PAM4 Modulation & PAM4 vs NRZ, How Much Do You Know?
The deployment of cutting-edge technologies such as cloud computing, big data, Internet of Things (IoT), and artificial intelligence drives the growing data traffic. Then the connected world is moving forward at higher speeds 100G, 200G, 400G, and 800G Ethernet. PAM4 — the new signal modulation technique is to address the high-bandwidth next-generation Ethernet.
PAM4 has been used in 100G PAM4 QSFP28 and 200G, and 400G transceiver interfaces to bring high-speed and long-haul data center interconnects. This post will introduce the PAM4 knowledge basis by comparing PAM4 vs NRZ (PAM2) to understand how PAM4 works, the difference between PAM4 and NRZ, and their advantages and disadvantages.
What is PAM4?
PAM4 refers to Pulse Amplitude Modulation 4-level, which is the most widely used digital modulation method. In PAM4 modulation, the amplitude of the carrier signal can take four different levels, which can encode two bits of data per symbol. PAM4 signaling is widely used in short-range and high-speed optical interconnect applications.
What is NRZ?
NRZ refers to Non-Return to Zero, also known as the Pulse Amplitude Modulation 2-level (PAM2), which is a traditional digital signal coding method. In NRZ, the amplitude of the carrier signal remains constant during each bit time, and there is no transition in the middle of the bit time.
PAM4 vs NRZ Encoding
PAM4 encoding uses four different voltage levels (symbol), each carries 2 bits of information: 00, 01, 10, or 11. NRZ encoding has two signal levels to represent logical 0 or 1, with each symbol only representing 1 bit of logical information.
Image Source: AN 835: PAM4 Signaling Fundamentals
Since PAM4 coding represents two bits per symbol, the baud rate (the number of symbols transmitted per second) is half of the bit rate (the number of bits transmitted per second). For instance, for the same baud rate, 28 GBaud PAM4 = 56 Gbs whereas 28 GBaud NRZ = 28 Gbs.
Therefore, PAM4 has twice the throughput of NRZ for the same baud rate.
PAM4 vs NRZ Spectrum
Compared to NRZ, PAM4 has half the Nyquist frequency of NRZ at 56 Gbps.
This feature brings many advantages to PAM4, including:
• Double the data bandwidth.
• Lower noise power in bandwidth by distributing the total noise power into wider frequencies.
• Higer Resolution over same oversampling rate.
PAM4 vs NRZ Eye Diagram
As a distinction in Power Spectrum Density, PAM4 has three eye patterns while NRZ has one big eye. PAM4 eye diagram leads to PAM4 limitations on Signal-to-Noise Ratio (SNR) and Bit Error Rate (BER). Given that the signal amplitude (eye height) of PAM4 is only 1/3 of NZR, the space between different voltage levels is narrow, which makes PAM4 more sensitive to noise. Finally, PAM4 has higher SNR and BER for remaining signal quality. Besides, the PAM4 signal will degrade over long-distance transmission for low noise tolerance. Therefore PAM4 is better for short-range data center interconnects.
To resist noise interference, advanced equalization is required to be added in PAM4 transceivers. Consequently PAM4 100G QSFP28 construction is more complex and requires more power consumption than other 100G QSFP28 optics.
PAM4 vs NRZ Advantages and Disadvantages
The main advantage of PAM4 over NRZ is that it has a high bandwidth efficiency. This is because, in PAM4, each symbol encodes two bits of information, while in NRZ, each bit only encodes one bit of information. Thus PAM4 signaling achieves twice the data density of NRZ signaling. Besides, it saves system costs by reducing the cost per bit and the number of required transceivers, cables, and connectors.
However, with poor noise tolerance, the disadvantage of PAM4 is that it has high power consumption and a limited transmission distance.
Advantages of NRZ over PAM4 include better error correction performance and more resistance to noise.
The table below lists the pros and cons of PAM4 vs NRZ.
PAM4 and NRZ are high-speed Ethernet enabling technologies for transmitting bytes of data through fiber, Coax, or PCB. Compared to NRZ, PAM4 signaling double the data capacity per lane and reduce system cost. PAM4 modulation is most suitable for high-speed and short-range data center interconnects. The PAM8 with 3 bits, 8 voltage levels, and 7 eye patterns per UI is introduced. It can be expected that higher bandwidth efficiency will become possible soon if PAM8 is applied in 100G, 200G, 400G, or even higher speed transceiver interfaces.
QSFPTEK manufactures and sells 100G PAM4 DWDM transceivers with 4Tbps total Capacity, 40-channel 100G Links, and maximum 80km reach. This QSFP28 PAM4 is a cost-effective solution for building an embedded DWDM network. As WDM capability is integrated directly into system equipment.
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