quirements will be different depending on which radio interfaces are being used. All CDMA transition plans have some things in common. The transition will be a lot easier to manage with multimode user terminals. Having a large community of dual- mode cell phones means that we can introduce CDMA more slowly than the user ter- minals. The CDMA system may have high blocking, but subscribers will not experience ineffective attempts because their calls will overflow onto the old system. As the popu- lation of dual-mode user terminals increases, we can increase the share of CDMA radio spectrum. Over time, the system will be nearly all CDMA, the result of a seam- less transition. Changing a wireless air interface is a complex task. Both the legacy air interface and CDMA have engineering issues, reuse rules, and interference management that have to continue during the migration for a seamless transition. 33.6.1 Greater radio efficiency with CDMA The principle behind the migration to CDMA is that the FDMA and TDMA air inter- faces are far less efficient than CDMA. They support fewer calls and serve less demand per megahertz of bandwidth per cell, making the migration to CDMA economical. Any FDMA or TDMA system can benefit from the increased efficiency of CDMA. Any of these technologies can allow partial radio spectrum conversion so that we can intro- duce CDMA in part of the bandwidth. We start by introducing dual-mode CDMA user terminals into the market. Once there is a substantial community of CDMA-ready sub- scribers, we can begin to convert radio spectrum. The idea is to take some piece of radio spectrum from the existing system and turn it into a CDMA carrier. The remaining spectrum should be enough to handle the re- maining legacy subscribers. As the legacy population decreases, we can convert another piece of spectrum into another CDMA carrier. Let us take an example, a 50-MHz GSM system with 25 MHz in each direction, 125 two-way GSM carriers with eight channels each. Using N = 4 with three-sector cells, we have 10