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Dual-stream Beamforming Technology—TD-LTE Innovative Technology of Datang Mobile 3 July.2011

As the main technology for TD-LTE network construction, dual-stream beamforming technology is a multi-antenna enhancement technology. Combining both smart antenna beamforming technology and MIMO spatial multiplexing technology, it is the innovation achieved by China Mobile and Datang Mobile together, presenting the technologies of Chinese communication industry.


1. Requirement analysis on introduction of 8-antenna dual-stream beamforming technology 


Multi-antenna technology is the developing trend of antenna technology. 8-antenna configuration has been introduced for existing TD-SCDMA, 8-sending-2-receiving-antenna for TD-LTE, and 8-sending-8-receiving antenna configuration will be introduced for TD-A.


Concerning improving coverage and reducing CAPEX imported into TD-LTE, 8-antenna configuration was introduced in TD-LTE system. With 8-antenna, TD-SCDMA could evolve into TD-LTE smoothly with TDD system remained, providing its advantages in beamforming.


a) Requirement on system smooth evolution


Currently, TD-SCDMA network spreads around China in high speed. TD-LTE—evolution technology of TD-SCDMA has been listed onto the schedule of future development of mobile communication network construction. How to guarantee smoothly evolution into TD-LTE when TD-SCDMA network construction is underway is the common concern for both operators and equipment providers.


Out of consideration for system smooth evolution, Datang Mobile issued the solutions of product equipment sharing the common platform, which maintains the existing investment on network construction and guarantees the convenience and efficiency of network updates. During smooth evolution of main equipment, out of consideration for reducing network construction cost and difficulties of base station construction, the existing antenna system of TD-SCDMA network should be remained the same for further application in TD-LTE. To complete smooth evolution of antenna system, 8-antenna, applied for macro coverage in TD-SCDMA network, will keep in use in TD-LTE network.


b) Requirement on technology evolution


Beamforming technology is a linear preprocessing technology based on close-spaced antenna display. Beamforming is conducted according to users’ channel with features of expanding coverage, increasing system capacity and reducing interference. As the core technology of TD-SCDMA, beamforming technology is widely applied in 3G network of China Mobile.


In Release 8 of LTE technology standard, single-stream beamforming technology is introduced for increasing cell’s average throughput and edge throughput as well as reducing interference between cells. However, it is necessary to expand beamforming technology for further requirement on high speed for LTE Release 9 and TD-Advanced system. Take 8-antenna—the most efficient antenna defined by LTE for example. In multi-antenna theory, single user with 8X2-antenna system is capable transmitting two data stream in the same time, which means channel capacity isn’t fully developed in single-stream beamforming technology in LTD Release 9 standard. According to channel capacity related theory, channel capacity is the logarithm of SNR; increase of capacity becomes slower with SNR’s increasing. For high SNR, reducing power of certain data stream into half won’t cause huge reduction to the data stream. Under this condition, the other half power could be applied for sending a new data stream, which would hugely increase transmitting capacity.


To meet the requirement on capacity increase with 8-antenna application and beamforming technology expansion, China Mobile and Datang Mobile cooperate to introduce the double-stream beamforming technology applied with 8-antenna plus 3.


2. Introduction on double-stream beamforming technology


Double-stream beamforming technology is applied in the condition with sufficient signal scatterers, combining both smart antenna beamforming technology (i.e. single-stream beamforming technology) and MIMO. In TD-LTE system, with symmetry of TDD channel, two beamforming data stream are transmitted in the same time for spatial multiplexing. The new technology remains features of the traditional single-stream beamforming technology including, wide coverage, cell capacity increasing and interference reducing. By the new technology, both reliability of edge users and throughput of central users of cell are increased.


Based on multi-antenna theory, number of receiving antenna cannot be less than number of data stream in spatial multiplexing. For application of 8-antenna double-stream beamforming technology, there should be 2-antennas in receiving end at least.


According to different scheduled users, double-stream beamforming technology could be divided into two categories—single-user double-stream beamforming technology and multi-user double-stream beamforming technology.


a) Single-user


For single-user double-stream beamforming technology, uplink channel is measured by base station; after receiving uplink channel status information, base station calculates two beamforming vectors according to uplink channel information; with the beamforming vectors, downlink beamforming is conducted for two data stream to send (See Table 1).


Table 1


With single-user double-stream beamforming technology, single user could transmit two data stream at one certain time, receiving beamforming gain and spatial multiplexing gain; transmitting rate is faster than single-stream beamforming technology, so as to increase system capacity.


b) Multi-users


For multi-user double-streaming beamforming technology, base station conducts multiple users’ match according to uplink channel information or UE feedback; after finishing multiple users’ match, beamforming vectors are generated according to certain criteria; and conducts forming for each UE and stream (See Table 2). With smart antenna beam direction theory, multi-user double-streaming beamforming technology achieves space division multiple address of multiple users.


Table 2


c) Standard development


Standardization of Double-stream beamforming technology is a project contributed with efforts from China Mobile and Datang Mobile.


Between 2007 and 2008, China Mobile and Datang Mobile submitted full solutions of double-stream beamforming technology to IMT-Advanced technology tem and standard sub-team separately, and both received approval.


Before 3GPP double-stream beamforming technology get approval, Datang Mobile began to push this technology toward in the technical range of LTE-Advanced (Release 10) since June 2008.


In 2009 March, double-stream beamforming technology received approval in 3GPP and discussion on relevant standardization works—RAN 1 began.


In 2009 December, double-stream beamforming standardization works almost finished; and the latest version of relevant protocol specifications (TS 36.211, TS 36.212, TS36.213 and TS 36.331) released in December included standards related to double-stream beamforming technology. Currently, only radio frequency related indexes are left for further standardization, expected to finish in the first quarter of 2010.


d) Implementation in practice


4+4 bipolarization antenna is a classic 8-antenna form and is suitablefor double-stream beamforming technology. In the existing network of TD-SCDMA, 4+4 bipolarization antenna supports F+A+E frequency range; if TD-LTE is supported within this frequency range, TD-SCDMA could smoothly evolve into TD-LTE with existing 4+4 bipolarization antenna, which therefore could keep applied in TD-LTE system.


To analyze the difficulty of implementation of double-stream beamforming technology, from analysis angle, double-stream beamforming technology could be recognized as the classical application of multi-antenna channel singular value decomposition, with little new theory problems to solve. From angle of signal processing, its implementation system is almost mature, and work is focus on calculation optimization. Therefore, based on existing theory analysis and signal processing technology as well as Datang Mobile’s in-depth understanding in smart antenna beamforming technology, the implementation of double-stream beamforming technology is quite easy to practice and will be applied in TD-LTE system soon.


e) Applications scenarios


8-antenna double-stream beamforming technology is the main technology for TD-LTE network construction, applied in macro cell coverage outdoors. It could increase space isolation efficiently and reduce interference between data stream.


Network construction of TD-LTE issued by Datang Mobile is shown in the chart below. With 4+4 bipolarization antenna, double-stream beamforming technology could cover macro cell outdoors. To fill the gap left by macro coverage, 1+1 bipolarization is applied in coverage on street stations outdoors. 2X2 MIMO is applied for micro cell coverage indoors (See Chart 3).


3. Advantages of 8-antenna double-stream beamforming technology


1) Increasing system throughput and building high-quality TD-LTE network


Double-stream beamforming technology could increase throughput of TD-LTE system efficiently. Compared with 2X2 MIMO—the basic antenna configuration of TD-LTE, both sector throughput and edge throughput are hugely increased in 8X2 double-stream beamforming technology. According to evaluation result from IMT-Advanced, compared with 2-antenna MIMO, the maximum of sector throughput is increased by around 80% and maximum of edge throughput is increased by around 130% in 8-antenna double-stream beamforming (Chart 4).


Table 4


2) Radius of cell increases and CAPEX investment of network construction reduces


According to TD-LTE coverage theory, coverage of 2-antenna of TD-LTE is limited to uplink service channel. Based on simulation evaluation, take uplink service channel edge rate of 64Kbps as prerequisite, cell coverage’s radius of 8-antenna of TD-LTE is about twice as 2-antenna of TD-LTE. Compared with network construction of 2-antenna of TD-LTE, implementation of 8-antenna efficiently reduces stations of TD-LTE and CAPEX of TD-LTE network construction.


The coverage radium of 8-antenna applied by TD-LTE is similar to TD-SCDMA’s. It shares the common coverage and station address with TD-SCDMA. In the smooth evolution process from TD-SCDMA to TD-LTE, resources like existing station address and antenna system could be reused, further reducing CAPEX of TD-LTE network construction.


Table 5


According to experience of TD-SCDMA network construction, coverage radium of single station may only reach 500m or even less in network construction in practice. Take the actual coverage radium of TD-SCDMA as the premise of evaluation of TD-LTE coverage, the coverage performances of 2-antenna and 8-antenna of TD-LTE are analyzed.


Based on simulation evaluation, 8-antenna of TD-LTE could efficiently improve the limited issues—increasing edge rate and expanding coverage radium at the same time (See Table 6).


Table 6


Analysis of coverage performance of 2-antenna and 8-antenna of TD-LTE is shown as above. When the coverage radium is 500m, uplink edge rate of 8-antenna of TD-LTE is 500Kbps, and 2-antenna’s is 64Kbps. Therefore, with the same actual coverage radium with TD-SCDMA, obvious advantages of edge rate of 8-antenna of TD-LTE highlighted compared with 2-antenna of TD-LTE; however, there isn’t obvious increase of edge rate from 2-antenna of TD-SCDMA to TD-LTE. If 2-antenna of TD-LTE reaches edge rate of 500Kbps, its coverage radium will only be around 280m, unable to share the common station address and coverage with TD-SCDMA system.


Based on analysis above, 8-antenna applied by TD-LTE could improve coverage performance dramatically; high rate of TD-LTE is highlighted with premise guaranteeing common coverage with TD-SCDMA.


4. Datang Mobile released double-stream beamforming technology in advance 

In 2009 December, TD-LTE new technology news conference “Achieving Dreams with innovative Technology” was held by Datang Mobile in headquarter of Datang Telecom Group in Beijing. In the conference, Datang Mobile released double-stream beamforming technology in advance, set up TD-LTE memo network, and conducted 8-antenna double-stream beamforming technology performance presentation.


The coverage of TD-LTE demo network presented in the conference ranged street from Xueyuan Bridge to Xuezhi Bridge on Xueyuan Road in Haidian District. During the demonstration car’s travel, throughput advantages of 8-antenna double-stream beamforming technology of TD-LTE were highlighted compared with 2-antenna MiMO technology.


In the presentation, coverage performance of 8-antenna was much better than 2-antenna. The two ends of demonstrated street were close to coverage edge of 2-antenna, where throughput of 2-antenna was declined, yet 8-antenna remained stable. When the demonstrated car traveled under the overpass at the edge of the road, signal receiving was affected. 2-antenna was hugely declined with obvious decreasing throughput; 8-antenna was affected as well, yet its throughput decreased slightly with stable performance.


According to analysis of value change during the whole demonstration, the average advantage of spectral efficiency increased by 40%-50% from 2-antenna MIMO to 8-antenna double-stream beamforming technology.


5. Conclusion 

Introduction of 8-antenna double-stream beamforming technology provides technical guarantee to smooth evolution from TD-SCDMA system to TD-LTE system. Its ability of efficiently improving throughput and edge coverage makes it focus in TD-LTE innovative technology.

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