Mobile WiMAX is entering practical use, enabling portable devices to send and receive data as fast as ADSL and WLAN. Device makers, however, are still struggling to solve the low PAE of the power amps in those devices.
Mobile
WiMAX (IEEE802.16e) is the next-generation broadband standard designed
to bring high-speed data communications on a par with asynchronous
digital subscriber lines (ADSL) and wireless local area networks (LAN)
to the mobile environment. In the US, Sprint Nextel Corp of the US has
announced commercial service will start between the summer and fall of
2008, while UQ Communications Inc of Japan has already boosted its
capital by more than Yen16 billion in preparation for service launch in
2009, and selected the base station equipment manufacturer. Commercial
rollout is coming fast.
People in charge of developing terminal equipment for Mobile WiMAX,
however, are beginning to get worried about one key device: the power
amplifier used in the terminal transmitter (Fig 1).
The general opinion is that the power added efficiency (PAE) of the power amps used in Mobile WiMAX terminals is far too low. Power amps for mobile phones, for example, offer PAEs of 40% to 45% for wideband code division multiple access (W-CDMA), and 50% to 55% for Global System for Mobile Communication (GSM). For Mobile WiMAX, though, the PAE is only 10% to 20%.
The PAE is the ratio between the power input into the power amp, and the signal output power, and is a key performance index for evaluating power amps. A low PAE means that a large fraction of input power is consumed as heat or otherwise wasted. An engineer at one equipment manufacturer explained, "Existing power amps have pretty low efficiency, and therefore consume quite a bit of power. That translates directly to shorter battery drive time for a WiMAX mobile terminal."
A low power amp PAE
also means that mobile handsets will not offer all the convenience they
otherwise might. Power consumed by the power amp is about 1W to 1.5W at
present, which seriously cuts into battery drive time. This problem
will become especially critical in about 2010, when Mobile WiMAX
functionality is implemented in compact mobile phones (Fig 2). Handset
manufacturers are doing their best to find high-efficiency power
amplifiers now in preparation, but with less than stellar results
(Table 1).
The fundamental cause of the low PAE of power amps for Mobile WiMAX is the transmission method used: orthogonal frequency division multiplexing (OFDM) uses multiple carriers, and imposes a major load on the amp. The primary reason is that OFDM requires that the power amp deliver very high linearity, and that can only be ensured by boosting power consumption.
A look at the OFDM signal waveform on the time axis reveals a major difference between the mean and peak amplitudes (Fig 3). OFDM is multi-carrier, and when multiple signals are multiplexed at a given time amplitude can jump to several times the mean. Power amps have to offer the ability to accurately track these input signals - in other words, linearity - over a wide dynamic range.
There
are limits to the linearity that power amps can provide, though,
because above a certain point the result will be output saturation. In
this region problems can occur such as signal waveform distortion and
leakage between adjacent channels.
Nikkei Electronics |
Nikkei Monozukuri |
Nikkei Automotive Technology |
|---|---|---|
 |
 |
 |
| The magazine for electronics designers and managers. | Design You Won't Regret - How to Reduce the Risk of Accidents | The comprehensive automotive technology magazine. |
Copyright © 1995-2010 Nikkei Business Publications, Inc. All rights reserved.
All editorial content and graphics on this Web site may not be reproduced in whole or in part without the express permission of the copyright owner.
