Initial Ranging procedure
in WiMAX standard
Student:Chung-ming Li
Outline

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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
Initialization AGC Stage
DL
UL
Normal AGC Stage
DL
DL
UL
UL
5ms
Power-on happens
at a random time
DL
UL
DL
5ms
5ms
Enable a 5ms
counter
Measure power
Energy detection
Yes
Several
tunes
AGC
Power on
Yes
Packet Detection
ED
AGC
Yes
AGC
PKT
Enable a 5ms counter
UL
t
Down-link timing
synchronization
 Oversampling
e
4
44.8
50
 j 2 2.5(109 )20(106 ) t
Channel
50Mhz
to 44.8Mhz
ADC
Mixer
AWGN
fA/D
LO
PLL
SRRC
4
Down-link timing
synchronization
 SRRC
GT ( f )  GR ( f )  G( f )
4

g[ n]  
1  ,



2

2


[(1  )sin( )]  (1  cos( ))],


4

4
2

 sin[ (1   ) (n  k ) ]  4 (n  k ) cos[ (1   ) (n  k ) ]

p
p
p

(n  k )
(n  k ) 2


[1  (4
)]

p
p

0,

for (n  k )  0
for ( n  k )  
P
4
for n  k 
( N  1)
2
for n  k 
( N  1)
2
Down-link timing
synchronization
Down-link timing
synchronization
sn 
N 1
S
k 0
k
e
 j
2
kn
N
if
  0,
Sk 
 =0,
k  iM (i  interger)
k  otherwise
then
sn 

N 1
S
k 0
k
e
2
 j
kn
N
k 0
 j
kM

k 0
N
1
M
S

N
1
M
e
 Sn  Sn N
M
2
kn
N
M
 S n  Ns
X kM e
 j
2
kMn
N
N s must be
integer !!
Down-link timing
synchronization
 Packet detection by cyclic prefix
 XY 
cov( X , Y )
 XY
LCP 1


i 0
LCP 1
E (( X  u X )  (Y  uY ))
E ( X 2 )  E 2 ( X )  E (Y 2 )  E 2 (Y )
rk i rki  FFT _ size
r
i 0

2
k  i  FFT _ size

k is the scanning sample offset
FFT_size
CP
CP
Down-link timing
synchronization
L 1
r k    h sk     nk ,
 0
where is thechanneldelay - profilefrom0 to L  1, nk  is theAWGN;
 L 1




r
w

k
f
w

h

s
w

k



n
w

k

      
 
  f  w

w 0
w 0    0

W 1
W 1
L 1
W 1
W 1
 0
w 0
w 0
  h    s  w  k     f  w    n  w  k   f  w ,
where k  is the scanning sample offset,
Down-link timing
synchronization
 Matched filter is with an eye to maximizing SNR
W 1
then  n  w  k  f  w   0,
w 0
if s(w  k1   )  f   w  for k1    0,
and E  s( w  k1   ) f  w   0 for k1    0
L 1
W -1
L 1
W -1
L 1
W -1
 0
w 0
 0
k1 
w 0
 0
k1 
w 0
then  h    s  w  k1    f  w    h    s  w  k1    f  w    h    s  w  k1    f  w 
L 1
W -1
 0
k1 
w 0
  h    f  w    h  k1 
2
Down-link timing
synchronization
2
L 1
W -1
h    f  w 


0
k1 
w0
where k1  0,
2
  h  k1  ,
, L 1
2
2
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
Delay
Cyclic
Prefix
Receive
Cyclic
Prefix
data
FFT
window
BS
clock
 j 2 n0k
x(n  n0 ) 
e
DFT
X (k )
Timing offset
CDMA Code with phase shift e j 2 n1k X (k )
Signal in
x(n  n0 )
FFT
e
 j 2 n0k
X (k )
e j 2 (n1 n0 )k X (k ) X (k )
144
j 2 ( n1  n0 ) k
e
X (k ) X (k )

k 1
For n  n0 , and X (k )  X (k ) ,k=1,2,.....144
We get a peak!
Timing offset
without timing advance
with timing advance
21
System Design
 (105  60) 106 
d air


8
2
 3 10  

d air  6.75 Km
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.