Super-Channel

What Is Super-Channel?

Before diving into super-channel, let's first understand an optical channel. An optical channel is a transmission path in an optical communication system that carries light signals. Each channel typically corresponds to a specific wavelength or frequency band, used to transmit digital information. Multiple channels can be transmitted simultaneously through the same optical fiber, distinguished by different wavelengths.

The development of optical communication technology has always focused on five key dimensions. The high demand for network bandwidth has made high-speed, long-distance, and large-capacity transmissions crucial goals for optical fiber communication.

5 Dimensions of Optical Communication Development

In early systems with data rates of 2.5 Gb/s, 10 Gb/s, and 40 Gb/s, issues like dispersion and other types of fiber optic channel impairments were the primary concerns. However, as we transitioned from 40 Gb/s to 100 Gb/s, enhancing spectral efficiency also became a significant focus. With the arrival of 1Tb/s nodes, improving transmission efficiency has become the central concern in optical fiber communication.

One of the critical technologies enabling 1Tb/s high-speed transmission is the super-channel. A super-channel combines multiple optical channels into a single, coherent channel through orthogonal multiplexing. This packaging forms a fully independent high-capacity channel. The generation, multiplexing, transmission, and reception of these subchannels are performed in a coordinated manner, facilitating efficient data transfer.

Since the spacing between subchannels is less than or equal to their Nyquist bandwidth (the maximum rate at which signals can be transmitted without introducing symbol interference), the signal transmission between each optical channel does not interfere with each other. This ensures that as many signals as possible can be transmitted effectively. As a result, the transmission efficiency of light signals within a super-channel is guaranteed.

How Do Super-Channel Achieve Efficient Transmission?

Super-channel technology is essentially to obtain more effective transmission channels by reducing the protection interval between signal bands. Achieving this requires advanced techniques to minimize spacing and effectively prevent interference.

In the super-channel, two main technologies are employed to meet these criteria: Super-Nyquist WDM and OFDM.

1. Super-Nyquist WDM: Super-Nyquist Wavelength Division Multiplexing (WDM) improves system spectral efficiency by sacrificing the resources of digital signal processing (DSP) to counteract the introduced inter-symbol interference. This enhances the overall spectral efficiency of the system. However, DSP demands high-performance hardware, and the system's performance is limited by the hardware conditions.

2. OFDM Technology: Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique widely used in both wireless and wired communication systems. The basic idea of OFDM is to break a high-speed data stream into multiple lower-speed subchannels, each orthogonal to the others, transmitting data on different frequencies. The orthogonality of these subchannels ensures that the system is not affected by inter-channel crosstalk.

By leveraging these two technologies, the super-channel can minimize the spacing between channels while ensuring they do not interfere with one another. This allows for the flexible transmission of optical signals efficiently.

Differences Between Super-Channel and WDM

super-channel technology essentially aggregates multiple optical channels, which seems somewhat similar to the principle of Wavelength Division Multiplexing (WDM). However, they differ in terms of independence and integration:

1. Channel Independence: In super-channel technology, collaborative modulation and demodulation occur across multiple wavelengths to achieve higher bandwidth and more optimized transmission performance. In contrast, WDM transmits multiple signals independently on different wavelengths, treating each wavelength as an independent channel.

2. Integration: super-channel offers a higher degree of integration compared to WDM. They reduce the spacing and redundancy between wavelengths, achieving greater spectral efficiency.

3. Application Scenarios: Super-channel technology is often utilized in scenarios requiring high-capacity transmission, such as data center interconnections and high-capacity backbone networks. WDM technology, on the other hand, is widely used across various fiber optic communication systems.

In summary, super-channel technology can be seen as an advanced application of WDM. By closely grouping multiple adjacent optical carriers together, the super-channel achieves higher spectral efficiency than traditional WDM.

Additional Ultra-High-Speed Transmission Technologies

In the era of pursuing high-capacity transmission, beyond spectral super-channel technology, there are two more technological paths to achieving ultra-high-speed, ultra-large-capacity, and ultra-long-distance transmission.

• High-Order Modulation Technology

This technology aims to maximize signal transmission rates within a given bandwidth by increasing both the number of symbols transmitted per second and the amount of data each symbol represents. This means improving the symbol rate (the number of symbols transmitted per second) and the modulation order (the amount of data each symbol represents) to achieve the highest possible signal transmission rate per unit of optical bandwidth.

However, due to limitations in core components such as digital-to-analog converters (DACs), the increase in symbol rate is limited. Additionally, as the modulation order increases, the requirements for laser linewidth and stability become more stringent, and the demands on the sampling rate and precision of the receiving end's analog-to-digital converter (ADC) increase. The system also becomes more sensitive to various impairments. Achieving high spectral efficiency and resilience to impairments in high-order modulation is the key to high-speed, large-capacity coherent optical transmission.

• Space Division Multiplexing (SDM)

Spatial multiplexing divides the data to be transmitted into multiple data streams, which are then encoded and modulated through different antennas before being transmitted, thereby enhancing the transmission efficiency of the system.

Spatial multiplexing divides the data to be transmitted into multiple data streams, which are then encoded and modulated through different antennas before being transmitted, thereby enhancing the transmission efficiency of the system.

Looking ahead, global network traffic is expected to continue growing. For optical networks, this will entail continuously optimizing each node and exploring more flexible and efficient solutions.