Why Aren’t Walsh Codes Used for Reverse Channelization ?
There are two key reasons that Walsh codes are not used for the channelization in the reverse link: Mobile transmission are not time aligned and therefore cannot be orthogonal, and Walsh codes do not provide enough unique channels (causing more network overhead to manage Walsh code handoffs).
- All Walsh Codes Arrive Together in Time to All Mobiles From the Base Station
- However, Transmissions from Mobiles DO NOT Arrive at the Same Time at the Base Station
In the forward link, a point source (the base station), transmits a composite signal containing the Walsh encoded channels for many users. Since the various coded channel are sent together, they arrive at a phone together no matter where the phone is.
Phones close to the base station receive all channels simultaneously as do phones far from the base station The only difference is that is that the composite signal arrives later for the phone far from the base station. However, the reverse link does not share this property because each phone sends its own signal back to the base station.
Since the phones are different distances from the base station, their signals arrive at different times, thus precluding orthogonality. No attempt is made in EIA/TIA-95-B to time align the phones transmissions since achieving the required accuracy for good orthogonality is cost prohibitive.