Carl-Erik W. Sundberg

Multiuser MIMO downlink with limited inter-cell cooperation: Approximate interference alignment in time, frequency and space

We consider a realistic albeit simplified scenario for wireless cellular systems of the next generation (4G and beyond), where MIMO-OFDM, opportunistic scheduling, channel state information at the transmitter and limited base-station cooperation are envisaged. We propose two strategies with limited base-station cooperation that can be easily implemented with todays technology and achieve an approximate form of inter-cell interference alignment. The first strategy consists of imposing a ldquopower maskrdquo in frequency such that adjacent cooperative clusters of base stations generate different interference levels in different frequency subchannels. The second strategy consists of switching between different cooperative clusters such that no user is in a permanently disadvantaged location.We compare single-user and multiuser MIMO systems in terms of average throughput as a function of the user location.

Hierarchical QAM BICM MIMO systems with iterative decoding and applications to media broadcast

The paper presents an approach to the design of layered BICM MIMO systems using multi-stage iterative decoding and Soft Output M-Algorithm (SOMA) based component decoders. The systems consider transmission based on multilevel coding linked to hierarchical QAM constellations. The receivers are iterative designs with a view to supporting high-rates. These receiver designs can also be used to obtain reduced complexity. The performance of the SOMA schemes are compared to achievable rate bounds for multi-level D-BLAST based schemes. The D-BLAST receivers illustrate general limitations of non-iterative approaches. The multi-layer transmission approach provides a natural support for unequal error protection (UEP), and can be used to balance tradeoffs between robust coverage vs quality in media broadcast applications. Rough predictions on such tradeoffs are given using layered MPEG2 video as a media example.

Wideband space-time coded systems with non-collocated antennas

In this paper we consider space-time coded wideband transmission from multiple base-stations. These systems can provide improved resistance to shadowing and extended range. An inherent challenge with these systems, however, is asynchronous reception of the signal elements. We describe three full space diversity approaches for wideband transmission from non-collocated antennas, with throughput, diversity, receiver-complexity trade-offs. Common to all systems are OFDM-type signaling, bit interleaved coded modulation, and iterative decoding. The lowest data-rate lowest complexity system employs an inner orthogonal space- time block code with embedded OFDM transmission, while the highest data-rate highest complexity scheme does not employ an inner code. The latter scheme can provide even higher data rates at reduced space diversity.

Iterative SOMA-Based SU-MIMO Receivers with Extrinsic-Information Adaptive Front-Ends

In this paper we present reduced-complexity high- performance iterative receivers for single-user MIMO systems that employ OFDM with bit-interleaved coded modulation. The proposed inner-outer decoder structures exploit the Soft-Output M algorithm (SOMA) to calculate bit-likelihood estimates via a limited search on a tree. A key element of the proposed designs is the way the SOMA front-end adapts at each iteration to new extrinsic information. This front-end defines the tree, both in terms of the order in which symbols are considered, as well as in terms of the subspaces used in the likelihood calculations at each depth in the tree. The proposed SOMA front-ends are different from the conventional QR-decomposition front-ends and provide a systematic way to adapt both the order and the subspaces at each iteration in an iterative decoding structure. Simulations for 4 × 4 16 QAM systems show that the proposed iterative receivers can yield significant performance gains over receivers using non-adaptive conventional front-ends. Index Terms—MIMO, iterative decoding, OFDM, BICM, MMSE, reduced-complexity receivers. I. INTRODUCTION

Network Coding over Time-Varying Networks by Means of Virtual Graphs

We present a method for network coding over time-varying networks with attractive tradeoffs between throughput and decoding-delay. We model the time-varying networks as a sequence of topology graphs. The method relies on designing a deterministic network code for a fixed virtual graph derived from the actual sequence of topologies, and on exploiting proper buffering in order to implement the code over the actual network. We show that, under certain conditions, this approach can achieve the maximum achievable throughput over the given time- varying network. We also present simulation results that validate the theoretical findings.

Iterative SU-MIMO receivers based on SOMA demappers with improved survivor selection

We present a new class of reduced-complexity highperformance iterative receivers for single-user MIMO/OFDM systems with bit-interleaved coded modulation (BICM). The proposed inner-outer decoder structures rely on joint demappers which generate soft-output via an M-algorithm limited tree-search. A key element of the proposed designs is the way the M-algorithm chooses its survivor candidates during the tree search. In particular, at every depth in the tree the survivor list comprises the best M candidates as well as additional good candidates which are chosen to enable high-quality soft-output information for every bit. In the context of iterative inner/outer decoders, simulations show that the proposed survivor selection criteria result in superior performance/complexity trade-offs as compared to a conventional M-algorithm search.