vada lab.sungkyunkwan univ. 1 l38: viterbi decoder 저전력 설 계 성균관대학교...

SungKyunKwan Univ.

1VADA Lab.

L38: Viterbi Decoder 저전력 설계

성균관대학교 전기전자 및 컴퓨터공학부

조 준 동

SungKyunKwan Univ.

2VADA Lab.

Viterbi Decoder◈ Convolutional Encoder

K = 3 (Constraint Length) R = 1/2 (Rate)

+ +

+

Informationsequence

U uj a j b j

A1 A0

V

C odeword

a j=uj+uj- 1+uj- 2

b j=uj+uj- 2

A(3,1/ 2) C onvolutional encoder

SungKyunKwan Univ.

3VADA Lab.

Viterbi Decoder

00

10

01

11

00

11

00

11

.......

10

01

00

11

10

01

10

01

00

11

State

Time 0 1 65432

Fig. 2. Trellis diagram for a (2,1/ 2) convolutional code

Information sequence : U = (0,0,1,0,1,0,...) Output codeword : V = (00,00,11,10,00,10,...)

SungKyunKwan Univ.

4VADA Lab.

◈ Viterbi Decoder

Viterbi Decoder

BMU SMUAC SU

PMM

Rec eivedSignal

BM SP Dec odedData

Viterbi decoder struc ture

SungKyunKwan Univ.

5VADA Lab.

Branch Metric Unit(BMU) : The branch metrics measure the difference the received symbol and the symbol that causes the transitions between states in the trellis.

Add-Compare-Select Unit(ACSU) : To find the survivor path entering each state, the branch metric of a given transition is added to its corresponding partial path metric(PM) stored in the path metric memory (PMM). This new partial path metric is compared with all the other new partial metric corresponding to all the other transitions entering that state. The transition that has the minimum partial path metric is chosen to be the survivor path of the state. The path metric of the survivor path of each state is updated and stored back into the PMM.

Survivor memory Unit(SMU) : The survivor path are stored in the SMU. A traceback mechanism is applied on the SMU during the decoding stage to output the decoded data.

Viterbi Decoder

SungKyunKwan Univ.

6VADA Lab.

⑴ Low power ACSU VLSI architecture▶ Conventional ACSU VLSI architecture

Butterfly structure

Viterbi Decoder

s a

sb sb

s aS0

S0

S1

S0

SungKyunKwan Univ.

7VADA Lab.

Viterbi Decoder

Architecture of conventional ACSU

Adder

Adder

C omp

Adder

Adder

C omp

(sa,S0)

BM i

PM i- 1

BM i

BM i

PM i- 1

BM i

(sa)

(sb,S1)

(sb)

(sa,S1)

(sb,S0)

M i

M i

(S0)

(S1)

SungKyunKwan Univ.

8VADA Lab.

― Algorithm

Viterbi Decoder [SKKU. Solution]

☞ The area and power of the lower power ACSU design are reduced by

20% and 30%, respectively, comparing with the conventional ACSU

design

>PM i- 1(sa) (sa,S0)

BMi+ BMi(sb,S0)

PM i- 1(sb)

+

>PM i- 1(sa)

PM i- 1(sb)

-(sa,S0)

BMiBMi(sb,S0)

-

SungKyunKwan Univ.

9VADA Lab.

▶ Low power ACSU VLSI architecture [C-Y Tsui, ISLPED’99]


SungKyunKwan Univ.

10VADA Lab.

※ Glitch minimization [Raghunathan, DAC’96]

(a) Lower power ACSU architecture (b) Conventional ACSU architecture

☞ The power consumption of architecture (a) is larger than that of architecture (b) by more than 17% because of glitch power dissipation


Y

X

+

+

0

1

<

A

B

D

C

(a) compare- add (b) add- compare

+

0

1

0

1

<

A

B

D

C

Y

X

SungKyunKwan Univ.

11VADA Lab.

※ Glitches in control logic


C LK

+

0

1

0

1

<

A

B

D

C

Y

X&

S

C

D

S

Fs=0 Fs=1 = A B. .

SungKyunKwan Univ.

12VADA Lab.

⑵ Low power traceback VLSI architecture▶ Systolic Viterbi, traceback decoder[J. Sparso’91]

Viterbi Decoder

ACSUTrace-BackUnit

1

Trace-BackUnit

2

Trace-BackUnit

3

Trace-BackUnit10

.....

Trace- Back Units

The struc ture of systolic Viterbi decoder

SungKyunKwan Univ.

13VADA Lab.

Viterbi Decoder

00

10

01

11

.......

0

10

State

Time 0 1 65432

2 2

2

2

1

3

3

2

2

1

2

2

4

1

1

2

2

3

3

2

3

1

1

1

0

1

1

0

0

0

0

0

0

0

0

0

0

1

0

0

0

1

0

path metric

dec ision vector

Sequence of staes of the traceback methode

Received codeword : V = (00,00,11,10,00,10,...)

SungKyunKwan Univ.

14VADA Lab.

Viterbi DecoderTime unit

ACSU

0000

00XX

ACSU

00XX

2

1

ACSU

0000

00XX

3

0000

ACSU

0000

0000

00XX

4

1101

dec ision vec tor state with smallest path metric

SungKyunKwan Univ.

15VADA Lab.

Viterbi Decoder....

Time unit

ACSU

1000

0000

0100

1011

0000

1101

1101

0000

0000

00xx

11

10

ACSU

1000

0000

0100

1011

0000

1101

1101

0000

0000

00xx

01 10

11

1110

ACSU

1000

0000

0100

1011

0000

1101

1101

0000

0000

00xx

0100

10 11 00

12

1110

survivor depth = 5K

T10 T1T2T3T4T5T6T7T8T9

T10 T1T2T3T4T5T6T7T8T9

T11

"0"

"1"

10

11

SungKyunKwan Univ.

16VADA Lab.

Viterbi Decoder

ACSU

1111

0000

0000

1011

0001

1000

1101

0001

0100

1110

1000

0000

0100

1011

0000

01 00 10 11 00 01 1

24

ACSU

1110

1000

0000

0100

1011

0000

1101

1101

0000

0000

00xx

0000

0000

1011

0001

1000

1101

0001

0100

11 00 10 01 01 10 00 10 10 00

19

ACSU

1111

0000

0000

1011

0001

1000

1101

0001

0100

1110

1000

0000

0100

1011

0000

1101

1101

0000

0000

01 10 01 00 10 11 00 01 01 00 0

20

.

.

.

.

.

.

SungKyunKwan Univ.

17VADA Lab.

※ Systolic array decoder 의 문제점

The systolic array viterbi decoder is organized to input the decision vector and the smallest path metric out of the ACSU and to output the decode bit by shifting every register for every cycle.

This system consumes a great dynamic power consumption due to switching activities of registers which is almost 80% of the total power consumption because every data in TBU shifts for every cycle.

Viterbi Decoder

SungKyunKwan Univ.

18VADA Lab.


▶ Our low power trace-back unit

C ONTROL BLOC K

0000

0000

00XX

C ONTROL BLOC K

ACSU

C ONTROL BLOC K

Time unit

1

3

2

0

000

ACSU

ACSU

00XX

00XX

SungKyunKwan Univ.

19VADA Lab.


C ONTROL BLOC K

0000

1101

0000

1101

0000

1011

0100

0000

ACSU 9

00XX

C ONTROL BLOC K

0000

1101

0000

1101

0000

1011

0100

0000

1000

11

ACSU 10

00XX

C ONTROL BLOC K

0000

1101

0000

1101

0000

1011

0100

0000

10

1000

ACSU 11

1110

01

00XX

.

.

.T1 T9T8T7T6T5T4T3T2

Trace- back

SungKyunKwan Univ.

20VADA Lab.


.

.

.

.

C ONTROL BLOC K

0000

10

1101

0000

1101

10

0000

1011

00

0100

0000

10

1000

0100

0000

1011

00

0001

1000

10

1101

0001

01

0000

11

ACSU 19

1110

01

00XX

00

C ONTROL BLOC K

0000

1101

01

0000

00

1101

01

0000

1011

0100

00

0000

1000

11

0100

10

0000

10

1011

0001

01

1000

1101

00

0001

1111

01

0000

0ACSU 20

1110

C ONTROL BLOC K

0000

10

1101

1101

10

0000

1011

00

0100

0000

10

1000

0100

0000

1011

00

0001

1000

10

1101

0001

01

1111

0000

110

ACSU 21

1110

01

SungKyunKwan Univ.

21VADA Lab.

After decision vector and the smallest path metric generated from ACSU

are transferred to the Control Block (CB), the CB outputs the decision ve

ctor and the smallest path metric with the right cycle using a counter and

a multiplexer.

The register array, which stores the value of trace-back from the CB, was

provided to finally output decoded bit, not by shifting all higher 4-bit d

ecision vector as in the classical TBU, but by shifting the lower 2-bit

only, which is the smallest path metric, to the left


SungKyunKwan Univ.

22VADA Lab.

◈ Experimental Result (area 11% , power 40% )


A r e a

0

1000

2000

3000

4000

5000

6000

7000

8000

2 3 4

K

gate

s

Trace- back Unit Low Power Trace- back Unit

Power Dissipation

0

200

400

600

800

1000

1200

1400

1600

2 3 4

K

pow

er(

uW

)

Trace- back Unit Low Power Trace- back Unit

SungKyunKwan Univ.

23VADA Lab.

⑶ Low Power Asynchronous Viterbi Decoder [Y.h.Lee , Stanford] ▶ Algorithm

Viterbi Decoder [Stanford Solution]

time ntime

n+1

Traceback processing

converge point

SungKyunKwan Univ.

24VADA Lab.

① 초기화 : 구속장의 5 배의 trellis 를 traceback 하고 , 그 경로를 저장한다 .

② Loop

A. 추적과 비교 : 임의의 초기 스테이트를 선택해 trace back 을 시작

한다 . 동시에 , route 를 추적해 나가면서 각 node 에서

저장된 route 와 비교한다 .

B. 비교 값이 같으면 추적을 멈추고 저장된 route 를 버린다 . 같지 않

을 때는 A 과정을 반복한다 .

③ 각각의 입력 신호에 대해 ② 과정을 반복한다 .


SungKyunKwan Univ.

25VADA Lab.

▶ Implementation

Self-timed TBU block diagram


Previous path

Input Port

AddressRD/WR Control

Shift ReisterMUX

M em ory M anagem entUnitAddress RD/WR

Control

SurvivingPath

M em ory

Self-precharge &Self-requesting

if not found

TraceBackUnit Oscillator

RingComparison

Logic

Requestif Path is not

found

RequestformACS

Acknowledge toACS

if path is found

SungKyunKwan Univ.

26VADA Lab.

① Self-timed TBU 가 request 신호를 기다리는 동안 전력 소모가 없다 .

② ACS 는 스테이트 결정 데이터를 버리기 위해 request 신호를 내보낸

다 .③ TBU 는 이전의 surviving path memory 와 previous path memory 를 읽어 들여 비

교한다 .

④ 같지 않으면 , TBU 는 previous path memory 를 update 하고 self- precharging, self-requesting 을 한 다음 ③ 과정을 반복한다 . 같으면 , ⑤ 과정으로 간다 .

⑤ TBU 는 ACS 에 scknowledgement 신호를 보내고 , 다음 ACS 의 request

신호를 위해 self-precharge 한다 .

Viterbi Decoder

SungKyunKwan Univ.

27VADA Lab.

Low-Power Bit-Serial Viterbi DecoderH. Suzuki, Y. N. Chang, K. K. Parhi “Low-Power Bit-Serial Viterbi Decoder for 3rd Generation W-CDMA System”, 1

999, CICC

◈ Abstract

This paper presents a low-power bit-serial Viterbi decoder chip with the coding rate =1/3 and the constraint length K=9(256 states)

The Add-Compare-Select(ACS) units have been designed using bit-serial arithmetic and a power efficient trace-back scheme and an application-specific memory have been developed for the trace-back operation.

The chip was implemented using 0.5m CMOS technology and is operative at 20Mbps under 3.3V and 2Mbps under 1.8V. The power dissipation is only 9.8mW at 2Mbps operation under 1.8V

SungKyunKwan Univ.

28VADA Lab.

Low-Power Bit-Serial Viterbi Decoder

◈ Architecture Overview

256 bit-serial ACS units are placed in parallel and each ACS unit include state metrics storage

Trace-back block, a 256 x 48 bit memory is required for the survivor path length of 48

SungKyunKwan Univ.

29VADA Lab.


Bit-Serial Viterbi Decoder Chip Diagram

SungKyunKwan Univ.

30VADA Lab.


◈ Bit-Serial ACS Unit

Bit-serial ACS unit

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31VADA Lab.


Each ACS unit has three full-adders.

Two of them are used to add the state metric and the branch metric and the third one is used to compare two new state metrics

Reducing the overhead down to 17% of the whole area of the ACS unit

SungKyunKwan Univ.

32VADA Lab.


◈ Trace Back Strategy

Trace Back operation

SungKyunKwan Univ.

33VADA Lab.


The memory size required in this paper is twice as large as the minimum memory size(256 x 2).

After 48 “TRACE BACK” operations, 24 decoded bits are obtained consecutively.

Two separate pointers, namely, a read pointer and a write pointer are required and the speed of the read pointer should be three times as fast as that of the write pointer

This operation was implemented with single-port memories using a time-multiplexed access method.

vada lab.sungkyunkwan univ. 1 l38: viterbi decoder 저전력 설 계 성균관대학교...

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