Abstract:
A smart antenna calibration system is disclosed for calibrating an antenna array having a plurality of antennas. Each antenna has a calibration coupler for providing a monitoring signal indicative of a signal passing through a transceiver associated thereof, and a processing unit including at least one signal splitter that splits at least one monitoring signal and a combiner array comprising one or more combiners for combining at least two split monitoring signals from first and second antennas to produce a first combined signal representing an in-phase sum and a second combined signal representing a quadrature sum. A power detector is in communication with the processing unit, which is configured to estimate a power level of the signal passing each of the first and second antennas and the in-phase power and quadrature power of the in-phase and quadrature sums for determining a phase difference of the signal on the antennas.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is related to, and claims the benefits of U.S. Provisional Patent Application Ser. No. 60/606,534, which was filed on Aug. 31, 2004. 

   BACKGROUND 
   The present invention relates generally to wireless communications systems, and more particularly to the calibration of one or more smart antennas for wireless communications to ensure the performance and signal quality of wireless communications systems. 
   Smart antenna technology can significantly improve performance and economics of wireless communications systems. It can enable PCS, cellular, and wireless local loop operators to gain significant increases in signal quality, capacity, and coverage area. Operators often require different combinations of these advantages at different times. Systems that offer the most flexibility in terms of configuration and upgradeability are often the most cost-effective, long-term solutions. 
   Smart antennas, also known as adaptive antennas, have a distinct advantage in modern wireless communications systems. A smart antenna is capable of beam forming or directing a beam of radiated energy toward a desired receiver. The dual purpose of a system deploying one or more smart antennas in an antenna array is to augment the signal quality of the radio based system through a more focused transmission of radio signals without reducing signal capacity. One advantage of this capability is to direct more power to the desired receiver. Another advantage of using smart antenna arrays for beam forming is the ability to reduce the transmitted power due to the more directional nature of smart antennas. Finally, a system deploying smart antenna arrays typically improves the channel conditions, such as a signal-to-interference ratio of the received signal, between any smart antenna array and the desired transceiver. 
   In order to accurately form a desired beam, the amplitude and phase of each component of the adaptive array sub-system should be known with a reasonable degree of precision. Un-compensated differences in gain and phase in a system with smart antennas degrade the antennas&#39; performance. Ideally, the gain and phase characteristics are predetermined at the time of manufacture and are environment-invariant. However, in reality these characteristics vary over time and different environments. Thus, a method and apparatus to calibrate the antenna array is needed. Specifically, a method to determine variations in gain and phase of a system having one or more smart antennas and a method to compensate for those variations are needed. 
   One conventional calibration method is the “remote subscriber/transponder assisted calibration”. This approach requires the assistance from a remote subscriber/transponder unit with a predetermined location. A set of N orthogonal calibration signals needs to be generated and then transmitted from each antenna to allow the subscriber/transponder to calculate the phase and power of each signal from each antenna, where N is the number of the antennas in an antenna array. Furthermore, the subscriber/transponder shall be placed at a line of sight (LOS) location to the antenna array, otherwise the air channel effects due to multi-path may significantly degrade calibration accuracy. 
   Another conventional calibration method is the “on-site calibration with a collocated calibration unit”. This approach requires a special collocated calibration unit and involves generating and injecting special calibration signals into the transmitter and receiver chains. The collocated calibration must have the ability to compute the phases and powers of multiple signals (calibration signals). 
   Both conventional approaches require a special calibration period during which special calibration signals are generated and injected or transmitted to the calibration unit. This causes a disruption to the normal system operation. Moreover, both approaches require a special calibration unit or subscriber unit/transponder that has the capability to detect simultaneously both the phase and power of multiple calibration signals. Both the said disruption and the need for special equipment can be prohibitively costly in certain wireless communications system designs. 
   Therefore, desirable in the art of smart antenna array designs are improved array calibration systems and methods that ensure the performance and signal quality of wireless communications systems. 
   SUMMARY 
   In view of the foregoing, the following provides a system and method ensuring the performance and signal quality of wireless communications systems with smart antenna arrays. 
   In one embodiment, a smart antenna calibration system is disclosed for calibrating an antenna array having a plurality of antennas. Each antenna has a calibration coupler for providing a monitoring signal indicative of a signal passing through a transceiver associated thereof, and a processing unit including at least one signal splitter that splits at least one of the monitoring signals and a combiner array comprising one or more combiners for combining at least two of the split monitoring signals from a first and a second antennas to produce a first combined signal representing an in-phase sum and a second combined signal representing a quadrature sum thereof. A power detector is in communication with the processing unit, which is configured to estimate a power level of the signal passing each of the first and second antennas and the in-phase power and quadrature power of the in-phase and quadrature sums for determining a phase difference of the signal on the two antennas. The power and phase differences can be used to calibrate one or more antennas in the antenna array. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  presents a conventional antenna array. 
       FIG. 2  presents a conventional quadrature combiner. 
       FIG. 3  presents a calibration system for a new smart antenna array in accordance with one embodiment of the present invention. 
       FIG. 4  presents a new smart antenna array with calibration couplers in accordance with one embodiment of the present invention. 
       FIG. 5  presents a RF routing and processing structure in accordance with one embodiment of the present invention. 
       FIG. 6  presents another RF routing and processing structure in accordance with one embodiment of the present invention. 
       FIGS. 7A and 7B  present flow charts summarizing the calibration method in accordance with one embodiment of the present invention. 
   

   DESCRIPTION 
   The following will provide a detailed description of improved array calibration systems and methods that ensure the performance and signal quality of wireless communications systems. 
     FIG. 1  presents a conventional antenna array  100 . For simplicity, an array having four antennas is presented. Each of the four antennas (labeled “a 1 ”, “a 2 ”, “a 3 ” and “a 4 ”) receives a transmit (Tx) input from a transceiver  102  and releases a receive (Rx) output to the transceiver  102 . The antenna gain and phase characteristics are determined at the time of manufacture. However, due to both time and environmental effects, the antenna gain and phase characteristics may change and the antennas may need to be tested. In addition, additional calibration may need to be performed in the field. Since the gain and phase of each antenna may vary significantly from the others, systems and methods need to be developed to economically perform the antenna test and calibration without disrupting normal communications traffic. 
     FIG. 2  presents a conventional quadrature combiner  200 . Based on  FIG. 2 , it is understood by those skilled in the art that the conventional quadrature combiner  200  is a 2×2 device consisting of three 2:1 combiners/splitters  202 ,  204 , and  206 , and one 90-degree hybrid combiner/splitter  208 . An input signal Va 1  is split into two equal signals  210  and  212  that are in phase. Also, an input signal Va 2  is split into two signals  214  and  216 , both equal in power, but with the signal  214  in phase with the signals  210  and  212 , and with the signal  216  in quadrature (90 degrees out of phase) with the signals  210 ,  212 , and  214 . An output signal Vb 1  of the conventional quadrature combiner  200  is the in-phase sum of the signals  210  and  214 , while an output signal Vb 2  of the conventional quadrature combiner  200  is the quadrature sum, of the signals  212  and  216 . In mathematical terms, the output signals Vb 1  and Vb 2  are represented as follows:
   Vb 1= A *( Va 1+ Va 2)   Vb 2= A*[Va 1+ Va 2*exp( j 90°)] 
where Va 1 , Va 2 , Vb 1 , and Vb 2  are complex voltages containing both power and phase. A is a constant determined by predetermined hardware calibration.
 
   It is noted that the detected powers Pb 1  and Pb 2  that are respectively associated with Vb 1  and Vb 2  are defined by the following power equations: 
                   P   b1     =       A   I     ·     {              V   a2          2     +            V   a1          2     +     2   ·          V   a2          ·          V   a1          ·     cos   ⁡     (       ϕ   a2     -     ϕ   a1       )           }                     P   b2     =       A   Q     ·     {              V   a2          2     +            V   a1          2     +     2   ·          V   a2          ·          V   a1          ·     cos   ⁡     (       90   ⁢   °     +     ϕ   a2     -     ϕ   a1       )           }                           ⁢     =       A   Q     ·     {              V   a2          2     +            V   a1          2     +     2   ·          V   a2          ·          V   a1          ·     sin   ⁡     (       ϕ   a2     -     ϕ   a1       )           }                     
where φa 1  and φa 2  are the phases of Va 1  and Va 2 . A 1 , which is the in-phase gain, and A Q , which is the quadrature gain, are constants that are determined by predetermined hardware calibration, and |V a1 | and |V a2 | are respective power levels for the input signals.
 
   One aspect of this invention simplifies smart antenna calibration by using the power equations defined above to calculate the phase difference between two antennas (φa 1 −φa 2 ). The calculation is made simple when the input signal power to the quadrature combiner  200  |V a1 | and |V a2 | can be determined by simple derivation based on some predetermined calibration data and measurements of P b1  and P b2 . Therefore, the power gain and phase for each antenna of the array can be compensated using the measured power and calculated phase values for each of the antennas. This is significantly different from conventional calibration methods in which signals at the carrier frequency have to be converted (e.g., demodulated by a demodulation module) to base band frequency for further processing, and both the power and phase detection have to be carried out separately. The present invention derives the phase difference through the power measurement and additional calculations based thereon, it is a “scalar detection” and avoids a complicated “vector detection”. 
     FIG. 3  presents a calibration system  300  for a new smart antenna array in accordance with one embodiment of the present invention. It includes a new smart antenna array  400  with new antenna signal couplers, a base transceiver station (BTS)  302 , a power detector (PD)  304  connected, via a connection  306 , to a RF signal routing and processing structure  308 . It is understood that the connection  306  may be a wired or a wireless connection. The PD  304  can be a standard mobile terminal or any other custom design that has a power detection and output power control capability. 
   In this embodiment, although an array having four antennas is used for simplicity, it is understood that a different number of antennas may be included in the array without deviating from the spirit of this invention. The smart antenna array  400  is similar to the conventional antenna array  100  except that the smart antenna array  400  has a plurality of calibration couplers, to be further described in  FIG. 4 , added in series between each of the antennas (labeled “a 1 ”, “a 2 ”, “a 3 ” and “a 4 ”), and its associated transceiver. The calibration couplers allow the transmitted signal going to, or the receive signal coming from, each antenna to be monitored without interrupting the normal communications traffic. Monitoring signals H 1 , H 2 , H 3 , and H 4  of the smart antenna array  400  are sent to the RF signal routing and processing structure  308 , to be further monitored by the PD  304  via the connection  306  to derive the in-phase and quadrature combined powers for each antenna. This information is then sent to the BTS  302  for the calculation of the gain and phase values for each of the four antennas to be used for calibration of the smart antenna array  400 . 
   The RF signal routing and processing structure  308  and the PD  304  utilize each of the monitoring signals H 1 , H 2 , H 3 , and H 4  to estimate the downlink transmitted power and calculate the phase difference between the antennas by using the above-mentioned power equations. The estimated power and calculated phase values for each antenna is then sent to and used by the BTS  302  to calibrate each antenna for consistent power and phase results. 
     FIG. 4  presents a new smart antenna array  400  with calibration couplers in accordance with one embodiment of the present invention. As shown, the new smart antenna array  400  is similar to the conventional antenna array  100 , except that calibration couplers  402  are added between each of the antennas and its corresponding transceiver  102 . The calibration couplers  402  allow a portion of the transmitted or received signals to be monitored without interrupting the normal communications traffic. In this manner, a known scaled version of the signals can be utilized for the measurement and calibration methodology used in this invention. The calibration couplers  402  should be placed physically as close as possible to each of the antennas (labeled “a 1 ”, “a 2 ”, “a 3 ” and “a 4 ”). The monitoring signals H 1 , H 2 , H 3 , and H 4  are then routed to a RF signal routing and processing structure as previously shown in  FIG. 3 . 
     FIG. 5  presents a RF routing and processing structure  500  in accordance with one embodiment of the present invention. It is understood that the RF routing and processing structure  500  can be implemented into the calibration system  300  as the RF routing and processing structure  308  of  FIG. 3 . The RF routing and processing structure  500  includes a first switch array  502  having RF switches S 1 , S 2 , S 3 , and S 4 , a splitter array  504  having four 2:1 splitters, a combiner array  506  having three quadrature combiners  512 ,  514 , and  516 , a second switch array  508  having RF switches W 1 , W 2 , W 3 , W 4 , W 5 , and W 6  for switching output power signals T 1 , T 2 , T 3 , T 4 , T 5 , and T 6 , and a 6:1 power combiner module  510 . When implemented into the calibration system  300 , the RF routing and processing structure  500  is connected to the PD  304 , not shown, via the connection  306 . The PD  304  (as shown in  FIG. 3 ) can be a mobile receiver having a received signal strength indicator (RSSI) power detection capability used for transmitter calibration. The PD  304 , not shown, may also need the capability to set its output power at a predetermined level if the antenna array receiver chain requires calibration at a predetermined level. The RF signal routing and processing structure  500  further receives from the smart antenna array  400  the monitoring signals H 1 , H 2 , H 3 , and H 4 . The key components in the RF routing/process structure  500  are the quadrature combiners  512 ,  514 , and  516 , where two signals are combined in phase and in quadrature (e.g., 90 degrees out of phase) simultaneously. 
   The RF routing and processing structure  500  routes and processes selected RF signals on the monitoring signals H 1 , H 2 , H 3 , and H 4  through the first switch array  502 , the splitter array  504  and the combiner array  506  to allow the PD.  304  to detect the in-phase/in-quadrature (I/Q) power levels. The detection of output powers and phase differences between antennas transmitter chains may be periodical or activated by commands from BTS  302 . In the case of periodical calibration, the PD  304  detects the powers between antennas, and sends the information back to BTS  302  periodically based on a certain protocol. When PD  304  sends back the transmitter information back, it could go through normal communication protocol. Through the uplink signal that used by the PD  304  to send back the transmitter information, BTS  302  will obtain the array receiver chain calibration data. 
   The phase levels between the antennas can then be calculated from the combined I/Q power levels using the above-mentioned power equations. For example, the phase differences can be calculated as follows. When the input powers, such as |V a1 | 2  and |V a2 | 2 , are known by measurement, the power equations will determine the phase difference between two antennas. 
   For example, a phase equation embodying the phase difference between antenna a 1  and antenna a 2  is as follows: 
   
     
       
         
           
             
               ϕ 
               a2 
             
             - 
             
               ϕ 
               a1 
             
           
           = 
           
             
               ± 
               a 
             
             ⁢ 
             
                 
             
             ⁢ 
             cos 
             ⁢ 
             
               { 
               
                 
                   
                     P 
                     b1 
                   
                   - 
                   
                     
                       A 
                       I 
                     
                     · 
                     
                       ( 
                       
                         
                           
                              
                             
                               V 
                               a2 
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               V 
                               a1 
                             
                              
                           
                           2 
                         
                       
                       ) 
                     
                   
                 
                 
                   2 
                   · 
                   
                     A 
                     I 
                   
                   · 
                   
                      
                     
                       V 
                       a2 
                     
                      
                   
                   · 
                   
                      
                     
                       V 
                       a1 
                     
                      
                   
                 
               
               } 
             
           
         
       
     
   
   The ambiguity of the sign in the phase equation can be removed by using a checking equation, as shown as follows, for checking the sign of sin(φa 2 -φa 1 ) obtained from the second of the power equations: 
   
     
       
         
           
             sin 
             ⁡ 
             
               ( 
               
                 
                   ϕ 
                   a2 
                 
                 - 
                 
                   ϕ 
                   a1 
                 
               
               ) 
             
           
           = 
           
             
               
                 P 
                 b2 
               
               - 
               
                 
                   A 
                   Q 
                 
                 · 
                 
                   ( 
                   
                     
                       
                          
                         
                           V 
                           a2 
                         
                          
                       
                       2 
                     
                     + 
                     
                       
                          
                         
                           V 
                           a1 
                         
                          
                       
                       2 
                     
                   
                   ) 
                 
               
             
             
               2 
               · 
               
                 A 
                 Q 
               
               · 
               
                  
                 
                   V 
                   a2 
                 
                  
               
               · 
               
                  
                 
                   V 
                   a1 
                 
                  
               
             
           
         
       
     
   
   If sin(φa 2 -φa 1 ) given by the checking equation is greater than zero, the plus sign is taken in the phase equation. Otherwise, the negative sign is taken. 
   With a similar procedure, the phase difference between antennas a 2  and a 3 , and between antennas a 3  and a 4  can be calculated. Where Δφ a2     —     1  is the phase difference between antennas a 2  and a 1 , Δφ a3     —     2  is the phase difference between antennas a 3  and a 2 , and Δφ a4     —     3  the phase difference between antennas a 4  and a 3 . In this embodiment, if antenna a 1  is to be used as a reference antenna, the phase difference between any other ones and a 1  can be derived. For example, if the phase difference Δφ a2     —     1  between a 1  and a 2  is known, and the phase difference Δφ a3     —     2  between a 2  and a 3  is known, a simple mathematical summation should render the phase difference between a 1  and a 3 . 
     FIG. 6  presents another RF routing and processing structure  600  in accordance with one embodiment of the present invention. The RF routing and processing structure  600  is similar to the RF routing and processing structure  500 , except for the interconnections of the quadrature combiner inputs. It is understood that the RF routing and processing structure  600  can be implemented into the calibration system  300  as the RF routing and processing structure  308  of  FIG. 3 . In the structure  600 , each of the quadrature combiners  512 ,  514 , and  516  receives their in-phase inputs from a 3:1 splitter  601  in a splitter array  602 . Because the 3:1 splitter  601  splits the monitoring signal H 1 , which in turn comes from the antenna a 1 , the antenna a 1  is thus utilized as the reference antenna, while all other antenna values are referenced to the antenna a 1 . Also, three attenuators  604  in the splitter array  602  also send one or more signals to the quadrature combiners  512 ,  514 , and  516  to maintain consistent signal paths for all four paths (including both downlink and uplink calibration paths) to improve the calibration accuracy. It is understood that these attenuators  604  are optional, and that each of these attenuators  604  exhibits attenuation characteristics similar to that of the 3:1 splitter  601 . As such, the attenuated signals can also be viewed as the split monitoring signals. The RF routing and processing structure  600 , when implemented to the calibration system  300 , is further understood to yield calibration performance characteristics and advantages similar to when the calibration system  300  is implemented with the RF routing and processing structure  500 . 
     FIG. 7A  presents a process flow  700  for a calibration method to derive the necessary calibration data according to one embodiment of the present invention. The flow  700  begins at a step  702 , where one or more smart antenna arrays are pre-calibrated to eliminate the hardware caused variations. This process can be done in the manufacture&#39;s factory or lab before the antenna array is deployed. Then the flow  700  proceeds to a step  704 , where a transmit calibration process is performed to determine both the transmit path loss and phase results. Alternatively, the flow  700  can also proceed to a step  706 , where a receive calibration process is performed to determine both the receive path loss and phase results. It is understood that the transmit path calibration and the receive path calibration are independent and they don&#39;t have to be performed together. 
   With reference to  FIGS. 3 ,  4  and  7 A, the step  702  is necessary to eliminate or compensate the undesired power and phase variations caused by the calibration hardware such as the calibration couplers  402  and the RF signal routing and processing structure  308 . For the precalibration, first, one can inject a calibration signal with a known power and phase at H 1  and detect the power and phase of the output at the output  306  without being interfered by other sources of signals from H 2 , H 3 , or H 4  (e.g., by only turning on switch W 1 ). This will determine the power and phase variations caused by the calibration hardware between H 1  and W 1 . 
   Then, one could detect the power and phase of the output  306  with only one selected switch among W 2 , W 3 , and W 4  turned on sequentially and by injecting the same calibration signal correspondingly at H 2 , H 3  or H 4 . By detecting the power and phase of the output at the output  306  with the known switch W 1 , W 2 , W 3 , W 4  on, the power and phase variations caused by the calibration hardware through these routes can be identified. Since the calibration signal fed into H 1 –H 4  are known, the pre-calibration of the RF signal routing and processing structure  308  also generates the A 1 , or the in-phase gain, and A Q , or the quadrature gain, which are needed by the power equations and the phase equation to calculate the antenna phase difference. Collectively, the information obtained is referred to as the pre-calibration data, and will be provided to and stored in the BTS  302  for removing the power loss and phase variations caused by the calibration hardware from the final calibration data in the calibration process. 
   In the step  704 , the transmit path calibration is conducted. The PD  304  monitors the down link power levels of the output power signals T 1  through T 6  and phase differences among different antennas from the same antenna array. The calibration includes two parts, the power detection and the phase difference detection parts. For the power detection, the PD  304  monitors a non-traffic channel such as a pilot channel, paging channel, access channel, or any other physical channel that maintains a constant power over time. The output powers of the antennas are estimated based on measurements of the power levels detected by the power detector (which will be described below). Once the antenna output powers P 1 , P 2 , P 3 , and P 4  corresponding to the antennas a 1 , a 2 , a 3 , and a 4  are known, the phase difference estimate can be executed. After both the corresponding in-phase and quadrature powers (P 1  and P Q ) are measured, the phase differences between the antennas can be mathematically determined. 
   In  FIG. 7B , the step  704  is further broken down into three steps: a step  708  for obtaining antenna output power values, a step  710  for obtaining the in-phase and quadrature power values, and a step  712  for obtaining the phase difference using the power values calculated in the steps  708  and  710 . 
   It is now assumed that either the structure  500  or the structure  600  is implemented into the calibration system  300 . With reference to  FIGS. 3 to 7B , and in the step  708 , the output power measurement of the antenna a 1  is initialized by turning on the switches S 1  and W 1 . In this case, only the antenna a 1  is being analyzed. The PD  304  detects the monitored output power signal T 1  after it passes through the power combiner module  510 . The output power of the antenna a 1  is then calculated from the equation:
 
 P   1   =C   1   *P   T1 
 
where P 1  is the transmit output power of the antenna a 1 , P T1  is the output power signal T 1  monitored by the PD  304  and C 1  is a constant. C 1  represents the influence of the calibration system including the calibration coupler  402  associated to the antenna a 1  and the RF signal routing and processing structure  308  which may comprise factors such as the coupling coefficient (loss) of the calibration coupler  402  corresponding to the antenna a 1 , the loss between the monitoring signal H 1  and the output power signal T 1 , the loss of the combiner module  510 , the power ratio between the signal power on the monitored channel that is used for power detection by the PD  304  and the total transmitted power of the antenna a 1 . It is understood that these factors can all be determined in the pre-calibration process mentioned above or are known to the calibration system. For example, the coupling coefficient (loss) of the calibration coupler  402  corresponding to the antenna a 1 , the loss between the monitoring signal H 1  and the output power signal T 1 , and the loss of the combiner module  510  can be obtained in the pre-calibration step  702 , while the power ratio is pre-set and known to the calibration system. For example, if the coupling coefficient is 20 dB, the loss between H 1  and T 1  is 32 dB, the loss of the 6:1 combiner module  510  is 8 dB, and the power ratio of the transmitted power to the monitored power is 20%, then the constant C 1  will be equal to:
 
 C   1 =20+32+8+10*log10(1/0.2)(dB).
 
   The power detection and calculation processes for the antennas a 2 , a 3  and a 4  are similar. The antenna a 2  output power is derived by first turning on switches S 2  and W 3  only, after detecting the monitored output power signal T 3  by the PD  304 , and the antenna a 2  output power can be calculated. Similarly, the antenna a 3  output power is derived by first turning on switches S 3  and W 5  only. After detecting the monitored output power signal T 5  by the PD  304 , the antenna a 3  output power can be calculated. The similar process will apply to calculate the output power of the antenna a 4 . After first turning on switches S 4  and W 5  only, the monitored output power signal T 5  is detected by the PD  304 , and the antenna a 4  output power is calculated. 
   The step  712  for the phase difference determination depends on step  710  which is performed to determine the combined in-phase and quadrature signals from each pair of antennas. For example, in order to analyze the in-phase and quadrature signals from antennas a 1  and a 2 , the switches S 1 , S 2 , and W 1  are turned on. Now, T 1  represents the combined signal. The PD  304  first detects the monitored in-phase power (P I ) of the combined signal through the power combiner module  510 . While switches S 1  and S 2  remain on, the switch W 1  is turned off and the switch W 2  is turned on. The PD  304  then detects the monitored quadrature power (P Q ) of the combined signal. The phase difference between the antennas a 2  and a 1  can then be calculated by utilizing the following equations, which are just another representation of the power equations:
 
 P   I   =A   I   ·{P   1   +P   2 +2·√{square root over ( P   1 )}·√{square root over ( P   2 )}·cos(φ 2 −φ 1 )}
 
 P   Q   =A   Q   ·{P   1   +P   2 +2·√{square root over ( P   1 )}·√{square root over ( P   2 )}·sin(φ 2 −φ 1 )}
 
where P I  is the in-phase and P Q  is the quadrature power of the combined signal, which have just been measured, P 1  and P 2  are the antenna a 1  and antenna a 2  output powers calculated previously in step  708 , and A I  and A Q  are constants that are determined in the pre-calibration, which depend upon the coupling coefficients of the calibration couplers, the losses between H 1  to T 1  and H 2  to T 2 , and the loss of the combiner module  510 . Since the only unknown in these two equations is the phase difference, and there are two equations available for determination, the phase difference between a 1  and a 2  can then be obtained by solving any of these two equations.
 
   The phase difference between the antennas a 3  and a 2 , as well as between the antennas a 4  and a 3  can then be calculated by utilizing the same process. For example, the phase difference between the antennas a 3  and a 2  can be derived by first turning on switches S 2 , S 3  and W 3 , when the PD  304  detects the in-phase power (P I ) of the output power signal T 3 . While switches S 2  and S 3  are kept on, the switch W 3  is turned off and the switch W 4  is turned on. The PD  304  then detects the monitored quadrature power (P Q ) of the output power signal T 4 . The phase difference between the antennas a 3  and a 2  can then be calculated. 
   Similarly, the phase difference between the antennas a 4  and a 3  can be derived by first turning on switches S 3 , S 4  and W 5 , when the PD  304  detects the in-phase power (P I ) of the output power signal T 5 . While the switches S 3  and S 4  are kept on, the switch W 5  is turned off and the switch W 6  is turned on. The PD  304  then detects the monitored quadrature power (P Q ) of the output power signal T 6 . The phase difference between the antennas a 4  and a 3  can then be calculated. 
   It is preferred that the power detection of any two antennas be performed followed by the phase difference derivation of the same two antennas. For example, the powers of the antennas a 1  and a 2  could be detected first, and then the phase difference between antennas a 1  and a 2  could be derived. After that, the derivation of the antennas a 3  and a 2  power and phase difference is carried out. However, it is understood that it is possible to perform the power detection of all antennas followed by the phase difference derivation of each antenna pair. It is also understood that usually the BTS transmits the same signals through different antennas in the antenna array, but if these signals has power or phase variation to start with, the information should be passed on the calibration system so that these factors are considered and removed from the calibration process. 
   In the step  706 , the receive path calibration is conducted. It is basically the same as the transmit path calibration except that it examines the calibration information for the receive path. The step  706  can also be divided in to steps  708 ,  710 , and  712 . For example, the process can be initiated by sending a calibration signal to the antenna array  400 . An in-phase calibration signal can be sent to all the antennas simultaneously. The calibration signal can be sent by the PD  304  or some other signal source and could be a normal traffic signal, access signal, or other uplink signal. When the smart antenna array  400  receives the signal, the calibration couplers  402  will generate the monitoring signal H 1 , H 2 , H 3 , and H 4 , which are representative copies of the original calibration signals. Each of these signals will represent the antenna power and phase characteristics. Like the transmit path calibration, the BTS  302  will calculate the phase difference based on power measurements. 
   The power received by the receiver chains is a function of the PD  304  transmitted signal power and the transmission loss between the PD  304  output and the antenna input. For example, the level of the power received by the antenna a 1  receiver (P R1 ) is calculated by the equation:
 
 P   R1   =C   1   *P   ST 
 
where P ST  is the PD  304  output power, and C 1  is a constant representing the losses between the PD output port and the input port of the antenna a 1 .
 
   If it is required that each of the antenna array receiver chains be calibrated at a predetermined input signal level, the PD  304  or any other device that transmitting the signal must have the capability to set its output power at a pre-determined level. A power module with the capability to map its output power with an internal parameter, such as an automatic gain control (AGC), can be utilized. With this mapping capability, the power module can set the internal parameter through the AGC to an appropriate value to generate the desired output power from the PD  304 . 
   Further, the calibration can be activated on an as-needed basis or programmed to happen periodically. Both the BTS and the PD can initiate the process as long as both have communicated to cooperate in the calibration process. For example, the PD  304  listens to a non-traffic channel such as a paging channel or the access channel or any channels that are used for paging/broadcast purpose for monitoring a calibration activation signal disseminated by the BTS  302 . Once the calibration activation information is detected, the PD  304  will start the power and phase difference detection. It is further understood that there are data processing required for this invention, but the processing can be performed on the antenna side as well as the PD side. For example, the RF signal routing and processing structure  308  can be attached to the smart antenna, but can also be in communication with the antenna but not physically attached thereto. Similarly, the mathematical calculations can be performed in the PD or in the BTS depending on the resource allocation. 
   This invention has distinct advantages over conventional smart antenna array calibration methodologies. For example, since no dedicated calibration periods are required, the normal communications traffic will not be interrupted. Since no dedicated down link calibration signals are required, individual downlink signals for each antenna to track the phase differences of each antenna may be eliminated. Finally, since no dedicated calibration module is required, a power detector such as a standard mobile terminal with power detection capability and the capability to set output power at a predetermined level can be used, the total calibration equipment cost is reduced. 
   The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
   Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.