Sixth generation promises to have a variety of applications, as well as network entities, in terms of power, range, data rates, latency, and propagation environment. The overall heterogeneity of the network poses significant challenges in terms of physical (PHY) layer design. This article first identifies some of the areas where the 5G multinumerology orthogonal frequency division multiplexing (OFDM) falls short before discussing the possible approaches toward PHY design. In particular, we discuss a backward-compatible, forward-looking, and extendable PHY layer framework that can be extended as new requirements arise to enable a graceful and sustainable network evolution. This is illustrated with a simple case study, where users with different requirements are allowed to use the same time-frequency (T-F) resources in a cohesive manner. The article concludes with the discussion of some critical research problems/areas pertaining to the said framework.

The recently popularized ISAC paradigm attempts carry out both communication and sensing functionalities uses the same time-frequency resources to combat the scarcity of these resources. However, high-resolution range and velocity radars require wideband long-duration transmission, which implies complex, costly receivers to sample at a high-frequency rate. In this paper, we propose an orthogonal time-frequency space (OTFS)-based ISAC system which enables achieving highly accurate range-velocity profiles without the need for large bandwidth transmissions or long-duration frames. This approach relaxes the constraints on bandwidth and time while still providing precise sensing information. The proposed scheme exploits a single OTFS carrier with rectangular pulse shaping as a pilot to estimate both simultaneous accruing delay and Doppler, thereby determining range and velocity, respectively. By leveraging the sidelobes of the physical pulse shape of the pilot signal, we propose an algorithm that allows the detection of the range and the velocity of radar targets beyond the resolution limitation set by the time duration and the bandwidth of the transmitted signal. The conducted simulation results along with the real experimental results demonstrate that the proposed design can achieve accurate low-complexity radar parameter estimation.

Orthogonal chirp division multiplexing (OCDM) is a new multi-carrier scheme that has been emerging as a new candidate for 6G waveform taking advantage of the unique features of the chirp spread spectrum that makes it immune to intersymbol interference raised due to delay spread. However, a thorough analysis of OCDM under doubly-dispersive channels has not been conducted yet to verify its robustness against Doppler spread as well. In this paper, we investigate the input-output relationship of an OCDM system under doubly-selective channel, where we demonstrate that the circular convolution property of OCDM partially holds even under Doppler spread. Building on previous results, we show the difficulties and problems associated with estimating and equalizing the channel at the receiver side in conventional OCDM systems, especially in case of having fractional delay and Doppler shifts. Then, we propose a new OCDM transceiver by adding fast Fourier transform (FFT) and windowing blocks to ensure channel tap separability and reduce the effect of fractional Doppler shift, respectively. Accordingly, a new channel estimation scheme is developed for the proposed OCDM system. The numerical and simulation results validate the advantages of the proposed OCDM system performance under doubly-dispersive channels over the conventional where the proposed leverages a bit-error-rate (BER) gain in perfect channel state information (CSI) of 2 dB and 3 dB for minimum mean squared error (MMSE) and message passing (MP) equalizers, respectively, and show that it holds great promise as an emerging radio access technology for 6G wireless systems.

In practical transmission, orthogonal time-frequency-space (OTFS) is implemented using different prefix configurations including reduced-cyclic prefix (RCP) and full-CP (FCP). This letter presents a novel OTFS structure, which is a modification of the FCP structure in OTFS modulation. The proposed approach leverages the FCP structure in the delay-Doppler domain to decode the information in the CP block rather than discarding it at the receiver to enhance the system performance. This is validated via simulations in terms of the reduction in channel estimation overhead as well as the improved signal-to-noise ratio (SNR) of the received signal.

The orthogonal time-frequency space (OTFS) waveform exceeds the challenges that face orthogonal frequency division multiplexing (OFDM) in a high-mobility environment with high time-frequency dispersive channels. Since radio frequency (RF) impairments have a direct impact on waveform behavior, this paper investigates the experimental implementation of RF-impairments that affect OTFS waveform performance and compares them to the OFDM waveform as a benchmark. Firstly, the doubly-dispersive channel effect is analyzed, and then an experimental framework is established for investigating the impact of RF-impairments, including non-linearity, carrier frequency offset (CFO), I/Q imbalances, DC-offset, and phase noise are considered. The experiments were conducted in a real indoor wireless environment using software-defined radio (SDR) at carrier frequencies of $2.4$ GHz and 5 GHz based on the Keysight EXG X-Series devices. The comparison of the performances of OFDM and OTFS in the presence of RF-impairments reveals that OTFS significantly outperforms OFDM.

Orthogonal time-frequency space (OTFS) modulation is fast becoming a popular modulation scheme for high-mobility wireless communication due to its ability to render time-varying channels invariant and sparse. However, due to the limited time duration and frequency bandwidth of the transmitted OTFS signal, the occurrence of fractional delay and Doppler shifts becomes inevitable, causing both inter-delay interference (IDeI) and inter-Doppler interference (IDI). In this paper, an impulse-based channel estimation method is used, which takes advantage of the fact that the pilot impulse is a pulse tone (pulsone). A pulsone maintains its shape despite time delay and Doppler shift operations, as well as being self-dual between time and frequency domains. Exploiting these two features, a low-complexity algorithm is proposed to estimate the fractional delay and fractional Doppler channel. We derive the Cramer-Rao lower bound (CRLB) of the estimated fractional channel and show that the developed estimator is efficient. Then, we provide the computational complexity analysis of the proposed algorithm. After that, we show and stress that the proposed channel estimation scheme is better in terms of the channel normalized mean squared error (NMSE) and the bit-error rate (BER). Lastly, the simulation results are confirmed via real experimental results.