Optical Transceiver Modules for 5G Fronthaul

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Launched in 2019, 5G quickly gained momentum in Asia, North America, and Europe. Global System for Mobile Communications (GSMA) predicts a continual growth of 5G connections in the next five years. Global operators will invest about US$1.1 trillion in mobile communications between 2020 and 2025, of which approximately 80% will be 5G CAPEX.

5G Wireless Fronthaul Interface

5G wireless communication demands more spectrum resources than 4G, for enhanced mobile broadband (eMBB), ultra-reliable and low-latency communication (URLLC), and massive machine type communication (mMTC). Currently, 5G uses the sub 6GHz FR1 spectrum that supports a maximum bandwidth of 100Mb/s, five times that of 4G LTE. When there are 64 channels and the bandwidth is 100MHz, the common public radio interface (CPRI) protocol requires at least 100Gb/s for fronthaul channels. However, in 2017, the industry was not ready for 100Gb/s optical transceiver modules. Therefore, the enhanced CPRI (eCPRI) protocol was developed.

The eCPRI protocol defines multiple split modes. An interface of a higher protocol layer requires a lower transmission bandwidth. In mainstream split scenarios, some physical-layer signal processing functions are transferred from the baseband to the antenna side, requiring only 25Gb/s from the fronthaul interface. In recent years, the requirements for mainstream fronthaul optical transceiver modules have evolved from 10Gb/s in the 4G era to 25Gb/s in the 5G era.

Considering that the low and medium frequency bands in the wireless spectrum are already crowded, 3GPP allocates a higher-frequency band for 5G. However, this results in a higher signal loss. Therefore, to ensure good communication quality, the construction of 5G base stations requires higher density than 4G, as well as higher optical transceiver module requirements. LightCounting predicts that of all optical transceiver modules sold in the next five years, over 50% will be 25G optics for 5G fronthaul.

25G optical transceiver modules are mainly used by wireless fronthaul. As such, reusing the existing resources in the 25G Ethernet industry can help telecom operators significantly reduce costs and enhance the efficiency of optical solutions.

Typical 5G Wireless Fronthaul Scenarios

The typical architecture for wireless fronthaul is either distributed RAN (DRAN) or centralized RAN (CRAN). In CRAN mode, BBUs are located in a central office. This significantly reduces space and power consumption of auxiliary equipment, especially air conditioners, thereby lowering CAPEX and OPEX. In addition, centralized BBUs form a BBU baseband pool, which can be centrally managed and scheduled for different network requirements.

Due to added base stations, the cost of 5G network construction is much higher than 4G, and site acquisition is challenging. Therefore, CRAN is preferred for large-scale deployment.

DRAN fronthaul is a simple scenario, where AAUs and DUs are deployed on and under the tower respectively, at a distance of 300m or less. In a CRAN scenario, the maximum distance between two units is 10km. Considering cost-effectiveness and easy maintenance, direct fiber connection is used for both DRAN and CRAN. In this case, 25G grey optical transceiver modules are required.

Direct fiber connections in CRAN scenarios require many optical fibers and cables. Where fiber resources are insufficient, 10km grey bi-directional (BiDi) transceiver modules are used, as they require half the number of fibers, and therefore reduce costs. If necessary, required fiber resources can be further reduced by using passive WDM and (semi) active WDM devices. In this case, 25G colored optical transceiver modules are required.

For a single 5G macro base station, a 100MHz spectrum requires three 25Gb/s eCPRIs. In China, China Mobile has 160MHz of 5G spectrum, while China Telecom and China Unicom share a total of 200MHz of 5G. If the interface rate remains at 25Gb/s, the number of interfaces increases from 3 to 6.

To meet interface transmission requirements, each macro base station requires six pairs of 25G optical transceiver modules. In this case, you can use a single set of 12-wavelength colored optical transceiver modules (one fiber per site) or two sets of 6-wavelength colored optical transceiver modules (two fibers per site).

To sum up, both DRAN and CRAN scenarios will skyrocket the demand for 5G fronthaul optical transceiver modules.

Different Types of 5G Fronthaul Optical Transceiver Modules

5G was released in 2019 and quickly adopted for commercial use. By the end of November 2020, millions of 5G base stations were already deployed allover the world. To cope with the rapid and widespread construction of base stations, operators opt for colored optical transceiver modules, saving costs and commercializing quickly. In addition, based on existing WDM standards, different organizations proposed coarse WDM (CWDM), micro-optic WDM (MWDM), LAN WDM (LWDM), and dense WDM (DWDM) standards.

25G SFP28 SR/LR/BiDi Grey Optical Transceiver Modules

25G SFP28 grey optical transceiver modules make use of the existing resources from the 10Gb/s SFP+ technologies:

  • 300m SR transceiver modules use the 850nm wavelength vertical-cavity surface-emitting laser (VCSEL).
  • 10km LR transceiver modules use the 1310nm wavelength distributed feedback (DFB) laser.
  • 10km BiDi transceiver modules use DFB lasers (1330nm upstream, 1270nm downstream).

The commercial chips that feature these wavelengths are readily available. Some chip suppliers can also provide industrial chips suitable for wireless fronthaul applications.

25G SFP28 CWDM/MWDM/LWDM/DWDM Colored Optical Transceiver Modules

Based on the principle of reusing WDM standards, the industry is discussing a variety of solutions for 25G colored optical transceiver modules. The CWDM standard is defined in ITU-T G.694.2. There are 18 wavelengths with 20nm spacing. CWDM transceiver modules are directly installed on DUs and AAUs, and external CWDM multiplexers/demultiplexers are used. In the wireless fronthaul scenario with three channels, six wavelengths are required, preferably CWDM6 of 1271nm, 1291nm, 1311nm, 1331nm, 1351nm, and 1371nm. As the first four wavelengths are the same as those of CWDM4 DML in data centers, chip suppliers only need to develop for industrial temperatures and the last two wavelengths. In the case of six channels, 12 wavelengths are required. Two CWDM6 and two fibers can be selected for transmission, or CWDM12 and one fiber can be used by adding the last six wavelengths of 1471nm, 1491nm, 1511nm, 1531nm, 1551nm, and 1571nm.

MWDM is a China Communications Standards Association (CCSA) standard proposed at the end of 2019. In MWDM, each of the CWDM6 standard wavelength is expanded through the thermoelectric cooler (TEC) to obtain 12 wavelengths with unequal spacing of 7nm. The MWDM12 wavelengths include 1267.5nm, 1274.5nm, 1287.5nm, 1294.5nm, 1307.5nm, 1314.5nm, 1327.5nm, 1334.5nm, 1347.5nm, 1354.5nm, 1367.5nm, and 1374.5nm.

Compared with CWDM6, the MWDM12 solution needs to add the TEC to the optical component and the TEC driver to the transceiver module circuit.

The channel spacing of the LWDM technology is 800GHz (approximately 4.4nm). More wavelengths can be obtained in the O-band with only a small dispersion penalty. IEEE 802.3 defines the 400GBASE-LR8 interface based on LWDM8 of 1273.54nm, 1277.89nm, 1282.26nm, 1286.66nm, 1295.56nm, 1300.05nm, 1304.58nm, and 1309.14nm. The last four wavelengths are used for 100GBASE-LR4. Therefore, the industry can readily support the last four wavelengths. If expanding to 12 wavelengths, the CCSA adds four wavelengths (1269.23nm, 1291.10nm, 1313.73nm, and 1318.35nm) to the LWDM8 wavelengths to form LWDM12. The only difference between LWDM12 and MWDM12 is the optical chip.

The DWDM technology is based on ITU-T G.698.4 and widely used in backbone and metro networks. The wavelength ranges from 1529nm to 1567nm, with a spacing of approximately 0.78nm. The number of wavelengths can be 6, 12, 20, 40, 48, or 96. However, DWDM transceiver modules are expensive and usually deployed in areas with insufficient fiber resources.

Due to narrow wavelength spacing, MWDM requires TEC controllers and more likely customized wavelength chips. The industry chain of direct modulated laser (DML) optical chips at the bottom layer of LWDM is immature, the electro-absorption modulated laser (EML) cost is high, and TEC controllers are required for LWDM. DWDM chips are costly, and DWDM requires TEC controllers. Only CWDM6 does not require TEC controllers and has abundant DML resources. Therefore, CWDM6 is recognized as the most cost-effective solution for carriers.

The transmission distance of a standard wireless fronthaul optical transceiver module is limited to 10km. With wider adoption of CRAN deployment, longer transmission distance may be required on the aggregation fronthaul networks. According to LightCounting, in the next 5 years, 3% of all grey optical transceiver modules will require a transmission distance greater than 10km. However, industry vendors continue to focus on 10km optical transceiver modules.

100G DSFP Optical Transceiver Modules

As 5G develops, the fronthaul communication capacity will need to gradually increase. However, for a wireless base station, the panel ports of the baseband board are fixed. Wireless equipment vendors need to find ways to improve the receiving and transmitting capacities of the ports.

The dual small form-factor pluggable (DSFP) optical transceiver module is a good solution. The DSFP standard released in 2018 supports a maximum rate of 100Gb/s and is mainly used for Ethernet protocols. It is also suitable for wireless eCPRI fronthaul scenarios. The DSFP transceiver module is compatible with the SFP transceiver module structure. With the integrated encapsulation inside the DSFP transceiver module, two signal channels can be transmitted, doubling the transmission and receiving capacity. Currently, 25G SFP28 transceiver modules are the standard. However, with the increasing demand for fronthaul bandwidth and evolution of baseband chips on the BBU side, more DSFP transceiver modules may be required.

25G SFP28 Tunable Colored Optical Transceiver Modules

CRAN is playing a bigger part in the deployment of 5G infrastructure. The three main carriers in China expect CRANs to make up 80% of 5G infrastructure by 2020, so there will be more demand for colored optical transceiver modules. At first, CWDM6 transceiver modules are widely deployed, as they are cheap and readily available. However, during base station construction and maintenance, the wavelength configuration requires a lot of time and effort. Therefore, the tunable DWDM colored optical technology is proposed.

The tunable DWDM system has the wavelength range and spacing as the fixed DWDM system. The only difference is that a wavelength-tunable DWDM transceiver module supports automatic configuration of 12 or 48 wavelengths. Currently, a tunable DWDM standard is initiated in CCSA, and the ITU-T G.698.x standard is being revised. Earlier, the DWDM tunable technology was applied to the transport network, but it was much more expensive than the CWDM6. Therefore, the industry has been working on reducing the cost of this solution.

GIGALIGHT's 5G Fronthaul 25G SFP28 Optical Transceiver Modules Solutions

GIGALIGHT offers both 25G SFP28 grey optical transceiver modules and 25G SFP28 colored optical transceiver modules, which are all industrial grade. All the 25G optical transceiver modules comply with the SFP28 protocols SFF-8419 and SFF8472. Electrical ports comply with CEI-28G-VSR. The transceiver modules comply with 5G fronthaul CPRI/eCPRI specifications and IEEE 802.3 Ethernet standards, to support both 24.33Gb/s and 25.78Gb/s data rates. Customers can choose different options based on their performance needs and budget. The full series of 5G fronthaul 25G SFP28 optical transceiver modules cover various DRAN and CRAN application scenarios.

GIGALIGHT 25G SFP28 Grey Optical Transceiver Modules for 5G Fronthaul

The 25G SFP28 grey optical transceiver modules include dual-fiber portfolio and single-fiber portfolio, which can meet customers' requirements for direct fiber connection in a wide range of scenarios.

  • The dual-fiber portfolio can offer 850nm SR solution for 100m transmission, 850nm CSR solution for 300m transmission, 1310nm LR Lite solution for 300m transmission, 1310nm LR solution for 10km transmission, 1310nm ER Lite solution for 30km/40km transmission, and 1310nm ER solution for 40km transmission.
  • The single-fiber portfolio can offer 1270/1330nm BiDi solution for 300m, 10km, 20km, and 40km transmission.

GIGALIGHT 25G SFP28 Colored Optical Transceiver Modules for 5G Fronthaul

The 25G SFP28 colored optical transceiver modules include CWDM portfolio, LWDM portfolio, and DWDM portfolio. These can meet customers' requirements for WDM connections in a wide range of scenarios.

  • The CWDM portfolio can offer CWDM12 solution for 10km transmission and CWDM6 solution for 15km transmission.
  • The LWDM portfolio can offer LWDM12 solution for 10km/20km transmission and LWDM9 solution for 40km transmission.
  • The DWDM portfolio can offer 6CH, 12CH, 20CH, and 40CH DWDM solutions for 10km transmission.

Why GIGALIGHT

To provide high-performance and low-cost optical transceiver modules for 5G fronthaul networking, GIGALIGHT uses the following key technologies:

High-integrated oRFIC Chipset

The DRV Combo chip and TIA ensure cost savings without compromising performance. The DRV Combo overcomes technical difficulties such as transceiving crosstalk and heat dissipation by design. It also integrates functional transceiver modules such as the receiving CDR, transmitting CDR, DML DRV, LA, and APC into a single chip.

Single-chip integration reduces power consumption and the PCB layout area, and improves reliability. Compared with the GeSi process used by other vendors, GIGALIGHT adopts the CMOS process to implement the high-current drive of the DFB and achieve mass production. Furthermore, the CMOS process is cheaper and faster.

High-performance Optical Chip

The transmit end uses the DFB, specifically for 25G SFP28 LR Lite 300m and 25G SFP28 LR 10km, 25G BiDi SFP28 300m/10km/20km/40km, and 25G CWDM SFP28 10km/15km transceiver modules.

DFB lasers boast good performance and do not require cooling, reducing costs.

High-sensitivity PIN PD

A PIN-PD is a photodiode with a P-I-N semiconductor structure, which is formed by adding an intrinsic (i) region between the N- and P-doped regions. The sensitivity of a PIN PD is critical to the performance of an optical transceiver module. The high-performance PIN PD features low noise and high sensitivity. In addition, it supports 10km transmission.

Conclusion

The eCPRI standard clarifies the 5G fronthaul interfaces. 25G fronthaul interfaces comply with Ethernet protocols and provide abundant O&M methods. In addition, the existing resources of the 25G Ethernet optical transceiver modules can be reused. 25G fronthaul interfaces have become industry standard. As CAPEX for 5G base station construction increases, operators are looking for more cost-effective 25G fronthaul optical transceiver modules. At the same time, the limited fiber resources drive the demand for colored optical transceiver modules. With decades of investment and innovation in the optical communication field, GIGALIGHT has launched a complete solution of 25G SFP28 grey and colored optical transceiver modules to build diversified access pipes for 5G wireless communication.