Nokia Solutions and Networks (NSN) recently announced their latest basestation product, an LTE only small cell aimed primarily at outdoor urban districts where traffic levels continue to grow rapidly. This is part of their Flexi Zone solution, aimed at sitting underneath an existing LTE macrocell layer (from NSN or another vendor), and meeting the high traffic demands through densification from large numbers of small cells.
We clarify the capabilities of the product, discuss why it's LTE only at this stage, and examine the reasoning behind a two-stage densification in urban areas.
Some small cell designs have been "bottom-up", with their origins in low cost and highly automated residential femtocells. These seek to achieve not just a low price point for the hardware but use specially designed smart software with high levels of automation to reduce the total cost of ownership. Several platform designs have increased in scope and capacity to handle large indoor/enterprise zones and outdoor street canyons, typically being based on 3G.
Three (High) Fives
NSN's approach has been to shrink their LTE macrocell down into a unit with the "three fives" metrics – 5 litres in volume, 5 Watts RF transmit power, 5 kg weight. I recall that their previous microcell product was "three sevens".
Think of the size and weight of a 5 litre oil carton, and you won't be far off.
Inside, a new chipset from the same family as NSN's macrocell products also runs the same software, benefitting from extensive field deployments, interoperability and maturity. The chipset is said to have sufficient processing power to handle hundreds of active sessions simultaneously, although in such extreme cases each user would only receive a relatively small proportion of bandwidth. There's also enough headroom to cope with future LTE-Advanced features from subsequent software upgrades. The software has had a few tweaks to handle small cell use cases, facilitating rapid deployment and SON (Self-Organising Network) optimisation.
It's a single LTE carrier product, supporting FDD bandwidth up to 20MHz. There's no 2G, 3G or Wi-Fi built-in, although Wi-Fi is in the roadmap and it can detect nearby 3G cells.
Other features include built-in GPS and 1588 timing sync, plus a Bluetooth connection for technicians to remotely configure the box from the ground nearby rather than having to physically plug a cable in.
Outdoor urban use
The relatively high RF power of 5 Watts means it's at the top end of what might be termed a small cell. By comparison, indoor enterprise units are typically 250mW or less. NSN really don't like using the term metrocell (perhaps because it's more closely associated with a competitor) and instead use the term picocell or microcell.
With the higher RF power, think of it covering a "hot zone" rather than a "hot spot".
As traffic levels continue to grow significantly, especially in demanding urban areas, planners recognise that they need to adopt small cells once the limits of their existing macrocell sites have been reached. I've heard many would prefer to do this using LTE rather than 3G for outdoor access, because investments would have a longer lifetime and LTE is technically more suited to mixing small/macrocells in an outdoor HetNet environment.
However, most users are still on 3G and their needs must be met also. Over the past year, we've heard a common requirement from operators to be for a combined 3G/LTE/Wi-Fi small cell. Several silicon vendors are actively bringing chipsets to market which support this, some capable of doing so within a low overall power budget. It may be that these will be more relevant for lower power small cells, say 250mW RF, used indoors and/or specific hotspot locations.
While it could be configured with a maximum power of 1W or 250mW for public indoor use, it would be physically larger than products specifically designed for that purpose. Perhaps some innovative pricing scheme will be used to compensate for that and make low power a more attractive option.
Two stage densification as outdoor small cells are adopted
As we start to see the densification of high traffic areas in our urban areas, it's natural that planners will deploy more and more small cells. NSN argue that initially these would be higher power, with a consequential larger footprint/coverage area.
This has two benefits:
- The precise location of the small cell isn't so critical
- Backhaul technology isn't so constrained
They argue that the larger coverage footprint of the higher RF power means that the cell can capture traffic from more users and so doesn't need to be positioned exactly in a given location. Conversely, it could be argued that peak data rates and high quality of service would be better if the resources were dedicated to a smaller number of users per cell (i.e. deploying larger numbers of lower powered small cells).
Subsequently, as more and more small cells are deployed, they would be configured with lower RF power and smaller coverage footprints. Their location is likely to need to become more precise to capture hotspot traffic, but they should be lower cost.
Consequences for urban backhaul
During this first stage of densification where there is more flexibility of the location, backhaul needs can be accommodated more easily. Fibre connections would be first choice when accessible, alternatively a clear Line-of-Sight wireless link using 60GHz or similar could be a good second choice.
Subsequently, as numbers grow and location is more important, we may expect greater use of Non-Line-of-Sight (NLoS) backhaul equipment. NSN offer the Dragonwave sub-6GHz NLoS in a separate box as an option today. NLoS backhaul is likely to be integrated into a future product.
Of course, other backhaul vendors may take a different view on the choice and timing of backhaul architecture evolution.
NSN have taken a strategic decision to focus on LTE rather than 3G for their urban small cell product. The high RF power and compact size will be attractive to network planners looking to take their first steps in densifying their network capacity. It sets the benchmark for higher power LTE small cells in terms of size, weight and form factor.
Whether an approach focussed entirely on LTE and using relatively higher RF power is the best (or only) strategy remains to be seen. It seems likely to me that some deployment of lower power indoor and/or 3G small cells would also be required, depending on the circumstances and strategy of each mobile operator.
Our thanks to Stephane Daeuble, Senior Product Marketer for Small Cells at NSN, for briefing us on this new product.