A Manifesto for better DAS

DAS_Close_SMateoNote: This article, by Real Wireless’s Managing Consultant Oliver Bosshard, was originally published in RCR Wireless

The traffic demands users are placing upon networks continues to accelerate rapidly. In order to cope with this trend, debates have focused on network offload; the benefits, options available and their respective merits.

Despite plenty of discussion, we’re still no closer to solving this problem. 80% of all data traffic is generated indoors, from office employees, visitors, customers and the like. Yet remarkably few buildings feature indoor mobile infrastructure installations; typically only the newest and largest (e.g. airports, stadia, shopping malls), ignoring the vast majority of people. Clearly there is a huge opportunity here and a number of technologies are jostling to be the solution – including Small Cells, Radio Remote Heads, Wi-Fi and optical or passive Distributed Antenna Systems (DAS), each with their own merits, weaknesses and use cases. Ideally we would have one standardized solution with standardized interfaces for all (large to small) indoor coverage solutions.

The oldest solution for indoor coverage is DAS. Originally an analogue, single-operator, single-technology solution, it has evolved to encompass digital multi-operator, multi-technology solutions – as well as supporting MIMO (Multiple Input – Multiple Output).

Despite these upgrades there remain a number of significant limitations to the technology – and a lot of room for improvement. Worse still, it now faces stiff competition from rival technologies, in particular from small cells. A number of analyses show that these might be more cost-effective than DASinsome cases and some companies and pundits have been making announcements about the end of DAS. With the massive reduction of base station (BS) pricing, operators might even opt for more dispersed Base Stations (BS) with a passive DAS, instead of an active DAS.

But DAS continues to be used widely, and in fact the number of deployments continues to grow. No surprise: it has a lot of compelling attractions, especially for multi-technology and multi-operator support (they can share one infrastructure, as opposed to one small cell for each). Indeed, it was striking how at Mobile World Congress 2014 the explosion of activity and news came from DAS vendors, demonstrating a number of new products and innovative ideas.

Certainly, all is not lost for DAS. If it could continue to evolve into a smart, digital solution – offering flexible sectorisation, intelligent/dynamic capacity steering, digitalization, package switching at a more competitive price – it could become the ideal solution.

Here is my manifesto for a better DAS.

(Note: In the interests of complete fairness, several companies are currently working on – or planning – some of these ideas. However, I’m not yet aware of anyone that has announced they intend to combine them all, and I see this as the real opportunity here.)

Better RRUs through equipment adjustments

Compared to conventional radio remote heads, the radios used in the Radio Remote Units (RRU) of DAS are technology-agnostic. Typically RRUs will feature modular support for all 5 bands and technologies, and are remarkably straightforward in their composition: one power supply, one Fibre or Cat6/7 connection, one RF output to an antenna or passive DAS.

Ideally, the RRUs should evolve in to full 2×2 MIMO remote units. To achieve this, the equipment required needs to become slightly more complex, with 2 RRUs in tandem fed by two Fibres or Cat7 cables with two RF outputs to a MIMO antenna or DAS. If the total spectrum of both streams together is less than 270 MHz, only one Fibre / Cat 7 connection may be used.

Increase signal capacity and noise cancellation via digital transmission

At present, the fiber connections between Master Units (MUs) and Radio Remote Units (RRUs) typically support a sampled analogue RF signal input of up to 10 Gbps in capacity.

With 270 MHz of cellular spectrum available across all 2G to 4G bands and technologies, and 30 MHz of sampled spectrum typically requiring 1 Gbps of digital capacity, this means 9 of the 10 Gbps available is required for cellular; leaving only 1Gbps spare for other technologies such as Wi-Fi.

But sampling of an analogue RF signal is not the most efficient usage of a transport medium. Imagine if the digital bit stream from the CPRI interface could be used. The CPRI data stream does not need sampling and therefore can be transmitted as it is, using the transport medium in an efficient way. As a result, conventional fibre connections could be replaced with Cat7 cabling, in conjunction with standard SFPs on the MUs and RRUs.

The other benefit of digital transmission is that the digital signal can be transported, amplified and distributed without the typical signal losses and noise creation. This would mean that RRUs could be situated far away from the MU and daisy chained as required.

Finally, by digitizing the transmission, the current issues DAS faces with signal loss over distance are rendered irrelevant, as the signal can be amplified without the risk of increased noise. Thisalsostartstoleadtowardsahybrid between DASandthenewer CRAN architectures beingcontemplatedforthewide-areanetwork.

Reduce costs, simplify and increase efficiency by connecting to CPRI

At present, DAS uses standard RF interfaces for BS – MU connections. This results in OEMs needing to purchase and produce additional BS hardware for compatibility. This increases the complexity of the solution, whilst adding additional costs for manufacturing, stocking and shipping.

Using CPRI – oranotherevolvedoptimizedinteroperabledigitalinterface- would do away with the need to include the radio in the BS, reducing hardware requirements, power consumption and the use of external directional couplers and termination loads between the BS and the MU. The need for less up-conversion and final amplification in the base stations would reduce hardware costs, power, UPS and air-conditioning significantly and avoid RF noise creation.

Finally, by digitizing the transmission, the current issues DAS faces with signal loss over distance are rendered irrelevant, as the signal can be amplified without the risk of increased noise. This would mean that RRUs could be far away from the MU and daisy chained as required.

But at the same time, thanks to our newly supported CPRI connectivity, the signal conversion at the RRU end becomes simpler and cheaper, thanks to a direct digital / RF conversion.

Take full advantage of routing and switching capabilities

Another benefit of CPRI interface is that the data is presented as a digital data stream. As a result, the data stream could be switched and routed by proprietary switches supplied by the DAS manufacturers, using either Cat 7 (up to 100m) or Fibre networks for longer MU – RU distances (up to 40 km is possible).

Doing so would not only allow full flexibility in traffic allocation to end points, shaping traffic to meet demand for capacity, but also smarter switching of unused or underused repeaters. By manually or dynamically switching off unused repeaters, more effective management of uplink and downlink noise pollution and power consumption is enabled.

Unfortunately CPRI implementation differs from OEM to OEM: it is one of those “not quite standardized standards”. We need to achieve open interfaces and (perhaps) cross-vendor interoperability if we are to get the best possible use out of DAS and a more open market. In the meantime, DAS manufacturers can create their own CPRI at the master unit output, in order to take advantage of the benefits digital transmission offers.

Another issue is that CPRI, while digital, is not compatible with Ethernet or current installed networks.

That too is changing, however. While there are issues with carrying these signals for their transportation over standard TCP/IP switching and routing networks these can be addressed. By standardising the CPRI interface across OEMs and encapsulating the cellular data packets in standard IP packets, traffic could be switched and routed via conventional routers instead of proprietary units supplied by DAS manufacturers – of course, this is reliant upon transmission requirements being met, such as synchronization and a jitter free constant serial data streams. However, several vendors are now demonstrating products that can, indeed, carry digitized RF over standard Ethernet. 

By adopting these proposed changes, we would see massive capital and operational savings in the use of DAS systems. Standard infrastructure would be able to be used for switching and routing, whilst larger areas would be able to be covered by a DAS system.

To end with two thoughts about the implications for business models and the industry.

It is notable that these changes to DAS, coming from an in-building context are very similar to, and probably converge with, the activities around virtualization and Cloud RAN that are happening elsewhere in the network. Again, the move to transporting digital radio signals, to support multiple services in a flexible way are similar. We may see some intriguing overlaps between DAS companies and Cloud RAN suppliers.

Finally, and worth noting, the term “neutral host” may well receive a completely new meaning and present a new opportunity for groups such as MSOs. Interestingly, there has always been a difference in neutral host between Europe & USA, and that difference could change in various ways. Beyond cost and efficiency savings, the proposed changes could actually catalyse new business models that could change the industries structure.