table 1. On three conditions as shown, the receiver sensitivity of good channels always has around 2 dB improvement over the bad ones. Meanwhile, the laser linewidth, lasing power and loss budget also get worse when the length of drop fiber increases. The loss budget of the system could reach more than 18 dB for Channel 8 with 0 km drop fiber, but only 8 dB budget is observed with 5 km drop fiber.

Fig. 4. WDM-PON prototype system.

Table 1. Measurement results of received power sensitivity, laser linewidth, bit error ratio, lasing power and loss budget for different channels with different drop fiber length

Drop fiber length (km)

Channel number

Received power

sensitivity (dBm)

Laser linewidth

(nm)

Without data package loss

Lasing power (dBm)

Loss budget (dB)

0 Ch 1 −24.18 0.05 Yes −7.28 16.89

0 Ch 8 −24.35 0.045 Yes −5.69 18.65

0 Ch 16 −22.63 0.0482 Yes −6.11 16.51

1 Ch1 −21.15 0.075 Yes −7.95 13.19

1 Ch 8 −21.01 0.0785 Yes −6.57 14.43

1 Ch 16 −19.48 0.089 Yes −6.98 12.49

3 Ch 1 −17.9 0.104 Yes −8.68 9.21

3 Ch 8 −18.4 0.112 Yes −7.41 10.98

3 Ch16 −15.96 0.116 Yes −7.75 8.20

5 Ch 1 −16.12 0.114 No −9.56 6.55

5 Ch 8 −16.54 0.113 Yes −8.44 8.09

5 Ch 16 −13.8 0.125 No −8.67 5.12


4. Trial setup with commercial LTE environment

To facilitate WDM-PON’s scenario of fronthaul, the WDM-PON prototype system is applied to carry CPRI signal in a commercial LTE environment. The system contains 16 channels and 200GHz AWG is used. If 2G, 3G and LTE are installed in a single station, 16 channels are reasonable to carry all connections to the remote units. The operation wavelengths are set in C band from 1530 nm to 1565 nm for both upstream and downstream. The trial setup is shown in Fig. 5. In the OLT, 4 among 8 SFP ports are used to communicate with the ONUs. Each SFP port can be inserted with 4 in 1 WDM-PON colorless optical module hence there are 16 channels in total. The other 4 SFP ports are inserted with normal SFP optics to connect with the BBU. Each ONU has a port for the 4 in 1 WDM-PON optical module and 4 other ports with normal SFP to connect with RRUs. The transmission distance is 20km while the fiber length between the AWG at ONU side and ONUs (drop fiber) is 1 km which can cover most of the fronthaul scenarios and take full advantage of installed FTTH fiber infrastructure. Moreover, up to 3 km drop fiber won’t affect the feasibility of using WDM-PON to carry CPRI. In the trial, 2 wavelengths between the OLT and ONU are activated to carry CPRI between the BBU and 2 RRUs which are Channel 1 and 3. The data rate is 2.5 Gbit/s complied with CPRI option 3 which is sufficient for frequency division duplexing LTE (FDD-LTE) with 2 × 2 antennas. The FDD-LTE environment is comprised of commercially available devices including a data card and a cell phone as terminals, 4 antennas working at 1800 MHz band, 2 RRUs, 1 BBU and the complete core network connecting with the FTP server and the video server. The trial is conducted in non-shielded environment to emulate a real implementation.

RRUWDM-PONONU

SFP

WDM-PON

Data card

Antenna

WDM-PONONU

SFP

Antenna

20km

…AWG

AWG

WDM-PON

OLT

SFP4 in1 colorless optics

FTP and Video Server

SAE

Gateway

LTE

LTE

Cell Phone

RRU

LTEBBU

Gateway

HSS

MME

Fig. 5. Trial setup (MME: Mobility Management Entity; HSS: Home Subscriber Server).

5. Trial results

Table 2. CPRI requirement and Performance of WDM-PON to carry CPRI

Parameters CPRI requirements WDM-PON with 20km fiber

Maximum roundtrip latency 400us 241us

Frequency error ± 2ppb ± 1.7ppb (with 20km fiber)

Latency accuracy without fiber

± 16.276ns <1ns

Maximum roundtrip latency without fiber 5us 4.9us

BER 10e-12 72 hours error free (with

20km fiber)


Table 2 lists the general physical requirements of CPRI based on bare fiber (several meters long). WDM-PON transparently transfers CPRI frames without any encapsulation methods. Therefore, WDM-PON doesn’t introduce any significant impairment to the trial system. Back-to-back WDM-PON induces latency of 2465 ns for upstream and 2458 ns for downstream while the latency with 20 km fiber is 120686 ns for upstream and 120687 ns for downstream. Both roundtrip latency with and without fiber could meet CPRI requirements. The frequency error is tested based on different LTE modes including TM2/3.1/3.2/3.3 which means 64QAM/64QAM/16QAM(QPSK)/QPSK(16QAM) is applied, respectively. Compared with bare fiber which frequency is very stable, WDM-POM didn’t bring more errors but add a fluctuation of around 2 Hz. However, the frequency error is still within the maximum allowed 2 ppb even for 64 QAM. Meanwhile, 72 hours long term test is performed with error free result which is sufficiently long to prove that the system can meet 10e-12 BER requirement.

EVM is considered as a general measurement of CPRI’s amplitude error and phase error. EVM of CPRI over both bare fiber and WDM-PON is given in Table 3. No worse performance of WDM-PON is observed for all the modes of CPRI signal.

Figure 6 demonstrates the phase noise of CPRI signal carried by both bare fiber and WDM-PON with 20 km fiber. An Agilent 5052B signal source analyzer is used to test the recovery clock of Serializer/Deserializer (SERDES) extracted from RRU’s circuit. The phase noise of both bare fiber and WDM-PON shows similar trend while WDM-PON induces several more peaks on the phase noise curve. Table 4 lists the phase noise of CPRI over both bare fiber and WDM-PON systems under different frequency conditions. The jitter is measured in the same way. The jitter of CPRI over WDM-PON is 6.2 ps compared with 1 ps of CPRI over bare fiber. The small amount of extra phase noise and jitter introduced by WDM-PON doesn’t bring any critical impacts on the CPRI performance.

Table 3. CPRI EVM for bare fiber and WDM-PON with 20 km fiber

EVM TM2(64QAM) TM3.1(64QAM) TM3.2(16QAM/QPSK) TM3.3(QPSK/16QAM) Bare fiber 1.67% 2.99% 4.02%/2.73% 5.15%/2.81%

WDM-PON

1.66% 2.9% 3.95%/2.65% 5.24%/2.9%

(a) Bare Fiber (b) WDM-PON

Fig. 6. Phase noise of (a) bare fiber and (b) WDM-PON with 20 km fiber.

Table 4. CPRI phase noise of bare fiber and WDM-PON with 20 km fiber

Phase noise (dBc/Hz)

10Hz 100Hz 1KkHz 10kHz

Bare fiber −77.2 −82.9 −87 −107.5

WDM-PON −60.4 −74.4 −86.6 −106.6


(a) (b) (c)

Bare fiber

WDM-PON

Fig. 7. Service performance comparison of bare fiber and WDM-PON with 20 km fiber (a) UDP upstream transmission; (b) UDP downstream transmission; (c) FTP downstream transmission.

To further evaluate WDM-PON system, real services are demonstrated with bare fiber and WDM-PON with 20 km fiber. The server is connected at the output of SAE and all data transmission goes through the whole LTE environment to the data card or cell phone. The trial is optimized to reach the FDD-LTE peak data rate (upstream 50 Mb/s and downstream 150 Mb/s) to better reflect the performance. As illustrated in Fig. 7, no differences between bare fiber and WDM-PON are shown for user datagram protocol (UDP) upstream and downstream, FTP transmission from the server to the data card. FTP transmission didn’t reach the peak data rate because UDP is a simple transport layer protocol intended for data packets. It just sends out the data but doesn’t have any mechanism to make sure the data can reach the destination successfully. However, FTP provides a bit stream service based on reliable connection. Customers have to establish a stable connection with the server before data exchange. Therefore, if there is any failure occurring to the communication link, FTP can accordingly turn to carry out the emergency mechanisms such as delay data retransmission, data detection and data flow control, so as to assure the reliable data transmission from end to end. Therefore, FTP has to spend much extra time to operate the emergency mechanisms. High definition video is also launched both on the data card and cell phone while using bare fiber and WDM-PON. The same user experience further indicates the feasibility of WDM-PON to carry CPRI.

6. Conclusion

The 16 × ch self-seeded WDM-PON prototype system with high density integrated QSFP module is thoroughly investigated and installed in the commercial LTE environment between BBU and RRU to carry CPRI. In the non-shielded environment, CPRI physical parameters while using WDM-PON meet the requirements. Real services also indicate WDM-PON has the same performance as bare fiber. The full feasibility test of CPRI over WDM-PON confirms the fronthaul scenario and brings a new solution to CPRI transmission.

Acknowledgments

This work is partly supported by Beijing Key Laboratory (No. BZ0268).


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