A A DVANCED W IRELESS W C SS T ECHNOLOGIES T ECHNOLOGIES Aditya - - PowerPoint PPT Presentation

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ADVANCED WIRELESS TECHNOLOGIES TECHNOLOGIES

Aditya K. Jagannatham Indian Institute of Technology Kanpur Indian Institute of Technology Kanpur Commonwealth of Learning Vancouver

MOOC on M4D 2013

Wireless Signal Fast Fading Wireless Signal Fast Fading

• The wireless signal can reach the receiver via

direct and scattered paths. p

• As a result, the receiver sees the

superposition of multiple copies of the superposition of multiple copies of the transmitted signal.

– Multipath Propogation

• These signal copies experience different

These signal copies experience different attenuations, delays.

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Wireless Signal Fast Fading Wireless Signal Fast Fading

• Results in interference, amplifying or

attenuating the signal power seen at the Rx. g g p

– This phenomenon is termed as fading.

• Strong destructive interference is referred to
• Strong destructive interference is referred to

as a deep fade.

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Techniques to Combat Fast Fading Techniques to Combat Fast Fading

• Several techniques can be employed to

Several techniques can be employed to improve performance in a wireless fading channel channel.

– Forward Error Correction. – Interleaving. – Hybrid ARQ (HARQ). – Diversity.

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Forward Error Correction (FEC) Forward Error Correction (FEC)

S f l f d i i

• System of error control for data transmission.

– Coding the data stream to correct at receiver.

• Sender adds redundant data to its messages

also known as ‘parity’ bits. p y

• Examples of forward error correction codes,

– Block Codes – Block Codes. – Convolutional Codes. Turbo Codes – Turbo Codes.

• FEC typically uses a large overhead.

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Interleaving Interleaving

• Symbol blocks to be transmitted.

– Each symbol block is coded to protect against y p g symbol errors (Ex. Convolutional Coding).

• Symbol blocks after Interleaving.

– Interleaving arranges data in a non contiguous – Interleaving arranges data in a non‐contiguous fashion.

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Interleaving Interleaving

• Deep fade results in a ‘Burst Error’ in the symbol

p y block affected by fading channel.

• Symbol blocks after Deinterleaving.

Symbol blocks after Deinterleaving.

– Erroneous symbols are spread across multiple blocks.

• This results in better error correction

performance for the block code.

– It can correct a fixed number of errors per block.

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Hybrid Automatic Repeat reQuest Hybrid Automatic Repeat reQuest

• It is an error‐control method for packet data

transmission.

• Uses ACKs/NACKs and timeouts to achieve

reliable data transmission reliable data transmission.

• An ACK is sent by the receiver to indicate that

it has correctly received a data frame or packet. p

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Hybrid Automatic Repeat reQuest Hybrid Automatic Repeat reQuest

• In case of a NACK, the receiver has two
• ptions in H‐ARQ.

p

– Send the complete packet (Chase Combining). – Send only the parity bits (Incremental – Send only the parity bits (Incremental Redundancy).

It t b d f t i i f l

• It cannot be used for transmission of real‐

time information (Ex audio/ video).

• Suited for non real‐time applications such as

data e‐mail

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data, e mail.

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BER of a Rayleigh Fading Channel BER of a Rayleigh Fading Channel

BER in a Fading Wireless Channel

• n

Detectio BER in a wired r BPSK D Channel BER for SNR

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Antenna Diversity Antenna Diversity

• Consider a wireless signal received using

multiple antennas at the receiver (Rx) i.e. p ( ) employing receive antenna diversity.

• Let the number of receive antennas be L
• Let the number of receive antennas be L.
• Hence, the receiver (Rx) sees L copies of the

transmitted wireless signal, each traveling through an independent Rayleigh flat‐fading through an independent Rayleigh flat fading channel.

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Schematic of a Rx Diversity System Schematic of a Rx Diversity System

Tx Tx Rx = Antenna

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Prerequisites for Diversity Gain Prerequisites for Diversity Gain

• Diversity implies the receiver is provided

y p p with multiple copies of the transmitted signal.

• The multiple signal copies should experience

independent levels of fading in the wireless p f f g channel.

• This is because only in that case the
• This is because only in that case the

probability that all signal copies fade simultaneously is reduced dramatically simultaneously is reduced dramatically.

– Leads to a significant reduction in the bit error t

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rate.

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BER of a Rayleigh Fading Channel BER of a Rayleigh Fading Channel

BER in a Fading Wireless Channel

• n

Detectio

L = 1

BER in a wired r BPSK D

L = 2 L = 4

Channel BER for

L 4 L = 8

SNR With Rx Antenna

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Diversity

15

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Diversity Diversity

• Examples of diversity techniques

– Transmit/Receive Diversity. / y – Temporal Diversity. Frequency Diversity – Frequency Diversity. – Multipath Diversity.

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Spatial Diversity Spatial Diversity

A th d t di it b bt i d

• As the name denotes, diversity can be obtained

by transmitting the wireless signal across independently fading spatial channels independently fading spatial channels.

• This implies there are several receiving and/or

transmitting antennas that are spaced transmitting antennas that are spaced sufficiently far apart.

• Spatial separation should be sufficently large to

Spatial separation should be sufficently large to reduce correlation between the different antennas or diversity branches.

• Spacing guideline is approximately λ/2. At 2 GHz,

the spacing is roughly 5 cm.

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Temporal Diversity Temporal Diversity

l di i i hi d h h

• Temporal diversity is achieved through

transmission of same wireless signal at different times i.e. through temporal spacing.

• The time separation between the signal

p g copies should be larger than the coherence time of the channel for the different copies to p experience independent fading.

• For instance at 2 GHz 60 Km/Hr the
• For instance, at 2 GHz, 60 Km/Hr, the

temporal spacing should at least be 2 ms.

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Frequency Diversity Frequency Diversity

F di i i hi d h h

• Frequency diversity is achieved through

transmission of same wireless signal in different i d d tl f di f b d i independently fading frequency bands i.e. through frequency spacing.

• The frequency separation should be larger than

the coherence bandwidth Bc of the channel.

• For cellular communications this is

approximately 300 KHz, since the delay spread is

• f the order of 3μs.

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Multipath Diversity Multipath Diversity

Si l li i d i d diff

• Signal replicas received are received at different

delays and phase factors at the receiver.

• If these different replicas are spaced sufficiently

far apart so that they can be distinguished and they experience independent levels of fading, they can be used to exploit multipath diversity.

• Receiver structures such as RAKE receiver in

CDMA and equalizers such as Maximum q Likelihood Sequence Estimator (MLSE) in a TDM/TDMA system provide multipath diversity.

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y p p y

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MIMO Communication Systems MIMO Communication Systems

• A MIMO system has multiple (nt > 1) transmit

y p ( t ) and multiple (nr > 1) receive antennas.

• MIMO wireless systems are a revolutionary

MIMO wireless systems are a revolutionary breakthrough because they offer Linear increase in throughput for the same – Linear increase in throughput for the same transmit power C b t f di th h i d t it – Combats fading through receive and transmit diversity.

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MIMO System Schematic Diagram MIMO System Schematic Diagram

Rx TX Rx

X

Transmitter Receiver Transmitter Receiver = Antenna

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MIMO Capacity vs SNR #Antennas MIMO Capacity vs. SNR, #Antennas

MIMO Capacity vs. SNR (dB) for Different No. of Antennas 10

2

r = t = 1 r = t = 2 r = t = 4 r = t = 8 /Hz) r = t = 8 10

1

Capacity (b/s/ C 5 10 15 20 25 30 35 40 45 50 10

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SNR (dB)

MIMO System Model MIMO System Model

 

 

      y y

1

      x x

1

       y 

2

y

       x 

2

x

MIMO System

     

r

n

y

     

t

n

x

• The MIMO system model can be represented as,

. ) ( ) ( ) ( k k k n Hx y  

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MIMO Capacity Schematic MIMO Capacity Schematic

• The MIMO system can be schematically represented

h ll l h l as having nt parallel channels.

– Spatial Multiplexing

Spatial nels rallel S Chann Pa

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SPACE‐TIME BLOCK CODES

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Alamouti Code Alamouti Code

• The Alamouti Code or the Alamouti Scheme can
• The Alamouti Code or the Alamouti Scheme can

be employed to obtain transmit diversity in a 2 transmit antenna system transmit antenna system.

• It was described by Siavash Alamouti in his

i i 1998 k “A i l t it pioneering 1998 work “A simple transmit diversity technique for wireless communications” communications .

• This powerful scheme, has been included in all

th 3G d 4G i l ll l d LAN the 3G and 4G wireless cellular and LAN standards.

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Alamouti Code Alamouti Code

• Alamouti invented the first Orthogonal Space

Alamouti invented the first Orthogonal Space Time Block Code (OSTBC) in 1998.

• It was designed for a two‐transmit antenna

system and achieves second order diversity y y (L=2) using a very simplistic symbol transmit scheme scheme.

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Alamouti Code Alamouti Code

• The Alamouti symbol transmit structure is

given as, g

   

 

       

       1 2 2 1 2 , 1

* *

x x x x

Space

   

 

   

   1 2

*

x x

S Time MOOC on M4D 2013