e fficient cooperative diversity schemes and radio resource allocation for ieee 802.16j

Efficient Cooperative Diversity Schemes and

Radio Resource Allocation for IEEE 802.16j

Department of Electronic Systems, Aalborg University, DenmarkDepartment of Systems and Computer Engineering, Carleton University, Canada

Speaker: Chan-Ying Lien

WCNC 2008

WCNC 2008

Basak Can, Halim Yanikomeroglu, Furuzan Atay Onat, Elisabeth De Carvalho and Hiroyuki Yomo

WCNC 2008

WCNC 2008

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OutlineOutline

• Introduction• System Model• Cooperative Diversity Schemes• Scheduling And Radio Resource Allocation

For Multi-hop Cellular Networks• The Frame Structure• Performance Evaluation• Conclusions And Future Works

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IntroductionIntroduction

BSBSBSBS

RSRSRSRS

MSMSMSMS

• In IEEE 802.16j:

MSMSMSMS MSMSMSMS MSMSMSMS MSMSMSMS MSMSMSMS

RSRSRSRS

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IntroductionIntroduction

BSBSBSBS

MSMSMSMS

• In IEEE 802.16j:

RSRSRSRS

Source

Relay

Destination

First phaseSR

SD

Second phaseRD

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System ModelSystem Model

• IEEE 802.16j based two-hop cellular network• A single cell with

– multiple fixed relays– multiple users

• low mobility users

• Channel gains of each sub-channel remain unchanged during one frame– consists of a certain number of OFDM symbols

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• AMC– The considered modulation modes

• BPSK, QPSK, 16-QAM and 64-QAM

– The considered FEC• 1/2, 2/3, 3/4, 5/6, 7/8 and 1

• Scheduling– A modified version of Proportional Fair Scheduling

(PFS)

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• j {1, 2, ..., J}∈ – denotes the sub-channel index in the frequency

domain

• u {1, 2, ...,U}∈– denotes the MS index

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• The end-to-end throughput with AMC is given by

ρ(γ) = R(γ)(1 - pe(γ))

• SNR γ• R(γ) represents the nominal rate (in b/s/Hz) of the selected AMC mo

de based on γ

• pe(γ) represents the block error rate with the selected AMC mode

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• R(γ) = M×η– AMC mode is 16-QAM M=4– Coding rate η = ½

• R(γ) = 4 * (1/2) = 4/2 (b/s/Hz)

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• Define the coverage area with radius r• The user throughput is above 0.5 b/s/Hz with pro

bability p

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Cooperative Diversity SchemesCooperative Diversity Schemes

• A. Cooperative Transmit Diversity–1– First phase

• MS and RS listen to the transmission of the BS

– Second phase• both BS and RS transmit simultaneously to the MS

– The post–processing instantaneous SNR at each sub-channel j achieved after space time decoding at the MS

same AMC mode is used

BSBSBSBS

MSMSMSMS

RSRSRSRS

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• A. Cooperative Transmit Diversity–1– With such link adaptation at a sub-channel j, the end-to-end

throughput per channel use is given by

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• B. Cooperative Transmit Diversity–2– The cooperative diversity–2 is a subset of cooperative

diversity–1– The main difference is that, the MS does not exploit th

e signal received during the first phase

– The AMC mode to be used in the first phase is chosen based on γSR,j for each sub-channel j BSBSBSBS

MSMSMSMS

RSRSRSRS

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• B. Cooperative Transmit Diversity–2– For the second phase, the AMC mode for each sub-channel j is

chosen based on the post–processing SNR given by

– the end-to-end throughput per channel use is given by

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• C. Cooperative Receive Diversity– In the first phase

• the source transmits at a particular AMC mode while both the relay and the destination receive

– In the second phase• the relay repeats with the same AMC mode and the BS rema

ins silent

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• C. Cooperative Receive Diversity– A potentially higher multiplexing loss due to the need f

or identical AMC modes and hence equal–duration phases

– Hence, cooperative receive diversity cannot outperform cooperative transmit diversity–2.

BSBSBSBS

MSMSMSMS

RSRSRSRS

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BSBSBSBS

MSMSMSMS

RSRSRSRS


• D. Cooperative Selection Diversity– With conventional relaying, the S → R transmissions occur in the

first phase

– During the first phase• The destination chooses not to receive

– In the second phase• only the relay transmits

• The destination relies solely on the signals received via the R → D link

– BS dynamically chooses between conventional relaying and direct transmission

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• D. Cooperative Selection Diversity– When the BS chooses to use conventional relaying

• the post–processing SNR at the MS is equal to γRD,j

• otherwise it is equal to γSD,j

– For the first phase of conventional relaying• the AMC mode is determined based on γSR,j

– For the second phase based on γRD,j

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• D. Cooperative Selection Diversity– Hence, the end-to-end throughput with conventional r

elaying is given by

– The end-to-end throughput with cooperative selection diversity is then given by

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• E. Adaptive Cooperative Diversity Scheme– Adaptive cooperative diversity scheme chooses the

best scheme (in terms of end-to-end throughput)• direct transmission • the aforementioned cooperative diversity schemes

– If the two schemes have the same performance the one with less complexity is selected BSBSBSBS

MSMSMSMS

RSRSRSRS

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• E. Adaptive Cooperative Diversity Scheme– We order the schemes with increasing complexity as f

ollows:• direct transmission• conventional relaying• cooperative transmit diversity–2 • cooperative transmit diversity–1

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Scheduling And Radio Resource Allocation Scheduling And Radio Resource Allocation For Multi-hop Cellular NetworksFor Multi-hop Cellular Networks

• The scheduling and the radio resource allocation are performed at the BS

• For each sub-channel j and for each user (i.e., MS) u, the BS calculates the post–processing SNR with the relay, i.e.,

• Let γSD,u,j denote the instantaneous SNR the user u experiences on a subchannel j in the S → D link

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• The BS plugs in γSD,u,j , γSR,j and to the look-up table and reads the corresponding throughput and nominal rate for each of them

• It calculates the end-to-end throughput with the relay,i.e.,

• Let = ρ(γSD,u,j) define the throughput that user u can obtain on sub-channel j w/o relay. For each user and for each sub-channel

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• BS first decides on to relay or not by

• For each sub-channel j, BS calculates the PFS metric for each user

ρu[k − 1] represents the past average throughput of user u at DL frame k−1.

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• for each sub-channel, the BS schedules the user who has the maximum PFS metric, i.e.,

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• Once the users are scheduled, the past average throughput for each user is updated by using a low pass filter with a time constant of T slots.

• This update is done according to

– cu,j is equal to one if user u is scheduled on subchannel j, otherwise it is equal to zero

– The time constant T adjusts the level of fairness of the scheduler. T should be long enough to provide fairness to the users

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The Frame StructureThe Frame Structure

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Performance EvaluationPerformance Evaluation

• A. Simulation Setup– An FEC block

• 96 coded bits

– One sub-channel • 8 data sub-carriers

• 1 pilot subcarrier

• over t consecutive OFDM symbols

– t {2, 3, 6, 12}∈ represents the number of OFDM symbols required to transmit one FEC block

– First phase can use up to 12 OFDM symbols– Second phase is fixed to 12 OFDM symbols– The scalable OFDMA mode with 1024 sub–carriers with a

system bandwidth of 10 MHz is considered

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• 60 users– with speeds up to 7.7 km/h

• 60 sub-channels• Frames

– 5 ms• For the S → R links the wireless channel model developed is used

with a path-loss exponent of 3

• Rician K factor of 10• For the R → D and S → D links the Non-LOS (NLOS) channel mode

l presented in is used with a path-loss exponent of 3.5

• Carrier frequency is 2.5 GHz• The effect of shadowing is not considered

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• The BS is at the center of the cell• All the relays are positioned symmetrically at a distance

of 10.4 km to the BS• The relays improve the coverage and system throughput while still

maintaining a reliable and high speed (using 64-QAM) link with the BS

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• B. Relative Performance Evaluation of the Cooperative Diversity Schemes

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Conclusions And Future WorksConclusions And Future Works

• Efficient radio resource allocation and user scheduling techniques have been developed for the DL transmissions in a two-hop cellular network using the emerging IEEE 802.16j standard

• Cooperative selection diversity scheme is a promising cooperative diversity scheme compared to the other more complex cooperative diversity schemes which require coherent signal combining at the MS

• Future work:– multi-cell– users with high mobility

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Thank You

e fficient cooperative diversity schemes and radio resource allocation for ieee 802.16j

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