Specifications

In the systems, where the transmitter power efficiency is of high importance, the transmitter nonlinearity

also creates an important issue. Generally speaking, the higher the transmitter nonlinearity, the higher

the transmitter efficiency can be reached. Unfortunately, the device with a nonlinear transfer function

also tends to distort the spectrum of the transmitted signal, especially if the modulated signal exhibits

the non-constant modulation envelope. In contrast, it is also true that only the non-constant envelope

modulation can withstand a strict band limitation by means of modulation filtering – characterized by

the roll-off parameter α in the following text. In other words, if the signal has a constant modulation

envelope, it has an unlimited spectrum, and, if it has a band limited spectrum, it experiences the amp-

litude variations, which after passing through the nonlinear power amplifier, would be suppressed, but

would also regenerate the side-lobes of the modulated signal spectrum. The phenomenon is known

as the spectral re-growth, and it depends mainly on the three transmitter characteristics. Those are

peak to average power ratio (PAPR) of the digital modulation scheme in use, transmitter nonlinearity

and the efficiency of the power amplifier linearization or pre-distortion technique and all have to be

considered when selecting the digital modulation technique for the system, where both power and

spectrum are the key issues.

In light of these facts one can arrive at the conclusion that setting up the limit at −60 dB

rather than

−70 dB was a reasonable step, while the initial limit has been left to be beyond the state of the present

linearization technology for equipments production which in turn hindered the use of spectrally more

efficient modulation techniques.

Fig. 3.2: Modulated signal spectrums. (left) 4CPFSK with R=10.4 kBaud, modulation index h~0.3.

(right) 4CPFSK with R=17.3 kBaud, modulation index h~0.1.

3.1.3. Transmitter power efficiency

In this section, the measurement results concerning the overall narrowband transmitter power efficiency

are presented. It is no ambition however, to provide exact power efficiency analysis of the particular

high power amplifier̶ with the selected linearization circuit proceeded. It is rather to give the example

of the practically achievable overall transmitter power efficiencies and to show the differences related

to selected digital modulation formats of each of the linear/nonlinear class.

The standard [2] specifying the conformity testing for TETRA-like devices allows -55 dBc in normal

or -50 dBc in extreme temperature conditions, assuming channel separation of 25 kHz.

Data speed and Modulations