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- Agrell, Erik, 1965, et al.
(författare)
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Gray coding for multilevel constellations in Gaussian noise
- 2007
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Ingår i: IEEE Transactions on Information Theory. - 0018-9448 .- 1557-9654. ; 53:1, s. 224-235
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Tidskriftsartikel (refereegranskat)abstract
- The problem of finding the optimal labeling (bit-to-symbol mapping) of multilevel coherent phase shift keying (PSK), pulse amplitude modulation (PAM), and quadrature amplitude modulation (QAM) constellations with respect to minimizing the bit-error probability (BEP) over a Gaussian channel is addressed. We show that using the binary reflected Gray code (BRGC) to label the signal constellation results in the lowest possible BEP for high enough signal energy-to-noise ratios and analyze what is "high enough"in this sense. It turns out that the BRGC is optimal for PSK and PAM systems whenever the target BEP is at most a few percent, which covers most systems of practical interest. New and simple closed-form expressions are presented for the BEP of PSK, PAM, and QAM using the BRGC. © 2007 IEEE.
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| 5. |
- Agrell, Erik, 1965, et al.
(författare)
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On optimal constellations for BICM at low SNR
- 2009
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Ingår i: Proceedings IEEE Information Theory Workshop ITW 2009, Taormina, Italy [Best Poster Award]. - 9781424449835 ; , s. 480-484
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Konferensbidrag (refereegranskat)abstract
- In this paper we study the problem of finding capacity-maximizing constellations in BICM for asymptotically low signal-to-noise ratios (SNRs). We base our analysis on the so-called Hadamard transform and on a linear approximation of the BICM capacity for asymptotically low SNRs. We fully characterize the set of constellations, input distributions, and binary labelings that achieve Shannon's limit E_b/N_0=-1.59 dB. For equiprobable input distributions, a constellation achieves this limit if and only if it is a linear projection of a hypercube.
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| 7. |
- Agrell, Erik, 1965, et al.
(författare)
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Power-efficient modulation formats in coherent transmission systems
- 2009
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 27:22, s. 5115-5126
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Tidskriftsartikel (refereegranskat)abstract
- Coherent optical transmission systems have a four-dimensional (4-D) signal space (two quadratures in two polarizations). These four dimensions can be used to create modulation formats that have a better power efficiency (higher sensitivity) than the conventional binary phase shift keying/quadrature phase shift keying (BPSK/QPSK) signals. Several examples are given, with some emphasis on a 24-level format and an 8-level format, including descriptions of how they can be realized and expressions for their symbol and bit error probabilities. These formats are, respectively, an extension and a subset of the commonly used 16-level dual-polarization QPSK format. Sphere packing simulations in 2, 3, and 4 dimensions, up to 32 levels, are used to verify their optimality. The numerical results, as the number of levels increases, are shown to agree with lattice-theoretical results. Finally, we point out that the use of these constellations will lead to improved fundamental sensitivity limits for optical communication systems, and they may also be relevant as a way of reducing power demands and/or nonlinear influence. © 2009, IEEE. All rights reserved.
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| 8. |
- Alvarado, Alex, 1982, et al.
(författare)
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Correcting Suboptimal Metrics in Iterative Decoders
- 2009
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Ingår i: IEEE International Conference on Communications. - 1550-3607. - 9781424434350
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Konferensbidrag (refereegranskat)abstract
- In this paper the issue of improving the performance of iterative decoders based on sub-optimal calculation of the messages exchanged during iterations (L-values) is addressed. It is well known in the literature that a simple---yet very effective---way to improve the performance of suboptimal iterative decoders is based on applying a scaling factor to the L-values. In this paper, starting with a theoretical model based on the so-called consistency condition of a random variable, we propose a methodology for correcting the L-values that relies only on the distribution of the soft information exchanged in the iterative process. This methodology gives a clear explanation of why the well-known linear scaling factor provides a very good performance. Additionally, the proposed methodology allows us to avoid the exhaustive search required otherwise. Numerical simulations show that for turbo codes the scaling factors found closely follow the optimum values, which translates to a close-to-optimal BER performance. Moreover, for LDPC codes, the proposed methodology produces a better BER performance compared with the known method in the literature.
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| 9. |
- Alvarado, Alex, 1982, et al.
(författare)
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On the Capacity of BICM with QAM Constellations
- 2009
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Ingår i: Proceedings International Wireless Communications and Mobile Computing Conference, IWCMC'09, Leipzig, Germany [Invited]. - New York, NY, USA : ACM. - 9781605585697 ; , s. 573-579
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Konferensbidrag (refereegranskat)abstract
- In this tutorial paper we analyze the capacity of bit-interleaved coded modulation (BICM) with quadrature amplitude modulation (QAM) constellations, and we pay special attention to different bit-to-symbol labeling strategies. The relation between the BICM capacity and the capacity of other CM schemes such as trellis coded modulation (TCM) and multilevel codes (MLC) is analyzed. Motivated by the fact that for BICM with some particular labelings, the same Eb/No maps to more than one BICM capacity value, we study the relation between the capacity and Eb/No. In particular, we present some analytical results on this relation, and we also give an intuitive explanation for the somehow contradictory behavior of these curves.
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| 10. |
- Alvarado, Alex, 1982, et al.
(författare)
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On the Design of Interleavers for BICM Transmission
- 2008
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Ingår i: Proceedings European Wireless Conference 2008, EW2008, Prague, Czech Republic. - 9783800731022
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Konferensbidrag (refereegranskat)abstract
- In this paper we design interleavers for bit-interleaved coded modulation (BICM) based on popular convolutional codes and quadrature amplitude modulation with Gray mapping. We analyze the so-called modular interleavers where the outputs of the convolutional encoder are appropriately matched with the input bits of the modulator. To quantify the achievable improvements we develop bounds on the coded BER using themultidimensional weight distribution spectrum of a code together with an equivalent QAM channel model. Based on these bounds, we show that the assignment of the encoder's output to the bit positions in the symbol significantly affects the system performance. The analytical developments are contrasted with numerical simulations. The improvements obtained through the proposed approach do not change the receiver's complexity and, depending on the system's parameters (rate, modulation, code's memory), they may be up to 1.7 dB!
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