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I. Introduction


A new era has started in the broadcast industry and is characterized by the introduction of digital technology. This digitalization involves both the signal generation (source coding) and the modulation (channel coding).

New standards have been proposed, and almost accepted, for both video and audio transmissions:

The Grand Alliance HDTV uses the 8VSB modulation, while in Europe DVB and Eureka-147 are using OFDM. These modulations do not have a constant envelope, implying higher sensitivity to non linear distortions. This fact has a great impact on the High Power Amplifier (HPA) linearity requirements in order to achieve acceptable intermodulation distortion.

The non linearity effects on the output signal are not a new problem: satellites, cellular, radio relay links, radar must also take into account the HPA non linearity. However the broadcast applications exacerbate this aspect due to the wide band spectra and the required high power compared with other applications.

Two kind of distortion are to be considered: amplitude and phase. Real amplifiers have a maximum output power (saturation level) and an input-output power relationship that will depart from a straight line as the output power approaches the saturation level: this is referred as AM/AM distortion. Similarly a phase shift depending on the power level will also occur, generating AM/PM distortion.

The final effect of AM/AM and AM/PM distortion is the generation of unwanted spectral energy both in-band and out-of-band. The in-band energy will cause distortion of the transmitted signal and out-of-band energy will cause ACI (Adjacent Channel Interference).

One way to reduce the effects of non linearity is to drive the HPA with a high back-off and a consequent lower power efficiency. In order to improve the efficiency and consequently reduce size and cost of the broadcast transmitters, the linearity of HPA must be increased: one solution is the use of pre-distortion techniques.

A pre-distorter is a device that generates a distortion that compensates for the HPA distortion. The result of a pre-distorter is that the HPA can operate at higher power with the same level of distortion or the same power with lower distortion.

Following are several types of pre-distortion techniques that have been proposed:

The feed-forward approach takes a sample from the amplifier output and removes the main signal leaving only the distortion products and subtracts this distortion signal from the output. This system is quite complex, requiring extra components, fine adjustments, extra power for the losses introduced on the main signal and additional power to generate the distortion signal.

Data pre-distortion is a technique that considers the data vector space (constellation), pre-distorting it in order to counteract the distortion introduced by the HPA. It only compensates the distortion at the sampling instant and it does not eliminate out-of-band distortion. Data predistortion is dependent on the modulation type and should not be applied to complex modulations such as OFDM.

Finally, the signal pre-distortion method generates a pre-distorted signal which, when passed through the HPA, will emerge without distortion. The total effects of an ideal signal pre-distorter combined with an HPA is a linear function up to the HPA saturation level. For this reason, a signal pre-distorter eliminates both in-band and out-of-band distortions, as long as the signal does not exceed the saturation level.

Theoretically the signal pre-distortion can be realized at RF, at IF or at the Base-Band. The RF approach is not desiderable because it requires a specific circuitry for each frequency band as well as alignment at that frequency and stability of the environmental parameters.

The IF and Base Band approaches solve these problems by being independent from the final frequency. They are more stable, with regard to environmental parameters, due to the lower operating frequency. The only critical aspect for these approache is the increased linearity requirements from the pre-distorter output up to the HPA. Since the pre-distorter generates some out-of-band energy, the bandwidth of the signal increases and the intermediate stages must avoid linear distortions over a wider bandwidth up to the HPA.

ITELCO has developed its own solution based on a Signal Base Band Pre-distorter with a complete digital realization. The following are the advantages of this approach:


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