Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-01-07
2002-08-20
Cangialosi, Salvatore (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S337000
Reexamination Certificate
active
06438117
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless telecommunications, and specifically to advanced cellular telephone networks.
BACKGROUND OF THE INVENTION
The Global System for Mobile (GSM) telecommunications is used in cellular telephone networks in many countries around the world. GSM offers a useful range of network services and standards. Existing GSM networks are based on time-division multiple access (TDMA) digital communications technology. In a TDMA-based cellular network, each mobile subscriber unit communicates with only a single base station at any given time. When a subscriber moves from one cell to another, a “hard handover” takes place, in which the base station with which the subscriber has been communicating breaks off its link with the subscriber, and a new base station takes over.
Code-division multiple access (CDMA) is an improved digital communications technology, which affords more efficient use of radio bandwidth than TDMA, as well as a more reliable, fade-free link between cellular telephone subscribers and base stations. The leading CDMA standard is IS-95, promulgated by the Telecommunications Industry Association (TIA). This standard provides “soft handover” (or “handoff”) capability, wherein in moving from one cell to another, the subscriber unit is temporarily in contact with two or more base stations at the same time. This soft handover, which is made possible by the code-division approach, decreases the likelihood of a loss of connection, which can happen frequently in hard handovers.
PCT patent application PCT/US96/20764, which is incorporated herein by reference, describes a wireless telecommunications system that uses a CDMA air interface (i.e., basic RF communications protocols) to implement GSM network services and protocols. Using this system, at least some of the TDMA base stations (BSSs) and subscriber units of an existing GSM network would be replaced or supplemented by corresponding CDMA equipment. CDMA BSSs in this system are adapted to communicate with GSM mobile switching centers (MSCs) via a standard GSM A-interface. The core of GSM network services is thus maintained, and the changeover from TDMA to CDMA is transparent to users.
Hybrid cellular communications networks, incorporating both GSM and CDMA elements, are also described in PCT patent publications WO 95/24771 and WO 96/21999, and in an article by Tscha, et al., entitled “A Subscriber Signaling Gateway between CDMA Mobile Station and GSM Mobile Switching Center,” in Proceedings of the 2nd International Conference on Universal Personal Communications, Ottawa (1993), pp. 181-185, which are incorporated herein by reference. None of these publications deals with specific issues of how to perform efficient handovers of subscriber units between different base stations in such hybrid networks.
PCT patent application PCT/US97/00926, which is also incorporated herein by reference, describes methods of intersystem handover between CDMA and TDMA BSSs in a hybrid GSM/CDMA telecommunications system. A GSM/TDMA BSS generates pilot beacon signals in accordance with CDMA technology. During a telephone call, a subscriber unit detects the pilot signals and notifies a base station controller that the signals have been detected. The subscriber unit is then handed over from the CDMA to the TDMA BSS without interrupting the call.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods and apparatus for use in a mixed TDMA/CDMA cellular communications network.
It is a further object of some aspects of the present invention to provide improved methods and apparatus enabling handover of a subscriber unit between TDMA and CDMA base stations without interrupting communications.
In preferred embodiments of the present invention, a mixed GSM/CDMA cellular communications system includes both TDMA and CDMA base stations, jointly controlled by a mobile switching center (MSC). Systems of this type are described generally in the above-mentioned PCT patent applications, which are incorporated herein by reference. A subscriber unit in the system, also referred to herein as a mobile station (MS), is capable of communicating with both types of base stations, by appropriately switching between TDMA and CDMA air interfaces, while preferably using GSM network protocols over both types of interface. It is a feature of preferred embodiments of the present invention that the communications system may be based on an existing GSM/TDMA infrastructure, with the addition of CDMA BSSs, and with substantially no other modification to the existing infrastructure.
In order to determine when a handover should take place, a MS in communication with a current base station of one type (CDMA or TDMA) monitors RF signals originating from another base station, which may be a base station of the other type (TDMA or CDMA, respectively). A message sequence between the current base station and the MS enables the MS to acquire appropriate synchronization information with regard to the new base station, and report back on this information to the current base station. The information is used by the system to enables the MS to establish an air interface with the new base station, whereupon the handover takes place without substantially interrupting communications between the MS and the network.
In the context of the present patent application, such handovers between base stations are referred to as “mobile-assisted handovers.” Mobile-assisted handover is used in GSM and in CDMA systems known in the art, wherein a mobile station measures and reports on the strength of signals received from a base station transceiver in a neighboring cell before being handed over to that cell. In hybrid GSM/CDMA systems that have been proposed to date, however, mobile stations are presumed to be capable of receiving signals from either a CDMA or a TDMA base station at any given time (or a CDMA beacon associated with a TDMA base station, as in the above-mentioned PCT patent application PCT/US97/00926), but not both, and are therefore not capable of providing this type of assistance. The provision of mobile-assistance in accordance with the principles of the present invention enables handovers to be conducted more smoothly and reliably than would otherwise be possible.
In some preferred embodiments of the present invention, the MS switches between TDMA and CDMA operation in the course of a telephone call, according to instructions received from the base station with which the unit is in communication. Before the handover is to take place, the MS receives signals from both TDMA and CDMA base stations, and reports back to the base station regarding the signals it is receiving. The information thus reported is reported back to and used by the BSC to initiate the handover. Preferably, the MS comprises a single radio transceiver, and therefore, at any given moment the MS can communicate with either the TDMA or CDMA base station, but not both. (In accordance with the principles of IS-95, however, as described hereinabove, the unit can communicate with more than one CDMA base station at once.) It is noted further that each GSM/TDMA base station has its own synchronization clock, to which the MSs in communication therewith are synchronized, while the CDMA base stations are mutually synchronized to a real time of day. Therefore, in switching between the TDMA and CDMA stations, the MS in each case acquires and synchronizes its operation to the appropriate clock signal without substantially interrupting the telephone call.
In some of these preferred embodiments, the MS is in communication with a CDMA base station, when it is determined that the unit may be handed over to a GSM/TDMA base station. CDMA transmission by the MS transceiver is interrupted temporarily, during which time the unit performs a GSM neighbor scan, generally in accordance with GSM standards, to acquire and synchronize to the TDMA base station. Preferably, the CDMA transmission is interrupted for a single frame, typically 20 ms long, creating an idle
Grilli Francesco
Jain Avinash
Cangialosi Salvatore
Minhas Sandip S.
Ogrod Gregory D.
Qualcomm Incorporated
Wadsworth Philip R.
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