Application of Passive WDM Technology in 5G Fronthaul

Labs Guide

Passive WDM technology is the main technology used in 5G fronthaul at this stage. Since the speed of 5G fronthaul is as high as 25G, dispersion becomes the main factor affecting 5G fronthaul. This paper introduces the technical principles of passive WDM fronthaul solutions and the wavelength allocation of common models. Through the study of the transmission and dispersion penalty (TDP) index of optical modules, the various influencing factors of TDP and their impact on 5G fronthaul are analyzed. Finally, combined with the transmission index requirements of optical fiber links, deployment recommendations for passive WDM technology in 5G fronthaul are proposed.

1. Introduction

5G fronthaul refers to the transmission between DU (distribution unit) and AAU (active antenna unit) in the 5G wireless access network. Currently, 5G fronthaul solutions mainly include: fiber direct drive, passive WDM, semi-active WDM and active WDM.

Among various fronthaul solutions, the direct fiber drive solution consumes a large amount of fiber core resources, and the construction of new optical cables requires a certain period of time and is also limited by pipeline and pole resources; the semi-active WDM solution is not yet mature; the active WDM solution is too expensive; and the passive WDM solution has become the main solution for 5G fronthaul at this stage due to its advantages of occupying less optical cable cores, low price, and rapid deployment.

2. Introduction to Passive WDM Fronthaul Solution

2.1 Technical Principles of Passive WDM Fronthaul Solution

Passive WDM uses wavelength division multiplexing technology, using passive MUX/DEMUX (multiplexer/demultiplexer) to combine multiple optical signals with different wavelengths into one optical fiber for transmission. For example, a 5G macro base station usually has 3 AAUs, and the number of transceiver ports from DU to AAU is 6. Using 1 6-way MUX/DEMUX on the DU side and the AAU side respectively can combine the transceiver signals between DU and AAU into one optical fiber for transmission, as shown in Figure 1.

Since MUX/DEMUX is a passive device, each service port needs to use an optical module with a different transmission wavelength, that is, a color optical module. The passive WDM system consists of two parts: MUX/DEMUX and color optical module. Since DU and AAU equipment are usually configured with optical modules with a specific wavelength (usually 1310nm), when using them, the existing optical modules with specific wavelengths (also called gray optical modules) of DU and AAU need to be replaced with color optical modules with the same rate.

The commonly used networking structures of passive WDM fronthaul solutions are divided into two types: dual star (see Figure 1) and bus (see Figure 2). The dual star networking structure is mainly used in the fronthaul scenario of 5G macro base stations, and the bus type is mainly used in scenarios such as highways, high-speed railways, and tunnels.

2.2 System Model of Passive WDM Fronthaul Solution

Based on cost considerations, passive WDM generally uses CWDM (coarse wavelength division) technology. CWDM supports a total of 18 wavelengths. The commonly used models for 5G fronthaul are 6-in-1 (i.e., 1 fiber transmits 6 wavelengths) and 12-in-1. The wavelengths used by each model are shown in Table 1.

Although the 18-in-1 model can be used for 5G fronthaul, the 18-in-1 model uses more optical module models, which increases the difficulty of maintenance. In addition, in a few access networks that use G.652B optical fiber, it will also be affected by the optical fiber E-band attenuation water peak, so it is not recommended.

In order to avoid the influence of the E-band attenuation water peak of G.652B optical fiber, the wavelengths of the 12-in-1 model are divided into the first 6 wavelengths (1271nm~1371nm) and the last 6 wavelengths (1471nm~1571nm), while the 6-in-1 model uses the first 6 wavelengths.

3. Impact of dispersion on 5G fronthaul

3.1 Transmission and Dispersion Penalty (TDP) of Optical Modules

Dispersion refers to the signal distortion caused by pulse broadening after different frequency components of an optical signal (pulse) propagate at different speeds over a certain distance. The effect of dispersion on transmission can be reflected by the Transmitter Dispersion Penalty (TDP) parameter of the optical module.

The TDP parameter reflects the degree of signal degradation caused by dispersion after the signal is modulated by the optical module and transmitted by the optical fiber. In actual testing, it is measured by the degree of degradation of the sensitivity of the standard receiving module. TDP is the difference in sensitivity between the following two situations (TDP = S2 - S1):

• The standard sensitivity S1 of an ideal reference transmitter;
• The sensitivity S2 of the transmitter under test + optical link of specified length.

3.2 TDP index of 5G fronthaul color optical module

The TDP index of the 5G fronthaul color optical module is related to the modulation method used by the optical module. The optical module is mainly divided into two types according to the modulation method: DML (Directly Modulated Laser) and EML (Electrro-absorption Modulated Laser).

DML emits light of different intensities by directly controlling the current passing through the laser. EML passes a constant current through the laser, and changes the ratio of light passing through an external modulator to obtain light of different intensities. It includes two parts: the light source (laser) and the modulator. When modulating directly, the laser always works in an unstable state, and there are many nonlinear effects that affect the quality of the output. Therefore, DML is generally used in low-speed and short-distance communication systems. Long-distance, high-speed communication systems generally use EML.

In addition, the TDP index of the 5G fronthaul color optical module (transmission rate is 25G) is also related to the central wavelength of the laser and the nominal transmission length. The TDP reference value of the color optical module with a nominal transmission length of 10km is shown in Table 2.

Due to the limitation of dispersion, DML can only be used for the first 6 waves. Although EML has a smaller TDP, its cost is much higher than DML. Therefore, 5G fronthaul mainly uses DML. However, 4G fronthaul (transmission rate is 10G) is less affected by dispersion. For the 12-in-1 model, the first 6 waves can be used to open 5G fronthaul, and the last 6 waves can be used to open 4G fronthaul.

3.3 Effect of ambient temperature on TDP

When the color optical module of 5G fronthaul works in a high temperature environment, the TDP will increase significantly. The longer the wavelength of the color optical module, the more obvious the impact of high temperature on TDP. When the ambient temperature exceeds 70℃, the TDP of the optical module with wavelengths of 1351nm and 1371nm will increase by 2 to 3dB compared to normal temperature (15℃ to 35℃).

To reduce the impact of high temperature on TDP, the short-wavelength optical modules of the first 6 wavelengths can be used on the AAU side, and the long-wavelength optical modules can be used on the DU side, as shown in Figure 3.

The operating temperature of the optical module is divided into commercial grade: 0 ~ +70 (℃) and industrial grade: -40 ~ +85 (℃). Since AAU is usually installed outdoors, the operating temperature range of the fronthaul optical module is more stringent, so the temperature characteristics of the AAU side optical module should meet the industrial grade requirements. DU is usually installed in the equipment room, and the temperature characteristics of the DU side optical module only need to meet the commercial grade requirements.

4. Fiber optic link transmission indicators

4.1 Fiber Link Attenuation

The total attenuation of the optical fiber link from DU to AAU includes: optical fiber and fusion attenuation, active connection attenuation and MUX/DEMUX insertion loss. The total attenuation of the optical fiber link can be calculated according to Table 3.

Table 3 5G fronthaul fiber link full-length attenuation calculation table

The transmission reference model of the 5G fronthaul fiber link is shown in Figure 4. Based on the fiber link length of 10.0 km and the number of active connections in the fiber link of 7, the maximum full-length link attenuation of the system is 10.5 dB.

Figure 4 Transmission reference model of optical fiber link in 5G fronthaul

4.2 System Optical Power Budget

The system optical power budget is mainly determined by the optical power parameters of the optical module, which is "minimum OMA transmit optical power" - "maximum transmit and dispersion penalty (TDP)" - "maximum OMA receive sensitivity". The main optical power indicators and optical power budget of the 5G fronthaul optical module (10km) can be found in Table 4.

Table 4 Main optical power indicators and optical power budget of 5G fronthaul color optical module (10km) (Note)

The optical power budget of the system should be greater than the full attenuation of the optical fiber link and reserve a maintenance margin of 2 to 3 dB. From the "optical power budget" value in Table 4, it can be seen that the color optical module with a nominal transmission length of 10 km cannot meet the attenuation index requirements of a 10 km optical fiber link in actual scenarios. According to the optical power budget of the system and Table 3, it can be calculated that when reserving a 2 dB maintenance margin, the ideal transmission distance of the color optical module (10 km) under the transmission model of Figure 4 does not exceed 6.3 km.

5. Conclusion and Recommendations

In summary, due to the high rate of 5G fronthaul, the system will generate a large TDP when the passive WDM solution is adopted. Therefore, the following measures are recommended in engineering implementation:

• The system models should be mainly 6-in-1 and 12-in-1, and 18-in-1 should not be used.
• 5G fronthaul color optical modules should use DML modulation and the first 6 waves. If only the fronthaul of 5G base stations is to be solved, a 6-in-1 system is suitable; since 5G stations are mostly co-located with 4G stations, a 12-in-1 system can be used, with the first 6 waves transmitting 5G and the last 6 waves transmitting 4G; if a physical station needs to solve the fronthaul of 5G and 4G 3D-MIMO (rate of 24.33Gbps) at the same time, it is recommended to use 2 sets of 6-in-1 systems. When the distance is short, a set of 12-in-1 system can also be used.
• The AAU side should use industrial-grade optical modules, and the wavelengths of the optical modules should be 1271nm, 1291nm, and 1311nm; the DU side should use commercial-grade optical modules, and the wavelengths of the optical modules should be 1331nm, 1351nm, and 1371nm.
• The ideal transmission distance of commonly used color optical modules (10km) in typical scenarios is 6.3km. When the distance exceeds this, it is recommended to use color optical modules with a larger optical power budget (15km).

[This article is an original article by 51CTO columnist "Mobile Labs". Please contact the original author for reprinting.]

Click here to read more articles by this author