Abstract:
In a cell phone telecommunication system having a macrocell and a small cell coextensive with or adjacent to the macrocell and sharing a common frequency band, interference can occur between a user device operating in the small cell, the local base station operating the user devices in the small cell on one hand and another user device serviced by a main base station for the macrocell. In order to reduce this interference, the shape of the transmission beam from at least one of the user devices or base stations is shaped into a narrow beam steered to be directed at the respective receiving unit. Alternatively, the shape of the transmission beam has a null point and the beam is steered so that its null point is directed toward the affected unit.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/515,521 filed on Aug. 5, 2011 and incorporated herein in its entirety. 
    
    
     FIELD 
     This application pertains to devices operating in a mobile telecommunication system including a small cell, and more particularly to devices operating in or adjacent to the small cell, the devices having antennas that are adaptively optimized to reduce interference between such devices while they are uploading or downloading signals. The application further covers methods for reducing the interference. 
     BACKGROUND 
     Mobile telephone systems have become very popular in recent years to the point where they are preferred over standard landline telephone systems by individuals and enterprises alike. One problem with mobile telephone systems is that they rely on cells extending over well-defined geographic areas traditionally served by a main base station. However, service for devices operating at the fringe of a cell is frequently intermittent or has a low quality. Moreover, as more and more devices are active within a cell, the cell&#39;s capacity to service the devices is strained and communication with the devices becomes poor. A further disadvantage is that because of local conditions within a cell area (for example, inside a building, or in an underground location), communications with certain devices may be severely attenuated and it is difficult if not impossible to service such devices from the main base station. 
     Moreover, some entities, including enterprises, may find it desirable to control communications with certain devices within a limited section of the cell, for example, inside a factory, or other facility. The standard protocols used for communications within a traditional cell area do not permit such functionality. 
     One solution to reduce these problems involves providing one or more small cells that are disposed physically either within a main cell or at its fringe. Each small cell is provided with a low power local base station communicating with the main base station via DSL, cable or broadband or other conventional communication channels. Devices within the small cell normally communicate with the local base station and not the main base station. Importantly, all communications between the devices and both the main and the local base stations are performed within the same frequency channels using a conventional protocol, such as WDCMA, GSM, CDMA2000, TD-SCDMA, WiMax, LTE, etc. Small cells can be categorized based on their operating range as femtocells (with a range of about 10 m), picocells (with a range of about 10-200 m) or microcells (with a range of 200-2000 m). In this environment, a main cell is termed a macrocell. The type of small cell depends obviously on the size of the respective area that must be serviced by it. For example, picocells are suitable for residences or a floor in a multi-storied building. 
     Small cell service is available from many mobile operators, such as Vodaphone, AT&amp;T, Sprint, Verizon, etc. A small cell service is advantageous to a mobile operator because it improves the coverage and increases the capacity of the respective macrocell. Consumers additionally benefit from a small cell because they get better voice quality; an increase in the battery life in their devices because of lower power requirements, and can use them to define a closed network. 
     Small cells can be operated in an open mode in which the small cell is available to any device within its coverage area, or in a closed mode in which devices must be registered or subscribed with the local base station before they can operate with the small cell. In other words, in an open small cell, a device within the small cell coverage area is serviced by the local small cell base station. As the device moves out of the small cell into the macrocell, it is seamlessly handed off to the main base station. An unsubscribed device in a closed small cell is ignored by the local base station and must be serviced by the main base station, if possible. 
     A problem with a mobile telephone system with a macrocell and one or more small cells is that the imbalance between the EIRP (Effective Isotropically Radiated Power) of devices served by the different base stations can cause serious interference between the two devices. Until now, this interference could be reduced only by reducing the range of the small cell so that both devices are served by the main base station. Obviously this is unsatisfactory because it defeats the purpose of defining a small cell. 
     In a picocell, this interference may be very pronounced, appearing as a mutual interference resulting in a poor signal-to-noise ratio during downloads to a device as well as uploads to the main base station. 
     Similarly, an unsubscribed device within a femtocell must be serviced by the main base station. This causes interference both for downloads by other subscribed devices and uploads to the local base station. 
     The present disclosure solves these problems, so that the benefits of small cells are not lost. 
     SUMMARY 
     In a system providing cell phone communications using a macrocell and a small cell sharing a common frequency band, there may be several units, including a first unit exchanging signals with a second unit within said small cell, and a third unit being serviced within said macrocell. The term unit is used here to refer generically either to a user device within a macrocell or small cell, or a base station servicing one or more user devices. 
     As described herein, the first unit includes a composite antenna generating a transmission beam to the second unit, the transmission beam having a beam direction; and a signal processor receiving input signals and generating transmission signals to be transmitted to the second unit by the composite antenna as said transmission beam. The signal processor is adapted to select the beam direction to minimize interference between said transmission beam and download signals to the third unit. The first unit could be a user device or the local base station. The third unit is normally a user device. 
     The processor is adapted to shape the transmission beam to have a narrow beam shape oriented in a beam direction related to the apparent position of the second unit with respect to the first unit. 
     Alternatively, the processor is adapted to shape the beam to have a null point at the apparent position of the third unit with respect to the first unit. 
     In one example, the signal processor is adapted to determine the apparent direction of the second unit with respect to the first unit, or the apparent direction of the third unit with respect to the first unit. It cannot determine direction or apparent direction. It only looks are a set of “beam pointing parameters” that produce an effect (e.g., power down commands) and compares with another. 
     In another aspect of the disclosure, there is described a user device operating in a cell phone system including a macrocell serviced by a main base station and a small cell sharing a frequency band with the macrocell and being serviced by a local base station. The user device includes a multiphase antenna; and a signal processor receiving transmission signals and cooperating with the antenna to generate a transmission beam having a beam directed to said small cell, said transmission beam corresponding to said transmission signals. The signal processor determines the beam direction based on the relative direction of another user device serviced by said main base station. More specifically, the transmission beam is selected to minimize interference with the transmissions from the other unit to the base station. 
     In another aspect of the disclosure, there is a cell communication system including a macrocell and a small cell operating in a common frequency range. There is also a first and second unit operating in the small cell and a third unit being serviced from the third cell. A method of reducing interference between units operating within said small cell and the third unit operating in the macrocell, includes the steps of determining the apparent direction of one of the second and third unit from said first cell; and transmitting signals from said first cell to said second cell using a transmission beam having a beam shape and a beam direction, said beam direction being selected based on said apparent direction. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a mobile telephone system having a macrocell and a small cell; 
         FIG. 2  shows a block diagram of the main and local base stations; 
         FIG. 3A-3D  show somewhat diagrammatically some characteristic beams shaped and steered to eliminate or reduce interference between two devices. 
     
    
    
     DETAILED DESCRIPTION 
     Transmission beams of various shapes are shown and discussed as being directed toward a particular unit of a communication system, or being directed away from another unit, wherein a unit can be either a user device or a base station. Of course, it should be understood that, because of reflections by various objects between two units of any system, as well as other often variable factors, in most instances the beams do not travel in a straight line and, therefore, it is more convenient to think of these beams as being directed at the apparent positions and/or directions of the respective units rather than repeatedly referring to their actual transmission paths. 
     As shown in  FIG. 1 , a mobile telephone communication system  100  includes a macrocell  10  having a range R. Devices  12 ,  14  within the macrocell  10  are serviced by a main base station  23  including a main server  20  and a main antenna  22  connected to the main server by a cable  24 . The system  100  further includes a small cell  50  with a range r servicing several devices  52 ,  54 ,  56 ,  58  with a local base station. The local base station  63  includes a local server  60  and a local antenna  62  connected to the local server  60  by a cable  64 . The local server  60  is connected to the main server by a broadband communication channel  66 . As can be seen in this Figure, mobile device  12  may become located very close to the edge of small cell  50  and therefore interference may occur as various devices communicate with the respective base stations, as discussed in more detail below. 
     As shown in  FIG. 2 , the local server  50  includes a signal processor  68  for processing the signals to and from the antenna  62 . Signals for the main processor  20  are exchanged through a communication gate  70 . A local manager  72  keeps track of the devices  52 - 58  within the range r serviced by the local base station. 
     The main server  20  includes a signal processor  30  processing the signals to and from the antenna  22 . Information about visitors are maintained in a databank  32  while a home subscriber server  34  keeps track of subscriber devices for the macrocell  20 . In addition, the server  20  provides internet access through an Internet gate  36  and Internet connection  38 . Mobile switching center  40  provides connection to devices served by other systems through a standard landline telephone connection  42 . 
     As previously discussed, devices in or near the range of small cell  50  can cause various interferences. If the range r of small cell  50  is reduced temporarily to avoid this interference, a local subscriber device  54  or  56  is left behind and can no longer be served by the local base station but must be served by main base station  23 . 
     In the present system, the devices and base stations use multiple antennas (not shown) and they operated in a manner so as to customize the shapes and the directions of the beams generated by the antennas. These beams are optimized to eliminate, or at least reduce interferences. The beam shaping and steering techniques used here are well known and include controlling the amplitudes and the relative phases of the signals applied to the antenna elements. 
     One well known technique for generating and steering an effective but narrow beam is known as transmission diversity (TD) as described in U.S. Pat. No. 7,633,905. 
     Another technique referred to as zero-forcing or null-steering (NS) technique is used to generate a null point transmission beam. This technique is used to generate a beam having a null (i.e. nodal) point in a direction in which the beam intensity must be minimized. Other beam shaping techniques are known in the art, including both cooperative (closed loop) and non-cooperative (open loop) techniques (such as Open- and Closed-Loop Transit Diversity—OLTD, CLTD) and can be used as well. 
       FIGS. 3A-3D  show how some of these techniques are used to eliminate or reduce interference. For the sake of clarity, the base stations  23 ,  63  are represented in these figures as a respective antenna. 
     In  FIG. 1 , the small zone  50  was disposed near the edge of macrocell  20 ; however, the processes described are also applicable for small cells that are contained inside a macrocell. One such configuration is shown in  FIG. 3A . In this configuration, the macrocell  20  surrounds the small cell  50  and is generally serviced by master base station  23 , while the small cell  50  is serviced by local base station  63 . 
     The cells may include several devices, however only two such devices  12 ,  54  are shown. Small cell  50  is a closed cell with device  54  being a subscriber device and device  12  being a nonsubscriber device. Therefore, while both devices are physically inside small cell  50 , only device  54  is serviced by local base station  63 . Device  12  is serviced through master base station  23 , however, the local base station  63  eavesdrops on the transmissions between device  12  and master base station  23  and sends commands to device  12  to reduce interference within the small zone  50  as discussed below. 
     Each device  12 ,  54  receives control commands, such as power up/down directions from both the master base station  23  and the local base station  63  and can identify which is sending them. When the local base station  23  the device  12  serviced by the master base station  23  to go down in power, the device  12 , rather than reducing its power, uses the NS technique to generate a signal having a null point N1 that is directed toward the local base station  63 . Then, if necessary, it may still be able to maintain or increase the power of the upload signal UL3 to talk to its master base  23  station in spite of the fact its beam is no longer pointing directly to it. 
     As described above, the device  12  can be adversely affected because the signal it receives from base station  23  (hereinafter the DL signal) could be degraded by jamming from the base station  63  (e.g., the DL signal from base station  63  to device  54 ). In addition, the UL signals from the device  54  can be degraded by jamming (UL signals) from device  12 . 
     As is conventional in the art, the transmission beams of various shapes are shown and discussed as being directed toward a particular unit of a communication system, or being directed away from another unit thereof. Of course, as stated above, it should be understood that because of reflections by various objects between two units of any system as well as other factors, in most instances the beams do not travel in a straight line and, therefore, it is more convenient to think of these beams as being directed at the apparent positions or directions of the respective units rather than actual transmission paths. 
     One embodiment has one or both devices  12 ,  54  shape the UL beams using mobile transmit diversity (MTD) techniques or other similar techniques generating relatively narrow focused beams, directed at the respective antenna. When the MTD technique is used, the interference between signals from the devices is reduced by approximately 3 db compared to a standard omnidirectional beam. The directions of the respective UL beams are determined using conventional quality parameters, such as the ones discussed in more detail below. 
       FIG. 3A  shows a configuration for minimizing interference for UL transmissions using MTD or other techniques for generating a generally narrow directional beam. Normally, device  12  generates an omnidirectional beam BR1 to reach master base station  23 . Unfortunately, this beam BR1 also encompasses device  54  and the antenna of base station  63  and creates interference in both UL and DL transmissions. To reduce this interference, at the request of the local base station  63 , device  12  shapes its UL transmission through the main base station  23  to generate a beam UL1. This beam UL1 avoids both device  54  and the antenna of local base station  63  and hence avoids interference with the transmissions between device  54 . 
     In the configuration of  FIG. 3A , the device  54  can maintain its omnidirectional UL beam. However, if devices  12  and  54  get too close to each other, UL transmissions from device  54  may interfere with transmissions involving device  12 . Therefore, at the request of the master base station  23 , device  54  can shape its beam, using for example MTD, to form a narrow beam UL2 directed at local base station  63 . 
     Even better results are expected if an NS (Null Steering) beam shaping technique is used with the null point being directed at the competing signal source. Using this technique, it is expected that interference can be reduced by 6-10 db. This technique requires a non-cooperative mode. 
     The NS technique, as previously discussed requires that a minimum signal quality be achieved in a selected direction. For example, for systems using CDMA, UMTS or HSPA protocols the quality indicator could be the fast control power emitted by the competing (or victim) signal source. A beam may be steered until the ratio of the PCB or TPC signals is maximized. 
     The GSM and LTE protocols do not use spread spectrum as a multiple access method and therefore, for these protocols, another quality parameter may be selected. For example the pilot signals/reference signals of the competing transmitter may be maximized. 
     Another quality indicator that can be used for steering beams from the local antenna may be determined by the local base station eavesdropping on UL transmissions from device  12  to the main base station  23 . For example, the CQI parameter can be monitored and the beams from device  12  can be steered using the NS technique to maximize this parameter. Alternatively, the number of retransmission requests can also be monitored and the beam can be steered to minimize this parameter. For example, the UL beam from device  12  can be steered until the number of retransmission requests from the master base station attains a level at which the device  12  is requested to retransmit a communication no more than 2 or 3 times. This information is obtained by the local base station  63  from the master base station  23 . 
       FIG. 3B  shows a configuration similar to that of  FIG. 2A . In this configuration, instead of using a narrow beam UL1, device  12  generates an upload beam UL3 that has been shaped and steered using the NS steering technique discussed above. As can be seen in  FIG. 3B , the beam UL3 has a null point in the direction of the antenna  62  of local base station  63  and therefore if the device  54  generates an uploading beam BR2 (e.g., an omnidirectional beam), interference between UL4 and UL3 is minimized at the antenna of the base station  63 . 
     There are at least two significant differences between the approaches in  FIGS. 3A and 3B . In  FIG. 3A , beam UL1 is a narrow beam directed towards the antenna of the intended target, in this case the antenna of main base station  23 . In  FIG. 3B , beam UL4 is shaped and steered so that it has a null point in the direction of the unintended or victim antenna, in this case the antenna of local base station  63 . 
       FIGS. 3C and 3D  show how the device  12  may be protected from interference due to download signals from the local base station  63  to device  54 . In  FIG. 3C , antenna  62  generates a download beam DL1 shaped using TD and steered toward device  54  as show. The antenna  22  downloads signals DL2 to device  12  at the same time however because of the shape of beam DL1, the interference at device  12  is reduced.  FIG. 3D  shows a configuration similar to  FIG. 3C  except that instead of generating a beam DL1, the antenna of local base station  63  generates a beam DL3 shaped and steered using an NS steering technique. In this case the null point N2 of beam DL3 is directed toward device  12 . 
     Numerous modifications may be made to the system and methods described herein without departing from the scope as this application as set forth in the appended claims.