A twelve-channel plan that doubles the low CWDM channels by shifting each by ±3.5 nm — created for 5G fronthaul.
Twelve Channels Made Out of Six
MWDM is an arithmetic trick rather than a new grid. Take the first six CWDM channels — 1271, 1291, 1311, 1331, 1351 and 1371 nm — and build two lanes around each by shifting 3.5 nm down and 3.5 nm up. Twelve channels come out: 1267.5, 1274.5, 1287.5, 1294.5, 1307.5, 1314.5, 1327.5, 1334.5, 1347.5, 1354.5, 1367.5 and 1374.5 nm.
The spacing is deliberately uneven. Two lanes in a pair sit 7 nm apart; consecutive pairs are 13 nm apart. Averaged over the plan that is roughly 10 nm, half of CWDM, which is where the name comes from — medium, between coarse and dense.
Why the Lasers Need Half a Cooler
An uncooled DFB drifts about 0.1 nm per degree Celsius, so over an outdoor cabinet's temperature range it moves five or six nanometres. That is fine against a CWDM channel and fatal against a 7 nm one, where the filter passband is only about 2.5 nm either side of centre.
MWDM optics therefore carry a small thermoelectric cooler, but nothing like a DWDM transmitter's. The DWDM job is to hold an absolute frequency to within about ±0.1 nm for the life of the module; the MWDM job is only to stop a DFB wandering out of a band several nanometres wide. Semi-cooled is the usual term, and it lands the cost much closer to CWDM than to DWDM.
The Problem It Was Built For
MWDM came out of Chinese operator requirements for 5G fronthaul, China Mobile in particular. A dense radio deployment needs a great many 25G links back from cell sites over fibre that is already in the ground and cannot be added to cheaply.
Eighteen CWDM channels sounds like plenty until you notice that the useful ones on legacy fibre are the low O-band group, and that fronthaul wants pairs of lanes per site. Twelve semi-cooled lanes across 1267.5 to 1374.5 nm solved it without asking anyone to install DWDM line systems at a base station.
What to Check Before You Buy
MWDM sits close to the O-band dispersion zero, so chromatic dispersion is not the constraint; loss is. Budget about 3 dB for a twelve-channel mux at each end and put the real numbers into the link budget calculator before promising a distance.
Confirm the industrial temperature rating if the module goes in a street cabinet, confirm the operating wavelength against the port label rather than the colour code, and check the DDM page after the first hot afternoon. A semi-cooled laser whose TEC control loop is marginal fails exactly the way you would expect it to.
MWDM reference
| Channels | 12 |
|---|---|
| Derivation | CWDM 1271 to 1371 nm, each shifted ±3.5 nm |
| Span | 1267.5 to 1374.5 nm |
| Spacing | 7 nm within a pair, 13 nm between pairs |
| Channel passband | about ±2.5 nm on typical filters |
| Mux insertion loss | about 3 dB for 12 channels |
| Isolation | ≥ 25 dB adjacent, ≥ 35 dB non-adjacent |
| Laser | semi-cooled DFB — small TEC, not a wavelength locker |
| Typical service | 12 × 25G 5G fronthaul on one fibre pair |
Related Terms
CWDM, DWDM, LAN-WDM and OADM. Our 12-channel MWDM modules carry this plan, and the grid calculator lists the CWDM channels the shift is built from.