Chapter: 5 p 5 Why wider-band single carrier transmission? - - PowerPoint PPT Presentation

SLIDE 1

3G Evolution

Chapter: 5 p 5

Wider-band single-carrier Wider band single carrier transmission

Payam Amani P A i@ it lth

Department of Electrical and Information Technology

Payam.Amani@eit.lth.se

3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 1 1

Outline

• Why wider-band single carrier transmission?
• Equalization against radio- channel frequency selectivity

– Time domain linear equalization – Frequency domain equalization – Other equalizer strategies

• Uplink FDMA with flexible bandwidth assignment
• DFT- spread OFDM

– Basic principles DFTS OFDM i – DFTS-OFDM receiver – User multiplexing with DFTS-OFDM – Distributed DFTS-OFDM

3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 2 2

Why wider-band single carrier transmission

• OFDM

Advantages: – Provides overal very high transmission bandwidth. – Robust to signal corruption due to radio channel frequency selectivity. g p q y y Drawbacks: – Large variations in the instantaneous power of transmitted signal.

• Reduced power amplifier efficiency
• High power amplifier cost
• Critical for uplink

– Some methods to reduce this power variations discussed in chapter 4 Some methods to reduce this power variations discussed in chapter 4.

• Limitations on the amount of reduction in these variations.
• Significant computational complexity and/or reduced link performance.
• wider-band single carrier transmission as an alternative for multicarrier

transmission especially for Uplink.

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Equalization against radio- channel frequency selectivity selectivity

• Equalization: main method to handle signal corruption due to radio

channel frequency selectivity channel frequency selectivity

• Time domain linear equalization

q

– Rake receiver in DS-CDMA

• Channel mached filtering with filter response as the complex conjugate of the

time reversed channel impulse response. p p

• Also called Maximum Ratio Combining (MRC)

) ( ) (

*

τ τ − = h w

• Maximizes post filter signal to noise ratio
• No compensation for radio channel frequency selectivity

Transmitter Channel Model Receiver

) (t s

) (τ h ) (t n ) (t r ) (τ w

) ( ˆ t s

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SLIDE 2

Time domain linear equalization

– Zero Forcing (ZF) equalizer: – Select the receiver filter to fully compensate for the radio channel frequency selectivity. Denotes linear convolution

1 ) ( ) ( = ⊗ τ τ w h

– Denotes linear convolution. – Suppression of any signal corruption caused by radio channel frequency selectivity

⊗

selectivity. – Large or potentially very large increase in noise level after equalization. – Overal degradation of the link performance.

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– Especially for channel with large variations in frequency response.

MMSE Equalization

• Time domain linear equalization

– MMSE Equalization

• A trade-off between signal corruption due to radio channel frequency selectivity

and noise/ interference.

{ }

2

) ( ) ( ˆ t s t s E − = ε

• Select a filter to minimize the mean squared error between the equalizer output

and the transmitted signal.

{ }

) ( ) (

– Linear equalization implemented as time discrete FIR filter. q p

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Time domain linear equalization

– Complexity of such time discrete equalizer grows relatively rapidly equalizer grows relatively rapidly with bandwidth of the sigal to be equalized:

w ) ( ˆ t s ) (t r

s

nT

– More wide band signal is subject t l ti l di h l

w ) (

to relatively more radio channel selectivity

}

n

r

– Requires the equalizer to have a larger span to be able to

1 − L

w

1

w

w

g p compensate it. s ˆ

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– More wideband signal leads to a correspondingly higher sampling rate for the recei ed signal

s

nT

rate for the received signal. Filter processing shall run with higher sampling rates.

w ) ( ˆ t s ) (t r

}

n

r – High complexity in the equalization and also calculating the inverse of channel output

1 − L

w

1

w

w

s ˆ

p autocorrelation matrix.

n

s

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SLIDE 3

Frequency domain equalization

• Reduce complexity of equalization.
• Carried out block wise in block size

N.

s

nT

n

r

n

s ˆ

k

S ˆ

k

R ) (t r

• Equalization as frequency domain

filtering.

{

• Block size N preferably selected as

for some integer n to allow for computational efficient

w

n

N 2 =

p radix-2 FFT/IFFT implementation of DFT/IDFT.

1 − N

w R

ˆ s

• For channels with extensive

frequency selectivity frequency domain equalization is less complex

1 − N

R

1

ˆ

− N

s

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complex.

Frequency domain equalization

• Time domain filtering

implements a time discrete implements a time discrete linear convolution.

s

nT

n

r

n

s ˆ

k

S ˆ

k

R ) (t r

• Frequency domain filtering

corresponds to circular convolution in the time domain

{

convolution in the time domain.

• First L-1 samples at the output

w

• f the frequency domain

equalizer will not be identical to the corresponding output of the

1 − N

w R

ˆ s

p g p time domain equalizer.

1 − N

R

1

ˆ

− N

s

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Frequency domain equalization

O l f t l t L 1 l

• Overlap of at least L-1 samples.
• Discard first L-1 samples at

Discard first L 1 samples at

• utput of the frequency domain

equalizer as they are also provided as the last part of the

1 − ≤ L

provided as the last part of the previously received / equalized block.

• Computational overhead or

higher receiver complexity.

CP Insertion Single carrier signal generation D/A Conversion

+ +

) (t x

Transmitter

higher receiver complexity.

• Cyclic prefix insertion.

generation Conversion

+

N Samples N+Ncp Samples

+ +

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Frequency domain equalization

• Cyclic prefix channel will seem to

the receiver as circular convolution

• ver a receiver processing block of

size N.

• Frequency domain taps can be

calculated directly.

) (t x

• Estimate of MMSE equalizer in

frequency domain.

• Overhead in power and bandwidth.
• Less overhead increase block

2 *

H W

K

=

size : channel shall be constant in duration of a block size ; upper limit for the block size

2

N H W

K K

+

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SLIDE 4

Other equalizer strategies

D i i f db k li ti (DFE)

• Decision feedback equalization (DFE)

– Previously detected symbols are fed back and used to cancel the contribution of the corresponding transmitted symbols to the overal signal

• corruption. Used in combination with time domain linear filtering. Also used

in combination with frequency domain linear equalization.

• Minimum Likelihood (ML) Detection or Maximum Likelihood Sequence

Estimator (MLSE)

Uses the entire received signal to decide on the most likely transmitted – Uses the entire received signal to decide on the most likely transmitted sequence, taking into account the impact of time dispersion on the signal. – Viterbi Algorithm – Used widely in 2G but too complex for 3G evolution (much wider transmission bandwidth, much more channel frequency selectivity, much higher sampling rate )

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Uplink FDMA with flexible bandwidth assignment assignment

• Share uplink radio resource using plink

intra-cell multiple access scheme.

cy

• High rate packet data transmission

Assign the entire system bandwidth to a terminal

Frequency

terminal.

• Burstiness of most packet data

services in most cases mobile

Frequency

terminals do not have anything to send in uplink. TDMA required.

• Just TDMA is not bandwidth efficient
• Just TDMA is not bandwidth efficient.
• Uplink is power limited.
• Allocating the entire system bandwidth to
• ne terminal is inefficient in terms of

bandwidth utilization.

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DFT Spread OFDM

• Small variations in the instantaneous power of the transmitted signal.
• Possibility of low complexity and high quality equalization in frequency domain.
• Possibility for FDMA with flexible bandwidth assignment.
• Uplink transmission scheme for LTE

Uplink transmission scheme for LTE.

Transmitter

CP OFDM (IDFT) D/A Conversion

) (t x

Size -M DFT

1 1

,..., ,

− M

a a a M N

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N

Basic principles

• Small variations in the instantaneous power of the transmitted signal.
• PAR distribution is independent of modulation in OFDM.
• Cubic metric: a measure of the additional back off required for a certain

Cubic metric: a measure of the additional back off required for a certain signal wave form relative to the back off needed for some refrence wave form.

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SLIDE 5

DFTS OFDM Reciever

• DFTS-OFDM spread signal is single carrier wideband signal which will be

corrupted in case of time dispersive channel. p p

• If channel is frequency selective over span of DFt, the inverse DFT at the

receiver will not be able to correctly reconstruct the original block of transmitted y g symbol.

• Need for an equalizer

q

Receiver

OFDM (IDFT) Size-M

Receiver

CP removal

1 1

ˆ ,..., ˆ , ˆ

− M

a a a

) (t r

n

r

Discard

(IDFT) Size M IDFT removal

Discard

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User multiplexing with DFTS-OFDM

• By dynamically adjusting the transmitter DFT size and the size of block
• f modulation symbols the nominal bandwidth of the DFTS OFDM
• f modulation symbols, the nominal bandwidth of the DFTS-OFDM

signal can be dynamically adjusted.

• By shifting the IDFT inputs to which the DFT outputs are mapped , the

exact frequency domain position of the signal to be transmitted can be adjusted.

• Allows for uplink FDMA with flexible bandwidth assignments

Allows for uplink FDMA with flexible bandwidth assignments.

1 1

ˆ ,..., ˆ , ˆ

− M

a a a

) (t r

n

r

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User multiplexing with DFTS-OFDM

1

M

1

M

2

M M

2

M

2 1

M M >

2 1

M M = 3/26/2009 3G Evolution - HSPA and LTE for Mobile Broadband 19

Localized and distributed DFTS-OFDM

Size-M DFT

}

DFT OFDM

5 = M

Localized DFTS

}

} } }

5 = M

OFDM

}

}

}

5 M

}

}

Distributed transmission Distributed transmission Localized Localized t i i

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transmission transmission

SLIDE 6

Questions: Thanks for your attention :

Q ti ? Questions?

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