Evolving Technologies for Seamless Mobile Front and Backhauling

Evolving Technologies for Seamless Mobile Front and Backhauling

Introduction


A recent Cisco VNI Global Mobile Data Traffic Forecast reported that in the next five years there will be 4 billion more mobile-ready devices and connections and the average mobile connection speed will increase 2.4 fold [1].

The Citrix mobile consumer survey in 2014 revealed that 54% of mobile subscribers would abandon a slow-loading page in less than 10 seconds. Furthermore, 63% of millennials were more likely to blame the mobile network for mobile video stalling [2].

With the advent of smart devices, cloud services, newer technologies for fixed and wireless connectivity and very impatient consumers, there is tremendous pressure to strengthen the access and mobile backhaul segment of the network. The copper and microwave connections to the towers can no longer endure the exploding capacity requirements. Fiber is fast becoming the de-facto solution to meet these demands [3]. 

This paper provides a primer addressing the current technological developments, for an economic passive fiber-based backhauling and fronthauling architecture. The various network elements namely; transceivers, multiplexers, monitoring and protection systems are discussed in detail clarifying how these technologies overcome the challenges depicted in Fig. 1.

The paper introduces the possible fronthaul and backhaul layout when looking at centralized and/or distributed architectures. It delves deeper into fully passive solutions for the remote sites that reduce latency, space, power and finally cost requirements.

Since fiber convergence to mobile networks is a demanding application with quickly changing requirements, it is a much debated subject matter for all mobile service operators.

 Centralized Architecture


While fibers are mostly being deployed in the backhaul networks, which interconnects the baseband unit (BBU) to the core network, a novel approach of building flexible mobile networks has been pushed forward since a couple of years where fiber is also used from the base station to the antenna, which is called fronthaul [4].

Traditionally, as depicted in Fig. 2(a), the BBU and the remote radio head (RRH) are collocated inside a cabinet close to the antenna and a coax cable is used to connect the RRH to the antenna located at the top of the cell site. With migration to fiber based connection, the RRH is placed close to the antenna at the top of the cell site and connected to the BBU as shown in Fig. 2(b). Fiber overcomes any limitation imposed by Coax, such as distance, weight and energy. Due to the possibility of longer distances, one can design the fronthaul network with centrally located base station at a central office location as depicted in Fig. 2(c) equipped with the number of baseband units for several base stations.
The architecture with the stacked BBUs at the central office or hostel to form a centralized BBU (C-BBU) is referred to as centralized architecture. This architecture aids in easy maintenance at the single location and provides improved security (no cabinets to break into) and reduces energy utilization. Furthermore, in LTE networks, the collocation of BBUs simplifies the X2 interface and also makes the associated latency and synchronization issues insignificant. The X2 interface provisions information exchange between BBUs for smooth handover and coordination.

While the connection between each RRH and BBU can be deployed with a dedicated fiber, the most efficient way would be via the deployment of wavelength multiplexing over a single fiber. An active wavelength division multiplexing (WDM) solution in fronthaul would have stringent signal synchronization requirements. Additionally, space and power limitation would dominate the design of the active system based network.

Passive WDM on the other hand provides low latency solution where colored transceivers are directly deployed in the RRH to provide the necessary WDM wavelength signal.

Network Offloading via Small-Cells and DAS


Once the mobile service provider (MSP) wants to deepen coverage and improve capacity in a targeted area, network offloading via distributed antennas systems (DAS) and small cells is recently gaining popularity as the alternative architecture to macro site expansion. DAS solution is also important where macro coverage is inferior.

A DAS network has a spatially separated antennas connected individually via fibers to the basestation or central office where the baseband processing occurs. The advantage of a DAS system is that it can be shared by multiple operators. Each of the operators then connect their own basestations to the shared distribution system [5]. A DAS can be implemented both indoors (iDAS) and outdoors (oDAS).

Small cells as depicted in Fig.3 are fully equipped to perform the processing at the same location. Most of the small cells today are designed as single frequency band for a single operator. Similar to macro site back/fronthauling, fiber can be optimized to DAS and small cells backhauling.

As depicted in Fig. 3, due to low loss and multiplexing facility of fiber, multiple small cells and macro cells can be linked over several km to the central office while still performing the processing at the central location.
The fiber-based access technology has matured over years with passive optical network (PON) fiber-to-the-home applications. Though the penetration of fiber to wireless backhaul or fronthaul network is very recent, the technological maturity is well proven. One must however note that the optical elements as discussed in the following sections need to fulfill certain conditions specific to wireless environment to make the fiber-based wireless network resilient and future-proof.  

WDM for Higher Capacity


Multiplexing can be achieved in different dimensions: time, wavelength, modulation/phase, polarization and also space. A wavelength division multiplexing (WDM) as the name reflects is the multiplexing of different channel wavelengths into a single fiber and is one of the best ways to easily expand the network capacity. Depending on the fiber availability, services and locations of connecting base stations and central office, a fronthaul/backhaul network can have variety of topologies i.e. ring, bus, daisy chain or point-to-point, use of single fiber or fiber pairs; all reinforced with multiplexing. 
Depending on the wavelength or frequency spacing between the neighboring channels, WDM is subdivided into two major types, dense WDM (DWDM) and coarse WDM (CWDM). Because of the tightly spaced channels in DWDM and lower linewidth requirements of DWDM system, more stable lasers like DFB or ECL are essential.
A CWDM system on the other hand uses FP laser, which costs lower than DWDM counterparts. The transmit laser might often be used with a temperature controller, to assure that the central wavelength of the laser does not drift far from the operational bandwidth. The choice of CWDM or DWDM during the network design depends mainly on distance, number of channels and data rate required. Additionally, add/drop multiplexers make them flexible with respect to locations. SK Telecom in South Korean was one of the first operators to use WDM in fronthaul networks.

CWDM provides up to 16 channels while DWDM can carry 80 wavelengths (in C band), that can be extended to 80 more channels when considering the L band. With WDM technology, the capacity increase is limitless. However as cost plays a vital role in deployments, low cost DWDM technology with outdoor-hardened specification is still in research. CWDM based fronthauling is much more economically beneficial and as CWDM transceivers withstand the extreme conditions, are therefore today used in most of the new fronthaul installations.

Passive WDM is also gaining interest as it requires no power and management at the remote sites and thus does not generate extra OPEX. WDM can be built in extremely compact and robust housings as shown in Fig. 5 and is outside plant (OSP) compliant.

Conclusion


In tomorrow’s online and interconnected world, we will see an unprecedented growth in data traffic requiring deployment of novel front/backhaul technologies with improved performance in harsh environments and with lower power consumption and lower cost.

Addressing these requirements, this paper sheds light on different temperature hardened colored transceivers, multiplexers in different form factors together with protection planning and monitoring that are an integral part of network design and planning while expanding wireless capacity and coverage. There is no single easy solution to every upgrade, but tackling the technical and business challenges step-by-step makes the back- and fronthaul evolution more simple and efficient.
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