| 1. |
- 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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| 2. |
- Alvarado, Alex, 1982, et al.
(författare)
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Distribution of L-values in Gray-mapped M^2-QAM: Closed-form Approximations and Applications
- 2009
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Ingår i: IEEE Transactions on Communications. - 1558-0857. ; 57:7, s. 2071-2079
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we develop closed form approximations for theprobability density function (PDF) of the reliability metrics (L-values) in bit-interleaved coded modulation (BICM). The expressions are valid for M^2-ary quadrature amplitude modulations (M^2-QAM) with Gray mapping when the metrics are calculated using the so-called max-log approximation. Based on the developed expressions, we also propose two simple Gaussian mixture approximations that are analytically tractable. We apply our developments to efficiently calculate the BICM channel capacity,and to develop bounds on the coded bit-error rate when aconvolutional code is used. The coded performance of an hybrid automatic repeat request (HARQ) based on constellation rearrangement is also evaluated.
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| 3. |
- Alvarado, Alex, 1982, et al.
(författare)
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Distribution of L-values in Gray-mapped M^2-QAM Signals: Exact Expressions and Simple Approximations
- 2007
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Ingår i: IEEE Global Telecommunications Conference, GLOBECOM 2007. - 1424410436 ; , s. 1759-1764
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Konferensbidrag (refereegranskat)abstract
- In this paper we develop formulas for the probability density function (PDF) of the reliability metrics in bit-interleaved coded modulation (BICM) for arbitrary M^2-ary quadrature amplitude modulation (M^2-QAM) with Gray mapping. The advantage of this approach over an entirely general one proposed previously, is that the resulting formulas are obtained without resorting to any algorithmic steps. We also propose Gaussian mixture approximations that are analytically tractable, and we analyze their accuracy when evaluating uncoded bit error rate (BER) and BICM capacity.
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| 4. |
- Alvarado, Alex, 1982, et al.
(författare)
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Exploiting UEP in QAM-based BICM: Interleaver and Code Design
- 2010
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 58:2, s. 500-510
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we formally analyze the interleaver and code design for QAM-based BICM transmissions using the binary reflected Gray code. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds the optimum design of interleaver and code is found, and numerical results for representative configurations are presented. When the new design is used, the improvements may reach 2 dB, and they are obtained without any increase on the transceiver's complexity. We also introduce the concept of generalized optimum distance spectrum convolutional codes, which are the optimum codes for QAM-based BICM transmissions.
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| 5. |
- Alvarado, Alex, 1982, et al.
(författare)
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On BICM-ID with Multiple Interleavers
- 2010
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Ingår i: IEEE Communications Letters. - 1558-2558 .- 1089-7798. ; 14:9, s. 785-787
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Tidskriftsartikel (refereegranskat)abstract
- In this letter, we study the performance of BICM-ID with multiple interleavers (BICM-ID-M) in terms of bit-error rate (BER), and show that BICM-ID-M is well-suited to exploit the unequal error protection (UEP) caused by the binary labeling. We show that BICM-ID-M should always be the preferred alternative for BICM-ID and that the gains obtained appear even for the simplest configuration (0.5-0.75 dB for a BER of 10-7). It is found that conventional design paradigms such as maximizing the free distance of the code should be modified.
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| 6. |
- Alvarado, Alex, 1982, et al.
(författare)
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On BICM receivers for TCM transmission
- 2011
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 59:10, s. 2692-2702
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Tidskriftsartikel (refereegranskat)abstract
- Recent results have shown that the performance of bit-interleaved coded modulation (BICM) using convolutional codes in nonfading channels can be significantly improved when the interleaver takes a trivial form (BICM-T), i.e., when it does not interleave the bits at all. In this paper, we give a formal explanation for these results and show that BICM-T is, in fact, the combination of a TCM transmitter and a BICM receiver. To predict the performance of BICM-T, a new type of distance spectrum for convolutional codes is introduced, analytical bounds based on this spectrum are developed, and asymptotic approximations are presented. It is shown that the free Hamming distance of the code is not the relevant optimization criterion for BICM-T. Asymptotically optimal convolutional codes for different constraint lengths are tabulated and BICM-T is shown to offer asymptotic gains of about 2 dB over traditional BICM designs based on random interleavers. The asymptotic gains over uncoded transmission are found to be the same as those obtained by Ungerboeck's one-dimensional trellis-coded modulation (1D-TCM), and therefore, in nonfading channels, BICM-T is shown to be as good as 1D-TCM.
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| 7. |
- 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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| 8. |
- Alvarado, Alex, 1982, et al.
(författare)
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On the Distribution of Extrinsic L-values in Gray-mapped 16-QAM
- 2007
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Ingår i: AMC International Wireless Communications and Mobile Computing Conference 2007, IWCMC 2007. - New York, NY, USA : ACM. - 1595936955 ; , s. 329-336
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Konferensbidrag (refereegranskat)abstract
- In this paper we address the issue of probabilistic modelling of the extrinsic L-values, used as reliability metrics in the context of bit interleaved coded modulation with iterative demapping (BICM-ID). Starting with a simple piece-wise linear model of the L-values obtained via the max-log approximation, we derive the expressions for the cumulative distribution functions of the L-values, that differentiated produce the desired forms of the probability density functions for Gray-mapped 16-QAM. To illustrate the usefulness of our analytical expressions we applied them to efficiently compute the so-called EXIT functions of the demapper for different values of SNR. The proposed analytical expressions are also compared to the histograms of the L-values obtained through time-consuming simulations.
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| 9. |
- Alvarado, Alex, 1982, et al.
(författare)
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On the performance of BICM with trivial interleavers in nonfading channels
- 2011
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Ingår i: IEEE International Conference on Communications ICC 2011. - : IEEE. - 1550-3607. - 9781612842332
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Konferensbidrag (refereegranskat)abstract
- Recent results have shown that the performance of bit-interleaved coded modulation (BICM) using convolutional codes in nonfading channels can be greatly improved if the bit-level interleaver takes a trivial form (BICM-T), i.e., if it does not interleave the bits at all. The reported gains reach a few decibels and are obtained using a less complex BICM system. In this paper, we give a formal explanation for these results and show that BICM-T is in fact the combination of a TCM transmitter and a BICM receiver. Analytical bounds that predict the performance of BICM-T are developed and a new type of distance spectrum for the convolutional code is introduced.
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| 10. |
- Alvarado, Alex, 1982, et al.
(författare)
-
Unequal Error Protection in BICM with QAM Constellations: Interleaver and Code Design
- 2009
-
Ingår i: IEEE International Conference on Communications. - 1550-3607. - 9781424434350
-
Konferensbidrag (refereegranskat)abstract
- In this paper we present a general methodology for the interleaver and code design for QAM-based BICM transmissions. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds, the optimum design of interleaver and code is presented. The improvements obtained reached 2~dB for the analyzed cases, and are obtained without complexity increase. Although previous works noted the influence of the interleaver design and the UEP, to the best of our knowledge, this paper is the first to analyze formally this problem for BICM transmissions.
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