Control Voltage Generator in a Transmitter Arrangement for Digitally Modulated Signals

1. Field of the invention

The invention relates to a control voltage generator in a transmitter arrangement for digitally modulated signals, and, more particularly, to a transmitter arrangement, whereby the modulation is a non-constant envelope modulation and the transmitter arrangement comprises a class C output amplifier.

2. Description of the prior art

In digital radio systems that do not use constant envelope modulation, the radio transmitter usually comprises a linear class A, AB or, 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 for use in the future North-American digital mobile radiotelephone system.

Linear amplifiers have a poor efficiency compared to a nonlinear amplifier operating in class C mode. On the other hand, linearized class C amplifiers generally have complicated designs.

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 a kind of 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 radiotelephone 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 a control voltage generator for digitally modulated signals, whereby the modulation is a non-constant envelope modulation and the transmitter arrangement comprises a class C output amplifier and a control voltage generator, wherein the output signal of the control voltage generator acts on the amplifier output envelope level or gain.

In digital modulation the modulating signal is used to create I,Q -waveforms (In-phase, Quadrature; i.e., two signals wherein one has the same phase as the modulating signal and the second is shifted by 90 degrees). 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.

Preferably the control voltage generator comprises a memory with a look-up table, whereby the I,Q-signals of the modulating signal are connected to the input of the look-up table to select a cell, the non-varying information of which is used to create a control voltage generator output signal in correspondence to the actual input signal.

The control voltage generator can be realized in several ways. The information initially stored in the cells of the look-up table is modified to compensate for the measured non-linearity of the control part of the amplifier. The control voltage generator can also be used to compensate for temperature dependent nonlinearities in the amplifier.

Advantageously the look-up table further has an input for a signal from a temperature sensor that senses the amplifier temperature. The sensor signal can be used to continuously modify the information in the cells or to select the correct cells into which the information has been stored in accordance with a temperature compensation algorithm.

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 control voltage generator and the connections to the transmitter arrangement.

The accompanying drawing shows the control voltage generator (CVG 10), comprising a digital look-up table (ROM 12), a digital to analog converter (D/A 14) and a filter (F 16).

The modulating signal DM (digital modulating signal input) is connected to the input of the I,Q-generator (GEN 20). 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 22,24), which operate at the sampling frequency Fs. The sampling frequency Fs is, e.g., four times the bit rate (Fb) of the input binary signal DM. The analog output signals from the digital to analog converters (D/A 22,24) are connected to the inputs of the up-converter/amplifier (AMP 26), operating in class C. The radio frequency output signal RF from the amplifier (AMP 26) is connected to the antenna (not shown) of the transmitter arrangement.

The look-up table ROM 12 is a read-only memory comprising a quantity of cells. Control words corresponding to every amplitude value of the I,Q signals are stored in the cells. The I,Q signals are branched from the main signal stream and connected to the inputs of the look-up table ROM. The I,Q signals are used to select an individual cell, i.e. to address a cell with its control word. The control words are read at the same sampling frequency as the I,Q signals are sampled in the main signal stream (in the digital to analog converters D/A 22,24) and supplied to the input of the digital to analog converter D/A 14. The sampling frequency is, e.g., four times the bit rate of the input binary signal DM.

The analog signal corresponding to the control word from the digital to analog converter D/A 14 is filtered in the low-pass filter F 16 and then supplied as a control signal CS to the transmitter amplifier AMP 26. The control signal CS will act on the output envelope level of the amplifier AMP by controlling a characteristic such as the collector voltage.

The cut-off frequency (B) of the filter F is chosen to be equal or greater than the bit rate (Fb), thereby attenuating the harmonics of the sampling frequency. Moreover, the lowpass filter F reduces the quantizing noise to the required level, this noise resulting from the limited accuracy of the control words (e.g. with a resolution of 8 bits) stored in the look-up table ROM 12. The required noise level will vary in accordance with the individual system requirements, an also in accordance with the required linearity.

The operation of the look-up table ROM 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 26 has a linear relation to the control signal CS, omitting any compensations. Between the input and output signals of the look-up table ROM we then have the interdependence:

where:

CS(n Ts) is the control signal at time instant n Ts;

I(n Ts) is the value of the I signal at the time instant T s;

Q(n Ts) is the value of the Q signal at the time instant n Ts;

n is an integer number corresponding to the number of the sampling point; and

Ts is the sampling interval corresponding to the frequency Fs; and W is the signal used to create the I,Q signals.

In the above case the control signal CS is given a value, according to the information stored in the look-up table ROM 12 selected by the respective I,Q signal, which is used to address the table.

The information in the look-up table cells can be modified to compensate for the non-linearities of the amplifier control part. 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 known temperature sensor connected to the amplifier AMP 26. The output of the sensor is connected to a third input of the look-up table ROM, which in this case includes a set of cells with information containing correction values. The sensor signal (TS) addresses a correction value, which is added to the cell values of the I,Q signals, in accordance with an algorithm stored in the memory (ROM 12). Another way is to have different sets of cells including control information for different temperature ranges.

The information contents of the look-up table ROM in 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.

Applications of the principles of the invention in this and similar manners are all intended to fall within the scope of the invention.