The Experts below are selected from a list of 591 Experts worldwide ranked by ideXlab platform
Miaowen Wen - One of the best experts on this subject based on the ideXlab platform.
-
Distributed Processing for Multi-Relay Assisted OFDM With Index Modulation
IEEE Transactions on Wireless Communications, 2019Co-Authors: Shuping Dang, Miaowen Wen, Shahid MumtazAbstract:Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth-generation (5G) wireless communications and has thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual Subcarriers. In this regard, we propose a distributed processing scheme for multi-relay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDM-IM systems. Specifically, a single selected relay only needs to decode partial information carried on certain Active Subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on Active Subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.
-
Layered Orthogonal Frequency Division Multiplexing With Index Modulation
IEEE Systems Journal, 2019Co-Authors: Shuping Dang, Miaowen Wen, Xueqin Jiang, Yuyang Peng, Han HaiAbstract:In this paper, we propose a novel scheme termed layered orthogonal frequency division multiplexing with index modulation (L-OFDM-IM) to increase the spectral efficiency (SE) of OFDM-IM systems. In L-OFDM-IM, all Subcarriers are first divided into multiple layers, each determining the Active Subcarriers and their modulated symbols. The index modulation (IM) bits are carried on the indices of the Active Subcarriers of all layers, which are overlapped and distinguishable with different signal constellations so that the number of the IM bits is larger than that in traditional OFDM-IM. A low-complexity detection is proposed to alleviate the high burden of the optimal maximum-likelihood detection at the receiver side. A closed-form upper bound on the bit error rate, the achievable rate, and diversity order are derived to characterize the performance of L-OFDM-IM. To enhance the diversity performance of L-OFDM-IM, we further propose coordinate interleaving L-OFDM-IM (CI-L-OFDM-IM), which interleaves the real and imaginary parts of the modulated symbols over two different subchannels. Computer simulations verify the theoretical analysis, and results show that L-OFDM-IM outperforms the conventional OFDM-IM scheme. Moreover, it is also confirmed that CI-L-OFDM-IM obtains an additional diversity order in comparison with L-OFDM-IM.
-
Enhanced Orthogonal Frequency Division Multiplexing With Index Modulation
IEEE Transactions on Wireless Communications, 2017Co-Authors: Miaowen Wen, Ertugrul BasarAbstract:Index modulation concept has attracted considerable research interest in the past few years. As a realization of index modulation in the frequency domain, orthogonal frequency division multiplexing with index modulation (OFDM-IM) has recently been proposed, which conveys information bits through both the subcarrier activation patterns and the amplitude phase modulation constellation points. This paper proposes two enhanced OFDM-IM schemes aimed at achieving higher spectral efficiency and diversity gain, respectively. The first one, termed OFDM with hybrid in-phase/quadrature index modulation (OFDM-HIQ-IM), explores the I- and Q- dimensions jointly for index modulation, allowing transmission of more index modulation bits in each subcarrier group. The second one, termed linear constellation precoded OFDM-IQ-IM (LP-OFDM-IQ-IM), spreads information symbols across two adjacent Active Subcarriers through linear constellation precoding to harvest additional diversity gain. By maximizing the minimum squared Euclidean distance, two different realizations of LP-OFDM-IQ-IM are derived, which leads to a rotated and a diamond-shaped constellation, respectively. The proposed OFDM-HIQ-IM and LP-OFDM-IQ-IM, as revealed by both theoretical analyses and computer simulations, enable low-complexity detection and exhibit superior error rate performance over the existing OFDM-IM schemes.
-
Vector OFDM With Index Modulation
IEEE Access, 2017Co-Authors: Yun Liu, Miaowen Wen, Dehuan Wan, Beixiong ZhengAbstract:The concept of index modulation (IM) has gained increased attention in recent years. As a member of the IM family, orthogonal frequency division multiplexing with index modulation (OFDM-IM), which is able to convey additional information bits using the indices of the Active Subcarriers, has emerged as an attrActive physical layer transmission technique. Vector OFDM (V-OFDM) has inherently good peak-to-average-power ratio (PAPR) feature and good bit error rate (BER) performance due to the nearly full diversity gains achieved by most of its sub-blocks. In this paper, in order to improve the BER performance of OFDM-IM and reduce the PAPR of the transmit signal, we propose an enhanced OFDM-IM scheme termed vector OFDM with index modulation (V-OFDM-IM). In addition to the structure of the transceiver, a complexity reduced maximum likelihood detector is presented. To further reduce the detection complexity, a near-optimal detector based on sequential Monte Carlo technique is introduced. The average bit error probability of the proposed scheme is derived in closed form over frequency-selective block Rayleigh-fading channel. Both the theoretical and the simulation-based results show that, under the same spectrum efficiency, V-OFDM-IM outperforms the conventional OFDM-IM and OFDM.
-
index modulated ofdm for underwater acoustic communications
IEEE Communications Magazine, 2016Co-Authors: Miaowen Wen, Xiang Cheng, Liuqing Yang, Yuke Li, Xilin Cheng, Fei JiAbstract:UWA channels exhibit time-varying multipath characteristics. To this end, OFDM is well known for its robustness against multipath channels but is prone to ICI induced by time variation. More recently, inspired by spatial modulation, the so-called IM-OFDM has also been proposed to provide higher system throughput than plain OFDM under certain conditions. A key feature of IM-OFDM is that partial Subcarriers are kept inActive. This could potentially improve system performance in the presence of ICI. Leveraging on this, we are the first to propose IM-OFDM for UWA communications. On the other hand, however, we realize that ICI could potentially lead to energy leakage from Active Subcarriers to inActive ones, and impair the demodulation of IM-OFDM. In this article, we introduce IM-OFDM for UWA communications and propose a hybrid IM-OFDM scheme with improved spectral efficiency. We then review existing ICI self-cancellation techniques for generic OFDM, and propose a new ICI cancellation method for IM-OFDM.
Ertugrul Basar - One of the best experts on this subject based on the ideXlab platform.
-
Fading-aligned OFDM with index modulation for mMTC services
Physical Communication, 2019Co-Authors: Ebubekir Memisoglu, Ertugrul Basar, Huseyin ArslanAbstract:Abstract 5th generation (5G) of wireless networking is coming with diverse use cases, such as enhanced-Mobile BroadBand (eMBB), Ultra Reliable and Low Latency Communications (URLLC), and massive Machine Type Communications (mMTC). As a result, 5G wireless networks require flexible physical layer solutions through new radio access technologies (RATs). At this point, orthogonal frequency division multiplexing with index modulation (OFDM-IM) appears a flexible solution to satisfy the diverse user demands. Considering the strict requirements of mMTC services, such as low throughput, low power consumption, and low cost design, we propose fading-aligned OFDM-IM for more spectrum- and energy-efficient communication. In the proposed method, inActive Subcarriers in OFDM-IM are cleverly utilized to avoid deep fading sub-channels, because the deep fading of the Active Subcarriers decreases bit error rate (BER) performance significantly. Computer simulation results demonstrate that more than 10 dB gain is obtained for a reference BER value of 1 0 − 4 at the same spectral efficiency with conventional OFDM. Moreover, the proposed method is compared with convolutional coded (CC) OFDM at the same spectral efficiency, and it is shown that the proposed scheme performs better in terms of BER performance. Furthermore, theoretical error performance of the proposed method is investigated to support our computer simulations.
-
Adaptive dual-mode OFDM with index modulation
Physical Communication, 2018Co-Authors: Aldirmaz Colak, Yusuf Acar, Ertugrul BasarAbstract:Abstract With the integration of the index modulation concept, the popular OFDM has gained an appealing new dimension. However, OFDM with index modulation (OFDM-IM) causes a decrease in data rate for higher order modulations due to its unused Subcarriers. Recently, dual-mode OFDM with index modulation (DM-OFDM-IM) has been proposed to prevent this decrease in data rate. The spectral efficiency (SE) of both techniques depends on the selection of different parameters, such as modulation types and number of Active Subcarriers. However, the quality of the wireless channel has not been taken into account yet to increase the data rate. In this paper, we propose a new adaptive DM-OFDM-IM (A-DM-OFDM-IM) system to enhance the bit error probability (BER) performance considering the channel conditions and obtain a substantial increase in SE. Unlike the previous DM-OFDM-IM and the classical OFDM-IM schemes, our proposed system enables the use of different types of modulations for different subblocks in one OFDM signal. We demonstrate that the SE of the proposed system is comparable with that of its OFDM-IM and DM-OFDM-IM counterpart under frequency selective Rayleigh channels. Moreover, computer simulations corroborate that the derived theoretical results are considerably accurate for the SE of A-DM-OFDM-IM systems.
-
Differential Subcarrier Index Modulation
IEEE Transactions on Vehicular Technology, 2018Co-Authors: Saud Althunibat, Raed Mesleh, Ertugrul BasarAbstract:One of the main challenges in orthogonal frequency division multiplexing (OFDM)–subcarrier index modulation (SIM) systems is the huge amount of resources required to obtain channel state information at the receiver side. In this paper, a differential subcarrier index modulation (DSIM) scheme is proposed, which entirely avoids the need for any channel knowledge at the receiver side. As such, time and energy resources spent in the channel estimation process are perceived. In DSIM, part of the transmitted block is modulated through ordinary signal modulation, whereas the second part is transmitted by selecting a specific permutation of the Active Subcarriers. The transmitted signals are designed to facilitate differential demodulation at the receiver side. A derivation is conducted for the average bit error probability of DSIM and an upper bound expression is obtained. Derived theoretical expression is substantiated through Monte Carlo simulation results. Reported results reveal that differential demodulation degrades the error performance of coherent SIM by nearly 4 dB in signal-to-noise-ratio.
-
Enhanced Orthogonal Frequency Division Multiplexing With Index Modulation
IEEE Transactions on Wireless Communications, 2017Co-Authors: Miaowen Wen, Ertugrul BasarAbstract:Index modulation concept has attracted considerable research interest in the past few years. As a realization of index modulation in the frequency domain, orthogonal frequency division multiplexing with index modulation (OFDM-IM) has recently been proposed, which conveys information bits through both the subcarrier activation patterns and the amplitude phase modulation constellation points. This paper proposes two enhanced OFDM-IM schemes aimed at achieving higher spectral efficiency and diversity gain, respectively. The first one, termed OFDM with hybrid in-phase/quadrature index modulation (OFDM-HIQ-IM), explores the I- and Q- dimensions jointly for index modulation, allowing transmission of more index modulation bits in each subcarrier group. The second one, termed linear constellation precoded OFDM-IQ-IM (LP-OFDM-IQ-IM), spreads information symbols across two adjacent Active Subcarriers through linear constellation precoding to harvest additional diversity gain. By maximizing the minimum squared Euclidean distance, two different realizations of LP-OFDM-IQ-IM are derived, which leads to a rotated and a diamond-shaped constellation, respectively. The proposed OFDM-HIQ-IM and LP-OFDM-IQ-IM, as revealed by both theoretical analyses and computer simulations, enable low-complexity detection and exhibit superior error rate performance over the existing OFDM-IM schemes.
-
Equiprobable Subcarrier Activation Method for OFDM With Index Modulation
IEEE Communications Letters, 2016Co-Authors: Miaowen Wen, Ertugrul Basar, Yuekai Zhang, Fangjiong ChenAbstract:Orthogonal frequency division multiplexing with index modulation (OFDM-IM) conveys additional information bits via the indices of Active Subcarriers. Consequently, the determination of the Active Subcarriers according to the incoming bits arises as a challenging problem. The existing solution resorts to the lexicographic ordering, which leads to a low implementation complexity but an unequal subcarrier activation probability. This letter proposes a distinct solution that allows the equiprobable activation of all OFDM-IM Subcarriers with comparable implementation complexity. The signal-to-noise ratio gain achieved by the proposed solution is also analyzed. Computer simulations reveal the advantages of the proposed solution.
Mikko Valkama - One of the best experts on this subject based on the ideXlab platform.
-
Frequency-Selective PAPR Reduction for OFDM
IEEE Transactions on Vehicular Technology, 2019Co-Authors: Selahattin Gokceli, Toni Levanen, Taneli Riihonen, Markku Renfors, Mikko ValkamaAbstract:In this correspondence paper, we study the peak-to-average power ratio (PAPR) problem in orthogonal frequency-division multiplexing systems. In conventional clipping and filtering based PAPR reduction techniques, clipping noise is allowed to spread over the whole Active passband, thus degrading the transmit signal quality similarly at all Active Subcarriers. However, since modern radio networks support frequency multiplexing of users and services with highly different quality-of-service expectations, clipping noise from PAPR reduction should be distributed unequally over the corresponding physical resource blocks (PRBs). To facilitate this, we present an efficient PAPR reduction technique, where clipping noise can be flexibly controlled and filtered inside the transmitter passband, allowing to control the transmitted signal quality per PRB. Numerical results are provided in 5G new radio mobile network context, demonstrating the flexibility and efficiency of the proposed method.
-
Frequency-Selective PAPR Reduction for OFDM.
arXiv: Signal Processing, 2019Co-Authors: Selahattin Gokceli, Toni Levanen, Taneli Riihonen, Markku Renfors, Mikko ValkamaAbstract:We study the peak-to-average power ratio (PAPR) problem in orthogonal frequency-division multiplexing (OFDM) systems. In conventional clipping and filtering based PAPR reduction techniques, clipping noise is allowed to spread over the whole Active passband, thus degrading the transmit signal quality similarly at all Active Subcarriers. However, since modern radio networks support frequency-multiplexing of users and services with highly different quality-of-service expectations, clipping noise from PAPR reduction should be distributed unequally over the corresponding physical resource blocks (PRBs). To facilitate this, we present an efficient PAPR reduction technique, where clipping noise can be flexibly controlled and filtered inside the transmitter passband, allowing to control the transmitted signal quality per PRB. Numerical results are provided in 5G New Radio (NR) mobile network context, demonstrating the flexibility and efficiency of the proposed method.
-
Compressive Identification of Active OFDM Subcarriers in Presence of Timing Offset
2015 IEEE Global Communications Conference (GLOBECOM), 2015Co-Authors: Alireza Razavi, Mikko Valkama, Danijela CabricAbstract:In this paper we study the problem of identifying Active Subcarriers in an OFDM signal from compressive measurements sampled at sub-Nyquist rate. The problem is of importance in Cognitive Radio systems when secondary users (SUs) are looking for available spectrum opportunities to communicate over them while sensing at Nyquist rate sampling can be costly or even impractical in case of very wide bandwidth. We first study the effect of timing offset and derive the necessary and sufficient conditions for signal recovery in the oracle-assisted case when the true Active sub-carriers are assumed known. Then we propose an Orthogonal Matching Pursuit (OMP)- based joint sparse recovery method for identifying Active Subcarriers when the timing offset is known. Finally we extend the problem to the case of unknown timing offset and develop a joint dictionary learning and sparse approximation algorithm, where in the dictionary learning phase the timing offset is estimated and in the sparse approximation phase Active Subcarriers are identified. The obtained results demonstrate that Active subcarrier identification can be carried out reliably, by using the developed framework.
-
GLOBECOM - Compressive Identification of Active OFDM Subcarriers in Presence of Timing Offset
2015 IEEE Global Communications Conference (GLOBECOM), 2014Co-Authors: Alireza Razavi, Mikko Valkama, Danijela CabricAbstract:In this paper we study the problem of identifying Active Subcarriers in an OFDM signal from compressive measurements sampled at sub-Nyquist rate. The problem is of importance in Cognitive Radio systems when secondary users (SUs) are looking for available spectrum opportunities to communicate over them while sensing at Nyquist rate sampling can be costly or even impractical in case of very wide bandwidth. We first study the effect of timing offset and derive the necessary and sufficient conditions for signal recovery in the oracle-assisted case when the true Active sub-carriers are assumed known. Then we propose an Orthogonal Matching Pursuit (OMP)- based joint sparse recovery method for identifying Active Subcarriers when the timing offset is known. Finally we extend the problem to the case of unknown timing offset and develop a joint dictionary learning and sparse approximation algorithm, where in the dictionary learning phase the timing offset is estimated and in the sparse approximation phase Active Subcarriers are identified. The obtained results demonstrate that Active subcarrier identification can be carried out reliably, by using the developed framework.
Kamran Kiasaleh - One of the best experts on this subject based on the ideXlab platform.
-
CCNC - Subcarrier-Index Modulation for Reed Solomon Encoded OFDM-Based Visible Light Communication
2019 16th IEEE Annual Consumer Communications & Networking Conference (CCNC), 2019Co-Authors: Nima Taherkhani, Kamran KiasalehAbstract:In this paper, we propose the subcarrier-index modulation with Reed Solomon encoded Optical Orthogonal Frequency Division Multiplexing with index modulation (IM-RS-OFDM) scheme for visible light communication (VLC). In this technique, the data is encoded using an RS codeword and then part of the redundancy introduced by the frame which exceeds the clipping range are punctured and their corresponding Subcarriers are set as inActive, where the indices of these inActive Subcarriers are used to transmit extra information bits. The puncturing of the redundancy aids in mitigating clipping noise generated due to the dynamic range constraints of the optical transmitter by shortening the number of Active Subcarriers, while the locations of the punctured Subcarriers are exploited to convey more bits in order to compensate for the reduction in spectral efficiency caused by coding redundancy. In the proposed scheme, the bipolar transmitting signal is clipped and biased according to DC bias optical OFDM (DCO-OFDM) system, and a Log-Likelihood ratio (LLR) calculation based detector is used to find the indices of punctured symbols in the codeword. Our simulation results show that the new scheme offers a better bit error rate performance compared to the conventional coded OFDM-based visible light communication.
-
ICNC - Reed Solomon Encoding for the Mitigation of Clipping Noise in OFDM-Based Visible Light Communications
2018 International Conference on Computing Networking and Communications (ICNC), 2018Co-Authors: Nima Taherkhani, Kamran KiasalehAbstract:In this paper, we present a clipping noise mitigation technique in orthogonal frequency-division multiplexing (OFDM) based visible light communication (VLC) system employing Reed Solomon (RS) coding. In this technique, the data is encoded using an RS codeword and then part of the redundancy introduced by the frame which exceeds the clipping range are removed by applying different number of puncturing, and the redundancy left in the data are used at the receiver side to retrieve the data block. The puncturing of the redundancy aids in mitigating clipping noise generated by truncating the OFDM symbols at the transmitter by shortening the number of Active Subcarriers, while the residual redundancy are exploited at the receiver for the reconstruction and correction of errors.
Shuping Dang - One of the best experts on this subject based on the ideXlab platform.
-
binary tree encoding for uniform binary sources in index modulation systems
arXiv: Information Theory, 2019Co-Authors: Justin P. Coon, Mihaialin Badiu, Ferhat Yarkin, Shuping DangAbstract:The problem of designing bit-to-pattern mappings and power allocation schemes for orthogonal frequency-division multiplexing (OFDM) systems that employ subcarrier index modulation (IM) is considered. We assume the binary source conveys a stream of independent, uniformly distributed bits to the pattern mapper, which introduces a constraint on the pattern transmission probability distribution that can be quantified using a binary tree formalism. Under this constraint, we undertake the task of maximizing the achievable rate subject to the availability of channel knowledge at the transmitter. The optimization variables are the pattern probability distribution (i.e., the bit-to-pattern mapping) and the transmit powers allocated to Active Subcarriers. To solve the problem, we first consider the relaxed problem where pattern probabilities are allowed to take any values in the interval [0,1] subject to a sum probability constraint. We develop (approximately) optimal solutions to the relaxed problem by using new bounds and asymptotic results, and then use a novel heuristic algorithm to project the relaxed solution onto a point in the feasible set of the constrained problem. Numerical analysis shows that this approach is capable of achieving the maximum mutual information for the relaxed problem in low and high-SNR regimes and offers noticeable benefits in terms of achievable rate relative to a conventional OFDM-IM benchmark.
-
Distributed Processing for Multi-Relay Assisted OFDM With Index Modulation
IEEE Transactions on Wireless Communications, 2019Co-Authors: Shuping Dang, Miaowen Wen, Shahid MumtazAbstract:Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth-generation (5G) wireless communications and has thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual Subcarriers. In this regard, we propose a distributed processing scheme for multi-relay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDM-IM systems. Specifically, a single selected relay only needs to decode partial information carried on certain Active Subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on Active Subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.
-
Layered Orthogonal Frequency Division Multiplexing With Index Modulation
IEEE Systems Journal, 2019Co-Authors: Shuping Dang, Miaowen Wen, Xueqin Jiang, Yuyang Peng, Han HaiAbstract:In this paper, we propose a novel scheme termed layered orthogonal frequency division multiplexing with index modulation (L-OFDM-IM) to increase the spectral efficiency (SE) of OFDM-IM systems. In L-OFDM-IM, all Subcarriers are first divided into multiple layers, each determining the Active Subcarriers and their modulated symbols. The index modulation (IM) bits are carried on the indices of the Active Subcarriers of all layers, which are overlapped and distinguishable with different signal constellations so that the number of the IM bits is larger than that in traditional OFDM-IM. A low-complexity detection is proposed to alleviate the high burden of the optimal maximum-likelihood detection at the receiver side. A closed-form upper bound on the bit error rate, the achievable rate, and diversity order are derived to characterize the performance of L-OFDM-IM. To enhance the diversity performance of L-OFDM-IM, we further propose coordinate interleaving L-OFDM-IM (CI-L-OFDM-IM), which interleaves the real and imaginary parts of the modulated symbols over two different subchannels. Computer simulations verify the theoretical analysis, and results show that L-OFDM-IM outperforms the conventional OFDM-IM scheme. Moreover, it is also confirmed that CI-L-OFDM-IM obtains an additional diversity order in comparison with L-OFDM-IM.
-
Lexicographic Codebook Design for OFDM With Index Modulation
IEEE Transactions on Wireless Communications, 2018Co-Authors: Shuping Dang, Gaojie Chen, Justin P. CoonAbstract:In this paper, we propose a novel codebook design scheme for orthogonal frequency-division multiplexing with index modulation (OFDM-IM) to improve system performance. The optimization process can be implemented efficiently by the lexicographic ordering principle. By applying the proposed codebook design, all subcarrier activation patterns with a fixed number of Active Subcarriers will be explored. Furthermore, as the number of Active Subcarriers is fixed, the computational complexity for estimation at the receiver is reduced and the zero-Active subcarrier dilemma is solved without involving complex higher layer transmission protocols. It is found that the codebook design can potentially provide a tradeoff between diversity and transmission rate. We investigate the diversity mechanism and formulate three diversity-rate optimization problems for the proposed OFDM-IM system. Based on the genetic algorithm, the method of solving these formulated optimization problems is provided and verified to be effective. Then, we analyze the average block error rate and bit error rate of the OFDM-IM systems applying the codebook design. Finally, all analyses are numerically verified by the Monte Carlo simulations. In addition, a series of comparisons are provided, by which the superiority of the codebook design is confirmed.
-
Adaptive OFDM With Index Modulation for Two-Hop Relay-Assisted Networks
IEEE Transactions on Wireless Communications, 2018Co-Authors: Shuping Dang, Justin P. Coon, Gaojie ChenAbstract:In this paper, we propose an adaptive orthogonal frequency-division multiplexing with index modulation (OFDM-IM) for two-hop relay networks. In contrast to the traditional OFDM-IM with a deterministic and fixed mapping scheme, in this proposed adaptive OFDM-IM, the mapping schemes between a bit stream and indices of Active Subcarriers for the first and second hops are adaptively selected by a certain criterion. As a result, the Active Subcarriers for the same bit stream in the first and second hops can be varied in order to combat slow frequency-selective fading. In this way, the system reliability can be enhanced. In addition, considering the fact that a relay device is normally a simple node, which may not always be able to perform mapping scheme selection due to limited processing capability, we also propose an alternative adaptive methodology in which the mapping scheme selection is only performed at the source and the relay will simply utilize the selected mapping scheme without changing it. The analyses of average outage probability, network capacity, and symbol error rate are given in closed form for decode-and-forward relaying networks and are substantiated by numerical results generated by Monte Carlo simulations.
