Multicarrier transmission

Abstract

A multicarrier-radio transmitter has a digital signal processor to produce a multicarrier signal at IF, and a transmit amplifier circuit to amplify and transmit the multicarrier signal at RF. A feedback loop of the transmit amplifier circuit has a subtractor, an I/Q demodulator in the forward path, a loop-filter system in the forward path at baseband, an I/Q modulator in the forward path, a power amplifier in the forward path, a pick-off node to pick off the multicarrier RF signal, and a down converter in the reverse path to down-convert the picked-off multicarrier RF signal to IF.

Claims

1. A multicarrier-radio transmitter internally using at least three frequency levels, baseband, an intermediate frequency (IF) and a radio frequency (RF) and comprising: a digital signal processor to produce a multicarrier signal at the IF, and a transmit amplifier circuit to amplify and transmit the multicarrier signal at the RF, the transmit amplifier circuit forming a feedback loop comprising a forward path and a reverse path, wherein the multicarrier signal at the IF is an input signal to the transmit amplifier circuit; wherein the feedback loop comprises: a subtractor to receive the multicarrier IF input signal from the digital signal processor and a fed-back signal from the reverse path at the IF and to provide a feedback-corrected signal at the IF; an I/Q demodulator in the forward path to down-convert the feedback-corrected multicarrier signal from the IF to baseband; a loop-filter system in the forward path at baseband; an I/Q modulator in the forward path to up-convert the baseband multicarrier signal from baseband to the RF; a power amplifier in the forward path to amplify the RF signal to be transmitted via an antenna; a pick-off node to pick off the RF multicarrier signal after the power amplifier; a down converter in the reverse path to down-convert the picked-off RF multicarrier signal from the RF to the IF.

2. The multicarrier-radio transmitter according to claim 1, wherein the loop-filter system in the forward path at baseband comprises a loop filter for the I component and a loop filter for the Q component.

3. The multicarrier-radio transmitter according to claim 1, wherein the loop-filter system comprises low-pass filters to ensure that a loop gain is below unity at 180 phase shift.

4. The multicarrier-radio transmitter according to claim 1, wherein the I/Q modulator is performing the up-conversion by means of a local oscillation signal, wherein the local oscillation signal is suppressed in the up-converted multicarrier signal at the RF.

5. The multicarrier-radio transmitter according to claim 1, wherein the I/Q demodulator, the I/Q modulator, and the down converter are performing by means of local oscillation signals, wherein the local oscillation signal for the I/Q demodulator has a frequency at the IF, the local oscillation signal for the I/Q modulator has a frequency at the RF, the local oscillation signal for the down converter has a frequency corresponding to the sum or the difference of the frequency of the local oscillation signal for the I/Q modulator and the frequency of the local oscillation signal for the I/Q demodulator.

6. The multicarrier-radio transmitter according to claim 5, further comprising local oscillators to produce the local oscillation signals, and a local-oscillator network connecting the local oscillators, wherein at least one of the local oscillators is frequency and phase controlled relative to the other local oscillator, or oscillators, to ensure that a sum or difference of the frequencies of the local oscillation signal for the down converter and the local oscillation signal for the I/Q demodulator equals the frequency of the local oscillation signal for the I/Q modulator.

7. The multicarrier-radio transmitter according to claim 6, wherein the frequency- and phase-control of at least one of the local oscillators also ensures that a phase of a difference signal of the local oscillation signal for the down converter and the local oscillation signal for the I/Q demodulator relative to a phase of the local oscillation signal for the I/Q modulator is constant.

8. The multicarrier-radio transmitter according to claim 6, wherein there are three local oscillators, and two of the three local oscillators are free-running oscillators, and the third local oscillator is frequency- and phase-controlled by an output signal of a phase-locked loop, wherein the phase-locked loop receives a signal representative of the local-oscillation signal of one of the two local oscillators, and wherein a feedback-signal of the phase-locked loop is a frequency-converted combination of the local-oscillation signal of the other of the two local oscillators and the local-oscillation signal of the frequency- and phase-controlled local oscillator.

9. The multicarrier-radio transmitter according to claim 5, further comprising a phase adjuster to adjust a local-oscillation-signal phase.

10. The multicarrier-radio transmitter according to claim 1, radio signals according to at least one of TETRA, TETRA-2, TEDS, DMR, and an analog trunked radio system.

11. The multicarrier-radio transmitter according to claim 10, having a signal bandwidth sufficient to transmit at least two DMR channels with 12.5 kHz bandwidth, or at least two TETRA channels with 25 kHz bandwidth.

12. A method of generating amplified multicarrier-radio signals with a multicarrier-radio transmitter internally using at least three frequency levels, baseband, an intermediate frequency (IF) and a radio frequency (RF); and comprising a feedback loop with a forward path and a reverse path, the method comprising: producing the multicarrier signal at the IF by a digital signal processor, receiving, at a subtractor, the multicarrier IF input signal and a fed-back IF signal from the reverse path to provide a feedback-corrected multicarrier signal at the IF; down-converting, in the forward path, the feedback-corrected multicarrier signal from the IF to baseband by an I/Q demodulator; filtering, in the forward path, the multicarrier signal at baseband by a loop-filter system; up-converting, in the forward path, the baseband multicarrier signal from baseband to the RF by an I/Q modulator; amplifying by a power amplifier, in the forward path, the RF signal to be transmitted via an antenna; picking off, by a pick-off node, the RF multicarrier signal after the power amplifier; down-converting the picked-off RF multicarrier signal from the RF down to the IF by a down converter in the reverse path.

13. The method of claim 12, wherein in the multicarrier signal produced by the digital signal processor, which is input into the subtractor, uses only a sub-set of carriers including at least two non-contiguous carriers, or a sub-set of carriers, that is asymmetric with respect to an RF-modulation carrier frequency used by the I/Q modulator to up-convert the baseband multicarrier signal from baseband to the RF.

14. The method of claim 12, wherein the I/Q modulator is performing the up-conversion by means of a local oscillation signal, wherein the local oscillation signal is suppressed in the up-converted multicarrier signal at the RF.

15. The method of claim 12, wherein the I/Q demodulator, the I/Q modulator, and the down converter perform conversions by means of local oscillation signals, wherein the local oscillation signal for the I/Q demodulator has a frequency at the IF, the local oscillation signal for the I/Q modulator has a frequency at the RF, and the local oscillation signal for the down converter has a frequency corresponding to the sum or the difference of the frequency of the local oscillation signal for the I/Q modulator and the frequency of the local oscillation signal for the I/Q demodulator.

16. The method of claim 15, wherein a sum or difference of the frequencies of the local oscillation signal for the down converter and the local oscillation signal for the I/Q demodulator equals the frequency of the local oscillation signal for the I/Q modulator.

17. The method of claim 16, wherein a phase of a difference signal of the local oscillation signal for the down converter and the local oscillation signal for the I/Q demodulator relative to a phase of the local oscillation signal for the I/Q modulator is constant.

18. The method of claim 12, wherein the multicarrier-radio transmitter transmits radio signals according to at least one of TETRA, TETRA-2, TEDS, DMR, and an analog trunked radio system.

19. The method of claim 18, wherein the multicarrier-radio transmitter has a signal bandwidth sufficient to transmit at least two DMR channels with 12.5 kHz bandwidth, or at least two TETRA channels with 25 kHz bandwidth.

Description

DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments are now described with reference to the accompanying drawings, wherein

(2) FIG. 1 illustrates an exemplary multicarrier-radio transmitter receiving a data signal, which is converted, amplified and fed to an antenna;

(3) FIG. 2 illustrates a transmit amplifier circuit of the exemplary multicarrier-radio transmitter of FIG. 1 in more detail;

(4) FIG. 3 illustrates the multicarrier-radio transmitter of FIG. 1 with a more detailed representation of the transmit amplifier circuit;

(5) FIG. 4 illustrates an exemplary local-oscillator network for the transmit amplifier circuit of FIG. 3;

(6) FIG. 5 illustrates a spectrum of a multicarrier RF signal without artifacts;

(7) FIG. 6 illustrates a spectrum of a multicarrier RF signal with artifacts that could be produced by a conventional transmit amplifier with Cartesian feedback, i.e. with an I/Q demodulator located in the reverse path of a feedback loop;

(8) FIG. 7 illustrates an exemplary digital signal processor.

(9) The drawings and the description of the drawings are of examples of the invention and are not of the invention itself.

DETAILED DESCRIPTION

(10) An exemplary multicarrier-radio transmitter 1 with a digital signal processor 100 (also referred to as DSP), which produces a multicarrier IF input signal IF.sub.IN from a data signal SP e.g. representing a plurality of communication channels with human speech and/or other payload data, and a transmit amplifier circuit 10 connected to an RF antenna 80 is illustrated in FIG. 1.

(11) The multicarrier-radio transmitter 1 of FIG. 1 may be part of a base station of a mobile-radio system serving a plurality of mobile terminals simultaneously. The multicarrier-radio transmitter 1 of the base station receives a plurality of signals, for example signals representing speech and/or other payload data signals, collectively referred to as SP as an input to the digital signal processor 100. Upon receipt of the signal SP at the DSP 100 a CPU 105 of the DSP 100 generates a digital multicarrier signal that is converted to the analog IF.sub.IN by a digital-to-analog converter (DAC) 108. The IF.sub.IN is output by the DSP 100, as is shown in FIG. 7 in more detail, to a transmit amplifier circuit 10 connected to an RF antenna 80.

(12) The transmit amplifier circuit 10 converts the multicarrier IF input signal IF.sub.IN to a multicarrier RF output signal RF.sub.TX suitable for transmittance via the RF antenna 80. The transmit amplifier circuit 10 has an input 2 and an output 3 and is formed by a feedback loop 11 between the input 2 and the output 3 as shown in FIGS. 2 to 4.

(13) The incoming multicarrier signal is amplified in the transmit amplifier circuit 10 by a power amplifier 50, which is linearized by the feedback loop 11. The amplified multicarrier signal RF.sub.TX is fed to the RF antenna 80 for transmittance, for example, over a radio network.

(14) According to FIG. 2 the feedback loop 11 of the transmit amplifier circuit 10 has a forward path 12 and a reverse path 13. A subtractor 5 and a pick-off node 60 constitute the two elements where the forward path 12 and the reverse path 13 are connected to each other to form the feedback loop 11.

(15) The multicarrier IF input signal IF.sub.IN from the digital signal processor 100 enters the feedback loop 11 by an input of the subtractor 5. The subtractor 5 generates an error signal that represents the difference between the IF.sub.IN and a fed-back signal applied to another input of the subtractor 5. The error signal is also referred to as a feedback-corrected multicarrier signal IF.sub.FBC that exits an output of the subtractor 5.

(16) In some examples an input amplifier 16, which may then be arranged in the forward path 12 (see FIGS. 3, 4) amplifies the IF.sub.FBC to condition the signal strength to the subsequent elements.

(17) Further in the forward path 12 an I/Q demodulator 20 for down-converting the IF.sub.FBC from IF to baseband (also referred to as BB) is provided. By this down-conversion the feedback-corrected multicarrier IF input signal is also split into an in-phase component I and a quadrature component Q.

(18) Following the I/Q demodulator 20 in the forward path 12 a loop-filter system 30 at baseband with low-pass characteristic provides stability to the feedback loop 11. The loop-filter system 30 may be comprised of two loop filters 30a, 30b for the I component and the Q component, respectively (see FIGS. 3 and 4). Thus, the I component and the Q component are low-pass filtered by loop filters 30a and 30b, respectively. The cut-off frequency of the loop filters 30a and 30b may be chosen such that the loop filters 30a, 30b filter out the frequencies with a phase shift close to 180 with regard to the baseband signal so that the loop gain for those frequencies is less than unity. Hence, frequencies at counter-phase (i.e. signals with a phase shift at around 180) are effectively attenuated so that the feedback loop 11 is stable.

(19) After the filtering by the loop filters 30a and 30b, the filtered multicarrier baseband signal I, Q is up-converted from baseband to RF by an I/Q modulator 40. The up-conversion by the I/Q modulator 40 includes the combination of the I and Q components into the common multicarrier RF signal (denoted RF in FIG. 2).

(20) The multicarrier RF signal is subsequently amplified by a power amplifier 50. The power amplifier 50 may be a single-step amplifier or may be comprised of a plurality of amplifiers in series. The amplified multicarrier RF signal (also referred to as RF.sub.TX) is output at an antenna connector, also referred to as output 3 and thereby fed to an RF antenna 80 (FIGS. 1 and 3) for transmittance over the air interface of the radio network.

(21) The RF.sub.TX is picked-off by the pick-off node 60 between the power amplifier 50 and the output 3. By way of the pick-off node 60 a feedback signal at the RF corresponding to RF.sub.TX is branched off from the amplified multicarrier RF signal RF.sub.TX so as to obtain a fed-back signal at the RF (also referred to as RF.sub.FB). This is the starting point of the reverse path 13.

(22) The fed-back multicarrier RF signal RF.sub.FB is down-converted in the reverse path 13 to IF by a down converter 70, thereby producing a fed-back multicarrier signal at the IF (also referred to as IF.sub.FB).

(23) The fed-back multicarrier IF signal IF.sub.FB is fed to an input of the subtractor 5, where it is subtracted from the multicarrier IF input signal IF.sub.IN that is fed into another input of the subtractor 5, as described above. Hence, the subtractor 5 is at the end point of the reverse path 13.

(24) Alternatively, the subtractor 5 can be implemented as a summer that adds an inverted version of the fed-back multicarrier IF signal IF.sub.FB.

(25) FIG. 3 again shows the complete multicarrier-radio transmitter 1 including the DSP 100 and the transmit amplifier circuit 10, as in FIG. 1. The loop-filter system 30 is presented in more detail in the form of two separate low-pass loop filters 30a, 30b for the I and Q components, respectively, as already explained above. In addition, FIG. 3 introduces various local oscillators.

(26) A local oscillator 21, also designated LO.sub.2, provides a local-oscillation signal to the I/Q demodulator 20 to down-convert the IF.sub.FBC. The local-oscillation signal produced by the local oscillator 21, LO.sub.2, has a frequency corresponding to the IF.

(27) A local oscillator 41, also designated LO.sub.TX, provides a local-oscillation signal to the I/Q modulator 40 to up-convert the filtered components I and Q of the multicarrier baseband signal from BB to RF. The local-oscillation signal produced by the local oscillator 41, LO.sub.TX, has a frequency corresponding to the RF.

(28) A local oscillator 71, also designated LO.sub.1, provides a local-oscillation signal to the down converter 70 to down-convert the RF.sub.FB from the RF to the IF. The local-oscillation signal produced by the oscillator 71, LO.sub.1, has a frequency corresponding, for example, to the sum or the difference of the RF and the IF.

(29) FIG. 4 illustrates an exemplary local-oscillator network 14 to coordinate the LO.sub.1, LO.sub.2, and LO.sub.TX so that the total effect of the stepwise down-conversion from RF to IF and from IF to baseband should be complementary to that of the single-step up-conversion from baseband to RF. Put differently, the local-oscillators, although all produce local-oscillation signals with different frequencies, are correlated such that they produce their local-oscillation signals in a manner that the frequencies and phases of the local-oscillation signals are matched to properly combine the fed-back multicarrier IF signal IF.sub.FB with the multicarrier IF input signal IF.sub.IN from the DSP 100, i.e. by eliminating IF frequency offset and phase shift at the inputs of the subtractor 5.

(30) For this purpose the local oscillator LO.sub.1 is frequency and phase controlled relative to the other local oscillators LO.sub.2, and LO.sub.TX, that are free-running, to ensure that the sum or the difference of the frequencies of the local-oscillation signals for the down converter 70 and the local-oscillation signal for the I/Q demodulator 20 equals the frequency of the local-oscillation signal for the I/Q modulator 40.

(31) The local oscillator LO.sub.1 is frequency- and phase-controlled by an output signal of a phase-locked loop 15 in the local-oscillator network 14. The phase-locked loop 15 comprises a phase determiner 18 that produces, for example, an output voltage that controls the local oscillator LO.sub.1. The local-oscillation signal produced by the local oscillator LO.sub.1 is fed back to the phase determiner 18 via an oscillation-signal mixer 19. The oscillation-signal mixer 19 mixes the local-oscillation signals from the local oscillator LO.sub.1 and the local oscillator LO.sub.TX so as to produce an instance of an intermediate frequency related to the two local-oscillation signals mixed. This produces the feedback signal of the phase-locked loop 15, also referred to as PLL feedback signal. Hence, the feedback loop of the phase-locked loop 15 also comprises the local oscillator LO.sub.1 and the mixer 19 that mixes the oscillation signal for I/Q modulator 40 produced by the local oscillator LO.sub.RX into the feedback loop of the phase-locked loop 15.

(32) The signal produced by the oscillation-signal mixer 19 contains a difference signal of the local oscillation signal produced by the local oscillator LO.sub.1 and the local oscillation signal produced by the local oscillator LO.sub.TX, which is the instance of an intermediate frequency mentioned above. The frequency- and phase-control for the local oscillator LO.sub.1 also ensures that the relative phase between this signal, that is an instance of an intermediate frequency, and the local-oscillation signal from the local oscillator LO.sub.2 is constant.

(33) The phase-locked loop 15 receives, as a control-input signal, a signal representative of the local-oscillation signal of the local oscillator LO.sub.2 associated with the I/Q demodulator 20. On the basis of any deviation in the frequency and/or phase between the control-input signal and the PLL feedback signal the phase-locked loop 15 produces a control signal for the frequency- and phase-controlled oscillator LO.sub.1. If the frequencies of the control-input signal and the PLL feedback signal are not the same, the control signal will increase (or decrease) in time, and if the frequencies of the control-input signal and the PLL feedback signal are the same, but the control-input signal and the PLL feedback signal being phase shifted by a value constant in time, the control signal will take a constant value representative of the phase shift. In response to this control signal the frequency- and phase-controlled oscillator LO.sub.1 adjusts the frequency and phase of the local-oscillation signal produced by it so that the frequency and the relative phase of the control-input signal and the PLL feedback signal become identical and zero, respectively.

(34) As explained above, the oscillation-signal mixer 19 of the PLL 15 produces a frequency-converted version of the local-oscillation signal of the controlled local oscillator LO.sub.1. It mixes the local-oscillation signals from the local oscillator LO.sub.1 and the local oscillator LO.sub.TX so as to produce an instance of an intermediate frequency related to the two local-oscillation signals mixed. As usual the oscillation-signal mixer 19 will produce two frequencies, corresponding to the sum and the (absolute value of) the difference of the two frequencies mixed. The difference of the two signals mixed can be used as the PLL feedback signal. If the oscillation frequency of the local oscillator LO.sub.1 is chosen to be the difference or the sum of RF and IF then the difference of the two frequencies mixed is an instance of an intermediate frequency (because RF(RFZF)=ZF, and (RF+ZF)RF=ZF).

(35) A phase adjuster 17 is provided in the local-oscillator network 14 to adjust the phase of the local-oscillation signal, in this example, of the local oscillator LO.sub.2, that forms the control-input signal to the phase-locked loop 15, e.g. by introducing a phase shift . The phase adjuster 17 is therefore inserted between the local oscillator LO.sub.2 and the phase determiner 18 that compares the control-input signal and the PLL feedback signal of the phase-locked loop 15 to control the local oscillator LO.sub.1. As the phase-locked loop 15 ensures that the relative phase of the control-input signal and the PLL feedback signal becomes zero, any desired constant relative phase between the signals (a) and (b) can be adjusted by means of the phase adjuster 17, wherein the signal (a) is the frequency-converted combination of the oscillation signal from the local oscillator LO.sub.TX and that from the controlled local oscillator LO.sub.1 (e.g. the difference signal produced by the oscillation-signal mixer 19 mentioned above), and the signal (b) is the local oscillation signal produced by LO.sub.2. By allowing the phase of the local-oscillation signal produced by LO.sub.2 to be adjusted, the phase adjuster 17 enables signal-propagation delays in the feedback loop 11 of the transmit amplifier circuit 10 to be compensated at the level of the IF, so that the region of instability (at around 180 phase shift in the IF signal) is also avoided at the IF level.

(36) An example of a spectrum of a multicarrier signal that can be produced, amplified and transmitted with the examples of the multicarrier-radio transmitter 1 is illustrated in FIG. 5. In this example, the frequency range in which the multicarrier-radio transmitter 1 can simultaneously transmit I/Q-modulated signals extends over six frequency channels, or carriers .sub.ch1 to .sub.ch6, of a given radio system, such as TETRA, TETRA-2, DMR; each having a channel bandwidth CH.sub.BW of 12.5 kHz or 25 kHz, for example. FIG. 5 also illustrates the position of the frequency of the local-oscillation signal for the I/Q modulator 40 in the center of the frequency range in which the multicarrier-radio transmitter 1 can transmit simultaneously with the label .sub.c, although this frequency is actually not (or not a significant) part of the spectrum. The parts of the spectrum below and above the center frequency .sub.c are labelled as LSB (lower side band) and USB (upper side band), respectively. The number six of frequency channels, or carriers, is of course only exemplary; in other implementations the number could be any natural number N.

(37) In the instance shown, however, only a subset of the N (for example: six) frequency channels, or carriers, is used, namely a subset of frequency channels/carriers that is not contiguous, and that is not symmetric to the frequency .sub.c of the local-oscillation signal for the I/Q modulator 40 in the center of the frequency range in which the multicarrier-radio transmitter 1 can transmit simultaneously.

(38) Due to the fact that in the multicarrier-radio transmitter 1 there is no I/Q demodulator outside the forward path 12 of the feedback loop 11 (i.e. no I/Q demodulator in the reverse path 13 of the feedback loop 11, and no I/Q modulator before the subtractor 5 where the signal to be transmitted enters the feedback loop 11) any artifacts in the transmitted multicarrier radio signal RF.sub.TX are suppressed by the feedback. In particular, there is no significant sideband mirroring and no significant feedthrough of the local-oscillation signal at .sub.c into the transmitted multicarrier RF signal RF.sub.TX, as shown in FIG. 5.

(39) This is further explained by FIG. 6, which schematically shows what could happen with a conventional RF transmitter with an I/Q demodulator in the reverse path of the feedback loop (Cartesian feedback) or an I/Q modulator before the input to the feedback loop.

(40) As shown schematically in FIG. 6, significant artifacts could then occur in the transmitted RF signals; i.e. mirror images 300 of the asymmetrically used frequency channels, mirrored at .sub.c (in the example shown these mirror images are at .sub.ch4 and .sub.ch6, and a feedthrough 200 of the local-oscillation signal at .sub.c into the transmitted multicarrier RF signal (the feedthrough 200 and the mirror images 300 are depicted as dashed lines and the mirror axis at .sub.c is marked with an arrow in FIG. 6)). Such artifacts might be acceptable under symmetric and contiguous use of the available frequency channels, because in symmetric use the mirror images 300 would fall onto another channel used by the same transmitter (rather than an alien frequency channel used by another operator), and in non-contiguous and symmetric use (i.e. use centered around the center frequency .sub.c) the feedthrough 200 of the I/Q modulator's local oscillator (i.e. the local-oscillation-signal frequency) would fall between two adjacent frequency channels (rather than between, or adjacent to, alien frequency channels used by other operators).

(41) Hence, the suppression of the artifacts shown in FIG. 6 enables available frequency channels to be used in a more flexible way, and is therefore useful for frequency economy.

(42) An exemplary digital signal processor 100 (DSP as an abbreviation) of a multicarrier-radio transmitter 1, which produces a multicarrier IF input signal IF.sub.IN for the transmit amplifier circuit 10 from data signals 110 (designated SP for speech), e.g. representing a plurality of communication channels with human speech and/or or other payload data, is illustrated in FIG. 7.

(43) The DSP 100 has an input 102 for receiving the data signals 110 and an output 103 for providing the IF.sub.IN to the input 2 of the transmit amplifier circuit 10. The DSP 100 has a CPU 105 to encode the received SP onto an IF signal using software 107 stored on memory 106. The memory 106 may be volatile random access memory (RAM) or non-volatile memory, e.g. read-only memory, Electrically Erasable Programmable Read-Only Memory (EEPROM), flash memory, or ferroelectric RAM (F-RAM). Thereby the CPU 105 generates a digital multicarrier IF signal that is converted by a digital-to-analog converter 108 (abbreviated DAC) to the multicarrier IF input signal IF.sub.IN, which is fed to the input 2 of the transmit amplifier circuit 10.

(44) All publications and existing systems mentioned in this specification are herein incorporated by reference.

(45) Although certain methods and products constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.