This popular US event came to Europe for the first time and drew a larger than expected crowd. Although the full event title includes Small Cells, I think it's fair to say the content and exhibits were very heavily DAS focussed. It provided a good perspective of how DAS products are evolving, but I felt was still somewhat US market oriented and lacked a deeper European operator perspective. Frankly, I'd hoped for a bit more lively debate with the small cell community too. I attended only the first day of this two-day event, which seemed oversubscribed and more popular than anticipated.
Analyst Earl Lum (EJL Wireless) opened with an excellent "state of the nation" view of DAS and related technologies. Distributed Antenna Systems combine and relay the RF signals from a centralised set of macrocell basestations throughout a building or campus. This competes with a Small Cell architecture that requires only simple Ethernet backhaul but is typically tied to a single network operator.
He highlighted the gross inefficiency of connecting full size basestations to Active DAS systems, pointing out that Alcatel-Lucent and TE Connectivity had removed the RF encode/decode stages by directly connecting CPRI. Jim Parker from AT&T stated this had saved CAPEX costs of $70k at one venue, with further OPEX savings from lower power consumption. However, Earl thought this might be a "one hit wonder" because CPRI is 90% standard and 10% proprietary. Others I spoke to thought there was a 50/50 or greater chance of other vendors providing CPRI direct interfaces in future. A possible concern is who would be ultimately responsible for the end-to-end RF performance – is it the basestation equipment vendor or the DAS vendor?
There are many new types of Digital DAS systems, where the RF signal is digitised. Some allow reconfiguration of the macrocell sector capacity to different RF heads. These are more expensive than simpler systems and it remains to be seen how strong market demand will be.
With deployment of LTE, some operators want to make full use of MIMO which requires double the capacity and double the cabling to the radio heads. Some of the latest DAS systems can concentrate all of this into a single fibre, while older systems need a separate fibre run per radio. Modern MIMO antenna designs are available in compact size. The cost of retrofitting this additional cabling can be significant. A lot of DAS revenue comes from such upgrades rather than just the initial deployment.
As we move into LTE-Advanced, the issue of phase synchronisation becomes more significant. This isn't required for Carrier Aggregation or VoLTE, but is needed for eICIC, MBMS and CoMP. Delegates I spoke with didn't all agree that these features were urgently needed for many in-building deployments. Earl does seem demand and thought that 2015 would be the year of in-building phase sync.
Independent In-Building Wireless Providers
Scott Coates, CEO of UK based Wireless Infrastructure Group, explained how their business has evolved from running towers (about 3,000 in 3 countries today) into installing and operating DAS systems. They invest and own in the DAS infrastructure, leasing out connectivity to the mobile operators.
What's new is that building owners, who would previously insist on charging rental fees to operators, are now prepared to contribute towards the cost of the system. This makes the business case viable for Tier 2 and 3 sized buildings where this previously wasn't possible.
When explaining a case study of a large installation for 2 million square feet, he also mentioned how the system was upgraded at considerable expense because Vodafone wanted use MIMO for LTE. This needed additional cabling, and antenna swapouts. The cost was born by that operator alone, although there is the opportunity for reduction in future if another operator decides to make use of it. In future installations, they'll almost certainly install MIMO capability from the outset, even where operators don't initially ask for it.
An advantage of there being a neutral party hosting the DAS system means that agreements can be reached where one operator wants additional features (such as MIMO) or only to cover a smaller part of the building(s). This may not be so easy when a competitor owns the DAS system and sublets it to others.
Focussing on easier deployment
iBwave explained how the same overall processes are used to design, plan and commission in-building systems, whether DAS or Small Cell. Their tablet based planning App is being integrated with other Apps that can measure and capture RF details during the site visit. One example is PCTel, who were showing some pretty nifty/compact/sophisticated walk test/measurement scanners. With costs coming down for all types of in-building wireless solutions, there is need for greater focus on deployment costs and skills which tools like these are achieving.
Benoit Fleury also reported some lively debate at their recent European user conference earlier in the week. It was clear to him there is much less consensus among European operators than in the US on how to tackle in-building cellular demand, something that didn't (for me) come out quite as strongly at this event.
Some technical stuff
Commscope gave a detailed technical presentation on just how different GSM and LTE are – you can't just upgrade the GSM basestation driving an existing indoor DAS and expect it to deliver high LTE data throughput. One reason is the spectral power density – a GSM signal uses 0.2MHz of bandwidth compared to 10MHz or 20MHz for LTE, a factor of 100. This means the radio signal is spread much more widely. In addition, a much better signal to noise ratio is needed for the high modulation rates to achieve the high speeds from LTE. This is further compounded by outdoor cell dominance, which can be a problem at the edge of the building and easily mitigated in GSM through simple configuration but is more complex when sharing the same frequency.
To compensate for these factors, you have to allocate most of the RF transmission power for LTE to achieve the same footprint from a radio node. For a high power 20 Watt RF unit, that would be 0.8W GSM vs 19.2W LTE.
Microlab discussed the issue of PIM (Passive Inter Modulation). This is an issue for higher powered (say 5W and above) RF units, caused by imperfections in any of the metal components or materials in the signal path. It's not really relevant to low power (sub 1W) small cells. Broadly speaking, as we stretch RF signals towards the theoretical limits of Shannons Law, degradation of the signal anywhere makes a noticeable difference to system performance. One large US operator is insisting that all basestation components are independently tested for PIM, and then a further system test when assembled.
It boils down to whether in-building cellular is about coverage or capacity
I heard some surprisingly different views about what the industry is really trying to achieve. Some feel that Enterprise Wi-Fi will satisfy the majority of high speed data traffic, especially in offices. This leaves the in-building cellular system providing excellent voice service and reasonable data service when moving around, enough for email but perhaps not excessive high speed video.
Others want to deliver outstanding LTE throughput everywhere in-building, with data speeds of multiple 10Mbps or more. LTE at over 50Mbps speeds can be achieved with carefully designed radio heads (DAS or Small Cells) positioned every 20 metres apart, ideally using 2x2 MIMO.
Circumstances will vary widely depending on building size, use case and budget.
Some other assorted nuggets
- Many retailers are asking (and prepared to pay for) in-house systems, especially where they are demonstrating/selling wireless systems. This includes Apple stores, operator's own stores, even car dealerships selling the latest "connected cars". EE has equipped some stores with 4G femtocells where there is not yet coverage. Venue owners (eg hotels) are also much more open to contributing towards the costs of good cellular serivce for their properties.
- AT&T's small cells deployed from next year will be multi-mode (3G, LTE and Wi-Fi). When fully accepted, existing 3G only small cells will be upgraded to the new product. I checked later to confirm this didn't apply to residential Femtocells, he was referring to operator deployed Enterprise and Urban ones.
- After DAS vendor Powerwave went out of business, few wanted to maintain those systems. AT&T procurement had to resort to eBay to source parts!
- EE is trialling a small cell solution for rural communities. LTE is their primary driver for small cells, although they do supply 3G residential small cells today for consumers and small businesses.
- China's huge LTE rollout (both FDD and TDD) is placing enormous strain on the component supply chain, and will continue to do so through 2016. Beware long lead times.