ECEN5533     Exam #2         13 November 2007 

1) A 580 Kbps random binary square wave m(t) is multiplied by x(t) = 1.7cos[2π49(10⁶)t], yielding an output y(t) = m(t)x(t). The symbol for a logic 1 is a 4 volt peak square pulse. The symbol for a logic 0 is a -1 volt peak square pulse.

[25] Find and Sketch the power spectrum GY(f). [Answer: You should sketch sinc2 functions centered at + & - 49 MHz, with peak value of 7.785 μ watts/Hz, and nulls at integer multiples of + & - 580 KHz from the center frequency. At + & - 49 MHz there should also be two delta functions, each with area 1.626.]

<<<<<>>>>>

2) An On-Off ASK signal is transmitted over the 55 mile link shown. The average output power is 3 watts. The receiver antenna gain is 10.1, and the antenna temperature is 299 degrees Kelvin. 

[25] Compute the P(BE) for this system. [0.3449]

<<<<<>>>>>

3) A 5 watt (average total power), +1.1 volt DC, 80 Kbps random binary square wave m(t), contaminated by zero mean additive white Gaussian Noise n(t), is input into a Ten Sample Detector which samples each bit ten times, computes a ten point average, and then compares the average to a threshold α to decide whether a Logic 0 or Logic 1 was transmitted. The P(Bit Error) is known to be 0.4562.

[25] Compute the power of the noise signal n(t). [3,133 watts]

<<<<<>>>>>

4) Given the 400 Mbps fiber optic system shown, if the desired P(BE) = 10^-9, how long can the distance "d" be, if the fiber optics cable has a loss of 0.7 db/Km? Hint: Q(5.997) = 10-9. [56.09 Km]

<<<<<end>>>>>