Assignment Requirements:
This Course Work has been designed based on and very similar to those of your lab sessions throughout the course. The objective is to use MATLAB and its Simulink toolbox to design and simulate the most common wireless communication systems.
Problem 1:
(i) Explain the use of carrier sense multiple access (CSMA) in communication networks, explain what the cause of a possible collision can be when CSMA is used.
(ii) Explain the difference between CSMA/CD (collision detection) and CSMA/CA (collision avoidance).
(iii) Provide the CSMA/CA step-by-step procedure to prevent a collision in the network. Also, by further research, clarify how CSMA/CA can monitor the traffic within the network.
(iv) Using a basic CSMA, assuming the packet lengths to be 2 x 10-3 sec, and the channel capacity of 10 Mbits/sec. What should be the maximum propagation delay to maintain the network efficiency above 80%?
Problem 2:
(i) Provide the advantages of CDMA (code division multiple access) over other multiple access channel sharing systems (e.g. TDMA, FDMA).
(ii) With the help of an example, explain how CDMA (code division multiple access) is used to multiplex three binary signals, each with 2 bits only. In your example use different spreading (chipping) codes of 8 bits each.
Problem 3: This is to design a PSK (phase shift keying) system and assess the effects of number of constellation as well as communication channel.
(i) Use Simulink to design a Binary PSK (BPSK) communication system as in Figure 1.
Figure 1
Use random binary source generator (you may use the source setup in your Lab 5 - Extra experiment) and AWGN (additive white Gaussian noise) as your channel effect. Use oscilloscopes and constellation maps to inspect the signals at right points of the system. Increase the noise level, draw the bit error rate (BER) versus signal to noise ratio (SNR), and specify at which noise level the demodulator is not able to recover the original signal accurately anymore. Draw your schematic and provide the constellation maps together with the input and output binary signals.
(ii) Use QPSK instead of BPSK in part (i). Demonstrate the results and compare it with those of BPSK. You may use the symbol rates similar to those suggested in the lab exercises.
Problem 4: Orthogonal frequency division multiple access (OFDM) is popular as a high-fidelity multiplexing system for wireless communication systems. Briefly describe the block diagram of the overall (transmitter - receiver) system, explain its advantages, and clarify why the OFDM system is an efficient modulation system.
Problem 5: For a line-of-sight (LoS) wireless transmission, the carrier frequency is 0.5 GHz and the distance between the transmitter and receiver antennas is 2 Km. In the transmitter we use a parabolic antenna with the face area of 0.8m2 and in the receiver an infinitesimal dipole is used.
Considering the path loss and antenna gains, calculate the overall gain of the (transmitter- receiver) system in decibel (dB).
Problem 6: Provide the importance of evaluation of Link Budget is satellite communication systems and by doing some research explain various factors and parameters in the budget. Clearly explain what each block and its value/parameter refer to.
Problem 7: This is the extension of Q3. Use MATLAB and Simulink.
(i) In Figure 1. Use QPSK as your PSK modulator and replace the AWGN channel with a Rayleigh fading channel with 5 paths. Increase the fading level and specify at which fading level the demodulator is not able to fully recover the original signal anymore.
(ii) Replace QPSK with 64-QAM, see the results, and explain the advantages and disadvantage of this system over BPSK and QPSK. Explain the differences between the two systems in terms of type of the channel and the level of noise and fading they can tolerate.
For each case register the numbers you used as gains, delays, and maximum Doppler frequency and provide the settings you have chosen/made for any of the Simulink module blocks. Also, provide the constellations diagrams before and after the channel and the signals in the inputs and outputs.
Problem 8: Here, an OFDM system is designed and evaluated. Using Simulink, setup the system according to Figure 2.
Figure 2
Select a four-path channel with discrete delays (0:4:12)*1e-6 Sec, average path gains of [0 -3 -6 -9] dB and sample rate of 50 KHz. Explain what the four paths can be related to.
(i) Use "MATLAB Function" in Simulink to generate a random complex-valued input as Data = complex(rand([256 4 1]), rand([256 4 1]));
Explain what the digits 4 and 1 in the Data argument refer to.
(ii) Apply orthogonal frequency division modulation/multiplexing (OFDM) to Data. Consider a Rician fading channel between the transmitter and the receiver. To evaluate the system for different fading levels, use constellation maps (why?) before the modulator and after the demodulator.
(iii) Explain how Doppler shift relates to mobile systems. Change the Doppler shift value in the "Channel" block from 0 to 150 Hz and compare the results for different settings.
Hint: For the OFDM modulator and demodulator you may choose discrete Fourier transform (DFT) length 267, number of guard bands [6; 5], and cyclic prefix length 16.
(iv) Change the data to
Data = complex(randn([256 10 2]), randn([256 10 2]));
Explain what the arguments [256 10 2] refer to and repeat (ii) and (iii). Can you still use constellation maps to view the data? Why? Try to find and explain a way to view and compare the input and output.
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