Experiment Demonstration of Optical Fronthaul

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capacity for future 5G mobile system. By experiment, transmission of 20MHz bandwidth PDM-. OFDM signal is verified in BtB and 20-km SSMF cases.
Experiment Demonstration of Optical Fronthaul Transmission System based on Power Division Multiplexing for Future 5G Scenarios Longsheng Li1, Meihua Bi1,2, Weikang Jia and Weisheng Hu1 1

State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China, [email protected] 2 College of Communication Engineering, Hangzhou Dianzi University, Xiasha Gaojiaoyuan 2rd Street, Hangzhou, Zhe Jiang province, 310018, China, [email protected]

Abstract: In this letter, we propose an analog RoF fronthaul scheme based on power division multiplexing for the purpose of enlarging the massive terminal devices access and transmission capacity for future 5G mobile system. By experiment, transmission of 20MHz bandwidth PDMOFDM signal is verified in BtB and 20-km SSMF cases. OCIS codes: (060.2330) Fiber optics communications; (060.4250) Networks;

1. Introduction Centralized access network architecture, such as cloud radio access network (C-RAN) [1], is considered to be an attractive candidate for future 5th generation mobile networks(5G). In this architecture, traditional baseband units (BBUs) are physically centralized. The fiber link between BBU and remote radio unit (RRU) is defined as fronthaul, through which radio signal is transmitted either in analog format or in digitized format. While, for future 5G scenarios, demands increase in both bandwidth and the number of access terminals, which would bring high data rate to mobile access network, especially in dense deployment cases such as massive multiple input multiple output (MIMO). With regard to this high data rate transmission, due to the limitation of the bandwidth of optical devices and wavelength number, traditional digital Radio over Fiber (DRoF) transmission such as widely deployed common public radio interface (CPRI) [2] is hardly able to carry high–speed sampled and digitized signal. Meanwhile, for analog radio over fiber (ARoF) transmission technology, since analog wireless signal is directly modulated into optical domain, it is commonly considered as a promising bandwidth efficient, simple and low cost solution. To further improve the bandwidth efficiency of equivalent high data rate transmission, several multiplexing methods are applied. As demonstrated in [3], ARoF combining subcarrier multiplexing (SCM) and wavelength division multiplexing (WDM) is used to increase transmission capacity. However, in this scheme, the system performance will be degraded with the increase in the number of SCM. To solve this problem, we introduce the non-orthogonal multiple access (NOMA) concept in analog fronthaul transmission system. NOMA provides a new dimension of multiplexing [4], which is the power of radio signal. Radio signals overlapped in time and frequency dimension can be separated in power dimension. In addition, NOMA technique has been widely investigated in wireless field and shown some advantages as [5]. In this paper, we firstly demonstrate the power division multiplexing (PDM) in fronthaul system based on fiber transmission link. By multiplexing additional power dimension, single wavelength can carry more user equipments (UEs) and equivalent carrier capacity. In this experiment, we verify the feasibility of this scheme by transmitting one LTE-like 20MHz bandwidth PDM-OFDM ARoF signal of 1-GHz center frequency over both back to back (BtB) and 20km standard single mode fiber (SSMF). Two signals at different power levels are multiplexed, and they all show encouraging performance even with low received optical power. 2. Principle of PDM in optical fronthaul transmission system Based on NOMA technique, the conception of PDM is introduced in this letter, and the corresponding procedures of PDM are illustrated in figure 1. In this figure, for the sake of explanation, we assume that U (U=3) UEs are multiplexed in power dimension. At the transmitter, data from U users is firstly modulated to the traditional OFDM baseband signals respectively. Then, according to U UEs’ channel gain [4], different signal is allocated with different electrical launched power so that to form a new multiplexed dimension. As with NOMA, the channel gain in our system can also be considered as power loss caused by different transmission distance. Note that, in this letter, the principle of mapping between power level and UE is that UE with lower channel gain, should be assigned to higher power level. To realize this processing, we distribute different weighted factor for different UE signal in baseband and directly add them together as transmitted signal s(t). Throughout the amplifier (AMP) and electrical-

to-optical converter (E/O), s(t) is modulated into optical PDM signal and injected into the fiber link. Passing through different distance fiber, this signal reaches the RRU module, and then is detected by optical-to-electrical converter (O/E). After that, the electrical signal is up-converted into intermediate frequency (IF), amplified by power amplifier (PA) and transmitted into air by antenna. At UE side, successive interference cancellation (SIC) receiver is need for demodulation. The receiver block shown in figure 1 gives the principle of SIC receiver. As shown in this figure, UE signals are demodulated in the order of decreasing power level. For instance, UE1 data is firstly demodulated. It is noted that, for the signal being demodulated, all signals at power levels lower than it are considered as noise. After UE1 data is demodulated, it is further used to realize SIC, which is composed of OFDM modulation and channel distortion. Here channel distortion acts as channel response. After these two blocks, current power level portion of s(t) is reconstructed and denoted as s(t)px. Interference cancellation is achieved after s(t)px is subtracted from s(t), and the rest is used to demodulate lower power level signals. In addition, channel gain in this system includes both wireless and optical power loss. In our experiment, to verify the initial PDM-based optical fronthaul transmission system, we only take the optical power loss and the scheme into account. Upconverter

O/E

Downconverter

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Equalization & OFDM demodulation

UE1 data

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UE3 signal Power allocation

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Fig. 1. Transmitter and receiver structure of PDM optical fronthaul transmission system

3. Experimental setup and results 20MHz bandwidth signal @1GHz AWG

14dBm Laser @1550nm

Offline DSP

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PC

MZM

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Fig. 2. Experiment setup

Figure 2 illustrates the experiment setup for downlink, where two power level signals with 20MHz bandwidth are multiplexed. These two signals are firstly generated in term of LTE-like baseband OFDM signal, assigned to different power levels, and then added together. All these offline processings are accomplished on MATLAB program. Note that 16QAM is employed at higher power level, while 4QAM is employed at lower one. This is because that lower level can suffer from worse signal noise rate (SNR). Except for the order of QAM, these two signals have the same parameters. In addition, due to air transmission line not for consideration in this experiment, we set the channel gain as 0dB and -3dB respectively. Based on this setup and normalized operation, 95% power is assigned to higher power level signals, and 5% is assigned to the lower one. After that, the combined signal is digitally up-converted within 1-GHz IF, and uploaded into the arbitrary waveform generator(AWG) with 5Gsa/s sampling rate. The output signal of AWG is amplified by a radio frequency amplifier and used to drive MachZehnder modulator (MZM) to generated the optical PDM signal. Passing through BtB/20km fiber and an attenuator, this optical signal is injected into a positive-intrinsic-negative (PIN) photodiode to realize the detection of signal. After PIN, the generated electrical signal is amplified by an AMP and sampled by oscilloscope (OSC) at 5Gsa/s for further digital signal processing (DSP). Noted that the power of laser injected into MZM is 14dBm at 1550nm, and 5dBm modulated optical signal is transmitted over fiber. To quantify the transmission performance, we give the results of error vector magnitude (EVM) and bit error rate (BER) as functions of received optical power for both BtB and 20km cases. Figure 5 shows the measured results. As

mentioned above, lower power level signal is considered as noise when higher one is being demodulated. In our experiment, OFDM symbols of two power levels are strictly aligned, thus the constellation diagram of power level1 in figure 3(a) interfered by level2 is like 64QAM constellation. But it is actually demodulated as traditional 16QAM. Because required EVM for 64QAM-OFDM is 8%, two constellation diagrams are given at -27dBm, corresponding to level1 EVM of 7.1% and level2 EVM of 33.7% for 20km case. Note that BtB and 20km cases have almost the same curve, which means OPDM do not suffer from fiber transmission at IF of 1GHz. The power spectrum density (PSD) of received signal is given in figure 4.

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Fig. 3. (a)Measured EVM vs. received optical power at BtB and 20 km. (b) Measured BER vs. received optical power at BtB and 20 km.

Fig. 4. PSD of received electrical OFDM signal

4. conclusion We firstly propose the optical fronthaul transmission system based on PDM technique, and experimentally demonstrate it by multiplexing two power levels of RoF signals. Both of them can be demodulated properly by SIC receiver even with relatively low received optical power. PDM can provide a new multiplex dimension for fronthaul transmission, and improve the spectrum efficiency for future 5G scenarios, so that to satisfy the rapidly increasing access capacity and the number of access terminals. Acknowledgement The work was jointly supported by the National Nature Science Fund of China (No. 61501157, No. 61431009), and Natural Science Foundation of Zhejiang Province, China (Grant No. LQ16F050004). 5. References [1] Mobile, China. “C-RAN: the road towards green RAN.” White Paper, version 2.5 (2013). [2] CPRI Specification V6.0, “Common Public Radio Interface (CPRI); Interface Specification,” Aug. 2013. [3] Murakoshi A, Tsukamoto K, Komaki S. High-performance RF signals transmission in SCM/WDMA radio-on-fiber bus link using optical FM method in presence of optical beat interference[J]. IEEE transactions on microwave theory and techniques, 2006, 54(2): 967-972. [4] Saito Y, Benjebbour A, Kishiyama Y, et al. System-level performance evaluation of downlink non-orthogonal multiple access (NOMA)[C]//PIMRC. 2013: 611-615. [5] Ding Z, Yang Z, Fan P, et al. On the performance of non-orthogonal multiple access in 5G systems with randomly deployed users[J]. IEEE Signal Processing Letters, 2014, 21(12): 1501-1505.