3 High data rate in interference limited senarios High data rates - - PowerPoint PPT Presentation

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Department of Electrical and Information Technology

3G Evolution

Chapter:

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3

High data rates in mobile communication

1

Payam Amani Payam.Amani@eit.lth.se

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Outline

• High data rates: Fundamental constraints

– High data rate in interference limited senarios

– High data rates in noise limitted senarios

• Higher data rates within a limited bandwidth : Higher order

modulation

– Higher order modulation in combination with channel coding

• Variations in instantaneous transmit power
• Wider bandwidth including multi-carrier transmission

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Rate control or power control [stefan Parkval]

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Rate control [stefan Parkval]

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Higher data rates

• Higher end user data rates compared to first 3G standards:
• ne of the main targets for LTE
• What do we mean by high data rates?

– Higher peak data rates – Higher data rates over the entire cell area – Higher data rates on the cell edge

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Higher data rates: Fundamental constraints

• Shannon channel capacity:

– Channel only impared by additive white Gaussian noise – Main factors limiting the channel capacity:

• Available signal power to

noise power ratio

• Bandwidth

⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⋅ = N S BW C 1 log2

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⋅ ⋅ + ⋅ = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⋅ = ≤ BW N R E BW N S BW C R

b 2 2

1 log 1 log

BW R = γ ) 1 ( log

2

N Eb γ γ + ≤

γ

γ

1 2 min − = ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ ≥ N E N E

b b

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Higher data rates: Fundamental constraints

– Information rate cannot exceed channel capacity. – Radio link bandwidth utilization . – Lower bound on the required received energy per information bit, normalized to the noise power density for a given bandwidth utilization. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⋅ = N S BW C 1 log2

⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⋅ ⋅ + ⋅ = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⋅ = ≤ BW N R E BW N S BW C R

b 2 2

1 log 1 log

BW R = γ ) 1 ( log

2

N Eb γ γ + ≤

γ

γ

1 2 min − = ⎭ ⎬ ⎫ ⎩ ⎨ ⎧ ≥ N E N E

b b

γ

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Minimum required Eb/N0 at receiver as a function of bandwidth utilization

• Bandwidth utilization significantly smaller than one :

– Relatively constant minimum required Eb/N0 regardless of bandwidth utilization.

• Bandwidth utilization larger than one (constant N0 and

bandwidth):

– Eb/N0 increases rapidly by bandwidth utilization. – Increase in data rate: much larger increase in the minimum required signal power at receiver.

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Minimum required Eb/N0 at receiver as a function of bandwidth utilization

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10

• 1

10 10

1

• 5

5 10 15 20 25 Bandwidth Utilization γ Minimum required Eb/N0 (dB) Minimum required Eb/N0 at the receiver as a function of bandwidth utilization Power Limited Region Bandwidth Limited Region

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High data rates in noise-limited senarios

• When noise is the main source of radio-link impairment:

– Increase of achievable data rates in a given bandwidth requires at least the same relative increase of signal power. – Low bandwidth utilization:

• Power limited operation : increase in the available bandwidth does not

substantially impact what received signal power is required for a certain data rate.

– High bandwidth utilization

• Bandwidth limited operation : Furthere increase in data rate requires a

much larger relative increase in the receive signal power unless the bandwidth is increased in proportion to the increase in the data rate.

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High data rates in noise-limited senarios

• To make use of the available SNR

efficiently; transmission bandwidth should at least be the same order as the data rate to be provided.

• By reducing the range in theory we

can provide higher data rates.

• Requiring bandwidth efficiency

greater or equal to one leads to significant cell range reduction.

• High data rates only available for

centre of the cell.

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High data rates in noise-limited senarios

• How to increase overal received

signal power for a given transmit signal power?

– Add antennas to the receiver side (Receiver antenna diversity)+ proper combining

• Can increase the signal to noise

ratio after combining in proportion to the number of antennas.

• Allows for larger data rates for the

same cell range.

– Add antennas to the transmitter (typically base station); focus a given total transmit power in the direction of the receiver (beam forming).

• Can increase signal power and allow

for higher data rates for the same cell range.

Proper combination Proper combination Proper combination Proper combination

SIMO-MISO SIMO-MISO MIMO

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High data rates in noise-limited senarios

– SIMO and MISO efficiently provide higher data rates up to a certain level (saturation), i.e as long as the data rates are power limited rather than bandwidth limited. – The mentioned saturation can be avoided by means of spatial multiplexing or MIMO. – Details in chapter 6. – Alternatively one can reduce noise power by designing a more advanced receiver with a smaller noise figure.

Proper combination Proper combination Proper combination Proper combination

SIMO-MISO SIMO-MISO MIMO

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Receive diversity SIMO case

• Addition of receiver antenna yields only a logarithmic increase in

channel capasityin SIMO channels. Knowledge of channel information at the transmitter provides no capacity benefits.

R i F s

M i h N E C ,..., 2 , 1 , 1 1 log

2 2 2

= = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + = h

⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

R s SIMO

M N E C

2 1

log ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

R s SIMO

M N E C

2 1

log

SIMO

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Transmit diversity – MISO case

• In case of abcense of channel knowledge in transmitter there is no

benefit for MISO channels over SISO channels in sence of capacity. However in fading environment there is some benefits for MISO over

• SISO. If the channel is known to the transmitter the capacity is similar

to the SIMO case. ) 1 ( log ) ,..., 2 , 1 ( 1

2 2

N E C M i h if

s MISO T i

+ = = =

⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

2 2 1

log

F T s MISO

M N E C h ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

2 2 1

log

F T s MISO

M N E C h ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

2 2 1

log

F s MISO

N E C h Channel known to transmitter Channel unknown to transmitter MISO ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

2 2 1

log

F T s MISO

M N E C h

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MIMO system

• MIMO system :
• Channel has no prefered direction and is completely unknown to the

transmitter Rss= IMT .

• Signals are independent and equi-powered at the transmit antennas.

MIMO

2 2 2 , 2

, 1 , ) 1 ( log M H H M M M N E M C

F j i R T s MIMO

= = = = + =

Orthogonal channels maximize capacity ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ + =

H T s M MIMO

HH M N E I C

R

2 det

log

n Hs y + =

T s

M E

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Higher data rates in interference limited senarios

• Dominant source of link impairment

in mobile communications is usually inter-cell or intra-cell interference.

• Similar results to the noise limited

senarios.

• Interference is usuall structured and

can be suppressed by means of spatial processing.

• Maximum achievable data rates in a

given bandwidth is limited by available SIR.

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Higher data rates in interference limited senarios

• To achieve higher data rates:

– Reduce cell size, reduce number of users, reduce the

• veral trafic in the cell, reduce

the interference level in the cell. – Proper combination of the received signals in SIMO case increases SIR after antenna combining. – Beam forming in MISO case will focus the transmit power in the direction of the target receiver thus reduces the interference to

• ther radio links, thus improves
• veral SIR in the system.

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Higher data rates within a limited bandwidth Higher order modulation

• Providing data rates larger than the available bandwidth is

fundamentally inefficient.

• Bandwidth is a scares and expensive resource.
• In some senarios in mobile communication high SNR and

high SIR can be made available (users near centre of cell and small cells with low traffic).

• LTE shall support providing high data rates in a limited

bandwidth for users in such conditions.

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Signal Constellations for Higher order Modulations

• Increasing the modulation alphabet to send more bits per modulation

symbol.

• QPSK in first 3G systems.
• Bandwidth of transmitted symbol is in principle independent of order of

modulation and mainly depends on modulation rate.

• Bandwidth utilization of 16QAM and 64QAM are 2 and 3 times of
• QPSK. (less robust to noise and interference)

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An application:

IPTV over WiMAX: Key Success Factors, challenges, and Solutions Communications Magazine, IEEE. vol. 45, pp.87-93, August 2007.

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Higher order modulation in combination with channel coding

• In general, 16QAM and 64QAM require higher received

Eb/N0 for a given BER compared to QPSK.

• By means of channel coding sometimes 16QAM or 64QAM

require less Eb/N0 for a given BER compared to QPSK. (in cases that the target bandwidth utilization implies that with lower order modulation no or very little coding gain can be achieved.)

• For a given SNR/SIR a certain combination of modulation

scheme and channel coding rate is optimal in sence of providing highest bandwidth utilization.

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Variations in instantaneous transmit power

• In case of higher modulation
• rder + coding we will have

larger variations in amplitude of the modulated signal and thus higher instantaneous power peaks.

• Increase the dynamical range of

the power amplifier.

• Reduction in power amplifier

efficiency and more expensive to build.

• Reduction of mean transmit

power (smaller cells)

• More suitable for downlink than

uplink.

Same average power

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Wider bandwidth including multi-carrier transmission:

• Transmission bandwidth at least as large as the required data rates is

needed to provide high data rates as efficiently as possible in terms of required SNR/SIR.

• Providing high data rates + good coverage is one of the main aims of

LTE support for wider transmission bandwidth. – Spectrum is scarce and expensive – Complex radio equipment (sampling rates, DA and AD complexity and power consumption and signal processing complexity, more complex and expensive RF parts.)

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Wider bandwidth including multi-carrier transmission:

– Signal corruption due to time dispersion – Multipath propagation – Time dispersive channel non- constant frequency response of the channel (frequency selectivity). – Larger impact for wideband transmissions.

Frequency A snapshot of Channel frequency response

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Wider bandwidth including multi-carrier transmission:

– Dependent on propagation

• environment. Small cells and

rural areas have less frequency selectivity. – Receiver side equalization used to conteract signal corruption due to radio- channel frequency

• selectivity. Shown good

results till 5MHz transmission bandwidth. – Higher bandwidth : very complicated equalizers.

Frequency A snapshot of Channel frequency response

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Wider bandwidth:

• Options:

– Usage of less optimal equalization worse radio link performance – Use special transmission schemes and signal designs :

• Multi-carrier transmission: transmitting overal wider-band signal as

several more narrowband frequency- multiplexed signals such as

• OFDM. More discussed in chapter 4.
• Use of specific wider-band single carrier schemes such as DFTS-

OFDM more discussed in chapter 5.

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Multicarrier transmission:

• Provides very smooth evolution in terms of both radio equipment and

spectrum of an existing radio- access technology to wider transmission bandwidth and corresponding possibility for higher data rates especially for downlink.

• Backward compatibility.

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Theoretical WCDMA spectrum

• Spectrum shaping increases the spectrum from its theoretical value.
• Spectrum widenning due to the transmitter imperfections.
• Avoid inter-subcarrier interference (can be accepted to some level).
• Large variations in the instantaneous transmit power.
• Inefficient power amplifier.
• More suitable for downlink.

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Refrences :

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Questions: Thanks for your attention :

Questions?