Review Questions

1.        List the main areas of application for wireless communications channels.

2.        What are the advantages and drawbacks of wireless information transmission when compared to the transmission methods using guided media?

3.        How is it possible to organize omnidirectional propagation of radio waves and microwaves?

4.        What factors allow radio waves of frequencies ranging from 2 MHz to 30 MHz to propagate for hundreds of kilometers?

Signals of frequencies from 2 MHz to 30 MHz are reflected by the Earth's ionosphere.

5.        What spectrum is used for satellite communications?

1.5 ­- 30.5 GHz

6.        Which atmospheric conditions impede the propagation of microwaves?

Rain, fog, dew

7.        What devices are used for omnidirectional propagation of infrared waves?

A.      Laser diodes

B.       Systems of lenses - correct

C.       Reflection from the ceiling

D.      Heat antennas

1.        What barriers cause diffraction of electromagnetic waves? What barriers
cause scattering?

When the signal encounters an impenetrable barrier (such as a plate of metal) whose size significantly exceeding the signal wavelength, then diffraction takes place. When the signal encounters a barrier whose size is comparable to the wavelength, it becomes scattered and starts to propagate at different angles.

2.        When is it necessary to use the elliptic orbits of telecommunications satellites?

To provide with communication the regions close to the North and South Poles.

3.        What are the drawbacks of a geostationary satellite?

-          long signal delays;

-          high signal attenuation;

-          poor connectivity for the regions close to the North and South Poles.

4.        In your opinion, what are the reasons for the commercial failure of the Iridium project?

5.        What condition must be observed for the FHSS technology to be fast?

If a high-speed encoding method is used  for every frequency used.

6.        Which property of the Barker sequence is the reason for its use in DSSS technology?

Fast synchronization between a transmitter and a receiver.

7.        What is the main property of the spreading sequences used in CDMA?

A mutual orthogonality of the spreading sequences.



15.     Is it possible to use the values 1 0 0 … 0, 0 1 0 0 … 0, 0 0 1 0 … 0, 0 0 0 1 0 …0, and so on as spreading sequences of the nodes of a network supporting CDMA based
on DSSS?

No, these sequences are not orthogonal relatively to the operations of DSSS; furthermore their inversions are not orthogonal as well. 


16.     Suggest an 11-bit spreading sequence, different from the Barker sequence, and provide the possibility of reliably detecting the starting instance of transmission of the next bit of source information.

Example:   01001000111