5G is coming: analyzing the fronthaul solution of high-density wireless systems

The LTE era of wireless communications has been fully opened. As 5G approaches, the bandwidth of a single base station will increase significantly, and the density of base station deployment will increase. The existing base station network construction method has shown some problems, such as difficult base station site selection, high equipment room costs, low base station resource utilization, and heavy maintenance workload.

With the separation of BBU and RRU and the gradual maturity of RRU remote technology, 5G in the future will inevitably adopt a BBU centralized deployment solution. The existing fiber direct connection fronthaul solution is no longer applicable. Therefore, for the future 5G ultra-high density wireless network, centralized deployment of fronthaul bearer has become a research focus.

The main advantages of BBU centralized deployment

The main advantages of centralizing BBUs at the site include:

1. Save construction investment: Centralized placement of BBU can reduce the number of baseband hardware devices and reduce equipment costs. At the same time, there is no need to build a new base station room and related supporting facilities. The space, power supply, air conditioning and other resources in the existing station room can be fully utilized, greatly saving investment in the room and supporting facilities.
2. Reduce operation and maintenance costs: BBU equipment is installed centrally, reducing the maintenance workload of wireless and bearer professionals; BBU is deployed centrally, which improves the efficiency of power supply and is conducive to energy conservation and emission reduction; GPS equipment can also be shared and centralized, reducing construction and maintenance costs.
3. Realize fast network construction: RRU is a completely outdoor design. It can directly introduce the mains power and install an outdoor integrated power box to meet the construction requirements, greatly reducing the difficulty of base station site selection and construction, and improving the speed of network construction. The original base station addressing, building or leasing a wireless base station requires civil engineering (not required for leasing), decoration, power supply and air conditioning, and main equipment procurement, construction and installation. The centralized BBU method will greatly shorten the base station construction cycle.
4. Improved wireless network quality: In the future, BBU baseband pooling will enable sharing of baseband resources among multiple wireless sites, effectively responding to the traffic tidal effect. Based on dynamic changes in traffic, baseband resources can be flexibly configured to improve resource utilization. It can eliminate interference at the edge of the RRU, improve switching performance in densely populated areas, and reduce dropped calls. It can achieve intra-site carrier aggregation to increase rates. It can achieve inter-site CoMP (cooperative multi-point) to increase inter-site edge rates.

Among the above advantages, the cost advantage of BBU concentration is the most intuitive and can be seen in the short term. According to the calculation of domestic operators, the average investment of each macro base station can be reduced by 184,600 yuan. Based on the scale of 5,000 stations, a total of 923 million yuan can be saved, and 162 million yuan of operating costs (mainly base station room rental and maintenance costs) can be saved each year.

Fronthaul bearer interface

After BBU centralization, the structure of the mobile bearer network will undergo significant changes. The new network structure is shown in Figure 1.

Mobile bearer is divided into two parts: mobile fronthaul and mobile backhaul. Fronthaul refers to the network between the base station RRU and the BBU, and backhaul refers to the network between the BBU and the S-GW/MME.

In terms of fronthaul, the CPRI interface has gained support from more mainstream equipment manufacturers and defined higher rates to meet LTE requirements. Therefore, the CPRI interface has become mainstream in China and is widely adopted.

CPRI (Common Public Radio Interface) can be used in various wireless network standards (GSM, cdma2000, LTE, etc.), and has been released to version 7.0, with a rate of 25Gbit/s. CPRI links are used between REC (Radio Equipment Control, i.e. BBU in LTE) and RE (Radio Equipment, i.e. RRU in LTE).

Since CPRI uses RF analog signal sampling and digitization, the bandwidth requirement is dozens of times higher than the service bandwidth. For a 4G system, under a 20M carrier configuration, the bearer bandwidth requirement of a 3-carrier RRU base station is:

• FDD LTE: 3×2.5Gbit/s (2T2R), 3×4.9Gbit/s (2T4R)
• TD-LTE: 3×4.9Gbit/s (4T4R), 3×9.8Gbit/s (8T8R)

In order to ensure that data between RRU and BBU can be transmitted normally and its performance can meet the application requirements of base stations, CPRI specifies physical layer indicators such as synchronization, delay accuracy, link quality, and alarms. For the current 4G system, it mainly includes:

Table 1 CPRI main physical layer indicators

Bandwidth requirements, physical indicator requirements, etc. are important bases for operators to evaluate whether the fronthaul technology solution is feasible.

CPRI color optical solution: The base station CPRI interface uses a color optical module, which implements wavelength division multiplexing through passive combiners and splitters, without the need to configure bearer equipment. The advantages of the CPRI color optical solution are as follows:

1. Compared with the common CRPI fiber direct connection solution, the CPRI color optical module has a longer transmission distance and can save fiber resources through WDM;
2. Compared with the WDM solution, the number of white light interconnection optical modules between BBU/RRU equipment and WDM equipment can be reduced, thus reducing construction costs.

However, the CPRI color optical solution is still in its infancy, and its network management and protection capabilities are still unknown. Whether maintenance issues can be resolved is also a key factor affecting its future application. However, due to the advantages of the color optical solution such as being passive throughout, easy to use, and supporting wavelength division multiplexing, it is currently the industry's more optimistic fronthaul solution.

3. WDM-PON bearer solution:

WDM-PON has been progressing slowly due to insufficient bandwidth demand from access users. Recently, with the rapid growth of CPRI bearer and user bandwidth demand, the industry has also begun to consider WDM-PON as one of the technical solutions for carrying CPRI signals.

One of the most important features of WDM-PON is wavelength independence. Different wavelength channels can use different protocols, rates, and code types to carry different services, which will reduce the optical fiber and special equipment for CPRI. In terms of performance, the requirements of CPRI are much stricter than those of ordinary user access. Therefore, WDM-PON needs to simplify the protocol and encapsulation process to meet CPRI's requirements for latency and jitter. For CPRI signals, direct transparent transmission can be used, or the CPRI signals can be simply processed and encapsulated into Ethernet frames for transmission. At present, the technical solution for WDM-PON to carry CPRI is still in its infancy, and Q2 of ITU-T15 group is carrying out relevant standardization work.

The bandwidth per base station of 5G is 1000 times that of 4G = 10x base station density x 10x carrier bandwidth x 10x spectrum efficiency. Generally speaking, ignoring the base station density factor, the bandwidth required per base station is 30 to 50 times that of 4G. According to the usual 5G model analysis, if the fronthaul interface in CPRI format continues to be used, the fronthaul bandwidth may reach more than 1Tbit/s, which will become unaffordable both economically and technically. Therefore, it is urgent to optimize the CPRI interface.

NGFI refers to the fronthaul interface between the baseband processing function and the remote radio frequency processing function in the next generation wireless network main equipment. The basic feature is to redefine the functions of BBU and RRU, move some BBU processing functions to RRU, and then change the form of BBU and RRU. After reconstruction, they are redefined as RCC (Radio Cloud Center) and RRS (Radio Remote System), and the fronthaul is redefined from the 4G point-to-point interface to a multi-point to multi-point mobile fronthaul network based on the packet switching protocol. The architecture of NGFI is shown in Figure 2.

There are many ways to classify RCC and RRS, including:

• 1) Internal division of 2 layers - LOW MAC/HIGH MAC
• 2) MAC/PHY Division
• 3) Bit-level/Symbol-level division
• 4) Symbol-level/Sample-level
• 5) Baseband/RF division

Different division methods will bring different fronthaul bandwidth and latency requirements, and generally the two cannot be met at the same time. The division method that requires a smaller bandwidth will have stricter latency requirements. In addition, since more functions are transferred to remote devices, it will also bring complexity to remote devices. Therefore, when using it in the future, it is necessary to weigh various solutions according to specific needs. At present, mainstream operators at home and abroad tend to use fronthaul bandwidth not exceeding 25Gbit/s, so that they can make full use of the upcoming 25G Ethernet standard and the mature 100G optical module industry chain (4×25G).

In terms of standardization, the IEEE1914 Task Group was established in October 2015 to study the requirements and architecture of NGFI. The Task Group is led by China Mobile and has 45 members, including domestic manufacturers such as FiberHome and ZTE. Many ideas have been put forward on application scenarios, application requirements, key indicators, etc. In addition, the IEEE defines the IEEE1904.3 standard for the frame structure of CPRI signals transmitted over Ethernet. This year, the standard will be transferred to the IEEE1914 Task Group. This is also a standard that the industry is optimistic about using the Ethernet frame format to carry 5G fronthaul with slight modifications.

Fronthaul technology solution 1: Fiber direct connection solution:

As the name implies, the CPRI interface between the RRU and BBU is directly connected by optical fiber. Currently, CPRI optical modules are provided by base station equipment suppliers, so there is no need to consider the interoperability of optical modules.

In order to save half of the optical fiber resources, the industry has proposed a single-fiber bidirectional solution. Single-fiber bidirectional means that the data signals in both directions are transmitted in one optical fiber using different wavelengths. Since a wavelength selection filter must be added in each direction to transmit the transmitted wavelength and refract the received wavelength, the cost is higher than that of a single-fiber unidirectional optical module.

The fiber direct connection solution is simple to implement, but the biggest problem is that it occupies a lot of fiber resources. Even if a single-fiber bidirectional solution is used, the amount of fiber resources occupied is still not negligible. In areas where fiber resources are scarce, this solution is not suitable. In addition, the current base station's management function for the CPRI optical interface is still relatively weak, and the protection mechanism is not perfect.

2. WDM solution:

This solution uses wavelength division multiplexing to carry multiple CPRI links using different wavelengths and then multiplex them into one optical fiber to save optical fiber resources. There are two implementation options for the WDM solution.

WDM bearer equipment solution: Use miniaturized WDM bearer equipment to carry CPRI links. Since the BBU and RRU are directly connected to the WDM bearer equipment through the CPRI interface, the base station equipment only needs to use short-distance optical modules. The WDM bearer equipment has networking capabilities and can provide a protection mechanism for the CPRI link.

There are two ways to implement this solution. One is the framing solution, which maps the CPRI signal to the OTN frame structure and then converts it into a color optical signal to achieve wavelength division multiplexing. The other is the unframing solution, which directly converts the CPRI signal into color optical signal to achieve wavelength division multiplexing. Due to the different implementation methods, the two types of equipment have differences in function and performance.

1. The implementation of signal encapsulation into OTN frames can support better management and improve the manageability and reliability of the fronthaul network; at the same time, due to the framing operation, the absolute delay and delay accuracy are degraded.
2. The implementation scheme of not encapsulating CPRI signals into OTN frames can ensure very small absolute delay and delay accuracy, but since it is impossible to monitor bit errors, it is impossible to perform switching when line performance deteriorates.