Polyphase Scrambled Walsh Codes for Zero-Correlation Zone Extension in QS-CDMA

Polyphase complex scrambling patterns are designed to extend the zero-correlation zone (ZCZ) of a family of user spreading sequences, called properly scrambled Walsh-Hadamard (PSW) codes, recently proposed for quasi-synchronous (QS) code-division multiple-access systems. The merits of the binary (ZCZ equals 2) and quadri-phase (ZCZ equals 4) PSW codes over the existing QS spreading codes are still retained in the proposed polyphase PSW codes. Scrambling patterns for the case of ZCZ of 8 are derived first. Then, the scrambling pattern design rules are generalized for greater ZCZs and a structured procedure to generate polyphase PSW codes for any ZCZ is developed. Finally, the cross-correlation properties of the proposed spreading codes are evaluated.This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by IEEE-Institute of Electrical and Electronics Engineers and can be found at: http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=4234. ©2012 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.Keywords: zero-correlation zone (ZCZ), orthogonal spreading codes, Quasi-synchronous CDMA, scrambling.Keywords: zero-correlation zone (ZCZ), orthogonal spreading codes, Quasi-synchronous CDMA, scrambling

Iterative detection for frequency-asynchronous distributed Alamouti-coded (FADAC) OFDM

We propose a near intercarrier interference (ICI)-free and very low complexity iterative detector for frequency-asynchronous distributed Alamouti-coded (FADAC) orthogonal frequency division multiplexing (OFDM). In the previous cancelation schemes, the entire subcarrier signals from one transmit (TX) antenna are estimated and canceled in the received signal from the other TX antenna and vice versa. However, the reliability of the estimated symbols are revealed to significantly vary across the subcarriers and thus, the poorly estimated symbols lead to the incorrect cancelation. Motivated from this, we first propose a scheme which does not cancel the interfering subcarrier(s) at the half band edges which undergo very high interference in FADAC-OFDM. For further improvement, we propose a so-called selective scheme which instantly measures the reliability of the detected symbols at each iteration and then exclude the unreliable symbols in the estimated interference generation. Moreover, the proposed scheme has a drastically reduced complexity by converting the cancelation process from the subcarrier domain to the time domain. In accordance with the analysis on the considered reliability measures, the numerical results show that the proposed scheme achieves the near ICI-free level only within three or four iterations for wide ranges of SNR, frequency offset, and delay spread.1

A Very Low Complexity QRD-M MIMO Detection Based on Adaptive Search Area

We propose a low complexity QR decomposition (QRD)-M multiple input multiple output (MIMO) detection algorithm based on adaptive search area. Unlike the conventional QRD-M MIMO detection algorithm, which determines the next survivor path candidates after searching over the entire constellation points at each detection layer, the proposed algorithm adaptively restricts the search area to the minimal neighboring constellation points of the estimated QRD symbol according to the instantaneous channel condition at each layer. First, we set up an adaptation rule for search area using two observations that inherently reflect the instantaneous channel condition, that is, the diagonal terms of the channel upper triangle matrix after QR decomposition and Euclidean distance between the received symbol vector and temporarily estimated symbol vector by QRD detection. In addition, it is found that the performance of the QRD-M algorithm degrades when the diagonal terms of the channel upper triangle matrix instantaneously decrease. To overcome this problem, the proposed algorithm employs the ratio of each diagonal term and total diagonal terms. Moreover, the proposed algorithm further decreases redundant complexity by considering the location of initial detection symbol in constellation. By doing so, the proposed algorithm effectively achieves performance near to the maximum likelihood detection algorithm, while maintaining the overall average computation complexity much lower than that of the conventional QRD-M systems. Especially, the proposed algorithm achieves reduction of 76% and 26% computational complexity with low signal to noise ratio (SNR) and high SNR, compared with the adaptive QRD-M algorithm based on noise power. Moreover, simulation results show that the proposed algorithm achieves both low complexity and lower symbol error rate compared with the fixed QRD-M algorithms. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.1

Waveform Design for 5G and beyond Systems

Currently, 5G communication systems are being commercially deployed in many countries [...

Performance Analysis of Rate Splitting in Massive MIMO Systems with Low Resolution ADCs/DACs

In this paper, we propose rate-splitting (RS) multiple access to mitigate the effects of quantization noise (QN) inherent in low-resolution analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). We consider the downlink (DL) of a multiuser massive multiple-input multiple-output (MIMO) system where the base station (BS) is equipped with low-resolution ADCs/DACs. The BS employs the RS scheme for data transmission. Under imperfect channel state information (CSI), we characterize the spectral efficiency (SE) and energy efficiency (EE) by deriving the asymptotic signal-to-interference-and-noise ratio (SINR). For 1-bit resolution, the QN is very high, and the RS scheme shows no rate gain over the non-RS scheme. As the ADC/DAC resolution increases (i.e., 2–3 bits), the RS scheme achieves higher SE in the high signal-to-noise ratio (SNR) regime compared to that of the non-RS scheme. For a 3-bit resolution, the number of antennas can be reduced by 27% in the RS scheme to achieve the same SE as the non-RS scheme. Low-resolution DACs degrades the system performance more than low-resolution ADCs. Hence, it is preferable to equip the system with low-resolution ADCs than low-resolution DACs. The system achieves the best SE/EE tradeoff for 4-bit resolution ADCs/DACs

Three-Mode Opportunistic Full-Duplex Switching

This paper proposes a novel opportunistic full-duplex (FD) scheme switching among three duplex modes (DMs) of half duplex (HD), three node FD (TNFD) and bidirectional FD (BiFD) for the cellular systems. To this end, we first derive the conditions, by which one of the three DMs achieves larger average spectral efficiency (ASE) than the other two DMs. Specifically, we derive the minimum required self inteference (SI) cancellation factors of user equipments (UEs)/base station for BiFD/TNFD to achieve the highest ASE among the three duplex modes respectively. We also show that they are given as an explicit function of the static system parameters: cell radius, path loss exponent and target SNR. Notably, for the typical small cell radii, BiFD with the state-of-art SI cancellation techniques significantly outperforms TNFD (which undergoes high inter-UE interference in small cell systems). Compared to the single mode or two mode duplex schemes, substantial ASE gains are achieved by three mode duplex switching according to the aforementioned static system parameters. Moreover, the proposed slow (i.e., a static system parameter-dependent) switching scheme achieves almost identical ASE to that of the fast switching scheme that requires high complexity to calculate and compare the instantaneous spectral efficiencies (SEs) of the three DMs

Problem-based learning in communication systems using MATLAB and Simulink

xxvii, 366 p. : ill. ; 24 cm

Over-sampling effect in distributed Alamouti coded OFDM with frequency offset

Unlike the conventional distributed Alamouti coded orthogonal frequency division multiplexing (OFDM), over-sampling substantially improves the performance of frequency asynchronous distributed Alamouti coded (FADAC) OFDM. This exclusive effectiveness of over-sampling in FADAC-OFDM comes from two factors. One is the unique characteristics of residual inter carrier interference term in FADAC-OFDM and the other factor is moving away the cyclic harmonic inter-carrier interference (ICI) by over-sampling. In addition, the authors confirm that over-sampling factor of 2, i.e. 2N point fast Fourier transform (FFT) is sufficient to move the cyclic harmonic ICI. From this investigation, the authors propose 2N point FFT FADAC-OFDM. The proposed scheme achieves the significantly improved performance not only in the flat fading channel but also in the selective fading channels. For example, while the non-oversampled FADAC-OFDM has the unacceptable error rate level near to or above 0.1 at the band edges even with high signal to noise ratio, the proposed scheme achieves near intersymbol interference-free performance. This leads the proposed scheme to achieve 50% reduction of the required number of null subcarriers to achieve ICI free performance compared with the non-oversampled FADAC-OFDM. Moreover, in terms of implementation feasibility and computational complexity, the proposed scheme is shown to be acceptable. © 2016 The Institution of Engineering and Technology.1

Path loss measurement and modeling of 5G network in emergency indoor stairwell at 3.7 and 28 GHz.

Research on path loss in indoor stairwells for 5G networks is currently insufficient. However, the study of path loss in indoor staircases is essential for managing network traffic quality under typical and emergency conditions and for localization purpose. This study investigated radio propagation on a staircase where a wall separated the stairs from free space. A horn and an omnidirectional antenna were used to determine path loss. The measured path loss evaluated the close-in-free-space reference distance, alpha-beta model, close-in-free-space reference distance with frequency weighting, and alpha-beta-gamma model. These four models exhibited good compatibility with the measured average path loss. However, comparing the path loss distributions of the projected models revealed that the alpha-beta model exhibited 1.29 dB and 6.48 dB for respectively, at 3.7 GHz and 28 GHz bands. Furthermore, the path loss standard deviations obtained in this study were smaller than those reported in previous studies