nteroperability and migration. This system, designed by a collaboration called the 3rd Generation Partnership Project 2 (3GPP2), was known as CDMA2000 and was also adopted by operators across several other countries such as Korea, China, Japan, and Canada. While CDMA radio has some advantages, it requires phase synchronization to ensure correct opera- tion, especially for transfer of handsets from the radio in one cell site to the next (a process called handover). This presented operators with the challenge of how to achieve very accurate phase syn- chronization with cell towers spread all around the huge landmass of North America. The simple answer was to use the Global Positioning System (GPS). Most of the cell towers then were large transmitter towers, sometimes called macro cells. Because the frequency of the original 2G spectrum was quite low (~800-850 MHz), these radios gave very good coverage over a wide area and great reception inside buildings-as low frequencies tend not to be readily absorbed. This combination of long range and good propagation properties allowed operators quite a deal of flexib- ility in positioning their radio towers. Because of the open locations, all that was required was for the operator to put a GPS antenna on top of the tower and connect a GPS receiver to the CMDA radio. This was a widely implemented approach in North America at the time. When later generations of mobile required phase synchron- ization, operators outside the U.S. were somewhat hesitant about adopting a timing solution (GPS) seen to be controlled by the U.S. government. This reluctance to overly rely on GPS gave operators and standards organizations an incentive to develop alternative systems to distribute phase and frequency. One other factor determined whether phase was required for a mobile network. There are two tech- niques (see Figure 2-5) for separating the full-duplex channels (one uplink and one downlink) for a radio system. One technique (and historically the most common) was frequency division duplex (