Superposition coded modulation with peak-power limitation

We apply clipping to superposition coded modulation (SCM) systems to reduce the peak-to-average power ratio (PAPR) of the transmitted signal. The impact on performance is investigated by evaluating the mutual information driven by the induced peak-power-limited input signals. It is shown that the rate loss is marginal for moderate clipping thresholds if optimal encoding/decoding is used. This fact is confirmed in examples where capacityapproaching component codes are used together with the maximum a posteriori probability (MAP) detection. In order to reduce the detection complexity of SCM with a large number of layers, we develop a suboptimal soft compensation (SC) method that is combined with soft-input soft-output (SISO) decoding algorithms in an iterative manner. A variety of simulation results for additive white Gaussian noise (AWGN) and fading channels are presented. It is shown that with the proposed method, the effect of clipping can be efficiently compensated and a good tradeoff between PAPR and bit-error rate (BER) can be achieved. Comparisons with other coded modulation schemes demonstrate that SCM offers significant advantages for high-rate transmissions over fading channels

Link-to-System Interfaces for System Level Simulations Featuring Hybrid ARQ

Projecte de final de carrera fet en col.laboració amb Technische Universität München. Lehrstuhl für KommunikationsnetzeWithin the continuous evolution of wireless communications, new and ambitious requirements are planned to be met by next generation of mobile communications. In order to achieve those requirements, new technologies and mechanisms that work well over broadband frequency, like OFDM and OFDMA, need to be investigated, developed and tested in simulators. New cellular systems designs are based on exploiting instantaneous channel conditions, improving the system performance. Due to this, system level simulations must support a Physical Layer (PHY) abstraction which accurately predicts the instantaneous performance of the link layer. In order to accomplish this, a new link-to-system (L2S) interface has been developed and implemented in OpenWNS, a system level simulator for evaluation of OFDM systems developed at RWTH, Aachen. This interface is mainly realized through a set of mapping mechanisms, used to provide a BLER given a received coded block SINR. Together with the theoretical study and evaluation of these mappings mechanisms, this thesis will analyze and evaluate Hybrid Automatic Repeat Request mechanisms (H-ARQ), and how these protocols can be implemented in system level simulators working together with the L2S interface. This thesis shows how the new L2S interface is more accurate than past approaches, providing a gain of around 3 dB. Concerning H-ARQ protocols, the results show how the use of these new techniques provides a considerable gain with respect to normal ARQ or not using any such techniqu

Physical-Layer Cooperation in Coded OFDM Relaying Systems

Mobile communication systems nowadays require ever-increasing data rate and coverage of wide areas. One promising approach to achieve this goal is the application of cooperative communications enabled by introducing intermediate nodes known as relays to support the transmission between terminals. By processing and forwarding the receive message at the relays, the path-loss effect between the source and the destination is mitigated. One major limit factor for relay assisted communications is that a relay cannot transmit and receive using the same physical resources. Therefore, a half-duplex constraint is commonly assumed resulting in halved spectral efficiency. To combat this drawback, two-way relaying is introduced, where two sources exchange information with each. On the other hand, due to the physical limitation of the relays, e.g., wireless sensor nodes, it's not possible to implement multiple antennas at one relay, which prohibits the application of multiple-input multiple-output (MIMO) techniques. However, when treating multiple relays as a cluster, a virtual antenna array is formed to perform MIMO techniques in a distributed manner. %This thesis aims at designing efficient one-way and two-way relaying schemes. Specifically, existing schemes from the literature are improved and new schemes are developed with the emphasis on coded orthogonal frequency division multiplexing (OFDM) transmissions. Of special interest is the application of physical-layer network coding (PLNC) for two-phase two-way relaying. In this case, a network coded message is estimated from the superimposed receive signal at the relay using PLNC schemes. The schemes are investigated based on a mutual information analysis and their performance are improved by a newly proposed phase control strategy. Furthermore, performance degradation due to system asynchrony is mitigated depending on different PLNC schemes. When multiple relays are available, novel cooperation schemes allowing information exchange within the relay cluster are proposed that facilitate distributed MIMO reception and transmission. Additionally, smart signaling approaches are presented to enable the cooperation at different levels with the cooperation overhead taken into account adequately in system performance evaluation

Link abstraction models based on mutual information for LTE downlink

Postprint (author’s final draft