Diversity Combining Techniques for Bandwidth ... - Semantic Scholar

1 downloads 0 Views 56KB Size Report
Daniel J. Costello Jr. Department of EE,. University of Notre Dame,. Notre Dame, IN, U.S.A. e-mail: Costello[email protected]. Thomas E. Fuja. Department of EE,.
ISIT2001, Washington, DC, June 24–29, 2001

Diversity Combining Techniques for Bandwidth-Efficient Turbo ARQ Systems Adrish Banerjee1

Daniel J. Costello Jr.

Thomas E. Fuja

Department of EE, University of Notre Dame, Notre Dame, IN, U.S.A. e-mail: [email protected]

Department of EE, University of Notre Dame, Notre Dame, IN, U.S.A. e-mail: [email protected]

Department of EE, University of Notre Dame, Notre Dame,IN, U.S.A. e-mail: [email protected]

Abstract — A type-I hybrid-ARQ scheme using multiple turbo codes is proposed. It uses turbo coded bit interleaved modulation for high bandwidth efficiency. The structure of parallel concatenated codes is exploited for diversity combining.

I. Introduction We present a framework for turbo coded bit interleaved modulation utilizing its structure for diversity combining. This work is motivated by recent results for binary modulation, where it has been shown that a bank of parallel low complexity encoders connected by several interleavers can perform as well as a conventional turbo code of the same rate, and it can do so with less complexity [1].

obtained by puncturing information and parity bits from a 2 rate 1/4 multiple turbo code, with a 4-state [1 1+D+D ] con1+D stituent code, in conjunction with the Gray mapped 16-QAM signal constellation. A 24 bit CRC is used for error detection. A coding gain of about 1.5 dB over the conventional diversity combining scheme was observed at low SNR. The smaller coding gain at higher throughputs is due to the better performance of the partially systematic rate 1/2 multiple turbo code compared with the conventional rate 1/2 turbo code for higher bandwidth efficiencies. 2

1.6

1.4

III. Numerical Results The following example compares the performance of a turbo diversity combining scheme with a conventional diversity combining scheme for an AWGN channel. Example: Scheme A uses a rate 2/4 turbo code obtained by 1+D2 puncturing a rate 1/3 turbo code, with a 4-state [1 1+D+D 2] constituent code which is Gray mapped to 16-QAM. Scheme B uses a rate 2/4 partially systematic multiple turbo code 1 This

work is supported by Motorola Inc., NASA grant NAG 5-8355, NSF grant CCR00-75514 and NSF grant CCR-9996222

Throughput(bits/s/Hz)

II. System Description The underlying turbo coded modulation scheme uses a multiple turbo code [2], bit interleaved and mapped to a signal constellation after its output bits are suitably punctured and multiplexed to achieve the desired number of information bits per transmitted symbol. Gray mapping is used between the coded bits and the modulation symbols. For multiple turbo codes, the decoder based on APP decoding works in extended serial mode [3], where the decoders take turns, each accepting the most recent extrinsic information from the other decoders. Type-I hybrid-ARQ (HARQ) diversity combining can be categorized into two types as follows: Scheme A: (Conventional diversity combining) The same packet is retransmitted until the receiver accepts it as error free or until a preset maximum allowed number of retransmission attempts is reached. For decoding, the soft values from the current as well as the previously received errorneous packets are combined. Scheme B: (Turbo diversity combining) On every retransmission, the same information bits are transmitted with different parity sequences. At the receiver, soft values of the information bits and different parity sequences are concatenated to form a lower rate code [4].

Scheme A Scheme B

1.8

1.2

1

0.8

0.6

0.4

0.2

0

0

1

2

3

4

5 Es/No(dB)

6

7

8

9

10

Fig. 1: Throughput comparison of different diversity combining turbo-ARQ schemes over an AWGN channel, N=512 symbols.

IV. Conclusions A new structure for bandwidth efficient coding is proposed which can be used with diversity combining techniques for rate adaptive bandwidth efficient hybrid-ARQ schemes. The combining of the received bits from different transmissions to form a more powerful lower rate code results in improved throughput and error performance at low SNR’s.

References [1] P. Massey, “A 4-state alternative code to the proposed 8-state turbo coding standard,” IEEE Communication Theory Workshop, (Borrego Springs, California), May 2001. [2] D. Divsalar and F. Pollara, “Multiple turbo codes,” in Proc. 14th Military Communications Conference, MILCOM, pp. 279–285, 1995. [3] J. Han and O. Y. Takeshita, “On the decoding structure of multiple turbo codes,” in Proc. IEEE International Symposium on Information Theory, (Washington D.C.), June 2001. [4] S. A. Barbulescu, M. Rice, and S. Pietrobon, “Turbo diversity combining scheme,” in Turbo Coding Seminar, pp. 51–58, Aug. 1996.