In this paper, a novel modulation scheme called two-dimensional discrete cosine transform orthogonal frequency-division multiplexing (2D-DCT-OFDM), which extends the principles of OFDM modulation to a 2D domain is proposed. Unlike traditional OFDM scheme, which relies solely on time-frequency processing, 2D-DCT-OFDM exploits an alternate 2D transform domain for information mapping and employs 2D-DCT operations along these dimensions for signal processing. Here, the design principles and performance analysis of the proposed 2D-DCT-OFDM modulation scheme are presented. Finally, the BER performance of the proposed 2D-DCT-OFDM scheme for different user equipment velocities are obtained and compared with the prior 1D and 2D modulation schemes (such as DFT-s-OFDM and OTFS) to get pertinent candidate waveform suggestions for beyond 5thgeneration (B5G) and 6G cellular systems. Simulation results show that the 2D modulation schemes provide better BER performance in high-mobility scenarios. In addition, 2D-DCT-OFDM provides slightly better BER performance than other 2D modulation schemes under extreme mobility with significantly less computational complexity. Due to the simplicity, computational efficiency, ease of implementation, and power efficiency of 2D-DCT-OFDM, it is suitable for beyond 5G and 6G.

Reconfigurable intelligent surface (RIS) is a promising technology to enhance the coverage and cover the areas under blockage. However, the efficient utilization of RIS resources during idle periods remain a challenge. The conventional RIS configuration inadvertently leads to signal reflection even during the idle periods, resulting in potential signal degradation and interference that can impede the direct and/or neighboring communication links. Due to hardware limitations, it is practically impossible for the RIS to completely absorb the unwanted signal, resulting in minimal reflection amplitude. Therefore, this paper presents the innovative operational mode called “scatter mode” for the RIS, a concept that intelligently controls the RIS behaviour during idle period by scattering incident signals in all spatial directions. By mitigating unnecessary signal reflections and reducing interference, the scatter mode enhances signal quality and network reliability. To realize the proposed scatter mode, two solutions are proposed, namely “random phase selection” and “sub-RIS”. With random phase selection and sub-RIS, the minimum interference is reduced by 12 dB and 25 dB compared to optimized phase to reflect. Finally, the signalling flow and procedure to seamlessly integrate the proposed configuration into the existing RIS deployment strategies and its practical implications are shown.

In this paper, a downlink synchronization technique in an orthogonal time-frequency space (OTFS) cellular system at the physical layer is proposed. The primary synchronization signal (PSS) detection is the most computationally complex algorithm during synchronization and cell search. Therefore, a faster and low-complexity algorithm based on the overlap-add method is proposed. Here, PSS is a Zadoff-Chu sequence (ZCS) carrying a primary cell identity (PCID) mapped in the two dimensional (2D) delay-Doppler domain is broadcast by the base station (BS) using OTFS modulation, which is non-coherently detected at the user equipment (UE). Further, the effect of OTFS modulation on 2D delay-Doppler ZCS is investigated and analyzed to show that the duality property of ZCS is conserved between delay-Doppler to time-frequency domains and between time-frequency and time domains. Finally, the detection probabilities with multiple UE velocities are shown in multi-cell environment to show the robustness of the proposed technique. In addition, the OTFS performance is compared with OFDM for third generation partnership project (3GPP) defined tapped delay line (TDL) channel model. Simulation results show that when the UE speed is 540 km/h ($\equiv \mathbf{14}\mathbf{kHz}$Doppler frequency), a 99% detection probability is achieved at -13 dB SNR using the proposed technique, whereas the detection probability for OFDM drops to zero.

In this paper, a non-coherent downlink synchronization technique in an orthogonal time-frequency space (OTFS) cellular system at the physical layer is proposed. Here, a Zadoff-Chu sequence (ZCS) carrying a secondary cell identity (SCID) is mapped in the two dimensional (2D) delay-Doppler domain is broadcast by the base station (BS) using OTFS modulation, which is non-coherently detected at the user equipment (UE). Unlike OFDM-based fourth (4G) or fifth generation (5G) systems, the detection of SCID with the proposed technique does not require channel estimation and equalization. This enables the UE to detect the SCID from multiple BSs with lower computational complexity. Further, the effect of OTFS modulation on 2D delay-Doppler ZCS is investigated and analyzed to show that the duality property of ZCS is conserved between delay-Doppler to time-frequency domains and between time-frequency and time domains. Unlike conventional 1D correlation properties, 2D correlation properties of ZCS are evaluated. Finally, to show the robustness of the proposed technique, the detection probabilities with multiple UE velocities are shown. Simulation results show that when the UE speed is 150 m/s (= 14 kHz Doppler frequency), a 100% detection probability is achieved at -21 dB SNR using the proposed technique, whereas the detection probability for OFDM drops to zero.

In this paper, we design a Joint Communication and Sensing (JCAS) waveform for Multiple Input Multiple Output (MIMO) wireless systems. The JCAS waveform is formed by combining Frequency Modulated Continuous Wave (FMCW) and Orthogonal Frequency Division Multiplexing (OFDM) waveforms. Specifically, it can be viewed as a combined MIMO-OFDM and MIMO-Radar waveform. The proposed waveform is used by the Base Station (BS) of the JCAS system, which consists of communication and sensing subsystems. The communication subsystem is used to transmit information to the desired user equipment (UE), while the sensing subsystem is used to detect multiple objects in the vicinity of the BS. For the communication subsystem, OFDM sub-carriers are used to carry UE’s data, while the FMCW acts as pilot signals and enable frequency domain channel estimation. For the sensing subsystem, after receiving the echos of JCAS signal, we remove the OFDM part of it and then form a Radar virtual array at the BS. The data from the virtual array is used to estimate the target parameters of interest, i.e., distance and velocity. Numerical results are presented to demonstrate the performance of both communication and sensing subsystems with the proposed JCAS waveform.

To compensate for high pathloss and improve coverage in millimeter-wave (mmWave) frequencies, highly directional transmit (Tx) and receive (Rx) beamforming is required. In addition, mmWave frequencies are extremely susceptible to blockages, and when blockage occurs, the link between Tx and Rx may be terminated and an alternate path is required to retain communication. Reconfigurable intelligent surface (RIS) can be considered as a possible solution to not only solve the blockage problem but also to cover dead spots. However, using conventional exhaustive search protocol, the overhead of beam training increases exponentially with the number of RIS reflective beams, as it requires finding the optimal beam pair in the Tx-RIS-Rx link. In this paper, a low-latency retro-reflective (LLRR) beam training protocol for RIS-assisted cellular system is proposed to reduce the overall beam training time by reflecting the RIS beams to the source. Furthermore, the feasibility of the RIS element to provide 180° phase shift is analyzed. Simulation results show that the proposed LLRR beam training significantly reduces the training time compared to the state-of-the-art and achieves gains of ≈ 1.5 bps/Hz and ≈ 9.8 bps/Hz at signal-to-noise ratio of 15 dB when the number of beams at RIS is 64 and 256, respectively.

The segmentation of images under biased conditions such as low contrast, high-intensity inhomogeneity, and noise is a challenge for any image segmentation model. The ideal image segmentation model must be capable of segmenting images with maximum accuracy and a minimum false-positive rate under biased conditions. In this paper, we propose a region-based active contour model (ACM), called global signed pressure and K-means clustering based on local correntropy with the exponential family (GSLCE), to address segmentation challenges under biased conditions. An adaptive weighted function is formulated based on the global and local image differences such that a single weighted function can drive both the global and local intensities. Further, the Riemannian steepest descent method is used for convergence of the proposed GSLCE energy function, and a Gaussian kernel is applied for spatial smoothing to obviate the computationally expensive level-set re-initialization. The experimental results show that, compared with state-of-the-art ACMs, the proposed GSLCE model obtained the best visual image segmentation results for synthetic and real images under biased conditions. Further, the qualitative and quantitative experimental results validate that the proposed model outperforms the state-of-the-art ACMs by yielding higher values of performance metrics. Moreover, the proposed GSLCE model requires substantially less processing time compared to the state-of-the-art ACMs.

In this paper, a random-access preamble (RAP) for the user equipment (UE) experiencing high Doppler in millimeter-wave (mmWave) cellular systems is proposed. After demonstrating the inefficiency of the conventional long-term evolution (LTE)-based RAP for fast-moving UEs (FUEs) in mmWave systems, linear-frequency-modulation-based RAP (LFM RAP) is proposed for the detection of FUEs. The correlation of LTE-based RAP, and ambiguity and extended-ambiguity functions of LFM RAP are analyzed to examine their properties affected by the high Doppler. The performance of RAPs is evaluated through detection and false alarm probabilities in a high Doppler and multiuser environment. The results show that the LFM RAP is appropriate for FUEs in mmWave cellular systems because of its high detection probability in a high-Doppler environment.

In this paper, a new cell selection technique for millimeter-wave (mmWave) cellular systems considering a hybrid beamforming structure in data transmission stage is proposed. Using the proposed technique with cell and beam synchronization signals (CBSS) transmitting simultaneously from multiple beams, the processing time required for channel estimation and cell selection can be reduced significantly. It is shown by simulation that additional gains in channel capacity can be achieved as compared to conventional technique.

A small-cell network (SCN) constructed by splitting a macro-cell into numerous small cells using an active antenna array system is studied. A synchronization signal appropriate for the SCN, virtually generated by a base station (BS) with 3D beamforming, is proposed for efficient handover in SCNs. The virtual cell synchronization signal (VCSS) carries a macro-cell ID (MCID) and virtual- cell ID (VCID) in a hierarchical manner, allowing us to distinguish between an intra-cell handover and inter-cell handover in SCNs. Using the signal metrics obtained by the VCSS, an efficient handover measurement technique is proposed which can significantly reduce the processing time and overhead by distinguishing between the intra- cell/inter-cell handovers.