[Show abstract][Hide abstract] ABSTRACT: We compare LDPC block and LDPC convolutional codes with respect to their decoding performance under low decoding latencies. Protograph based regular LDPC codes are considered with rather small lifting factors. LDPC block and convolutional codes are decoded using belief propagation. For LDPC convolutional codes, a sliding window decoder with different window sizes is applied to continuously decode the input symbols. We show the required Eb/N0 to achieve a bit error rate of 10-5 for the LDPC block and LDPC convolutional codes for the decoding latency of up to approximately 550 information bits. It has been observed that LDPC convolutional codes perform better than the block codes from which they are derived even at low latency. We demonstrate the trade off between complexity and performance in terms of lifting factor and window size for a fixed value of latency. Furthermore, the two codes are also compared in terms of their complexity as a function of Eb/N0. Convolutional codes with Viterbi decoding are also compared with the two above mentioned codes.
[Show abstract][Hide abstract] ABSTRACT: The spectral efficiency of today's cellular networks that feature small inter-site distance and high spectral reuse is limited by inter-cell interference. An effective means to cope with the signal radiation across cell boundaries in the cellular uplink is joint detection of multiple users at cooperative base stations (BSs), a concept known as network MIMO or coordinated multipoint (CoMP). However, it is well known that the cluster size of cooperating base stations is limited in a real system due to backhaul, latency and signaling constraints. Thus, cooperation of base station needs to be applied jointly with other methods for inter cell interference reduction. An important lever is the usage of antenna downtilt to control the direction of the vertical antenna pattern and, therefore, the distance of signal propagation. In this work, we investigate the effect of the antenna downtilt on the performance of cooperative uplink detection in a large scale field trial and show the importance of downtilt optimization for cooperative systems.
[Show abstract][Hide abstract] ABSTRACT: This paper introduces a failure aware dynamic task scheduling approach for unreliable heterogeneous MPSoCs. Global and local errors are sporadically injected in the system. Two dynamic task scheduling modes are newly introduced to compensate these errors, one for each error injection method. Error free processing elements are favored, faulty ones are isolated. In case of an error the erroneous task is detected and dynamically compensated to guarantee an error free execution. Different applications are used to prove the feasibility of this approach. The failure aware dynamic task scheduling approach assures an error free execution of all applications.
[Show abstract][Hide abstract] ABSTRACT: The dramatic increase of network infrastructure comes at the cost of rapidly
increasing energy consumption, which makes optimization of energy efficiency
(EE) an important topic. Since EE is often modeled as the ratio of rate to
power, we present a mathematical framework called fractional programming that
provides insight into this class of optimization problems, as well as
algorithms for computing the solution. The main idea is that the objective
function is transformed to a weighted sum of rate and power. A generic problem
formulation for systems dissipating transmit-independent circuit power in
addition to transmit-dependent power is presented. We show that a broad class
of EE maximization problems can be solved efficiently, provided the rate is a
concave function of the transmit power. We elaborate examples of various system
models including time-varying parallel channels. Rate functions with an
arbitrary discrete modulation scheme are also treated. The examples considered
lead to water-filling solutions, but these are different from the dual problems
of power minimization under rate constraints and rate maximization under power
constraints, respectively, because the constraints need not be active. We also
demonstrate that if the solution to a rate maximization problem is known, it
can be utilized to reduce the EE problem into a one-dimensional convex problem.
Full-text · Article · Oct 2011 · IEEE Transactions on Wireless Communications
[Show abstract][Hide abstract] ABSTRACT: With the opening up of white spaces, efficient use of the fragmented spectrum - TV white space in particular - has become an extremely important focus of research. Apart from efficient spectrum usage, special care needs to be taken to maintain low out-of-band radiation to avoid harmful interference to incumbent services like TV signals and wireless microphones. For this reason, a flexible digital radio with multicarrier modulation scheme is the only solution. In this paper, we show the performance of two innovative multicarrier systems, Filter Bank Multi Carrier (FBMC) and Generalized Frequency Division Multiplexing (GFDM). A simple interference cancellation technique called serial inter-carrier interference cancellation scheme has been used to improve performance of the GFDM system. Channel equalization techniques have been implemented for FBMC.
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a detailed analysis into the benefits and costs of merging decoders for different channel code types such as convolutional, turbo and LDPC codes. An ASIP (application-specific instruction set processor)-based framework for multi-code forward error correction (FEC) architectures is applied to implement three dedicated decoders for convolutional, turbo and LDPC codes respectively as well as one decoder capable of decoding all three. Synthesis results and performance estimations for all architectures are presented and used to draw a clear and fair comparison between single-mode and multi-mode decoders.
[Show abstract][Hide abstract] ABSTRACT: High detection complexity is known to be one of the major challenges in MIMO communications based on spatial multiplexing. Tuple Search Detector (TSD) was recently introduced, significantly reducing detection complexity in comparison to conventional algorithms while achieving close to full max-log-APP BER performance. Irregular control flow and sequential nature of depth-first-based detectors frustrate efficient application of parallelization techniques, typically leading to inefficient realizations. This work presents a novel TSD implementation, based on a scalable and parallelizable pipelined ASIP architecture. The proposed VLSI design is implemented for 4×4 MIMO transmission using 64-QAM constellation on 65-nm CMOS technology. In low SNR scenarios, proposed detector achieves 403.6 Mbps throughput at 454 MHz clock frequency. TSD can be moreover adjusted according to transmission conditions, reaching >1 Gbps. A silicon area of 0.14 mm2 (98.9 kGEs) is occupied by the TSD core, reporting low power dissipation (57.94 mW) under typical case operating conditions. Proposed detector implementation achieves close to full max-log-APP BER performance and high detection throughput with reasonable hardware complexity, by far outperforming state-of-the-art realizations.
[Show abstract][Hide abstract] ABSTRACT: In recent years, chip design complexity is further increasing through multi-processor system-on-chip built up from macro blocks. System-level design promises to close the growing productivity gap between hardware and software design but more sophisticated design models are needed. Therefore, we developed a new model of system-level design abstraction. Therein, three views divide the design space to reduce design complexity. A unified design process with five steps has been defined for the design views. Furthermore, we show that the model is able to formalize system-level design exploration. Finally, exploration has been considered as walk through the design views connected via inter-view links. With the proposed system-level model, the authors provide conceptual foundations to more holistic design and introduce formalization of design exploration.
[Show abstract][Hide abstract] ABSTRACT: Message passing schedules that reduce the decoding complexity of terminated LDPC convolutional code ensembles are analyzed. Considering the AWGN channel, various schedules are compared by means of density evolution. The results of the analysis together with computer simulations for some (3,6)-regular codes confirm that sliding window decoding is an attractive practical solution for low-latency and low-complexity decoding.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes two new arbitration techniques to enable fair link bandwidth allocation. One technique is a weighted round-robin scheme with weights based on the number of contending flows at the input port. The second technique is an age-based scheme with probabilistic arbitration where the traversed packet distance approximates age. Opposed to existing work, both schemes reach almost absolute fairness of link bandwidth allocation and simplify calculation of arbitration metrics.
[Show abstract][Hide abstract] ABSTRACT: Coordinated Multi-Point (CoMP) appears to be an effective option to combat inter-cell interference in mobile communications. Previous field trials for uplink CoMP have shown that large improvements in spectral efficiency and fairness that are promised by theoretical work can also be achieved in real-world scenarios. However, these results only consider systems with single antenna base stations. We extend this work by presenting field trial results for a system with multi antenna base stations, and we show that this change of the system setup has a strong impact not only on the throughput but also on the relative performance of a cooperative compared to a non-cooperative system. Based on the presented results suggestions for further research and field trials are derived.
[Show abstract][Hide abstract] ABSTRACT: The energy consumption of wideband communications receivers depends highly on the parametrization of the A/D conversion stage. The design of energy-efficient receivers requires an optimal parametrization. This paper investigates the power dissipation of state-of-the-art A/D converters and studies the optimal parametrization from an information theoretic perspective. The results show that a large sampling rate and very low quantization resolution will usually be most energy-efficient.
[Show abstract][Hide abstract] ABSTRACT: High detection complexity is known to be one of the major challenges in MIMO communications based on spatial multiplexing. Tuple Search Detector (TSD) was recently introduced, significantly reducing detection complexity in comparison to conventional algorithms while achieving close to full max-log-APP BER performance. Besides high computational complexity, irregular control flow and sequential nature of the tree search represent major limitations of depth-first-based detectors, frustrating efficient application of parallelization techniques and hence leading to inefficient realizations with regard to most practical applications. This work presents a novel TSD implementation, scalable in constellation size and number of antennas and mapped to a highly parallel and pipelined ASIP architecture. Major challenges and key strategies enabling a high-throughput and low-complexity realization are presented and performance of the resulting flexible and efficient detector implementation is evaluated. Proposed realization is shown to achieve >; 300 Mbps throughput at a reference clock frequency of 400 MHz (regarding 4×4 MIMO transmission with 16-QAM), by far outperforming comparable state-of-the-art realizations.
[Show abstract][Hide abstract] ABSTRACT: This paper analyzes the impact of dynamic task scheduling, processing element allocation and data transfer management on system performance of heterogeneous MPSoCs. Therefore, all parts of a runtime scheduling unit are analyzed. Bottlenecks are identified and their complexity evaluated. Furthermore, traced information is processed with two newly introduced tools. The first one generates an annotated SDF3 file. The second one creates a static schedule which applies the same scheduling and allocation decision as the dynamic scheduler. The execution of the static schedule reduces the burden of task management to a minimum. The resulting static execution is compared with the execution of the dynamic schedule. Hence, runtime overhead of dynamic scheduling is unveiled.
[Show abstract][Hide abstract] ABSTRACT: This article quantifies the global carbon footprint of mobile communication systems, and discusses its ecological and economic implications. Using up-to-date data and life cycle assessment models, we predict an increase of CO2 equivalent emissions by a factor of three until 2020 compared to 2007, rising from about 86 to 235 Mto CO2e, suggesting a steeper increase than predicted in the well-known SMART2020 report. We provide a breakdown of the global carbon footprint, which reveals that production of mobile devices and global radio access network operation will remain the major contributors, accompanied by an increasing share of emissions due to data transfer in the backbone resulting from rising mobile traffic volumes. The energy bill due to network operation will gain increasing importance in cellular business models. Furthermore, technologies to reduce energy consumption are considered a key enabler for the spread of mobile communications in developing countries. Taking into account several scenarios of technological advancement and rollout, we analyze the overall energy consumption of global radio access networks and illustrate the saving potential of green communication technologies. We conclude that, conditioned on quick implementation and alongside other "classical" improvements of spectral efficiency, these technologies offer the potential to serve three orders of magnitude more traffic with the same overall energy consumption as today.
Full-text · Article · Aug 2011 · IEEE Communications Magazine
[Show abstract][Hide abstract] ABSTRACT: Coordinated multi-point (CoMP) has been selected as a key technology feature of LTE-Advanced, as it enables the exploitation of inter-cell interference in order to significantly increase spectral efficiency, especially at the cell-edge. While first field trials on CoMP schemes have delivered the proof-of-concept and shown that a moderate extent of theoretically predicated CoMP gains can indeed be achieved in practical systems, the implementation of these schemes has revealed many practical challenges. One central question is, for example, how small cooperation clusters can be extracted from large cellular systems, such that major portions of potential CoMP gains can be obtained at minimum signaling overhead. This paper deals with static clustering concepts, and shows that both in a hexagonal cell layout and under a realistic deployment and signal propagation scenario, static clustering concepts can perform close to optimal UE-specific clustering, while being easy to use and requiring negligible signaling overhead.
[Show abstract][Hide abstract] ABSTRACT: Mobile internet devices are among the fastest growing markets in consumer electronics. A big issue with these devices is the low battery capacity that requires the user to recharge the device regularly. This paper aims to reduce the energy consumption of LTE modems used in future devices. For this, the LTE HARQ process is modeled as a Markov chain and a simple generic model for energy consumption is introduced that divides energy consumption in a static and a dynamic part. Using these and the results of link-level simulations it is found that throughput maximization will also maximize the energy efficiency if the static energy consumption dominates. However, if the energy consumption also contains a dynamic part, the energy efficiency will severely degrade unless another optimization approach is used.
[Show abstract][Hide abstract] ABSTRACT: Energy-efficient link adaptation is studied on a slowly varying Rayleigh fading channel assuming perfect CSI at the receiver but no CSI at the transmitter, except for the statistical characterization. Both circuit power and power dissipated in the power amplifier are considered and the energy consumption per bit is minimized over bandwidth, transmission power, and transmission rate. It is shown that the joint optimization of power and rate leads to a point on a curve that connects the two extreme situations that correspond to ignoring the power dissipated in the power amplifier and ignoring the circuit power dissipation, respectively. The first case leads to throughput maximization whereas the second case leads to transmit power minimization. The global optimum depends on the ratio of the power dissipated in the power amplifier to the total power dissipation.
[Show abstract][Hide abstract] ABSTRACT: Opportunistic use of White Spaces (WS) has opened up a whole new paradigm in research on cognitive radio (CR). One of the issues addressed by the QoSMOS project is the design of a flexible and efficient physical layer (PHY) for CR systems. To this aim, different modulation techniques are being investigated and are presented in this paper. They fulfill the low out of band leakage and spectral efficiency requirements of a CR operating in fragmented WS. The design of reconfigurable and flexible radio-frequency (RF) front-end is also introduced, since its performance is tightly linked to these parameters and to the implementation feasibility and constraints. Emphasis is put on benchmarking the performance of the proposed schemes over classical approaches.
[Show abstract][Hide abstract] ABSTRACT: Coordinated Multi-Point (CoMP) is known to be a key technology for next generation mobile communications systems, as it allows to overcome the burden of inter-cell interference. Especially in the uplink, it is likely that interference exploitation schemes will be used in the near future, as they can be used with legacy terminals and be based on operator-proprietary signal processing concepts, hence requiring no or little changes in standardization. Major drawbacks, however, are the extent of additional backhaul infrastructure needed, and the sensitivity to imperfect channel knowledge. This paper jointly addresses both issues in a new framework incorporating a multitude of proposed theoretical uplink CoMP concepts, which are then put into perspective with practical CoMP algorithms. This comprehensive analysis provides new insight into the potential value of different uplink CoMP concepts in next generation wireless communications systems, and reveals the subset of schemes that are most likely to be used in practice.