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resistance to multipath. This was a major issue in the development of digital cellular standards, where it was necessary that each standard be written to accommodate the worst-case multipath conditions likely to be encountered by users over the entire geographic region of usage for that standard. Considerable attention is also being given to multipath speci cations as part of the standardization of the air interface for 2-GHz PCS systems in the United States [JTC94a,b]. Constant Envelope Modulation. Most mobile radio products are designed with class C power ampli ers, which provide the highest power ef ciency among the common types of power ampli ers. However, class C ampli ers are highly nonlinear, so it is necessary that the signal to be ampli ed is constant-envelope or as nearly so as is practical. The reason for this is that any amplitude uctuations in the input signal will give rise to spectral widening of the output signal, in turn causing increased ACI. It is because of these considerations that frequency modulation has remained in widespread use in the mobile radio industry. Although analog FM mobile radio systems were originally designed for analog voice, they have been extended to data service simply by feeding baseband digital data streams to the frequency modulator. This in effect is a method of FSK, where input amplitude levels correspond to transmitted tones. An FM signal is by its very nature constant-envelope; however, it is not spectrally ef cient, due to its high sidelobes. Thus, as the needs for greater bandwidth ef ciency have grown, efforts have been made to design modulation schemes that are less wasteful of bandwidth while preserving (or nearly so) the constant-envelope nature of FM. To conform to spectrum constraints, it is necessary in some systems to apply ltering to the modulated waveform before power ampli cation, and the ltering produces amplitude variations. In order that undesirable out-of- band spectral components not be generated, it is then necessary that the ampli cation be linear. Consequently, such non-constant-envelope ltered signals are commonly referred to as linear modulation systems. In the subsections that follow, we describe three modulation techniques that are speci ed in prominent standards for radio modems: four-level-FM, GMSK, and /4shift QPSK. We shall see that the rst two are constant-envelope modulations, while the third, which is speci ed in a ltered version in the IS-54 TDMA digital cellular standard, is a form of linear modulation. Although today s implementations of linear modulation schemes require linear ampli ers, and thus suffer a loss in ef ciency relative to class C ampli cation, much research work is in progress to develop new methods of power ampli cation that combine near-linearity with power ef ciency approaching class C characteristics. 7.4.2 Digital Frequency Modulation As we noted earlier in this section, FM is the predominant form of modulation used in the mobile radio industry. Although FM has long been used for carrying analog voice over radio systems, newer digital systems have also been based on FM, speci cally multilevel digital FM. A widely used format is four-level FM, which is equivalent to 4ary FSK. A typical modulator implementation will use direct modulation, in which the four-level baseband digital signal is applied directly to the voltage-controlled oscillator (VCO). This provides a relatively simple design that is compatible with class C power ampli cation and permits demodulation with a simple frequency discriminator followed by a sampler. A disadvantage of digital FM is that the spectral skirts are relatively high, crystal reports c# barcode c39 use .net vs 2010 crystal report barcodes creator to receive barcode for .net address KeepDynamic.com/ barcodesgenerate, create barcodes valid none for visual c#.net projects KeepDynamic.com/ bar codeExample of Huffman coding based on probabilities of symbols to be coded.
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codes generating barcode vb.net generate, create bar code guide none on visual basic.net projects KeepDynamic.com/barcodegenerate, create bar code number none on .net projects KeepDynamic.com/barcodebaseband or bandpass equalization. In radio modems operating at high carrier frequencies, surface acoustic wave (SAW) devices sometimes provide a cost-effective solution for implementation of the equalizer. The SAW devices can operate at the high frequencies required; and given their cost advantage, passband equalization at either IF or RF is preferred to baseband equalization. Example 9.2: Equalization of a Null in the Passband Here we provide some numerical examples to compare the effectiveness of different equalizers on frequency-selective fading channels. As an example applicable to radio communications, consider a frequency-selective fading channel whose frequency response is given by H (j ) = A + B(j ) where A and B are complex numbers. This model is used to represent frequencyselective fading in microwave line-of-sight (LOS) channels. The transfer function of this channel has one zero at s = A/B, which results in a null at the frequency = Im[ A/B]. The depth of the notch is determined by Re[ A/B]. Figure 9.14 shows plots of the inverse of the MMSE versus normalized timing error for A = 1, B = 0.4, and various numbers of taps for a DFE working with an SQPR modem. The inverse of the MMSE is a measure of SNR after equalization, which can be viewed as a performance criterion for an equalizer. The SNR before equalization is 23 dB. The forward tap gains are associated with = T /2, and plots include various numbers of forward (N ) and backward (M) tap gains. As the number of forward tap gains increases, the at region of the curve widens, indicating a larger span of time in which the sampling can take place without signi cant performance degradation. The insensitivity to sampling time will signi cantly reduce the complexity of the timing recovery circuit. An increase in the number of backward taps will increase the SNR. For this particular example, three forward and three feedback taps are adequate to provide a wide region of allowed sampling times and a maximum SNR. Figure 9.15 shows the probability of error versus location of a deep null in the passband of the channel for a QPSK modem. The channel model is the same as above, but the parameters are adjusted to force a null on frequency axis. The probability of error is calculated by determining the overall sampled impulse response after equalization and calculating the average error rate for all possible combinations of the ISI [Pah85a]. Plots include the modem without an equalizer, an FSE with ve T /2-spaced taps, and a DFE modem with three forward and three feedback taps. For the deep null near the center (FT 0), only the DFE shows an acceptable level of performance. As the null moves toward the edges of the passband, the performance of the other two modems improves rapidly. As a result, one can conclude that for robust performance over a range of channel conditions, a DFE is required. 9.5.2 Adaptive Algorithms for Equalizers In Section 9.5.1 we analyzed the effectiveness of various equalizers under the MMSE criterion and found that the optimal tap gains in each case are determined from a set of linear equations. Direct solution of these equations requires measurement of the instantaneous overall channel impulse response, measurement of the variance of qr code gen.net 2003 crystal report generate, create quick response code api none in .net projects KeepDynamic.com/qr bidimensional barcodeto add qr-codes and qr-code data, size, image with visual basic.net barcode sdk byte KeepDynamic.com/qr codesTIP Many Travan drives don t perform Read-After-Write (RAW) verification, an
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