While most of the industry agrees we'll need many more and smaller cellsites in our urban streets, three competing approaches include Urban small cells, Cloud-RAN (C-RAN) and Outdoor DAS (Distributed Antenna Systems). We talked to three different backhaul and fronthaul service providers seeking to address this market, contrasting their approach and learning what's on offer today.
There are three broad categories of backhaul and fronthaul to the smaller urban cellsites which we've classified as:
- Urban small cells, with standard broadband IP to the core network
- Cloud-RAN (C-RAN) & Digital DAS, using Digital Radio over Fiber (D-RoF) technologies such as CPRI and OBSAI to pipe the baseband signals to central baseband processing sites. These can be short range to nearby macrocells or several kilometers to larger so-called "baseband hotels".
- Outdoor Analogue DAS, using dedicated fibre or wavelengths to relay the analog RF signals
The bandwidth for each option grows by an order of magnitude, leading to different technical approaches.
Small cells might be adequately served by 100Mbps or so, while each CPRI connection needs up to 9.8Gbps and outdoor analogue DAS must have dedicated fibre. Small cell connections are termed backhaul while Cloud RAN and DAS connections are usually named fronthaul. Fronthaul usually means fibre, although there are a few point to point wireless CPRI products on offer.
|Architecture||Connectivity to site||Bandwidth|
|Cloud-RAN/Digital DAS||Fronthaul||600Mbs to 9.8Gbps|
|Outdoor Analog DAS||Fronthaul Analog RF||Analog|
A critical factor to make Cloud RAN and Outdoor DAS viable is whether the higher cost and time to install these high bandwidth fronthaul links offsets any additional capacity and compatibility they deliver. The extra capacity comes from spectrum efficiencies by tightly co-ordinating with macrocells. If the gains are not substantial, it would surely make more sense to install low cost small cells – even if more sites are required overall.
Some companies are working hard to serve demand for fibre connectivity in these urban areas, building on services already in place for fixed internet provision. Clearly these investments demonstrate they believe there is business to be had and profits to be made.
Manhattan, New York – A co-located Cloud RAN approach
I spoke with Ray La Chance, President and CEO of ZenFi Networks. His background includes many years delivering fibre-optic service to enterprises throughout New York and one of his former businesses, Lexent (sold in 2010), focused on providing regional neutral dark fibre network infrastructure to large enterprises and telecom carriers wishing to deploy their own high capacity metropolitan networks. He's witnessed the evolution of fibre connectivity migrate from the preserve of a few very large downtown corporate buildings to become available for many of the medium sized offices and neighbourhoods throughout the city.
He notes that New Media centres are springing up in locations previously used for warehouses or manufacturing, seeing these younger type of companies avoiding costly Fifth Avenue office buildings but valuing high speed internet connections much more than a premium location. Districts such as Brooklyn and Queens are developing ecosystems of technology based companies not seen before.
His latest venture, ZenFi, was founded in 2013 to focus on providing fibre connectivity for mobile networks requiring fibre-to-the-antenna (FTTA) fronthaul, distributed colocation and high capacity backhaul. By focussing exclusively on this specific market niche, he expects to get ahead of the larger cable and fixed network providers who have a much wider portfolio of products to manage.
His view is that today's MetroEthernet services aren't technically sufficient to support fronthaul for advanced LTE – they lack the bandwidth, jitter and latency tolerances required. He believes that dedicated dark fibre links will be needed serving up to 10Gbps per site. His plan is to build a series of co-location centres where mobile operators can install their own "baseband hotels", sharing fibre to many thousands of sites across Manhattan.
By providing a shared facility, costs can be apportioned across multiple operators in the same way that DAS is done for in-building today. The scope of their service involves only dark fibre links and secure equipment rooms – Zenfi don't provide RF expertise or manage the network. This enables each mobile operator with the flexibility to provide their own transmission equipment, radio heads and baseband servers.
Atlanta to Orlando – Dedicated Fibre Backhaul
Tower Cloud Inc. provide mobile backhaul links throughout the South Eastern United States, connecting some 1,700 tower sites. George Townsend, their Business Development Director, told me that their proportion of wired vs wireless connections has evolved from 80/20 to 95/5 in recent years, with virtually every new connection being made by fibre. Average macrocell site backhaul bandwidths are 100Mbps, with a few at 200-250 and some with 1Gbps. Anything over 300Mbps justifies fibre. Although there is some suburban infill being built out, the majority of new business is to be found in the metropolitan areas.
George sees real momentum building up towards densification of mobile networks, with both Tier 1 and 2 operators having aggressive build plans. There's still considerable difference in the requirements for each site, and some thought is required to design each installation taking a number of factors into account. These include RF planning constraints, choice of available street furniture and type of cells used together with location and availability of the nearest fibre connections.
Typical backhaul requirements for individual urban small cells are in the region of 50Mbps. This could also be served wirelessly and Tower Cloud would use that technology when appropriate in due course.
Tower Cloud also provides dark fibre connections when their customers request it, but it seems to me that their managed backhaul serves today's needs.
UK – Managed CPRI Fronthaul
Openreach (the functionally separate part of BT Plc) provides wholesale fibre and wireline connections to both fixed and mobile operators across the UK, commonly as a managed Ethernet service.
They recently announced successful tests of a managed CPRI service. In this case, rather than provide dark fibre, they'll deliver a managed service that can cope with the full bandwidth of one or more CPRI links used to connect remote radio heads. This service is independent of any specific RAN equipment vendor and transparent to CPRI proprietary extensions or features. It will meet the tight jitter and latency requirements needed to support LTE.
One of the key advantages of urban small cells is that they can be deployed quickly and easily, requiring only a suitable site, adequate power and some form of IP backhaul. The plethora of wireless and wireline backhaul products available is huge.
Cloud RAN will require multiple Gigabit links which can't use existing MetroEthernet and will require either dedicated CPRI managed service, such as BT Openreach is proposing, or dark fibre to each site. There are also a few short-range point-to-point wireless CPRI fronthaul products on the market, but the choice is more limited.
Outdoor DAS will certainly require dark fibre per site, and some companies are gearing up to be able to deliver that.
A recent analyst report on Cloud RAN highlighted that the costs and benefits of this approach are not yet fully quantified or understood. These case studies show that there are companies who believe it is both commercially viable and there is an opportunity to serve the market. Whether this approach is limited to existing urban macrocell sites or extends further as more smaller sites are installed remains to be seen.
At the end of the day, the business case will decide which wins. There may be a place for both, with fibre connecting many existing urban macrocell sites surrounded by small cells with wireless and/or simpler wired backhaul.
My own view is that the lower cost and simplicity of small cell backhaul provides a significant practical advantage. Whether this overcomes the conservative nature of mobile operators to adopt new technology (and new vendors) remains to be seen.