Analog Transport - An Alternative for Mobile Fronthaul?
Christina Lim, Yu Tian and Ampalavanapillai Nirmalathas, Department of Electrical and Electronic Engineering, The University of Melbourne, Australia
IEEE 5G Tech Focus: Volume 2, Number 2, May 2018
As the wireless data traffic shows no signs of slowing down, this creates a significant challenge for the next generation wireless systems with the aggregated data in the fronthaul easily exceeding the practical limits of current CPRI-based mobile fronthaul, making the capacity of the fronthaul as the key bottleneck for the next generation wireless systems. With many alternatives currently being investigated, analog transport for mobile fronthaul emerges as a simple and practical solution to address the capacity and low-latency requirements while supporting a centralized architecture.
The wireless landscape has experienced transformational changes over the last two decades and is further shaped by the rapid growth of affordable smart portable devices with an expected number of mobile-connected devices to reach 11 billion by 2021 . The next generation wireless communications (5G and beyond) adopting a centralized radio access network architecture (CRAN) is expected to support enhanced capacity, connectivity and low latency . CRAN enables pooling of processing modules (baseband units (BBUs)) in a centralized location which would serve a large number of radio processing units (remote radio heads (RRHs)) in remote locations . The centralized nature also enables high-level coordination functionalities such as coordinated multi-point (CoMP) transmission and massive multiple-input-multiple-output (MIMO).
Current CRAN relies on fiber-based fronthaul to provide the physical connectivity between the RRHs and BBU-pool and bases on Common Public Radio Interface (CPRI) technology . CPRI uses uncompressed digitisation of the wireless signals supporting up to 12 Gb/s for use in 4G LTE-Advanced mobile systems . Unfortunately current mobile CPRI fronthaul technology is not economically sustainable as it does not scale with the future wireless demands. Such an explosion of bandwidth will create a front-haul bottleneck with CPRI style approaches only resulting in a significant wastage of optical resources. Few alternatives have recently been proposed: using data compression techniques to minimize the required fronthaul optical bandwidth [5-6], functional splits by moving more higher layer operations into the RRHs [7-9] and using analog optical transport in place of CPRI in the fronthaul links [10-18].
3rd Generation Partnership Project (3GPP) has defined eight options for functional split between the BBU and RRH . High-level functional splits improves bandwidth utilization but decentralizes control functionality and increases latency while low-level splits maintains the centralization capability but increases the bandwidth utilization. There is a tradeoff between latency, throughput and centralization capavility. One example of low-level splits is eCPRI with split point within the physical layer .
The analog transport of the wireless signals either at an intermediate frequency (IF) or at the radio frequency (RF) over the fronthaul link emerges as a promising option with capability to overcome the issues arising from functional splits. This scheme maintains the wireless spectral bandwidth thus reducing the requirement for CPRI transmission capacity, while enabling centralized control capability with minimal latency. Despite the simplicity of analog optical transport of the wireless signals, this approach is currently not popular among network providers. The main reason being that analog signals are more prone to noise and nonlinearity impacting the dynamic range of the link. This is true for large macrocell sites that require a large dynamic range to satisfactorily service users at cell edges. Wireless infrastructure evolving towards smaller cell sizes relaxes this stringent requirement on the dynamic range. Analog transport may now emerge as a serious contender for mobile fronthaul.
2. Analog Transport of Wireless Signals
Traditionally the concept of analog transport of wireless signals is shown in Fig. 1. The wireless signals are modulated onto an optical carrier and then distributed optically via an optical distribution network. The wireless signals are then detected using a photodetector before they are distributed wirelessly to the end users. The wireless signals can be transported at intermediate frequencies over fiber (IFoF) (Fig. 1a) or at the designated wireless frequency over fiber (RFoF) (Fig. 1b).
Figure 1: Schematic of (a) IFoF and (b) RFoF transport schemes
RFoF transport is the simplest technique as the detection in the RRH only requires direct detection using a photodetector and does not require additional frequency translation stages. It has the advantage of realizing a small footprint RRH with full centralized control architecture. Despite the simplistic design, this scheme suffers from RF power fading due to fiber chromatic dispersion  and the requirement for optical devices with speeds matching that of the wireless carrier frequency which becomes more stringent for wireless signals in the millimeter-wave region.
IFoF transport overcomes the limitations of RFoF where relatively low-speed optoelectronic devices are required as the wireless signals are distributed optically at a much lower frequency and hence, has much reduced fiber chromatic dispersion effects. On the contrary, the RRH requires frequency translation stages that demand stable local oscillators and linear mixers. The complexity increases with wireless carrier frequency and may impede future network upgradability.
3. Demonstration of IFoF for 5G Fronthauls
There have been a number of reported demonstrations of IFoF for mobile fronthaul transport [10-13]. These demonstrations include the distribution of 128 x 100 MHz signals over 20 km of fronthaul link based on sub-Nyquist sampling technique , 32 x 200 MHz signals over 1 km of fronthaul link , bi-directional transmission of 20 x 80 MHz downlink signals and 16 x 80 MHz uplink signals over 25 km of fronthaul link based on carrier aggregation  and 60 GHz using leaky wave antenna . These demonstrations focus on pushing the boundary of capacity improvement.
Ishimura et. al. has demonstrated an IFoF transport scheme that supports wideband operation and long-distance transmission by overcoming the impact of dispersion-induced RF power fading for transporting double sideband formatted wireless signals . The scheme is based on a parallel intensity modulation (IM)/phase modulation (PM) transmitter that exploits the complementary relationship of the IM and PM signals response of the link. Therefore by assigning the aggregated IF signals to the appropriate modulators during electrical-to-optical conversion, the optically modulated IF signals will experience a relatively flat response over the fronthaul link thus overcoming the impact of fiber chromatic dispersion. They have demonstrated a transmission of 20 x 360 MHz orthogonal-frequency division multiplexed (OFDM) signals over 40 km of single-mode fiber (SMF)  and 14 x 1.2 GHz OFDM signals over 20 km of SMF .
Sung et. al. has demonstrated the feasibility of an IFoF transport for mobile fronthaul for 28 GHz wireless operation with real-time processing . In the proposed scheme, the clock signal was simultaneously transmitted with the wireless signals in the form of an IF carrier. The clock was then used to convert the IF signals to the 28 GHz wireless signals, and hence reducing the implementation cost of the RRH. The results showed that simultaneous clock transmission resulted in performance degradation of <0.5% in error vector magnitude. A peak data rate of 1.5 Gb/s per user was demonstrated in this transmission .
4. Demonstration of RFoF for 5G Fronthauls
Apart from IFoF transport, there are also reported RFoF transport for mobile fronthaul targeting towards mm-wave wireless signals transmission for 5G [17-18]. Dat et.al. demonstrated the transmission of 2 x 20MHz LTE-A and 4 x 800MHz F-OFDM for 90GHz wireless transmission using dual optical carriers with photonics upconversion .
Tian et. al. has demonstrated an RFoF transport scheme for the distribution of 60 GHz wireless signals in a multi-user mobile fronthaul link incorporating non-orthogonal multiple access (NOMA) scheme . This highlights the centralized controlled capability of BBU pool with a proof-of-concept demonstration for a two-user scenario. Figure 2 shows the architecture of the 60 GHz RFoF fronthaul with the RRH serving two users spatially separated within the same cell – one user closer to the RRH (NU) and one user at the cell edge (FU). To ensure the far user (FU) has similar performance as the near user (NU), the centralized BBU jointly process and multiplex the signals from the two users in the power domain using NOMA scheme with significant different electrical launched power favoring the FU. Tian et. al. proposed a NOMA scheme using a novel multilevel code (MLC) scheme that ensured no error propagation from the far user to the near user, enabling more flexible power allocation ratio design at the centralized BBU . A total of 8 Gb/s was demonstrated over 3 km of SMF and up to 2.5m wireless link at 60 GHz to simultaneously serve a near user located at 0.38 m and a far user at 2.5m from the RRH.
Figure 2: System architecture for 60 GHz wireless with RFoF fronthaul incorporating NOMA scheme
CPRI-based mobile fronthaul will be a bottleneck for the next generation wireless systems. To maintain a bandwidth-efficient centralized architecture supporting latency sensitive applications, analog transport technology emerges as a potential solution for mobile fronthaul. Despite the elegance and simplicity, there are still many technical challenges including implementation cost, interoperability and compatibility, that need to be further addressed.
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Christina Lim received the Ph.D. degrees in Electrical and Electronic Engineering from the University of Melbourne, Australia in 2000. She is a Professor and currently the Deputy Head of Department at the Department of Electrical and Electronic Engineering, the University of Melbourne, Australia. She was awarded the Australian Research Council (ARC) Australian Research Fellowship from 2004-2008 and the ARC Future Fellow (2009-2013). She was an elected member of the IEEE Photonics Society Board of Governors (2015-2017) and currently serving in the IEEE Photonics Society Conference Council. She is also a member of the Steering Committee for the IEEE Topical Meeting on Microwave Photonics Conference. She is an Associate Editor for IEEE Photonics Technology Letters and IET Electronics Letter. Her research interests include fiber-wireless access technology, modeling of optical and wireless communication systems, microwave photonics, and optical network architectures.
Yu Tian received the B.S. degree in Optoelectronic Information Engineering from Huazhong University of Science and Technology, Wuhan, China, in 2014. Since 2014, she has been working towards the Ph.D. degree in fiber wireless communications in the University of Melbourne, Melbourne, Australia. Her main research interests are primarily in the area of 60 GHz radio-over-fiber fronthaul communications with an emphasis on physical layer network implementation, coordinated multipoint transmission, non-orthogonal multiple access schemes, and system level simulation.
Thas A Nirmalathas is a Professor of Electrical and Electronic Engineering at the University of Melbourne and also Director of the Networked Society Institute – an interdisciplinary research institute focusing on challenges and opportunities arising from the society’s transition towards a networked society. Prof Nirmalathas obtained his PhD in Electrical and Electronic Engineering from the University of Melbourne. Over the past two decades, he has held many senior leadership positions at the University of Melbourne including Head of Department, Electrical and Electronic Engineering. He has also held visiting scientists appointments at NICT Japan and I2R Singapore. Thas has written more than 450 technical articles. His current research interests include energy efficient telecommunications, access networks, optical-wireless network integration and broadband systems and devices.
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