- Q1 I Figure 1 1 Shows The Functional Block Diagram For A Generic Communication System Between An Information Source 1 (126.36 KiB) Viewed 62 times
Q1 (ii) (a) Provide an expression to calculate the energy of the signal s(t) and determine whether this signal is a power-type or energy-type signal. [3 marks] (b) Further suppose that the maximum amplitude of the information signal is +2 volts, the minimum amplitude is -2 volts, and a modulation index of 0.667 is used. (i) Determine the carrier frequency fe and find the expression of the output signal of the AM-DSB-C modulator and Fourier Transform of the output signal. (ii) Draw the spectrum of the modulated signal. [Hint: Give the answer in terms of s(t) and SGD).] [8 marks] (c) Discuss the difference between the bandwidth of the input signal s(t) and the bandwidth of the modulated signal Smod (t). [2 marks] (d) At the transmitter, design and draw the modulator. At the receiver, design and draw a demodulator. [4 marks]
Q2 (i) A binary on-off keying (OOK) modulator uses binary signalling to transmit symbols 1 and 0. Symbol 1 is represented by a rectangular pulse of amplitude A volts while Symbol O is represented by 0 volts. Both the symbols have the same duration T. The additive noise at the receiver input is white and Gaussian, with zero mean and power spectral density of N, /2 volts/Hz. (a) For the binary OOK modulator, draw the waveforms representing 1 and 0, respectively. Design and draw the optimal receiver for the binary OOK. [6 marks] (b) Assuming that the symbols 1 and 0 occur with equal probability, find the expression for the average energy of the modulated signal, and use conditional error probability to find the expression for average probability of error at the optimal receiver when a matched filter is used: and give the answer in terms of A, No, T and the Q-function. [8 marks] (c) Given the symbol duration is 2x104 seconds and N./2=2x100 volts2/Hz. What value of A is required to achieve a bit error rate (BER) of 10-3. (Hint: Use Table 2.1 for the Q-function to find the solution.) [6 marks] Note: the standard expression for the Q-function is 1 и: A(-)-bad, couple du Qz=- ES V21 2
Tabulation of the Q-function z Q(z) z Q(z) 0.0 0.50000 2.0 0.02275 0.1 0.46017 2.1 0.01786 0.2 0.42074 2.2 0.01390 0.3 0.38209 2.3 0.01072 0.4 0.34458 2.4 0.00820 0.5 0.30854 2.5 0.00621 0.6 0.27425 2.6 0.00466 0.7 0.24196 2.7 0.00347 0.8 0.21186 2.8 0.00256 0.9 0.18406 2.9 0.00187 1.0 0.15866 3.0 0.00135 1.1 0.13567 3.1 0.00097 1.2 0.11507 3.2 0.00069 1.3 0.09680 3.3 0.00048 1.4 0.08076 3.4 0.00034 1.5 0.06681 3.5 0.00023 1.6 0.05480 3.6 0.00016 1.7 0.04457 3.7 0.00011 1.8 0.03593 3.8 0.00007 1.9 0.02872 3.9 0.00005
Q3 Suppose that the bandwidth of a linear time-invariant (LTI) baseband channel is 20 MHz. A bit sequence, “11010010", generated at the data rate Ro is modulated by using 4-ary PAM. The transmit power is 5 Watt. (i) If 90% in-band power of the signal can go through the channel, plot the transmitted signal and calculate the generation data rate Ry of the given bit sequence, “11010010”. For this transmitted signal, what is the required channel bandwidth for 95% in-band power to go through? [9 marks] (ii) At the receiver side, the power spectral density of the additive white Gaussian noise is 2x10-8 volts/Hz. After the modulated signal goes through the 20 MHz baseband channel, calculate the received signal-to-noise ratio (SNR) and bit error rate. (Hint: The BER of 4-ary PAM is given by @(70.8y) where y is the received SNR. Use Table 2.1 for the Q-function to find the solution.) [4 marks] (iii) Given a bandpass channel with the same channel bandwidth of 20 MHz, determine the minimum value of M if M-ary PSK is now used to transmit the bit sequence with the generation data rate Ry, allowing 90% in-band power to go through the channel. Determine the symbol rate. [5 marks] (iv) Use the result in (iii) to explain the difference in the bandwidth efficiency between Binary PSK and M-ary PSK (M>2). [2 marks]
Q4 (1) Describe the difference between real and virtual information paths from a transmitting node to a receiving node in a layered network architecture. [2 marks] (ii) Calculate the load in b/s observed on a server that handled 240 files in a 30 second period with an average file size of 400 kbytes. What normalised load would this place on a 100 Mb/s Ethernet link if protocol overheads are ignored? [3 marks] (iii) Describe the layering structure of the IEEE Local Area Network (LAN) architecture and compare this to the equivalent layers in the Open Systems Interconnection (OSI) architecture reference model, explaining the reason for any differences. Explain the technique used by Ethernet (802.3) to manage access onto physical layer connection shared by multiple devices. [5 marks] (iv) Explain the reasons for deploying Network Address Translation (NAT) on an Internet Protocol (IP) router. Identify the address field in the IPv4 header that NAT will modify for a packet leaving a private network and explain your answer. [4 marks]
Q4 (v) In the network shown in Figure 4.1, an Ethernet switch is being used. PC 1 is sending IP packets to PC 3. The Ethernet frames transporting the packets are sniffed at PC 1 and PC 3. (a) State the source and destination MAC (Ethernet) and IP addresses sniffed at PC 1 and at PC 3, and explain your answer in your own words. (b) Comment on how the answer would be different if an Internet Protocol (IP) switch were used in place of the Ethernet switch. [6 marks] PC 1 switch PC3 10.0.0.1 16.b8.af.b9.d6.3d 10.0.0.3 2d.f6.76.34.43.67 10.0.0.0 a3.66.82.e1.cb.38 PC 4 PC2 10.0.0.2 a5.56.bf.7e.a3.61 10.0.0.4 bc.53.93.6b.5f.2c Figure 4.1