||1.1 : Devices for Mobile Phones and WiMAX
As Fig. 1.2 shows, with each type of wireless communications system introduced, there has been an increase in communication data transmission speed. For mobile phone services, HSDPA*6 currently provides a transmission speed of approximately 14Mbps; however, advancements are being made towards the actual use of LTE*7 , with the aim of achieving a transmission speed of 100Mbps. Additionally, for WLAN*8 , which is used for communications in the home, changes towards the actual use of WiMAX*9 are progressing with the aim of expanding the limits of usage to include outdoor communications as well.
In order to achieve these high communication speeds, a linear amplifier that enables accurate control of the wireless signals is required. One problem is, however, using a linear amplifier requires that power consumption be increased. To resolve this problem, Mitsubishi Electric is engaged in improving the power efficiency of its full range of linear amplifiers. Additionally, with the increasing functions being introduced for mobile phones, such as digital terrestrial TV reception, taking photos and movies, GPS*10 and games, the company is making progress in response to the growing demand for smaller wireless communications components, smaller power amplifiers and greater functionality.
*6 HSDPA : High Speed Downlink Packet Access
*7 LTE : Long Term Evolution
*8 WLAN : Wireless Local Area Network
*9 WiMAX : Worldwide Interoperability for Microwave
*10 GPS : Global Positioning System
*11 HSPA : High Speed Packet Access
*12 FWA : Fixed Wireless Access
*13 WMAN : Wireless Metropolitan Area Network
*14 WPAN : Wireless Personal Area Network
Figure 1.3 outlines the structure of a mobile phone and its built-in power amplifier. This phone uses two frequency bands, so two power amplifiers are installed. In order to support expanding functionality such as increasing communications capacities and multiple overseas frequencies, some phones are equipped with more than four power amplifiers. For these devices too, the demand for smaller devices that are compatible with various countries' systems continues to grow.
*15 CODEC : COder/DECoder
To meet these demands, Mitsubishi Electric produces a broadband amplifier capable of amplifying two frequency bands and a device with two circuits embedded to amplify two bands. Fig. 1.4 illustrates the power amplifier circuit and shows the evaluation results. Utilized to control power output across a wide range, the amplifier circuit provides a circuit switching function that operates in response to power output. As an example, at a power output of 15dBm, a significant improvement in efficiency, from approximately 8% to over 20%, is obtained.
Figure 1.5 shows the trend in the development of a device to realize these functions. It is a GaAs system incorporating a HBT with superior power amplification and a HEMT with outstanding switching and low-noise operation. Expectations are high for the introduction of a device that integrates these features. The realization of such a device will lead to wireless communications equipment with even higher performance capabilities.
*16 BiFET : Bipolar Field-Effect Transistor