GSM and LTE, 2 technologies in 1 base station

LTE for bandwidth and GSM for voice are a match made in heaven for subscribers. The roll-out however, not so much. Running them both from the same radio equipment (BTS) can be the answer. SatSite can run both YateBTS (GSM) and YateENB (LTE) at the same time, in the same spectrum, using the same radio hardware.

Software-defined BTS

This is made possible by replacing commonly used FPGA and DSP boards with one Intel Atom chipset. Both the GSM YateBTS and the LTE YateENB are modules implemented in software, allowing the base station to be reprogrammed or reconfigured to support new protocols. A base station can run GSM at first, and can be later software-upgradeable to LTE, running multiple air interface protocols using the same radio, at the same time.

Mixed 2G/4G spectrum allocation

From a spectrum point of view, as seen in the image below, the mixed GSM/LTE technology enables a base station to be software-configurable for up to 4-TRX/ARFCN. A base station can use the 850, 900, 1800, and 1900 MhZ bands for both GSM and LTE, meaning that it will allocate two ARFCN to GSM and will use the remaining spectrum for LTE.

ss_mix_spectr_2015-10-6_pic1_version1.1Based on the subscribers’ activity (data vs. voice), operators can assign in software the spectrum priority for either LTE or GSM, so LTE gets a higher priority if there is a lower use of voice services. This optimizes the resources allocation in the network and supplies better access to users.

YateBTS and YateENB – Yate modules

Yate is an underlying part of the software architecture of our mixed 2G/4G RAN. It has a highly expandable architecture that provides unified management and monitoring. Both YateBTS and YateENB are software modules based on Yate. Yate’s SDR architecture enables the LTE and the GSM modules to use the same radio hardware. You can find out more about Yate’s multiple modules here.

ss_mix_spectr_2015-10-6_pic2_version1.1Yate’s SDR architecture also enabled us to replace the conventional, special purpose equipment combination of a baseband unit (BBU) + a remote radio unit (RRU), with a single unit. With this technology we implemented all the functions of both a conventional base station and a base station controller, eliminating the costly Abis interface for traffic and signaling, as well as partial functions of an Mobile Switching Center (MSC), in terms of mobility, power and frequency management and handover.

The mixed 2G/4G RAN technology is embodied in our SatSite base station. SatSite acts more like a conventional eNodeB, even when running on GSM, because it uses IP backhaul for both 2G and 4G. It also contains the IP list of all neighboring SatSite units.

Using off-the-shelf hardware and a generic operating system, SatSite embraces everything SDR stands for, and is the solution for an easy adoption of new standards or technologies, even 5G in the future.

Unified Core Network Demo with iPhone 6

Recently, we verified the interoperability of the new iPhone 6 with the Unified Core NetworkTM, by performing the industry’s first VoLTE call from a GSM mobile phone to iPhone 6, through a single unified switch. This is a Big Deal. Why?

First, Some Background on the Problem

One of the changes 4G LTE is forcing on mobile operators is the elimination of older SS7 MAP core networks of 2G and 3G in favor of IMS. However, many critical services, like roaming are not yet “fully baked” in IMS, so operators will probably continue to run 2G and 3G networks for the foreseeable future. In fact, mixed 2G/4G deployments are happening in many places right now and those operators are in the situation of installing and managing two nearly independent core networks.

The Solution

SS7Ware’s Unified Core Network (UCN), along with YateBTS is the answer to the 2G/4G mixed network problem. The UCN provides a “packet core” for routing internet traffic and an IMS/VoLTE core for handling calls and text messages. It works with YateBTS to support 2.5G GSM/GPRS handsets and with any standard eNodeB to support 4G LTE devices . We first introduced it with this video. (In that first video we referred to “OpenSAE” and “OpenVoLTE” as two different things, but we have since combined them into a single UCN server.)

Unified Core Network The switching, routing and mobility management functions of the core network (4G SAE/IMS and 2G Mobile Switching Center/Visitor Location Register/GPRS support node) are implemented in a single UCN server. This approach offers many advantages:

  • Simplified architecture; even a large network is just many copies of an identical box.
  • Simple scalability; just add more servers.
  • Increased network resiliency: there is a many-to-many relationship between radio sites and UCN servers, with seamless failover and load balancing.

And now the Demos!

Most recently, we have been testing the UCN with the IMS client in the new iPhone 6. (Unlike  over-the-top applications like Skype, a true IMS client is implemented in the baseband processor, so using a true VoLTE-capable handset is critical.) We used the UCN with an an off-the-shelf LTE eNodeB to provide a 4G radio network for the iPhone and we used YateBTS to provide a 2G radio network for a second test phone. In this first demo video, the iPhone registers to the UCN through the 4G radio network and the GSM phone registers to the UCN through the 2G radio network.

The two phones register to the same HLR using SS7-MAP. Then, we exchanged text messages between the two handsets. If you are a hardcore techie, here is a ladder diagram: 2g_4g_register_sms_sip_map-video And here is the signaling trace from the UCN server console. If you look at the ladder diagram, you see that we are using conventional SS7-MAP to register an LTE iPhone6 to the HLR. This solves the LTE roaming problem, and we can do it thanks to the Yate messaging engine, which is the basis of the UCN server.

In the second demo video, we make phone calls between the two handsets.

For the 2G phone, this is an ordinary circuit-switched GSM call. For the 4G phone, this is a VoLTE call. What is special here is that these calls are being handled by a single switch in the UCN, behaving as a 4G IMS and a 2G MSC at the same time. We can do this because the UCN server is built on Yate, which combines one of the most solid SIP implementations in the industry with carrier-certified SS7-MAP support, and because YateBTS gives us a very LTE-like RAN for 2.5G.

We have already had the opportunity to present this technology to mobile operators. Their first reaction is disbelief, followed by a lot of excitement. “You mean we can use CAMEL on a VoLTE call?”  “You mean we can authenticate 4G handsets in an ordinary HLR?” “You mean we can run GSM and LTE out of the same core network?” And to this we say, “Yes, you can!”