Patent Publication Number: US-7720441-B2

Title: Apparatus and method for minimizing inter-signal interference in a wireless communication system

Description:
PRIORITY 
   This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Sep. 27, 2005 and assigned Serial No. 2005-90151, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to signal transmission in a wireless communication system, and in particular, to an apparatus and method for minimizing inter-signal interference in a wireless communication system. 
   2. Description of the Related Art 
   In the field of wireless communications, new communication systems have recently been developed and new frequency bands have been allocated. Also, new technologies have emerged to achieve miniaturization and digitization beyond conventional analog wireless communication schemes. The significance of such wireless communication systems is increasing in view of the nature of free propagation of waves in the air. 
   Due to the feature of a wireless propagation path, a wireless communication system can adopt a variety of signal transmission schemes. Among them, there is a polarization-based signal transmission scheme. Polarization is a property that the direction and magnitude of an oscillated electric field is related to a predetermined direction in electromagnetic propagation. Now a description will be made of a wireless communication system utilizing polarization, starting with electric propagation. 
     FIG. 1  shows propagation of electric waves in a typical wireless communication system. The phase of electric waves is propagated across space according to the polarization of an antenna. Assuming that the electric field E of the electric waves is propagated in a z-axis direction, the x-y plane electric field E denoted by reference numeral  101  at a predetermined position has an x-axis electric field E x , a y-axis electric field E y , and a particular polarization phase θ. The electric field  101  is determined according to the polarization of the antenna, the spatial position of the antenna, and reflection and diffraction in surroundings. 
   If a receive antenna is polarized with a predetermined phase, it has a maximum reception power when receiving waves polarized with the polarization phase. Therefore, the polarization can be utilized to increase efficiency by distinguishing an intended wave or adjusting the polarization angle of a transmit/receive antenna in the wireless communication system using antennas. Although a random polarization phase is propagated instantaneously due to a phase delay caused by the polarization phase θ of the transmit antenna, and reflection and diffraction from a surrounding object, the average reception power measured for a predetermined period reveals that electric waves are propagated and polarized constantly irrespective of frequency or the velocity of a receiver. Polarization-based signal transmission and reception will be described with reference to  FIG. 2 . 
     FIG. 2  shows polarization-based signal transmission and reception in a typical wireless communication system. A Base Station (BS)  200  includes first and second polarization antennas  201  and  203 , for polarization-based signal transmission, and Mobile Stations (MSs)  230  and  260  receive signals based on polarization. 
   The first and second polarization antennas  201  and  203  are perpendicular to each other. The BS  200  communicates with the MSs  230  and  260  based on the above-described polarization property. Thus, the MSs  230  and  260  receive signals from the polarization antennas  201  and  203  of the BS  200  through antennas with predetermined polarization phases. 
   For example, on the downlink, the polarization antennas  201  and  203  send transmission polarization phases in accordance with the received polarization phases of the MSs  230  and  260 . The MSs  230  and  260  have a maximum reception power, for communications with the BS  200 . 
   If the polarization difference between two users is adjusted to be orthogonal, as shown in  FIG. 2 , each polarization has no effect on the other polarization. Thus, it is possible to provide a service to the two users simultaneously. 
   This polarization-based signal transmission scheme is efficient to a communication system relying on point-to-point communication or using two terminals with orthogonal reception polarization phases. However, if the communication system services two or more users, transmitted/received signals interfere with one another. The inter-signal interference with MSs will be described with reference to  FIG. 3 . 
     FIG. 3  shows the polarization phase of each MS in a typical wireless communication system using polarization. A first MS (MS  1 ) has a polarization phase  301  and a second MS (MS  2 ) has a polarization phase  303 . When the BS sends a signal with the polarization phase  301  to MS  1 , part of the polarized signal acts as an interference component  305  to the polarization phase  303  of MS  2 . 
   A polarization phase varies with the polarization phase of an antenna at an MS and an environment, even if it is the average of polarization phases measured for a long time. As a consequence, there may exist no MSs having mutually orthogonal polarization phases, resulting in interference. Inter-signal interference is one obstacle to signal restitution. Accordingly, a need exists for solving the inter-signal interference problem. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, the present invention provides an apparatus and method for minimizing inter-signal interference in a wireless communication system. 
   The present invention provides an apparatus and method for minimizing inter-signal interference using polarization in a wireless communication system. 
   According to one aspect of the present invention, in a method of minimizing inter-signal interference in a wireless communication system, the polarization phases of MSs are measured. An MS group including a predetermined number of MSs is determined and a polarization phase of the MS group is compensated. The polarization phase of a transmission signal is matched to the compensated polarization phase and the matched signal is sent to the MS group. 
   According to another aspect of the present invention, in an apparatus for minimizing inter-signal interference in a wireless communication system, a polarization phase measurer measures the polarization phases of MSs. A decider determines an MS group including a predetermined number of MSs. A polarization phase compensator compensates a polarization phase of the MS group. A polarization phase matcher matches a polarization phase of a transmission signal to the compensated polarization phase. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates electric propagation in a typical wireless communication system; 
       FIG. 2  illustrates polarization-based signal transmission and reception in a wireless communication system; 
       FIG. 3  illustrates the polarization phase of each MS in a typical wireless communication system using polarization; 
       FIG. 4  is a block diagram of a transmitter and a receiver in a BS operating based on polarization in the wireless communication system; 
       FIG. 5  illustrates polarization phase compensation in the wireless communication system according to the present invention; 
       FIG. 6  is a block diagram of a signal transmitter for minimizing inter-signal interference in the wireless communication system according to the present invention; 
       FIG. 7  is a flowchart illustrating a signal transmission operation for minimizing inter-signal interference according to the present invention; 
       FIG. 8  is a flowchart illustrating a signal transmission operation for minimizing inter-signal interference according to the present invention; and 
       FIG. 9  is a flowchart illustrating a signal transmission operation for minimizing inter-signal interference according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
   The present invention provides an apparatus and method for sending signals, minimizing interference between Mobile Stations (MSs) in a wireless communication system. Specifically, the polarization phases of MSs are measured and an MS group is set which includes a predetermined number of MSs. Polarization phase compensation is performed for the MS group and the polarization phase of a transmission signal is matched to a compensated polarization phase. In this way, the Base Station (BS) communicates with the MS group. The MS group includes one or two MSs. 
   Each MS reports its current channel quality by, for example, a Signal-to-Noise Ratio (SNR) a Carrier-to-Interference Ratio (CIR), a Signal-to-Interference Noise Ratio (SINR) or a Carrier-to-Interference Noise Ratio (CINR). The BS selects an MS group with the best channels and allocates channels to the MS group. 
   With reference to  FIG. 4 , a description will be made below of the structures of a transmitter and receiver for sending and receiving signals based on polarization in a BS. The BS includes a receiver  410  and a transmitter  450 . The receiver includes two orthogonal receive polarization antennas  411  and  413 , electric field measurers for measuring the electric fields E x  and E y  received at the receive polarization antennas  411  and  413 , i.e. an E x  measurer  415  and an E y  measurer  417 , and a polarization phase measurer  419 . 
   The two receive polarization antennas  411  and  413  receive uplink electric fields from an MS  401 . Specifically, the x-axis polarization antenna  411  receives an x-axis electric field E x  and the y-axis polarization antenna  413  receives an y-axis electric field E y . The E x  measurer  415  and E y  measurer  417  measure the x-axis electric field E x and the y-axis electric field E y , respectively. The polarization phase measurer  419  calculates a polarization phase by Equation (1) 
   
     
       
         
           
             
               
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                       - 
                       1 
                     
                   
                   ( 
                   
                     
                       E 
                       y 
                     
                     
                       E 
                       x 
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   After the polarization phase measuring, the BS compensates the polarization phase φ and receives a signal from the MS  401  with the compensated polarization phase φ. Therefore, the reception SNR is increased. 
   The transmitter includes two orthogonal transmit polarization antennas  451  and  453 , and electric field polarization matchers  455  and  457  (i.e. an E x  polarization matcher  455  and an E y  polarization matcher  457 ). For downlink transmission to the MS  401 , the polarization matchers  455  and  457  match electric fields E x  and E y  to the polarization phase φ measured by the receiver  410 . The transmit polarization antennas  451  and  453  send the phase-matched electric fields E x  and E y  to the MS  401 , thereby maximizing the reception power of the MS  401 . Polarization phase compensation for minimizing inter-signal interference will be described with reference to  FIG. 5 . 
     FIG. 5  shows polarization phase compensation in the wireless communication system according to the present invention. For a BS, there are two MSs (MS  1  and MS  2 ) having a polarization phase  511  and a polarization phase  551 , respectively. In downlink transmission, these polarization phases  511  and  551  are not orthogonal to each other. 
   As a result, as much interference as mutual projection between polarized signals occurs to signals sent to the MSs. 
   However, if the BS has knowledge of the polarization phases  511  and  551  of MS  1  and MS  2 , it can send signals to them so that the polarization phase  511  is orthogonal to the polarization phase  551 , thus avoiding the mutual interference. For this purpose, the BS compensates the polarization phase  511  and generates the compensated polarization phase  515  for MS  1  to be orthogonal to the polarization phase  551 . While the reception power  513  of MS  1  is decreased by the use of the compensated polarization phase  515 , the orthogonality between MS  1  and MS  2  prevents inter-signal interference for them. 
   On the contrary, no mutual interference occurs either when the polarization phase  551  is rendered orthogonal to the polarization phase  511 . Thus, the BS compensates the polarization phase  551  of MS  2  with respect to the polarization phase  511  and generates the compensated polarization phase  555  for MS  2 . The use of the polarization phase  555  decreases the reception power  553  of MS  2 , but the resulting orthogonality prevents inter-signal interference for MS  1  and MS  2 . 
   Therefore, the polarization phase of at least one MS is compensated so that polarization phases are mutually orthogonal to each other between MSs. A BS according to the present invention will be described with reference to  FIG. 6 . 
     FIG. 6  shows a signal transmitter for minimizing inter-signal interference in the wireless communication system according to the present invention. A signal transmitter  600  can be implemented in the BS. It includes a decider  601 , a polarization phase measurer  603 , a channel quality measurer  605 , a polarization phase compensator  607 , multipliers  609  and  611 , adders  613  and  615 , and transmit polarization antennas  617  and  619 . The signal transmitter  600  communicates with MSs  660  and  690 . 
   The polarization phase measurer  603  measures the polarization phases φ of MSs within its service area, for example, within a cell. The polarization phase measuring is performed every predetermined interval and the polarization phases are updated correspondingly. 
   The channel quality measurer  605  measures the channel qualities of the MSs. The channel qualities can be SNRs, CIRs, SINRs, or CINRs. 
   The decider  601  receives the polarization phase measurements and the channel qualities from the polarization phase measurer  603  and the channel quality measurer  605 , and determines an MS group that the signal transmitter  600  will send signals based on the received information. Since the signal transmitter  600  communicates with the MSs  600  and  690  using polarization phases, it communicates with two or less MSs to reduce inter-signal interference. Thus, the MS group includes one or two MSs. The decider  601  outputs transmission signals s 1  and s 2  for the decided MS group among received transmission signals and provides information about the MSs of the MS group to the polarization phase compensator  607 . How the decider  601  selects MSs to be grouped into an MS group will be described later. 
   The polarization phase compensator  607  may receive the polarization phase measurements directly from the phase measurer  603  or through the decider  601 . The polarization phase compensator  607  compensates the polarization phases of the MS group such that inter-signal interference is minimized and outputs the compensated polarization phases φ 1  and φ 2  to the multipliers  609  and  611 . 
   The multipliers  609  and  611  multiply the compensated polarization phases φ 1  and φ 2  by the transmission signals s 1  and s 2 . 
   The adders  613  and  615  each combine the transmission signals or the polarization phase-compensated signals. Thus, the transmit polarization antennas  617  and  619  send the resulting transmission signals whose polarization phases have been matched by the multipliers  609  and  611 , and the adders  613  and  615 . 
   In other words, the multipliers  609  and  611 , and the adders  613  and  615  collectively form a polarization phase matcher. Specifically, in the polarization phase matcher, the multipliers  609  and  611  multiply the transmission signals received from the decider  601  by the compensated polarization phases, the first adder  613  combines the signals received from the multipliers  609  and  611 , and the second adder  615  combines the transmission signals received from the decider  601 . 
   The signal from the first adder  613  is sent through the first polarization antenna  617  and the signal from the second adder  615  is sent through the second polarization antenna  619 . 
   The decider  601  determines the MS group including one or two MSs to which the signal transmitter  600  will send signals. Assuming that three MSs M 1 , M 2 , and M 3  exist within the cell, the decider  601  may consider six MS groups, {M 1 }, {M 2 }, {M 3 }, {M 1 , M 2 }, {M 1 , M 3 } and {M 2 , M 3 }. Then, the polarization phases of a selected one of the MS groups are compensated in the polarization phase compensator  607 . 
   The MS grouping in the decider  601  can be considered in three ways. 
   In one of them, the decider  601  receives the channel qualities of the MSs and calculates the transmission capacities of the MS groups using the channel qualities. Given M 1 , M 2  and M 3  within the cell, the transmission capacity of the MS group {M 1 } is calculated by Equation (2)
 
log 2 {1+SNR 1 }  (2)
 
where SNR 1  represents the SNR of M 1 .
 
   The transmission capacity of the MS group {M 2 , M 3 } is calculated by Equation (3)
 
log 2  {1+SNR 2,3 }+log 2 {1+SNR 3,2 }  (3)
 
where SNR i,j  represents the SNR of Mi in an MS group including MSi and MSj.
 
   The decider  601  calculates the transmission capacities of the six MS groups using the channel qualities of the MSs. Then the decider  601  selects an MS group having the largest transmission capacity. 
   Another way is that the decider  601  receives the channel qualities of the MSs, selects a first MS having the best channel quality, and selects a second MS having a polarization phase within a predetermined polarization phase range with respect to the polarization phase of the first MS. 
   The polarization phase range can be set based on a polarization phase (+90 or −90 degrees) orthogonal to that of the first MS. For example, if the polarization phase of the first MS is 0 degrees and the polarization phase range covers 20 degrees, the polarization phase range is from +80 to +100 degrees for +90 degrees orthogonal to the polarization phase of the first MS, or from −80 to −100 degrees for −90 degrees orthogonal to the polarization phase of the first MS. Thus, an MS having a polarization phase falling within the polarization phase range is selected as the second MS. Yet, the second MS should have a channel quality equal to or better than a channel quality threshold. For example, the second MS should have an SNR equal to or better than an SNR threshold. If an MS below the channel quality threshold has a polarization phase within the predetermined polarization phase range, it cannot be selected as the second MS. The MS group includes only one MS having the best channel quality. If a plurality of MSs have polarization phases within the predetermined polarization phase range, an MS having the highest of SNRs equal to or higher than the SNR threshold is selected as the second MS for the MS group. 
   The other way is that the decider  601  selects a first MS according to a rule preset considering proportional fairness. Then a second MS is selected, which has a polarization phase within a predetermined polarization range set with respect to the polarization phase of the first MS. 
   The predetermined polarization phase range is set to be orthogonal to the polarization phase of the first MS. Thus, an MS having a polarization phase falling within the polarization phase range is selected as the second MS. Yet, the second MS should have a channel quality equal to or better than a channel quality threshold. For example, the second MS should have an SNR equal to or higher than an SNR threshold. If an MS below the channel quality threshold has a polarization phase within the predetermined polarization phase range, it cannot be selected as the second MS. In this case, the MS group includes only one MS having the best channel quality. If a plurality of MSs have polarization phases within the predetermined polarization phase range, the second MS is selected for the MS group, considering the predetermined polarization phase range and the channel quality threshold. 
   Assuming that the BS is the signal transmitter for minimizing inter-signal interference, a method of minimizing inter-signal interference according to the present invention will be described with reference to  FIGS. 7 ,  8  and  9 . 
     FIG. 7  shows an example of a signal transmission operation for minimizing inter-signal interference according to the present invention. The BS measures the polarization phases of MSs in step  701 . The polarization phases are not instantaneous values but the averages of polarization phases measured for a predetermined time period. The BS updates the polarization phases for every predetermined time period. 
   In step  703 , the BS measures the channel qualities of the MSs, such as SNRs, CIRs, SINRs, or CINRs. The polarization phase measuring and the channel quality measuring may take place simultaneously, or the channel quality measuring may precede the polarization phase measuring. 
   The BS calculates the transmission capacities of all MS groups that can be created with the MSs within the cell area of the BS in step  705 . 
   The BS selects an MS group having the maximum transmission capacity in the manner described with reference to  FIG. 6  in step  707 . 
   In step  709 , the BS compensates the polarization phases of the MS group. The MS group may include one or two MSs and the polarization phase compensation is performed so the polarization phases of the MSs are orthogonal to each other and thus there is no mutual interference between the MSs. 
   The BS matches the polarization phases of transmission signals for the MSs of the MS group to compensated polarization phases and sends the polarization phase-matched transmission signal to them in step  711 . 
     FIG. 8  shows another example of a signal transmission operation for minimizing inter-signal interference according to of the present invention. The BS measures the polarization phases of MSs in step  801 . The polarization phases are not instantaneous values but the averages of polarization phases measured for a predetermined time period. The BS updates the polarization phases for every predetermined time period. 
   In step  803 , the BS measures the channel qualities of the MSs, such as SNRs, CIRs, SINRs, or CINRs. The polarization phase measuring and the channel quality measuring may take place simultaneously, or the channel quality measuring may precede the polarization phase measuring. 
   The BS selects a first MS having the best channel quality in step  805  and determines whether there is any MS with a polarization phase falling within a predetermined polarization phase in step  807 . The polarization phase range is set based on a polarization phase orthogonal to that of the first MS. 
   In the absence of such an MS, the BS goes to step  811 . In the presence of such an MS, the BS goes to step  809 . 
   In step  809 , the BS determines whether the MS has a channel quality equal to or better than a channel quality threshold, for example, an SNR threshold. If the channel quality of the MS is equal to or greater than the channel quality threshold, the BS selects the MS as a second MS to be included in an MS group. On the contrary, if the channel quality of the MS is less than the channel quality threshold, the BS does not select the MS. When a plurality of MSs have a channel quality equal to or greater than the channel quality threshold, an MS with the best channel quality among them is selected as the second MS. 
   In step  811 , the BS determines the MS group through step  807  or step  809 . 
   The BS performs polarization phase compensation for the determined MS group in step  813 . The MS group may include one or two MSs. That is, the MS group includes only the first MS with the best channel quality or both the first and second MSs. The polarization phase compensation is carried out so the polarization phases of the MSs in the MS group are mutually orthogonal to thereby avoid inter-signal interference. 
   The BS matches the phases of transmission signals for the MSs of the MS group to compensated polarization phases and sends the signals in step  815 . 
     FIG. 9  shows another example of a signal transmission operation for minimizing inter-signal interference according to the present invention. The BS measures the polarization phases of MSs in step  901 . The polarization phases are not instantaneous values but the averages of polarization phases measured for a predetermined time period. The BS updates the polarization phases for every predetermined time period. 
   In step  903 , the BS selects a first MS according to a predetermined order. The order is preset considering proportional fairness. 
   The BS determines whether there is any MS with a polarization phase falling within a predetermined polarization phase in step  905 . In the presence of such an MS, the BS goes to step  907 . 
   In step  907 , the BS determines whether the MS has a channel quality equal to or greater than a channel quality threshold, for example, an SNR threshold. If the channel quality of the MS is equal to or greater than the channel quality threshold, the BS selects the MS as a second MS to be included in an MS group. On the contrary, if the channel quality of the MS is less than the channel quality threshold, the BS does not select the MS. When a plurality of MSs have a channel quality equal to or greater than the channel quality threshold, an MS with the best channel quality among them is selected as the second MS. 
   In step  909 , the BS determines the MS group through step  905  or step  907 . 
   The BS performs polarization phase compensation for the determined MS group in step  911 . The MS group may include one or two MSs. That is, the MS group includes only the first MS with the best channel quality or both the first and second MSs. The polarization phase compensation is carried out so the polarization phases of the MSs in the MS group are mutually orthogonal to thereby avoid inter-signal interference. 
   The BS matches the phases of transmission signals for the MSs of the MS group to compensated polarization phases and sends the signals in step  913 . 
   In accordance with the present invention as described above, in a wireless communication system, an MS group to communicate with a BS is decided according to polarization phases and channel qualities of MSs, and the polarization phases of transmission signals for the MSs of the MS group are compensated in a manner that ensures orthogonality between the MSs. Therefore, inter-signal interference is minimized during communications between the BS and the MSs. The cancellation of interference impeding signal recovery increases overall system efficiency. 
   While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.