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