S-72.3260 2008 L1 - TKK Tietoli · PDF filej S-72.3260 Radio Resource Management Methods 3 op TKK Communications Laboratory 6 ... Minimum signal-to-noise-ratio ... the jamming

1

S-72.3260 Radio Resource Management Methods 3 cr

LecturerProf. Riku JänttiE-mail: [email protected]

Tel. +358 9 451 2353E219

AssistantLic. Tech. Mohammed Al Rawi

E-mail: [email protected]. +358 9 451 4905

316 (OK7)

http://tll.tkk.fi/en/Studies/S-72.3260/

S-72.3260 Radio Resource Management Methods 3 op TKK Communications Laboratory 2

Tentative schedule

Week Day Lectures Exercises12 Tue 1 Introduction

Wed 2 Handovers, dynamic channel allocationThu

13 TueWedThu 3 Power control

14 Tue 4 Admission and load controlWed 5 Schedulign principlesThu 1 Handovers, dynamic channel allocation

15 Tue 6 Slow schedulingWed 2 Power controlThu 3 Admission and load control

16 Tue 7 Channel adaptive schedulingWed 4 Slow schedulingThu 8 Multicarrier systems: sub-channel allocation, bit loading

17 Tue 9 Mesh networks: topology control Wed 5 Opportunistic schedulingThu 6 Multi carrier & mesh systems

18 Tue 7 Old exam

2


Course material• Requirements

– J. Zander and S.-L. Kim, Radio Resource Management for Wireless Networks, Artech House, 2001. Chapters 1 – 7

– Lecture notes• Slides (available in internet)• Notes (paper copy)

– Tutorial notes • Recommended reading

– Selected articles


Radio Resource Management• Radio spectrum is a scarce resource that should be

utilized as efficiently as possible. Consequently, the spectrum must be shard among many users.

• In practice, all sharing (multiple access) techniques causes interference among the users.

• The objective of Radio Resource Management is to control the division of resources (power, time slots, frequency channels, spreading codes etc.) among the users such that – The quality of service demands of the users are

satisfied– The spectral efficiency is maximized– Interference is mitigated– Operator revenue is maximized

3


Radio Resource ManagementCellular network

To each active terminal assign• Base station• Waveform (“Channel”)• Transmitter powersuch that link quality & power constraints are satisfied for as many terminals as possible

i

j


Radio Resource ManagementAd hoc / mesh network

• Define the network topology (connectivity, routers)• Find routes between source and destination• Assign waveforms to links (“channel”)

Such that the end-to-end quality of service constraints are met.

4

Wireless access technologies


Wireless access technologies• Frequency division multiple access (FDMA)• Time division multiple access (TDMA)• Frequency hopping (FH)• Direct sequence spread spectrum (DSSS)• Code division multiple access (CDMA)• Orthogonal frequency division multiplexing (OFDM)• Spatial Division Multiple Access (SDMA)

5


F/TDMA• FDMA: Each user has a dedicated frequency channel• TDMA: Each user has a dedicated time slot• In practice, combination of these two are needed

– A single frequency band is divided among the users in time.– Uplink and donwlink is separated either in frequency (FDD:

Frequency division duplex) or in time (TDD: Time division duplex)– Adjacent cells (coverage areas of base station antennas) are

separated in frequency in order to decrease co-channel interference.

– Cells utilizing the same frequency are placed in distance D fromeach other which depends the number of frequency channels available for the network operator.

– The greater the reuse-distance D is, the less the cells interfere with each other, but the less bandwidth is available per cell.


F/TDMA

• Each cell is using a dedicated frequency channel and each user in a cell has a dedicated time slot.

• Utilized e.g. in GSM• Example

– Consider downlink of cell S– Other cells using the same frequency

cause co-channel interference:

DS

( )( ) 2

0

3

31 366 3

m m

S m

k

D R kR k

R k Nγ

−

−

=

=

= ≈+

R

Minimum signal-to-noise-ratioif all the interfering base stationstransmit with fixed power

Frequency reuse factor (cluster size)

6


2

Timing• Different users have different propagation times• In multi-path fading channels, also the delay spread

must be taken into account

1 23 km

12 km

≈ 10 μs≈40 μs

1 2

≈ 60 μs

1 22Time slots

Guard timeSlot 1 Slot 2


Frequency Hopping• Slow Frequency Hopping

– The utilized frequency changes from slot-to-slot based on predefined hopping sequence

– Averages the interference in frequency domain

1Cell1

3 4t

f

1Cell2

2 3 4t

f

12 3

4t

f

12

34

t

f

2

TDMA SFH-TDMA

7


F/TDMA RRM challenges• Admission and handover control

– Admission control based on available channels– Reservation of guard channels for handoff calls– …

• Dynamic channel allocation– Fixed channel allocation is based on conservative

estimates on the interference and traffic load leading to trunking losses (the number of channels per cell is low)

– By controlling the reuse of channels based on instantaneous traffic load and interference capacity can be enhanced.

• Slow power control– Fixed transmit power can cause unnecessary large

interference leading to long reuse distance.– By controlling the power dynamically to meet the SINR

requirements, interference can be mitigated and reuse distance decreased


DSSS• Direct sequence spread spectrum• Spreading

– Single symbol is represented by a pulse of length Tb

– Before transmission the pulse is multiplied with a spreading sequence consisted of W shorter pulses chips of length Tc.

Data symbols{-1,1}

Spreading code

noise

kT

k kT

b

b( +

z1)

+1

-1

Equivalent lowpass model in BPSK caseCorrelaror receiverChannel

8


DSSS

Power spectrum of the signalafter spreading

Power spectrum of the signalbefore spreading

Jamming signal

Despreading concentrates the symbolEnergy into narrow band and dividesthe jamming signal energy to wide band


DSSS

-10 -8 -6 -4 -2 0 2 4 6 8 1010-6

10-5

10-4

10-3

10-2

10-1

100

fTb

Pow

er S

pect

ra

Before spreadingAfter spreading

9


DSSS

• Narrowband vs Broadband.|H(f)|

f

|H(f)|

f

Power spectrumof the channel

Narrow bandsignal is attenuated

Wideband signal energyis divided into larger band.Attenuation on narrow bandhas very little effect on thesignal.


CDMA• CDMA (Code Division Multiple Access) different users

are separated from each other using a spreading code:– DS-CDMA (Direct Sequence) Spreading is done in

time domain.– FH-CDMA (fast Frequency Hopping) Spreading is

done in frequency domain.• Frequency reuse factor k is one. All cells use the same

frequency band.• All users transmit simultaneously.

t

f

KoodiCode words

10


DS-CDMA• Multi-user interference appears as noise. • Bit error probability on a Gaussian channel depends on

the signal-to-noise+interference ratio (SINR)

• Capacity is interference limited. – With fixed bit energy Eb, the bit error rate grows as

the number of interfering users increase,– Alternatively for fixed bit error probability, the

required bit energy grows as the number of users incerease.

– Pole capacity: Number of users for which Eb →∞

{ } ( )0 0

Pr error 1 12 2

b b

b b

E WEQ Q Q SINRM E N ME NW

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟

= = =⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟+ +⎜ ⎟⎜ ⎟

⎝ ⎠⎝ ⎠


CDMA RRM challenges• Fast power control

– Fast fading causes the received signal to fluctuate.• Smaller SINR-margin is needed if the fading is

compensated by using fast power control.– Near-far problem: Unless power is controlled in a cell,

power received from users close to the base station can be several orders of magnitude larger than the power from users on the edge of the cell. Transmit power control is needed to balance the power levels at the receiver

• Admission and load control based on interference power– Admission and load control should keep the transmit

power control loop stable• Rate control

– In 3G CDMA systems, the data rate of the users can be controlled dynamically based on the traffic demand and available resources (in terms of interference level)

11


OFDM• Orthogonal Frequency Division Multiplexing

– Symbol of length T is modulated using e.g. BPSK.– T is relatively large => Narrow band signal.– Ns (even) symbols are transmitted in parallel such

that the carrier frequencies are 1/T apart from each other.

12

22

1( ) exp 2

s

ss

N

NkN

k

t ks t I i tT TT

π−

+=−

⎛ ⎞⎛ ⎞ ⎛ ⎞⎜ ⎟= Π⎜ ⎟ ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠⎜ ⎟

⎝ ⎠∑

2 12

sN tiTe

π ⎛ ⎞−⎜ ⎟⎝ ⎠

22

sNi t

eπ ⎛ ⎞−⎜ ⎟

⎝ ⎠

∑{ }nI

PSKmodulator

Serial to parallel

2 22

sN tiTe

π ⎛ ⎞−⎜ ⎟

⎝ ⎠

Inverse Fouriertransform of Nsconsecutive symbols

1I

2I

sNI


OFDMSpectrum of OFDM modulated signal

-15 -10 -5 0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Carrier

8sN =

12


OFDM44 Hz

s

c

Nf

==

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-3

-2

-1

0

1

2

3

4

OFDM modulated signal

Subcarriers


OFDM• Adjacent channels are orthogonal to each other (do not

interfere with each other)

• Receiver

( )( )( )0

1 1 1exp 2 exp 2 exp 2 ,

1 exp 2 1 02 ( )

Tt k t l k li t i t i t dt k lT T T T T TT T

i k li k l

π π π

ππ

⎛ ⎞ −⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ ⎛ ⎞Π Π = ≠⎜ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎝ ⎠

= − − =−

∫

2 12

sN tiTe

π ⎛ ⎞− −⎜ ⎟⎝ ⎠

22

sNi t

eπ ⎛ ⎞− −⎜ ⎟

⎝ ⎠

2 22

sN tiTe

π ⎛ ⎞− −⎜ ⎟

⎝ ⎠

∫

∫

∫

T

1I

2I

sNI

13


OFDM• OFDM modulator can be realized using Inverse Fast

Fourier Transform (IDFT) and Digital-to-Analog D/A converter.

• Receiver can be realized using Analog-to-Digital circuit and Fast Fourier Transform (FFT)

• Digital signal processing is fast and cheap. No need for separate oscillators for every sub-channel.


OFDM• The advantage of the OFDM

is that the narrow band sub-channels are more robust against multi-path propagation (channel bandwidth < coherence bandwidth) compared to the case in which the data would have been transmitted directly using single wideband channel.

|H(f;t)|

f

Pow

ersp

ectru

mof

the

chan

nel

14


OFDM• Intersymbol interference ICI caused by multipath propagation

can be removed partly by using a cyclic prefix. • Channel equalization is much simpler than in wideband case.

(Smaller number of taps is needed)

End of symbolDelayed replicaof the transmittedsignal.

Cyclic prefix Integration time of FFT(Receiver only considers this part of the signal)

Phase change caused by change of bit

(Nonlinear distortion)

Delayed replica appears as a phase shift


OFDMA• Orthogonal Frequency Division Multiple Access

– Different users are multiplexed together

IFFT

User 1

User 2 D/A

User N

Seria

l to

para

llel

15


OFCDMA• Orthogonal Frequency and Code Division Multiplexing

OFCDM/ Multi-carrier CDMA (MC-CDMA)– Combination of OFDM and CDMA– High spectral efficiency– Robustness against multi-path propagation – Multiple access flexibility, good robustness against

inter-cell interference.


OFDMA RRM Challenges• Sub-band and transmit power allocation

– Sub-bands can be allocated to the users based on their channel responses. Allocate channel to the user who benefits the most from it.

– Transmit power could be allocated to maximize the system capacity by allocating power inversely proportional to the channel gain (waterfilling)

• Time-frequency domain scheduling– Packet scheduling allocates time-frequency chunks to

the users • Curse of dimensionality

– It is not possible to have individual channel state feedback from all sub-carriers. Hence, the carriers need to be grouped and joint feedback for a group is generated.

16


SDMA• Multi-antenna (MIMO, Multiple Input –

Multiple Output) system, in which multiple users share the same frequency band.

• Data streams are separated using spatial processing.

SDMA capacity region (MMSE-SIC receiver)

2 22

1 10

2

0

1log 1

1log 1 , 1,2

i i ii i

i i i

r pN

r p iN

= =

⎛ ⎞≤ +⎜ ⎟

⎝ ⎠⎛ ⎞

≤ + =⎜ ⎟⎝ ⎠

∑ ∑ h

h1r

2r


SMDMA RRM Challenges• Curse of dimensionality

– Channel state for each user and each sub-carrier is nr x nt matrix where nr is the number of receive antennas and nt is the number of transmit antennas

– In TDD mode, the transmitter can have channel state information due to channel reciprocity. However, the interference seen by the receivers cannot be known.

– Optimization of feedback information is a challenging problem. That is finding the tradeoff between capacity gain and signalingcost in terms of number of required feedback bits.

• Power allocation problem– How should the transmit power be divided among the various data

streams (among antennas and sub-carriers) such that quality of service constraints are met and capacity is maximized.

• Schedulign problem– How to allocate the users on the various sub-carriers such that

their transmissions can be orthogonalized. Now up to nt data streams could be served simultaneously on a given sub-carrier.

17


Relay and mesh networks• In traditional wireless

networks the terminals are connected to access point through one hop.

• In mesh networks, there can be multiple hops between source and destination.

• The relay nodes can be either fixed or mobile.

R32 relay

relayrelay

relay

relayrelay

BS/AP


RRM for Relay and mesh networks• Topology control

– Changing the transmit power controls the connectivity of the nodes as well as data rates of the links

• Routing and load balancing– There can be multiple paths between source and

destination. Selection of which path to use affects the network load distribution

• Link scheduling / frequency assignment– In single frequency network, a node cannot transmit

and receive simultaneously. Also links nearby can interfere with each other so their transmission must be coordinated.

18


Integrated reconfigurable networks• Paradigm of the traditional cellular

– Anytime anywhere (with fixed [low] quality)– Always connected– Single operator, single access technology networks

with global roaming• Paradigm for the future cellular

– Anytime anywhere (with varying service quality)– Always Best Connected (ABC)– Multi-access networks, seamless internetworking

with IP based core networks– Integration and reconfiguration– Cognitive interference aware radios


Integrated reconfigurable networks• Multi-access

– Fixed network (ISP)– GSM/GPRS– WCDMA– WiFi WLAN– WiMax WMAN– Etc.

• Multi-hop– Mesh networks– Ad hoc access

• Multi-operator– Licensed bands– Unlicensed bands– Spectrum pooling– Dynamic spectrum assignment

PAN 1

IP core

CorporateIntranetISP

PAN1

RAN1

RANn

PAN2 PAN

3

RAN1

RANn

Radio accessOperator 2

19


RRM challenges for reconfigurable wireless

• Dynamic spectrum allocation– Co-existence strategies– Resource allocation between competing networks– Spectrum sharing & trading

• End-to-end QoS management– Dynamic network selection– User terminal driven RRM decisions

Interference

20


Interference• Intersymbol interference (ISI)

– In frequency selective fast fading channel, the received signal contain the transmitted symbol and delayed replicas of previously transmitted symbols. This form of interference can be mitigated e.g. by using Viterbi equalizer.

• Co-channel interference– In F/TDMA system interference occurs due to reuse

of the same channel in other cells.

111

1 1 1

111

D

√3DTDi Di+1

i i+1


Interference• Adjacent frequency channels are not completely orthogonal

due to nonlinear distortions caused e.g. by the power amplifier. Also the Doppler effect causes frequency shift that needs to be taken into account.

• A guard band is needed to keep the adjacent channel interference at tolerable level.

• In WCDMA: Adjacent Channel Leakage Ratio (ACLR): Uplink: 33 dB and 43 dB, Downlink: 45 dB and 50 dB

3.84 Mhz

5 Mhz

n200 kHz

Guard band

P

Nf 1Nf + 2Nf +

1ACKR 2ACKRfN center frequency of channel N

21


Interference• In CDMA systems the interference can be divide into

two parts– Intracell interference

• Interference due to simultaneous transmissions taking place in the same cell

– Intercell interference• Interference due to simultaneous transmissions in

neighboring cells– The ratio between intercell and intracell interference

depends on overlap between the cells and the utilized handover strategy (hard/soft). Typically the intracell interference is dominating.


Interference• Downlink interference

– Interference is generated by the co-channel base stations.

– The interference power level generated by a cell depends on its aggregate traffic. => Interference power is slowly changing and can be predicted.

22


Interference• Uplink interference

– In F/TDMA and OFDMA systems the uplink interference is generated by single mobile in each co-channel cell. The interference is highly random, since it depends on the mobile positions and scheduling decisions of the individual cells.

– In CDMA case, the interference is generated by the aggregate of the users making it less random. However, the smaller the traffic the more unpredictable is the interference.


Interference• In TDD systems it could happen that different cells have

different frame lengths for uplink and downlink transmissions or they are out of sync. Hence, there can be interference between uplink and downlink transmissions of different cells. This can cause very high interference, since transmitter in one cell can be in the close proximity of a receiver of another cell. Downlink transmission is more sensitive to uplink interference than downlink is to uplink.

• In ad hoc networks, the interference is generated by simultaneously active radio links. Here too the transmitter of one link can be in close proximity of another. This situation isusually handled by scheduling the transmission of the interfering links e.g. by using CSMA/CA random access protocol.

23


Signal-to-interference+noise ratio (SINR)

• Signal-to-interference+noise ratio is a quality metric for the quality of the received signal.

• It is defined as the ratio between received signal power S and received interference power I + receiver noise power σ2

SINR = S/(I+σ2)

• Another closely related quality metric is the received bit energy Eb to interference + noise spectral density I0 + N0

• Bit energy S=r Eb where r is the bit rate • Spectral density of the noise and interference can be obtained

by dividing them by the bandwidth W

Eb/(I0+N0) = W/r·SINR

• In SINR equation, we have σ2=N0W and I = I0W


Quality of Service• Channel quality in terms of bit error rate (BER), block

error rate (BLER) and frame error rate (FER) are monotonically decreasing or at least non-increasing functions of SINR.

• The channel capacity (in terms of bit rate) is typically non-decreasing function of SINR.

Anders Furuskär, Radio Resource Sharing and Bearer Service Allocation for Multi-Bearer Service, Multi-Access Wireless Networks, PhD Thesis, TRITA-S3-RST-0302, Radio Communication Systems, Dept of S3, KTH, April 2003.

R code rate

24


Interference modeling • Consider a discrete time equivalent low pass model for a

narrowband signal• The signal is sampled with sampling time higher than the

symbol duration– A denotes the set of active transmitters– bi denotes the complex symbol transmitted by user i

E{bi}=0, E{|bi|}=1– wi =[wi1

* wi2* … wiW

*]* is the sampled waveform. – W is the number of samples per symbol– Cross correlation between waveforms wi*wj

=ρij

– pi denotes the transmit power of the signal– hij denotes the channel response in a channel between user j and

receiver i– zi =[zi1

* zi2* … ziW

*]* denotes the white thermal noise vector at receiver i zik~N(0,σ2)

E{zi}=0, E{zizi*}=Iσ2/W


Interference modeling• Transmitted signal

• Received signal at receiver i

• The interference signal at the receiver is given by

i i i ip b=s w

noisesignal

interference

i ij j j j i ii i i i ij j j j ij A j A

j i

h p b h p b h p b∈ ∈

≠

= + = + +∑ ∑r w z w w z

i ij j j jj Aj i

h p b∈≠

= ∑x w { } { }

{ } 2*

i ij j j jj Aj i

i i ij jj Aj i

E h p E b

E h p

∈≠

∈≠

= =

=

∑

∑

x w 0

x x

25


Interference modeling• Matched filter output

* *

* * *

noisesignal

interference

i ii i i

i ii ii i i ii ij j ij j ii ij Aj i

r h

r h h p b h h p b h zρ∈≠

=

= + +∑

w r


SINR• Signal-to-interference+noise (power) ratio SINR:

– Squared cross-correlation θij=E{ρij2 }

– Link gain gij=|hij|2

• General form of SINR

{ }{ } { }

2* 4 2

2 2 22 22 2 2 2* *

ii ii i iii i ii i

i

ii ij ij j ii ij ij jii ij ij j j ii i j ì j ì

j ì

E h h p b h p h p

h h E p h h E pE h h p b h z ρ σ ρ σρ≠ ≠

≠

Γ = = =⎧ ⎫⎪ ⎪ + ++⎨ ⎬⎪ ⎪⎩ ⎭

∑ ∑∑

2

ii ii

ij ij jj Aj i

g p

g pθ σ∈≠

Γ =+∑

26


SINR• Given SINR requirement γi, we can rewrite the SINR

equation as follows

• Define – a normalized link gain matrix H=[hij]

hij=gijθij/gii, i≠ j and hii=0 – normalized noise vector η=[ηi], ηi=γiσ2/gii

• The SINR equation can be written in matrix form as follows:

p=Hp+η

2ij ij

i i jj ì ii ii

gp p

g gθ σγ

≠

⎛ ⎞= +⎜ ⎟

⎝ ⎠∑


Controlling the interference• Transmit power control:

– Find p such that

• Rate control (link adaptation). – Find ri such that Eb/(I0+N0) can be supported

2

ii ii i

ij ij jj Aj i

g p

g pγ

θ σ∈≠

Γ = ≥ ⇔ ≥ ++∑

p Hp η

0 0

tgt

bi

i

EWr I N

γ⎛ ⎞

= ⎜ ⎟+⎝ ⎠i iγΓ ≥

27


Controlling the interference• Admission and load control

– Control the set of active users A such that the power such that

• Packet scheduling– Control A(t) and ri(t) as a function of time such

and E{ri(t)}>Rreq

i iγΓ ≥

( ) ( )i it i A tγΓ ≥ ∈


Time scales of RRM• Slow mobility, fast RRM

• Fast mobility, fast RRM Time scale of interest

Time scale of interest

Channel stays essentially constant during the observationtime. RRM algorithms have time to converge beforethe channel state changes.

Channel stays essentially constant during theone control interval (e.g. TTI). Channel is (slowly) changing, but the channel processcan be assumed to be stationary

Mean

28


Time scales of RRM• Fast mobility, slow RRM

Channel obtains variations are very fast compared to the control interval. Channel is averaged out

– RRM is slow compared to the fading but not the path loss change due to mobility: Channel is stationary

– RRM is slow compared to fading and mobility: Channel is non-stationary

– RRM is very slow: Mobile position gets averaged out


Conclusions• All practical multiple access schemes cause multi-user

interference (MUD) to appear• Signal-to-noise+interference ratio is one of the key

parameters to model the impact of the MUD on the system performance.

• Consequently, many RRM schemes try to control the SINR by controlling the rate (required SINR), transmit power, number of simultaneously active users etc.

Documents

S-72.3260 2008 L1 - TKK Tietoli · PDF filej S-72.3260 Radio Resource Management Methods 3 op TKK Communications Laboratory 6 ... Minimum signal-to-noise-ratio ... the jamming