Typography

The rapid development of wireless services proposes higher transport requirements for bearer networks. Apart from the OTN or PTN which can be used as a LTE/5G bearer, PON (Passive Optical Network) technologies not only provide broadband access, but also can be deeply integrated with mobile access to offer an efficient fronthaul and backhaul solution.

1. Background of PON Technologies

PON is widely used in optical broadband access because of its multi-service, low investment, and easy maintenance features. With continuous evolution on bandwidth, latency, reliability, it is extended to backhaul bearing for LTE/5G. Since PON ODN tree structure matches the trend of C-RAN in densely populated areas, PON is also a good candidate for 5G fronthaul.

2. Evolution of PON Technologies and Wireless Bearer

Single-wavelength rate of PON continues to grow. 50G-PON standard is under discussion and is expected to be finalized by 2020.

Figure 1 shows the evolution of PON bandwidth. The single wavelength rate evolves from GPON (2.5 G/1.5 G) to XGS-PON (10 G/10 G) and to 50G-PON, which is consistence with the growth of backhaul bandwidth.

Figure 1: PON Has Similar Evolution Road to That of 5G

Another PON research direction is to use the multi-wavelength multiplexing to save fiber resource.   At the end of 2018, China Telecom completed 25G WDM-PON 5G live network verification in Suzhou city, China. The current obstacles of this technology include high cost and high power consumption of the tunable optical modules, while the cost is expected to decrease if they were put into large scale deployment.

3. PON-Based Backhaul Bearer Technology

5G NR base station greatly improves the backhaul bandwidth than LTE. For example, a typical 5G NR S111 macro station backhaul bandwidth is about 6 Gbps, while that of 5G NR small base station is 1-2Gbps. 

XGS-PON has proved to be successful in LTE small cell and macro site backhaul, it also satisfy the backhaul of 5G small cell or macro with a few number of sites. In the D-RAN scenarios where the sector is larger than S111 or has more mobile sites, the next-generation 50G-PON technology will apply.

In terms of networking, 5G backhaul generally adopts the IPRAN technology, which requires perfect Layer 3 functions and IP FRR service protection functions. New-generation OLT equipped with these capabilities will meet IPRAN demands.

4. PON-Based Fronthaul Bearer Technology

WDM-PON is a better solution than low latency TDM-PON in terms of bandwidth. e.g. the typical bandwitdh of 5G fronthual is 25G either in eCPRI or CPRI protocol.

Figure 2: WDM Overlay Network for Bearing 5G Fronthaul

Another type of PtP WDM Overlay network architecture is shown in Figure 3. The trunk fiber and ODN network are multiplexed with TDM-PON signals and WDM signals (25Gbps per lambda) by introducing an external wavelength division multiplexer (WDM) filter called WDM1r. WDM signals are (de)multiplexed by WDM filters based on arrayed waveguide grating (AWG) devices at the OLT and the first level feeder fiber. The architecture supports the coexistence of WDM, GPON and XG(S)-PON and enables transmission of wireless and wireline services over a same OLT platform. 

Key technologies of the WDM Overlay include, wavelength planning to ensure ODN coexistence, 25G WDM optical components power budget for coexistence, and the low latency.

4.1 PtP WDM Overlay Wavelength Planning

Figure 3: PtP WDM Overlay Wavelength Planning

PtP WDM wavelength planning shall avoid conflict with GPON and XG(S)-PON. In Figure 4, the wavelength plan of PtP WDM locates in the C-Band is from 1528nm to 1560nm, ensuring sufficient guard bands between PtP WDM signals and GPON or XG(S)-PON signals. The PtP WDM signals adopt standard DWDM grids with 100GHz channel spacing defined in ITU-T G.694.1. The upstream and downstream are both in the proposed wavelength band, where it support up to 20 pairs of wavelengths operating over duplex modules. The wavelength plan also shares the same wavelength range with WDM-PON and G.Metro (ITU-T G.698.4), benefits include sharing same industry chain and being migratable to tunable modules.

4.2 Optical Power Budget and 25G WDM Optical Components Performance

4.2.1 WDM Overlay Optical Power Budget

The link loss is estimated at 23.5dB including the Optical Power Penalty (OPP) which is mainly induced by chromatic dispersion in the C-Band. The link loss of the PtP WDM overlay network in Figure 3 is calculated from the optical losses listed in Table 1 and Table 2. Table 1 shows the link loss of the PtP WDM overlay network with 10km fibers is 23.5dB. Table 2 shows the loss with 5km fibers is 20.5dB.

Eval items

6-wavelength  AWG1

WDM1r

10km fiber

1: 4

splitter

AWG2

Connector

OPP

Engineering Margin

Total

Link

Loss

(dB)

3.5

1.5

3

7

3.5

3

3

2

23.5

Table 1: Link loss of PtP WDM Overlay Network with 10km fiber

 

Eval items

6-wavelength  AWG1

WDM1r

5km fiber

1: 4

splitter

AWG2

Connector

OPP

Engineering Margin

Total

Link

Loss

(dB)

3.5

1.5

1.5

7

3.5

3

1.5

2

20.5

Table 2: Link Loss of PtP WDM Overlay Network with 5km Fibers

GPON and XG(S)-PON will add 1dB loss induced by WDM1r to the existing optical power budget.

4.2.2 25G WDM Optical Components Performance

The characterized transmission performance of 25G WDM optical components is shown in Figure5. The receive sensitivity of the 25G avalanche photodiode (APD) receiver at the condition of 5E-5 errors is about -22.5dBm, when the C-Band WDM signals are transmitted back-to-back without fiber dispersion. The 25G WDM optical component in the C-band usually has a transmitter optical power of about 0 dBm. So it can support the optical link loss of 22.5dB when FEC RS(528,514) is enabled, which is about 1dB lower than the link loss with 10km fibers in Table1, but about 2dB higher than the link loss with 5km fibers in Table2. If the fiber distance is greater than 10km, there are two technical paths to reach the 22.5dBm of the APD receiver. One is to increase the APD receiving sensitivity or transmit optical power, another is, uses an amplifier at the central office to increase the link budget. As shown in the Figure 5, when the erbium doped fiber amplifier (EDFA) is used for amplification, the PIN receiver can achieve the 26 dB link budget (suppose the transmitting end is 0dBm and the dispersion cost of 10km fibers is included).

Figure 5: BER Curve of the 25 G WDM Optical Component

4.3 Transmission Latency Evaluation

Through real test, it turns out no latency impact compared with using dark fiber after WDM overlay introduced. As shown in Table 3 and Table 4, the average result of the ping core network server is similar.

Serial Number

32Byte

64Byte

256Byte

1000Byte

1500Byte

1

7.3177

7.4293

7.2689

7.3804

7.4816

2

7.14495

7.239

7.2775

7.3576

7.5732

3

7.2145

7.3445

7.3184

7.4096

7.4296

4

7.1219

7.2366

7.2874

7.4607

7.5991

5

7.2118

7.1653

7.3398

7.258

7.6664

Average Value

7.20217

7.28294

7.2984

7.37326

7.54998

Table 3: WDM Overlay-based packet delay test (UE-CN, unit: ms)

 

Serial Number

32Byte

64Byte

256Byte

1000Byte

1500Byte

6

7.1942

7.2671

7.3095

7.4553

7.5928

7

7.2478

7.2588

7.2918

7.4067

7.5925

8

7.2094

7.2787

7.2467

7.3283

7.4641

9

7.1456

7.3139

7.3272

7.2899

7.5478

10

7.2118

7.3688

7.2737

7.4232

7.6694  

Average Value

7.20176

7.29746

7.28978

7.38068

7.57332  

Table 4: Packet Delay Test Based on Dark Fiber (UE-CN, unit: ms)

5. Conclusions

PON ODN-based access network can fully meet the deployment demands of 5G fronthaul and backhaul through continuous evolution. It will bring better service experience to home access and mobile access via offering lower latency and higher bandwidth capabilities.

 

Ariyanto, AVP Access Network Strategy, PT. Telkomunikasi Indonesia, Tbk