corDECT Wireless in Local Loop system based on Digital Enhanced Cordless Telecommunications (DECT) standard or European Telecommunications Standards Institute (ETSI). DECT defines a wireless communication standard between a Fixed Part (FP) and Portable Part (PP). The communication takes place using Gaussian Frequency Shift Keying (GMSK) Modulation. The access technique used in Multi-Carrier Time Division Multiple Access (MC-TDMA), a new technique, in which frequency can change from one time slot to another. DECT uses a frame of 10 msec in which 24 time slots are defined. The communication between FP and PP uses time-division duplex in which 12 time slots are used for each way transmission. In a 20 MHz frequency spectrum used by a DECT system. 10 carriers are defined. Thus for communication from FP to PP, 10 carriers can be used in each of the 12 time-slots, amounting to 120 channels. In DECT, no channel is assigned to any FP or PP. Any PP can select any of the 120 channels to communicate to a FP using Dynamic Channel selection (DCS) algorithm. The algorithm requires the PP to measure Receive Signal Strength (RSSI) on all the 120 channels. It locks to the CBS Providing strongest signal and then maintains a RSSI table for all other channels. When a PP wants to establish a connection. It chooses a channel providing lowest interference. Even when the communication is taking place, the RSSI table is periodically updated and if a channel with less interference than the current one is found, a seamless handover to the new channel takes place.

ETSI has chosen 1880-1935 MHz band for DECT operation. Normally any 20 MHz band out of this spectrum can be chosen for operation. Each DECT channel can carry 32 kbps of payload, and the voice coding used in DECT is 32 kbps ADPCM. It is possible for two DECT channels to be combined to provide 64 kbps data communication between PP and FP. corDECT uses DECT interface Unit and Compact Base Station to implement DECT Fixed Part and a Wallset and Handset to implement DECT Portable Part.

Network Interface to corDECT

The corDECT system has been designed such that it could be easily integrated into any country’s network. The system interfaces to the network on EI (2.048 Mbps) lines as per ITU-T G703 standard. In one of the configuration the corDECT system acts like a switch with its local loop. The numbering plan is flexible so that it can be modified as per the requirements. The tones, announcements, metering charging, switching, routing and special services are provided by the switch part of the corDECT system. Alternatively the corDECT Wireless in Local Loop can be configured as a Access Network connected to main exchange on ITU-T specified access protocol.

The corDECT Wireless in Local Loop has been designed to be a modular system. The basic unit provides service to 1000 subscribers. Multiple corDECT systems can be connected together using a transit switch. The system has been designed in such a way that the initial investment for fixed part is very low and most of the cost is incurred on the subscriber unit, which needs to be obtained only when the operator signs up a subscriber. Further, since corDECT Wireless in local Loop does not require frequency planning the installations need not be coordinated. Thus the low cost makes the system one of the most versatile Wireless in Local Loop system available today.

corDECT Wireless in Local Loop

System Architecture

The corDECT system is designed to provide a cost-effective wireless high-quality voice data connection in dense urban as well as sparse rural areas. It has eight major subsystems. There are :

DECT Interface Unit (DIU) Performs switching system control and Interfaces corDECT WLL system to telecom Network

Compact Base Station (CBS) Provides wireless access in an area and supports
twelve simultaneous full duplex channels.

Wallset (WS)                                           A wireless fixed terminal, connected to any standard
                                                                   telephone, modem or fax machine and or to a
                                                                   personal computer.

Handset (HS) A portable telephone providing voice

[Under Development] service to a user

Multiwallset (MWS) Wireless fixed terminal, providing

[Under Development] four independent (telephone, fax, modem) connection in a building.

Base Station Distributor (BSD) Connects four remotely-located

[Under Development] CBS’s to the DIU using the EI links on radio or fiber.

Relay Base Station (RBS) A relay device to extend the CBS

[Under Development] wireless coverage to 25 Kms.

Network Management System Manages multiple DIU’s and their

(NMS) [Under Development] associated CBS, Wallset, Multiwallset, BSD’s, RBS.

1. DECT Interface Unit (DIU)

The DIU is a DECT exchange for wireless subscribers and provides an interface to Public Switched Telephone Network (PSTN). Functions such as call processing. CBS powering, and PCM/ADPCM transcoding. DECT Network layer and Link layer functions are handled by the DIU. System operation and maintenance (O&M) and remote fault monitoring can be performed from the DIU or alternatively from a remote location using the Network Management System. The interface to the PSTN is via EI digital links. An optional subscriber max (SMUX) unit in the DIU converts the EI interface to 30 junction lines which can be connected to two-wire subscriber lines of an exchange.

2. Compact Base Station (CBS)

The CBS is a small, unobtrusive pole-mounted or wall-mounted unit. Each CBS serves, one cell, providing upto 12 simultaneous speech channels. The cell radius depends on the propagation environment and antenna gain of the handset Wallset. Typically, it ranges from 150m- 5Kms. The CSB has two antenna for diversity. A directional antenna with significant gain can be used when coverage required is either confined to certain directions, or the coverage is divided into sectors covered by different CBS’s. An omni-directional antenna could be used, such omni-directional antenna with 2dB, 4 dB and 6dB gain is available. The CBS is interface to the DIU using 3 standard subscriber pairs from the existing loop plant. The CBS’s are interfaced to the DIU through the Base Station Distributor (BSD) unit.

3. Base Station Distributor(BSD)

The Base Station Distributor is an optional unit used when a cluster of CBS’s are to be located some distance away from the DIU. The BSD is connected to the DIU or EI lines and each EI line carries signals for four CBS’s. the BSD demultiplexes the signal on the EI line and feeds it to the four CBS’s. the four CBS’s are connected to BSD, each using 3 pair of 0.6 mm twisted pair copper wires. The maximum distance supported is 1 Km. The copper wires carry both power and signals from BSD to CBS.

4. Wallset (WS)

The Wallset is a small wall-mounted unit with an external antenna and powered from a/c mains. An internal battery provides back-up in case of power failure. The external antenna provides gain and extends the range of a CBS in area where CBS density is low. The Wallset provides a standard RJ-11 telephone socket so that any telephone, fax machine, modem or even a payphone can be connected to it. The data rate supported on modem is typically 9600 kbps as the voice is coded before transmission on air.

5. Handset (HS)

The Handset is a small, lightweight, portable unit operated from rechargeable battery. It allows the user to call from with in the coverage area of any of the CBS connected to one DIU. The Handset has intelligent to handover seamlessly from the CBS to another. The same IIS can be used within different DIU’s by appropriate re-registration when moving from one location to another.

6. Multi-Wallset (MWS)

To cater to multiple subscriber at the same geographical location, an office or apartment block, a Multi-Wallset can be used. This unit support upto four subscribers thereby substantially reduce cost per subscriber. Further, the installation of a Multi-Wallset for four subscriber is on different from that of a signal Wallset for one subscriber. The Multi-Wallset provides each subscriber an independent line.

7. Network Management System (NMS)

The corDECT system provides a Network Management System (NMS) which could be used to manage upto 30 DIU’s and their subscribers. The NMS is interface to the PSTN on an EI link and could be located anywhere on the network. A 64 Kbps link with x.25 protocol is provided between the NMS and every DIU that it managers. The NMS performs the operations and maintenance function for all the DIU’s and the subscribers connected to them. It monitors the health of every card on each of the DIU’s as well as the CBS. Using silent polling, it can find out the health of any Wallset or handset. The NMS carries out subscriber management for all subscribers. When the DIU’s are configured as independent exchange, the NMS also provides billing for all the subscribers.