What is TD-LTE and Why the sudden interest in TD-LTE?

TDD (time division duplex) version of LTE is known as TD-LTE. Recently operators and vendors across world have requested the 3GPP standards body to begin working on specifications that would enable TD-LTE to be deployed in the 2.6 GHz band of spectrum as well. This frequency band is currently used for WiMAX and would allow operators like Clearwire to make it possible to deploy TD-LTE at 2.6GHz in the US.

A recent article from Fierce broadband wireless puts reason of renewed interest as below

• The FDD LTE and TD-LTE versions of the 3GPP standard are very similar. As a result, devices can support both the FDD and TDD interfaces through a single chipset--i.e., without any additional cost. This is a hugely important new development: TD-LTE will benefit from the wide availability of FDD LTE devices that will be able to support TD-LTE as well. Unlike WiMAX, TD-LTE does not need to prove to have a substantial market share to convince vendors to develop devices. Vendors do not need to develop new devices, they simply need to add TD-LTE support to the existing ones.
• There is a lot of TDD spectrum available, and in most cases it is cheaper and under-utilized. 3G licenses frequently have TDD allocations and upcoming 2.5 GHz auction in most cases contemplate TDD bands.
• The increasing availability of base stations that can be cost-effectively upgraded will make it possible and relatively inexpensive for WiMAX operators to transition to TD‑LTE using the same spectrum allocation. The transition will still require substantial efforts and be justified only in some cases, but it will make it easier for WiMAX operators to have roaming deals and to have access to the same devices that LTE operators have.
• Industry commitment to WiMAX 16m, the ITU-Advanced version of WiMAX and successor to the current WiMAX 16e, is still limited.

The proposal is to adopt the 2496MHz-to-2690MHz frequency band in the US for TD-LTE. Part of the 2.6GHz band is already specified for TDD, namely the 2570MHz-to-2620MHz band.

Currently, the LTE standards support both FDD and TDD operation. Fifteen paired (for FDD operation) and eight unpaired (for TDD operation) spectrum bands have already been identified by the 3GPP for LTE as shown below.

E‑UTRA Operating Band	Uplink (UL) operating band BS receive UE transmit			Downlink (DL) operating band BS transmit UE receive			Duplex Mode
	FUL_low – FUL_high			FDL_low – FDL_high
1	1920 MHz	–	1980 MHz	2110 MHz	–	2170 MHz	FDD
2	1850 MHz	–	1910 MHz	1930 MHz	–	1990 MHz	FDD
3	1710 MHz	–	1785 MHz	1805 MHz	–	1880 MHz	FDD
4	1710 MHz	–	1755 MHz	2110 MHz	–	2155 MHz	FDD
5	824 MHz	–	849 MHz	869 MHz	–	894MHz	FDD
6	830 MHz	–	840 MHz	875 MHz	–	885 MHz	FDD
7	2500 MHz	–	2570 MHz	2620 MHz	–	2690 MHz	FDD
8	880 MHz	–	915 MHz	925 MHz	–	960 MHz	FDD
9	1749.9 MHz	–	1784.9 MHz	1844.9 MHz	–	1879.9 MHz	FDD
10	1710 MHz	–	1770 MHz	2110 MHz	–	2170 MHz	FDD
11	1427.9 MHz	–	1447.9 MHz	1475.9 MHz	–	1495.9 MHz	FDD
12	698 MHz	–	716 MHz	728 MHz	–	746 MHz	FDD
13	777 MHz	–	787 MHz	746 MHz	–	756 MHz	FDD
14	788 MHz	–	798 MHz	758 MHz	–	768 MHz	FDD
…
17	704 MHz	–	716 MHz	734 MHz	–	746 MHz	FDD
...
33	1900 MHz	–	1920 MHz	1900 MHz	–	1920 MHz	TDD
34	2010 MHz	–	2025 MHz	2010 MHz	–	2025 MHz	TDD
35	1850 MHz	–	1910 MHz	1850 MHz	–	1910 MHz	TDD
36	1930 MHz	–	1990 MHz	1930 MHz	–	1990 MHz	TDD
37	1910 MHz	–	1930 MHz	1910 MHz	–	1930 MHz	TDD
38	2570 MHz	–	2620 MHz	2570 MHz	–	2620 MHz	TDD
39	1880 MHz	–	1920 MHz	1880 MHz	–	1920 MHz	TDD
40	2300 MHz	–	2400 MHz	2300 MHz	–	2400 MHz	TDD

What is difference in between LTE FDD & TDD?

In both LTE FDD and LTE TDD, the transmitted signal is organized into subframes of 1 millisecond (ms) duration and 10 subframes constitute a radio frame. Each subframe normally consists of 14 OFDM symbols (12 OFDM symbols in case of the so-called “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. Another difference is the other subframes are allocated either for uplink transmission or for downlink transmission.

In case of FDD operation, there are two carrier frequencies, one for uplink transmission (fUL) and one for downlink transmission (fDL). During each frame, there are consequently 10 uplink subframes and 10 downlink subframes and uplink and downlink transmission can occur simultaneously within a cell.

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

Further readings: 3GPP LTE for TDD Spectrum in the Americas, 3GPP standards