Initial Ranging procedure in WiMAX standard

Student:Chung-ming Li

Outline

Introduction WiMAX frame structure Down-link timing synchronization Initial ranging Timing offset System design conclusion

Introduction

In OFDMA, it is required that all transmission from MS should arrive BS at the same time.

Imagine a cell size of 10Km, we have round trip time of 66.6 us.

UL data must arrive BS within 0 to 66.6 us.

The whole network synchronization refers to BS clock.

Introduction

MSs are distributed all over the cell. A MS at cell edge receive data quite

late and transmits data very early , being compared with MS close to BS.

When a new MS is seeking entry into the network, the MS has no idea as to what time or power should be used for transmitting the initial signal.

Introduction

This is the BS’s job to detect this new MS.

Find the misalignment between the new MS ,and then send response to correct it.

The following slides start with a brief description about how WiMAX handles this situation.

WiMAX frame structure

TTG and RTG are 105.714 μs and 60 μs, respectively.

WiMAX frame structure

Down-link timing synchronization

DL UL

5ms

DL UL

5ms

DL UL

5ms

DL ULDL U

L

Initialization AGC Stage Normal AGC Stage

t

Power-on happens at a random time

Enable a 5ms counter

Measure power

Energy detection

AGC

Packet DetectionYes

Yes

YesSeveral tunes

Enable a 5ms counter

ED

AGCPKT

AGC

Power on

Down-link timing synchronization

Oversampling

Mixer

4 Channel ADC 50Mhzto 44.8Mhz SRRC

LO PLL

9 62 2.5(10 ) 20(10 )j te

fA/D

4

44.850

AWGN

Down-link timing synchronization

SRRC

)()()( fGfGfG RT

2

4[ ] 1 ,

2 2[(1 )sin( )] (1 cos( ))],4 42

( ) ( ) ( )sin[ (1 ) ] 4 cos[ (1 ) ]

( ) ( )[1 (4 ) ]

0,

g n

n k n k n kp p pn k n kp p

( ) 0for n k

( )4Pfor n k

( 1) 2Nfor n k

( 1) 2Nfor n k

Down-link timing synchronization

Down-link timing synchronization

21

0

0, ( interger)=0, otherwise

N j knN

n kk

k

s S e

ifk iM i

Sk

12 21

0 0

21

0

NN Mj kn j kMn

N Nn k kM

k k

Nj knM NM

kMk

n N n Nsn M

then

s S e X e

S e

S S S

must be integer !!sN

Down-link timing synchronization

Packet detection by cyclic prefix

2 2 2 2

1

_0

12

_0

(( ) ( ))cov( , )

( ) ( ) ( ) ( )

CP

CP

X YXY

X Y

L

k i k i FFT sizeiL

k i FFT sizei

E X u Y uX Y

E X E X E Y E Y

r r

r

is the scanning sample offsetk

CP

CP

FFT_size

Down-link timing synchronization

AWGN; theis ,1 to0 from profile-delay channel theis where

,1

0

knL

knkshkrL

1 1 1

0 0 0

1 1 1

0 0 0

,

where is the scanning sample offset,

W W L

w w

L W W

w w

r w k f w h s w k n w k f w

h s w k f w n w k f w

k

Down-link timing synchronization

Matched filter is with an eye to maximizing SNR

1 1

1

1

0

1 1

1 1

1 1 1 1 1 1

1 1 10 0 0 0 0 0

1 1 2

0 0

then 0,

if ( ) for 0,

and E ( ) 0 for 0

then

W

w

L W - L W - L W -

w w wk k

L W -

wk

n w k f w

s w k f w k

s w k f w k

h s w k f w h s w k f w h s w k f w

h f w

1h k

Down-link timing synchronization

1

2

1 1 2 221

0 0

1

,

where 0, , 1

L W -

wk

h f w h k

k L

Initial ranging

In WiMAX, code division multiple access( CDMA) codes are used to detect new MS.

A new MS will transmit one CDMA code.

The BS is not only to detect the new MS but also to calculate its timing,frequency and power offset.

Initial ranging

Code length=144, 256 sets, using BPSK modulation

Timing offset

CyclicPrefix

FFT windowCyclicPrefix

Delay

BS clock

Receive data

020( ) ( )j n kDFTx n n e X k

Timing offset

Signal in FFT

0( )x n n 02 ( )j n ke X k

12CDMA Code with phase shift ( )j n ke X k

1 02 ( ) ( ) ( )j n n ke X k X k

1 0

1442 ( )

1

( ) ( )j n n k

k

e X k X k

0For , ( ) ( ) ,k=1,2,.....144

We get a peak!n n and X k X k

21without timing advance with timing advance

Timing offset

System Design

6

8

(105 60) 1023 10

6.75 Km

air

air

d

d

Conclusion

The BS detects the new MS and calculates the timing offset, then corrects MS transmitting parameters.

Taking the round trip time intoconsideration,the maximal cell size should be 6.75 Km.