King Fahd University of Petroleum and Minerals Electrical

King Fahd University of Petroleum and Minerals Electrical Engineering Department Mr. Ali Hussain Mugaibel Dr. Maan Kousa 16 March 1999 CETEM’ 99 Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Topics of Discussion n Turbo Codes Introducing turbo codes. u Turbo Encoders u Turbo Decoders u n n Promises Challenges Can we Improve TC ? Conclusion Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Codin g Source Channel Convolutional Block 101101 X 0 + Input 1001011 0 0 10 D 1 0 00 D D 1 D 0110100 011011 011010 0 + (a) Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99 X 1

What are turbo codes ? n New class of CC. Introduced in 1993 by Berrou, Glavieux and Thitimajashima. n n Perform close to Shannon limit in terms of BER. Use iterative scheme with reference to the turbo engine principle. n Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Turbo Encoder n More than one CC are linked in parallel by interleavers. Information Source PAD X 0 Interleaver Enc 1 X 1 Enc 2 X 2 Puncturing & Parallel/Serial MUX Figure 1: Simplified Turbo Encoder Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99 to the channel

Turbo Encoder n Important factors : RSC encoders u Termination u Puncturing u Input X 0 + Input D D D + (a) D X 0 + X 1 D D D + (b) Figure 2: (a) Classical Non-Recursive Non- Systematic Code (b) Recursive Systematic Code Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99 D X 1

Turbo Decoder n SOVA: SOVA is supposed to be the least computation intensive method compared with MAP, Log. MAP, . . . )Interleaver 1)-1 Decoder 1 r 0 r 1 Interleaver 1 Decoder 2 DEMUX Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Turbo Decoder n Important factors : Iteration u DEMUX u Synchronization u n Main idea u The decoder will pass the hard decision together with a reliability estimate of this decision to the next decoder Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Promises n n n The major promise of turbo codes is their astonishing performance of bit error rate (BER) at relatively low Eb/N 0. For AWGN channel: for a frame size of 256=65536 bits we can achieve a BER=10 -5 at only Eb/N 0=0. 7 d. B, which is very close to Shannon limit. For a Rayleigh fading channel: a BER=10 -5 can be achieved at Eb/N 0=4. 3 d. B which represents a gain of 2. 3 d. B as compared to classical convolutional codes with similar complexity. Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Challenges n n n Delay: associated with interleaver and the iterative decoding algorithm. Complexity: in the optimal decoding algorithm. Can we use Turbo Codes for real time applications ? Can we improve Turbo Codes ? Can we introduce a mathematical model for the evaluation of different Turbo Codes ? Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Can We improve Turbo Codes ? A good understanding of the code ingredients will help to improve the code. n If we understand what the interleaver is doing we can introduce short frames suitable for real time applications. n Puncturing is very important to increase the rate to an applicable value. n The decoding algorithm can be sufficiently simplified yet with very minor degradation. n Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99

Conclusion We have reviewed Turbo Codes and some of their promises and challenges. n n Turbo Codes are attracting more and more applications. More work can be done in understanding the effects of the codes ingredients on the performance. n n The decoding delay is a major challenge in turbo coding Turbo Coding can be applied as a case study for many of the existing systems ( GSM, IS(. … n Mugaibel/Kousa Turbo Codes: Promises and Challenges , CETEM' 99