As networks grow and need to add capacity, there are several different ways to achieve frequency reuse. We compare and contrast four options.
First, a quick recap of the three ways to increase capacity on mobile networks...
These haven't changed since mobile networks were invented:
- Adding more spectrum. Not only is this scarce, but its often very expensive – a natural resource that governments find easy to tax. Quite a lot of new spectrum has been found for LTE, but there are so many frequency bands and modes (over 50) that this adds further complexity to both network and handset equipment. Compatibility becomes a major issue.
- Increasing spectral efficiency. LTE is close to reaching the limits of Shannon's law on what is physically possible. 3G/HSPA isn't that far behind. While there may be innovative techniques that can stretch this further, I can't help thinking these might place heavy technical constraints on the network. In the medium term, refarming existing 2G and 3G bands for use by 3G and 4G achieves much of this goal.
- Spatial frequency reuse. This is where the largest gains are to be found, hence the growing number of basestations and interest in Small Cells. This means adding more transmitters/receivers, allowing the same spectrum to be reused again nearby.
Four ways to add more sectors
1) Sector splitting
The simplest basestation configuration is a single sector uni-directional type. This is ideal for large rural areas with little traffic, where coverage is key. These can also be used to fill in coverage holes, again where high capacity isn't required.
When traffic levels grow at a site and it can't meet demand, additional antenna and baseband/RF modules are installed to divide the area covered into three equally sized sectors, a bit like cutting a cake. Different frequencies or spreading codes are assigned to each sector, effectively tripling the capacity. Individual sectors can be split further, say when there is a lot of traffic arising from a particular direction.
Sometimes up to six sectors are used. Each sector needs its own set of antenna (at least two for diversity) physically aligned, which can make them unsightly for the largest cell towers. Occasionally even more sectors are used, such as for special events/festivals, when they would each have a small coverage footprint and low RF power. In urban environments, high levels of multi-path signal reflection and other constraints make it difficult to predict the effects.
The main advantage is that further/additional cellsites aren't needed, saving on site rental, backhaul, site visits and additional equipment. So it's generally the first choice option for planners. Disadvantages include additional antenna (which may not be permitted for zoning/planning approval), less than ideal RF signal path (leading to higher RF power used, higher equipment cost, lower network efficiency, slower speeds) and higher cost of outdoor/high specification cellsites.
2) Remote Radio Heads
Rather than co-locate the additional sectors at the cellsite, it's possible to position them nearby. This allows additional antenna to be pointing more directly at the traffic sources, bypassing obstructions and other sources of interference.
In the simplest case, the Remote Radio Heads (RRH) might be fitted to each side of a large building. They can be as much as 1km away (or further). Usually they are outdoors but one example is to provide coverage inside a small shopping mall which would otherwise be inefficient to penetrate from the outside.
RRH effectively partition the traditional basestation into two halves. The RRH includes all the RF aspects and is connected by a high capacity link to the cellsite, where the baseband processing and control is handled. There is a standard for the RRH interface, CPRI (Common Public Radio Interface), which allows multi-vendor interoperability. The CPRI signal is typically around 50x the user data rate, so an LTE RRH would need about 2Gbps. This is normally connected via fibre, but there are point-to-point wireless links which are compatible.
The use of RRH gets around the issues of poor RF antenna positioning and can re-use the existing cellsite and backhaul. They are also seen as lower technical risk because they are an integral part of the basestation and tightly co-ordinated with it. Disadvantages are the additional sites (needing site rental, power, more site visits), high speed backhaul and relatively high cost of equipment.
3) Distributed Antenna System
DAS systems are mostly (but not always) deployed indoors, usually for large buildings, shopping malls, stadia and other locations with large numbers of pedestrian or stationary users.
As with RRH's, these transport and project the RF signal through a range of radio transceivers deployed throughout the building. Unlike RRH's, the input signal to a DAS system is normally the RF signal that would normally go directly to the external antenna. Passive DAS systems take this RF feed and distribute it through heavy duty co-axial cables to multiple antenna throughout a building. These can be shared by several different mobile network operators by connecting their own basestations through an RF combiner. This was often fairly inefficient, leading to the expense of a high powered RF basestation as well as costly, inconvenient cabling throughout. Specialise RF design engineering and configuration skills were essential. Multiple zones or sectors could be achieved by separately cabling up different floors of a building or areas of a mall independently.
There has been quite some advancement in DAS systems over recent years, and the distinction between an RRH and DAS has become somewhat blurred. Active DAS systems allow the RF signal to be re-amplified, extending the reach. The RF signal can be transmitted over fibre and even (for short distances) over CAT6 copper wire. Zones/sectors can be reconfigured to handle different times of day.
Many have been specifically tailored to cater for defined frequency bands. This can cope with future technology changes (e.g. refarming/reassigning 3G to LTE) but may require a swap-out when new LTE bands are introduced. Few would support the 5GHz Wi-Fi bands. Some of the latest DAS kit works around these issues by catering for a wide range of bands (700MHz through 2.7GHz) and also transporting IP backhaul for standalone Wi-Fi access points.
A key advantage touted by DAS vendors is that these systems can handle multiple network operators. The rebuttal from Small Cell vendors is that the higher cost of DAS is often only viable when shared between operators.
A few vendors offer DAS systems that are much more like RRH architecture. These distribute the signal from a single large basestation to multiple radio heads over fibre/copper cables.
Some RF design skill is required to design, commission and optimise these DAS systems. In the past, project deployment costs can be 50% of more of the total budget. DAS vendors have worked hard to simplify and reduce the cost of deployment, relying on the built-in features of the macrocell to adapt and tune the system to the building environment.
4) Small Cells
Small Cells take advantage of great advances (and investment) in silicon and software technology to miniaturise a complete cellular basestation onto a chip. Each is a completely functional single sector cellsite in its own right. These are much more autonomous than an RRH, using built-in SON (Self Optimising Network) features to detect, adapt and optimise to their environment. For standalone units, such as residential femtocells, virtually no technical skill is required for installation.
A second key advantage is that the backhaul link can be almost any IP capable link, whether making use of existing office broadband internet or a dedicated point-to-point wireless link. The comparatively lower data rates (say 100Mbps instead of 2Gbps) makes these easier to engineer.
There is also no need for any extra equipment in the nearby basestation.
There are several different ways to increase the capacity of mobile networks. Spatial frequency reuse by adding more sectors or cells is by far the most effective.
The four different ways of achieving this will all be used to some extent in almost every network. Sectorisation and RRH deployments have been popular in the past but are reaching their limits. Over the coming few years, we can expect to see a transition across to Small Cells as the primary means of adding extra capacity onto networks.