carcarx
03-18-2009, 08:19 AM
Hopefully none of you ClearWire subscribers have experienced this yet, but don't be surprised if you do. First, a little background:
WiMax and LTE both use OFDM as their radio transport. The US digital TV standard, VSB8, also uses OFDM. Data to be transmitted via OFDM is doled-out to multiple RF carriers and then recombined at the receiver. (For those of you who used dial-up modems, remember how you'd hear discrete tones, then as the modem discovered the line quality the sound would become more like white-noise? More and more audio carriers were added until there were so many it sounded like white noise - all those audio carrier tones at once.)
It's the same principle for both DSL and OFDM.
With radio signals bouncing off objects not all of a transmission arrives from the same direction at exactly the same time. For those of you who use antennas you could see this as "ghosts" on analog TV transmissions.
Now, with US digital TV the multipath problems cause some digital TV streams that are badly ghosted to be unrecoverable at the receiver. The receiver can't get a single signal strong enough to recover a usable datastream, even though the total signal strength is excellent.
The same can happen with WiMax and LTE.
Until "5G" comes around, in which each OFDM "carrier" is a cdma RF stream, this will be a problem.
WiMax and LTE both use OFDM as their radio transport. The US digital TV standard, VSB8, also uses OFDM. Data to be transmitted via OFDM is doled-out to multiple RF carriers and then recombined at the receiver. (For those of you who used dial-up modems, remember how you'd hear discrete tones, then as the modem discovered the line quality the sound would become more like white-noise? More and more audio carriers were added until there were so many it sounded like white noise - all those audio carrier tones at once.)
It's the same principle for both DSL and OFDM.
With radio signals bouncing off objects not all of a transmission arrives from the same direction at exactly the same time. For those of you who use antennas you could see this as "ghosts" on analog TV transmissions.
Now, with US digital TV the multipath problems cause some digital TV streams that are badly ghosted to be unrecoverable at the receiver. The receiver can't get a single signal strong enough to recover a usable datastream, even though the total signal strength is excellent.
The same can happen with WiMax and LTE.
Until "5G" comes around, in which each OFDM "carrier" is a cdma RF stream, this will be a problem.