Due:
(Due in class
at the start of the day)
Instructor: T. Kunz
1) Spectrum Allocations and Frequency Reuse (10 marks)
a) One of the major advantages
(I claimed) of the ISM band at 2.4 GHz is its global availability. Discuss
whether it is absolutely necessary to achieve spectrum harmonization (i.e.,
assign the same frequency bands everywhere) on a global scale. Even if spectrum
harmonization were not to be absolutely necessary, what are its advantages?
b) Assume a regular cell layout
(i.e., cells are all identical in size and hexagonal in shape, as used in the
course notes). We discussed the idea of frequency reuse and the notion of reuse
patterns in class in the context of AMPS. The basic idea is that you would not
like to use the same set of frequencies in two adjacent cells, due to
interference that could cause. AMPS uses a reuse pattern of 7, I also mentioned
that another typical reuse pattern is 3 cells (i.e., each cell gets to use a
third of the spectrum). Would it make sense to base frequency reuse on groups
of 2, 4, 5, or 6 cells? Explain your answer.
2) Media Access Control (10 marks)
CDPD and
GPRS (among others) employ a random access strategy. In essence, a node that
wants to transmit data, competing for access to a shared channel with other
nodes. As a result, collusions can occur. For a cellular network, could you
envision other approaches for packet data transmission over a shared channel
that would
· Reduce/minimize/do away with collisions
· Allow an arbitrary (or at least large number)
of nodes in a cell to transmit some data
· Does not pre-allocate resources to each node
(i.e., nodes will only access the channel if and when they have data to send
Are there
any disadvantages to your proposal?
3) CDMA (20 marks)
a) You are to assign 4 DS-CDMA
senders a chip sequence. Below, three potential sets of 4 chip sequences are
given. Which of these four sets would you choose, and why?
(i) -1 +1 +1 -1 +1 +1 +1 -1 -1 -1 -1 -1 -1 +1 +1
+1
+1 +1 +1
+1 -1 -1 -1 -1 -1 +1 +1 -1 -1 +1 +1 -1
+1 +1 +1
-1 -1 +1 +1 +1 +1 +1 -1 -1 -1 +1 -1 +1
+1 -1 +1
-1 -1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1
(ii) +1 +1 +1 -1
+1 +1 -1 -1 -1 -1 -1 -1 +1 +1 +1
+1 +1 +1
+1 -1 -1 -1 -1 -1 +1 +1 -1 -1 +1 +1 -1
+1 +1 +1
-1 -1 +1 +1 +1 -1 -1 -1 -1 -1 -1
+1 -1 +1
-1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1
(iii) -1
+1 +1 -1 +1 +1 +1 -1 -1 -1 -1 -1 -1 +1 +1 +1
+1 +1 +1
+1 -1 -1 -1 -1 -1 +1 +1 -1 -1 +1 +1 -1
+1 +1 +1
-1 -1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1
+1 -1 +1
-1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1
b) If a 5th sender
were to join this group, could you assign it a chip sequence, based on your
answer to a)? If so, what would that chip sequence be? If not, what would you
have to do to accommodate the additional station?
c) Assume a receiver receives
the signal:
(+1 -1
+1 -3 +1 +1 +3 +1 +3 -1 -1 -1 +1 -3 -1 -1).
Which station send what data
bit?
d) Assume a receiver receives
the signal
(+1 +2
+2 0 0 -1 0 -2 -2 0 0 -1 -2
+1 +2 0).
Which stations did send what
data bit?
e) In both frequency and time
division multiplexing, there is a need for guard “space” to make sure that
“adjacent” users do not interfere with each other. In frequency division
multiple access, this is done by not using small guard bands separating the
individual channels. In time division multiple access, guard bands exist in the
time domain (i.e., between one sender stopping transmission and the next sender
starting up, there is a brief pause). Does DS-CDMA require guards? If so, where
are they? If not, why not?
4) Location Management (10 marks)
Both CDPD
and GSM deal with mobile devices that move between cells. Briefly outline the
similarities and the differences between the two approaches? Do they “really”
solve the same problem?