David is very upbeat about prospects for metrocells in the near future. With a growing pipeline of trials around the world, Sprint have just announced that Alcatel-Lucent will be one of their key suppliers for LTE. I asked him clarify some of the issues around metrocells.
What's the difference between a metrocell and other types of small cell?
Metrocells are an operator planned deployment located in unsupervised areas and so must be more robust to avoid tampering or abuse. Outdoor versions must also be fully weatherproofed, not just to account for rain but also a wide temperature range. In all cases, these devices need to be unobtrusive – typically looking about the size and shape of a burglar alarm box.
This contrasts with a residential femtocell which are unplanned deployments and where cost has been a primary driver. Those units do not need to cater for such a wide temperature range, weatherproofing or physical security implications.
What is the scope of the recent Sprint deal just announced?
This is exclusively for LTE, which Sprint are aggressively rolling out nationwide. They are already promoting the technology hard and have said they expect to install 10's of thousands of small cells. Alcatel-Lucent small cell portfolio doesn't include CDMA technology which Sprint also use today, so the deal doesn't impinge on the almost 1 million residential 3G CDMA femtocells which Sprint have already deployed.
Alcatel-Lucent's solution builds on many years of experience deploying small cells of all types. Our Metrocell Express isn't just about buying the box, but includes all the other services from business strategy, system architecture, design and deployment.
The deal includes many products from our LightRadio portfolio, so that Sprint can call off whichever products suit their network deployment needs at that time. So I can't tell you what the mix of indoor vs outdoor, macro versus small cell that the network will evolve to – you'll have to ask Sprint themselves for that detail.
Would you recommend that the small cells have their own dedicated spectrum?
Of course, it's ideal to be able to allocate dedicated spectrum, such as a 3G carrier, exclusively for the small cells. Many operators today are constrained with limited amount of spectrum, so share the same frequencies (carriers) between both macro and small cells. Our solution supports both approaches. In the medium to long term, as a higher percentage of the traffic is handled through small cells, I expect more operators will be able to adopt the approach of dedicated spectrum.
We are already seeing some operators target Metro small cells for data only traffic from slow moving or stationary users. Voice calls and faster moving (e.g. vehicular traffic) is handled on the wide area macrocells.
What is the mix between indoor and outdoor metrocells?
Coverage in public spaces is the key aspect here – we aren't talking about residential or enterprise areas. In some cases, the distinction isn't entirely clear – for example, is the Olympic Stadium indoor or outdoor when much of the audience is sitting under a roof?
What is needed is a toolkit approach, where there are range of options and choices. For example, we've seen greater interest in our "Cube" technology rather than our older omni-directional metrocells. This is because they radiate the RF signal directionally, which means they can be located closer to the macrocell center – . This is key to opening up more potential locations for Metro small cells which would otherwise be rejected.
What Backhaul and Power is required
The two critical external capabilities required when installing any metrocell are power (which can be as much as 60 W peak for the largest metrocell) and backhaul (to take full advantage of small cell capability, typically of the order of 20 to 100Mbit/s depending on 3G or LTE).
The high data throughput of Metrocells justifies the use of Ethernet for the backhaul (the connection from the small cell to the central network switching centres). This can be carried over a wide range of different technologies, from DSL, Wi-Fi, microwave or fibre.
In some cases, Wi-Fi can be suitable for the last few hundred meters if there is no significant contention – this approach has been used successfully in a number of deployments and Point-to-Point microwave also have an important part to play. What is key is to use publicly available technologies.
We've found that DSL is also a possible backhaul solution, but have found cable modems can prove to be more of a challenge to provide consistant QoE as bandwidth is shared so, during peak usage the "speed" can be significantly degraded.
Timing and Synchronisation for metrocells
Timing and synchronization methods which use the backhaul may also affect the choice of transmission technology. However, in the US, all small cells (femtocells and metrocells) have GPS built-in. This is essential to meet the E911 emergency calling requirements, where an accurate location for the device is needed. As a by-product, very accurate timing and synchronization signal is provided.
In most cases, the GPS signal can be received sufficiently – it doesn't need to be continuous – but some indoor locations do require an external GPS antenna with its associated cable run.
What happens when LTE Advanced comes along?
Our metrocells are designed with a modular approach – the radio units themselves are plug-and-play modules which can be quickly and easily added or swapped during an upgrade process. This means that only a short site visit is required to make the change, with remote management handling interim software upgrades and configuration changes.
This ensures that operators who install metrocells today, gaining experience and securing the premium on-street locations, are well positioned to expand and upgrade cost effectively in the future.
Will they blend in?
Just look below at a picture of a white metrocell unobtrusively positioned on a supporting girder, similar in size and format to the loudspeaker system.
