Why backhaul/fronthaul for Small Cells is still a challenge?

Small-cells are considered as one of the key architectural enablers to the challenging demands such as high spectral and energy efficiency of 5G and beyond mobile networks. Although the small-cell technology concept has been articulated and studied for many years within the 4G LTE framework, the concept has never found the widespread application mainly due to the cost of operational and capital expenses.

In the conventional wireless networks, the cost of the macro cell base station (macro-BS) has been a dominant factor that includes both the operational and capital expenses. The cost of a small-cell base station deployment, on the other hand, is much lower in comparison to that of a macro-BS; but efficient and satisfactory operation of all these densely deployed small cells strictly hinges on the availability of the smart backhaul and fronthaul networks provisioning ultra-low latency, high data rate and high reliability. Such backhaul and fronthaul networks will guarantee the global information and communication requirements in future smart and resilient cities and solve the ubiquitous and global connectivity problems in many challenging scenarios which include but not limited to:

• coverage or capacity enhancements for remote or sparsely populated areas where infrastructure is not available or expensive to deploy, and
• disaster affected areas where conventional cellular networks are either damaged or congested.

There are considerable market interests on the development of small cell backhaul and fronthaul solutions that could be an evolution of the existing backhaul and fronthaul technologies, i.e., Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Multiprotocol Label Switching (MPLS) and Ethernet or an integration of emerging technologies such as Integrated Access and Backhaul over millimeter-wave (mm-wave) and backhaul/fronthaul over free space optical (FSO) which is also knonw as fiber less technology.

The data rate requirements of 5G and beyond networks would necessitate that the fronthaul be fiber optic [1]. However, it’s unlikely that fiber will be economical for all installation sites and will face deployment restrictions to lay the fibre in many hyper-dense small cells deployment areas [2].  A more cost-effective alternative is wireless fronthaul, where small cell's traffic is carried over microwave or FSO links. However, such wireless solutions are based on delivering the traffic of small cells to a central fronthaul hub, which would be ideally located at an existing macro-BS where high-speed and reliable backhaul is already available.

Now, the key challenge in designing wireless fronthaul network is how to optimize the location of these central fronthaul hubs that connect the small cells to the core network. Such a problem has been shown to be NP-complete problem which even may not have optimal solution since some small cells are/may be located in hard to reach areas and LOS propagation is/may be impossible [3].

In such scenarios, Non-LOS (NLOS) solutions which rely on licensed sub-6 GHz spectrum or microwave frequencies could be used at the expense of severe interference, traffic congestion and high operational cost [4]. Line-of-sight (LOS) FSO has also recently gained attention as a promising fronthaul technology as it relies on license free point-to-point (PtP) narrow beams to deliver the traffic at higher data [5]. Although, the terrestrial FSO seems to be an attractive solution as it provides high data rate with no interference compared to NLOS solution, it may not be a cost-effective candidate for fronthauling small cells deployed in ultra-dense urban areas as LOS may not always be available. Therefore, more FSO equipment may be required to fronthaul the small cells which could significantly increase the fronthaul capital cost.

New backhaul/fronthaul solutions could include hybrid radio links such as RF/FSO or RF/mm-wave and Networked flying platforms (NFPs) where unmanned aerial vehicle or unmanned balloons will route the small cells traffic via flying hubs [8].

In short, to meet the underlying challenges and constraints for backhaul and fronthaul and associated networking, communications and signal processing issues, some novel paradigm shift in backhaul and fronthaul network design is needed.

References:

[1].            M. Z. Shakir, M. A. Imran, X. Wang, et al., “Smart fronthauling and backhauling for 5G systems: from precoding to network architecture,” IEEE Wireless Communs. Mag., vol. 22, no 1, Oct. 2015. [IEEE Xplore].

[2].           H. Dahrouj, A. Douik, F. Rayal, T. Y. Al-Naffouri, and M.-S. Alouini, “Cost-effective hybrid RF/FSO backhaul solution for next generation wireless systems,” IEEE Wireless Commun., vol. 22, no. 5, pp. 98–104, Oct. 2015. [IEEE Xplore].

[3].            E. Karamad, R. Adve, Y. Lostanlen, et al., “Optimizing placements of backhaul hubs and orientations of antennas in small cell networks,” in Proc. IEEE ICC Workshop Next Generation Backhaul/fronthaul Networks (BackNets’2015), pp. 68–73, London, UK, Jun. 2015. [IEEE Xplore].

[4].           U. Siddique, H. Tabassum, E. Hossain, and D. I. Kim, “Wireless backhauling of 5G small cells: challenges and solution approaches,” IEEE Wireless Commun., vol. 22, no. 5, pp. 22–31, Oct. 2015. [IEEE Xplore].

[5] M. Khalighi, M. Uysal, “Survey on Free Space Optical Communication: A Communication Theory Perspective,” IEEE Commun. Surveys & Tutorials, vol. 16, no. 4, pp. 2231-58, Fourthquarter 2014. [IEEE Xplore].

[8].            M. Alzenad, M. Z. Shakir, H. Yanikomeroglu, and M.-S. Alouini, “FSO-based vertical backhaul/fronthaul framework for 5G+ wireless networks,” IEEE Communs. Mag., vol. 56, no. 1, Jan. 2018, DOI: 10.1109/MCOM.2017.1600735 [IEEE Xplore].

Notes:

IEEE ComSoc Emerging Technologies Initiative on Backhaul/fronthaul Networking & Communications (ETI BNC) is a technical forum to put forward IEEEs’ agenda and contribution in the research and standardization activities on future backhaul/fronthaul networking and communications. ETI BNC is a forum for researchers, developers and practitioners from both academia and industry to identify and discuss the backhaul/fronthaul requirements, challenges, recent development and smart end-to-end solutions pertaining to fifth-generation (5G) of mobile communication networks. ETI BNC is a prolific opportunity to educate about, promote and accelerate the evolution of next generation of backhaul/fronthaul networking and communications by fostering technical activities in the related areas. Follow the ETI BNC website for activities and news, subscribe to the mailing list and engage with the members.