Mobile Communications Engineering: Theory and Applications by William C.Y. Lee

From the Publisher

From one of the field's foremost educators, here is the classic guide to mobile communication—fully revised for the 1990s and beyond. It is unique because it shows readers how to understand the differences in applying technologies between wireline communications and wireless communications. The new second edition extensively updates the basics. It also coves traffic and capacity analysis on mobile communications networks and addresses rapidly expanding new technologies, such as digital cellular, PCS, and multiple access techniques not only including FDMA, TDMA, CDMA, and SDMA, but also applying the techniques on the virtual channels.

Booknews

In this update of the 1982 edition intended for engineering seniors and first year graduate school students, Yee (VP and chief scientist, Applied Research and Science, AirTouch Communications; Walnut Creek, CA) covers the first 100 years, and theoretical and practical considerations, of diverse radio communication links between two terminals of which at least one is in motion. Includes a chapter on new concepts (e.g. data transmission via cellular systems), problem exercises, and references. Annotation c. by Book News, Inc., Portland, Or.

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Antenna terminal mismatch

When the antennas and the antenna terminal are mismatched, a small portion of the transmitting power is reflected back into the power amplifier. If the power amplifier is not perfectly linear, additional intermodulation interference will be generated. To correct this situation, an improved matching device is needed to minimize the reflected power and thus eliminate the additional intermodulation interference caused by mismatching between the power amplifier and the antenna feed.

Colocated transmitting antennas

When two transmitting antennas are placed close together, the signal of one antenna becomes a source of interference for the other system. Signals from an adjacent antenna transmission are strong enough to be picked up at the input to the power amplifier of the desired signal transmitter, which results in intermodulation. interference. One method that can be used to reduce the effects of this type of intermodulation interference is to install an interference-cancellation system, such as the one shown in Fig. 12.10.

In Fig. 12.10, the interfering signal ... becomes ... upon arrival at the antenna of the desired signal. The change in amplitude is the effect of signal loss within the transmission medium, whereas the change in phase is the result of propagation delay. The magnitude of the changes in signal amplitude and phase is dependent upon the separation distance d between the two colocated antennas. The cancellation circuit provides an internal path for the interfering signal that forms an error-cancellation loop between the antenna for the interfering signal (antenna1), and the antenna for the desired signal (antenna 2). The objective of the interference-cancellation circuit is to cancel the interfering signal components as they enter antenna 2, prior to reaching the power-amplifier stage, where intermodulation can occur. To accomplish this action, a portion of the interfering signal is directionally coupled to a phase- and amplitude-adjusting circuit and then combined with the interfering signal picked up by antenna 2. If the amount of phase shift and attenuation imparted to the cancellation signal is correct, then the signal from the cancellation circuit will cancel out the interfering signal picked up by antenna 2. Additional details on interference-cancellation systems are available in published books [11].

12.6 Intersymbol Interference

If a digital transmission system is linear and distortionless over all frequencies, then the system has a bandwidth that is theoretically infinite. However, in practical terms, every digital transmission system has a finite bandwidth with a certain varying amount of frequencyresponse distortion. In a distortionless system, one with an infinite bandwidth, a basic pulse s(t) would suffer no degradation as a consequence of frequency response. For instance, let x(t) represent a pulse train of N pulses:...

where the pulse-to-pulse spacing is 1/R and R is the signaling rate for binary pulses ak = ± 1. Since s(t) suffers no distortion, an increased signaling rate can be achieved by decreasing the pulse width and increasing the number of pulses transmitted within a given interval of time. However, when a practical system with finite bandwidth and frequency-response distortions is considered, the individual pulses tend to spread out and overlap, which results in the condition known as "intersymbol interference" (ISI). Therefore, in a practical system it is necessary to employ techniques to shape the pulse waveforms of the transmitted signal, as was previously described in Sec. 11.2 of Chap. 11. Other techniques that can be used to minimize intersymbol interference due to overlapping of pulses, while at the same time increasing the signaling rate, are dependent upon the use of equalizers [12] or special digital coding circuits [13].

Another consideration that is independent of the signaling rate and transmitting frequency is the effects of time-delay spread ... due to the mobile-radio environment. Time-delay spread is affected by bandwidth limitations, and by multipath signal reflections that arrive at the receiving antenna at different times. Since it is impractical to selectively choose the mobile-radio environment, the improvements that can be obtained through more conventional techniques must be evaluated.

When the signaling rate is relatively low, as when...

then the effects of time-delay spread have a negligible effect on received pulses. For instance, if the delay spread in a typical suburban area is 0.5 ... and the signaling rate is 16 kb/s, then 1/R = 6.25 ..., which is much greater than the delay spread. In this situation, no intersymbol interference (ISI) exists. When the signaling rate increases, the ISI increases; the ISI can be greatly reduced by using signal waveform-shaping techniques, and by using an equalizer. There are many reference publications that deal with this type of problem [5, 14,15,16].

Figure 12.11 is a conventional "eye" diagram that can be used to evaluate the causes of intersymbol interference in a mobile-radio environment. The effects of ISI on signal degradation can be classified as amplitude and timing degradation, corresponding to the vertical and horizontal displacements shown in Fig. 12.11. Signal amplitude degrading is caused by the summed effects of Gaussian noise, limited bandwidth distortions, variations in local oscillator stability, decisionthreshold uncertainties, and Rayleigh fading. Signal timing degradations are caused by limited frequency-response distortions, time slippage due to clock error, time jitter due to clock instability, static misalignment of timing signals, and time-delay spread in the mobileradio medium. The effects of Gaussian noise and Rayleigh fading on signal amplitude were described in Secs. 8.2 and 8.3 of Chap. 8. The effects of timing distortion due to limited frequency response are described by Sunde [5], Bennett [14], Ho [15], and Taub [16].

Degradation effects of time-delay spread on signal timing

The following analysis of the effects of time-delay spread on signal timing is based on the assumption that the probability of error due to Gaussian noise and/or Rayleigh fading is the same as it would be for the case where there is no delay spread. Therefore, in the presence of time-delay spread, the carrier-to-noise ratio must be increased by...

where A is the vertical opening of the eye diagram in the no-delayspread condition, case 1, Fig. 12. 11(a). A represents the signal amplitude degraded either by Gaussian noise alone or by Gaussian noise and Rayleigh fading combined. The term a is the vertical opening of the eye diagram in the presence of significant delay spread, case 2, Fig. 12.11(b). Under this condition, Eq. (12.58) can be calculated by using the method applied in the following example....