Who will be conducting the 5G symphony?

If the 20th century was the century of electricity, then the 21st century is the century of light. Electricity solved the challenge of power transmission in the last century. Now, we are using light for the transmission of information. In the face of an explosive growth of data, market demand for data communication bandwidth has risen dramatically. In order to meet the market’s requirements for transmission efficiency, power consumption and cost efficiency, optical interconnection technology using lightwaves as the transmission medium has become the current and future mainstream technology for realizing medium- and long-distance data communication.

From NRZ to PAM4

No discussion of important technologies in the field of optical communication would be complete without mentioning NRZ and PAM4.

Non-Return-to-Zero (NRZ) code uses NRZ-coded signals and high/low signal levels to represent digital logic signals for transmitting information. NRZ has two baseband transmission modes for digital signals: unipolar non-return-to-zero code and bipolar non-return-to-zero code.

Following the development of NRZ, PAM4 has become a popular signal transmission technology. PAM4 refers to 4-level pulse amplitude modulation (4 Pulse Amplitude Modulation), which is a modulation technology that uses four different signal levels for signal transmission. It has been widely adopted in the field of high-speed signal interconnection.

What are the advantages of PAM4 over NRZ?

Many industry insiders have noticed an energy loss of signals when using NRZ when the transmission rate reaches 28Gbps, and subsequently proposed the use of the PAM4 scheme for transmission. However, NRZ can still generally meet the demands of 28Gbps, and migrating from NRZ to PAM4 needs to be supported by compatible chipsets and test systems, all of which incurs extra cost. Hence, the adoption rate of PAM4 to date has been relatively slow to date.

When the transmission rate was increased to 56Gbps, NRZ’s loss-of-signal problem reached a point where it could no longer be ignored. This prompted the use of higher-order modulation technology to overcome NRZ’s deficiencies, thus garnering the industry’s support for PAM4.

Comparing these two technologies, NRZ signals use high/low signal levels to represent the 1 and 0 of digital logic signals, and each clock cycle can transmit 1 bit of logic information. On the other hand, PAM4 signals adopt 4 different signal levels for signal transmission, and each clock cycle can transmit 2 bits of logic information, namely 00, 01, 10 and 11. Therefore, under the same serial transmission rate, the bit rate of PAM4 signals is twice that of NRZ signals, effectively doubling its transmission efficiency.

If optical signals could also be transmitted using PAM4, when the electro-optical conversion is performed within optical modules, the clock recovery and pre-emphasis of PAM4 signals could be directly implemented. This would eliminate the need to convert PAM4 signals into 2 times the serial transmission rate of NRZ signals first before continuing to related processes, thus reducing the cost of chip design. Thanks to the favorable conditions facilitated by PAM4, the IEEE 802.3 Working Group has set PAM4 as the encoding technology at the physical layer for 400Gbps/200Gbps/50Gbps interfaces.

Why does 5G need PAM4?

One of the most important goals of 5G networks is to support large-bandwidth applications, such as online viewing of 4K or even 8K videos, and applications such as VR/AR that require high speed and low latency. For end users, while these changes may only translate into better viewing experiences or a few more apps, they represent a substantial upgrade in the overall network environment.

For example, the 5G spectral width should start from 100MHz, which is 5 times higher than that of 4G in its initial stage, and will be 15 to 25 times higher than that of 4G when it reaches the Sub6G frequency stage. In terms of the network’s load-bearing capacity, it is likely that there will be an improvement of 2 to 3 orders of magnitude.

However, signal transmission is naturally followed by the problem of energy loss. Nowadays, the parameters of 5G networks have to be improved considerably all round, but if loss rates remain unchanged, energy loss will also increase proportionally. Therefore, the premise of improvement is to “stop loss” first. Think of the process of filling a pool with water. If there is a leakage problem, it will be an exercise in futility.

As mentioned, since PAM4 signals can transmit 2 bits of information per symbol period, to achieve the same signal transmission capability, the symbol rate of PAM4 signals only needs to reach half that of NRZ signals.

Taking the single-channel 50GPAM4 or 50GE optical module as an example, in terms of signal emission, the PAM4 encoding chip would first convert the 2×25G NRZ signal into a 1×25GBaud PAM4 signal, and then the laser driver chip would amplify the PAM4 signal and drive the 25G laser, converting the electrical signal into a single wavelength 25GBaud (50Gbps) optical signal. Secondly, in terms of signal reception, a detector would convert the above optical signal into an electrical signal, which would be shaped and amplified to become output at the receiving end of the PAM4 decoding chip. Finally, the PAM4 decoding chip will convert the signal into a 2×25GNRZ signal.

In doing so, the loss caused by the transmission channel can be greatly reduced, effectively improving the efficiency of bandwidth utilization.

In the construction of 5G networks, cost is also a major challenge for operators to overcome. In particular, due to the characteristics of the 5G signal frequency, the number of base stations needs to be at least twice that of 4G in order to achieve ideal network coverage and speed. This number is expected to reach 4 to 5 times that of 4G after the 5G network is fully rolled out. Furthermore, the power consumption of each base station is higher, and the network requires more optical fibers. However, operators cannot increase users’ fees according to this ratio. In fact, they might even have to increase the speed and reduce fees. Hence, cost savings can only be achieved through the utilization of more cost-efficient equipment.

The reduced costs of optical modules brought about by the improvement of the PAM4 broadband utilization rate is a key factor in realizing low cost and wide coverage for 5G. As the basic unit of the network’s physical layer, optical modules account for an increasing proportion of the costs of mobile-bearing network equipment, with some equipment even exceeding 50-70% of total costs. According to the research published by the Yole analysis agency, from 2019 to 2025, the demand for optical modules from the data communication market will achieve a compound annual growth rate of approximately 20%. Meanwhile, the telecommunications market will achieve a compound annual growth rate of approximately 5%.

Following the continuous development of 5G technology and the further increase of users’ network requirements, higher levels such as PAM6 or PAM8 may be used for signal transmission in the future. The 5G “symphony” needs not only optical interconnection technology as its “conductor,” but also the perfect coordination of all “musicians,” the core networks and the backhaul and fronthaul wireless connections, to perform this spectacular production.