The invention relates to a radio transmitter and a transmission method.
In a radio transmitter applying IQ-modulation, local oscillator (LO) leakage is due to errors in the I and Q branches. One means of cancelling the LO leakage is to null it during the manufacture of the transmitter. However, this approach does not take into account aging and the effect environmental factors may have on the leakage. It is thus clear that improved ways of cancelling LO leakage are needed.
An object of the present invention is to provide an apparatus, comprising a first local oscillator configured to generate a first oscillation signal, a first mixer configured to generate a transmit signal by mixing an input signal and the first oscillation signal, an observation receiver configured to receive a portion of the transmit signal, apply, in a first operation mode of the apparatus, the received portion of the transmit signal in the observation receiver such that a transmit signal component on a frequency of the first oscillation signal present in the received portion of the transmit signal falls into a band-pass frequency of the observation receiver, shift, in a second operation mode of the apparatus, the received portion of the transmit signal such that an oscillation leakage signal component of the first oscillation signal present in the received portion of the transmit signal falls into the band-pass frequency of the observation receiver; and which apparatus comprises a compensation unit configured to generate a compensation signal on the basis of the band-pass signal of the observation receiver for compensation of the input signal.
In another aspect, there is provided an apparatus, comprising means for generating a first oscillation signal, means for generating a transmit signal by mixing an input signal and the first oscillation signal, means for receiving a portion of the transmit signal, means for applying, in a first operation mode of the apparatus, the received portion of the transmit signal in the observation receiver such that a transmit signal component on a frequency of the first oscillation signal present in the received portion of the transmit signal falls into a band-pass frequency of the observation receiver, means for shifting, in a second operation mode of the apparatus, the received portion of the transmit signal such that an oscillation leakage signal component of the first oscillation signal present in the received portion of the transmit signal falls into the band-pass frequency of the observation receiver, and means for generating a compensation signal on the basis of the band-pass signal of the observation receiver for compensation of the input signal.
In another aspect, there is provided a method, comprising generating a first oscillation signal, generating a transmit signal by mixing an input signal and the first oscillation signal, receiving a portion of the transmit signal in an observation receiver, applying, in a first operation mode, the received portion of the transmit signal such that a transmit signal component on a frequency of the first oscillation signal present in the received portion of the transmit signal falls into a band-pass frequency of the observation receiver, shifting, in a second operation mode of the apparatus, the received portion of the transmit signal such that an oscillation leakage signal component of the first oscillation signal present in the received portion of the transmit signal falls into the band-pass frequency of the observation receiver, and generating a compensation signal on the basis of the band-pass signal of the observation receiver for compensation of the input signal.
In another aspect, there is provided a computer-readable medium having computer-executable components comprising generating a first oscillation signal, generating a transmit signal by mixing an input signal and the first oscillation signal, receiving a portion of the transmit signal in an observation receiver, applying, in a first operation mode, the received portion of the transmit signal such that a transmit signal component on a frequency of the first oscillation signal present in the received portion of the transmit signal falls into a band-pass frequency of the observation receiver, shifting, in a second operation mode of the apparatus, the received portion of the transmit signal such that an oscillation leakage signal component of the first oscillation signal present in the received portion of the transmit signal falls into the band-pass frequency of the observation receiver, and generating a compensation signal on the basis of the band-pass signal of the observation receiver for compensation of the input signal.
In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which
In the following, the invention is mainly explained in conjunction with a predistortion transmitter. However, the invention is not limited to predistortion transmitters but may also be applied in other transmitter types having an observation/feedback path. Thus, instead of or in addition to a predistortion feedback path, the transmitter may include a feedback path for observation of power levels in the transmit path or measurement of timings between a plurality of transmit paths, for instance.
In
The output of the predistortion unit is passed on to a digital-to-analogue converter unit 112, in which the digital I and Q signals are converted to analogue signals. A low pass filter 114 filters undesired signal components, which are introduced by the digital to analogue conversion unit 112.
In the mixer 116, the I and Q signals output by the low-pass filter 114 are mixed with a first oscillation signal from the first local oscillator 130. The output of the mixer 116 is a signal on a radio frequency (RF) to be transmitted by the transmitter.
A band-pass filter 118 filters undesired components introduced by the mixer 116. The output of the band-pass filter is forwarded to a power amplifier 120, which amplifies the signal for transmission. The components 110 to 120 form a transmission path of the transmitter.
A coupler 126 is coupled to the output of the power amplifier 120. One output of the coupler is forwarded to radio frequency modules for radio transmission (not shown) and the second output is coupled to an observation receiver of the transmitter. Practically, the observation receiver includes a feedback path including the components 140 to 144.
The first unit in the observation receiver/path is a mixer 140 for down-converting the transmit signal received from the coupler 122. For the down-conversion, the mixer receives an oscillation signal either from a first local oscillator 130 or a second local oscillator 132. The transmitter includes a mechanism 134 for selecting the source of the oscillation signal. In an embodiment, the mechanism is a switch. In another embodiment, the mechanism may based on attenuation of one of the signals. That is, the coupling may be such that both of the oscillation signal sources LO1, LO2 are connected to the mixer 140 all the time but one of the sources may be attenuated such that feeding of one of the signals only is allowed to the mixer.
In the embodiment of
The band-pass filter 142 filters undesired components of the down-converted signal and the analogue to digital converter 144 converts the signal into digital form, which is fed to the compensation unit 110.
When the mixer 140 is coupled to the first oscillation unit, the transmitter is in a first operation mode, in which mode the compensation unit 110 forms a compensation signal for compensation of the predistortion caused by the amplifier 120. In a second operation mode, when the mixer is coupled to the second oscillation unit, the compensation unit generates a compensation signal for compensation of a leakage signal generated by the first oscillation unit 130 to the transmit path. The compensation signal for the purpose of cancellation of the leakage signal is generated such that the second oscillation unit 132 generates an oscillation signal on such a frequency that the leakage signal is shifted to a band-pass frequency of the observation receiver and a compensation signal may then be generated of it in the compensation unit 110.
The compensation unit 110 compares, in the first operation mode, the signal to be transmitted by the predistortion unit with the signal received via the observation receiver. Ideally these signals should be the same. Based on the comparison, the predistortion unit calculates correction coefficients to be applied to the transmit signal such that the transmit signal and the feedback signal received via the feedback path would be as similar as possible. During the first operation mode, the compensation unit uses previously estimated and stored values for compensation of oscillation leakage.
In the second operation mode, the compensation unit 110 may use previously stored values for the compensation of the predistortion. In the second operation mode, the unit generates a compensation signal for the compensation of the oscillation leakage signal.
In a second operation mode, the switch 234 may forward a portion of a transmit signal, extracted by the coupler 222, to the integrated circuit 236 including a second local oscillator (LO2) and a mixer for mixing the inputted transmit signal with the second oscillation signal. Upon mixing of the extracted transmit signal portion and the second oscillation signal, an LO leakage signal component present in the transmit signal portion falls into a band-pass frequency of the observation receiver. Thereby, in the second operation mode, the leakage signal passes the band-pass filter 242, and an inverse compensation signal to the leakage signal may be generated in the analogue to digital converter 244 and the compensation unit 210.
In the embodiment of
The compensation unit may include or be coupled to two databases, a predistortion database 452 including a compensation signal with respect to different amplifying values of the power amplifier, and a leakage signal compensation database 454. Instead of databases, the compensation values may also be stored in lookup tables, for instance.
In a predistortion transmitter, the compensation of the predistortion may be applied continuously. In a first operation mode, the transmitter measures the predistortion and applies the compensation signal online. In this mode, the inversion unit 450 may update the predistortion values to the database if they have changed, and forward them to a summing unit 456 to be applied online to the incoming signal for compensation of the predistortion. However, in a second operation mode, when the transmitter generates an LO leakage compensation signal, the predistortion compensation signal is not available online. The controlling unit 458 may then, during the generation of an LO leakage compensation signal, order the summing unit to read the predistortion compensation values from the table/database 452. Typically the second mode, that is calculation of a leakage compensation signal, takes only a few milliseconds.
In the first operation mode, the compensation unit compensates for the LO leakage by reading the current LO leakage compensation values from the table/database 454. Updating of the LO leakage compensation signal may be needed when the environmental conditions, such as temperature, for instance, of the apparatus change.
Thus, practically, the apparatus may be most of the time in the first operation mode when the predistortion compensation signal is generated online, and an LO leakage compensation signal is read from memory such as a table or a database. The second operation mode is applied fairly seldom, and during the mode the predistortion compensation signal may be read from the memory, and the new LO leakage compensation signal is written to the memory.
The extracted signal portion may include several signal components, such as one on a centre frequency of the local oscillator, and a signal component on an LO leakage frequency of the first oscillation signal. The LO leakage depends on an IQ imbalance of the transmitter components.
In 506, the method branches on the basis of an operation mode of the transmitter. The first mode denotes a normal operation mode of a transmitter. In the case of a predistortion transmitter, the normal mode means online generation of a predistortion compensation signal. The second mode means a mode in which an LO leakage compensation signal is generated and a predistortion compensation signal is read from memory.
In 508, the observation receiver uses a first oscillation signal, which may be generated by the same first local oscillator that also feeds the transmit path. The transmit signal conveyed to and processed by the observation receiver will thus be on a band-pass frequency of the observation receiver, whereby frequency components outside the centre frequency of the transmit band (such as a leakage component of the first local oscillator) will be filtered out. In 510, a compensation signal for compensation of the distortion will be generated. As shown by 512, during the first mode LO leakage is compensated for using a compensation signal read from a memory.
In the second operation mode in step 514, the observation receiver uses a second oscillation signal whereby the LO leakage signal component in the extracted transmit signal portion is shifted onto a band-pass frequency of the observation receiver. In a first embodiment, the second oscillation signal is generated from a first oscillation signal by performing an additional mixing step. In another embodiment, a separate second oscillator is provided specifically for generating the second oscillation signal.
In 516, the LO leakage signal is on a band-pass frequency of the observation receiver, whereby the observation receiver is capable of generating a compensation (inverse) signal of the LO leakage signal. During the second mode, the predistortion compensation signal is read from memory as shown by step 518.
In
Embodiments of the invention or parts of them may be implemented as a computer program comprising instructions for executing a computer process for implementing the method according to the invention.
The computer program may be stored on a computer program distribution medium readable by a computer or a processor. The computer program medium may be, for example but not limited to, an electric, magnetic, optical, infrared, or semiconductor system, device or transmission medium. The computer program medium may include at least one of the following media: a computer readable medium, a program storage medium, a record medium, a computer readable memory, a random access memory, an erasable programmable read-only memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunications signal, computer readable printed matter, and a computer readable compressed software package.
Other than computer program implementation solutions are also possible, such as different hardware implementations (entities or modules), such as a circuit built of separate logics components or one or more client-specific integrated circuits (Application-Specific Integrated Circuit, ASIC). A hybrid of these implementations is also feasible.
It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.
Number | Date | Country | Kind |
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20075763 | Oct 2007 | FI | national |