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ISSN: 2277-3754ISO 9001:2008 Certified

International Journal of Engineering and Innovative Technology (IJEIT)

Volume 2, Issue 6, October 2012

125

Abstract LTE (Long Term Evolution) is the upcomingstandards towards 4G, which is designed to increase the capacityand throughput performance when compared to UMTS andWiMax. Turbo codes are the recent development in the area offorward error correction codes that achieve near-Shannon limitperformance.This codes have been successfully implemented insatellite and video conferencing systems and provision has beenmade in 3rd generation mobile systems. The decoder systems arecompared for complexity as well as for equal numbers ofiterations. The following figure shows the bit error rateperformance of the parallel concatenated coding scheme in anAWGN channel over a range of Eb/No values for two sets of code

block lengths and number of decoding iterations. Results showthat less complex decoder strategies produce good results for voicequality bit error rates.

Index Terms Convolutional Interleaver, Turbo encoder

Turbo decoder, MAP decoder, 3GPP LTE.

I.INTRODUCTION

Long Term Evolution [1] has long been seen as the first

advancement towards stronger, faster and more efficient 4G

data networks. The technology under LTE can currently reach

downlink peak rates of 100Mbps and uplink speeds of

50Mbit/s. The LTE technology is also a scalable bandwidth

technology for carriers operating anywhere from 20 MHz

town to 1.4 MHz. Long Term Evolution offers some excellent

advantages over current 3G systems including higher

throughput, plug and play compatibility, FDD (Frequency

Division Duplexing) and TDD (Time Division Duplexing),

low latency and lower operating expenditures. It also offers

legacy modes to support devices operating on GPRS systems,

while supporting seamless pass-through of technologies

operating on other older cellular towers. The authors of the

(Global system for mobile communications) and UMTS

(Universal Mobile Telecommunications System). The

technologies put forth by LTE will not only be implemented

over time, they are designed to be scalable. This scalability

means the company can slowly introduce LTE technologies

over time, without disrupting current services.

Table I. Lte Performance Requirements [5].

Metric Requirements

Spectral Flexibility 1.4, 3, 5, 10, 15 and 20 MHz

Peak data rate 1. Downlink (2 Ch MIMO): 100 Mbps

2. Uplink (Single Ch Tx): 50 Mbps (20

MHz ch)

Supported antennaconfigurations.

1. Downlink: 4x2, 2x2, 1x2, 1x12. Uplink: 1x2, 1x1

Spectrum efficiency 1. Downlink: 3 to 4 times HSDPA Rel. 6

2. Uplink: 2 to 3 times HSUPA Rel. 6

Latency 1. Control-plane: Less than 100 msec to

establish U-plane

2. User-plane: Less than 10 msec from UE

to Server

Mobility 1. Optimized for low speeds (0-15 km/hr)

2. High performance at speeds up to 120

km/hr

3. Maintain link at speeds up to 350 km/hr

Coverage 1. Full performance up to 5 km

2. Slight degradation 5 km30 km

3. Operation up to 100 km should not be

precluded by standard

II. TURBO CODES

Turbo codes were first introduced in 1993 by Berrou,

Glavieux, and Thitimajshima, [2] where a scheme is

described that achieves a bit-error probability of 10-5 using a

rate 1/2 code over an additive white Gaussian noise (AWGN)

channel and BPSK modulation at an Eb/N0 of 0.7 dB. The

codes are constructed by using two or more component codes

on different interleaved versions of the same information

sequence. Turbo codes are a high performance forward error

correction at a given code rate. These codes are especially

used in deep space satellite communication and the

application, which requires reliable transformation of

information over the communication links in the presence of

data corrupting noise. At present, these codes are competing

with Low Density Parity Check (LDPC) codes, which

produce similar performance. Turbo code implementation is

by a parallel concatenation of two recursive systematic

convolutional encoder codes depend on pseudo-random

permutation (the interleaver). The encoder performs a long bit

information frame. The interleaver to produce permuted

frame interleaves this input bit. The first encoder RSC1

encodes the original input and the interleaved frame

(permuted frame) is encoded by RSC2. Then the two encoded

bits are merged together with the real input bits to produce the

output.

III.FUNDAMENTALTURBOCODES

The fundamental turbo code encoder is built using two

identical recursive systematic convolutional (RSC) codes

with parallel concatenation [3] as shown in Figure 1. An RSC

encoder is typically a rate 1/2 encoder and is termed a

constituent encoder. The input to the second constituent

encoder is interleaved using an internal turbo codeinterleaver. Only one of the systematic outputs from the two

component encoders is used. This is because the systematic

Performance of Turbo Encoder and Turbo

Decoder for LTEPatel Sneha Bhanubhai, Mary Grace Shajan, Upena D. Dalal

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output from the other component encoder is just a permuted

version of the chosen systematic output.

Fig 1: Turbo Code Encoder [3]

Fig 2: Conventional Convolution Encoder and Equivalent RSC

Encoder [3]

IV.MATH

A conventional convolution encoder shown in [Fig. 2] is

represented by the generator sequences

g0(D) = 1+D2+D3 andg1(D) = 1+D+D3..(1)

It can equivalently be represented in a more compact form as

G = [g0(D),g1(D)].(2)

The RSC encoder of this conventional convolutional encoder

is represented as

G(D) =[1,g1(D)/g0(D)]..(3)

where the first output represented byg0(D) is fed back to the

input. In the above representation, 1 denotes the systematic

output, g1(D) denotes the feed-forward output and g0(D) is

the feedback to the input of the RSC encoder.

V.CHANNELCODINGSCHEMEINLTEThe major channel coding schemes [3] used for different

transport channels in the LTE system are summarized in

Figure 3. The turbo coding is used for large data packets,

which is the case for downlink and uplink data transmission,

paging and broadcast multicast (MBMS) transmissions. A

rate 1/3 tail biting convolutional coding is used for downlink

control and uplink control as well as broadcast control

channel (BCH).

Fig 3: Channel Coding Schemes in the LTE System [3].

VI.TURBODECODERSTRUCTURE

The basic structure of a Turbo decoder is functionally

illustrated in Fig.2. A turbo decoder consists of two maximum

a posteriori (MAP) decoders separated by an interleaver that

permutes the input sequence. The decoding is an iterative

process in which the so-called extrinsic information is

exchanged between MAP decoders. Each Turbo iteration isdivided in to two half iterations. During the first half iteration,

MAP decoder 1 is enabled. It receives the soft channel

information (soft valueLs for the systematic bit and soft value

Lp1 for the parity bit) and the a priori information La1 from

the other constituent MAP decoder through deinterleaving to

generate the extrinsic informationLe1 at its output. Likewise,

during the second half iteration, MAP decoder 2 is enabled,

and it receives the soft channel information (soft valueLs for a

permuted version of the systematic bit and soft valueLp 2 for

the parity bit) and the a priori information La2 from MAP

decoder1 through interleaving to generate the extrinsic

informationLe2 at its output. This iterative process repeatsuntil the decoding has converged or the maximum number of

iterations has been reached.

Turbo Coding

(1/3)

DL-SCH

PCH

MCH

Tail biting

Convolutional

Coding (1/3)

BCH

DCI

UCIBlock Code w/

variable Rate

Block Code 1/16 CFI

RSC encoder 1

Xk

Zk

Turbo code

internal interleaver RSC encoder 2 Zk

Ck

UL-SCH

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Fig 4: Basic Structure of an Iterative Turbo Decoder [4].

VII. CONVOLUTIONALINTERLEAVER

A convolutional interleaver consists of N rows of shift

registers, with different delay in each row. In general, each

successive row has a delay which is J symbols duration higher

than the previous row as shown in Fig. 5. The code word

symbol from the encoder is fed into the array of shift registers,

one code symbol to each row. With each new code word

symbol the commutator switches to a new register and the new

code symbol is shifted out to the channel. The i-th (1 i

N-1) shift register has a length of (i-1)J stages where J = M/N

and the last row has M-1 numbers of delay elements.

Fig 5: Convolutional Interleaver [4]

The convolutional deinterleaver performs the inverseoperation of the interleaver and differs in structure of the

arrangement of delay elements. Zeroth row of interleaver

becomes the N-1 row in the deinterleaver. 1st row of the

former becomes N-2 row of later and so on.

Fig 6: Block diagram of 8 bit convolution interleaver [4].

In order to verify the Verilog HDL models for the

interleaver and deinterleaver the authors have developed

another top level Verilog HDL model, combining interleaver

and deinterleaver [8]. The scrambled code words from the

output of the interleaver is applied as input to thedeinterleaver block along with clock as synchronization

signal. It is observed in Fig 4 that the scrambled code word is

converted into its original form at the output of the

deinterleaver block.

VIII.SOFTWAREMODEL

Fig 7: Parallel Concatenated Convolutional coding: Turbo

codes [7]

Table II. RESULTS

Block

length

512 BER-1024 BER-2048

Eb/No BER

0 0.4825 0.4875 0.4906

3 0.4757 0.4829 0.4879

6 0.4655 0.4765 0.4829

9 0.4519 0.4668 0.4755

12 0.4327 0.4517 0.4657

[A]. Turbo Encoder Software Model

[B]. Turbo Decoder Software Model

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Fig 8: The plot of BER along Y-axis and SNR along X-axis.

VIII.CONCLUSION

The decoding performance improves with an increase in

the block lengths. For larger block lengths the BER is found to

be less for a given value of Eb/No .Here the computation time

varies as the number of samples per frame increases; these

cases also provide the BER of order 10-3

.

REFERENCES[1] ERICSSON press backgrounder on mobile broadband and

LTE February 2011 http:/www.3gpp.org/LTE[2] Barnard Sklar Fundamental and Application Second

Edition (Prentice-Hall, 2001, ISBN 0-13-084788-7).

[3] Farooq Khan,Samsung Telecommunications America. LTEfor 4G Mobile Broadband http://www.cambridge.org

[4] Manjunatha K N, Kiran B, Prasanna Kumar.C/ Design andASIC Implementation of a 3GPP LTE- Advance Turbo

Encoder and Turbo Decoder International Journal of

Engineering Research and Applications (IJERA) ISSN:

2248-9622 www.ijera.com Vol. 2, Issue 4, July-August 2012,

pp.006-010.

[5] S M Chadchan ,Member IEEE and C. B.Akki 3GPPLTE/SAE International Journal of computer and Electricalengineering Vol 2,NO 5 October, 2010.

[6] 3GPP TS 36.212 version 9.2.0: Evolved Universal TerrestrialRadio Access (E-UTRA); Multiplexing and channel coding,

May 2010 http:/www.3gpp.org

[7] Copyright 2010 The mathworks, Inc.Published [email protected] www.mathworks.com/trademarks.

AUTHORS PROFILE

Patel Sneha B.received BE in Electronicsand communication from Charotar Institute

of Technology, Changa.Currently she is

studying ME EC at Parul Institute of

Technology, Baroda in Gujarat Technology

University.

Mary Grace Shajan Reader in Electronics and

communications, Parul Institute of

Engineering and Technology, Completed

Under graduation in Electronics from M.S

University Baroda in 1985. Post graduation

in Electronics and Communications from

Dharmsinh Desai Institute of Technology

Nadiad in 2004. Professional Experience of

26 Years in the field of Communications

Area of Wireless Communication.

Dr. Upena D. Dalal, M. E (EC and Comm.

Systems) and Ph.D. in Wireless

Communication at National Institute of

Technology Surat. Currently she is anAssociate Professor in Electronics

Department at Sardar Vallabhbhai National

Institute of Technology, Surat.
mailto:[email protected]:[email protected]

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IJEIT1412201212_22