Wireless Communications: Multipath, Signal Power, Spread Spectrum, Link Margin, WLAN Design, Encrypt

Multipath and Its Impact

1. (20 Marks) 

Note: Word limit for each question part is 150 words excluding references, equations and diagrams. (Total word count – 450)

a) In wireless communications, multipath is a common phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Consider an example scenario (Figure below) in which a pair of wireless transmitter and receiver are operating at 2.5 GHz. A transmission from node A takes two paths, a direct path, and an indirect path after bouncing from the ceiling. As a result, node B receives two copies of the same signal, however, the one resulting from the indirect path is separated by a distance of 6 cm. The propagation velocity of the RF wave is 3 x 108 m/s. Critically analyse the impact of multipath in this case. Show your workings. (8 Marks)

b) In wireless communications, how does the power of a signal change with respect to distance? Critically analyse the relationship between power seen at the receiver and the distance ‘r’ between transmitter and receiver.  If the distance ‘r’ is reduced by half, how would it affect the signal power seen at the receiver? Show your workings. (6 Marks)

c) Assume the following scenario: John Doe an Engineer from the Newcastle City Council installed a new wireless LAN in the Eldon Square shopping mall after performing a wireless site survey. After installation he measured RF signal strength and coverage and found it adequate. During the first two weeks of December 2020 (busy Christmas shopping season), bad weather forced many shoppers from the street into the mall such that it was crammed with people. Many complained about the poor quality of wireless network. During this time, the signal strength and quality of WLAN was less than desirable in many areas. What do you think is the reason(s) for poor wireless signals? Which RF signal property might have contributed to the poor signal reception? (6 Marks)

2. (20 Marks)

Note: Word limit for each question part is 200 words excluding references, equations, and diagrams. (Total word count – 600)

a) Suppose that a signal travels through a wireless transmission medium and its power is reduced to 1/8th of its original transmit power. How much of the signal power is lost in dB? Show your workings in the answer. (6 Marks)

b) Critically compare Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) with respect to key issues such as narrowband interference and colocation. (8 Marks)

Signal Power and Distance Relation

c) Consider the following RF transmission system (Figure below) which is designed with the impedance of the load as 40.5 Ohm and that of the source and transmission line as 50 Ohm. Find the Voltage Wave Standing Ratio (VSWR). How much Return Loss is expected for this system? By examining the values obtained for VSWR and Return Loss, evaluate the operational efficiency of this system. (6 Marks)

3. (15 Marks): 

Note: Word limit for each question part is 200 words excluding references, equations, and diagrams. (Total word count – 400)

a) Assume you are working as a Network Engineer for a Telecommunications service provider and your task is to estimate the feasibility of a 20 km link between two buildings with one access point and one client radio.  The access point is connected to an antenna with 12 dBi gain, with a transmitting power of 500 mW and a receive sensitivity of -84 dBm.  The client is connected to an antenna with 10 dBi gain, with a transmitting power of 16 dBm and a receive sensitivity of -82 dBm. The cables in both systems are short, with a loss of 3 dB at each side at the 900 MHz frequency of operation. What is the link margin? By examining the link margin, comment on the reliability of the link. (10 Marks)

b) IEEE 802.11n introduced few significant enhancements to the previous version (802.11g). Critically explain how these enhancements resulted in producing a wireless standard that could achieve much higher data rates? (5 Marks)

4. (15 Marks)

Note: Word limit for each question part is 150 words excluding references, equations and diagrams. (Total word count – 450)

a) Consider the scenario shown in the figure below.  Any two adjacent nodes are within their transmission ranges. Let us assume that 802.11 CSMA/CA is used. Node A transmits RTS to B, which is successfully received by node B.  Node B starts transmitting the CTS. At the same time when node B is transmitting, node D sends RTS to node C. Would this cause a problem? Will D be able to transmit its data to node C? If so, when and how? If not, why? (5 Marks)

b) In a WLAN system which is using DSSS, following data 11010010001011011101 is transmitted. If 1101 is used as the spreading chip code, what was the original data? Show your workings. (5 Marks)

c) Briefly explain which factors are important for planning a Wireless LAN design for a campus network. (5 Marks)

5. (15 Marks)

Note: Word limit for part (a) is 200 words excluding references, equations, and diagrams. Word limit for part(b), (c) is 150 words. (Total word count – 500).

a) Critically analyse the effectiveness of security enhancement proposed in the Temporal Key Integrity Protocol (TKIP). Discuss its key features and process for encryption. (8 Marks)

b) Consider a message M which is to be encrypted with RSA algorithm. Describe the steps in the encryption and decryption algorithms, including an explanation of the symbols and terms used paying particular attention to the keys employed in encryption and decryption and the size of the message? (3 Marks)
                                                
c) In the case of a public key with n = 55 (with p=5 and q=11) and e = 3, determine the value of private key d. Show all steps. (4 Marks)

6. (15 Marks)

Note: Word limit for each question part (a) is 300 words excluding references, equations and diagrams. Word limit for part(b), (c) is 150 words. (Total word count – 500).

a) List and critically explain at least five (05) security mechanisms that are important for wireless communications? (10 Marks)

b) Analyse what can happen when an intruder compromises the pre-shared key (PSK) or passphrase used during WPA/WPA2-Personal authentication? (5 Marks)