Abstract:
A wireless communication system base station uses a dual-polarized antenna array to receive two signal groups having orthogonal polarizations. In a preferred embodiment, a first signal group is received using a first sub-array of odd-numbered antenna elements, while a second signal group is received using a second sub-array of even-numbered antenna elements. Dipoles within the odd-numbered elements are selected so that they have a common polarization orthogonal to the dipoles selected in the even-numbered elements. The two signal groups thus have orthogonal polarizations. Base station signal processing hardware performs separate spatial channel estimation for these two groups, and uses the polarization diversity to improve performance in both the uplink and downlink. The use of polarization diversity allows the physical dimensions of the antenna array to be kept to a minimum.

Description:
This application is a combination-in-part of U.S. patent application Seer. No. 09/229,482 filed Jan. 13, 1999 now U.S. Pat No. 6,347,234, which is a continuation-in-part of U.S. patent application Ser. No. 08/929,638 filed Sep. 15, 1997, both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of wireless communications systems. More specifically, it relates to cellular communications system base stations that use adaptive antenna arrays to increase system performance. 
     BACKGROUND OF THE INVENTION 
     Wireless communication system base stations conventionally have multiple antennas for communicating with mobile transceivers. Normally, however, only one antenna is selected at one time for communicating with a given mobile. More recently, researchers have been investigating the use of adaptive (smart) antenna arrays at base stations. Using sophisticated signal processing, multiple antennas of the array can be used simultaneously for communicating with a given mobile. Such smart antenna arrays, however, generally require more antenna elements than normally exist on conventional antenna towers. Moreover, in order to provide sufficient channel estimation, beam forming, and spatial diversity, conventional adaptive antenna array techniques require that the physical dimensions of the antenna array be significantly larger than the antenna platforms of existing antenna towers. Unfortunately, increasing the size of the antenna array increases the cost in setting up and maintaining the antenna array. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a wireless communications base station is provided having a compact dual-polarized antenna array. The dual polarized antenna array is characterized by a small antenna cross-section, thereby making it very suitable for base stations located in urban areas where zoning is a major issue. Although the dual polarized antenna array is compact, it nonetheless provides sufficient diversity, especially in an urban environment where signal polarization is received in a highly random manner. Compared with spatial diversity, polarization diversity allows the use of a much smaller antenna array. The present invention provides techniques for implementing both polarization diversity and direction finding in a single antenna array. 
     According to the present invention, each antenna element of the array is dual-polarized, i.e., each antenna element has two orthogonal polarizations that can be independently selected. These dual-polarized antenna elements are adaptively controlled during transmission and reception to provide increased performance using a compact antenna array. 
     For signal reception, the antenna elements of the antenna array are divided into two groups, a first group and a second group. All antenna elements in the first group have a first polarization, and all antenna elements in the second group have a second polarization orthogonal to the first polarization. In other words, each antenna in the first group has a polarization orthogonal to each antenna in the second group. Since the first and second groups of antenna elements have orthogonal polarizations, reception polarization diversity can be advantageously used for direction finding and fading mitigation (fading diversity) in a single small antenna array. For direction finding, each group of antenna elements can be used independently, resulting in a first direction from the first group and a second direction from the second group. The first and second directions can then be combined to estimate the direction of arrival of received signals. For fading mitigation, signals can be received separately at the first and second antenna groups, then adaptively combined to mitigate the effects of fading. 
     For signal transmission, all the antenna elements in the antenna array are grouped together and set to have the same polarization. Because signals are then transmitted with the same polarization, the antenna gain for signal transmission is maximized. The common polarization can be set to either one of the two orthogonal polarizations. Alternatively, both polarizations can be simultaneously selected in each antenna element to produce a circular polarization for the signals transmitted from the antenna elements. 
     In addition to the above techniques, the polarizations of the antenna elements in the array can be selected adaptively in transmission or reception according to the specific signal polarization characteristics of each mobile. For example, in reception, the combination of antenna polarizations can be selected that maximizes the signal-to-interference ratio (SIR) for a given mobile. These polarization settings can then be used for transmission as well to improve reception at the mobile. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective illustration of a dual-polarized antenna array according to a preferred embodiment of the present invention. 
     FIG. 2 is a front view of a dual polarized antenna element according to a preferred embodiment of the present invention. 
     FIGS. 3A and 3B illustrate the division of the antenna array into two groups, a first group (shown in FIG. 3A) containing those elements with odd antenna element indices, and a second group (shown in FIG. 3B) containing those elements with even antenna element indices, according to a preferred embodiment of the invention. 
     FIG. 4 is a schematic diagram of the elements of the antenna array, illustrating that an odd group of elements is selected to have a first polarization, while an even group is selected to have a second polarization orthogonal to the first polarization. 
     FIG. 5 is a block diagram illustrating a base station comprising the dual-polarized antenna array and the method of using same, according to a preferred embodiment of the present invention. 
     FIG. 6 is a block diagram detailing the components of a multichannel RF front end shown in FIG.  5 . 
     FIG. 7 is a block diagram illustrating the details of a channel module shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the invention will now be described with reference to the figures. Although the following description and figures contain various specifics, these details are provided for the purpose of illustrating the general principles and techniques of the invention. Accordingly, the invention includes within its scope many variations and alterations of the following preferred embodiments. 
     FIG. 1 is a perspective illustration of a dual-polarized antenna array according to a preferred embodiment of the present invention. The array  10  comprises a collection of K antenna elements  12 . 1 ,  12 . 2 , . . . ,  12 .k . . . ,  12 .K arranged in a circle. The array shown in this particular example has twelve elements, i.e., K=12. Each antenna element in the circular array  10  is a dual-polarized antenna element having two independently selectable polarizations. The diameter of the circular array  10  depends on the number K of array elements and the spacing between the elements. Preferably, the spacing between adjacent antenna elements is about 0.5 to 1 wavelengths, and the number of array elements K is an even number in the range of 6 to 20. For example, a typical 12-element antenna array  10  designed for operation in the PCS frequency spectrum around 2 GHz preferably has a diameter of about 0.4 to 0.6 meters, and a preferred spacing between adjacent antennas of about 10 to 16 cm. 
     FIG. 2 is a front view of a dual polarized antenna element according to a preferred embodiment of the present invention. The antenna element contains a first set of N dipole antennas  14 . 1 ,  14 . 2 , . . . ,  14 .N having a first common polarization, and a second set of N dipole antennas  16 . 1 ,  16 . 2 , . . . ,  16 .N having a second common polarization, where the first polarization is orthogonal to the second polarization. The first and second sets of dipole antennas are paired, forming a set of N dual polarized dipole pairs ( 14 . 1  and  16 . 1 ,  14 . 2  and  16 . 2 , . . . ,  14 .N and  16 .N) which are mounted in a linear column, as shown. Each set of the two dipoles  14 .n and  16 .n has separate conductive leads (not shown), allowing them to be used independently for reception and transmission. The dipoles  14 .n and  16 .n are mounted with +45 degree and −45 degree orientation to the vertical, giving them a relative orientation of 90 degrees (orthogonal polarization). 
     Reverse Link and Channel Estimation 
     During signal reception (i.e., reverse link, or uplink), the antenna array is preferably used as follows. The K antenna elements are divided into two groups: a first (odd) group and a second (even) group. Each group thus has K/2 antenna elements. The first group (shown in FIG. 3A) contains those elements  12 . 1 ,  12 . 3 , . . . , 12 .K- 1  with odd antenna element indices k. The second group (shown in FIG. 3B) contains those elements  12 . 2 ,  12 , 4 , . . . ,  12 .K with even antenna element indices k. These groups form two circular sub-arrays. As shown in FIG. 4 (where the K antennas are arranged linearly for illustration purposes only), the odd group is selected to have a first polarization, while the even group is selected to have a second polarization orthogonal to the first polarization. For example, the odd group is set to have only the +45 degree dipoles in the antenna elements selected, and the even group is set to have only the −45 degree dipoles selected. Alternatively, the odd group can have the −45 degree dipoles selected, and the even group can have the +45 degree dipole group selected. 
     FIG. 5 is a block diagram illustrating a base station comprising the dual-polarized antenna array  10  described above. The antenna elements of antenna array  10  are connected to a multichannel RF front end  18 , which will be further described below in relation to FIG.  6 . The RF front end  18 , is connected to a reception (RX) interface module  20  and a transmission (TX) interface module  22 . A first data bus  24  connects the RX and TX interface modules  20  and  22  with a set of M channel modules  26 . 1 , . . . ,  26 .M. The number M of channel modules is proportional to the total number of concurrent users that the base station is capable of serving. In current implementations, M is preferably 32, 64, or 128. These modules, in turn, are connected via a second data bus  28  to a BTS controller  30 . As will be described in more detail below in relation to FIG. 7, the channel modules perform beam forming and channel estimation functions. 
     Signals detected by the antenna array  10  are processed through RF front end  18  and RX interface module  20 , after which they enter channel modules  26 . 1 , . . . ,  26 .M. During this processing, the signals remain separated into two groups, corresponding to the even and odd antenna sub-arrays selected for reception as described above. The two signal groups, corresponding to two orthogonal polarizations, are then separately processed by the channel modules. The channel modules perform spatial processing to estimate spatial channel properties of the two groups, such as the uplink spatial beam for a given mobile. Two sets of spatial information are thus derived from the two groups of orthogonally polarized signals. These two sets of information are then used to provide improved reception. 
     FIG. 6 is a block diagram detailing the components of the multichannel RF front end  18 . A set of K signals from the antenna array  10  are modified by a reception (RX) compensation block  32 , and then processed in parallel by a set of K amplifiers  34 . 1 , . . . ,  34 .K and K receivers  36 . 1 , . . . ,  36 .K. The signal from each antenna element is separately processed by these parallel chains so that the relative phase information between the K signals is preserved. The resulting K signals are then made available to the RX interface module  20  via a receiver bus  38 . 
     Adaptive antenna array techniques in transmit/receive base stations require simultaneous reception and transmission of signal replicas through multiple channels. This function requires known receiving and transmitting channel behavior, namely, known amplitude and phase response. Analog parts of transmitting and receiving channels are typically subject to non-predictable changes due to various causes: temperature change, aging of components, receive and transmit power fluctuation, etc. Hence, a method to measure the phase and amplitude response of the channels for both receive and transmit paths is preferred. This transceiver compensation is accomplished in the present invention by incorporating frequency converter modules (FCMs  60  and  66 , operating together with transmitter  62  and receiver  64 , respectively) in the multichannel RF front end  18 . One technique for accomplishing such transceiver compensations is described in detail in U.S. patent application Ser. No. 08/929,638 filed Sep. 15, 1997. 
     After passing through the RX interface module  20 , the K received signals are sent to a set of M channel modules  26 . 1 , . . . ,  26 .M for spatial processing. FIG. 7 is a block diagram illustrating the details of a channel module  26 .m. Each such module contains a set of J beam director ASICs  40 . 1 , . . . ,  40 .J and a corresponding set of J CSM processing blocks  42 . 1 , . . . ,  42 .J. The value of J corresponds to the total number of concurrent users served by one channel module. The product of M and J (i.e., MJ) corresponds to the total number of concurrent users served by one base station, and is typically 32, 64, or 128. The beam directors perform beam-forming and channel-estimation functions, which are described in detail in U.S. patent application Ser. No. 09/229,482 filed Jan. 13, 1999 and U.S. patent application Ser. No. 08/929,638 filed Sep. 15, 1997. These spatial correlation (SPC) processes are separately and jointly performed on the two groups of signals from the two antenna groups. The results from the two groups are used to form AOA odd  and AOA even  histograms that are then used to determine the mean AOA and multipath for each polarization group. The AOA histogram can be computed either by counting the number of occurrences of the output from the channel estimator or by accumulating the magnitude of the output from the channel estimator. Magnitude accumulation is the preferred method. The two AOA histograms for the two groups can be combined linearly to determine the direction and width of the forward link beam. The ratio of the AOA odd  histogram peak power to the AOA even  histogram peak power is used to determine the preferred transmitting signal polarization. 
     Using the AOA histograms of the two signal groups, six reverse link beams can be formed in a beam director ASIC following these rules: 
     1. Select the two strongest peaks in AOA odd  histogram (i.e., reject the second highest peak if it is located in the first side-lobe angle distance of the first highest peak, in which case the third highest peak is selected instead; reject the third highest peak if it is located in the first side-lobe angle distance of the first highest peak, in which case the forth highest peak is chosen instead). 
     2. Form a single sector-wide beam and two narrow beams using the odd-polarized sub-array in the direction of the selected AOA peaks. 
     3. Repeat for the AOA even  histogram. 
     The narrow spatial beams provide for antenna gain, while the wide beams are used in case of failure in estimating the uplink channel. The failure may happen, for example, when the mobile is too close to the base station, when there is strong signal interference, or when there are other dynamic changes in the channels. The wide beam thus provides a signal even when the narrow beams are unintentionally misdirected. 
     U.S. patent application Ser. No. 09/229,482 filed Jan. 13, 1999 and U.S. patent application Ser. No. 08/929,638 filed Sep. 15, 1997 describe AOA histogram analysis techniques in detail, as well as the operation of BTS controller  30 . 
     Forward Link 
     In transmission (i.e., forward link or downlink) operation, all the antenna elements are preferably used as one single group, i.e., all K antennas are selected to have the same polarization. All the transmitting elements thus use either the +45 degree polarized dipoles, the −45 degree polarized dipoles, or both the +45 and −45 degree dipole pair simultaneously to effectively provide a single circular polarized or vertically polarized element. A set of K downlink signals generated by the channel modules pass through the TX interface module  22  and are made available on a transmission data bus  44  of the multichannel RF front end  18 , as shown in FIG.  6 . These K signals are processed in parallel by a set of K transmitters  46 . 1 , . . . ,  46 .K and corresponding linear power amplifiers (LPAs)  20   48 . 1 , . . . ,  48 .K. The signals then are modified by a transmission compensation circuit block  50 . The FCM and RX RF are described above in relation to the reverse link and channel estimation.The downlink operation is otherwise identical to that described in U.S. patent application Ser. No. 09/229,482 filed Jan. 13, 1999. This arrangement provides the same performance as a vertically polarized array since the polarization of the mobile antenna and the electromagnetic field near it are typically random. 
     In an alternative embodiment of the downlink operation, rather than using a single polarization for all antenna elements in the array, the system adaptively adjusts the polarizations of the various antenna elements in the array. This adjustment can be accomplished by feeding both polarization dipoles (including both +45 and −45 degree polarization dipoles) in each antenna element to the transmitters. This operation requires doubling the number of transmitters to 2K. If signals arriving at base station have distinct polarization characteristics, beam forming coefficients controlling both +45 and −45 degree polarization dipoles can be adjusted so that the transmitted signals have the same polarization characteristics as the received signals. One way to estimate the polarization characteristics of the received signals is by computing the ratio of the AOA odd  histogram peak power to the AOA even  histogram peak power. An similar adaptive technique can be used in the uplink operation as well. In particular, the 2K receiver chains can be used simultaneously to receive from both polarizations of the K antennas, in analogy to the technique described above for transmission. 
     Since the diversity here is provided by polarization, the antenna array does not have to be large since very narrow beams are not required. The size of the array can be determined to provide sufficient beam shape with low side lobes. Since the beam shape quality is also desired for the downlink (because it allows for good beam shaping flexibility), the same array can be used for both uplink and downlink. The downlink can use either one of the polarized sub-arrays, or a combination of the sub-arrays (using hybrids to form circular polarized beams or linearly combined to form vertically polarized beams, etc.). This arrangement results in a much more compact antenna array.