With the rollout of LTE networks and an increasing number of new LTE devices coming to market, the emergence of a second variant of the standard, TD-LTE may seem confusing. So what is TD-LTE, how will it impact the mobile world and what will the implications be for test engineers working in device development?
To date, most of the high profile news around LTE has concerned its FDD - frequency division duplexing - variant. This technology has been deployed by operators including Verizon and Europe's TeliaSonera.
In FDD, a carrier frequency is used for data uplink and downlink respectively. By contrast, TD-LTE uses a single carrier frequency for both uplink and downlink, dividing the radio frame into subframes that can be allocated to either uplink or downlink as per the immediate user need. This dynamic approach is well suited to the changeable data profile of today’s users – who for the most part download more than they upload, but will occasionally upload data like photos or videos .
Originally, TD-LTE had been championed principally by China Mobile, which saw TD-LTE as an evolutionary path from the TD SCDMA technology deployed as a Chinese 3G standard. As such, TD-LTE was initially perceived as a China-specific standard. However, spectrum for mobile communications is a valuable commodity and TD-LTE's use of single frequency rather than paired spectrum is advantageous to any operator in countries where spectrum is limited or where an operator only has access to a single unpaired frequency.
In meeting a worldwide need, TD-LTE has all the potential to be much more than a Chinese solution. This is exactly what happened at the start of 2011 with the launch of the Global TD-LTE Initiative (GTI), through which many of the leading global operators, including Vodafone, Bharti Airtel and Softbank joined with China Mobile to declare their interest in the technology. Indeed, many operators see TD-LTE as being a complementary technology to be deployed alongside FDD: According to research group Informa’s 2011 LTE Study, 33% of 250 global operators surveyed are considering deploying TD-LTE as well as FDD networks. With a further 19% considering TD-LTE alone, over half of operators are looking at rolling out TD-LTE. However, despite this level of support, TD-LTE is still a little way behind FDD. A key reason for this is the limited availability of mobile devices but this is set to change now that the technology can rely on a global market.
In test and measurement, TD-LTE’s move into the mainstream has seen intense activity over the past year to make available the test infrastructure necessary to bring new devices to the market. Test solution providers such as Anite having recognised the potential of TD-LTE back in 2009 have been able to quickly respond and accelerate the availability of test cases based on the 3GPP standard for protocol conformance testing.
Test case availability is crucial as it will enable formal certification of devices by international industry bodies such as the Global Certification Forum (GCF). While devices can start to ship before certification is possible – and indeed may work as intended on a given network – certified compliance with the 3GPP TD-LTE standard confirms that the device will also operate when roaming on other networks – crucial for a standard that now has such global appeal. The availability of certified mobile devices is therefore a key next step in building market confidence in TD-LTE.
The number of test cases validated by the Global Certification Forum for TDD bands is reaching parity with the number for FDD bands. This table shows test cases provided by Anite, which currently leads in test case provision.
Creating protocol conformance tests for TD-LTE has however been an evolutionary process that has been able to build on the work already conducted for FDD. The key differences in the time division structure between FDD and TD-LTE described above reside at the physical layer, with both variants sharing a common protocol stack and the majority of tests for FDD can therefore be adapted for use on TD-LTE. Despite this, the process of validating test cases is time consuming, with each test case requiring validation for each LTE variant separately, as well as for each band of carrier frequencies that the technology will be deployed on. This work needs to be conducted by an independent and certified testing organisation and includes running the test case on at least two devices and checking the test case code against the original 3GPP specification. Naturally, access to devices at the very early stages is a challenge. This is overcome by close collaboration between test providers and device manufacturers who provide access to prototype designs to their key testing partners.
The good news is that in protocol testing, the availability of test cases for TD-LTE is now virtually at parity with FDD, much thanks to leading players in the test industry that have helped lead the development and validation of these test scripts within the 3GPP standards. While work is on-going to provide a set of test cases for either variant of LTE that is as comprehensive as say – 3G standards – both TD- and FDD LTE have sufficient test cases to prove basic compliance. In this respect, TD-LTE is now as well-equipped as FDD to succeed.
Indeed, the maturation of testing has helped TD-LTE reach a key milestone – the extension of the GCF certification scheme for TD-LTE devices. While certification of FDD devices started at the end of 2010, July 2011 saw the announcement that GCF will begin certification of TD-LTE devices operating in the 2570-2620 MHz band, which is allocated for TD-LTE in several important global markets. This is just the start as further operating bands for TD-LTE are added to the scheme. The GCF has heralded this news as “a globally-recognised means of demonstrating interoperability between LTE devices and networks”. Undoubtedly, TD-LTE is now ready for the mainstream.