PAPER CODE NO. EXAMINER: Prof Y. Huang
ELEC 411 DEPARTMENT: EE&E TEL. NO.: 44521
SECOND SEMESTER EXAMINATIONS 2017/18
RADIO PROPAGATION for Wireless Systems
TIME ALLOWED: Two Hours
INSTRUCTIONS TO CANDIDATES
The numbers in the right hand margin represent an approximate guide to the marks available for that question (or part of a question). Total marks available are 100.
Answer All Questions.
The use of a calculator IS allowed.
Additional Information:
Please see the attached “Constants and Equations”
1.
We have studied wave polarisations in this module.
a)
Explain why polarisation is important for a radio system, and why elliptical polarisation is not used in practice.
5
b)
Write down the mathematical expression of the E-field of an x-polarized plane wave with an amplitude of 5 V/m travelling towards y-direction at 500 MHz in free space, obtain the magnetic field and power density of this wave, and then sketch this wave with the correct coordinates.
10
c)
Give the mathematical expression of the E-field of a right-hand circularly polarised wave with an amplitude of 5 V/m travelling towards y-direction at 500 MHz in free space. If the wave is reflected by a perfect conducting ground plane, what is the mathematical expression of the reflected wave? And its polarisation?
10
Total
25
2.
A WiFi signal at 2.45 GHz is reflected by a glass window with a thickness of 1.5 cm, a relative dielectric constant of 4 and conductivity of 0. The incident angle is assumed to be 30-degree:
a)
Calculate the reflection coefficient for the case when the wave is TM (parallel) polarization.
13
b)
What are the possible measures one could use to minimize the reflection from the window? Justify your answer using relevant equations.
7
c)
What are the possible measures one could use to improve WiFi signal coverage inside a house?
5
Total
25
3.
a)
Radio wave scattering plays a very important role in radio communications. With the aid of a diagram, explain the concept of wave scattering, and then explain why the sky is blue on a sunny day using scattering theory.
12
b)
The Global Positioning System (GPS) uses 1575.42 MHz (L1) band for commercial applications. If the transmitted power from a satellite (which is 20,200 km above the Earth and circles the Earth twice a day) is 27 W and the gain of the antenna is 13 dBi, find the received power at a GPS receiver with an antenna gain of 3 dBi.
Furthermore, explain what the ionosphere is, and then discuss how it may affect the GPS signal in terms of the polarisation and attenuation.
13
Total
25
4.
Mobile communications are now moving towards the 5th generation (5G) which will significantly increase the data rate and reduce the latency. The information on the communication channel is vital for such a system.
a)
One of the most important parameters is called path loss. Explain the concept of path loss, and then use a popular path loss model for urban environment as an example to explain what parameters determine the path loss.
•
7
b)
Another important parameter is called delay spread. Explain the concept of delay spread, and then explain how to combat this problem in real-world applications.
7
c)
A typical impulse response of a radio channel has three received pulses with power levels of -10 dB, -20 dB and -13 dB at 2 s, 3 s, and 4 s, respectively, calculate the mean excess delay and RMS delay spread. Then estimate the coherent bandwidth of the channel.
11
Total
25
Important Constants and Equations
o=1.25710-6 H/m, 0 =8.85410-12F/m
Reflection coefficient:
Input impedance:
VSWR:
Intrinsic impedance:
Snell’s law:
Equivalent impedance:
Loss tangent:
Skin depth:
Friis’ transmission formula:
Maxwell’s equations:
Radiation from a current Il:
Mean access delay and delay spread: