Technical Field [0001] The present invention relates to generating precoders for joint transmission (JT) from multiple transmission points to multiple user equipments in a Downlink Coordinated Multi-Point transmission/reception (DL CoMP) communications system. Background Art [0002] The following abbreviations are used herein: CoMP Coordinated Multi-Point (the abbreviation CoMP often also means Coordinated Multi-Point transmission/reception, as will be evident from the context) CQI Channel Quality Indicator CSI Channel State Information - CSI includes PMI, RI, CQI (see below) DL Downlink DMRS Demodulation Reference Signal eNodeB Evolved NodeB (i.e. evolved base station) JT Joint Transmission MMSE Minimum Mean Squared Error PMI Precoder Matrix Indicator SINR Signal to Interference plus Noise Ratio RI Rank Indicator TP Transmission Point UE User Equipment [0003] Also, the following mathematical notations are adopted herein: ■ \a\ denotes the absolute value of a ■ ||a|[2 = |a(l)| +...+|a (N)| 2 (unless stated otherwise); ■ Ea denotes the expectation (or expected value) of a ; and ■ For any matrix A, A denotes the conjugate transpose of A, and r A represents the operation of taking the trace of A. [0004] Joint Transmission Downlink Coordinated Multi-Point transmission/reception (JT - DL CoMP). [0005] Fig. 1 schematically represents JT in a DL CoMP system. The system includes multiple TPs (these may be eNodeBs), each TP being equipped with multiple antennas, and multiple UEs where each UE is also equipped with multiple antennas. The multiple TPs transmit data to the multiple UEs on the same time-frequency. Generally, to minimise interferences between TPs and between UEs, the transmission is carried out with CoMP precoders which are generated based on (i.e. generated from the knowledge of) the channel state information (CSI). [0006] Each UE feeds back CSI (which includes RI, PMI and CQI) to its serving TP via uplink, as illustrated in Fig. 2. [0007] In CSI measurement, for each UE there are as many CSI configurations as there are TPs involved in JT - DL CoMP. Figs. 3A and 3B show CSI measurement for a UE involved in JT - DL CoMP with two TPs. The CSI config#0 is for TP#0 (the serving TP) and the CSI config#l is for TP#1 (the neighbouring TP). [0008] System Description A JT DL CoMP system having N T TPs and N UE UEs may be described mathematically as set out below. [0009] Let t denote the number of antennas at the n -th TP. The total number of transmit antennas V ¾ used in DL CoMP transmission is: [0010] Let N denote the number of receive antennas at each UE, and let H ,„ (size RX n ) deno e the channel between the n-th TP and the -th UE. [0011] Then the DL CoMP channel of the i -th UE (size x N TX ) is: H , = [H 1 , H ,2 H iN i = 1,.., N UE Equation (1) [0012] Let V, (size N x RI ) denote the precoder for the i -th UE. [0013] The received signal at the i -th UE ( y , ) is given by: y , = <å V X + . = N UE Equation (2) 1 where n , is additive Gaussian noise. Note that, from the DMRS, the i -th UE can find the effective channel H ,V; to generate a decoder. [0014] Precoding Precoding is dependent on PMI which is part of the CSI. (Recall that CSI is fed back by a UE to its serving TP via uplink.) Let p in denote the PMI corresponding to H,„ . Note that in a 2-stage PMI codebook system, p in is a pair PMI#1 and PMI#2. [0015] According to the 3GPP standard (TS 36.211), the precoder W,„ (of size t , c ΐ ) associated with the reported PMI p n is used for precoding data to send from the n-th TP to the i -th UE. The total DL CoMP precoder is therefore given by: Equation (3) = xW Note that, in a 2-stage PMI codebook system, 2) associated with PMI#1 and PMI#2. [0016] Precoding in this way is not optimal and it may be desirable to provide an improved or at least an alternative way of generating precoders. [0017] It is to be clearly understood that mere reference herein to previous or existing systems, methods, models, processes, procedures, practices, publications or other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things individually or in any combination formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art. Summary of Invention [0018] In one broad form, the invention provides a method for generating precoders for joint transmission (JT) in a downlink coordinated multi-point (DL CoMP) wireless communications system, the system including a plurality of transmission points (TPs) operable to communicate with a plurality of user equipments (UEs) wherein each UE has one of the TPs as its serving TP, and the method comprises: transmitting channel state information (CSI) from each UE to its serving TP, wherein the transmitted CSI includes precoder matrix indicators (PMI), and using the PMI to generate precoders for transmission of data from the plurality of TPs to the plurality of UEs. [0019] The use of the PMI to generate precoders may involve using the PMI to find a representative matrix ( n ) representing the channel (H ) between an n-th TP and an i-th UE. In some embodiments, a fixed codebook (W ) of representative matrices may be generated from PMI codebook(s), the CSI transmitted from each UE to its serving TP may include a rank indicator (RI), and W may be different for different RI. In such embodiments, if the RI for the i-th UE ( RI ) is equal to the number of receive antennas of the UE ( N ) (i.e. if RI,, = N ) then may contain matrices H(m), m = l,... of size N R n , where th is the number of antennas at the -th TP. Alternatively, if RI i is less than N (i.e. if RI i < ) then W may contain vectors {m), m = \,... of size r l . Proposals for the way in which H may be calculated in specific embodiments of the invention, both for the case where RI, = N R and also the case where RI, < , are discussed below. [0020] It is envisaged that non-coherent precoding may be used in some embodiments (or some embodiments may operate or be used in systems where non-coherent precoding is used), and where this is so the method for generating precoders may further comprise using the representative matrix H,„ , a Lagrange multiplier n and a noise variance estimate s, to compute the precoders (Vi ) . The precoders V,„ may be computed using an iterative procedure. Proposals for the way in which the precoders \ i and the Lagrange multiplier vn may be calculated for the case of non-coherent precoding in specific embodiments of the invention are discussed below. [0021] Whilst non-coherent precoding may be used in some embodiments of the invention, in other embodiments coherent precoding may be used (or embodiments may operate or be used in systems where coherent precoding is used). Where coherent precoding is used, the method for generating precoders may involve finding the representative matrix (H,„ ) in the manner described for the case of non-coherent precoding (as discussed above and also in further detail below), and then further finding a representative matrix H representing the total channel as follows: H,. = \ , i2 ,..., iN i =l,..,N UE [0022] In the case of coherent precoding, the method for generating precoders may further comprise using the said representative matrix H , a Lagrange multiplier u and a noise variance estimate s,2 to compute the precoders (\¾. Like in the case of non-coherent precoding, for the case of coherent precoding the precoders V- may be computed using an iterative procedure. Proposals for the way in which the precoders έ and the Lagrange multiplier u may be calculated for the case of coherent precoding in specific embodiments of the invention are discussed below. [0023] Regardless of whether coherent precoding or non-coherent precoding is used, the CSI transmitted from each UE to its serving TP may include (in addition to the PMI) a channel 2 quality indicator (CQI), and the above-mentioned noise variance estimate ' may be found using the CQI by i) finding the signal to interference plus noise ratio ( SINR ) based on thresholds in the CQI table; and ii) calculating s using the SINR and the serving TP's transmit power Ps .A specific proposal for the way in which this might be done is discussed below. [0024] As mentioned above, the CSI transmitted from each UE to its serving TP may include a rank indicator (RI). Suitably, from the up to N TP reported RI in , the majority may be selected as a single common RI for the i -th UE. In this case, it may be that only CQI (/) associated with the selected RI are candidates for CQI selection. The selection may be carried out per codeword independently, and the majority among the candidates may be selected as a common CQI for the / -th codeword CQI (/) . [0025] In another broad form, the invention provides a downlink coordinated multi-point (DL CoMP) wireless communications system in which joint transmission (JT) is performed between a plurality of transmission points (TPs) and a plurality of user equipments (UEs), wherein each UE has one of the TPs as its serving TP, channel state information (CSI) is transmitted from each UE to its serving TP, the transmitted CSI includes precoder matrix indicators (PMI), and the PMI is used to generate precoders for transmission of data from the plurality of TPs to the plurality of UEs. [0026] Aspects and features described herein with reference to one form of the invention (e.g. the method form) may also form part of, or be used (in any combination) in any other form of the invention (e.g. the system form). In fact, more generally, any of features or aspects described herein can be combined in any combination with any one or more other features or aspects described herein within the scope of the invention. Brief Description of Drawings [0027] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows: [0028] [Fig- 1] Fig. 1 is schematically represents a JT - DL CoMP system. [Fig. 2] Fig. 2 is schematically represents the way each UE feeds back CSI to its serving TP via uplink. [Fig. 3A] Fig. 3A schematically illustrates CSI measurement for JT - DL CoMP. [Fig. 3B] Fig. 3B schematically illustrates CSI measurement for JT - DL CoMP. [Fig. 4] Fig. 4 is a flowchart illustrating, for the case of non-coherent precoding, a method for generating j-MMSE precoders in accordance with the embodiment of the invention discussed below. [Fig- 5] Fig. 5 is a flowchart illustrating, for the case of non-coherent precoding, a method for computing the Lagrange multiplier for the n-th TP. [Fig. 6] Fig. 6 is a flowchart illustrating, for the case of coherent precoding, a method for generating j-MMSE precoders in accordance with the embodiment of the invention discussed below. [Fig. 7] Fig. 7 is a flowchart illustrating, for the case of coherent precoding, a method for computing the Lagrange multiplier for all TPs. [Fig. 8] Fig. 8 schematically illustrates the estimation of a UE's noise variance based on the reported CQI of the serving TP. [Fig. 9] Fig. 9 schematically illustrates R and CQI collection from the reported RI and CQI for transmission to the i-th UE. Description of Embodiments [0029] Joint transmit & receive optimisation methods have previously been proposed. See, for example, Sampath H. and Paulraj A., "Joint Transmit and Receive Optimization for High Data Rate Wireless Communication Using Multiple Antennas", Thirty-Third Asilomar Conference on Signals, Systems, and Computers, 1999, and Zhang J., et. al., "Joint Linear Transmitter and Receiver Design for Downlink of Multiuser MIMO Systems", IEEE Communications Letters, Vol. 9, No. 11, November 2005. [0030] Embodiments of the present invention provide MMSE precoders based (at least somewhat) on the joint transmit & receive optimization methods discussed in the above academic papers. However, unlike the methods in these academic papers, the present invention does not require knowledge of the channel to generate MMSE precoders. Instead (and in contrast), embodiments of the invention require only the PMI, which is fed back by UEs to serving TPs, as shown in Fig. 2. The precoder according to the particular embodiments of the invention discussed below will be referred to as the j-MMSE precoder. [0031] A) Non-coherentpreceding In the case of non-coherent precoding, the individual j-MMSE precoder V,„ is computed using the joint transmit and receive MMSE optimization as follows. [0032] Finding representative channels Let W denote the fixed codebook of representative channel matrices which is generated from the PMI codebook(s). There are different W for different RI. - For I = , the W contains matrices H( ), m =\,... of size Nk th - For RI,, < Vffi , the W contains vectors ( ), m =\,... of size h c [0033] Let H,„ be the representative for the channel H,„ . The representative channel is obtained as follows: [0034] If RI^ N , then H =H(ffl*)eil ffl , i =\,...,N UE , n =l,...,N P with m* argmax tr {H( w)W f[H (m)W,„ H(m) W Equation [0035] If RI Cin (m2) >... >Cn (mN ) and the N corresponding (m ),h(m 2),...,h(m N ) to form the channel matrix Equation (6) [0036] Here W,„ (of size th RI ) is the precoder in the 3GPP standard (TS 36.211) associated with the PMI p n . Note that, if the PMI consists of PMI#1 and PMI#2, then = [0037] Generating thej-MMSE precoder Vi (see Fig. 4) Let (m) denote the m -th iteration of the procedure. The precoder is generated as follows: 1) (401) Initialize Gi (m = ) = in , i =l,...,N UE . Here in is a I i - n matrix with the ( ,b) -th element being zero for a ¹ b and being 1 for = b . 2) (402) Compute in {m +\) using G ,„( ) and the Lagrange multiplier u for i =1,..., N as follows. Equation (7) 3) (403) Compute G,„ {m +1) using V ( +1) and the given noise variance estimate s ,2 for = 1,..., NUE as follows. uation (8) 4) 5) convergent threshold. 6) (406) Output V ( + 1), i = %...,N . [0038] Computing the Lagrange multiplier uh (see Fig. 5) For each of the n -th TP, the Lagrange multiplier vn is computed as follows. 1) (501) Compute as Equation (9) 2) (502) Set min and u 3) (503) Set ¾=( +¾ / 2 . 4) (504) Compute the following quantity . 5) (505) Check if Pn >Pn and if so set =uhotherwise set max = uh. Here P„ is the transmit power of the n -th TP. 6) (506) Repeat step 3), step 4) and step 5) until P„ ~ P„ P and if so set » = otherwise set »max = . Here P is the N P total transmit power, = · 6) (706) Repeat step 3), step 4) and step 5) until P-P (m ) > · > in NRX ) the corresponding vectors h(m, ), (m2),..., (mN ) to form the channel matrix [Claim 8] The method as claimed in claim 7, wherein non-coherent precoding is used and the method further comprises using the representative matrix H,„ , a Lagrange multiplier uh and a noise variance estimate s,2 to compute the precoders (Vi ). [Claim 9] The method as claimed in claim 8, wherein precoders Vi are computed using an iterative procedure. [Claim 10] The method as claimed in claim 9, wherein precoders V are computed using the following iterative procedure where denotes the m-th iteration: a) initialize a quantity G in m = 0) = J jn , i = l,...,N UE , where J,„ is a RIi n matrix with the ( ,b) -th element being zero for a ¹ b and 1 for a = b b) compute Vin (m + Y) using G in (m) and the Lagrange multiplier uh for i = 1,..., NUE as follows: Vi ( + ) = å H;G¾W)G ( «)H „+ I G ( ) c) compute G n (m + 1) using \ in (m + 1) and the noise variance estimate s for i = 1,..., NUE as follows: d) co e) rep < e , where . denotes the Frobenius norm and e is a convergent threshold; and f) output Vi + 1), UE- [Claim 11] The method as claimed in claim 10, wherein the Lagrange multiplier vn for the n -th TP is computed using the following procedure: a) compute as U U = ¾ H G (m)G,„(m)H,, b) set i and ; e) check if P > Pnand if so set v = vn otherwise set = n , where Pn is the transmit power of the n -th TP; f) repeat step c), step d) and step e) until P - P < e , where e is a convergent threshold; and g) output uh [Claim 12] The method as claimed in claim 8, wherein the CSI transmitted from each UE to its serving TP includes a channel quality indicator (CQI) and the noise variance estimate f is found using the CQI as follows: a) find the signal to interference plus noise ratio ( SINR ) based on thresholds in the CQI table; and b) calculate s using the SINR and the serving TP's transmit power P as follows. where is the number of codewords used for the -th UE. [Claim 13] The method as claimed in claim 1 wherein, from up to NTP reported RIin , the majority is selected as a single common RI i for the z' -th UE. [Claim 14] The method as claimed in claim 13, wherein only CQIifi (/) associated with the selected RJi are candidates for CQI selection, the selection is carried out per codeword independently, and the majority among the candidates is selected as a common CQI for the / -th codeword CQI l) . [Claim 15] The method as claimed in claim 7, wherein coherent precoding is used and the method further comprises finding a representative matrix H representing the total channel as follows: I = [H ,H,.2,...,H W , i = \,..,N [Claim 16] The method as claimed in claim 15, wherein the method further comprises using the representative matrix H , a Lagrange multiplier u and a noise variance estimate s, to compute the precoders (V ) . [Claim 17] The method as claimed in claim 16, wherein precoders are computed using an iterative procedure. [Claim 18] The method as claimed in claim 17, wherein precoders are computed using the following iterative procedure where ) denotes the m-th iteration: a) initialize G(m = ) = J I , i=\,...,NUE ,where J, is a RJ N matrix with the ( ,b)-th element being zero for a¹ b and 1 for a=b,and NTX is the total number of transmit antennas of all TPs; b) compute Y m+Y) using G m and the Lagrange multiplier u for = 1,...,NUE as follows: V,( +1) = ¾ « + H G ( ) c) compute G,( + 1) using V,( + 1) and the noise variance estimate s,2 for i =,...,NUE as follows. 'N , G,( +1) = "(m +l)H H,V, +1)V/ (m+l)Hf + , I .7=1 d) compute E +Y)- G ( ) e) repeat step b), step c) and step d) until l ,(m +1) - =l f) output V;(m +1), = 1,...,NUE [Claim 19] The method as claimed in claim 18, wherein the Lagrange multiplier u is computed using the following procedure: a) compute as ) set i and max ; c) set =( S! +u i ) d) compute the following quantity P = ; e) check if P >P then set min = u otherwise set a = u, where is the N P total transmit power P = P„ , f repeat step c), step d) and step e) until P-P