Subcarrier Activation Patterns Research Articles

A new subcarrier‐index modulation schemes for downlink NOMA systems

SummaryIn this paper, we propose two downlink multiple access architectures for networks where human‐type communication users (HTCUs) and machine‐type communication devices (MTCDs) coexist. The proposed schemes combine non‐orthogonal multiple access (NOMA), orthogonal frequency division multiplexing (OFDM), and index modulation (OFDM‐IM) concepts. In the first scheme, the base station (BS) transmits bits of HTCUs using modulated symbols and bits of MTCDs by the subcarrier activation pattern (SAP). This approach called IM‐NOMA with null subcarriers (IM‐NOMA‐NS) ensures that inactive subcarriers are always null, which improves the system bit error rate (BER) performance. To improve the spectral efficiency (SE), we propose a second approach, termed IM‐NOMA with dual‐mode modulation (IM‐NOMA‐DM), in which the HTCUs' bits are transmitted using two‐dimensional modulation and the MTCDs' bits are transmitted using one‐dimensional modulation and the SAP. For each proposed system, a near‐optimal low‐complexity detector, based on the energy‐detection (ED) and the log‐likelihood ratio (LLR) criterion, is provided to mitigate the detection burden of the optimal maximum‐likelihood (ML) detector. The BER performances and SE of the proposed schemes are investigated. The average BERs of IM‐NOMA‐NS and IM‐NOMA‐DM are derived in closed‐form expressions corroborated by the simulation results. We have proved numerically that the proposed schemes achieve a good trade‐off between BER performance, SE, and the number of supported users, making them more suitable for Internet of Things (IoT) applications.

Coordinate Interleaved OFDM With Repeated In-Phase/Quadrature Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits through ordinary constellation symbols and indices of active subcarriers, is a promising multicarrier transmission scheme and has attracted the attention of researchers due to numerous benefits such as flexibility and simplicity. Nonetheless, OFDM-IM cannot satisfy the needs of future wireless communication services such as superior reliability, high data rates, and low complexity. In this article, we propose a novel OFDM-IM scheme named coordinate interleaved OFDM with repeated in-phase/quadrature IM (CI-OFDM-RIQIM), which provides superior error performance and enhanced spectral efficiency due to its diversity order of two and clever subcarrier activation pattern (SAP) detection mechanism, respectively. In addition, CI-OFDM-RIQIM is further extended to coordinate interleaved OFDM with in-phase/quadrature IM (CI-OFDM-IQIM) by doubling information bits transmitted by IM. Furthermore, log-likelihood ratio (LLR) based low-complexity detectors are designed for both proposed schemes. Theoretical analyses are performed and an upper bound on the bit error probability is derived. Comprehensive computer simulations under perfect and imperfect channel state information (CSI), are conducted to compare the proposed and reference schemes. It is shown that CI-OFDM-RIQIM and CI-OFDM-IQIM show superior results and can be considered promising candidates for next-generation wireless communication systems.

Dual-Mode Index Modulation for Non-Orthogonal Frequency Division Multiplexing

This paper presents dual-mode index modulation for spectral efficient frequency division multiplexing (SEFDM-DM). SEFDM is a non-orthogonal multicarrier technique created by compressing the subcarrier spacing of classical orthogonal frequency-division multiplexing (OFDM). SEFDM provides high spectrum efficiency (SE) at the expense of increased inter-carrier interference (ICI). SEFDM with index modulation (SEFDM-IM) has two information-bearing units, that is, a subcarrier activation pattern and modulated symbols, which reduce ICI at the expense of an SE deficit owing to inactive subcarriers. The proposed SEFDM-DM uses all available subcarriers while retaining the diversity gain of index modulation (IM). It enhances SE by transmitting distinct constellation modes through a subcarrier index selection mechanism. This study also presents SEFDM-DM with coordinate interleaving (SEFDM-CDM), which introduces space-time block codes with coordinate interleaving to enhance transmission diversity by sending real and imaginary parts of the constellation symbols over different subcarriers. Analytical and simulation results corroborate the benefits of the suggested work in terms of SE and bit error rate.

Composite Multiple-Mode Orthogonal Frequency Division Multiplexing With Index Modulation

In this paper, we propose a composite multiple-mode orthogonal frequency division multiplexing with index modulation (C-MM-OFDM-IM) scheme to increase the spectral efficiency (SE) of OFDM-IM systems by extending the indexing to the energy and constellation domains. In C-MM-OFDM-IM, the information bits are mapped to not only the subcarrier activation patterns (SAPs) and modulation symbols, but also the energy allocation patterns (EAPs) and constellation activation patterns (CAPs). To cope with the practical situations, we propose a variant IM scheme named C-MM-OFDM-IM-II to build a new mapping rule between information bits and the increased CAPs, capable of further increasing the SE of C-MM-OFDM-IM. Upper-bounded bit error rate (BER) and lower-bounded achievable rate are both derived in closed-form to evaluate the performance of C-MM-OFDM-IM(-II). Moreover, we further propose two enhanced schemes, named generalized C-MM-OFDM-IM(-II) and C-MM-OFDM with in-phase/quadrature IM(-II), where the former jointly considers all SAPs, EAPs, CAPs and modulated symbols, while the latter expands the index implementation to the in-phase and quadrature constellation domains. Simulation results show that C-MM-OFDM-IM(-II) outperforms the conventional OFDM-IM related schemes, especially in the high signal-to-noise ratio (SNR) region, and verify the accuracy of the theoretical analysis for the upper-bounded BER and achievable rate.

Index Modulation OFDM Systems with Direct Pattern of Subcarrier Activation

OFDM-IM is a spatial modulation technique applied to OFDM's block subcarriers, resulting in a modified version of OFDM. Compared to traditional OFDM, OFDM-IM boasts several benefits, such as enhanced bit-error rate (BER) performance and power efficiency. When working with OFDM-IM, the specification of subcarrier-activation-patterns (SAPs) is done by utilizing either look-up tables (LUTs) or a combinatorial approach to conform with data. However, the data-to-SAP mapping methods of both suffer from increased complexity as the number of subcarriers expands. The paper proposes a direct data-to-SAP mapping that eliminates the need for any type of look-up tables. Through the presented analysis and computer simulation results, it is demonstrated that the OFDM-IM system utilizing the proposed mapping scheme outperforms equivalent traditional OFDM-IM systems in both complexity and bit error rate (BER) performance.

Generalized Quadrature Space-Frequency Index Modulation System With Low-Complexity Detection

In this article, we propose a generalized quadrature space-frequency index modulation (GQSFIM) system that maps the information bits to the subcarrier activation pattern (SAP), the two antenna activation patterns (AAPs), and the modulated symbols. Specifically, two independent AAPs are utilized for the modulated symbols in a GQSFIM subblock to get higher spectral efficiency. With the help of the union bounding technique, the closed-form average bit error probability (ABEP) expression of the GQSFIM system with the maximum-likelihood (ML) detection is derived. Additionally, two low-complexity step-by-step signal detection algorithms, local ML based minimum mean-squared error (MMSE) and local ordered block based MMSE, are provided to reduce the high computational complexity of the ML detection, where the SAP, the two AAPs, and the modulated symbols are detected separately. Numerical results verify the correctness and accuracy of the theoretical ABEP of the GQSFIM system, especially in the high signal-to-noise ratio region. Moreover, the proposed GQSFIM system achieves better spectral efficiency and ABEP performance compared with the typical space-frequency index modulation systems.

On capacity and performance of noncoherent OFDM-IM systems with binary codes

This paper studies the capacity and error performance of a noncoherent index-modulated orthogonal frequency division multiplexing (OFDM-IM) system with binary coded subcarrier activation patterns. To that end, soft-decision (i.e., unquantized) and hard-decision (i.e., quantized) channels are considered, where the latter is equivalent to a binary asymmetric channel (BAC). By using maximum likelihood and maximum a posteriori criteria, crossover probabilities of the quantized channel are derived in order to characterize its capacity and probability of error in detecting IM signals. Capacity expressions are then given for both channels. Using the Blahut-Arimoto algorithm, an iterative algorithm is developed to find the optimal input distribution for the unquantized channel, while for the quantized channel, it is found by solving a nonlinear equation for its zero. It is shown that using a uniform input distribution instead of the optimal (nonuniform) one results in a negligible capacity loss, therefore binary codes with a uniform distribution of 1s and 0s are nearly optimal. Compared to the unquantized channel, it is shown that an approximate loss less than 2dB is incurred when quantization is used and this loss diminishes rapidly as signal-to-noise ratio (SNR) increases. In addition, it is demonstrated that the gap (in dB) between uncoded noncoherent OFDM-IM system and the capacity limit is generally wide, and the gap narrows when subcarrier clustering is considered. The findings in this paper can have theoretical and practical significance for understanding capacity limits and can provide some guidance for coding design in noncoherent OFDM-IM systems.

A novel orthogonal frequency division multiplexing with index modulation waveform with carrier frequency offset resistance and low peak‐to‐average power ratio

SummaryIn this paper, we propose a novel orthogonal frequency division multiplexing (OFDM) scheme with high carrier frequency offset (CFO) resistance and low peak‐to‐average power ratio (PAPR). In this scheme, we consider a hybrid model with two subblock types, namely, pilot subblocks and standard subblocks. In pilot subblocks, active subcarriers are utilized for PAPR reduction while inactive carriers generated by the index modulation (IM) are utilized for the coarse CFO estimation. For standard subblocks, we consider unique subcarrier activation patterns (SAPs) with high‐diversity IM to enhance the bit error performance of the overall system. Additionally, the inactive data tones in standard subblocks are utilized for fine CFO estimation, which enhances the CFO estimation quite significantly. Furthermore, in this paper, we show that proposed hybrid OFDM‐IM (H‐OFDM‐IM) scheme can outperform conventional OFDM‐IM and classical OFDM both in CFO estimation and PAPR reduction without requiring transmission of any side information. Finally, we show both mathematically and through computer simulations that proposed H‐OFDM‐IM can achieve a satisfactory bit error rate (BER) performance under high CFO scenarios.

General Carrier Index Aided Dual-Mode Differential Chaos Shift Keying With Full Mapping: Design and Optimization

In this paper, we propose a general carrier index assisted dual-mode differential chaos shift keying with full mapping (GCI-DM-DCSK-FM) system, where a pair of distinguishable modem-mode signals is designed with the aid of general carrier index modulation. Specifically, the selected and unselected subcarriers are used to carry differential chaos shift keying (DCSK) and quadrature chaos shift keying (QCSK) signals, respectively, thereby obtaining higher data rate and spectral efficiency. To tackle the issue of discarding the subcarrier activation pattern (SAP), an effective full mapping method is proposed for the GCI-DM-DCSK-FM system to harvest better bit error rate (BER) performance and higher data rate. Moreover, a noise decontamination method is designed for the GCI-DM-DCSK-FM system to enhance the BER performance. To confirm the validness for our proposed systems, we derive the BER expressions of the GCI-DM-DCSK-FM system over additive white Gaussian noise (AWGN) and multipath Rayleigh fading channels. Finally, plenty of simulation results show the excellent BER performance of the GCI-DM-DCSK-FM system.

Protection scheme of subcarrier index in OFDM with index modulation aided by LDPC coding

The receiver of OFDM with Index Modulation (OFDM-IM) usually adopts a Log Likelihood Ratio (LLR) detection algorithm based on the activation state of subcarriers. However, the LLR detection algorithm will cause detection errors in subcarrier activation pattern (SAP) or get illegal SAP. Consequently, further errors occur in demodulation, increasing the bit error rate (BER). To solve this problem, we propose the protection scheme of subcarrier index aided by LDPC coding, which reduces the SAP detection errors by encoding the index information bits. On the receiver, the LDPC Coding Aided (LA) detection algorithm is designed, and the formula of LLR of index information bits is derived in detail. Monte Carlo simulation is carried out over multi-path fading channel by MATLAB software. The results show that under the condition that the spectrum efficiency is not lower than the classical OFDM-IM scheme, the proposed protection scheme can obtain a gain of about 5~9 dB when the BER is 10-4, effectively improving the BER performance of OFDM-IM scheme.

Joint-Mapping Orthogonal Frequency Division Multiplexing With Subcarrier Number Modulation

Orthogonal frequency division multiplexing with subcarrier number modulation (OFDM-SNM) has been recently proposed to improve the spectral efficiency (SE) of the traditional OFDM system. In this paper, we propose a joint-mapping OFDM-SNM (JM-OFDM-SNM) scheme to transmit the signal vector with a constant length of information bits by jointly considering the subcarrier activation patterns and constellation symbols. A low-complexity detection scheme based on log-likelihood ratio criterion is proposed to relieve the high computational complexity of the maximum-likelihood detection at the cost of a negligible performance loss. Upper-bounded bit error rate (BER) and lower-bounded achievable rate are both derived in closed-form to evaluate the performance of JM-OFDM-SNM. To suit different application scenarios, we further propose two enhanced schemes, named adaptive JM-OFDM-SNM (AJM-OFDM-SNM) and JM-OFDM with in-phase/quadrature SNM (JM-OFDM-IQ-SNM), where the former adjusts the constellation orders for different numbers of active subcarriers, and the latter extends the indexing to in-phase and quadrature domains. Simulation results corroborate the tightness of the derived BER expression in the high signal-to-noise ratio region and show that (A)JM-OFDM-SNM improves the performance of OFDM-SNM, while both AJM-OFDM-SNM and JM-OFDM-IQ-SNM schemes perform better than JM-OFDM-SNM at the same SE.

Coordinate interleaved orthogonal frequency division multiplexing with index modulation‐based orthogonal cooperative system

AbstractIn this paper, coordinate interleaved orthogonal frequency division multiplexing with index modulation‐ (CI‐OFDM‐IM) based orthogonal cooperative system (OCS) is proposed, where the source activates half of the subcarriers to convey the symbols in the first time slot (TS). The subcarrier activation patterns (SAPs) are also exploited to convey additional information. In the second TS, the relay amplifies and forwards the received signal to the destination, meanwhile, the source activates the remaining half of the subcarriers to convey the new symbols to the destination. Since the source and relay transmit different symbols to the destination on the different subcarriers in the second TS, the mutual interference is avoided at the destination. Compared with the current OFDM based OCS, the proposed CI‐OFDM‐IM‐based OCS can achieve a higher spectral efficiency, since the SAPs are exploited to convey additional information. Furthermore, the CI‐OFDM‐IM‐based OCS can achieve additional power gain for a certain average transmit power, since the power of inactive subcarriers can be reallocated on the active subcarriers. The optimal constellation rotation, in the sense of maximizing the minimum Euclidean distance between the symbols conveyed on a subcarrier over two TSs, is provided to optimize the performance of CI‐OFDM‐IM‐based OCS. Simulation results show that the proposed CI‐OFDM‐IM‐based OCS achieves a better trade‐off between spectral efficiency and bit error ratio performance.

Quantum Learning Based Nonrandom Superimposed Coding for Secure Wireless Access in 5G URLLC

Secure wireless access in ultra-reliable low-latency communications (URLLC), which is a critical aspect of 5G security, has become increasingly important due to its potential support of grant-free configuration. In grant-free URLLC, precise allocation of different pilot resources to different users that share the same time-frequency resource is essential for the next generation NodeB (gNB) to exactly identify those users under access collision and to maintain precise channel estimation required for reliable data transmission. However, this process easily suffers from attacks on pilots. We in this article propose a quantum learning based nonrandom superimposed coding method to encode and decode pilots on multidimensional resources, such that the uncertainty of attacks can be learned quickly and eliminated precisely. Particularly, multiuser pilots for uplink access are encoded as distinguishable subcarrier activation patterns (SAPs) and gNB decodes pilots of interest from observed SAPs, a superposition of SAPs from access users, by joint design of attack mode detection and user activity detection though a quantum learning network (QLN). We found that the uncertainty lies in the identification process of codeword digits from the attacker, which can be always modelled as a black-box model, resolved by a quantum learning algorithm and quantum circuit. Novel analytical closed-form expressions of failure probability are derived to characterize the reliability of this URLLC system with short packet transmission. Simulations how that our method can bring ultra-high reliability and low latency despite attacks on pilots.

Super-Mode OFDM With Index Modulation

Orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) appears as a promising multi-carrier waveform candidate for beyond 5G due to its attractive advantages such as operational flexibility and ease of implementation. However, OFDM-IM may not be a proper choice for 5G services such as enhanced mobile broadband (eMBB) since achieving high data rates is challenging because of its null subcarriers. One solution to enhance the spectral efficiency of OFDM-IM is the employment of multiple distinguishable constellations (modes) by also exploiting its null subcarriers for data transmission. This article proposes a novel IM technique called super-mode OFDM-IM (SuM-OFDM-IM), where mode activation patterns (MAPs) and subcarrier activation patterns (SAPs) are jointly selected and conventional data symbols are repetition coded over multiple subcarriers to achieve a diversity gain. For the proposed scheme, a low-complexity detector is designed, theoretical analyses are performed and a bit error rate (BER) upper bound is derived. The performance of the proposed system is also investigated through real-time experiments using a software-defined radio (SDR) based prototype. We show that SuM-OFDM-IM exhibits promising results in terms of spectral efficiency and error performance; thus, appears as a potential candidate for 5G and beyond communication systems.

Coded OFDM-IM with iterative detection☆

Coded Orthogonal frequency division multiplexing with index modulation (OFDM-IM) has been proposed, where channel coding and soft decision are employed to exploit the multipath diversity for the information conveyed by the constellation symbols. However, the performance of coded OFDM-IM is dependent on the detection performance of subcarrier activation pattern (SAP) and the diversity order of the SAPs requires further investigation. In this paper, a lower-bound of LLR detector is derived to show that the maximum achievable diversity order is the minimum Hamming distance between the SAPs for the LLR detector. Based on the analyses, iterative detection is proposed to improve the performance of coded OFDM-IM, where information-assisted LLR detector is proposed to improve the detection performance of SAP by using the soft information of the constellation symbols. Simulation results are provided to corroborate the diversity analyses and show the advantage of the coded OFDM-IM with iterative detection.

Enhanced Huffman Coded OFDM With Index Modulation

In this paper, we propose an enhanced Huffman coded orthogonal frequency-division multiplexing with index modulation (EHC-OFDM-IM) scheme. The proposed scheme is capable of utilizing all legitimate subcarrier activation patterns (SAPs) and adapting the bijective mapping relation between SAPs and leaves on a given Huffman tree according to channel state information (CSI). As a result, a dynamic codebook update mechanism is obtained, which can provide more reliable transmissions. We take the average block error rate (BLER) as the performance evaluation metric and approximate it in closed form when the transmit power allocated to each subcarrier is independent of channel states. Also, we propose two CSI-based power allocation schemes with different requirements for computational complexity to further improve the error performance. Subsequently, we carry out numerical simulations to corroborate the error performance analysis and the proposed dynamic power allocation schemes. By studying the numerical results, we find that the depth of the Huffman tree has a significant impact on the error performance when the SAP-to-leaf mapping relation is optimized based on CSI. Meanwhile, through numerical results, we also discuss the trade-off between error performance and data transmission rate and investigate the impacts of imperfect CSI on the error performance of EHC-OFDM-IM.

Concatenated Graph Coding on Bandwidth Part for Secure Pilot Authentication in Grant-Free URLLC

Grant-free multiple access is a critical mechanism introduced in 5G new radio (NR) to support ultra-reliable low-latency communication (URLLC) services. Pilot authentication (PA) is a key security mechanism to guarantee reliable performance of grant-free URLLC. However, PA can be easily paralyzed by pilot-aware attack since pilot signals are usually publicly known, and unprotected. To solve this, we develop the concatenated graph coding (CGC) theory by which time-frequency resources on bandwidth part (BWP) can be encoded flexibly to protect PA securely. Particularly, we use bipartite graph, and multigraph theory to model PA on BWP as transmission, and retrieval of pilot (TRP). Each transmitter in the uplink needs transmit a unique random pilot sequence as subcarrier activation pattern (SAP) on BWP. After observing SAPs from multiple transmitters, the receiver decodes a pilot sequence of interest, and tests its authenticity. The retrievability of authentic pilots is defined, and formulated analytically. We also derive the analytical closed-form expression of system failure probability, and accessibility in the regime of large-scale antenna arrays, and short data packets. Interestingly, we find that four trade-offs exist: retrievability-latency, retrievability-accessibility, reliability-latency, and reliability-accessibility. Simulation results show the security advantage of our proposed theory in grant-free URLLC system.

Outage Performance of Multi-Carrier Relay Selection for Full-Duplex OFDM With Index Modulation System

This letter focuses on the multi-carrier relay selection (RS) problem in a two-hop full-duplex relay-assisted orthogonal frequency-division multiplexing with index modulation system which employs a decode-and-forward communication protocol. Specifically, two widely used multi-carrier RS schemes termed as bulk RS and per-subcarrier RS are adopted in the proposed system. The exact expressions and asymptotic expressions of the average outage probabilities over all subcarrier activation patterns are theoretically derived. All the analyses are numerically verified by the Monte Carlo simulation results, which proves that the system can achieve the full diversity gain that equals the number of relays by multi-carrier RS schemes, thus greatly improving the system reliability.

Subcarrier Subset Selection-Aided Transmit Precoding Achieves Full-Diversity in Index Modulation

Index modulation (IM) is a recently proposed multi-carrier transmission scheme, which conveys information both by conventional symbols as well as by the specific subcarrier activation patterns conveying them. However, an impediment of IM is that it lacks transmit diversity gain. In this paper, we circumvent this limitation by proposing a limited-feedback assisted IM transmission scheme. Specifically, Euclidean distance based subcarrier subset selection (ED-SSS) is proposed and its attainable transmit diversity order is shown to be $N_c-N_{IM}+1$ , where $N_c$ is the total number of subcarriers in an IM block and $N_{IM}$ is the number of subcarriers used for IM. Furthermore, the ED-SSS is shown to be amenable to low-complexity implementation owing to the orthogonality of its subcarriers. In order to attain the maximum transmit diversity order of $N_c$ , ED-SSS is further extended with the aid of transmit precoding and its transmit diversity order is quantified. The proposed precoding assisted ED-SSS is shown to subsume several of the existing precoding aided IM transmission schemes. Simulation studies are conducted for validating our theoretical claims and also for quantifying the attainable performance gains of the proposed schemes. Specifically, at a BER of $10^{-3}$ an SNR gain as high as 8 dB is observed in case of precoding when compared to its counterpart operating without precoding.

Enhanced Orthogonal Frequency-Division Multiplexing With Subcarrier Number Modulation

A novel modulation scheme termed orthogonal frequency-division multiplexing with subcarrier number modulation (OFDM-SNM) has been proposed and regarded as one of the promising candidate modulation schemes for next generation networks. Although OFDM-SNM is capable of having a higher spectral efficiency (SE) than OFDM with index modulation (OFDM-IM) and plain OFDM under certain conditions, its reliability is relatively inferior to these existing schemes, because the number of active subcarriers varies. In this regard, we propose an enhanced OFDM-SNM scheme in this paper, which utilizes the flexibility of placing subcarriers to harvest a coding gain in the high signal-to-noise ratio (SNR) region. In particular, we stipulate a methodology that optimizes the subcarrier activation pattern (SAP) by subcarrier assignment using instantaneous channel state information (CSI) and therefore the subcarriers with higher channel power gains will be granted the priority to be activated, given the number of subcarriers is fixed. We also analyze the proposed enhanced OFDM-SNM system in terms of outage and error performance. The average outage probability and block error rate (BLER) are derived and approximated in closed-form expressions, which are further verified by numerical results generated by Monte Carlo simulations. The high-reliability nature of the enhanced OFDM-SNM makes it a promising candidate for implementing in the Internet of Things (IoT) with stationary machine-type devices (MTDs), which are subject to slow fading and supported by proper power supply.