When it comes to deciding on deploying small cell base stations, one is faced with a few options. One option is based on cloud RAN architecture with remote radio heads connected through optical fiber to a central base station housing the baseband processing. A second option is that of a compact base station which includes both the radio frequency and baseband processing functions. The compact base station is connected to the core network by a number of different backhaul technologies.
The availability of low cost fiber is a gating factor in deploying cloud RAN architecture. Remote radio heads require very high capacity links to support modern air interface features such as multiple antennas for MIMO. CPRI and OBSAI interfaces run at between 3 and 6 Gbps depending on the number of supported antennas. The compact base station on the other hand requires much lower capacity for backhaul – on the order of tens to over a hundred Mbps. Low backhaul throughput requirements should translate into lower deployment cost to the advantage of compact base stations.
Distributed Base Station Architecture is the Fundamental Building Block of Cloud-RAN
Compact Base Station Architecture
But cloud RAN architecture can have one significant advantage over compact base stations when it comes to performance in interference environment as is typically the case with small cell deployments. The availability of central baseband processing coupled with fast, high capacity fiber backhaul allows different LTE-Advanced features to run resulting in better overall capacity than loosely coupled compact base station architecture with distributed baseband processing. For example, it becomes practical to deploy soft-cell scheme to maximize cell-splitting gain.
Shared or soft-cell techniques allow the small cells to broadcast the same control channels and synchronization signals as the macro cell. The mobile would be able to get its control and data channels from either the macro cell or small cell or from both simultaneously. This feature is enabled in LTE Release 10 with further enhancements in Release 11. It requires tight coupling between different layers in the heterogeneous network to keep control and data channels emanating from them in sync.
Soft cell deployment scenario: small cell control signalling can be from the small cell, marco cell, or both.
What does all this mean? If you are an operator, you have to carefully consider your small cell deployment strategy in a holistic manner. For example, LTE features have special requirements on backhaul. This interrelationship of requirements is more complex than it has ever been. The bifurcation of technology options should be considered in an integrative manner: silos addressing different functions of the wireless network (e.g. radio access network, core network, transmission network, etc.) can lead to detrimental incompatibility.
Frank, all things considered Cloud RAN can provide a lot of superior features envisioned through the proposals in LTE Rel 10-12. Since <10 Gbps filber links are fairly cheap these days.. I guess its the digging and laying that will burn up a lot of cash in greenfield deployments. HetNets have a lot of issues to deal with in just in terms of mobility of users in terms of Macro to small and vice-versa, eliminating backhaul issues in terms of wireless spectrum and or interference might just help the case. There are also some proposals for separating C-plane and U-plane traffic in such a hierarchical set up which are better served by a Cloud RAN. May be the Google fiber project can help 🙂
Jeebak, I agree that given the tight coupling between the RF and baseband in a C-RAN system, more features can be incorporated to increase capacity (CoMP being a major one). Interestingly, despite the high cost of fiber (especially if it’s underground as required by municipalities in many developed markets), fiber networks keep on growing in part to support the wireless network (AT&T’s CEO recently commented on this). There is much value in having fiber because it provides the option to expand service in a way that we may not know of now. Nevertheless, fiber is not ‘ubiquitous’ and I believe C-RAN in the outdoors will see limited deployment.
After I wrote the reply, the following news story came to my attention: Telefonica competitors ask Spain to intervene in fibre rollout
http://uk.reuters.com/article/2013/04/19/uk-spain-telecoms-telefonica-idUKBRE93I0GB20130419. It fully illustrates how fiber is used in practice for competitive reasons.
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