At the recent CES a number of companies seem to have shown the first CDMA/LTE smartphones to be launched at some point later in the year. From a telecoms point of view the interesting thing is how these phones will do voice calls while connected to an LTE network. The issue here is that LTE doesn’t have a built in voice service like GSM and UMTS. While most of the press mostly commented around the design, screen size, processor and other features, this topic was completely overlooked.
Steve, over at the Voice over LTE blog, however, comes up with some interesting answers on how voice service is implemented. According to his post, these LTE phones will be connected to LTE and the CDMA network simultaneously so IP based traffic can use the LTE network while voice calls and text messages can be handled simultaneously by the CDMA network.
From a 3GPP GSM/UMTS engineer’s point of view this is, well, lets say, outmost sub-optimal. In the world of GSM and UMTS a lot of care is taken that voice calls and data sessions can move seamlessly between the two technologies. Ongoing calls will be handed over between the technologies when running out of UMTS coverage, for example, and the IP address and session contexts will also be preserved when hopping between the two radio networks. Voice and IP services even run concurrently, something that doesn’t work in the CDMA world at all. For LTE, the same mechanisms have been standardized for IP sessions and a number of different solutions exist to hand over ongoing LTE voice calls to GSM and UMTS.
So why are those phones not using these solutions? For one thing, they are designed for CDMA 2.5 / 3G network operators and from what I can tell they can’t use CS fallback, because that’s specified for fallback to GSM and UMTS only (please correct me if I’m wrong here). Also, Voice over LTE via GAN (VOLGA) is not an option for them as it is based on GSM and UMTS technology. That leaves them with VoLTE, which is an IMS solution with a specified feature set. Talked about in the industry for pretty much a decade it’s still nowhere to be seen and the announcement of dual-active-radio phones for LTE indicates that showtime for that technology won’t be anytime soon. Hence, their move to dual-active-radio phones.
Apart from the opinion of a 3GPP engineer, is a dual-active-radio device a solution that the customer could like? After all, the customer isn’t interested in sleek designs under the hood, but whether the device works well or not. So from my point of view, I as a customer want my smartphone to do two major things: It has to provide me with a highly reliable voice service anytime, anywhere and it has to provide me with a fast and reliable Internet connectivity for my IP based applications such as email, web browsing, Twitter, etc. etc.
So the question is, will those dual-active-radio phones provide that? From what I can tell the answer is yes, with some restrictions. The first one is the potentially higher power requirements to keep two radios listening to two networks at the same time. However, if I get a full day of use out of the device then I, as a user, don’t care to how many networks the device is connected to simultaneously. If those devices can do that is something yet to be seen. Secondly, voice service. Yes, the CDMA network layer provides that and as the radio is turned on all the time, my voice service in terms of availability and call setup time should be in the same ballpark as current 2G/3G phones. Furthermore, I wonder if the first generation of the CDMA/LTE networks and the devices themselves can switch the IP context between the different networks. If not, then applications will loose their connection to the network every time a switch between LTE and CDMA is done for data services.
Loosing the IP context between CDMA and LTE might come in as a big argument against such a solution. But is it really an issue? Today, most smartphones can use both 3G and Wi-Fi for internet connectivity and automatically switch between the two radio technologies. Here, IP connectivity is also lost and applications have to re-connect to their server on the Internet each time the switch it made. Not very pretty from a design point of view but it seems to work in practice already today without anyone loudly complaining.
Oh, yes, and before I receive some comments on the Skype implementation on the phones that was reported, yes, I’ve seen that, too and I very much like it as an added goodie. However, I’d never rely on it as my main telephony service for a number of reasons. The most important one is that I often make calls while moving and often implicitly use, what I think is the network operators biggest asset, the capability to hand over ongoing calls from 3G to 2G.
I find the thought around Wi-Fi very interesting. In effect, if the IP context can’t be taken over from LTE to CDMA, then the LTE network can be seen to do the same thing Wi-Fi already does today in 3G phones. So instead of having only a Wi-Fi chip, LTE smartphones have an LTE chip in addition that can also provide IP connectivity. And that, I think, is a crucial point in the discussion and way of thinking of whether it’s a good idea to have dual-active-radio LTE phones, which in fact will be triple-active-radio phones, as Wi-Fi is switched on all the time as well.
So the final train of thought for this post is whether this approach could and should work in the GSM and UMTS world as well!? Let’s first look at the “should”. From what I can tell, CDMA operators are keen on launching LTE because they depend on it as an upgrade path from their CDMA network technology that is not evolving anymore and is capable, from what I hear, to deliver around 1.5 MBit/s but not much more. Also, from a capacity point of view, it’s much more limited to what’s currently going on in UTMS networks. On the UMTS front, there’s a healthy evolution program in place. Many networks are already upgraded for speeds up to 7.2 or 14.4 MBit/s in the downlink, with 21 MBit/s in the downlink and dual-carrier 42 MBit/s on the way to reality. Also, network capacity keeps rising with more advanced devices coming to the market all the time that make better use of the resources, use of several carriers and densification of the network, all while preserving backwards compatibility with GSM and concurrent voice and IP based operation. So why go for the compromises of a GSM/UTMS/LTE phone that has to have two radios switched on at the same time? You can have it all on the evolution of UMTS for smartphones, while LTE is being used for non-voice devices such as netbooks, LTE USB sticks, pad computers, etc. For me, a GSM/UMTS/LTE phone only makes sense once voice over LTE works as good as voice over GSM/UMTS, including, and that’s most important, handover to GSM and UMTS. As an operator, anything less doesn’t cut it, Skype and others are here today. Whether an operator bets on IMS VoLTE or VOLGA, or a combination of both will be sorted out by competition.
And finally for today the “could” part. Running GSM/UMTS and LTE separately at the same time is going to be a bit of an engineering challenge. As per design, a single SIM card can only supply the credentials to be active in one radio network at the same time. So to be active in GSM and LTE at the same time would require two IMSIs (International Mobile Subscriber Identities). You can’t do that with standard SIM cards today. Of course one could think about solutions such as one SIM card having two IMSIs and two secret keys but why work on this when the real issue is to have Voice on LTE with seamless interworking with GSM and UMTS?
In summary, I think the answer for CDMA networks is to have dual-active-radio CDMA / LTE phones until they’ve got the voice question solved as they don’t have many other alternatives to evolve their networks and live with the downsides for the moment. For UMTS network operators, I think the equation is different as the strong evolution path of UMTS and using LTE for non-voice centric devices makes dual-active-radio designs not very attractive.