Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1996-11-15
2001-01-16
Dolinar, Andrew M. (Department: 3747)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S560000, C455S562100
Reexamination Certificate
active
06175737
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the field of two-way wireless communication systems and more specifically to methods and apparatus for wireless communications in mobile telephone systems.
Conventional Cellular Systems
Cellular mobile telephone systems have developed due to a large demand for mobile services. Cellular systems “reuse” frequency within a number of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users (mobile stations). Each cell covers a small geographic area and collectively groups of adjacent cells cover a larger geographic region. Each cell has a fraction of the total amount of RF spectrum which is available to support cellular users located in the cell. Cells are of different sizes (macro-cell or micro-cell) and are generally limited to a fixed capacity. The shapes and sizes of cells are functions of the terrain, the man-made environment, the quality of communication and user capacity. Cells are connected to each other via land lines or microwave links and to the public-switched telephone network (PSTN) through telephone switches. The switches provide for the hand-off of users from cell to cell and thus from frequency to frequency as mobile users move between cells.
Base Station (BTS)
In conventional cellular systems, base stations, or base transceiver stations (BTS), are the interface between mobile stations and the rest of the communications system. A base station is usually located in the center of a cell. The transmitting power of a base station determines the cell size. A base station typically has between one and sixteen transceivers where each transceiver uses separate RF channels. Base stations have RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users (mobile stations) in the cell. The base stations employ forward RF frequency bands (forward carriers) to transmit forward channel communications to users and employ reverse RF bands (reverse carriers) to receive reverse channel communications from users in the cell. Conventional forward channel communications are static in that they employ fixed power, at fixed frequencies and have fixed sectors if sectorized antennae are used.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency domain duplex (FDD) signaling. Although time domain duplex (TDD) signaling, in which the forward and reverse channels take turns using the same frequency band is possible, such operation is not part of any widespread current cellular implementation.
The base station in addition to providing RF connectivity to users also provides connectivity to a Mobile Telephone Switching Office (MTSO). In a typical cellular system, one or more MTSO's will be used over the coverage region. Each MTSO can service a number of base stations and associated cells in the cellular system and supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
Base Station Controllers
In conventional cellular systems, base station controllers (BSC) monitor and control one or more base stations. The number of base stations controlled typically is between several tens and several hundreds. The principal tasks of the base station controllers are frequency administration, the control of a base station, and exchange functions. The base station controllers assign RF carriers to support calls, coordinate the handoff of mobile users between base stations, and monitor and report on the status of base stations. The base station controllers can be located at the same site as the base stations or at a different site. Base station controllers and base stations together form a functional unit referred to as the base station subsystem (BSS).
Mobile Services Switching Center
The mobile services switching center (MSC) is the interface between the cellular system and the PSTN. The MSC is a switching exchange (switch) for routing calls from the fixed PSTN network through the base station controllers (BSC) and the base stations (BTS) to individual mobile stations (MS). The MSC switch provides the network with data about individual mobile stations. Depending on the cellular network size, one or more interfaces to the fixed PSTN network may exist through one or more switches. The number of base stations controlled by a single MSC depends upon the traffic at each base station, the cost of interconnection between the MSC and the base stations, the topology of the service area and other similar factors.
Operation and Maintenance Center
The operation and maintenance center (OMC) has access to both the MSC switches and the base station controllers in order to process error messages coming from the network and to control the traffic load of the BSC controllers and the BTS base stations. The OMC configures the BTS base stations through the BSC and allows components of the system to be checked.
A handoff between base stations occurs, for example, when a mobile user travels from a first cell to an adjacent second cell. Handoffs also occur to relieve the load on a base station that has exhausted its traffic-carrying capacity or where poor quality communication is occurring. The handoff is a communication transfer for a particular user from the base station for the first cell to the base station for the second cell. During the handoff in conventional cellular systems, there is a transfer period of time during which the forward and reverse communications to the mobile user are severed with the base station for the first cell and are not established with the second cell. A typical conventional cellular system has the transfer period designed to be less than 100 milliseconds.
Conventional cellular implementations employ one of several techniques to reuse RF bandwidth from cell to cell over the cellular domain. The power received from a radio signal diminishes as the distance between transmitter and receiver increases. All of the conventional frequency reuse techniques rely upon power fading to implement reuse plans. In a frequency division multiple access (FDMA) system, a communications channel consists of an assigned particular frequency and bandwidth (carrier) for continuous transmission. If a carrier is in use in a given cell, it can only be reused in cells sufficiently separated from the given cell so that the reuse site signals do not significantly interfere on the carrier in the given cell. The determination of how far away reuse sites must be and of what constitutes significant interference are implementation-specific details. The cellular Advanced Mobile Phone System (AMPS) currently in use in the United States employs FDMA communications between base stations and mobile cellular telephones.
In time division multiple access (TDMA) systems, multiple channels are defined using the same carrier. The separate channels each transmit discontinuously in bursts which are timed so as not to interfere with the other channels on that carrier. Typically, TDMA implementations also employ FDMA techniques. Carriers are reused from cell to cell in an FDMA scheme, and on each carrier, several channels are defined using TDMA methods.
In code division multiple access (CDMA) systems, multiple channels are defined using the same carrier and with simultaneous broadcasting. The transmissions employ coding schemes such that to a given channel on a given carrier, the power from all other channels on that carrier appears to be noise evenly distributed across the entire carrier bandwidth. One carrier may support many channels and carriers may be reused in every cell.
In space division multiple access (SDMA) systems, one carrier is reused several times over a cellular domain by use of adaptive or spot beam-forming antennae for either terrestrial or space-based transmitters.
TDMA Conventional Cellular Architectures
In TDMA systems, time is divided int
Dolinar Andrew M.
Law Office of Imam
Lovejoy David E.
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