What is Digital Pre-Distortion?

Digital Pre-Distortion (DPD) is a technique to increase linearity or compensate for non-linearity in power amplifiers. Power amplifiers are critical in communication systems and it is crucial to improve their efficiency and overall performance. DPD is a cost-effective linearization technique which aims to provide improved linearity, better efficiency, and take full advantage of power amplifiers.

Digital Pre-Distortion applies inverse distortion, using a pre-distorter, at the input signal of the PA to cancel the distortion generated by the power amplifier. This requires knowing the PA characteristics accurately with effective implementation for successful DPD functioning. Proper implementation can show improvements of up to 40% in PA efficiency used in wireless base stations.

Ideally, all amplifiers should be perfectly linear, which means the output signal should be an exact copy of the input signal. However, it is not possible to make a perfectly linear amplifier because amplifying devices, such as transistors or vacuum tubes, are non-linear by nature which introduces some amount of non-linearity in the output/amplified signal. Non-linearity generates spectral re-growth, which leads to interference and can also lead to violation of emissions standards set by regulatory bodies. It can also lead to the degradation of bit-error rate (BER) and data throughput of the communication system.

Drawbacks of Digital Pre-Distortion:

• Higher design complexity
• Higher bandwidth requirement

Another technique of reducing non-linearity in a power amplifier is known as "Back off". In this technique, the amplifier is operated at a lower power level which helps ensure that the power amplifier operates in the linear region. However, backoff reduces the efficiency of amplifiers, more so in new transmission formats such as WCDMA, OFDA, etc. DPD allows power amplifiers to be used at their actual saturation power to maximize efficiency and performance.