563489578934

Sec. 8–8 Cellular Telephone Systems 621
TABLE 8–6 TYPICAL SPECIFICATIONS FOR A LIGHTWAVE SYSTEM †
Maximum capacity for two-fiber
bidirectional rings
Line rate
Line code
Wavelength
Optical source
Optical detector
Optical fiber
Transmitted power
1.3 mm +5.5 dBm
1.55 mm +4.0 dBm
Receiver sensitivity
-27.0 dBm
24 DS-3 equivalents (16,128 full-duplex VF circuits
per fiber pair) per span
48 DS-3 equivalents (32,256 full-duplex VF circuits)
per terminal
2.488 Gb>
s
Scrambled unipolar NRZ
1310 nm or 1550 nm
Distributed feedback (DFB) laser
Avalanche photodiode (APD)
Single mode
Maximum receiver power (no overload) -10 dBm for 1.31 mm, -9.0 dBm for 1.55 mm
Bit error rate (BER)
610 -10 for -27 dBm receiver input
610 -9 accumulated BER for systems up to 400 km
(250 miles)
Maximum repeater spacing
1.31 mm 60 km (0.45 dB>
km loss fiber)
1.55 mm 92 km (0.25 dB>
km loss fiber), dispersion limited
† Google “fiber optic specifications” for current fiber-optic systems.
TABLE 8–7
LINK BUDGET ANALYSIS FOR A 1.55-mm FIBER SPAN
Description
Value
Maximum transmitted power
+4.0 dBm
Receiver sensitivity
-27.0 dBm
Available margin
31.0 dB
Losses in a 57-mile fiber link
Optical path penalty
2.0 dB
Transmitter> receiver connector loss (0.7 dB each)
1.4 dB
Fiber attenuation (92 km × 0.25 dB> km) †
23.0 dB
System margin
4.6 dB
Total loss
31.0 dB
† The 0.25-dBkm loss includes splicing losses.
This cellular radio concept is illustrated in Fig. 8–29. (In practice, the cell towers are
usually placed at the corner of the hexagonal cell where three cells intersect. Three 120 degree
beamwidth directional antennas are placed on the tower—one directional antenna covering
each of the three cells. This provides coverage of the cells using one tower, instead three
towers.) Each user communicates via radio from a cellular telephone set to the cell-site base