Transmitter arrangement for digitally modulated signals

Abstract: A radio transmitter arrangement for e.g. QAM signals compri­ sing a class C output transistor amplifier (AMP). The control voltage generator (CVG) comprises a memory, whereby the I,Q -signals (I,Q) of the modulating signal (DM) address the control voltage generator (GEN), the information of which is used as a control signal (CS) acting as the collector voltage to control the amplifier output level (RF). The information can be modified to compensate for temperature variations and other non-linearities.

Description: BACKGROUND OF THE INVENTION Field of the invention The invention relates to a transmitter arrangement for digitally modulated signals, and especially in a transmitter arrangement, whereby the modulation is a non-constant envelope modulation and the transmitter arrangement comprises a nonlinear class C output amplifier and a control voltage generator. Description of the prior art In digital radio systems that do not use constant envelope modulation, the radio transmitter usually comprises linear class A or AB, in some instances linearized class C amplifiers. Such modulation schemes include e.g. schemes of the QAM-type (Quadrature Amplitude Modulation), where the modulated information exists both in the signal phase and in the signal amplitude. A modulation scheme of this kind has been proposed to be used in the future North-American digital mobile radiotelephone system. Linear amplifiers have a poor efficiency compared to a nonlinear amplifier operating in class C. On the other hand, linearized class C amplifiers generally lead to complicated designs. A transmitter arrangement, which comprises a non linear-call C output amplifier and a control voltage generator is known from FR-A-2564658. The amplifier output level is controlled by the output signal of the control voltage generator. However, the circuit for taking the modulation signals at control of the output level into consideration is rather complicated. Summary of the invention It is an object of the present invention to provide a transmitter arrangement with features that overcome the drawbacks of the prior art, and provides kind of a linearization, especially in such radio systems where the requirements on linearity are not too stringent. This is the case for a subscriber set in the above mentioned digital mobile radio-telephone system. A not too stringent requirement on linearity means in this case an intermodulation distortion of the order -30 dB. According to the invention the transmitter arrangement is provided with an amplifier wherein the output level is controlled by the output signal of the control voltage generator. In a preferred embodiment of the transmitter arrangement the amplifier comprises a transistor amplifier and the output signal of the control voltage generator is the collector voltage of the amplifier. In digital modulation the modulating signal is used to create I,Q -waveforms (In-phase, Quadrature; i.e. two signals of which one has the same phase as the modulating signal and the second is 90 degrees phase shifted). The I,Q -waveforms are multiplied with the sin and cos -waveforms of a signal that has a frequency equalling e.g. the transmission frequency. The resulting signal is a modulated signal, e.g. the radio frequency transmission signal. Advantageously the controlling output signal of the control voltage generator is generated from the I,Q-signals in the modulator in correspondence to the actual input signal, and suitably the control signal is modified by information stored in a look-up table in the control voltage generator, to compensate for non-linearities in the control part of the amplifier. The transmitter arrangement advantageously further comprises a temperature sensor measuring the amplifier temperature, whereby the output of the sensor is connected to the control voltage generator to continuously modify the controlling output signal in accordance with a temperature compensation algorithm stored in the control voltage generator. BRIEF DESCRIPTION OF THE DRAWINGS The invention is further described by way of example with reference to the accompanying drawing, in which the only figure is a block diagram showing the main parts of the inventive transmitter arrangement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the block diagram of figure 1 the modulating signal DM (digital modulating signal input) is connected to the input of the I,Q-generator GEN. The outputs I and Q of the generator GEN carry an in-phase signal I and a 90 degrees phase-shifted signal Q respectively. In the main signal stream the signals I, Q are connected to the input of the digital to analog converters D/A, which operate at a sampling frequency e.g. four times the bit rate of the input binary signal DM. The analog output signals from the digital to analog converters D/A are connected to the inputs of the up-converter CONV, which is supplied with a modulation frequency signal LO. The output from the up-converter CONV is connected to the radio frequency input IN of the amplifier AMP. The amplifier AMP is a transistor amplifier operating in class C. The radio frequency output signal RF from the amplifier AMP is connected to the antenna of the transmitter arrangement. The amplifier AMP comprises one or more stages, as is indicated by the dashed lines, supplying the control signal CS to three different amplifier stages. The I,Q-signals are branched from the main signal stream and connected to the inputs of the control voltage generator CVG. The I,Q -signals address a control word, and are then supplied as a control signal CS to the transmitter amplifier AMP. The control signal CS is supplied to the amplifier AMP as the supply voltage, i.e. the collector voltage. The operation of the transmitter arrangement can be described by signal equations. In the following it is assumed that the level of the output signal RF from the transmitter amplifier AMP has a linear relation to the control signal CS, omitting any compensations. We then have the interdependence: I = a(t)·sin F(t), Q = b(t)·cos F(t), LO = cos Wot, IN = c(t)·cos[Wot + F(t)], RF = K·c(t)·cos[Wot + F(t)], and CS = f[c(t)]+ correction, i.e. CS is a signal containing a part, which depends on c(t), and a compensating part. In the above case the control signal CS is given a value, according to the information stored in the control voltage generator GEN and selected by the respective I,Q -signals. The information in the control voltage generator GEN can be modified to compensate for the non-linearities in the control part of the amplifier. The values to be put in the cells are then calculated according to the above equations and modified with results from actual measurements. This is the case for temperature compensation as well. The temperature compensation of the amplifier requires a temperature sensor (not shown) connected to the amplifier AMP. The output of the sensor is connected to an input of the control voltage generator, which in this case includes a set of cells with information containing correction values. The sensor signal addresses a correction value, which is added to the cell values for the I,Q -signals, in accordance with an algorithm stored in the memory. The information contents of the control voltage generator is preferably programmed (stored) during the final production tests of the transmitter arrangement equipment, when the individual variations in the amplifier characteristics, such as non-linearities and temperature dependency, can be accurately measured.

Claims: Transmitter arrangement for digitally modulated signals, whereby the modulation is a non-constant envelope modulation and the transmitter arrangement comprises a non-linear class C output amplifier (AMP) and a control voltage generator (CVG), the output signal (CS) of the control voltage generator (CVG) being connected to the amplifier (AMP) so as to control the output level (RF) thereof, characterized in that the controlling output signal (CS) of the control voltage generator (CVG) is generated from the I,Q-signals (I, Q) in the modulator in correspondence to the actual input signal (DM). Transmitter arrangement according to claim 1, characterized in that the controlling output signal (CS) of the control voltage generator (CVG) is modified by information stored in the control voltage generator, to compensate for non-linearities of the amplifier (AMP). Transmitter arrangement according to claim 1, characterized in that it comprises a temperature sensor measuring the amplifier (AMP) temperature, whereby the output of the sensor is connected to the control voltage generator (CVG) to continuously modify the control signal (CS) in accordance with a temperature compensation algorithm stored in the control voltage generator.

Inventors: KIUSMA ERKKI, KIUSMA, ERKKI
Applicants: NOKIA MOBILE PHONES LTD, NOKIA MOBILE PHONES LTD.
Application year: 1990
Main classification: H04L27
IPC codes: H03F3, H03F1, G05B11, H04L27, H04B1