Sara Sandberg

Design of bandwidth-efficient unequal error protection LDPC codes

This paper presents a strategy for the design of bandwidth-efficient LDPC codes with unequal error protection. Bandwidth efficiency is obtained by appropriately designing the codes for higher order constellations, assuming an AWGN channel. The irregularities of the LDPC code are designed, using the Gaussian approximation of the density evolution, to enhance the unequal error protection property of the code as well as account for the different bit error probabilities given by the higher order constellation. The proposed code design algorithm is flexible in terms of the number and proportions of protection classes. It also allows arbitrary modulation schemes. Our method combines the design of unequal error protection LDPC codes for the binary input AWGN channel with the code design for higher order constellations by dividing the variable node degree distribution into sub-degree distributions for each protection class and each level of protection from the modulation. The results show that appropriate code design for higher order constellations reduces the overall bit-error rate significantly. Furthermore, the unequal error protection capability of the code is increased, especially for high SNR.

UEP concepts in modulation and coding

First unequal error protection (UEP) proposals date back to the 1960s (Masnick and Wolf; 1967), but now with the introduction of scalable video, UEP develops to a key concept for the transport of multimedia data. The paper presents an overview of some new approaches realizing UEP properties in physical transport, especially multicarrier modulation, or with LDPC and Turbo codes. For multicarrier modulation, UEP bit-loading together with hierarchical modulation is described allowing for an arbitrary number of classes, arbitrary SNR margins between the classes, and arbitrary number of bits per class. In Turbo coding, pruning, as a counterpart of puncturing is presented for flexible bit-rate adaptations, including tables with optimized pruning patterns. Bitand/ or check-irregular LDPC codes may be designed to provide UEP to its code bits. However, irregular degree distributions alone do not ensure UEP, and other necessary properties of the parity-check matrix for providing UEP are also pointed out. Pruning is also the means for constructing variable-rate LDPC codes for UEP, especially controlling the check-node profile.

On the UEP Capabilities of Several LDPC Construction Algorithms

This paper analyzes construction algorithms for low-density parity-check (LDPC) codes with respect to their unequal error protection (UEP) capabilities. We show that the choice of code construction algorithm highly affects the performance and UEP properties of LDPC codes with identical degree distributions. Our results provide an explanation to disagreements in earlier research.

Design of Unequal Error Protection LDPC Codes for Higher Order Constellations

We present an optimization method for unequal error protection (UEP)-LDPC codes with higher order constellations. By modifying the density evolution algorithm under the Gaussian approximation, we propose a flexible code design algorithm for a variable number of protection classes and arbitrary modulation schemes with Gray mapping. Our results show that appropriate code design for higher order constellations reduces the overall bit-error rate. Furthermore, the influence on the UEP capability of the code, that is, the difference in bit-error rate between the protection classes, is investigated.

Jointly optimized rate-compatible UEP-LDPC codes for half-duplex co-operative relay networks

This paper addresses the design of low-density parity-check (LDPC) codes for half-duplex co-operative relay networks. Structured rate-compatible codes with unequal error protection (UEP) are designed through joint optimization of the codes for the channels between source and relay and source and destination. The proposed codes clearly outperform simpler LDPC codes which are not optimized for relay channels and puncturing-based rate-compatible LDPC codes, and they show a significant performance improvement over the direct link communication depending on the position of relay. The optimization algorithm for the proposed codes is based on density evolution using the Gaussian approximation and optimal variable node degree distributions are found through iterative linear programming. Interestingly, they anyhow show performance which is almost comparable to the performance of codes optimized through a more complex non-linear optimization algorithm. We analyze the performance of our proposed codes with short, medium and long block lengths, and with low and high rates under realistic assumptions, i.e., imperfect decoding of the codeword at relay and variant signal-to-noise ratio within a single codeword.

Improved design of unequal error protection LDPC codes

We propose an improved method for designing unequal error protection (UEP) low-density parity-check (LDPC) codes. The method is based on density evolution. The degree distribution with the best UEP properties is found, under the constraint that the threshold should not exceed the threshold of a non-UEP code plus some threshold offset. For different codeword lengths and different construction algorithms, we search for good threshold offsets for the UEP code design. The choice of the threshold offset is based on the average a posteriori variable node mutual information. Simulations reveal the counter intuitive result that the short-to-medium length codes designed with a suitable threshold offset all outperform the corresponding non-UEP codes in terms of average bit-error rate. The proposed codes are also compared to other UEP-LDPC codes found in the literature.

Interference mitigation based on precoded SRS

This paper illustrates an interference mitigation method based on the estimation of an equivalent channel using Sounding Reference Signal (SRS) sequences in Time Division Duplex (TDD) multi-cell systems. The key idea is to precode the SRS sequence with a pre-coder that is computed based on the choice made by the User Equipment (UE) of its preferred signal subspace. Assuming channel reciprocity in TDD systems, the interfering Base Stations (BSs) can estimate the equivalent precoded channels and avoid sending signal to their own UE in that direction, thus reducing interference, without any need for feedback. We investigate this method with varying degrees of available perfect channel state information at the transmitter and show that it can surpass a Maximum-Ratio-Transmission (MRT) transceiver.

Transmission Strategy for Interference Alignment in a System-Level Perspective

The paper builds up on the idea of separating, in the time dimension, between conventional uncoordinated MIMO and IA transmission. The former usually utilizes all available dimensions (antennas) to send useful information, while the latter mitigates inter-cell interference. Due to the multitude of users in cellular networks, the transmission schemes are suitable for different groups of users. Therefore, in order to take advantage of both schemes, the users are classified into two groups: one with those users that supposedly benefit from cooperation and the other with those that should not cooperate. The proposed method for grouping the users has the advantage of being based on a simple decision metric, which avoids having to calculate precoders for all different possible combinations. Since each group must apply the proper transmission scheme, a fraction of the temporal resources is dedicated to each type of transmission. An automatic reservation of the temporal resources and an adaptive adjustment of the grouping threshold for each user, which avoids having to empirically determine a single threshold, are also proposed. These proposed mechanisms are shown to provide performance gains of up to 30% over the conventional transmissions.