In a communication system, a transmitting device serves for converting a useful signal into an output signal. For this purpose, the output signal is modulated in a manner dependent on the information contained in the useful signal. The modulated output signal is transmitted as transmission signal via a communication channel, such as, for example, via a data cable or a radio channel. In a receiver, the received transmission signal is demodulated and the information of the original useful signal is provided in a form that enables further signal processing.
In the communication channel, the transmission signal is disturbed by a multiplicity of influences. The latter include already a thermal noise present in the transmitting device. Other transmission signals on the communication signal or interference signals from electrical apparatuses are likewise responsible for any disturbance or degradation of the transmission signal. An additional factor is attenuation of the transmission signal by a spatial separation of transmitting device and receiver. For a successful transmission of the information contained in the useful signal, the output signal must have a signal level that is large enough in order that the received transmission signal can be processed by the receiver.
Modern communication systems, and here in particular wireless communication systems, have a large dynamic range for the signal level. This is the case for amplitude-modulated transmission systems, inter alia. One example is a communication system defined in accordance with the so-called Universal Mobile Telecommunication Standard (UMTS). Another example thereof is the so-called High Speed Uplink Packet Access (HSUPA) Standard.
The dynamic range for an amplitude-modulated transmission system is illustrated below. The RMS power (Root-Mean-Square power) of the transmitting device is used to specify the average power of the transmitting device without interference or overdriving. In a UMTS or HSUPA system, said power, in relation to a minimum required output power, is approximately 35-40 dB. In addition, various requirements imposed by the standards on a power level of a transmitter give rise to a further adjustment range of up to 80 dB. The total dynamic range of the transmitter is therefore approximately 115-120 dB.
In a known transmitting device, an amplifier unit is provided for providing the required output power. Thus, in a polar transmitter architecture, for example, the setting of the output power is effected in the amplitude branch of the transmitter. If appropriate, a further setting is effected by adjusting an operating point of an amplifier in the phase branch of the transmitter, for example by adjusting a DC voltage or bias voltage of a transistor. For the case of a so-called polar modulator architecture, the power can likewise be implemented by means of a setting of the output power in the amplitude branch of the transmitter. If appropriate, a further setting is provided by a preamplification by means of a control amplifier or Variable Gain Amplifier (VGA). In this case, obtaining a high dynamic range is associated with particular expenditure. By way of example, the setting of the output power with the use of a VGA requires an additional control of the temperature response of the VGA.