Cell stations of the Personal Handy-phone System (PHS) consist of the cell station equipment and antennas, and form service areas with microcells having a radius of several hundred meters. The system configuration assures large capacity for accommodating subscribers because frequencies can be utilized repeatedly between microcells. On the negative side, however, it requires the installation of many cell stations. This means that cell stations must be compact in size and easy to install.
A new compact cell station has now been developed that weighs only 3-5 kg and has a volume of just 3-4 liters. Several types of cell station equipment and antennas are available, allowing various combinations for meeting the type of cell station installation requirements. As a result, cell stations can now be installed economically in a wide variety of locations.

Cell Station Installation Concept

The equipment cost has been reduced by sharing constituent units as much as possible among different types of installations. Moreover, future increases in traffic can be accommodated by installing additional units to expand the number of available communications channels. Construction work has been simplified by employing easy-to-use connectors, developing dedicated fixtures for each type of installation and adopting a unified interface between fixtures and cell station equipment. In addition, since most installations are outdoors, thoroughgoing measures have been taken to make sure the equipment is waterproof and dustproof.
Low-power cell station equipment has a volume of just 3-4 liters (A4 size) and weighs a light 3-5 kg, allowing easy installation in many different places.

Outdoor Installation Types

Cell station equipment for outdoor installation includes types that can be installed on public telephone booths, utility poles and building roofs. Digital circuits from the public switched telephone network are connected to the cell station equipment via an arrester. The equipment operates on a commercial AC 100 V power supply. If necessary, power can be supplied to the cell station equipment via a non-fuse breaker (NFB). A centralized power supply system using DC power supplied from the switching office is also possible.

Indoor Installation Types

PHS service areas also include indoor locations, such as underground shopping malls and department stores, in addition to outdoor service.
Indoor installations use two rod antennas about 20 cm in length. A flat antenna resembling a fire alarm is also available for inconspicuous installation on the ceiling.
Indoor service areas, such as in department stores, are much smaller per cell station than their outdoor counterparts, because it is more difficult for radio waves to be propagated through enclosed spaces partitioned by floors or walls. This makes it necessary to have a higher density of cell station equipment, including a cell station installation on every floor.
For that reason, compact indoor cell station equipment and a line concentration controller are used to configure service areas economically.

Basic Configuration and Functions of Cell Station Equipment

Cell station equipment primarily consists of an RF unit, a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) processing unit, an ADPCM (Adaptive Differential Pulse Code Modulation) transcodec unit, a digital network interface unit and others.
In addition to such basic capabilities as a function for terminating the line from NTT's digital network, other functions provided include one for remote control by a maintenance center and one for monitoring the overall operation of the equipment.

1) RF unit

The radio signal interface supports transmission and reception of /4 QPSK modulated signals using a 4-channel TDMA/TDD system that conforms to the RCR STD-28 specification formulated by ARIB's predecessor, the Research and Development Center for Radio Systems (RCR). The cell station equipment adopts a diversity system that assures stable transmission and reception even in environments susceptible to fading. It also incorporates a function for detecting interference on radio channels, which gives each cell station autonomous and distributed control over the assignment of radio control and radio communications channels. Other functions are provided for monitoring and assuring speech quality. These include a function that constantly monitors speech quality on the wireless circuit to a personal station (PS) and switches a call to another channel if line quality deteriorates due to interference, and one that works to restore speech quality following handover of a call to another cell station.

2) TDMA/TDD processing unit

This unit assigns communications channels to time slots on radio circuits and disassembles and assembles radio frames on control channels and communications channels.

3) ADPCM transcodec unit

This unit handles the conversion of the 32 kbps ADPCM signals used on the PHS radio links and the 64 kbps µ-low PCM signals used on NTT's digital network. Signal conversion is performed in accordance with ITU-T Recommendation G.726.

4) Digital network interface unit

PHS cell stations are interconnected to NTT's digital network via the I'-interface standardized by the Telecommunication Technology Committee (TTC). In addition to information concerning location registration, authentication and call setup, they also exchange information with the maintenance center using D-channel data packets. Because one slot is used for the radio circuit control channel in the basic configuration, each cell station accommodates three communications channels.

5) Monitoring and other functions

The cell station equipment incorporates a self-diagnostic function for automatically detecting any hardware failures. In the event a failure occurs, it is reported to the maintenance center via a D-channel data packet.

Types of Cell Station Equipment

1) Low-power cell station equipment

The low-power cell station equipment (20 mW) is used when the service area per cell station forms a microcell having a radius of 100-300 m. It is equipped with two antennas for diversity transmission and reception.
This unit can be mounted with a battery pack as an emergency power supply to assure continued use even during commercial power outages.

2) High-power cell station equipment

The high-power cell station equipment (100 - 500 mW) expands the service area per cell station to a radius of 300 - 500 m, by increasing transmitting power and improving antenna gain.
This unit is equipped with four antenna branches for improved receiver sensitivity and enhanced diversity reception against fading.

3) Compact indoor cell station equipment

The compact indoor cell station equipment is used in combination with a line concentration controller for installations inside buildings.
This compact cell station equipment provides transmitting power of 10 mW, which is sufficient to cover indoor service areas that generally require a cell radius of less than 50 m.
One compact cell station equipment can support up to three communications channels, enabling simultaneous communications on all the circuits accommodated in the line concentrator controllers located throughout the entire building.

Future Activities

Cell stations with higher transmission power and capable of supporting more communications channels are now being researched for future implementation in suburban areas. These include cell stations with fewer channels yet wide area coverage for use in low-traffic areas and cell stations with more channels plus wide-area coverage for installation in high-traffic areas. Cell stations with very high transmission power capabilities are also being examined for use in areas with extremely low traffic volumes.