Aggregation is being used to meet demand for faster data rates, by combining multiple channels at different frequencies and even different radio technologies. LTE, Wi-Fi and LTE using Wi-Fi spectrum are all actively being developed. We look at the options on offer and consider the tradeoffs.
LTE Carrier Aggregation
One of the most popular features of LTE-Advanced has been Carrier Aggregation. This combines two data streams from two or more different frequencies to achieve higher peak speeds. There’s been a competitive element with vendors and operators keen to announce and launch ever faster service. This article outlines ten operators who deployed it before the end of 2014.
For those with more spectrum, it can be relatively easy to achieve very high rates. EE UK combine 20MHz bands at 1800MHz and 2600MHz to deliver 300Mbps in certain areas of Central London. Singapore and Switzerland have also benefitted with a similar approach.
For those with less spectrum, or with smaller allocations split between different bands, it can be more awkward. AT&T and Verizon are combining 700MHz and AWS spectrum which should deliver 225Mbps. South Korean operators have achieved a similar configuration but are further ahead with deployment.
We have also seen aggregation of both FDD and TDD modes, and some using up to four bands.
Keysight Technologies published an excellent webinar slidedeck explaining the standards and technology operation in more detail.
And just to show how far this could be taken, Nokia combined 10 different bands in their lab comprising 200MHz of bandwidth to achieve almost 4Gbps. However, I doubt if this would be much use for portable devices due to the high power consumption.
Using three or four bands to achieve 300Mbps requires the latest Class 6 smartphone devices such as the Samsung Galaxy S5 or Tri-Band Note 4 and so are not immediately available to all. Dual carrier aggregation up to 150Mbps is more common in many existing Class 4 products based on Qualcomm’s Snapdragon 801 chip.
Several small cell vendors already offer dual carrier LTE aggregation, such as Spidercloud who announced it at Mobile World Congress earlier this year.
Two ways of combining LTE and Wi-Fi
Alcatel-Lucent made a big splash at Mobile World Congress this year, creating some clear marketing terms for the two main ways of combining licenced and unlicensed spectrum. They termed these:
- Wi-Fi Boost: Combining and co-ordinating standard Wi-Fi with LTE, each running in their own spectrum. Also known as LWA (LTE Wi-Fi Aggregation)
- Carrier Boost: LTE Carrier Aggregation with LTE using unlicenced spectrum. This is more commonly known as LAA (LTE Assisted Access).
In both cases, the primary connection remains with an LTE channel running in licenced spectrum. The additional data paths are used to increase bandwidth only when required, such as when watching video or other higher bandwidth services.
LWA = Wi-Fi Boost – A faster route to market
Mike Schabel, VP Small Cells at Alcatel-Lucent, points out that many of the challenges with Wi-Fi relate to the uplink. Using Wi-Fi for downlink and LTE mostly for uplink brings together the best of both worlds. This can be done by routing traffic across both wireless interfaces and through a central anchor point in the mobile core network (EPC). Traffic can be steered between both modes.
Their initial studies indicate that moving the Wi-Fi uplink traffic to LTE doubles the range and increases downlink throughput by anything from 20 to 70% depending on congestion levels. Cell edge performance improves dramatically by 10 to 50 times.
Perhaps the best thing about this approach is that it shouldn’t require any new hardware. While adding this mode isn’t trivial, it should be feasible to implement with a software update to existing smartphones and software features in the network.
Trials of this architecture are planned for Q2 2015 with commercial support later in the year. Alcatel-Lucent are partnering with Ruckus Wireless on this, with both seeing it as a major opportunity.
Trials using this or a very similar architecture have just been announced by Google with their Project Fi service. This combines Wi-Fi with LTE from Sprint or T-Mobile, choosing the best path for traffic at any time. Voice and data would be routed via a selection of validated Wi-Fi hotspots where quality of service is good enough, and through LTE when moving around. The service only works with a specially customised Nexus smartphone.
Cellular Boost = Licenced Assisted Access, bringing LTE to unlicenced spectrum
Several companies are working hard to promote LAA, which runs additional LTE channels in the unlicenced frequencies at 5GHz. This is perfectly legal in several countries (USA, China, South Korea etc.), but Europe and elsewhere will require technical changes (such as Listen Before Talk) before it will be permitted. Some critics are concerned that the performance of LTE adapted in this way will be so limited as not to be worthwhile, but this is yet to be proven.
From a network viewpoint, this initial configuration is little more than running LTE at yet another frequency band and should be fairly simple to implement. Ericsson has confirmed that all its small cell designs can support this mode and promised commercial product before the end of the year. Qualcomm has been championing this approach for some time and developing product to support it.
Again, suitable handsets will be required which support this mode. This isn’t a simple software upgrade and will require physically different RF hardware.
Summary of terms, changes and timescales
|LTE Carrier Aggregation||CA||Recent Class 4 phone models 150Mbps;
New Class 6 required for 300Mbps
|Usually only a software update||150Mbps and 300Mbps already commercially deployed.|
|LTE/Wi-Fi Aggregation||LWA||Software update to smartphone||Software update and extra capacity in EPC (Core Network)||Trials during 2015|
|LTE Assisted Access (US/China)||LAA||Hardware changes will require new smartphones||Add new LTE frequency band. Some small cells already designed to support this.||Planned deployment late 2015|
|LTE Assisted Access (Europe etc.)||LAA||Hardware change will require new smartphones||Add Listen-Before-Talk capability in hardware/software||Standards not yet agreed.|
Some considerations of the choices to be made
LTE carrier aggregation uses spectrum below 3GHz which has longer range and is more predictable. Where bands are closer together, radio planning and coverage footprints should be similar. It may be most difficult to achieve when combining 700MHz and 2600MHz.
Wi-Fi Boost makes use of a relatively unpredictable and diverse data paths. Variable delay will impact the peak performance especially for latency intolerant applications. This may be a good compromise for watching video while still being connected with a good uplink for voice or gaming interaction. The high frequency spectrum at 5GHz makes this more suitable for short range, indoor applications. It will probably work better in controlled environments in an office or large dwelling rather than in congested public gatherings.
Cellular Boost (LAA) should deliver a fast and good quality experience, achieving further range and good uplink performance. Again, the high frequency 5GHz makes it more suited for indoor use. Performance in Europe and elsewhere, once LBT is added, may diminish some of its advantages.
Many indoor small cells already have Wi-Fi in the box. It should not be difficult to evolve these to more integrated use, and many vendors have announced plans to do so. The coverage footprint may be more limited for the 5GHz band and this may encourage more to be installed. I would expect this to be most popular for use indoors.