Patent Publication Number: US-2010120441-A1

Title: Increasing a sectorization order in a first sector of an antenna array

Description:
TECHNICAL FIELD 
     The present invention relates to a network node in a wireless communication system and a method therein. More particularly the present invention relates to increasing a sectorization order in a first sector of an antenna array. 
     BACKGROUND 
     The Third Generation (3G) of wireless telecommunication standards and technologies is based on the International Telecommunication Union (ITU) family of standards under the International Mobile Telecommunications program, “IMT-2000”. The 3G technologies enable network operators to offer the users a wider range of more advanced services while achieving greater network capacity through improved spectral efficiency. The services include wide-area wireless voice telephony, high-speed Internet access, video telephony and broadband wireless data, all in a mobile environment. Typically, they provide service at 5-10 Mb per second. 
     The IMT-2000 comprises several radio interfaces such as e.g. Wideband Code Division Multiple Access (W-CDMA), Code Division Multiple Access (CDMA2000), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) and Digital Enhanced Cordless Telecommunications (DECT). 
     TD-SCDMA is also called the low-chip-rate (LCR) Universal Terrestrial Radio Access (UTRA) Time Division Duplex (TDD). It has been adopted by 3rd Generation Partnership Project (3GPP) as part of Universal Mobile Telecommunications System (UMTS) release 4, becoming in this way a global standard, which covers all radio deployment scenarios, from rural to dense urban areas, from pico to micro and macro cells, from pedestrian to high mobility. TD-SCDMA uses Time Division Duplexing (TDD). By dynamically adjusting the number of timeslots used for downlink and uplink, the system can more easily accommodate asymmetric traffic with different data rate requirements on downlink and uplink than Frequency Division Duplex (FDD) schemes. Since TDD does not require paired spectrum for downlink and uplink, spectrum allocation flexibility is also increased. Also, using the same carrier frequency for uplink and downlink means that the channel condition is the same on both directions, and the base station can deduce the downlink channel information from uplink channel estimates. The “S” in TD-SCDMA stands for “synchronous”, which means that uplink signals are synchronized at the base station receiver, achieved by continuous timing adjustments. This reduces the interference between users of the same timeslot using different codes by improving the orthogonality between the codes, therefore increasing system capacity. 
     TD-SCDMA may be used for supporting huge sport events, such as e.g. the Olympic Games. The network deployments adopt smart antenna technology with typically eight antenna elements in each sector and most of them are configured with three sectors per site. As a key technology in TD-SCDMA to reach high coverage of the data transmission, beam forming characterized by single-stream transmission with user-specific beam adaptation based on the closely spaced antenna array is used. 
     The next step in wireless communications is the Fourth-Generation Communications System (4G) which is also known as Beyond the Third Generation (B3G). A 4G system will be able to provide a comprehensive Internet Protocol (IP) solution where voice, data and streamed multimedia can be given to users on an “Anytime, Anywhere” basis, and at higher data rates than previous generations. There are certain objectives that are projected for 4G, such as that 4G will be a fully IP-based integrated system. This will be achieved after wired and wireless technologies converge and will be capable of providing between 100 Mbit/s and 1 Gbit/s speeds both indoors and outdoors, with premium quality and high security. 4G will offer all types of services at an affordable cost. 
     Moving towards 4G, quite many different multi-antenna technologies are discussed and evaluated as pre-4G wireless systems, such as e.g. 3GPP Long-Term Evolution of UMTS (LTE) and Wireless INitiative NEw Radio (WINNER). In contributions to the 3rd Generation Partnership Project (3GPP) multi-antenna schemes with more adaptivity at the transmitter may result in more serious variation of the inter-cell interference at high load scenarios, i.e. suffers more from the feedback delay, due to the multi-user scheduler and user-specific transmission weights or power. In addition, the beam forming and/or precoding schemes suffers a lot from the non-stationary interference, comparing to some more robust Multiple-Input Multiple-Output (MIMO) schemes, such as e.g. Per Antenna Rate Control (PARC). Accordingly, the same problem exists in TD-SCDMA system and any other systems with beam forming. 
     Further, as of today, LTE defines up to four reference signals using four antenna elements per sector cell. The extension to even more antennas per sector may be troublesome since it could require even more reference signals per sector which would require more overhead. This will most probably complicate mobility management and quality measurements for adaptation. Thus the support of more than four antennas per sector may pose certain problems if antenna common pilots are used. 
     The transition from the 3G antenna systems with more than four antennas, such as e.g. the 3G TD-SCDMA, to the four antenna systems of 4G, such as e.g. LTE, may be challenging due to the fact that it may also require that the physical antennas are exchanged in order for the four antenna system of 4G to operate properly. This would involve high costs due to labor costs and hardware purchases. 
     SUMMARY 
     It is therefore an object of the present invention to provide a method and arrangement enabling a smooth and less costly transition from a 3G system such as TD-SCDMA, to a 4G system such as LTE. 
     According to a first aspect of the present invention, the object is achieved by a method in a network node for increasing a sectorization order in a first sector of an antenna array. The antenna array is deployed with a first sectorization. The network node and antenna array are comprised in a wireless communication system. The network node comprises a first number of signal connections. Each signal connection is adapted to transmit and receive signals. The method comprises the step of splitting the first number of connections into a number of groups wherein each one of the respective connection groups corresponds to one respective virtual sector resulting in a number of virtual sectors. The method comprises the further step of mapping the number of virtual sectors to the first sector such that the first sector is split into the number of virtual sectors. 
     According to a second aspect of the present invention, the object is achieved by a network node. The network node and antenna array are comprised in a wireless communication system. The antenna array is deployed with a first sectorization. The network node is adapted to be connected to the antenna array. The network node comprises a first number of signal connections. Each signal connection is adapted to transmit and/or receive signals. The network node further comprises a splitting unit adapted to split the first number of connections into a number of groups. Each one of the respective connection groups corresponds to one respective virtual sector. This results in a number of virtual sectors. The network node also comprises a mapping unit adapted to map the number of virtual sectors to the first sector. The mapping is performed such that the first sector is split into the number of virtual sectors. 
     Since the existing antenna elements in the wireless communication system are re-used when splitting the existing physical sector with more than four transmit antenna elements, to two or more smaller virtual sectors, each virtual sector having fewer transmit antenna elements, the costly and cumbersome exchange of antenna elements due to the wireless communication migration from 3G to 4G may be avoided. 
     An advantage of the present invention is that the system capacity and the coverage are remarkably improved as a result of the separation of the sectors which increases the resource upper bound by almost the double. 
     A further advantage of the present invention is that the system robustness is improved against the feedback delay and the non-stationary interference by generating less varying interference. 
     A yet further advantage of the present invention is that re-use of installed antenna elements at a base station is easy to upgrade. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described more in detail in relation to the enclosed drawings, in which: 
         FIG. 1  is a block diagram illustrating embodiments of a wireless communication network. 
         FIG. 2  is a block diagram illustrating embodiments of an antenna array. 
         FIG. 3  is a block diagram illustrating embodiments of a network node. 
         FIG. 4   a  is a block diagram illustrating embodiments of a wireless communication network as seen from above. 
         FIG. 4   b  is a block diagram illustrating embodiments of a wireless communication network as seen from above. 
         FIG. 5   a  is a block diagram illustrating embodiments of a wireless communication network as seen from above. 
         FIG. 5   b  is a block diagram illustrating embodiments of a wireless communication network as seen from above. 
         FIG. 6  is a flow chart illustrating method steps according to embodiments of the present solution. 
         FIG. 7  is a flow chart illustrating method steps according to embodiments of the present solution. 
         FIG. 8  is a block diagram illustration embodiments of a network node. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is defined as a network node in a wireless communication system and a method therein, which may be put into practice in the embodiments described below. 
     This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It should be understood that there is no intent to limit the present method or arrangement to any of the particular forms disclosed, but on the contrary, the present method and arrangement is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the claims. 
     When using the word “comprise” or “comprising” it shall be interpreted as non-limiting, in the meaning of “consist at least of”. The solution is defined as methods and arrangements which may be put into practice in the embodiments below. 
       FIG. 1  discloses a wireless communication system  100  according to the present solution. The wireless communication system  100  is adapted to provide wireless communication over a radio link  105  to a wireless communication device  110  such as e.g. a cellular telephone, a Personal Digital Assistant (PDA), a wireless computer etc. The wireless communication system  100  may e.g. be a cellular communication system. 
     The wireless communication system  100  may further be connected to an infrastructure network  115  such as e.g. the Public Switched Telephone Network (PSTN), an IP network, a wireless core network or a radio access network. The present solution may be applicable to any wireless communication system with more than four transmit antenna elements at the base station, such as e.g. GSM, TD-SCDMA, WCDMA, CDMA-2000, LTE, IEEE802.16, IEEE802.11n and standards alike. 
     The wireless communication system  100  comprises an antenna array  120  and a network node  130  as depicted in  FIG. 1   
     The antenna array  120 , also known as the existing antenna array, may e.g. be arranged at an antenna site in an antenna tower as seen in the example of  FIG. 1  or on a high building. The antenna array  120  may be internal or external to a base station  140 , such as e.g. a Node-B, an enhanced Node-B, a Radio Base Station (RBS), a Base Transceiver Station (BTS), a Radio Remote Unit (RRU), a Main Remote Unit (MRU) or any other suitable node adapted to communicate over a radio link.  FIG. 2  depicts the antenna array  120  according to some embodiments of the present solution. The antenna array  120  is deployed with a first sectorization. Examples of sectorization orders include e.g. one, two, three or six. 
     The antenna array  120  may comprise a number n of antenna elements. The number n of antenna elements must be two or larger. According to some embodiments the antenna array  120  comprises at least four antenna elements. As depicted in  FIG. 2  the antenna array  120  may comprise eight antenna elements  201 - 208 . 
     The antenna elements  201 - 208  may be adapted to transmit and receive signals. At transmission, the signals transmitted from the different antenna elements can be formed by encoding and scaling the underlying signals with signal and antenna specific complex weights. The weights can be selected according to the multi antenna mode used, such as e.g. beamforming, precoding, spatial multiplexing, spatial diversity, etc. Such schemes are suitable for varying degrees of channel knowledge and channel conditions, the signals are transmitted in the best directions in multiple directions and in all directions respectively. At reception, different versions of the same transmitted signal and/or signals are received by the antenna elements and these can be combined taking the channel properties into account. This is also known by receive diversity and is commonly used to improve the received signal quality since by using more antennas, more energy is collected and the reliability can be improved since it can be less probable that the signal received by all antennas are of low quality at the same time. 
     The antenna elements  201 - 208  of the antenna array  120  may be configured to have a circular or linear or any two dimensional or three dimensional appearances and/or shape adapted for wireless communication such as e.g. a beam-forming Time Division Duplexing (TDD) system, a GSM. 
     The antenna elements  201 - 208  are arranged for a geographical coverage of a first sector, such as e.g. an area or a physical sector. According to some embodiments the antenna array  120  serves a cell  150 , as depicted in  FIG. 1 . The cell  150  depicted in  FIG. 1  is omni directional. 
       FIG. 3  depicts the network node  130 , also known as a beamforming network, according to some embodiments. The network node  130  is adapted to be connected to the antenna array  120 . The network node  130  comprises a first number n of signal connections, also known as outputs, where each of the first signal connections is adapted to transmit and receive signals e.g. to the antenna array  120 . As depicted in  FIG. 3  there may be eight signal connections  301 - 308 . The number n of signal connections may be connected to the number n of antenna elements of the antenna array  120 . As depicted in  FIG. 3 , the antenna element  201  may be connected to the signal connection  301 , the antenna element  202  may be connected to the signal connection  302 , etc. The network node  130  may thus be adapted to transmit and receive signals to and from each antenna element of the antenna array  120  via the first number of signal connections. 
     The network node  130  further comprises a second number n of signal connections, also known as inputs. Each of the second signal connections is adapted to transmit and/or receive signals for one of the respective number n of the antenna elements. The second number n of signal connections may be adapted to be connected to the base station  140 . The second number n of signal connections may be adapted to be connected to the antenna ports of two virtual sectors served by the base station  140 . The second number of signal connections may be eight. The exemplary scenario of eight second number of signal connections  311 - 318  is shown in  FIG. 3 .  FIG. 3  further depicts the example of how a signal coming from the base station  140  intended for the first antenna element  201  of the antenna array will be received from the base station  140  by the connection  311  to be signal processed by the network node  130 , as will be further described below, thereafter transmitted via the connection  310  to the antenna element  201 . The same principle is valid for the signals coming from the base station  140  intended for the antenna elements  202 - 208  respectively of the antenna array  120  which will be received from the base station  140  by the connections  312 - 318  respectively, as will be further described below. 
     The network node  130  may be represented by a base band signal processing unit. 
     The network node  130  may be co-located with the antenna array  120 , or located between the antenna array  120  and the base station  140 , or the network node  130  may be co-located with the base station  140 . According to some embodiments the network node  130  performs pre-antenna processing. The network node  130  may be realized as a hardware element in the shape of e.g. a connection net which may be located after a power amplifier when the network node  130  is co-located with the base station  140 . The network node  130  may be implemented digitally as part of the baseband processing internal to the base station  140 . 
     The present solution provides a method in the network node  130  for increasing a sectorization order in a first sector, known as an original sector, of the antenna array  120 , wherein the network node  130  and the antenna array  120  are comprised in the wireless communication system  120 , as mentioned above. According to some embodiments a method in the network node  130  for sector splitting in a beam-forming Time Division Duplexing (TDD) network with more than four transmit antennas at base station is provided. 
     This is highly attractive for a telecommunication operator who wants to migrate e.g. his existing TD-SCDMA system into a LTE by reuse of the existing antenna elements at the antenna sites. Each TD-SCDMA physical sector may be deployed with eight antenna elements  201 - 208  in the antenna array  120 , whereas the present specifications of the LTE system only has support of four antenna connections. This is due to the fact that common reference signals are used and there are only four of them in the LTE system. The LTE system with its common reference signal approach is designed with only four antenna elements in each antenna array in mind. 
     Thus the present solution presents a way to utilize all eight existing antennas of the antenna array  120  by reusing the antenna elements of the 3G TD-SCDMA system when migrating to a 4G system such as e.g. LTE. The present solution comprises creating a two, or more, virtual sectors out of each original physical sector. This will be explained in further detail below and depicted in accompanying Figures. Each virtual sector may then have four, or less, virtual antennas. This sector splitting method results in that there are twice as many, or more, virtual sectors as there are original physical sectors. Each original physical sector may be split in half, or even smaller sections, and covered by two, or more, corresponding virtual sectors. 
     The present solution is highly advantageous since the physical elements, i.e. the antenna elements of the antenna array, may be in continuous use thus avoiding the high costs of investing in new antenna elements and the corresponding costs of labor performing the antenna exchange. 
     As is well known, higher order sectorization improves performance similar to spatial multiplexing since the same time and frequency resource may be used in both sectors serving different users in the two cells at the same time. Ideally, the system capacity could be doubled by splitting the sector in two virtual sectors ad using the same resource in both sectors at the same time. However, in practice there some interference between the sectors will exist, which makes the gain less than a factor of two. 
     Thus the antenna element reuse of the present solution is also advantageous when experiencing a shortage of capacity. In the high load scenarios, sometimes a resource upper bound might be reached before the system is limited by the performance, i.e. the interference, which ends to resource limit within a cell. The separation of the sectors increase the resource upper bound by almost the double, thus improve the capacity significantly. Thus the present solution may be very useful in urban areas experiencing capacity shortage. 
     Additionally, the intercell interference variations may be lower, for example due to the fact that a less adaptive transmitter scheme is used within each virtual sector with a smaller number of antennas. 
     The solution will hereafter be described referring to e.g. the TD-SCDMA system of eight antenna elements in each antenna array and the LTE system of four antenna elements in each antenna array, although it is applicable to other standards as well, as mentioned above. The present solution is also applicable to other antenna arrangements. 
     The present solution will now be described with reference to  FIG. 3 ,  FIGS. 4   a - 4   b ,  FIGS. 5   a - 5   b  and  FIG. 6 . 
       FIGS. 4   a - 4   b  depict embodiments of an single sector wireless communication network  100 , as seen from above, wherein the antenna array  120  is adapted to transmit and receive signals omni-directionally in the first sector  410 .  FIG. 4   a  depicts the communication network  100  before the method according to the present solution is preformed.  FIG. 4   b  depicts the communication network  100  after the method according to the present solution is preformed. The first sector  410  is a physical sector such as e.g. a cell. The created virtual sectors  411  and  412  are delimited by dash dot lines in  FIG. 4   b.    
       FIGS. 5   a - 5   b  depict embodiments of an existing three-sector wireless communication network  100  as seen from above, wherein a first antenna array  120  is adapted to transmit and receive signals in a first sector  510 , a second antenna array  120  is adapted to transmit and receive signals in a second sector  520  and a third antenna array  120  is adapted to transmit and receive signals in a third sector  530 . The first sector  510 , the second sector  520  and the third sector  530  are physical sectors, such as e.g. cells. The physical sectors  510 ,  520 ,  520  are delimited by dashed lines in  FIG. 5   b . The created virtual sectors  511 ,  512  and  521 ,  522  and  531 ,  532  are delimited by dash dot lines in  FIG. 5   b.    
       FIG. 6  presents a flow chart illustrating method steps according to the present solution.  FIG. 6  refers to embodiments concerning transmission.
           601 . The first step of the method is for the network node  130  to perform splitting of the first number n of connections into a number k of groups. Each one of the respective connection groups corresponds to one respective virtual sector, resulting in a number k of virtual sectors.   According to some embodiments the n connections may be split into k groups with n/k connections each. According to other embodiments the n connections may be distributed and/or used in each of the k groups.   According to some embodiments each virtual sector corresponds to a cell in a cellular communication system.   According to some embodiments as depicted in  FIG. 3  and  FIGS. 4   a  and  4   b  n equals eight and k equals two. As depicted in  FIG. 3  the eight connections  301 - 308  are split into two groups  321  and  322 . As seen in  FIG. 4   b  the first group  321  may then correspond to the respective virtual sector  411  and the second group  322  may correspond to the respective virtual sector  412 . Thus the first sector  410 , being depicted in  FIG. 4   a , may be split into two virtual sectors  411 ,  412 , as depicted in  FIG. 4   b , and each said connection group  321 ,  322 , as depicted in  FIG. 3 , may comprise four connections. The first connection group  321  may comprise the four connections  311 - 314  and the second connection group  322  may comprise the four connections  315 - 318 .   The method according to the present solution may also be performed as described above in the three sector wireless communication system being depicted by  FIG. 5   a  to result in the six virtual sector wireless communication system of  FIG. 5   b . Thus the first sector  510  may be split into two virtual sectors  511 ,  512  and the second sector  520  may be split into two virtual sectors  521 ,  522 , and the third sector  530  may be split into two virtual sectors  531 ,  532 , whereby the respective connection groups comprise four connections each, as described above.   According to some embodiments k may equal four. In the embodiment wherein there are eight antenna elements  201 - 208  this results in that the first sector may be split into four virtual sectors and each said connection group may comprise two connections (not shown in any Figure).     602 . Referring again to the flowchart of  FIG. 6  the method may also comprise the step of signal processing the signals corresponding to said first number n of signal connections. The signal processing may enable each said group of signals to be transmitted in one respective virtual sector.   The signal processing may be performed by altering the phases and/or the amplitudes of the signals corresponding to said first number n of signal connections and forming linear combinations of said altered signals.   According to some embodiments depicted in  FIG. 3  and  FIGS. 4   a - 4   b  n equals eight and k equals two. Thus the signals corresponding to the eight connections  301 - 308  may be signal processed. The performed signal processing may enable the first group  321  of signals, as depicted in  FIG. 3 , to be transmitted in the first virtual sector  411 , as depicted in  FIG. 4   b , and the second group  322  of signals, as depicted in  FIG. 3 , is enabled to be transmitted in the second virtual sector  412 , as depicted in  FIG. 4   b.      The signal processing required to enable the splitting and mapping to the first sector when transmitting may be described mathematically as follows. According to some embodiments the antenna array  120  consists of eight antennas  201 - 208  and the first sector  410  is to be split into two virtual sectors  411 ,  412  of four virtual antennas each  311 - 314 ,  315 - 318 . Furthermore, let the vector y represent the transmitted signal from the antenna array  120  covering the first sector  410  and let the vector       
     
       
         
           
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             and is used for altering the phases and/or amplitudes of the signals in x and forming linear combinations of the altered signals. By an appropriate choice of the complex-valued scalar elements {W kl }, the signals for the first (second) virtual sector can be steered to geographically cover the area intended for the first (second) virtual sector while at the same time keeping the interference between the two virtual sectors at a low level. 
             According to some embodiments depicted in  FIG. 3  and  FIGS. 5   a - 5   b  the present solution enables a virtual split from three physical sectors  510 ,  520 ,  530  to six virtual sectors  511 ,  512 ,  521 ,  522 ,  531 ,  532 , wherein the virtual sectors  511 ,  512 ,  521 ,  522 ,  531 ,  532  may correspond to six cells. To properly reuse the existing antenna arrangement of the antenna array  120 , the signal processing requires to take into account that the antenna diagrams are tailored for a three-sector deployment, i.e. corresponding to a coverage of 120 degrees, while conventional six sectors systems should ideally use antennas with antenna diagrams aimed for half as wide sectors, i.e. corresponding to a coverage of 60 degrees. Thus, the signal processing takes this into account by designing beam weights used to form the linear combinations carefully to control the isolation between the resulting sectors. That results in that the interference between any two neighboring virtual sectors is small even though the antenna diagrams of the n antenna elements of the antenna array  120  cover both virtual sectors. 
             Further may each split small virtual sector adopt some less adaptive multi-antenna or even single-antenna technologies with improving both the system performance and the system robustness against the feedback delay and the non-stationary interference. 
               603 . Referring again to the flow chart of  FIG. 6 , the network node  130  performs mapping of the number k of virtual sectors to the first sector  410 , such that the first sector  410  is split into the number k of virtual sectors. The mapping may further comprise mapping of virtual antennas to the physical antennas. In the example of  FIG. 3  the four virtual antennas  311 ,  312 ,  313 ,  314  of the first virtual sector are mapped to the four antenna elements  201 , 201 ,  203 ,  204 . 
             The lobes of the k virtual sectors may be narrower than the lobe of the first physical sector  410 , since the k virtual sectors are adapted to cover a part of the same area as the first physical sector  410 . A narrower lobe is advantageous since it results in an enhanced array gain and it also enables a reduced interference. 
             According to some embodiments as depicted in  FIG. 3  and  FIGS. 4   a - 4   b  n equals eight and k equals two. Thus the two virtual sectors  411 - 412  are mapped to the first sector  410  such that the first sector  410  is split into a first virtual sector  411  and a second virtual sector  412 . 
             According to some embodiments k may equal four. In the embodiment wherein there are eight antenna elements  201 - 208  this results in that four virtual sectors are mapped to the first sector  410  such that the first sector  410  is split into a first virtual sector, a second virtual sector, a third virtual sector and a fourth virtual sector (not shown in any Figure). 
             Each virtual sector of the k virtual sectors may thereby be operated by n/k virtual antenna elements. According to some embodiments wherein n equals eight and k equals two the first virtual sector  411  may be operated by four virtual antenna elements  311 ,  312 ,  313 ,  314  corresponding to the antenna elements  201 - 204  and the second virtual sector  412  may be operated by four virtual antenna elements  315 ,  316 ,  317 ,  318  corresponding to the antenna elements  205 - 208 . 
             According to some embodiments, not shown in any Figure, the first virtual sector  411  may be operated with four virtual antennas  311 ,  312 ,  313 ,  314  which in turn use all the physical antenna elements  201 - 208  with one set of complex beam forming weights. Further the second virtual sector  412  may be operated with four virtual antennas  315 ,  316 ,  317 ,  318  which also use all the physical antenna elements  201 - 208  with a second set of complex beam forming weights. 
           
         
       
    
     The method may also be performed when receiving, i.e. in the reverse direction. 
       FIG. 7  presents a flow chart illustrating method steps according to the present solution.  FIG. 7  refers to embodiments concerning reception.
           701 . The first step of the method, when being performed for reception, is for the network node  130  to perform mapping by mapping the first sector  410  to the number of virtual sectors  411 - 412  such that the first sector  410  is split into the number of virtual sectors  411 - 412 . The mapping may further comprise mapping of physical antennas to the virtual antennas. This step is described in reverse in step  603  above.     702 . Referring again to the flowchart of  FIG. 7  the method may also comprise the step of signal processing the signals from the antenna array  120  corresponding to the first sector  410  to signals of the first number n of signal connections. The signal processing may enable each said group  321 - 322  of signals is adapted to be receiving signals in one respective virtual sector  411 - 412 .   The signal processing may be performed by altering the phases and/or the amplitudes of the signals corresponding to said first number n of signal connections and forming linear combinations of said altered signals. Examples of receiving algorithms are e.g. x_est=f(W)y, e.g. f(W)=ŴT, or f(W)=(ŴT(ŴT W+var(N)I)̂−1) x, x=ŴT y.   This enables the reuse of exactly the same analog radio frequency hardware, such as e.g. attenuators and phase shifters, in both transmitting and receiving directions.     703 . Referring again to the flow chart of  FIG. 7 , the network node  130  performs splitting of the first number n of connections into a number k of groups. Each one of the respective connection groups corresponds to one respective virtual sector, resulting in a number k of virtual sectors. This step is described in further detail in section  601  above.       
     To perform the method as described above, the network node  130  comprises the following arrangements, as depicted in  FIG. 8 . 
     As depicted in  FIG. 8  the network node  130  comprises a first number n of signal connections. Each signal connection is adapted to transmit and/or receive signals. 
     Further the network node  130  may comprise a second number n of signal connections. Each signal connection is adapted to transmit and/or receive signals for one of the respective number n of antenna elements. 
     The network node  130  further comprises a splitting unit  810  adapted to split the first number n of connections into a number k of groups. Each one of the respective connection groups corresponds to one respective virtual sector, resulting in a number k of virtual sectors. 
     The network node  130  may further comprise a signal processing unit  820  adapted to signal process the signals corresponding to said first number n of signal connections, such that each group of signals is adapted to be transmitted in one respective virtual sector. The signal processing unit  820  may further be adapted to signal process the signals from the said antenna array  120  corresponding to the first sector  410 , such that each said group  321 - 322  of signals is adapted to be receiving signals from one respective virtual sector  411 - 412 . 
     The signal processing unit  820  may be adapted to alter the phases and/or the amplitudes of the signals corresponding to said first number n of signal connections and by forming linear combinations of said altered signals. 
     The network node  130  further comprises a mapping unit  830  adapted to map the number k of virtual sectors to the first sector  410 , such that the first sector  410  is split into the number k of virtual sectors. The mapping unit  830  may be further adapted to map the first sector  410  to the number of virtual sectors  411 - 412  such that the first sector  410  is split into the number k of virtual sectors  411 - 412 . 
     The present method for increasing a sectorization order in a first sector  410  of an antenna array  120  may be implemented through one or more arrangements such as the network node  130  as depicted in  FIGS. 3 and 8 , together with computer program code for performing the functions of the invention. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the present method when being loaded into the base station  140  and/or the antenna array  120  of the wireless communication network  100 . One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code can furthermore be provided as pure program code on a server and downloaded to the antenna array  120  and/or the base station  140  remotely. 
     The present invention is not limited to the above-describe preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims. 
     One exemplary embodiment of the method is increasing the sectorization order using existing antenna arrays deployed with a certain sectorization. The increase in sectorization is realized by a beamforming network mapping a number of virtual sectors to a single original sector. The beamforming network comprises a number of outputs connected to the existing antenna array and a number of inputs divided into groups and where each group corresponds to a virtual sector. 
     One exemplary embodiment is a beamforming network comprising a number of connections for receiving or outputting base band signals for a corresponding number of antenna elements, and wherein the antenna elements are arranged for a geographical coverage on a first sector. The beamforming network may further comprise a number of connections for outputting or receiving a first and a second group of signals; wherein the beamforming network is arranged to signal processing the first group of signals to be received or transmitted in the first virtual sector and the second group of signals to be received or transmitted in the second virtual,sector thereby splitting the first sector into at least the first and second virtual sectors.