Dr. Tripathi, a Principal Consultant at Award Solutions, joined Award Solutions in March 2004, bringing his knowledge and experience in mobile wireless technologies to facilitate the planning, development and delivery of technical training seminars. He teaches and consults on various technologies including, LTE E-UTRAN and EPC, WiMAX, UMTS R99, HSDPA, HSUPA, HSPA+, 1xEV-DO, IMS, and WiMAX. He has taught various aspects of 3G and 4G commercial cellular technologies including but not limited to network operations, network planning, and network optimization.
Since receiving his doctorate in Wireless Communications from Virginia Tech, Dr. Tripathi has held several strategic positions in the wireless arena. For Nortel Networks, he worked to analyze and optimize the performance of CDMA networks, in such areas as load balancing, handoff, power control, supplemental channel management, and switch antenna diversity. As a Senior Systems Engineer and Product Manager for Huawei Technologies, Dr. Tripathi worked on the infrastructure design and optimization of CDMA2000, 1xEV-DO, and UMTS radio networks. He has significant experience designing, analyzing, and field-testing Radio Resource Management algorithms for CDMA2000 and 1xEV-DO.
In 2001, he co-authored a book on Radio Resource Management, and he is the author of numerous research papers and patent submissions. He has contributed chapters to two books on applications of fuzzy logic to communications and applicability of network neutrality principles to wireless systems. He is a co-author of an upcoming book on cellular communications (to be published by IEEE/Wiley).
Dr. Tripathi's position at Award Solutions puts him at the forefront of emerging technologies. He has authored courseware related to LTE, WiMAX, 1xEV-DO, HSUPA, UMTS optimization, 1xEV-DO RF optimization, advanced antenna techniques, and IP convergence. In addition to teaching the students in the Industry, he also trains his colleagues (i.e., instructors) on various technologies (e.g., LTE, WiMAX, 1xEV-DO, HSDPA, HSUPA, 802.11n, and advanced antenna techniques). His extensive knowledge, hands-on experience with commercial deployments, and enthusiasm for the subject matter, coupled with a passion for teaching, provide the foundation for consistently enjoyable, informative, and effective classes.
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
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...