In this blog a brief description of the motivations for LTE TDD (Time Division Duplexing) will be presented.

LTE supports three duplexing techniques:

a) FDD: In FDD mode, a pair of spectrum chunks is provided for in the uplink and downlink. At a given instant, transmission occurs in one spectrum chunk, and reception occurs in another spectrum chunk.

b) TDD: In this mode, the available frame duration is divided into different parts in the time domain for the uplink and downlink. The number of uplink and downlink subframes are (statically, in practice) configurable in TDD mode.

Half-FDD:  H-FDD is a special case of FDD where an H-FDD device uses different frequencies to transmit and receive while communicating with an FDD eNB. However, it transmits and receives at different times.

  

For all the three modes of duplexing, OFDMA-based multiple access technology is used for downlink transmission. The SC-FDMA multiple access technology is used for uplink transmission in all the FDD, TDD and H-FDD modes.

So… Why TDD?

If FDD works, then why do we need LTE TDD? Well, there are clearly several advantages of TDD over FDD. Additionally, there are several reasons and motivations to deploy such system.

  • TDD Spectrum: TDD LTE (or TD-LTE) can be deployed using an unpaired band where both uplink and downlink communication will share a common frequency spectrum. Like in the case of LTE FDD, TD-LTE supports scalable bandwidths from 1.4 MHz to 20 MHz depending on the frequency band. The availability of huge chunks of unpaired spectrum for broadband allows TD-LTE to exploit the available resources at a lower cost when compared with that of the FDD spectrum. An example of such band is the 2.4 to 2.59 GHz band in the USA. This is, perhaps, the most common reason why network operators have decided to deploy LTE-TDD: the lack of paired spectrum.
  • Network Architecture: The overall network architecture remains the same, while very few changes are required in the protocol stack as compared to FDD systems. Both FDD and TDD systems are harmonized using the same OFDMA and SC-FDMA access solutions and the protocol stack, except for the minor changes due to the switching between the UL and DL.
  • Potential for Lower Cost eNB and UE: For TDD systems, the devices do not require duplexer systems to allow simultaneous transmission and reception like FDD systems. This enables simpler and economical eNBs and UEs.
  • Efficiency of LTE: TD-LTE uses the same bandwidth and access methods as the FDD systems. This allows TD-LTE to provide high spectral efficiency like FDD, but the maximum data rates achievable will be less compared to FDD due to the sharing of radio resources between the UL and DL.
  • Potentially Attractive for Asymmetric Services: Since broadband communications are data-centric and essentially asymmetric in nature, TD-LTE can exploit efficient scheduling schemes to support data services.
  • Evolution Path for Legacy TDD Systems: TD-LTE supports an evolution path for both High Chip Rate (HCR) UMTS TDD and Low Chip Rate (LCR) UMTS TDD systems (or TD-SCDMA).