Abstract:
An active antenna controller, arranged to provide a control signal for dynamic adjustment of the beam shape of an associated active antenna based upon a quality measurement for a signal communicated through the active antenna.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Spanish Application Number 201230704, filed on May 10, 2012, the entirety of which is incorporated herein by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The invention concerns an active antenna controller, an active antenna comprising such a controller and a method of controlling an active antenna for an active antenna, preferably as part of a cellular network station. 
       BACKGROUND TO THE INVENTION 
       [0003]    Active antennas are increasingly being used, particularly within cellular networks. These antennas may comprise dedicated transmission and reception boards per dipole, allowing a network operator to configure the antenna beam shape. In particular, the tilt angle can be adjusted. Moreover, different tilt angles can be set for uplink and downlink paths. 
         [0004]    The tilt of the beam is conventionally set up by the design engineer in a static way. This process is carried out in an optimization phase, during installation, periodically or on an ad-hoc basis. Once set during optimisation, the tilt angle remains constant. 
         [0005]    One reason for this is as follows. Adjustment of the beam shape may change the coverage area of the cell sector. The optimal coverage area is normally determined during network planning. This takes into account cell capacity, neighbouring cell coverage and capacity, radio propagation effects, terrain and other factors in order to provide a minimum Quality of Service (QoS) across a wide geographical area. Moreover, current network planning is also based on assumptions for a large number of factors, including the distribution of mobile stations within a cell, typical proportions of active users, type of service being requested (voice or data), propagation channel modelling. Thus, keeping the beam shape and especially tilt angle constant has been considered desirable, to avoid any negative impact on minimum QoS. 
         [0006]    Improving the efficient use of active antennas without disrupting QoS levels is a significant challenge in radio system design. 
       SUMMARY OF THE INVENTION 
       [0007]    Against this background, there is provided an active antenna controller, arranged to provide a control signal for dynamic adjustment of the beam shape of an associated active antenna based upon a quality measurement for a signal communicated through the active antenna. 
         [0008]    The active antenna controller therefore dynamically adjusts the beam shape, preferably the tilt angle of the active antenna, based on feedback from a signal transmitted by or received at the same active antenna. This dynamic (optionally, closed-loop) approach contrasts with the static control taken by existing systems, in which the beam shape (tilt angle) is set up during an initial configuration phase and then remains constant. The dynamic adjustment may take place with a predetermined frequency, is advantageously automatic and may be effected using a processor configured accordingly. 
         [0009]    The quality measurement may capture the variation in distribution of mobile stations within a cell, proportions of active users, type of service being requested (voice or data), channel propagation and therefore improves efficiency. By optimally selecting the beam shape, the noise in the system can thereby be reduced and the signal strength (and signal-to-noise ratio or equivalently, signal-to-noise-plus-interference ratio) for signals received by the active antenna may be increased. This may increase the capacity of the system using the active antenna, especially if it is a cellular network base station and consequently, the experience provided to users may also be improved. 
         [0010]    An active antenna typically comprises a plurality of dipoles, each dipole being coupled to a respective RF module. The control signal may therefore be provided to the plurality of RF modules to effect the desired beam shape. The control signal is preferably a signal provided to one or more RF modules in order to generate the selected beam shape. More preferably, the control signal is configured to select a phase vector, thereby setting the relevant phase offset on each RF module. This may determines the resulting beam shape, such as tilt angle. 
         [0011]    Preferably, the quality measurement is for a signal (which may comprise one or more signals in practice) received at the active antenna. Additionally or alternatively, the dynamic adjustment relates to signals received at the active antenna. 
         [0012]    Preferably, the quality measurement may be based on a signal communicated between the active antenna and a plurality of differently located mobile stations. More preferably, the quality measurement is based on a signal received at the active antenna from a plurality of differently located mobile stations (termed the uplink). Although the term mobile stations is used, this is used within its normal context within cellular networks, such mobile stations being equivalently referred to as User Equipment (UE). Thus, some stations may not necessarily be moving or even movable. Advantageously, the differently located mobile stations may each comprise a receiver distinct from any receivers coupled to the active antenna. The differently located mobile stations are beneficially located over a predetermined geographical area. This allows the quality measurement to indicate the effectiveness of the active antenna beam shape. The quality measurement preferably comprises one or more of: at least one signal strength of the communicated signal; at least one signal to noise ratio for the communicated signal; and an error rate of data contained in the communicated signal. 
         [0013]    Optionally, the active antenna controller further comprises an input for receiving information relating to the quality measurement for one or more signals communicated through the active antenna. This information relating to the quality measurement may be for one or more signals received at the active antenna. In the latter case, the information may be provided to the active antenna controller by another station that has received the signal transmitted by the active antenna. 
         [0014]    In the preferred embodiment, the quality measurement is for a signal communicated between the active antenna and a plurality of differently located mobile stations. The quality measurement may then be based on a signal-to-noise ratio for a respective portion of the communicated signal associated with each of the plurality of differently located mobile stations. A signal-to-noise ratio (SNR) may be a signal-to-noise-plus-interference ratio (SINR), where there is interference present. 
         [0015]    In some embodiments, a processor may be adapted to determine the quality measurement based on one or more signals received at the active antenna. 
         [0016]    Preferably, the active antenna controller also comprises a processor, configured to generate the control signal for dynamic adjustment of the beam shape of the associated active antenna based upon the quality measurement. More preferably, the active antenna controller further comprises an output for providing the control signal to the associated active antenna. There may optionally be a plurality of control signals, each of the plurality of control signals relating to a respective RF module for the plurality of RF modules comprised in the active antenna. 
         [0017]    The active antenna controller may control the beam shape of the active antenna essentially continuously. In other words, each time a new quality measurement for a signal communicated through the active antenna is obtained, the beam shape may be adjusted. 
         [0018]    However, the preferred embodiment operates slightly differently. Here, the active antenna controller is configured to provide the control signal during a first time period so as to adjust the beam shape of the associated active antenna to each of a plurality of configuration beam shapes, to establish a respective quality measurement for a signal communicated through the active antenna when each of the configuration beam shapes is set. In other words, the first time period is a configuration period, used identify a desirable or optimum beam shape. Then, the active antenna controller may be configured to determine a desired beam shape based on the plurality of established quality measurements. This allows the active antenna controller to use the plurality of established quality measurements (each of which is related to a respective beam shape) to find the beam shape which optimises or maximises the quality measurement. In these embodiments, the active antenna controller may be further configured to provide the control signal during a second time period so as to adjust the beam shape of the associated active antenna to only the desired beam shape. In this second time period (subsequent and distinct from the first time period), the beam shape of the active antenna may be fixed by the active antenna controller and no further quality measurements are taken in this second time period. Most preferably, the first and second time periods are repeated with a predetermined frequency, a new first time period following the completion of the second time period. It will be appreciated that the second time period is advantageously significantly longer than the first time period. For example, the second time period may be 5, 10, 15, 20, 50 or 100 times longer than the second time period. 
         [0019]    Optionally, the plurality of configuration beam shapes are predetermined. In such cases, the desired beam shape may be determined by selecting the one of the plurality of configuration beam shapes having the optimum associated quality measurement. Alternatively, the plurality of configuration beam shapes may be determined iteratively. This may be implemented by the active antenna controller being configured repeatedly to: set a configuration beam shape; establish the associated quality measurement for the set configuration beam shape; and determine a subsequent configuration beam shape on the basis of the established associated quality measurement. The desired beam shape may then be determined on the basis of the difference between the subsequent configuration beam shape and the previous configuration beam shape or based on the number of repetitions or another factor that the skilled person would understand to be used in iterative determinations. 
         [0020]    In embodiments, the active antenna controller may be further configured to provide the control signal based upon whether the active antenna is being used for transmission or reception. In other words, a different control signal may be provided for transmission and reception. This can be beneficial in stations (base stations or mobile stations) of cellular networks, so that the beam shape can be different between uplink and downlink. In some embodiments, the beam shape for either transmission or reception may be dynamically adjusted based upon the quality measurement, whereas the beam shape for the other may be fixed or set in a different way. 
         [0021]    As noted above, the associated active antenna may comprise an array of antennas. In such cases, the active antenna controller is preferably further configured to generate the control signal for separate adjustment of each of the antennas in the associated active antenna. The associated active antenna may comprise in particular cases at least one antenna formed of a plurality of dipoles. The plurality of dipoles may be arranged in cross-polar form. The associated active antenna can optionally comprise a plurality of antennas, each antenna being formed of a respective plurality of dipoles. Then, the active antenna controller may be further configured to generate the control signal for separate adjustment of each of the plurality of dipoles in the associated active antenna. This may allow improved reception diversity. Each of the plurality of dipoles may have a different polarisation. Typically, each of the plurality of dipoles is assembled as a pair of dipoles, with orthogonal polarization. This may allow transmit and receive diversity to be effected. Thus, the tilt angle of each polarisation may be adjusted independently. 
         [0022]    In the preferred embodiment, the active antenna controller is adapted for control of an associated active antenna for use in a station (especially a base station) of a cellular radio network, preferably for control of the active antenna in a reception mode. The present invention may also be provided as a component (or part) of a station for a cellular radio network comprising the active antenna controller as described herein. Preferably, the station is a base station of a cellular radio network, such as a Node B of a UTRAN cellular network or eNodeB of an E-UTRAN cellular network. 
         [0023]    The active antenna controller may be part of the active antenna (for example, within a housing of the active antenna) or separate from it. In the former case, there may be provided an active antenna comprising the active antenna controller as described herein. In the latter case, there may be provided a component of a cellular network station comprising the active antenna controller as described herein. For example, the component may be a base band unit of a base station. 
         [0024]    In a further aspect, the present invention may provide a method for controlling an active antenna comprising dynamically adjusting the beam shape of the active antenna based upon a quality measurement for a signal communicated through the active antenna. In another aspect, the present invention may be found in a computer program, configured to carrying out a method described herein when operated on a processor. 
         [0025]    It will be understood that this method can optionally comprise steps or features used to carry out any of the actions described in connection with the active antenna controller detailed above. Also, any combination of the individual apparatus features or method features described may be implemented, even though not explicitly disclosed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention may be put into practice in various ways, a number of which will now be described by way of example only and with reference to the accompanying drawings in which: 
           [0027]      FIG. 1  shows a schematic diagram of an existing active antenna system implemented for reception; 
           [0028]      FIG. 2  shows a schematic diagram of an active antenna system according to a first embodiment of the present invention, implemented for reception; 
           [0029]      FIG. 3  shows a schematic diagram of an active antenna system according to a second embodiment of the present invention, implemented for reception; 
           [0030]      FIG. 4A  depicts a graph of phase vector against time in line with a mode of operation according to the present invention; and 
           [0031]      FIG. 4B  illustrates an expanded portion of the graph of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    Referring first to  FIG. 1 , there is shown a schematic diagram of an existing active antenna system  10 , implemented for reception. Within the active antenna system  10 , there is shown a first antenna  30  and a second antenna  50 , although there may be more than these two antennas in a practical implementation of this system. An active antenna controller  60  is also provided. 
         [0033]    Each of the first antenna  30  and second antenna  50  comprise a cross-polar array of dipoles. This provides two-way receiver diversity, as each dipole can be coupled to a separate receiver. 
         [0034]    Coupled to a first dipole of the first antenna  30  is a first receiver  22 . The output of the first receiver  22  is provided to a first multiplier  20 . The other input to the first multiplier  20  is a first phase offset signal  21 . The form of the first phase offset signal  21  is e j.Φ1 , causing the output of the first receiver  22  to be adjusted by a phase angle of φ 1 . The output of the first multiplier  20  is a first output signal  23 . The first receiver  22  and first multiplier  20  together form a first RF module. 
         [0035]    The second dipole of the first antenna  30  is coupled to a second receiver  27 . The second receiver  27  can be used for diversity reception. The output of the second receiver  27  is provided to a second multiplier  25 . The other input to the second multiplier  25  is a second phase offset signal  26 , of the form e j.φ1 . The output of the second multiplier  25  is a second output signal  28 . The second receiver  27  and second multiplier  25  together form a second RF module. 
         [0036]    An equivalent configuration exists for the second antenna  50 . A first dipole of a second antenna  50  provides a signal to a third receiver  42  and the output of the receiver is provided to a third multiplier  40 . The other input to the third multiplier  40  is a third phase offset signal of the form e j.φ     n   . The output of the third multiplier  40  is a third output signal  43 . The second dipole of the second antenna  50  is coupled to a fourth receiver  47 , the output of which is provided to a fourth multiplier  45 . The other input to the fourth multiplier  45  is a fourth phase offset signal  46 , of the form e j.φn . The output of the fourth multiplier  45  is a fourth output signal  48 . The third receiver  42  and third multiplier  40  together form a third RF module and the fourth receiver  47  and fourth multiplier  45  together form a fourth RF module. 
         [0037]    It will be noted that the first phase offset signal  21  and the second phase offset signal  26  are the same and the third phase offset signal  41  and the fourth phase offset signal  46  are also the same. Thus, the antenna patterns of both reception paths of the respective dipoles are the same for each antenna. 
         [0038]    As explained above, only two antennas are shown in this schematic diagram, but it will be appreciated that more than two antennas may be provided. This will be understood by the dotted lines separating the components coupled to the first antenna  30  from the components coupled to the second antenna  50 . 
         [0039]    The active antenna controller  60  is configured such that the tilt of the beam, in uplink and downlink, is set by the design engineer in a static way. This occurs at a single time, for example during installation or during periodical optimisation or on an ad hoc basis. Then, the tilt angle remains constant independently of the network load, the user distribution and any other factor. Moreover, the tilt angle configured in the downlink path is the same as that in the uplink path. 
         [0040]    It has been recognised that this static approach is inefficient. Factors such as user distribution, type of service and network load vary with the time. Ignoring these variations reduces efficiency. Rather, it is possible to take advantage of the active antenna architecture to adjust the antenna tilt dynamically in order to maximise the quality of the signal communicated through the antenna. This is especially the case where the active antenna is used in the uplink of a base station for a cellular network and the quality of the received signal can be maximised. 
         [0041]    In particular, the target tilt angle can be one that maximises the signal-to-noise ratio in the reception chain of the base station, but without degradation of performance. This may be achieved by ensuring that a minimum signal-to-noise ratio can be set per user, such that all active users within the cell of the base station are guaranteed a minimum service level. Approaches to implement this dynamic adjustment of beam shape are now described. 
         [0042]    Referring next to  FIG. 2 , there is shown a schematic diagram of an active antenna system according to a first embodiment of the present invention, implemented for reception. Most of the components of this active antenna system  11  are identical to the active antenna system  10  shown in  FIG. 1 . In such cases, the same reference numerals have been used. The main difference lies in the replacement of active antenna controller  60  by a new active antenna controller  70 . In this approach, the first output signal  23 , second output signal  28 , third output signal  43  and fourth output signal  48  are provided to the active antenna controller  70 , which dynamically adjusts the phase offset signals (specifically, the first phase offset signal  21 , the second phase offset signal  26 , the third phase offset signal  41  and the fourth phase offset signal  46 ) applied to the multipliers. In this approach, the phase angle (φ 1 ) applied to the dipoles of the first antenna  30  and the phase angle (φ 2 ) applied to the dipoles of the second antenna  50  can be adjusted in accordance with the output signals received at the active antenna controller  70 . A beamforming algorithm is used for this purpose. 
         [0043]    Referring next to  FIG. 3 , there is shown a schematic diagram illustrating an active antenna system in accordance with the second embodiment of the present invention. Again, this system is similar to the systems shown in  FIGS. 1 and 2  and identical components are indicated by the same reference numerals. The active antenna controller  80  is different from the active antenna controllers shown in the other diagrams, though. In this embodiment, the first phase offset signal  21  is different is different from the second phase offset signal  26 . Similarly, the third phase offset signal  41  is different from the fourth phase offset signal  46 . This allows a variable tilt angle to be applied independently to each diversity branch. 
         [0044]    Referring next to  FIG. 4A , there is depicted a graph of phase vector against time in line with a mode of operation according to the present invention. Due to the multiplicity of tilt options, the algorithm for selection and application of the phase offset is designed to maximise performance and minimise overhead. Thus, tilt sweeping is used to discover the most appropriate tilt to apply. 
         [0045]    In a tilt scanning period  110 , the phase offset or equivalently tilt angle is adjusted over a predetermined tilt range  150 . The algorithm measures a received signal-to-noise ratio (or multiple signal-to-noise ratios in the uplink with multiple transmissions from different mobile stations) for each tilt angle value. Preferably, the signal-to-noise-plus-interference ratio (SINR) is used rather than SNR, although either may be employed in practice. At time  115 , the phase sweeping is completed and this is the end of the tilt scanning period  110 . Then, the tilt angle value providing the optimal performance is selected. The algorithm selects the tilt angle providing the maximum SINR, whilst ensuring that a minimum quality of service level for each user within the cell is guaranteed, by checking that the respective SINR is at least at a minimum level or threshold. 
         [0046]    This is desirable in the uplink, because the SINR per user can be monitored for each tilt angle. The tilt selected is that which maximises the average (mean, median or mode) SINR over all users and simultaneously ensure that all users are above the minimum quality threshold. The minimum threshold can depend on the type of service (for example, voice or data), so each user may have a different minimum quality threshold. 
         [0047]    A tilt application period then begins. At an initial part  120  of the tilt application period, the tilt angle determined during the tilt scanning period is converged towards and this conversion is completed at time  125 . During time period  130 , the tilt angle remains constant. It will be seen that the tilt application period  130  is much longer in duration than the tilt scanning period  110 . 
         [0048]    At time  140 , a new tilt scanning period begins. This may be initiated at a predetermined time following time  115  or time  125 . Alternatively, it can be initiated by an event occurring, such as a new user becoming active in the cell. 
         [0049]    Referring next to  FIG. 4B , there is illustrated an expanded portion of the graph of  FIG. 4A . The expanded portion  160  shows the tilt scan step  170  and a time unit  173  for tilt offset. Starting from a minimum allowed tilt  171 , the tilt angle is increased stepwise over time by increments of tilt scan step  170  every time unit  173 . This continues until a maximum allowed tilt  172  is reached. 
         [0050]    As the numbers of different phase combinations are limited to the electrical tilt range supported by the active antenna, an exhaustive method can be used. This may be implemented whilst still maintaining a reasonable tilt scanning period  110 . Typically, the electrical tilt range of base station antenna, as maximum, has 15 different values. In the example where 15 possible phase vectors are selected, this will lead to a phase sweeping period of 150 ms, assuming a time unit  173  of 10 ms per tilt value. In addition, the tilt range suitable for scan be set by an operator with a minimum and maximum tilt value. 
         [0051]    This approach is applicable to the implementations described with respect to both  FIGS. 2 and 3 . However, it will be recognised that for the embodiment shown in  FIG. 3 , where the tilt angle may be selected independently for each dipole, the number of phase vectors selected in the tilt scanning period  110  will need to be greater than in the embodiment shown in  FIG. 2 . This creates a further challenge, which can be resolved in a number of different ways. The following are some possible approaches. 
         [0052]    1. A tilt sweeping can firstly be carried out for all of the reception paths simultaneously, followed by a single tilt sweeping in the additional reception branches. 
         [0053]    2. An exhaustive tilt sweeping can be performed for all possible tilt combinations between the diversity branches in an initial tilt scanning period. However, in subsequent tilt scanning periods, the tilt sweeping may be limited to a range of 2 or 3 tilt values around the last optimum tilt combination. 
         [0054]    3. Either option 1 or 2 above may firstly be used. This can then be followed by tilt sweeping per group of cross polar dipoles. 
         [0055]    Although embodiments of the invention have been described above, the skilled person will recognise that various modifications or adjustments can be made. For example, although the tilt angle has been specified for adjustment in the embodiments described above, it will be understood that the beam shape can be adjusted differently and in different ways. Also, the specific configuration of active antenna, having multiple antennas each with two cross polar dipoles may be varied. The number of antennas and the number of dipoles per antenna may be different. The number of dipoles per antenna may vary between different antennas. 
         [0056]    The tilt angles applied in the downlink may be different from those used in the uplink or the same. It is believed that having a fixed tilt angle in the downlink (that is, for use in transmission from a base station) is preferable. 
         [0057]    Although signal-to-noise ratio is used as a measurement of quality, it will be understood that a signal-to-noise-plus-interference ratio can equivalently be used. It will also be appreciated that other measurements may be used, such as signal strengths or error rates. Also, optimising the quality measurement can be performed in a variety of different ways. For example, an iterative approach may be taken, in which the tilt angle (or other shape) is adjusted according to a quality measurement determined in respect of one or more previous tilt angles. This can be done during an initialisation phase, as described above, or on a continuous basis. This latter approach is more difficult and less preferred.