Telcom 2110 Network Design
Homework #6
Spring 2010
1.
Consider the design of a point to point link
of .5 Km distance using 802.11a equipment
in the 5GHz band.
Assume a line of sight exists between the transmitter and receiver and
the link uses channel 36 (i.e., carrier frequency is 5.180 GHz). The transmit power is
15dBM and the required signal receiver signal strength for 54 Mbps operation
is 78
dBm.
If the build in antenna on the 802.11 equipment has a antenna gain of 2.2 dB
at
both the transmitter and receiver will the link operate at 54 Mbps while allowing for a 10
dB system margin? If not what must the gain of external antennas be to operate the link?
Given:
d = .5km = 500 m
f = 5.180 GHz = 5.180 x 10
9
Hz
P
t
= 15 dBm
Required P
r
= 78 dBm
G
t
= G
r
= 2.2 dB
From Free Space Loss model we have
Path Loss(P
L
)
= 21.98 – 20log(
) + 20log(d)
= 21.98 – 20log(
3*10
8
5.18*10
9
) + 20log(500)
= 100.704 dB
Since P
r
= P
t
+ G
t
– P
L
+ G
r
= 15 + 2.2 – 100.704 + 2.2
= 81.304 dB
The expected received power is 81.304 dB
Then we have expected system margin of 81.304 – (78) = 3.304 dB which is less than 10 dB
Therefore, the system will not be able to operate at 54Mbps.
In order to effectively operate the system at 54Mbps we need 10dB system margin. Thus, we
need to have received power of 10 + (78) = 68 dB which means we need to have additional gain
from implementation of external antenna.
So, from P
r
= P
t
+ G
t
– P
L
+ G
r
+ G
external antenna
68 = 15 + 2.2 – 100.704 + 2.2 + G
G = 13.304 dB
Therefore, external antennas need to have gain of at least 13.304 dB in order that the system will
be able to operate at 54Mbps
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2.
The Figure below shows the layout of the 4
th
floor of the SIS building. The scale is 1 cm.
in the drawing
represents 2.5 m.
The power levels and frequency channels of the IEEE 802.11b wireless LAN access
points are given
in the table below. Assuming a loss of 6 dB per wall and shadow
fading of 13 dBm. Use the partition dependent path loss model to answer the
following
Power level
Channel
AP1
20 dBm
2.412 GHz
(ch1)
AP2
20 dBm
2.437 GHz
(ch6)
a.
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 Spring '10
 D>Tipper
 AP, Decibel, IEEE 802.11, signal strength

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