System and method for determining and controlling gain margin in an RF repeater

Abstract

An apparatus for repeating signals includes a receive antenna for capturing a receive signal, processing circuitry for processing the receive signal to form a repeated signal, and a transmit antenna for transmitting the repeated signal. The processing circuitry includes gain circuitry for gain in the repeated signal and decorrelation circuitry configured for modifying the repeated signal with respect to the receive signal to thereby decorrelate the repeated signal from the receive signal. The processing circuitry further comprises circuitry configured for calculating a gain margin for the apparatus utilizing the decorrelated receive and repeated signals.

Description

The output signal, or repeated signal 37, is then set forth by Equation 2:
tx(t)=G·rx(t−d)·e^{j·2·π·fshift·t}   EQ. 2
Therefore, the repeated signal tx(t) reflects the frequency shift of the received signal provided in accordance with one aspect of the present invention by frequency-shifting circuitry 44.

To determine the relative level of the feedback signal 34 in the overall receive signal 38, the repeated signal 37 is cross-correlated with the receive signal 38. Correlation is a linear operation, therefore this cross-correlation is equivalent to the sum of the input signal 31 in(t) cross-correlated with the repeated signal 37 tx(t) and the feedback signal 34 F·tx(t−p) cross-correlated with the repeated signal 37 tx(t). Assuming all signals are WSS over the measurement interval, the cross correlation of in(t) with tx(t) will have an average value of zero because the signals are uncorrelated due the frequency shift. The cross correlation F·tx(t−p) with tx(t) will have an average value of F·tx_rms² at t=p.

To determine the relative level of the input signal in(t) in the receive signal 38, the frequency shift is mathematically removed from the repeated signal 52 tx(t) prior to calculating the cross-correlation, and the unshifted repeated signal is then cross-correlated with the receive signal 38. The frequency shift can be mathematically removed by multiplying it with a complex exponential with the negative of the frequency shift as shown in EQ. 3.
tx_unshift(t)=tx(t)·e^{−j·2·π·fshift·t}   EQ. 3
Again, since correlation is a linear operation, rx(t−d) can be split into its components, in(t) and F·tx(t−p). Since the frequency shift has been removed, tx_unshift(t) is uncorrelated with F·tx(t−p) and has an average value of zero, while tx_unshift is correlated with in(t) and has an average value proportional to G⁻¹·tx_rms² at t=d.

Therefore, the gain margin may then be calculated as a ratio of the cross correlations of rx(t) with tx_unshift(t) at t=d and tx(t) with rx(t) at t=p.

$\begin{array}{cc}\frac{\frac{1}{G}{\mathrm{tx}}_{rms}^2}{F·{\mathrm{tx}}_{rms}^2}=\frac{1}{G·F}& \mathrm{EQ}.4\end{array}$

In another embodiment of the invention, the ratio of the input and feedback signals is determined by other methodology that relies upon the fact that the input signal and feedback signal are decorrelated due to the applied frequency shift of the repeated signal. In an alternative embodiment, the average power of the receive signal can be calculated, rx_rms². The relative level of the feedback signal in the receive signal is then calculated as the cross-correlation of the receive signal with the receive signal shifted by the negative of the frequency shift provided by the transmission circuitry of the invention. For ease of understanding, the feedback signal in this case is represented as the delayed input signal multiplied by the loop gain of the repeater multiplied by the frequency shift, G·F·rx(t−d−p)·e^{j·2·π·fshift·t}. The receive signal can then be represented as the sum of the input signal and the feedback signal as shown in EQ. 5.
rx(t)=in(t)+G·F·rx(t−d−p)·e^{j·2·πfshift·t}   EQ. 5
The receive signal shifted by the negative of the frequency shift fshift is shown in EQ. 6
rx_unshift(t)=in(t)·e^{−j·2·π·fshift·t}+G·F·rx(t−d−p)   EQ. 6

Again, since cross correlation is a linear operation, the cross correlation of rx(t) with rx_shift(t) is equivalent to the sum of the input signal 31 in(t) with a negative frequency shift applied cross-correlated with the receive signal rx(t) and the feedback signal G·F·rx(t−d−p) cross correlated with rx(t). Assuming all signals are WSS over the measurement interval, and G and F are linear, time-invariant systems, then the cross correlation of in(t)·e^{−j·2·π·fshift·t} with rx(t) will have an average value of zero because the signals are uncorrelated. The cross correlation G·F·rx(t−d−p) with rx(t) has an average value of G·F·rx_rms² at t=d+p. The gain margin can then be calculated as the ratio of the average power of rx(t) to the cross-correlation of rx(t) and rx_shift(t) as shown in EQ. 7.

$\begin{array}{cc}\frac{{\mathrm{rx}}_{rms}^2}{G·F·{\mathrm{rx}}_{rms}^2}=\frac{1}{G·F}& \mathrm{EQ}.7\end{array}$

In accordance with another aspect of the present invention, the correlation circuitry insures that the average phase of the cross-correlation is zero, while minimizing the number of computations. If the frequency shift is small and the bandwidth of the signal is very large, then the calculations are performed over a large number of samples. However, to minimize the number of computations, one embodiment of the invention performs a windowed cross-correlation. The windows are equally distributed throughout one period of the frequency shift f_shift. For instance, if the frequency shift was one Hertz, the correlation would have to be performed over a one-second period. However, if a window were applied from 0-0.1 seconds and 0.5-0.6 seconds, then the average phase of the cross-correlations would still sum to zero. Generally, any number of windows can be used as long as the average phase of the cross-correlations equals zero.

In one embodiment, the correlation circuitry or processing circuitry assumes that the magnitude of the repeated signal is approximately constant throughout the measurement period. If the magnitude of the signal's envelope varies greatly during the measurement, the sum of the correlations may not add to zero. In accordance with another embodiment of the invention, to compensate for the problem, the receive and repeated/transmit signals are normalized so that they have a constant envelope prior to the correlation calculations. Normalization does not change the ratio of the input signal to the feedback signal. If the receive signal and repeated signal are normalized by the same function, the methodology of the invention continues to provide the desired results.

In accordance with another feature of the invention, one embodiment of the invention may provide a constant frequency shift that is always applied to the repeated signal. Alternatively, the frequency shift feature is selectable and may be selectively turned ON or OFF selectively as the gain margin measurement is needed.

Furthermore, in another embodiment of the invention, the frequency shift may be selectively varied both in the amount of the frequency shift, and also the sign of the shift. For example, the invention may alternate between a positive frequency shift and a negative frequency shift so that the overall average frequency shift utilized in the invention is zero.

Turning now to FIGS. 3 and 4, those figures set forth schematic diagrams with respect to various embodiments of the invention. In FIG. 3, the frequency shift is supplied after the gain block within the repeater circuitry. In FIG. 4, the frequency shift is applied prior to the gain block. However, it will be understood that the frequency shift circuitry might be implemented anywhere between the receive input and the transmit output in accordance with the principles of the invention.

Turning to FIG. 3, where like reference numerals are utilized, repeater 30a incorporates a receive antenna 32 and transmit antenna 36 coupled with appropriate repeater circuitry 40a. As will be understood by a person of ordinary skill in the art, the components are shown in a downlink path 60 in the repeater 30a. Similar components will exist in the uplink path 62 for handling uplink traffic between wireless devices and a base station for example. Accordingly, components within the downlink path 60 will be described herein in further detail with the assumption that similar functionality and components would be utilized in the uplink path 62.

Receive antenna 32 receives both the input signal and frequency shifted or modified feedback signal. That receive signal is coupled to a low noise amplifier (LNA) 64 for amplifying downlink RF receive signals from a base station. A mixer component 66 is fed by an appropriate local oscillator (LO) signal and converts the RF receive signal 38 to an IF signal at a different IF frequency or a frequency at or near the baseband frequency for ease of later processing in the repeater. The signal is then filtered by a filter component or circuitry 68. In the embodiments illustrated in FIGS. 3 and 4, the repeater circuitry incorporates both analog and digital components. Digital signal processing circuitry 70 is implemented for adjusting gain as well as for providing the necessary modification, such as a frequency shift f_shift, to the repeater signal before it is transmitted as a repeated signal. Appropriately, an A/D converter circuit 72 converts the analog signal to an appropriate digital signal for further digital processing. The digital signal is sent to DSP circuitry 70 that might be an FPGA, digital signal processor or other such element. The DSP circuitry might include an additional digital mixer circuit 74 fed by a suitable numerically-controlled oscillator (NCO) signal to provide digital downconversion for ease of further processing. The signal might also be filtered by an appropriate digital filter 76. Filter 76 might also change the amplitude of the signal. Component 78 represents suitable circuitry for adjusting the gain within repeater 30a. Although as noted, the gain component might be implemented together with the filter 76. In accordance with one noted aspect of the invention, frequency-shifting circuitry 80 provides the desired frequency shift within the repeated signals in order to provide the proper decorrelation between the feedback signal 34 and the input signal 31. The signal has a frequency shift added by mixing it with the frequency shift oscillator. The signal might then be digitally upconverted by appropriate digital upconversion circuitry 82 fed by a transmit NCO. The signal may then be converted to back to an analog signal by D/A circuitry 84.

The analog signal, such as at analog IF, is further upconverted with mixer circuitry 86 fed by an appropriate transmit LO to an appropriate RF signal. The RF signal is filtered by filter circuitry 88, and then fed to an RF power amplifier 90 before being transmitted as a repeated signal through the transmit antenna 36.

The mixing elements are typical of a repeater. There can be more or fewer mixing elements than illustrated in the examples and still implement a functional repeater. In one embodiment, the frequency shift mixing operation can be combined with one or more of the other mixers if desired. However, the mixing operations must be implemented such that the frequency of the input signal and the frequency of the transmit or repeated signal differ by the amount of the frequency shift. The frequency shift mixer is shown after the filter; however, t can be placed anywhere between the receive and transmit antennas. In FIG. 4 it is shown prior to the filter and/or gain block.

In the embodiments illustrated in FIGS. 3 and 4, the noted correlation functionality is provided by suitable correlation circuit 100 within the digital signal processing (DSP) circuitry 70. The correlations to determine gain margin are performed by a capturing samples of the signal in the digital path and performing the described calculations. The capture point may be anywhere in the signal path, either before or after the frequency shift circuit because the circuit has the ability to add or remove the applied frequency shift by mathematical computation. In the illustrated embodiments, the correlation circuit samples the receive signal 38 via suitable connections 102 and also is coupled to sample the frequency-shifted repeated signal in the repeater path as illustrated by the appropriate connection 104. Correlation circuit 100 may also provide the necessary functionality for automatically controlling the gain through the gain component 78 by way of line 106 based on the measured correlations and determined gain margin. It would be understood by a person of ordinary skill in the art that the various functionality within the digital signal processing circuitry 70 might be implemented in a number of different ways to achieve the functionality of the invention. Accordingly, the illustrations in FIGS. 3 and 4 are not limiting. That is, the specific details regarding how the various components are utilizing and arranged within the DSP circuitry 70 and the overall computer circuitry 40a are illustrative, and not meant to be limiting.

Turning to FIG. 4, like reference numerals are utilized with respect to the components in FIG. 3. FIG. 4 illustrates a repeater 30b of the invention wherein the frequency shift circuitry 80 is positioned prior to the gain block or gain circuitry 78 within the repeater circuitry 40b.

In a static case, the frequency shift of the feedback signal 34 would be identical to the frequency shift of the repeated signals 37. However, in some implementations of the invention, there might be additional frequency shifting between the repeated signal 37 and the feedback signal 34 due to Doppler shifting, or other parameters and conditions within the installation and operation of the repeater. To that end, in one embodiment, the invention measures and accounts for the additional frequency shifting in the cross-correlation calculations. For example, this might be done by examining or measuring how the phase of the cross-correlation changes throughout the correlation period. By finding constant changes in the correlation phase during the correlation period, the phase change due to any additional frequency shift can be readily determined. If more or less of an integer number of phase rotations are observed in the measurement, then the correlation result can be truncated or extrapolated respectively to account for the additional frequency shift. In an alternative embodiment of the invention, the circuitry provides a compensating amount of frequency shift that is added or subtracted during the correlation calculation to negate the affect of any additional frequency shift associated with the environment and installation. This functionality will be implemented in the digital signal processing circuitry as suitable in the circuits of FIGS. 3 and 4.

In one aspect of the invention, once the gain margin measurement is determined, the gain is automatically adjusted by the DSP circuitry 70, and specifically gain block or component 78. The gain may be automatically adjusted to ensure that the gain margin is above an acceptable level to insure proper operation. As will be understood, the gain margin might be adjusted through the DSP control circuitry as well as the specific gain adjustment parameters. The gain margin is usually greater than 0 dB in order to prevent oscillation. In fact, it is usually kept well above 0 dB to allow for variation in repeater gain, and antenna isolation. The present invention, by constantly measuring the gain margin as noted herein, provides automatic gain adjustment so that the repeater can compensate for any variation in the gain margin. In that way, the minimum threshold that has to be maintained might be reduced as the repeater is able to constantly automatically adjust the gain margin. As discussed above with respect to the correlation calculations, one embodiment of the invention might use a complex sinusoid to frequency shift the repeated signal that is output. However, one of ordinary skill in the art would realize that other signals might be used to modify the repeated signals with respect to the received signal to thereby decorrelate the repeated signal from the receive signal. Such modification must have minimal affect on the repeated or transmitted output of the repeater.

The invention, as described with respect to various embodiments herein, wherein the frequency shift and correlation calculations are implemented in the digital domain, such as through DSP circuitry 70. However, such frequency shifting and decorrelation might also be implemented in the analog domain. Alternatively, a mixed signal implementation using both analog and digital components might be utilized to provide the desired signal decorrelation functions and correlation calculations.

The invention, as described with respect to various embodiments herein, wherein the decorrelating function applied to the repeated signal is a frequency shift, could apply other decorrelating functions that cause minimal degradation of the repeated signal. The methods described herein could be readily adapted by a person of ordinary skill in the art to use alternate decorrelation functions to measure gain margin and control the gain of a repeater to maintain a minimum gain margin.

As noted above, while a repeater is described herein as an exemplary embodiment, the invention might be applied to any type of signal repeating system wherein some part of the transmitted or repeated signal is fed back or finds its way into the input on the receive side as a feedback signal.

Having described this invention in its various embodiments and parameters, other variations will become apparent to a person of ordinary skill in the art without deviating from the scope of the described embodiments and the invention.

Claims

1. An apparatus for repeating signals, the apparatus comprising:

a receive antenna for capturing a receive signal that includes an input signal and a feedback signal;

processing circuitry coupled with the receive antenna for processing the receive signal to form a repeated signal;

a transmit antenna coupled with the processing circuitry for transmitting the repeated signal;

the processing circuitry for processing the receive signal including:

gain circuitry to provide gain in the repeated signal;

decorrelation circuitry including frequency shifting circuitry that is configured for decorrelating the input signal of the receive signal and the repeated signal by introducing a frequency shift in the repeated signal to form the repeated signal that is decorellated and frequency-shifted from the input signal of the receive signal;

gain margin circuitry configured for calculating a gain margin for the apparatus by utilizing samples of the decorrelated receive signal and the frequency-shifted repeated signal and utilizing samples of the repeated signal wherein the frequency shift that is used to decorrelate the receive and repeated signals has been removed.

2. The apparatus of claim 1 wherein the frequency shifting circuitry is operable for creating the frequency shift by multiplying the input signal with a complex sinusoid.

3. The apparatus of claim 1 wherein the processing circuitry is further operable for adjusting the gain based upon the calculated gain margin.

4. The apparatus of claim 1 wherein the gain margin circuitry determines the gain margin by comparing a cross-correlation of the receive signal and the repeated signal with a cross-correlation of the receive signal and the repeated signal wherein the frequency shift used to decorrelate the receive signal has been removed.

5. The apparatus of claim 4 wherein the receive signal includes both the input signal and the feedback signal, cross-correlations being performed over a sufficient correlation length so that an average phase of the feedback signal relative to the input signal is around zero degrees.

6. The apparatus of claim 4 wherein cross correlations are performed over a correlation length that is an integer number of periods of the frequency shift.

7. The apparatus of claim 4 wherein cross correlations are windowed, the windowing being performed so that an average phase of the cross correlations is around zero degrees.

8. The apparatus of claim 4 wherein the processing circuitry normalizes the input signal to have generally a constant envelope.

9. The apparatus of claim 5 wherein the processing circuitry is further configured for determining additional frequency shifting in the feedback signal and providing a compensating amount of frequency shift for cross-correlations to reduce the effect of the additional frequency shifting.

10. The apparatus of claim 1 wherein the processing circuitry is configured to dynamically increase or decrease the amount of frequency shift provided by the frequency shifting circuit to the input signal.

11. The apparatus of claim 1 wherein the decorrelation circuitry is selectively turned ON and OFF for selectively calculating the gain margin.

12. The apparatus of claim 1 wherein the processing circuitry is implemented at least partially with digital circuitry.

13. A method for repeating signals comprising:

capturing a receive signal with a receive antenna, the receive signal including an input signal and a feedback signal;

processing the receive signal with processing circuitry to form a repeated signal;

transmitting the repeated signal;

the processing steps including:

providing gain in the repeated signal;

decorrelating the input signal of the receive signal and the repeated signal by introducing a frequency shift in the repeated signal to form the repeated signal that is decorrelated and frequency-shifted from the input signal of the receive signal;

calculating a gain margin for the apparatus by utilizing samples of the decorrelated receive signal and the frequency-shifted repeated signal and utilizing samples of the repeated signal wherein the frequency shift that is used to decorrelate the receive and repeated signals has been removed.

14. The method of claim 13 further including creating a frequency shift by multiplying the input signal with a complex sinusoid.

15. The method of claim 13 further comprising adjusting the gain based upon the calculated gain margin.

16. The method of claim 13 wherein calculating the gain margin includes comparing a cross-correlation of the receive signal and the repeated signal with a cross-correlation of the receive signal and the repeated signal wherein the frequency shift used to decorrelate the receive signal has been removed.

17. The method of claim 16 wherein the receive signal includes both the input signal and the feedback signal, and further comprising performing cross-correlations over a sufficient correlation length so that an average phase of the feedback signal relative to the input signal is around zero degrees.

18. The method of claim 16 further comprising performing cross correlations over a correlation length that is an integer number of periods of the frequency shift.

19. The method of claim 16 further comprising windowing cross correlations so that an average phase of the cross correlations is around zero degrees.

20. The method of claim 16 further comprising normalizing the input signal to have generally a constant envelope.

21. The method of claim 16 further comprising determining additional frequency shifting in the receive signal and providing a compensating amount of frequency shift for cross correlations to reduce the effect of the additional frequency shifting.

22. The method of claim 13 further comprising dynamically increasing or decreasing the amount of frequency shift provided by the frequency shifting circuit to the input signal.

23. The method of claim 13 further comprising selectively turning the decorrelation circuitry ON and OFF for selectively calculating the gain margin.

24. An apparatus for repeating signals, the apparatus comprising:

a receive antenna configured to capture a receive signal;

a transmit antenna configured to transmit a repeated signal formed from the receive signal;

gain circuitry configured to provide gain in the repeated signal;

decorrelation circuitry configured to modify the repeated signal and including frequency shifting circuitry operable to create a frequency shift in the repeated signal with respect to the receive signal, wherein the repeated signal is frequency shifted and decorrelated from the receive signal; and

gain margin circuitry configured to calculate a gain margin for the apparatus utilizing the decorrelated receive and repeated signals, the gain margin circuitry utilizing a repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive and repeated signal.

25. The apparatus of claim 24, further comprising processing circuitry configured to adjust the gain based upon the calculated gain margin.

26. The apparatus of claim 24, wherein the gain margin circuitry is configured to calculate the gain margin by comparing a cross-correlation of the receive signal and the repeated signal with a cross-correlation of the receive signal and the repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive signal and repeated signal.

27. The apparatus of claim 26, wherein the receive signal includes both an input signal and a feedback signal, the cross-correlations being performed over at least one of a length of the correlation that is an integer number of periods of the frequency shift or a length of the correlation that is a sufficient length so that an average cross-correlation phase of the feedback signal relative to the input signal is around zero degrees.

28. The apparatus of claim 26, wherein the cross-correlations are windowed, the windowing being performed so that an average phase of the cross-correlations is around zero degrees.

29. The apparatus of claim 26, further comprising processing circuitry configured to normalize the receive and repeated signals to have generally a constant envelope.

30. The apparatus of claim 26, further comprising processing circuitry configured to determine additional frequency shifting in the feedback signal and provide a compensating amount of frequency shift for the cross-correlations to reduce the effect of the additional frequency shifting.

31. The apparatus of claim 24, further comprising processing circuitry configured to dynamically increase or decrease the amount of frequency shift provided by the frequency shifting circuitry to the repeated signal.

32. The apparatus of claim 24, wherein the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in different circuits.

33. The apparatus of claim 24, wherein at least two of the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in the same circuit.

34. The apparatus of claim 24, wherein the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in the same circuit.

35. The apparatus of claim 24, further comprising:

a first mixer configured to down-convert the receive signal from a radio frequency to an intermediate frequency or baseband frequency;

an analog-to-digital converter configured to convert the down-converted receive signal to a digital signal, wherein the gain circuitry provides gain to the digital signal, wherein the decorrelation circuitry shifts the frequency of the digital signal; and

a digital-to-analog converter configured to convert the digital signal to an analog signal;

a second mixer configured to up-convert the analog signal to a radio frequency signal, wherein the radio frequency signal is provided to the transmit antenna for transmission as the repeated signal.

36. The apparatus of claim 24, wherein the gain circuitry, decorrelation circuitry, and the gain margin circuitry are implemented in the analog domain.

37. A method for repeating signals comprising:

capturing a receive signal;

transmitting a repeated signal formed from the receive signal;

providing gain in the repeated signal;

modifying the repeated signal by creating a frequency shift in the repeated signal with respect to the receive signal, wherein the repeated signal is frequency shifted and decorrelated from the receive signal;

calculating a gain margin for the apparatus utilizing the decorrelated receive and repeated signals and utilizing a repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive signal and repeated signal.

38. The method of claim 37, further comprising adjusting the gain based upon the calculated gain margin.

39. The method of claim 37, wherein calculating the gain margin includes comparing a cross-correlation of the receive signal and the repeated signal with a cross-correlation of the receive signal and the repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive signal and repeated signal.

40. The method of claim 39, wherein the receive signal includes both an input signal and a feedback signal, and further comprising performing the cross-correlations over at least one of a length of the correlation that is an integer number of periods of the frequency shift or a length of the correlation that is a sufficient length so that an average cross-correlation phase of the feedback signal relative to the input signal is around zero degrees.

41. The method of claim 39, further comprising normalizing the receive and repeated signals to have generally a constant envelope.

42. The method of claim 39, further comprising determining additional frequency shifting in the receive signal and providing a compensating amount of frequency shift for the cross-correlations to reduce the effect of the additional frequency shifting.

43. The method of claim 37, further comprising dynamically increasing or decreasing the amount of frequency shift provided by the frequency shifting circuit to the repeated signal.

44. The method of claim 37, further comprising:

down-converting the receive signal to an intermediate frequency or baseband frequency;

converting the down-converted receive signal to a digital signal, wherein gain is provided to the digital signal, wherein the digital signal is frequency shifted;

converting the digital signal to an analog signal; and

up-converting the analog signal to a radio frequency signal, wherein the radio frequency signal is transmitted as the repeated signal.

45. The method of claim 37, wherein providing gain in the repeated signal, modifying the repeated signal, and calculating the gain margin are implemented in the analog domain.

46. An apparatus for repeating signals, the apparatus comprising:

a receive input to couple the apparatus to a receive antenna, wherein the receive antenna is configured to capture a receive signal, wherein the receive input is configured to obtain the receive signal from the receive antenna;

a transmit output to couple the apparatus to a transmit antenna, wherein the transmit output is configured to provide a repeated signal formed from the receive signal to the transmit antenna, wherein the transmit antenna is configured to transmit the repeated signal;

gain circuitry configured to provide gain in the repeated signal;

decorrelation circuitry configured to modify the repeated signal and including frequency shifting circuitry operable to create a frequency shift in the repeated signal with respect to the receive signal, wherein the repeated signal is frequency shifted and decorrelated from the receive signal; and

gain margin circuitry configured to calculate a gain margin for the apparatus utilizing the decorrelated receive and repeated signals, the gain margin circuitry utilizing a repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive and repeated signal.

47. The apparatus of claim 46, further comprising processing circuitry configured to adjust the gain based upon the calculated gain margin.

48. The apparatus of claim 46, wherein the gain margin circuitry is configured to calculate the gain margin by comparing a cross-correlation of the receive signal and the repeated signal with a cross-correlation of the receive signal and the repeated signal that is shifted in frequency by the negative of the frequency shift used to decorrelate the receive signal and repeated signal.

49. The apparatus of claim 48, wherein the receive signal includes both an input signal and a feedback signal, the cross-correlations being performed over at least one of a length of the correlation that is an integer number of periods of the frequency shift or a length of the correlation that is a sufficient length so that an average cross-correlation phase of the feedback signal relative to the input signal is around zero degrees.

50. The apparatus of claim 48, wherein the cross-correlations are windowed, the windowing being performed so that an average phase of the cross-correlations is around zero degrees.

51. The apparatus of claim 48, further comprising processing circuitry configured to normalize the receive and repeated signals to have generally a constant envelope.

52. The apparatus of claim 48, further comprising processing circuitry configured to determine additional frequency shifting in the feedback signal and provide a compensating amount of frequency shift for the cross-correlations to reduce the effect of the additional frequency shifting.

53. The apparatus of claim 46, further comprising processing circuitry configured to dynamically increase or decrease the amount of frequency shift provided by the frequency shifting circuitry to the repeated signal.

54. The apparatus of claim 46, wherein the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in different circuits.

55. The apparatus of claim 46, wherein at least two of the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in the same circuit.

56. The apparatus of claim 46, wherein the gain circuitry, the decorrelation circuitry, and the gain margin circuitry are implemented in the same circuit.

57. The apparatus of claim 46, further comprising:

a first mixer configured to down-convert the receive signal from a radio frequency to an intermediate frequency or baseband frequency;

an analog-to-digital converter configured to convert the down-converted receive signal to a digital signal, wherein the gain circuitry provides gain to the digital signal, wherein the decorrelation circuitry shifts the frequency of the digital signal; and

a digital-to-analog converter configured to convert the digital signal to an analog signal;

a second mixer configured to up-convert the analog signal to a radio frequency signal, wherein the radio frequency signal is provided to the transmit antenna for transmission as the repeated signal.

58. The apparatus of claim 46, wherein the gain circuitry, decorrelation circuitry, and the gain margin circuitry are implemented in the analog domain.