C;ankaya University - ECE Department - ECE 376

Student Name:Student Number:

Questions

Open source examDuration: 2 hours

1. (70 Points) A DS spread spectrum communication system with two users (with twomessage signals of VI (t) and v2 (t) ) is given. Each user transmits at a bit rate of 4 kbits /sec. For this communication system, to provide a Pe (probability error) of 10-4

, find theSNR (due to interference between users arising from the common use of time andfrequency axis) and Le (processing gain) and Te (chip duration of the PN sequences). Statewhat assumptions you have made in these calculations.

Now assuming that these two users are assigned two of the following four spreading (PN)sequences,

C-[-l -1 1 -1 -1 1 -1]I -

2C = [-1 -1 1 1 -1 -1 1]

C-[-l -1 1 -1 1 1 1]3 -

C-[-l -1 1 1 1 -1 1]4 -

Explain which of the IC, 2C, 3C and 4C you would choose as PN sequences of the twousers bearing in mind that the time shifted autocorrelation function of the chosen PNsequences should make the chosen PN sequence maximum length. Find the new Le and Te•

Find the time shifted autocorrelation and time shifted crosscorrelation of the chosen PNsequences and plot them. Note that autocorrelations and crosscorrelations can be obtainedfrom matrix multiplication of PN sequences.

For the above DS spread spectrum system, draw transmitter and receiver block diagramsand plot time waveforms and spectrums of signals, labelling these plots with numericvalues.

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2. (30 Points) Answer the following questions as True or False. For the False ones givethe correct answer or the reason. For the True ones justify your answer.

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c::D ECE 376 Final Exam 01.06.2011 Sayfa 2