Thus, for most commercial applications, direct upconversion is the most common approach because of its simplicity and cost-effectiveness. As mentioned previously mentioned, direct upconversion is subject to a several sources of error because of part-to-part variation and non-linearity of analog components.
Efficient Compensation of Transmitter and Receiver IQ Imbalance in OFDM Systems
In this section, we will evaluate the affects of DC offsets, phase noise, quadrature skew, and gain imbalance on the performance of a communications system. These are illustrated in the Figure 9. Communications systems require accurate transitions in phase, amplitude, and frequency of a modulated RF signal. In fact, errors due to DC offset, phase noise, quadrature skew, or gain imbalance each impair the accuracy of the signal generated.
Because the accuracy of each signal transition determines factors such as bit error rate BER and order of modulation scheme 4-QAM versus QAM , they have a direct affect on the potential system throughput. When characterizing modulation measurements, it is important to note that the phase, amplitude, and frequency of the RF signal translated to the amplitude of each I and Q vector.
Thus, errors in the baseband signal translate to errors in the RF signal. As a point of reference, the graph below illustrates two time-domain baseband signals, I and Q I in red, Q in blue. When recovering actual symbols, I and Q must be precisely synchronized and sampled at appropriate intervals.
In the time-domain graph below, the ideal symbol locations for the given baseband signals are illustrated by the large dots. Thus, the recovery of actual symbols depends upon various characteristics of the IQ symbol. These characteristics include accuracy of the amplitude, phase, and frequency of each baseband signal.
For these modulation schemes, small errors of DC offset, phase noise, quadrature skew, or IQ gain imbalance can make the transitions of the RF signal too difficult to distinguish. In these situations, the receiver is unable to accurately assess the relative phase and frequency of the baseband waveform and the constellation plot will begin to spin. For example, consider several cases of baseband signals with significant error. For a 4-QAM modulation scheme, the DC error is insignificant and the signal can be successfully demodulated.
However, for a QAM modulation scheme, the error is significant enough to prevent carrier recovery. Thus, the constellation plot begins to spin when the demodulation algorithm is unable to accurately estimate the phase or frequency of the baseband signal. In addition, a similar affect can be observed when phase noise is present in the LO. Phase noise creates instantaneous frequency error of the baseband signal. For higher order modulation schemes, phase noise also prevents carrier recovery and causes the constellation plot to spin.
For the examples below, we illustrate the constellation plots of two modulation schemes when a phase noise of dBc at a 1 KHz offset is observed. Next we evaluate quadrature skew as source of error.
This source of error is typically observed in the LO splitter, which divides the LO into an in-phase and a quadrature-phase signals. While an ideal system would result in each of these being exactly 90 degrees out of phase, some systems specify up to 3 degrees or more of skew. For lower order modulation schemes, quadrature skew has relatively little affect on system throughput.
However, like other sources of error, higher order modulation schemes such as QAM are significantly impaired. Finally, we evaluate the affects of IQ gain imbalance.
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Again, for higher order modulation schemes such as QAM IQ gain imbalance of even a few dB can prevent proper demodulation of the signal. This is illustrated in the figures below:. Thus, as the figures above illustrate, we can error in each component of a direct upconverter results has a significant affect on the ability of a receiver to successfully demodulate a signal.
Thus, when characterizing an RFIC, it is important to characterize overall performance with some of the measurements shown above. A second method that can be used to characterize DC, gain, or phase error in an IQ-based RF transmitter is by generating single tones at 90 deg of phase skew into the and Q inputs. In general, upconversion subsystem, which contains two mixers and a LO conversion, can cause significant error. One way test this subsystem is characterize the carrier and sideband or image suppression of the system, as illustrated in the diagram below.
As the figure illustrates, both I and Q inputs are supplied with single tones that are 90 deg out of phase. While this can be proved mathematically, it is sufficient to represent this graphically as well in the following diagram. In addition, this occurs for both mixers and each I and Q baseband signal. In a real system, errors such as quadrature skew and IQ gain imbalance will prevent the LO — I or Q from completely cancelling.
As a result, we are left with an unsuppressed sideband, also known as an unsuppressed image.
Bandwidth‐constrained digital pre‐compensation technique for multi‐carrier satellite communications
This is referred to as an unsuppressed carrier. Both of these tones are frequently expressed in dBc, which is the amplitude difference between the tone and the LO. This is illustrated in the graph below:. As the figure above illustrates, the sideband and carrier suppression are measured in dBc, relative to the original carrier. In addition to evaluating system error in a communications system, you can also generate baseband signals that can either simulate system error or compensate for it.
Adjustments to baseband I and Q signals typically involve tweaking the gain, phase, or amplitude. In this section, we describe two scenarios where applying adjustments to the baseband signal is necessary. In a typical RF transmitter implementation, individual components, including the digital-to-analog-converter are subject to slight errors in gain and DC offset.
Thus, when considering a DAC or direct quadrature modulator, it is important to simulate the error of the DAC signals by applying gain or DC offset adjustments to the baseband I or Q signals. Error in DAC output is illustrated by the diagram below:. As mentioned in a previous section, errors in DC offset or gain of each baseband input to a direct quadrature modulator results in an unsuppressed carrier and sideband. In addition, these errors affect the quality of the modulated signal.
Thus, in order to verify whether a given subsystem, such as a direct quadrature modulator, is capable of use with a particular standard such as WiMAX, the device can be tested using baseband inputs that have been adjusted to simulate the accuracy of the baseband DACs. These adjustments can typically be performed in software by creating an impaired waveform in LabVIEW, or in hardware by applying DC or gain impairments to the arbitrary waveform generator.
A second instance where it is important to apply baseband adjustments is when configuring systems to generate wideband RF signals. With these systems, a benchtop vector signal generator VSG can be used to generate wideband RF signals using external baseband I and Q inputs.
US20070165745A1 - Adaptation of iq-error compensation - Google Patents
An implementation of a typical system is shown below:. As the figure illustrates, two PXI arbitrary waveform generators AWGs are synchronized to generate I and Q signals for the external baseband input to the benchtop vector signal generator. Using this approach, you can compensate for errors in the vector signal generator. By adjusting AWG characteristics such as DC offset, gain, and skew, you can compensate for the gain and phase errors of the vector signal generator.
As a result, you can generate the most accurate signal possible. As an example, the following sideband and carrier suppression measurements were made using an Agilent Ex series VSG with two PXI arbitrary waveform generators. By adjusting the DC offset, gain, and phase of the arbitrary waveform generators, you can compensate for the VSG to generate the most accurate RF signal possible.
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This is illustrated below:. As the image above illustrates, generation of a 1GHz RF signal at -5 dBm can be done with better than dBc of sideband suppression and dBc of carrier suppression by manually tweaking the baseband signal. Thus, for test cases which require highly accurate RF signals, small adjustments of the baseband signal can be implemented to improve the accuracy of the RF signal. As communications systems continue to evolve, the affects of error in IQ-based signal generation continues to become more prevalent.
For example, while 3 deg of quadrature skew might be acceptable for a cell phone using MSK modulation, it is entirely unacceptable for a WiMAX transceiver using QAM as a modulation scheme. Thus it is important to understand and characterize system error in communications transceivers. As a result, modulation and spectral measurements are an important part of system characterization. NI modular instruments can be used with the LabVIEW modulation toolkit to implement a complete communications sytem, from baseband waveforms to RF signals.
In addition, these products can test devices used in both homodyne and heterodyne communications systems. As an example, common NI products include the following:. The IF digitizer can be used to acquire both intermediate frequency and baseband signals. For baseband acquisition, OSP enables decimation of baseband waveforms to the appropriate symbol rate.
Communications System Architectures Modern digital communications systems require significant digital signal processing and sophisticated analog circuitry. This is illustrated in the diagram below: Figure 1. Block Diagram of Communications System As the block diagram above illustrates, communications signals require several stages of signal processing, both in the digital domain and analog domain.
To start with, let us expand our diagram above to example a typical RF transmitter, shown below: Figure 2. Figure 3. Block Diagram of Direct Upconversion RF Transmitter As the diagram above illustrates, a typical transmitter is required to perform signal processing both to digital signals and analog signals.
Back to Top 2. The DPU 28 uses the feedback supplied from the output of transmit filter 36 by coupler 38 to control the correction of errors in the RF output 30 due, in the main part, to the RFPA 34 and the transmit filter From the perspective of the invention, the two main processes performed by the DPU 28 on the base band input 27 are an equalisation filtering algorithm 48 and a predistortion algorithm The predistortion algorithm 50 is a type of lineariser that modifies the base band input 27 to counteract distortion produced by the RFPA The digital predistortion algorithm 50 distorts the base band input 27 in such a way as to counteract the distortion that the RFPA will introduce to the RF output The predistortion algorithm 50 is adaptive and responds to the feedback from coupler 38 to adjust the degree of predistortion applied to the base band input 27 in order to minimise the distortion caused to the RF output 30 by the RFPA Digital predistortion algorithms are known and an example can be found in International patent application no.
The equalisation filtering algorithm 48 is a compensating filter that addresses errors introduced by the transmit filter As mentioned earlier, the transmit filter 36 is, in practice, subject to pass-band ripples in both the amplitude and phase domains and also suffers from variations in group delay across its pass-band. The equalisation filtering algorithm 48 ameliorates these problems associated with the pass-band in a manner that will be described now with reference to FIG.
A filter 52 with a transfer function G z represents the equalisation filtering algorithm 48 , the predistortion algorithm 50 , the upconverter 29 , the RFPA 34 , and the transmit filter The filter 52 receives the signal x and outputs a signal y likewise comprising a sequence of samples, y[n] that represents the RF output The model includes another filter 54 , which represents the downconverter The filter 54 has a transfer function H z and its input is signal y and it produces an output signal y fb likewise comprising a set of samples y fb [n] representing the signal acquired by the DPU 28 from the downconverter Accordingly, the DPU 28 needs to estimate G z.
In order to do this, the DPU 28 assumes that the filter 52 in the model is a finite impulse response FIR filter with an impulse response g comprising a series of samples. A series of values for x and y fb can be acquired from the baseband input 27 and the downconverter output, respectively, and placed into the equation for calculating E. In controlling the equaliser algorithm 48 to steer G z to 1, the DPU 28 minimises phase and amplitude ripples and group delay variation in the pass-band and the transition bands of the transmit filter As described above, DPU 28 functions as both i an assessor that compares the signal upstream from the transmit filter with the signal downstream from the transmit filter in order to provide an indication of residue of at least one of phase ripple, amplitude ripple, and group delay variation within said band and ii a controller that adjusts the equalisation filter, under the guidance of the assessment performed by the assessor, to reduce the residue.
In a variation on the scheme described above with reference to FIG. Instead, the DPU 28 uses a fast fourier transformation FFT technique to produce a frequency spectrum of each of the base band input 27 and the output of the downconverter In each of FIGS. In certain circumstances, it may be undesirable to control the digital predistortion algorithm 50 on the basis of feedback obtained from coupler This is because the signal obtained by the DPU 28 through coupler 38 has been modified by the transmit filter 36 and might not contain an accurate picture of the spectrum of residual distortion in the output of RFPA That is to say, the distortion spectrum of the RFPA 34 may be significantly distorted by the action of the transmit filter For example, the transmit filter 36 might significantly suppress one IMD side band of the signal being amplified with respect to the other IMD side band.
USB2 - Analog power amplifier predistortion methods and apparatus - Google Patents
In FIG. A further coupler 64 is provided between the RFPA 34 and the transmit filter Switch 62 allows the DPU 28 to selectively sample either the output of the RFPA 34 at coupler 64 or the output of the transmit filter 36 at coupler The operation of the switch is controlled by the DPU As mentioned above, the digital predistortion algorithm 50 might not obtain a true picture of the residual distortion appearing in the output of RFPA by monitoring a signal returned through coupler Therefore, when the DPU 28 is to adapt the digital predistortion algorithm 50 on the basis of feedback from the RFPA 34 , the switch 62 is controlled to connect the DPU 28 to coupler 64 so that the transmit filter 36 is by-passed in the feedback arrangement so that the transmit filter 36 does not bias the spectrum of the distortion that is created by the amplifier When the DPU 28 is to adapt the equalisation filtering algorithm 48 , the DPU 28 controls the switch 62 to connect the DPU 28 to coupler 38 so that the feedback signal acquired by the DPU 28 contains information about the affect of the transmit filter 36 and the effectiveness of the equalisation filter In operation, the switch 62 is usually set to connect coupler 64 to the DPU 28 for adaption of the predistortion algorithm The switch 62 is occasionally switched over to coupler 38 to allow the equalisation filtering algorithm 48 to be updated.
Generally, the equalisation filtering algorithm needs to be updated much less frequently than the digital predistortion algorithm 50 because the properties of the transmit filter 36 change only slowly over time e. Like the embodiment of FIG. The second equalisation filtering algorithm 66 is a correcting filter that corrects the feedback signal as used by the digital predistortion algorithm 50 to compensate for the alteration of the distortion spectrum of the RFPA 34 resulting, e.
If desirable, the second equalisation filter 66 can be rendered adaptive based on a sounding signal generated by the DPU The sounding signal is periodically sent through the loop comprising the upconverter 29 , the RFPA 34 , the transmit filter 36 , and the downconverter The diplexer module 68 also includes a receive filter 70 connected to the antenna path for isolating signals received at the antenna for use by the receiver circuits of the transmit-receiver of FIG.
The purpose of the receive filter 70 is to isolate signals with frequencies that fall within the band that has been allocated to the transmitter-receiver for signal reception. In the embodiment of FIG. Although the present invention has been described in the context of an amplifier system that relies on pre-distortion to compensate for amplifier distortion, the invention can also be implemented in the context of amplifier systems that utilize feed-forward compensation schemes in addition to or instead of pre-distortion.
Although the present invention has been described in the context of implementations involving a processing unit i. In alternative embodiments, a processing unit may operate 1 in the digital domain on signals other than base band signals, such low digital IF signals, or 2 in the analog domain. The present invention may be implemented in the context of wireless signals transmitted from a base station to one or more mobile units of a wireless communication network. In theory, embodiments of the present invention could be implemented for wireless signals transmitted from a mobile unit to one or more base stations.
The present invention can also be implemented in the context of other wireless and even wired communication networks to reduce spurious emissions. Embodiments of the present invention may be implemented as circuit-based processes, including possible implementation on a single integrated circuit such as an ASIC or an FPGA , a multi-chip module, a single card, or a multi-card circuit pack. As would be apparent to one skilled in the art, various functions of circuit elements may also be implemented as processing steps in a software program.
Such software may be employed in, for example, a digital signal processor, micro-controller, or general-purpose computer. It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
Effective date : Year of fee payment : 4. Year of fee payment : 8. A radio transmitter for transmitting signals in a predetermined frequency band has a transmit filter and a compensating filter. The transmit filter filters a signal to be transmitted from the transmitter to suppress the transmission of parts of the signal outside the band.
The compensating filter, e. Some further embodiments of the invention will now be described. Apparatus for generating a transmission signal in a frequency band, the apparatus comprising: a compensating filter;. The invention of claim 1 , wherein: the compensating filter is adapted to operate at baseband;. The invention of claim 1 , further comprising a linearizer connected upstream of the amplifier and adapted to predistort the outgoing signal to reduce distortion introduced into the transmission signal by the amplifier.
The invention of claim 3 , wherein the linearizer is connected to receive the first feedback signal and adapted to predistort the outgoing signal to reduce distortion introduced into the transmission signal by the amplifier and by the transmit filter.
The invention of claim 3 , further comprising: a second sampler connected between the amplifier and the transmit filter and adapted to sample the amplified outgoing signal to generate a second feedback signal;. The invention of claim 3 , further comprising a correcting filter connected to receive the first feedback signal and adapted to correct the first feedback signal as used by the linearizer for a roll-off effect in the characteristic of the transmit filter.
The invention of claim 1 , further comprising a transmit band cover filter connected in the feedback path between the first sampler and the compensating filter. The invention of claim 1 , wherein the one or more features comprise at least one of a phase ripple, an amplitude ripple, and a group delay variation of the transmit filter within the band.
The invention of claim 8 , wherein the one or more features comprise the phase ripple of the transmit filter within the band. The invention of claim 8 , wherein the one or more features comprise the amplitude ripple of the transmit filter within the band. The invention of claim 8 , wherein the one or more features comprise the group delay variation of the transmit filter within the band.
The invention of claim 8 , wherein the one or more features comprise at least two of the phase ripple, the amplitude ripple, and the group delay variation of the transmit filter within the band. The invention of claim 12 , wherein the one or more features comprise the phase ripple, the amplitude ripple, and the group delay variation of the transmit filter within the band. The invention of claim 1 , further comprising: an antenna connected downstream of the transmit filter and adapted to transmit the transmission signal from the apparatus and receive a received signal transmitted to the apparatus;.
A method for generating a transmission signal in a frequency band, the method comprising: amplifying an outgoing signal;.
The invention of claim 15 , wherein the one or more features comprise at least one of a phase ripple, an amplitude ripple, and a group delay variation of the transmit filter within the band. The invention of claim 15 , further comprising predistorting the outgoing signal based on the feedback signal, prior to amplifying the outgoing signal, to reduce distortion introduced into the transmission signal by the amplifying and the transmit filtering. Apparatus for generating a transmission signal in a frequency band, the apparatus comprising: means for amplifying an outgoing signal;.
The invention of claim 18 , wherein the one or more features comprise at least one of a phase ripple, an amplitude ripple, and a group delay variation of the transmit filter within the band.
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The invention of claim 18 , further comprising means for predistorting the outgoing signal based on the feedback signal, prior to amplifying the outgoing signal, to reduce distortion introduced into the transmission signal by the means for amplifying and the means for transmit filtering. CN CNC en USB2 en. EPA1 en. KRA en. CNC en. Method and system for radio frequency rf group delay compensation in a broadcast system.