wireless industry
W
e are in the midst of a connectivity and mobility revolution. This is driving a
dramatic change in communication technologies throughout the world, and the engineers that are responsible for networks and the development of mobile devices are facing tremendous technical challenges.
These challenges are spurred by soaring sales of hand- held smart devices, which are ramping up the number of high-speed data connections. According to Cisco, wireless data usage is going to increase at a compound annual growth rate of almost 80 percent through 2016, as more consumer use handheld devices for social networking and entertainment. This explosion of wireless traffic will be driven by the uptake of sleek-form- factor devices that combine seamless connections with an increase in time between charges.
To keep pace with this rocketing demand for data, smart devices and networks are migrating to broader frequency ranges, wider signal bandwidths and higher data rates. This is fuelling innovation in the RF front-end industry, with engineers developing new designs for making best use of the limited energy stored in the battery.
RF system challenges
Efforts to increase data rates and improve the network’s spectral efficiency have included the introduction of new standards employing advanced coding techniques, such as orthogonal frequency-division multiplexing (OFDM). One of the downsides of these complex coding techniques is that the modulations can lead to a significantly increased variation in the amplitude of the modulated signal – this is needed to encode more information for each transmitted symbol (see Figure 1). In addition, improvements in spectral efficiency diminish the efficiency of the RF transmission system. This reduces in-use time for the mobile device, and also heats it up.
Increasing spectral efficiency is only going be part of the solution, due to the relentless pace of growth of wireless data. To help to meet this demand, additional frequency spectrum is being allocated to networks. However, these allocations to digital networks are haphazard. They are regulated by independent governments, and the decision makers are not concerned with designing a cost-effective global radio system. What is happening is that the frequency bands being given over to digital networks differ greatly from region to region. This uncoordinated approach means that smart devices have to cope with an increasing number of frequency bands of varying bandwidths – there are now more than 36 cellular transmit bands of
various bandwidth, ranging from 695 MHz to 3800 MHz. Compounding this issue are the incremental, yet challenging, RF specifications for ensuring that digital networks can operate without interference with other wireless systems, such as public safety bands, global positioning systems, and wireless LAN (WiFi).
One of the demands that the latest digital network modulation schemes place on smart devices is the requirement for linear amplification. To ensure a high- quality wireless network, the RF transmit system must accurately and proportionally amplify the input signal – significant distortions in either phase or amplitude cannot be tolerated. Excessive distortion has two unwanted consequences: Data errors in the transmitted signal; and a spilling of the transmission energy into other bands, which results from modulation products. In the latter case, this leads to interference with other data devices.
In general, improvements to the spectral efficiency of the modulation scheme increase the crest factor or peak-to-average ratio (PAR) of the signal (see Figure 2). PA efficiency peaks when this amplifier is at or near its
Figure 1. An example showing modulation constellations with increasing amplitude variation
Figure 2. Increase in peak-to-
average ratio (PAR) for
common digital network modulation schemes
July 2012
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