Backward compatible - a complex topic. I'm not an engineer, but I rub elbows with them a lot, and can usually ask the right question and understand the answer. I believe IS-95 only relates to the air interface, i.e., how the BTS and the handset talk to each other. The rest of the network is manufacturer proprietary, i.e., LU, MOT, NT/QCOM all do it differently. The design of that rest of the network is probably more similar to each company's respective GSM or TDMA designs than to those of the other manufacturers. So compatibility with the other technologies for much of the system is manufacturer dependent, and that is what I imagine Qualcomm is doing with Vodaphone, utilizing a common switch to manage a CDMA overlay.
As to the air interface, that's another matter altogether. Current CDMA technology utilizes only 5 MHz per carrier, 2.5 MHz each for the uplink and downlink. The PCS A and B block holders have 30 MHz slices of spectrum to play with. An advantage of CDMA is its ability to hear over noise from cells it's not interested in; so all cells in an area use the same 5 MHz of spectrum. You space your cells appropriately, more or less on a grid, to get the quality of coverage you want, i.e., vehicle or residential or commercial building penetration and overcoming shadowing by hills, etc. Spacing for conceivable loads should not be dependent upon capacity requirements, as you can add second carriers using another 5 MHz to clusters of cells to up the capacity. The second carrier utilizes some of the existing infrastructure already in place at the cell site, but results in nearly the same revenues to the BTS vendor as the first carrier. The savings are on everything else it takes to put a cell site in place. Other technologies require "splitting" cells, often removing an existing cell and replacing it with two others to add capacity. There is a big balancing act because you can't reuse the same frequency in adjacent cells, and hence use up your available frequency in a more profligant manner.
Adding wideband CDMA to an IS-95 system would probably mean in essence adding another carrier utilizing, I've heard said on these threads, 15 MHz. It would probably use more for the downlink than the uplink because there would likely be a greater need for capacity in that direction. The A/B band carriers will have to make a choice as to whether three carriers will be sufficient to service those with only voice requirements allowing them to utilize half their spectrum to pursue the data market. Allowing for handoff (mobility) also eats up a lot of capacity. My suspicion is any wideband solution won't allow for handoff, just the ability to make large data transfer untethered but not while cruising down the freeway at 70 mph - though I could be wrong.
Adding any sort of CDMA, wideband or otherwise, raises the same sort of issues the operators have been facing with their overlays of analog cellular. The system spacing is not designed for CDMA, though much of it can probably be adapted sub-optimally. You've got the issue of migration of your existing customer base as you take spectrum away from the incumbent technology, probably the most intractable problem the operator faces. But for the most part, you just put in new equipment, maybe reusing the existing switch (a la the Vodaphone trial), possibly reusing some of the BTS control equipment with modifications. Of course, the vendors prefer to sell lots more equipment.
A question the analog operators have to be asking themselves now is do they continue to invest in capacity as the wireless market enjoys the spurt of growth from the price wars with PCS and risk seeing that investment stranded as their churn increases with the expiration of one-, two- and three-year contracts and defections to PCS?
Chris Reeder, does this explanation pass the sniff test? To all, I'm at a lake with no wideband CDMA to hook a computer up to next week, so hold the questions. |
