Joe Neil, Director of Solution Architecture at Microsemi, travels the world working with mobile operators of all shapes and sizes. His role is primarily focussed on their network architecture and especially the design for timing and synchronisation. With most networks either trialling or actively operating LTE services today, he reveals some of his findings and expectations as the industry migrates towards more sophisticated LTE-Advanced features, HetNets and Small Cell densification.
How far down the road is LTE deployment today?
Pretty much all mobile operators are either trialling or actively rollout out LTE service today. All are FDD, apart from a few significant exceptions (China Mobile, Softbank Japan), and complement the existing 2G/3G service. The difficult issue isn't about data, it's voice service. Voice is heavily commoditised – 50% of infrastructure worldwide is still 2G GSM and that's where a lot of the voice remains. There's no pressing reason for operators to tear that investment out of the ground until they have consistent, reliable and easy to provision 4G voice service to replace it with.
So operators are primarily rolling out LTE to satisfy growing demand for broadband data. Some price plans are relatively data rich and bring in good levels of revenue (especially US and some APAC countries). Operators in other regions are still capturing the majority of their revenue from voice services.
Most LTE deployments are at higher RF frequencies, such as 2300MHz and above. This results in problems with in-building penetration from outdoor cellsites and limited macrocell range. One Indian operator highlighted these two specific issues at a conference recently, and intends their urban Small Cell deployment to be initially for coverage rather than capacity.
This still leaves the issue of how best to deal with in-building service – do you try to penetrate from outside or install equipment indoors? We clearly see strong competition between Remote Radio Heads (RRH), Distributed Antenna Systems (DAS) and Enterprise Small Cells who are all chasing this lucrative market.
There's no doubt we will need much a more dense deployment of radio heads, the debate is what physical form these might take and the marketing terminology used.
Using time division effectively at the cell edge
Rather than looking purely at the product architecture, I believe we should be focussing most on how to increase the efficiency of the system overall. The LTE Advanced standards have done that by developing features for both the FDD and TDD modes which add a time scheduling dimension. For example, ABS (Almost Blank Subframes) turn off the macrocell signal for short periods and allow the underlying Small Cells to transmit at the same shared frequencies. eICIC (enhanced Inter Cell Interference Co-ordination) synchronises the transmissions from hierarchical cells to maximise efficiency and effectiveness.
Alternatively, TD-LTE could be used to add capacity in dense urban and enterprise environments where the most capacity is required, complementing or co-ordinated with the macrocell layer. I've spoken to many operators who plan to evolve by deploying FDD mode first, then following up with additional TDD mode later.
The phase and time delivery work at the ITU-T has been working towards the goal of 1.5us phase accuracy at the small cell, which would be applicable to both FDD and TDD modes.
How do these features impact network design?
The use of TD-LTE or LTE-Advanced features such as these requires a carefully designed architecture to deliver phase synchronisation. TD-LTE networks have had to incorporate these from the outset. FDD-LTE networks can initially launch with just frequency sync, but will need to ensure an end-to-end phase sync solution is in place before they can fully adopt many LTE-Advanced features.
Synchronous Ethernet (SyncE) is popular and widely available in some countries, but less so in others. It hasn't taken off at all in the USA – there is just too much transmission equipment in the access network/at the edge which doesn't support it. In other countries, it's made its way to almost every macrocell. I believe that it's still up for discussion whether this will be the main solution for Small Cells.
Timing solutions based on packet transmission are greatly affected by jitter (variation in packet delay/latency). Classic DSL probably won't support phase timing to the required tolerance. Native GPON doesn't easily support PTP transfer. GPON 2 deals with this by injecting a timing signal at the physical level of the frame structure (similar to CPRI). By contrast, a managed Gigabit Ethernet, especially indoors, makes the transport of FDD timing signals relatively easy.
In the outdoor scenario, some of today's microwave backhaul products are already capable of transporting a frequency timing signal natively. That industry sector is working hard to achieve reliable delivery of PTP packets across the radio. We are still way off having an easy solution that works over multiple hops.
So if we want to deliver these LTE-Advanced capabilities, we need to find ways of capturing timing from GPS in a robust, telecom-grade way and propagating that to nearby Small Cells. If you can't put a GPS antenna on the roof (such as in very high rise urban zones with many skyscrapers), other methods are required. This is an area where Microsemi brings expertise. Fundamentally we have invested in solutions that bring frequency/timing and power (over Ethernet) to Small Cells and will continue to do so.
So why not just use Wi-Fi instead of complex phase-timed Small Cells?
Wi-Fi is also looking to increase its efficiency through MIMO (Multiple Input/Multiple Output) and other techniques. These will also require phase timing of a similar performance to cellular Small Cells. There is a trend to look at how Wi-Fi can be more closely integrated with LTE, and this could significantly change how Wi-Fi is deployed. There are many issues around logistics and regulations – for example, will network operators be allowed to deploy Wi-Fi access points with these functions, especially those with LTE built it. LTE-U, using LTE in the unlicensed Wi-Fi frequency bands, is another option being proposed. All of these options will need phase alignment to achieve their maximum potential.
We at Microsemi are thinking ahead on how that could work and ensuring that our strategy evolves to deliver the best power/sync/timing solutions for the industry.
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For more information about Microsemi's Small Cell solutions, visit www.microsemi.com