Layer 2 protocol in a cellular communication system

Multiplex communications – Communication over free space – Portable address responsive receiver

Reexamination Certificate

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Details

C370S311000, C455S039000, C340S870030

Reexamination Certificate

active

06175557

ABSTRACT:

BACKGROUND
The present invention relates to a method for transmitting messages between mobile stations and a central switching system, and more particularly to a method for transmitting these messages using a more efficient communications link protocol over the air-interface of a cellular telephone system.
In a typical cellular radio system, a geographical area, e.g., a metropolitan area, is divided into several smaller, contiguous radio coverage areas called “cells” . The cells are served by a series of fixed radio stations called “base stations” . The base stations are connected to and controlled by a mobile services switching center (MSC). The MSC, in turn, is connected to the land-line (wire-line) public switched telephone network (PSTN). The telephone users (mobile subscribers) in the cellular radio system are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units (mobile stations) which communicate voice andlor data with the MSC through a nearby base station. The MSC switches calls between and among wire-line and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, and provides for the operation, maintenance and testing of the system.
FIG. 1
illustrates the architecture of a conventional cellular radio system built according to the Advanced Mobile Phone Service (AMPS) standard. In
FIG. 1
, an arbitrary geographic area is divided into a plurality of contiguous radio coverage areas, or cells, CL
1
-C
10
. While the system of
FIG. 1
is, for illustration purposes, shown to include only ten cells, the number of cells may be much larger in practice. Associated with and located in each of the cells C
1
-C
10
is a base station designated as a corresponding one of a plurality of base stations B
1
-B
10
. Each of the base stations B
1
-B
10
includes a plurality of channel units, each comprising a transmitter, a receiver and a controller, as is well known in the art.
In
FIG. 1
, the base stations B
1
-B
10
are located at the center of the cells C
1
-C
10
, respectively, and are equipped with omni-directional antennas transmitting equally in all directions. In this case, all the channel units in each of the base stations B
1
-B
10
are connected to one antenna. However, in other configurations of the cellular radio system, the base stations B
1
-B
10
may be located near the periphery, or otherwise away from the centers of the cells C
1
-C
10
and may illuminate the cells C
1
-C
10
with radio signals directionally. For example, the base station may be equipped with three directional antennas, each one covering a 120-degree sector cell as shown in FIG.
2
. In this case, some channel units will be connected to one antenna covering one sector cell, other channel units will be connected to another antenna covering another sector cell, and the remaining channel units will be connected to the remaining antenna covering the remaining sector cell. In
FIG. 2
, therefore, the base station serves three sector cells. However, it is not always necessary for three sector cells to exist and only one sector cell needs to be used to cover, for example, a road or a highway.
Returning to
FIG. 1
, each of the base stations B
1
-B
10
is connected by voice and data links to an MSC
20
which is, in turn, connected to a central office (not shown) in the public switching telephone network (PSTN), or a similar facility, e.g., an integrated system digital network (ISDN). The relevant connections and transmission modes between the mobile switching center MSC
20
and the base stations B
1
-B
10
, or between the mobile switching center MSC
20
and the PSTN or ISDN, are well known to those of ordinary skill in the art and may include twisted wire pairs, coaxial cables, fiber optic cables or microwave radio channels operating in either analog or digital mode. Further, the voice and data links may either be provided by the operator or leased from a telephone company (telco).
With continuing reference to
FIG. 1
, a plurality of mobile stations M
1
-M
9
may be found within the cells C
1
-C
10
. Again, while only nine mobile stations are shown in
FIG. 1
, the actual number of mobile stations may be much larger in practice and will generally exceed the number of base stations. Moreover, while none of the mobile stations M
1
-M
9
may be found in some of the cells C
1
-C
10
, the presence or absence of the mobile stations M
1
-M
9
in any particular one of the cells C
1
-C
10
depends on the individual desires of each of the mobile subscnbers who may travel from one location in a cell to another or from one cell to an adjacent or neighboring cell.
Each of the mobile stations M
1
-M
9
includes a transmitter, a receiver, a controller and a user interface, e.g., a telephone handset, as is well known in the art. Each of the mobile stations M
1
-M
9
is assigned a mobile identification number (MIN) which, in the United States, is a digital representation of the telephone directory number of the mobile subscriber. The MIN defines the subscription of the mobile subscriber on the radio path and is sent from the mobile station to the MSC
20
at call origination and from MSC
20
to the mobile station at call termination. Each of the mobile stations M
1
-M
9
is also identified by an electronic serial number (ESN) which is a factory-set, “unchangeable” number designed to protect against the unauthorized use of the mobile station. At call origination, for example, the mobile station will send the ESN to the MSC
20
. The MSC
20
will compare the received ESN to a “blacklist” of the ESNs of mobile stations which have been reported to be stolen. If a match is found, the stolen mobile station will be denied access.
Each of the cells C
1
-C
10
is allocated a subset of the radio frequency (RF) channels assigned to the entire cellular system by the concerned government authority, e.g., the Federal Communications Commission (FCC) in the United States. Each subset of RF channels is divided into several voice or speech channels which are used to carry voice conversations, and at least one paging/access or control channel which is used to carry supervisory data messages, between each of the base stations B
1
-B
10
and the mobile stations M
1
-M
9
in its coverage area. Each RF channel comprises a duplex channel (bi-directional radio transmission path) between the base station and the mobile station. The RF channel consists of a pair of separate frequencies, one for transmission by the base station (reception by the mobile station) and one for transmission by the mobile station (reception by the base station). Each channel unit in the base stations B
1
-B
10
normally operates on a preselected one of the radio channels allocated to the corresponding cell, i.e., the transmitter (TX) and receiver (RX) of the channel unit are tuned to a pair of transmit and receive frequencies, respectively, which is not changed. The transceiver (TXIRX) of each mobile station M
1
-M
9
, however, may tune to any of the radio channels specified in the system.
In typical land-line systems, remote stations and control centers are connected by copper or fiber optic circuits which have a data throughput capacity and performance integrity that is generally significantly better than the data throughput capacity and performance integrity provided by an air interface in a cellular telephone system. As a result, the conciseness of overhead required to manage any selected communication link protocol for land-line systems is of secondary importance. In cellular telephone systems, an air interface communications link protocol is required in order to allow a mobile station to communicate with a cellular switching system. A communications link protocol is used to initiate and to receive cellular telephone calls.
The electromagnetic spectrum available for use by cellular telephone systems is limited and is portioned into units called channels. Individual channels are used as communication links either on a shared basis or on a dedicated or reserved basis. When indi

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