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Mestrado em
Engenharia Electrotécnica e de Computadores
Redes Móveis e Sem Fios
Exame de Recurso – 1a parte
30 de Junho de 2015
Duração 1h15
In order to avoid grading mistakes, please answer each question on a different page and keeping the order as
much as possible.
1) In a mobile network using CDMA with 8-symbol chip sequences, there are three mobile stations trying to transmit
to a base station with keys S1= -1, -1, +1, +1, +1, +1, -1, -1, S2= -1, -1, -1, -1, +1, +1, +1, +1 and S3=+1, +1, -1, -1, +1, +1, -1, -1. The time of one bit corresponds to 8 chips. Assume that the logical value “0” is represented by -1
and logic value “1” is represented by +1. The sequence that was received at the base station was the following: 0,
+1, -1, -2, 0, 0, +2, +2. The transmissions are affected by the following noise pattern: 0, +1, +1, 0, 0, 0, 0, 0. The
decoding thresholds are -2 and +2, respectively for logical “0” and logical “1”.
a) Which data bits were transmitted by S1, S2 and S3? (2,0 val)
b) In a), the base station is unable to decode the signal from one of the mobile stations. Assuming that the external
noise/interference is low, what is the most plausible cause? (1,0 val)
c) Assuming that the situation addressed in b) is due to external noise/interference and that the affected mobile
station is transmitting a logical “1”, find a noise/interference pattern that provides the same result. (2,0 val)
2) In a Wireless Sensor Network, the sensor nodes are equipped with radio transmitters that operate in the 868 MHz frequency band, using a 200 kHz wide RF channel. The sensor nodes perform magnetic field measurements and
send them to a monitoring station nearby. The sensor nodes are very basic and have no battery. Instead, a small
solar panel allows them to accumulate energy in a super-capacitor, just enough for the next packet transmission.
The packet size is 15 octets. The employed modulation is BPSK and the transmit power is 2 mW. The noise
spectral density is -140 dBm/Hz. The receiver sensitivity is considered to be the received power at which a BER of
1% is attained without interference. Assume that both the sender and receiver antennas are isotropic and the
deployment area is flat. Two-Ray propagation model is assumed with antenna height of 10 cm.
a) Assuming that there is no energy expenditure other than that due to the packet transmissions, calculate the
minimum interval between packets when the solar panel is able to generate 0.002 mW of power. (1,0 val)
b) Calculate the receiver sensitivity. (1,5 val)
c) Assuming that there is no interference, what is the FER when the sensor node is deployed at maximum range?
(1,5)
3) Consider an IEEE 802.11b cell configured to operate at 11 Mbit/s, where all the stations are receiving a multicast
video stream. Consider that video frames have a fixed size of 10000 octets and that the video stream has a frame
rate of 25 frames per second (fps). The RTP+UDP+IP headers together have a length of 40 octets. Additional data are as follows: RTSThreshold=500 octets, SIFS=16us, DIFS=34us, PHY overhead=96us, MACh=34bytes, avg.
Backoff =67us, ACK=14bytes, FER=0.01. ACK frames are transmitted with the minimum bitrate supported by the
technology. Note: the PHY bitrates supported by IEEE 802.11b are 1, 2, 5.5 and 11 Mbit/s.
a) What is the fraction of the cell’s PHY layer capacity occupied by the video stream transmission? Justify. (2,0
val)
b) What is the average bitrate actually received by the receiver’s video application? (1,5 val)
c) Assuming that the channel is BSC and that all bit errors take place in the PHY payload, calculate the BER. (1,5
val)
4) Consider the IPv4 network depicted in the figure below, where H1 is communicating with H2. The packets from
H1 follow the following path: R3-R1-R2-R4-R5-H2. The routing tables are incomplete.
a) Complete the routing tables with entries that result in the routing path from H1 to H2, such that the lengths of
the subnetwork masks are minimized. (2,0 val)
b) Are the routing tables that result from a) enough to support TCP flows between H1 and H2? (1,0 val)
c) Consider that an IP tunnel is established from H1 to H2. For the traffic in each link between H1 and H2,
indicate the source and destination addresses of the inner and outer IP headers. Note: when the IP address of
the node is not known, indicate the name of the node.
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
20.1.0.0/16
10.4.0.0/16
direct
R3
R3
R3
R3
R3
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
10.4.0.0/16
20.2.1.0/24
direct
direct
R4
direct
R4
R4
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
20.1.0.0/16
20.2.1.0/28
10.4.0.0/16
R3
R3
R2
direct
direct
R5
direct
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
20.2.0.0/16
30.1.1.0/28
R3
direct
direct
R3
R3
R2
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
20.1.0.0/16
20.2.1.0/28
20.2.0.0/16
R1
R1
direct
R4
direct
direct
R4
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.1.0/24
20.1.0.0/16
20.2.1.0/28
10.4.0.0/16
R2
R2
R2
R2
R2
direct
R5
Destination Next Hop
10.1.0.0/24
10.1.2.0/24
10.2.1.0/24
10.3.0.0/16
10.4.0.0/16
R4
R4
R2
R4
direct
Propagation Models Channel Capacity
Fresnel Zone
Radius 𝒓(𝑭𝒏) = √𝒏 ∙ 𝝀 ∙ 𝒅𝟏 ∙ 𝒅𝟐𝒅𝟏 +𝒅𝟐
Shannon-Heartley
Theorem 𝑪 = 𝑩 ∙ 𝐥𝐨𝐠𝟐 (𝟏 +
𝑺
𝑵)
Friis Model 𝑷𝒓 = 𝑷𝒕 ∙
𝑮𝒕 ∙ 𝑮𝒓 ∙ 𝝀𝟐
(𝟒 ∙ 𝝅 ∙ 𝒅)𝟐 ∙ 𝑳
Bit Error Rate
(DQPSK) 𝑩𝑬𝑹𝑫𝑩𝑷𝑺𝑲 = 𝟎.𝟓 ∙ 𝒆−𝑬𝒃𝑵𝟎
Two-Ray Model 𝑷𝒓 = 𝑷𝒕 ∙
𝑮𝒕 ∙ 𝑮𝒓 ∙ (𝒉𝒕 ∙ 𝒉𝒓)𝟐
𝒅𝟒
Bit Error Rate (BPSK) 𝑩𝑬𝑹𝑩𝑷𝑺𝑲
= 𝑸(√𝟐 ∙ 𝑬𝒃𝑵𝟎
)
Crossover
Distance (Two-
Ray Model)
𝒅𝒄 = (𝟒𝝅 ∙ 𝒉𝒕 ∙ 𝒉𝒓)/𝝀 Bit Error Rate (QPSK) 𝑩𝑬𝑹𝑸𝑷𝑺𝑲
= 𝑸(√𝟐 ∙ 𝑬𝒃𝑵𝟎
)
Log-Distance
Model w/
Lognormal
Shadowing
𝑷𝑳 = 𝑷𝑳𝟎 + 𝟏𝟎 ∙ 𝜸
∙ 𝒍𝒐𝒈𝟏𝟎 (𝒅
𝒅𝟎)
+ 𝚾𝒈
Q function 𝑸(𝒌) = 𝑷(𝑿 > 𝝁 +
𝒌𝝈) =𝟏
√𝟐𝝅∫ 𝒆−𝝀
𝟐/𝟐𝒅𝝀+∞
𝒌