UWB Synchronization Hui-Min Yeh. Wireless Access Tech. Lab. CCU Wireless Access Tech. Lab. Outline Introduction Code synchronization Typical acquisition

UWB Synchronization

Hui-Min Yeh

Wireless Access Tech. Lab.

CCU Wireless Access Tech. Lab.

Outline

Introduction Code synchronization Typical acquisition scheme Typical search strategies Definition of ‘Hit set’

Transmitter design Hybrid TH/DS signal format

Receiver design Proposed search algorithms

Joint BRS-NCS search Joint RPS-NCS search

Proposed acquisition scheme

Simulation resultsConclusions



0

Power delay profile

Time

The objective of the signal synchronization is to properly align the received signal by correctly estimating the random delay 0



Introduction

Code synchronization (TH code; DS code): To enable the lower BER of data demodulation, the template signal

should be aligned with the received signal, the code alignment is so-called “code synchronization” process.

Code synchronization can be spilt into two parts Code acquisition

It is a coarse code acquisition, to resolve the code phase error within certain range.

Code tracking It is a fine tuning process, to guarantee the timing error below an

acceptable level.



Acquisition strategy: The search for acquisition is based on the auto-correlation properties of the applied the codes, the auto-correlation is high if the receiver is synchronized and low in other situations [5].

MAX criterion (Maximum selective):

TC criterion (Threshold Crossing):

MAX/TC criterion:

( ) ( ) ( )choose cell if , 1,...,j j icC u u for i N

( ) ( )choose cell if , else to to next cellj jC u

( ) ( ) ( )

( ) ( )

in a sector, select cell if , 1,..., ;

choose cell if , else to to next sector.

j j i

j j

C u u for i M

C u



Before any data can be received in an UWB communication, the receiver must synchronize on the transmission.

Receiver structures Noise template signal Reference template signal

Packet data Synchronization on (known) code Reception and processing data

Preamble Data

Sequence for synchronization

Data can have specific coding-> communication can

not be tapped



Code synchronization

Find the best fit between the code and the received signal



Typical acquisition system The receiver’s reference signal generator will shift and guess the

code boundary of received signal, and the AWGN noise will effect the performance.

Correlator

3 4 & 5



Why use RAKE receiver?

More than one hypothesized phase can be considered as an estimated timing delay for a coarse acquisition



Definition of ‘Hit set’

Hit set definition:

The definition of ‘Hit set’ is a critical issue . When the threshold setting is low, the noise effect on the false alarm probability will increase. Oppositely, when the threshold setting is high, the mean acquisition time will increase.

2 2ˆ{ :[ ( ; ) (0; )]}h b bS R AR h h

where being the vector of channel gains

ˆ = - h



Ex: The periodicity of TH code =4, frame duration , that is Nh=128, Tc = 1 nsec, normalized pulse energy =1,CM1(noise free),

search space = 512 cells, Th=2, EGC receiver (20 fingers)

Peak value

Partial correlation energy

128 secfT n

1 2 NTrue phas

e

Hit set



Rake fingers = 20, search space = 512 cells, CM1



Search strategies

What is the efficient search algorithm? Serial or Parallel

Typical serial search algorithms [1] Linear Search (LS) Random Permutation Search (RPS) Bit Reversal Search (BRS)

Ideal mean stopping time:

Linear Search (LS)

Random Permutation Search (RPS)

Bit Reversal Search (BRS)

2( ) (3 )( )

2

N H N HE M

N

1( )

1

NE M

H

1( ) ( 1)

2

NE M

H

( ) : ( )E M Mean Stopping Time number of locations searched: H : N search space hit set



Bit Reversal Search The search algorithm is described by assuming that N is a power of 2. The order of search positions in the bit reversal search

algorithm is decided by “bit reversing”

For example: Let the integer of N is equal to 8 (2^3)

Search locations for N = 23

Reorder search locations by ‘bit reversing’

‘Bit Reversing’

Decimal 0 1 2 3 4 5 6 7

Binary 000 001 010 011 100 101 110 111

Decimal 0 4 2 6 1 5 3 7

Binary 000 100 010 110 001 101 011 111



The ideal and normalized mean stopping time vs. the parameter H/N for three serial search algorithms (N=512 cells)



Bit Reversal Search algorithm , code length=4, Nh=128,

CM1(noise free), search space = 512 cells, EGC receiver (20 fingers)



Outline

Introduction Code synchronization Typical acquisition scheme ‘Hit’ definition Typical search strategies





Simulation resultConclusions



Frame (C0) Frame (C1) Frame (CNth-1)

(TH-sequence) (Repeat NT times)

1’s Symbol (Nds*Tf)

Tf

Next symbol

mod( , )( ) ( )thl th l N c

l

x t P a t lT c T

ds

mod( , ) th

where

p is the transmitted power

is the periodic DS spreading sequence with period N

{ 1,1} and {c } is the periodic TH with period N

th

l

l l N

a

a

Transmitter design: Hybrid TH/DS signal format

0a1a 1dsNa



The variation of power spectral density (PSD) between the pure time hopping sequences and hybrid TH/DS signals No TH coding The periodicity of TH code is equal to the number of pulses pe

r symbol Hybrid TH/DS signal

The hybrid TH/DS signal format method not only speeds up the code acquisition, but also improves the interference to other co-existing systems [7].

( )thN



UWB signals with no Time-Hopping coding

Parameters: 10 symbols; number of

pulses per symbol ( ) =64; pulse repetition time ( ) =10 nsec; chip duration ( ) = 1 nsec

The transmitter power is concentrated at multiples of pulse repetition frequency.

Time (ns)

Am

plitu

de

10 secfT n

sN

fT

cT

1/ 0.1 fT GHz



UWB signals with Time-Hopping coding

Consider the same parameters The periodicity of TH code ( ) is equal to the number of pulses per

symbol, that is , The PSD of the signals is composed of spectral lines at the distances of

The transmitted power is concentrated at multiples of symbol repetition frequency.

thN

64.th sN N

1/ 1.5625s fN T MHz



UWB signals with hybrid TH/DS signals

Consider the same parameters

a) Short TH code with ; repeat NT =8 It will increase a lot of spectral lines due to the repeat operation

b) Short TH code with ; repeat NT =8; The polarity of pulses per symbol is scrambles by DS code

8thN

8thN 64dsN {1, 1}

( )a ( )b



Outline






Simulation resultConclusions



Proposed Joint Search Algorithms Joint BRS-NCS Search Joint RPS-NCS Search

Non consecutive search (NCS) criterion: The non consecutive search strategy is proposed owing to more than

one HIT phase in multi-path channel models. The non consecutive search can achieve a rapid acquisition by

testing search locations with a step size D which is greater than one cell.

The cells in total search space are so called uncertainty region that can be divided into blocks

This will reduce the search time required to acquire the location of hit set from an initial search cell, but it could also increase the miss probability.

Np ( 1,2..., )rB r p

D where D denotes the number of cells in rN p B



LI-NCS (Linear Increasing NCS)

In order to avoid the miss search, we propose a linear increasing NCS (LI-NCS) approach.

The search equation can be expressed as

The LI-NCS approach can be described as following:

① Set initial loop =1, and check blocks. The number of test cells are

② If the correlation output of each block in this loop is lower than threshold, then go to the step (3). If the correlation output of one block in this loop is higher than threshold. then the search procedure is DONE.

③ Set loop = loop + 1, and also check blocks, the number of cells are

, then go to step (2)

{( 1)D } 1, 2,..., in each and 1,2,...,D-1r loop where r p loop loop

{( 1)D 1},r 1,2,...,r p

{( 1)D },r loop 1,2,...,r p

p

p



An example of the LI-NCS search approach

Assume that search space N=12, hit set =[7 8 9 10 11], block size D = 4,

0 1 2 3 4 5 6 7 8 9 10

11

1 2 33 ~ { , , }p B B B



Joint BRS-NCS search procedure a) search space N=48, hit set H=8, b) divided into 8 blocks, each block with D = 6

CorrectSymbol

Boundary

D cells

=: New search location(a)

(b)

(0)

(4)

(2)

(6)

(3)

(1)

(5)

(7)

Search space N



Receiver design : Acquisition scheme

Received signal

Find DS-boundary(get symbol boundary)

Coarse TH-boundaryacquisition

Fine acquisition

(First stage)

(Second stage)ACQ1

th

ACQ2

Two-stage acquisition scheme TH code acquisition DS code acquisition



Set new search location based on Joint Search Algorithm (new shift )

Template signal generator

Hit

No Hit

Z

(RAKE receiver with parallel correlators)

TC #1

Second Stage

th fN Tdt

th fN Tdt

th fN Tdt

( )r t

First Stage

0j

1j

1j M

1 ˆ( )s t

th fkN T

(Coarse Acquisition)

th

11

10 0

( ) ( )thNM

f l c cj l

s t t lT c T jT

One branch correlator

Maximum path amplitude selective

1 ˆ( )coarses t

Roughly find TH-boundary

coarse

(Fine Acquisition)



Fine Acquisition Procedure:

Left direction Right direction

Starting point

Step1: The terminating point of coarse acquisition process is the starting point of fine acquisition.

Step2: The new search window in fine acquisition process is equal to the double Hit set size.

Step3: The fine acquisition process searches both right and left directions from the starting

point of fine acquisition with one branch correlator to acquire the strongest path. The search range of each direction is equal to the size of hit set.

Step4: We utilize the maximum selective criterion to select the strongest path amplitude

Step5: Finally, the fine acquisition process passes the parameter on to the second stage.

coarse

th



Set step size=: time-hopping code length and new search location based on Joint Search Algorithm location (new shift )

Template signal generator

Hit

No Hit

Z

(RAKE receiver with parallel correlators)

TC #2

( )r t

0j

1j

1j M

2 ˆ( )s t

T th fN N Tdt

T th fN N Tdt

T th fN N Tdt

Found DS-boundary(Found symbol

boundary simultaneously)

T th fkN N T

Second Stage

11

20 0

( ) ( )dsNM

f l c cj l

s t t lT c T jT



Outline


Transmitter design Hybrid TH/DS signal format Power spectral density (PSD)




Simulation results



Simulation cases

Two-stage vs. one stageComparison of search strategiesTwo-step approachMAI case



Channel model 1&4

The first path of CM1 we consider has the strongest amplitude gain, and the tenth path of CM4 has the strongest amplitude gain in our simulation.

The strongest path The strongest

path



SNR vs. Detection probability for the second stage in CM1

Assume that the first stage finds the correct boundary.



Mean stopping time of two acquisition schemes as a function of threshold parameter A

Simulation parameters:

T16, 64 sec, 1 sec, 4, N 4 for two-stage

16, 16 for one stage acquisition scheme

s f c th

th ds

N T n T n N

N N



The mean stopping time of Joint BRS-NCS /Joint RPS-NCS method versus threshold setting for step size D=4, D=8, and D is approximated to hit set.



Comparison of performance in terms of Mean stopping time versus threshold setting for proposed search algorithms and bit reversal search algorithm.



A statistical number as function of offset value for 5000 trials, a) SNR = 10 dB, coarse acquisition result, b) considering fine acquisition result, in CM1

( )a

( )b

( )b( )a

( )a ( )b

Single-user

Multi-user



SNR = 20 dB, 5000 trials, CM4

( )a

( )b( )a

( )b

Single-user

Multi-user

( )a



MAI vs Detection probability (CM1)

Simulation results for detection probability of the UWB channel model (CM1), as a function of the number of interfering users Nu-1



MAI vs Detection probability (CM4)

Simulation results for detection probability of the UWB channel model (CM4), as a function of number of interfering users Nu-1



Simulation results of detection probability for different lengths of in UWB channel model (CM1), as a function of interfering users (Nu-1), SNR=20dB

fT



Conclusions

The mean stopping time of two-stage acquisition scheme is simulated and it is evident to see that the two-stage acquisition scheme outperforms the conventional one stage acquisition scheme.

By simulation results, we can see that the “Joint Search Algorithms” have lower mean stopping time.

We propose a two-step approach which is composed of coarse step and fine step to achieve a fine code acquisition in first stage.

Adding more users to the systems is investigated by computer simulations.



References

[1] E. A. Homier and R. A. Scholtz, “Rapid acquisition of ultra-wideband signals in the dense multipath channel,” Proceeding of 2002 IEEE Conference Ultra Wideband Sys. Tech, (Baltimore, MD), pp. 105-109, 2002.

[2] J. Oh, S. Yang, and Y. Shin, “A Rapid Acquisition Scheme for UWB Signals in indoor wireless channels,” IEEE WCNC ’04, Wireless Commun. and Networking Conference, vol. 2, 2004.

[3] S. Gexici, E. Fishler, F. Kobayashi, H. V. Poor, and A. F. Molisch, “A rapid Acquisition Technique for Impulse Radio,” in Proc. IEEE Pacific Rim Conference Commun., Comput., Signal Process., pp. 627-630, Aug 2003.

[4] L. Reggiani, G. M. Maggio,”Rapid Search Algorithms for Code Acquisition in UWB Impulse Radio Communications,” IEEE Commun. Journal, vol. 23, pp. 898-908, May 2002.

[5] G. E. Corazza, “On the MAX/TC criterion for code acquisition and its application to DS-SSMA systems,” IEEE Trans. on Commun., vol. 44, pp. 1173-1182, Sept. 1996.

[6] M. Z. Win, R. A. Scholtz, “Impulse Radio : How It Works,” IEEE Commun. Letters, vol. 2, No. 1, pp. 36-38, Jan. 1998.



[7] S. Aedudodla, S. Vijayakumaran, T. F. Wong, “Rapid Ultra-wideband Signal Acquisition,” IEEE Wireless Commun. and Networking Conference, vol. 2, pp 1148-1153, March 2004.

[8] O. S. Shin and K. B. Lee, “Utilization of multi-paths for spread-spectrum code acquisition in frequency-selective rayleigh fading channels” IEEE Journal Trans. on Commun., Apr. 2001.

[9] S. L. Han, S. O. Hyun, E. K. Chang, “Code acquisition for the DS-CDMA RAKE receiver in a multi-path fading channel,” Proceeding of IEEE Singapore International Conference, pp. 215-219, July 1995.

[10] V. Saravanan, F. W. Tan,”Equal gain combining for acquisition of UWB signals,” MILCOM ’03, IEEE Military Commun. Conference, vol. 2, pp.880-885, Oct. 2003.

Documents

UWB Synchronization Hui-Min Yeh. Wireless Access Tech. Lab. CCU Wireless Access Tech. Lab. Outline Introduction Code synchronization Typical acquisition