DAVID LEWIS 5G FRONTHAUL
bidirectional fiber. It does this by transmitting six wavelengths up and six wavelengths down in a bidirectional fashion. The six receivers at each end of the link have the same 13 nm passband as the CWDM grid. The modification made by MWDM is to have two channels, each offset from the standard CWDM channels by ± 3.5 nm. For reaches beyond 5 km, the longer wavelengths of CWDM and MWDM are subject to high chromatic dispersion. Thus, for longer reach systems of 5 to 15 km, it is necessary to move away from wavelengths significantly above the fiber zero dispersion wavelength. This is achieved by using LAN WDM (LWDM), which is considered by the ITU-T to be dense WDM (DWDM) with channels on an 800 GHz spacing. For systems where more channels per fiber are required, the next step up is dense WDM (DWDM) in the C-band (1530 to 1565 nm). With 100 GHz channel spacing, WDM links support up to 40 channels per fiber. This makes large, centralized RAN (CRAN) fronthaul networks in fiber-constrained applications possible. The basic parameters of the four grids are listed in Table 1. Table 2 compares typical implementations of the four grids in terms of the type of transmitter device.
As can be seen, the trade-offs between fiber count, dispersion penalties, and requirements for laser temperature control mean that “one size does NOT fit all”. Given the range of topologies and applications demands, it is likely that
Figure 3: Lumentum DFB laser chip photograph and schematic
TUNABLE SFP+ TECHNOLOGIES FOR 5G FRONTHAUL Lumentum has a long history supplying tunable transceiver modules with reaches up to 80 km (10 Gb/s) or 15 km (25 Gb/s). Industrial temperature versions operate from -40 to 85 °C and have power consumption less than 2.5 W. With over two million tunable XFP and SFP+ modules shipped, Lumentum technology has been proven in the field many times over. These transceivers are built around Lumentum digital supermode distributed- Bragg-reflector (DSDBR) wideband tunable laser. The laser is monolithically integrated with an InP Mach-Zehnder modulator (MZM). An integrated semiconductor optical amplifier (SOA) provides optical power control and acts as a shutter to allow dark tuning when reverse biased. In addition, automatic wavelength tuning allows for software-controlled, full C-band tunability. Thus, any single module can flexibly support any channel. Because the modules are plug-and-play, setup requirements are greatly reduced. There is no need to track fiber and identify the proper add/drop channel on the mux/demux facility. This means a single TSFP module with automatic wavelength tuning replaces a large list of part numbers when fixed DWDM transceivers are used. This provides further operational and supply chain efficiencies with improvements in both OpEx and CapEx for operators. The result is lower module cost, simplified maintenance, and reduced inventory investment and management. CONCLUSION 5G fronthaul networks are being deployed on a variety of WDM wavelength grids. Low channel count networks with 6 or 12 wavelengths are in the O-band and employ three standard approaches. For higher channel count networks, C-band DWDM on 100 GHz using tunable modules is gaining traction. Automatic wavelength tuning modules offer significant OpEx and CapEx savings for mobile operators needing high channel counts.
all grids will co-exist in 5G fronthaul networks. Thus, mobile operators will be evaluating and deploying all of the above approaches for fronthaul connectivity. ENABLING 5G FRONTHAUL WITH DIRECTLY MODULATED LASERS Directly modulated lasers (DMLs) are used in high volume for optical transceiver modules such as SFP28 and QSFP28. In the datacom market, standards such as 25GBASE-LR and 100G-CWDM4 utilize uncooled 25 Gb/s DMLs within the data center environment. These same 25 Gb/s DML chips can be used for 5G fronthaul on CWDM, MWDM and LWDM grids with temperature control in mobile outdoor plant environments. Lumentum is a leading supplier of 25 Gb/s DMLs providing market-leading power efficiency, wide temperature
Table 1: Wavelengths, channels, and spacing for 5G fronthaul grid plans
NO. CHANNELS
WAVELENGTH RANGE (NM)
FREQUENCY RANGE (THZ)
CHANNEL SPACING
GRID TYPE
CWDM
6
1271 – 1371
20 NM
MWDM
12
1267.5 – 1374.5
7 NM
LWDM
12
1269.23 – 1318.35 227.4 – 236.2 800 GHZ
DWDM
40
1529.55 – 1560.61
192.1 - 196.0 100 GHZ
Table 2: Comparison between grids for 25 Gb/s transceivers on bidi fiber
BANDWIDTH PER DIRECTION ON BIDI FIBER (GB/S)
MAX REACH (KM)
GRID TYPE
TX DEVICE
TEMPERATURE CONTROL
TRANSCEIVER COST
CWDM DML
UNCOOLED 5 – 10
75
$
MWDM DML
COOLED 5 – 10
150
$$
LWDM DML / EML COOLED 10 – 20
150
$$
MZM / TUNABLE
DWDM
COOLED 10 - 20
500
$$$
operation, self-hermetic chip designs, and high bandwidth enabled by a corrugation pitch modulated (CPM) design (see Figure 3).
DML: Directly modulated laser, typically a direct feedback laser (DFB) EML: Externally modulated laser, typically a DFB plus electro-absorption (EA) modulator MZM: Mach-Zehnder modulator Uncooled: DFB center wavelength varies by ~0.1 nm/°C. Temperature control keeps each channel within the tolerance of the grid. Max reach: The combination of transmitter chirp and fiber dispersion limits transmission distance for 25 Gb/s DML transmitters at wavelengths above 1330 nm.
Figure 4: Lumentum has a long history supplying tunable transceiver modules
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ISSUE 25 | Q3 2021
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