The Evolution of LTE TDD

With the emergence of smart devices, people are downloading content in unprecedented volumes, putting stress on the network. As a result, wireless operators globally are facing increasing demand for high speed mobile broadband services. More and more users are flocking to such bandwidth-consuming applications as YouTube and Netflix, leaving operators searching for technology to stay ahead of this ever-growing demand.

Many operators are looking to LTE as the de facto global standard for mobile broadband technology due to its cost savings, high spectral efficiency, mobility and interoperability. Even with LTE, however, operators see a need to offload their data traffic in order to provide users with wireline-like speed and capabilities. According to a recent report by Qualcomm, while LTE allows operators to use new, wider spectrum and complements existing 3G networks to handle even more mobile traffic, radio link improvement is fast approaching the theoretical limit and the spectrum available to operators is often limited and expensive¹.

In a race for providing a wireline-like experience to wireless users, operators are not leaving any stone unturned. Operators are already offloading data traffic via small cells and Wi-Fi, but have found that these solutions lack mobility. Wi-Fi is effective in improving user experience when a significant portion of users are located in the vicinity of hotspots such as residential homes, airports and coffee shops, and the resulting data traffic can be offloaded to Wi-Fi applications, however it’s not mobile. A mix of macro cells and small cells, also referred to as a Heterogeneous Network (HetNet), as well as small cell value added services like Local IP Access (LIPA) can ease pressure, but these solutions are also constrained to a specific location and number of users.

Just when operators are at a point where they have exhausted all possible data offload approaches, Time Division Duplex (TDD) in the form of LTE shines through. TDD has the potential to be positioned as a complementary solution to Frequency Division Duplex (FDD) networks, bringing additional capacity to congested areas, opening up a new way of data offload and backhaul for small cell deployments.

What is TDD?

There are two modes of operation for LTE technology: FDD and TDD, which are technically very similar and part of the same radio access specification. LTE FDD and TDD were both defined and introduced as part of the 3GPP specification in 2009 to make efficient use of paired and unpaired spectrum allocations over a common, core network architecture. The main differences are around the duplex method used.

In both LTE FDD and LTE TDD, the transmitted signal is organized into subframes of one millisecond (ms) duration and 10 subframes constitute a radio frame. Each subframe normally consists of 14 orthogonal frequency division multiplexing (OFDM) symbols (12 OFDM symbols in an extended cyclic prefix). Although the frame structure is, in most respects, the same for LTE FDD and LTE TDD, there are some differences between the two—most notably the use of special subframes in TDD. The subframes in TDD are allocated either for uplink (UL) or downlink (DL) transmission.

In the case of FDD operation, there are two carrier frequencies, one for UL transmission and one for DL transmission. During each frame, there are consequently 10 UL subframes and 10 DL subframes, and UL and DL transmission can occur simultaneously within a cell.

In TDD operation, there is only a single carrier frequency, and UL and DL transmissions in the cell are always separated in time. As the same carrier frequency is used for UL and DL transmission, both the base station and the mobile terminals must switch from transmission to reception and vice versa. Thus, as a subframe is either a UL subframe or DL subframe, the number of subframes per radio frame in each direction is less than 10.

Figure 2. FDD can cover a larger area with the fixed DL/UL on different frequencies, but TDD can provide more DL capacity with the flexible DL/UL ratio².

LTE TDD’s relationship to FDD

TDD makes it relatively easy to dynamically change the capacity ratio between UL and DL to reallocate time slots, which makes it well suited for today’s DL-heavy traffic pattern. In most instances, network operators will desire more DL capacity than UL since users more frequently download content like video and web pages than upload content they’ve created.

Beyond the regional deployments of TD-SCDMA, TDD wasn’t deployed widely in 3G networks, but it has great potential in LTE. The operator community was originally hesitant to adopt this new technology due to its similarity to WiMAX, but has since discovered that TDD and FDD technologies can co-exist nicely and is now supportive of a new market with TDD LTE. Because they have common core network architecture, there is no additional CAPEX and the two technologies interoperate seamlessly. The main difference is the need for a specific radio frequency (RF) unit. Another significant difference is in the physical layer definition; the higher layers and the rest of network architecture remain applicable for FDD.

FDD is still leading the game, however. Most commercial LTE networks are based on FDD because the FDD ecosystem is more mature and is still where most of the spectrum allocation is done. All major operators around the world are already acquiring wide bands of FDD spectrum for their 4G LTE networks, which is well suited for voice because it is inherently symmetric in the UL and DL. In addition, FDD can provide better coverage of a larger area due to the fixed DL/UL on different frequencies.

However, some operators are able to exploit the TDD advantage by deploying the two technologies in tandem to offload traffic for very asymmetrical applications such as video or even newer areas like machine-to-machine (M2M) applications. For example, Vodafone has developed an innovative use case of LTE TDD being used as a backhaul for small cell deployments. As multimedia broadcast and multicast services (MBMS) pick up traction, it makes even more sense to effectively deliver this broadcast information in the DL using the unpaired TDD without impacting the user services delivered on FDD in parallel.

Existing FDD networks can leverage LTE TDD for targeted capacity expansions, ensuring a larger economy of scale by utilizing common EPC network architecture wherever possible. TDD is excellent for hot-spot expansions (picocells and femtocells) and new LTE TDD networks plan for small nodes from day one. LTE TDD is an excellent indoor complement for small nodes because it does not interfere with the FDD network. It is the ideal technology to leverage unpaired spectrum, which is typically available at higher frequency bands optimal for capacity expansion, making it less fragmented. Most FDD deployments use 2.6GHz, but some of the largest rollouts used other bands. The risk associated with a technology deployed in many different spectrum bands is that its adoption is hindered because manufacturers will not create as many devices to support it. TDD is in a much better position since most of the rollouts are expected in only two frequency bands: 2.3GHz and 2.6GHz. Chipset manufacturers are especially interested in 2.3GHz deployments for well-populated countries. TDD also enables a number of smart-antenna technologies such as beamforming. A mix of LTE TDD hot-spots with LTE FDD macrocells will boost capacity and expand coverage.

Most vendors in network infrastructure equipment and device chipsets support both TDD and FDD in their commercial products, indicating that they see a strong market potential for both flavors. This simplifies implementation and minimizes the additional OPEX/CAPEX costs to deploy LTE TDD. TDD is comparable to FDD in data throughput as well as latency measures, and handover (HO) procedures can be enabled from FDD to TDD and vice versa.

This is the new beginning of the hybrid LTE TDD/FDD deployment model.

The current state of TDD technology adoption

LTE TDD is expected to be widely adopted in 2015, reaching 89 million connections and representing roughly 25 percent of the total forecasted LTE connections for that year.

Figure 3. Ovum estimates that LTE FDD will take-off around 2012–13, while TDD will take off around 2013–14 for LTE TDD³.