Radio Resource Management for a Two-Radio Resource Management for a Two-hop OFDMA Relay System in Downlinkhop OFDMA Relay System in Downlink

Mi Kyoung Kim and Hwang Soo LeeMi Kyoung Kim and Hwang Soo LeeSchool of EECS, Division of Electrical Engineering, KAIST, KoreaSchool of EECS, Division of Electrical Engineering, KAIST, Korea

IEEE Symposium on Computers and Communications (ISCC 2007)IEEE Symposium on Computers and Communications (ISCC 2007)

Outline

IntroductionSystem modelProblem formulationSimulation resultsConclusions

Introduction

• The relay system, with its many merits, is one of the most promising techniques for the future of mobile communications systems.

• The resource allocation mechanism of the relay system is divided into the centralized and distributed methods, according to the role of resource manager.

• In the centralized method, BS controls all information from BS-MS, BS-RS, RS-RS, and RS-MS links.

Introduction

Motivation If more RS is added in the system, BS must control more

information and the information collection time and algorithm execution time will be longer.

It generates too much signaling overhead and complexity.

Goal To propose methods that have almost the same performance as the

centralized method but lower complexity and higher flexibility compared to the centralized method.

System Model

BS

MS

RS1

MS MS

RS2

RSM

(1) RS is a fixed station.

(2) The channel state information (CSI) is known to the transmitter (BS or RS).

(3) There is a protocol to gather CSI and broadcast the allocation results and the protocol uses separate control channel.

(4) The mobility velocity of MS is slow so that the BS-MS or RS-MS connection is not altered during resource allocation.

(5) One MS can communicate with only one BS or RS.

Problem formulation

BS allocates resources to the directly connected mobile stations and relay stations, first.

Separate and Sequential Allocation (SSA): RS uses the assigned subcarriers from BS.

Separate and Reuse Allocation (SRA):All system subcarriers independently.

Problem formulation – Inputs and decision variables

Parameter Value

K Users. K=Kb+Kr.

N Subcarriers.

M Relay Stations.

PbThe total transmit power of BS.

PrmThe total transmit power of each RS m.

Each subcarrier n of user k is assigned an equal power.

pbThe transmit power for subcarrier n of BS. pb=Pb/N

pr The transmit power for subcarrier n of RS m. pr=Prm/N

Problem formulation – Inputs and decision variables

Parameter Value

B Channel bandwidth.

N0 Noise power spectral density. (W/Hz)

ΓkA constant SNR gap of M-QAM modulation for the BER requirement.

Γk = -ln(5BERk) / 1.6

RkMinimum data rate requirement of user k.

PrmThe total transmit power of each RS m.

hk,nThe channel gain of user k in subcarrier n.

Hk,n The corresponding signal-to-noise ratio (SNR) of user k in subcarrier N.

Hk,n= |hk,n|2/ N0(B/N)

H’k,n The effective SNR of user k in subcarrier n. H’k,n= Hk,n / Γk

Problem formulation – Inputs and decision variables

Parameter Value

ck,n The subcarrier assignment indicator.

ck,n=1, if subcarrier n is assigned to user k.

rk,n The achievable data rate of user k in subcarrier n.

rk,n=(B/N)log2(1+pk,nH’k,n)

rk, The achievable data rate of user k. nknkNnk rcr ,,1

Centralized method

)'1(log*

)'1(log*

,,2,11

,,2,11

,,11

nknknkNn

Kk

nknknkNn

Kk

nknkNn

Kk

HpcN

B

HpN

Bc

rc

Overall achievable capacity:

BS

MS1 MS2MS3

RS1

MS4

MS5

RS2

MS6

MS7

Centralized method

C1 is a constraint for the correct value of the subcarrier assignment indicator

Centralized method

C2 is a constraint so that subcarrier n is not shared by several MSs.

Centralized method

C3 is the rate requirement for each MS.

rk: The achievable data

rate of user k.

Rk: Minimum data rate

requirement of user k.

Semi-distributed methods

BS

MSMS MS

RS

Step-I Step-II

SSA

SRA

Semi-distributed methods - Step-I

BS

MSMS MS

RS

The reported rate requirement of RS is the sum of the rate requirements of the MSs connected to RS.

RS

MSMS

Semi-distributed methods - Step-II (SSA)

SSA: RS m allocates subcarriers using only assigned Nrm

subcarriers from BS.

The real number of used

subcarriers in RS.

Semi-distributed methods - Step-II (SRA)

SRA: RS m allocates all N subcarriers to MSs.

Simulation results

Parameter Value

The radius of BS 1000 m

The distance between BS and RS

700 m

Channel model IEEE 802.16d

The total transmit power of BS and RS

BS: 20 W

RS: 10 W

The total bandwidth of the system

10 MHz

The total number of subcarriers

1024

The QoS requirement of user k

BERk < 10-3, rk 64 kbits/s

Simulation results

Simulation results

Simulation results

Simulation results

Conclusions

• In this paper, they suggest two semi-distributed methods, the SSA and the SRA methods, for an OFDMA relay system.

• These methods reduce the large burden of the BS and offer an easy extension of the traditional BS based system.

Thank you!