Abstract:
The present invention is related to a method and apparatus for reducing antenna correlation between multiple antennas. A transmitter generates at least two beams with a plurality of antennas. The generated beams are spatially separated to point away each other. Therefore, the transmitted signals travel through different channel conditions and arrive at a receiver mutually uncorrelated. The beams may be generated by antennas having different antenna pattern, or by an array antenna. The beams may be polarized differently. The schemes may be implemented on a subcarrier basis in an orthogonal frequency division multiplexing (OFDM) system. Trellis coded mapping may be utilized for adjacent symbols to be mapped to antennas with low correlation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/601,674 filed Aug. 12, 2004, which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     The present invention is related to wireless communication systems. More particularly, the present invention is related to a method and apparatus for reducing antenna correlation between multiple antennas. 
     BACKGROUND 
     A scheme for utilizing multiple antennas, such as multiple-input multiple-output (MIMO) system, has been developed. By utilizing multiple transmit and receive antennas, a capacity and throughput of the wireless communication system can be enhanced. The performance enhancement is restricted as a correlation of the signals between the plurality of antennas becomes higher. If the correlation of antennas is high, the advantage of a multiple antenna system may be lost. 
     In a MIMO system, data is converted to a plurality of parallel data streams and the parallel data streams are transmitted simultaneously from different antennas. The data stream may be transmitted only from a subset of the antennas. Selection of antennas for transmission is often based on quality of the link seen at the antenna or other relevant quality indicators. The performance of the MIMO system degrades as signals become more correlated between antenna transmissions. 
     In general, the antenna correlation depends on factors such as distance between the antennas and channel state including scatterings conditions. Signals received at the receiver are subject to multipath fading and the antenna correlation becomes lower as the influence of fading differs each other. Therefore, it is desirable to lower the antenna correlation in wireless communication systems utilizing multiple antennas. 
     SUMMARY 
     The present invention is related to a method and apparatus for reducing antenna correlation between multiple antennas. A transmitter generates at least two beams with a plurality of antennas. The generated beams are spatially separated to point away each other. Therefore, the transmitted signals travel through different channel conditions and arrive at a receiver mutually uncorrelated. The beams may be generated by antennas having different antenna pattern, or by an array antenna. The beams may be polarized differently. The schemes may be implemented on a subcarrier basis in an orthogonal frequency division multiplexing (OFDM) system. Trellis coded mapping may be utilized for adjacent symbols to be mapped to antennas with low correlation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a diversity transmission system with directional antenna elements. 
         FIG. 2  is a simplified diagram of a diversity transmission system utilizing beam/pattern forming. 
         FIG. 3  is a simplified diagram of a diversity transmission system utilizing polarization. 
         FIG. 4  is a simplified diagram of a diversity transmission system utilizing polarization in an OFDM system. 
         FIG. 5  is an illustration of one possible assignment of subcarriers. 
         FIG. 6  is a simplified diagram of a diversity transmission system using subcarrier grouping in an OFDM system. 
         FIG. 7  is an illustration of one possible assignment of subcarrier groups. 
         FIG. 8  is a simplified diagram of a diversity transmission system using trellis mapping. 
         FIG. 9  is a simplified diagram of a diversity transmission system using time frequency multiplexing. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is applicable to any wireless communication systems employing multiple transmit or receive antennas for transmission and reception including, but not limited to, MIMO antenna schemes for OFDM systems. 
     The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components. 
     Hereafter, a wireless transmit/receive unit (WTRU) includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes but is not limited to a Node-B, site controller, access point or any other type of interfacing device in a wireless environment. The transmitting and receiving features of the following embodiments can be utilized in a WTRU, base station or both. 
     Although the following embodiments emphasize transmit diversity, the same principals can be applied to reception diversity. 
       FIG. 1  is a simplified block diagram of a diversity transmission system with directional antenna elements. A diversity transmitter  20  produces a signal or signals for transmission over an antenna array. The antenna array has antenna elements  22   1  to  22   N  that are capable of producing their own antenna beam or pattern  26   1  to  26   N . These beams/patterns  26   1  to  26   N  may differ in the azimuth, elevation or both. 
     As illustrated in  FIG. 1 , these different beams/patterns  26   1  to  26   N  result in the transmitted signals following different paths  28   1  to  28   N . Path  28   1  results from the transmission from antenna element  22   1  through beam/pattern  26   1  and being scattered off of scatterer  24   1 . Path  28   N  results from the transmission from antenna element  22   N  through beam/pattern  26   N  and being scattered off of scaterer  24   N . The different paths  28   1  to  28   N  are received by an antenna or antenna array  30  and processed by a receiver  32 . For simplicity,  FIG. 1  illustrates one path per beam/pattern. In practice, each beam/pattern results in various multipath scattering arrangements. 
     The unique beams/patterns formed by the antenna elements  22   1  to  22   N  can be produced by various techniques. One technique is to alter the physical arrangement of the elements to create the differing beams/patterns. To illustrate, some beams may be planar and some may have an uptilt. Additionally, metamaterials may be used for the antenna elements. These materials can be used to create highly directional antenna elements that may be oriented to “point away” from each other. Since the antenna patterns “point away” from one another, the signals transmitted by these antennas experience different channel conditions. The channel conditions are mainly determined by the scattering environment of the channel through which the signal travels. Signals in different scattering environments have different multipath fading patterns. As a result, the transmitted signals by each transmit antenna are received mutually uncorrelated. Such an arrangement is desirable for antenna arrays with little separation between the elements. These closely packed elements can be “pointed away” from one another to reduce or eliminate cross correlation. 
       FIG. 2  is a simplified diagram of a diversity transmission system utilizing beam/pattern forming. The diversity transmitter  20  produces a signal or signals for transmission over an antenna array. A beamformer/pattern former  34  is used to produce distinct beams/patterns over the array. As illustrated in  FIG. 2 , an array having N elements  36   1  to  36   N  may be used to form N−1 distinct beams/patterns  26   1  to  26   N−1 . These distinct beams/patterns allow for additional transmission diversity. The signals from these distinct beams/patterns  26   1  to  26   N−1  are received by an antenna or antenna array  30  of a receiver  32 . 
     As illustrated in  FIG. 2 , a first beam/pattern  26   1  directs the signal towards a scaterer  24   1  so that one path of the signal follow path  28   1 . Also, an N−1 beam  26   N−1  directs the signal towards a scaterer  24   N−1  so that one path of the signal follows path  28   N−1 . 
     One technique to generate the beams are as follows. First, a maximal set of low correlation antenna elements are identified by calculating correlation values between antennas based on received signals from the receiver on either base band or radio frequency (RF). The identification may be based on channel state information (CSI) feedback from the communication entity. The high correlation set is used to enhance the low correlation set via beamforming or other technique. 
     To further augment spatial diversity, reflector and isolators between antenna elements may be used to separated transmission and reception spatially. These reflectors and isolators may include the use of native geography, such as buildings. 
       FIG. 3  is a simplified diagram of a diversity transmission system utilizing polarization. The diversity transmitter  20  produces a signals or signals for transmission. An antenna mapper  38  maps these signals to appropriate polarized antennas  40   1  to  40   N  of an antenna array. Although various polarization techniques may be used, in  FIG. 3 , the polarization is simply illustrated by a “−” or “+” sign by each element  40   1  to  40   N . Although illustrated in the Figure as alternating polarization, the polarization may be done in a variety of manners. The polarization adds additional diversity to the transmissions, since the differing polarizations will typically result in different scattering patterns. Such a technique is desirable for arrays with little spatial diversity between the antenna elements, such as small footprint arrays. To illustrate, the use of an alternating polarization as illustrated in  FIG. 3  allows for the effective spatial separation between elements of a same polarization to be doubled, decreasing the correlation between antenna elements. The polarized transmissions are sent through the air interface  42  to an antenna or antenna array  30  of a receiver  32 . 
       FIG. 4  is a simplified diagram of a diversity transmission system utilizing polarization in an OFDM system. An OFDM processor  44  produces subcarriers for transmission. A subcarrier mapper  46  maps corresponding subcarriers to corresponding polarized element/elements  40   1  to  40   N  of the antenna array. The subcarrier mapper  46  maps subcarriers that are close to each other to antennas  401  to  40 N having different polarization. These polarized subcarriers are sent through the air interface  42  to an antenna or antenna array  30  of a receiver  32 . 
       FIG. 5  is an illustration of one possible assignment of subcarriers. As illustrated in  FIG. 5 , the subcarriers are divided into odd and even subcarriers with respect to their frequency. The odd subcarriers are illustrated with a narrow line and the even with a thick line. The odd subcarriers are sent over antenna TX 1  and the even over antenna TX 2 . Antenna TX 1  has a vertical polarization and TX 2  has a horizontal polarization. 
       FIG. 6  is a simplified diagram of a diversity transmission system using subcarrier grouping in an OFDM system. Correlation between antenna elements tends to be frequency dependent. As a result, the embodiment of  FIG. 6  groups subcarriers having a similar frequency and maps them to antennas based on the antenna correlation of the respective frequencies. 
     An OFDM processor  44  produces subcarriers for transmission. A subcarrier group mapper  48  maps subcarrier groups to a corresponding antenna/antennas  36   1  to  36   N . The mapping is based on antenna correlation information. Such information may be signal from the receiver or derived, such as by using channel reciprocity. The OFDM transmission is sent through the air interface  42  to an antenna or antenna array  30  and a receiver  32 . 
       FIG. 7  is an illustration of one possible assignment of subcarrier groups. The subcarriers are illustrated as horizontal line segments. The subcarriers are grouped as illustrated by the brackets “}”. The odd groups of subcarriers as illustrated with the thinner lines are transmitted over antenna TX 1  and the even group with the think line are transmitted over antenna TX 2 . 
       FIG. 8  is a simplified diagram of a diversity transmission system using trellis mapping. In the embodiment of  FIG. 8 , Trellis coding is used to map signals to antennas to reduce the likelihood of adjacent symbols being mapped to highly correlated antennas. Trellis coded modulation is traditionally used to increase the free distance in coding. The same principle is applied to increase the “correlation distance” between adjacent symbols when mapping onto antennas. The symbols are mapped to antenna elements with respect to a Trellis code and transmitted. 
     A diversity transmitter  20  produces symbols of a signal/signals for transmission. A trellis mapper  50  identifies the antennas having a high correlation and utilizes a trellis code to map adjacent symbols to uncorrelated antennas  36   1  to  36   N . The antenna correlation information may be signaled from the receiver or derived, such as by using channel reciprocity. The trellis mapped transmissions are sent through the air interface  42  to an antenna or antenna array  30  to a receiver  32 . 
       FIG. 9  is a simplified diagram of a diversity transmission system using time frequency multiplexing. A diversity transmitter  20  produces signals for transmission. These signals are time and/or frequency multiplexed by a time/frequency multiplexer (MUX) prior to transmission over the elements  36   1  to  36   N  of the antenna array. To illustrate, in the frequency domain, if the transmission and reception paths of two signals are separated spatially, they can be transmitted within the same frequency band or time slot. Alternately, these signals can partially overlap without degrading each other&#39;s performance significantly. The multiplexed signals/signals are sent through the air interface  42  to an antenna or antenna array  30  to a receiver  32 . 
     Although the embodiments of  FIGS. 1 through 9  are described separate from one another, they can be used in combination with one another. To illustrate, the time/frequency multiplexing of  FIG. 9  can be used with the directional elements of  FIG. 1 . Additionally, the following embodiments are generally described as relating to an entire antenna arrays. However, the described embodiments may be utilized between a subset of the antennas/antenna elements.