563489578934

662
Wire and Wireless Communication Applications Chap. 8
The range of this wireless link could be increased by increasing the transmit power,
reducing the receiver noise figure, or implementing a spread spectrum (SS) system. If direct
sequence SS is used with a r = 4 stage shift register, as shown in Fig. 5–37, then the PN code
length is N = 15. Assuming that code length spans one bit of data, then the chip rate is R c = NR.
From Eq. (5–131), this SS system would provide a processing gain of G p = R c R = N = 15, or
G pdB = 11.76 dB. If despreading at the receiver is implemented after the IF stage (i.e., after the
internal noise source of the receiver), this processing gain would increase the CNR at the detector
input by 11.76 dB. This would increase the useful range to around 500 feet. That is, referring
to Fig. 8–40 at d = 500 feet, we know that a processing gain of 11.76 dB would result in a CNR
of 13.0 dB, which corresponds to a BER of 2.1 × 10 -5 .
PROBLEMS
★ 8–1 A remote terminal for a telephone company services 300 VF subscribers and 150 G.Lite DSL
subscribers (1.5 Mbs data plus a VF signal that is converted to a DS-0 signal). Compute
the minimum data rate needed for the receive fiber-optic line that terminates at the RT
from the CO.
8–2 A 50-pair line provides telephone service to 50 subscribers in a rural subdivision via local loops
to the CO. How many subscribers can be served if the 50 pairs are converted to T1 lines and a
remote terminal is installed in the subdivision?
8–3 Assume that a telephone company has remote terminals connected to its CO via T1 lines. Draw a
block diagram that illustrates how the T1 lines are interfaced with the CO switch if the CO uses:
(a) An analog switch.
(b) An integrated digital switch.
8–4 Indicate whether a conference-call connection is better or may be worse than a single-party call if
a digital switch is used (at the CO) instead of an analog switch. Explain your answer.
★ 8–5 Full-duplex data of 24 kbs in each direction from a personal computer is sent via a twisted-pair
telephone line having a bandpass from 300–2700 Hz. Explain why modems are needed at each
end of the line.
8–6 A satellite with twelve 36-MHz bandwidth transponders operates in the 64-GHz bands with
500 MHz bandwidth and 4-MHz guardbands on the 4-GHz downlink, as shown in Fig. 8–10.
Calculate the percentage bandwidth that is used for the guardbands.
8–7 An Earth station uses a 3-m-diameter parabolic receiving antenna to receive a 4-GHz signal
from a geosynchronous satellite. If the satellite transmitter delivers 10 W into a 3-m-diameter
transmitting antenna and the satellite is located 36,000 km from the receiver, what is the
power received?
8–8 Figure 8–12b shows an FDMFM ground station for a satellite communication system. Find
the peak frequency deviation needed to achieve the allocated spectral bandwidth for the
6240–MHz signal.
★ 8–9 A microwave transmitter has an output of 0.1 W at 2 GHz. Assume that this transmitter is used in
a microwave communication system where the transmitting and receiving antennas are parabolas,
each 4 ft in diameter.
(a) Evaluate the gain of the antennas.
(b) Calculate the EIRP of the transmitted signal.
(c) If the receiving antenna is located 15 miles from the transmitting antenna over a free-space
path, find the available signal power out of the receiving antenna in dBm units.