The Wireless Signal
Wireless signals are an integral part of many of today's embedded system designs. Mobile computer providers talk of media convergence, where consumers will be able to browse the Internet or watch live sports on a wireless computer, mobile telephone, portable digital television, or personal digital assistant (PDA).
Put simply, the media will be transparent to the wireless technology. Nevertheless, media convergence is the precursor to a myriad of complex technological issues, such as enhanced data compression, interoperability, propagation, and interference. Numerous other wireless uncertainties, such as the large number of international standards and media formats, deserve a book of their own. This chapter, in keeping with signal integrity engineering, is less concerned with media, standards, and the peculiarities of wireless propagation; it focuses on measuring and analyzing wireless signals. Wireless signals and spectrum analysis are wide-ranging subjects with several specialist areas and it could be argued that such topics are better suited to dedicated wireless books. However, because wireless is becoming so prevalent in embedded system design and there are so many fresh wireless issues, the wireless environment deserves valuable thinking time from the signal integrity engineer. Consequently, this book would be incomplete without an explanation of modern wireless signals and their measurement. Therefore, it is the aim of this chapter to help you understand some of the new techniques in wireless signal measurement. This chapter also offers a few thought-provoking ideas about signal analysis in the modern wireless environment.
With such a rich and diverse subject as wireless signals and their measurement, it will always be debatable which wireless instruments and applications to include in a broad SI book. Nevertheless, this topic is somewhat straightforward, because it can be argued that the spectrum analyzer (SA) is the principal tool for evaluating radio frequency (RF) signal characteristics. Moreover, spectrum analysis is the dominant test setting for a wide range of wireless systems and device designs. Also, spectrum analysis currently supports research and development applications ranging from low-power radio frequency identification (RFID) systems to high-power radar and RF transmitter measurements.
RADIO FREQUENCY SIGNALS
An RF carrier signal is like a blank piece of paper on which a message can be written and dispatched. RF carriers can transport information in many ways based on variations in the carrier's amplitude or phase, where modulation is simply a change in the shape of a wireless carrier signal. In practice, we talk of amplitude modulation (AM)and frequency modulation (FM), but to be pedantic, frequency modulation is the time derivative of phase modulation (PM). Combinations of AM and PM lead to numerous variations of modulation schemes,such as Quadrature Phase Shift Keying (QPSK),a digital modulation format in which the symbol decision points occur at multiples of 90 degrees of phase. Quadrature Amplitude Modulation (QAM) is a high-order modulation format in which both amplitude and phase are varied simultaneously to provide multiple states. Even highly complex modulation formats such as Orthogonal Frequency Division Multiplexing
(OFDM) can be decomposed into magnitude and phase components. Most elementary texts on wireless provide comprehensive illustrative examples that make plain the methods used to modulate a carrier signal. In the case of understanding modulation, a picture really is worth a thousand words.
However, to understand the digital representation of a modulated wireless carrier, you must be familiar with the vector model that is commonly used to represent a signal's amplitude and phase, as shown in Figure 10-1. A signal vector can be thought of as representing the instantaneous value of the magnitude (amplitude) and phase of a signal as the length and angle of a vector, respectively.
In a polar coordinate system, the same point could be expressed on a graph in traditional Cartesian coordinates, or rectangular horizontal X and vertical Y coordinates. In a digital representation of RF signals, an in-phase (I) and quadrature (Q) format of time samples are commonly used. These are mathematically equivalent to Cartesian coordinates, with I representing the horizontal or X component and Q the vertical or Y component. Figure 10-2 illustrates the magnitude and phase of a vector, along with corresponding I and Q components.
For example, consider the demodulation of an AM signal that is to be represented in I and Q components. This basically requires computing the instantaneous carrier signal magnitude for each I and Q sample, where each result is digitally represented and stored in memory. A plot of the stored data (amplitudes) over time would give a representation of the original modulating signal. However, PM demodulation is more complex and consists of computing the phase angle of I and Q samples, storing the results in memory, and performing some trigonometric computations to correct the data. Then the data is used to reconstruct the original modulating signal. Understanding quadrature I and Q signals appears difficult, but in reality it's similar to understanding the representation of the value of a sinusoid at any point in time using a vector's X and Y coordinates.
However, the ideal signals represented in Figures 10-1 and 10-2 are seldom realized in practice. As mobile telephony and various wireless systems extend throughout the modern world, a problem with radio interference emerges. Products such as mobile phones normally operate within a licensed spectrum. As a rule, manufacturers of mobile telephony equipment and other wireless devices are legally bound to operate within a specific frequency band. Such devices have to be designed to prevent the transmission of adjacent channel RF energy. This is especially challenging for complex multi-standard devices that switch between different modes of transmission and maintain simultaneous links to different telephone networks. Simpler devices that operate in unlicensed frequency bands also have to be designed to function properly in the presence of interfering signals.
Government regulations often dictate that these devices are allowed to transmit in
only short bursts at low power levels. Accurately detecting, measuring, and analyzing “bursty”-type wireless signals are tasks of significant concern in SI engineering.
Next: FREQUENCY MEASUREMENT
About the Authors
Dr. Geoff Lawday is Tektronix Professor in Measurement at Buckinghamshire New University, England. He delivers courses in signal integrity engineering and high performance bus systems at the University Tektronix laboratory, and presents signal integrity seminars throughout Europe on behalf of Tektronix.
David Ireland , European and Asian design and manufacturing marketing manager for Tektronix, has more than 30 years of experience in test and measurement. He writes regularly on signal integrity for leading technical journals.
Greg Edlund Senior Engineer, IBM Global Engineering Solutions division, has participated in development and testing for ten high-performance computing platforms. He authored Timing Analysis and Simulation for Signal Integrity Engineers (Prentice Hall).
Title: Signal Integrity Engineer's Companion ISBN 0131860062, Prentice Hall, Chapter 10: The Wireless Signal.
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