Apart from the role of LTE as boon for exponential data consumption worldwide, there is no clear –cut standard for the voice over LTE. Voice over LTE is important as people like to use voice services while access data over LTE on the move.
Worldwide state of LTE is similar to CDMA in India – LTE is used mostly for data using dongle while voice services on LTE are still not in mature shape.
In United States CDMA players – Verizon and MetroPCS and GSM players – AT&T and Rogers are offering voice on LTE smartphones. They are using Circuit Switched Fallback (CSFB) as the phones have multiple radio support but only one radio band active at a time, so while the user makes a call the device is forced off the LTE network and goes to 2G/3G network to make phone calls.
Also LTE spectrum is available in more than 40 bands over the world and thus to make a LTE chip supporting all bands for worldwide roaming is a problem. Also high speed data & LTE/3G(EVDO & HSPA)/2G(GSM/GPRS/EDGE & 1X) switching makes the battery backup unstable compared to HSPA/EVDO phones.
But as LTE coverage is scarce over the globe, operators have to use this CSFB technique to offer roaming to remote parts which has only GSM/1X coverage.
Over the past two years, the industry has identified four main tracks to offer voice over LTE:
a) Circuit Switched Fallback (CSFB) to 2G/3G CS.
b) IMS-based, such as 3GPP IP Multimedia Subsystem (IMS) Multimedia Telephony (MMTel) over LTE.
c) Over-the-top (OTT) – for example, Skype.
d) Circuit Switched over Packet Switched (CS over PS) – for eg, voice over LTE via Generic Access (VoLGA).
Among this four options only first one is taken as intrim solution for voice over LTE and used by major LTE operators as said before. TeliaSonera, world’s first LTE operators is gearing up to roll out voice services on LTE using CSFB. According to them both CSFB & VoLTE is to be there as CSFB will help users to roam outside 4G coverage.
There is also another optimization technique called network-controlled packet-switched handover, which will work in parallel with CS fallback, that will reduce the data session setup times down to a few hundred milliseconds, it will come in 2012.
To bring the voice over LTE, The GSM Association (GSMA) had partnered with U.S. operator Verizon and conducting trials on the network equipped Alcatel-Lucent using LG Revolution smartphone. In the demonstration voice calls over LTE, which is completely IP-based and uses SIP (Session Initiation Protocol) to set up the call, was made over HD voice codec as the smartphone has the feature. But the test fails basically while the user moves from one cell to another and call drops or call quality goes down. So true VoLTE has to go long way.
In India BWA operators like Airtel and Aircel are already in voice business, and they will adopt CSFB to offer voice over LTE devices. While smaller ones like Tikona, Augere & Qualcomm may be focused on data services only targeting individuals and enterprise segments.
There are reports that Qualcomm will tie up existing CDMA-EVDO players to offer pan India data roaming. But largest BWA player Reliance Infotel may buy a 2G player after M&A policy comes with new telecom policy or simply tie up with RCom to offer voice on LTE using CSFB.