PRODUCT HOW-TO: Simplify the design of the cellular base station digital predistortion subsystems

While digital techniques enable systems to run faster and consume less power using smaller package sizes and at higher reliability with each generation, these systems place new demands on the RF and analog signal acquisition portion of the system. The complex modulation and wide bandwidth of base station transmitters result in higher crest factors for the power amplifier (PA).

To meet the more stringent requirements in the presence of higher crest factors, the PA is typically oversized to enable operation in the linear region. Without digital correction, the PA efficiency can be on the order of 10 percent - meaning 200W is required to run a 20W PA.

The PA is the largest consumer of electrical power in a base station and therefore a significant factor in the operating expense for the cellular service provider. To improve PA efficiency, digital techniques are used for crest factor reduction (CFR) and digital predistortion (DPD).

While an amplifier is most efficient when driven into saturation, it is also highly nonlinear in saturation. Complex digital modulation requires extremely high linearity from the PA, which consequently means that it must be driven well below saturation.

Operating the PA just below saturation offers good efficiency if there is a way to compensate for the inherent nonlinearity of the amplifier. Digital predistortion has emerged as the preferred method of PA linearization.

DPD is a feedback technique where the output of the PA is sampled and converted to digital data. A distortion-free transmit signal stored in a FIFO is compared to the feedback signal, creating an inverse of the transfer function.

This is summed with the transmit data after CFR to reduce the nonlinearity of the PA output. An adaptive algorithm or a lookup table may be used to produce the compensating digital signal or the two methods can be combined, but this is beyond the scope of this article. Here, we will focus on the analog requirements for the receiver that samples the PA output.