Integrating Mediumband with Emerging Technologies: Unified Vision for 6G and Beyond Physical Layer

TL;DR

Deep fading is identified as a dominant bottleneck in wireless links for 6G and beyond. The paper introduces mediumband wireless communication as an operating regime defined by $T_m < T_s < 10T_m$ and advocates a unified PHY that integrates reflecting surfaces, sensing, digital twins, ray-tracing, and AI to place links into this regime. It provides design-time and real-time strategies to create mediumband channels, including a PDS metric and RS-assisted CIR lengthening, with a note that real-time gains persist as long as MPCs exist. The framework applies to both narrowband and broadband scenarios and envisions anticipatory networks that dynamically manage the environment to sustain mediumband operation, promising improvements in reliability, spectral/energy efficiency, and resilience for 6G and beyond.

Abstract

In this paper, we present a vision for the physical layer of 6G and beyond, where emerging physical layer technologies integrate to drive wireless links toward mediumband operation, addressing a major challenge: deep fading, a prevalent, and perhaps the most consequential, obstacle in wireless communication link performance. By leveraging recent insights into wireless channel fundamentals and advancements in computing, multi-modal sensing, and AI, we articulate how reflecting surfaces (RS), sensing, digital twins (DTs), ray-tracing, and AI can work synergistically to lift the burden of deep fading in future wireless communication networks. This refreshingly new approach promises transformative improvements in reliability, spectral efficiency, energy efficiency, and network resilience, positioning 6G for truly superior performance.

Figures (6)

• Figure 1: On the top is a typical propagation environment, where $s(t)$ is the transmit signal and $r(t)$ is the receive signal. On the bottom-left is a depiction of a typical $s(t)$. In digital wireless communication, not all parts of $s(t)$, but only the points regularly separated in time (i.e. RED dots), carry information. This separation time is the symbol period, $T_s$. On the bottom-right is a depiction of a multipath delay profile as an impulse train, where the $x$--coordinate and the height of the impulses represent the excess delay, $\tau_n$, and the strength, $\gamma_n$, of the corresponding MPC respectively, where $T_m$ is a suitable measure of the time spread of MPCs like maximum excess delaySarkar03. A wireless communication system, whose $T_m$ and $T_s$ approximately satisfy the condition: $T_m < T_s < \lambda T_m$, which is represented by the GREEN region in Fig. \ref{['fig:fig0']}, is said to be operating in the mediumband.
• Figure 2: Bas2023BasMag2024 Main regions on the $T_mT_s$--plane representing three classes of wireless channels, where the exact value of the constant, $\lambda$ is dependent on the exact definition of $T_m$. For instance, if $T_m$ is the maximum excess delay, $\lambda \approx 10$, but if $T_m$ is defined differently $\lambda$ would be different. Importantly, whatever the lines of separation of different regions, mediumband on the $T_mT_s$--plane refers to the region between narrowband and broadband regions where something called "the effect of deep fading avoidance" is most prominent. Henceforth, $\lambda=10$ is assumed.
• Figure 3: On the left is a depiction of a typical multipath delay profile of a mediumband channel, where $\hat{\tau}$ is the time offset and also the location of the fading factor denoted by $g$ and shown in light GREEN. Typically, it is the MPCs further away from this fading factor that contribute the most to the effect of deep fading avoidance in $g$. On the right is a PDF of the desired fading factor (only the real part) of mediumband wireless communication in NLoS propagation. The Gaussian PDF is also shown for comparison.
• Figure 4: On the left is a depiction of a wireless link where suitably selected reflecting surfaces artificially lengthen the CIR. Unlike in intelligent reflecting surfaces, herein, no adjustments to the phase of the MPCs and the estimation of the fading factors between nodes and the reflecting surfaces are necessary. On the right is a depiction of a typical multipath delay profile of, as it is called a generalized mediumband channel, where $\hat{\tau}$ is the time offset. The two fading factors, denoted by $g_1$ and $g_2$ shown in light GREEN are modelled to be located at $t=\hat{\tau}$ and $t=\hat{\tau}+T_s$ respectively. Typically, it is the MPCs located further away from these fading factors (circled in RED) that contribute the most to the effect of deep fading avoidance in $g_1$ and $g_2$.
• Figure 5: Top view of a mediumband wireless communication system aided by reflecting surfaces, where artificial reflectors located between two ellipses introduce three MPCs to lengthen the multipath delay profile. Simple geometric arguments ensure that their corresponding propagation delays satisfy: $\frac{2a_1}{c} \leq \tau_1, \tau_2, \tau_3 \leq \frac{2a_2}{c}$, where $c$ is the speed of light, and $2a_1$ and $2a_2$ are the lengths of the semi-major axes of the inner and outer ellipses. Importantly, the precise knowledge of the location of reflectors: $(x_i,y_i)$ are not needed here. For graphical simplicity, it is ellipses that we considered here, but in practice, considering ellipsoids with corresponding geometric arguments would be more appropriate.