How Do You Hand Over A 4G Voice Call to 2G?

WiMAX, LTE, UMB, etc. etc., buzz words in the emerging 4G wireless space. Different interests, standardization groups and politics but they all have one thing in common: All are based on IP and all will rely on Voice over IP (VoIP) in one form or another (e.g. IMS or SIP) to carry voice calls. With sheer bandwidth, IP header compression and optimized handover strategies between cells I can imagine it happening. But what happens when you run out of network coverage and only a GSM network is available to continue the call in?

A number of alternatives exist. The first one might be evolved EDGE which could deliver GPRS data rates high enough to sustain a VoIP call begun in a 4G network on the packet switched side of the network. However, I wouldn’t bet on this one happening everywhere. It’s more likely that the VoIP call must be continued in the circuit switched side of the GSM network. But how can that be done?

Voice Call Continuity (VCC) could come to the rescue. A first version is already standardized in 3GPP TS 23.206 and it can do this and many other interesting things. I’ve done a short intro on VCC before, take a look here. Yes, it’s standardized but it’s not a home run:

One of the problems with VCC is that the mobile needs to be connected to both the 4G network and the GSM network at the same time to perform a handover. This consumes more energy then only being connected to one network at a time. Furthermore, such a dual connection might be difficult to establish if the two networks use the same frequency band. If the 4G network is deployed in the 2.5 or 3.5 GHz band then this is not going to be a problem. In case classic 2G frequency bands (850, 900, 1800, 1900 MHz) are partly re-farmed and the GSM network to be handed over to is nearby then VCC will become a challenge. 3GPP Release 8 might yet get a work item to study the possibility of single radio VCC (SR-VCC) to deal with these issues and I am looking forward to see how handover speeds in the order of a few hundred milliseconds can be achieved.


All-IP wireless networks will be a great thing to have but solving the handover to legacy wireless networks to prevent calls from dropping is going to be a difficult thing.

8 thoughts on “How Do You Hand Over A 4G Voice Call to 2G?”

  1. if 4G and GSM use the same frequency bands, then not only a dual connection might be a problem, but a single connection as well, due to interference, or I got something wrong?

  2. Mikalaj,

    Both GSM and 4G could use for example the 900 MHz band (if regulators approve) at the same time but on different parts of it. To stay with the example: The 900 MHz band is 25 MHz wide in each transmission direction. Let’s say a 4G channel uses 5 MHz of that spectrum, another 4G channel another 10 MHz. So GSM would still have 10 MHz left.

    In practice it might not be so simple to do because the 900 MHz band today is already used by at least 2 or sometimes even more GSM networks in most countries. So operators who would like to put 4G into 900 MHz must remove some of their GSM channels in order to put their new 4G system there.


  3. And as a further note: LTE is quite flexible and will also allow channels as small as 1.25 MHz. These might be more suitable for the 900 MHz band. However, the bandwidth of such a channel is rather low compared to an LTE channel with 10 MHz…


  4. Regarding 4G/2G Handover (HO).
    2G (GSM) is FDD/TDMA whereas 4G seems to be (S)OFDM(A).

    So it seems to me that the signal of (4G) would only be interference/noise for the other (2G).
    Furthermore, at cell boundary (assuming HO is triggered due to low 4G received signal strength at MS) the signal strength of the 4G would be to weak to bother the 2G HO target Base Station.

    Certainly this is the reason why 3G can coexist on 2G bands. As (W)CDMA would receive all non intened signals as low level noise. But to what extent would the same apply for (S)OFDM(A)?



  5. Jason, you point out an interesting issue.

    3G systems make use of spreading codes, which “dilute” signal down to (or below) the thermal noise. Then, the receiver de-spread the useful signal, and at the same time, spread (and dilute) the interfering signals. 3G systems are therefore strong against interferences, and are likely to create less interferences.

    For 2G and 4G system, multiple access is done by using multi-frequency time division multiple access (wether the different carriers are orthogonal or not). So, there is no spreading tricks in these systems, they are just travelling the air at their full power, thus creating interferences.

    In any case, interferences are mitigated mainly by deploying networks in different frequency bands, although these bands can be adjacent.

    The issue presented by Martin in this post is about seamless handover, which is to be addressed by the every layer. For the PHY layer, “seamless-ness” can only be obtained by doing “make-before-break” hand-overs, which may be tricky over heterogeneous networks. This implies that the terminal is connected to the two networks before doing the HO.

    Then, wether the two networks are interfering with each other is another topic, which is targeted at the radio-planning/regulation level.

  6. Hello Jason and plutonheaven,

    I don’t think the networks themselves would interfere with each other when frequency refarming is used. However, the mobile has to communicate with two networks at the same time during the VCC handover which might create the following problems in the handset itself:

    -Power: Communicating with two CELLULAR networks at the same time requires more power in case base stations are too far away than what is allowed by safety standards.

    -The Mobile could interfere with itself: I can remember that I once read somewhere that 3G to 2G handover can become a problem when the 2G network is in the 1800 MHz band. In this case the UMTS uplink band is very close to the GSM downlink band. Thus the constant UMTS uplink transmission of the mobile induces energy into the downlink reception of GSM in the mobile’s antenna. The mobile thus starts to interfere with itself. There seem to be no effects of this kind for UMTS to 900 MHz GSM handover.


  7. Interesting–however your analysis precludes the fact that GSM will still be available by the time there are all IP 4G networks carrying traffic. One might think that as 4G becomes commercially dominant, the vast majority of traffic would have been migrated from GSM 2G to UMTS. Of course we are talking 1o years or more, no?

  8. Hi Phil,

    thanks for you comment on my blog. You raise a good question for which I have a two part answer/opinion and further questions 🙂

    4G and ten years from now is a tricky thing. The WiMAX protagonists, who count that system into the 4G domain would like to deploy their kit yesterday. If they want to do more than to compete with DSL and cable at home they need to have smartphones or other mobile devices that can do voice. So the question is how to do voice with 4G if not yesterday but then at least tomorrow and not ten years from now!? VoIP + handover to 2G when coverage is lost is one option, or partnering with a GSM operator right away for the voice part. It seems to me the required multimode devices are currently under development and some companies have already made announcements for WiMAX/2G devices. Doing voice exclusively over 2G is probably not the first choice of 4G operators as they see their technology as superior and having voice on GSM they would have to admit that their technology is not yet where it should be. Also, they wouldn’t make much money they can keep this way.

    The second part is what will be in ten years from now. I am not sure. I see European operators refreshing their GSM networks today so obviously they have no intention of switching them off anytime soon. Also as long as UMTS is not on the 900 MHz band and devices are not generally available for that band there is little chance the coverage will come close to the GSM level. But that can obviously change in the future.

    This also raises the question if operators will live with three deployed systems simultaneously (2G for coverage and roaming, 3.9G for the mainstream data stuff and 4G as the bleeding edge) for some time. I think they have to since switching off either 2G or 3G for the next generation technology is likely out of the question. Good for network vendors that can offer integrated base stations that do all three standards simultaneously. So that’s the scenario I see in ten years from now and to me that asks for VoIP handover to circuit switched.

    All the best,

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