Gottfried Lechner

Testbedding MIMO HSDPA and WiMAX

Abstract—Modern wireless communication systems employMIMO and feedback—two properties that make it especiallydifficult to measure the performance of such systems withreasonable effort in actual outdoor scenarios.In this paper, we will present a time, cost, and manpower effi-cient measurement approach to evaluate the throughput achievedby such systems. Summer/winter, large distance outdoor-to-indoor/outdoor, urban/alpine measurements have been carriedout to successfully test this approach. Exemplarily, we report onthe throughput gains (over TX power and base-station-antenna-rotation) of standard compliant 2x2 MIMO HSDPA and IEEE802.16-2004 WiMAX when, for example, employing improvedchannel coding methods. I. M OTIVATION Modern wireless communication systems rely on complexalgorithms to squeeze out the last bit of performance froma radio link. They do so by exploiting the characteristics of achannel in an ingenious way. In the end—although sometimesa good benchmark—it is not the BER or the theoreticallyachievable capacity that counts, but the throughput actuallyachieved by the system under investigation.Determining the throughput performance of ingenious trans-mission schemes, on the other hand, is a different story. Todo so, usually, the communication system is simulated in e.g.M

Efficient DSP implementation of an LDPC decoder

We present a high performance implementation of a belief propagation decoder for decoding low-density parity-check (LDPC) codes on a fixed point digital signal processor. A simplified decoding algorithm was used and a stopping criteria for the iterative decoder was implemented to reduce the average number of required iterations. This leads to an implementation with increased throughput compared to other implementations of LDPC codes or turbo codes. This decoder is able to decode at 5.4 Mbps on a Texas Instruments TMS320C64xx DSP running at 600 MHz.

Optimization of LDPC Codes for Receiver Frontends

The degree distribution of low-density parity-check (LDPC) codes is optimized for systems that iterate over the receiver frontend, e.g., soft detector, demodulator, equalizer, etc., and the LDPC decoder. The overall extrinsic information transfer (EXIT) function of an iterative LDPC decoder is computed, based on the codes own EXIT chart, under the Gaussian assumption. While the optimization of the variable node distribution is a nonlinear problem, the optimization of the check node distribution is shown to be a linear problem. This fact is exploited to design codes where both the variable and the check node distributions are optimized, resulting in more robust constructions. The technique presented requires only knowledge of the measured EXIT function of the receiver frontend

Spatially Coupled Repeat-Accumulate Codes

In this paper we propose a new class of spatially coupled codes based on regular repeat-accumulate codes. We show that these codes have several advantages over spatially coupled regular low-density parity-check codes including simpler encoders and slightly higher code rates when the thresholds and decoding complexity (as measured by the Tanner graph edge density) are similar.

Design of Multi-Edge-Type Bilayer-Expurgated LDPC Codes for Decode-and-Forward in Relay Channels

We consider the design of bilayer-expurgated low density parity-check (BE-LDPC) codes as part of a decode and-forward protocol for use over the full-duplex relay channel. A new ensemble of codes, termed multi-edge-type bilayer expurgated LDPC (MET-BE-LDPC) codes, is introduced where the BE-LDPC code design problem is transformed into the problem of optimizing the multinomials of a multi-edge-type LDPC code. We propose two design strategies for optimizing MET-BE-LDPC codes; the bilayer approach is preferred when the difference in SNR between the source-to-relay and the source to-destination channels is small, while the bilayer approach with intermediate rates is preferred when this difference is large. In both proposed design strategies multi-edge-type density evolution is used for code optimization. The resulting MET-BE-LDPC codes exhibit improved threshold and bit-error-rate performance as compared to previously reported bilayer LDPC codes.

Multi-Way Relay Networks: Orthogonal Uplink, Source-Channel Separation and Code Design

We consider a multi-way relay network with an orthogonal uplink and correlated sources, and we characterise reliable communication (in the usual Shannon sense) with a single-letter expression. The characterisation is obtained using a joint source-channel random-coding argument, which is based on a combination of Wyner et al.s Cascaded Slepian-Wolf Source Coding and Tuncels Slepian-Wolf Coding over Broadcast Channels. We prove a separation theorem for the special case of two nodes; that is, we show that a modular code architecture with separate source and channel coding functions is (asymptotically) optimal. Finally, we propose a practical coding scheme based on low-density parity-check codes, and we analyse its performance using multi-edge density evolution.

EXIT Chart Analysis of Binary Message-Passing Decoders

Binary message-passing decoders for LDPC codes are analyzed using EXIT charts. For the analysis, the variable node decoder performs all computations in the L-value domain. For the special case of a hard decision channel, this leads to the well-know Gallager B algorithm, while the analysis can be extended to channels with larger output alphabets. By increasing the output alphabet from hard decisions to four symbols, a gain of more than 1.0 dB is achieved using optimized codes. For this code optimization, the mixing property of EXIT functions has to be modified to the case of binary message-passing decoders.

Implementation of an LDPC decoder on a vector signal processor

A parallel processor architecture-a vector signal processor (VSP), which consists of independent computation units is presented. This architecture is used to implement the sum-product algorithm to decode low-density parity-check codes. The VSP is well suited for this parallel decoding algorithm which results in a scalable decoder that allows a tradeoff between chip area and data throughput. With increasing number of computation units a data throughput of up to 36.1 MBit per second can be achieved which outperforms existing implementations on digital signal processors.

Anytime characteristics of spatially coupled code

In this paper, we propose an efficient channel coding scheme for anytime communications. We design this anytime code based on spatially coupled LDPC codes and investigate the performance over binary erasure channels. Through density evolution analysis, we asymptotically show the desired anytime properties of the proposed code. We also numerically predict the finite-length performance of the anytime code. We develop an adaptive window decoding technique to ensure a low complexity anytime receiver. Through simulation, we compare the performance of our proposed code with existing anytime codes.

Check splitting of root-check LDPC codes over ARQ block-fading channels

In this paper we extend the concept of check splitting to root-check LDPC codes, thus providing an incremental redundancy code construction specifically for the ARQ block-fading channel. By construction, the proposed coding scheme achieves a high level of diversity and effectively adapts the transmission rate to the instantaneous channel conditions.