LTE TDD (TD-LTE): How much different from LTE FDD?

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LTE TDD (TD-LTE): How much different from LTE FDD?

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By Dr. Nishith D. Tripathi

While initial commercial deployments have focused on FDD (Frequency Division Duplex) version of LTE (Long Term Evolution), interest in the TDD (Time Division Duplex) version of LTE has been rising.  The TDD-based LTE is also known as TD-LTE (Time Division- LTE).  We will discuss TD-LTE from the eyes of LTE FDD; the reader is assumed to be familiar with LTE FDD.

First and foremost, TD-LTE shares the same channel bandwidth between the uplink (UL) and the downlink (DL).  As an example, the LTE FDD uses paired spectrum such as 10 MHz in the UL and separate 10 MHz in the DL.  In contrast, the TD-LTE would use the same 10 MHz for both UL and DL.  While FDD allows simultaneous transmission and reception at an entity such as the eNodeB or the UE, TDD involves either transmission or reception at a given time instant.  The main implications of using TDD instead of FDD are that the service operator does not need large amount of spectrum to deploy TDD and that the average throughput is slightly lower in TDD due to relatively higher overhead.  We summarize below key differences between TD-LTE and LTE FDD related to the frame structure, radio channels and signals, data transmission in UL and DL, and deployment aspects

LTE FDD uses Type 1 Frame structure, whereas TD-LTE uses a Type 2 Frame Structure.  While traditional FDD systems use symmetric spectrum in UL and DL (LTE FDD does allow asymmetric bandwidth), TDD has inherent support for an asymmetric use of UL and DL.  The Type 2 frame structure defines various configurations that basically specify how much time is dedicated to the DL and to the UL.  The UL:DL ratio varies from 3:2 ("uplink-heavy") to 1:9 ("downlink-heavy").  Within a 10 ms frame, subframe 0 and subframe 5 are always used for the DL in TD-LTE.  TD-LTE defines one or two special subframes in a 10 ms frame.  The special subframe has three parts- DwPTS (Downlink Pilot Time Slot), GP (Guard Period), and UpPTS (Downlink Pilot Time Slot).  DwPTS and UpPTS are legacy terms from TDD version of UMTS (Universal Mobile Telecommunication System); formally, there is no "pilot" channel in LTE.  The traditional "pilot" channel is called Reference Signal in LTE.  DwPTS facilitates downlink synchronization, and UpPTS facilitates uplink synchronization.  GP helps avoid interference between the uplink and the downlink and provides the transceiver adequate time to switch from transmit function to receive function and vice versa. 

Roles of radio channels and signals remain the same in TD-LTE.  However, structures of certain signals and channels are different in TD-LTE.  The main reason for structure differences is to support different UL:DL ratios.  The primary synchronization signal is sent in the third OFDM symbol in slot 2 (subframe 1) and slot 12 (subframe 6), and the secondary synchronization signal is sent in the last OFDM symbol of slot 1 (subframe 0) and slot 11 (subframe 5).  Recall that the primary synchronization signal is sent in the last OFDM symbol of slot 0 (subframe 0) and slot 10 (subframe 5) and the secondary synchronization signal is sent in the second last OFDM symbol of these slots/subframes in LTE FDD.  Multiple PRACHs (Physical Random Access Channels) (up to six) may be present in a given UL subframe in TD-LTE, whereas LTE FDD supports zero or one PRACH in a given subframe.  While four random access preamble formats are available to TD-LTE and LTE FDD, an additional fifth format is also available for use in TD-LTE for small cells.  The PDCCHs (Physical Downlink Control Channels) can use up to 2 OFDM symbols in subframes 1 and 6.  In other downlink subframes, up to 3 or 4 OFDM symbols can be occupied by the PDCCHs in LTE TDD just like LTE FDD.  Number of PHICH (Physical HARQ Indicator Channel) groups differs as a function of UL:DL ratio.  HARQ is Hybrid Automatic Repeat reQuest.  Sounding reference signal in the UL has different configurations in TD-LTE.

The overall DL/UL data transmission approach remains the same for TD-LTE and LTE FDD.  There are additional parameters in the DCI (Downlink Control Information) messages carried over the PDCCHs to support resource allocation in TD-LTE.  The UL resource allocation (and UL power control command) specified by the PDCCH in subframe "n" is valid for the UL subframe "(n+4)" in LTE FDD and subframe "(n+k)" in TD-LTE, where k ranges from 4 to 7.  HARQ and semi-persistent scheduling are also affected due to different UL:DL ratios.  While DL HARQ supports up to 8 HARQ processes in LTE FDD, TD-LTE supports up to 15 HARQ processes in the DL.  To support DL transmission, the TD-LTE UE could use ACK/NACK bundling to send a single response to multiple processes or use ACK/NACK multiplexing to provide process-specific HARQ responses.  While UL synchronous HARQ has eight HARQ processes in LTE FDD, the number of TD-LTE HARQ processes ranges from 1 to 7.  Semi-persistent scheduling has additional constraints in LTE TDD.  Due to the channel reciprocity in TD-LTE, the channel conditions in the UL and DL are likely to be similar.  Such knowledge can be exploited by the eNodeB scheduler to make decisions about the DL packet scheduling by observing the UL.

From the perspective of deployment, the availability of unpaired spectrum is needed for TD-LTE.  Due to the tight timing synchronization requirements for TDD, eNodeBs would need a network synchronization mechanism such as GPS (Global Positioning System).  LTE FDD may or may not use GPS.  In addition to the inter-cell interference "management" schemes of LTE FDD such as adaptive modulation and coding, HARQ, UL power control, and ICIC (Inter Cell Interference Coordination), LTE-TDD can exploit GP to reduce inter-cell interference.

In summary, TD-LTE utilizes unpaired spectrum and reuses many of the LTE FDD features and mechanisms.  Differences between LTE FDD and LTE TDD are primarily due to the fundamental TDD/FDD difference and different UL:DL ratios supported by LTE TDD.  Many of the LTE FDD and LTE TDD differences exist at the physical layer, allowing LTE FDD and LTE TDD to benefit from the same LTE ecosystem.  Countries such as India and China may see early widespread deployments of TD-LTE.  TDD-based legacy networks, such as TD-SCDMA (Time Division- Synchronous Code Division Multiple Access) in China, can evolve to TD-LTE to achieve higher spectral efficiency.

  • Really very nice and gives a good overview of TDD and FDD...