WO2008131593A1

WO2008131593A1 - Method and system for managing control information

Info

Publication number: WO2008131593A1
Application number: PCT/CN2007/001454
Authority: WO
Inventors: Mattias Wennstrom
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2007-04-29
Filing date: 2007-04-29
Publication date: 2008-11-06
Also published as: US8238225B2; US20100046461A1; CN101669297B; CN101669297A

Abstract

The present invention relates to the management of control information in a multi-user multi-input, multi-output system (1) with one base unit (2) and one or several user units (31, 32), where respective user unit sends an index representing a vector from a user codebook (311) belonging to the user unit (31). The base unit (2) co-schedules compatible user units (31) to simultaneously share one time-frequency recourse by selecting a precoding matrix from a transmitter codebook (21) based on the indices received from co-scheduled user units (31, 32), according to a predefined function. The base unit (2) transmits control information regarding selected precoding matrix to each co-scheduled user unit (31, 32). The predefined function is known to each user unit (31, 32) and the control information (23r, 23r') to each user unit (31, 32) only contains information on the index from every other user unit. Respective user unit (31, 32) regenerates the precoding matrix according to its own index, other user units indices and the predefined function.

Description

Method and System for managing control information

Field of the Technology

The present invention relates to a method of managing control information in a multi-user multi-input, multi-output system with one base unit and one or several user units, where respective user unit sends an index representing a vector from a user codebook belonging to said the unit, where the base unit co-schedules compatible user units to simultaneously share one time-frequency recourse, where the base unit selects a precoding matrix from a transmitter codebook based on the indices received from co-scheduled user units, the selection being made according to a predefined function, and where the base unit transmits control information regarding selected precoding matrix to each co-scheduled user unit.

The present invention also relates to a multi-user multi-input, multi-output system, different units belonging to such a system, and to various computer program elements whereby an inventive method or inventive units can be realised.

Background of the Invention

In a multiple-input, multiple-output (MIMO) communication system using wireless orthogonal frequency division multiplexing (OFDM), the use of multiple antennas is much preferred in order to increase the performance of the system. When multiple antenna transmission at the transmitter side, either the base station or the mobile station, is used, then the space dimension can be exploited for scheduling of the transmission to the different users in the system.

To increase the throughput of the cell, an OFDM time-frequency resource can simultaneously be shared by multiple users. This is described in publications "Philips, 3GPP Technical document Rl-071403", "Samsung, 3GPP Technical document Rl- 060335", "On the optimality of multiantenna broadcast scheduling using zero forcing beamforming" by T.Yoo and A. Goldsmith, IEEE journal on selected areas in communications, Vol.24, No.3, March 2006, pp.528-541. When two or more users share the same time-frequency resource, ^' the transmissions to the co-scheduled users will mutually interfere with each other at the receiver side.

To minimize this interference between the co-scheduled users, precoding at the base station is performed in form of a linear mapping of each users signal to the transmitter antennas. This linear mapping is easily described by a matrix multiplication, where a precoding matrix, is jointly selected by the scheduled mobile users to minimize the mutual interference. Hence, precoded MU-MIMO is a closed loop MIMO scheme where the transmission is adjusted based on the channel to the co-scheduled mobile users.

In more detail, each mobile user selects a channel quantization vector (CQV) from a finite mobile user codebook of precoding vectors. The CQV is a quantization of the estimated channel vector from the N_T base station transmitter antennas to the receiver antenna at the mobile user. If the mobile user has multiple receive antennas, the mobile user can find a receiver filter to combine the multiple receive antennas to obtain a virtual single antenna. The CQV is then estimated as the channel from the N_T base station transmitter antennas to the single virtual receive antenna, i.e. after receiver filtering.

The selected CQV from the mobile user codebook is signalled to the base station. The scheduler in the base station then determines which mobile users are to be co- scheduled in an OFDM time frequency resource and selects the precoding matrix based on the reported CQV from these users.

For correct reception and demodulation of the transmitted signals, the mobile users need to have knowledge of the used transmitter precoding matrix and this information needs to be signalled to the mobile users.

Furthermore, knowledge of the full precoding matrix at the mobile user enables the use of interference suppression algorithms to minimize the residual interference from co-scheduled mobile users.

In "Philips, 3GPP Technical document Rl-071403", a method to solve the large signalling overhead problem is discussed where it is suggested to use dedicated precoded reference signals, one set of reference signals per scheduled mobile user.

The overhead of this method is compared with the overhead of signalling the used base station precoding matrix index. However, no attempt was made in the comparison to reduce the signalling overhead in the downlink from the upper limit value of \og₂(K^L) bits where K is the user codebook size and L is the number of co- scheduled users. One problem with MU-MIMO signalling lies in the overhead of signalling the used precoding matrix at the base station transmitter to all the scheduled mobile users. If L mobile users are scheduled together in a mutual OFDM time-frequency resource, and there is a one-to-one correspondence between the selected vectors in the mobile user codebook and the selected base station precoding matrix, then there are K^L possible base station transmitter precoding matrices where K is the number of vectors in the mobile user codebook. To signal the used transmitter precoding matrix in the downlink, L-log₂(K) bits are needed, which generally is too large to be practical.

Summary of the Invention

With the purpose of solving one or more of the above indicated problems, and from the standpoint of the above indicated field of invention, the present invention teaches that the predefined function is known to each user unit, that the control information to each user unit only contains information on the index from every other user unit, and that respective user unit regenerates the precoding matrix according to its own index, other user units indices and the predefined function. Since the base unit must have used the index of the user unit in the construction of the precoding matrix, the user unit has a prior knowledge of how the precoding matrix is constructed and this need not to be re-confirmed, only the residual information needs to be signalled to user unit. This will limit the required number of bits to represent the precoding matrix in the control message from L-log₂(K) bits to (L- l)-log₂(K) bits, which is a significant reduction of the control signalling. The index from the user codebook could for instance correspond to a requested channel quantization vector.

One embodiment of the present invention teaches that the base unit and respective user unit agrees on forbidden combinations of indices from co-scheduled user units and takes these forbidden combinations into consideration when selecting/regenerating the precoding matrix. Defined forbidden combinations of indices limits the possible amount of precoding matrices and thus limits the number of bits required to represent all possible precoding matrices to (L-l)-log₂(S) bits where S<K.

One proposed way of deciding on forbidden combinations is to have the base unit and respective user units to agree on a real-valued positive threshold (T), where a first user unit (A) and a second user unit (B) is only allowed to be co-scheduled if their respective indices represents vectors ( u_a , u_b ) that fulfils the condition

«_β «4 ≤ T .

There are different functions that can be used to create the precoding matrix and one proposed predefined function is the Zero Forcing precoder, which for L co- scheduled user units can be written as:

/(H) = H(Tf /?)^"1 where H = [x\ X₂ ... XL ]

and the output of the function/is a NT by L precoding matrix.

The present invention teaches that if a user unit is not co-scheduled with another user unit, then the base unit transmits a single stream and indicates this in the control information to the user unit. It is also taught that if the least amount of bits required to represent the possible amount of precoding matrices has at least one free bit combination, then it is possible to allow one bit combination to represent single stream transmission. The advantages of a method, system, a base unit, a user unit and a computer program element according to the present invention are that less number of bits are required to represent the used precoding matrix in the control messages form the base unit to the user units. This will significantly reduce the signalling overhead for MU- MIMO communication systems.

Brief Description of the Drawings

A method, a system, units and various computer program elements according to the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a schematic and simplified illustration of a MU-MIMO system where indices are sent from the user units to the base unit, and

Figure 2 is a schematic and simplified illustration of a MU-MIMO system where control information is sent from the base unit to the user units.

Detailed Description of the Invention

An inventive method of managing control information will now be described with reference to figure 1 showing in a multi-user multi-input, multi-output (MU- MIMO) system 1 with one base unit 2 and one or several user units 31, 32, where the base unit has N_T antennas. The user units 31, 32 are for instance mobile telephones or other mobile communication units present within a cell serviced by the base unit 2. A user unit may also be any stationary unit that uses the MU-MIMO system for communication.

Respective user unit 31, 32 sends an index α_/ representing a vector u from a user codebook 311 belonging to the user unit 31. A user codebook 311 can be defined consisting of K vectors

^UN_T = KL and each user unit 31, 32 reports its preferred vector u through their respective index α/, a 2.

Assume that L users are MU-MIMO co-scheduled which have reported indices a i, ύ₂, ..., a_h from the user codebook 311 where 1 < α_/ < K. It is well known that users which have reported the same user unit codebook index can not be MU-MIMO co- scheduled together since it makes user separation with linear receiver impossible, so all ai are unique. Without loss of generality, rearrange the user indices and define a new set as a_/, ..., ai such that 1 < α_/ < a^ < ... < K. Similar to the user codebook 311, a base unit precoder codebook 21 is defined as

Ψ = \w Ϋ

where each precoding matrix W_p is a NT x L matrix. The size of the precoder code book 21 is generally much larger than the user code book 311. The precoding matrix used at the base unit is a function of the selected user unit codebook vectors, hence

where the function / is unique the mapping function from the set of vectors indicated by the scheduled user unit codebook indices ai, ..., at. The unique mapping between the subset of user unit codebook indices aj, ..., ai and a transmitter precoding index/* can be defined as/? -I(aι, ..., a_t).

The mapping assures that for each subset α_/, ..., a_t one and only one transmitter precoding matrix p is defined.

The mapping also assures that for each precoding matrix p, one and only one subset a i, ..., ai is defined. Each of the allowed combinations α_/, ..., ai gives a unique base unit precoder index p and thus define the base unit precoder codebook.

Furthermore, for some particular sets ai, ..., a_t, no transmitter precoding matrix is defined, thereby the particular ai, ..., ai is in this case a forbidden selection. The forbidden selections of α_/, ..., ai are known in the system to both the transmitter and the receivers. For instance, we have already introduced the forbidden selection aj = ci2

= ... = a_t above.

As described above, the base unit 2 co-schedules compatible user units 31, 32 to simultaneously share one time-frequency resource, and in doing so the base unit 2 selects a precoding matrix from the transmitter codebook based on the indices α/, «2 received from co-scheduled user units, the selection being made according to a predefined function/

Figure 2 shows that the base unit 2 transmits control information regarding selected precoding matrix to each co-scheduled user unit 31, 32.

The purpose of the control signalling is to indicate to each scheduled user unit, which precoding matrix from the precoder codebook is used by the base unit. Since the size of the precoder codebook 21 is P matrices, log₂(P) bits are needed for this control signalling, and in prior art the same message is sent to each user.

However, the aim of the present invention is to reduce this overhead. The invention relies in that each scheduled user unit get residual information only, that is, only new information. Therefore, the message 23r, 23r' to each user unit will be different. For a certain user A, which has reported a user unit codebook index CIA, there exists an ordered subset of the base unit precoder code book 21. This subset can be described as { W_b] W_b2 ... W_bQ).

The subset can be identified using the Q indices to the set precoder codebook

b_q = l(a_v - - - _l a_A, - , a_L) q = l,...,Q

Therefore, for each user unit codebook vector defined by an index a, there exists a subset of base unit codebook indices as

[C )

B_a = ⁰Q J

So, when a user unit has selected a vector a in the user unit codebook, it knows, that if it is scheduled, the base unit precoding matrix will have an index belonging to B_a. This fact is the key which is used to reduce the control signalling, since it is only necessary to inform this user which of the indices in B_a is used.

The number Q and the subsets B_a are determined by obeying the restrictions

^■ The subset α_/, ..., ai must contain a ■ The subset α_/, ..., ai must not be a forbidden selection.

When the used base unit precoding matrix is signalled to user unit 31 , only the index q from the ordered in B_a needs to be signalled and thus log₂(0 bits needs to be signalled to the user unit 31. The procedure is repeated for the other user unit 32 and log₂(0 bits are thus also signalled to the user unit 32, however, the bits to the user unit 31 is different to those to the user unit 32.

Since the predefined function / is known to each user unit 31, 32 it is thus enough that the control information to each user unit, for example user unit 31 only contains information on the index az from every other user unit 32. This enables respective user unit to regenerate the precoding matrix W_p according to its own index aj, other user units indices α_? and the predefined function/

It is proposed that the index from the user codebook 311 corresponds to a requested channel quantization vector (CQV).

To reduce the signalling even more it is proposed that the base unit 21 and respective user unit 31, 32 agrees on forbidden combinations of indices from co- scheduled user units and takes these forbidden combinations into consideration when regenerating said precoding matrix.

One way of using forbidden combinations of indices is to introduce a restriction on the relation between reported user unit codebook vectors in a co-scheduled resource, i.e. forbidden selections of user codebook indices are introduced. If a first user unit 31 , A and a second user unit 32, B are co-scheduled and the first user unit A has reported the vector u_a from the user codebook 311 and the second user unit B has reported u_h , the restriction could be that the two user units A, B can only be co- scheduled in a time-frequency resource if u ^*u_b ≤ T where 7Ms a real-valued positive threshold known in the system. With this restriction, the size of the base unit precoder codebook 21 is reduced and it will also impact the number Q above.

A practical example of the present invention will now be disclosed. A user unit codebook 311 consisting of CQVs is defined. Also a base unit precoder codebook 21 consisting of precoding matrices is defined. Each co-scheduled user unit has reported an index to a vector in the user unit codebook 311. A scheduling unit 22, belonging to the base unit 2, selects L users and finds the matrix p in the base unit precoder codebook by p = I(aj, .... a_t). To signal the selection /? to the L scheduled users, L individual control messages are assembled as follows: To a first user unit which has reported user codebook index a^ the set

K = [^ W) is generated where

b^^] = l{a_v - , a_A,- , a_L)

for all allowed combinations of {a_v- - -,a,} which includes a A. The first user unit A is thus signalled the position number q in B_a where bM _{= p} q

Since the first user unit knows OA, and that the subset of indices aj, ..., ai are ordered such that 1 < a_\ < a_∑ < ... < aι < K it can generate B_a and from q it can obtain p. Also, from the ordering cti, ..., au the first user unit A gets information on which of the columns of the precoding matrix is the column intended for the first user unit A. Assume that the number of transmitter antennas N_τ is 4 at the base unit, that the user unit codebook size is K= 16 and 1=2 user units that are possible to MU-MIMO co-schedule. In total, there are 16²=256 "possible" precoding matrices which then will require 8 bits to signal to the two user units which base unit precoding matrix is used.

Assume a threshold of T=0.25 to introduce forbidden selections of indices. This gives P=56 allowed precoding matrices assuming a Discrete Fourier Transform mobile user codebook according to "Samsung, 3GPP Technical document Rl- 060335" and by a method which indicates which one of the allowed base unit precoding matrices is used, the required signalling is 6 bits.

The signalling can be reduced further according to the present invention by only signalling the uplink reported user unit codebook index of the MU-MIMO co- scheduled users. Due to the forbidden selections of indices, for each particular vector u_a there are only Q=I "allowed" vectors u_h that can be paired with u_a . Therefore, only 3 bits needs to be signalled.

For the example above, the mapping p=I(a,b) is given in table 1.

Table 1 Example mapping between base unit precoder codebook index p=I(a,b) and user unit codebook indices a,b.

Furthermore, for the control signalling, for each selection of user unit codebook index a, there exists a table of the possible co-scheduled vectors b, defined as C₃ and shown for this example and the cases a=l and b=14 in table 2 below. These sets are easily derived from table 1.

Table 2 The sets of C, and Cj₄

If the first user unit A has reported a user unit codebook index a-\ and it is MU- MIMO co-scheduled with a second user unit B which has reported b=\A, then from table 1, P=I(I, 14)=6 is the index of the used base unit precoding matrix. It is regenerated as

Furthermore, since the first user unit A signalled a=\ in the uplink it knows that Ci will be used when indicating the used precoding matrix so r=^β is signalled in the downlink and the user first unit is thereby informed that its co-scheduled user has b=14. It can thereby deduce the index to the precoder as /7=7(1, 14)=6. Since it has the lowest reported user codebook index of the two co-scheduled users units, it knows that the first column of We is the precoding vector for the first user unit A.

To the second user unit B, which has signalled b=14, r-1 is signalled in the downlink, hence, from Cj ₄, it know that it has been co-scheduled with a user unit which has reported a=l. The second user unit B then orders the pair according to a<b and finds the index to the precoder as /7=7(1, 14)=6. Also, since it has the higher reported user codebook index of the two co-scheduled user units, it knows that the second column of We is the precoding vector for the second user unit B. The example above shows one way of sending information on the index from every other user unit, which is to send the index r for the other user unit, whereby it is possible to recreate the precoding matrix. It is obvious to the skilled person that there are other ways of sending information on the indices from the other users.

Another example using table 1 above shows that the information from the base unit to each user unit could be to send the index p, where p-I(a,b). Using the same selections as in the example above with the cases a=l and b~14, a table 3 shows possible matrices p depending on selection a, b would then be:

Table 3 The sets of b\ and b\*

Furthermore, since the first user unit A signalled α=l in the uplink it knows that Bi will be used when indicating the used precoding matrix so r=6 is signalled in the downlink and the user first unit is thereby informed that _p=6=/(l, 14). It can thereby deduce that its co-scheduled user has b=14. Since it has the lowest reported user codebook index of the two co-scheduled users units, it knows that the first column of We is the precoding vector for the first user unit A. To the second user unit B, which has signalled b=14, r=\ is signalled in the downlink, hence, from B ₁4, it know that p=6=I(l, 14), and that it has been co- scheduled with a user unit which has reported a=l. Also, since it has the higher reported user codebook index of the two co-scheduled user units, it knows that the second column of W ₆ is the precoding vector for the second user unit B. There are different functions / and one proposed function to use is the Zero

Forcing precoder, which for L co-scheduled user units can be written as:

/(H) = H(H^* H)^"1 where H = [Xi Xi ... x_L ] The present invention also teaches that if a user unit 31 is not co-scheduled with another user unit, then the base unit 2 transmits a single stream and indicates this in the control information to the user unit 31.

If there is room in the sets B_a = jδ,^{(a ]} bj^{{ ]} J, it is possible to signal that only a single stream will be transmitted, i.e. Z=I. For instance, in the example above, table 2 will in this case be modified to table 4 below.

Table 4

With renewed reference to figure 1, an inventive multi-user multi-input, multi- output system 1 will now be described. The system comprises one base unit 2 and one or several user units 31, 32, where respective user unit 31, 32 is adapted to send an index α_/, ci₂ representing a vector from a user codebook 311, 312 belonging to the user unit 31, 32.

A scheduling unit 22, belonging to the base unit 2, is adapted to co-schedule compatible user units 31, 32 to simultaneously share one time-frequency recourse, where the base unit 2 is adapted to select a precoding matrix from a transmitter codebook 21 based on the indices aι, a_∑ received from co-scheduled user units 31, 32. The scheduling unit 22 is adapted to make said selection according to a predefined function. Figure 2 shows that a transmitting unit 23 is adapted to transmit control information 23r, 23r' regarding selected precoding matrix to each co-scheduled user unit 31, 32. The present invention teaches that the predefined function is known to each user unit 31, 32, that the control information 23r, 23r' to each user unit only contains information on the indices from every other user unit, and that respective user unit is adapted to regenerate the precoding matrix according to its own index, other user units indices and the predefined function.

It is proposed that the index α/, ci2 from the user codebook corresponds to a requested channel quantization vector (CQV).

It is also proposed that the base unit 2 and respective user unit 31, 32 is adapted to take agreed forbidden combinations of indices from co-scheduled user units into consideration when selecting/regenerating the precoding matrix.

One way of agreeing to forbidden combinations is that the base unit and respective user unit are adapted to agree on a real- valued positive threshold T, and that the scheduling unit is adapted to co-schedule a first user unit A and a second user unit B only if their respective indices represents vectors u_a , u_h that fulfils the condition u_au_b ≤ T .

The scheduling unit and respective user unit can be adapted to use the Zero Forcing precoder as a predefined function, which for L co-scheduled user units can be written as:

/(H) = H(H^* /?)^"1 where # = [*i Xi ... XL ]

It is also proposed that if a user unit 31 is not co-scheduled with another user unit, then the base unit 2 is adapted to transmit a single stream and to indicate this in the control information to the user unit 31.

The present invention also relates to a base unit 2 adapted to communicate with one or several user units 31, 32 in a multi-user multi-input, multi-output system 1, where the base unit 2 is adapted to receive, from each user unit 31, 32 an index α/, a2 representing a vector from a user codebook belonging to respective user unit 31, 32.

A scheduling unit 22, belonging to the base unit 2, is adapted to co-schedule compatible user units 31, 32 to simultaneously share one time-frequency recourse, where the scheduling unit 22 is adapted to select a precoding matrix from a transmitter codebook 21 based on received indices from co-scheduled user units. The scheduling unit 21 is adapted to make the selection according to a predefined function.

A transmitting unit 23, belonging to the base unit 2, is adapted to transmit control information 23r, 23r' regarding selected precoding matrix to each co-scheduled user unit 31, 32.

The present invention specifically teaches that the control information 12r, 12r' to each user unit only contains information on indices from every other user unit.

It is proposed that the index from respective user unit corresponds to a requested channel quantization vector (CQV). An inventive base unit 2 can be adapted to take agreed forbidden combinations of indices from co-scheduled user units 31, 32 into consideration when selecting said precoding matrix.

Such an agreement could be that the scheduling unit 22 is adapted to co-schedule a first user unit A and a second user unit B only if their respective indices relates to vectors (u_a ,ύ_b ) that fulfils the condition ύ^*u_b ≤ T , where T is a real-valued positive threshold.

It is proposed that the scheduling unit 22 is adapted to use the Zero Forcing precoder as said predefined function, which for L co-scheduled user units can be written as:

/(H) = H(H^* H)^"1 where H = [xi Xi ... XL ]

The present invention also teaches that if a user unit is not co-scheduled with another user unit, then the base unit is adapted to transmit a single stream and to indicate this in the control information to the user unit.

The present invention also relates to an inventive user unit 31 adapted to communicate with a base unit 2 in a multi-user multi-input, multi-output system 1, where the user unit 31 is adapted send an index ai to the base unit, the index representing a vector coming from a user codebook 311 belonging to the user unit 31.

The user unit 31 is adapted to receive control information 23r regarding a selected precoding matrix related to possibly other co-scheduled user units 32 from the base unit 2. The present invention specifically teaches that a predefined function used by the base unit 2 to select the precoding matrix is known to the user unit 31, and that the control information only contains information on indices «₂ from other co-scheduled user units 32.

The user unit 31 is adapted to regenerate the precoding matrix according to its own index α_/, other user units indices ci₂ and the predefined function.

It is proposed that the index corresponds to a channel quantification vector, CQV, and that the user unit 31 is adapted to make a request of a desired channel quantization vector by sending the index α_/ to the base unit 2. It is also proposed that the user unit 31 is adapted to take agreed forbidden combinations of indices from other co-scheduled user units into consideration when regenerating the precoding matrix. One way of realising this is to adapt the user unit 31 to take a real-valued positive threshold T into consideration when regenerating the precoding matrix, the threshold representing a value where a first user unit A and a second user unit B is only allowed to be co-scheduled if their respective indices represents vectors u_a , u_b that fulfils the condition <_β «4 ≤ T ,

The user unit 31 may be adapted to use the Zero Forcing precoder as predefined function, which for L co-scheduled user units can be written as:

f{H) = Ε{H^*Εy^λ where H = [xι Xi .^,. XL ]

It is also proposed that if the control information indicates that the user unit 31 is not co-scheduled with another user unit, then the user unit 31 is adapted to communicate with the base unit 2 through a single stream.

The present invention also relates to a number of different computer program elements. One inventive computer program element 4 comprises computer program code means 41 to make a computer unit execute the steps of a base unit 2 according to the inventive method.

Another inventive computer program element 5 comprises computer program code means 51 to make a computer unit execute the steps of a user unit 31 according to the inventive method. Another computer program element 6 comprises computer program code means 61 which when executed by a computer unit enables the computer unit to function as an inventive base unit 2.

Another computer program element 7 comprises computer program code means 71 which when executed by a computer unit enables the computer unit to function as an inventive user unit 31.

It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiments thereof and that modifications can be made within the scope of the inventive concept as illustrated in the accompanying Claims.

Claims

1. A method of managing control information in a multi-user multi-input, multi- output system with one base unit and one or several user units, where respective user unit sends an index representing a vector from a user codebook belonging to said user unit, where said base unit co-schedules compatible user units to simultaneously share one time-frequency resource, where said base unit selects a precoding matrix from a transmitter codebook based on the indices received from co-scheduled user units, said selection being made according to a predefined function, and where said base unit transmits control information regarding selected precoding matrix to each co- scheduled user unit, characterised in that, said predefined function is known to each user unit, that said control information to each user unit only contains information on the index from every other user unit, and that respective user unit regenerates said precoding matrix according to its own index, other user units indices and said predefined function.

2. A method according to claim 1, characterised in that said index from the user codebook corresponds to a requested channel quantization vector (CQV).

3. A method according to claim 1 or 2, characterised in that said base unit and respective user unit agrees on forbidden combinations of indices from co-scheduled user units and takes said forbidden combinations into consideration when selecting/regenerating said precoding matrix.

4. A method according to claim 1, 2 or 3, characterised in that said base unit and respective user units agrees on a real-valued positive threshold (T), that a first user unit (A) and a second user unit (B) is only allowed to be co-scheduled if their respective indices represents vectors (u_a ,u_b ) that fulfils the condition U_n Uv ≤ T .

5. A method according to any preceding claim, characterised in that said predefined function is the Zero Forcing precoder, which for L co-scheduled user units can be written as:

f(H)

X₂ ... XL ]

6. A method according to any preceding claim, characterised in that if a user unit is not co-scheduled with another user unit, then said base unit transmits a single stream and indicate this in said control information to said user unit.

7. A multi-user multi -input, multi-output system comprising one base unit and one or several user units, where respective user unit is adapted to send an index representing a vector from a user codebook belonging to said user unit, where a scheduling unit, belonging to said base unit, is adapted to co-schedule compatible user units to simultaneously share one time-frequency recourse, where said scheduling unit is adapted to select a precoding matrix from a transmitter codebook based on the indices received from co-scheduled user units, where said scheduling unit is adapted to make said selection according to a predefined function, and where a transmitting unit, belonging to said base unit, is adapted to transmit control information regarding selected precoding matrix to each co-scheduled user unit, characterised in that, said predefined function is known to each user unit, that said control information to each user unit only contains information on the indices from every other user unit, and that respective user unit is adapted to regenerate said precoding matrix according to its own index, other user units indices and said predefined function.

8. A system according to claim 7, characterised in that said index from the user codebook corresponds to a requested channel quantization vector (CQV).

9. A system according to claim 7 or 8, characterised in that said base unit and respective user unit is adapted to take agreed forbidden combinations of indices from co-scheduled user units into consideration when selecting/regenerating said precoding matrix.

10. A system according to claim 7, 8 or 9, characterised in that said base unit and respective user unit are adapted to agree on a real-valued positive threshold (T), and that said scheduling unit is adapted to co-schedule a first user unit (A) and a second user unit (B) only if their respective indices represents vectors (u_a ,u_b ) that fulfils the condition W_n M,, ≤ T .

11. A system according to any one of claims 7 to 10, characterised in that said base unit and respective user unit are adapted to use the Zero Forcing precoder as said predefined function, which for L co-scheduled user units can be written as: f(H)

Xi ... χ_L ]

12. A system according to any one of claims 7 to 11, characterised in that if a user unit is not co-scheduled with another user unit, then said base unit is adapted to transmit a single stream and to indicate this in said control information to said user unit.

13. A base unit adapted to communicate with one or several user units in a multiuser multi-input, multi-output system, where said base unit is adapted to receive, from each user unit, an index representing a vector from a user codebook belonging to respective user unit, where a scheduling unit, belonging to said base unit, is adapted to co-schedule compatible user units to simultaneously share one time-frequency recourse, where said scheduling unit is adapted to select a precoding matrix from a transmitter codebook based on received indices from co-scheduled user units, where said scheduling unit is adapted to make said selection according to a predefined function, and where a transmitting unit, belonging to said base unit, is adapted to transmit control information regarding selected precoding matrix to each co-scheduled user unit, characterised in that, said control information to each user unit only contains information on indices from every other user unit.

14. A base unit according to claim 13, characterised in that said index from respective user unit corresponds to a requested channel quantization vector (CQV).

15. A base unit according to claim 13 or 14, characterised in that said base unit is adapted to take agreed forbidden combinations of indices from co-scheduled user units into consideration when selecting said precoding matrix.

16. A base unit according to claim 13, 14 or 15, characterised in that said scheduling unit is adapted to co-schedule a first user unit (A) and a second user unit (B) only if their respective indices represents vectors (u_a ,u_b ) that fulfils the condition

W_n H* ≤ T , where T is a real-valued positive threshold.

17. A base unit according to any one of claims 13 to 16, characterised in that said base unit is adapted to use the Zero Forcing precoder as said predefined function, which for L co-scheduled user units can be written as:

/(//) = 7/(77^* /Tf¹ where H xi ... Xi ]

18. A base unit according to any one of claims 13 to 17, characterised in that if a user unit is not co-scheduled with another user unit, then said base unit is adapted to transmit a single stream and to indicate this in said control information to said user unit.

19. A user unit adapted to communicate with a base unit in a multi-user multi- input, multi-output system, where said user unit is adapted send an index to said base unit, said index representing a vector coming from a user codebook belonging to said user unit, and where said user unit is adapted to receive control information regarding a selected precoding matrix related to possibly other co-scheduled user units from said base unit, characterised in that, a predefined function used by said base unit to select said precoding matrix is known to said user unit, that said control information only contains information on indices from other co-scheduled user units, and that said user unit is adapted to regenerate said precoding matrix according to its own index, other user units indices and said predefined function.

20. A user unit according to claim 19, characterised in that said index corresponds to a channel quantification vector (CQV) and that said user unit is adapted to make a request of a desired channel quantization vector (CQV) by sending said index to said base unit.

21. A user unit according to claim 19 or 20, characterised in that said user unit is adapted to take agreed forbidden combinations of indices from other co-scheduled user units into consideration when regenerating said precoding matrix.

22. A user unit according to claim 19, 20 or 21, characterised in that said user unit is adapted to take a real-valued positive threshold (T) into consideration when regenerating said precoding matrix, said threshold representing a value where a first user unit (A) and a second user unit (B) is only allowed to be co-scheduled if their respective indices represents vectors (u_a , u_b ) that fulfils the condition u_au_b ≤ T .

23. A user unit according to any one of claims 19 to 22, characterised in that said user unit is adapted to use the Zero Forcing precoder as said predefined function, which for L co-scheduled user units can be written as:

/(H)

X₂ ... XL ]

24. A user unit according to any one of claims 19 to 23, characterised in that if said control information indicates that said user unit is not co-scheduled with another user unit, then said user unit is adapted to communicate with said base unit through a single stream.

25. A computer program element comprising computer program code means to make a computer unit execute the steps of a base unit according to anyone of claims 1 to 6.

26. A computer program element comprising computer program code means to make a computer unit execute the steps of a user unit according to anyone of claims 1 to 6.

27. A computer program element comprising computer program code means which when executed by a computer unit enables said computer unit to function as a base unit according to anyone of claims 13 to 18.

28. A computer program element comprising computer program code means which when executed by a computer unit enables said computer unit to function as a user unit according to anyone of claims 19 to 24.