Exam text content

 

ELT — 41206 Basic Course on Wireless Communications, Exam 17 March 2016, Please give the
answers in English. The attached 2-page formulas are allowed. Only basic calculators (scientific function

calculators) are allowed. Exam's compiler: Elena Simona Lohan, TG315.
: Please fit ALL your answers in maximum 4 single-

sided pages (i.e., 2 double-sided pages); there will be 1 point loss for each extra single-sided page).
Also, please answer in the given order (from 1 till 4), not in a random order. There will be 1 point loss if
answers are not in increasing order.

1. (5 p) a) Assume that the transmitted power of an isotropic radiator is 5 kW and the operating
freguency is £= 2.5 GHz.
a. Whatis the power density at the receiver 10 km away?
b. 1fthereceiver antenna is isotropic, what is the total received power?
c. Whatis the transmission loss, assuming a free space loss model?
d. What is the transmission loss L,, , assuming a simplified path loss model of the following
type, with path loss coefficient n=4:

L x 40.4+10m10g,, d,,

e. What happens with the transmission loss if the path loss coefficient n increases to 10?

(5 p) b) You are walking downtown Barcelona with your mobile navigator and you have just entered
the Gothic district, having narrow streets and relatively tall building. Suddenly your GPS signal is
lost. Enumerate 4 possible causes which might explain the loss of your navigation signal. Please
avoid reasons such as 'my battery went off? or 'my phone has broken”. Also briefly explain how
such causes could be alleviated or mitigated. Mention at least 4 methods which might be used to
improve the navigation on your mobile device. Explain also briefly the main difference between
navigation and communication signals.

2. (2p)a) Which is the spreading factor for the seguence illustrated in the figure below? 1f the symbol
rate is 100 kbps, what is the minimum bandwidth needed to transmit the spreading seguence with the
spreading factor of this figure?

  

1 symbol

(3p) b) Regarding WLAN architectures: i) What is an Independent Basic Service Set (IBSS) in the
context of WLAN communications? ii) What is the Hidden Node Problem? Draw an illustrative
example. iii) What are the main differences between IBSS and the Basic Service set (BSS)?
3. (2p) a) Draw a cluster of hexagonal cells, which has a re-use factor of 0.16667. Assuming that you
can choose from a range of freguencies fi, i=1,2,..., show what freguency is to be used in each
cluster cell and in each cell of 2 neighbour clusters (with same cluster size)

(3p) b) Explain the near-far problem in CDMA systems. Give illustrative examples from both uplink
and downlink situations. How you can alleviate the near-far problem in CDMA systems?

4. (4p)a) Compare UMTS with GSM according to the following table. If the results are not presented
in a table of this form, they will not be considered

 

GSM UMTS

 

Modulation types(s)

 

Multiple access types(s)

 

Order of magnitude of maximum
supported data rates

Packet-switching support : Yes/No; add
explanations if needed

 

 

 

 

 

 

(3p) b) — In the following plot, the autocorrelation of 2 CDMA code seguences is shown. Write which
seguence has better auto-correlation properties and explain why. Draw an example of a good cross-
correlation function between 2 pseudorandom seguences

4!

 

(3p) c) Make a judgment on the evolution and challenges on the future wireless communications (your
ideas what on what will be next and what are the challenges we have to cope with)