5G Fronthaul Solution

The 5G fronthaul solution provided by HYD TECHNOLOGY ,which is serving the users with features such as high reliability, high speed transmission, high security, low latency, and long battery life.

  • 1U Passive Optical Fiber Expansion Equipment

The 5G fronthaul bearer solution is mainly divided into the optical fiber direct drive solution and the WDM equipment bearer solution. Among them, the WDM equipment bearer solution is divided into three types: passive WDM solution, OTN/WDM solution and WDM-PON solution, which essentially use WDM technology to carry multiple eCPRI links with different wavelengths. , and then multiplexed into one fiber to achieve the purpose of saving fiber resources

  • 3U Passive Optical Fiber Expansion Equipment

In the WDM scenario, it is more appropriate to use the semi-active B-type solution to lay out optical modules, and a single station will use 12 optical modules.
One of the single station optical module solutions is a passive solution, which uses 12 colored light modules; the semi-active A-type solution uses 12 colored light modules and 12 gray light modules; the semi-active B-type solution uses 12 A colored light module.
Among them, the passive solution cannot effectively monitor the status of optical modules and optical fibers, the cost of the semi-active A-type solution is relatively high, and the semi-active B-type solution becomes a more suitable method.

  • 1U & 3U Passive Fiber Expansion Equipment 

Passive WDM have been widely used in scenarios that require fiber savings, such as 5G fronthaul solution and other scenarios where distribution cables and backbone cables are insufficient.

Passive WDM schemes are divided into two types: coarse WDM and dense WDM according to the wavelength interval. Since the cost of implementing a single optical path in a dense WDM system is several times that of a coarse WDM system, and the wavelength allocation complexity of the dense WDM solution is greatly increased when it is used for fronthaul, it is recommended that the fronthaul adopt the coarse WDM solution.

  • 1U Compact Wavelength Division Equipment

As the wireless network deployment is gaining depth, the large capacity distributed base stations have also been constructed, and to cater to this, base stations are also expanding. However, these stations require locators to be close to users.

The front haul scheme uses the connection between RRU and BBU, which is a direct fiber connection and also comprises serious problems like consumption of serious fiber and expansion difficulties. 

HYD Technology has launched this 1U compact WDM module which resolves many problems like the lack of fiber transmission resources between C-RAN and BBU-RRU architecture. It also helps meet the coverage and expansion of operator stations.

  •  2U Semi-active Equipment

HYD Technology‘s  2U Semi-active Equipment is a crucial component of the advanced fronthaul WDM system, designed to enhance signal transmission efficiency by combining active and passive elements, ensuring seamless and cost-effective deployment of cutting-edge networks.

It goes well with the metropolitan area network as a convergence layer, allowing the long-distance network transmission applications to work efficiently. It has a 2U chassis and hence comprises 8 slots for the general service, with the single slot performance being highly feasible that it meets the needs of different services. 

5U Semi-active Equipment
  • 5U Semi-active Equipment

The 5G fronthaul solution is developing leaps and bounds, and hence there is a requirement to deploy the 5G base stations on a larger scale. The base station’s coverage needs the deployment locations to be close to users. 

In the case of a front haul solution, the direct driver fiber between AAU and DU leads to problems like lack of fiber resources, high cost of cable construction, and time and difficulty in the cable expansion.


  •  OLPM Card

The advantage of optical fiber direct drive solution in the 5G fronthaul solution is to increase the optical module rate from 4.9G/10G of 4G to 25G. At present, the industry generally adopts the method of increasing the single-wavelength baud rate to 25G Baud to increase the rate of the fronthaul optical module.

The technical difficulty of the optical fiber direct drive solution is lower than that of the WDM equipment carrying solution.


  • Passive Fiber Expansion Card 

In 5G Fronthaul, the baseband unit (BBU) is divided into a central unit (CU) and a distributed unit (DU). The 5G fronthaul interconnects the active antenna unit (AAU) with the DU, and the midhaul interconnects the CU with the DU. Backhaul Refers to the CU.

Optical modules with a rate of 800Gbps have become a necessity for the new trend of 5G fronthaul, midhaul and backhaul networks in the future

  • 5G Front-haul Semi-active Module 

5G fronthaul has the industry characteristics of short distance and cost sensitivity

The amount of optical modules and optical fibers in different base station networking structures will increase in 5G fronthaul

· At present, gray optical modules are the main application for 5G fronthaul applications, but the proportion of color optical modules and wavelength division multiplexing technology in the CRAN architecture is gradually increasing

5G fronthaul solution

What is 5G fronthaul, midhaul and backhaul?


In 5G and other mobile communications technologies, the terms “Fronthaul” and “Backhaul” are used to describe different parts of the network connection. These two concepts are crucial to understanding the 5G network structure and how it works.


1. Prequel (Fronthaul):

    – Fronthaul refers to the connection from the Centralized Unit (CU) or Distributed Unit (DU) to the Remote Radio Head (RRH) or Remote Radio Unit (RRU).

    – In 5G, due to the adoption of the cloud RAN (cRAN) structure, the baseband processing part and the radio frequency part of the base station are physically separated. Fronthaul connections typically require very high bandwidth because they transmit raw, unencoded RF signals.

    – Fronthaul connectivity can be achieved via fiber optic, microwave, or millimeter wave.


2. Backhaul:

    – Backhaul is the connection from the base station to the core network. This is the path from a mobile user terminal (such as a smartphone, tablet, etc.) through a wireless network to the Internet or other services.

    – Traditionally, backhaul can be achieved through various means, including wired such as fiber optic, DSL, or wireless such as microwave links.

    – In order to meet the high bandwidth and low latency requirements of 5G, 5G backhaul usually requires higher transmission capabilities.


In general, both fronthaul and backhaul are critical parts of data transmission in mobile communication networks. They ensure that user data can flow from the terminal device through the wireless network to the core network and ultimately reach the target service or the Internet.


What are the differences between 5G fronthaul, midhaul and backhaul?

The fronthaul (Fronthaul), midhaul (midhaul) and backhaul (Backhaul) of 5G represent different network segments, which connect different functional parts in the mobile communication network. Here is a brief explanation of these three concepts and the differences between them:


1. Prequel (Fronthaul):

    – Connection method: Connect the Remote Radio Unit (RRU) or Remote Radio Head (RRH) to the Distributed Unit (DU) or Centralized Unit (CU).

    – Features: Due to the transmission of unencoded raw IQ data or baseband signals, the required bandwidth is very large. It usually requires very low latency.

    -Transmission medium: Optical fiber is usually used for connection to meet the requirements of high bandwidth and low latency.


2. Midhaul:

    – Connection method: connect distributed unit (DU) and central processing unit (CU).

    – Features: This is a relatively new concept in 5G networks, especially as base station functions are further decomposed. The data has already been partially processed, so the bandwidth requirements for midhaul may be lower than for fronthaul.

    – Transmission medium: Optical fiber or microwave can be used.


3. Backhaul:

    – Connection method: Connect the central processing unit (CU) or base station to the core network.

    – Features: It is mainly responsible for transmitting user data and control information. As 5G develops, backhaul connections need to support higher bandwidth and lower latency.

    – Transmission medium: It can be optical fiber, DSL, satellite, microwave, etc.



– Fronthaul mainly involves the connection from the physical layer of the base station to the radio frequency part.

– Midhaul involves connections within the base station, especially in more complex and distributed 5G base station designs.

– Backhaul involves the connection from the base station to the core network.


The main differences between the three are where they sit in the network, the devices they connect to, and their bandwidth and latency requirements.


What is the fronthaul interface in 5G?

In the 5G network, the fronthaul interface is used to connect the Centralized Unit (CU) or Distributed Unit (DU) to the Remote Radio Head (RRH) or Remote Radio Unit. RRU). This physical separation allows the baseband processing and RF processing of the base station to be performed at different locations, resulting in flexibility and economies of scale.


The key interfaces used in 5G fronthaul are eCPRI (Enhanced Public Radio Interface) and Fronthaul Gateway Interface in OpenRAN. Here is a brief description of eCPRI:


  eCPRI (Enhanced Public Radio Interface):

– eCPRI is an enhanced version of CPRI (Public Radio Interface), designed to meet the needs of 5G fronthaul. Compared with traditional CPRI, eCPRI optimizes bandwidth utilization, supports higher data rates, and is more flexible.

– eCPRI defines user plane and control plane data transmitted between the physical layer and higher layers.

– eCPRI allows for more flexible data packaging, meaning different information can be transmitted as needed, from raw IQ samples to fully processed RF signals.

– eCPRI supports larger bandwidth in the fronthaul, which is to meet the high data rate requirements of 5G.


With the development of 5G and Open Radio Access Network (OpenRAN), fronthaul interfaces and related technologies are still constantly evolving and improving to meet the challenges of future communication needs.

5G fronthaul solution

What are the requirements for 5G fronthaul solution?

5G fronthaul solution has strict requirements for network performance, reliability and efficiency, especially since 5G promises to provide higher data rates, lower latency and more device connections than previous mobile communications technologies. The following are the main requirements for 5G fronthaul solution:


1. High bandwidth:

Due to the high data rate requirements of 5G, fronthaul links must provide sufficient bandwidth to support large amounts of data transmission, especially when using eCPRI or other efficient fronthaul protocols.


2. Low latency:

 To support real-time applications and services such as augmented reality, virtual reality, and autonomous vehicles, fronthaul must have extremely low latency.


3. Flexibility:

 Fronthaul solutions for 5G networks must be flexible to support different deployment scenarios, frequencies and bandwidths.


4. High reliability and availability:

To ensure continuous service quality and meet the needs of critical applications, fronthaul links must be highly reliable.


5. Support multiple transmission media:

The fronthaul solution should be able to support multiple transmission media, such as optical fiber, microwave and millimeter wave.


6. Synchronization capabilities:

 Due to 5G’s TDD (Time Division Duplex) operation and massive MIMO technology, fronthaul solutions must have precise synchronization capabilities.


7. Network slicing support:

 5G supports network slicing, allowing customized network performance for different applications and services on the same physical network. The prequel must be able to support this functionality.


8. Energy efficiency:

 Given the scale and density of 5G networks, energy efficiency is a key consideration. Fronthaul solutions must provide the required performance while also considering power consumption.


9. Cost-effectiveness:

 Although 5G fronthaul solution has many requirements, it still needs to be economically feasible, especially when deployed on a large scale.


10. Open and standardized:

 To support multi-vendor environments and avoid vendor lock-in, fronthaul solutions should be based on open standards.


These requirements ensure that 5G networks can meet the needs of various applications and services, while also meeting the expectations of operators and end users.


What is the application of 5G fronthaul solution in data centers?

The application of 5G fronthaul solution in data centers is mainly related to edge computing, virtualization and centralized radio access network (C-RAN) architecture. Here is a brief overview of these use cases and how they relate to the data center:


1. Edge computing:

    – With the rollout of 5G, edge computing has become a strategy to achieve low latency and high reliability. Edge computing places computing resources at the edge of the network, close to end users or “things”, thereby reducing the distance and time of data transmission.

    – In this scenario, data centers are no longer just large facilities concentrated in one place, but become multiple small, geographically dispersed data centers distributed across the network.

    – 5G fronthaul  can link these edge data centers and remote wireless units (RRU), bringing data processing closer to users and devices.


2. Centralized Radio Access Network (C-RAN):

    – C-RAN is an architecture that separates the baseband processing unit (BBU) of the base station from the wireless unit (such as RRU) and concentrates processing in a central location (or data center).

    – The 5G fronthaul solution is responsible for connecting these centralized BBUs and decentralized RRUs.

    – By centralizing processing within the data center, resource sharing, dynamic allocation and economies of scale can be achieved.


3. Network functions virtualization (NFV):

    – 5G encourages the adoption of NFV, which means traditional physical network functions such as firewalls, load balancers, etc., are converted into virtualized applications running on standard server hardware.

    – These virtual network functions (VNFs) can be dynamically deployed and scaled anywhere within the data center.

    – 5G fronthaul can interact with VNFs within these data centers to provide the flexibility and scalability required for 5G networks.


4. Real-time data analysis and artificial intelligence (AI):

    – 5G networks are expected to generate large amounts of data. Data centers, especially those located at the edge of the network, enable real-time data analysis and processing.

    – 5G fronthaul solution provides these data centers with fast, low-latency connections from network equipment (such as RRUs) to the data center.


In general, the application of 5G fronthaul solution in data centers is related to the geographical location, architecture and services of the data center to meet the high bandwidth, low latency and high reliability requirements of the 5G network.


What does fhgw 5G mean?

“FHGW” usually stands for “Fronthaul Gateway”. In the context of 5G networks, Fronthaul Gateway is a key device that plays a bridging role in the 5G fronthaul solution , connecting a central processing unit (CU) or a distributed processing unit (DU) with multiple remote radio units (RRUs). or Remote Radio Heads (RRHs).


Here are some key points about FHGW in 5G:


1. Interface conversion:

 FHGW can provide conversion functions between different fronthaul interface protocols (such as CPRI, eCPRI, etc.), so that equipment from different manufacturers can work together in the same network.


2. Aggregation:

 FHGW allows multiple RRUs to connect to a DU or CU through a single fronthaul. In this way, there is no need to deploy a separate fronthaul link for each RRU, thereby reducing the cost of network construction and maintenance.


3. Clock synchronization:

 For TDD (Time Division Duplex) and other 5G applications that require precise clock synchronization, FHGW can provide the necessary clock signals for RRUs.


4. Security:

By encrypting fronthaul traffic, FHGW can improve the security of 5G networks.


5. Network optimization:

FHGW can perform traffic management and optimization to ensure efficient use of bandwidth resources in the fronthaul network.


In short, the FHGW in 5G serves as a key component in the fronthaul network. It ensures efficient, secure and coordinated communication between different network devices.


The Application of CWDM in 5G fronthaul solution

In the 5G fronthaul solution, in order to meet the needs of high data rates and large capacity, a technology called optical wavelength division multiplexing (WDM) is used. CWDM (Coarse Wavelength Division Multiplexing) is a variant of WDM that allows multiple optical signals of different wavelengths to be transmitted simultaneously in a single optical fiber, thereby greatly increasing the transmission capacity of the optical fiber.


The following are the applications of CWDM in in 5G fronthaul solutions :


1. Increased bandwidth capacity:

 By transmitting optical signals of multiple wavelengths simultaneously in a single optical fiber, CWDM significantly increases the total bandwidth of the fronthaul link.


2. Simplified deployment:

Using CWDM, multiple channels can be combined onto one fiber link, which means the number of physical fibers can be reduced, thereby simplifying deployment and reducing costs.


3. Flexibility:

As the network grows and needs change, channels can be easily added or removed from the CWDM system.


4. Compatibility:

CWDM systems are generally compatible with existing fiber optic networks, meaning that 5G fronthaul CWDM solutions can be deployed on existing fiber optic infrastructure without the need for costly upgrades or replacements.


5. Cost-Effectiveness:

CWDM is a more economical solution compared to DWDM (Dense Wavelength Division Multiplexing). Their widely spaced wavelengths generally mean that lower cost components and equipment are required.


In practical applications, CWDM solutions are usually used for short- or medium-distance fronthaul links, such as in urban or suburban areas. This is because CWDM’s widely spaced wavelengths and the fact that these wavelengths are relatively insensitive to losses and dispersion in the fiber make them particularly suitable for these applications.


In general, the CWDM in 5G fronthaul solution provides an efficient, economical and flexible method to meet the high bandwidth and large capacity requirements of 5G networks.


The Application of LWDM in 5G fronthaul solution

LWDM (Lan Wavelength Division Multiplexing) is a relatively new wavelength division multiplexing technology, between CWDM and DWDM. LWDM is designed to provide a balance between cost, performance and bandwidth for local network environments (LAN). In the 5G fronthaul solution, LWDM emerged to meet the demand for higher data rates and low latency, while also taking into account cost factors.


The following are the applications of LWDM in 5G fronthaul solution :


1. High bandwidth and low latency:

 LWDM allows multiple wavelengths to be transmitted in one fiber, thus providing higher overall bandwidth. Since data can be transmitted on every wavelength, this approach reduces latency and increases throughput.


2. Cost-effectiveness:

 Compared with DWDM, LWDM provides a more economical option, especially in fronthaul scenarios that require high bandwidth but do not require long-distance transmission.


3. Effective utilization of optical fiber resources:

By using LWDM, multiple data channels can be obtained on a single optical fiber, thereby making more effective use of existing optical fiber resources.


4. Flexible deployment:

 LWDM provides a flexible way to allow operators to increase or decrease wavelengths when needed to meet real-time data needs.


5. Compatibility:

LWDM systems are generally compatible with existing fiber optic networks, which allows them to be deployed on existing fiber optic infrastructure without the need for costly upgrades or replacements.


The application of LWDM in 5G fronthaul solution is based on its unique advantages compared with CWDM and DWDM. It provides an economical solution that meets the high bandwidth and low latency requirements of 5G fronthaul solution.


What are the conditions required for the 5G fronthaul solution to adopt a semi-active wavelength division solution?

The “semi-active wavelength division solution” of 5G fronthaul usually refers to a solution that combines active (such as amplifiers or adjustable devices) and passive (such as splitters or combiners) optical components in the fronthaul network. This combination is designed for performance optimization and cost-effectiveness.


The following are the conditions required to adopt a semi-active wavelength division solution:


1. High bandwidth requirements:

With the deployment of 5G, the bandwidth requirements for data transmission have increased significantly. Semi-active solutions allow multiple wavelengths to be multiplexed in the same fiber, providing high bandwidth.


2. Optical link distance:

Considering the presence of active components such as amplifiers, this solution is suitable for long-distance transmission beyond the capabilities of passive solutions.


3. Dynamic network requirements:

If the fronthaul network needs to be dynamically adjusted or optimized, such as through adjustable wavelength equipment, then active components are required to support these functions.


4. Power management:

 In links, especially in long links or multi-wavelength systems, optical amplifiers may be needed to compensate for losses or provide the required signal quality.


5. Network scalability:

 As the network grows and 5G is further deployed, the semi-active wavelength division solution can be easily expanded to meet the needs of more wavelengths or links.


6. Cost-Effectiveness:

 Although active components may increase the cost of the initial investment, they can provide higher performance, longer link distances, and greater bandwidth and may be cost-effective in the long run.


7. Equipment Compatibility:

Considering the mixed use of active and passive components, it is necessary to ensure that all equipment and components are compatible in operation and performance.


8. Maintenance and monitoring:

Due to the inclusion of active components, maintenance and remote monitoring capabilities need to be considered to ensure the normal operation of the network.


Using a semi-active WDM solution is a way to balance performance, cost and flexibility. When considering this option, operators need to evaluate the needs and limitations of their specific network to determine whether this is the best option.

5G fronthaul solution

What are the two possible transmission methods in 5g fronthaul solution?

In the 5G fronthaul solution , the two most common transmission methods are:


1. Fronthaul based on CPRI/eCPRI:

    – CPRI (Common Public Radio Interface) is a fronthaul protocol for 4G LTE networks. It defines the baseband unit (BBU) and the remote radio unit (RRU) or remote radio head (Remote Radio). Head, RRH) interface.

    – With the advancement of 5G, eCPRI (enhanced CPRI) is introduced as a more efficient fronthaul interface, which has lower latency and higher bandwidth efficiency.


2. Ethernet-based fronthaul:

    – This method uses standard Ethernet protocols to transmit fronthaul traffic, usually combined with Time-Sensitive Networking (TSN) or other QoS (Quality of Service) policies to ensure on-time transmission and priority processing of data.

    – The advantage of Ethernet fronthaul is its versatility and compatibility with a variety of network devices and topologies.


The two methods can be chosen based on specific network requirements, device compatibility, and economic considerations. However, with the development of 5G fronthaul solution, more and more operators and equipment suppliers are turning to more flexible and efficient Ethernet fronthaul solutions.


What components are needed for 5g fronthaul solutions?

5G fronthaul solutions typically involve multiple devices and components to ensure efficient, low-latency, and high-bandwidth communications. The following are some key components commonly used in 5G fronthaul solutions:


1. RRU/RRH (Remote Radio Unit/Remote Radio Head):

This is the radio frequency part of the base station, usually installed near the antenna, and is responsible for radio frequency communication with mobile devices.


2. BBU (Baseband Unit):

It processes baseband signals, performs modulation/demodulation, encoding/decoding and other tasks, and communicates with RRU/RRH through the fronthaul link.


3. Optical modules:

These modules are used to convert and transmit signals in optical fibers. Including SFP, SFP+, QSFP, QSFP28, etc.


4. WDM/CWDM/DWDM Mux/Demux:

These are multiplexers and demultiplexers, used to transmit multiple signals of different wavelengths simultaneously in the same optical fiber to increase transmission capacity.


5. Optical Amplifiers:

In long-distance fronthaul links, optical amplifiers may be required to compensate for fiber losses.


6. Optical switches and routers:

Used to switch and route signals in optical networks.


7. Fronthaul Gateway:

Used to aggregate multiple fronthaul links and may provide conversion between different fronthaul interface protocols.


8. Clock and synchronization equipment:

For TDD and some FDD scenarios, clock synchronization is necessary, such as a GPS receiver or other synchronization equipment.


9. Transmission equipment:

including Ethernet switches, routers and other equipment used for data transmission and routing.


10. Control and management system:

used for network configuration, monitoring and maintenance.


11. Security equipment:

such as firewall or encryption equipment, used to ensure the security of the fronthaul link.


The devices and components listed above are common parts of 5G fronthaul solutions. Specific configurations and requirements may vary based on different network designs, geographies, and service provider policies.

HYD Technology has developed its 1000 series products which are remote-based stations and tend to resolve problems that consist of limited fiber transmission in a remote manner, which could be between BBU-RRU/DU-AAU present within the C-RAN architecture. It also meets the base station expansion requirement and produces many solutions for accessing full scenarios and end-to-end carriers.

Application 1: 5G Semi-active Solution

The 5G semi-active device by HYD Technology is deployed on the side of DU and AAU. A 2-core fiber between 5G devices is required, providing one-on-one protection for the primary and backup lines. The AAU and DU’s eCPRI optical interference must be replaced with the HYD Technology’s colored transceiver module. 

Application 2: Passive WDM in 5G fronthaul Solution

Passive optical fiber expansion equipment is deployed for the passive solutions. It is deployed between the 4G BBU/5G DU pool and 4G RRU/5G AAU between the passive optical fiber equipment, and just one pair is needed. The SFP color transceiver module has to be replaced by the CRU port of the 4G BBU/5G DU and 4G RRU/ 5G AAU module from HYD Technology. 

Application 3: Active Solution

Active solutions are also needed if the active optical fiber expansion equipment has to be deployed between BBU/DU and RRU/AAU. Hence the actual devices present don’t need to be changed as it offers mixed transmissions for all the other service types.

If any enquiry ,please contact HYD TECHNOLOGY .

The Overview of 5G Fronthaul Solution

The fronthaul solution was also discussed a lot during the 4G construction period.

At that time, there were also solutions based on optical fiber direct connection (gray light solution) and wavelength division multiplexing technology.

At the same time, there were also fronthaul problems such as 3G/4G mixed transmission and 4G independent construction.
Similarly, in the 5G fronthaul solution, there are currently more discussions and there is also direct fiber connection (gray light solution), but there are many variants of the wavelength division multiplexing solution, from the original CWDM coarse wave solution to LWDM and MWDM.

It is dazzling, and also faces the problem of 4G transformation and 5G mixed transmission. 

(1)Fiber optic direct connection gray light scheme


First of all, it is emphasized that fiber optic direct connection does not mean not to use optical modules, but to use gray optical modules.

In areas with rich fiber resources, this solution is more convenient to open in the early stage, without adding any transmission equipment, and the BBU and the antenna RRU are connected through a 1310nm gray light module (for gray light and color light, you can see the previous write-up article “Understanding Light”).


If our dual-fiber bidirectional system is used at this time, it is calculated based on a total of 3 AAUs in 3 directions of a base station, one receiving and one sending, and a total of 3 optical modules and 6 optical fibers are required.

Consider a BBU pool with 6 base stations, as shown in the figure below.

For an access ring, the end-to-end connection between the AAU side and the CO equipment room requires 6 fibers, and at least 36 fibers are required.

5G fronthaul

Therefore, the consumption of optical fiber resources by the dual-fiber bidirectional solution is a terrible thing, especially for operators who rely on renting optical fiber resources, which will lead to a huge initial investment.

In addition, from the perspective of time synchronization requirements, compared with 4G, which requires 1588 time synchronization, for 5G, single-fiber bidirectional is the key requirement for its fronthaul key capability.

Therefore, we start from the transceiver optical module, and change the dual-fiber bidirectional to single-fiber bidirectional optical module BiDi, that is, the number of optical modules has not changed, and it is still 1 port of the BBU and 1 optical module for each AAU. , but the transmit and receive wavelengths are transmitted in the same core fiber.
The 5G 25G single-fiber bidirectional optical module adopts 1270nm/1330nm wavelength, which is the same wavelength as the 10G single-fiber bidirectional optical module.

It should be noted here that if the 1270nm/1330nm wavelength is used at the BBU end, then the 1330nm/1270nm wavelength optical module must be used at the AAU end. Using BiDi single-fiber bidirectional can save half of the optical cable resources.

5G fronthaul
The advantage of the above optical fiber direct connection solution is that no additional transmission equipment is required, and the initial investment is low.

The disadvantage is that the fiber resource consumption is still very large, and there is no effective link protection and operation and maintenance management capabilities. If there is a problem in the middle, you have to rely on the wireless side or manually to find it.


Since the optical fiber direct connection solution is too wasteful of optical fiber resources, let’s use the wavelength division multiplexing solution. There are two types of wavelength division multiplexing: CWDM (coarse wavelength division) and DWDM (dense wavelength division).

The cost of DWDM is relatively high, and at the same time, under the transmission requirements of 5-10km in front transmission, using DWDM feels a bit overkill. Let’s take a look at how the CWDM solution works.


(2)5G Fronthaul Using

Using CWDM passive solution


The 5G fronthaul CWDM solution is actually a color light passive solution. The feature of the solution is to configure a set of passive multiplexers and demultiplexers near the AAU base station and DU equipment room.

When dual-fiber bidirectional is used, each of the three AAU services passes through a pair of optical fibers, and is converged into a pair of optical fibers by the splitting and multiplexing at the base station side.

This greatly saves the optical fiber resources of wiring and backbone optical cables. As shown below.

As can be seen from the figure above, the AAU and the multiplexer/demultiplexer connected to the base station use dual-fiber bidirectional, that is, the same wavelength is used for sending and receiving, and one fiber must be used for sending and receiving, and of course only 3 wavelengths are required.

In the current 5G fronthaul solution construction, as emphasized above, single-fiber bidirectional is a key technical requirement for 5G fronthaul. At the same time, in order to save wiring and backbone cable resources, 25G single-fiber bidirectional BiDi optical module solutions are generally used.

The configured optical modules have different transmit and receive wavelengths, so each AAU can only need one optical fiber, and then converge into one optical fiber through splitting and multiplexing devices.

Compared with the above dual-fiber bidirectional solution, this saves half of the optical cable. Here, when a single fiber is bidirectional, 6 wavelengths are required, which is the currently discussed 6-wavelength CWDM solution.


How to choose the wavelength of the 6 waves here? In consideration of cost and performance, the six wavelengths of 1271, 1291, 1311, 1331, 1351, and 1371 are commonly used in the relatively mature 25G optical modules.

The positions of these 6 wavelengths in the spectrum, the first 5 waves belong to the O-band, and the sixth wave belongs to the E-band. Why are the 6 wavelengths of CWDM used?


The main reason is that the 20nm wavelength interval of CWDM brings great temperature adaptability. The currently commonly used laser is a temperature-sensitive device, and there are requirements for the operating temperature of the business board in project bidding.

Taking G.652D optical fiber as the transmission medium as an example, 25G optical modules are commonly used in the market, and its working temperature ranges from -40 to +85 degrees Celsius.

This span is a full 125 degrees, and the wavelength-to-temperature drift of the optical module DFB laser is about 0.1nm/°C. That is to say, 125 degrees will cause a wavelength shift of 12.5nm.

Therefore, when a 20nm CWDM wavelength is used, there is sufficient isolation. Reflected in the optical module, there is no need for a special TEC temperature controller, which also has advantages in cost.


From the perspective of dispersion control, the dispersion penalty of 1271-1311nm is about 1dB, and the dispersion penalty of 1331nm is about 3dB.

According to the estimation of laser luminous power, the PIN receiver (maximum receiving sensitivity of about -14dBm) can also meet the demand.

However, for 1351-1571nm, the dispersion cost reaches 4.5dB, and the PIN tube is unable to do what it wants.

Therefore, it is necessary to use an APD tube with better performance to compensate for the dispersion cost. Good performance means higher cost.

The above is based on the situation of completely new 5G, and 3 pairs of wavelengths can meet the demand.

But in fact, the current 4G network is still and will be the main force for a long time.

There is an unavoidable problem of co-station with 4G base stations. We also consider a base station with three 4G RRU antennas, plus three 5G AAUs, a total of six pairs of optical modules are needed.


For BiDi single-fiber bidirectional, another 6 waves are needed to realize the fronthaul. The current mainstream is to increase the last 6 waves of CWDM.

The result of this is that the EML laser with high cost is used at the sending end, and the APD tube is used at the receiving end, thereby further driving up the cost. The above is the 12-wave 5G fronthaul solution based on CWDM.

(2) Adopt LWDM 12-wave semi-active scheme

In order to speed up the deployment of 5G and save costs, we reuse the old ones as much as possible.

The LWDM 12-wavefront IPL solution is proposed under such circumstances. The full name of LWDM is LAN-WDM, which conforms to the IEEE 802.3BA standard.

The spectrum of the O-band is divided according to 800 GHz, and there are 8 standard wavelengths in total. In the 4*25G QSFP28 LR4 optical module, 4 LAN-WDM wavelengths are used. These 4 wavelengths are the first 4 waves of the LWDM standard.

First 4 waves:

1295.56nm, 1300.05nm, 1304.58nm, 1309.14nm.

The last 4 waves:

1273.54nm, 1277.89nm, 1282.26nm, 1282.66nm.

But there are only 8 waves above, where will the other 4 waves come from? These 4 waves are the 4 wavelengths of multiplexing CWDM:

1269.23nm, 1332.41.nm, 1313.73nm, 1291.10nm.

From the above figure, we can see that the wavelength interval of LWDM is narrower than that of CWDM (20nm), and all of them are located in the low dispersion area near 1310nm.

That is to say, at the receiving end, the optical module can use PIN tube can meet the transmission requirements.

At the same time, due to the narrowing of the wavelength interval, a relatively large wavelength shift is necessary.

In order to meet the environmental requirements of -40°C+85°C, TEC temperature control becomes indispensable.


Compared with the pure CWDM solution, there are not many advantages in cost.

CWDM does not use TEC due to the large wavelength interval, while LWDM uses TEC due to the small wavelength interval; the second two waves of CWDM have large dispersion and require APD reception, while LWDM only needs PIN reception because the center wavelength is in a lower dispersion band.


(3) Adopt MWDM 12-wave semi-active scheme


MWDM was proposed by China Mobile, and in order to meet the current urgent 5G deployment requirements, some CWDM wavelengths are reused.

Its solution mainly includes AAU colored optical module, passive wavelength division multiplexer on AAU side, and active WDM equipment on DU side.

It includes not only the active color light module and active WDM equipment, but also the passive wavelength division multiplexer on the AAU side. In other words, MWDM is also a semi-active WDM solution.

So how does MWDM realize 12 wavelengths?


On the basis of multiplexing the first 6 waves of 25G CWDM, by adding TEC temperature control, the wavelength is shifted left and right by 3.5nm to form 12 wavelengths.

Among them, the first 8 wavelengths are matched with DML+PIN+TEC, and the last 4 wavelengths are matched with DML+APD+TEC. From this we can see that DML is used at the sending end, but both APD and TEC at the receiving end are indispensable. Therefore, in terms of cost, MWDM has no advantage over the previous two solutions.


But regardless of cost, since the central office is an active device, it can provide alarm performance detection, and any fiber fault location can also be realized, and module-level monitoring is supported, which means that remote module faults can be monitored.

In addition, because it is active, it can provide 1+1 protection for optical lines by detecting changes in optical power.


Finally, use a table to summarize the WDM solution for the above 5G fronthaul solution:



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