Robust Digital Retrodirective Beamforming Technique for Multipath Channel Environment

Orthogonal Frequency Division Multiplexing (OFDM) is employed in the current 4G cellular technologies. Because of its resistance to multipath propagation channels, OFDM is a popular multicarrier modulation technique. Although OFDM is one of the effective approaches for a 4G network, out-of-band radiation, inter-symbol interference (ISI), and inter-carrier interference make this method unsuitable for 5G. (ICI). A new approach or next-generation cellular network, i.e., very high data rate, low latency, greater throughput, more device connectivity, and improved device-to-device communication, should be used to deliver effective and dependable communication. The system introduces generalized frequency division multiplexing (GFDM). In GFDM, a single cyclic prefix is utilized throughout the frame and an adjustable pulse-shaping filter can be used to monitor the transmitted signal’s out-of-band emission. Using this ISI, the multipath channel can be controlled. In the proposed system, with large signal constellations, higher-order modulation schemes, and advanced receiver structure, a very high data rate can be achieved. By using various iterative receiver structures the performance of the GFDM system in 5G cellular networks can be analyzed.

Current studies on modulation and detection schemes in molecular communication mainly focus on the scenarios with static transmitters and receivers. However, mobile molecular communication is needed in many envisioned applications, such as target tracking and drug delivery. Until now, investigations about mobile molecular communication have been limited. In this paper, a static transmitter and a mobile bacterium-based receiver performing random walk are considered. In this mobile scenario, the channel impulse response changes due to the dynamic change of the distance between the transmitter and the receiver. Detection schemes based on fixed distance fail in signal detection in such a scenario. Furthermore, the intersymbol interference (ISI) effect becomes more complex due to the dynamic character of the signal which makes the estimation and mitigation of the ISI even more difficult. In this paper, an adaptive ISI mitigation method and two adaptive detection schemes are proposed for this mobile scenario. In the proposed scheme, adaptive ISI mitigation, estimation of dynamic distance, and the corresponding impulse response reconstruction are performed in each symbol interval. Based on the dynamic channel impulse response in each interval, two adaptive detection schemes, concentration-based adaptive threshold detection and peak-time-based adaptive detection, are proposed for signal detection. Simulations demonstrate that the ISI effect is significantly reduced and the adaptive detection schemes are reliable and robust for mobile molecular communication.

수중통신에서는 다중경로로 인해 심벌간의 상호간섭이 발생하여 시스템의 성능을 저하시킨다. 최근 수중 통신시스템에서는 이러한 열악한 채널환경을 극복하고 전송률을 향상시키기 위하여 등화기 및 채널 부호화와 시공간 부호화 방식의 효율적인 연접 구조가 수중통신에서 MIMO시스템으로 응용되어 연구되고 있다. 수중 MIMO 환경에서 시공간 부호화 방식 중에서는 시공간 블록 부호화 방식과 시공간 격자 부호화 방식이 있으며, 시공간 격자 부호화 방식이 등화기 및 채널 부호화 방식에 최적이다. 따라서 본 논문에서는 다중경로 채널 환경에서 효율적인 전송을 위해 터보 Pi 부호화기를 외부부호로 사용하고, MIMO시스템에서 전송 다이버시티와 부호화이득을 동시에 가지는 시공간 격자부호를 내부부호로 구성한다. 그리고 심볼간 상호 간섭 제거를 위해 Zero Forcing 등화기법을 사용하여 수중채널에서의 시뮬레이션을 통해 성능을 확인하였다. In the underwater communication, the performance of system is reduced because of the inter-symbol interference occur by the multi-path. In the recent years, to deal with poor channel environment and improve the throughput, the efficient concatenated structure of equalization, channel codes and Space Time Codes has been studied as MIMO system in the underwater communication. Space Time Codes include Space Time Block Codes and Space Time Trellis Codes in underwater communication. Space Time Trellis Codes are optimum for equalization and channel codes among the Space Time Codes to apply in the MIMO environment. Therefore, in this paper, turbo pi codes are used for the outer code to efficiently transmit in the multi-path channel environment. The inner codes consist of Space Time Trellis Codes with transmission diversity and coding gain in the MIMO system. And Zero Forcing method is used to remove inter-symbol interference. Finally, the performance of this model is simulated in the underwater channel.

Code division multiple access (CDMA) based on spread spectrum technique has become one of the most important multiple access technologies in contemporary communication systems. The properties of spreading codes largely govern the performance of CDMA systems. Therefore, CDMA can insure interference free operation when code design methodology takes into account of major impairing factors such as intersymbol interference (ISI), asynchronous transmission and random signs in consecutive symbols. In this study, the authors propose a design scheme for CDMA signal reception with zero ISI in the multipath environment. Cyclic shift property of maximal length sequence (m‐sequence) is utilised in the design to avoid ISI. The proposed scheme exploits inherent diversity present in the multipath channel and can be seen as antenna diversity. This scheme avoids the use of complex detectors and the performance is compared by simulations with conventional CDMA system using Walsh–Hadamard codes and m‐sequence. The impact of the Rician factor on the end‐to‐end performance is also studied in detail and suitable receiver to be used has been suggested for different fading scenarios. Simulations results presented indicate that the proposed scheme provides superior performance over traditional systems.

The orthogonal frequency division multiplexing (OFDM) transceiver has enjoyed great success in many wideband communication systems. It has low complexity and robustness against multipath channels. It is also well-known that the OFDM transceiver has poor frequency characteristics. To get transceivers with better frequency characteristics, filterbank transceivers with overlapping-block transmission are often considered. However these transceivers in general suffer from severe intersymbol interference (ISI) and high complexity. Moreover costly channel dependent post processing techniques are often needed at the receiving end to mitigate ISI. We design discrete Fourier transform (DFT) modulated filterbank transceivers for multipath fading channels. The DFT modulated filterbanks are known to have the advantages of low design and implementation cost. Although the proposed transceiver belongs to the class of overlapping-block transmission, the only channel dependent part is a set of one-tap equalizers at the receiver, like the OFDM system. We show that for a fixed set of transmitting or receiving filters, the design problem of maximizing signal-to-interference ratio (SIR) can be formulated into an eigenvector problem. Experiments are carried out for transmission over random multipath channels, and the results show that satisfactory SIR performance can be obtained.

Inter-Symbol Interference (ISI) is one of the challenges of bio-inspired diffusion-based molecular communication. The degradation of the remaining molecules from a previous transmission is the solution that biological systems use to mitigate this ISI. While most prior work has proposed the use of enzymes to catalyze the molecules degradation, enzymes also degrade the molecules carrying the information, which drastically decreases the signal strength. In this paper, we propose the use of photolysis reactions, which use the light to instantly transform the emitted molecules so they no longer be recognized after their detection. The light will be emitted in an optimal time, allowing the receiver to detect as many molecules as possible, which increases both the signal strength and ISI mitigation. A lower bound expression on the expectation of the observed molecules number at the receiver is derived. Bit error probability expression is also formulated, and both expressions are validated with simulation results, which show a visible enhancement when using photolysis reactions. The performance of the proposed method is evaluated using Interference-to-Total-Received molecules metric (ITR) and the derived bit error probability.

In this communication, we present a new method of encoding /decoding information contained in a wave packet using multipath effect. A signal is mixed with an auxiliary signal and sent into an enclosed cavity. The strongly distorted signal transmitted through the cavity is then sent to a receiver, where it is time reversed and sent to a second enclosed metallic cavity that is strictly identical to the first one. As a result, the waveform of the initial signal is reconstructed at a point of the second cavity that is equivalent to the point of the first cavity due to the spatial and temporal focusing. Using the mechanism, secure wireless data transmission is carried out. Finally, the impact of the size of the cavity on intersymbol interference and the frequency dispersion caused by frequency selective fading are discussed. Such a communication system ensures a high level of security.

The paper studies the capacities of time hopping ultra wideband (TH-UWB) systems by adopting orthogonal pulse-based modulation schemes in a multipath environment with the consideration of multiple access interference (MAI) and inter symbol interference (ISI). Recently, orthogonal pulse-based modulation schemes such as pulse shape modulation (PSM), biorthogonal PSM (BPSM) and the combination of orthogonal pulse position modulation (OPPM) and BPSM have been proposed for UWB system to reduce ISI and MAI. In this paper, we first investigate the influences of ISI and MAI on the capacities of TH-UWB systems in terms of signal-to-interference and noise ratio (SINR) for high-order modulation schemes based on various sets of orthogonal pulses. How mutual information varies with the number of multipath components (MPCs) is then analyzed. The complete mathematical analysis and simulation results are provided in detail. When maximizing the mutual information, the Gaussian distribution approximate is assumed to describe the characteristics of MAI and ISI. Simulation results are provided to verify the theoretical analysis. Some conclusions are drawn thereafter, such as that capacity depends on the pulse patterns adopted and mutual information varies with inversely proportional to the number of MPCs. We also observed that the capacities of UWB systems for OPPM-BPSM schemes are larger than those of PSM and BPSM schemes.

An adaptive transmission scheme using variable-length spreading sequences (VLSS) is proposed. The system is based on an impulse direct sequence ultra-wideband (DS-UWB) approach for wireless communication where the number of active users is variable. In contrast to the conventional DS-UWB system, which employs a fixed length spreading code, the proposed system changes the length of spreading code according to the system load adaptively, which is proven to be able to reduce the inter-chip interference, inter-symbol interference and multiple-access interference and thus improve the system performance in terms of bit error rate. The proposed system is simulated in multipath channels with a high level of multipath fragments, modelled by the standard IEEE 802.15.3a indoor channel model 1 (CM1) and channel model 2 (CM2). Numerical results demonstrate that when RAKE receivers are employed, the proposed VLSS DS-UWB system outperforms the conventional system by appropriately allocating the spreading sequences.

Underwater acoustic (UWA) communication in shallow water has been challenging owing to multipath-induced signal distortion and motion-induced Doppler distortion. In this study, we investigate the performance of a UWA communication system using orthogonal signal division multiplexing (OSDM) on intersymbol interference (ISI) and Doppler shift in a simulation, by comparing it with an existing communication system that uses coherent modulation with an adaptive filter, to clarify whether OSDM is suitable for UWA communication in shallow water. The obtained results clarify that OSDM is robust to heavy ISI, and it is expected to be suitable for a rapidly time-varying channel. For example, OSDM can improve the output signal-to-noise ratio (SNR) of 9.4 dB in a shallow water channel with a multipath delay time dispersion of 20 symbols, compared with the UWA communication system with decision feedback equalizer (DFE). However, it is also clarified that OSDM is more sensitive to the Doppler effect compared with the existing communication system, and there is the necessity of applying frequency-offset compensation, because the permissible Doppler shift, when the output SNR falls to -3 dB, for OSDM is less than half that for the UWA communication system with DFE.

Intersymbol Interference (ISI) always appears when the communication channel is a multi-path channel. Many methods are used to reduce the effect of ISI. In this paper we focus on the autocorrelation property of the spreading codes and the role it plays in minimizing the ISI effect, where we obtain the spreading codes with minimum autocorrelation property. The results of comparing the average autocorrelation of the obtained codes with that of the well known Hadamard codes show a great enhancement in the performance, where for example, for codes of length 8 a gain of 408% was achieved at one shift, and for codes of length 16 a gain of 530% was achieved at one shift.

In this paper, we propose a generic receiver based on optimum maximum likelihood sequence estimation (MLSE) employing a generalized Ungerboeck metric with significantly reduced complexity for the general class of M-ary quasi orthogonal signaling (MOS) in severe multipath fading channels. First, by using the factor graph (FG) of the obtained generalized Ungerboeck metric for MOS and sum-product algorithm (SPA) framework, a highly efficient reduced state sequence estimation (RSSE) algorithm is proposed that operates on forward and backward directions and is directly applied to unwhitened channel matched filter (CMF) and code matched filter outputs at symbol rate. The proposed structure, generating the a posteriori probabilities (APP) by bidirectional RSSE recursions, are substantiated with symbol rate bidirectional decision feedback (BDF) based on surviving paths in order to eliminate the post- and pre-cursor inter-symbol interference (ISI) as well as multi code interference (MCI) due to the non-ideal properties of the signaling waveforms and multipath channel. It appears as the unification of Ungerboeck type reduced trellis equalization framework for MOS and achieves near-optimum performance especially for channels with large dispersion. Second, we identify a bias term through an error probability analysis which helps us improve the proposed receiver's performance. Furthermore, a tight approximation for the bit error rate (BER) of the proposed receiver is derived. These analyses lead to significant insight on the selection of system parameters and clearly demonstrate the high performance and efficiency of the proposed scheme by a proper choice of the signaling parameters.

This paper proposes a combination of adaptive equalizer and Least Mean Square-RAKE (LMS-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by mitigating the multipath effect using LMS-RAKE receiver and suppressing the ISI effect with the adaptive equalizer. Simulation results verify that significant equalization gain can be obtained by the proposed system especially in UWB multipath channel models such as channel CM3 and channel CM4 that suffered severe ISI effect.

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

The multiple-chirp-rate signal has been proved a good orthogonal characteristic in BFT (biorthogonal Fourier transform) domain. In this work, it is adopted to improve the modulation efficiency with BFT demodulator. However, the performance of BFT demodulator in real wireless environment was not evaluated yet. By our derivation, unfortunately, the orthogonality of multiple-chirp-rate chirp in BFT domain is proved to be damaged over multipath channel. Furthermore, the interference model is constructed, and the linear MMSE detection is proposed to recover the M-ary symbols. Numerical results indicate that the algorithm can simultaneously mitigate the inter-symbol interference (ISI) and the inter-rate interference of the BFT output in multipath environments.