ENCS300 Communication Systems Assessment 2026 | University of Dubai

ENCS300 Assessment

Course Learning Outcomes (CLO)

• Identify building blocks of communication systems and the basic principles that are used in the analysis and design of analogue communication systems
• Identify building blocks of communication systems and the basic principles that are used in the analysis and design of analogue communication systems

Question 1 [20 points]

(1)An available channel bandwidth is 1.2 MHz. Each voice channel requires 25 kHz of bandwidth. In addition, a 5 kHz guard band is required between adjacent channels. Assume no guard band is required at the two outer edges of the 1.2 MHz band.

•  A.[2 points] Derive an expression for the total required bandwidth in terms of the number ofchan- nelsN.
• B.[2 points] Determine the maximum integer number of channels that can fit.

(2)A receiver has a sensitivity of−72 dBm when operating at a bit rate ofRb =2 Mb/s. Assume the sensitivity is the minimum average received power required for reliable detection.

• (1)[2 points] Convert the sensitivity−72 dBm to watts.(2)
• (2) [2 points] Compute the minimum bit energy Eb at this operating point using

E b =PminTb,Tb =1\Rb

3)[2 points] A second system operates atRb =8 Mb/s with sensitivity−66 dBm. Compute its minimum Eb.

(4) [2 points] Which system requires a lower energy per bit? By what factor?

(5) A point-to-point microwave communication link operates at a carrier frequency of fc = 12 GHz and a data rate of Rb = 150 Mb/s. The transmitter and receiver are separated by a line-of-sight distance of 45 km through free space. Use c = 3 × 10⁸ m/s.

• A. [2 points] Compute the carrier wavelength λ.
• B. [2 points] Compute the one-way propagation delay tprop.
• C. [3 points] Compute the number of bits in flight on the link (the bandwidth–delay product) Nflight = Rb tprop.
• D. [1 point] Briefly interpret what Nflight means in words.

Question 2 [24 points]

(1) Let the message signal be

m(t) = 2 cos(2πfmt), fm = 2 kHz,

and let the carrier frequency be fc = 100 kHz. A DSB-SC (double-sideband suppressed-carrier) signal is generated as

sDSB-SC(t) = m(t) cos(2πfct).

• A. [2 points] Write sDSB-SC(t) as a sum of cosines at the appropriate frequencies using trigonometric identities.
• B. [2 points] From your expression in (a), identify all spectral lines (their frequencies and amplitudes) in the spectrum SDSB-SC(f).
• C. [2 points] Sketch the amplitude spectrum |SDSB-SC(f)|. Clearly label the frequency axis and the line amplitudes.
• D. [2 points] Sketch the power spectrum SDSB-SC(f) (power per spectral line). Clearly label the frequency axis and the line powers.

(2) A conventional AM signal is given by

sAM(t) = Ac [1 + μ cos(2πfmt)] cos(2πfct),

with Ac = 5, μ = 0.6, message frequency fm = 3 kHz, carrier frequency fc = 100 kHz, and load resistance R = 1Ω.

A. [2 points] Expand sAM(t) into a sum of cosines and identify:

• the carrier component (frequency and amplitude).
• the upper sideband (USB) and lower sideband (LSB) components (frequencies and amplitudes).

B. [2 points] Sketch the amplitude spectrum |SAM(f)|, showing the carrier and the two sidebands at fc ± fm.

C. [2 points] Compute the carrier power Pc, the total sideband power Pside, and the total transmitted power PT using the standard AM power formulas in terms of the modulation index μ.

D. [2 points] Determine the percentage transmission efficiency, defined as

η = (Psideband / PT) × 100%.

(3) Let the carrier be given by

c(t) = 8 cos(2πfct),

and let the message signal be

m(t) = cos(40πt).

Further assume that the message signal is used to frequency modulate the carrier with kf = 30, and that the transmitted power is 31.7 W. Find:

A. [1 point] The modulation index
B. [1 point] The expression for the modulated signal
C. [3 points] The number of transmitted harmonics
D. [3 points] Express the modulated signal in terms of the selected harmonics