RTX Implements DECT Wireless Local Loop (WLL)

Over 120 million DECT systems have been sold over the last 12 years. No other technology is as reliable or as cheap. DECT offers wireless communication for simultaneous transfer of both voice and data through a wireless network. By combining IP and DECT technology, RTX has implemented a Wireless Local Loop technology that can be used on a nationwide scale.

The DECT Standard

The Digital Enhanced Cordless Telecommunications (DECT) standard is a radio technology designed for voice and data networking applications. A DECT sy-stem is made up of a DECT Fixed Part, a connection to one or more base stations (FP’s), and one or more DECT Portable Parts (PP’s); handsets or cordless phones. There is virtually no limit to the size of the infrastructure in terms of base stations and cordless terminals.

The DECT standard was originally designed with the Air interface as well as the protocol SW that interfaces to the backbone net optimized for Wireless Local Loop as well as PABX systems and dome-stic cordless phones. All applications can make use of the same RF spectrum in the same locations, but only because the hand-over mechanisms are dealt with in the PP’s and not the FP’s. This is unlike the cell phone systems in which all these features are controlled by the base stations and the backbone net. WLL infrastructure based on DECT is therefore very easy to install and maintain as there is no need for specifically detailed frequency and network planning.

The DECT radio interface is based on Frequency Division Multiple Access, Time Division Multiple Access, Time Division Duplex (FDMA/TDMA/TDD) radio access methodology. Basic DECT uses 10 carrier frequencies in the radio spectrum. The time spectrum is subdivided into 10ms timeframes. Each frame is split into 24 timeslots. This structure allows for 12 simultaneous 32kbit/s full duplex voice connections encoded using the G.726 codec. A total spectrum of 120 duplex channels is therefore available to a DECT base station. Infrastructures using the DECT technology can support traffic comparable to a large number of users in office environments.

The capacity in a DECT system is very high and has been calculated to be 6000 erlang/km²/floor. All communications in a DECT system are secure and PIN numbers are used in challenge response mechanism permitting users access to a DECT network. The same mechanism can be used to create a cipher key for the encryption of voice and data sent over the air.

Roaming

As the DECT portable device moves, the signal strength from surrounding base -stations is measured. When a device moves towards a new base station, a second full duplex channel is opened, and data is transmitted in parallel to both base stations. When the new communication is established, the original channel is closed. The diagram on page 7 shows an RTX IP DECT WLL network for both rural and urban configurations.

RTX System Components for WLL System

 - RTX Sync and Sync/Switch Unit

This Sync Unit provides the interface between a number of base stations and the backbone net. One Sync Unit supports up to 24 base stations and provides GPS synchronisation in order to synchronise the entire system. The communication between this Sync Unit and the base stations is done using a VDSL link. The data transfer rate downstream from the Sync Unit to each FP is 20 M bit/s, and it is 6 M bit/s upstream from FP to Sync Unit. This can be achieved at a distance of 1.5 km between base stations and Sync Units. The connection to the IP network is made by use of a 24 port VDSL modem. The Sync/Switch Unit provides the interface between 6 base stations and the backbone net, like the RTX Sync Unit it also provides GPS synchronisation for the Base stations. Connections to the IP based backbone net and the base stations are made by use of Ethernet cables.

Multiple DECT cells and clusters are connected using IP communication, and are synchronized by the built-in GPS receivers in the Synch and Synch/Switch Unit.

- RTX base stations

The air interface is designed as a zero blind slot RF solution, which means that all the 24 timeslots as defined in the DECT standard can be used. A connector for a WLAN Access point board is integrated. Additional RF filtering is designed into the system in order to deal with potential RF system issues, when integrating the two very different RF solutions. The connection to the backbone net is made via an RTX Sync Unit or an RTX Sync/Switch Unit, the connection to the Sync/Switch Unit is made using a normal Ethernet connection, and the connection to the RTX Sync Unit is done using a VDSL connection.

- Outdoor Repeater

A DECT repeater is simply used to extend the range of the FP’s for use in areas where high capacity is not needed. In this way, the range extension can be achieved without any additional wiring apart from a mains supply to the repeater. The repeater is designed using built-in directional antennas. By integrating the directional antennas into the repeater housing and implementing timing advance, we support long range. A 12 km range between an RTX Base station and an outdoor repeater has been successfully tested.

- WTLU, wireless telephone extension unit

The WTLU is simply providing a normal PSTN interface, enabling users to connect normal PSTN phones to the WLL systems. From a DECT WLL point, the unit works like a PP.  The WTLU can also be configured to function as an indoor repeater. The WTLU is plugged directly into a mains socket, and no additional mains net adaptor is needed.

- DECT desktop terminal

This phone looks like a traditional PSTN phone, but the inside reveals a DECT PP with normal DECT PP hardware. The power supply is normally provided by means of a mains net adaptor, however a back-up -battery is included to allow for emergency calls in case the mains supply is missing.

Why RTX IP dect WLL

DECT is a mature technology for use in WLL systems, as it has been in use for such applications for more than 10 years now. One of the new contributions from RTX in this market place is the use of “off-the-shelf” optimized DECT specific components that were originally designed for domestic use, in cost optimized products for the consumer market. This is partly due to the use of some very flexible CMOS, BB IC’s and RF IC’s developed by SiTel Semiconductor. These standard DECT chips have been combined with very power-full general purpose DSP’s and other system components. The combination of the flexible com-ponents and RTX’s skills and -experience in developing SW and HW for DECT systems has enabled RTX to design and manufacture very competitive products for this market place.

The mix of an IP based Core Network and the use of VoIP and SIP gives substantial savings compared to traditional WLL systems.