Abstract:
A communication system with a multi-channel array antenna utilizes a receiver matching process that adapts the pass band frequency response of each channel to a selected reference channel. This process is implemented digitally by inserting a tapped delay line filter in each channel, selecting one of the channels as a reference, and adapting the others to match the reference in both phase and amplitude. The process is performed for each system calibration cycle, which occurs just before receive data is captured and processed. The improvements include an apparatus and an algorithm that select a reference channel in the adaptive process during each system calibration cycle, producing optimal, or near optimal, channel matching.

Description:
DEDICATORY CLAUSE  
       [0001]     The invention described herein may be manufactured, used and licensed by or for the Government of the United States of America for governmental purposes without the payment to the inventors of any royalties thereon. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Multi-channel array antennas in various systems have been used to digitally form antenna beams for the past few decades. By digitally forming the antenna beam, adaptive processing techniques can be implemented to shape beam patterns so that energy received in the direction of interference is minimized while energy received in the direction of the desired signal is maximized. This process is typically referred to as adaptive beam forming. Some examples of applications implementing adaptive beam forming techniques include radar, sonar, seismic, imaging, cellular, satellite, and global positioning systems.  
         [0003]     Adaptive beam forming processes attenuate interference signals impinging on the antenna array by placing beam pattern nulls in the direction of the interference. However, the amount of attenuation is limited by the level of matching between the channels. A relatively recent method for achieving high levels of receiver channel matching employs the use of adaptive channel equalization. In such a process, analog signals are received by individual antennas, propagated through the receiving hardware of each channel, converted to digital signals, equalized, and finally combined to form the antenna beam. Therefore, any system employing adaptive beam forming can implement adaptive channel equalization techniques for receiver channel matching.  
         [0004]     The receiver channel matching is accomplished digitally by inserting a tapped delay line filter in each channel, selecting a reference channel, and generating equalization tap weights to match the channels to the reference in both phase and amplitude. This process is performed for each system calibration cycle, which occurs just before receive data is captured and processed. The equalization is typically performed every system operational cycle, or dwell. Matching the channels in this manner improves the channel matching results and reduces the cost and complexity of receiver hardware by decreasing manufacturing tolerances. The level of matching achieved is determined by the number of filter taps, the signal to noise ratio (SNR) of the channels, and the reference channel characteristics. Given that the number of filter taps is limited by the processing throughput and the SNR is sufficient, improved matching can be achieved by intelligently selecting the reference channel. Previous implementations of this matching procedure chose an arbitrary, but properly functioning channel, as a reference during system initialization. The reference channel could then fail or degrade over time and greatly reduce the channel matching. One such method of channel equalization is described in U.S. Pat. No. 5,357,257. In that patent at page 7, lines 63-66, the inventor states that any operating channel can be chosen as the reference channel. The present invention improves upon this prior art by choosing a reference channel during system calibration cycles that produces the optimal, or near optimal, channel matching.  
       SUMMARY OF THE INVENTION  
       [0005]     An improved channel matching apparatus and process that matches the pass band frequency response of each channel to a selected reference channel is disclosed for systems with a multi-channel array antenna. The preferred embodiment of this invention is implemented digitally by inserting a tapped delay line filter in each channel, selecting one of the channels as a reference, and adapting the others to match the reference in both phase and amplitude. The improved matching is performed for each system calibration cycle, which occurs just before receive data is captured and processed. The improvement consists of an apparatus and a method that intelligently select a reference channel for the adaptive process during system calibration cycles, producing optimal, or near optimal, channel matching. Once the channel matching is performed, the adaptive beam forming is completed.  
         [0006]     Accordingly, one object of the present invention is to provide an apparatus and method for improving the channel equalization in a multi-channel communication system by intelligently selecting during each calibration cycle an optimal, or near optimal, reference channel.  
         [0007]     Another object of the present invention is to provide an apparatus and method for selecting the reference channel sufficiently fast so that the reference channel selection can be performed during system calibration cycles.  
         [0008]     Still, another object of the present invention is to provide an apparatus and method for selecting the reference channel that doesn&#39;t impose a severe drain on system resources. 
     
    
     DESCRIPTION OF THE DRAWING  
       [0009]      FIG. 1  is a block diagram of a typical multi-channel communication system that employs adaptive equalization to achieve the system channel matching and shows the improved reference channel selection processor and the initial reference selection by a greatest means calculator.  
         [0010]      FIG. 2  is a block diagram of the preferred embodiment of the improved reference channel selection processor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]     Referring now to the drawings wherein like numbers represent like parts  FIG. 1  shows a multi-channel communication system that performs equalization using adaptive processing techniques. During system operation, signals are received through inputs  100 ,  200 , and  300 , which correspond to receiver  120 , receiver  220 , and receiver  320 . During system calibration, a white noise generator  50  is switched so that input  110 A goes to receiver  120 , input  210 A goes to receiver  220 , and input  310 A goes to receiver  320 . Any number of channels may be employed as is appropriate for a given application.  
         [0012]     The first receiver channel includes a switch controlling inputs  100  and  110 A which in turn are coupled to receiver  120 . The output of receiver  120  is the input of analog-to-digital (A/D) converter  130  for sampling the signal received by receiver  120  and providing resultant samples to primary equalization filter  140 , to the greatest mean calculator  400 , to the primary adaptive equalization processor  500 , and to the reference channel selection processor  600 . The characteristics of the equalization filter  140  can be changed by the weight vectors output W from the primary adaptive equalization processor  500 , which will be discussed later.  
         [0013]     The second receiver channel includes a switch  210 , a receiver  220 , an A/D converter  230 , and a primary equalization filter  240  which are all substantially identical to the respective corresponding components of the first receiver channel. The third receiver channel also includes a switch  310 , a receiver  320 , an A/D converter  330 , and a primary equalization filter  340  which are all substantially identical to the respective corresponding components of the other two receiver channels. Again the outputs of the A/D converters  130 ,  230 , and  330  are coupled to, and become the inputs for: the primary equalization filters  140 ,  240 , and  340 ; the greatest mean processor  400 ; the primary adaptive equalization processor  500 ; and the reference channel selection processor  600  such that samples from the three receiver channels are provided to each processor. The greatest mean processor  400  determines the channel with the greatest mean calibration signal for an initial reference input to the primary adaptive equalization processor  500 . After the initial reference input, the reference channel selection processor  600 , provides subsequent updated reference inputs to the primary adaptive equalization processor  500  as explained below. The primary adaptive equalization processor  500  calculates weight vectors, w 1 , w 2 , - - - , w N , which are provided as inputs to the equalization filters  140 ,  240 , and  340 . These weight vectors contain the equalization filter coefficients such that when applied to the A/D data input to the primary equalization filters, the data is then equalized.  
         [0014]     The outputs of primary equalization filters  140 ,  240 , and  340  constitute the equalized output of the respective receiver channels. These equalized outputs from the receiver channels are coupled to, and become the inputs for, an adaptive beam forming processor  1400 . Conventional adaptive beam forming processors which generate beam weights that attenuate signals in the direction of interference are well known in the art and are not shown here.  
         [0015]     In the prior art such as U.S. Pat. No. 5,357,257, the channel match processor (numbered  500  in that patent) employs any of the “operating channels” without discrimination as the reference channel. There is no suggestion or teaching as to the advantages of intelligently selecting a specific operating channel so as to achieve the objectives noted above. The present invention improves upon the prior art by employing a greatest mean component,  400 , to initially select a reference channel and then employs the reference channel selection processor,  600 , to intelligently update that reference channel during every cycle. It is this apparatus and method for intelligently choosing a reference channel during each cycle which represents the improvement over the prior art.  
         [0016]     A block diagram of the preferred embodiment of an improved reference channel selection processor  600  is shown in  FIG. 2 . The equalized data from primary equalization filters  140 ,  240 , and  340  is coupled to and becomes the input for, the primary cancellation ratio calculator  700 . This primary calculator determines the cancellation ratio of each channel. The output of the primary cancellation ratio calculator  700  is coupled to, and becomes the input for channel selector  800 , which selects a number of channels, X, beginning with that channel having the largest cancellation ratio and continuing with the next largest, and so on (R 1 , R 2 , . . . R x ). The number X is limited by the processing throughput of the system and the time between calibration cycles. The number X should be chosen to be as large as the system can tolerate. A practical rule is to choose X to be approximately 5% of the number of channels in the system. The output of the channel selector  800  is the X channels to be used for references in the same number of corresponding secondary adaptive equalization processors represented here by components  900 ,  1000 , and  1100 .  
         [0017]     The A/D converter data input into the reference channel selection processor  600  is coupled to, and becomes inputs for, the secondary adaptive equalization processors  900 ,  1000 , and  1100 . These secondary adaptive equalization processors calculate weighting vectors for inputs to the secondary equalization filters. For example secondary adaptive equalization processor  900  calculates the weight vectors for secondary equalization filters  910 ,  920 , and  930 , secondary adaptive equalization processor  1000  calculates the weight vectors for secondary equalization filters  1010 ,  1020 , and  1030 , and secondary adaptive equalization processor  1100  calculates the weight vectors for secondary equalization filters  1110 ,  1120 , and  1130 . These secondary adaptive equalization processors  900 ,  1000 , and  1100 , operate in the same manner as the primary adaptive equalization processor  500  of  FIG. 1 . In each secondary adaptive equalization processor, a portion of the sampled data from each channel, as determined by the system equalization process design, is used to calculate the equalization filter weights. The rest of the samples are vectors (d 1 , d 2  . . . d n ) which are inputs to the corresponding secondary equalization filters. Additional inputs to the secondary equalization filters are the adaptive weight vectors, which are denoted as w 910 , w 920  . . . w 1130 , in  FIG. 2 . This equalization process is fully discussed for  FIG. 1  and not repeated for  FIG. 2 . Specifically, the remaining samples for each channel from secondary adaptive equalization processor  900  are equalized by equalization filters  910 ,  920 , and  930 , the remaining samples for each channel from secondary adaptive equalization processor  1000  are equalized by secondary equalization filters  1010 ,  1020 , and  1030 , and the remaining samples for each channel from secondary adaptive equalization processor  1100  are equalized by secondary equalization filters  1110 ,  1120 , and  1130 .  
         [0018]     The outputs of secondary equalization filters  910 ,  920 , and  930  are the inputs to the secondary cancellation ratio calculator  940 , whose output is the input to the average cancellation ratio calculator  950 . The output of the average cancellation ratio calculator  950  is the average of the cancellation ratios output from secondary cancellation ratio calculator  940 . This structure and process is repeated for the remaining secondary equalization filters.  
         [0019]     The outputs of the secondary average cancellation ratio calculators  950 ,  1050 , and  1150  are inputs to the comparator  1200 . This comparator selects the channel with the largest average cancellation ratio as the updated reference channel for the primary adaptive equalization processor  500  of  FIG. 1 .