DAS (Distributed Antenna Systems) have come under a lot of criticism from the small cell community in the past, being positioned as expensive, difficult and time-consuming to install. We thought it time for a DAS vendor to put their side of the story, and spoke with Tony Lefebvre, Director Product Management at TE Connectivity, to highlight the tradeoffs.
We concluded that both DAS and Small Cells are needed to meet future needs, with different scenarios where each could be more suited.
Comparing the fundamental architecture and components.
In both cases, cellular signals are transmitted from multiple RF radio heads spread around high traffic areas.
- DAS systems comprise relatively dumb radio units connected usually by dedicated fibre cables to a central machine room/hosting location, equipped with racks of traditional macrocell basestations. The baseband processing happens in the basestations, allowing additional capacity to be installed at the central location. DAS systems can be shared by multiple operators, each connecting their own basestations to the shared RF distribution system. Multiple radio heads can simulcast the same RF signal, allowing the same capacity to be delivered over multiple floors of a building or campus areas. Wideband designs allow each DAS radio head to transmit across all licenced bands and for multiple operators simultaneously.
- Small cells are designed with full basestation capabilities and processing onboard, reducing the requirements for backhaul to IP broadband, usually over Ethernet. They implicitly provide additional capacity through frequency reuse, with each unit providing an additional sector. Small cells are usually single operator and single frequency band today, although multi-band/multi-technology products are becoming available.
Initial installation cost/time/expertise
Tony Lefebvre, Director Product Management at TE Connectivity, a DAS equipment vendor, gave his view of the trade-off.
"If existing Ethernet cabling is already available, then obviously using it with pico/femto would give an installation advantage. But if it needed additional Power over Ethernet (PoE) or separate backhaul cabling, then it may make little difference whether the new cable is pulled through to support DAS or a small cell. So the 'Day 1' installation cost can vary significantly depending on whether reusing existing cabling or not.
"Careful technical planning and design of DAS installations is required, which certainly requires specific DAS design expertise. A lot of that deals with planning for future growth – looking at the number of bands, services and multi-operator sharing. While this involves more design effort up front, it allows each installation to grow more easily and achieve a longer and more profitable lifetime.
"Onsite, installation of a 'box on the wall' requires comparable skill levels whether for a DAS radio head or a full Small Cell. You still need to use some common sense about where to position the antenna. We continue to make great strides to simplify the commissioning of DAS products, but a lot of the complexity arises from the need to support multiple services and basestation types throughout a single installation.
"Recently, we've noticed that LTE is becoming more straightforward in this respect – for example, the macrocell basestations are easier to configure."
Comparing the cost of a DAS radio head vs Small Cell
"In simple terms, the cost of a DAS radio head may be more expensive than a single small cell. But you have to ensure you are comparing 'apples with apples' – the total traffic capacity of each individual DAS radio head is much higher because it can handle multiple bands for multiple operators simultaneously, where a small cell might only support two or three bands for a single operator.
"Perhaps a fairer comparison would be measured in cost-per-Mbps of total throughput or cost per operator. I think it's incorrect to say DAS is expensive or doesn't handle capacity. Of course, if you are only going to do a single band, e.g. 2.6GHz with a single protocol, then certainly DAS doesn't make sense – you still need a base station source and another device to transmit at the antenna - but once you add additional bands and serve multiple operators, that's when cost per Mbps starts to decrease."
Comparing equipment lifetime
"Some of the latest Small Cells are engineered with enough processing overhead and capability to support future releases of the standards, such as LTE and LTE-Advanced. They can also be remotely updated and configured from 3G to LTE in the same frequency band. However, more significant changes, such as introducing new bands or new technologies may require hardware swap-outs.
"If designed properly, DAS systems can cater for frequency refarming and future changes of bandplans without the need to update the remote units, giving a lifetime of 5 years or more. Most systems installed in the last 2-3 years cater for today's LTE allocated band plans, although it would be fair to say that some of the older sites may not."
Examples of situations we think DAS would be more suitable
"Looking ahead, we foresee a broad ecosystem for the wireless systems. There are certainly going to be many places where small base stations make most sense but there will also be locations where a combination of small base stations, DAS and Wi-Fi would be more appropriate. When we look at small businesses of 100K sq feet and below, these would be good areas for Small Cells, because a full DAS would simply not be economic.
"As you get into larger buildings, DAS begins to become a more attractive option because you're looking at coverage (rather than capacity). At the next level up, in large public venues and stadiums, that's where we believe small cell and DAS certainly makes a lot of sense.
"In the past, some DAS systems were also used to provide Wi-Fi coverage. Today, for capacity we'd recommend installing remote APs (Access Points) throughout the venue and using our system to backhaul that traffic. This allows maximum capacity and best use of our resources.
"The rationale is that you are serving more operators and multiple frequency bands, providing a much more complex system than for in-building. The crossover point in the US is where we get into multiple band scenarios, e.g. where Verizon and ATT are co-operating in a single venue and want to apply all their frequency assets via the DAS. This allows them both to have flexibility to independently add capacity.
"Launching LTE at 700MHz with a 10MHz channel and then adding a 1900MHz band later would effectively double capacity at each radio head without needing to make any hardware changes at the remote end – just adding baseband processing at host location/data centre. We are also seeing an uptick of neutral hosted systems for public venues, typically using a higher power radio head and efficiently distributing over multiple floors.
"Smaller service areas (sector sizes) can be achieved using lower power radio heads (e.g. 23dBm) which we use a lot in large sports stadiums, creating a lot of independent radio sectors – we've seen 20 and sometimes even up to 30 sectors of capacity in a US football stadium."
Key Feature Comparison: DAS and Small Cells
|Main components||Radio head, dedicated cabling, traditional macrocells||Small cells, shared IP, (Ethernet), central gateway|
|Radio Head||Dumb radio head, wideband handling most cellular frequencies||Fully capable basestation, manufactured for one or two specific bands|
|Power supply||Local power required if fibre connection, or via power over co-ax from nearby distribution cabinet||Power over Ethernet or local power|
|Backhaul||Dedicated fibre, optionally co-ax for last 10-15 metres||Any IP, typically Ethernet, better if assigned QoS/bandwidth|
|Host site||Usually onsite, equipped with large macrocell basestations, but can be located at Central Office many miles away||Usually not required for smaller systems; larger buildings may have small cluster controller box onsite.|
|Wi-Fi integration||Standalone Wi-Fi Access Points which can share DAS backhaul||Integrated Wi-Fi or standalone co-located APs|
|Installation expertise required onsite||Building electrician||Building electrician|
|Cost per radio head||Medium||Low|
|Method of capacity increase||Extra spectrum, then additional sectors with own radio heads, by adding extra basestation equipment in hosting location||Install extra small cells|
|Multi-operator support||Yes, in own spectrum||Typically no, except via roaming|
|Large public venues||Yes||Yes|