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Key Characteristics and Future Prospects of TD-LTE-Advanced 3 July.2011

Background

In the 6th ITU-R WP5D Meeting held in Dresdner, Germany in October 2009, ITU received six IMT-Advanced candidate technical proposals from different countries and standardization organizations. After ITU collecting 3G global mobile communication technology standard IMT-2000, a new round of global mobile communication technology standard fight begins. TD-SCDMA candidate technology submitted by Datang Telecom on behalf of China in 1999 was later adopted as one of the five IMT-2000 global core standards. In 2009, its follow-up evolution technology TD-LTE-Advanced submitted to ITU by China became one of six candidate IMT-Advanced technology proposals. This article focuses on the details of TD-LTE-Advanced candidate technology submitted by China, such as: key technology characteristics, the conclusion on the technology and our prospects.

 

1.     TD-LTE-Advanced technology proposal

1.1          Background of TD-LTE-Advanced submission 

After TD-SCDMA joined 3GPP standardization organization, Chinese enterprises and research institutes started works on standardization of TD-SCDMA and its follow-up evolution and enhancement in 3GPP with more international enterprises’ participation. In 2007, under the joint effort from Chinese operators, equipment enterprises and research institutes led by the government, 3GPP completed combination of two frame structures— LTE and TDD with TDD’s specific timeslot structure remained. This action pushed TD-LTE into one of 3GPP LTE technology standards. Therefore, Chinese enterprise took the leadership in TD-LTE standards and its enhancement technology, and Datang was believed as the main contributor to TD-LTE standard.  

 

In the 6th ITU-R WP5D Meeting held in Dresdner, Germany in October 2009, TD-LTE-Advanced technical proposal with self-owned intellectual property right was submitted by China and adopted as one of the candidate technical proposals for further technical evaluation.

 

1.2          TD-LTE-Advanced key technology

Since the strong compatibility between LTE system and TDD mode, TD-LTE-Advanced key technology characteristics are similar with FDD. TD-LTE R8 key technology parameter and TD-LTE-Advanced key technology will be elaborated below.

 

TD-LTE R8 basic parameter is shown in the table below.

Table 4-1 TD-LTE R8 Basic Parameter

 

character

specification

Access technology

UL

DF TS -OFDM

DL

OFDMA

bandwidth

1.4,3,5,10,15,20MHz

Minimum TTI

1ms

Sub carrier interval

15kHz

CP length

Short

4.7us

long

16.7us

modulation

QPSK,16QAM,64QAM

 

Space mutilplex technology

Up to 1 layer for UL per UE

Up to 4 layers for DL per UE

MU-MIMO FOR UL and DL

 

TD-LTE-Advanced frame structure

For frame structure, TD-LTE-Advanced keeps the special sub-frames for TD-SCDMA and TD-LTE. The typical frame structure is shown in the below chart—10ms wireless frame contains two sub-frames.

 

For TDD, uplink and downlink are separate in time domain, yet with same carrier frequency, which means there are 10 sub-frames available in uplink and downlink for each 10ms period, and each sub-frame could go either uplink or downlink.

 

 

Carrier convergence technology

To meet the minimum bandwidth 40MHz according to ITU-R and IMT-Advanced, TD-LTE-Advanced adopted carrier convergence technology with following characteristics:

-      meeting maximum 100MH bandwidth requirement through multiple carrier convergence;

-      Supporting convergence of continuous and discontinuous frequencies;

-      Keeping compatibility between R8 and R9 on each carrier;

-      Each transmission block has one corresponding carrier transmission, multiple carriers transmit parallelly;

-      Supporting PDCCH channel cross carrier scheduling;

-      Terminal supports any bandwidth level of single carrier and multi -carrier;

 

Downlink MIMO enhancement

To further improve peak rate and system spectrum efficiency, downlink MIMO enhancement includes following features:

-      Downlink supports maximum of 8×8 MIMO and 8 layers of transmission;

-      On the basis of satisfying ITU peak spectral efficiency, further achieving maximum peak efficiency of 30/Hz;

-      Non-transparent MU-MIMO makes best use of multi-channel spatial multiplexing to improve cell’s average spectral efficiency;

-      Supporting maximum of 4 user pairing;

-      Improving performance channel estimation and receptivity through optimizing CSI-RS and DM-RS;

-      Optimizing CSI feedback and improving downlink precoding performance;

-      Further improving precoding performance of TD-LTE-Advanced system through channel reciprocity enhancement;

-      Enhancing reference mark design;

-      Improving performance of channel measurement and estimation through CSI-RS to support CSI feedback optimization and reduce reference mark expenditure to minimum level;

-      Achieving non-transparent MU-MIMO through designing 4 orthogonal-port DM-RS to reduce the interference among  multi-user efficiently and improve demodulation performance;

 

Uplink MIMO technology

To further improve uplink peak rate and system spectral efficiency and to satisfy requirement on ITU uplink peak spectral efficiency, enhancement for TD-LTE-Advanced system’s uplink MIMO includes following features:

-        Uplink PUCCH channel transmission diversity;

-        Improving edge users’ uplink control channel performance;

-        Supporting uplink users’ maximum MIMO transmission with 4 stream;

-        Matching the requirement of maximum of 15bps/Hz;

-        Improving uplink users rate and cell throughput;

-        Supporting 2 stream and 4 stream transmission through design of 2 stream and 4 stream transmission code;

-        Supporting uplink MU-MIMO;

Further improving average spectra frequency of uplink MU-MIMO;

Supporting uplink MU-MIMO through enhancing uplink reference mark design;

-        Supporting uplink receiver enhancement;

Improving uplink MIMO performance by Turbo-SIC receiver to reduce interference between stream, between users and between cells;

 

ComP technology

ComP (Coordinated Multiple Points Transmission/Reception) technology is adopted by TD-LTE-Advanced, so as to reduce interference between cells of LTE system, realize network combined with same frequency and improve system spectra efficiency. ComP means that multiple transmission points separated physically coordinate to attend data transmission for one terminal (PDSCH) or coordinate to receive data from one terminal (PUSCH). Coordinated multiple transmission points normally means base stations of different cells. Generally speaking, uplink ComP has little effect on physical layer standard, so unless indicated particularly, all refer to downlink ComP in the following text. ComP technology includes several major characteristics as below:

-        It could be sorted into joint transmission (JP) and co-scheduling/ beamforming (CS-BF) according to coordinating transmission pattern;

-        JP turns Interference cell into useful signal with complexity increased;

-        CS-BF avoids and reduces interference between cells by coordination;

-        Concentration method of JP

-        For multiple users joint transmission, Intra-NodeB and MU-JP-ComP

Self-evaluation result shows that it could improve system average spectral efficiency and edge efficiency at the same time;

-        For single user joint transmission, Intra-NodeB SU-JP

Self-evaluation result shows that it could improve edge performance, but cannot improve average throughout;

-        For multiple points coordination transmission among basestations, ComP of Intra-NodeB

High requirement for X2 port causes high practice complexity;

-        For ComP technology based on RRH distribution antenna:

This method could further improve performance, yet has high requirement on transmission and baseband process. 

 

Dual- stream beamforming technology

Dual-stream beamforming technology takes best use of channel reciprocity of TDD system for higher spectral efficiency. TD-LTE-Advanced supporting dual-stream beamforming contains major characteristics as below:

-        Utilizing correlation and non-correlation of channel at the same time;

-        Typical application 1- bipolarization antenna;

-      Typical application 2-grouping antenna;

-        Two feedback patterns;

ž   No-PMI: applies TDD channel reciprocity and estimates through uplink SRS for channel estimation;

ž   PMI: beamforming through terminal feedback channel information, mainly used for FDD;

-        Dedicated reference mark DMRS design supports four patterns:

ž   Single user double streams (orthogonal DMRS);

ž   Double users single stream (orthogonal DMRS);

ž   Double users double streams (semi-orthogonal DMRS);

ž   Four users single streams (semi-orthogonal DMRS);

 

Hierarchical heterogeneous network, relay and Femtocell

To meet increasing requirement on data service density, TD-LTE-Advanced system applies macro cell to complete basic coverage, and  hot micro cell, indoor femtocell and relay station hierarchically, increases low power node density, so as to provide more cell split gain. TD-LTE-Advanced system has layered heterogeneous network features as below:

-        Hierarchical network (Het-Net)

-        70% data service happens in 10% hotspot area according to statistics;

-        Multiple nodes combination networking of Macro, Pico, Femto and Relay as required;

-        The point is interference coordination end, managing to ensure control channel and sevice channel in the same frequency for network combination;

-        Relay;

ž   Relay base station efficiently expands coverage to enhance flexibility of network construction;

ž   TD-LTE-Advanced currently preferentially supports Typel Relay, i.e., level 3 Relay transmitted by wireless Back-Haul;

ž   Mainly involves control channel of Back-Haul link and service channel design;

-        Femto

ž   Efficiently increasing throughout of indoors users data by Femto;

ž   Femto normally applies small-capacity and low-cost CSG mode;

ž   Efficiently manage and control Femto by Femto gateway access;

ž   Equipment simplifying and Cost control are key points of Femto;

ž   Need analyzing Femto’s self configuration and self optimizing technology;

ž   Need solving interference coordination between cells under the condition of intensive Femto.

 

To sum up, on the basis of LTE-R8, TD-LTE-Advanced applies layered network technologies, including: carrier covergence technology, uplink MIMO technology, downlink MIMO enhancing technology, ComP technology, double stream beamforming technology, Relay and Femto. With these technologies, TD-LTE-Advanced performance is enhanced efficiently covering areas including: system bandwidth, peak value rate, edge performance, spectral efficiency, system throughout and networking’s flexibility. All these meet every aspect of requirements for ITU IMT-Advanced candidate technologies.     

 

2.     Conclusion and prospects

IMT-Advanced standardization lasting for 10 year is close to the end. With submitting 6 candidate technologies, the 4th generation global mobile telecommunications IMT-Advanced standard pattern became quite clear. There is potentially no suspense who will win IMT-Advanced standard in Oct. 2010. These two standards are quite similar regarding technology, complexity and theoretical performance; therefore, the true competition will be revealed in future industry development.  

 

China, as the proposer of 3G technology TD-SCDMA and the leader of 4G technology TD-LTE-Advanced, confirmed its leadership in international standardization. As one of the pioneers in TD-LTE-Advanced technologies, we will pay more efforts on new technologies R&D, keeping the leadership in international standardization. On the other hand, cooperating with operators, we will make further progress on commercialization of 3G and 4G technologies, and expand TD-SCDMA and TD-LTE industry all over the globe.  

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