The traffic demands users are placing on networks continues to rapidly accelerate. In order to cope with this trend, debates have focused on network offload – the benefits and options available and their respective merits.
Despite plenty of discussion, we’re still no closer to solving this problem. Eighty percent 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, such as airports, stadiums, shopping malls, provide this service, ignoring the vast majority of people. Clearly there is a huge opportunity here and a number of technologies are jostling to be the solution: small cells, remote radio heads, Wi-Fi and optical or passive distributed antenna systems, each with their own merits, weaknesses and use cases. Ideally we would have one standardized solution with standardized interfaces for all indoor coverage solutions.
The oldest solution for indoor coverage is DAS. Originally an analog, single-operator, single-technology solution, DAS has evolved to encompass digital multi-operator and multitechnology solutions, as well as supporting multiple-input/multiple-output technology.
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 analysis show that these might be more cost-effective than DAS in some cases and some companies and pundits have been making announcements about the end of DAS. With the massive reduction of base station pricing, operators might even opt for more dispersed base stations with a passive DAS.
But DAS continues to be widely used with the number of deployments continuing to grow. No surprise: DAS has a lot of compelling attractions, especially for multitechnology and multi-operator support since 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 who demonstrated a number of new products and innovative ideas.
Certainly, all is not lost for DAS. If it continues to evolve into a smart, digital solution – offering flexible sectorization, intelligent/dynamic capacity steering, digitalization and 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 announcing their intentions to combine them all, and I see this as the real opportunity here.)
Better RRUs through equipment adjustments
Compared to conventional remote radio heads, the radios used in the radio remote units of DAS are technology-agnostic. Typically RRUs will feature modular support for all five bands and technologies, and are remarkably straightforward in their composition: one power supply, one fiber or Cat 6/7 connection, one RF output to an antenna or passive DAS.
Ideally, the RRUs should evolve into full 2×2 MIMO remote units. To achieve this, the equipment required needs to become slightly more complex, with two RRUs in tandem fed by two fiber or Cat 7 cables with two RF outputs to a MIMO antenna or DAS. If the total spectrum of both streams together is less than 270 megahertz, only one fiber/Cat 7 connection may be used.
Increase signal capacity and noise cancellation via digital transmission
At present, the fiber connections between master units and RRUs typically support a sampled analogue RF signal input of up to 10 gigabits per second in capacity.
With 270 megahertz of cellular spectrum available across all 2G-4G bands and technologies, and 30 megahertz of sampled spectrum typically requiring 1 Gbps of digital capacity, this means 9 of the 10 Gbps available is required for cellular, leaving only 1 Gbps spare for other technologies such as Wi-Fi.
But sampling of an analogue RF signal is not the most efficient use 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 fiber connections could be replaced with Cat 7 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. This also starts to lead toward a hybrid between DAS and the newer cloud radio access network architectures being contemplated for the wide-area network.
Reduce costs, simplify and increase efficiency by connecting to CPRI
At present, DAS uses standard RF interfaces for BS/MU connections. This results in vendors needing to purchase and produce additional BS hardware for compatibility, increasing the complexity of the solution while adding additional costs for manufacturing, stocking and shipping.
Using CPRI – or another evolved optimized interoperable digital interface – 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 100 meters) or fiber networks for longer MU/RU distances (up to 40 kilometers 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 vendor to vendor: 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. But that, too, is changing. While there are issues with carrying these signals for their transportation over standard TCP/IP switching and routing networks, they can be addressed. By standardizing the CPRI interface across vendors 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. This is reliant upon transmission requirements being met, of course, 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, while larger areas would be able to be covered by a DAS system.
Parting thoughts
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 C-RAN happening elsewhere in the network. Again, the move to transport digital radio signals to support multiple services in a flexible way are similar. We may see some intriguing overlaps between DAS companies and C-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 mobile service operators. Interestingly, there has always been a difference in neutral host between Europe and the U.S., and that difference could change in various ways. Beyond cost and efficiency savings, the proposed changes could actually catalyze new business models that could change the industry’s structure.
Oliver Bosshard is a managing consultant at Real Wireless with extensive experience in DAS, propagation planning, backhaul, project management and rollout of 2G-4G and TETRA technologies. Before joining Real Wireless, Bosshard was in charge of product management and strategy at Bluwan (2011-2014) and prior to that, he was the CTO at Urban Wimax (2007-2011). Bosshard started his career as electrical engineer in Switzerland. From there he moved into BTS and BSC site build management for Orange Switzerland, where he also started his RF planning and optimization career before moving into the lead of the national special projects team (key achievements, bringing multi-operator coverage to airports, trains and 200 km-plus of railway tunnels for all Swiss operators, using active and passive DAS) and representing Orange Group in special projects working groups internationally (1999-2004). He then founded his consultancy company working in different projects, technical consultancy in Africa and network rollout management in various Caribbean islands (2004-2005). At Andrews (Commscope), he was head of the “Wireless Innovations Group U.K. and Ireland” where he built a team for design, deployment, project management and monitoring of Wembley Stadium O2 Dome and Glasgow Underground DAS, among others.
Editor’s Note: Welcome to Analyst Angle. We’ve collected a group of the industry’s leading analysts to give their outlook on the hot topics in the wireless industry.
