Abstract:
A wireless communications system supporting multiple MIMO transmission modes supporting both diversity and directional transmissions under a plurality of different transmission modes comprises a plurality of transmit and receive antenna elements where the transmit antenna elements are arranged to provide polarization diversity. The transmitting station derives actual knowledge of the forward channel by feeding back certain information such as a preferred beam index and a channel quality indicator figure of merit for that beam from the receiving station to the transmitting station along a reverse channel. The receiving station knows the beam weights used by the transmitting station. The transmitting station applies the fed back information to transmit user data intended for the receiving station in the optimal fashion, such as along the preferred beam and at a time when forward channel conditions are satisfactory. The system provides robust single or multiple stream diversity transmission, together with the option of single user or multi-user beamforming to allow on-the-fly trade-offs between coverage gain and capacity in a wireless telecommunications system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This Application claims priority from Canadian Patent Application No. 2,525,337 filed Oct. 28, 2005  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT  
       [0003]     Not Applicable  
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC  
       [0004]     Not Applicable  
       BACKGROUND OF THE INVENTION (1) Field of the Invention  
       [0005]     The present invention relates to multiple input multiple output (MIMO) wireless communication systems. More particularly, the present invention relates to MIMO wireless communication systems with support for both directional transmission and space or polarization diversity at the transmitting station, and feedback from the receiving station to the transmitting station, to increase the coverage and capacity of the MIMO wireless communication system. (2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98.  
         [0006]     In the field of wireless communications, as consumer usage patterns and multi-media applications evolve, there is a need for wireless communications providers to deploy wireless communications equipment providing improved data throughput or capacity, and coverage.  
         [0007]     As is well known in the art of wireless communications, both directional transmission and diversity transmission are important in improving capacity and coverage.  
         [0008]     Directional transmission and diversity transmission each provide link-level gains that may be leveraged to enhance coverage and/or capacity in a wireless network, depending on the data transmission strategy employed.  
         [0009]     Diversity transmission refers to the use of one or more distinct, or diverse, propagation channels to send information. By exploiting the diverse propagation channels, information is transmitted more rapidly and/or more reliably.  
         [0010]     Directional transmission refers to the shaping of a radiation pattern to be stronger in some directions than in others. Beamforming is a particular type of directional transmission in which the radiation pattern forms a beam shape. The beam is characterized by a predominant direction in which the energy is maximum, denoted the beam point direction or beam direction.  
         [0011]     When diversity transmission and directional transmission are combined, there are many different ways in which the antenna system can be used to pass data between a base station and one or more user terminals. These different data transmission methods, which are henceforth referred to as transmission modes or simply modes, have different advantages and disadvantages depending on channel conditions and user traffic patterns. It is therefore advantageous for an antenna system to support several transmission modes. It is further advantageous for the network equipment to be able to select between modes according to channel conditions and user traffic patterns.  
         [0012]     It is a requirement that both the transmission mode and the directional radiation patterns must be known by both the transmitting station and receiving station. The control of these aspects by the transmitter requires that the transmitter have knowledge of the “forward” channel, which extends from a transmitter to a receiver. This knowledge is not available directly at the transmitter, since the only channel it is able to monitor is the “reverse” channel. In order to enable mode selection and control of directional transmission, and communication of the selected mode and directional pattern, methods of providing the required information from the receiving station to the transmitting station are proposed. For example, some systems assume that the forward and reverse channels are approximately equal and use the reverse channel characteristics to estimate the forward channel characteristics. This assumption is generally not accurate except possibly in a general sense.  
         [0013]     To increase the coverage and capacity of wireless communication systems without increasing the amount of wireless equipment required to serve users, multiple input multiple output (MIMO) models have been developed. An example of a MIMO system is disclosed in U.S. Pat. No. 6,870,515 issued to Kitchener et al. Kitchener et al. discloses a MIMO wireless communication system comprising a plurality of transmit and receive antenna elements, where the transmit antenna elements are arranged so as to provide polarization diversity and to avoid spatial diversity. In such a MIMO system, the antenna polarizations at the transmitter are chosen to be orthogonal (i.e., +45° slant or vertical and horizontal pairs) in order to reduce the physical footprint of transmit antenna elements, thereby limiting the coverage and capacity of the system.  
         [0014]     Furthermore, polarization diversity is a well known concept, which has been used to increase the coverage and capacity of a wireless communication system, and has been combined with beamforming. The disadvantages of such systems is that they do not include multimodal support or any feedback of the actual forward channel characteristics.  
         [0015]     What is therefore needed is a wireless communication system that supports multiple transmission modes exploiting both diversity and directional transmission, and the exchange of the required information between transmitting and receiving stations.  
         [0016]     What is further needed is a means for using directional transmission to maximally improve signal to interference ratios (SNIR) on a network-wide basis.  
       SUMMARY OF INVENTION  
       [0017]     The present invention seeks to provide a wireless communication system that supports both diversity transmission and directional transmission for transmissions from a centralized access point, sometimes known as a base station, to one or more terminal devices, commonly known as user equipment (UE).  
         [0018]     The communications link formed when a base station transmits to a UE is referred to as the downlink (DL).  
         [0019]     The communications link formed when a UE transmits to a base station is referred to as the uplink (UL). In some wireless communication systems, multiple base stations may be joined to a core network, to create a wireless network.  
         [0020]     The present invention may be applied to an isolated link (whether DL, UL or both) or within a wireless network.  
         [0021]     The system of the present invention supports both diversity and directional transmissions under a plurality of different transmission modes. Different transmission modes are used because one may be better suited to a certain propagation channel condition. The transmitting station derives actual knowledge of the forward channel by feeding back certain information to the transmitting station. This feedback allows the transmitting station to control its diversity mode and radiation pattern in a manner that is optimized for the current channel conditions. Feedback may also be used to support transmit scheduling and radio link control decisions that are specified for certain wireless communications networks.  
         [0022]     The advantage of the present invention is that it provides robust single or multiple stream diversity transmission, together with the option of single user or multi-user beamforming to allow on-the-fly trade-off between coverage gain and capacity in a wireless communication system.  
         [0023]     According to a first broad aspect of an embodiment of the present invention, there is disclosed a multiple-input multiple-output (MIMO) wireless communications system comprising a transmitter and a receiver, the transmitter comprising: a plurality of transmit antenna elements having a plurality of diversity characteristics for transmitting user data to the receiver; a directional transmitter for acting on a first set of weight parameters to coherently combine those transmit antenna elements having a first common diversity characteristic into a first set of directional beams having the first diversity characteristic and for acting on a second set of weight parameters to coherently combine those antenna elements having a second common diversity characteristic into a second set of directional beams each having the second diversity characteristic, the first and second sets providing a plurality of independent MIMO channels for transmission of the user data between the transmitter and the receiver; and a pilot generator associated with each transmit antenna element, for introducing a mutually orthogonal pilot symbol into the user data transmitted by its associated transmit antenna element along each beam in the associated set of directional beams; and the receiver comprising: at least one receive antenna element for receiving the first and second sets of directional beams; a memory for storing the first and second sets of weight parameters; a receive processor for determining a preferred beam in the first and second sets of directional beams based on the stored first and second sets of weight parameters; and a reverse channel signaler for communicating to the transmitter the preferred beam; wherein the transmitter may transmit the user data intended for the receiver along the preferred beam to the receiver.  
         [0024]     According to a second broad aspect of an embodiment of the present invention, there is disclosed a transmitter for a multiple-input multiple-output (MIMO) wireless communications system comprising: a plurality of transmit antenna elements having a plurality of diversity characteristics for transmitting user data to a receiver; a directional transmitter for acting on a first set of weight parameters to coherently combine those transmit antenna elements having a first common diversity characteristic into a first set of directional beams having the first diversity characteristic and for acting on a second set of weight parameters to coherently combine those antenna elements having a second common diversity characteristic into a second set of directional beams each having the second diversity characteristic, the first and second sets providing a plurality of independent MIMO channels for transmission of the user data between the transmitter and the receiver; and a pilot generator associated with each transmit antenna element, for introducing a mutually orthogonal pilot symbol into the data transmitted by its associated transmit antenna element along each beam in the associated set of directional beams whereby the receiver may determine a preferred beam from the received pilot symbols and communicate it to the transmitter for use with the user data intended for the receiver.  
         [0025]     According to a third broad aspect of an embodiment of the present invention, there is disclosed a receiver for a multiple-input multiple-output (MIMO) wireless communications system comprising: at least one receive antenna element for receiving, from a transmitter, a first set and a second set of directional beams containing user data coherently combined using sets of weight parameters, each beam further comprising a series of initially orthogonal pilot symbols associated with each antenna element of the transmitter; a memory for storing the first and second sets of weight parameters; a receiver processor for determining a preferred beam in the first and second sets of directional beams based on the stored first and second sets of weight parameters; and a reverse channel signaler for communicating to the transmitter the preferred beam; whereby the receiver may determine a preferred beam and communicate it to the transmitter wherein the transmitter may transmit the user data intended for the receiver along the preferred beam to the receiver.  
         [0026]     According to a fourth broad aspect of an embodiment of the present invention, there is disclosed A method of multiple-input multiple-output (MIMO) wireless communications between a transmitter and a receiver comprising the steps of: the transmitter: acting on a first set of weight parameters to coherently combine transmit antenna elements having a first common diversity characteristic into a first set of directional beams each having the first diversity characteristic; acting on a second set of weight parameters to coherently combine antenna elements having a second common diversity characteristic into a second set of directional beams each having the second diversity characteristic; providing the first and second sets of directional beams as a plurality of independent MIMO channels for transmitting user data between the transmitter and the receiver; and introducing a mutually orthogonal pilot symbol into the user data transmitted by its associated transmit antenna element along each beam in the associated set of directional beams; and the receiver: receiving the first and second sets of directional beams at at least one receive antenna element; storing the first and second sets of weight parameters; determining a preferred beam in the first and second sets of directional beams based on the stored first and second sets of weight parameters; and communicating to the transmitter the preferred beam; wherein the transmitter may transmit the user data intended for the receiver along the preferred beam or beams to the receiver. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0027]     The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which:  
         [0028]      FIG. 1  shows the transmitter&#39;s pre-coder architecture according to an embodiment of the present invention;  
         [0029]      FIG. 2  shows an example of beam patterns resulting from two antenna elements spaced at one half of the signal wavelength according to an embodiment of the present invention;  
         [0030]      FIG. 3  shows an example of beam patterns resulting from two antenna elements spaced at ten wavelengths, according to an embodiment of the present invention; and  
         [0031]      FIG. 4  shows a MIMO channel between the transmitter of the embodiment of  FIG. 1  with two receiver antennas according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The invention will be described for the purposes of illustration only in connection with certain embodiments; however, it is to be understood that other objects and advantages of the present invention will be made apparent by the following description of the drawings according to the present invention. While a preferred embodiment is disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present invention and it is to be further understood that numerous changes may be made without straying from the scope of the present invention.  
         [0033]     Referring to  FIG. 1 , there is shown a basic transmitting station pre-coder architecture, shown generally at  100 , according to the present invention.  
         [0034]     The transmitting station  100  comprises a spatial grouper  110 , a scheduler  120 , a plurality of adaptive modulation coders (AMCs)  130 - 135 , a plurality of linear transformers or pre-coder matrices  150 ,  160 , a plurality of pilot tone mixers  170 - 173 , a cross-polarized antenna array  180  and a controller  190 .  
         [0035]     The transmitting station  100  accepts as input a plurality of user streams  101 - 102  of data to be transmitted along the forward channel and a feedback signal from one or more receiving stations along the reverse channel and outputs a plurality of shaped beams of energy  185 - 187  containing the data for receipt by one or more of the receiving stations  420  ( FIG. 4 ).  
         [0036]     The spatial grouper  110  accepts as input each of the plurality of user streams  101 - 102  and a feedback grouping control signal  192 . It allocates the data to be transmitted along a plurality of beam streams  111 - 113  to the scheduler  120 . The beam streams  111 - 113  correspond respectively to the shaped energy beams  185 - 187  output by the pre-coder  100  along the forward channel. In the exemplary embodiment discussed herein, where there are three shaped energy beams  185 - 187 , designated Left (L)  185 , Centre (C)  186  and Right (R)  187  respectively, there would be three corresponding beam streams of user data, respectively  111 ,  112  and  113 .  
         [0037]     While conceptually, as shown in  FIG. 1 , each user data stream  101 - 102  may be allocated to each of the beam streams  111 - 113 , typically, at any given point in time, a user data stream will be allocated to only a single beam stream  111 - 113 . However, changes in the operating environment, including the transmitting channel characteristics and the user traffic pattern, may mandate a change at a later period in time between a first beam stream and a second beam stream, as discussed below. Such a change may be initiated by the spatial grouper  110  in response to the feedback grouping control signal  192  received from the controller  190 . This feedback grouping control signal  192  is derived from, inter alia, the index of a preferred beam corresponding to a particular receiving station and the channel quality indicator (CQI) associated with the preferred beam. For example, if the controller  190  determines that a particular UE i  has indicated a preference for the Right (R) beam for its user data as described below, and that data from user stream j is intended for UE i , the controller  190  instructs the spatial group  110  to group user stream j into the Right (R) beam stream  113 .  
         [0038]     The scheduler  120  accepts as input each of the beam streams  111 - 113  and a feedback scheduler control signal  193  and generates a plurality of scheduled data streams  121 - 126  and a plurality of precoder selection control signals  127 - 128 . In the simple 2-element 3-beam example shown in  FIG. 1 , the precoder selection control signals  127 - 128  may not be required. In a more complex arrangement involving a larger number of elements and a greater number of beams, it may be more appropriate to partition the architecture using a number of selections to precoder matrices. In such a case, different precoder matrices can be used on the different antenna polarizations.  
         [0039]     The scheduler  120  orders the data that arrives along the beam streams  111 - 113  and generates a scheduled data stream for processing by a corresponding AMC  130 - 135 . The ordering established may change in response to changes in the operating environment, including the transmitting channel characteristics and the user data traffic environment. For example, if there is not a favourable channel available for transmission, the scheduler  120  may inhibit data transmission until conditions improve sufficiently. Such a change may be initiated by the scheduler  120  in response to the feedback scheduler control signal  193  received from the controller  190 . This feedback scheduler control signal  193  is derived from, inter alia, a channel quality indicator (CQI) figure of merit.  
         [0040]     In addition to ordering the data into a scheduled data stream, the scheduler  120  may implement a multiplexing scheme in order to improve the channel utilization. Those having ordinary skill in this art will readily recognize which multiplexing schemes may be most suitable. The notion of packet wireless access with scheduling is a form of time division multiplexing (TDM) which is already described in the context of the precoder solution. Alternatively, the system may use an orthogonal frequency division multiplexing (OFDM) access technology and different users may be allocated different segments of the available spectrum, that is, a sub-block of the available OFDM sub-carriers. Other potential candidates will readily come to mind to those having ordinary skill in this art.  
         [0041]     The number of scheduled data streams  121 - 126  corresponds to the number of adaptive modulation coders  130 - 135 , which in turn corresponds to the product of A, the number of pre-coder matrices  150 ,  160  and B, the number of shaped energy beams  185 - 187 . In the exemplary embodiment shown in  FIG. 1 , there are two pre-coder matrices  150 ,  160  (A=2) and three shaped energy beams  185 ,  186 ,  187  (B=3), so that there are six (A×B=6) AMCs  130 - 135  and six scheduled streams.  121 - 126 . Each of the scheduled streams  121 - 126  feed into a corresponding AMC  130 - 135 . The pre-coder selection control signals  127 - 128  generated by the scheduler  120  control one of the pre-coder matrices.  
         [0042]     Each adaptive modulation coder  130 - 135  accepts as input a corresponding scheduled data stream  121 - 126  generated by the scheduler  120  and a feedback coder control signal  194 - 196  (only three such signals are shown for clarity) and generates a corresponding coded data stream  137 - 142  for input into one of the pre-coder matrices  150 ,  160 .  
         [0043]     Each AMC  130 - 135  modulates and encodes the data in accordance with one of a plurality of known modulation and/or encoding schemes. Those having ordinary skill in this art will readily recognize that the choice of modulation and/or coding scheme may vary according to the transmitting channel characteristics and the user data traffic environment in response to the feedback coder control signals  194 - 196  received from the controller  190 . These feedback coder control signals  194 - 196  are derived from, inter alia, one or more channel quality index (CQI) figures of merit.  
         [0044]     As indicated, the number of AMCs  130 - 135  corresponds to the product of A, the number of pre-coder matrices  150 ,  160  and B, the number of shaped energy beams  185 - 187 .  
         [0045]     Thus, of the A×B (=6 in the illustrative embodiment of  FIG. 1 ) coded data streams, B (=3 in the illustrative embodiment of  FIG. 1 ) coded data streams  137 - 142 , corresponding to each of the B shaped energy beams  185 - 187  will be allocated to each of the A (=2 in the illustrative embodiment of  FIG. 1 ) pre-coder matrices  150 ,  160 . For example, coded data streams  137 ,  139 ,  141  are generated for processing by pre-coder matrix  150  and coded data streams  138 ,  140 ,  142  are generated for processing by pre-coder matrix  160 .  
         [0046]     Diversity is achieved through a plurality of polarizations. Each polarization implements a separate diversity branch. A, the number of pre-coder matrices  150 ,  160 , also denotes the number of diversity branches.  
         [0047]     The pre-coder matrices, designated X  150  and Y  160 , respectively, are used on the two diversity branches.  
         [0048]     The pre-coders  150 ,  160  provide complex weightings of the antenna inputs arriving at inputs  151 - 153 ,  161 - 163  corresponding to the Left, Centre and Right beams respectively that shape the radiation pattern, which in turn control directional transmission. The pre-coders  150 ,  160  may not necessarily be identical. They may be different if the antenna element patterns are different for the two polarizations. For simplicity, in the illustrated embodiments, identical element patterns are assumed for the two polarizations so the pre-coders  150 ,  160  are identical. Each pre-coder matrix  150 ,  160  generates a plurality of beams (in the illustrated embodiment of  FIG. 1  there are three beams) of encoded data  154 ,  155 ,  164 ,  165 , labeled as the Left (L), Centre (C) and Right (R) beams. A person of ordinary skill in the art will readily recognize that the beamweight parameters submitted to each pre-coder may be different, and that, subject to some limitations, any number of beams may be constructed. The beams are constructed to provide good, unambiguous coverage across the sector with minimum beamwidth designs consistent with the dimension of the available antenna array  180  dimensions. If the antenna array  180  comprises more than two antennas, more and more complex beam patterns may be possible with the provision of suitable beamweight parameters.  
         [0049]     Independent (orthogonal) pilots  174 - 177  are applied to the individual antennas at the transmitting station through a plurality of mixers  170 - 173 . In multi-user wireless networks, these pilot transmissions are used by a receiving station  420  to determine a preferred serving cell (not shown). The pilot signal transmissions also enable the receiving station  420  to make channel quality indicator (CQI) measurements, select the best beam or beams within the serving cell, and perform MIMO transmission modes, as discussed below. Pilot transmissions, unlike data transmissions, do not undergo the pre-coding operation.  
         [0050]     The transmitting station is equipped with a plurality of groups of antenna elements [ 181 ,  183 ], [ 182 ,  184 ] with one group being used for each diversity branch. In the illustrative embodiment of  FIG. 1 , group [ 181 ,  183 ] corresponds to pre-coder matrix Y  160  (for diversity branch Y), while group [ 182 ,  184 ] corresponds to pre-coder matrix X  150  (for diversity branch X). The preferred configuration is to have sub-wavelength spacing of the array elements within each diversity branch. However, the same approach is applicable to arrays with wider spacing, in which case spatial diversity may be employed in addition or in substitution for polarization diversity. For illustration,  FIG. 1  shows a dual column cross-polar antenna array  180 , however a person of ordinary skill in the relevant art will readily recognize that the discussion presented here may be generalized to larger array sizes and arbitrary array geometries.  
         [0051]     The controller  190  accepts the feedback signal  191  from one or more receiving stations  420  along a reverse channel (not shown) and generates the feedback grouping control signal  192 , the feedback scheduler control signal  193  and a plurality of feedback coder control signals  194 - 196  that are fed to the spatial grouper  110 , scheduler  120  and adaptive modulation coders (AMCs)  130 - 135  respectively.  
         [0052]     The feedback signal  191  comprises a preferred beam index from the responding receiving station  420  that is indicative of the receiving station  420 &#39;s preference, based upon the existing conditions along the forward channel  410  between the transmitting station  100  and the receiving station  420 , of along which beam, user data intended for it should be transmitted.  
         [0053]     Preferably, the feedback signal  191  also comprises a channel quality indicator (CQI) figure of merit corresponding at least to the designated preferred beam.  
         [0054]     In operation, user data streams  101 - 102  are grouped by the spatial grouper into an appropriate beam stream  111 - 113 , corresponding to a preferred beam index designated by the intended recipient receiving station  420  and fed back to the transmitting station  100  in the feedback signal  191  along the reverse channel (not shown) extending between the transmitting station  100  and the receiving station  420 .  
         [0055]     Each beam stream  111 - 113  is scheduled and multiplexed by the scheduler  120 , in accordance with the existing conditions of the forward channel  410  extending between the transmitting station  100  and the intended recipient receiving station(s)  420  as denoted by the corresponding CQI figure(s) of merit fed back to the transmitting station  100  in the feedback signal  191  along the reverse channel (not shown) extending between the transmitting station  100  and the receiving station(s)  420 .  
         [0056]     The scheduler  120  generates a plurality of scheduled streams  121 - 126  corresponding to a unique pair of the A diversity branches and B beams in the transmitter station  100  and feeds each scheduled stream  121 - 126  into a corresponding AMC  130 - 135 .  
         [0057]     Each AMC  130 - 135  encodes and modulates the corresponding scheduled stream  121 - 126  in accordance with a modulation and encoding scheme that is appropriate having regard to the existing conditions of the forward channel  410  extending between the transmitting station  400  and the intended recipient receiving station(s)  420  as denoted by the corresponding CQI figure(s) of merit for the associated beam, fed back to the transmitting station  100  in the feedback signal  191  along the reverse channel (not shown) extending between the transmitting station  100  and the receiving station  420 .  
         [0058]     The AMCs  130 - 135  corresponding to a particular diversity branch generates a coded data stream  137 - 142  that is provided to the associated pre-coder matrix  150 ,  160 .  
         [0059]     Each pre-coder matrix  150 ,  160  applies a complex weighting using pre-determined beamforming weights to generate directional beams from the input coded data streams and outputs a plurality of beams of encoded data  154 ,  155 ,  164 ,  165 .  
         [0060]     The encoded beams of data  154 ,  155 ,  164 ,  165  are each mixed with an orthogonal pilot signal  174 - 177  by mixers  170 - 173  respectively and are output to the compact multi-element cross-polarized array  180  along the A diversity branches (in the illustrative embodiment of  FIG. 1 , A=2).  
         [0061]     The orthogonal pilot signals  174 - 177 , which are not beamformed in the present embodiment, may be monitored by the receiving station(s)  420  in order to determine the channel quality indication (CQI) figure(s) of merit and the preferred beam index, which information may be fed back to the transmitting station  100 &#39;s controller  190  as the feedback signal  191  along the reverse channel (not shown) extending between the receiving station(s)  420  and the transmitting station  100 .  
         [0062]     In relation to  FIG. 1 , which shows a two-element array on each diversity branch with each array using an identical three-column (three beam) preceding matrix, the pre-coder transformation may be expressed by:
 
E=[e L e C e R ]  (1)
 
 where  e   L ,  e   C  and  e   R  are the fixed beamformer weights needed to form the three beams (denoting Left, Centre and Right pointing directions). Accordingly, the two-element cross-polarized antenna configuration may be expressed as:  
             E   =     [           e     L   ⁢           ⁢   1             e     C   ⁢           ⁢   1             e     R   ⁢           ⁢   1                 e     L   ⁢           ⁢   2             e     C   ⁢           ⁢   2             e     R   ⁢           ⁢   2             ]             (   2   )             
 
         [0063]     Based on this definition, there is no need for the pre-coder transformation, E to be orthogonal. Preferably, the pre-coder transformation E will be designed so as to make the overall beam envelope match, as closely as possible, the required reference sector coverage pattern and to give each of the resulting beams a similar ground footprint (for example, equal coverage and user traffic loading). The beam shapes may be optimized by adjusting a number of variables, including but not limited to element spacing, element patterns, scan angles of all beams (about boresight) and amplitude weighting of all beams relative to a central beam.  
         [0064]      FIG. 2  illustrates an exemplary three-beam pattern shown as dB gain (Y-axis) as a function of beam angle (X-axis), based upon a two-element array with elements spaced at one-half wavelength, such as is shown in the illustrative embodiment of  FIG. 1 . The element pattern reflects the relation:
   G (θ)=cos 2 θ  (3) 
 where θ is the angle and G(θ) is the gain (linear scale). 
 
         [0065]     Here, left  210  and right  220  beams are steered by −28° and +28° respectively relative to the centre beam  230 . The pattern is shown clipped at −20 dB below the peak ( 211 ,  212 ,  221 ,  222 ) for formatting reasons. In fact, the realized pattern would not experience such clipping.  
         [0066]     Ref i  is the target sector coverage envelope, that is, the azimuthal response to which the coverage of the multibeam patterns are attempted to be matched. Ref i  is the standard sector beam pattern specified in the 3GPP/PP2 standards and is universally adopted for performance simulations.  
         [0067]     E i  is the azimuthal pattern of the assumed element response, that is, for a single element of the two-element antenna array  180 . It is assumed that E i  is the same on both polarizations.  
         [0068]     The corresponding weighting matrix is:  
                   E   =     [           e     L   ⁢           ⁢   1             e     C   ⁢           ⁢   1             e     R   ⁢           ⁢   1                 e     L   ⁢           ⁢   2             e     C   ⁢           ⁢   2             e     R   ⁢           ⁢   2             ]                 =     [           0.500   -     j   ⁢           ⁢   0.448           0.707         0.500   -     j   ⁢           ⁢   0.448                 0.55   +     j   ⁢           ⁢   0.448           0.707         0.500   -     j   ⁢           ⁢   0.448             ]                   (   4   )             
 
         [0069]     For the case of widely spaced array elements, for example, where spatial diversity is used instead of or in addition to polarization diversity, the same parameters may be adjusted to achieve different design objectives. For example, the element spacing may be chosen so that the lobe width is smaller than the per-mobile multipath angle scatter, the scan angles of the first lobes of the left (L) and right (R) beams may be adjusted so that the grating lobes of all three beams are evenly spaced and uniformly cover the field of coverage, and the element pattern may be designed so that the beam envelope is a reasonably close match to the required reference sector coverage patterns.  
         [0070]      FIG. 3  shows an exemplary three-beam pattern, again shown as dB gain (Y-axis) as a function of angle (X-axis), based on a two-element array with elements spaced at ten wavelengths. The element pattern again reflects the relation shown in equation (3). Here, however, the left  310  and right  320  beams are steered by −2° and +2° respectively relative to the centre beam  330  and the corresponding weight matrix is given by:  
                   E   =     [           e     L   ⁢           ⁢   1             e     C   ⁢           ⁢   1             e     R   ⁢           ⁢   1                 e     L   ⁢           ⁢   2             e     C   ⁢           ⁢   2             e     R   ⁢           ⁢   2             ]                 =     [           0.323   -     j   ⁢           ⁢   0.629           0.707         0.323   -     j   ⁢           ⁢   0.629                 0.323   +     j   ⁢           ⁢   0.629           0.707         0.323   -     j   ⁢           ⁢   0.629             ]                   (   5   )               
         [0071]     The pre-coder transformation, E needs to be known at the receiving station  420  as well as at the transmitting station  100 . Ideally, the pre-coder dictionary should be configurable to the receiving station  420  to accommodate different pre-coder designs optimized according to deployment or network. This configuration may be done by signaling between the transmitting station  100  and the receiving station  420 . In the case of a pre-coder matrix used by the base station for downlink transmissions, this may be communicated to the UE, either once on initialization for fixed pre-coding arrays, or more often in a system supporting dynamic or variable pre-coding arrays. In a cellular network, it may be desirable for different base stations to use different pre-coder arrays; in this case pre-coder matrices would need to be signaled to the UE at handover.  
         [0072]     The transmitting station  100  may also adaptively tune or learn its pre-coder matrix such that the matrix is tailored to the forward channel  410  conditions and spatial traffic distribution in the area affecting signal quality at that transmitting station  100 . This is done by measuring the reverse channel (not shown) transmissions over a fairly long period of time. This adaptive pre-coder matrix tuning makes us of a capability to signal new pre-coder matrices to the receiving station. Control signals  197  and  198  extend from the controller  190  to the precoder matrices  150 ,  160  to allow the precoder coefficients to be updated.  
         [0073]     In the two-element cross-polarization antenna shown in  FIG. 1 , the pre-coder beam patterns are constructed so that the central beam provides coverage in-fill around the cusp region of the left and right beams. This ensures that the best beam selection produces negligible cusping loss. A person of ordinary skill in the relevant art will readily recognize that larger array sizes, such as those with 4, 6 or 8 spatial elements, may be used for significantly improved performance of the system. However, these configurations will use an increased number of orthogonal pilots for the forward channel  410  transmission, with corresponding increased feedback for best beam indication, and involve a physically larger antenna structure.  
         [0074]     Numerous modes, or methods of transmitting data over the diversity branches and beams, are possible. Depending on the mode of transmission, the receiving station  420  can have one, two or more signal paths, commonly known as receive diversity branches. Each receive data path may originate from an individual antenna, or a group of closely spaced antennas having like polarizations (cf. eg.  421 ,  422 ).  FIG. 4  shows a representation for the MIMO forward channel  410  for the two-column cross-polarized array  180  discussed in  FIG. 1  assuming two receiver antennas  421 ,  422 . Based on the representation shown in  FIG. 4 , the channel matrix may be defined as follows:  
             H   =     [           h   11           h   12               h   21           h   22               h   31           h   32               h   41           h   42           ]             (   6   )             
    where h 11  represents the channel defined by the path  411  between transmitter antenna  1   182  and receiver antenna  1   421 ;     h 21  represents the channel defined by the path  413  between transmitter antenna  2   181  and receiver antenna  1   421 ;     h 31  represents the channel defined by the path  415  between transmitter antenna  3   184  and receiver antenna  1   421 ;     h 41  represents the channel defined by the path  417  between transmitter antenna  4   183  and receiver antenna  1   421 ;     h 12  represents the channel defined by the path  412  between transmitter antenna  1   182  and receiver antenna  2   422 ;     h 22  represents the channel defined by the path  414  between transmitter antenna  2   181  and receiver antenna  2   422 ;     h 32  represents the channel defined by the path  416  between transmitter antenna  3   184  and receiver antenna  2   422 ; and     h 42  represents the channel defined by the path  418  between transmitter antenna  4   183  and receiver antenna  2   422 .    
 
         [0083]     In operation, the UEs determine an estimate of the forward MIMO channel  410  described by H by detecting the received amplitudes and phases of the four orthogonal pilots, P 1   174 , P 2   175 , P 3   176  and P 4   177  transmitted from the transmitting station  100 . The H matrix may be partitioned into two sub-matrices which describe the propagation paths between the transmitter and the receiving station  420  using polarization X at the transmitting station  100 :  
               H   X     =     [           h   11           h   12               h   31           h   32           ]             (   7   )             
 
 and the propagation paths between the transmitting station  100  and the receiving station  420  using polarization Y at the transmitting station  100 :  
               H   Y     =     [           h   21           h   22               h   41           h   42           ]             (   8   )             
 
         [0084]     The vector e S  represents the pre-coding transformation corresponding to the selected beam which will be applied to the data transmissions from the transmitting station  100  on both diversity branches. The equivalent MIMO channel matrix, including the effect of the pre-coding transformation, is given by:  
                   G   =     [           g   11           g   12               g   21           g     g   ⁢           ⁢   22             ]                 =     [           (         e     S   ⁢           ⁢   1       ⁢     h   11       +       e     S   ⁢           ⁢   2       ⁢     h   31         )           (         e     S   ⁢           ⁢   1       ⁢     h   12       +       e     S   ⁢           ⁢   2       ⁢     h   32         )               (         e     S   ⁢           ⁢   1       ⁢     h   21       +       e     S   ⁢           ⁢   2       ⁢     h   41         )           (         e     S   ⁢           ⁢   1       ⁢     h   22       +       e     S   ⁢           ⁢   2       ⁢     h   42         )           ]                   (   9   )             
 
 where S is the index of the selected beam (i.e., S is either .L, C or R). 
 
         [0085]     Here, g ij  is the channel between transmit diversity branch i and receive branch j. The matrix G is used in the UE receiver to recover data intended for it, from the total signal impinging upon it.  
         [0086]     Now let:  
               X   =       [           x     L   ⁢           ⁢   1             x     L   ⁢           ⁢   2                 x     C   ⁢           ⁢   1             x     C   ⁢           ⁢   2                 x     R   ⁢           ⁢   1             x     R   ⁢           ⁢   2             ]     =       E   T     ⁢     H   A           ⁢     
     ⁢   and           (   10   )               Y   =       [           y     L   ⁢           ⁢   1             y     L   ⁢           ⁢   2                 y     C   ⁢           ⁢   1             y     C   ⁢           ⁢   2                 y     R   ⁢           ⁢   1             y     R   ⁢           ⁢   2             ]     =       E   T     ⁢     H   B                 (   11   )             
 
         [0087]     These are the combined precoding and channel transformations for all beams of the two transmit diversity branches. The components of matrices X and Y represent the effective complex beam amplitudes received by the two receive branches for each of the two transmitted polarizations. The receiving station  420  may select one beam based on average measurements made over both diversity branches, or may select a different beam for each diversity branch.  
         [0088]     The receiving station  420  may select the preferred beam for transmission of data by comparing channel quality or power estimates from each of the three (Left, Center and Right) beams.  
         [0089]     For example, if a single beam per user is selected (that is, the same beam is applied on both transmitter polarizations), the receiving station  420  determines the following:
 
Power_Beam_L=|x L1 | 2 +|x L2 | 2 +|y L1 | 2 +|y L2 | 2   (12)
 
Power_Beam_C=|x C1 | 2 +|x C2 | 2 +|y C1 | 2 +|y C2 | 2   (13)
 
Power_Beam_R=|x R1 | 2 +|x R2 | 2 +|y R1 | 2 +|y R2 | 2   (14)
 
         [0090]     In this particular example, the best beam corresponds to the selection which provides maximum summed received power.  
         [0000]     Feedback from Receiving Station to Transmitting Station  
         [0091]     Depending on the outcome of the receiving station&#39;s beam selection procedure, the receiving station  420  feeds back a preferred beam index or preferred beam indices to the transmitting station  100 . This requires the transfer of [log 2 C] bits of information per feedback interval on the uplink to indicate the best beam from those available, where C is the number of beams and [x] denotes the smallest integer that is not less than x.  
         [0092]     The transmitting station  100  will transmit to the receiving station  420  on the selected beam using one of the transmission modes as discussed below.  
         [0093]     Alternatively, the receiving station  420  may feed back the full channel matrix H. Such full channel feedback would allow the transmitting station  100  to select a column from a predetermined pre-coding matrix using the above equation, transferring some complexity from the receiving station  420  to the transmitting station  100 . This is desirable for the downlink case, since base station complexity is preferable to UE complexity. Full channel feedback would also allow the transmitting station  100  to instantaneously formulate a pre-coding matrix optimized for the fed back user channel. One cost of this flexibility would be an increased number of feedback bits and the introduction of additional signaling to inform the receiving station  420  of the beam selection decision.  
         [0094]     It should be noted that in the above description, a channelization resource refers to the smallest transmission resource that can be assigned to a single user in a multiple access system. Simple examples include a frequency band in an FDMA system or a timeslot in a TDMA system. In a system supporting multiple users through a multiple access scheme with multiple channelization resources, there may be many concurrent users. The single user/multi-user distinction and the descriptions used therein are with respect to one channelization resource, but can then be applied across all available multiple access channels. There are many alternative multiple access technologies. The present invention may be used with any of them.  
         [0095]     In the case in which the transmitting station  100  is a base station, in addition to beam selection feedback, the UE (receiving station  420 ) supplies channel quality indicators (CQI) to the base station. Either a single “joint CQI” describing the average channel quality for both diversity branches on the preferred beam, or else a CQI for each diversity branch, is provided, depending upon the transmission mode. CQI feedback enables scheduling and radio link control features in some wireless networks. Furthermore, as described above, CQI feedback is used within the proposed system to select certain aspects of some of the transmission modes.  
         [0000]     MIMO Transmission Modes  
         [0096]     Numerous transmission modes are possible using the system of the present invention. The transmission modes may be roughly grouped into those that support a single user per channelization resource, and those that support multiple users on a single channelization resource. In a system supporting multiple channels, there may be many concurrent users. Tables 1 and 2 below describe transmission modes that are supported by a base station using the present invention when each user is constrained to use the same beam for both diversity branches. Single user transmission modes are described in Table 1, multi-user modes are described in Table 2. It should be noted that the single user/multi-user distinction and the descriptions provided here are with respect to one channel, but can be used for all available channels and that it is possible to use different modes for different channels or users. There may be alternative multiple access technologies.  
         [0097]     Those having ordinary skill in this art will readily recognize that all modes may be generalized to the case in which different transmit diversity branches directed at a single user may use different beams, with the addition of per-polarization beam selection and CQI feedback in the uplink. Furthermore, such a person will readily recognize that the single-user modes presented for the downlink in Table 1 may be generalized to uplink transmissions for a UE using the system of the present invention.  
         [0098]     The single user modes tend to decrease interference and provide link gain, resulting in improved coverage for the wireless communication system. Multi-user modes do not decrease interference but support more total throughput, which is a capacity improvement.  
         [0099]     Referring to Table 1, modes A 1 , A 2 , and A 3  from Table 1 have been previously discussed in the art in the context of a pure diversity system, that has no directional transmission, for CDMA systems cf Derryberry, R.T. et al, “Transmit Diversity in 3G CDMA Systems”,  IEEE Communications , April 2002, pp 68-75). Mode A 4  is related to the Per Antenna Rate Control scheme, which has also been described in the art in the context of pure diversity.  
         [0100]     The UE provides channel feedback such as channel quality indication (CQI) to the base station, which may be used by the base station in selecting a suitable transmission mode. In many wireless networks, CQI may also enable the base station to execute link control features such as modulation, coding, or power control, and scheduling to multiple users. In the present invention, the UE reports back CQI measurement for both diversity branches using the preferred beam(s) at the serving base station.  
         [0101]     The intrinsic low fading correlation resulting from diversity transmissions allows the base station to simply estimate link capacity using one or both transmission streams.  
         [0102]     The CQI estimation at the UE is based on pilot measurements, which the UE pre-processes with the known preceding transformation. This leads to a potential issue when pilot transmissions from other base stations in a network are measured, to estimate interference since the other station&#39;s pilots are not transformed through any preceding. To avoid undue pilot processing at the UE, the effects of beamforming would only be seen on the wanted signal and not interference from other base stations. The CQI derivation may therefore include a small compensation factor to reduce the estimate of other cell interference to account for the beam gain. This compensation factor can be derived from computer simulation.  
                         TABLE 1                           Modes supporting single-user per channelization       resource            Transmission Mode Option   Implications for UE Receiver               A1. Single beam &amp; user w/   Single antenna UE is possible.       Selection Diversity. User   Multi-antenna UE could improve       gets polarity with best   performance. UE feeds back       channel   beam index + CQI for each           polarity       A2. Single beam &amp; user w/   Single antenna UE is possible.       STTD - user stream decomposed   Multi-antenna UE could improve       into two substreams which are   performance. UE feeds back beam       sent on the two polarities   index and joint CQI covering       with, e.g., Alamouti coding   both div. branches       A3a. Single beam &amp; user   Single antenna UE is possible.       w/ closed loop Tx Diversity   Multi-antenna UE could improve       (TxAA). Both diversity   performance. UE feeds back       branches transmit the same   beam index + complex channel       user stream, with relative   measurements to derive optimal       scaling and phase shift   weights and optionally channel       determined from UE&#39;s DL   quality information depending on       channel estimate.   the exact nature of the receiver           design. Additional CQI           information may be fed back if           required to support other           network features such as link           control and scheduling.       A3b. Single beam &amp; user w/   Single antenna UE is possible.       transmit polariation   Multi-antenna UE could improve       selection. All transmit   performance. UE feeds back beam       power is switched onto the   index + one bit to select the       polarization with highest   polarization. Additional CQI       received power as determined   information may be fed back if       from UE&#39;s DL channel   required to support other       estimate.   network features such as link           control and scheduling.       A4. Single beam &amp; user w/   UE needs minimum of two antennas       MIMO. User stream is   to separate two substreams.       decomposed into two   Additional UE antennas could       substreams which are   improve performance 1 . UE feeds       independently coded and sent   back beam index + CQI for each       on the two polarities   polarity on that beam                  
 
         [0103]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 Modes supporting two or more users per channelization resource 
               
             
          
           
               
                 Transmission Mode Option 
                 Implications for UE Receiver 
               
               
                   
               
               
                 B1a) Single beam, 2 users. 
                 UE needs two antennas to 
               
               
                 Each user generates a single 
                 separate two user streams. UE 
               
               
                 stream that is independ-ently 
                 feeds back beam index + CQI for 
               
               
                 coded and sent on one polarity. 
                 each polarity on that beam 
               
               
                 B1b) Two beams, 2 users. Each 
                 Single antenna UE is possible 
               
               
                 user generates a single stream 
                 for case of spatially distinct 
               
               
                 that is independently coded and 
                 beams. Multi-antenna UE to 
               
               
                 sent on one polarity. 
                 improve performance. UE feeds 
               
               
                   
                 back beam index and CQI for 
               
               
                   
                 each polarity. 
               
               
                 B1c). Two beams, 4 users-2 
                 UE needs minimum of two 
               
               
                 users per beam. Each user 
                 antennas for X-pol 
               
               
                 generates a single stream that 
                 cancellation. Multi-antenna UE 
               
               
                 is independently coded and sent 
                 to improve performance. UE 
               
               
                 on one polarity. 
                 feeds back beam index and CQI 
               
               
                   
                 for each polarity 
               
               
                 B2a) Single beam, 2 users. Two 
                 Single antenna UE is possible. 
               
               
                 user streams are mixed onto 
                 Multi-antenna UE to improve 
               
               
                 both polarities with STTD 
                 performance. UE feeds back 
               
               
                 coding. The streams have a 
                 beam index and single joint CQI 
               
               
                 common block coding and 
                 for both polarities 
               
               
                 modulation scheme. 
               
               
                 B2b) Two beams, 4 users-2 
                 UE needs minimum of two 
               
               
                 users per beam. Streams from 
                 antennas to separate streams 
               
               
                 two users on a given beam are 
                 from 4 users. Multi-antenna UE 
               
               
                 mixed onto both polarities via 
                 to improve performance. UE 
               
               
                 STTD coding. 
                 feeds back beam index and 
               
               
                   
                 single joint CQI for both 
               
               
                   
                 polarities 
               
               
                 B3a. Two beams, two users, 
                 Double antenna UE needed to 
               
               
                 with closed loop Tx diversity 
                 separate user data streams. 
               
               
                 (TxAA). Both polarities 
                 More UE antennas to improve 
               
               
                 transmit the same stream to one 
                 performance. UE feeds back 
               
               
                 user on one beam on both 
                 beam index + complex channel 
               
               
                 diversity branches, with 
                 measurements to derive optimal 
               
               
                 relative scaling and phase 
                 weights and optionally channel 
               
               
                 shift determined by UE&#39;s 
                 quality information depending 
               
               
                 channel DL estimate. Same is 
                 on the exact nature of the 
               
               
                 done for a second user on a 
                 receiver design. Additional 
               
               
                 different beam. 
                 CQI information may be fed back 
               
               
                   
                 if required to support other 
               
               
                   
                 network features such as link 
               
               
                   
                 control and scheduling. 
               
               
                 B3b. Two beams, two users, w/ 
                 Double antenna UE needed to 
               
               
                 transmit polarization 
                 separate user data streams. 
               
               
                 selection. Feedback from each 
                 More UE antennas to improve 
               
               
                 user selects one polarization 
                 performance. Each UE feeds 
               
               
                 for that user; all power for 
                 back beam index + polarization 
               
               
                 that user is concentrated on 
                 selection index. Additional 
               
               
                 that user&#39;s selected 
                 CQI information may be fed back 
               
               
                 polarization. 
                 if required to support other 
               
               
                   
                 network features such as link 
               
               
                   
                 control and scheduling. 
               
               
                 B4. Two beams, 2 users w/MIMO. 
                 UE needs minimum of two 
               
               
                 Each user transmission is 
                 antennas to separate two 
               
               
                 decomposed into two substreams 
                 substreams. More UE antennas to 
               
               
                 which are independently coded 
                 improve performance. UE feeds 
               
               
                 and sent on the two polarities 
                 back beam index + CQI for each 
               
               
                   
                 polarity on that beam 
               
               
                   
               
             
          
         
       
     
         [0104]     It should be noted that in the above tables, performance improvements from additional UE antennas could improve link gain and improve the ability to reject interference.  
         [0105]     The present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combination thereof. Apparatus of the invention can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and  
         [0106]     methods actions can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The invention can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language.  
         [0107]     Suitable processors include, by way of example, both general and specific microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; CD-ROM disks; and buffer circuits such as latches and/or flip flops. Any of the foregoing can be supplemented by, or incorporated in ASICs (application-specific integrated circuits), FPGAs (field-programmable gate arrays) or DSPs (digital signal processors).  
         [0108]     Examples of such types of computers are the spatial grouper  110 , scheduler  120 , AMCs  130 - 135 , pre-coder matrix  150 ,  160  and pilot tone mixers  170 - 173  contained in the pre-coder  100 , suitable for implementing or performing the apparatus or methods of the invention.  
         [0109]     The system may comprise a processor, a random access memory, a hard drive controller, and an input/output controller coupled by a processor bus.  
         [0110]     It will be apparent to those skilled in this art that various modifications and variations may be made to the embodiments disclosed herein, consistent with the present invention, without departing from the spirit and scope of the present invention.  
         [0111]     Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein.  
         [0112]     Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.