The RISE-MM project was divided into three distinct phases. In the first phase, I explored various wireless network architectures and deployment scenarios, including near-field and far-field, as well as indoor and outdoor environments. This investigation provided a deep understanding of the technical requirements and system specifications for each architecture and scenario, which proved essential for the analytical modeling of RISE-MM channels in the next phase. To conclude the first phase, I identified a RISE-MM system model that will be used for analysis in the second phase of this project. The basic schematic of this system model is presented in Figure 1 and the polar representation of the system model is presented in Figure 2. In the second phase, building on the insights from the first, I examined how the channel state information (CSI) in RISE-MM networks degrades over time under different network conditions. I also analyzed the impact of the age of channel information on the network's communication, quality of service (QoS), and secrecy performances. This research was pioneering, leading to three novel contributions that laid the foundation for a new research area: "Age of Channel Information and Its Impact on Emerging Massive Wireless Networks." These contributions are briefly outlined below.
1- On the effective capacity of RIS-enabled mmWave networks with outdated CSI [R1]
This work investigates the impact of outdated CSI on the effective capacity of a RISE-MM downlink network, considering statistical QoS constraints. It provides practical insights for optimizing system parameters to balance performance with minimal complexity and cost in channel estimation, based solely on outdated channel information. The analysis derives a closed-form expression for the effective capacity that accounts for both the optimism in packet scheduling and the correlation between instantaneous and outdated CSI. This work identifies that RIS beam patterns significantly affect the correlation between outdated and instantaneous CSI, with narrower beams requiring more frequent reconfiguration to maintain capacity, raising channel estimation costs. Each beam pattern has an optimal beam training frequency that maximizes effective capacity. Selecting the appropriate beam pattern, based on user velocity and distance from the RIS, improves capacity while minimizing estimation overhead.
2- On the impact of age of channel information on secure RIS-assisted mmwave networks [R2]
This work investigates the impact of outdated channel information on the secrecy performance of a RISE-MM network. Specifically, it explores how aging channel information affects the joint optimization of transmit beamforming and RIS configuration, with the goal of improving network secrecy. In this study, I derived a closed-form expression for the cumulative noise effect caused by outdated channel information and inter-user interference. I then analyzed how this affects the received SINR at both the legitimate user's location and potential eavesdropping locations. Monte-Carlo simulations showed that the proposed system can still achieve adequate secrecy capacity even when outdated channel information is used for RIS configuration. This insight allows for tuning of system parameters to enhance secrecy performance while keeping the complexity and costs of channel estimation and frequent RIS reconfiguration to a minimum.
3- Optimizing QoS in Secure RIS-Assisted mmWave Network With Channel Aging [R3]
This work explores the impact of the age of channel information on secrecy performance when a RISE-MM network operates under statistical QoS constraints. Specifically, this work optimizes the QoS performance of a RISE-MM network given only outdated channel estimates. To this end, it proposes a technique for the joint optimization of transmit beamforming and RIS configuration, along with a closed-form solution for the optimal transmit power control policy. I investigate the impact of channel aging on the performance of these techniques. I first identify the factors influencing the aging process of a RISE-MM channel in both the near and far fields of the RIS. Subsequently, I examine the impact of channel aging on secrecy capacity and demonstrate that adequate secrecy capacity can still be achieved even when channel information is outdated, reducing the need for frequent RIS configuration. Moreover, the proposed optimal power control policy results reveal that operating in a high SNR regime does not necessarily increase the achievable effective secrecy capacity when the system operates under stricter QoS constraints. These findings allows system designers to adopt a more pragmatic system design approach that consumes less energy while maintaining the required QoS and secrecy performance.
Aside from the RISE-MM channel estimation analysis, I also performed work on waveform design for joint communication and sensing in RISE-MM networks. Specifically, I explore two waveform designs based on "Orthogonal Time Frequency Space (OTFS)" and "Unique Word Orthogonal Frequency Division Multiplexing (UW-OFDM)". Brief details are in the following.
4- A Low-Complexity Standard-Compliant PAPR Reduction Scheme for OTFS Modulation [R4]
OTFS modulation is known for its benefits in radar and communication waveform design, particularly for reliable transmission in high Doppler channels by mapping information symbols in the Delay-Doppler domain. However, the inverse-discrete Fourier Transform operation in OTFS results in a high Peak-to-Average Power Ratio (PAPR) in the transmitted frames. This work introduces a novel symbol pre-distortion algorithm to reduce PAPR, constrained by Error Vector Magnitude (EVM) limits. The method leverages the EVM tolerance used in wireless standards, avoiding the need for side-channel information. The pre-distortion for each symbol is determined by its contribution to peak values in the output. The approach is simple, flexible, and adds no complexity to symbol detection at the receiver.
5- UW-OFDM based Bi-Static Joint Communication and Sensing in 5G-NR Systems
This work explores the potential of using UW-OFDM for joint communication and sensing. In this scheme, the guard interval is replaced with a deterministic sequence, known as a UW, instead of the typical random Cyclic Prefix (CP). While the UW offers the same benefits as a CP, its deterministic nature allows it to be tailored for sensing applications, such as detecting multiple targets. This study designs a first-of-its-kind 5G-NR standard-compliant UW-OFDM framework for bi-static, multi-target sensing, ensuring that the system's communication performance remains unaffected.
References
[R1] Syed Waqas Haider Shah, S. P. Deram, and J. Widmer, “On the Effective Capacity of RIS-enabled mmWave Networks with Outdated CSI”, in Proc. of IEEE International Conference on Computer Communications (IEEE INFOCOM 2023), New York, United States, 2023.
[R2] Syed Waqas Haider Shah, M. Qaraqe, S. Althunibat, and J. Widmer, “On the Impact of Age of Channel Information on Secure RIS-enabled mmWave Networks”, in Proc of IEEE Vehicular Technology Conference (IEEE VTC- Spring 2024), Singapore, 2024.
[R3] Syed Waqas Haider Shah, M. Qaraqe, S. Althunibat, and J. Widmer, “Optimizing QoS in Secure RIS-Assisted mmWave Network With Channel Aging”, IEEE Transactions on Vehicular Technologies, 2024.
[R4] S. Sharma, Syed Waqas Haider Shah, and J. Widmer, “A Low-Complexity Standard-Compliant PAPR Reduction Scheme for OTFS Modulation”, in Proc of IEEE Vehicular Technology Conference (IEEE VTC-Spring 2024), Singapore, 2024.
