Wireless telecommunication system

#This invention relates generally to a telecommunication system and more particularly to a wired telecommunication system that can establish a communication path between a selected wired communication path and a cordless telephone.

#A wired telecommunication system such as, for example, a Private Branch Exchange (PBX) has attracted widespread use for business communications because of its flexibility of allowing one to design specific services in accordance with one's business needs. Nevertheless, the equipment with which the PBX users communicate are traditional telephones which are hardwired to the PBX switch. This undesirably limits the mobility of the user who is tethered by the cord connecting the handset of a telephone to its fixed base-unit. Although the traditional telephones can be replaced with cordless telephones to increase the mobility, this replacement, however, does not provide a desirable solution because the portable handset of a cordless telephone must interact with the fixed base unit associated therewith. This being so, as the distance between the portable handset and the base unit increases, the signals to and from the handset are accordingly attenuated. In addition, significant interference is caused by the signals from other cordless telephones which are also connected to the PBX switch. As such, the mobility gained by using a cordless telephone is not only limited, but also results in a compromise of signal quality.

#Attempts have been made to solve the above-identified problems. One such attempt involves the use of a pico-cellular communications system such as described in the publication by D. .ANG.kerberg, "Properties of a TDMA Pico Cellular Office Communication system," IEEE ICC 1989, pages 186-191. In such a system, fixed base units are connected to a centralized system manager to provide a transceiving function for the various different pico-cell sites. The system users are supplied with portable handsets which can interact with any one of the fixed base units. As a mobile handset moves from a pico-cell site of a first base unit to another pico-cell site of a second base unit, the provision of the transceiving function is switched from the first to the second base unit under the control of a system manager. Although this switching scheme is intended to solve the signal attenuation problem, it does, however, still require relative high transmission power. The fact that the transmitted signals contain relatively high power may preclude such system from operating in certain radio-frequency bands. Moreover, although this system manages to eliminate the above-noted interference by using a time-division-multiple-access (TDMA) scheme (i.e., each portable handset can only transmit and receive signals during its preassigned time slots), the received signals in this system are corrupted by another type of degradation, namely, multipath distortion. Such distortion occurs when various mutually delayed version of the signal travel along various paths of different lengths. Furthermore, to achieve a significant capacity a substantial spectrum allocation may be required.

#FIG. 1 is a block diagram of a Time-Division Duplex digital system in accordance with the principles of the invention;

#FIG. 2 is a block diagram of the fixed radio station of FIG. 1;

#FIG. 3 is a block diagram of FIG. 1 including structure of the digital router,

#In this invention, a wired telecommunication system such as, for example, a Private Branch Exchange (PBX) is now provided with a wireless capability. More specifically, a PBX is provided with wireless/portable access to permit a user to roam freely over the area served by the wired PBX system. The system uses existing redundant wiring available in a typical PBX installation to subdivide the covered area into a number of small cells, typically, one cell per room. Radio and switching techniques for this system can result in several benefits such as: A) Low initial cost for a system with limited coverage/capabilities, with expansion costs being proportional to the capability being added; B) Simple, FM-based radio interface which is not impaired by multipath propagation and, through extensive frequency reuse, providing good system capacity at low cost; and, C) Low power operation that allows the use of the ISM band (902-928 MHz) under Part 15 of the FCC rules without spread-spectrum techniques and with low risk of interference to or from the system.

#The concept is based on a so-called "femtocellular" architecture. The prefix "femto" is the next one down after "pico" and is used to emphasize the fact that the coverage area is subdivided into cells that are even smaller than those for so-called "pico cellular" systems as proposed in the publication by D. .ANG.kerberg entitled "Properties of a TDMA Pico Cellular Office Communication System," 39th IEEE Veh. Tech. Conf., San Francisco, Calif. (May 1988) pp. 186-191. Typically, in this invention, there are one or more cells per room, and the interconnections among the various cell sites and the central PBX switch are accomplished through existing PBX wiring.

#In this system, the covered area is subdivided into very small cells, even smaller than those proposed for so-called "picocellular" systems. Thus, the system here disclosed is referred to as a "femtocellular" system. Many existing PBX's have redundant wiring in the walls; typically, there are redundant jacks in each room, and each jack carries four wire pairs, of which only two are used by most terminals. Additionally, these wires can easily support a bit rate of 1-2 Mbit/s. While this may not be very much as an aggregate bit rate, a femtocellular architecture would allow this bit rate to be re-used many times over the area covered and thus allow the wireless adjunct to provide the required capacity. Indeed, since PBX wiring is available in essentially every room, a cell can typically be a single room. Because signal propagation from room to room is not very good in the average building, the full spectrum can be reused in rooms which are only a few rooms away.