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1、58High Frequency Electronics High Frequency Design SIGNAL PROPAGATION Signal Propagation in the 900 MHz to 5 GHz Wireless Bands By Gary Breed Editorial Director T he effects of signal propagation are part of the trans- mission path for all radio signals. However, with the proliferation of digi- tal
2、modulation formats (some very complex), pro- pagation effects play a much larger role. In fact, dealing with such issues as fading and multi- path are primary considerations in baseband signal processing. This brief tutorial identifies the issues in signal propagation that affect todays wireless com
3、munications systems. Basic Path Loss Characteristics All radio signals have their field intensity reduced with increasing distance, as the wave front spreads out like the surface of an inflat- ing balloon. Microwave system designers will be familiar with the free-space loss equation: = 36.6 + 20logf
4、 + 20logd where is the attenuation in dB, f is the fre- quency in MHz and d is the distance in miles. To obtain a result in dB, the above equation uses log notation, and is another form of the following: PTX/PRX= 4.56 103f2d2 where PTXand PRXare the transmitted and received power respectively. The l
5、atter equa- tion shows that the loss is a function of both f2 and d2. The dependence on frequency is, essentially, a conversion factor to express dis- tance in wavelengths. The main characteristic to note is that attenuation increases as the square of the distance. However, in the near- fieldwhich i
6、s often considered to be zero dis- tance to 10 times the largest dimension of the antennathe attenuation increases approxi- This tutorial article reviews the major propagation characteristics at the lower microwave frequencies, and their effects on wireless voice and data communication systems opera
7、ting in that range Propagation factors Free-space path loss Solely distance-relat- ed; loss vs. distance is 1/d3in the near-field, 1/d2in the far-field. AbsorptionAtmospheric absorption a rela- tively small factor; water vapor effects increase with frequency; vegetation and structures (walls, window
8、s, etc.) have signif- icant effects. Natural and man-made noiseNatural noise is very small; man-made noise varies with location and type of environment (e.g. residential vs. commercial). Reflection,refraction and multipath Effects increase with frequency due to shorter wavelengths; time-delay effect
9、s require compensation in signal processing. Specific areas of interest for wireless The mobile environmentMotion results in changing patterns of fades and time delays. The indoor environmentSystem design must accommodate variability in building materials, furnishings and building layout. Statistica
10、l analysisA proven tool for deal- ing with complex propagation issues. Table 1. A summary of key propagation characteristics and related issues in wireless communications. Copyright 2003 Summit Technical Media, LLC From September 2003 High Frequency Electronics RadioFans.CN 收音机爱 好者资料库 60High Frequen
11、cy Electronics High Frequency Design SIGNAL PROPAGATION mately as the cube of the distance. With this information, we can make our first-order estimates of signal loss due to distance at the various frequencies of operation. The next propagation characteris- tic to consider is absorption. Although a
12、tmospheric absorption is small up to 5 GHz (increasing at higher frequencies), other absorption types are part of the wireless envi- ronment. These mechanisms include the attenuation due to lossy dielectrics in structures, such as walls, windows, doors, cubicle parti- tions etc. The effects of each
13、type of material can be evaluated individual- ly, but measurement is the usual method of evaluating this type envi- ronment. More about this later. Noise is the next characteristic to note.At microwave frequencies, natu- ral noise sources are trivial. Atmospheric noise is predominant at lower freque
14、ncies; solar and galactic noise is low-level and further attenu- ated by the atmosphere. This is not the case for man-made noise sources. Noise is usually considered to be random and wideband. Many electri- cally-operated devices have the potential to create wideband noise that reaches to the GHz ra
15、nge. Any “spark” or fast rise time voltage tran- sition generates unwanted signals over a wide frequency range. Although usually small, the proximi- ty of these devices to our wireless equipment can result in detectable, or even interfering,noise levels (remember the 1/d2and 1/d3relation- ships). Fi
16、nally, all the discrete-fre- quency low-level signals from clock oscillators, leakage and other low- level artifacts of normally-operating high frequency circuitry can add up to a measurable total power, with noise-like wideband characteristics. Direct Paths and Multipath The final major part of sig
17、nal propagation at 900 MHz to 5 GHz is the behavior of the radio waves as they travel from the transmitter to receiver. Signals are reflected from the ground, buildings and other objects. At distinct edges (e.g. corners of buildings) the signals will be diffracted, and variations in the atmosphere c
18、ause refraction. In other words, the transmitted signal is scat- tered, bent and bounced before it reaches the receiver. The Mobile Environment At the receive antenna, the direct signal and these modified signals are summed, which can result in either reinforcement or cancellation. If our radios and
19、 the environment are sta- ble (e.g. point-to-point microwave over short distance), all we need to do to avoid signal problems is move the antenna to a spot where the signal is strong. But if we are moving, the reinforcements and cancellations will be constantly varyingthe single most important matte
20、r in mobile wireless communications. The Indoor Environment Although a self-contained indoor wireless system like a WLAN has very little motion to deal with, the environment is much more cluttered than that between a cellular base sta- tion and a mobile handset. Walls, floors, ceilings, cubicles, of
21、fice furni- ture and people are present. As noted earlier, this environment is too com- plex to model as a collection of dis- crete objects, in the same way that the constantly-varying mobile envi- ronment cannot be modeled. Measurements and Statistics Why do we need to understand propagation so tho
22、roughly? Because of the high performance of our wire- less systems. An analog voice-only radio can tolerate a lot of variation we may know what the other party means even if there is a fade, or we can ask for it to be repeated. If the system operates 99 percent of the time, one percent isnt missed.
23、But in a digital system, 1 percent is a lot of data. Detecting missing data, requesting a repeat and re- sending the data take up even more time. Also, because they are numer- ically-based, digital systems do not tolerate errors. 1 percent outage may result in 20 or percent or greater reduction in t
24、hroughput. To achieve higher reliability, we need to know what conditions our digital system must tolerate. Since we cant model the exact propagation path, we must have an alternative method for per- formance prediction. Thus, statistical methods have been developed. Based on mathematical models and
25、 measured data, these methods are now the basis of evaluating the reliability of communications in both mobile and indoor environments. While work continues, the most sig- nificant work was done in the 1980s, providing many graduate students with topics for their theses! The mobile environment gener
26、al- ly uses a combination of Rayleigh fading model and Additive White Gaussian Noise (AWGN).The Rayleigh model determines the relia- bility of the path by providing a rela- tionship between the average signal- to-noise ratio and the number of expected errors. AWGN is used to set specific signal-to-n
27、oise ratios. Both computer simulation and simulated- path test equipment use these mod- els to evaluate the quality of particu- lar system, circuit and signal process- ing schemes. The indoor environment remains more complex. Measurements remain an important technique, with several vendors offering
28、test sources and receivers for relatively fast on-site evaluation of a proposed system. There are valid models for the prediction of indoor system perfor- mance, some of them proprietary. In general, these use a floor plan and basic structural data to estimate indoor wireless performance. These models are usually backed up with measurements, which usually can be coupled to the software models to refine the prediction. RadioFans.CN 收音机爱 好者资料库
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