Ultra-modern stadium build-outs are becoming a common occurrence. Just look at the investment in the recent Sochi Olympics and the upcoming FIFA World Cup. These events are driving the construction of new venues with modern infrastructure, and they attract a clientele who fully expect to engage in mobile-savvy activities – from texting to video phone calls to sharing the game live with friends.
This kind of scenario, where we have both modern infrastructure – like abundant fiber – and dynamic user expectations, creates one example of a compelling use case for an emerging technology: the Cloud RAN. As this type of architecture becomes more popular, the mobile backhaul and fronthaul must keep pace. In particular, there are multiple options for fronthaul. In this blog, I will review the attributes of the Cloud RAN architecture and the options available for fronthaul transport of the Common Public Radio Interface (CPRI).
Cloud RAN approach
A distributed radio access network (RAN) architecture includes remote radio heads (RRHs) connected to the baseband unit (BBU) using the CPRI. The RRHs include the radio itself, associated amplification and filtering, plus the antenna. The BBU is implemented separately and carries out the centralized signal processing functions. This separation and the associated centralization of processing in the BBU, can enable improved coordination of radio capabilities across a set of RRHs. This becomes increasingly important in LTE and especially LTE-Advanced where such techniques can bring increased efficiency through interference mitigation, for example. There are also operational efficiencies to be gained from the flexible sharing of centralized processing resources. However, CPRI traffic must be transported (fronthauled) efficiently and within tight quality constraints between the RRH and BBU locations.
CPRI is a digital interface standard for encapsulating radio samples between a radio and a digital baseband processing unit. The interface is not packet based; rather signals are multiplexed in a low latency timeslot-like fashion. CPRI defines a maximum latency, a near zero jitter, and a near zero bit error rate. In practice, a value of .4 milliseconds for transport leaves an acceptable delay budget for processing requirements and propagation delay. The capacity required is up to 10 Gb/s, with distances of up to 40km between the remotes and the BBU.
CPRI fronthaul options
A number of transport options are potential candidates for CPRI transport, including the following:
- Dedicated fiber: This can be an attractive option for scenarios where an operator has a large installed base of available fiber. Even when fiber is available it must be used prudently, the cost associated with deploying new fiber limits the broad applicability of this option.
- Optical transmission network (OTN): OTN brings forward error correction (FEC) and can increase the reach of metro optical networks. Utilizing OTN for CPRI transport does require careful consideration as a number of the highly valuable features of OTN also add latency.
- Passive optical network (PON): PON is a potentially attractive option for CPRI transport in high traffic areas (e.g. dense urban neighborhoods), where small cell deployment is most likely to occur. Careful design engineering is required to accommodate the power loss budget and prevent additional latency being incurred which would limit the cell radius.
- Microwave: For short distances (1km or less), microwave transport is a potential option. Currently this transport technology could only support of a subset of the CPRI line bit rate options.
- Wavelength based systems: WDM offers a good combination of characteristics for CPRI transport. In particular, coarse wavelength-division multiplexing (CWDM) brings low delay and high throughout yet is economical, both in equipment costs and in its use of fiber resources.
CWDM for fronthaul transport
The use of CWDM entails deploying CWDM optics in each RRH and the use of passive optical filters to combine multiple client CPRI signals (different, standardized wavelengths) onto a single optical fiber pair. These individual wavelengths are broken out again by similar CWDM filters at the BBU location.
CWDM technology fits well with the unpredictable nature of remote placement, often outdoors e.g. in stadiums and on street “furniture”. With no active electronics, and relatively simple optics, a CWDM transport solution can be deployed in all outdoor environments providing significant CAPEX/OPEX savings. There are no specialized enclosures, no requirement for battery backup and minimum power is consumed. CWDM technology provides a cost-effective transport option necessary to support the projected RRH small cell roll-outs.
The stringent delay requirements of the CPRI protocol are also well supported by using CWDM. The lack of active electronics along the optical path between the RRH and the BBU means that the only source of transport-incurred latency is due to signal propagation. This allows the operator to maximize the distance between the RRH and BBU.
Fronthaul architectural advantage
Simple, cost-effective CPRI fronthaul enables an increased range of use cases for RRH deployments. While a number of techniques and media options are being explored, passive CWDM has many advantages in cost-effectively transporting the CPRI interface with strong performance (low delay and high capacity). As fronthaul is deployed as an extension and adjunct to backhaul, operators should look for a platform that delivers a consistent seamless solution across the combined architecture. For more information, read the Mobile Fronthaul For Cloud-RAN Deployment Application Note.
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