Compensation of I-Q imbalance in digital transceivers

• US 20040203472A1
• Filed: 09/05/2002
• Published: 10/14/2004
• Est. Priority Date: 09/05/2002
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: G-Plus, Inc. (Microchip Technology Incorporated)
• Inventors: Chien, Charles
• Granted Patent: US 7,248,625 B2
• Time in Patent Office: Days
• US Class Current: 455/68
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22
• CPC Class Codes: H04B 17/20 of receivers
• CPC Class Codes: H04L 2027/0016 Stabilisation of local oscillators
• CPC Class Codes: H04L 2027/0024 at the receiver end
• CPC Class Codes: H04L 27/0014 Carrier regulation of chaotic carriers H04L27/001; for multicarrier receivers H04L27/2657

Abstract

A transceiver includes a switching unit configurable for isolating an input of a receiver from an output of a transmitter during a local calibration mode. A known signal present at the output at a first power level during the calibration mode will also be present at the input at a second power level lower than the first power level and will be converted by the quadrature demodulator. A compensation factor is estimated for compensating the receiver section for imbalances in the in-phase and quadrature phase signals resulting from conversion of the known signal. Remote calibration is implemented using a method for remotely compensating for I−Q imbalance wherein a data packet having a known signal is transmitted to a receiver for conversion by a quadrature demodulator and compensation factors are estimated for compensating for imbalances in the in-phase and quadrature phase signals resulting from conversion of the known signal.

Claims

1. A radio transceiver comprising:

at least one antenna;

a receiver section having a quadrature demodulator for receiving various signals and converting the various signals into in-phase and quadrature phase signals;

a transmitter section for transmitting at least one known signal to an input of the quadrature demodulator for conversion by the quadrature demodulator;

a switching unit configurable for providing an amount of isolation between an output port of the transmitter section and an input port of the receiver section such that when the at least one known signal is present at the output port at a first power level, the at least one known signal is also present at the input port at a second power level lower than the first power level and is converted by the quadrature demodulator; and

at least one receiver compensation factor estimation unit coupled to an output of the quadrature demodulator for estimating, after conversion of the at least one known signal into in-phase and quadrature phase signals, at least one receiver compensation factor for compensating the receiver section for an imbalance in subsequently received in-phase and quadrature phase signals.

2. The radio transceiver recited in claim 1, wherein the receiver section further comprises a receiver I−

Q compensation unit coupled to the at least one receiver compensation factor estimation unit for applying the at least one receiver compensation factor to subsequent in-phase and quadrature phase signals.

3. The radio transceiver recited in claim 2, wherein the receiver section further comprises a receiver memory coupled to the at least one receiver compensation factor estimation unit and the receiver I−

Q compensation unit for storing the at least one receiver compensation factor and for providing the at least one receiver compensation factor to the receiver I−

Q compensation unit.

4. The radio transceiver recited in claim 2, wherein the receiver I−

Q compensation unit determines the receiver compensation factors using tan φ

_r and (γ

_r cos φ

_r)<sup>−

1</sup>, where γ

is a value of amplitude imbalance and φ

_r is a value of phase imbalance.

5. The radio transceiver recited in claim 2, wherein the receiver I−

Q compensation unit determines the receiver compensation factors using γ

_r sin φ

_r and (γ

_r cos φ

_r)<sup>−

1</sup>, where γ

_r is a value of amplitude imbalance and φ

_r is a value of phase imbalance.

6. The radio transceiver recited in claim 2, wherein the receiver section further comprises:

a low pass filter having an input coupled to the output of the quadrature demodulator;

a receiver variable gain amplifier having an input coupled to an output of the low pass filter; and

an analog-to-digital converter having an input coupled to an output of the receiver variable gain amplifier and an output coupled to inputs of the at least one receiver compensation factor estimation unit and the receiver I−

Q compensation unit.

7. The radio transceiver recited in claim 2, wherein the receiver section further comprises:

a receive modem processor for receiving and processing an output of the receiver I−

Q compensation unit; and

a first serial/parallel converter for receiving and converting an output of the receive modem processor.

8. The radio transceiver recited in claim 1, wherein the imbalance is at least one of a gain imbalance and a phase imbalance.

9. The radio transceiver recited in claim 1, wherein the at least one compensation factor includes at least one of a gain compensation factor and a phase compensation factor.

10. The radio transceiver recited in claim 1, wherein the at least one receiver compensation factor estimation unit estimates an amplitude imbalance (γ

_r) according to the equation

11. The radio transceiver recited in claim 10, wherein the at least one receiver compensation factor estimation unit estimates a phase imbalance (φ

_r) according to the equation

12. The radio transceiver recited in claim 1, wherein the at least one receiver compensation factor estimation unit estimates an amplitude imbalance (γ

_r) according to the equation

13. The radio transceiver recited in claim 12, wherein the at least one receiver compensation factor estimation unit estimates a phase imbalance (φ

_r) according to the equation

14. The radio transceiver recited in claim 1, further comprising a pre-processor coupled to the at least one receiver compensation factor estimation unit for at least one of removing DC offsets and reducing noise.

15. The radio transceiver recited in claim 1, further comprising a transceiver controller for controlling the transceiver such that during a local calibration mode the switching unit is configured to provide the amount of isolation.

16. The radio transceiver recited in claim 15, wherein the at least one receiver compensation factor comprises a plurality of receiver compensation factors and wherein the transceiver controller determines if the plurality of compensation factors have converged to within a given tolerance.

17. The radio transceiver recited in claim 1, wherein the switching unit comprises a first switch for coupling the output port to the at least one antenna and a second switch for coupling the input port to the at least one antenna.

18. The radio transceiver recited in claim 17, wherein the switching unit further comprises a third switch for coupling the at least one antenna to ground.

19. The radio transceiver recited in claim 18, wherein the switching unit further comprises a fourth switch for coupling the output port to a pre-defined impedance.

20. The radio transceiver recited in claim 19, wherein the pre-defined impedance is 50 ohms.

21. The radio transceiver recited in claim 18, wherein the switching unit further comprises a fourth switch for directly coupling the output port to the input port.

22. The radio transceiver recited in claim 21, wherein the switching unit further comprises a fifth switch for coupling the output port to a pre-defined impedance.

23. The radio transceiver recited in claim 1, wherein the at least one antenna comprises:

a plurality of antennas and wherein the switching unit comprises a first plurality of switches for coupling the plurality of antennas to at least one of the input port and the output port; and

a second plurality of switches for coupling the plurality of antennas to ground.

24. The radio transceiver recited in claim 1, wherein the switching unit comprises at least one of a micro-electromechanical system (MEMS), a field effect transistor (FET), bipolar junction transistors (BJT'"'"'s), hetero junction transistors (HBT'"'"'s), GaAs transistors, and a pin-diode.

25. The radio transceiver recited in claim 1, wherein the amount of isolation is approximately 20-40 dB.

26. The radio transceiver recited in claim 1, wherein the at least one known signal is at least one of a single complex tone c₁e^jω

^₁t and a single complex tone c₁e<sup>−

jω

₁t</sup>, where c₁ is a constant.

27. The radio transceiver recited in claim 1, wherein the at least one receiver compensation factor estimation unit comprises a plurality of receiver compensation factor estimation units.

28. The radio transceiver recited in claim 27, wherein the at least one known signal is at least one of a plurality of complex tones {c_ke^jω

^_kt} and a plurality of complex tones {c_je<sup>−

jω

₁t</sup>}, where c_k and c_j are constants, and j and k index at least one of different frequencies and different constants.

29. The radio transceiver recited in claim 1, wherein the receiver section further comprises a low noise amplifier for amplifying the various received signals, the low noise amplifier having an input connectable to the switching unit and an output coupled to the input of the quadrature demodulator.

30. The radio transceiver recited in claim 1, wherein the transmitter section comprises a transmitter compensation factor estimation unit coupled to the output of the at least one receiver compensation factor estimation unit for determining at least one transmitter compensation factor for compensating the transmitter for an imbalance in transmitted in-phase and quadrature phase signals.

31. The radio transceiver recited in claim 30, wherein the transmitter section further comprises an inverse Fast Fourier Transform (IFFT) unit for generating a multi-carrier signal and wherein the transmitter compensation factor estimation unit estimates, after conversion of the multi-carrier signal into in-phase and quadrature phase signals, at least one transmitter compensation factor for compensating the transmitter for an imbalance in transmitted in-phase and quadrature phase signals.

32. The radio transceiver recited in claim 30, wherein the at least one transmitter compensation factor estimation unit estimates an amplitude imbalance (γ

_t) according to the equation

33. The radio transceiver recited in claim 30, wherein the at least one transmitter compensation factor estimation unit estimates a phase imbalance (φ

_t) according to the equation

34. The radio transceiver recited in claim 30, wherein the transmitter section further comprises a transmitter I−

Q compensation unit for applying the at least one transmitter compensation factor to the transmitted in-phase and quadrature phase signals.

35. The radio transceiver recited in claim 34, wherein the transmitter I−

Q compensation unit determines the at least one transmitter compensation factor using tan φ

_t and (γ

_t cos φ

_t)<sup>−

1</sup>, where γ

_t is a value of amplitude imbalance and φ

_t is a value of phase imbalance.

36. The radio transceiver recited in claim 34, wherein the transmitter I−

Q compensation unit determines the at least one transmitter compensation factor using γ

_t sin φ

_t and (γ

_t cos φ

_t), where γ

_t is a value of amplitude imbalance and φ

_t is a value of phase imbalance.

37. The radio transceiver recited in claim 34, wherein the transmitter section further comprises a transmitter memory coupled to the transmitter compensation factor estimation unit and the transmitter I−

Q compensation unit for storing the at least one transmitter compensation factor and for providing the at least one transmitter compensation factor to the transmitter I−

Q compensation unit.

38. The radio transceiver recited in claim 34, wherein the transmitter section further comprises:

a second serial/parallel converter for converting transmission data; and

a symbol mapping unit having an input coupled to an output of the second serial/parallel converter and an output coupled to an input of the transmitter I−

Q compensation unit for receiving and mapping the converted transmission data to the transmitted in-phase and quadrature phase signals and providing the transmitted in-phase and quadrature phase signals to the input of the transmitter I−

Q compensation unit.

39. The radio transceiver recited in claim 34, wherein the transmitter section further comprises a transmit modem processor for receiving and processing an output of the transmitter I−

Q compensation unit.

40. The radio transceiver recited in claim 34, wherein the transmitter section further comprises:

a digital-to-analog converter having an input coupled to an output of the transmitter I−

Q compensation unit;

a filter having an input coupled to an output of the digital-to-analog converter;

a transmitter variable gain amplifier having an input coupled to an output of the filter; and

a quadrature up-converter having an input coupled to an output of the transmitter variable gain amplifier.

41. The radio transceiver recited in claim 40, wherein the transmitter section further comprises a power amplifier having an input coupled to an output of the quadrature up-converter and an output coupled to the switching unit.

42. The radio transceiver recited in claim 41, further comprising a transceiver controller for controlling the transmit modem processor to generate the at least one known signal and for controlling the transmitter section to transmit the at least one known signal to the output port of the transmitter section at a power level suitable for an input range of the receiver section.

43. The radio transceiver recited in claim 42, wherein the transceiver controller controls the transmitter section to transmit the at least one known signal to the output port of the transmitter section at a power level suitable for an input range of the receiver section by setting gains of the transmitter variable gain amplifier and the power amplifier.

44. The radio transceiver recited in claim 42, wherein the receiver section includes a low noise amplifier and a variable gain amplifier and wherein the transceiver controller adjusts gain of at least one of the low noise amplifier and the variable gain amplifier during reception of the at least one known signal.

45. The radio transceiver recited in claim 42, wherein the transceiver controller sets the at least one known signal to a specified amplitude value generated digitally in the transmit modem processor.

46. The radio transceiver recited in claim 43, wherein a determination of gain settings is performed using at least one of an existing power control loop in the transmitter and an existing AGC control loop in the receiver.

47. The radio transceiver recited in claim 43, wherein a determination of gain settings is performed using a look up table having entries determined apriori based on transceiver gain and power specifications.

48. The radio transceiver recited in claim 41, wherein the switching unit comprises a duplexor for coupling the at least one antenna to the output of the power amplifier for a first frequency band and to the input of the low noise amplifier for a second frequency band.

49. The radio transceiver recited in claim 48, wherein the switching unit further comprises a first switch for coupling the output of the quadrature up-converter to the input of the power amplifier.

50. The radio transceiver recited in claim 49, further comprising a second switch for bypassing the duplexor by directly coupling the output of the quadrature up-converter to the input of the low noise amplifier.

51. The radio transceiver recited in claim 50, further comprising a third switch for coupling the input of the power amplifier to ground.

52. The radio transceiver recited in claim 1, wherein the receiver section comprises a direct conversion receiver.

53. The radio transceiver recited in claim 1, wherein the receiver section comprises a wideband intermediate frequency receiver.

54. The radio transceiver recited in claim 1, wherein the receiver section comprises a superheterodyne receiver.

55. The radio transceiver recited in claim 1, wherein the receiver section comprises a low intermediate frequency receiver.

56. A communication system comprising:

at least one antenna;

a receiver including a quadrature demodulator for converting various signals into in-phase and quadrature phase signals;

a transmitter for transmitting a data packet including at least one known signal for conversion by the quadrature demodulator;

at least one compensation factor estimation unit for estimating, after conversion of the at least one known signal into in-phase and quadrature phase signals, at least one compensation factor for compensating for an imbalance in the in-phase and quadrature phase signals resulting from conversion of the known signal.

57. The communication system recited in claim 56, wherein the data packet comprises at least one of a preamble portion, a midamble portion and a postamble portion for including the at least one known signal.

58. The communication system recited in claim 56, further comprising at least one I−

Q compensation unit for applying the at least one compensation factor to subsequent in-phase and quadrature phase signals.

59. The communication system recited in claim 56, further comprising a memory for storing the at least one compensation factor and for providing the at least one compensation factor to the at least one I−

Q compensation unit.

60. The communication system recited in claim 56, wherein the imbalance is at least one of a gain imbalance and a phase imbalance.

61. The communication system recited in claim 56, wherein the at least one compensation factor includes at least one of a gain compensation factor and a phase compensation factor.

62. The communication system recited in claim 56, wherein the transmitter comprises a power detector for receiving the at least one known signal and estimating at least one intermediate value required to compute an I−

Q imbalance in the transmitter based on the known signal.

63. The communication system recited in claim 56, wherein the data packet comprises at least one of a data portion, a header portion and a cyclic redundancy check portion.

64. The communication system recited in claim 56, further comprising at least one message buffer for storing data.

65. The communication system recited in claim 56, wherein the at least one known signal comprises a plurality of known signals having different baseband symbol values.

66. In a communication system comprising at least one antenna, a transmitter and a receiver having a quadrature demodulator for receiving various signals and converting the various signals into in-phase and quadrature phase signals, an output of the transmitter connectable to an input of the receiver by a controllable switching unit, a method for compensating for I−

Q imbalance in the in-phase and quadrature phase signals, the method comprising;

providing at least one compensation factor estimation unit for estimating at least one compensation factor;

controlling the switching unit to provide an amount of isolation between an output of the transmitter and an input of the receiver;

controlling the transmitter to generate at least one known signal at the output of the transmitter;

setting a gain of the communication system such that the at least one known signal is coupled from the output of the transmitter to the input of the receiver at a power level that is within an operating range of the input;

converting the at least one known signal into in-phase and quadrature phase signals at the quadrature demodulator;

estimating at least one compensation factor at the at least one compensation factor estimation unit based on an imbalance in the at least one known signal; and

applying the at least one compensation factor to subsequently received signals, transmitted signals or both received and transmitted signals.

67. The method recited in claim 66, further comprising storing the at least one compensation factor in a memory.

68. The method recited in claim 67, wherein setting a gain of the communication system comprises setting an initial gain of the communication system and wherein the method further comprises:

before controlling the transmitter to generate the at least one known signal, setting a value for at least one of an initial temperature of the communication system and an initial frequency of the at least one known signal;

after estimating the at least one compensation factor, storing the at least one compensation factor in the memory;

after storing the at least one compensation factor in the memory, performing at least one of setting a new gain value of the communication system, setting a new temperature value and setting a new frequency value; and

estimating at least one additional compensation factor based on the at least one new value.

69. The method recited in claim 68, further comprising estimating a plurality of additional compensation factors for at least one of a plurality of gain settings of the communication system, a plurality of temperatures of the communication system and a plurality of frequencies of the at least one known signal.

70. The method recited in claim 67, wherein applying the at least one compensation factor to subsequently received signals comprises:

controlling the switching unit to couple the input of the receiver to the at least one antenna;

synchronizing with radio frequency signals received from a remote transmitter;

performing automatic gain control to determine an appropriate receiver gain setting;

retrieving from the memory an appropriate compensation factor for the receiver based on at least one of a current gain setting, a frequency, and a temperature;

converting the received radio frequency signals into in-phase and quadrature phase signals at the quadrature demodulator; and

performing I−

Q compensation based on the appropriate compensation factor to compensate for an imbalance in the in-phase and quadrature phase signals resulting from converting the received radio frequency signals.

71. The method recited in claim 70, further comprising performing automatic frequency control, phase recovery, and channel estimation.

72. The method recited in claim 71, wherein at least one of synchronization, automatic gain control, I−

Q compensation, automatic frequency control and phase recovery are performed in a separate control channel.

73. The method recited in claim 70, further comprising:

after performing I−

Q compensation, again performing automatic gain control;

after again performing automatic gain control, determining if a receiver gain setting has changed; and

if the receiver gain setting has changed, again performing I−

Q compensation.

74. The method recited in claim 66, wherein controlling the transmitter to generate at least one known signal at the output of the transmitter comprises:

controlling the switching unit to couple the output of the transmitter to the at least one antenna;

performing power control to determine an appropriate transmitter gain setting;

retrieving from the memory an appropriate compensation factor for the transmitter based on at least one of a current gain setting, a frequency, and a temperature; and

performing I−

Q compensation based on the appropriate compensation factor.

75. The method recited in claim 74, further comprising:

after performing I−

Q compensation, again performing power control;

after again performing power control, determining if a transmitter gain setting has changed; and

if the transmitter gain setting has changed, again performing I−

Q compensation.

76. The method recited in claim 66, wherein the switching unit is controlled to provide an isolation of approximately 20-40 dB between the output of the transmitter and the input of the receiver.

77. The method recited in claim 66, wherein the at least one known signal is at least one of a single complex tone c₁e^jω

^₁t and a single complex tone c₁e<sup>−

jω

₁t</sup>, where c₁ is a constant.

78. The method recited in claim 66, wherein the at least one compensation factor estimation unit comprises a plurality of compensation factor estimation units.

79. The method recited in claim 66, wherein the at least one known signal is at least one of a plurality of complex tones {c_ke^jω

^_kt} and a plurality of complex tones {c_je<sup>−

jω

_jt</sup>}, where c_k and c_j are constants, and j and k index at least one of different frequencies and different constants.

80. The method recited in claim 66, wherein controlling the transmitter to generate the at least one known signal at the output of the transmitter comprises controlling a transmit modem processor to generate the at least one known signal.

81. The method recited in claim 66, wherein setting a gain of the communication system comprises setting gains of a variable gain amplifier and a power amplifier in the transmitter such that the at least one known signal is coupled from the output of the transmitter to the input of the receiver at a power level that is within an operating range of a low noise amplifier at the input of the receiver.

82. The method recited in claim 81, further comprising adjusting a gain of the low noise amplifier and a variable gain amplifier in the receiver during reception of the at least one known signal.

83. The method recited in claim 66, wherein setting a gain of the communication system comprises setting the at least one known signal to a specified amplitude value generated digitally in the transmit modem processor.

84. The method recited in claim 66, wherein a determination of gain settings is performed using at least one of an existing power control loop in the transmitter and an existing AGC control loop in the receiver.

85. The method recited in claim 66, wherein a determination of gain settings is performed using a look up table having entries determined apriori based on transceiver gain and power specifications.

86. The method recited in claim 66, wherein estimating at least one compensation factor comprises estimating at least one of a gain compensation factor and a phase compensation factor.

87. The method recited in claim 86, wherein the at least one compensation factor estimation unit estimates a receiver gain compensation factor (γ

_r) according to the equation

88. The method recited in claim 86, wherein the at least one compensation factor estimation unit estimates a receiver phase compensation factor (φ

_r) according to the equation

89. The method recited in claim 66, wherein the at least one compensation factor estimation unit estimates a transmitter amplitude imbalance (γ

_t) according to the equation

90. The method recited in claim 66, wherein the at least one compensation factor estimation unit estimates a transmitter phase imbalance (φ

_t) according to the equation

91. The method recited in claim 66, wherein the at least one compensation factor comprises a plurality of compensation factors and wherein estimating the at least one compensation factor includes determining if the plurality of compensation factors have converged to within a given tolerance.

92. The method recited in claim 91, wherein determining if the plurality of compensation factors have converged to within a given tolerance comprises determining if a predetermined time has elapsed.

93. The method recited in claim 66, wherein the at least one compensation factor estimation unit estimates a receiver gain compensation factor (φ

_r) according to the equation

94. The method recited in claim 93, wherein the at least one compensation factor estimation unit estimates a receiver phase compensation factor (φ

_r) according to the equation

95. In a communication system comprising at least a first transmitter and a first receiver having a quadrature demodulator for receiving various signals and converting the various signals into in-phase and quadrature phase signals, a method for compensating for I−

Q imbalance in the in-phase and quadrature phase signals, the method comprising;

transmitting from the first transmitter to the first receiver a data packet having at least one known signal for conversion by the quadrature demodulator;

estimating, after conversion by the quadrature demodulator of the at least one known signal into in-phase and quadrature phase signals, at least one compensation factor for compensating for an imbalance in the in-phase and quadrature phase signals.

96. The method recited in claim 95, wherein the data packet further includes at least one of a preamble portion, a midamble portion and a postamble portion for including the at least one known signal.

97. The method recited in claim 95, wherein the first transmitter is located remotely from the first receiver.

98. The method recited in claim 95, further comprising storing the at least one compensation factor in memory.

99. The method recited in claim 95, wherein the data packet further comprises at least one of a data portion, a header portion and a cyclic redundancy check portion.

100. The method recited in claim 95, wherein the at least one known signal is included in the preamble portion such that after estimation of the at least one compensation factor, the at least one compensation factor is immediately applied to the data portion.

101. The method recited in claim 99, further comprising storing the data portion in a buffer.

102. The method recited in claim 101, wherein the at least one known signal is included in at least one of the midamble portion and the postamble potion such that the data portion is first stored in the buffer before estimation of the at least one compensation factor.

103. The method recited in claim 102, wherein after estimation of the at least one compensation factor, the at least one compensation factor is applied to the data portion.

104. The method recited in claim 95, wherein the at least one compensation factor is used to compensate for an imbalance in in-phase and quadrature phase signals in at least one of the first receiver and the first transmitter.

105. The method recited in claim 95, wherein the communication system further comprises a second transmitter located locally with the first receiver and wherein the method further comprises using the at least one compensation factor to compensate for an imbalance in in-phase and quadrature phase signals in the second transmitter.

106. The method recited in claim 105, wherein the communication system further comprises a second receiver located locally with the first transmitter and wherein the method further comprises transmitting from the second transmitter to the second receiver a return message including the at least one compensation factor.

107. The method recited in claim 106, wherein the at least one compensation factor is used to compensate for an imbalance in in-phase and quadrature phase signals in the first transmitter.

108. The method recited in claim 106, wherein the at least one compensation factor is used to compensate for an imbalance in in-phase and quadrature phase signals in the second receiver.

109. The method recited in claim 95, wherein the at least one known signal is further used to perform automatic gain control, automatic frequency control and phase recovery.