⚡ electrical engineering

RSMA-Assisted OFDM-OTFS Hybrid Framework for Mixed-Mobility Multiuser Systems

This paper proposes a novel RSMA-assisted OFDM-OTFS hybrid framework for 6G vehicular networks that employs a common stream to mitigate inter-carrier interference for OFDM users while transmitting private streams over disjoint bands, demonstrating superior outage probability and rate performance over orthogonal multiplexing even under channel estimation errors.

Original authors: Wafa Hedhly, Leila Musavian, Nikolaos Thomos

Published 2026-07-10

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Wafa Hedhly, Leila Musavian, Nikolaos Thomos

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a future highway for data where two very different types of cars are trying to drive at the same time. On one side, you have OFDM cars: these are the reliable, old-school vehicles that everyone is used to, but they get really dizzy and lose their way if they drive too fast (high mobility). On the other side, you have OTFS cars: these are futuristic, super-fast racers designed specifically to handle high speeds without getting dizzy, but they are huge and take up a lot of road space.

The problem? If you try to put them all on the same road using the old rules, the fast racers crash into the dizzy cars, and the dizzy cars spin out of control. This is what happens in current 6G networks when users moving at different speeds (like a slow pedestrian and a fast train) try to share the same signal.

The New Traffic Plan: RSMA

The authors of this paper, Wafa Hedhly and her team, propose a clever new traffic system called RSMA (Rate-Splitting Multiple Access). Think of RSMA as a smart traffic controller that doesn't just tell cars to stay in separate lanes (which is the old, boring way called "Orthogonal Multiplexing"). Instead, it splits the message into two parts: a Common Stream and Private Streams.

Here is how the magic happens:

The Common Stream (The Safety Net): The controller broadcasts a special "safety message" to all the OFDM cars. This message is spread across the entire road (the whole bandwidth). It's like a giant, glowing guide rail that helps the dizzy OFDM cars stay on track even when the road is bumpy and fast (high Doppler effects). The OTFS racers don't need this guide rail; they are too cool for that.
The Private Streams (The VIP Lanes): Once the OFDM cars have grabbed the safety message, they get their own specific lanes (private streams) to carry their personal data. The OTFS racers get their own separate, exclusive lanes too.
The Cleanup Crew (SIC): Before the OTFS racers can zoom off with their private data, they have to do a little cleanup. They listen to the "safety message" (the common stream), figure out what it is, and cancel it out of their signal so it doesn't interfere with their own racing data. This process is called Successive Interference Cancellation (SIC).

The Catch: Imperfect Eyes

The paper points out a crucial detail: the cars' eyes (the receivers) aren't perfect. They sometimes misjudge the road conditions. This is called channel estimation error. If the OTFS racers misjudge the safety message while trying to cancel it out, they leave behind a little bit of "residual interference" (a messy smudge on their windshield). The paper shows that if the road is too bumpy or the eyes are too blurry, this smudge can actually make things worse.

What the Simulations Showed

The authors didn't just guess; they ran 100,000 computer simulations (Monte Carlo iterations) to see how this new system would work. They set up a scenario with 4 OFDM users (moving between 40 km/h and 120 km/h) and 2 OTFS users (moving between 120 km/h and 300 km/h) on a road with 24 subcarriers (lanes).

Here is what they found in these simulations:

The Sweet Spot: When they gave the "safety message" (common stream) a lot of power (specifically 0.8 of the total power), the OFDM cars became incredibly reliable. They could handle the high speeds without crashing, and the whole system moved much faster than the old "separate lanes" method.
The Trade-Off: However, there was a catch. Because the OTFS racers had to spend time canceling out the safety message, and because less power was left for their own private data when the safety message was too strong, their speed dropped. If the safety message took up 0.8 of the power, the OTFS racers were slower than they would have been in the old system.
The Blurry Vision Problem: When the "eyes" were very blurry (high channel estimation error), the cleanup crew (SIC) made mistakes. If the safety message was too strong in these conditions, the leftover smudges (residual interference) actually hurt the OFDM cars more than the old system would have. But, if the power was tuned just right, the system could still beat the old method even with imperfect eyes.

The Bottom Line

The paper suggests that this new RSMA framework is a powerful tool for mixing old and new technologies in 6G networks. It proves that by using a shared "safety stream" to help the struggling OFDM users, you can get a much better overall performance. But it's not a magic wand that fixes everything instantly. The authors show that you have to be very careful with how much power you give to the safety stream. If you give it too much, the OTFS racers slow down; if you give it too little, the OFDM cars get dizzy.

In short, the paper demonstrates that this hybrid framework can improve the total speed and reliability of the network, but only if you balance the power carefully and accept that there is a trade-off between helping the slow cars and keeping the fast racers happy. It's a simulation-based finding, meaning it works beautifully in the computer model, waiting for real-world testing to confirm the ride.

Technical Summary: RSMA-Assisted OFDM–OTFS Hybrid Framework for Mixed-Mobility Multiuser Systems

Problem Statement
Future 6G vehicular networks will necessitate the coexistence of Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Time Frequency Space (OTFS) waveforms to accommodate users with diverse mobility profiles. While OFDM is compatible with existing infrastructure, it suffers from severe inter-carrier interference (ICI) under high-mobility conditions due to Doppler effects. Conversely, OTFS is robust in double-dispersive channels but typically consumes larger spectrum resources. The challenge lies in efficiently managing interference and spectrum utilization when these heterogeneous users—some operating in the time-frequency (TF) domain and others in the delay-Doppler (DD) domain—share the same network resources. Existing Rate-Splitting Multiple Access (RSMA) studies have largely focused on either all-high-mobility OTFS users or OFDM-only scenarios, lacking a general framework for mixed-mobility, mixed-waveform coexistence.

Methodology
The authors propose a novel downlink RSMA-assisted framework designed for a base station serving $K_1$ OFDM users and $K_2$ OTFS users simultaneously. The system model incorporates the following key mechanisms:

Stream Allocation: The transmitted signal is split into a common stream and private streams.
- Common Stream: Contains the common messages of all OFDM users. It spans the entire available bandwidth to assist OFDM users in managing ICI induced by high Doppler shifts. OTFS users do not receive common messages but must decode and cancel this stream.
- Private Streams: The private messages of OFDM users and the streams of OTFS users are transmitted over disjoint frequency bands.
Signal Processing:
- OFDM Users: Receive the common stream across all subcarriers and their private streams on dedicated subcarriers. They employ Successive Interference Cancellation (SIC) to decode the common stream first, then remove it to decode their private streams.
- OTFS Users: Operate in the DD domain. They decode and cancel the common stream (which acts as interference) before decoding their own private DD-domain signals using Minimum Mean Squared Error (MMSE) equalization.
Imperfect CSI: The system model explicitly accounts for channel estimation errors during the SIC process. Residual interference resulting from imperfect channel state information (CSI) is mathematically derived for both OFDM and OTFS receivers, affecting the Signal-to-Interference-plus-Noise Ratio (SINR) and outage probability.

Key Contributions

Hybrid Framework Design: The paper introduces a specific RSMA architecture that enables the coexistence of OFDM and OTFS users with mixed mobility profiles, addressing a gap in prior literature which often assumed homogeneous user mobility or waveform types.
ICI Mitigation Strategy: By allocating a common stream that spans the whole bandwidth, the framework specifically targets the mitigation of ICI for high-mobility OFDM users, a known weakness of standard OFDM in Doppler channels.
Analytical Modeling with Imperfect CSI: The authors derive instantaneous SINR and outage probability expressions that explicitly incorporate channel estimation errors and residual interference from imperfect SIC, providing a realistic performance evaluation.
Trade-off Analysis: The study quantifies the inherent trade-off between the reliability of OFDM users and the throughput of OTFS users, governed by the power allocation factor of the common stream.

Simulation Results
Numerical results, based on Monte Carlo simulations over doubly selective channels, demonstrate the following:

Total Sum-Rate: The proposed RSMA framework outperforms Orthogonal Multiple Access (OMA) benchmarks, particularly at high Signal-to-Noise Ratios (SNR) and when a significant portion of power (e.g., $p_c = 0.8$ ) is allocated to the common stream.
OFDM Performance: Increasing the common stream power significantly improves the sum-rate and outage probability of OFDM users by enhancing robustness against ICI. However, this benefit is contingent on the accuracy of channel estimation; severe estimation errors can degrade performance due to residual interference, though high power allocation can eventually compensate for these errors.
OTFS Performance: Conversely, OTFS users experience a degradation in sum-rate and outage performance as the common stream power increases. This is because OTFS users do not benefit from the common stream and must dedicate power to cancel it, reducing the power available for their private messages.
Trade-off: The results confirm a clear trade-off: optimizing for OFDM reliability via high common-stream power comes at the cost of OTFS throughput and reliability.

Significance and Claims
The paper claims that the proposed RSMA-assisted framework is a viable solution for future mixed-mobility vehicular networks where legacy OFDM devices and emerging OTFS devices must coexist. The authors assert that the framework offers superior rate and outage performance compared to orthogonal multiplexing, specifically for OFDM users suffering from high-mobility ICI. However, the paper modestly notes that this improvement is not universal; it depends heavily on the power allocation strategy and channel estimation quality. The framework is presented as a flexible tool that allows network operators to balance the competing needs of OFDM robustness and OTFS throughput, rather than a solution that maximizes both simultaneously without cost.

The New Traffic Plan: RSMA

The Catch: Imperfect Eyes

What the Simulations Showed

The Bottom Line

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