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Français CWDM in access & mobile fronthaul: CPRI/eCPRI, MWDM, LWDM
Outside data centres, the biggest consumer of CWDM-class optics is the mobile network. Every 4G/5G radio on a tower needs a fibre link to its baseband unit — fronthaul — and towers rarely have spare fibre. Passive WDM lets a dozen radios share one strand, which is why the industrial-temperature 10G/25G coloured SFPs on the market exist. This page covers the fronthaul interfaces, the WDM plans used there (CWDM, MWDM, LWDM, DWDM) and the operational constraints that make fronthaul optics different from enterprise ones.
Fronthaul interfaces
| Interface | Rate | Nature | Optics |
|---|---|---|---|
| CPRI option 3 / 6 / 7 / 8 | 2.5 / 6.1 / 9.8 / 10.1 Gb/s | constant bit-rate digitised radio (4G) | 1G–10G SFP/SFP+, 8B/10B or 64B/66B per option |
| CPRI option 9 / 10 | 12.2 / 24.3 Gb/s | 4G/5G legacy | 25G-class SFP28 |
| eCPRI | 10 / 25 Gb/s Ethernet | packetised (5G, functional split 7.2x) | 10G SFP+, 25G SFP28 — the 5G workhorse |
| Backhaul / midhaul | 10–100G | IP/Ethernet | standard optics |
Fronthaul is latency-bound: a one-way budget of roughly 100 µs (5G split 7.2x) including fibre (5 µs/km) limits radios to about 10–20 km from the baseband — hence the 10/20/40 km reach classes and the absence of anything longer.
Why WDM
| Option | Fibres per site (12 radios) | Notes |
|---|---|---|
| Dedicated fibre pairs | 24 (or 12 single-fibre BiDi) | needs plenty of fibre |
| Passive CWDM, dual fibre | 2 | 12 channels 1271–1491 (or 1271–1611) |
| Passive CWDM, single fibre | 1 | 6 links on 12 channels |
| MWDM / LWDM, dual fibre | 2 | 12 channels in the O-band, 25G-friendly |
| DWDM (fixed or tunable) | 2 | 40+ channels; tunable SFP28 reduces inventory |
| Semi-active / active WDM | 1–2 | passive at the tower, active (with monitoring) at the hub |
Wavelength plans
| Plan | Channels | Wavelengths | Rate | Notes |
|---|---|---|---|---|
| CWDM 6 | 6 | 1271–1371 nm (O-band) | 10/25G | low dispersion near 1310 — good for 25G; avoid the 1383 nm water peak |
| CWDM 12 | 12 | 1271–1491 or 1291–1511 nm | 10G; 25G on O-band channels | E/S-band channels have dispersion issues at 25G over 10 km |
| CWDM 18 | 18 | 1271–1611 nm | 1–10G | full grid; 1550–1610 dispersion-limited at 10G/40 km (Channel plan) |
| MWDM (medium WDM) | 12 | the first six CWDM channels split ±3.5 nm: 1267.5 / 1274.5, 1287.5 / 1294.5, 1307.5 / 1314.5, 1327.5 / 1334.5, 1347.5 / 1354.5, 1367.5 / 1374.5 | 25G | O-band only → dispersion-safe 25G at 10–20 km; TEC-stabilised DFB lasers; China Mobile-driven |
| LWDM (LAN-WDM) | 12 | 800 GHz grid around 1269–1318 nm (extends the 100GBASE-LR4 LAN-WDM set) | 25G | O-band; cooled DFB; tighter filters than CWDM |
| DWDM | 40–96 | C-band, 100/50 GHz | 10/25G | tunable SFP28 fronthaul modules; cooled; higher cost and power |
Wavelength references: Wavelength bands, ITU grid.
Fronthaul module requirements
| Requirement | Why | Typical value |
|---|---|---|
| Industrial temperature | outdoor cabinets and tower-mounted radios | −40 … +85 °C (I-temp) |
| Reach 10 / 20 / 40 km | tower-to-hub distances within the latency budget | 10G: 10–40 km; 25G: 10–20 km |
| Dispersion tolerance at 25G | 25G NRZ over 10–20 km on E/S/C-band CWDM channels fails; O-band plans or dispersion-tolerant lasers needed | MWDM/LWDM, or 25G CWDM limited to 1271–1371 |
| Low power | radios have limited power budgets | uncooled where the plan allows (CWDM), TEC where not (MWDM/LWDM/DWDM) |
| DDM with remote monitoring | no technician at the tower | Rx/Tx/temperature via the radio's or hub's management |
| Vendor validation | radio and baseband vendors whitelist optics | Vendor lock |
| Tunable option | one spare covers all channels | tunable DWDM SFP28 (Tunable transceivers) |
Semi-active WDM
To monitor a passive tower site from the hub, semi-active systems keep the tower passive (mux only) and put an active unit at the hub that terminates and re-transmits each channel, adding OTDR-like supervision, per-channel power monitoring and protection. Fully active WDM puts equipment at both ends. The choice trades tower power and space against observability.
Design and commissioning checklist
- Count radios per site and the growth plan → channel plan (6/12/18, MWDM/LWDM, DWDM).
- Distances → reach class and dispersion check per wavelength at the interface rate.
- Budget per channel including mux/demux, OADMs and connectors (Mux/demux & link budget).
- Choose I-temp modules validated for the radio and baseband vendors; verify wavelength coding and rate codes on the bench (CWDM transceivers).
- Label modules by channel; single-fibre plans need pair labels (Single-fibre CWDM & OADM).
- Record per-channel Rx at both ends at commissioning; monitor via the radio's DDM.
Enterprise and campus CWDM
The same passive kits serve campus rings and metro links for 1G/10G services where fibre is leased by the strand: a 8- or 16-channel mux pair replaces 8–16 fibre pairs, OADMs serve intermediate buildings, and 40 km-class modules cover most metro distances (CWDM deployment).
In CodingBox
Fronthaul optics are read like any CWDM/DWDM SFP: the wavelength field, rate codes (CPRI options are coded as their bit rate; eCPRI as 10G/25G Ethernet), temperature grade in the vendor PN and live DDM. Coding wavelength and rate consistently with the real laser is what keeps a radio vendor's whitelist and a mux channel plan in agreement (Check transceiver, EEPROM editor).