Note: Descriptions are shown in the official language in which they were submitted.
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LAYER 2 PROTOCOL IN A CELLULAR C014~CATION SYSTEM
This application is related to U.S. Patent 5,603,081 entitled "A Method for
Communicating in a Wireless Communication System," which issued February 11,
1997.
$ 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 (PSTI~. 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 wire-Iine 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, C 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 C1-
C10
is a base station designated as a corresponding one of a plurality of base
stations
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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 B1-B10 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 B1-B10 may be
located near the periphery, or otherwise away from the centers of the cells C
1-
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-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 B1-B10 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 (PSTI~, 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).
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With continuing reference to FIG. 1, a plurality of mobile stations M1-
M9 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 M1-M9 may be found in some of
the cells C1-C10, the presence or absence of the mobile stations Ml-M9 in any
particular one of the cells C 1-C 10 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-M9 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-M9 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-M9 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-
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M9 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 B1-B10 normally operates on a preselected one of the
radio channels allocated to the corresponding cell, i.e., the transmitter ~
and
receiver (RX) of the channel unit are tuned to a pair of transmit and receive
frequencies, respectively, which is not changed. The transceiver (TX/RX) of
each mobile station M1-M9, 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 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
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single mobile station is assigned the exclusive use of the channel for as long
as it
needs it.
The continued need to serve existing analog-only mobile stations has led
to the specification in IS-54B of an analog control channel (ACC) which has
been
$ inherited from the prior AMPS or the equivalent EIA/TIA-553 standard.
According to EIA/TIA-553, the analog forward control channel (FOCC) on the
down-link from the base station to the mobile stations carries a continuous
data
stream of messages (words) in the format shown in FIG. 3. Several different
types (functional classes) of messages may be transmitted on the analog FOCC.
These messages include a system parameter overhead message (SPOM), a global
action overhead message (GAOM), a registration identification message
(REGID), a mobile station control message, e.g., a paging message, and a
control-filler message. The SPOM, GOAM and REGI17 are overhead messages
which are intended for use by all mobile stations in the coverage area of the
base
station. Overhead messages are sent in a group called ari overhead message
train
(OMT). The first message of each OMT must always be the SPOM which is
transmitted every 0.8 t 0.3 seconds.
The format of the analog FOCC shown in FIG. 3 requires an iiile mobile
station listening to the FOCC to read all the messages transmitted in each OMT
(not just paging messages) even though the information contained in these
messages may not have changed from one OMT to the next OMT. This
requirement tends to unnecessarily limit the mobile station battery life. One
of
the goals of the next generation digital cellular systems is to extend the
"talk
time" for the user, that is, the battery life of the mobile station. To this
end,
2$ U.S. Patent 5,404,355 which issued April 4, 1995 _.
discloses a digital FOCC which can carry the types of messages
specified for the analog FOCC, but in a format which allows an idle mobile
station to read overhead messages when locking onto the FOCC and thereafter
only when the information has changed, and to enter "sleep mode" at all other
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times. While in sleep mode, the mobile station turns off most internal
circuitry
and saves battery power.
The above-referenced U.S. Patent 5,404,355 shows how
a digital control channel (DCC) may be defined alongside the digital traffic
channels (DTC) specified in IS-54B. Referring to FIG. 4, a half rate DCC
would occupy one slot, while a full-rate DCC would occupy two slots, out of
the
six slots in each time-division-multiple-access (TDMA) frame of duration 40
milliseconds (msec). For additional DCC capacity, additional half rate or full-
rate DCCs may be defined in place of the DTCs until there are no more
available
slots on the carrier (DCCs may then be defined on another carrier if needed).
Each IS-54B RF channel, therefore, can carry DTCs only, DCCs only, or a
mixture of both DTCs and DCCs: Within the IS-54B framework, each RF
channel can have up to three full-rate DTCs/DCCs, or six half rate DTCs/DCCs,
or any combination in-between, for example, one full-rate and four half rate
DTCs/DCCs.
In general, however, the transmission rate of the DCC need not coincide
with the half rate and full-rate specified in IS-54B, and the length of the
DCC
slots may not be uniform and may not coincide with the length of the DTC
slots.
FIG. 5 shows a general example of a forward (or downlink) DCC configured as
a succession of time slots l, 2, ~. . . , N, . . . included in the consecutive
time
slots 1, 2, . . . serit on a carrier frequency. - These DCC slots may be
defined on
a radio channel such as that specified by IS-54B, and may consist, as seen in
FIG. 5 for example; of every n-th slot in a series of consecutive slots. Each
DCC slot has, a duration that may or may not be 6.67 msec, which is the length
of a DTC slot according to the IS-54B standard. ('There are six DTC slots in
each 40-msec TDMA frame.) Alternatively (and without limitation on other
possible alternatives), these DCC slots may be defined in other ways known to
one skilled in the art.
As shown in FIG. S, the DCC slots may be organized into superframes
and each superframe may include a number of logical channels that carry
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different kinds of information. One or more DCC slots may be allocated to each
logical channel in the superframe. The exemplary downlink superfrariie in
FIG. 5 includes three logical channels: a broadcast control channel (BCCH)
including six successive slots for overhead messages; a paging channel (PCH)
including one slot for paging messages; and an access response channel (ARCH)
including one slot for channel assignment and other messages. The remaining
time slots in the exemplary superframe of FIG. S may be dedicated to other
logical channels, such as additional paging channels PCH or other channels.
Since the number of mobile stations is usually much greater than the number of
slots in the superframe, each paging slot is used for paging several mobile
stations that share some unique characteristic, e.g., the last digit of the
MIN.
For purposes of efficient sleep mode operation and fast cell selection, the
BCCH may be divided into a number of sub-channels. U.S. Patent
5,404,355 discloses a BCCH structure that allows the mobile station to read
a minimum amount of information when it is switched on (when it locks onto a
DCC) before being able to access the~system (place or receive a cali). After
being switched on, an idle mobile station needs to regularly monitor only its
assigned PCH slots (usually one in each superframe); the mobile can sleep
during
other slots. The ratio of the mobile's time spent reading paging messages and
its
time spent asleep is controllable and represents a tradeoff between call-set-
up
delay and power consumption.
Since each TDMA time slot has a certain fixed information carrying
capacity, each burst typically carries only a portion of a layer 3 message as
noted
above. In the uplink direction, multiple mobile stations attempt to
communicate
with the system on a contention basis, while multiple mobile stations listen
for
layer 3 messages sent from the system in the downlink direction. In known
systems, any given layer 3 message must be carried using as many TDMA
channel bursts as required to send the entire layer 3 message.
The communication link protocol is commonly referred to as a layer 2
protocol within the communications industry and its functionality includes the
n
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limiting or framing of higher level messages. Traditional layer 2 protocol
framing mechanisms or bit stuffing in 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.
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 (physical layer), layer 2, and layer 3. The physical layer (L1)
defines
the parameters of the physical communications channel, e.g., RF spacing,
modulation characteristics, etc. Layer 2 (L2) defines the techniques necessary
for the accurate transmission of information within the constraints of the
physical
channel, e.g., error correction and detection, etc. Layer 3 (L3) defines the
procedures for reception and processing of information transmitted over the
physical channel.
FIG. 6 schematically illustrates pluralities of layer 3 messages 11, layei 2
frames 13, and layer 1 channel bursts, or time slots, 15. In FIG. 6, each
group
of channel bursts corresponding to each layer 3 message may constitute a
logical
channel, and as described above, the channel bursts for a given layer 3
message
would usually not be consecutive slots on an IS-54B carrier. On the other
hand,
the channel bursts could be consecutive; as soon as one time slot ends, the
next
time slot could begin.
Each layer 1 channel burst 15 contains a complete layer 2 frame as well
as other information such as, for example, error correction information and
other
overhead information used for layer 1 operation. Each layer 2 frame contains
at
least a portion of a layer 3 message as well as overhead information used for
layer 2 operation. Although not indicated in FIG. 6, each layer 3 message
would
include various information elements that can be considered the payload of the
message, a header portion for identifying the respective message's type, and
possibly padding.
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Each layer 1 burst and each layer 2 frame is divided into a plurality of
different
fields. In particular, a limited-length DATA field in each layer 2 frame
contains the layer
3 message 11. Since layer 3 messages have variable lengths depending upon the
amount
of information contained in the layer 3 message, a plurality of layer 2 frames
may be
needed for transmission of a single layer 3 message. As a result, a plurality
of layer 1
channel bursts may also be needed to transmit the entire layer 3 message as
there is a one-
to-one correspondence between channel bursts and layer 2 frames.
As noted above, when more than one channel burst is required to send a layer 3
message, the several bursts are not usually consecutive bursts on the radio
channel.
Moreover, the several bursts are not even usually successive bursts devoted to
the
particular logical channel used for carrying the layer 3 message.
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.
Thus, there is a need for a layer 2 header which describes what is contained
in the
time slot, how it is contained in the time slot and how the information should
be
2 0 interpreted.
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SUMMARY
Therefore, in accordance with a first aspect of the present invention there is
s provided a mobile station for use in a cellular communication system
comprising:
means for receiving signals from the cellular system, wherein a plurality of
frames
are contained within the signals; and means for distinguishing between
different
types of frames by examining cyclic redundancy checks assigned to each frame
by
the cellular system.
1 o In accordance with a second aspect of the present invention there is
provided a cellular communication system with a plurality of mobile stations
and
at least one base station, comprising: means for assigning different cyclic
redundancy checks to different types of frames; and means for transmitting a
plurality of different frames using a layer 2 protocol to the mobile stations
wherein
15 the mobile stations distinguish between the different frames by examining
the
cyclic redundancy checks.
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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 shows the architecture of a conventional cellular radio system;
FIG. 2 shows a three sector cell which may be used in the system shown
in FIG. 1;
FIG. 3 shows the format of a forward analog control channel;
FIG. 4 shows the structure of a forward TDMA channel according to IS-
54B;
FIG. 5 is a generalized view of a digital control channel having time slots
which are grouped into superframes;
FIG. 6 illustrates a plurality of layer 3 messages, layer 2 frames, and
layer 1 channel bursts in a communication system;
FIG. 7 illustrates a block diagram of an exemplary cellular mobile radio
telephone system;
FIG. 8 illustrates the logical channels which make up the digital control
channel according to one embodiment of the present invention;
FIG. 9 shows a hyperframe structure;
FIGS. l0a-loo illustrate various SPACH layer 2 protocol frames
according to one embodiment of the present~invention; and
FIG. 11 shows an exemplary slot format on the forward DCC.
DETAILED DESCRIPTION
Although the description hereinafter focuses on systems which comply
with IS-54B, the principles of the present invention are equally applicable to
a
variety of wireless communication system, e.g., cellular and satellite radio
system, 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,
CA 02282892 1999-09-22
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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 channel
level (layer 1 ). The physical channel may be, for example, a relatively
narrow RF band
(FDMA), a time slot on a radio frequency (TDMA), a 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 U.S. Patent No.
5,353,332 to Raith et al, entitled "Method and Apparatus for Communication
Control in a
Radiotelephone System"; to U.S. Patent 5,404,355 entitled "Digital Control
Channel,"
issued April 4, 1995; to U.S. Patent 5,610,917 entitled "Layer 2 Protocol for
the Random
Access Channel and the Access Response Channel," issued March 11, 1997; to the
aforementioned U.S. Patent 5,603,081; to U.S. Patent 5,745,523 entitled "Mufti-
mode
Signal Processing," issued April 28,-1998; and to U.S. Patent 5,420,864
entitled "A
Method of Effecting Random Access in a Mobile Radio System" issued May 30,
1995.
FIG. 7 represents a block diagram of an exemplary cellular mobile
radiotelephone
2 0 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 MSC 140 which in turn is
connected to
the PSTN (not illustrated). General aspects of such cellular radiotelephone
systems are
known in the art.
2 5 The base station 110 for a cell includes a plurality of voice channels
handled by
voice channel receiver 150 which is controlled by the control and processing
unit 130.
Also, each base station includes a control channel
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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 absolute information (information
about the particular cell corresponding to the control channel on which the
information is being broadcast and may include the service profile of that
cell,
the control channel organization, and the type of cell) and relative
information
(generally the same kind of information as absolute information but is
information concerning the characteristics of other cells) 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.
According to the present invention, the digital control channel DCC
comprises the logical channels shown in FIG. 8. The DCC logical channels
include: a broadcast control channel (BCCH), comprising a fast broadcast
control
channel F-BCCH, an extended broadcast control channel E-BCCH, and a
broadcast short-message-service control channel S-BCCH; a short-message-
service/paging/access channel SPACH, comprising a point-to-point short-
message-service channel (SMSCH), the paging channel (PCH), and an access
response channel (ARCH); the random access control channel (RACH); and the
reserved channel. The DCC slots can be organized into higher level structures
called superframes as illustrated in FIG: 5, or as preferably illustrated in
FIG. 9,
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which depicts the frame structure of a forward (base station to mobile
station)
DCC and shows two successive hyperframes, each of which preferably comprises
a respective primary superframe and a respective secondary superframe.
Three successive superframes are illustrated in FIG. 9, each comprising a
plurality of time slots that are organized as the logical channels F-BCCH, E-
BCCH, S-BCCH, and SPACH. In general, one or more DCC slots may be
allocated for each logical channel in the superframe. Each superframe in a
forward DCC preferably includes a complete set of F-BCCH information (i.e., a
set of layer 3 messages), using as many slots as are necessary, and each
superframe preferably begins with a F-BCCH slot. After the F-BCCH slot or
slots, the remaining slots in each superframe include one or more (or no)
slots
for the E-BCCH, S-BCCH, and SPACH logical channels.
The BCCH, which in the example shown in FIG. 5 is allocated six DCC
slots, carries overhead messages. One of the overhead messages is used to
define the end of the BCCH section within the superframe. The PCH, which is
allocated one DCC slot, carries paging messages. The ARCH, which is also
allocated one DCC slot, carries channel assignment and other messages. The
exemplary superframe of FIG. 5 may contain other logical channels, including
additional paging channels, as indicated by FIG. 9. If more than one PCH is
defined, different groups of mobile stations identified by different traits
may be
assigned to different PCHs.
The BCCH acronym is used to refer collectively to the F-BCCH, E-
BCCH, and S-BCCH logical channels. These three logical channels are used, in
general, to carry generic, system-related information. The attributes of these
three channels are that they are unidirectional (downlink), shared, point-to-
multipoint, and unacknowledged. The fast BCCH is a logical channel used to
broadcast time critical system information. The extended BCCH is a logical
channel used to broadcast system information that is less critical than the
information sent on the F-BCCH. The broadcast short message service S-BCCH
CA 02282892 1999-09-22
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is a logical channel that is used to broadcast short messages used for an SM~
broadcast service.
The SPACH channel is a logical channel that is used to send information
to specific mobile stations regarding SMS point-to-point, paging and to
provide
an access response channel. The SPACH channel may be considered to be
further subdivided into three logical channels: SMSCH, ARCH, and PCH. The
paging channel PCH is a subset of the SPACH dedicated to delivering pages and
orders. The access response channel ARCH is a subset of the SPACH to which
the mobile station autonomously moves upon successful completion of an access
on the random access channel. The ARCH may be used to convey analog voice
channel or digital traffic channel assignments or other responses to the
mobile
access attempt. The SMS point-to-point channel SMSCH is used to deliver short
messages to specific mobile stations receiving SMS services, although the
messages could also be addressed to more than one mobile. Similarly, the
paging messages on the PCH may also be directed to more than one mobile.
The SPACH is unidirectional (downlink), shared, and unacknowledged. The
PCH is generally point-to-multipoint, in that it can be used to send paging
messages to more than one mobile station, but in some circumstances the PCH is
point-to-point. The ARCH and SMSCH are generally point-to-point, although
messages sent on the ARCH can also be addressed to more than one mobile
station.
For communication from the mobile stations to the base stations, the
reverse (uplink) DCC comprises a random access channel RACH, which is used
by the mobiles to request access to the system. The RACH logical channel is
unidirectional, shared, point-to-point, and acknowledged. All time slots on
the
uplink are used for mobile access requests, either on a contention basis or on
a
reserved basis. Reserved-basis access is described in U.S. Patent
5,420,864 entitled "Method of Effecting Random Access in a Mobile
Radio System"which issued May 30, 1995. One feature of RACH operation is that
CA 02282892 1999-09-22
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reception of some downlink information is required, whereby mobile stations
receive real-time feedback for every burst they send on the uplink. This is
known as Layer 2 ARQ, or automatic repeat request, on the RACH, and may be
provided by a flow of information called shared channel feedback on a downlink
sub-channel.
It may be important sometimes to be able to distinguish between the
BCCH slots and the SPACH slots within a superframe. For example, upon
being switched on, a mobile station does not know which slots are BCCH slots
and which slots are SPACH slots. The mobile station needs to find the overhead
information at the beginning of the BCCH section to be able to determine its
paging slot. Also, the boundary between the BCCH section and the SPACH
section may have changed for a variety of reasons. For example, if a system
has
been using twelve slots of a thirty-two-slot superframe for the BCCH and wants
to use thirteen slots for the BCCH, mobile stations assigned to the first
paging
slot after the BCCH slots must be informed that they should monitor another
paging slot.
According to one aspect of the present invention, one way to distinguish
between BCCH slots and SPACH slots is to use different cyclic redundancy
check (CRC) bits in these channels. For example, the CRC bits in the Layer 2
frames sent in the BCCH slots may be inverted, while the check bits in the
Layer 2 frames sent in the SPACH slots are~not inverted. Thus, when a mobile
reads the CRC bits, it obtains an indication of the kind of slot it has read.
Using
the check bits in this way is advantageous in some situations where it is
necessary to re-assign a mobile station to another paging slot. The mobiles
could
obtain this information by decoding one or two bits that would identify the
type
of slot being decoded, but at a cost of reduced bandwidth. In Applicants'
system, the mobile stations will recognize that something has changed when
they
spot the inverted CRC bits, and in response they will re-read the F-BCCH,
including the new DCC structure and paging slot assignment.
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Furthermore, as illustrated in FIG. 8, the DCC logical channels may
include reserved channels that make the communication system more flexible:
new features, services, or functions can be added at a later time without
affecting
existing mobiles. According to this embodiment of the present invention, the
BCCH overhead messages include a field which indicates where the reserved
channels are located in the superframe. These reserved channels have a
potentially wide variety of uses, such as carrying messages specific to a
system
operator and/or mobile station manufacturer. While existing mobile stations
may
not be able to use the new features described in the reserved channels, the
existing mobile stations will take the location and number of reserved slots
into
consideration when determining the location of their respective paging
channels.
The SPACH layer 2 protocol is used whenever a TDMA burst, or time
slot, is used to carry point-to-point SMS, paging, or ARCH information. A
single SPACH layer 2 protocol frame is constructed so as to fit within a 125-
bit
envelope. An additional five bits are reserved for use as tail bits, resulting
in a
total of 130 bits of information carried within each slot assigned for SPACH
purposes. FIGS. l0a-loo show a range of possible SPACH layer 2 protocol
frames under various conditions. A summary of the possible SPACH formats is
provided in the first table below. A summary of the fields comprising layer 2
protocol frames for SPACH operation is provided in the second table below.
Similar frame formats are used for all SPACH channels such that all
frames have a common Header A. The contents of the Header A determine
whether or not a Header B is present in, any given SPACH frame. The
Header A discriminates among hard page frames (containing no layer 3
information), PCH frames, ARCH frames and SMSCH frames. A Hard Triple
Page frame containing three 34-bit mobile station identifications (MSIDs) can
be
sent on the PCH (burst usage (BU) = Hard Triple Page). A Hard Quadruple
Page frame containing four 20-bit or 24-bit MSIDs can be sent on the PCH
(BU = Hard Quadruple Page).
CA 02282892 1999-09-22
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One or more L3 messages may be transmitted in one frame, or continued
over many frames. It is currently preferred that MSIDs are only carried within
frames where BU = PCH, ARCH or SMSCH with the burst type (BT) = Single
MSID, Double MSID, Triple MSID, Quadruple MSID, or automatic
retransmission request ARQ Mode BEGIN. The mobile station identity type IDT
field identifies the format of all MSIDs carried within a given SPACH frame
(i.e., no mixing of MSID formats is allowed). Pages carried on the PCH are
preferably not allowed to continue beyond a single SPACH frame, although the
protocol allows for it. All other PCH messages may continue beyond a single
SPACH frame.
For non-ARQ-mode operation, the L2 SPACH protocol supports sending
a single L3 message to multiple MSIDs in addition to the fixed one-to-one
relationship between MSIDs and L3 messages. A Message Mapping field (MM)
is used to control this aspect of the layer 2 frame operation. A valid SPACH
frame requires that all L2 header information pertinent to a given L2 frame be
included entirely within that frame, i.e., the L2 header from a given SPACH
frame cannot wrap into another SPACH frame. An Offset Indicator field (On is
used to allow both the completion of a previously started layer 3 message and
the
start of a new layer 3 message to occur within a single SPACH frame.
The following table summarizes the possible SPACH formats:
CA 02282892 1999-09-22
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SMS PCH ARCH CAN BE CONTINUED
Single MSID Y Y Y Y
Double MSID N Y Y Y
Triple MSID N Y Y Y
Quadruple MSID N Y Y Y
Hard Triple Page N Y N N
(MIN)
Hard Quadruple N Y N N
Page
(MINI)
Continue Y Y Y Y
ARQ Mode BEGIN Y N Y Y
ARQ Mode CONTINUEY N Y Y
Group ID I Y Y I Y I Y
I
FIG. l0a illustrates the SPACH Header A according to one embodiment
of the present invention. The SPACH Header A contains burst usage (BU)
information and flags for managing mobile stations in a sleep mode. The BU
field provides a high-level indication of burst usage. According to the
present
invention, the operation performed on each SPACH channel is not
predetermined. The BU field indicates whether the burst is being used for
paging, access response, or short message services. The flags indicate changes
in sleep mode configuration as well as in broadcast control channel
information.
This header is always present in all possible SPACH frame types.
FIG. lOb illustrates the SPACH Header B according to one embodiment
of the present invention. The SPACH Header B contains supplementary header
information used to identify the remaining contents of the layer 2 frame. This
' header is present when Header A indicates a burst usage of type PCH, ARCH or
SMSCH. In one alternative, the bit used for the offset indicator OI shown in
FIG. lOb as part of the Header B may be used as a SPACH response mode SRM
indicator, i.e., as information about the layer 2 access mode (contention or
reservation) to be used in the next access attempt made by the receiving
mobile
CA 02282892 1999-09-22
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station. The SRM indicator indicates how a mobile is to respond once it has
received all frames associated with a given SPACH message.
FIG. lOc illustrates a Null Frame. The Null frame is sent when
necessary by the cellular system when there is nothing else to be transmitted
for
any given SPACH burst. The Null Frame also contains a Go Away GA flag
which will be described below.
FIGS. lOd, l0e illustrate a Hard Triple Page Frame and a Hard
Quadruple Page Frame. A Hard Triple Page is a single frame page message
containing three 34-bit MINs. A Hard Quadruple Page is a single frame page
message containing four 20-bit or 24-bit MINs as determined by the mobile
station identity type.
A Single MSID frame, as illustrated in FIG. lOf, is used for starting the
delivery of ARCH or SMSCH L3 messages in a non-ARQ mode. In addition,
this frame may also be used for sending L3 PCH messages (pages or otherwise),
which are non-ARQ by definition. Page messages sent using a Single MS>D
frame cannot be continued into another frame.
If an ARCH or SMSCH L3 message is too long to fit into a Single MSID
frame then the remaining L3 information is carried using additional COrfZ'INUE
frames or MSID frames as necessary. If a complete ARCH or SMSCH L3
message does fit within a Single MSID frame, it is padded with filler, i.e.,
bits
having a predetermined value like zero, as necessary.
If a non-page PCH L3 message is too long to fit into Single MSID frame
then the remaining L3 information is carried using additional CONTINUE frames
or MSID frames as necessary. If a complete PCH L3 message does fit within a
Single MSID frame, it is padded with FILLER as necessary. A Double MSID
frame, as illustrated in FIG. lOg, is used for starting the delivery of two
ARCH
messages in a non-ARQ mode or two PCH L3 messages. The number of MSIDs
is indicated in the BT field with the same IDT format used for both instances
of
MSID. Page messages sent using a Double MSID frame cannot be continued
into another frame. FIG. lOh shows a Double MSID frame with continuation.
CA 02282892 1999-09-22
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FIG. l0i shows a CONTINUE frame. FIG. lOj shows an Offset Single MSID
frame.
A Triple MSID frame, as illustrated in FIG. lOk, is used for starting the
delivery of three ARCH L3 messages in a non-ARQ mode or three PCH L3
messages. The number of MSIDs is indicated in the BT field with the same
mobile station identity type format used for all instances of MSID. Page
messages sent using a Triple MSID frame cannot be continued into another
frame. A Quadruple MSID frame is used for starting the delivery of four ARCH
L3 messages in non ARQ mode or four PCH L3 messages. The number of
MSIDs is indicated in the BT field with the same mobile station identity type
format used for all instances of MSID. Page messages sent using a Quadruple
MSID frame cannot be continued into another frame.
A CONTINUE frame, as illustrated in FIG. 101, is used for continuation
of the L3 messages which are too long to fit into the previous frame. Note
that a
L2 header which is specific to any given SPACH frame must always be carried
entirely within that frame (i.e., the L2 header associated with a given SPACH
frame is not completed by using a subsequent SPACH frame).
An ARQ Mode BEGIN frame, as illustrated in FIG. lOm, is used for
starting the delivery of a L3 ARCH or SMSCH message in the ARQ mode. The
ARQ Mode BEGIN frame contains only one MSID within its L2 header as well
as a portion of the L3 message itself. If the L3 message is too long to fit
into a
single ARQ Mode BEGIN frame, then the remaining L3 information is carried
using additional ARQ Mode CONTINUE frames as necessary. If the L3
message does fit within a single ARQ Mode BEGIN frame, it is padded with
filler as necessary.
The PE field in conjunction with the transaction identifies TID field
identifies the transaction initiated by the ARQ Mode BEGIN frame and serves to
associate any subsequent ARQ Mode CONTINUE frames with this same
transaction. An ARQ Mode BEGIN frame has an implicit frame number FRNO
value of zero associated with it.
CA 02282892 1999-09-22
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The ARQ Mode CONTINUE frame, as illustrated in FIG. lOn,, is~used
for continuing a L3 ARCH or SMSCH message which is too long to fit into the
previous ARQ Mode frame (BEGIN or CONTINUE). The frame number FRNO
field identifies the CONTINUE frames within the context of the overall L3
message. The FRNO field value is incremented for each CONTINUE frame sent
in support of a given transaction (i.e., multiple CONTINUE frames may be sent
to complete the transaction initiated by the ARQ Mode BEGIN frame). The
ARQ Mode Continue frame is also used to repeat any previously sent ARQ
Mode CONTINUE frames received incorrectly by the mobile station.
According to one embodiment of the present invention, a group identity
field (GID) can be included in the SPACH~ layer 2 protocol. The group identity
field indicates that a mobile is part of a group. By using this group
identity, the
communication system can page the entire group using one page. A Group ID
frame is illustrated in FIG. 100. The Group ID frame is used for starting the
delivery of ARCH or SMSCH L3 messages in a non-ARQ mode. In addition,
this frame may also be used.for sending L3.PCH messages (p ges or otherwise),
which are non-ARQ by definition. Page messages sent using a Group ID frame
cannot be continued into another frame. If an ARCH or SMSCH L3 message or
a non-page PCH L3 message is too long to fit into a Group ID frame, then the
remaining L3 information is carried using an END frame or additional
CONTINUE frames as necessary. If a complete ARCH or SMSCH L3 message
or a non-page PCH L3 message does fit within a Group ID frame, it is padded
with filler as necessary.
According to another embodiment of the present invention, a go-away flag
GA can be included in the SPACH layer 2 protocol for example in the Null
Frame illustrated in FIG. lOc. The GA flag can be used by the cellular system
to indicate that the mobile stations should not attempt to use a certain cell.
For
example, this would permit a system operator to test a base station without
risk
of mobile stations trying to lock onto it.
The following table summarizes the SPACH Layer 2 Protocol fields:
CA 02282892 1999-09-22
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Field Name Length Values
(bits)
BU = Burst Usage 3 000
=
Hard
Triple
Page
(34
bit
MSID)
001
=
Hard
Quad
Page
(20
or
24
bit
MSID)
010
=
PCH
Burst
011
=
ARCH
Burst
100
=
SMSCH
Burst
101
=
Reserved
110
=
Reserved
111
=
Null
PCON = PCH Continuation1 0
=
No
PCH
Continuation
1
=
PCH
Continuation,
Activated
BCN = BCCH Change Notification1 Transitions
whenever
there
is
a
change
in
F-BCCH
information.
SMSN = SMS Notification1 Transitions
whenever
there
is
a
change
in
S-BCCH
information.
PFM = Paging Frame Modifier1 0
=
Use
assigned
PF
1
=
Use
one
higher
than
assigned
PF
BT = Burst Type 3 000 = Single MSID Frame
001 = Double MSID Frame
010 = Triple MSID Frame
011 = Quadruple MSID Frame
100 = Continue Frame
I01 = ARQ Mode Begin
110 = ARQ Mode Continue
111 = Reserved
IDT = Identity Type 2 00 = 20 bit TMSI
O1 = 24 bit MINI per IS-54B
10 = 34 bit MIN per IS-54B
11 = 50 bit IMSI
MSID = Mobile Station 20/24/3 20 bit TMSI
Identity
4/50 _ 24 bit MINI
34 bit MIN
50 bit IMSI
1~ GID = Group Identity 24/34/5 24 bit MIN 1
0 34 bit MIN
50 bit IMSI
MM = Message Mapping 1 0 = One instance of L3LI
and
L3DATA per instance of
MSID.
1 = One instance of L3LI
and
L3DATA for multiple MSIDs.
CA 02282892 1999-09-22
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OI = Offset Indicator 1 0 = No message offset included.
1 = Message offsetiacluded.
or
SRM = SPACH Response Mode 0 = Nezt access attempt
made on
RACH to be contention-based.
1 = Nezt access attempt
made oa
RACH to be reservation-based.
CLI = Continuation Length 7 Number of bits remaining
Indicator in the
previous L3 message.
GA = Go Away 1 Indicates if the cell is
barred
0 = cell not barred
1 = cell barred
L3LI = Layer 3 Length Indicator8 Variable length layer 3
messages
supported up to a maximum
of Z55
octets.
L3DATA = Layer 3 Data VariableContains a portion (some
or all) of the
layer 3 message having
an overall
length as indicated by
L3LI. The
portion of this field not
used to carry
layer 3 information is
filled with
zeros.
PE = Partial Echo 7 The 7 least significant
bits of the
mobile station IS-54B MIN.
TID = Transaction Identity 2 Indicates which ARQ mode
transaction
is being transmitted on
the ARCH or
SMSCH.
FRNO = Frame Number 5 Uniquely identifies specific
frames
sent is support of as ARQ
mode
transaction.
FILLER = Burst Filler VariableAll filler bits are set
zero.
CRC = Cyclic Redundancy Code 16' Same Generator polynomial
as IS-54B
(includes DVCC)
According to the present invention, the mobile station can be in any of a
plurality of states. For example, a mobile station would be in a "start random
access" state before the first unit of a message that is to be transmitted by
a
random access has been transmitted. The mobile station would be in a "start
reserved access" state before the first unit of a message that is to be
transmitted
by a reservation-based access has been transmitted. The mobile station would
be
in a "more units" state if there are more units associated with the same
access
event pending for transmission. The mobile station would be in a "after last
CA 02282892 1999-09-22
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burst" state if the last unit of an access event has been transmitted.
Finally, the
mobile station would be in a "success" state after a message has been sent
successfully.
The layer 2 protocol also provides for a plurality of flags. Forward
shared control feedback (SCF) flags are used to control the reverse channel,
i.e.,
the RACH, as noted above. These SCF flags are a BRI flag, a R/N flag, and a
CPE flag that are interleaved and transmitted in two fields in each downlink
slot
(layer 1); the total length of the two fields is twenty-two bits. A preferred
information format in the slots of the forward DCC is shown in FIG. 11. This
IO format is substantially the same as the format used for the DTCs under the
IS-
54B standard, but new functionalities are accorded to the fields in each slot
in
accordance with Applicants' invention. .In FIG. 11, the number of bits in each
field is indicated above that field. The bits sent in the SYNC field are used
in a
conventional way to help ensure accurate reception of the CSFP and DATA
fields, and the SYNC field would be the sarrie as that of a DTC according to
IS-
54B and would catty a predetermined bit pattern used by the base stations to
find
the start of the slot. The CSFP field in each DCC slot conveys a coded
superframe phase (SFP) value that enables the mobile stations to find the
start of
each superframe.
The busy/reserved/idle (BRI) flag is used to indicate whether the
corresponding uplink RACH slot is Busy, Reserved or Idle for reserved-basis
accesses, which is described in U.S. Patent 5,420,864. S~
bits are used for these flags and the different conditions are encoded as
shown in
the table below:
BRIS BRh BRI3 BRI2 BRh BRIO
Busy 1 1 1 I 0 0
Reserved 0 0 1 1 1 1
Idle 0 0 0 0 0 0
CA 02282892 1999-09-22
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The received/not received (R/N) flag is used to indicate whether or not
the base station received the last transmitted burst. A five-times repetition
code
is used for encoding this flag as shown in the table below:
f1 R/N4R/N3 R/Nz R/Nl R/No
Received 1 1 1 1 1
Not Received 0 0 0 0 0
According to the present invention, partial echo information is used to
identify which mobile station was correctly received after the initial burst
of
random access and/or which mobile station is intended to have access to the
reserved slot. For example, the seven least significant bits of an IS-54B-type
MIN can be assigned as the partial echo information, and these are preferably
encoded in a manner similar to the manner in which the digital verification
color
code (DVCC) is encoded under IS-54B, i.e., a (12,8) code, producing eleven
bits
of coded partial echo information.
The following table shows how the mobile decodes received flags
according to the layer 2 state. Note that only the flags relevant to the layer
2
state are shown. In the "start random access" state, the BRI flag is the only
relevant flag. During a multiburst message transmission, both the BRI and R/N
flags are relevant. In the summary in the following table, b; is the bit
value.
Layer 2 StateBusy/Reserved/Idle Received/Not
' Received
Busy Reserved Idte Received Not
received
111100001111 000000 11111 00000
Start random NIA NIA
6
access Idle
IF
~
b,
<
2
~iND~
b,
<
2
~.i
i.a
Start reservedReserved N/A N/A
IF
<
3
bits
difference
to
access Reserved
flag
code
value
CA 02282892 1999-09-22
-27-
More units Busy IF < 4 bits difference
to Busy flag
code value
~btz4 ~br<4
rr
After last Busy IF < 4 bits difference
burst to Busy flag
code value
~b,z4 ~b,<4
r.r r.i
The mobile station interprets a received coded partial echo value as
having been correctly decoded if it differs by less than three bits from the
correct
coded partial echo (CPE). This is referred to as PE match.
A mobile station is allowed a maximum of Y+l, where Y=(0, 1, ..., 7),
transmission attempts before considering the attempt to transfer a message as
a
failure. The random delay period used in the mobile station after a Not Idle
condition or after a transmission attempt is uniformly distributed between
zero
msec and 200 msec with a granularity of 6.667 msec (the duration of a time
slot). A mobile station is preferably not allowed to make more than Z
consecutive repetitions of an individual burst, where Z=(0, 1, ..., 3).
According to one embodiment of the present invention, the BMI (base
station, mobile switching center and interworking function) can page a mobile
station by using SPACH Notification and thereby save much system bandwidth in
some situations. For example, when a SPACH. message is to be delivered to a
mobile in the system illustrated in FIG. 1, all ten base stations would
transmit it
since the system would generally not know in which cell the mobile was
located.
If the SPACH message required a total of ten slots to transmit, 100 slots
would
be used by the system to send the SPACH message, ten slots per base station.
To avoid this waste, a SPACH Notification message would be broadcast
in all ten cells, or whatever the appropriate number of cells for the mobile
station
happened to,be, rather than the entire SPACH message. In essence, the SPACH
Notification message asks the mobile station if it is able to receive a
message.
CA 02282892 1999-09-22
-28-
When the mobile station responds (on the RACH), the BMI can determine in
which cell the mobile station is located and thus can send the SPACH message
through that cell's base station.
In addition, the SPACH Notification message may also indicate what type
of SPACH message will be sent to the mobile station. For example, if the
mobile station receives a SPACH Notification which indicates that an SSD
(Shared Secret Data) Update is coming, the mobile station issues a response
containing a SPACH confirmation and starts a timer. The BMI then transmits
the SSD Update Order message. Upon receipt of the message, the mobile stops
the timer and enters the SSD Update Proceeding State. However, if the timer
expires prior to receiving the SSD Update Order message, the mobile returns to
the DCCH camping state. The SPACH Notification could also be used to notify
the mobile that a SMS message is coming.
In another aspect, the system may dynamically assign temporary mobile
station identities (TMSIs) to the mobile stations. Such a TMSI would be a 20-
bit
or 24-bit MSID sent by the system over the air interface to a mobile. The TMSI
would be used by the network to page or deliver a message to the corresponding
mobile station on the SPACH, and the TMSI would be used by the mobile station
to make accesses on the RACH.
Using 20-bit TMSIs increases the paging capacity in comparison to using
24-bit TMSIs at the expense of reducing the address space, i.e., the number of
mobiles that can be paged, in the same way that using 24-bit MSIDs increases
paging capacity in comparison to using 34-bit MINs (compare FIG. l0e to
FIG. lOd, for example). As seen from FIG. 10e, a single layer 2 paging frame
can carry five 20-bit TMSIs, or pages, instead of four 24-bit TMSIs (or
MSB~s).
By providing a plurality of TMSI formats, one has the flexibility to trade off
address space for paging capacity.
It is currently preferred that the BMI assign a TMSI to a mobile in
response to the mobile's registration, in which case the TMSI can be provided
in
an information element called MSID Assignment that is included in a
CA 02282892 1999-09-22
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Registration Accept message sent on the SPACH. Advantageously, the mobile
station would treat the assigned TMSI as valid until it is switched off or
until it
decides to carry out any of the following system accesses: a new system
registration; a forced registration; a power-up registration; a TMSI timeout
registration; a deregistration registration; or the first system access of any
kind
made after receiving various other messages, such as a registration reject
message. A mobile station assigned a TMSI in a registration accept message
sent
by the BMI using ARQ mode advantageously would only treat the assigned
TMSI as valid if the ARQ transaction were completed successfully from a layer
2
perspective.
While a particular embodiment of the present invention has been
described and illustrated, it should be understood that the invention is not
limited
thereto since modifications may be made by persons skilled in the art. The
present application contemplates any and all modifications that fall within
the
spirit and scope of the underlying invention disclosed and claimed herein.
