Wireless Technology Prospects and Policy Options (2011) / Chapter Skim
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2 Key Technology Considerations
2 Key Technology Considerations
Pages 33-66
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From page 33... ...
It does not aim to describe the full range of technical challenges associated with wireless communications; the interested reader is referred to the 1997 NRC report The Eolution of Untethered Communications,1 which describes many of the fundamental challenges associated with wireless communications or, for a more recent view of the technology and its appli cations, several recent textbooks on wireless communications.2 1 National Research Council, The Eolution of Untethered Communications, National Acad emy Press, Washington, D.C., 1997. 2 See, e.g., Andrea Goldsmith, Wireless Communications, Cambridge University Press, Cambridge, England, 2005; David Tse and Pramod Viswanth, Fundamentals of Wireless Communication, Cambridge University Press, Cambridge, England, 2005; and Theodore S
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From page 34... ...
The shift relies on an important tradeoff: although the RF performance of analog components on a CMOS chip is worse than that of discrete ana log components, more sophisticated computation can compensate for these limitations. Moreover, the capabilities of radios built using CMOS can be expected to continue to improve.
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From page 35... ...
, frequency shift keying of the transmission frequency from f0 to f1 (Figure 2.1.3) , and phase shift keying of the phase by 180 degrees (Figure 2.1.4)
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From page 36... ...
Used with permission. FIGURE 2.1.2 Amplitude shift keying.
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From page 37... ...
Used with permission. Figure2.1.3 FIGURE 2.1.4 Phase shift keying.
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From page 38... ...
A series of dots approximately represents the continuous function shown in Figure 2.2.1. To find the frequency domain representation of this function, we can calcu late its Fourier transform.
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From page 39... ...
This capability creates new opportunities for modestlatency rather than realtime communication and may be of increasing impor tance to applications such as public safety communications. Digital signal processing of the audio can also, for example, be used to enhance under standability in (acoustically)
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From page 40... ...
Box 2.4 describes how waveforms can be constructed as a superposition of sinusoidal waves, and Box 2.5 describes several modern modulation schemes in use today. The introduction of the more sophisticated digital modulation schemes in widespread use today -- such as CDMA and OFDM, whereby different users using the same frequency band are differentiated using mathematical codes -- have further transformed radio communications (see Box 2.6)
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From page 41... ...
represents the signal amplitude in the frequency domain. A signal continued Amplitude Time FIGURE 2.4.1 Representation of square wave (solid line)
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From page 42... ...
Nevertheless, the spectra do illustrate the relationship between the coefficients of the time-domain harmonics in the Fourier series, and the frequency-domain components in the amplitude and power spectra. These are more clearly related by the Fourier transform, which accepts a time domain representation of a signal, such as x ( t )
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From page 43... ...
Amplitude Frequency FIGURE 2.4.3 Power spectrum representation of3 signal shown in Figure 2.2.1. Figure 2.4.
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From page 44... ...
is a technique that allows two bits of information to be sent concurrently. Two identical carriers 90 degrees out Q 01 11 I 00 10 Figure 2.5.1 FIGURE 2.5.1 Possible pairs of bits transmitted using quadrature phase shift keying.
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From page 45... ...
Q 0000 0100 1100 1000 0001 0101 1101 1011 I 0011 0111 1111 1011 0010 0110 1110 1010 FIGURE 2.5.2 Possible groups of four bits transmitted using quadrature ampli tude modulation. Figure 2.5.2
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From page 46... ...
Rather than a particular subcarrier being assigned rigidly to each user, as in FDMA, these subcarriers can be dynamically allocated among users, pro viding more subcarriers to different classes of users to give higher data rates, lower error rates, or other quality-of-service choices. Also, transmitting a given payload in a given period of time over multiple channels at a lower data rate (e.g., 1 Mbps over 16 channels)
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From page 47... ...
Although deep-space communication was the original source of these ideas, they have since been incorporated as a funda mental enabler of modern wireless communications ranging from wireless local area networking to mobile phones and to satellite radio and television. For four decades, the workhorse combination making these advances pos sible has been the convolutional coder and the Viterbi decoder, which remain the mainstay of many systems.
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From page 48... ...
That is, where nodes need to communicate only with nearby nodes, then relied on transmission from highpower central sites are giving way to more localized transmissions using eversmaller cells5 and mesh networks (Box 2.8) that provide much greater capacity by enabling frequencies to be reused at a finegrained level.
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From page 49... ...
Four independent degrees of freedom can be used to establish inde pendent channels -- frequency, time, space, and polarization.6 In the past, technology and the regulatory schemes that govern it have relied princi pally on a static separation by frequency and space. Advances in digital signal processing and control make it possible for radios to exploit the available degrees of freedom on a dynamic basis and to coordinate their own use of the various degrees of freedom available so as to coexist with 6 Polarization has seen practical application only for separating wireless signals for satel lite and pointtopoint microwave services.
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From page 50... ...
Digital logic advances make it possible for radios to incorporate significant and grow ing computing power that enables them to coordinate their own use of the various degrees of freedom available so as to coexist with each other and with uncoordinated spectrum occupants. Since much of the processing is performed digitally, the performance improvements popularly associated with Moore's law that characterize the computer industry are likely to apply to improvements in this type of processing.
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From page 51... ...
When the approach is used to increase data rates, it is called multipleinput, multipleoutput (MIMO) , and when it is used to extend range, it is called beam forming.
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From page 52... ...
On the other hand, operation at these frequencies also has some attractive properties. Only at these frequencies are very large bandwidths available, making them the only practical option to support wireless applications that require extremely high data rates.
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From page 53... ...
INTERFERENCE AS A PROPERTY OF RADIOS AND RADIO SYSTEMS, NOT RADIO SIGNALS It is commonplace to talk about radio signals interfering with one another, a usage that mirrors the common experience of broadcast radio signals on the same channel interfering with each another. Thus, the term "interference" might suggest that multiple radio signals cancel each other out, making their reception harder or impossible.
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From page 54... ...
Removing signal ambiguity thus entails investment in one or more of the following: better radio components, additional radio complexity, additional integrated cir cuit area, additional antennas, additional computation, and/or additional power consumption. Another area for potential improvement is in systems of radios.
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From page 55... ...
One might imagine building a radio that uses digital signal processing over very wide frequency ranges to separate out desired signals from potentially interfering signals. Doing so would allow one to leverage improvements in digital logic and better digital signal process ing techniques to mitigate interference.
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From page 56... ...
Some applica tions will have cost and/or power requirements that preclude the use of highly sophisticated radios that coordinate their behavior. For example, the constraints on cost and power consumption for embedded networked sensors preclude the use of highly sophisticated radios that are able to do very sophisticated signal processing or complex computation to coordi nate their behavior.
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From page 57... ...
Moreover, although great techni cal progress has been made in recent years, resulting in the deployment of new wireless services, wireless communications will remain a fertile environment for future basic research as well as product and service development. Timescales for Technology Turnover Different wireless services are characterized by the different timescales on which technology can be upgraded.
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From page 58... ...
,15 whereas private pilots would incur a large capital cost and have to learn a new system even though the existing technology already meets their requirements. TALENT AND TECHNOLOGY BASE FOR DEVELOPING FUTURE RADIO TECHNOLOGY The changing nature of radios is creating new demands for training and education.
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From page 59... ...
Frequency Allocation Chart that is published by the NTIA. The chart separates the spectrum from 30 MHz to 300 GHz into federal or nonfederal use and indicates the current frequency allocations for a multitude of services (cellular, radiolocation, marine, land mobile radio, military systems, and so on)
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From page 60... ...
Often designed to operate with very low average power levels, pointtopoint microwave 18 Robert Matheson, Spectrum Usage for the Fixed Serices, NTIA Report 00378, March 2000, p.
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From page 61... ...
22 Results from Some Measurement Activities The NTIA has a long history of spectrum measurement work going back to at least 1973.23 Those early efforts included federal land mobile radio measuring use in the 162174 and 406420 MHz range, and Federal Aviation Administration radar bands in the 2.72.9 GHz range. These projects were generally considered successful because the measurements focused on a definite problem and were able to address specific ques tions, such as whether claimed interference was real and whether minor changes to receivers could mitigate the problem of overcrowded use.
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From page 62... ...
26 Anticipated use of pointtopoint microwave has moved largely to optical fiber instead, although it is still used in many rural areas where the traffic does not justify the cost of laying fiber. A number of research projects have attempted to directly mea sure spectrum utilization.27 Shared Spectrum Company, a developer of spectrumsensing cognitive radio technology, has made several measure ment studies since 2000, including occupancy measurements in urban settings such as New York City and Chicago, suburban settings such as northern Virginia, and rural environments in Maine and West Virginia.28 Spectrum measurements for the New York City study were done during a period of expected high occupancy, the Republican National Conven tion.29 The studies aimed to determine how much spectrum might be allo cated for more sophisticated wireless applications and secondary users relative to primary (licensed)
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From page 63... ...
Use of White Space to Increase Spectrum Utilization The basic goal of "white space" utilization is to let operators with lower priority use the space when higherpriority users leave the spec trum unoccupied. From a technical perspective this approach requires adding sensing capability to devices to determine if a higherpriority user is using the spectral band (or bands)
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From page 64... ...
Second, considering frequency as the only degree of freedom available to separate users makes for simpler technical analysis but is highly limit ing. Radios built to perform dynamic beam forming, for instance, allow highly sophisticated spatial separation.
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From page 65... ...
. As desired data rates rise into the gigabit persecond range, adaptive antenna arrays will be used to obtain the necessary received power for both mobile and fixed devices.
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From page 66... ...
For example, • Engineering alone does not determine whether a service support ing aviation merits greater protection from interference than a service delivering entertainment. • The density and the distribution of a constellation of mobile devices (which affect their ability to interfere)
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