Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1996-11-14
2001-06-05
Olms, Douglas (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S324000, C370S503000, C375S145000, C375S149000, C375S356000, C375S357000, C455S502000
Reexamination Certificate
active
06243372
ABSTRACT:
FIELD OF INVENTIONS
The inventions herein pertain to the field of communication networks, including methods and apparatus for acquiring and maintaining synchronization in a wireless communication network.
BACKGROUND
In a typical wireless communication network, one or more base stations are selectively positioned within respective, defined geographic areas or cells, and are used to transmit and receive communication signals to and from, respectively, one or more remote stations, (e.g., mobile or cellular telephone handsets), located within the respective cell. In particular, the base stations act as both intermediary points by which a communication path may be periodically established and maintained with respective remote stations, as well as end points of a hierarchial stationary network, which also includes an overlay or backbone network, such as, e.g., a public switched telephone network (“PSTN”).
A selected communication protocol defines a method in which the various remote stations can communicate with one or more base stations of the communication network, e.g., in order to place and receive telephone calls. The communication protocol will preferably provide air-channel agility between respective base stations and remote stations, while also providing a secure voice or data link. A fundamental concern of the selected communication protocol for a network is the ability of the remote stations to communicate with the base stations in a simple, flexible and rapid manner, e.g., so that a remote station is not required to wait to establish a communication path, and/or so that a hand-off of an active call between base stations in a mobile network is transparent to a respective remote station. In this respect, the ability to acquire and maintain synchronization between a base station and a mobile station is an important consideration. Further, network-wide synchronization should be established and maintained for optimal operation of a mobile communication network, e.g., to minimize interference problems otherwise caused by non-synchronized base and/or mobile station transmissions in the same, or adjacent, cell location(s).
For example, in U.S. patent application Ser. No. 08/284,053, assigned to the assignee of the present application and which is hereby fully incorporated herein by reference, a protocol for a wireless communication network is described for use with a wireless communication network, wherein each base station transmits over a set of time-division air channels, or time slots, by transmitting in time slots in sequence, referred to herein as an over the air loop. Each base station time slot polling transmission is followed by a first gap (or “guard time”), a remote station transmission (if a remote station attempts to communicate), and a second (guard time) gap, before the base station transmits over the next time slot. A remote station receiving a base station polling transmission in an (unoccupied) time slot, may then transmit information to the base station over that respective channel, e.g., to establish a communication link with that base station via the respective polling path. Each base station may thereby maintain communication with as many remote stations as there are available time slots in its over the air loop.
In accordance with this protocol, handoffs between base stations are preferably initiated by the respective mobile station, which monitors available time slots from the same and competing base stations during unused time slots. A mobile station may handoff within the same over the air loop to establish communication in a new time slot of the same base station, or may handoff in such a manner to establish communication within a over the air loop of a different base station. In the latter case, a “base station controller” may assist in transferring the call from one base station to another.
Successful operation of the aforedescribed protocol depends on the stability of respective internal base station and mobile station transmission timing. In particular, in order for respective base station and mobile station transmissions to stay locked to each other within the respective time slot interval(s), the mobile station transmission timing must be synchronized with the base station transmission timing. For example, in U.S. Pat. No. 4,494,211, issued to Schwartz, a synchronization system for a “master/slave” satellite pair is described whereby each satellite of the pair transmits timing signals synchronized to its own respective clock, receives the timing signals from the other satellite and independently measures the difference between the transmission and reception of the respective timing signals. The time difference measured by each satellite is then transmitted to the other, wherein each utilizes the respective time difference measurements to calculate the asynchronism between the respective clocks and the range between the satellites. Based on those calculations, the “slave” satellite clock is then adjusted so that the calculated asynchronism is reduced to within an acceptable difference. As can be seen, the Schwartz synchronization system is both complex and requires periodic signaling bandwidth in both directions dedicated to the transmission of clock signals and respective measurement calculations.
Thus, it would be advantageous to provide a method and network architecture for the simple and rapid acquisition and maintenance of synchronization between respective base stations and mobile stations of a mobile communication network, without adversely impacting over-the-air bandwidth or the robustness of an established communication path. It would be further advantageous to provide the capability in a mobile communication network for network-wide synchronization, including both “intra-network” synchronization, i.e., between respective elements of the mobile network itself, and “inter-network” synchronization, i.e., between the mobile network and a respective linking overlay network.
SUMMARY OF THE INVENTIONS
The present inventions provide methods and corresponding network architectures for acquiring and maintaining synchronization between discrete elements of a communication network and between a communication network and a linking overlay network, such as, e.g., a public switched telephone network (“PSTN”) and achieving serial data synchronization from the overlay network to the serial audio/data interface in the remote station. As part of a preferred communication protocol, base stations which are synchronized to a PSTN of a wireless communication network periodically transmit a preamble. A remote detects the preamble and sets its counter to an initialized state based on the received preamble. An early/late analysis of subsequently received base station preambles is used to adjust both the remote station timing and to adjust the output frequency of the remote station master clock and codec clock to effectively maintain end to end synchronization with the respective base station and the PSTN throughout the duration of an established communication link.
In a preferred embodiment, each base station of a mobile communication network is provided with an adjustable master clock, which generates a respective base station clock signal. The base station clock signal is used to regulate a frame timing counter, which controls the timing for the respective transmission and reception of digital data frames within each time slot interval of the base station's over the air loop. In accordance with one aspect of the inventions, the base station master clock frequency is continually adjusted at a serial data rate level to match the serial data rate of the overlay network. By way of example, a low frequency component signal is derived from a higher frequency base station clock signal and compared to a reference signal derived from an incoming serial data link of an overlay PSTN network, such as, e.g., a T
1
facility. A signal representing the phase difference between the low frequency signal and the reference signal is then used as a feedback adjustment to the
Jensen Ryan N.
Lindsay Charles L.
Petch Byran K.
Hom Shick
Lyon & Lyon LLP
Olms Douglas
Omnipoint Corporation
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