GiLTE – GSM and LTE sharing the same spectrum

cellXica GiLTE graph 150CellXica have come up with an unusual concept, running both LTE and GSM simultaneously in the same spectrum band. While immediately migrating mobile networks to the much more efficient, faster and capable LTE technology would be considered ideal, support for legacy handsets using the baseline GSM technology embedded in almost all devices is necessary and more realistic. We investigate and consider the potential.

The global migration across to LTE

Each new generation of technology delivers higher performance, tolerates greater interference, accesses more spectrum and improves coverage. Fast, seamless data service and high definition voice are more widely available. As a global standard, it is widely compatible and enables high volume mass market which drives costs down.

Network operators naturally want to adopt LTE to make best use of the expensive licensed spectrum they own. Many have already invested heavily, with the result that over 1 Billion users are now served by LTE.

However, the majority of the global population remains on GSM today and 3G will peak around 2020. VoLTE (Voice over LTE) has proved complex to deploy, with few live VoLTE networks today. Many networks may avoid the complexity and choose to retain voice services on 2G/3G using CSFB (Circuit Switched Fallback). The premium price for LTE smartphones will delay widespread adoption in more price conscious markets for some years.

Several talk of switching off their 2G or 3G networks in the near future: Telstra Australia will terminate GSM at the end of 2016, claiming it only handles 1% of traffic; ATT USA, all three Singapore networks and Optus Australia have also announced plans to switch off 2G within the next year. Telenor plans to switch off 3G in 2020, keeping GSM running through until 2025.

Most European operators I’ve spoken to will retain the flexibility to operate a sliver of 2G service and plan to do so for some time. It’s the lowest common denominator, supported by almost every handset and remains fairly efficient for low power M2M applications.

Sharing the same spectrum

We’ve seen a few examples where network operators have actively used different generations of mobile technology in the same spectrum. One example was in the US, where ATT configured indoor 3G residential Femtocells to use the same spectrum as for outdoor GSM. Since the Femtocells were primarily deployed in areas with poor signal coverage and were low RF power, there should be relatively little interference between the two. I’m sure it made for some interesting corner cases.

The same spectrum can also be shared between network layers, so that macro and small cells both use exactly the same frequencies. LTE has built-in features to improve performance by co-ordinating transmissions between the two over the X.2 interface such as ABS and eICIC.

Combining GSM and LTE in the same small cell

CellXica, a UK small cell startup specialising in SDR (Software Downloadable Radio) modules, have gone a step further and integrated LTE and GSM within the same RF chain, using a single module to handle both simultaneously. They've branded this as GiLTE (GSM in LTE spectrum).

The concept is that a single 200kHz GSM carrier is embedded into a 3MHz or wider LTE channel so that smartphones in the coverage area could use either simultaneously. Older non-LTE capable phones would only see the GSM signal and operate using that totally unaware of the new technology in place.

cellXica GiLTE graph

This could be used in any frequency band that currently supports both LTE and GSM for that region. Today’s smartphones are capable of detecting and operating in either mode in several bands, adapting when bands are refarmed and repurposed from 2G, 3G to 4G. 

A single GSM carrier can handle up to 7 concurrent voice calls (14 using the halfrate codec). Their product literature does not mention GPRS although I see no reason why a data service could not be offered, albeit at comparatively slow data rates. Perhaps it would be more sensible for data users to switch to LTE, where data rates in excess of 50Mbps would be available.

Potential opportunity in the unlicensed bands

The UK is relaxing the licensing rules for a small part of the 1800MHz guard band spectrum that was originally allocated to partition and protect interference between DECT cordless phones and the outdoor GSM network. It’s thought this could be used with one of the standalone core network software products (e.g. Quortus, Druid, Attocore etc) to create a completely standalone mobile network that supports standard GSM and LTE smartphones.

CellXica are promoting use of their outdoor small cell to serve rural non-spots. The outdoor product has a nominal cell radius of 3km complemented by a separate indoor small cell for inside buildings. Traffic would be routed through their own MVNO service as a commercial proposition. Village trialists are sought and can apply through their WaveMobile associate company. The website is very basic with limited information and appears to be at a very early stage of development.

Outlook

I don’t see this being relevant for the higher frequencies being considered for LAA and MulteFire which use LTE technology alone. Frequency bands would have to be those which are assigned for both LTE and GSM, as embedded into today’s mass market smartphones.

It does open up an interesting solution that allows GSM to be retained as a background alternative. This is probably most likely to be attractive in remote, rural areas of undeveloped countries rather than in dense, high capacity urban environments where spectrum efficiency is critical.

Whether this remains a niche architectural variant or becomes more widely adopted remains to be seen. Nonetheless, it's exciting to see further innovation in this field, adapting and repurposing existing technology standards for use in novel ways.

More information and specifications can be found on the CellXica website

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