Multicarrier WCDMA Feasibility, Part 4

See Part 1 for an analysis of receiver performance specifications.
See Part 2 for more on the variable gain amplifier (VGA) implementation and spurious free dynamic range (SFDR).
See Part 3 to learn about key performance issues in the transmit path, including frequency error and power control.
See Part 5 for an an in-depth look at transmit modulation and direct conversion.

Power Amplifier Linearization
Another method for increasing the efficiency of the power amplifier is to allow the amplifier to move closer toward saturation, hence increasing efficiency, but also compensating for the resulting distortion that results. There are two main approaches to PA linearization.

Analog feedforward uses linear feedforward compensation amplifiers around the main power amplifier to counter the distortion problems and provide sufficient linearity so that spectral regrowth does not pollute adjacent channels. This approach typically results in efficiencies less than 10% and is a complicated, but tractable, analog problem where the feedforward amplifiers' linearity also need to be considered.

A second approach to PA linearization comes in the form of digital predistortion. This method uses the simple concept that a digital numerical representation is very linear and highly predictable, with no effect from environmental operating conditions. Thereby, if the transfer function of the PA can be determined, summation with an equal and opposite transfer function (Figure 12) results in a highly linear system response which introduces no noise or distortion.

Furthermore, the manufacture of the analog feedforward amplifiers is no longer needed and a cheaper digital process can be used. The impact on the converters for a system implementing digital predistortion should be considered. The forward path is considered first, as in Figure 13.

Any signal passed through a power amplifier is disturbed in two ways. First, additive noise is introduced to the signal, and second, a nonlinear PA transfer function leads to odd-order intermodulation products. For a WCDMA signal these effects lead to spectral regrowth in the adjacent and alternate channels. Third-order intermodulation products cause spreading of the distortion over three times the bandwidth of the carrier; fifth-order intermodulation gives fives times the bandwidth, and seventh-order intermodulation gives seven times the bandwidth. For a single carrier, having a wanted channel bandwidth of 3.84 MHz, third-order distortion occupies a band between 1.92 MHz and 5.76 MHz either side from the center of the wanted channel (Figure 14).

This appears in the adjacent channel together with the additive broadband noise. The first alternate channel is unaffected by third-order intermodulation but is still affected by the broadband noise. Similar consideration of the fifth- and seventh-order intermodulation products shows an additional channel is affected with increasing order of intermodulation.

With four carriers, the distorted signal bandwidth is now 18.84 MHz. Consequently, third-order intermodulation now affects a band 9.42 MHz to 28.26 MHz from the center of the signal bandwidth; third-order intermodulation affects significantly more alternate channels. Additionally, for a fixed digital/analog converter (DAC) intermodulation distortion (IMD) performance, as more carriers are added there is more energy in the alternate channel, which reduces the adjacent channel leakage power ratio (ACLR) by the factor 10log₁₀(#carriers) relative to the single carrier case. Recall that the intent of digital predistortion is to create antidistortion, a system employing digital predistortion needs 10log₁₀(#carriers) more IMD performance relative to the single carrier case to maintain the same ACLR as the single carrier case.