Ing. Jozef Lukáč

The article deals with single-carrier MAC (Multiple Access Channel), where the relative delay of sources is a fraction of the symbol period. We review the channel likelihood function and propose two approaches to deal with inter-symbol interference (ISI). We either track ISI or remove ISI by a one-step marginalization of the logarithm of the likelihood ratio (LLR). For ISI tracking, we model ISI as a virtual convolutional code observed in a multipath channel. We use both approaches to compare joint decoding (JD) of sources and hierarchical decoding (H-decoding) – decoding of XOR frame. We use concatenated convolutional codes as a channel code and BCJR (Bahl, Cocke, Jelinek, Raviv) decoding algorithm. The proposed methods are suitable for equally strong sources. In that case, we lose about 2 dB when using ISI elimination w.r.t. the ISI tracking.

In the report, we deal with the detection of symbolwise xor HNC map of 2 non-synchronous BPSK sources. The relative delay of frames is an integer multiple of the symbol period. We assume the second source to be delayed by n0 symbols w.r.t. the first source. It is shown that the detection can be decomposed to n0 independent subsequences/parts; further we propose three approximate detectors. Next we show that singular fading that is resolved by HNC map for synchronous sources is not resolved for channels with integer-symbol-period delay for the same HNC map. Neither simple differential encoding will help to resolve it. Finally, we mention that a HNC map should also resolve singular fading in the H-MAC channel with delay and we propose one such simple HNC map – the map with cyclic shift of one frame. A demonstration of the detection on experimental testbed is presented.

In the article, we present an ad hoc (AH) detector for two differentially encoded BPSK sources in the Hierarchical MAC (H-MAC), i.e., for the case when the receiver sees the superposition of non-orthogonal signals from individual sources. (Prefix “H-” means Hierarchical, it emphasizes that the entity is related to the many-to-one principle.) The AH detector decodes the XOR H-map of the two BPSK streams—in other words, it decides whether the transmitted symbols from the two sources are the same or opposite. The BER of the detection in H-MAC is denoted as H-BER. The H-BER is compared with the other two differential detectors, with the coherent (Coh) detector, and with an approximate coherent (ApC) detector. The exact analytical H-BER formula is derived for the ad hoc and coherent detectors. The proposed ad hoc detector is very simple for evaluation, does not require the estimation of subchannel phases, does not depend on noise variance, and it is uniformly only roughly 3.5 dB worse than the coherent one.

This paper addresses the problem of differential demodulation in a 2-source Hierarchical MAC (H-MAC) in the context of a Wireless Physical Layer Network Coded system with Hierarchical Decode and Forward relaying strategy. We consider a parametric H-MAC with known attenuation values but unknown phases. Component phases have generally a strong impact on the resulting superposed constellation and need to be accurately estimated to facilitate a coherent demodulation. The use of a differentially encoded modulation eliminates the need of precise channel phase tracking, which is a nontrivial problem in the WPNC scenario. At the same time it can operate with the same 3dB penalty as in a differential single user case. In this paper we summarize several differential demodulation procedures and additionally introduce new ad hoc methods, which can substantially reduce the computation complexity of the demodulation. We compare the various demodulation algorithms by means of evaluating the hierarchical bit error rate performance for different channel states. Additionally we analyze the robustness of differential demodulation w.r.t. channel phase noise.

This paper considers a noncoherent reception in a parametrized hierarchical MAC (H-MAC) stage in Wireless Physical Layer Network Coding (WPNC) networks with Hierar- chical Decode and Forward (HDF) relay strategy. We employ a differential PSK modulation on two source node transmitters and perform a hierarchical target symbol demodulation at a relay without the need of an explicit estimation of channel phases. Relative channel phase parametrization has generally a nonlinear impact on the resulting constellation, which poses high demands on its estimation accuracy in coherent reception settings. In this paper we present the results of an over-the-air verification of proposed differential demodulation algorithms together with a comparison with respect to results obtained by a computer simulation.

This paper considers a channel state estimation (CSE) pro- blem in a parametrized Hierarchical MAC (H-MAC) stage in Wireless Physical Layer Network Coding (WPNC) networks with Hierarchical Decode and Forward (HDF) relay strategy. The primary purpose is to present the results of a non-pilot based phase estimator performance evaluation. In particular, the performance comparison of a Matlab simulation and an over the air transmission using USRP N210 transceivers in terms of mean square error (MSE) and bit error rate (BER). Also, we analyze the properties of the Cramer Rao Lower Bound (CRLB) w.r.t. different channel parametrizations.