PAM4

What Is PAM4?

PAM4, or Pulse Amplitude Modulation with 4 levels, is a widely used modulation technology that employs four distinct signal levels for advanced high-speed signal connectivity. Unlike traditional modulation schemes like NRZ (Non-Return-to-Zero), which use high and low signal levels to represent 1 and 0 of a digital logic signal, PAM4 utilizes four different signal levels — 00, 01, 10, and 11 — to transmit signals. This allows PAM4 to encode two bits of information per symbol, effectively doubling the data transmission rate compared to NRZ at the same baud rate. PAM4 is increasingly important in high-speed communication systems, particularly in applications like data centers and 5G networks, where maximizing data throughput is crucial.

Demystifying the Need for PAM4

According to TeleGeography, the global bandwidth market continues to expand rapidly, placing greater demands on the signal transmission capabilities of current network technologies and infrastructure. Traditional NRZ signals face challenges in transmission efficiency under these conditions. PAM4 addresses this issue by offering improved efficiency, meeting the growing bandwidth needs while remaining cost-effective, making it a highly favorable solution.

• High cost-effectiveness: To enhance the transmission rate of an optical module, there are two approaches: increasing the number of signal transmission channels or boosting the rate of each channel. However, expanding channels incurs significant construction costs, making the latter option more favorable. PAM4, with its multiple signal levels, enhances transmission efficiency per channel per UI. By maintaining the current channel quantity and optical components, PAM4 doubles the network interface rate through internal electrical chip upgrades within the optical module. Thankfully, existing network control chips (e.g., X86 chips) and high-end device interfaces (e.g., SerDes interfaces) can process PAM4 signals without issue.

• Mature technology: In 2014, the IEEE 802.3bj standard for 100G backplanes introduced two signal transmission modes: NRZ signals at 4 x 25.78 Gbit/s baud rate and PAM4 signals at 4 x 13.6 Gbit/s baud rate. While NRZ quickly entered commercial use with advancements in chip, PCB, and connector technologies, PAM4 remained sidelined in 100G Ethernet due to its technological immaturity and high costs. However, as the industry moved towards the 200G/400G interface standard, the need to increase the data rate of each pair of differential lines to 50 Gbit/s or higher became evident. Continuing with NRZ technology would impose stricter time margin requirements for transceiver chips and transmission links, given each symbol period is less than 20 ps. Consequently, in this scenario, PAM4 emerged as the superior choice.

Discovering the Positive Aspects of PAM4

As information and network technologies progress rapidly, the demand for efficient information transmission increases. PAM4 emerges as the preferred choice for next-generation high-speed signal interconnection due to its high transmission efficiency and low construction costs.

• High transmission efficiency: PAM4 offers superior transmission efficiency compared to NRZ due to its additional signal levels. With twice the bit rate within the same symbol period, PAM4 achieves higher transmission efficiency. Specifically, PAM4 operates at half the baud rate of NRZ for the same bit rate, resulting in significantly reduced signal loss in transmission channels. In summary, PAM4 excels in improving signal transmission efficiency and minimizing signal loss.

• Low construction costs: With its higher bit rate, PAM4 enables enhanced transmission efficiency on 5G transport networks without requiring additional optical fiber devices. This allows for the use of fewer mature optical components, leading to significant reductions in construction and R&D costs.

Delving into the Challenges Presented by PAM4

While PAM4 signals can convey more logic information than NRZ signals within the same symbol period, their eye diagrams reveal smaller eye openings. Consequently, PAM4 signals are more susceptible to interference from external factors, particularly noise. This vulnerability compromises their performance in terms of transmission distance and heat dissipation compared to NRZ signals.

• Short transmission distance: PAM4 signals are prone to interference from external environments, leading to a higher bit error rate (BER) over long transmission distances. To maintain stability and accuracy in signal transmission beyond 5 km, amplifiers and forward error correction (FEC), among other measures, are necessary.

• Greater heat dissipation required: PAM4 signals, unlike NRZ signals, demand additional auxiliary devices to ensure stable and accurate signal transmission over long distances. This results in higher power consumption, leading to increased heat generation in transceivers at both ends of each link. Consequently, the use of radiators may be necessary when employing PAM4.

Applications of PAM4: Where It Finds Utility

As data communication technologies advance, PAM4 is prominently featured in various components such as 50G, single-wavelength 100G, and 400G (non-ZR) optical modules. These modules are integral to numerous routers and switches. Below, we outline how PAM4 is utilized across different network scenarios.

5G Mobile Transport Network

The emergence of eMBB, URLLC, and mMTC services has significantly increased bandwidth demands on 5G transport networks. Compared to 4G, 5G boasts three to five times higher spectral efficiency, with a spectrum width starting from 100 MHz, five times broader than early 4G stages. Sub-6GHz bandwidth has increased by 15 to 25 times compared to 4G. In high-frequency spectrum, 5G can exceed 800 MHz, further augmenting capacity. According to the Next Generation Mobile Networks (NGMN) Alliance's bandwidth assessment, 5G transport networks will evolve to 50GE/200GE during sub-6GHz deployment and up to 100GE/200GE/400GE in high-frequency stages, as illustrated in the following figure. PAM4 technology will underpin these high-bandwidth networks due to its efficient signal transmission capabilities.

Metro Fixed Network

A metro network serves to connect hosts, databases, and LANs across a city, functioning as a high-bandwidth, multi-service multimedia communication network that integrates data, voice, and video services. The interface bandwidth of this network directly impacts the performance of network services. PAM4 technology can enhance the capacities of these interfaces, including those at the core and aggregation layers, thereby improving information transmission efficiency across the entire network. This ensures an optimal network experience for LANs and mobile terminals on the metro network while providing the necessary bandwidth to support the growth of communication services within the metro system.

Currently, mainstream interfaces in metro fixed networks include 10GE and 40GE interfaces. However, with the rapid evolution of HD, 4K, 8K, and VR/AR technologies, these network interfaces are expected to progress to 50GE, 200GE, or even 400GE, as depicted below.

DCI or DCN

The operational efficiency of Data Center Interconnect (DCI) or Data Center Networking (DCN) systems relies heavily on the performance of switches and routers. High-performance optical modules play a crucial role in enhancing the data conversion and transmission capabilities of these devices. PAM4-encoding chips within optical modules can convert NRZ signals to PAM4 signals, thereby increasing the volume of information processed by switches and routers while minimizing signal space occupation. This enhancement better supports functions related to the OSI reference model's first three layers (physical, data link, and network layers), thereby empowering DCI scenarios such as cross-region operations, user access, and remote disaster recovery.

For instance, the adoption of 50G PAM4 technology exemplifies the swift evolution of data centers, leading to the migration of server interfaces, DCN interfaces, and DCI interfaces from 10GE/40GE to 50GE/200GE/400GE, as shown in below.