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Mobile Wireless Communications in .NET Encoder Code 128A in .NET Mobile Wireless Communications




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Mobile Wireless Communications use none none printer todeploy none on none Microsoft .NET same uplink and do none for none wnlink frequency bands, but channels, codes in this case, occupy much wider 1.25 MHz frequency bands. It uses QPSK for downlink communications and OQPSK for uplink transmission.

Both modulation techniques will also be described in this chapter. In Europe, the bands assigned to, and used by, GSM are 890 915 MHz in the uplink direction and 935 960 MHz in the downlink direction. The 25 MHz bands available in each direction are further broken into 124 frequency channels of 200 kHz each.

Each frequency channel in turn is accessed by up to eight users, using time slot assignments. The modulation technique adopted, to be described later in this chapter, is called 0.3 GMSK.

Additional wireless communication channels for cellular communications have been made available in the US in the 1.85 1.99 GHz band, the so-called PCS band, and bands ranging from 1.

71 to 1.9 GHz in Europe. For simplicity in providing examples in this chapter, we focus on the modulation techniques used in the 800 900 MHz bands.

The discussion in this chapter will focus on the digital modulation techniques noted above. Thus, DQPSK, GMSK, QPSK, and OQPSK will be described in some detail after rst providing some introductory material on digital modulation. It is to be noted, however, that multiple criteria are involved in the speci c choice of a modulation technique to be used.

Examples of such criteria include bandwidth ef ciency, power ef ciency, cost and complexity, and performance in fading channels. Note that the battery-operated mobile or subscriber terminal, in particular, must be reasonably inexpensive, small in size, and parsimonious in its use of power. Operation in a fading environment means that constantamplitude modulation techniques with nonlinear ampli ers used are favored.

This is one reason for the original choice of FM for the rst-generation analog systems. Our approach in this chapter will be to rst describe the simplest, basic digital modulation techniques. These include on off keying (OOK) or amplitude-shift keying (ASK), phase-shift keying (PSK), and frequency-shift keying (FSK).

We will then touch brie y on quadrature amplitude modulation (QAM), used, incidentally, in wireline modems to increase the transmission bit rate over a bandwidth-limited channel, such as the telephone access line from home or of ce to a telephone exchange or central of ce. Quadrature phase-shift keying (QPSK) is a special case of QAM. We will go on to discuss enhanced digital modulation techniques such as OQPSK or MSK, and GMSK, chosen to work relatively well in the constrained bandwidth, fading environment of wireless cellular systems.

We conclude this chapter with an introduction to orthogonal frequency-division multiplexing (OFDM). This scheme, which goes back historically at least to the 1960s, is being considered for use in advanced cellular systems. It has already been applied to high-speed wireless LANs and to high-speed DSL modems developed for use over telephone copper wire access lines to the home.

We discuss its application to wireless LANs as part of the IEEE 802.11g and a standards in 12..

5.1 Introduction to digital modulation techniques Consider an unmodu none for none lated sinusoidal carrier A cos 0 t operating continuously at a carrier frequency of f0 Hz, or 0 = 2 f0 radians/sec. Its amplitude, frequency, and phase may. Modulation techniques Acos 0t 1/R sec Figure 5.1 OOK sig none none nal corresponding to binary 1, 0, 1. individually be va ried or modulated in accordance with an information-bearing signal to be transmitted to provide, in the case of the rst two, the well-known AM and FM highfrequency signals used in (analog) radio broadcast systems with which we are all familiar. The simplest digital equivalents are called, respectively, as noted above, on-off keyed or amplitude-shift keyed transmission, OOK or ASK, frequency-shift keyed transmission, FSK, and phase-shift keyed transmission, PSK. In this digital case, successive binary digits (bits), 0 or 1, are used to vary or modulate the amplitude, frequency, and phase, respectively, of the unmodulated carrier.

The binary, information-bearing, digits are also referred to as comprising the baseband signaling sequence. In the simplest case, that of OOK transmission, a binary 0 will turn the carrier off; A cos 0 t will be transmitted whenever a binary 1 appears in the baseband signaling sequence. Say the binary sequence is being transmitted at a bit rate of R bits/sec.

Then each binary symbol or bit lasts 1/R sec. An example of an OOK signal corresponding to the three successive bits 1, 0, 1 appears in Fig. 5.

1. Note that OOK transmission is not suitable for digital mobile systems because of the variation in amplitude of the signals transmitted. Recall that we indicated earlier that constant-amplitude signals are preferred because of the fading environment, which, by de nition, introduces unwanted random amplitude variations.

We discuss OOK transmission here to provide a more complete picture of digital communication. In actuality, the instantaneous change in amplitude shown occurring in Fig. 5.

1, as the OOK signal turns off and then comes back on again, cannot occur in the real world. Such abrupt transitions would require in nite bandwidth to be reproduced. In practice, the signals actually transmitted are shaped or pre- ltered to provide a more gradual transition between a 0 and a 1, or vice-versa.

The resultant transmitted signal corresponding to a baseband binary 1 may be written Ah(t) cos 0 t. The low-frequency time function h(t) represents a signal-shaping function. This signal shaping is designed to allow the transmitted OOK sequence to t into a prescribed system bandwidth with little or no distortion.

We shall discuss signal shaping brie y in the next section. The rf carrier transmission bandwidth, the frequency spread of the modulated transmitted signal about the carrier, is readily shown to be given by 2B, with B the baseband bandwidth, the bandwidth required.
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