⚡ electrical engineering

Dual user wired or 60 GHz wireless selective access link with 10Gbit/s 4-QAM signal based on the upper and lower single sideband modulation

This paper proposes a hybrid optical link utilizing upper and lower single-sideband modulation to enable selective 10 Gbit/s 4-QAM access for two users via either wired or 60 GHz wireless connections, demonstrating robust transmission performance over 20 km of single-mode fiber without dispersion compensation.

Original authors: Ruijiao Zhang, Lei Xu, Guang Chen, Hechao Liu, Wei Liu, Wenxiao Liu

Published 2026-09-30

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CC BY 4.0

Original authors: Ruijiao Zhang, Lei Xu, Guang Chen, Hechao Liu, Wei Liu, Wenxiao Liu

Original paper licensed under CC BY 4.0 (https://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

In the modern world, our demand for data is insatiable. We stream high-definition video, conduct complex scientific research, and shop online, all of which require networks that can carry vast amounts of information quickly and reliably. Currently, we rely on two main types of connections to satisfy these needs. The first is the fiber-optic cable, a thin strand of glass that can transmit enormous quantities of data over long distances with incredible speed, but it is rigid; once the cable is laid, the connection is fixed to a specific location. The second is wireless access, which offers the freedom to connect from anywhere within a certain range, but it often struggles with the sheer volume of data required for next-generation services. The challenge for engineers is to merge these two worlds: to create a system that offers the massive capacity of fiber optics with the flexibility of wireless access, allowing users to switch between the two or use them simultaneously without complex infrastructure changes.

Researchers at the North China University of Water Resources and Electric Power and Beijing Information Science and Technology University have proposed a new way to achieve this fusion. They designed a system that can deliver data to two different users, giving each the choice to receive the information either through a wired connection or via a high-speed wireless signal, all while using a single stream of light traveling through a fiber-optic cable. The core of their idea involves a technique called single-sideband modulation. Imagine a light beam carrying data as a carrier wave. Instead of sending the data in a way that creates a messy, broad signal, the researchers shaped the light so that the information sits neatly on one specific side of the carrier frequency. They did this for two separate data streams, placing one on the "upper" side of a light frequency and the other on the "lower" side of a different light frequency. These two shaped signals were then combined into a single, unified beam of light.

This unified beam travels down a standard fiber-optic cable for up to 20 kilometers. At the end of the line, the signal reaches a receiving station equipped with two different types of access points for two separate users. The brilliance of the design lies in how the signal is split and processed at this destination. Depending on what the user needs, the system can filter the light to extract the data in one of two ways. If the user wants a wired connection, the system isolates a specific pair of light tones and converts them directly into an electrical signal that travels over a cable. If the user prefers wireless access, the system selects a different pair of light tones. When these two specific tones hit a detector, they interact to create a high-frequency radio wave, specifically a 60 gigahertz millimeter-wave signal, which can be broadcast wirelessly to a device. This process, known as heterodyne beating, effectively turns the light signal into a radio signal right at the point of delivery.

The researchers tested this concept using a sophisticated computer simulation to see if it would hold up under real-world conditions. They modeled the transmission of a 10 gigabit per second data stream, a speed sufficient for demanding modern applications, using a modulation format known as 4-QAM. This format encodes data by changing the phase of the signal, allowing for efficient use of the available bandwidth. The simulation showed that even after the light traveled 20 kilometers through the fiber, the system could successfully separate the signals for both users. Whether the data was delivered as a wired connection or converted into a 60 gigahertz wireless signal, the quality remained high. The researchers examined the clarity of the data by looking at visual representations of the signal, known as constellation diagrams and eye diagrams. These images showed that the data points remained distinct and the signal paths stayed open, even after the long journey through the fiber, indicating that the system could handle the transmission without needing extra equipment to correct for signal degradation.

The findings suggest that this approach offers a flexible and efficient path forward for next-generation networks. By keeping the signal unified while it travels through the fiber and only deciding whether to send it as wire or wireless at the very end, the system simplifies the network architecture. It eliminates the need for complex, separate infrastructures for wired and wireless services. The simulation results indicate that this method can support two users simultaneously, giving each the freedom to choose their preferred access mode without compromising speed or reliability. While this work remains a simulation and has not yet been built as a physical prototype, the theoretical performance demonstrates that such a hybrid network is feasible. It offers a promising solution for a future where the line between wired and wireless connectivity blurs, providing high-speed, adaptable access to the growing demands of a data-driven society.

Technical Summary: Dual User Wired or 60 GHz Wireless Selective Access Link with 10Gbit/s 4-QAM Signal

Problem Statement
The rapid expansion of "big data" and multimedia services has created a demand for access networks that combine the high bandwidth of fiber optics with the flexibility of wireless access. While Passive Optical Networks (PON) offer high capacity and cost-effectiveness, they lack access flexibility. Conversely, wireless networks provide flexible access but are often limited by lower transmission rates. Existing hybrid solutions, such as those integrating Radio over Fiber (RoF) and PON, often suffer from challenges including low network integration, opaque modulation formats, low spectrum utilization, fixed access modes, and complex network structures. There is a need for a fusion method that allows for high-speed, flexible, and selective access for multiple users without these limitations.

Methodology
The authors propose and simulate a hybrid optical access link architecture capable of providing either wired or 60 GHz millimeter-wave (MMW) wireless access to two distinct Hybrid Optical Network Units (HONUs). The system operates as follows:

Optical Line Terminal (OLT): Two optical carriers ( $C_1$ at $f_1$ and $C_2$ at $f_2$ ) are generated with a frequency spacing of $5f_{IF}$ (where $f_{IF}$ is the intermediate frequency). Two downlink 4-QAM data streams are modulated onto these carriers using nested Mach-Zehnder Modulators (MZMs).
- Data 1 is modulated onto $C_1$ using Upper Single Sideband (SSB) modulation, generating a signal at $f_1 + f_{IF}$ .
- Data 2 is modulated onto $C_2$ using Lower SSB modulation, generating a signal at $f_2 - f_{IF}$ .
- The resulting unified optical signal contains four optical tones: two unmodulated carriers ( $f_1, f_2$ ) and two data-carrying sidebands ( $f_1+f_{IF}, f_2-f_{IF}$ ).
Transmission: The unified signal is transmitted over 20 km of Single-Mode Fiber (SMF). The system is designed to operate without chromatic dispersion compensation.
Hybrid Optical Network Unit (HONU): At the receiver, the unified signal is decomposed based on user requirements using Fiber Bragg Gratings (FBGs) and Optical Band-Pass Filters (OBPFs).
- Wired Access: The system filters the data-bearing tone and its corresponding unmodulated carrier (e.g., $f_1+f_{IF}$ and $f_1$ ). These are detected by a photodiode (PD) to generate a 10 GHz Intermediate Frequency (IF) electrical signal via heterodyne beating, which is then demodulated to recover the baseband data.
- Wireless Access: The system filters the data-bearing tone from one carrier and the unmodulated carrier from the other source (e.g., $f_1+f_{IF}$ and $f_2$ ). Heterodyne beating between these tones generates a 60 GHz MMW signal ( $6f_{IF}$ ), which is radiated via an antenna for wireless user terminals.

Key Contributions

Selective Dual-User Access: The scheme enables two separate user terminals to independently choose between wired (10 GHz IF) or wireless (60 GHz MMW) access modes using a single unified optical downlink signal.
SSB Modulation Strategy: By utilizing upper and lower SSB modulation on two distinct carriers, the system efficiently packs two data streams and two reference oscillators into a unified signal, simplifying the OLT architecture.
Heterodyne Detection: The system employs heterodyne beating at the HONU to convert optical signals directly to either IF (for wired) or MMW (for wireless) electrical signals, eliminating the need for complex local oscillators at the user end.
Dispersion Tolerance: The architecture is demonstrated to maintain performance over 20 km of SMF without requiring dispersion compensation modules.

Results
The study utilized a simulation platform to validate the proposed link. Key findings include:

Signal Integrity: Constellation diagrams and eye diagrams for 10 Gbit/s 4-QAM signals were analyzed for both wired and wireless modes.
Transmission Performance: After 20 km of fiber transmission, the constellation points remained distinct without overlapping, and eye diagrams remained open, indicating robust transmission performance.
Noise Impact: While phase noise from fiber chromatic dispersion and polarization mode dispersion caused some expansion of constellation points and narrowing of eye diagrams as distance increased, the system maintained sufficient signal quality for successful demodulation at both 10 km and 20 km.
Dual-User Verification: Both HONU1 and HONU2 successfully recovered their respective data streams in both wired and wireless configurations.

Significance
The paper claims that the proposed hybrid optical access link offers a feasible solution for next-generation access networks by integrating the high capacity of fiber with the flexibility of wireless access. The significance lies in its ability to provide selective access (wired or 60 GHz wireless) for multiple users on demand while maintaining a unified optical format at the OLT. This approach simplifies the network structure, improves spectrum utilization, and demonstrates that high-speed (10 Gbit/s) 4-QAM signals can be effectively transmitted over 20 km of standard fiber without dispersion compensation, supporting the convergence of wired and wireless networks for future broadband services.

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