WO2008113210A1 - Évaluation de la qualité de canaux dans un système mimo sans fil à plusieurs utilisateurs. - Google Patents

Évaluation de la qualité de canaux dans un système mimo sans fil à plusieurs utilisateurs. Download PDF

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Publication number
WO2008113210A1
WO2008113210A1 PCT/CN2007/000909 CN2007000909W WO2008113210A1 WO 2008113210 A1 WO2008113210 A1 WO 2008113210A1 CN 2007000909 W CN2007000909 W CN 2007000909W WO 2008113210 A1 WO2008113210 A1 WO 2008113210A1
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WIPO (PCT)
Prior art keywords
precoding
vector
parameter
codebook
vectors
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PCT/CN2007/000909
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English (en)
Inventor
Mattias Wennstrom
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Huawei Technologies Co., Ltd.
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Application filed by Huawei Technologies Co., Ltd. filed Critical Huawei Technologies Co., Ltd.
Priority to CN2007800522796A priority Critical patent/CN101682909B/zh
Priority to PCT/CN2007/000909 priority patent/WO2008113210A1/fr
Publication of WO2008113210A1 publication Critical patent/WO2008113210A1/fr

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0417—Feedback systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W72/00—Local resource management
    • H04W72/50—Allocation or scheduling criteria for wireless resources
    • H04W72/54—Allocation or scheduling criteria for wireless resources based on quality criteria
    • H04W72/542—Allocation or scheduling criteria for wireless resources based on quality criteria using measured or perceived quality
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0452—Multi-user MIMO systems

Definitions

  • the present invention relates to a method in a Mobile Station (MS) for channel quality estimation in a multiple user Multiple Input Multiple Output (MIMO) system, said system including a Base Transceiver Station (BTS) communicating with a first MS and with at least one other MS, wherein the communication with said first MS uses a first precoding vector and the communication with each of said at least one other MS uses a separate interfering precoding vector being different from the first precoding vector, said first precoding vector and the at least one interfering precoding vector being part of a precoding codebook, said codebook comprising a set of possible interfering precoding vectors including both precoding vectors being orthogonal to said first precoding vector and precoding vectors being non-orthogonal to said first precoding vector.
  • BTS Base Transceiver Station
  • the present invention also relates to a method in a Base Transceiver Station (BTS) for assisting channel quality estimation in a multiple user Multiple Input Multiple Output (MIMO) system, said system including a BTS communicating with a first Mobile Station (MS) and with at least one other MS, wherein the communication with said first MS uses a first precoding vector and the communication with each of said at least one other MS uses a separate interfering precoding vector being different from the first precoding vector, said first precoding vector and the at least one interfering precoding vector being part of a precoding codebook.
  • BTS Base Transceiver Station
  • MIMO Multiple Input Multiple Output
  • the present invention also relates to a Mobile Station (MS) and a Base Transceiver Station (BTS), respectively, implementing the methods of the MS and BTS.
  • MS Mobile Station
  • BTS Base Transceiver Station
  • the present invention relates to a multi user (MU) wireless communication system with multiple antennas at a serving base transceiver station (BTS) and one or multiple antennas at receiving mobile stations (MSs). More specifically, the invention relates to a method for controlling the accuracy of the channel quality estimation in such a system.
  • MU multi user
  • BTS serving base transceiver station
  • MSs mobile stations
  • the invention relates to a method for controlling the accuracy of the channel quality estimation in such a system.
  • MIMO downlink multiple input multiple output
  • OFDM orthogonal frequency division multiplexing
  • MS is meant essentially any mobile station, mobile terminal (MT), User Equipment (UE) or the like present in such a system
  • BTS is meant essentially any node, such as a BTS or a Node B, in such a system controlling and communicating with these MSs.
  • the use of multiple antennas is favorable in order to increase the performance of the system.
  • the space dimension added to the signaling space by the use of multiple antennas may additionally be exploited for scheduling of transmission to different users in the system.
  • the use of multiple antennas increases the signaling space available for scheduling by adding a space dimension.
  • the further division of the available channels in the space dimension generates a set of channels that generally are not orthogonal to each other.
  • CCI co-channel interference
  • SINR signal to interference plus noise ratio
  • SIR Signal to Interference Ratio
  • SNR Signal to Noise Ratio
  • BLER Block Error Rate
  • BER Bit Error Ratio
  • Transport Block (TB) size or the like, which could all be used for determining channel quality according to the present invention.
  • the estimated channel quality value and desired precoding vector or quantized channel vector, i.e. spatial signature vector, will be fed back from the MS to the BTS and will be used in the precoding matrix generation.
  • the channel quality will also be used in the link adaptation to set the correct modulation constellation and code rate for the transmission to the mobile station.
  • the size of a precoding vector/channel quantization vector codebook used may have an influence on the interference estimation. If a small codebook of possible precoding vectors/channel quantization vectors is used, an error of the interference estimation may be small since there is little ambiguity of which precoding vector that is the interfering precoding vector. On the other hand, a small codebook also implies that a matching of a channel to a precoding vector estimated by a user will be poor, since the channel may have a large number of different characteristics that may be difficult to match, in an exact way, with a limited number of precoding vectors. Therefore, there will be a signal gain loss with a small codebook.
  • the precoding vectors are a priori paired two-by-two for a 2 transmitter antenna system and chosen orthogonal. In this way, if a UE selects a precoding vector, it knows the potential interfering precoding vector as the orthogonal complement and can thus estimate the signal to interference ratio perfectly.
  • a MU-MIMO scheduling method in which users are grouped based on their spatial correlation which is calculated based on their reported channels vectors. The strategy is to choose near-orthogonal users to improve performance of the zero forcing precoder by avoiding the signal attenuation when selecting near-parallel users.
  • grouping of near orthogonal users is used in order to simplify the scheduling algorithm used. Here, zero forcing semi-orthogonal user grouping is used.
  • Reference documents [4] and [5] disclose scheduling methods based on groups of near-orthogonal users and do not show channel quality estimation.
  • reference document [6] a resource allocation method similar to the one described in reference document [5] is presented, in which the scheduler assigns sub- carriers or groups of sub-carriers and time resource to certain users, for which the selected signature vectors simultaneously have low correlations. This grouping of resources is done by taking a predetermined and fixed correlation threshold into consideration. However, channel quality feedback and control signaling overhead is not mentioned in reference document [6].
  • the background art solutions presented above do either not disclose channel quality estimation at all or present inexact channel quality estimations resulting in less cell throughput than is necessary for some numbers of users in the system. They do also disclose static methods, not being able to adjust the methods to changing situations in the system.
  • the background art solutions may show solutions that apply to a few specific situations, but they all fail to show a general solution applicable to all varying kinds of situations that may occur in the system.
  • a radio communication system is in its nature highly varying, having a number of variable characteristics. Therefore, none of the background art solutions is capable to handle all the situations that may occur in such a system.
  • the present invention aims to provide a channel quality estimation that is more exact and results in a higher cell throughput than the channel quality estimations known in the background art.
  • the present invention further aims to provide a general channel quality estimation that is better able to handle the varying situations that may occur in the system than the channel quality estimations presented in the background art.
  • the object is achieved for a method implemented in a Mobile Station (MS), where a channel quality estimation performed in said first MS takes into account said first precoding vector as well as one of the following subsets:
  • restricted subset of the precoding vectors in said codebook said restricted subset having a predefined number of vectors including at least one but not all of the possible interfering precoding vectors in said codebook being non-orthogonal to the first precoding vector, or
  • variable subset of the precoding vectors in said codebook, said variable subset having a variable number of vectors including at least one of the possible interfering precoding vectors in the codebook, said variable number being derived from a parameter determined in said BTS.
  • BTS Base Transceiver Station
  • the object is also achieved for a Mobile Station (MS) and a Base Transceiver Station (BTS), respectively, implementing the above methods.
  • MS Mobile Station
  • BTS Base Transceiver Station
  • a parameter p representing a measure of how precoding vectors in the codebook relate to each other, for instance a correlation measure, a difference measure or an inner product measure, is known by both the BTS and the MSs in the system.
  • This parameter may then be used by the MS in the process of estimating channel quality, by restricting the subset of possible interfering precoding vectors to include in the estimation.
  • the number of possible interfering precoding vectors to include in the channel quality estimation calculation may be either predetermined or variable.
  • the parameter p from which the number of possible interfering precoding vectors to include in the subset, may be chosen in various ways.
  • the variable alternative all values of p are allowed. This makes it possible for the method according to the present invention to optimize channel estimation accuracy, cell throughput and co-scheduling in the system by choosing a suitable value for p .
  • an adaptive selection of a suitable p is performed in the BTS. This makes it possible for the BTS to select p in accordance with the present traffic load and interference conditions in the system. This further improves the possibility to smoothly adjust p in such a way that an optimized tradeoff between channel estimation accuracy, cell throughput and co-scheduling is achieved.
  • the choice of the parameter p in the BTS may be performed based on the number of users in the system, the number of co- scheduled users in the system or the number of co-scheduling possibilities per available radio resource.
  • the parameter p may then be given a relatively high value if the co-scheduling possibilities for MSs being in a cell are relatively small or if there is a small number of MSs being present in a cell.
  • the parameter p may then be given a relatively low value if the co-scheduling possibilities for MSs being in a cell are relatively large or if there is a large number of MSs being present in a cell.
  • the BTS may closely adapt to the present conditions in the system, which enhances the overall performance.
  • Fig. 1 shows a flow diagram for the method or the present invention.
  • Fig. 2 shows simulation results of the signal to noise plus interference (SINR) range of variation for different values of the parameter p.
  • SINR signal to noise plus interference
  • Fig. 3 shows a simulation of probability for MU-MIMO scheduling.
  • Fig. 4 shows a simulation of how cell throughput in the system varies with varying values of the parameter p.
  • interfering precoding vector is in this document meant a precoding vector in a precoding codebook that may, when it is used in a transmission to one MS, cause interference in another transmission to another MS.
  • This information is, according to the invention, a parameter p representing a measure of how precoding vectors in the codebook relate to each other, for instance a correlation measure, a difference measure or an inner product measure.
  • MS A when one MS (MS A ) calculates its channel quality information, for example SINR, it may according to the present invention assume that MU-MIMO transmissions simultaneously are taking place to several MSs in the system (MSA, MSB ,MSC , ⁇ •) and that the MSs other than MS A (MSB 5 MSC,..) are using precoding vectors that are related to the precoding vector used for the transmission to MS A in accordance with the parameter p known to both the BTS and all the MSs. MS A can then use this knowledge when estimating the channel quality estimation and thereby improving the estimation.
  • the MSA performing channel quality estimation knowing which precoding vector itself is using can from this knowledge restrict the set of interfering precoding vectors that may possibly be used by MS B , MSC, ⁇ , and can thus restrict the channel quality estimation calculations to these possible interfering precoding vectors.
  • the parameter p represents a measure of how precoding vectors in the codebook are spatially correlated to each other
  • the MS A knows that only precoding vectors in the codebook having a spatial correlation less than a measure p to the precoding vector itself is using are possible interfering precoding vectors and have to be included in the calculations.
  • the parameter p may be defined as essentially any parameter or information that gives information about how the transmission of MU-MIMO data to user (B, C, 7) impacts on the interference at user A.
  • the enumeration (A 5 B, C, ...) is arbitrary. Signaling of the parameter p can be made using a broadcast channel or by unicast transmission.
  • the parameter p may, according to an embodiment of the present invention, be adaptively adjusted by the base station and signalled to the MSs on a broadcast channel or on another channel.
  • This adaptive adjustment of the parameter p may, as an illustrative example, be performed as described in the following. If there are many users in the system, the BTS may broadcast a small value of p . This small value of p will reduce the probability that two users can be MU-MIMO scheduled together, but will also on the other hand improve the channel quality estimation accuracy, for example a SINR calculation. When the number of users in the coverage area is lower, the parameter p can be increased and broadcasted to the active users in the area. The increased value of parameter p increases the probability to find two users that can be scheduled together but reduces the accuracy of the channel quality estimation, for example a SINR calculation.
  • a parameter p is defined, which is known by both the BTS and the MSs.
  • the parameter p may, for instance, be defined as the maximum allowed spatial correlation between the reported quantized channel vectors from one MS (MS A ) and the co- scheduled MU-MIMO users (MS 3 , MS C , ...) when the channel quality information is calculated in the mobile station.
  • the parameter p may also be defined as the maximum allowed spatial correlation between the precoding vector used by MS A and the precoding vectors of co-scheduled MU-MIMO users (MS B , MS C , ••)•
  • MS B the precoding vector used by MS A
  • MS C the precoding vectors of co-scheduled MU-MIMO users
  • the parameter p and the vector quantization codebook (for the vector quantization case) or the parameter p and the precoding codebook (in the precoding case) are known, all possible pairs of mobile stations that can be co-scheduled in a MU-MIMO transmission are given.
  • the BTS may have the right to override the rule of maximum p when performing scheduling, but from a mobile station perspective, the channel quality should be calculated according to this rule.
  • the parameter p may be changed dynamically by the BTS and broadcasted to the MSs.
  • channel quality estimations may be calculated, including some practical examples where the SINR is used as a channel quality estimator.
  • the present invention is not limited to SINR as a measure of channel quality.
  • SINR as a measure of channel quality.
  • - P is the transmitted power per user in the system.
  • Equation 1 results in an average MU interference and therefore gives a SINR estimation similar to the one shown in reference documents [1] and [3].
  • a lower bound of SINR may be considered by:
  • Equations 1 to 3 above describe SINR estimation similar to the one performed in background art solutions.
  • examples of how channel quality may be estimated according to the present invention are shown.
  • SINR estimations are here given, but the principles of the present invention may be used for estimating essentially any channel quality measure known by a skilled person, such as Signal to Interference Ratio (SIR), Signal to Noise Ratio (SNR), Block Error Rate (BLER), Bit Error Ratio (BER), TB size or the like.
  • SIR Signal to Interference Ratio
  • SNR Signal to Noise Ratio
  • BLER Block Error Rate
  • BER Bit Error Ratio
  • a parameter p is introduced such that the interference in this example can be calculated as:
  • w k , h k , superscript H , , and P are defined as above, p is the parameter known by both the BTS and the MS, and is an arbitrary function of h k and precoding vectors W / that fulfil
  • p is the parameter known by both the BTS and the MS, and is an arbitrary function of h k and precoding vectors W / that fulfil
  • the parameter p may have a predetermined value known by both the MSs and the BTS having been determined in, for instance, a standardization agreement, thereby including in the calculation a restricted subset of the possible interfering precoding vectors in the codebook.
  • This restricted subset thus includes a number of precoding vectors corresponding to the value of the parameter p .
  • This restricted subset should further include some but not all of the precoding vectors in the codebook that are non- orthogonal to the precoding vector used by the MS performing the estimation (MS ⁇ ). This choice of precoding vectors to be included in the restricted subset improves the cell throughput in the system.
  • the setting of the parameter p may also be adaptive, thereby including a variable subset of the possible interfering vectors in the codebook in the calculation.
  • the parameter p is determined in the BTS and is signaled to the MSs in the system, by the use of, for example broadcasting.
  • the adaptive adjustment of parameter p allows for the BTS to adapt to varying characteristics of the radio communication system and also allows for tradeoff between the MU-MIMO scheduling and the channel quality estimation accuracy.
  • the corresponding SINR estimate for a MS may, assuming a linear receive filter v in the mobile station, be written as:
  • H k is the channel matrix for MS ⁇
  • v * is the corresponding receive filter in MS&.
  • p 2 and P are defined as stated above.
  • both predetermined and adaptive setting of the parameter p may be used, as was discussed above in connection with the estimation according to equation 4, each of the predetermined and adaptive setting having the same advantages as was stated above in connection with equation 4.
  • CVQ channel vector quantization
  • This quantized channel, or rather, an index of Iog 2 (i?) bits identifying the corresponding vector in the CVQ-codebook is signalled back to the BTS from the MSs.
  • ZF Zero Forcing
  • H is the assembled matrix of reported quantized channel vectors from the mobile stations.
  • This matrix H only contains vectors to the selected MSs for which the BTS scheduler has decided to perform the transmission.
  • the MSs may then perform the SINR estimation according to the equations 1 or 2 above by using equation 6 to obtain the precoding vectors w .
  • the MS performing the SINR estimation shall only consider precoding matrices (defined in equation 6) for which the spatial correlation is less than the parameter p between the quantized channel vector h k of the MS performing the estimation and
  • H , w k , h k , superscript " , , and P are defined as stated above.
  • both predetermined and adaptive setting of the parameter p may be used, as was discussed above in connection with the estimation according to equation 4, each of the predetermined and adaptive setting having corresponding advantages as stated above in connection with equation 4.
  • the precoding codebook itself may vary, depending on the value of the parameter p .
  • the size of the precoding codebook may vary in relation to the value of the parameter p . That is, based on the parameter p , different codebooks of different sizes can be used. It is also possible to have a mother codebook, (hereafter denoted "total codebook"), having a defined number of precoding vectors, from which precoding codebooks, having a limited part of the defined number of precoding vectors of the total codebook as its precoding vectors, can be derived.
  • the precoding codebook from which the variable subset used in the channel quality estimation is taken, may have varying sizes. This has the advantage that the precoding codebook does not have to have a bigger size than necessary, which is advantageous since large codebook sizes increases overhead signaling in the system.
  • information corresponding to a combination of the size of the precoding vector and the parameter p may be determined in the BTS and signaled to the MS performing the channel quality estimation. This information may, for instance, be signaled as a pair of bits. This information may then be used by the MS to derive the precoding codebook and the parameter to be used in the channel quality estimation.
  • table 1 an example of such information signaling is given. As is clear to a skilled person, there are many ways to construct such a table, and this is only one such example.
  • Precoding codebook size Parameter p used to find possible interfering precoding vectors in the precoding codebook
  • FIG. 1 shows a flow diagram for the embodiment of the invention utilizing adaptive setting of parameter p.
  • a parameter typically parameter p, relating to the number of possible interfering precoding vectors to be included in the channel quality estimation in the MS is determined in the BTS, based at least on the number of MSs present in the system.
  • the BTS signals the determined parameter p to the MS.
  • the MS determines a channel quality estimate taking a first precoding vector, i.e. the precoding vector used by the MS itself, and a number of precoding vectors in the codebook into account. The number of precoding vectors taken from the codebook being derived from the parameter received from the BTS.
  • One further aspect of the invention is the cell throughput, which the use of different values of the parameter p results in.
  • a simulation of this is illustrated in Figure 4.
  • the cell throughput in the system varies with varying values of p.
  • This choice of the parameter p may be performed based, for example, on the number of users in the system, the number of co-scheduled users in the system or the number of co-scheduling possibilities per available radio resource.
  • the parameter p may then be given a relatively high value if the co-scheduling possibilities for MSs being in a cell are relatively small or if there is a small number of MSs being present in a cell.
  • the parameter p may then be given a relatively low value if the co- scheduling possibilities for MSs being in a cell are relatively large or if there is a large number of MSs being present in a cell.
  • the determination of the value of the parameter p to use may either be predetermined, for instance set in a standardisation agreement, or adaptively set by a BTS and then signalled to the MSs in the cell.
  • the present invention has the possibility to set the parameter p to a value that optimizes a trade-off between cell throughput and channel quality estimation accuracy.
  • the present invention presents a general solution for these problems, excluding these values for fixed p since they result in less cell throughput compared to the other possible values of p as can be seen in Figure 4. Further, the present invention also presents a way to adaptively adjust p that optimize cell throughput, channel quality estimation accuracy and scheduling for all values of p .
  • An adaptive adjustment of p and thus an adaptive adjustment of which possible interfering precoding vectors to include in the channel quality estimation, is not shown in background art.
  • the MS and the BTS arranged for performing channel quality estimation according to the invention can be adapted to perform any of the steps of the method of the invention. A trivial requirement is of course that such a step does involve a MS or a BTS, respectively.

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  • Engineering & Computer Science (AREA)
  • Quality & Reliability (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

L'invention porte sur une méthode améliorée d'évaluation de la qualité de canaux dans un système MIMO (à plusieurs entrées et plusieurs sorties) à plusieurs utilisateurs. Le système inclut une station de base (BTS) communiquant avec une première station mobile (MS) et avec au moins une autre MS, la communication avec la première station mobile utilisant un premier vecteur de précodage, et la communication avec chacune des autres MS utilisant un vecteur de précodage interférant séparé différent, faisant partie d'un livre de codes de précodage. Ledit livre de codes comprend un ensemble de vecteurs de précodage interférant possibles orthogonaux avec le premier vecteur de précodage et des vecteurs de précodage non-orthogonaux avec le premier vecteur de précodage. Selon la méthode, une évaluation de qualité de canal exécutée dans la première MS prend en compte le premier vecteur de précodage ainsi que l'un des sous-ensembles suivants: un sous-ensemble restreint de vecteurs de précodage du livre de codes présentant un nombre prédéterminé de vecteurs incluant au moins un mais pas tous les vecteurs de précodage interférants du livre de codes non-orthogonal au premier vecteur de précodage, ou un sous-ensemble variable de vecteurs de précodage du livre de codes présentant au moins l'un des vecteurs de précodage interférants du livre de codes, ce nombre variable dérivant d'un paramètre déterminé dans la BTS.
PCT/CN2007/000909 2007-03-20 2007-03-20 Évaluation de la qualité de canaux dans un système mimo sans fil à plusieurs utilisateurs. WO2008113210A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
CN2007800522796A CN101682909B (zh) 2007-03-20 2007-03-20 多用户多输入多输出无线系统中的信道质量估计
PCT/CN2007/000909 WO2008113210A1 (fr) 2007-03-20 2007-03-20 Évaluation de la qualité de canaux dans un système mimo sans fil à plusieurs utilisateurs.

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