WO2009038509A1

WO2009038509A1 - Mimo scheme selection based on user terminal velocity

Info

Publication number: WO2009038509A1
Application number: PCT/SE2007/050669
Authority: WO
Inventors: Afif Osseiran; Magnus A Olsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2007-09-20
Filing date: 2007-09-20
Publication date: 2009-03-26
Also published as: WO2009038509A8

Abstract

A method, radio network node, adapting device, and base station for selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal in a telecommunications system based on a current velocity of said terminal is described. The method comprises deriving a transmission scheme indicator via a mobile terminal link and selecting one scheme from a prestored set of transmission schemes based on the indicator. Thereby a more optimal selection of the spatial transmission scheme may be accomplished for each user, something which enhances the user experience and also significantly improves the throughput/capacity and spectral efficiency of the system.

Description

MIMO scheme selection based on user terminal velocity

TECHNICAL FIELD

The present invention relates to a method and arrangement in a telecommunication system, in particular it relates to a method and arrangement for selecting multi-antenna transmission scheme in a telecommunication system.

BACKGROUND

In future wireless communication systems, e.g. targeting International Mobile Telecommunications Advanced (IMT- Advanced) , it is envisioned that multi-antenna techniques, e.g. Multiple Input Multiple Output (MIMO) and beamforming methods, will be of major importance to achieve high spectral efficiency and user data rates. However, there exist a vast number of different multi-antenna techniques realizing different combinations of spatial diversity, spatial multiplexing and beamforming. The optimal spatial transmission scheme, i.e. ir.ulti-antenna technique, in each situation depends on several parameters, e.g. scenario, Signal-to-Interference plus Noise-Ratio (SINR) , system load, etc.

For stationary or slowly moving users of a radio cell (pedestrian velocities, up to 5-10 km/h) , it has been shown, for example in the WINNER project [1] focusing on the development of radio technologies for mobile systems beyond the third generation of mobile communications, that multi-user (MU) MIMO precoding techniques (e.g. Successive Minimum Mean Square Error (SMMSE) [2] and Regularized Block Diagonalization (RBD) [3] ) performs very well and can provide exceptional downlink spectral efficiency. However, at higher velocities, e.g. 50 km/h, it has been shown that the performance of such techniques degrades significantly and provides no capacity gain compared to a traditional Single Input Single Output (SISO) system [4] . An alternative option is to choose a more adequate multi- antenna technique for high speed users, e.g. Linear Dispersion Codes (LDC) ^"5], which provides a better performance compared to a SISO system. The well-known Alamouti scheme described more in detail in [6] is one example of LDCs.

It is problematic to choose which antenna technique to use for accomplishing optimized performance in a radio cell harboring users with different mobility patterns and possibly also requiring different Quality of Service (QoS) . Especially so in metropolitan areas, typically city centers of large and medium size cities, where the user population is dense and expected to be a mix of stationary or low mobility users (e.g. sitting with their laptops at a cafe) , and users traveling in vehicles at velocities up to e.g. 50-70 km/h. As discussed above, in a microcellular scenario MU MIMO precoding techniques seem to be the best spatial transmission scheme for the stationary or low mobility users, while for the users with higher mobility more robust schemes, e.g. LDCs, are required.

SUMMARY

The present invention aims at providing a solution that at least to a certain extent alleviates the problems indicated above.

It is therefore an object of the present invention to propose an adaptation method and arrangement that estimates/measures the users' velocity/mobility, and adapts the spatial transmission scheme accordingly in a micro-cellular scenario.

The object is achieved with a method or arrangement according to the appended independent claims. Further objects and advantages are evident from the dependent claims.

A first aspect of the present invention relates to a method in a radio base station for selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal based on a current velocity of the terminal. The method steps involves deriving a transmission scheme indicator via a mobile terminal link and selecting one scheme from a prestored set of transmission schemes based on the indicator.

In one embodiment of this aspect of the invention the indicator comprises a mobile terminal velocity indication wherein a velocity value is calculated based on the velocity indication, and based on that value one scheme, from the prestored set of transmission schemes, is selected.

A further embodiment of this aspect of the invention further comprises comparing the velocity value with a prestored threshold value and selecting a low velocity transmission scheme, such as MU MIMO precoding with SMMSE or RBD if the velocity value is below the threshold value and selecting a high velocity scheme such as a LDC if the velocity value is above the threshold value. In another embodiment of this aspect of the invention the indicator is a 1 bit message, wherein a bit value 0 indicates selecting a low velocity transmission scheme, such as MU MIMO precoding with SMMSE or RBD and a bit value 1 indicates selecting a high velocity transmission scheme, such as a LDC. In a further embodiment of this aspect of the invention the transmission scheme indicator is based on GPS measurements .

Another embodiment of this aspect of the invention comprises that the transmission scheme indicator is a terminal velocity value.

In yet another embodiment of this aspect of the invention the transmission scheme indicator comprises a Doppler frequency. In still another embodiment of this aspect of the invention the Doppler frequency is computed based on UL channel estimation made on the terminal link.

A further embodiment of this aspect of the invention comprises estimating at least one of a SINR value and channel rank value and selecting transmission scheme based on at least one of said values.

A second aspect of the present invention relates to a radio network node capable of selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal in a telecommunications system based on a current velocity of said terminal. The node to this end has a radio signal transceiver arranged for performing the method steps of the first aspect of the invention.

In one embodiment of this aspect of the invention the node is a NodeB, an eNodeB or a mobile terminal.

A third aspect of the present invention relates to a transmission scheme adapting device comprising a radio signal transceiver, a mobile terminal velocity estimator and a memory space. The memory space has a set of prestored transmission schemes and a threshold velocity value, wherein velocity values below the threshold are mapable to low velocity schemes, such as MU MIMO precoding with SMMSE or RBD, and velocity values above the threshold are mapable to high velocity schemes, such as a LDC. The device furthermore comprises a transmission scheme selector for selecting a transmission scheme for transmissions to a mobile terminal based on one of mapping an estimated terminal velocity value to the prestored set of schemes and a transmission scheme indicating signal received from the mobile terminal.

In one embodiment of this aspect of the invention the velocity estimator comprises a channel estimator and uses output from the channel estimator for velocity estimation. A fourth aspect of the present invention relates to a radio-base-station capable of selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal in a telecommunications system based on a current velocity of said terminal. The base station comprises an adapting device as described above in the third aspect of the invention.

The proposed inventive solution provides the advantage of accomplishing a more optimal selection of the spatial transmission scheme for each user, something which enhances the user experience and also significantly improves the throughput/capacity and spectral efficiency of the system.

The features described above in relation to the method according to the invention may, where applicable, also be implemented in an arrangement according to the invention with the same advantages as described in relation to the method.

It goes without saying that the above aspects of the invention may be combined in the same embodiment. In the following, preferred embodiments of the invention will be described with reference to the drawings. The invention will now be described more in detail with the aid of preferred embodiments in connection with the enclosed drawing. BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 exemplifies schematically a UE to BS signaling link used for transmission scheme selecting.

Fig. 2 in a flowchart schematically illustrates method steps of the invention according to one embodiment thereof. Fig. 3 in a flowchart schematically illustrates method steps of the invention according to another embodiment thereof.

ABBREVIATIONS

BS= Base Station GPS= Global Positioning System

IMT= International Mobile Telecommunications

LDC= Linear Dispersion Code

MIMO= Multiple Input Multiple Output

MMSE= Minimum Mean Square Error MU= Multi-User

RBD= Regularized Block Diagonalization

SINR= Signal to Interference plus Noise Ratio

SISO= Single Input Single Output SMMSE= Successive Minimum Mean Square Error FDD= Frequency Division Duplex TDD= Time Division Duplex WINNER= Wireless world INitiative New Radio DETAILED DESCRIPTION

In the following, various embodiments of the invention will be described.

Fig. 1 depicts a first node 110 communicating with a second node 120 in a radio communication system. In some embodiments, the first node 110 may be a user equipment such as a mobile cellular radiotelephone, a Personal Digital Assistant (PDA), a laptop, a computer or any other kind of device capable of communicating radio resources, and the second node 120 a base station, a wireless communications station, a fixed station, a control station, a repeater or any similar arrangement for radio communication.

However, the situation may as well be the opposite, such as in some other embodiments, wherein the first node 110 may be a base station, a wireless communications station, a fixed station, a control station, a repeater or a similar arrangement for radio communication, and the second node 120 is a user equipment such as a mobile cellular radiotelephone, a PDA, a laptop, a computer or any other kind of device capable of communicating radio resources.

However, in this text the term user equipment 110 will be used for the first node 110 and the term base station 120 will be used for the second node 120 in order to facilitate the comprehension of the present method. The objective of the invention is to adapt the spatial transmission scheme in a cellular system according to the user velocity, or more specifically to the velocity of the user equipment (UE) sometimes also called mobile terminal or mobile station.

The actual adaptation/selecting/changing of multi-antenna technique may be triggered by a velocity threshold which depends on the radio environment. If, for example, the user velocity is below the desired threshold then MU-MIMO precoding is selected for transmission. On the other hand, if/when the user velocity exceeds the velocity threshold, the transmitter may switch to a more robust scheme e.g. LDC for ascertaining maintaining a transmission performance level or at least minimizing performance degradation.

The inventive solution targets microcellular deployments in metropolitan areas where the user population is a mix of stationary or low-mobility (pedestrian) users and higher mobility (up to 50-70 km/h) users traveling in vehicles.

For stationary or low mobility users, e.g. up to approximately 10 km/h, MU MIMO precoding schemes such as e.g. SMMSE or RBD is preferred. For higher mobility users the performance of these kinds of schemes drops drastically and hence more rcbust schemes, e.g. LDCs are preferred. An example of LDCs is Alamouti which is more robust against Doppler frequency compared to MU MIMO precoding schemes.

The selecting- or adaptation method or mechanism measures/estimates the velocity/mobility of the user via info from his/her UE or via info derived from the radio link. Some options on how this can be carried out exist. Fig. 2 exemplifies method steps of one embodiment of the invention wherein the UE is assumed equipped with a GPS receiver for positioning purposes, which receiver then also can be used for velocity measurements. The UE then derives a velocity value from the GPS data in a per se known way and in step 201 signals the velocity value to the BS, such as a NodeB station or eNodeB station (evolved NodeB) . The value is then in step 202 compared to a threshold value prestored in the BS and if found being lower than said threshold, MU MIMO precoding is applied in step 203. Otherwise step 204 follows wherein Linear Dispersion Codes (LDCs) are applied.

Alternatively, the BS receives 301 a 1 bit message from the UE, such as a Channel Quality Indicator (CQI) message, indicating the preferred transmission (Tx) scheme to be chosen/selected/applied. A bit value 1 for example indicates applying MU MIMO step 303, and value 0 indicates applying LDC step 304. This procedure is shortly exemplified in Fig. 3. In another embodiment the velocity or Doppler frequency is estimated based on a channel estimation process made on the radio link, where a channel estimate which is rather constant over time indicates low velocity and an estimate changing over time indicates a higher velocity. In a Frequency Division Duplex (FDD) system, the UE performs the channel estimation procedure and signals CQI to the BS for indicating which transmission scheme to be chosen (see above) .

In a Time Division Duplex (TDD) system, the BS may perform the estimation procedure on the UL channel and transform it to the DL utilizing the reciprocity principle. The maximum Doppler frequency fβ or velocity V _{can De} estimated at the BS based on the uplink channel (assuming a TDD system) . In fact f_D is proportional to the user velocity [7] and given by:

f _ ^V _ ^VC

Where λ is the wave length, C the speed of the light and

/the transmission frequency. Then the estimation of the velocity can be done in the following three stages:

Stage 1: Based on the synchronization process the BS will be able to notice a change in the Doppler frequency. Let

us assume " , the time instant of the synchronization.

Stage 2: Comparing the current channel estimate, using for instance an MMSE estimator (other estimators could also be used) , with previous estimates, the BS would estimate the coherence time T₀ which is the periodicity of the time variation of the radio channel. For instance it can easily be obtained by constantly comparing the auto-correlation functions of the estimated channel Λ(/Q) ^:

Where R_x(I^t₂) is the auto-correlation of the random variable X and equal to £[.v(/j).r^*(/₂)] . Δ is a time interval greater than zero, β is close to one and can be chosen according to the desired accuracy (e.g. granularity) of the estimated coherence time. Stage 3: Finally the Doppler frequency can be obtained by using [8]

The adaptation mechanism then takes the estimate/measurement and uses it for selection of spatial transmission scheme. If the user velocity is below a certain desired velocity threshold F , a MU MIMO precoding scheme (e.g. SMMSE) is applied. Otherwise a LDC is applied. Let S be the transmission scheme, then the selection process can be summarized as follows:

[MU-MIMO if v≤r s ,

[ LDC otherwise

It should also be mentioned that this adaptation mechanism can be part of another spatial adaptation mechanism. For example, for the higher mobility users it can be advantageous to adapt the LDC according to e.g. SINR and channel rank [4] . Nothing prevents the adaptation mechanism based on velocity to be combined with that approach. In that case, MU MIMO precoding is still applied for the low mobility users, while a second step is applied for the high mobility users where SINR and channel rank is considered when selecting the LDC.

The solution according to the present invention has the advantage of providing a more optimal selection of the spatial transmission scheme for each user, which enhances the user experience and also significantly improves the throughput/capacity and spectral efficiency of the system.

Any examples and terminology relating to 3GPP LTE standard being used herein should not be seen as limiting the scope of the invention, the methodology of which in principle can be applied to any communication system. The described subject matter is of course not limited to the above described and in the drawings shown embodiments, but can be modified within the scope of the enclosed claims.

REFERENCES

[1] WINNER, Wireless World Initiative New Radio, www.ist- winner . org

[2] V. Stankovic and M Haardt, "Multi-user MIMO Downlink Precoding for Users with Multiple Antennas", in Proc. of the 12-th Meeting of the Wireless World Research Forum (WWRF), Toronto, ON, Canada, Nov. 2004.

[3] V. Stankovic and M. Haardt, "Novel Linear And Non- Linear Multi-User Mimo Downlink Precoding With Improved Diversity And Capacity", WWRF#16, April 2006.

[4] IST-4-027756 WINNER II, "D3.4.1 The WINNER II Air Interface: Further Refinement Spatial-Temporal Processing Solutions," November 2006.

[5] B. Hassibi and B. M. Hochwald "High-Rate Codes That Are Linear in Space and Time," IEEE Transactions on Information Theory, Vol. 48, No. 7, July 2002.

[6] S. M. Alamouti, "A simple transmit diversity technique for wireless communications," IEEE Journal on Selected Areas in Comm. , vol. 16, no. 8, pp. 1451- 1458, Oct. 1998.

[7] W. Lee, Mobile Communications Engineering, McGraw- Hill, 1997.

[8] T. S. Rappaport, Wireless Communications, Prentice Hall, 2002.

Claims

1. A method in a radio base station for selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal based on a current velocity of said terminal, said method comprising:

- deriving a transmission scheme indicator via a mobile terminal link, selecting one scheme from a prestored set of transmission schemes based on said indicator.

2. A method according to claim 1, wherein said indicator comprises a mobile terminal velocity indication and the method further comprises:

- computing a velocity value based on said velocity indication, and selecting one scheme from said prestored set of transmission schemes based on said value.

3. A method according to claim 3 further comprising comparing said velocity value with a prestored threshold value and selecting a low velocity transmission scheme, such as MU MIMO precoding with SMMSE or RBD if the velocity value is below said threshold and selecting a high velocity scheme such as a LDC if the velocity value is above said threshold.

4. A method according to claim 1, wherein said indicator is a 1 bit message, wherein a bit value 0 indicates selecting a low velocity transmission scheme, such as MU MIMO precoding with SMMSE or RBD and a bit value 1 indicates selecting a high velocity transmission scheme, such as a LDC.

5. A method according to any one of claims 1-4 wherein the transmission scheme indicator is based on GPS measurements .

6. A method according to any one of claims 1-5 wherein the transmission scheme indicator is a velocity value.

7. A method according to any one of claims 1-3 wherein the transmission scheme indicator comprises a Doppler frequency.

8. A method according to claim 7 wherein the Doppler frequency is computed based on UL channel estimation made on said terminal link.

9. A method according to any one of claims 1-8 further comprising estimating at least one of a SINR value and channel rank value and selecting said transmission scheme based on at least one of said values.

10. A radio network node capable of selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal in a telecommunications system based on a current velocity of said terminal, said node comprising a radio signal transceiver arranged for performing the method according to one of claims 1-9.

11. A radio network node according to claim 10, wherein the node is a NodeB, an eNodeB or a mobile terminal.

12. A transmission scheme adapting device comprising:

- a radio signal transceiver,

- a mobile terminal velocity estimator, a memory space comprising a set of prestored transmission schemes and a threshold velocity value, wherein velocity values below said threshold are mapable to low velocity schemes, such as MU MIMO precoding with SMMSE or RBD, and velocity values above said threshold are mapable to high velocity schemes, such as a LDC. - a transmission scheme selector for selecting a transmission scheme for transmissions to a mobile terminal based on one of mapping an estimated terminal velocity value to said set of schemes and a transmission scheme indicating signal received from said terminal.

13. A device according to claim 12 wherein said velocity estimator comprises a channel estimator and uses output from said channel estimator for velocity estimation.

14. A radio-base-station capable of selecting a spatial transmission scheme to be used for multiple input multiple output (MIMO) transmissions to a mobile terminal in a telecommunications system based on a current velocity of said terminal, said base station comprising an adapting device according to one of claims 12-13.