1. What is a wireless local loop?
In a telephone network, a wireless local loop (WLL) is a generic term for an access system that uses a wireless link to connect subscribers to their local exchange in place of conventional copper cable. Using a wireless link shortens the construction period and also reduces installation and operating costs.
2. Market for WLL systems
The telephone penetration rate differs greatly from one country or region to another, with some areas of the world not even having one telephone per 100 population. There is an urgent need to eliminate the backlog for telephone service, now estimated at over 50 million lines, in order to make global communications a true reality. Moreover, as countries move ahead with plans to privatize telecommunications businesses and introduce competition, investments in telephone network facilities are continually increasing. Subscriber access lines account for a relatively large share of the total investment in outside plant and equipment. If the cost of the local loop could be reduced, it is believed that telephone penetration would advance dramatically at one stroke.
The cost breakdown of installing subscriber lines, including the construction work is in Fig. 1.
The “last 300-meter” segment to each subscriber’s premises accounts for a large share of the total, in terms of both the work and cost involved. Using a wireless link for subscriber access makes it possible to initiate service in a very short period of time, because the installation of the wireless facilities involves only a limited amount of small-scale construction. Wireless links are especially effective for the last 300 meters.
As a result of the explosive diffusion of cellular phones in recent years, wireless equipment prices have dropped dramatically. The cost of installing a WLL system for subscriber access lines is now virtually the same as that of copper cable, though it can vary depending on the subscriber line length and local conditions. In the coming years, the superiority of WLL systems will only increase.
3. Technical requirements of WLL systems
The following conditions are required of WLL systems which are looked to as a replacement for existing copper subscriber lines.
1) Communications quality
Since a WLL system serves as the access line for fixed telephone sets, it must provide the same level of quality as conventional telephone systems with respect to such aspects as speech quality, grade of service (GOS), connection delay and speech delay.
In addition, since radio waves are used, careful consideration must be given to protection of confidentiality and terminal authentication.
2) Short construction period
3) Low cost
The overall cost must be low, including equipment, construction and maintenance costs.
4) Absence of interference with other wireless systems
A WLL system must not cause any interference with the operation of existing systems, such as microwave communications and broadcasting systems.
5) High traffic volume
One characteristic of a WLL system is that it must support a larger traffic volume per subscriber than mobile communications systems.
4. Comparison of WLL system technologies
WLL systems incorporating various types of wireless technologies are already being provided primarily for use in rural areas. A comparison will be made here of a number of typical technologies.
1) TDM/TDMA and P-MP communications equipment
Communications equipment based on time division multiplex/time division access (TDM/TDMA) technology and a point-to-multipoint (P-MP) system is relatively well suited for rural use, because it provides service coverage over a wide area encompassing the base station, relay stations and subscriber stations.
2) Fixed use of cellular systems
This approach involves using the wireless equipment of a cellular phone system, thereby curtailing the mobile function. Using cellular phones as the subscriber terminals makes it possible to hold down the system cost. The use of advanced speech compression technology increases frequency utilization efficiency, but speech quality deteriorates as a consequence.
3) PHS-WLL system
Similar to 2) above, the system cost is reduced by using PHS terminal technology and wireless equipment. Since the voice encoding system uses 32 kbps adaptive differential pulse code modulation (ADPCM), speech quality on a par with that of fixed telephony is obtained.
It is clear that a PHS-WLL system is superior in terms of speech quality and economy, among other aspects. In addition, while fixed use is the main objective at the time of system implementation, extensibility to mobile service in the future is also demanded in many cases. PHS is also advantageous in this respect, because the system was originally designed to provide mobile service.
5. Scope of application of each technology
In order to optimize the investment in equipment, suitable WLL systems can be used selectively according to the subscriber density as a main consideration. Fig. 2 maps the optimum scope of system application in terms of the subscriber density and distance from the local switch (LS).
6. Moves toward PHS-WLL system standardization
For the purpose of supporting WLL applications of PHS, the Association of Radio Industries and Businesses (ARIB) completed studies on the items noted below during 1995 for inclusion in Version 2 of RCR STD-28.
- Mobility limitation
- Expansion of service coverage
- Supplementary services for supporting fixed telephony service, including public telephone service (metering pulse, subscriber line testing and hooking)
7. Product example
A PHS-WLL system has already been commercialized and is scheduled to be implemented for commercial service in several countries before the end of this year.
Shown here is an example of a PHS antenna and cell station