US20110176641A1

US20110176641A1 - D.C. Offset Estimation

Info

Publication number: US20110176641A1
Application number: US12/690,549
Authority: US
Inventors: Jonas Persson
Original assignee: ST Ericsson SA
Current assignee: ST Ericsson SA
Priority date: 2010-01-20
Filing date: 2010-01-20
Publication date: 2011-07-21
Also published as: WO2011089184A1; US8976692B2; CN102742165B; US20120314602A1; CN102742165A; KR20120112614A; EP2526625A1; KR101771272B1

Abstract

A combination of a phase shifter, a measurement receiver, and an offset estimator enable the d.c. offset in the transmit path of a quadrature transmitter to be distinguished from the d.c. offset in the measurement receiver. The measurement receiver performs a first measurement on the transmit path output with a “normal” phase shift of 0 degrees and 90 degrees for in-phase (I) and quadrature (Q) components, and a second measurement with a “special” phase-shift of 180 degrees and 270 degrees for the I and Q components, respectively

Description

TECHNICAL FIELD

This invention relates to electronic communication systems and more particularly to signal offset compensation in such systems.

BACKGROUND

Many current electronic communication systems use quadrature modulation schemes, which use in-phase (I) and quadrature (Q) signal components, and do not have constant envelopes. Examples of such communication systems are cellular radio telephone systems that use wideband code division multiple access (WCDMA), orthogonal frequency division multiple access (OFDMA), and their variants. Thus, part of the communicated information is encoded in the amplitude (envelope) of the transmitted signal and part is encoded in the phase of the transmitted signal.
To avoid distorting the communicated information, the power amplifier (PA) and various other components of the radio transmitter have to be linear, which is to say for example that the functional relationship between the output power of the PA and the input power of the PA is a straight line for all possible power levels. In addition, the phase shift of the input signal for example through the PA has to be constant for all possible power levels.
Departures from amplitude linearity and constant phase introduce distortion into the communicated signal, such as spectral broadening that can disturb adjacent channels. Amplitude/phase distortion (vector distortion) in the transmitter can also increase the bit error rate (BER) of the communication system, e.g., degrading the audio quality of a voice call or reducing the speed of an internet connection.
In general, the likelihood of proper performance can be increased by including in the transmitter a measurement receiver (MRX) that samples the transmitted signal and generates a compensation signal is fed back to the modulator, PA, and/or other transmitter components to correct the transmitter output signal. Such an arrangement 100 is depicted in FIG. 1, which shows an antenna 102, a coupler 104, an amplifier 106, a quadrature modulator 108, and an MRX 110. The amplifier 106 and modulator 108 can be considered the “transmit path” of the arrangement 100, which it will be understood typically includes oscillators and other components not shown. As seen in the figure, the MRX 110 samples the transmitted signal generated by the transmit path through the operation of the coupler 104 and provides a compensation signal to the modulator 108.
The MRX 110 can be used for several purposes, one of which is measurement, or more generally estimation, of the direct-current (d.c.) offset between I and Q components in the amplifier 106 and quadrature modulator 108. To achieve that purpose correctly, the I/O d.c. offset of the MRX itself typically must be negligible (ideally, it should be zero) or at least well known. Otherwise, the I/O d.c. offset of the transmit path will generally not be correctly estimated.
European Patent Application Publication No. EP 1 835 626 A1 by Ishikawa et al. describes a d.c. offset correction value estimating unit that estimates a d.c. offset correction value based on a transmit signal that is produced by a quadrature modulator. A signal level detecting unit detects the signal level of an input signal, a weight factor calculating unit computes a weight factor for the d.c. offset correction value in accordance with the signal level, and a weighting unit assigns a weight to the d.c. offset correction value in accordance with the weight factor. A d.c. offset in the transmit signal is compensated by using the thus weighted d.c. offset correction value.
U.S. Patent Application Publication No. US 2007/0092023 by Kang et al. describes a method for self-calibrating mismatch and d.c. offset in a mobile transceiver. The transceiver's transmitter is used as a signal generator and the transceiver's receiver is used to measure a response characteristic. A baseband processor calibrates the mismatch and the d.c. offset for the receiving and transmitting sides using a test signal received from the transmitter.
U.S. Pat. No. 7,266,359 to Chen et al. describes a method for removing d.c. interference from a signal received by a communication receiver that removes d.c. offsets induced by the receiver and the transmitter. The method includes removing an estimated d.c. offset from a received signal, correcting a frequency shift in the received signal, estimating a second d.c. offset signal induced by the transmitter, and removing the
estimated second d.c. offset from the received signal. The receiver d.c. offset is estimated and removed before performing a timing carrier offset correction using Barker code manipulation to remove receiver d.c. offset and to sum all Barker chips after effectively multiplying Barker codes to correlate to a Barker sequence unaffected by the receiver d.c. offset signal.
U.S. Patent Application Publication No. US 2008/0063113 by Gao et al. describes a method of correcting d.c. offset errors in a transmitter having an OFDMA-based quadrature modulator. A compensator before the modulator compensates the d.c. offset and is updated with estimated d.c. offset values obtained by performing a discrete Fourier transform in the digital baseband domain while sending a pair of orthogonal test tones to the modulator's inputs.
U.S. Patent Application Publication No. US 2009/0041161 by Jian et al. describes a d.c. offset estimation in an OFDMA system that includes a carrier frequency offset estimator receiving an input signal and estimating a carrier frequency offset value, a symbol timing recovery unit providing a symbol boundary of the input signal, and a d.c. offset estimator estimating a d.c. offset value based on the input signal, the carrier frequency offset value, and the symbol boundary.
Despite those and other previous attempts, the problem of d.c. offset estimation remains difficult to solve, and the requirements on an MRX remain difficult to meet.

SUMMARY

This invention enables the I/O d.c. offset in the transmit path to be estimated without prior knowledge of the I/O d.c. offset of the MRX itself. Moreover, this invention enables both the I/O d.c. offset of the transmit path and the I/O d.c. offset of the MRX to be estimated.
In accordance with aspects of this invention, there is provided an apparatus for estimating a d.c. offset in a transmitter having a transmit path for quadrature modulating a carrier with input I and Q component signals and generating a transmit signal. The apparatus includes a measurement receiver, a phase shifter, and an offset estimator. The measurement receiver is configured to quadrature demodulate a portion of the transmit signal to generate an I component measurement signal and a Q component measurement signal. The phase shifter is configured to generate a first pair of oscillator signals having a relative phase shift of substantially 90 degrees for quadrature demodulation in the measurement receiver and for quadrature modulation in the transmit path. The phase shifter is also configured to selectively generate a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a phase shift of 180 degrees with respect to the first pair of oscillator signals for quadrature demodulation in the measurement receiver. The offset estimator is configured to compute at least one of a d.c. offset of the transmit path and a d.c. offset of the measurement receiver based on the input I and Q component signals and on measurement I and Q component signals generated with the first and second pairs of oscillator signals.
In other aspects, there is provided a method of estimating a d.c. offset in a transmitter having a transmit signal generated by quadrature mixing input I and Q component signals with respective ones of a first pair of oscillator signals having a relative phase shift of substantially 90 degrees. The method includes generating a first pair of measurement I component and Q component measurement signals by quadrature demodulating a portion of the transmit signal with the first pair of oscillator signals; generating a second pair of measurement I component and Q component signals by quadrature demodulating a portion of the transmit signal with a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a relative phase shift with respect to the first pair of oscillator signals of substantially 180 degrees; and computing the d.c. offset based on the first and second pairs of measurement I and Q component signals and on the input I and Q component signals.
In other aspects, there is provided a computer-readable medium having stored instructions that, when executed by a computer, cause the computer to perform a method of estimating a d.c. offset in a transmitter having a transmit signal generated by quadrature mixing input I and Q component signals with respective ones of a first pair of oscillator signals having a relative phase shift of substantially 90 degrees. The method includes generating a first pair of measurement I component and Q component measurement signals by quadrature demodulating a portion of the transmit signal with the first pair of oscillator signals; generating a second pair of measurement I component and Q component signals by quadrature demodulating a portion of the transmit signal with a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a relative phase shift with respect to the first pair of oscillator signals of substantially 180 degrees; and computing the d.c. offset based on the first and second pairs of measurement I and Q component signals and on the input I and Q component signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The several objects, features, and advantages of this invention will be understood by reading this description in conjunction with the drawings, in which:

FIG. 1 is a block diagram of transmitter with a measurement receiver;

FIG. 2 is a block diagram of a portion of an improved transmitter with a measurement receiver;

FIG. 3 is a flow chart of method of d.c. offset estimation;

FIG. 4 depicts a communication network; and

FIG. 5 is a block diagram of a user equipment for the communication network.

DETAILED DESCRIPTION

This invention is applicable to any type of communication system and can be applied in any part of the system, e.g., uplink (UL) or downlink (DL), where d.c.-offset estimation is of interest.
FIG. 2 is a block diagram of a portion of an improved transmitter 200 with a measurement receiver in accordance with this invention. The transmitter 200 includes a coupler 202, a transmit path that has an analog part 204 and a digital part 206, a voltage-controlled oscillator (VCO) 208, a phase shifter 210, an MRX 212, and an offset estimator 214.
The digital part 206 of the transmit path includes a digital transmit signal waveform generator (WFG) 216 that produces an in-phase transmit signal i_txand a quadrature transmit signal q_tx, which are converted to analog form by respective digital-to-analog converters (DACs). The analog part 204 of the transmit path uses the analog transmit signal components produced by the DACs to quadrature-modulate a carrier signal generated by the VCO 208 or equivalent local oscillator (LO) in the usual way with two mixers fed respectively by an unshifted and 90-degree shifted LO signal. The phase shifter 210 implements the 0-degree and 90-degree phase shifts of the LO signal needed for the quadrature modulation in the transmit path, and for quadrature demodulation in the MRX 212 as described below. The modulated carrier is suitably amplified, for example by a power amplifier PA and variable-gain amplifier VGA in the analog part 204, and the complex transmitter output signal is passed to an antenna or other port (not shown) through the coupler 202.
The waveform generator 216 also provides an in-phase reference signal i_refand a quadrature reference signal q_refto the estimator 214. The reference signals i_refand ref are simply the transmit signals i_txand q_tx, respectively, and are used by the estimator 214 as described in more detail below. In general, the generator 216 generates signals that are compliant with the applicable modulation type and system standards in terms of data rate, pulse shaping filter, data (IQ) constellation, etc. The artisan will understand that the generator 216 can be implemented in a substantially conventional way, although a relevant aspect of the generator 216 is the time alignment between the signal that is sent (i_txand q_tx) and the signal that is measured (i_refand q_ref). In order to ensure a suitable time alignment, the generator 216 can include or implement a suitable variable delay element, or such a delay element can be provided elsewhere in the transmitter 200, to adjust the reference signals with respect to the transmit signal.
As depicted in FIG. 2, the MRX 212 receives from the coupler 202 a portion of the transmit signal generated by the transmit path. That portion is amplified by a suitable low-noise amplifier LNA and passed to a quadrature demodulator that includes two mixers fed by selectively phase-shifted LO signals from the phase shifter 210. The down-shifted (demodulated) I and Q component signals produced by respective ones of the mixers are low-pass filtered and converted to digital component measurement signals i_measand q_measby respective suitable analog-to-digital converters ADC.
In addition to nominal 0-degree and nominal 90-degree phase shifts used for the quadrature modulator in the analog part 204 and for the quadrature demodulator in the MRX 212, the phase shifter 210 also generates nominal 180-degree and nominal 270-degree phase shifts of the LO signal for the quadrature demodulator in the MRX 212. The MRX 212 performs a first measurement with the “normal” phase shift, i.e., 0 degrees and 90 degrees for the I and Q components, respectively, in the MRX quadrature demodulator, and a second measurement with a “special” phase-shift, i.e., 180 degrees and 270 degrees for the I and Q components, respectively.
It is currently preferred that the phase shifter 210 generates the shifts in successive pairs for particular time intervals, but it will be noted that the phase shifter 210 can be configured to provide four continuous outputs to the MRX 212, which could then have two parallel demodulators, each comprising two mixers, generating four continuous measurement signal components. Although such an arrangement has some advantages, it requires extra physical space, more power, and careful matching of the pairs of mixers.
In general, using the MRX 212 as little as possible is desirable in order to save power, which can be important in a battery-powered transmitter. It is currently expected that the MRX would be used for periods of 25-50 microseconds, with an approximately 50% duty cycle of the 0/180 and 90/270 phase shifts, which of course are synchronized to the transmit path because the transmit and MRX measurement signals have to be time-aligned as noted above. It is also currently expected that the noisiness of the MRX measurement signals should be reduced by suitable smoothing, for example by low-pass filters, resettable integrators, or simply averaging. It will be noted that the equations given below are in terms of discrete signal samples, and do not include such smoothing, which can be implemented in many suitable ways, for example by software programming in the offset estimator 214.
The combination of the phase shifter 210, MRX 212, and offset estimator 214 as described above enables the d.c. offset in the transmit path, comprising the analog and digital parts 204, 206, to be distinguished from the d.c. offset in the MRX 212. The estimator 214, which may be a suitably programmed digital processor or collection of logic gates, can compute either or both of the d.c. offsets according to the following equations and provide the computed offsets as results that can be used by other components in the transmitter 200.
As depicted in FIG. 2, an offset result generated by the estimator 214, which is an estimate of the d.c. offset, can be fed back to manipulate either the analog or digital parts 204, 206 or the MRX 212. For example, the offset result can be fed back to the transmit digital part 206 by providing the result to the transmit generator 216, which can then compensate the transmit signals i_tx, q_txit generates based on the offset result. Such compensation can include simply adding the offset result to the transmit signals either in the generator 216 by including suitable adders (not shown for clarity) external to the generator 216. For another example, the offset result can be combined with the analog transmit signals in the analog part 204 by suitable adders included before the mixers of the quadrature modulator. An offset result generated by the estimator 214 can also or instead be fed back to the MRX 212 as depicted in FIG. 2 through suitable adders included after the mixers in the quadrature demodulator. Also as depicted in FIG. 2, the offset result generated by the estimator 214 can be provided to other components (not shown) in the transmitter 200, for example, for inclusion in a report message.
Thus, FIG. 3 is a flow chart of a method of estimating a d.c. offset in a transmitter having a transmit signal generated by quadrature mixing input I and Q component signals with respective ones of a first pair of oscillator signals having a relative phase shift of substantially 90 degrees, e.g., 0-degree and 90-degree signals coming from the phase shifter 210. In step 302, a first pair of measurement I component and Q component measurement signals, such as i_measand q_measdescribed above, is generated by quadrature demodulating a portion of the transmit signal with the first pair of oscillator signals. In step 304, a second pair of measurement I component and Q component signals by quadrature demodulating a portion of the transmit signal with a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a relative phase shift with respect to the first pair of oscillator signals of substantially 180 degrees, e.g., 180-degree and 270-degree signals from the phase shifter 210. In step 306, the d.c. offset is computed based on the first and second pairs of measurement I and Q component signals and on the input I and Q component signals.
The complex transmitter output signal including d.c. offset of the transmit path can be written as follows:
z _TX =i _ref +q _ref +z _DC,TX =i _ref +q _ref +i _DC,TX +q _DC,TX Eq. 1
in which z_TXis the transmitter output signal, i_refis an I channel reference, q_refis a Q channel reference, and z_DC,TXis the d.c. offset of the transmit path, which can be separated as shown into d.c. offsets of the I and Q components of the transmit path, i_DC,TXand q_DC,TX, respectively.
In a similar way, the complex output signal including d.c. offset of the MRX 212 can be written as follows:
z _MRX =i _meas +q _meas Eq. 2
in which z_MRXis the MRX output signal, i_measis the I channel signal measured by the MRX 212, and q_measis the Q channel signal measured by the MRX 212. The combined d.c. offset z_DC,totof the transmit path and the MRX 212 can be written as follows:
z _DC,tot =Z _DC,TX +Z _DC,MRX =i _DC,TX +q _DC,TX +i _DC,MRX +q _DC,MRX Eq. 3
in which z_DC,MRXis the d.c. offset of the MRX 212 and the other parameters are as defined above.
With the usual 0-degree and 90-degree phase shifts in the transmit path and MRX 212, the 1-channel and Q-channel measurement signals generated by the MRX 212 can be written as follows:
i _meas =i _ref +i _DC,MRX +i _DC,TX Eq. 4
q _meas =q _ref +q _DC,MRX +q _DC,TX Eq. 5
and with the “special” 180-degree and 270-degree phase shifts in the MRX 212, the I-channel and Q-channel measurement signals generated by the MRX 212 can be written as follows:
î _meas =−i _ref +i _DC,MRX −i _DC,TX Eq. 6
{circumflex over (q)} _meas =−q _ref +q _DC,MRX −q _DC,TX Eq. 7
in which the “hat” indicates the “special” phase shifts.
Adding Eq. 4 and Eq. 6 yields the following:
i _meas +î _meas=2i _DC,MRX Eq. 8A
which can be re-arranged to give the 1-channel d.c. offset of the MRX 212 as follows:
$\begin{matrix} i_{D C, MRX} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2} & Eq . 8 B \end{matrix}$
In a similar way, adding Eq. 5 and Eq. 7 yields the following:
q _meas +{circumflex over (q)} _meas=2q _DC,MRX Eq. 9A
which can be re-arranged to give the Q-channel d.c. offset of the MRX 212 as follows:
$\begin{matrix} q_{D C, MRX} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2} & Eq . 9 B \end{matrix}$
Subtracting Eq. 6 from Eq. 4 yields the following:
i _meas −î _meas=2i _ref+2i _DC,TX Eq. 10A
which can be re-arranged to give the I-channel d.c. offset of the transmit path as follows:
$\begin{matrix} i_{D C, TX} = \frac{i_{meas} - {\overset{}{i}}_{meas} - 2 i_{ref}}{2} & Eq . 10 B \end{matrix}$
In a similar way, subtracting Eq. 7 from Eq. 5 yields the following:
q _meas −{circumflex over (q)} _meas=2q _ref+2q _DC,TX Eq. 11A
which can be re-arranged to give the Q-channel d.c. offset of the transmit path as follows:
$\begin{matrix} q_{D C, TX} = \frac{q_{meas} - {\overset{}{q}}_{meas} - 2 q_{ref}}{2} & Eq . 11 B \end{matrix}$
The offset estimator 214 can compute the individual d.c. offsets of the transmit path and the MRX 212 using Eqs. 8B, 9B, 10B, and 11B.
The estimator 214 and various other functional blocks of the transmitter 200 can be implemented by one or more suitably programmed electronic processors, collections of logic gates, etc. that process information stored in one or more memories. The stored information may include program instructions and data that enable the estimator 214 to implement the equations described above.
FIG. 4 is a diagram of an exemplary communication network 400, which may be, for example, a WCDMA communication system. Radio network controllers (RNCs) 402 a, 402 b control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs user calls via the appropriate RBSs, which communicate with user equipments (UEs) 300 a, 300 b through downlink (i.e., base-to-mobile, or forward) and uplink (i.e., mobile-to-base, or reverse) channels. RNC 402 a is shown coupled to RBSs 404 a, 404 b, 404 c, and RNC 402 b is shown coupled to RBSs 404 d, 404 e, 404 f. Each RBS, which can also be called a NodeB, serves a geographical area that can be divided into one or more cell(s). RBS 404 f is shown as having five antenna sectors S1-S5, all or some of which can be said to make up the cell of the RBS 404 f. The RBSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 402 a, 402 b are typically connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center and/or a packet radio service node (not shown). The artisan will understand that the components and arrangement depicted in FIG. 4 are examples and should not be construed as limiting the components and arrangement of an actual communication system.
FIG. 5 depicts a communication device 500, such as a mobile telephone, remote terminal, or equivalent device, that can communicate through a wireless link with a base station in a communication network. The device 500 can be a UE 300 in the network 400. Among other things, the UE 500 includes one or more programmable processors 502 or suitable logic that processes information stored in one or more memories 504, 506. The stored information may include, among other things, program instructions for computing the d.c. offsets as described above. It will be appreciated that the processor 502 typically includes timers, etc. that facilitate its operations. Transceiver (TRX) circuitry 508 provides for the reception and transmission of control and traffic signals on the link between the UE 500 and the base station, which can include similar transceiver circuitry. The TRX 508 includes the transmitter portion 200 described above that operates under the control of the processor 502.
It is expected that this invention can be implemented in a wide variety of environments, including for example mobile communication devices. It will be appreciated that procedures described above are carried out repetitively as necessary. To facilitate understanding, many aspects of the invention are described in terms of sequences of actions that can be performed by, for example, elements of a programmable computer system. It will be recognized that various actions could be performed by specialized circuits (e.g., discrete logic gates interconnected to perform a specialized function or application-specific integrated circuits), by program instructions executed by one or more processors, or by a combination of both. Many communication devices can easily carry out the computations and determinations described here with their programmable processors and application-specific integrated circuits.
Moreover, the invention described here can additionally be considered to be embodied entirely within any form of computer-readable storage medium having stored therein an appropriate set of instructions for use by or in connection with an instruction-execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch instructions from a medium and execute the instructions. As used here, a “computer-readable medium” can be any means that can contain, store, or transport the program for use by or in connection with the instruction-execution system, apparatus, or device. The computer-readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device. More specific examples (a non-exhaustive list) of the computer-readable medium include an electrical connection having one or more wires, a portable computer diskette, a RAM, a ROM, an erasable programmable read-only memory (EPROM or Flash memory), and an optical fiber.
Thus, the invention may be embodied in many different forms, not all of which are described above, and all such forms are contemplated to be within the scope of the invention. For each of the various aspects of the invention, any such form may be referred to as “logic configured to” perform a described action, or alternatively as “logic that” performs a described action.
It is emphasized that the terms “comprises” and “comprising”, when used in this application, specify the presence of stated features, integers, steps, or components and do not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof.
The particular embodiments described above are merely illustrative and should not be considered restrictive in any way. The scope of the invention is determined by the following claims, and all variations and equivalents that fall within the range of the claims are intended to be embraced therein.

Claims

1. An apparatus for estimating a direct-current (d.c.) offset in a transmitter having a transmit path for quadrature modulating a carrier with input in-phase (I) and quadrature (Q) component signals and generating a transmit signal, the apparatus comprising:

a measurement receiver, wherein the measurement receiver is configured to quadrature demodulate a portion of the transmit signal to generate an I component measurement signal and a Q component measurement signal;

a phase shifter, wherein the phase shifter is configured to generate a first pair of oscillator signals having a relative phase shift of substantially 90 degrees for quadrature demodulation in the measurement receiver and for quadrature modulation in the transmit path, and the phase shifter is configured to selectively generate a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a phase shift of substantially 180 degrees with respect to the first pair of oscillator signals for quadrature demodulation in the measurement receiver; and

an offset estimator, wherein the offset estimator is configured to compute at least one of a d.c. offset of the transmit path and a d.c. offset of the measurement receiver based on the input I and Q component signals and on measurement I and Q component signals generated with the first and second pairs of oscillator signals.

2. The apparatus of claim 1, wherein the offset estimator is configured to compute an I component d.c. offset of the measurement receiver according to:

i_{D C, MRX} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}

in which i_DC,MRXis the I component of the d.c. offset of the measurement receiver, i_measis the measurement I component signal generated with one of the first pair of oscillator signals, and i_measis the measurement I component signal generated with one of the second pair of oscillator signals having a 180-degree phase shift with respect to the one of the first pair of oscillator signals; and the offset estimator is configured to compute a Q component d.c. offset of the measurement receiver according to:

q_{D C, MRX} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}

in which q_DC,MRXis the Q component of the d.c. offset of the measurement receiver, q_measis the measurement Q component signal generated with the other one of the first pair of oscillator signals, and {circumflex over (q)}_measis the measurement Q component signal generated with the other one of the second pair of oscillator signals.

3. The apparatus of claim 1, wherein the offset estimator is configured to compute an I component d.c. offset of the transmit path according to:

i_{D C, TX} = \frac{i_{meas} - {\overset{}{i}}_{meas} - 2 i_{ref}}{2}

in which i_DC,TXis the I component of the d.c. offset of the transmit path, i_measis the measurement I component signal generated with one of the first pair of oscillator signals, î_measis the measurement I component signal generated with one of the second pair of oscillator signals having a 180-degree phase shift with respect to the one of the first pair of oscillator signals, and i_refis the I component signal; and the offset estimator is configured to compute a Q component d.c. offset of the transmit path according to:

q_{D C, TX} = \frac{q_{meas} - {\overset{}{q}}_{meas} - 2 q_{ref}}{2}

in which q_DC,TXis the Q component of the d.c. offset of the transmit path, q_measis the Q component measurement signal generated with the other one of the first pair of oscillator signals, {circumflex over (q)}_measis the measurement Q component signal generated with the other one of the second pair of oscillator signals, and q_refis the Q component signal.

4. A method of estimating a direct-current (d.c.) offset in a transmitter having a transmit signal generated by quadrature mixing input in-phase (I) and quadrature (Q) component signals with respective ones of a first pair of oscillator signals having a relative phase shift of substantially 90 degrees, the method comprising:

generating a first pair of measurement I component and Q component measurement signals by quadrature demodulating a portion of the transmit signal with the first pair of oscillator signals;

generating a second pair of measurement I component and Q component signals by quadrature demodulating a portion of the transmit signal with a second pair of oscillator signals having a relative phase shift of substantially 90 degrees and a relative phase shift with respect to the first pair of oscillator signals of substantially 180 degrees; and

computing the d.c. offset based on the first and second pairs of measurement I and Q component signals and on the input I and Q component signals.

5. The method of claim 4, wherein computing the d.c. offset includes computing an I component d.c. offset according to:

i_{D C, MRX} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}

in which i_DC,MRXis the I component d.c. offset, i_measis the measurement I component signal generated with one of the first pair of oscillator signals, and î_measis the measurement I component signal generated with one of the second pair of oscillator signals having a 180-degree phase shift with respect to the one of the first pair of oscillator signals; and computing the d.c. offset includes computing a Q component d.c. offset according to:

q_{D C, MRX} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}

in which q_DC,MRXis the Q component d.c. offset, q_measis the measurement Q component signal generated with the other one of the first pair of oscillator signals, and {circumflex over (q)}_measis the measurement Q component signal generated with the other one of the second pair of oscillator signals.

6. The method of claim 4, wherein computing the d.c. offset includes computing an I component d.c. offset of the transmit path according to:

i_{D C, TX} = \frac{i_{meas} - {\overset{}{i}}_{meas} - 2 i_{ref}}{2}

in which i_DC,TXis the I component of the d.c. offset of the transmit path, i_measis the measurement I component signal generated with one of the first pair of oscillator signals, î_measis the measurement I component signal generated with one of the second pair of oscillator signals having a 180-degree phase shift with respect to the one of the first pair of oscillator signals, and i_refis the I component signal; and computing the d.c. offset includes computing a Q component d.c. offset of the transmit path according to:

q_{D C, TX} = \frac{q_{meas} - {\overset{}{q}}_{meas} - 2 q_{ref}}{2}

7. A computer-readable medium having stored instructions that, when executed by a computer, cause the computer to perform a method of estimating a direct-current (d.c.) offset in a transmitter having a transmit signal generated by quadrature mixing input in-phase (I) and quadrature (Q) component signals with respective ones of a first pair of oscillator signals having a relative phase shift of substantially 90 degrees, wherein the method comprises:

8. The medium of claim 7, wherein computing the d.c. offset includes computing an I component d.c. offset according to:

i_{D C, MRX} = \frac{i_{meas} + {\overset{}{i}}_{meas}}{2}

q_{D C, MRX} = \frac{q_{meas} + {\overset{}{q}}_{meas}}{2}

9. The medium of claim 7, wherein computing the d.c. offset includes computing an I component d.c. offset of the transmit path according to:

i_{D C, TX} = \frac{i_{meas} - {\overset{}{i}}_{meas} - 2 i_{ref}}{2}

q_{D C, TX} = \frac{q_{meas} - {\overset{}{q}}_{meas} - 2 q_{ref}}{2}