Enaa Mari
The deployment of 4G has brought a new market segment to the optical transceiver industry, and the demand for 5G will give new impetus to the 5G transceiver market and further increase the space of this market segment. 5G network has the characteristics of high site density and high time synchronization accuracy, which puts forward new requirements for the function and performance of 5G optical transceivers. This article will make an in-depth analysis of the relationship between 5G and optical transceiver.
What type of transceiver can be applied for 5G network?
With the increasing demand for download speeds, wireless communications are increasingly dependent on fiber optic communications. The inherent advantages of fiber optic communications in terms of high capacity and long range are well suited to meet the requirements of 5G for carrier networks. In the pre-5G era, the driver for the rapid development of fiber optic communication technology and industry is wired access to homes and enterprises. The popularization and upgrading of “fiber to the home” and “fiber to the building” have led to comprehensive coverage and performance upgrades of access networks, metropolitan networks, and backbone networks. Each base station in 4G network has a BBU(Baseband Unit) and is directly connected to the core network through the BBU. In 5G network, however, the access network is no longer composed of BBU, RRU(Remote Radio Head), and antenna. Instead, it is reconfigured into the following 3 parts: CU(Centralized Unit), DU(Distribute Unit), and AAU(Active Antenna Unit). 4G transmission has only two parts: fronthaul and backhaul, which evolve into three parts in 5G network: AAU connects DU part called 5G Fronthaul, while 5G Middlehaul means DU connects CU part, and 5G Backhaul is the communication bearer between CU and core network.
Next, several representative examples will be given to illustrate the requirements of 5G for optical modules.
- 25G BiDi SFP28 optical transceiver: a solution for 5G Fronthaul
In the 4G era, the demand for optical modules in the fronthaul network is dominated by single-mode 10G Duplex. In the 5G era, the fronthaul network will still be dominated by direct driven optical fibers, but it puts forward new requirements for the speed and port density of optical module. Considering saving optical fiber resources and ensuring high-precision time synchronization by equidistant uplink and downlink, BiDi module is more advantageous than Duplex module. On the other hand, 5G is at least 10 times faster than 4G in terms of download rate, and 25Gbit/s is necessary under the Ethernet Common Public Radio Interface (eCPRI) protocol. Based on the above two considerations, the 25G BiDi SFP28 optical module can better meet the needs of 5G forward transmission: the transmission distance of 10km can cover most application scenarios, and the 1270/1330nm transmission wavelength will be conducive to the realization of low-cost employment.
- 50G PAM4 optical transceiver: a solution for 5G Middlehaul
5G Middlehaul network will have requirements for 50Gbit/s optical modules. Considering the low-cost implementation, based on 25G optical devices, supplemented by pulse amplitude modulation (PAM4) format will become a more attractive scheme. The 50GE PAM4 optical transceiver uses the QSFP28 encapsulation mode, LC optical interfaces, and single-mode optical fibers. The transmission distance is 10km or 40km, and the maximum power consumption is 4.5W. On the one hand, the 10km transmission can be realized by directly modulating the laser at 25G baud rate, and the 40km transmission requires the use of a 25G baud rate electrically modulated laser and an avalanche photodetector. On the other hand, both 10km and 40km specifications require high-linearity laser drivers and trans-impedance amplifiers. The QSFP28 PAM4 50G transceiver is a cost-saving solution to realize 5G Middlehaul network transmission.
- 100G coherent optical module: a low-cost solution for 5G Backhaul
The convergence layer and core layer of 5G Backhaul network have requirements for 100G, 200G and 400G coherent optical modules. The wavelength of coherent optical module is in the C-band, and the transmission distance is generally within 200km. Among them, the cost of 100G coherent optical module is lower, such as 100G CFP DCO module, which can achieve higher transmission bandwidth by wavelength division multiplexing technology. Perhaps you have never heard of 100G CFP DCO module. Compared with the more well-known 100G QSFP28 optical module, CFP coherent module is only used for coherent optical communication, which is a complex technology in development.
How to save the cost of 5G transceiver?
In the past few years, the market price of 4G optical modules has been decreasing by 40% almost every 2 years, which has contributed to the large-scale deployment of 4G. It can be argued that 4G optical modules are well suited to meet downstream customers’ requirements for low cost. One of the reasons for this is that the industry chain has appropriately defined the specifications of different types of optical modules to achieve the required functions with low-cost technology while meeting the performance specifications, e.g., 10G Vertical Cavity Surface Emitting Laser (VCSEL) technology for optical modules with transmission distance up to 150m, 10G Fabry-Perot (FP) laser technology for optical modules with transmission distances up to 1.4 km. For transmission distances of up to 10km, 10G Distributed Feedback Laser (DFB) technology is used. In short: for scenarios with high performance requirements, choose the more expensive technology; for scenarios with low performance requirements, choose the less expensive technology.
In the 5G era, the demand for optical modules will exceed that of 4G era, and low cost will still be the main demand for optical modules in the industry chain. According to the different application scenarios, several different specifications of optical transceivers are defined for different transmission distances, and the corresponding technical means are selected according to the specifications, which may continue to be an effective strategy to achieve low cost for 5G transceivers. Sharing the industrial chain will also help to achieve the goal of low cost 5G network. The demand for high bandwidth of data center optical interconnection is ahead of fixed network access and wireless access. Therefore, 25 Gbit/s optical devices have long been widely used in data communication optical modules. It is also one of the strategies to develop 5G transceivers based on optical devices and related technologies that have been widely used in data communication optical module. For example, the laser used in the 25G BiDi optical module is similar to the laser used in the 100G CWDM4 (Coarse Wavelength Division Multiplexer) optical module used in the data center, except for its different temperature range.
The most fundamental means to achieve low cost is technological innovation, including innovation in network architecture, optical devices of physical layer, network protocols and other aspects. For example, in the 4G era, the CPRI protocol was used for signal transmission between remote RF modules (RRUs) and base-band processing units (BBUs). In response to the higher bandwidth requirements of 5G, the 3rd Generation Partnership Project (3GPP) has proposed a new protocol standard—eCPRI, which compresses the bandwidth of the forward interface to 25G, thus reducing the cost of optical modules. This is an innovation at the network protocol level. Another example: Considering the immediate need for industrial grade temperature in wireless application scenarios, using it to replace commercial grade temperature range lasers will reduce the packaging cost of optical modules, which is an innovation at the physical layer of optical devices.
The key technology of 5G transceiver
The core technology of optical transceiver can be divided into packaging(form factor) technology and optical/electrical component technology. Technological innovation is also basically reflected in these two aspects. Most of the packaging technologies required for 5G transceivers can learn from the existing mature technologies. For example, the optical path structure of 25G BiDi module is similar to that of 10G BiDi module. For another example, the 200G QSFP-DD and 400G QSFP-DD optical modules are both based on 50G PAM4 modulation mode, supplemented by wavelength division multiplexing technology.
The key technologies required for 5G transceiver will be more in the area of optical/electronic devices. In other words, technology innovation in optoelectronics can be seen at three levels: expansion of functionality, increase in speed, and decrease in cost. For instance, laser chips with industrial temperature range will no longer need temperature control devices; laser chips for non-gas-tight environment will no longer need expensive gas-tight package shells; laser chips with small divergence angle will no longer need more expensive aspheric lenses; laser chips with anti-reflection will no longer need isolators. These types of innovations belong to the expansion of the laser chip function, which will simplify the optical module package form, not only to improve the reliability also at the same time reduce the cost.
To sum up, the key technology of 5G transceiver is more reflected in the innovation of optoelectronic chips which include:
- high speed laser chip technology in the industrial temperature range
- high linearity 25G baud rate DFB chip and EML chip technology
- low cost 25G wavelength-tunable laser chip technology
- low cost coherent 100G/200G/400G optical module technology
- high linearity 25G/50G electrical chip technology.
Summary: breakthrough technology and seize the opportunity
Compared with the 4G era, 5G network will occupy a more important position in the whole optical module market, and it will become the next windfall for the development of optical module industry. 5G wireless communication with high bandwidth, low latency, large connection characteristics put forward higher requirements for the function and performance of optical transceivers, which will promote the progress of optical module and optoelectronic chip technology. Technological innovation and localization substitution will be the main way to realize low-cost 5G transceivers. Whoever has mastered the core technology of optical chips can lead the future of 5G optical communication.
