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.