Modulation techniques in .NET Integrated Code 128 Code Set C in .NET Modulation techniques

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Modulation techniques use visual .net code 128 code set c generating toreceive code-128 in .net GS1 Bar Codes Glossary 4. 3 4 n (n) 3 4. Figure 5.20 4 DQPSK /. of QPSK. These variatio ns, just as those for 8-PSK, will be reduced as compared with QPSK, however. How would detection at the receiver be accomplished in this case It is clear that phase detection is necessary, since the information to be reproduced is carried in the phase of the received signal.

It was noted earlier that the receiver must generally be locked in precisely to the phase and frequency of the transmitted signal in the case of phase-shift keyed and FSK-type modulation. Such techniques then require the receiver to correctly track the absolute phase of the transmitted signal for correct detection to take place. Such absolute phase information is not needed, however, in using this type of differential phase modulation scheme.

Differential phase shift decisions only need be made, each T-sec interval, to correctly determine the (ai , bi ) pair generated at the transmitter, from which, in turn, the corresponding binary information sequence may be reconstructed. In essence, the transmitted signals, one after the other, provide the necessary phase reference for the signal following. This can simplify the phase detection process considerably.

Provision must be made, however, for detecting and correcting an erroneous phase decision; otherwise the phase error could perpetuate inde nitely!. 5.4 Orthogonal frequenc .net vs 2010 barcode code 128 y-division multiplexing (OFDM).

We conclude this chapte r with an introduction to OFDM. As noted in the introduction to this chapter, OFDM is currently used in high-speed DSL modems over copper-based telephone access lines. It has also been standardized as part of the IEEE standards 802.

11g and 802.11a for high bit rate, 54 Mbps, data transmission over wireless LANs (WLANs). Those applications are described as part of our WLAN discussion in 12.

OFDM has been proposed as well for advanced cellular systems. The application of OFDM to wireless systems is due to the increasing need for higher bit rate, higher bandwidth data transmission over radio-based communication systems. We noted, in 2, that, as the transmission bandwidth increases, frequency-selective.

Mobile Wireless Communications f1 a1 store s N N bits aN R bps t N transmit N carriers in parallel every Ts T N/RN/R = serial-to-parallel conversion (a) OFDM transmitter transmitter B f fN (b) OFDM spectrum spectrum f(Hz) Hz Figure 5.21 Orthogonal- frequency multiplexing (OFDM). fading and consequent s VS .NET Code 128 Code Set B ignal distortion are encountered. In the case of digital transmission, particularly, inter-symbol interference occurs, with successive digital symbols overlapping into adjacent symbol intervals.

OFDM mitigates this effect by essentially dividing the signal transmission spectrum into narrow segments and transmitting signals in parallel over each of these segments. If the bandwidth of each of these frequencyspectrum segments is narrow enough, at or non-frequency-selective fading will be encountered and the signal transmitted over each segment will be received non-distorted. It is this virtue that has resulted in OFDM taking on special signi cance for the wireless application.

Speci cally, consider the case of binary digits transmitted at a rate of R bps. The bandwidth B required to transmit these bits is, from (5.6a), R(1 + r), with r the Nyquist rolloff factor, or the order of R Hz.

Now consider a sequence of N of these bits stored for an interval TS = N/R. We call this interval the OFDM symbol interval. Serial-toparallel conversion is then carried out, with each of the N bits stored used to separately modulate a carrier.

All N modulated-carrier signals are then transmitted simultaneously over the TS -long interval. This procedure constitutes the essence of OFDM. Figure 5.

21(a) portrays this simple OFDM generation process. The parameters ak , 1 k N, represent the successive bits stored, while the frequencies fk , 1 k N, represent the N carrier. Modulation techniques frequencies transmitted Code 128B for .NET in parallel. Figure 5.

21(b) shows the resultant OFDM spectrum. To make the various carrier frequencies orthogonal to each other, it suf ces to have the spacing f between carriers equal to 1/TS . We thus have B = N f, B the transmission bandwidth as shown in Fig.

5.21(b). Note that f is effectively the bandwidth of each of the N parallel frequency channels as well.

It is thus clear that this signal-spreading process has reduced the transmission bandwidth of each of the signals transmitted in parallel by the factor of N. With N large enough, at fading, rather than frequency-selective fading, occurs on each of the frequency channels used, thus overcoming any frequency-selective fading incurred without this serial-toparallel process. (Note that this process of storing a sequence of bits and then transmitting N carriers in parallel differs from QAM in which one carrier only is used.

) What value of N or, alternatively, f, is desirable Recall from 2 that at fading occurs if the transmission bandwidth B and delay spread av are related by av < 1/2 B Hence we must have must have f < 1/2 av . In equivalent terms of time, using TS = N /R av (5.15) f = 1/TS , we (5.

16). Simply stated, then, to avoid inter-symbol interference the OFDM symbol interval must be much larger than the delay spread. This is what one expects intuitively. Consider some examples.

As the rst example, let the transmission bandwidth be 1 MHz, with 800 kbps data transmission attempted over this channel. (A Nyquist rolloff factor of 0.25 is assumed here.

) A delay spread of 1 msec would result in frequencyselective fading, with intersymbol interference encountered. Using OFDM with ten carriers spaced 100 kHz apart, the inter-symbol interference would be mitigated, with at fading encountered over each of the ten parallel 100 kHz-wide channels. (These OFDM carriers are often referred to as subcarriers.

) As the second example, consider the case of a 20 MHz-wide channel, the one adopted for the high data rate IEEE 802.11g and 802.11a wireless LAN standards.

Without the use of OFDM, delay spreads greater than 10 nsec would result in frequency-selective fading. As noted in 2, however, delay spreads encountered in various environments, including those of microcells and indoor environments, are much larger than this value. A technique such as OFDM is thus required in going to very high bandwidth systems.

A carrier separation f, and hence subcarrier bandwidth, of 312.5 kHz has, in fact, been adopted for the 20 MHz-wide wireless LAN systems, as we shall see in 12. With an OFDM subcarrier bandwidth of 312.

5 kHz, inter-symbol interference is incurred with delay spreads greater than 0.5 msec, a substantial improvement over the case without OFDM introduced. The implementation of OFDM using multiple carriers transmitted in parallel can, however, produce a problem in implementation.

We now show, by analysis, that this problem can be avoided by the use of the discrete Fourier Transform technique. Consider the N parallel output signals of Fig. 5.

21 transmitted over a TS -long symbol interval. Call the sum of these signals, the total signal transmitted, v(t). De ne the kth carrier frequency fk as fc + k f, 0 k N 1.

In effect, then, we are simply rede ning the carrier nomenclature, the lowest of the N parallel subcarriers written as fc , the rest all spaced intervals of f.
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