Between DAS and Small Cells – A Practical Guide
November 29, 2013 1 Comment
When small cells started appearing on the market, DAS was one of their targets. The thought was small cells would make DAS obsolete. After all, small cells are inexpensive and easy to deploy. This did not happen and not likely to happen for many reasons. DAS will be around as will small cells. While both serve similar functions, there are a few critical differences that ensure continuity for both types of technologies.
To see why, let’s first look at how DAS and small cells work. DAS systems are essentially transport systems: they take RF signals from one location and transport it to another. The means of transport is fiber. RF signals are modulated onto optical signals which are transported over fiber before being converted to RF for transmission. This leads to point 1: DAS provides a single point of interface for different RF sources to connect to. In other words, multiple operators can share the DAS systems (if they wish, of course! Interestingly, there are many operators who oppose sharing anything). Sharing DAS reduces the cost of deployment and operation. A complementary point is that DAS systems take on a full range of frequencies and channel which include those of public safety and emergency service operated by different levels of government. This leads to point 2: the DAS remote headends where optical is converted to RF are multiband, multicarrier modules. A single remote DAS headend which can transmit at high RF output power (20 W) can support 4 or more bands of multiple carriers. Point 3 is that DAS is oblivious to the air interface technology: it can be 2G, 3G or 4G (well, almost – many DAS systems don’t yet support TDD functionality). Because of these characteristics, DAS systems have found a home in solving wireless access in large venues where the business case is viable because the number of expensive active modules (remote DAS modules) is reduced.
Small cells on the other hand are a single operator play with a single band (some now coming on market support two band). Small cells covering indoor venues typically have much less RF power than DAS remote modules (order of 100 mW to 200 mW). Sharing small cells is a further step along the integration ladder between two networks which makes it see more resistance from operators, although some have devised a business model to share the small cells gateway to which small cells from different operator networks can connect to. For these reasons, the small cells business case works well in covering a small venue where clients of a single operator have a large presence.
The contrast does not stop there, but extends into practical issues. For example, when one uses low power modules in indoor applications, many such modules are required which means they have to be close to the user. This raises issues of availability of space for its deployment at the desired location and access to this module for maintenance in case of failure. Such practical issues had a large role to play in defining the markets for DAS and small cells.
For whatever reason, DAS has been labeled as a “coverage” solution, and I suspect the small cells camp had a hand in this to differentiate small cells as being a “capacity” solution. This is the wrong way to think about this because DAS can be as much about a capacity play as small cells can be a coverage play. After all, operators can keep connecting more sectors to the DAS systems just as small cells can be deployed in areas where there is no coverage.
Here, I need to say that I have been specifically amazed by how regulatory practices influence the success of business models which naturally lead to adopting certain technologies. This is addition to competitive practices between operators had a large impact on the traction that DAS and small cells see in the market.
To sum up, both these technologies are greatly complementary. Each has a certain business case and value proposition. More so, there is an evolutionary process taking place that will add to this mix of choices. Consider for example the recent Ericsson Dot product which is a remote radio head with Cat5 cable backhaul. Consider also the evolution of base station to cloud RAN architecture. Such developments are bound to change the landscape for both DAS and small cells, but there will be another occasion to talk about this.