CS431 Assignment 1 Solution and Discussion

[zareen]

Assignment No. 02
Semester: Fall 2019
Wireless Communications – CS431
Total Marks: 20
Due Date: Nov 29, 2019
Assignment Objectives:

The objective of the assignment is the enhance the capabilities of students about:
• To understand about the transponder method for multiple access feasibility and various techniques used in multiple access.
• To understand about the network services used in WAN wireless communication network expanded globally.

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For any query about the assignment, contact at cs431@vu.edu.pk

Assignment

Question#1 (15 Marks)
An organization is going to install a system to provide a satellite communication and wants to make it feasible for different type of communication traffic (multiple access). There are different schemes available to accomplish this purpose.
Being a network adviser, you are required to suggest atleast two most appropriate schemes that the organization should follow. Also explain their 2 benefits and 2 drawbacks?

Question#2 (5 Marks)
A cellular communication network is operating globally. You are required to identify which type of communication service network is providing? Explain with a simple example.

Best of Luck

[zareen]

Answer to Q#1
Modern transponders can carry many different types of communications traffic. They can also receive signals from multiple ground stations, combining (multiplexing) or splitting (de-multiplexing) them for onwards transmission to other multiple ground stations. This method, by which many users share a common satellite resource, is called Multiple Access. There are several schemes for accomplishing this, each with its benefits and drawbacks.

TDMA (Time Division Multiple Access) allocates a time slot to the user in a repetitive time frame. The signal is digitized and the data bits are stored in a buffer in a compressed time frame until their allocated time slot comes around when they are transmitted during their allocated time. At the receiver end of the link the bits are rearranged, spreading them out to reassemble the original digital signal and converted back to analogue form. The signal occupies the entire transponder bandwidth, but only during its allocated time slot. The rest of the time the bandwidth is available to other users. Digital signals typically have better noise immunity than analogue signals.

FDMA (Frequency Division Multiple Access) shares the bandwidth between the users, with each user allocated a unique, narrower section of the available bandwidth. It works with analogue signals and all users have uninterrupted use of their own narrow frequency band or channel with all users occupying the available bandwidth simultaneously, each within their own narrow channel. The sender’s signal, called the baseband signal, is frequency shifted into the allocated frequency band for transmission and the receiver restores it back to the baseband.

CDMA (Code Division Multiple Access) also known a Spread Spectrum, modulates the user’s signal with a pseudorandom code so that it occupies the full available spectrum, appearing as noise. The receiver uses the same pseudorandom code in an auto correlator device, which only recognizes a signal modulated with the same auto code and thus separates it from the noise. CDMA is more complex but has better noise immunity and provides greater security than the other two systems.

Answer to Q#2
• Mobile Service Satellites (MSS)
• Example: Satellite Phones