 they are able to keep most of their 2G and 3G equipment in place because LTE is backward compatible. Re-use of infrastructure for LTE depends on the age of the rest of the equipment in the cell site and the spectrum deployed. Technical details about LTE infrastructure are included in the next section. Carriers, particularly those using CDMA2000 technologies, will benefit from using a technology adopted by most carriers worldwide. One advantage is the potential to earn additional roaming revenue from carriers with compatible networks. Carriers make agreements with one another in which they receive payments when other sub- scribers use their network when they are out of range of their own carrier's network. In addition, many LTE handsets include HSPA chips SO that subscribers will be able to take their phones with them when they travel to GSM-based networks that use HSPA as well as on those with LTE. This is a particular advantage for business travelers who tend to generate more revenue than residential consumers. LTE is expected to be the predominant 4G protocol adopted by carriers worldwide. Manufacturers produce a wider choice of devices for networks which use the most prevalent air interfaces, because these networks represent a large pool of potential customers. Handset prices also tend to be lower because there are more choices and more competitive pressure. Frequency- and Time-Division Implementations of LTE The 3rd Generation Partnership Project (3GPP) standards group has defined two dif- ferent air interfaces for LTE. One of the air interfaces is based on Frequency-Division Multiplexing (FD-LTE) and the other is based on Time-Division Multiplexing (TD- LTE). In common usage, LTE refers to FD-LTE because it is expected to be imple- mented more widely. FD-LTE is also referred to as Frequency-Division Duplex (FDD), and TD-LTE is also referred to as Time-Division Duplex (TDD). Frequency-Division Multiplexing (FDM) requires paired frequencies, which means that one spectrum band is used for the downlink (from th