Small Cell timing requires more complex design engineering

Engineering DesignAs the Small Cell industry evolves from its roots in 3G Residential Femtocells, the selection and design of fundamental components has also become more complex. More vendors are developing their own Small Cell products, LTE technology is appearing on the scene and use cases have expanded to harsh outdoor environments.

These changes lead to greater challenges for engineers addressing the tight timing and synchronisation side of product designs who are seeking the sweetspot which balances between cost, performance and functionality.

We look more closely at the engineering issues involved, and spoke with Pletronics, who offer some of the world's leading oscillator engineering expertise, to understand the issues and how they can be addressed.

The backdrop of growing Small Cell use cases and technology

Pletronics has observed increased activity in Small Cell designs over the last couple of years. Three specific trends stand out:

  1. We can see a wider diversity of Small Cell designs are emerging with most activity aimed at Enterprise and Metrocell products.
  2. LTE Small Cell designs are now appearing for both FDD and TDD modes (although not simultaneously). This has become much more active during the last 6-9 months, especially for TD-LTE.
  3. Multimode (combined 3G and LTE) products are starting to appear, taking advantage of some of the latest silicon platforms. Fully integrated dual mode chipsets are still at the sampling stage, but will become more prevalent during 1H 2014.

Korea is a currently a hive of activity for Small Cell design driven by its lead on LTE, although Taiwanese ODMs continue to support 3G products in volume and are also moving to support these trends. China and the UK are also very busy with new development.

A change of frequency

According to Pletronics, the increased requirements have led to several Small Cell vendors switching to another chipset platform. The principal silicon chipset players are actively jockeying for position.

The visible evidence for this is silicon platform change is the master clock frequency. Where 3G residential Femtocells typically used a 19.0 to 26MHz oscillator, they now see parts being specified for 30MHz to 40MHz.

This reflects not just the higher processing performance required but also the wider RF frequency bands used for LTE, which can be 10MHz or 20MHz rather than 5MHz for 3G.

A wider range of operating environments

Dave Kenny, Pletronics VP of R&D states: One can easily understand the wider operating temperature range of an outdoor Small Cell compared to indoors. We are normally protected from the harsh cold or extreme heat when inside, with an assumption that the operating temperature wouldn't go outside a range of 0C to 60C. Sometimes you'd be surprised at how hot an indoor unit can get if left on a shelf exposed to direct sunlight and how quickly the temperatures can change. You can also find Small Cells used in unheated warehouses, where temperatures dip below zero.

All of the above leads not just to a diverse range of Small Cell product specifications, but also leads to a wider range of oscillator requirements. Exactly the same Small Cell performance requirement from an operator can lead to different designers specifying different parameters and/or choosing different oscillator technology. Where a more expensive OCXO might meet one customer's needs, another may select a lower cost TCXO alternative.

This is leading engineers to become more active when determining and refining their specifications.

A triple dog bone specification

Pletronics refers to a three tier tolerance as a "triple dog bone" specification. What this means is that a looser performance is acceptable at more extreme temperatures. There are three temperature bands, each with upper and lower boundaries, above or below which the frequency tolerance is reduced.

triple-dog-bone

This is a good tradeoff for the designer, saving the cost of a part which would otherwise be specified for tight tolerances throughout the entire temperature range, while performing admirably in the vast majority of use cases.

A typical "triple dog bone" specification might be:

40 to 60C 80ppb
0 to 70C 100ppb
-40 to 85C 250ppb

There are very few situations I can think of which combine extreme cold and very dense population usage. One example I can think of is game I saw recently where the snow was falling heavily in the stadium, but I wouldn't expect the supporters to be making quite as much use of their smartphones in these situations. They're more likely to keep their gloves on!

Greater level of engineering design, test and verification

Pletronics has been surprised at the range of requests they receive from their customers. The diversity requires greater involvement and co-operation between their engineering team and their customers to tailor products to meet the needs best suited for each customer's application. Pletronics is a valuable resource for the customers and can provide advice on the best performance, create custom fixtures (often turned around within days) and supply specific test and measurementdata. Such specialized engineering resources provide the expertise during the design phase, where a customer's in-house resources wouldn't be as cost effective or may not have seen such a wide range of applications.

Working with customers often leads to new insights on how and in what situations these products are used. This feedback provides an opportunity for Pletronics to review their own product test processes. In reviewing these outcomes Pletronics will update procedures to match even more closely the actual environment in which they will be deployed. .

Close partnerships with customers

An example of one customer Pletronics has worked closely with is iPosi, who has developed their own high performance embedded in-building GNSS/GPS receiver for small cells and other applications. They have very specific timing and frequency stability requirements which focus more on obtaining the highest degree of short term stability (sub second to 100 second range) than over longer periods. Not only were Pletronics able to adapt a standard product to meet these specifications, but also proactively suggested some custom fixes to make their design easier to implement. They could also use their extensive in-house test suite to make a range of measurements that wouldn't be cost effective for their customers to do.

The evolution of Small Cells is helping drive innovation in the timing and sync industry. The growing diversity of Small Cell use cases combined with the advent of LTE is creating a wide range of engineering challenges to achieve the right mix of cost, performance and functionality. Pletronics is one of the few companies able to contribute specialist engineering expertise and resources to assist those developing all kinds of Small Cell products.

Our thanks to the team at Pletronics and iPosi for their assistance in writing this article. Both Pletronics and iPosi are sponsors of ThinkSmallCell

For more information about Pletronics, visit their website at www.pletronics.com

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