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IT351 - Mobile & Wireless Computing

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1. Why can waves with a very low frequency be used for submarine communication? Why are they not used for data transmission in mobile phones and computer networks

2. Explain briefly what is meant by free space loss.

3. Although a Line-of-sight is required between a sender and receiver for radio waves with frequency > 30 MHz, mobile phones can work in big cities without such a LOS. Explain

4. Is a directional antenna useful for mobile phones? Why?

5. What are the main problems of signal propagation? Why do radio waves not always follow a straight line?

6. Explain how is reflection both useful and harmful

7. Wireless Communications have to tackle many problems at the physical layer due to the nature of the media (spectrum)

a. Modulation techniques can help to overcome interference. Draw the graphs for the number 001101 for ASK, FSK, and PSK

b. Why is digital modulation not enough for radio transmission? What are the general goals of digital modulation? What are typical schemes?

Homework

Search the Internet and describe briefly the License-free ISM band. Explain why is the International availability of the same ISM bands important.

Answers

1) Long waves can propagate long distances so they can be transmitted through the oceans to a submarine and can penetrate objects easily. They are not suitable for mobile/ computer networks for the following reasons:

i. Lower frequencies also mean lower data rates so the available bandwidth is less.

ii. Lower frequencies also require large antennas for efficient transmission and

reception. This might work for submarines, not for mobile phones.

iii. Lower frequencies penetrate material more easily. Thus frequency reuse is much more difficult.

2) Free space loss: Even if no matter exists between the sender and the receiver, the signal still attenuates and the received power is proportional to 1/d2. This is because the sending energy is equally distributed over a growing sphere surface as the distance increases between sender and receiver.

3) Short waves that are used for mobile phone systems are affected by reflection, refraction, scattering and diffraction. So signals transmitted from a sender may bounce off the walls of buildings and trees several times till it reaches a receiver.

4) Without any additional “intelligence” directional antennas are not useful in standard mobile phones as users may not want to direct the phone to a certain antenna. Users move, rotate, flip the phones etc. Phones are in bags, pockets, … while operated

hands-free. There is no chance of directed transmission.

5) Problems: attenuation, scattering, diffraction, reflection, refraction. Except for attenuation all other effects can divert the waves from a straight line.

6) Reflection is useful: Only in vacuum and without gravitational effects radio waves follow a straight line. Without reflection radio reception in towns would be almost impossible. A line.-of-sight almost never exists.

Reflection is harmful: Reflection is the main reason for multipath propagation causing ISI.

7)

a)

Amplitude Shift Keying (ASK), below, applies two different amplitudes, one each to 0 and1.

001101

Frequency Shift Keying (FSK) applies a different frequency, one each to 0 and 1.

Phase Shift Keying (PSK), below, applies a shift phase to each new bit. A phase shift is a realignment of the starting position of the wave.

b) Why after I convert from digital to analogue I need to do analogue Modulation.

Worldwide regulation always uses FDM for separating different systems (TV, WLAN, radio, satellite, …). Thus, all radio systems must modulate the digital signal onto a carrier frequency using analogue modulation. The most prominent system is the traditional radio: all music and voice use frequencies between, e.g., 10 Hz and 22

kHz. However, many different radio stations want to transmit at the same time.

Therefore, all the original signals (which use the same frequency range) must be

modulated onto different carrier frequencies.

Other reasons and motivations for analogue modulation are antenna and medium characteristics.

- The goals for digital modulation are to achieve spectral efficiency, power efficiency and robustness.

- Typical schemes are ASK, PSK, FSK.

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