Rajala - WLL vs full mobile:
1) WLL has considerably more robust link budgets than mobile, and it is not isolated to just 'soft-handoff'. For instance, in CDMA the near-far problem becomes much more tractable when the users aren't continually moving and effecting the received signal. Another issue in CDMA is synchronization and orthogonality are easier to guarantee when the the users aren't moving all over. TDMA also benefits from fixed users, although via different pathways. For instance, TDMA could get much better frequency re-use if those pesky users weren't continually moving between cells. Also, there is less need for buffers between the various time-slots and the link budget allotment for multipath can be made less. At the very least this is likely to result in 30% extra users. (In theory it should be substantially more - like 3g which has a factor of 6 improvement for fixed vs mobile users - but the current equipment and standards aren't built to take full advantage of fixed users.)
2) Fewer basestations and sectors are needed. The obvious example is that there wouldn't be the need for all of the towers along the freeways, but in addition there is less uncertainty about user location, so there is less need to overbuild 'in-case' the users clump at Joe's house for the superbowl (e.g. in CDMA there is less of the link-budget assigned to 'cell-breathing'). Again, this is not a huge savings, but it is probably on the order of 30% or more.
3) Finally, since the WLL systems do not take full advantage of the improved performance available in #1, they are easier to tune and require less adjustment. In this area I have to make an almost complete WAG as to the savings, but I would guess it to be 20 or 30%.
Thus, the initial capital cost (hardware + real-estate) of a mobile system is probably at least 70% more than for WLL system for the same number of users. In addition, the engineering cost is 25% less, so overall I would expect WLL costs to be about 50% less for the same number of users. That is not a huge savings, but it is substantial. I'd be interested in what you disagree with.
Clark
PS You quoted a GSM engineer as saying that the extra db's from eliminating handoff is small. Agreed. But link-budget savings for CDMA come in very very different areas. See item #1 above. |
