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Reflection modulation based on reconfigurable intelligent surface (RIS) is considered to be a promising information transfer mechanism without requiring any additional radio frequency chains. However, existing reflection modulation schemes consider manipulating the ON/OFF states of RIS elements, which suffers from power loss. In this paper, we propose a new scheme, called RIS-aided quadrature reflection modulation (RIS-QRM), for harvesting the reflection power based on an RIS-aided downlink multiple-input single-output (MISO) wireless system. To this end, RIS-QRM partitions RIS elements into two subsets for reflecting impinging signals into two orthogonal directions as well as passive beamforming and encodes its local data onto the element partition options. A closed-form expression for the unconditional pairwise error probability of RIS-QRM in Rician fading is derived assuming the maximum-likelihood detection. Moreover, we propose a low-complexity detection method with compromised performance for decoupling the joint search of the constellation symbol and the RIS element partition. Computer simulation results corroborate the effectiveness of RIS-QRM and validate the analytical results. It is shown that RIS-QRM is able to improve the error performance of the additional bits delivered by the RIS without deteriorating that of the bits carried on the constellation symbol, as compared to ON/OFF-based schemes.

Reconfigurable intelligent surfaces (RISs) have recently emerged as an innovative technology for improving the coverage, throughput, and energy/spectrum efficiency of future wireless communications. In this paper, we propose a new transmission protocol for wideband RIS-assisted single-input multiple-output (SIMO) orthogonal frequency division multiplexing (OFDM) communication systems, where each transmission frame is divided into multiple sub-frames to execute channel estimation simultaneously with passive beamforming. As the training symbols are discretely distributed over multiple sub-frames, the channel state information (CSI) associated with RIS cannot be estimated at once. As such, we propose a new channel estimation method to progressively estimate the associated CSI over consecutive sub-frames, based on which the passive beamforming at the RIS is fine-tuned to improve the achievable rate for data transmission. In particular, during the channel training, the RIS plays two roles of embedding training reflection states for progressive channel estimation and performing passive beamforming for data transmission on the data tones. Based on the estimated CSI in each sub-frame, we formulate an optimization problem to maximize the average achievable rate by designing the passive beamforming at the RIS, which needs to balance the received signal power over different sub-carriers and different receive antennas. As the formulated problem is non-convex and thus difficult to solve optimally, we propose two efficient algorithms to find high-quality solutions. Simulation results validate the effectiveness of the proposed channel estimation and beamforming optimization methods. It is shown that the proposed joint channel estimation and passive beamforming scheme is able to drastically improve the average achievable rate and reduce the delay for data transmission as compared to existing schemes.

To relieve the shortage problem of licensed spectrum, the aggregation of licensed and unlicensed carriers has emerged as an attractive solution for opportunistically increasing the throughput of LTE systems while enhancing the utilization of unlicensed spectrum. As one of the representative aggregation technologies, LAA is in favor of the harmonious coexistence with WiFi systems by leveraging the LBT mechanism and different pre-defined access priorities. In this article, we present a general overview of the attractive benefits and technical challenges for aggregating licensed carriers with unlicensed carriers. We then introduce some fundamental features of the LBT based LAA, including transmission modes, ED threshold, random backoff within the contention window, and access priority classes. Under multiple unlicensed channels, we propose two novel multi-carrier LBT mechanisms to improve the transmission behavior and coexistence performance of LAA and WiFi. It is verified in terms of successful transmission probability and carrier utilization rate that two proposed multi-carrier LBT mechanisms can achieve much better performance than the conventional counterpart using random carrier selection.

International audience; Reflection modulation based on reconfigurable intelligent surface (RIS) is considered to be a promising information transfer mechanism that conveys information by reconfiguring incident electromagnetic waves without using radio frequency chains. Existing reflection modulation schemes propose to convey additional information by manipulating the ON/OFF states of reflecting elements, which underuse the potential of the RIS. In this paper, we first propose a new reflection modulation scheme, called RIS-based quadrature reflection modulation (RIS-QRM), to achieve joint passive beamforming and information transfer for RIS-aided single-input single-output wireless communications by assuming that the direct link between the transceivers is blocked. We then extend the RIS-QRM scheme to the multiple-input single-output setting with the direct link between transceivers being taken into account. In this case, an optimization problem is formulated to jointly optimize the active beamforming and the phase shifts of the RIS in order to improve the received signal power at the user. Computer simulation results corroborate the effectiveness of the RIS-QRM scheme. It is shown that in contrast to ON/OFF-based schemes, the RIS-QRM scheme is able to improve the error performance of the additional bits delivered by the RIS without deteriorating that of the bits carried by the constellation symbols.

To enjoy advantages of both the single-carrier (SC) transmission and the generalized spatial modulation (GSM), a hybrid architecture of SC-GSM has been developed in the literature. However, low-complexity demodulation for SC-GSM signals with near-optimal performance is challenging, especially in soft decision making. To this end, based on the deterministic sequential Monte Carlo (SMC) principle, we propose two low-complexity demodulators for the SC-GSM system with zero-padding (ZP) in broadband channels. By exploiting the natural triangular structure of the multi-path channel to apply SMC sampling, a vector-level SMC demodulator without the need of the sequential construction process is proposed, significantly saving the computational burden. To lower the required computation during the sampling and weight updating processes of the vector-level SMC demodulator, a symbol-level SMC demodulator is further proposed, which takes samples from the expanded constellation including the origin. In particular, an efficient online legality check scheme, which eliminates illegal samples during each sampling step, is integrated into the symbol-level SMC demodulator to avoid error propagation and save the computational burden dramatically. Both proposed demodulators produce soft outputs, which can be used for soft demodulation in coded systems, especially for turbo receivers. The simulation results verify the near-optimal performance of the two proposed demodulators in the uncoded and coded SC-GSM system with ZP, respectively.

Recent considerations for reconfigurable intelligent surfaces (RISs) assume that RISs can convey information by reflection without the need of transmit radio frequency chains, which, however, is a challenging task. In this paper, we propose an RIS-enhanced multiple-input single-output system with reflection pattern modulation, where the RIS can configure its reflection state for boosting the received signal power via passive beamforming and simultaneously conveying its own information via reflection. We formulate an optimization problem to maximize the average received signal power by jointly optimizing the active beamforming at the access point (AP) and passive beamforming at the RIS for the case where the RIS's state information is statistically known by the AP, and propose a high-quality suboptimal solution based on the alternating optimization technique. We analyze the asymptotic outage probability of the proposed scheme under Rayleigh fading channels, for which a closed-form expression is derived. The achievable rate of the proposed scheme is also investigated for the case where the transmitted symbol is drawn from a finite constellation. Simulation results validate the effectiveness of the proposed scheme and reveal the effect of various system parameters on the achievable rate performance. It is shown that the proposed scheme outperforms the conventional RIS-assisted system without information transfer in terms of achievable rate performance., 31 pages; 7 figures; under minor revision for an IEEE journal

Multiple-input multiple-output orthogonal frequency division multiplexing with index modulation (MIMO-OFDM-IM) is a relatively new multicarrier transmission technique for the MIMO systems to achieve both energy and spectral efficiency. However, due to the dependence of subcarrier symbols introduced by the index modulation, existing MIMO detection techniques cannot be applied in MIMO-OFDM-IM as it will lead to erroneous detection on index information of the active subcarriers and further deteriorate the system performance. In this paper, we first develop an optimal detection algorithm with reduced complexity for MIMO-OFDM-IM by performing the subcarrier-wise detection under the constraint on the legal subcarrier combination within each OFDM-IM subblock. To further reduce the computational complexity, we then propose a low-complexity subcarrier-wise algorithm based on the deterministic sequential Monte Carlo technique to achieve near-optimal detection for MIMO-OFDM-IM. Specifically, the near-optimal detector applies the QR decomposition to generate an upper triangular structure and draw antenna-wise samples at the subcarrier level. Computational simulations and complexity analysis show that the proposed algorithms provide similar performance to optimal one with reduced computational complexity.

Adaptive linear equalizer, whose coefficients are designed to be adjustable to the channel impulse response, has emerged as a simple and efficient technique to adaptively compensate for the channel fading. However, conventional adaptive linear equalizers suffer from performance degradation and slow convergence in the underwater acoustic channel with large delay spread. To solve this problem, in this paper, we propose a novel adaptive decision-feedback equalizer (DFE) based on the minimum symbol-error rate (MSER) criterion. Specifically, by taking the sample-by-sample adaptation into account, the problem is first formulated as minimizing the norm between two consecutive adaptations under the constraint that the latest adaptation will provide correct detection for both the current and past symbols. Then we solve the optimization problem by using the Lagrange multiplier method to obtain the adaptive DFE that minimizes the sequential symbol detection error with a fast convergence rate. Simulation results show that the proposed MSER-based adaptive DFE significantly outperforms the existing equalizers in terms of convergence speed and steady-state performance for underwater acoustic channels.

Due to the interchannel interference and the varying active antennas in generalized spatial modulation (GSM), it is challenging to detect the transmitted data effectively while imposing low computational burden to the receiver. In this paper, we propose a novel algorithm based on fixed-complexity sphere decoding (FSD) for the soft detection of the GSM signal, which performs the breadth-first tree search in two stages sequentially. By exploiting the null space of the GSM channel, the inactive antenna searching is carried out antenna-wise in the first stage, in which a recursive algorithm for updating the QL decomposition is proposed to calculate the branch metrics and an effective examination scheme is developed to prune those illegal or repetitive child nodes. Based on the available QL decomposition structure that has been updated recursively during the first stage, the second stage of the proposed detector is then readily concatenated to search for the modulated symbols carried on the active antennas in a one-by-one manner. Simulation results in terms of bit error rate (BER) corroborate the superiority of the proposed soft detector for the GSM system.