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A hybrid downlink NOMA with OFDM and OFDM-IM for beyond 5G wireless networks
(IEEE - Institute of Electrical and Electronics Engineers, Inc., 2020) Tusha, Armed; Doğan, Seda; Arslan, Hüseyin
In this paper, a hybrid power domain non-orthogonal multiple accessing (NOMA) scheme by the superposition of orthogonal frequency division multiple accessing (OFDM) and index modulated OFDM (OFDM-IM) technologies is presented and named IM-NOMA. It is shown via both computer-based simulations and mathematical analysis that IM-NOMA outperforms the classical OFDM-NOMA in terms of bit error rate (BER) under a total power constraint and achievable sum rate. The system performance of IM-NOMA not only depends on the power difference between the overlapping users but also on features of the OFDM-IM signal. Hence, this scheme is robust against possible catastrophic error performance in case similar power is assigned to the users.
Control of fractional delay effect for SC transmission in beyond 5G networks
(Institute of Electrical and Electronics Engineers Inc., 2022) Yılmaz, Talha; Tusha, Armed; Arslan, Hüseyin
Wireless networks beyond 5G are expected to support a wide range of applications and use cases, and single-carrier (SC) based wireless technologies have been considered in the literature as a suitable solution for high-frequency bands and latency-critical applications. However, SC systems experience severe performance degradation under multipath wireless channels. In a realistic scenario, wireless channels can cause fractional delays that introduce devastating self-interference into the system. Therefore, in this paper, we present an extended channel model that considers pulse shaping and fractional delay. Moreover, we use the proposed model to analyze the performance of the SC transmission scheme with a root-raised-cosine (RRC) pulse shape over a fractional delay channel. In this context, we propose a flexible system design with the aid of the exhaustive search method to control the effect of self-interference on SC in the presence of a fractional delay wireless channel. Lastly, we provide bit error rate results (BER) to validate the accuracy of the proposed compensator for SC systems in practical scenarios.
Error performance enhancement and complexity reduction in OFDM systems via coordinate interleaving under practical impairments
(Turkiye Klinikleri, 2024) Reşat, Mustafa Anıl; Tusha, Armed; Doğan Tusha, Seda; Özyurt, Serdar; Arslan, Hüseyin
In this work, subcarrier coordinate interleaving (CI) is implemented to orthogonal frequency division multiplexing (OFDM) systems with the aim of both enhancing the error performance and reducing the implementation complexity. To this end, the modulated symbols are independently chosen from a modified M-ary amplitude-shift keying signal constellation under a specific CI strategy. In addition to doubling the diversity level of the original OFDM scheme, the adopted CI approach also drastically reduces the inverse fast Fourier transform (IFFT) size at the transmit side by guaranteeing the first half of the input vector to be identical with the second half at the input to the IFFT block. It is further demonstrated that the proposed system has the ability to enhance the robustness against common practical impairments such as insufficient cyclic prefix and phase noise. The closed-form expression of symbol error probability of the system is derived and confirmed with the simulation results.
Exploiting user diversity in OTFS transmission for beyond 5G wireless systems
(Institute of Electrical and Electronics Engineers Inc., 2022) Tusha, Armed; Althunibat, Saud; Hasna, Mazen O.; Qaraqe, Khalid; Arslan, Hüseyin
Orthogonal time frequency space (OTFS) modulation is a recent technology that offers a significant advantage on link reliability due to its strong delay-Doppler resilience. In classical cellular technology, overall system performance is limited by the fading and interference experienced by particular user equipment (UE) over its radio resources (RR). Besides, OTFS performance depends on the channel diversity of the UEs. Therefore, this letter examines the potential of multi-user diversity for OTFS systems considering practical scenarios. Specifically, we introduce a novel scheduling algorithm that proposes the assignment of the available RR of OTFS frame to the UE with the largest number of channel taps, resulting in enhanced channel diversity. It is shown via both mathematical analysis and simulation results that the proposed scheduling technique outperforms the scheme with random access to delay-Doppler RR in terms of system bit error rate (BER) with maximum likelihood (ML) receiver. The BER results validate the accuracy of the proposed technique for OTFS transmission with minimum mean square error (MMSE) detector considering various system configurations.
Index modulation-aided IQ imbalance compensator for OTFS communications systems
(Institute of Electrical and Electronics Engineers Inc., 2022) Tusha, Armed; Dogan Tusha, Seda; Althunibat, Saud; Başar, Ertuğrul; Qaraqe, Khalid; Arslan, Hüseyin
Design of simple transceiver architectures is inevitable in order to provide low computational complexity, low power consumption and affordable cost in beyond 5G (B5G) wireless systems, but it results in hardware impairments that significantly degrade the performance reliability of transmission. In this paper, among these hardware impairments, we discuss in-phase and quadrature (IQ) imbalance in orthogonal time frequency space (OTFS), which is a recent waveform considered as a potential candidate for B5G systems to relax the vulnerability against time-variant wireless channels. To mitigate the effect of IQ imbalance for OTFS, we propose an energy and spectral efficient IQ imbalance compensation scheme with the aid of index modulation (IM), which provides an attractive flexibility in the system design. In contrast to conventional solutions used in classical wireless technologies, such as iterative and pilot-based techniques, the proposed scheme avoids additional energy consumption and significant spectral efficient loss during the estimation and compensation of the IQ imbalance effect. The obtained bit error rate (BER) results validate the accuracy of the proposed compensator for different OTFS system configurations considering perfect/imperfect channel state information in practical scenarios.
Index modulation-based flexible waveform design
(Institution of Engineering and Technology, 2020) Tusha, Seda; Tusha, Armed; Başar, Ertuğrul; Arslan, Hüseyin
The service limitations of conventional orthogonal frequency division multiplexing (OFDM)-based technologies have motivated academia and industry to seek for new solutions in order to support the emerging services and use cases of future wireless networks. In this chapter, promising frequency-domain index modulation (IM) options, i.e., OFDM with IM (OFDM-IM), generalized OFDM with index modulation (OFDM-GIM), dual-mode OFDM (DM-OFDM), OFDM with interleaved subcarrier IM (OFDM-ISIM), are considered as complementary waveforms of classical OFDM. In frequency-domain IM, data information is sent not only via modulated subcarriers but also via proper activation of the subcarriers resulting in higher spectral efficiency (SE) and better error performance compared with OFDM-based schemes. Furthermore, features of OFDM, including intelligent subcarrier selection and adaptive activation ratio, are assessed. Lastly, the flexible utilization of these features is discussed to control channel effects, hardware impairments, asynchronicity, and to serve wide range requirements of fifth generation (5G) and beyond networks.
Intelligent spectrum occupancy prediction for realistic measurements: GRU based approach
(Institute of Electrical and Electronics Engineers Inc., 2022) Tusha, Armed; Kaplan, Batuhan; Çırpan, Hakan Ali; Qaraqe, Khalid; Arslan, Hüseyin
Cognitive radio (CR) technology has always been a research hotspot in the wireless communications field as it has the potential to significantly improve system capacity at the cost of increased processing time and power consumption, which represent highly critical performance indicators (CPI) towards next-generation wireless networks. In particular, the main problem in the CR-based communication links resides in the prediction of spectrum availability in accordance with strict secondary user (SU) CPIs requirements, which is not achievable through the traditional approaches. In this work, we design a novel hierarchical spectrum prediction model, taking advantage from the recurrent neural network (RNN) with the focus on the gated recurrent unit network (GRU). Specifically, the proposed system architecture offers an accrue prediction on the spectrum availability for the SU considering the prior information of the primary user (PU). The performance of the proposed design is illustrated through extensive simulation results. Specifically, real spectrum measurements gathered from Doha, in Qatar are performed to assess the performance accuracy of the designed architecture. In particular different from the conventional scheme that uses a binary representation of spectrum occupancy (idle is '0' and occupied is '1'), we perform training and prediction over the minimum and maximum recorded measurements.
Inter-numerology interference in OFDM-IM systems
(Wiley, 2021) Doğan Tusha, Seda; Tusha, Armed; Başar, Ertuğrul; Althunibat, Saud; Qaraqe, Khalid; Arslan, Hüseyin
In 5G and beyond communication systems, distinct numerologies can coexist to serve diverse requirements for users and applications. However, the inter-numerology interference (INI) is a main challenge that significantly impacts the system performance. Therefore, the performance under INI has become an essential evaluation metric for the suitability of the different transmission schemes in the future communication systems. This paper analyzes the impact of INI on the performance of orthogonal frequency division multiplexing with index modulation (OFDM-IM) systems. Specifically, an analytical expression of the INI level in OFDM-IM systems is presented as a function of the subcarrier activation ratio (SAR) and subcarrier activation probability (SAP). Furthermore, aiming at reducing the INI level, an adaptive subcarrier mapping scheme (SMS) is proposed based on the conventional combinatorial mapping scheme. Moreover, analysis and evaluation of SAR and SAP are performed regarding the requirements of 5G and beyond services. It is proved that the INI level in OFDM-IM systems is highly dependent not only on the number of active subcarriers but also on their position in an OFDM block.
Interference burden in wireless communications: a comprehensive survey from phy layer perspective
(2025) Tusha, Armed; Arslan, Hüseyin
Interference represents one of the most common barriers for the wireless communications society to bring the fully connected world to life, where everybody and everything is connected at any time, aiming to support a wide range of services and applications with increasing demand in terms of data rate with a higher degree of reliability and security, while keeping an affordable overall system capacity, complexity, and latency. Essentially, interference clearly explains the primitive nature of the wireless communications systems, where there is always an unwanted physical signal that disrupts the communication link, occurring from the physical layer (PHY) architecture of transmission signal, its interaction with the wireless channel and transceiver architecture in particular. Therefore, in past wireless technologies, waveform design along with wireless channel impairments and handset architecture define the main sources of interference, leading to inter-symbol interference (ISI), inter-carrier interference (ICI) and co-channel interference (CCI) types. In this line, recent advances in wireless technologies have revealed unprecedented interference types including inter-numerology interference (INI), inter-antenna interference (IAI), inter-waveform interference (IWI), cross-link interference (CLI) and inter-Doppler interference (IDI), while additional unique interference types are expected in near future. Consequently, a broader view of the interference has become a crucial need in order to avoid and relax its impact towards beyond 5G radio access technologies. Despite the extensive research in the literature performed by academia and industry, to the best of the authors' knowledge, there is no work that provides a comprehensive taxonomy framework of interference sources and types, and a review of management techniques from the perspective of the PHY layer. This work aims to fill this gap in the literature. With this notation, in this survey, we propose an intuitive, generic, and expandable framework that categorizes the interference sources and their corresponding management solutions. In particular, we split the interference sources into two main groups by taking into account the user of interest such as self-user-interference (SUI) and other-user-interference (OUI), which we further classify considering the user's intention about the presence of interference named intentional SUI (I-SUI), unintentional SUI (U-SUI), intentional OUI (I-OUI), and unintentional OUI (U-OUI). In line with this, we offer a classification of the interference management techniques regarding the source of interference. Lastly, the survey presents open research perspectives for beyond 5G wireless systems and concluding remarks.
IQI mitigation for narrowband iot systems with OFDM-IM
(Institute of Electrical and Electronics Engineers Inc., 2018) Tusha, Armed; Doğan, Seda; Arslan, Hüseyin
The crucial aim for 5G narrowband Internet of Things (NB-IoT) is to support massive connectivity with wide coverage area, low power consumption, low hardware complexity, and low data rate. Direct conversion receiver (DCR) has a simple structure to fulfill NB-IoT requirements. However, a significant degradation has been observed on the performance of orthogonal frequency division multiplexing (OFDM) systems with the DCR due to in-phase and quadrature imbalance (IQI) at RF front-end. Estimation and mitigation of IQI in OFDM systems require an iterative receiver at the expense of high complexity and power consumption. In this paper, OFDM with index modulation (OFDM-IM) is proposed as a promising candidate in order to meet the demands of NB-IoT use cases. OFDM-IM-based systems carry data information not only by modulated subcarriers but also by indices of fractionally used subcarriers. A non-iterative and efficient receiver that exploits inactive subcarriers is introduced to mitigate the effect of IQI for NB-IoT with OFDM-IM. Cyclic redundancy check algorithm is used to investigate the accuracy of the receiver under IQI. Theoretical analysis and computer-based simulations show that the proposed non-iterative receiver for OFDM-IM in NB-IoT under IQI works very well in various scenarios. Moreover, our findings show that the OFDM-IM under IQI provides higher signal-to-interference ratio in comparison with classical OFDM.
Low complex inter-doppler interference mitigation for OTFS systems via global receiver windowing
(Institute of Electrical and Electronics Engineers Inc., 2023) Tusha, Armed; Arslan, Hüseyin
Orthogonal time frequency space (OTFS) waveform has been considered as a potential candidate for affording a wide range of emerging communications and sensing applications in beyond 5G wireless technologies. OTFS takes advantage of both time and frequency selectivity of the doubly dispersive wireless environment, which has been considered as one of the limiting factors in classical wireless communication technologies. However, the performance of OTFS-based wireless systems is severely affected by fractional Doppler that arises due to inevitable and inherent characteristics of the wireless channel taps of not being always integer Doppler values in the delay-Doppler plane. Thus, this results in severe inter-Doppler interference (IDI) and a low channel sparsity in delay-Doppler (DD) domain. To this end, this work introduces a novel receiver (Rx) windowing technique to combat the impact of fractional Doppler on error performance of the OTFS-based transmission with practical pulse shaping filters at the transceiver. Specifically, we introduce a global cyclic prefix (CP) based raised cosine (RC) windowing at Rx in order to alleviate the effect of IDI on the received signal. To do so, the proposed scheme changes the pulse shape of the OTFS signal in DD domain that defines the IDI level and sparsity of the effective channel. Furthermore, we assess the performance of the proposed Rx windowed OTFS (RW-OTFS) scheme under various practical communication scenarios. The obtained numerical and simulation results validate the accuracy of the proposed IDI mitigator for OTFS systems in practical high-mobility scenarios.
Multidimensional index modulation for 5G and beyond wireless networks
(IEEE-Institute of Electrical and Electronics Engineers Inc, 2021) Doğan Tusha, Seda; Tusha, Armed; Başar, Ertuğrul; Arslan, Hüseyin
Index modulation (IM) provides a novel way for the transmission of additional data bits via the indices of the available transmit entities compared with classical communication schemes. This study examines the flexible utilization of existing IM techniques in a comprehensive manner to satisfy the challenging and diverse requirements of 5G and beyond services. After spatial modulation (SM), which transmits information bits through antenna indices, application of IM to orthogonal frequency-division multiplexing (OFDM) subcarriers has opened the door for the extension of IM into different dimensions, such as radio frequency (RF) mirrors, time slots, codes, and dispersion matrices. Recent studies have introduced the concept of multidimensional IM by various combinations of 1-D IM techniques to provide higher spectral efficiency (SE) and better bit error rate (BER) performance at the expense of higher transmitter (Tx) and receiver (Rx) complexity. Despite the ongoing research on the design of new IM techniques and their implementation challenges, proper use of the available IM techniques to address different requirements of 5G and beyond networks is an open research area in the literature. For this reason, we first provide the dimensional-based categorization of available IM domains and review the existing IM types regarding this categorization. Then, we develop a framework that investigates the efficient utilization of these techniques and establishes a link between the IM schemes and 5G services, namely, enhanced mobile broadband (eMBB), massive machine-type communications (mMTCs), and ultrareliable low-latency communication (URLLC). In addition, this work defines key performance indicators (KPIs) to quantify the advantages and disadvantages of IM techniques in time, frequency, space, and code dimensions. Finally, future recommendations are given regarding the design of flexible IM-based communication systems for 5G and beyond wireless networks.
NOMA with Index modulation for uplink URLLC through grant-free access
(Institute of Electrical and Electronics Engineers Inc., 2019) Doğan, Seda; Tusha, Armed; Arslan, Hüseyin
This paper proposes non-orthogonal sharing of available resources between latency-critical and latency-tolerant communication for fulfilling tight requirements of ultra-reliable low-latency communication (URLLC) as well as avoiding inefficient spectrum utilization of grant-based (GB) access for sporadic URLLC traffic. In the proposed system, grant-free (GF) access is adopted for URLLC to reduce transmission latency, while GB access is used for latency-tolerant communication. Due to GF access, collision emerges between the communications, and use of OFDM technology for both communications leads to wideband interference (WB-I) on URLLC. Therefore, a novel non-orthogonal multiple accessing (NOMA) scheme based on orthogonal frequency division multiplexing (OFDM) and OFDM with index modulation (OFDM-IM) is proposed in order to reduce the impact of the collision on URLLC, that requires 99.999% success probability within 1ms. OFDM-IM technology is used for latency-tolerant communication since WB-I is converted to either narrowband dominant interference (NB-DI) or narrowband interference (NB-I) by fractional subcarrier activation in OFDM-IM. In this way, URLLC is partially affected by latency-tolerant communication. It is shown that the proposed NOMA scheme significantly reduces the latency in comparison to classical NOMA scheme based on pure OFDM while guaranteeing $10^{-5}$ reliability for URLLC, via both computer-based simulations and theoretical analysis.
Non-orthogonal radio access technologies
(Institution of Engineering and Technology, 2020) Tusha, Armed; Tusha, Seda; Arslan, Hüseyin
The radio resource scarcity of classical orthogonal radio access-based technologies has encouraged academia and industry to search for a way out in order to support the exponential growth of data rate, system capacity, and communication latency in beyond 5G wireless communications. Recently, the concept of nonorthogonal radio accessing, where user equipment (UEs) with different services intentionally share the same radio resources, is proposed to compensate the radio resource scarcity. The focus of this chapter is to emphasize the potential of nonorthogonal access technologies in next generation networks. The fundamentals of power-domain non-orthogonal multiple accessing (PD-NOMA) are revised considering system sum-rate and user fairness and compared with conventional orthogonal frequency division multiple access (OFDMA). Furthermore, state-of-the-art NOMAbased schemes, including low-density spreading (LDS) and index modulation (IM), grant-free (GF) random access, and relatively novel concept of waveform coexistence for multiple accessing, are discussed. Finally, future directions and potential non-orthogonal radio access technologies are analyzed considering practical scenarios.
OFDM with index modulation for asynchronous mMTC networks
(MDPI, 2018) Doğan, Seda; Tusha, Armed; Arslan, Hüseyin
One of the critical missions for next-generation wireless communication systems is to fulfill the high demand for massive Machine-Type Communications (mMTC). In mMTC systems, a sporadic transmission is performed between machine users and base station (BS). Lack of coordination between the users and BS in time destroys orthogonality between the subcarriers, and causes inter-carrier interference (ICI). Therefore, providing services to asynchronous massive machine users is a major challenge for Orthogonal Frequency Division Multiplexing (OFDM). In this study, OFDM with index modulation (OFDM-IM) is proposed as an eligible solution to alleviate ICI caused by asynchronous transmission in uncoordinated mMTC networks. In OFDM-IM, data transmission is performed not only by modulated subcarriers but also by the indices of active subcarriers. Unlike classical OFDM, fractional subcarrier activation leads to less ICI in OFDM-IM technology. A novel subcarrier mapping scheme (SMS) named as Inner Subcarrier Activation is proposed to further alleviate adjacent user interference in asynchronous OFDM-IM-based systems. ISA reduces inter-user interference since it gives more activation priority to inner subcarriers compared with the existing SMS-s. The superiority of the proposed SMS is shown through both theoretical analysis and computer-based simulations in comparison to existing mapping schemes for asynchronous systems.
OFDM with index modulation for narrowband iot and mmtc
(İstanbul Medipol Üniversitesi Fen Bilimleri Enstitüsü, 2018) Tusha, Armed; Arslan, Hüseyin
The crucial aim for 5G narrowband Internet of Things (NB-IoT) is to support massive Machine-Type Communications (mMTC) with wide coverage area, low power consumption, low hardware complexity and low data rate. Direct conversion receiver (DCR) has a simple structure to fulfill NB-IoT requirements. However, a significant degradation has been observed on the performance of orthogonal frequency division multiplexing (OFDM) systems with DCR due to in-phase and quadrature-phase imbalance (IQI) at RF front-end. Estimation and mitigation of IQI in OFDM systems require iterative receiver at the expense of high complexity and power consumption. In this study, OFDM with index modulation (OFDM-IM) is proposed as a promising candidate in order to meet demands of NB-IoT use cases. OFDM-IM based systems carry data information not only by modulated subcarriers but also by indices of fractionally used subcarriers. A non-iterative and efficient receiver that exploits inactive subcarriers is introduced to mitigate the e?ect of IQI for NB-IoT with OFDM-IM. Cyclic Redundancy Check (CRC) algorithm is used to investigate accuracy of the receiver under IQI. Theoretical analysis and computer-based simulations show that proposed non-iterative receiver for OFDM-IM in NB-IoT under IQI works very well in various scenarios. Moreover, our findings show that OFDM-IM under IQI provides a higher signal-to-interference ratio (SIR) in comparison to classical OFDM. In mMTC systems, lack of coordination between the machine users and the BS in time destroys orthogonality between the subcarriers, and causes intercarrier interference (ICI). Unlike classical OFDM, fractional subcarrier activation leads to less ICI in OFDM-IM technology. Furthermore, a novel subcarrier mapping scheme (SMS) named as Inner Subcarrier Activation is proposed to further alleviate adjacent user interference in asynchronous OFDM-IM based systems.
Orthogonal time frequency space multiple access using index modulation
(Institute of Electrical and Electronics Engineers Inc., 2023) Doğan Tusha, Seda; Tusha, Armed; Althunibat, Saud; Qaraqe, Khalid
Orthogonal time frequency space (OTFS) is a recent two-dimensional modulation technique utilizing delay-Doppler domain and nominated as a promising candidate waveform for beyond 5G communication systems. By leveraging both time and frequency selectivity of doubly-dispersive radio channels, it has shown a better performance compared to traditional schemes. OTFS offers different options for accommodating multiple users on a given resource block. This article introduces a novel OTFS based uplink multiple access scheme by means of index modulation (IM) in delay-Doppler domain. IM based OTFS multiple access (OTFS-IMMA) proposes the coexistence of multiple users over the same delay-Doppler resources. Unlike conventional OTFS multiple access schemes, OTFS-IMMA allows users to employ IM to activate a subset of the available delay-Doppler resources without a coordination by the base station. Although data collision may occur between the users, our analysis shows that the achievable performance is still better than other schemes. Moreover, the proposed OTFS-IMMA scheme paves the way for flexible resource utilization in OTFS-MA to satisfy various users' demands in beyond 5G systems. It has been shown via both mathematical analysis and simulations that the proposed OTFS-IMMA scheme offers significant enhancement not only on bit error rate performance but also on the overall system efficiency. In addition, the achieved performance gain through OTFS-IMMA further increases as the number of channel taps and/or the number of delay-Doppler bins increase.
Performance analysis of frequency domain im schemes under CFO and IQ imbalance
(Institute of Electrical and Electronics Engineers Inc., 2019) Tusha, Armed; Doğan, Seda; Arslan, Hüseyin
Next generation of mobile networks is the key area of research with increasing demand for new applications and use cases. In order to satisfy the applications with high data rate, high reliability, and low latency, multicarrier transmission through index modulation (IM) has been considered as a promising candidate owing to its flexible structures. In general, existing IM schemes are only evaluated for ideal communication scenarios. In this work, error performance of three frequency domain IM schemes are investigated and compared in the presence of carrier frequency offset (CFO) and in-phase/quadrature (IQ) imbalance. Orthogonal frequency division multiplexing with IM (OFDM-IM), OFDM with generalized IM (OFDM-GIM), and OFDM with subcarrier number modulation (OFDM-SNM) are assessed. It is shown via computer based simulations that although OFDM-GIM and OFDM-SNM provide higher spectral efficiency compared with OFDM-IM, they are more sensitive against RF impairments. Among them, OFDM-IM provides the best error performance under CFO and IQ imbalance.
Performance analysis of OTFS under in-phase and quadrature imbalance at transmitter
(IEEE-Institute of Electrical and Electronics Engineers Inc., 2021) Tusha, Armed; Doğan Tusha, Seda; Yılmaz, Ferkan; Althunibat, Saud; Qaraqe, Khalid; Arslan, Hüseyin
The influence of in-phase and quadrature (IQ) imbalance on the performance of orthogonal time frequency space (OTFS) transmission is investigated in this work. OTFS is a recent two-dimensional transmission scheme with promising robustness against the time and frequency selectivity of the time-varying wireless channel, and IQ impairment arises from inevitable imperfections between in-phase and quadrature branches in the transceiver design. In this paper, the mathematical model is presented to illustrate the physical effect of the IQ mismatched transmitter in OTFS, which results in a new type of interference named mirror-Doppler interference (MDI), occurring only along the Doppler axis. Moreover, a closed-form expression for the average bit error rate (BER) of OTFS with IQ imbalanced transmitter is derived taking into account M-ary symbol with a constant envelope and maximum likelihood (ML) detection principle. Importantly, our findings prove that IQ imbalanced transmitter causes a significant reduction in the diversity gain of OTFS transmission, but does not lead to an error floor in the BER performance as in one-dimensional current communication systems. Monte-Carlo simulations are provided to validate the derived mathematical formulas for OTFS transmission with constant envelope M-ary symbols under different system configurations.
Physical effect of in-phase and quadrature imbalance in delay-doppler domain
(Institute of Electrical and Electronics Engineers Inc., 2021) Tusha, Armed; Doğan Tusha, Seda; Yılmaz, Ferkan; Althunibat, Saud; Qaraqe, Khalid; Arslan, Hüseyin
Orthogonal time frequency space (OTFS) technique is a recent two-dimensional (2-D) modulation aiming to exploit both time and frequency selectivity of doubly dispersive wireless channel, which significantly affects the reliability of conventional wireless communication systems. In OTFS transmission, delay-Doppler domain is utilized for conveying the data symbols. Besides channel characteristics, the performance of wireless communications systems is severely limited due to the impairments at the RF front-end. One of these impairments is the in-phase and quadrature (IQ) imbalance that occurs due to inevitable imperfections existing between in-phase and quadrature branches at either transmitter (Tx) or receiver (Rx) architecture. The aim of this paper is to reveal and investigate the physical effect of IQ impairment in delay-Doppler domain, which is unknown in the literature. Hence, we provide theoretical models to explicitly understand the impact of IQ imbalance at different stages of OTFS-based communication systems including Tx, Rx, and jointly Tx-Rx. Importantly, we show that although the performance of OTFS transmission depends on both delay and Doppler axes, IQ imbalance leads to an interference originating only from the mirror Doppler axes, which we name mirror Doppler interference (MDI), along with power degradation. Moreover, we assess the performance of IQ imbalanced OTFS under various communication scenarios considering ideal and practical pulse shapes with maximum likelihood (ML) and minimum mean square error (MMSE) detectors, respectively.

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