A COOPERATIVE BEAM FORMING METHOD  APPARATUS AND BASE STATION

FIELD OF THE INVENTION
[0001] The present invention relates to multimedia broadcast multicast field  more particularly  to a cooperative beam forming method  apparatus and base station based on fixed grid of beams GoB of MBMS(Multimedia Broadcast Multicast Service).

BACKGROUND OF THE INVENTION
[0002] The concept of BF (Beam Forming) derives from smart antenna (SA). The basic principle of SA is that forming a directional beam via process at transmitter using the correlation of channel impulse response of half wavelength-distance antennas array to increase the SNR at receiver and extend coverage area of system. Traditional BF is usually acting on single signal stream  that is  the transmit signal is multiplied with a weight factor and then is transmitted via a plurality of antennas. Normally such gain is called as array gain. Then  the detailed meanings of BF extends with the continuous deeper research of MIMO system  in which distance of antennas is not limited to half wavelength but may be 4 wavelength/10 wavelength  etc. In these cases  the correlation of channel matrix will weaken largely  and this non-correlation may bring diversity gain  the normal method is STBC (Space Time Block Code); this non-correlation may also bring multiplexing gain  the normal method is v-blast. In this sense  BF has the same meaning as precoding of MIMO  in other words  such BF may be seen as an implementing manner of precoding. The common BF operation manner  such as characteristic root BF  its typical meaning is that BF vector is determined via singular value decomposition (SVD) using the second order statistic information of channel (namely correlation matrix of channel). When single stream is sent  the characteristic vector corresponding to the maximum characteristic value is selected as BF vector; when multiple streams are sent  it is selected in sequence according to the order of size. For MU (Multiple Users)-MIMO  BF may be equal to SDMA (Space Division Multiple Access)  wherein BF matrix or precoding matrix needs to maximize SINR of single user  and decreases Interference among users as far as possible  and common algorithms is such as block diagonal (BD) zero forcing algorithms  etc.
[0003] In R8 of 3GPP LTE  MBMS has typically been deployed by two scenarios  that is  single-cell (SC) and Multimedia Broadcast Single Frequency Network (MBSFN) transmission. With the evolution of LTE to LTE-A (LTE-Advanced)  further enhanced MBMS is required in LTE-A requirement documents namely in TR 36.814 frame according to the conclusion of last Warsaw meeting on Jun.30-Jul.04  2008.
[0004] For the SC MBMS transmission  some advanced physical layer technologies  for example  HARQ  link adaptation and multi-antenna techniques  etc. may be used to improve the receiving performance of the UEs with some feedbacks over air interface from terminals (user equipment  UE) to base station (eNB). As one of the improved technologies  a “hybrid scheme” has been proposed in the PCT application for patent for invention with the application number PCT/CN2008/001443  and it may be viewed as the combinations of open-loop transmit diversity  Direction Of Arrival (DoA) based beam forming and the HARQ. Another typical example is to implement beam forming operation based on GoB (Grid of Beams) for SC MBMS as proposed by the PCT application for patent for invention with the application number PCT/CN2008/001441.
[0005] Compared with two kinds of existing schemes for SC MBMS  that is  open-loop transmit diversity and max-min beam forming  although obvious performance improvement has been obtained by those enhanced algorithms in the applications for patent for invention with the application numbers PCT/CN2008/001441 and PCT/CN2008/001443  the resulted performance improvement is still limited mainly because all of MBMS users share the same physical time-frequency resources so as to cause that any operation performed on the physical resource is synchronously acting on all of MBMS receiving users. For the reason of random character of users’ distribution  when technology such as max-min beam forming is used  while the main beam aims at the worst user  a plurality of other users may probably be located at side lobe of the main beam. This point may be further explained in Fig.s 1 and 2. Fig. 1 shows a schematic diagram of single cell MBMS transmission model. All of interested MBMS users receive MBMS service on the shared physical time-frequency resource as shown in Fig.1  thus any operation has effect on all MBMS users. The core of two enhanced algorithms in the applications for invention with the application numbers PCT/CN2008/001441 and PCT/CN2008/001443 is the smart usage of beam forming. The normalized average beam patterns under 4 and 8 transmit antennas at eNB are shown in Fig. 2. It is clear from Fig. 2 that the main beam pattern becomes narrower with the increase of antenna numbers. Whenever the above narrow beam is used to aim at some users or user group  other users or user group may be located at side lobe of the main beam so as to incur performance loss due to the scattering of users’ distribution within cell  namely random character.
[0006] As a result from above analysis  the performance improvement is still limited and more advanced technology is necessary to be investigated.

SUMMARY OF THE INVENTION
[0007] In the present invention  a new method is proposed for further performance improvement for SC MBMS: performing cooperative beam forming based on fixed grid of beams GoB  which may further improve the receiving performance of single cell MBMS.
[0008] According to the first aspect of the present invention  a cooperative beam forming method is proposed  wherein the method comprises following steps: grouping users into cell central user group and cell edge user group; performing cell central user group multiplexing based on superposition coding; performing cooperative beam forming operation based on fixed grid of beams GoB for cell edge user group.
[0009] Preferably  grouping user groups into cell central user group and cell edge user group based on measurement report.
[0010] Preferably  the measurement report is reference signal receiving power RSRP or reference signal receiving quality RSRQ.
[0011] Preferably  when RSRP or RSRQ measurement value in the measurement report is lower than predetermined threshold value thus the user is put into cell edge user group otherwise the user is put into central user group.
[0012] Preferably  the superposition coding method is power allocation superposition coding  constellation rotation superposition coding  transmission rate superposition coding  interleaving superposition coding and different bit rate superposition coding.
[0013] Preferably  the cooperative beam forming steps based on GoB comprises:
predefining beam cluster;
determining target sector serving cell edge users;
classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster;
for each cell edge MBMS user class 
dividing all of cell edge MBMS users in cell edge MBMS user class into sets  each cell edge MBMS user set having similar PMI measurement value;
target sector sending cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class  the cooperative request comprises information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve;
target sector forwarding  the common data needed to be transmitted  to each cooperative sector after successfully cooperative negotiation;
all cooperative sectors transmitting data to this cell edge MBMS user class using beam forming operation based on GoB.
[0014] Preferably  predefining beam cluster by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located.
[0015] Preferably  cell edge MBMS user class corresponding to a sector cluster is nearer to the sector cluster than other sector clusters.
[0016] Preferably  all edge MBMS users simultaneously measure PMI measurement values corresponding to target sector and all of sectors in sector cluster of the target sector to obtain optimum PMI of the MBMS user; and the edge MBMS user feeds back the optimum PMI to target sector.
[0017] Preferably  obtaining the optimum PMI based on long term SINR calculation.
[0018] Preferably  the cooperative request further comprises: information of PMI value application  indicating the cooperative sector the PMI value used during final cooperation.
[0019] Preferably  target sector forwards  the common data needed to be transmitted  to each cooperative sector via X2 interface between base stations.
[0020] According to another aspect of the present invention  a cooperative beam forming device is proposed  wherein the device comprises: a means for predefining beam cluster; a means for determining sector service  for determining target sector serving cell edge users; a means for classifying sector cluster  for classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster; a means for dividing cell edge MBMS user set  for dividing  all of cell edge MBMS users in cell edge MBMS user class  into sets  each cell edge MBMS user set having similar PMI measurement value; a means for sending cooperative request  target sector sends cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class  the cooperative request comprises information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve; a means for forwarding common data  target sector forwards the common data needed to be transmitted to each cooperative sector after successfully cooperative negotiation; a means for data transmitting of cooperative sectors  all cooperative sectors transmit data to this cell edge MBMS user class using beam forming operation based on GoB.
[0021] Preferably  the means for predefining beam cluster predefines beam cluster by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located.
[0022] Preferably  determining sector cluster of target sector  so that cell edge MBMS user class corresponding to a sector cluster is nearer to the sector cluster than other sector clusters.
[0023] Preferably  all edge MBMS users simultaneously measure PMI measurement values corresponding to target sector and all of sectors in sector cluster of the target sector through the means for classifying sector cluster  to obtain optimum PMI of the MBMS user; and the edge MBMS user feeds back the optimum PMI to target sector.
[0024] Preferably  the means for classifying sector cluster obtains the optimum PMI based on long term SINR calculation.
[0025] Preferably  the cooperative request sent by the means for sending cooperative request comprises: information of PMI value application  indicating the cooperative sector the PMI value used during final cooperation.
[0026] Preferably  target sector forwards  the common data needed to be transmitted  to each cooperative sector via X2 interface between base stations.
[0027] According to embodiments of the present invention  A base station is proposed  comprising: a grouping unit  for grouping users into cell central user group and cell edge user group; a processing unit for cell central user  for performing cell central user group multiplexing based on superposition coding; the above-mentioned cooperative beam forming device.
[0028] Compared with the prior art  the beneficial effects of the present invention are:
1) since all of MBMS user groups receive the same data on the common physical resources  all of cell edge MBMS users m-e may be taken as pure cell edge unicast users. Therefore  the two cooperative mechanisms do not have intrinsical difference.
2) accordingly  some cooperative mechanisms for network/cooperative MIMO are advantageous for cooperative beam forming operation  for example  physical resource coordination in cooperative sectors  reference signal definition for cooperation  etc.
3) however  cooperative beam forming still has some unique feature  for example  user grouping fed back based on PMI (Precoding Matrix Index)  etc.
[0029] Particularly  the beneficial effects according to the present invention are described in detail as follows:
i) for example  different MBMS user groups are mapped onto different physical resources based on sub-frame level  the different physical resources being different from the previous scheme in which all of MBMS users share the same resources. Therefore  different operations onto different user groups become possible and large performance improvement is anticipated;
ii) for cell central users  superposition coding could be used due to their higher SINRs; for those cell edge users (namely  m-e)  network/cooperative MIMO mechanism may be applied  that is  cooperative beam forming based on GoB may be used;
iii) for cell edge unicast user  signaling overhead is similar to network/cooperative MIMO to support this scheme.

BRIEF DESCRIPTION OF THE DRAWINGS
[0030] By the detailed description of the non-limiting embodiments with reference to the following drawings  the above and other objects  features and advantages of the present invention will become apparent. In drawings:
[0031] Fig.1 shows a schematic diagram of single cell MBMS transmission model;
[0032] Fig.2 shows the normalized average beam patterns under 4 and 8 transmit antennas at eNB;
[0033] Fig.3 shows a cell topology using cooperative beam forming method according to embodiments of the present invention;
[0034] Fig.4 shows a schematic diagram of user grouping according to embodiments of the present invention;
[0035] Fig.5 shows superposition coding for cell central users (u-c and m-c);
[0036] Fig.6 shows a schematic diagram of cooperative beam forming based on GoB of cell edge user (m-e);
[0037] Fig.7 shows a flowchart of cooperative beam forming method according to embodiments of the present invention;
[0038] Fig.8 shows the detailed steps of cooperative beam forming operation based on GoB;
[0039] Fig.9 shows cooperative beam forming device according to embodiments of the present invention; and
[0040] Fig.10 shows base station structure according to embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS
[0041] Hereinafter  the present invention is described with reference to the drawings. In the following description  some detailed embodiments are only used for the aim of description  which should not be understood as any limitation to the present invention  but only as examples. When it might cause ambiguous understanding of the present invention  the traditional structure or composition will be omitted.
[0042] Before concepts according to the present invention are introduced  firstly all of users (unicast + MBMS) are classified into the following four kinds: ? unicast UE near the eNB  for the purpose of convenience  this kind of user is denoted as “u-c”; ? SC MBMS UE near the eNB  for the purpose of convenience  this kind of user is denoted as “m-c”; ? unicast UE at cell edge  for the purpose of convenience  this kind of user is denoted as “u-e”; ? SC MBMS UE at cell edge  for the purpose of convenience  this kind of user is denoted as “m-e”.
[0043] It should be noted that u-c user and m-c user may be the same user  however for the convenience of understanding they may be assumed as different users  and this is suitable for u-e and m-e as well; for u-c and u-e  it is obvious that different data is transmitted to them  but for m-c and m-e  the same data is transmitted to them; here only three kinds of users  that is  u-c user  m-c user and m-e user  are considered. For those u-e users  other hot spot technologies being in discussion such as network/cooperative MIMO may be used.
[0044] The key points of concepts of the present invention are: different from the current scheme that all of MBMS users share the same resource based on sub-frame level  in the present invention  different MBMS user groups are mapped onto different physical resources so that operations of different user groups become possible and large performance improvement is anticipated.
[0045] The method according to embodiments of the present invention comprises following key steps:
[0046] Step 1. how to implement user grouping. For u-c or m-c/m-e users  serving eNB may classify users into different user groups based on measurement report  for example  based on RSRP (reference signal receiving power) and/or RSRQ (reference signal receiving quality). In other words  when measurement value (such as average SINR  RSRQ) is lower than predetermined threshold value  the user is put into cell edge user group otherwise the user is put into central user group. The predetermined threshold value may be dynamic or semi-static adjusted by the serving eNB according to MBMS user number in the whole cell. According to the present invention  only two kinds of user groups are considered  that is  central user group and cell edge user group.
[0047] Step 2. how to operate central user group (u-c + m-c): central user group further comprises u-c users besides m-c users. For this kind of user group  both m-c user and u-c user share the same physical resource. In other words  since central user group has higher SINR  superposition coding via appropriate power allocation or constellation rotation between u-c user and m-c user may be used to multiplex both kinds of users.
[0048] Step 3. how to operate cell edge user group (m-e): the cell edge user group only comprises m-e users. For this kind of user group  cooperative beam forming based on GoB similar to network/cooperative MIMO for cell edge unicast users may be defined. Due to the existence of cooperation among several cells  the common physical resources among coordinated cells may be deployed. It is still an open issue on how to configure the needed common physical resources until now but the mechanism similar to network/cooperative MIMO for unicast may be reused here. Since all MBMS users m-e receive the same transmit data on the common physical resources  all MBMS users m-e may be seemed as one cell edge unicast user for ease of understanding. As a result  the cooperative beam forming operation based on GoB for m-e users has the similar characteristic with that under discussion CoMP (Cooperative Multi-Port) MIMO technology for cell edge unicast user  so that cooperative beam forming based on GoB according to the method of the present invention may be proposed.
[0049] According to the present invention  different from that existing SC MBMS schemes of sharing the same physical resources  user group is considered here and different user groups are mapped onto different resources thus different operation is possible to different user groups. For cell central users  that is  m-c users and u-c users  superposition coding may be used due to their higher SINR. For those cell edge users  that is  m-e  cooperative beam forming based on GoB  similar to network/cooperative MIMO mechanism  may be used.
[0050] Fig.3 shows a cell topology. In Fig.3  only the central cell and the first tier neighbor cells are considered for simplification. As shown in Fig.3  cells 1 to 7 are illustrated  in which cell 1 is the central cell and cells 2 to 7 are the first tier neighbor cells. Each cell has three sectors. Here SC MBMS transmission means single-sector transmission with/without neighbor interference coordination.
[0051] User grouping is shown in Fig. 4. Here sector 1-2 of the central cell 1 is our interested target sector. All of users (that is  u-c  m-c/m-e) may be classified into two kinds based on the measurement report such as RSRP and/or RSRQ and the pre-determined threshold  as shown in Fig. 4. One kind is cell central users  that is  those u-c and m-c users within the dot line. Another kind is cell edge users  that is  those m-e users outside the dot line. After user grouping  different operations to different user groups become possible.

[0052] Operation on central user group (u-c + m-c):
[0053] Superposition coding for central users (u-c + m-c) is shown in Fig. 5. Here assuming there are three central unicast users and three MBMS users respectively. The number of central unicast users and the MBMS users is random. From Fig. 5 it may be seen that both u-c and m-c users share the same physical resources via superposition coding. For the superposition coding operation  two typical algorithms  for example  appropriate power allocation or constellation rotation between u-c and m-c may be used to multiplex both kinds of users. During the scheduling period  all central unicast users may be scheduled using PF (Proportional Fair) or RR (Round Robin) algorithms but the time interval parameter TM for MBMS may be determined by the amount of MBMS content and the configured MCS value.

[0054] Operation on cell edge user group (m-e):
[0055] Cooperative beam forming based on GoB for cell edge users (m-e) is shown in Fig. 6. Without loss of generality  here assuming there are 8 cell edge MBMS users  that is  from m-e1 to m-e8.
[0056] Before the cooperative beam forming is introduced  the communication system is in the following state :
1) sector 1-2 of the cell 1 is the interested target sector and it has 6 direct neighbor sectors  that is  1-1  3-3  2-1  2-3  7-1 and 1-3;
2) by using pre-configuration manner  these 6 direct neighbor sectors are divided into two sector clusters  that is  sector cluster-a denoted with left slash including the sector 1-1  the sector 3-3 and the sector 2-1  and cluster-b denoted with right slash including the sector 2-3  the sector 7-1 and the sector 1-3 in Fig. 6;
3) when the target sector is changed  the direct neighbor sectors are also changed and thus both clusters are redefined;
4) when all those cell edge MBMS users (that is  from m-e1 to m-e8) are nearer to cluster-a than cluster-b  cooperation is performed among target sector 1-2 and those sectors within cluster-a. When all those cell edge MBMS users are nearer to cluster-b than cluster-a  cooperation is performed among target sector 1-2 and those sectors within cluster-b. When these two clusters both have cell edge users  cooperation is performed among target sector 1-2 and all of 6 direct neighbor sectors (belonging to cluster-a and cluster-b respectively). The benefit of defining sector cluster is: cooperative signaling may be decreased especially when the number of cell edge MBMS users is small;
5) without loss of generality  only cell edge users m-e1  m-e2  m-e3 and m-e4 are considered here  in other words  only the cooperation between target sector 1-2 and cluster-a is considered.
In this case  the detailed procedure of cooperative beam forming based on GoB is explained as follows:
a) Firstly  the common codebook  that is  grid-of-beams (GoB)  is predefined at target sector 1-2 and those sectors within cluster-a.
b) At receivers  all cell edge MBMS users simultaneously measure the measurement value(for example long-term SINR) corresponding to target sector 1-2 and all those sectors in cluster-a so as to obtain the optimum PMI for this user  in other words  that which sector is optimal to serve the edge user is selected and obtained via measurement for this user. This user feeds back the optimum PMI to the target sector at the same time.
c) At target sector 1-2  based on PMI feedback values of all the aforesaid users  all of cell edge MBMS users are classified into a plurality of groups (in this embodiment  since there are only four edge users  these four edge users at most belong to four different groups respectively)  wherein each group users have the similar PMI measurement characteristics  for example  PMI difference among users belonging to the same user group is less predetermined threshold-1. Furthermore  when the user classification is performed  PMI difference between different user groups should be kept relative as large as possible (PMI difference of any two users respectively belonging to two user groups is larger than predetermined threshold 2). The maximum user group number is determined by the number of cooperative sectors;
d) after users classification  target sector1-2 sends cooperative request signaling to each sector in sector cluster-a  the cooperative request signaling comprises:
i. informing the cooperative sector that which user group should be served by it (for example  the 3rd user group including m-e1  m-e2  m-e3 and m-e5);
ii. at the same time  informing the cooperative sector of the used PMI value during the final cooperation.
e) after successfully cooperative negotiation  target sector 1-2 forwards  the common data needed to be transmitted  to each cooperative sector in sector cluster-a via X2 interface between base stations.
f) finally  all cooperative sectors transmit data to all of aforesaid cell edge MBMS user groups using beam forming operation based on GoB.
[0057] Fig.7 shows a flowchart of cooperative beam forming method according to embodiments of the present invention. As shown in Fig. 7  the cooperative beam forming method based on MBMS fixed grid of beams comprises the following steps: grouping users into cell central user group and cell edge user group (S701); performing cell central user group multiplexing based on superposition coding; performing cooperative beam forming operation based on GoB for cell edge user group (S703).
[0058] In step S701  user groups are classified into cell central user group and cell edge user group based on measurement report  for example  the measurement report may be reference signal receiving power RSRP  reference signal receiving quality RSRQ. When RSRP or RSRQ measurement value in the measurement report is lower than predetermined threshold value  the user is put into cell edge user group; otherwise the user is put into central user group. For example  the measurement value may be SINR measurement value.
[0059] In step S702  the superposition coding may be performed by using power allocation  or constellation rotation  interleaving superposition and bit rate superposition. The common superposition coding typically has four kinds: superposition by allocating different powers  by constellation rotation  superposition by different interleaving (for example IDMA)  superposition by different bit rates. These superposition methods may be all applied in step S702 according to the present invention. For example  in the step S702  cell central user group may be multiplexed with unicast service users based on superposition coding so as to share the same physical resources.
[0060] In step S703  users in cell edge user group is further divided into different sector clusters according to PMI  and at the same time cooperation is performed for cell target sector (each comprises users in a same sector cluster).
[0061] Furthermore  Fig.8 shows the detailed steps of cooperative beam forming operation based on GoB. As shown in Fig.8  the steps of cooperative beam forming based on GoB comprises the following sub-steps: predefining beam cluster (S801); at receiver  determining which sector serves cell edge users  that is  determining target sector serving cell edge users (S802); classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster (S803); after users classification  for each cell edge MBMS user class  dividing all of cell edge MBMS users in cell edge MBMS user class into sets  each cell edge MBMS user set having similar PMI measurement values (S804); target sector sending cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class (S805)  the cooperative request comprises information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve; target sector forwarding  the common data needed to be transmitted  to each cooperative sector after successfully cooperative negotiation (S806); all cooperative sectors transmitting data to this cell edge MBMS user class using beam forming operation based on GoB (S807).
[0062] In the step S801  the process of predefining beam cluster is achieved by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located.
[0063] In the step S802  cell edge MBMS user class corresponding to a sector cluster is nearer to the sector cluster than other sector clusters. In other words  all edge MBMS users simultaneously measure PMI measurement values corresponding to target sector 1-2 and all of sectors in sector cluster a in which target sector 1-2 is located  (for example  long term SINR) to obtain optimum PMI of the user; the edge MBMS user feeds back the optimum PMI to target sector 1-2 at the same time. Wherein  in other words  that which sector is optimal to serve the edge user is selected and obtained via user measurement of edge MBMS user.
[0064] In step S804  based on PMI feedback values of all the aforesaid users  all of cell edge MBMS users are classified into groups so that each group users have the similar PMI measurement value. When the user classification is performed  PMI difference between different user groups should be kept relative as large as possible. Besides  the maximum user group number is determined by the number of cooperative sectors.
[0065] In step S805  the cooperative request comprises: service information  informing the cooperative sector that which user group should be served by it; and information of PMI value application  informing the cooperative sector of the used PMI value during the final cooperation.
[0066] In step S806  target sector forwards  the common data needed to be transmitted  to each cooperative sector via X2 interface between base stations.
[0067] Fig.9 shows the detailed structure of cooperative beam forming device 900 according to embodiments of the present invention. The cooperative beam forming device may also comprise the following means: a means 901 for predefining beam cluster; a means 902 for determining sector service  for determining which sector serves cell edge users  that is  for determining target sector serving cell edge users; a means 903 for classifying sector cluster  for classifying all of cell edge MBMS users in target sector into different sector cluster  so that users in each sector cluster have similar predefined matrix index PMI measurement  that is  for classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster; a means 904 for dividing cell edge MBMS user set  for dividing  all of cell edge MBMS users in cell edge MBMS user class  into sets  each cell edge MBMS user set having similar PMI measurement value; a means 905 for sending cooperative request  target sector sends cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class  the cooperative request comprising information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve; a means 906 for forwarding common data  target sector forwards the common data needed to be transmitted to each cooperative sector after successfully cooperative negotiation; a means 907 for data transmitting of cooperative sectors  all cooperative sectors transmit data to this cell edge MBMS user class using beam forming operation based on GoB.
[0068] The means for predefining beam cluster predefines beam cluster by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located. While determining sector cluster of target sector  cell edge MBMS user class corresponding to a sector cluster is nearer to the sector cluster than other sector clusters. All edge MBMS users simultaneously measure PMI measurement values corresponding to target sector and all of sectors in sector cluster of the target sector through the means for classifying sector cluster  to obtain optimum PMI of the MBMS user; and the edge MBMS user feeds back the optimum PMI to target sector. The means for classifying sector cluster obtains the optimum PMI based on long term SINR calculation. The cooperative request signaling sent by the means for sending cooperative request comprises the following contents: information of PMI value application  indicating the cooperative sector the PMI value used during final cooperation. Target sector forwards  the common data needed to be transmitted  to each cooperative sector via X2 interface between base stations.
[0069] Fig.10 shows a base station 100 according to embodiments of the present invention  comprising: a grouping unit 1001  for grouping users into cell central user group and cell edge user group; a processing unit 1002 for cell central user  for performing cell central user group multiplexing based on superposition coding; the aforesaid cooperative beam forming device 1003.
[0070] Since network/cooperative MIMO mechanism should be defined in 3GPP LTE-A for cell edge unicast users  here the cooperative beam forming based on GoB for cell edge MBMS users has the following characteristics compared with network/cooperative MIMO:
1) since all MBMS user groups receive the same data on the common physical resources  all MBMS users m-e could be seemed as one pure cell edge unicast user. Therefore  both kinds of cooperative mechanisms don’t have any intrinsical difference.
2) correspondingly  some cooperative mechanisms used for network/cooperative MIMO are useful for cooperative beam forming operation. For example  physical resources coordination among cooperative sectors  reference signal definition used for cooperation etc.
3) But cooperative beam forming still has some unique characteristics  for example  user grouping based on PMI feedback etc. The beneficial effects according to embodiments of the present invention are summarized as follows: i) for example  based on sub-frame level  different MBMS user groups are mapped onto different physical resources different from the previous scheme in which all MBMS users share the same resources. Therefore  different operations onto different user groups become possible and large performance improvement is anticipated. ii) for cell central users  superposition coding could be used due to those higher SINRs; for those cell edge users (that is  m-e)  network/cooperative MIMO mechanism  that is  cooperative beam forming based on GoB may be used. iii) signaling overhead is similar to network/cooperative MIMO for cell edge unicat user to support this scheme.
[0071] MBMS performance will be especially enhanced especially for IMT-A which has been determined in the performance requirement of IMT-A. Many companies are considering how to improve the MBMS performance. Compared with both existing schemes for SC MBMS  that is  open-loop transmit diversity and max-min beam forming  although obvious performance improvement has been obtained by the proposed enhanced algorithms  the resulted performance improvement is still limited. The present invention has more attractive characteristics from implementation view. An improved SC MBMS technology is proposed according to embodiments of the present invention  the detailed algorithms program is analyzed  comprising: user grouping  different operations for different user groupings  and the unique characteristics of this method.
[0072] The above embodiment is only for an exemplarily purpose and not intended to limit the present invention. The skilled in the art should understand that the present invention may be modified  replaced without departing from the scope and spirit of the present invention  with these modifications and replacements falling within the scope limited by appended claims.

CLAIMS
1. A cooperative beam forming method  wherein the method comprises following steps:
grouping users into cell central user group and cell edge user group;
performing cell central user group multiplexing based on superposition coding;
performing cooperative beam forming operation based on fixed grid of beams GoB for cell edge user group.
2. The cooperative beam forming method according to claim 1  characterized in grouping user groups into cell central user group and cell edge user group based on measurement report.
3. The cooperative beam forming method according to claim 2  characterized in that  the measurement report is reference signal receiving power RSRP or reference signal receiving quality RSRQ.
4. The cooperative beam forming method according to claim 3  characterized in that  when RSRP or RSRQ measurement value in the measurement report is lower than predetermined threshold value thus the user is put into cell edge user group otherwise the user is put into central user group.
5. The cooperative beam forming method according to any one of claims 1 to 4  characterized in that  the superposition coding method is power allocation superposition coding  constellation rotation superposition coding  transmission rate superposition coding  interleaving superposition coding and different bit rate superposition coding.
6. The cooperative beam forming method according to any one of claims 1 to 5  characterized in that  the cooperative beam forming steps based on GoB comprises:
predefining beam cluster;
determining target sector serving cell edge users;
classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster;
for each cell edge MBMS user class 
dividing all of cell edge MBMS users in cell edge MBMS user class into sets  each cell edge MBMS user set having similar PMI measurement value;
target sector sending cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class  the cooperative request comprises information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve;
target sector forwarding  the common data needed to be transmitted  to each cooperative sector after successfully cooperative negotiation;
all cooperative sectors transmitting data to this cell edge MBMS user class using beam forming operation based on GoB.
7. The beam forming method according to claim 6  characterized in predefining beam cluster by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located.
8. The beam forming method according to claim 6  characterized in that  cell edge MBMS user class corresponding to a sector cluster is nearer to the sector cluster than other sector clusters.
9. The beam forming method according to claim 6  characterized in that  all edge MBMS users simultaneously measure PMI measurement values corresponding to target sector and all of sectors in sector cluster of the target sector to obtain optimum PMI of the MBMS user; and the edge MBMS user feeds back the optimum PMI to target sector.
10. The beam forming method according to claim 9  characterized in obtaining the optimum PMI based on long term SINR calculation.
11. The beam forming method according to claim 6  characterized in that  the cooperative request further comprises: information of PMI value application  indicating the cooperative sector the PMI value used during final cooperation.
12. The cooperative beam forming method according to claim 6  characterized in that  target sector forwards  the common data needed to be transmitted  to each cooperative sector via X2 interface between base stations.
13. A cooperative beam forming device  wherein the device comprises:
a means for predefining beam cluster;
a means for determining sector service  for determining target sector serving cell edge users;
a means for classifying sector cluster  for classifying all of cell edge MBMS users in target sector according to sector cluster of target sector  and forming cell edge MBMS user class corresponding to sector cluster;
a means for dividing cell edge MBMS user set  for dividing  all of cell edge MBMS users in cell edge MBMS user class  into sets  each cell edge MBMS user set having similar PMI measurement value;
a means for sending cooperative request  target sector sends cooperative request to each sector in sector cluster corresponding to the cell edge MBMS user class  the cooperative request comprises information that which cell edge MBMS user set that one sector in this sector cluster  as cooperative sector  should serve;
a means for forwarding common data  target sector forwards the common data needed to be transmitted to each cooperative sector after successfully cooperative negotiation;
a means for data transmitting of cooperative sectors  all cooperative sectors transmit data to this cell edge MBMS user class using beam forming operation based on GoB.
14. The cooperative beam forming device according to claim 13  characterized in that  the means for predefining beam cluster predefines beam cluster by predefining common codebook in target sector and sectors in sector cluster in which the target sector is located.
15. A base station  comprising:
a grouping unit  for grouping users into cell central user group and cell edge user group;
a processing unit for cell central user  for performing cell central user group multiplexing based on superposition coding;
the cooperative beam forming device according to any one of claims 13 to 14.

ABSTRACT
The present invention proposes a method  device and base station for cooperative beam forming based on MBMS fixed grid of beams GoB. The cooperative beam forming method comprises following steps: grouping users into cell central user group and cell edge user group (S701); performing cell central user group multiplexing based on superposition coding (S702); performing cooperative beam forming operation based on fixed grid of beams GoB for cell edge user group (S703).