**2.2 Techniques for enhancement of average power efficiency**

Modulation schemes and multiple access techniques allowing high data rates in wireless communication standards lead to non-constant signal envelope with high crest factor or peak to average power ratio (PAPR). Since a typical class AB power amplifier in mobile base station offers highest power added efficiency (PAE) about at one dB compression area in the power sweep plot, high peak to average power ratio leads to power back-off from the peak efficiency point which leads to efficiency reduction (see Fig. 3.). As a result, average efficiency over time is much lower than the peak efficiency. From the system point of view, reduction of peak to average power ratio can be done with different techniques at the cost of

Fig. 3. Typical power sweep plot of a class AB power amplifier showing efficiency degradation when the power is backed-off from 1 dB compression point.

reduced data rate, transmit signal power increase, BER performance degradation, computational complexity increase, and so on (Jiang and Wu, 2008). Independent of the reduction techniques, rest of the peak to average power ratio still exists, so that for further average efficiency improvement, power amplifier architecture which can keep power efficiency high also when the transmitted power is backed-off must be considered.

### **Envelope elimination and restoration (EER) or Kahn technique**

This average efficiency enhancement technique is based on the idea to separate the amplitude modulated envelope from the constant envelope, phase modulated carrier signal. The envelope is amplified with high efficiency envelope amplifier, whereas the carrier is amplified with nonlinear but highly efficient power amplifier. The output of the envelope amplifier is supplied to the carrier amplifier which reconstructs the typical signal with nonconstant envelope of modern wireless communication standards (Diet et al, 2004).

### **Envelope Tracking (ET)**

160 Wireless Communications and Networks – Recent Advances

being used in high frequency applications which are served with other device technology e.g. GaAs MESFET and HEMT. The research interest has been then attracted by widebandgap semiconductor materials for high frequency power devices. Silicon Carbide (SiC) is superior in thermal conductivity compared to other wide-bandgap semiconductors. However, the cost of SiC is relatively high. Moreover, this material is not appropriate for applications with very high operating frequencies. For Indium Phosphide (InP), another wide-bandgap compound semiconductor, the focus of research is on extremely high-speed

The most prominent wide-bandgap semiconductor is Gallium Nitride (GaN). Comparing with Silicon device technology which is mainly driven by microprocessor and computer industries, GaN found its applications in screen industries enabled by GaN OLED (organic light emitting diode) technology and data storage industries utilizing blue laser produced by GaN laser diode to read out the data from a Blue-ray DiscTM. In automotive applications and power electronics, GaN devices are attractive due to high operating temperature and high breakdown field for switching power supply. For RF power amplifiers, GaN-based power devices offer extremely large bandwidth, high power density, high operating frequency and high output impedance. The advantages of GaN-based power devices for

Modulation schemes and multiple access techniques allowing high data rates in wireless communication standards lead to non-constant signal envelope with high crest factor or peak to average power ratio (PAPR). Since a typical class AB power amplifier in mobile base station offers highest power added efficiency (PAE) about at one dB compression area in the power sweep plot, high peak to average power ratio leads to power back-off from the peak efficiency point which leads to efficiency reduction (see Fig. 3.). As a result, average efficiency over time is much lower than the peak efficiency. From the system point of view, reduction of peak to average power ratio can be done with different techniques at the cost of

Power added efficiency (PAE)

**PAE (%)** 

wireless communications will be discussed more thoroughly in the next section.

Fig. 3. Typical power sweep plot of a class AB power amplifier showing efficiency

degradation when the power is backed-off from 1 dB compression point.

**2.2 Techniques for enhancement of average power efficiency** 

digital applications where high power is not required.

Similar to Kahn technique, supply voltage level of the RF amplifier is dynamically modified depending on the level of the signal envelope. A slight difference is that the input of the RF amplifier is still amplitude and phase modulated. Only with excessive signal power, the supply voltage of the RF amplifier is modified. The RF amplifier of this technique operates also in a linear mode unlike the Kahn technique, where the RF amplifier operates solely in a nonlinear mode.
