Reducing the drive signal bandwidths of EER microwave power amplifiers

Abstract

High efficiency and linearity can be achieved using Kahn's envelope elimination and restoration (EER) technique. However, when used with modulation schemes such as CDMA and orthogonal frequency division multiplexing (OFDM), the resulting polar drive signals suffers from expanded bandwidth. Rudolph has shown that the bandwidth of the polar drive signals can be reduced by hole punching (limiting the minimum envelop of the modulated signal). This paper quantifies the error vector magnitude (EVM) caused by the ensuing distortion and compares the performance to a distortionless method based on the well known partial transmit sequence (PTS) method for OFDM. Although the hole punch process performs better than the PTS approach in terms of spectrum improvement, its EVM will limit the data throughput of the transmission.

A method for real-time generation of slew-rate limited envelopes in envelope tracking transmitters

“…Then, one of the main challenges in ET envelope drivers is to supply the power required by the transistor at the same speed of the signal envelope. In recent works, some solutions based in iteratively reducing the bandwidth envelope [2] or reducing the PAPR [3] were shown. Our proposal is to generate a slew-rate reduced version of the RF signal envelope, because the typical speed restriction in power devices, as is the case of power supplies, is the slew-rate.…”

Computationally efficient real-time digital predistortion architectures for envelope tracking power amplifiers

“…Focusing on this later objective, two different approaches based on SR and BW reduction of the RF signal's envelope showed that these strategies are suitable to adapt the envelope characteristics to the EA requirements or limitations at the expenses of having efficiency degradation. On the one hand, the method proposed in [5, 6] limits the BW of the envelope iteratively, which may represent an issue in real time applications. On the other hand, the method proposed in [8] consists of a real-time algorithm where the resulting signal is limited in SR but not in BW, making challenging its amplification if only a switched mode EA is considered or requiring a wide band if only a linear EA is considered.…”

“…In dual-band applications, for example, this becomes even more challenging since the combined envelope can present BWs more than 5 × the carrier separation. Therefore, in order to relax the EA requirements, some solutions have been proposed to reduce the BW and slew-rate (SR) of the original signal's envelope [5][6][7][8]. Unfortunately, the use of a slower version of the envelope to supply the PA drain not only degrades the overall efficiency but also results in nonlinear distortion amplification.…”

“…On the one hand, the method proposed in [5,6] limits the BW of the envelope iteratively, which may represent an issue in real time applications. On the other hand, the method proposed in [8] consists of a real-time algorithm where the resulting signal is limited in SR but not in BW, making challenging its amplification if only a switched mode EA is considered or requiring a wide band if only a linear EA is considered.…”