Measurement of voltage standing wave ratio of antenna system
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.
1. A measurement circuit for measuring a voltage standing wave ratio (VSWR) in an antenna system, the measurement circuit comprising:
a directional coupler connected between a transmitter and an antenna;
a receiver circuit to generate measurement signals for forward and reflected signals in a transmission line of the antenna system; and
a processing circuit configured to:
compute reflection coefficients for a plurality of frequency bins based on the measurement signals;
detect interference in one or more of the frequency bins by computing interference detection metrics for one or more frequency bins among said plurality of frequency bins as a function of the reflection coefficients, wherein the interference detection metrics comprises a forward coupling factor error or a reflection coefficient error, wherein it is determined that a corresponding frequency bin contains an interfering signal if the forward coupling factor error or the reflection coefficient error exceeds an error threshold;
discard reflection coefficients for one or more frequency bins in which interference has been detected; and
compute the VSWR of the antenna system based on the reflection coefficients for a remaining number of frequency bins.
2. The measurement circuit of claim 1 , wherein the processing circuit is configured to control, based on the computed VSWR, transmission of a signal on the transmission line.
3. The measurement circuit of claim 2 , wherein the processing circuit is configured to control by sending a control signal to refrain from transmitting the transmission signal on the transmission line.
4. The measurement circuit of claim 1 , wherein the processing circuit is configured to detect a transmission line or antenna fault based on the computed VSWR, and send a control signal for the antenna system responsive to the detection.
5. The measurement circuit of claim 1 , wherein:
the receiver circuit is configured to generate measurement signals by generating time-domain measurement signals; and
the processing circuit is configured to:
compute discrete Fourier transforms of the measurement signals for the forward signal and the reflected signal to convert the measurement signals to the frequency domain, and
compute the reflection coefficients based on the frequency-domain measurement signals.
6. The measurement circuit of claim 1 , wherein the processing circuit is configured to divide a magnitude of the measurement signal for the reflected signal by a corresponding magnitude of the measurement signal for the forward signal to compute the reflection coefficients.
7. The measurement circuit of claim 1 , wherein, to detect interference in one of more of the frequency bins, the processing circuit is configured to
compare the interference detection metrics for one or more frequency bins to the error threshold.
8. The measurement circuit of claim 1 , wherein the processing circuit is configured to determine measurement errors for each of the frequency bins as a function of its corresponding reflection coefficient, and wherein the measurement errors comprise interference detection metrics for determining interference in one or more of the frequency bins.
9. A method of measuring a voltage standing wave ratio (VSWR) of an antenna system, the method comprising:
generating measurement signals corresponding respectively to a forward signal and a reflected signal in a transmission line of the antenna system;
computing reflection coefficients for a plurality of frequency bins based on the measurement signals;
detecting interference in one or more of the frequency bins by computing interference detection metrics for one or more frequency bins among said plurality of frequency bins as a function of the reflection coefficients, the interference detection metrics comprising a forward coupling factor error or a reflection coefficient error, wherein it is determined that a corresponding frequency bin contains an interfering signal if the forward coupling factor error or the reflection coefficient error exceeds an error threshold;
discarding reflection coefficients for one or more frequency bins in which interference has been detected; and
computing the VSWR of the antenna system based on the reflection coefficients for a remaining number of frequency bins.
10. The method of claim 9 , wherein the method is implemented by a wireless terminal comprising the antenna system, and wherein the wireless terminal computes the VSWR to control a transmitter of the wireless terminal.
11. The method of claim 10 , wherein the method comprises controlling, based on the computed VSWR, transmission of a signal on the transmission line, wherein the controlling comprises refraining from transmitting the transmission signal on the transmission line.
12. The method of claim 10 , wherein the method comprises detecting a transmission line or antenna fault based on the computed VSWR, and sending a control signal to refrain from transmitting a transmission signal on the transmission line.
13. The method of claim 9 , wherein:
the generating measurement signals comprises generating time-domain measurement signals;
the method comprises computing discrete Fourier transforms of the measurement signals for the forward signal and the reflected signal to convert the measurement signals to the frequency domain; and
computing the reflection coefficients based on the frequency-domain measurement signals.
14. The method of claim 9 , wherein computing reflection coefficients for a plurality of frequency bins based on the reflection coefficients comprises dividing a magnitude of the measurement signal for the reflected signal by a corresponding magnitude of the measurement signal for the forward signal.
15. The method of claim 9 , wherein detecting interference in one or more frequency bins based on the interference detection metrics comprises
comparing the interference detection metrics for one or more frequency bins to the error threshold.
16. The method of claim 9 , wherein computing interference detection metrics for one or more of the frequency bins comprises computing measurement errors for each of the frequency bins as a function of its corresponding reflection coefficient, and wherein the measurement errors comprise the interference detection metrics.
17. The method of claim 16 , wherein discarding frequency bins comprises discarding frequency bins where the measurement error satisfies an error threshold.
18. The method of claim 16 , wherein computing measurement errors for each of the frequency bins as a function of its corresponding reflection coefficient comprises:
computing a forward coupling factor for each frequency bin as a function of the measurement signal for the forward signal and a reference signal; and
computing the measurement error as a function of the difference between the forward coupling factor and a reference value.
19. The method of claim 16 , wherein computing measurement errors for each of the frequency bins as a function of its corresponding reflection coefficient comprises computing the measurement error based on a difference between the computed reflection coefficient and an expected reflection coefficient.