Mobile Communications Networks Wireless Transmission Manuel P. - - PowerPoint PPT Presentation

Wireless Transmission 1

Mobile Communications Networks Wireless Transmission

Manuel P. Ricardo

Faculdade de Engenharia da Universidade do Porto

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♦ How is a signal affected when it propagates in a wireless link? ♦ How are bits transported by a carrier? ♦ What is the maximum bitrate transportable by a wireless link? ♦ Why do characteristics such as bitrate vary along the time? ♦ What will future radio functions look like?

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Electromagnetic Waves – Generation and Propagation

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Frequencies for Radio Transmission

♦ Frequency bands as defined by the ITU-R Radio Regulations

fc= 3 GHz λ λ λ λ = = = = 10 cm λ λ λ λ − − − − wave length

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Wireless Systems in Europe

• In Portugal

ANACOM attributes the frequencies http://www.anacom.pt

• FWA

Fixed Wireless Access

• ISM

Industrial, Scientific and Medical

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To Think About

♦ What factors may affect the power of the signal received by a

mobile phone?

♦ How does the power of a received signal depend on the

» distance? » wavelength (λ)?

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Signal Propagation and Wireless channels

Power of the signal received depends on 3 factors

– Path loss

Dissipation of radiated power; depends on the distance

– Shadowing

• caused by the obstacles between the transmitter and the receiver
• attenuates the signal absorption, reflection, scattering, diffraction

– Multipath

constructive and destructive addition of multiple signal components

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W, dBW, dBm, dB, Gain

W W dBW

r r r

P W P P log . 10 1 log . 10 =         = s J W time energy power P

W

r

1 1 1 , , =       =

( )

dBm dBm dBW dBW W W W W

s r s r s r s r dB

P P P P P P P P Gain − = − = − = = log . 10 log . 10 / log . 10

 

) log( * 10

1mW P r

W r dBm

P =

dBm dBm dBW dBW

r s r s dB dB dB

P P P P Gain Atenuation Loss − = − = − = =

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Path Loss – Free Space Model

x b d G PG

l dB

20 ) log( . 20 4 log . 20 − = −         = π λ

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Signal Propagation and Wireless Channels

PGdB

Path loss Shadowing + Path loss

log(d)

Shadowing + Path loss Multipath + Shadowing + Path loss

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Path Loss – Free Space Model

Assume a receiver needs to receive, at least, 0.1 µ µ µ µW PL increases 20 db per logd

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Path Loss – Other models

Two-ray model Simplified model

λ 10

0 ≈

d

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Path Loss – Indoor Factors

♦

Walls, floors, layout of rooms, location and type of objects

» Impact on the path loss » The losses introduced must be added to the free space losses

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Shadowing

♦ Signal traversing wireless channel suffers random attenuation ♦ Random attenuation

» described as a statistical process » having a log-normal distribution

♦ If the simplified path loss model is used, then

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Multipath

♦ Multipath multiple rays

» multiple delays from transmitter to receiver » time delay spread

♦ The time-varying nature of the multipath channel ♦ The time-varying nature of the multipath channel

» caused by the transmitter / receiver movements » location of reflectors which originate the multipath

τ

1

τ

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Multipath – Narrowband Channel

♦ For a narrowband channel

low B low symbol rate (symbol/s) large time/symbol multipath components arrive in the time period of their symbol

B 2B

♦ Narrowband channel has Rayleigh fading

The power received has an exponential probability density function

Pr

f fc Pr t

pdf

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Multipath – Wideband Channel

♦ Multipath components

» may arrive at the receiver within the time period of the next symbol » causing Inter-Symbol Interference (ISI).

♦ Techniques used to mitigate ISI

» multicarrier modulation » spread spectrum

transmitted signal received signal

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To Think About

♦ What is the difference betweeen B and fc?

f fc

B 2B

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1 1 B1 t 1 1 B2 B1 >, < B2 ? t

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Capacity of an Wireless Channel

♦ Assuming Additive White Gaussion Noise (AWGN)

» Given by Shannon´s law

(bit/s)

N0 – power spectral density of the Noise

♦ Capacity in a fading channel (shadowing + multipath)

usually smaller than the capacity of an AWGN channel

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Capacity of an Wireless Channel

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To Think About

♦ How can we transmit bits using a continuous carrier?

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Digital Modulation

♦ Digital modulation

» maps information bits into an analogue signal (carrier)

♦ Receiver

» determines the original bit sequence based on the signal received » determines the original bit sequence based on the signal received

♦ Two categories of digital modulation

» amplitude/phase modulation » frequency modulation

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MPAM MSK MPSK MSK

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Amplitude and Phase modulation

♦

sent over a time symbol interval

♦ Amplitude/phase modulation can be:

» Pulse Amplitude Modulation (MPAM)

information coded in amplitude

» Phase Shift Keying (MPSK),

information coded in phase

» Quadrature Amplitude Modulation (MQAM)

information coded both in amplitude and phase

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Differential Modulation

♦ Bits associated to a symbol

depend on the bits transmitted over prior symbol times

♦ Differential BPSK (DPSK)

» 0 no change phase » 0 no change phase » 1 change phase by π

♦ Diferential 4PSK (DQPSK) the bit

» 00 change phase by 0 » 01 change phase by π/2 » 10 change phase by -π/2 » 11 change phase by π

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Frequency Modulation, Minimum Shift Keying (MSK)

♦ Frequency modulation

» encodes information bits into the frequency of the carrier

♦ Minimum Shift Keying

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Coding for Wireless Channels

♦ Coding enables bit errors to be either

detected or corrected by receiver

♦ Codes designed for AWGN channels

» do not work well on fading channels » do not work well on fading channels » cannot correct the long error bursts that occur in fading

♦ Codes for fading channels are normally

» based on an AWGN channel code » combined with interleaving » objective spread error bursts over multiple codewords

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Convolutional Code; Interleaving

Example: convolutional code Interleaving

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To Think About

♦ Why does your WLAN interface change dynamically its working

bitrate? bitrate?

♦ What happens, from the modulation and coding points of view,

when the WLAN interface changes from 54 Mbit/s to 6 Mbit/s?

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802.11a – Rate Dependent Parameters

250 kSymbol/s

% of useful information

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Adaptive Modulation/Coding

♦ Adaptive transmission techniques

» aim at maintaining the quality low/stable BER » works by varying: data rate, power transmitted, codes

♦ Adapting the data rate

» symbol rate is kept constant » symbol rate is kept constant » modulation schemes / constellation sizes depend on γ multiple data rates

♦ Adapting the transmit power

» compensate γ=Pr/N0B variation due to fading » maintain a constant received γ

♦

Adapting the codes

» γ large weaker or no codes » γ small stronger code may be used

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Spread Spectrum

♦ Spread spectrum techniques

» hide the information signal below the noise floor » mitigate inter-symbol interferences » combine multipath components

♦ The spread spectrum techniques

» multiply the information signal by a spreading code

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Spread Spectrum – Direct Sequence

Modulator Information signal (Rb bit/s) Spread signal (Rc = N Rb chip/s) Pseudo-random sequence (Rc = N Rb chip/s) De-modulador Pseudo-random sequence Spread signal Information signal

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Direct Sequence Spread Spectrum – Immunity to Interferences

P P f signal wideband interference narrowband interference f

• riginal signal

f spread signal interferences f P f P Received signal f P received signal f Signal after de-spreading

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Spread Spectrum –Frequency Hopping

information signal (3 bits/hop) 1 tb 1 1 t f f3 td (3 bits/hop) (3 hops/bit) t f t td tb: bit period td: hop peridod f2 f1 f3 f2 f1

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Multicarrier Modulation

♦ Multicarrier modulation (e.g. OFDM) consists

» dividing a bitstream into multiple low rate sub-streams » sending sub-streams simultaneously over sub-channels

♦ Subchannel

» has bandwidth BN = B/N » has bandwidth BN = B/N » provides a data rate RN R/N » For N large, BN = B/N << 1/Tm

flat fading (narrowband like effects) on each sub-channel, no ISI

♦ Orthogonal sub-carriers

» space between carriers minimised » system capacity maximised

≈

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Multiple Antennas and Space-Time Communications

♦ Multiple Input Multiple Output combines

» signals generated by multiple transmit antennas » signals received by multiple receive antennas

♦ MIMO used to improve data rate (bits/s) or quality (BER) ♦ MIMO used to improve data rate (bits/s) or quality (BER)

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Ultra Wide Band

» UWB transmits very short duration pulses

– individual pulse much shorter than a single bit – very large bandwidth – very low transmission power low interference

1 1

OOK - On-Off Keying PAM – Pulse Amplitude Modulation PPP – Pulse Position Modulation BPSK – Binary Phase Shift Keying

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Software Defined Radio

♦ Software Defined Radio

aims at implementing the radio functions in software

♦ Digital Signal Processors being integrated with microcontroller

better integration of radio and communications functions better integration of radio and communications functions

(e.g. de-coding, de-framing, error detection, MAC, mobility management)

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Cognitive Radio

♦ Cognitive radio

» fills unused bands » avoids interferences » increases spectral efficiency

♦ Paves the way to

» dynamic spectrum licensing » secondary markets in spectrum usage

♦ SDR is the mean required by cognitive radio