Timing is Everything: Navigating important timing component choices for small cell designers

TimingThe small cell industry now addresses a much greater scope than just the initial residential femtocell. This has implications on one critical and potentially costly component which every small cell must have – a timing oscillator. The low cost residential femtocell oscillators aren't suitable for all applications; so what are today's choices available to small cell designers and what factors determine component selection?

Femtocells pioneered a very cost effective solution

In the early days of residential femtocells, it quickly became clear that a very low unit cost would be needed for the on-board frequency timing component. Even though the 3GPP standards body relaxed frequency tolerances for 3G femtocells from 50ppb (parts per billion) to 250ppb, with the industry typically specifying 100ppb - it's still a demanding goal to meet.

Traditional large macrocells used OCXO (Oven Controlled Crystal Oscillators) which can cost many hundreds of dollars each, so instead the industry turned to TCVCXO (temperature compensated/voltage controlled oscillators) which could meet both the price point and the technical performance.

[One of the key performance parameters in femtocells – the frequency stability – is supported by the oscillators. The primary component inside the oscillator is a crystal which exhibits frequency variations when subjected to temperature changes.] While OCXO's effectively contain their own thermal chamber that keeps the crystal at a constant temperature regardless of the outdoor conditions, a TCVCXO characterises the performance of the crystal over the operating temperature range and uses temperature compensation algorithms to achieve a much better performance. The voltage control function is used to keep it within parameters.

Rakon was one of the few companies to develop a product for that market, gaining many early design wins with their PLUTO oscillator which has been deployed in the majority of femtocells installed to date.

A wider range of small cell environments

The first major change in small cell requirements was when they began to be used outdoors. Typical operating temperature ranges for indoor femtocells and enterprise small cells would be 0-30C – few people like working below zero or above 30C. Once installed outdoors and exposed to direct sunlight and/or harsh cold weather, the operating range expands to anything up to 85C or more and down to -40C or less. Both these extreme conditions can be experienced daily, so that rather than a fairly constant temperature the oscillator is exposed to frequent and dramatic temperature changes.

Recognising the costs involved in meeting every corner of the performance envelope, we've heard of banded performance requirement specifications, where less accuracy is tolerated at the extreme ends of the temperature spectrum. This is a pragmatic approach for small cell designs which are exposed to extreme temperatures and such a wide range of operational environments.

LTE tightens the specifications further

LTE as initially deployed isn't perhaps quite as demanding, but some LTE-Advanced features and all TD-LTE modes require phase alignment, not just frequency tolerance. Future-proofing any LTE small cell product today involves taking a close look at this aspect, with phase accuracy specified to within 1.5us (microseconds) and some operators (e.g. EE UK) specifying 0.5us.

Maintaining phase accuracy when offline and disconnected from the main timing source is much harder for phase timing. Where an oscillator might retain frequency accuracy even after network synchronization is lost (a capability termed holdover) for up to a month, the same product could lose phase timing synchronisation within a few hours. Although base stations would go offline when the backhaul link fails (because it can't send/receive data for end users), alternative timing sources and/or longer holdover time are very important to ensure it can restore full service immediately the fault is rectified. Otherwise a cold start may be needed, taking precious minutes to reacquire synchronisation before coming back online.

Making the right design choice

To address these different and demanding requirements, oscillator vendors have launched a wider range of products to choose from, each with their own pros and cons. For example, Rakon's latest PLUTO+ product has both improved frequency vs. temperature performance and improved Phase Noise to meet the LTE RF requirements. Another recent product launched by Rakon for the evolving small cell applications is their MERCURY OCXO. This new OCXO technology bridges the gap between conventional OCXOs and TCXOs for performance, price, size and power consumption.

Here's an example of the product range offered by Rakon:

 Small-Cell-Timing-Requirements

Factors affecting component selection

Small cell designers are faced with a number of important design parameters when choosing timing and frequency control components and they are often required to assess the implications of the following competing factors:

Power Consumption: Small cells powered via Ethernet, or with tight energy budgets (including heat dissipation), may not cope with the higher power consumption of an OCXO.

Physical Size: Overall product form factor may encourage the use of a single small package rather than separate crystal and ASIC parts.

Holdover Times: Especially important when recovery time from backhaul outages is important, such as higher profile public venues and/or where covering "notspots" (coverage holes) rather than just adding capacity.

Component Lifetime: Small cells are a major investment of which the manpower cost to replace/upgrade them is a substantial part. Confidence that any installed equipment will continue to run to specification for years without site visits can justify a relatively minor additional component price that buys higher quality.

Partnerships: Oscillators, just like any other part of the solution, don't work on their own. Designers gain confidence when working with vendors who have developed partnerships, built or joined vendor eco-systems, and become embedded in reference designs.

Supply Chain: Procurement departments don't just want the best price, they want confidence that vendors have enough manufacturing capacity and cashflow to supply their needs. Access to multiple manufacturing facilities and the option to second-source key components helps address that concern.

Cost: Price is not the prime consideration in component selection, but it's an important factor. Cheapest may not be the best choice in the long run considering the importance of the competing design parameters listed above and the choices and trade-offs that small cell designers have to juggle.

An oscillator vendor's view of the market today

Mary Carbin, Business Development Manager at Rakon, has been involved with the small cell industry from the outset. She told me that the company had worked closely with most of the industry, developing specific products to meet the requirements, and meeting the (often highly unpredictable) order intake. Despite having to second guess the order volumes for months ahead, they've not failed to deliver.

She tells me that today's small cell designers are making different design decisions even when developing similar products. Some companies are choosing the more expensive and power consuming OCXOs, while others choose lower cost options. This means that operator procurement departments should take more notice of the underlying components in the products they buy, and not simply compare based on price and feature list. Just as when consumers buy any domestic electronic equipment, what initially appears to be the cheapest option may turn out to have hidden costs and consequences later.

Our thanks to Mary Carbin, Rakon, for assistance and insight when preparing this article. For more information about Rakon oscillators, read their small cell oscillator factsheet at www.rakon.com

Rakon is a sponsor of ThinkSmallCell.

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