Abstract:
The present invention provides methods and apparatus to detect the presence of interferers in a wideband digital VSWR measurement signal. Interferers cause power differences over different intervals of time and frequency, and may be detected by comparing the measured signals in both time and frequency domains with the original signal. Frequency components in the measured signals may be discarded if the interference is deemed too large. The remaining frequency components can then be used to compute the VSWR.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to measurement of the Voltage Standing Wave Ratio (VSWR) of antenna systems and, more particularly, to measurement techniques to reduce the impact of interfering signals in the calculation of the VSWR. 
       BACKGROUND 
       [0002]    Traditionally, the Voltage Standing Wave Ratio (VSWR) of antenna systems is measured using filtered or narrow-band power measurements. By measuring the power of the forward and reflected signals (P fwd  and P ref I) simultaneously, the scalar return loss (RL) and reflection co-efficient (Γ) can be calculated. 
         [0003]    Typical VSWR measurement systems consist of a directional coupler, and analog power detectors to measure the voltage of the forward signal, denoted V fwd , and the voltage of the reflected signal, dentoed V refl . A mixer circuit and low pass filter are used to measure the VSWR over a specific frequency and bandwidth. An analog to digital converter (ADC) quantizes the power detector value. High values of V refl  relative to V fwd  indicate potential transmission line and antenna faults. Both V refl  and V fwd  will be affected if there is an interference signal coupled into the measurement within or even adjacent to the measurement bandwidth. 
         [0004]    The current analog VSWR (AVSWR) and narrowband digital VSWR (DVSWR) measurement systems are unable to detect or account for interferers as they have no knowledge of the dynamic interferer. False alarms can be raised if the interference is large enough to raise the V refl  such that it looks like a cable fault. False alarms may cause outages in the network if the transmission signal is turned off by fault management software to prevent damage to the radio in the event of a cable fault. False alarms also may cause unnecessary maintenance on the radio and antenna system. Therefore, special tests are used to verify the sensitivity of the AVSWR measurement to adjacent interferers. 
         [0005]    Another drawback is that the AVSWR measurement can only be performed over a small bandwidth determined by the configured frequency of the mixer and low pass filter. The AVSWR method cannot measure the V fwd  or V refl  separately. Any voltage induced on the antenna system due to an interferer will be coupled into both the V fwd  and V refl  signals, and will impact the accuracy of the measurement. The level of coupling is based on the S-parameters of the directional coupler. 
         [0006]    The AVSWR measurement method also does not compare the V fwd  or V refl  with the original transmitted signal, denoted V ref , and thus does not determine if there is any interference that impacts the overall accuracy of the measurement. 
       SUMMARY 
       [0007]    The present invention provides methods and apparatus to detect the presence of interferers in a wideband digital VSWR measurement signal. Interferers cause power differences over different intervals of time and frequency, and may be detected by comparing the measured signals in both time and frequency domains with the original signal. Frequency components in the measured signals may be discarded if the interference is deemed too large. The remaining frequency components can then be used to compute the VSWR. 
         [0008]    Exemplary embodiments of the invention comprise methods of measuring the VSWR in a wideband communication system. In one exemplary embodiment, time-domain measurement signals for a forward wave and a reflected wave are converted to a frequency domain. Reflection coefficients are then computed for a plurality of frequency components in the measurement signals. The reflection coefficients are used to compute interference detection metrics for one or more of said frequency components in the measurement signals. Frequency components in the forward wave, the reflected wave, or both, may be discarded based on the interference detection metrics, and the the remaining frequency components in the forward and reflected waves are used to compute the VSWR. 
         [0009]    Other embodiments of the invention comprise a wireless terminal configured to compute a VSWR. The wireless terminal comprises a directional coupler connected between a transmitter and an antenna, a receiver circuit to generate time domain measurement signals for forward and reflected waves, and a processing circuit for computing the VSWR. The processing circuit is configured to convert time-domain measurement signals for the forward wave and the reflected wave to a frequency domain; compute reflection coefficients for a plurality of frequency components in the measurement signals as a function of the reflection coefficients, compute interference detection metrics for one or more of said frequency components in the measurement signals, discard one or more frequency components in the forward wave or the reflected wave, or both, based on the interference detection metrics, and compute a VSWR for the forward wave and the reflected wave based on the remaining frequencies in the forward and reflected waves. 
         [0010]    The present invention enables the detection of an interfering signal in measurement signals used to compute the VSWR. As a result, elements in the measurement signals containing interference may be discarded before the VSWR is computed. False alarms can be avoided, resulting in fewer service calls and network outages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates a wireless terminal including a measurement circuit for measuring the voltage standing wave ratio (VSWR) of the antenna system. 
           [0012]      FIG. 2  illustrates exemplary processing for computing the VSWR according to one exemplary embodiment. 
           [0013]      FIG. 3  illustrates exemplary processing for computing the VSWR according to a second exemplary embodiment. 
           [0014]      FIG. 4  illustrates an exemplary method of computing a VSWR for an antenna system. 
           [0015]      FIG. 5  is an exemplary graph of the measured coupling factor showing an interfering signal. 
           [0016]      FIG. 6  is an exemplary graph of the forward coupling factor error showing an interfering signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates an exemplary wireless terminal  10  including a measurement circuit  20  for determining a voltage standing wave ratio (VSWR). A signal source  12  generates a digital signal for transmission over a wireless channel. The digital signal is applied to the input of a transmitter  14 . After conversion to analog form, the transmitter  14  upconverts, filters, and amplifies the signal. The output of the transmitter  14  is coupled via a transmission cable  16  to a transmit antenna  18 . A measurement circuit  20  is coupled to the transmission cable  18  between the transmitter  14  and antenna  18  to measure the voltage standing wave ratio (VSWR) of the antenna system. As will be hereinafter described in greater detail, the measurement circuit  20  is configured to detect interferers in the signals on which measurements are made, and to remove frequency components of the measurement signals containing excessive interference. 
         [0018]    The measurement circuit  20  includes a directional coupler  22 , forward receiver  24 , a reverse receiver  26 , and a signal processing circuit  28 . The directional coupler  22  generates scaled versions of the transmitted signal, i.e the forward signal, and reflected signal. An interfering signal may be coupled by the directional coupler  22  into both the forward and reflected signals. The forward receiver  24  receives the scaled version of the transmit signal, referred to as the forward measurement signal (sigFwd). The forward receiver  24  demodulates and digitizes the forward measurement signal. The reverse receiver  26  receives a scaled version of the reflected signal, referred to as the reflected measurement signal (sigRefl). The reverse receiver  26  demodulates and digitizes the reflected measurement signal. The digitized measurement signals are input to the signal processing circuit  28 . Additionally, the original signal is applied to the signal processing circuit  28  and used as a reference signal (sigRef). 
         [0019]    The signal processing circuit  28  has two main functions. First, the signal processing circuit  28  detects the presence of interfering signals within or adjacent to the measurement bandwidth. As described in greater detail below, interfering signals may be detected by comparing the measurement signals with the original transmitted or reference signal in both the time and frequency domain. Second, the signal processing circuit  28  calculates the VSWR of the antenna system based on the measurement signals. For purposes of calculating the VSWR, frequency components in the measurement signals corrupted by interference are discarded. 
         [0020]      FIG. 2  illustrates exemplary processing steps  100  performed by the signal processing circuit  28  in one embodiment. The inputs to the signal processing circuit  28  include sigRef, sigFwd, and sigRefL. The signals sigRef, sigFwd, and sigRefL are sampled over a finite period of time. The measurement signals sigFwd and sigRefL may be sampled as RF signals and converted to the digital domain. The reference signal (sigRef) may comprise a baseband signal, or may be sampled as an analog RF signal in a manner similar to sigFwd and sigRefL. 
         [0021]    The signal processing circuit  28  cross correlates sigFwd with sigRef to find the peak correlation and determine the sample offset (block  102 ). Once the sample offset is known, the signal processing circuit  28  time-aligns sigFwd and sigRefL with sigRef (block  104 ). The signal processing circuit  28  then performs a Discrete Fourier Transform (DFT) or other transform operation to convert the time-aligned signals (sigRef, sigFwd′, sigRefl′) from the time domain to the frequency domain (block  106 ). The frequency domain signals (refSigFreq, rawFwdFreq, and rawReflFreq) represent the magnitude and the phase of the signals with respect to frequency. The signal processing circuit  28  compensates rawFwdFreq and rawReflFreq for nonlinearities in the analog circuits by applying calibration factors determined during radio calibration procedures (block  108 ). The signals output from the analog compensation function are the measured forward frequency (measFwdFreq) and measured reflected frequency (measReflFreq). 
         [0022]    The signal processing circuit  28  calculates a measured forward coupling factor, (fwdCfMeas) for each of a plurality of frequency bins (block  110 ). The forward coupling factor for a given frequency bin is equal to the measured forward frequency divided by the reference signal frequency (measFwdFreq/refSigFreq).  FIG. 5  shows a graph of the forward coupling factor for a white noise reference signal with a GSM interferer. The measured forward coupling factor (fwdCfMeas) is compared against the known coupling factor (fwdCfCal) determined at the time of calibration. The difference between the measured forward coupling factor and the calibrated coupling factor is computed to obtain an absolute forward coupling factor error (absFwdCfErr) (block  112 ).  FIG. 6  shows a graph of the forward coupling factor error for a white noise reference signal with a GSM interferer. 
         [0023]    The forward coupling factor error is used as an interference detection metric. A high forward coupling factor error indicates the presence of an interfering signal. Therefore, the forward coupling factor error computed for each frequency bin can be compared to an error threshold (block  114 ). If the forward coupling factor error is greater than or equal to the error threshold, it is determined that interference is present in the corresponding frequency bin (block  116 ). 
         [0024]    The signal processing circuit  28  also computes a measured reflection coefficient (measReflCoeff) for each frequency bin. The measured reflection coefficient is computed by dividing the measured forward frequency by the measured reflected frequency (measFwdFreq/measReflFreq). The measured reflection coefficients for frequency bins where interference is present are then discarded (block  120 ). The measured reflection coefficient for the remaining frequency bins are used to compute the VSWR (block  122 ). The VSWR is given by: 
         [0000]    
       
         
           
             VSWR 
             = 
             
               
                 
                   V 
                   max 
                 
                 
                   V 
                   min 
                 
               
               = 
               
                 
                   1 
                   + 
                   ρ 
                 
                 
                   1 
                   - 
                   ρ 
                 
               
             
           
         
       
     
         [0000]    where ρ=|Γ| and Γ is the measured reflection coefficient. 
         [0025]      FIG. 3  illustrates exemplary processing steps  150  performed by the signal processing circuit  28  in another exemplary embodiment. In this embodiment, the measured reflection coefficient is compared to an expected reflection coefficient to detect the presence of interfering signals in the measurement signals. The frequency components containing interference are discarded and the VSWR is calculated based on the remaining frequency components. 
         [0026]    The signal processing circuit  28  cross-correlates the forward signal with the reference signal to determine a sample offset (block  152 ). The sample offset determined by the cross-correlation is used to time align the forward signal and reflected signal (block  154 ). The time-aligned signals (sigFwd′, sigRefl′) are then converted from the time domain to the frequency domain by a DFT operation (block  156 ). Analog compensation is applied to the raw frequency domain signals (rawFwdFreq and rawReflFreq) as previously described to obtain a measured forward frequency (measFwdFreq) and measured reflected frequency (measReflFreq)(block  158 ). 
         [0027]    A measured reflection coefficient (measReflCoeff) is computed by dividing the measured forward frequency by the measured reflected frequency (block  160 ). The measured reflected coefficient is input to a trend-fitting algorithm to obtain an expected reflection coefficient (reflCoeffExp)(block  162 ). The signal processing circuit  28  determines a difference between the measured reflected coefficient and the expected reflected coefficient (block  164 ). This difference represents the absolute reflection coefficient error (absReflCoeffErr). Because interfering signals will result in a detectable error in the reflection coefficient over the bandwidth of the measured signals, the reflection coefficient error can be used as an interference detection metric. 
         [0028]    The reflection coefficient error is compared to an error threshold (block  166 ). If the reflection coefficient error exceeds the error threshold, it is determined that the corresponding frequency component contains an interfering signal (block  168 ). The frequency components containing the interfering signals are discarded (block  170 ). The measured reflection coefficients for the remaining frequency components are used to compute the VSWR as previously described (block  172 ). 
         [0029]      FIG. 4  illustrates a general method  200  performed by the signal processing circuit  28  to compute the VSWR. The signal processing circuit  28  converts time domain and measurement signals to the frequency domain (block  210 ). As previously noted, the frequency domain representation represents the magnitude and phase as a function of frequency. The bandwidth is divided into a plurality of frequency bins. The signal processing circuit  28  computes reflection coefficients for each of the frequency bins (block  220 ). The signal processing circuit  28  also computes an interference detection metric for each frequency bin (block  230 ). In the embodiment shown in  FIG. 2 , the interference detection metric comprises the forward coupling factor error. In the embodiment shown in  FIG. 3 , the interference detection metric comprises the reflection coefficient error. Based on the interference detection metric, the signal processing circuit  28  discards frequency components in the measurement signals corresponding to frequency bins where the interfering signal is detected (block  240 ). The VSWR is then computed based on the remaining frequency components (block  250 ). 
         [0030]    The techniques described herein enable the detection of an interfering signal in measurement signals used to compute the VSWR. As a result, frequency components in the measurement signals containing interference may be discarded. False alarms can be avoided, resulting in fewer service calls and network outages. Thus, network operators would see a significant savings. In cases where the interfering signal is persistent, the interfering signal can be identified and an appropriate response can be sent to the operator (alarms, warnings, or other corrective action). 
         [0031]    Thus, the foregoing description and the accompanying drawings represent non-limiting examples of the methods and apparatus taught herein. As such, the present invention is not limited by the foregoing description and accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.