English
Introduction
With the explosive growth of cloud computing, 5G communications, and AI technologies, global data traffic is expanding exponentially, driving an urgent need for optical transmission networks to upgrade from 100G to 400G and beyond. Traditional NRZ modulation, constrained by limited bandwidth efficiency and significant transmission rate bottlenecks, struggles to meet the demands of next-generation networks. In this context, PAM4 (4-Level Pulse Amplitude Modulation) technology—with its unique encoding mechanism and bandwidth advantages—has emerged as the core enabling technology for upgrading 100G Ethernet and realizing 400G optical transmission.
1. PAM4 Technology
1.1 What is PAM4?
PAM4 (4-Level Pulse Amplitude Modulation) is an advanced modulation technique that encodes data using four distinct signal amplitude levels, allowing 2 bits of data per symbol—doubling the efficiency of traditional NRZ (1 bit per symbol).
Key Features:
- Gray Code Mapping: Reduces bit error rates during signal transitions.
- Multi-Level Signal Waveform: Requires sophisticated signal processing for accurate demodulation.
- Standardized for High-Speed Networks: Adopted in IEEE 802.3 for 400GE, 200GE, and beyond, making it critical for data centers and 5G transport networks.
1.2 Advantages of PAM4
- Doubled Bandwidth Efficiency: Achieves 2x the bit rate at the same baud rate, reducing channel bandwidth requirements by 50% and minimizing signal loss.
- Cost-Effective Deployment: Leverages existing fiber infrastructure and optical components, significantly reducing hardware investment in network construction and upgrades. PAM4 demonstrates superior signal-to-noise ratio (SNR) performance over long distances. Combined with optimized equalization techniques, it achieves low bit error rates (BER) for reliable high-speed transmission.
- Future-Proof Scalability: Enables smooth transitions from 100G → 400G → 800G/1.6T without overhauling network architecture.
2.Comparisons of PAM4 in 100G & 400G Ethernet
2.1 PAM4 in 100G Ethernet
PAM4 technology is implemented in 100G Ethernet under the IEEE 802.3cd standard, serving as an upgrade to traditional NRZ modulation.
Key applications: 2.1.1 Data Center Short-Distance Interconnects (100G BASE-KP4)
- Uses 13.6 GBaud PAM4 to achieve 50 Gb/s per lane, with dual-lane aggregation for 100G.
- Reduces channel count by 50% vs. NRZ (4×25G), lowering optical module and link costs.
2.1.2 Medium/Short-Distance Fiber Transmission
- Single-wavelength PAM4 modulation enables stable 100Gb/s transmission over 2 kilometers of single-mode fiber, ideal for enterprise data centers & 5G network.
- Exhibits excellent compatibility in 100G Ethernet applications. By upgrading the internal electrical chips within optical modules, it enables a smooth transition of existing NRZ networks without requiring transmission link reconstruction, thereby reducing network upgrade complexity and investment costs.
2.2 PAM4 in 400G Ethernet
PAM4 technology is the core enabler for the commercialization of 400G Ethernet, demonstrating critical importance across three dimensions: speed breakthroughs, cost control, and standardized compatibility.
2.2.1 Rate Breakthrough
- NRZ would require >100 GBaud for 400G, exceeding optoelectronic limits.
- PAM4 reduces baud rate to 53.1 GBaud (4×53.1G) or 26.6 GBaud (8×26.6G), mitigating signal loss & noise.
2.2.2 Cost-Effective Deployment
Leverages existing single-mode fiber, cutting hardware costs by over 30% compared to NRZ solutions. Simultaneously, PAM4 has become the standardized modulation technology for 400G Ethernet. The IEEE 802.3bs standard explicitly designates it as the core encoding scheme for the 400GE physical layer, covering all scenarios from short-distance interconnects within data centers to long-distance transmission. This has accelerated the maturation and cost reduction of the supply chain for optical modules, switches, and other components, providing a cost-effective 400G solution for applications such as 5G transport networks and supercomputing center interconnects.
2.3 NRZ vs. PAM4: Core Differences
The core distinctions between NRZ (Non-Return-to-Zero) and PAM4 (4-Level Pulse Amplitude Modulation) technologies stem from their encoding mechanisms, which cascade into differences in bandwidth efficiency, signal integrity, and application domains.
| Features | NRZ Technology | PAM4 Technology |
|---|---|---|
| Encoding | 2 levels (0/1), 1 bit/symbol | 4 levels (-3,-1,+1,+3), 2 bits/symbol |
| Bandwidth Efficiency | 1x (Baseline) | 2x (Double NRZ at same baud rate) |
| Voltage Swing | Full-scale (High noise margin) | 1/3 of NRZ (Denser constellation) |
| Raw BER | 1e-12 (Excellent) | 1e-4–1e-6 (Requires correction) |
| Signal Processing | Basic equalization sufficient | Needs FFE/DFE + Advanced FEC |
| Power Consumption | Lower (Simple circuitry) | Higher (DSP-intensive) |
| Cost Factor | Low (Mature technology) | Moderate (Complex components) |
| Primary Applications | 25G/50E Ethernet, PCB traces | 100G+/400G+ optics, Data center fabrics |
| Future Roadmap | Legacy maintenance | Coherent PAM4 for 1.6T+ systems |
3.Core Technologies of PAM4
3.1 High-Performance Optoelectronic Devices and Drivers
PAM4 technology relies on breakthroughs in optoelectronic devices and driver circuits, with core requirements focusing on three key dimensions: bandwidth matching, linearity control, and low-noise characteristics—critical for mitigating the signal-to-noise ratio (SNR) degradation inherent in multi-level modulation.
3.1.1 Optical Transmitters
- Short-distance transmission primarily employs 20GHz-bandwidth VCSEL lasers, leveraging their narrow linewidth to minimize chromatic dispersion.
- Medium/long-distance transmission requires external modulators to ensure linear modulation at 53.1GBaud+ baud rates, preventing bit error rate increases caused by level distortion.
3.1.2 Optical Receivers
- High-sensitivity detection and linear amplification are achieved using PIN or APD photodetectors with high responsivity for weak-signal recovery.
3.1.3 Driver Circuits
- Analog Approach: Combines two NRZ signals to generate four-level waveforms via precision resistor networks—cost-effective but relying on precision resistor networks for linearity.
- Digital Approach: Utilizes high-speed DACs to directly output 0/1/2/3 levels, offering superior timing accuracy for ultra-high-speed scenarios exceeding 112Gbps.
- Both methods address impedance matching and power noise suppression, employing differential signaling to reduce crosstalk and ensure sharp, consistent level transitions.
3.2 Advanced DSP Technology
In PAM4 systems, DSP acts as a stabilizer, compensating for inherent physical limitations to build a robust signal bridge. While PAM4 doubles efficiency by transmitting 2 bits per symbol, it narrows vertical eye openings, exacerbating vulnerability to noise, inter-symbol interference (ISI), and channel loss.
3.2.1 Key DSP Functions
- Pre-Equalization (Tx): Pre-compensates for known channel impairments.
- Adaptive Post-Processing (Rx): Combats high-frequency attenuation, reflections, and crosstalk via ADC-digitized signal reconstruction, reopening collapsed eye diagrams.
- Symbol Decision: Precisely decodes distorted signals into correct 4-level symbols.
3.2.2 Core Value of DSP
DSP shifts from passive compensation to active optimization, unlocking PAM4's full potential:
- Forward Error Correction (FEC): Elevates raw BER from ~1E-4 to commercial-grade 1E-12.
- Advanced Clock Recovery algorithms and digital Phase-locked Loop (PLL): Extracts low-jitter clocks from degraded data streams, ensuring system synchronization.
- Power-Performance Tradeoffs: Next-gen DSP cores leverage advanced semiconductor processes to deliver trillion-operations-per-second throughput at minimal power, enabling deployable, nanosecond-latency systems.
DSP transcends auxiliary roles, defining PAM4's bandwidth-distance product, energy efficiency, and commercial viability as its intelligent core.
4.Frequently Asked Questions (FAQ)
Q1: Why is PAM4 modulation the mandatory technology for 400G Ethernet?A: 400G transmission requires four times the bandwidth of 100G. Using NRZ modulation would necessitate increasing the baud rate to over 100GBd, far exceeding the performance limits of existing optoelectronic components. PAM4, however, transmits 2 bits per symbol, reducing the required baud rate to 53.1 GBaud. This mitigates high-frequency channel loss while maintaining compatibility with existing fiber resources, making it the core enabler for 400G commercialization.
Q2: How does PAM4 ensure signal reliability?
A: This is primarily addressed through a dual approach of hardware optimization and DSP algorithm compensation: - Hardware: Selecting high-linearity optoelectronic components, optimizing PCB impedance matching, and implementing differential transmission design. - Algorithm: Leveraging DSP-integrated FFE/DFE equalizers and MLSE sequence estimators to cancel intersymbol interference. Combined with KP4RS-FEC technology, this reduces the raw bit error rate from 1e-4 to below 1e-12, ensuring transmission reliability.KP4 RS-FEC: Corrects errors, improving BER from 1e-4 → <1e-12 (meeting telecom standards).
This hybrid approach ensures reliable, high-speed PAM4 transmission.
