**1. Introduction**

156 Wireless Communications and Networks – Recent Advances

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Progress in wireless communication technology has enabled applications which were unthinkable as the first digital mobile phone came into the market. Integration of digital camera into a mobile phone was an important step of the convergence between telecommunication and information technology as users started to require transfer of digital pictures besides conventional voice and text information. In addition, fast progress in digital technology has been an immense driving force of the needs for high data rates in telecommunications. Digital multimedia contents e.g. pictures, music, video clips are expected to be available anytime and anywhere which results into tremendous requirements in research and development in wireless technology.

Even though the industry tends to be majorly driven by software applications as well as "look and feel" of mobile devices, enabling hardware technologies in the background also deserve appropriate attention from R&D engineers. As soon as the performance of mobile communication systems cannot fulfil the expectation of users in terms of data rate and error robustness, the importance of the enabling hardware technology becomes obvious.

In order to cope with the rapid growth of the needs in wireless data transmission with constantly increasing data rates, new technical challenges arise perpetually on every layers of the OSI reference model. Whereas new modulation and multiple access techniques e.g. OFDM and OFDMA are introduced to support higher data rates and intelligent network configuration deals with the optimization of routing to increase the capacity and to improve load distribution, progress in hardware components in mobile devices and mobile base stations on the physical layer is also required to serve the needs of the higher OSI layers. Such progress on the physical layer includes techniques and hardware architectures which can enhance power efficiency of the system components while still complying with other specifications regarding linearity, noise, interference, etc.. Also, novel semiconductor device technology provides improved power handling capability resulting in smaller hardware size and high impedance which simplifies the design of matching networks. Moreover, large bandwidth and high impedance offer the possibility to create multiband components by designing the matching networks to be reconfigurable (Fischer, 2004).

This chapter aims to review state-of-the-art research in power amplifiers for wireless communication infrastructure featuring advantages of Gallium Nitride (GaN)-based power devices including large bandwidth capability, high power density and high output impedance. Regarding the issues of power amplifier design, state-of-the-art power amplifier architectures will be discussed with various prospects. For wireless communication standards with high data rates e.g. WCDMA, WiMAX and LTE, their modulation schemes and multiple access techniques lead to non-constant signal envelope with high peak to average power ratio. As a consequence, power amplifiers in wireless communication infrastructure are required to operate in a wide dynamic range making it difficult to maintain high average efficiency over time. This chapter will discuss widespread techniques for average efficiency enhancement including Doherty power amplifier concept and envelope tracking (ET) with state-of-the-art results. Another possibility for power efficiency improvement is the switched-mode power amplifier where the waveforms of the voltage and current are optimized to achieve low power dissipation at the power transistor. GaNbased power transistors have demonstrated in numerous research works to be suitable power devices for the switched-mode architecture as well as for average efficiency enhancement techniques e.g. Doherty power amplifier and envelope tracking. As examples, results of 2.45 GHz GaN class AB power amplifier and GaN VHF class E power amplifier will be presented in this chapter. The wide band capability of GaN-based devices also supports design of reconfigurable and wideband power amplifiers. With all advantages of GaN-based devices, they are still not a mature technology in terms of reliability and memory effects. Results from investigation on memory effects and parasitics of GaN-based devices will also be discussed in the chapter showing promising improvements in these regards which make GaN-based devices interesting and promising power devices for future wireless communication infrastructure.

### **2. Power amplifiers in the wireless communication infrastructure**

In a mobile communication system, power amplifier is an important component which boosts the transmitted signal power before it is sent via the antenna to the receiving device through wireless channels (see Fig. 1.). In a base station for mobile communication standards e.g. GSM, UMTS or LTE, power amplifier is the part which consumes the largest portion of power. Thus, the efficiency of power amplifier has the greatest influence on the entire system's efficiency. In addition, cooling requirement of a base station is also dominated by its power amplifier. In terms of cost, power amplifier is also the most expensive part of a base station. For the first generation of UMTS base stations, the costs of power amplifier and cooling are about 30%-35% of the cost of an entire base station (Chalermwisutkul, 2007). Besides the efficiency, linearity is also an important specification of power amplifiers which ensures that the transmitted signal is not distorted by the nonlinearity to an unacceptable level causing excessive bit errors.

Fig. 1. Block diagram of a UMTS base station transceiver showing power amplifier and other system components.

devices including large bandwidth capability, high power density and high output impedance. Regarding the issues of power amplifier design, state-of-the-art power amplifier architectures will be discussed with various prospects. For wireless communication standards with high data rates e.g. WCDMA, WiMAX and LTE, their modulation schemes and multiple access techniques lead to non-constant signal envelope with high peak to average power ratio. As a consequence, power amplifiers in wireless communication infrastructure are required to operate in a wide dynamic range making it difficult to maintain high average efficiency over time. This chapter will discuss widespread techniques for average efficiency enhancement including Doherty power amplifier concept and envelope tracking (ET) with state-of-the-art results. Another possibility for power efficiency improvement is the switched-mode power amplifier where the waveforms of the voltage and current are optimized to achieve low power dissipation at the power transistor. GaNbased power transistors have demonstrated in numerous research works to be suitable power devices for the switched-mode architecture as well as for average efficiency enhancement techniques e.g. Doherty power amplifier and envelope tracking. As examples, results of 2.45 GHz GaN class AB power amplifier and GaN VHF class E power amplifier will be presented in this chapter. The wide band capability of GaN-based devices also supports design of reconfigurable and wideband power amplifiers. With all advantages of GaN-based devices, they are still not a mature technology in terms of reliability and memory effects. Results from investigation on memory effects and parasitics of GaN-based devices will also be discussed in the chapter showing promising improvements in these regards which make GaN-based devices interesting and promising power devices for future

wireless communication infrastructure.

system components.

**2. Power amplifiers in the wireless communication infrastructure** 

nonlinearity to an unacceptable level causing excessive bit errors.

In a mobile communication system, power amplifier is an important component which boosts the transmitted signal power before it is sent via the antenna to the receiving device through wireless channels (see Fig. 1.). In a base station for mobile communication standards e.g. GSM, UMTS or LTE, power amplifier is the part which consumes the largest portion of power. Thus, the efficiency of power amplifier has the greatest influence on the entire system's efficiency. In addition, cooling requirement of a base station is also dominated by its power amplifier. In terms of cost, power amplifier is also the most expensive part of a base station. For the first generation of UMTS base stations, the costs of power amplifier and cooling are about 30%-35% of the cost of an entire base station (Chalermwisutkul, 2007). Besides the efficiency, linearity is also an important specification of power amplifiers which ensures that the transmitted signal is not distorted by the

Fig. 1. Block diagram of a UMTS base station transceiver showing power amplifier and other
