What’s the Use-By date on your small cell?

UseByDateMany consumer products have a planned short lifetime. Infrastructure investments such as Small Cells have a much longer lifecycle, helping their financial business case and reducing the risk of customer dissatisfaction from unexpected disruption. What are the underlying causes of short product life?



A related example

My bathroom extractor fan stopped working after a couple of years. It’s a marvel of Italian engineering, sleek and physical well designed with a rated MTBF of 30,000 hours (at 2 hours per day = 40 years). Mechanically there wasn’t anything wrong with it. On closer inspection, the primitive electronic timer circuit had failed and the simplest solution was a complete replacement.

In many cases, that would have involved two visits by a qualified electrician (one to diagnose, one to fit the part) and a delay of a few days to source the new part (there are at least 10 variants of the fan from this manufacturer alone). The total cost could easily become several hundred dollars plus all the inconvenience.

It’s an example where shaving a few pennies off the cost of an otherwise highly reliable product results in customer dissatisfaction, unnecessary downtime and inevitable degrading of product reputation.

How long are Small Cells designed to last for?

After many years of 3G small cell evolution, we’ve reached a stage where the silicon platforms and software stacks are now very mature.  More than 10 million units deployed in the field, high levels of integration, low component counts and years of software debugging have made this just another consumer item.

Unlike our smartphones, which typically get upgraded every couple of years or so, small cells are planned to have a lifetime of many years. I’ve heard a figure of seven years is commonly specified in network operator RFPs (Requests for Proposals) for in-building products.

Although some consumers turn their residential small cells off overnight, the vast majority remain powered up continuously for years. They’re kept indoors, remain within a fairly regulated and reasonable temperature range and rarely suffer from physical shock or movement. Their low power means they don’t need much airflow.

This is an almost ideal environment for well designed electronic consumer products, and the silicon parts can be expected to continue working for decades.

Outdoor small cells, both urban and rural, justify higher specification parts – they carry more traffic, the cost of maintenance/replacement is much higher and the consequence of outages is more severe.  We’ll look at those issues at another time and focus on in-building products here.

What most affects the lifetime of an In-building Small Cell?

The one component which most limits lifetime is the crystal oscillator which has a quartz core. The natural physical properties of the quartz age over time resulting in small but discernable changes to the timing characteristics. These might not be noticeable in a watch, which measures time to a tolerance of a second or two per month, but are critical when acting as a source for network timing.

Frequency timing tolerances are measured in parts per billion for 3G, and phase timing is even more demanding.

A few pennies shaved off the product cost here would have significant consequences for the expected lifetime, shortening it by years. Low end GPS grade TCVCXOs would be lower cost for sure, but would likely fail the synchronisation and phase timing requirements after a short time.

Some react to this by saying it’s not feasible to withstand the much higher cost of a more complex part, such as a full OCXO (Oven Controlled Crystal Oscillator) rather than the much lower cost TCVCXO (Temperature Compensated Voltage Controller Crystal Oscillator). Mary Carbin, Business Development Manager at RAKON, tells me that’s quite unnecessary for low cost residential and SOHO products. Having pioneered a range of low cost TCVCXO’s for residential femtocells, RAKON has already delivered millions of products will exceed the lifetime without blowing the budget. Only TCVCXOs with tilt compensation and 100% FvT screening can achieve both performance and cost targets while avoiding the need for more expensive OCXOs.

It may simply be a case of being more rigorous when selecting the oscillator manufacturer to ensure that the long lifetime specifications will be met.

The financial consequences can be significant

As with my extractor fan example above, the financial costs of early product failures can mount up and would include:

  • Handling of one or more support calls to determine the root cause
  • Full cost of replacement unit
  • Possible site visit to diagnose/install/test unit (more likely for SOHO and small business users)
  • Consequences of reduced service quality and outages (potential for customer churn, loss of reputation)

The other main reason for replacement

The other limiting factor on product lifetime would include a technology refresh, such as upgrading consumers and businesses to LTE. Some existing 3G units may simply be thrown away, but many could be recycled for use in other countries or by other users. Their value would be more likely retained if they could be expected to have a longer working lifetime.

Just as many smartphones are recycled today, it’s not unreasonable to expect that to happen with residential small cells. Remember that 3G won’t peak worldwide until 2019, when 48% will be using it globally.

TD-LTE probably has the toughest and most demanding oscillator specification

Several Asian countries such as China, India and Japan have started deploying large TD-LTE networks and are expected to grow these with substantial volumes of small cells.

This technology is probably one of the most demanding for timing and sync, requiring phase synchronisation tolerance within 1.5uS. While again there are a variety of different oscillator technologies and products which meet that spec, it would be foolhardy to scrimp on the component cost without appreciating the consequences on product lifetime.


Today’s planned product lifetime for in-building small cells is at least 7 years.

The most critical component affecting that lifetime is the crystal oscillator.

Careful component selection and vendor evaluation is essential to ensure that a long lifetime can be met, avoiding substantial financial consequences in later years. This is an aspect which could be easily overlooked by zealous and highly cost conscious procurement departments.

It would be a shame if the Small Cell industry suffered the consequences of an unnecessarily poor reputation by shaving a few cents off the parts list.

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#1 Art King said: 

Fully agree. Everything has to be done with a long term sustaining view.

0 Quote 2015-07-23 19:33
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