Small cells installed inside buildings often depend on GPS to provide synchronisation, but indoor reception is not always possible. The cost of installing an external GPS antenna can be prohibitive. Microsemi explained the issue in detail and discussed ways around the problem in a recent webinar, which we summarise below.
The need for high precision in-building timing synchronisation
Small cells operate most effectively when they are tightly synchronised to a common timing source. While that’s important for 3G, it’s essential for LTE. Some of the more advanced LTE features such as CoMP (Co-ordinated Multi-path) specifically require phase timing, rather than just frequency synchronisation, and a typical specification is 1.5 microseconds or better.
Several different possible sources of phase timing could be used and ideally at least two alternatives should be available to handle resilience in the event of failure.
The main timing sources are:
- GNSS (usually GPS): Provides very high accuracy but potentially a problem indoors due to low signal strength
- NTP (Network Timing Protocol): Although adequate for simpler residential products, it is considered insufficient for larger, integrated Enterprise systems. The acquisition time to establish or recover synchronisation can take a long time.
- Macro Sniffing (sensing and slaving to external macrocell broadcasts): Not always possible, especially where in-building systems are solving coverage problems
- PTP 1588v2 (Packet sync using IP, often in conjunction with Synchronous Ethernet): Not always available but considered be a good backup.
A combination of GNSS and PTP/SyncE is a popular choice, balancing high performance with good resilience.
GPS synchronisation can achieve phase timing of as low as 0.03 microseconds, far better than the 1.5 microseconds typically specified.
High costs of external GNSS antenna
External GNSS (typically GPS) antennas are often used to support in-building systems of all architectures – Small Cell, Distributed Radio Systems and DAS. This usually involves mounting an external GPS antenna on a rooftop with unrestricted view of the sky, then connecting using thick coax cable to a receiver located in a machine room at or below ground level.
The technology isn’t complex or particularly expensive, but installation can be surprisingly costly. High expense arises from:
- Gaining permission to mount the antenna on the roof
- Ongoing roof rental
- Fitting lightning surge protection
- Ensuring adequate weather protection (temperature range, hail etc.)
- Running thick coax cable vertically throughout the building
- Ensuring cabling remains undisturbed and isn’t a trip hazard or unsightly
The total cost of an external GNSS antenna installation varies tremendously from region to region. A typical cost range of $10K to $25K in Europe increases to as much as $65K in some APAC cities where rooftop space commands a high premium.
|APAC||up to $65K|
Source: Microsemi 2015
In a recent ThinkSmallCell webinar, 73% of attendees indicated that for Enterprise Small Cell deployments, the cost of installing an external GPS antenna was moderately or extremely important
Integrated GNSS Master
Microsemi have launched a solution aimed at solving this problem. Their iGM-1100i is about the same size as a typical in-building small cell. It contains a full GNSS master clock with a built-in antenna, and is sensitive enough to be able to operate in the vast majority of cases. The solution employs assisted GPS, where ephemeris and satellite correction data is sent over the internet, plus some “secret sauce” engineering expertise to dramatically increase performance.
The unit is similar in many ways to a small cell, with straightforward “plug and play” installation, connected and powered over a single Ethernet cable. It would usually be installed in the server room, but doesn’t have to be. It can supply timing directly for four or eight small cells. A larger distributed small cell system, such as Spidercloud E-RAN or Airvana OneCell, would only require a single connection. DAS systems and DRS (Ericsson RadioDOT, Huawei Lampsite etc.) would require one per operator basestation.
Microsemi recommends the unit be located within 20 feet of an external window. Field trials have shown successful operation in over 90% of indoor locations; working even in rooms where smartphones could not determine their location using GPS.
In the case of GNSS outages, holdover using the internal oscillator can remain in tolerance for up to one hour. Our recommendation is to slave from another PTP master connected via the network, which provides a very solid and reliable solution.
Some operators believe strongly in the use of SyncE throughout their network. SyncE isn’t needed for this GNSS solution but would be useful in conjunction with PTP 1588.
It’s also possible to deploy multiple units in parallel.
Timing and synchronisation for Enterprise In-building small cell systems will become even more important with the evolution of LTE and LTE-Advanced features which require tight phase sync.
One of the highest deployment costs of Enterprise small cells relates to the installation and cabling of an external GNSS rooftop antenna. This can range from anything between $10K to $65K and installers confirm this to be a significant issue.
There are several technical solutions to provide a high quality timing source with a combination of GNSS and PTP 1588 is considered a popular choice. Microsemi’s new in-building GNSS solution avoids the high cost of an external GPS antenna, thus improving the viability and simplifying deployment of all types of Enterprise In-building wireless solutions.
Further reading (and viewing)
For more detail on this issue and Microsemi’s solution, view the slides or the full recording of their webinar