PHS-WLL system consists of inexpensive WLL Cell Stations (WCSs), which cover a relatively small area. PHS-WLL system can thus be installed economically and yet flexibly as a high-quality access line to main telecommunications networks in various locations, from urban to rural areas. On the other hand, CDMA-WLL system encompasses expensive Base Transceiver Stations (BTSs), which however can handle a very high volume of telecommunications traffic. CDMA-WLL system is suited for use as an access line from suburban areas to big towns, where there is high demand for large volume of telephone traffic.

Recent years have seen a variety of WLL/FWA (Fixed Wireless Access) systems being introduced into the telecommunications market. Because respective systems have strengths as well as weaknesses in comparison to others, there have been no clear definitions as to the most appropriate areas of application for these systems. Telecommunications administrations as well as carriers, therefore, have been calling for compilation of guidelines, which should help them in choosing the most appropriate system, when it comes to the expected installation costs, phone traffic volume, needs for data communications as well as geographical features in their planned installation districts.
In response to such voice, the PHS MoU Group’s Business WG (BWG) compiled the “Study Report of System Comparison between PHS-WLL and CDMA [IS-95]-WLL.” This report examines these two most popular WLL systems from multifaceted viewpoints. The highlights of the report are as follows.
This report in its entirety is also available at the PHS MoU Group’s website at http://www.phsmou.org.

1. Cell range

Range of a cell for WCS or BTS is determined by the station’s channel capacity and the density of demand for telephone lines in the area concerned. For example, in a flat suburban area with 50 telephone subscribers per sqkm, the range of a BTS with three sector-directional antennas using four radio frequencies (RFs) is calculated as a radius of 2.6 km. Under the same circumstances, a PHS-WLL cell station consisting of two WCSs can cover a radius of up to 1.3 km (see Fig. 1).
It is clear that, due to its limited channel capacity, CDMA-WLL system coverage is far smaller than its ability of reaching 30 to 40 km.

2. Size of WCS and BTS

WCS is a considerably small radio station that weighs 10 to 15 kg and consumes 40 to 50 W of electric power. On the other hand, four-RF BTS including the power supply unit and batteries weighs about 1 ton, and requires about 10 kW of electricity (see Fig. 2). PHS-WLL system, therefore, can easily respond to the demand for much larger traffic volume compared to CDMA-WLL, by installation of additional WCSs inexpensively and other means. Moreover, WCSs can be readily operated by solar power; but CDMA-WLL BTSs require constant supply of commercial electricity, in addition to a huge investment for the installation.

3. Comparison of system costs

It is not an easy task to compare the system costs for PHS-WLL and CDMA-WLL, because their networks and systems designs largely differ depending on the aforementioned circumstances. Nonetheless, calculations were made using known values of equipment in order to obtain cost curves for assumed cases of systems installation in three different locations ? namely rural areas, towns and suburban areas (see Fig. 3).
Fig. 3 indicates that PHS-WLL has cost advantage over CDMA-WLL in areas with not so much demand for traffic, and CDMA-WLL has cost advantage over PHS-WLL in areas with high telephone demand. Intensity of the demand is thus a crucial factor in choosing the most appropriate WLL system.

4. Appropriate areas of application for WLL systems, and conclusions

Not only in rural areas, but also in towns with telephone demand ranging approximately from 3,000 to 5,000 subscribers, PHS-WLL-based access lines can be constructed economically because of its low system costs. On the other hand, CDMA-WLL-based access lines, which are costly but yet capable of handling high traffic volume, generally require a high demand of more than 5,000 subscribers to minimize the system costs per line. Fig. 4 shows appropriate areas of application for PHS-WLL and CDMA-WLL, respectively.