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Description  |
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TECHNICAL FIELD
This invention relates to cordless, cellular, and PCS telephone systems and
more particularly to an arrangement for prioritizing access to a channel
shared by a plurality of wireless stations.
BACKGROUND OF THE INVENTION
In a wireless communication system, each mobile terminal must be able to
establish communications (commonly referred to as registration) with a
stationary base station in a cell. To allow this, some communication
method must be defined and be known by all terminals. Most cellular
communication systems in use today employ frequency division multiple
access (FDMA). One type of FDMA uses busy-tone multiple access (BTMA). The
digital equivalent of a BTMA uses either a repetitive bit or a repetitive
field in a transmitted bit stream instead of a busy tone for inhibiting
access, thus permitting the busy-tone channel to carry other data. The
U.S. domestic cellular telephone systems, in accordance with EIA standard
IS-3-D, employ BTMA and frequency re-use. Each cell is assigned one of a
plurality of forward control channels designated as paging channels,
paired in radio frequency with reverse control channels designated as
access channels, and reused in such a way as to minimize interference
between cells. Both the mobile terminals (conventional cellular
telephones) and the base stations operate full-duplex. Repetitive bits
called D-I bits positioned in the forward control channel message stream
provide the busy-idle indication. The mobile terminal, prior to seeking an
access channel, preselects which of the plurality of access channels it is
receiving the best. The mobile terminal then determines its D-I status; if
busy, it enters a random time-out to re-try, but if not busy, the terminal
starts sending a message on the corresponding reverse control channel.
Sometimes the reverse control channel in a first cell receives a message
from a remote mobile terminal, the FM receiver of which is undesirably
captured by a second cell using the same frequency pair. When this
happens, the forward control channel of the first cell will raise its busy
indication, and may actually attempt to exchange messages with the mobile
terminal; but, because the mobile terminal's receiver is captured by a
second cell, the protocol and control means ultimately protects the
cellular system. Still, this effect reduces the capacity of the signaling
channels. U.S. Pat. No. 5,047,762 of E. J. Bruckert proposes a scheme to
help control this problem. A second fixed station receiving the remote
mobile terminal's transmission uses the station identification and the
signal strength of the remote mobile terminal to determine whether
transmissions from other remote terminals should be inhibited. Depending
upon the geographical proximity of the first fixed station to the second
fixed station and the remote mobile terminal signal strength at the second
fixed station, a signal representative of the remote mobile terminals
signal strength at the second fixed station may be transmitted by the
second fixed station. This representative signal may be used by a second
remote mobile terminal to determine whether the second remote mobile
terminal may transmit.
The U. S. patent application of J. P. Cotsonas, et al., Ser. No. 07/704516,
filed on May 23, 1991, and assigned to the same assignee as the present
application, discloses resolving contention for a dedicated channel among
base units responding to a registration request from a mobile unit by
assigning a time also referred to as time slot at which the base unit will
respond as a function of the random number generated by the base unit. In
addition, that U.S. patent application also teaches resolving contention
by the base units responding at particular times based on the received
signal strength from a mobile unit requesting the service to the
individual base stations. The base unit receiving the greater received
signal strength will respond first.
While the foregoing arrangements assist in avoiding contention between base
stations in responding to a registration request from a mobile unit, the
generation of a random number in order to determine a time slot suffers
from the problem that there are a finite number of time slots available
which increases the probability that two or more base units will generate
a random number designating the same time slot. Further, the method of
determining the time slot on the basis of the received signal strength
also suffers from the problem that there will be locations where more than
one base station receives the same signal strength from a mobile unit;
hence, these base stations will be trying to respond in the same time
slot.
In addition, the foregoing ,arrangements do not solve the problem of
assigning a mobile unit to particular base station and giving that base
station priority over the other base stations in responding to a
registration request. This problem is an important one in personal
communication services (PCS) systems where it is desirable for traffic
control to assign mobile units to particular base stations and to utilize
one of those base stations to service an assigned mobile unit if possible.
SUMMARY OF THE INVENTION
This invention is directed to solving these and other problems and
disadvantages of the prior art. According to the invention, to register, a
mobile unit transmits a registration request message on a shared channel
which all base stations continuously monitor. All available base stations
receiving the message respond on another shared channel with the base
station to which the mobile unit is assigned transmitting in a first time
slot and the other base stations transmitting in a time slot determined by
the strength of the received signal from the mobile unit. This allows the
assigned base station to respond first and avoids any collisions between
the assigned base station and other base stations.
If the assigned base station responds, the mobile unit sends an acknowledge
message to the assigned base station which then registers the mobile unit.
However, if the assigned base station does not respond to the registration
request message, the mobile unit selects the base station which responded
in a time slot earlier than the response of the other base stations. The
mobile unit then sends an acknowledge message to the selected base
station, and the selected base station then registers the mobile unit. If
two base stations respond to the registration request message in the same
time slot, the mobile unit determines whether there is a response in a
time slot which is closer to the first time slot than the collided time
slot. If there is a time slot earlier in time than the collided time slot,
the mobile unit sends an acknowledge message to the base station
transmitting in this earlier time slot. However, if the collided time slot
is earlier in time then any other time slot in which a response is
received, the mobile unit transmits a second registration request message
specifying the collided time slot. Each of the base stations that had
responded in the collided time slot responds by sending a base reply
message in a random time slot, and the other base stations do not respond
to this second registration request message. The mobile unit selects one
of the base reply messages in a random time slot. The mobile unit
transmits an acknowledge message which includes the random time slot
number. The selected base station is responsive to that acknowledge
message to register the mobile unit.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a block diagram of a personal communications service system that
incorporates an illustrative embodiment of the invention;
FIG. 2 is a block diagram of a base station;
FIG. 3 is a block diagram of a mobile unit;
FIG. 4 is a table defining the dedicated channels shared by all base
stations;
FIG. 5 is a flow chart of the operations performed by the base controller
of a base station during the registration procedure in accordance with the
invention;
FIG. 6 illustrates a registration request frame;
FIG. 7 illustrates a base reply frame;
FIG. 8 illustrates an acknowledge frame; and
FIG. 9 is a flow chart of the operations performed by a mobile unit during
the registration procedure in accordance with the invention.
DETAILED DESCRIPTION
FIG. 1 illustrates a wireless communication system designed to provide PCS
type service for a plurality of mobile units such as mobile units 105 and
118. Base stations 102-104 are each, capable of maintaining radio links
such as RF links 113 and 120 with mobile units such as mobile units 105
and 118 respectively, and each base station is interconnected to the
system controller and switch 101 via a digital link such as link 109.
Advantageously, these links may be primary rate interface (PRI) links as
defined for ISDN communication. System controller and switch 101 provides
the control and switching facility for interconnecting base stations with
each other via links 108-110 or interconnecting a base station with
central office 115 via link 116 and the corresponding digital link for the
base station. A link such as link 108 can advantageously handle up to 23
different calls. Link 116 may be a plurality of PRI links.
Central office 115 is a central office such as the AT&T 5ESS. Central
office 115 is also interconnected to the public telephone system. System
controller and switch 101 may advantageously be the switching system
disclosed in B. M. Bales, et al. "A Switching System Using Identical
Switching Nodes", Ser. No. 07/636521, filed on Dec. 31, 1990, and assigned
to the same assignee as the present application. This application of B. M.
Bales, et al. is hereby incorporated by reference.
Wireless communication between a base station and a mobile unit, such as
base station 103 and mobile unit 105, is via a wireless communication link
such as RF link 113. This wireless communication is implemented utilizing
spread spectrum technology with frequency hopping. The specific
requirements for the frequency hopping are set forth in a Report and Order
in General Docket Number 89-354, this Report and Order being adopted by
the Federal Communications Commission on Jun. 14, 1990 and released on
Jul. 9, 1990. In one implementation of the invention, one hundred seventy
three possible channels are created in the 902-928 MHz frequency band and
each active telephone call transmits and receives audio information in a
random set of 50 of these channels. Transmission only occurs for 5
milliseconds in each channel. This type of transmission is disclosed in
greater detail in M. E. Gillis, et al. "A Cordless Telephone Arranged for
Operation in a Frequency Hopping System", filed Oct. 21, 1991, Ser. No.
07/779754 and assigned to the same assignee as the present application.
The U. S. patent application of Gillis, et al. is hereby incorporated by
reference. Audio information received by base station 103 from mobile unit
105 via RF link 113 is then transmitted to system controller and switch
101 via link 109.
Consider now a first example of how mobile unit 105 initially registers
itself with a base station and consequently system controller and switch
101. As previously mentioned, there are 173 channels available. Of the
available channels, eight channels are dedicated for specific purposes as
illustrated in FIG. 4, which gives a table of dedicated channel
assignments. All eight of the dedicated channels are commonly shared by
the base stations and the mobile units. The remaining 165 channels are
utilized for carrying audio information with a set of 50 channels being
utilized for each telephone call. Consider mobile unit registration with
respect to mobile unit 105 which is assigned to base station 103. Mobile
unit 105 transmits a registration request message, containing its handset
ID (also referred to as a security code) on the dedicated registration
request channel (channel 1). In addition, the registration request
contains the system ID and time slot number fields. This request is
illustrated in FIG. 6. Mobile unit 105 then tunes to the dedicated based
reply channel (channel 2) to await a response from the base stations. The
dotting sequence (also referred to as the synchronization pattern)
illustrated in the registration request frame of FIG. 6 allows idle radio
units in the base stations to come into synchronization with the frame
transmission of the registration request message. The idle radio units of
the base stations monitor the registration request channel looking for a
dotting sequence from any mobile unit. If a base station is receiving a
sufficiently strong signal, it is assumed that the base station will be
able to acquire synchronization within one dotting frame. If the base
station is not receiving a sufficiently strong signal, the base station
does not acquire synchronization, and the base station is not a good
candidate for registration.
Assume for this example, that the base stations receive descending levels
of signal strength in the following order: base station 104, base station
103, base station 102. The radio units in each base station are capable of
deciphering levels of signal strength. Since it is assumed that base
station 103 (the assigned base station for mobile unit 105) has an idle
receiver, base station 103 responds to the registration request message by
transmitting the base reply message in the base reply frame illustrated in
FIG. 7. The base reply frame is transmitted in the base reply channel in
time slot 0. Since base stations 102 and 104 are not assigned base
stations for mobile unit 105, those base stations each start a response
timer whose time interval is determined by signal strength of the
registration request. This time interval identifies a time slot in the
dedicated base reply channel during which the base station responds to the
mobile unit. The number of time slots available in the base reply channel
is determined by the granularity of the signal strength detector plus one
dedicated time slot for the assigned base station. The stronger the signal
is, the shorter the time interval. When the timer expires, the base
station sends a base reply message on the base reply channel containing
the mobile unit's ID.
In the present example, base station 103 responds first, followed by base
station 104, and then base station 102. Mobile unit 105 is responsive to
the base reply frame to select the base station in the earliest time slot.
In the present example, mobile unit 105 selects base station 103. In the
base reply frame, base station 103 does not include its own
identification; rather, base station 103 stores the time slot in which it
transmitted the base reply. This is done to save transmission capacity and
also to assure that mobile unit received the base reply. When mobile unit
105 receives the base reply frame, it responds by transmitting an
acknowledgement on the dedicated handset acknowledged channel (channel 3).
In addition to the mobile unit ID, the acknowledgement message also
carries the time slot number which designates that base station 103 has
been selected for registration. As illustrated in FIG. 8, the handset
acknowledgement frame is a total of 7.5 milliseconds.
After base station 103 receives the acknowledgement message, it transmits
to system controller and switch 101 the ID of mobile unit 105. During
transmission on the registration request, base reply, and handset
acknowledge channels, collisions can occur where other mobile units or
base stations are also attempting to transmit on these channels at the
same time as base station 103 and mobile unit 105. The problems arising
from these collisions are solved as set forth with respect to the
explanation of FIG. 5 which details the actions taken by a base station
during the registration process and FIG. 9 which details the action taken
by a mobile unit during the registration process. Once registration is
accomplished, the system controller and switch 101 has the information
required to route an incoming call for mobile unit 105 to base station 103
in the present example. Since a mobile unit may be moving, each mobile
unit implements the registration procedure every 30 seconds.
If base station 103 had no idle radio unit or had received the registration
request message from mobile unit 105 at too low a signal level, then base
station 103 would not have responded in the first time slot. In that
situation, base station 104 would respond before base station 102, and
mobile unit 105 would respond to base station 104 in the same manner as
previously described for base station 103. Mobile unit 105 would have
become registered with base station 104.
Consider now a second example of how mobile unit 105 initially registers
itself with a base station. In this example, it is assumed that mobile
unit 105 is assigned to base station 103, but base station 103 does not
respond to the registration request message. However, both base stations
102 and 104 respond with base reply messages in time slot 1. Mobile unit
105 is responsive to the base reply messages to determine that a collision
has occurred in time slot 1 (also referred to as a collided time slot). As
illustrated in FIG. 6, the registration request includes a time slot
number field. When mobile unit 105 sent out the initial registration
request message, it designated time slot 0 in the time slot field. A "0"
in the time slot field indicates that this is the first registration
request message. In response to detecting a collision in time slot 1,
mobile unit 105 sends out a second registration request message with a "1"
in the time slot field. This informs the base stations that a collision
has occurred in time slot 1. Both base stations 102 and 104 are responsive
to the second registration request message to each transmit a base reply
message in a random time slot as selected by a random number generator in
each base stations 102 and 104. Mobile unit 105 then selects either base
station 102 or 104 on the basis of earliest random time slot and transmits
the acknowledge message to that base station. The selected base station
then registers mobile unit 105. If the second registration request message
results in a collided time slot, then mobile unit 105 sends another
registration request designating the time slot on which the collision
first occurred which in this example is time slot 1.
FIG. 2 illustrates, in greater detail, base station 103. Base controller
201 controls the operations of radio units 202-204. Radio unit 202 is
shown in detail, and radio units 203-204 are similar in design. Radio unit
202 includes clock 216 for providing synchronization to Time Domain
Demultiplexer (TDD) 211 and protocol converter 215. Radio unit 202 also
includes radio frequency (RF) transceiver 213, antenna 217 and frequency
synthesizer 212. Transceiver 213 comprises both a RF transmitter and a RF
receiver. Transceiver 213 demodulates voice signals and control signals
transmitted by a mobile unit and couples the voice signals via protocol
converter 215 to base controller 201 via link 210. Base controller 201
provides all control for radio unit 202 via bus 210. Control signals
received from transceiver 213 are transferred through protocol converter
215 to base controller 201 via bus 210. Protocol converter 215 is also
responsive to digitally encoded voice signals received via bus 210 from
base controller 201 to convert those digital voice signals into the format
utilized for transmission to a mobile unit. Signal strength monitor 214 is
responsive to a signal from RF transceiver 213 to arrive at a digital
value representing the signal strength being received by RF transceiver
213 and to transmit this digital value to base controller 201 via bus 210.
Base controller 201 also controls the hopping sequence by generating a
pseudo-random data list of frequencies using a seed in a pseudo-random
generator as is well known in the art. The pseudo-randomly generated data
list is transferred to TDD 211 via bus 210 by base controller 201. TDD 211
controls the frequency selected in the frequency hopping cycle by
transmitting to frequency synthesizer 212 at appropriate times the values
stored in the data list generated by base controller 210. TDD 212 also
refreshes the frequency synthesizer 212 as the synthesizer progresses
through the frequency hopping cycle. In order to have RF transceiver 213
transmit and receive on one of the dedicated channels, base controller 201
transfers frequency data to TDD 211 which contains only one frequency,
e.g. the frequency of the dedicated channel. TDD 211 then controls
frequency synthesizer 212 so that transceiver 213 operates on this one
frequency. Clock 216 is used to achieve synchronization with the dotting
pattern previously described and to achieve Big Sync (Big Sync is
described in a later section) based on input received from transceiver 213
and TDD 211.
Consider the first example which was given with respect to FIG. 1 in light
of the detailed block diagram of radio unit 202 of FIG. 2. Further assume,
for the sake of the example, that radio unit 202 was idle when mobile unit
105 started the registration procedure. Since radio unit 202 was idle,
base controller 201 transmitted frequency data to TDD 211 which caused
frequency synthesizer 212 to control RF transceiver 213 such that RF
transceiver 213 monitored the registration request channel. When mobile
unit 105 transmits the registration request, transceiver 213 receives this
request and transmits a signal indicative of the signal strength to signal
strength monitor 214. In addition, transceiver 213, via protocol converter
215 and bus 210, transmits the registration request including the security
code which identifies mobile unit 105 to base controller 201. Base
controller 201 is responsive to the registration request from radio unit
202 to request the signal strength from signal strength monitor 205 via
bus 210. Base controller 201 is responsive to the signal strength from
signal strength monitor 205 to ascertain whether or not the signal
strength is strong enough so that base station 103 can handle calls from
mobile unit 105. Assuming that the signal strength is strong enough, base
controller 201 transmits frequency data to TDD 211 via bus 210 to tune
transceiver 213 to the base reply channel. Then, base controller 201
transmits via bus 210 the base reply message to protocol converter 215,
which causes transceiver 213 to transmit this message on the base reply
channel during time slot 1.
Base controller 201 tunes transceiver 213 to the acknowledge channel by
controlling TDD 211 and frequency synthesizer 212. Transceiver 213 is
responsive to the acknowledge message transmitted by mobile unit 105 to
transfer this message to base controller 201 via protocol converter 215
and bus 210. The acknowledge message contains the ID of mobile unit 105.
Base controller 201 transmits the mobile unit ID to system controller and
switch 101 as part of a registration message.
Clock 216, under control of base controller 201, performs the
synchronization process with respect to the dotting information--on both
the registration request and the acknowledge messages. Similarly,
transceiver 213 is controlled by base controller 201 via protocol
converter 215 and clock 216 to provide the dotting synchronization
information during the transmission of the base reply on the base reply
channel.
Base stations 102 and 104 have the same configuration as shown for base
station 103 in FIG. 2. For the first example, consider how base station
104 responds to the registration request from mobile unit 105 assuming
base station 103 had not re | | |
