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Description  |
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BACKGROUND
The present invention relates to a method for transmitting messages between
mobile stations and a central switching system, and more particularly to
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 landline (wireline) 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 and/or data with the MSC through a nearby base
station. The MSC switches calls between and among wireline 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 may be seen divided into a plurality
of contiguous radio coverage areas, or cells, C1-C10. 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 C1-C10 is a base station designated as a
corresponding one of a plurality of base stations B1-B10. Each of the base
stations B1-B10 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 B1-B10 are located at the center of the cells
C1-C10, 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 B1-B10 are connected to one antenna.
However, in other configurations of the cellular radio system, the base
stations B1-B10 may be located near the periphery, or otherwise away from
the centers of the cells C1-C10 and may illuminate the cells C1-C10 with
radio signals directionally. For example, the base station may be equipped
with three directional antennas, each one covering a 120 degrees 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
coveting 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 B1-B10 is connected by voice
and data links to a mobile switching center (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
B1-B10, 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 M1-M10
may be found within the cells C1-C10. Again, while only ten 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 M1-M10 may be found
in some of the cells C1-C10, the presence or absence of the mobile
stations M1-M10 in any particular one of the cells C1-C10 depends on the
individual desires of each of the mobile subscribers 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 M1-M10 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 M1-M10 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 the MSC 20 to the mobile station at call termination. Each of the
mobile stations M1-M10 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 C1-C10 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 B1-B10 and
the mobile stations M1-M10 in its coverage area. Each RF channel comprises
a duplex channel (bidirectional 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
B1-B10 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 does not change. The transceiver
(TX/RX) of each mobile station M1-M10, 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 divided into units called channels. Individual
channels are used as communication links either on a shared basis or on a
dedicated or reserved basis. When individual channels are used as
communication links on a shared basis, multiple mobile stations may either
listen to or contend for the same channels. In the contending situation,
each shared channel can be used by a plurality of mobile stations which
compete to obtain exclusive use of the channel for a limited period of
time. On the other hand, when individual channels are used as
communication links on a dedicated basis, a single mobile station is
assigned the exclusive use of the channel for as long as it needs it.
In light of the generally reduced data throughput capacity and performance
integrity afforded by an individual channel in a channel sharing situation
in a cellular telephone environment, the selection of an efficient air
interface protocol to serve as the basis of the communication link becomes
paramount.
The communication link protocol is commonly referred to as a layer 2
protocol within the communications industry and its functionality includes
the delimiting or framing of higher level messages. Traditional layer 2
protocol framing mechanisms of bit stuffing and flag characters are
commonly used in land line networks today to frame higher layer messages,
which are referred to as layer 3 messages. These layer 3 messages may be
sent between communicating layer 3 peer entities residing within mobile
stations and cellular switching systems.
In cellular systems, the likelihood of successfully sending a message over
a radio channel is inversely proportional to the length of the message
since the entire message will be considered to be in error even if only a
single bit of the transmitted message is received in error. In order to
address this problem, messages are first divided into small packets or
frames. Thus, it becomes important for the cellular system to know if all
of the transmitted packets are correctly received by a mobile station.
SUMMARY
According to the present invention, in order to make this determination a
base station may send a mobile station any given number of frames using
automatic retransmission request (ARQ) wherein the base station may ask
the mobile station to send a current status report on the frames it has
received and then resend any frames not received correctly. For example,
the mobile station can be asked to identify what frames it has received at
any point during an ARQ based transmission or the mobile station can
autonomously send a message to the base station stating what frames it has
received.
According to one embodiment of the present invention, a method for
obtaining a report from a mobile station on the status of frames
comprising an entire message transmitted to the mobile station is
disclosed. First, a status request is sent to the mobile station from a
base station. A status report is then sent to the base station. The status
request specifies whether the mobile station should send the status report
on a reservation basis (i.e., using a reserved frame) or on a contention
basis (i.e., using an idle frame). The mobile station then transmits a bit
map to the communication system to indicate which frames have been
correctly received by the mobile station at the point in time when it
received the status request.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described in more detail with reference
to preferred embodiments of the invention, given only by way of example,
and illustrated in the accompanying drawings, in which:
FIG. 1 illustrates the architecture of a conventional cellular radio
system;
FIG. 2 illustrates a three sector cell which may be used in the system
shown in FIG. 1;
FIG. 3 illustrates a block diagram of an exemplary cellular mobile radio
telephone system;
FIG. 4 illustrates the logical channels which make up the digital control
channel according to one embodiment of the present invention;
FIGS. 5a-b illustrate SPACH Header sections A and B, respectively,
according to one embodiment of the present invention;
FIGS. 6a-b illustrate the Random Access Procedures for a base station and a
mobile station according to one embodiment of the present invention;
FIGS. 7a-b illustrate a SPACH ARQ Mode Procedure for a mobile station and a
base station according to one embodiment of the present invention;
FIG. 8 illustrates an ARQ Mode Begin frame according to one embodiment of
the present invention; and
FIG. 9 illustrates an ARQ Mode Continue frame according to one embodiment
of the present invention.
DETAILED DESCRIPTION
Although the description hereinafter focuses on systems which comply with
IS-54B and its successors, the principles of the present invention are
equally applicable to a variety of wireless communication systems, e.g.,
cellular and satellite radio systems, irrespective of the particular mode
of operation (analog, digital, dual-mode, etc.), the access technique
(FDMA, TDMA, CDMA, hybrid FDMA/TDMA/CDMA,etc.), or the architecture
(macrocells, microcells, picocells, etc.). As will be appreciated by one
skilled in the art, the logical channel which carries speech and/or data
may be implemented in different ways at the physical layer level. The
physical channel may be, for example, a relatively narrow RF band (FDMA),
a time slot on a radio frequency (TDMA), a unique code sequence (CDMA), or
a combination of the foregoing. For purposes of the present invention, the
term "channel" means any physical channel which can carry speech and/or
data, and is not limited to any particular mode of operation, access
technique or system architecture.
This application contains subject matter which is related to co-pending
U.S. patent application Ser. No. 07/955,591, entitled "Method and
Apparatus for Communication Control in a Radiotelephone System," filed on
Oct. 2, 1992, to co-pending U.S. patent application Ser. No. 07/956,640,
entitled "Digital Control Channel," filed on Oct. 5, 1992, to co-pending
U.S. patent application Ser. No. 08/047,452, entitled "Layer 2 Protocol
for the Random Access Channel and the Access Response Channel," filed on
Apr. 19, 1993, to co-pending U.S. patent application Ser. No. 08/147,254,
entitled "A Method For Communicating in a Wireless Communication System,"
filed on Nov. 1, 1993, to co-pending U.S. patent application Ser. No.
07/967,027, entitled "Multi-Mode Signal Processing," filed on Oct. 27,
1992, and to co-pending U.S. patent application Ser. No. 08/140,467,
entitled "A Method of Effecting Random Access in a Mobile Radio System,"
filed on Oct. 25, 1993. These six co-pending applications are incorporated
herein by reference.
FIG. 3 represents a block diagram of an exemplary cellular mobile
radiotelephone system according to one embodiment of the present
invention. The system shows an exemplary base station 110 and a mobile
station 120. The base station includes a control and processing unit 130
which is connected to the mobile switching center MSC 140 which in turn is
connected to the public switched telephone network (not illustrated).
The base station 110 for a cell includes a plurality of voice channels
handled by voice channel transceiver 150 which is controlled by the
control and processing unit 130. Also, each base station includes a
control channel transceiver 160 which may be capable of handling more than
one control channel. The control channel transceiver 160 is controlled by
the control and processing unit 130. The control channel transceiver 160
broadcasts control information over the control channel of the base
station or cell to mobiles locked to that control channel.
When the mobile 120 is in an idle mode, the mobile periodically scans the
control channels of base stations like base station 110 to determine which
cell to lock on or camp to. The mobile 120 receives the absolute and
relative information broadcast on a control channel at its voice and
control channel transceiver 170. Then, the processing unit 180 evaluates
the received control channel information which includes the
characteristics of the candidate cells and determines which cell the
mobile should lock onto. The received control channel information not only
includes absolute information concerning the cell with which it is
associated, but also contains relative information concerning other cells
proximate to the cell which the control channel is associated.
For a better understanding of the structure and operation of the present
invention, the digital control channel (DCC) may be divided into three
layers: layer 1 (the physical layer), layer 2 and layer 3. The physical
layer defines the parameters of the physical communications channel, e.g.,
RF spacing, modulation characteristics, etc. Layer 2 (L2) defines the
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