Mobile operators are facing two important decisions when planning their small cell rollout in public areas. The first question to be answered is how to transport traffic from small cells to the edge of the mobile core, while the second question concerns interference management. Although these are two distinct decisions, they are closely related. The performance and functional capabilities of the network connecting small cells determines which solutions the mobile operator can use to mitigate interference. Vice versa, the choice of interference management tools drives the operator toward specific connectivity solutions.
Connecting small cells over different physical media, including the preferred fiber option, microwave and copper, constitutes only one degree of freedom. Mobile operators also have the option to choose between different architectures, namely backhaul and fronthaul. The traditional backhaul architecture connects the Radio Access Network (RAN) to the mobile core, with baseband processing taking place at the cell site directly. Fronthaul, in contrast, enables remote baseband processing, resulting in centralized RAN (C-RAN) architectures that improve the efficiency of interference management in Heterogeneous Networks (HetNets). As a result, the overall throughput and perceived data rates are increased in a combined macro and small cell RAN.
When deploying backhaul architecture, integrated, self-contained small cells need to be synchronized precisely to enable distributed interference coordination and mobility management to operate effectively. Because latencies and differences in phase have a significant negative impact on the performance of interference coordination, low-latency connectivity and accurate phase synchronization of small cells, potentially including the overlaying macro cell layer, become vital. Distributing precise phase synchronization information over the backhaul network is a challenging task not supported by many of the backhaul architectures deployed today. Existing networks need to be migrated.
Fronthaul transports the unprocessed Radio Frequency (RF) signal between remote radio heads (RRH) and the centralized pool of Baseband Units (BBU). While fronthaul requires higher bandwidth, lower latency and better jitter performance than backhaul, it enables a more efficient use of RAN resources. Interference and mobility management is simplified and by far more effective when implemented centrally, as opposed to the distributed approach in case of backhaul architecture. In addition to improving interference management, fronthaul reduces the amount of interfaces to the mobile core, is future-proof, simplifies site engineering, provides energy savings and predicts reduced costs for installation and maintenance.
The high bandwidth demand ranging up to multiple Gigabits per second and the stringent requirements imposed on delay and jitter performance limit the alternatives available to provide fronthaul connectivity. Optical fiber is the only physical medium capable of mastering these challenges with a wider scope. While copper is not an option, microwave might be used for small radio site configurations only. The availability of fiber has a significant impact on the business case for fronthaul deployment.
Many mobile network operators including Orange, T-Mobile, Verizon Wireless and others have upgraded a large percentage of their macro cell sites with fiber connectivity when rolling out LTE. There is, however, a huge regional difference in fiber availability in metro regions where small cells will be placed. In addition, managed fronthaul connectivity services are a portfolio element still to be developed by many alternative access vendors.
Multiple factors affect the choice between fronthaul and backhaul. A fundamental one is whether the mobile operator has access to fiber or fiber-based fronthaul services offered by a local access provider. Mobile operators face a tradeoff between the flexibility of backhaul architecture and the improved performance of fronthaul solutions. Recent studies conducted by mobile operators and industry analysts have shown that the cost of using backhaul and fronthaul is comparable. The advantage of fronthaul does not come from cheaper equipment, but from a more efficient way of managing spectrum resources in the macro and small cell layer. Cost per bit is therefore reduced by increasing RAN efficiency and capacity. Hence, having access to metro fiber or fronthaul connectivity services is a clear competitive advantage for mobile operators.
As a consequence, flexible fiber connectivity solutions fulfilling the intrinsic performance and cost requirements of backhaul and fronthaul are a mandatory technology for fiber access providers offering backhaul and fronthaul services for macro and small cells to mobile network operators.