The File - Dec 1, 2008 - (Page 6)

In Focus | Mobile handsets Meet WiMAX power amp challenges By Darcy Poulin Senior Systems Engineer SiGe Semiconductor, Inc. As designers turn their attention to mobile WiMAX devices, they are quickly learning that there are some specific design challenges regarding power amplifiers. For Wave 2 mobile WiMAX products, the mobile device needs to efficiently deliver +23dBm output power with high linearity from a 3.3 VDC supply. Managing power in mobile WiMAX is quickly shaping up to be vitally important as first-generation designs are tested and deployed. One of the challenges of designing for mobile WiMAX is its long range, since WiMAX networks typically achieve coverage of about 1 km per cell. To achieve these ranges, WiMAX must have an optimised power profile—from the base station right down to the components in the mobile device. High transmit power is therefore important. But how high can WiMAX go, and what are the limitations imposed by regulatory bodies, technological limits and usage models? Designers of the power amplifier (PA) and those selecting PAs need to find the optimal balance between high power and high efficiency in order to ensure robust links, high data rates and good range for their WiMAX services. The nature of WiMAX What makes WiMAX challenging for designers is that it is an access technology with a unique set of constraints. As a result, power amplification circuits that were used for cellular or Wi-Fi applications cannot simply be dropped into WiMAX designs and tweaked to perform adequately. In many ways, WiMAX can be considered a hybrid technology because it shares aspects of both cellular and Wi-Fi networks. Mobile WiMAX is very similar to cellular, in that it is meant for use in highly mobile devices and utilises licensed frequency bands for high reliability. It also employs transmit power control techniques, much like CDMA cellular does. However, it differs from cellular because it operates at much higher data rates (resulting in more stringent linearity requirements) and must simultaneously handle voice over Internet Protocol (VoIP), data and video transmissions. Managing the bandwidth and priority of transmission for these types of services requires a quality of service (QoS) component that is not required for mobile voice alone. On the other hand, WiMAX is also similar to Wi-Fi. For instance, it offers high data rates, uses orthogonal frequency division multiplexing (OFDM) with modulations from BPSK to 64-QAM, and is an all-IP-based network. However, it differs from Wi-Fi because it uses a fully scheduled service, unlike the collision-based carrier sense multiple access (CSMA) technique used by Wi-Fi. This gives WiMAX a significant advantage over Wi-Fi. As the number of users increases in a CSMA network, overall capacity drops dramatically since each collision requires a subsequent retransmission. With a scheduled service, overall network capacity is unaffected as the number of users increases, since the base station manages each user’s access to the network efficiently. WiMAX coverage Wi-Fi networks typically cover ranges that are measured in the tens or hundreds of meters for each access point. However, WiMAX networks will achieve coverage of about 1 km per BS. In order to achieve this, mobile WiMAX networks employ a number of techniques to achieve longer range, including high transmit power, subchannelisation and adaptive modulation. Simply put, RF power translates directly into range; thus higher power equals longer range. To achieve long range, WiMAX base stations transmit at power levels of approximately +43dBm Figure 1: Achievable modulation versus distance with +23dBm transmit power. Figure 2: Transmit power versus distance from base station. (20W), as compared to Wi-Fi APs, which typically transmit at +18dBm (60mW). A WiMAX mobile station (MS) typically transmits at +23dBm (200mW), as compared to +18dBm (60mW) for Wi-Fi. Cellular (CDMA) transmit powers for both the BS and MS are similar to those used in WiMAX. However, because WiMAX uses much higher modulation orders to achieve higher throughput, WiMAX requires a much better SNR than cellular. For the mobile transmitter, high modulation orders require a PA with much better linearity and greatly complicates PA design compared to GSM or CDMA. You might notice that there is a large difference (approximately 20dB) between downlink power (from the BS to the MS) and uplink power (from the MS to the BS), so mobile WiMAX networks are severely uplink limited. This is also the case for cellular networks. This means that while a mobile can easily receive transmissions from a BS, its relatively low transmit power makes it difficult for the BS to hear it. One way to combat this mismatch is by using a technique called subchannelisation, where only a subset of all of the available subchannels is used for any particular user. In effect, each mobile concentrates its power over a smaller range of frequencies, and the net signal gain is 10*log(Ntotal/Nused), continued on page 6 EE Times-India | December 1-15, 2008 | www.eetindia.com http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/RELEVANCE/CDMA.HTM?ClickFromNewsletter_081201 http://www.powerdesignindia.co.in/SEARCH/SUMMARY/technical-articles/%22power amplifier%22.HTM?ClickFromNewsletter_081201 http://www.wirelessnetdesignline.com/encyclopedia/defineterm.jhtml?term=WiMAX&x=&y http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/%22transmit power%22.HTM?ClickFromNewsletter_081201 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/SNR.HTM?ClickFromNewsletter_081201 http://www.eetindia.com/STATIC/REDIRECT/Newsletter_081201_EETI02.htm?ClickFromNewsletter_081201

Table of Contents for the Digital Edition of The File - Dec 1, 2008

EETimes India - December 1, 2008
Contents
National Semiconductor
WiMAX Goes Beyond Mobile Devices
Mobile Handsets Integrate WLAN
Meet WiMAX Power Amp Challenges
NGSA - 08, Energy INDIA, ICDCIT 2008, REA-2008, ICON 2008, ICPCM 2008

The File - Dec 1, 2008