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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to mobile communications systems such as
cellular telephone systems and, more specifically, to a system for
reducing power consumption in a mobile or portable transceiver of such a
system.
In many communications systems, the transceivers are only sporadically
active. For example, a cellular telephone remains idle for significant
periods of time when no call is in progress. During such idle periods the
cellular telephone consumes substantially the same amount of power as
during active periods. However, to ensure that a transceiver receives
sporadically transmitted messages, it must continuously monitor a channel.
In a digital cellular telephone system, such as that described in U.S.
Pat. No. 5,056,031 entitled "Method and Apparatus for Controlling
Transmission Power in a CDMA Cellular Telephone System" and copending U.S.
patent application Ser. No. 07/543,496, now abandoned, entitled "System
and Method for Generating Signal Waveforms in a CDMA Cellular Telephone
System," both assigned to the assignee of the present invention, messages
transmitted by a base station may include those for alerting the mobile
station to the presence of an incoming call and those for periodically
updating system parameters in the mobile station.
While a mobile station installed in a vehicle may be powered by the
vehicle's electrical system, prolonged use of the mobile station when the
vehicle is not operating may drain the vehicle's battery. Furthermore,
many mobile stations are portable and powered by an internal battery.
Personal Communications Systems (PCS) handsets are almost exclusively
battery powered. In any such stations it is desirable to minimize power
consumption to increase battery life.
A mobile station may consume significant amounts of power by continuously
monitoring the channel for incoming messages. The resulting power drain on
the battery reduces the time available for actively handling calls. A
system that reduces power consumption by periodically monitoring the
channel for incoming messages during idle periods would be highly
desirable. These problems and deficiencies are clearly felt in the art and
are solved by the present invention in the manner described below.
SUMMARY OF THE INVENTION
The present invention reduces receiver power consumption in a communication
system having a transmitter communicating with one or more remote
receivers on a channel. Each receiver periodically enters an "active
state" during which it can receive messages on the channel. The
transmitter sends one or more messages to each receiver during each
successive occurrence of the active state of the receiver. Although we
generally refer herein to a single receiver, it is understood that more
than one such receiver in a system may be active simultaneously. During
the "inactive state" of a receiver, the time period between successive
active states, the transmitter does not send any messages to that
receiver, although it may send messages to other receivers in the system
that are in the active state. In the inactive state, the receiver may
perform any action not requiring coordination with the transmitter. The
receiver may use the inactive state to reduce its power consumption by
removing power from one or more components, such as those components used
for monitoring the channel.
The channel is divided in the time dimension into a continuous stream of
"slots." The receiver has a "slot cycle," which comprises two or more
slots. The receiver is assigned one slot of its slot cycle during which it
must monitor the channel. The receiver is generally in the active state
only during its assigned slot and in the inactive state during the
remainder of its slot cycle. However, if the message itself directs the
receiver to perform some further action, it must remain in the active
state until it completes the action.
The transmitter and receiver slot timing should be aligned in the time
dimension to ensure that transmitted messages are not lost but are
received in the assigned slot. In certain embodiments, the transmitter and
receiver slot timing may be continuously synchronized. However, in other
embodiments, the receiver may operate independently during the inactive
state and some timing drift may occur relative to the transmitter. In such
embodiments, the receiver may periodically synchronize its slot timing to
that of the transmitter.
In a digital cellular telephone system, for example, the receiver may
acquire and track a pilot signal that the transmitter provides on a
separate pilot channel. In the inactive state, the receiver may conserve
power by removing power from the pilot signal tracking circuitry during
the inactive state. In the inactive state, the receiver may maintain its
slot timing using an internal clock source. A short time before the next
occurrence of its assigned slot, the receiver may apply power to this
circuitry and require the pilot signal. The receiver may then realign its
timing with that of the transmitter by synchronizing it to the pilot
signal. In addition to applying power and requiring a pilot signal, the
receiver may perform any other actions or initializations to prepare it to
receive a message at the beginning of its assigned slot.
Each message may also contain a field indicating whether another message is
forthcoming. If an additional message is forthcoming, the receiver remains
in the active state into the next slot. If there are no additional
messages, the receiver may immediately enter the inactive state for the
remainder of the slot cycle.
In a system having multiple receivers, each receiver is pseudorandomly
assigned a slot in its slot cycle. An identification number uniquely
associated with the receiver may be provided to a hash function, which
pseudorandomly produces the assigned slot number.
All receivers in the system need not have the same slot cycle. Furthermore,
the slot cycle of a receiver may change during operation. For example, the
receiver may select a new slot cycle and send a message to the transmitter
notifying it of the new slot cycle. Although either the receiver or
transmitter may change the slot cycle of the receiver, both must have the
slot cycle information.
The foregoing, together with other features and advantages of the present
invention, will become more apparent when referring to the following
specification, claims, and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of our invention, we now refer to the
following detailed description of the embodiments illustrated in the
accompanying drawings, wherein:
FIG. 1 illustrates the slotted transmission of messages in an embodiment of
the present invention having a transmitter and two receivers;
FIGS. 2(a-d) illustrate the timing relationship between transmitter slot
signals and receiver slot signals at successive points in time;
FIG. 3 illustrates an embodiment of the present invention having a message
channel and a pilot channel;
FIG. 4 illustrates the transition from the inactive state to the active
state at the assigned slot of a receiver; and
FIGS. .ident.(a-b)illustrate a message having a sequence number field.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, a transmitter 10 may send messages to two receivers 12 and 14.
Other embodiments may have a greater or lesser number of receivers. In a
digital cellular telephone system such as discussed in the
above-referenced U.S. Patent and copending application, transmitter 10 is
disposed in a base station or cell site (not shown) and transmits messages
to receivers 12 and 14, disposed in mobile stations (not shown). The
mobile stations may be cellular telephones or personal communications
system (PCS) handsets.
The base station transmits these messages, which may provide the mobile
station with an indication of an incoming telephone call, with a
requirement to take special control actions, or with updated system
parameters, on a "paging" channel. The paging channel transmissions are
represented by the broken lines in FIG. 1.
Receivers 12 and 14 have 32-bit electronic serial numbers (ESN) 16 and 18,
respectively. The ESN of each receiver is different from that of all other
receivers. In a cellular telephone system, a call to a cellular telephone
is routed to a mobile telephone switching office (MTSO, not shown). The
MTSO in turn routes the call to a base station within transmitting range
of the mobile station. Either the MTSO or the base station includes means
for converting the telephone number of the cellular telephone into the
mobile station ESN.
During a mobile station initialization or "registration" as it is known in
the cellular communications art, or at other times as required, receivers
12 and 14 each select a slot cycle index 20 and 22 respectively. Slot
cycle indices 20 and 22 determine the length of the slot cycles 24 and 26
of receivers 12 and 14 respectively. A processor in a mobile station may
select a slot cycle index using an algorithm or it may use a predetermined
value. For example, both slot cycle indices 20 and 22 have the value "1"
in FIG. 1. A range of 1-7 is preferred for slot cycle indices 20 and 22.
Thus, the maximum slot cycle index, MAX.sub.13 SSI is "7." A value of "0"
may be chosen to indicate that a receiver will continuously monitor the
channel, i.e., the slotted communication method of the present invention
will be bypassed. In a cellular telephone system, each mobile station
transmits the slot cycle index selected by its receiver to the base
station, which requires this information to access the receivers.
Receivers 12 and 14 compute slot cycles 24 and 26, which are
5.times.2.sup.(slot cycle index 20) and 5.times.2.sup.(slot cycle index
22) slots in length respectively. Transmitter 10 generates timing 28,
which comprises a stream of periodic slots 30. Similarly, receiver 12
generates timing 32, which comprises a stream of periodic slots 34, and
receiver 14 generates timing 36, which comprises a stream of periodic
slots 38. Slots 30, 34, and 38 are equal in length and are preferably 200
milliseconds (ms) in length. Thus, using a range of slot cycle indices of
1-7 in the above function yields a range of slot cycles of between 10 and
640 slots in length, which corresponds to a time range of between 2 and
128 seconds using 200 ms slots.
Receiver 12 monitors the channel during an active slot 40, which occurs
once in each slot cycle 24. Receiver 14 monitors the channel during an
active slot 42, which occurs once in each slot cycle 26. Assigned slots
are pseudorandomly selected to facilitate their even distribution among
the slots of a slot cycle having a given length. Although many
pseudorandom methods for selecting assigned slots are suitable, a method
using Equations 1 and 2, below, is preferred.
Equations 1 and 2 may be used by transmitter 10 and receivers 12 and 14 to
determine the periodic points in time, relative to "system time," at which
assigned slots occur. At the beginning of system time, the first slot
(slot.sub.0) of each possible slot cycle occurred simultaneously. System
time may be the current value of a counter (not shown) in each transmitter
10 and receiver 12 and 14. Such a counter (not shown) can run for
thousands of years without repeating if it has a sufficiently large number
of bits, and can easily be constructed by one skilled in the art. In
addition, transmitter 10 may synchronize its counter (not shown) to a
universal broadcast time source, such as that produced by the Global
Positioning System (GPS). Receivers 12 and 14 synchronize their counters
(not shown) to that of transmitter 10, as discussed below.
PGSLOT= 52.sup.MAX --.sup.SSI
x((40503x(L.sym.H.sym.D))mod2.sup.16)/2.sup.16 (1)
where:
MAX.sub.13 SSI is the maximum slot cycle index;
L is the least significant 16 bits of the ESN;
H is the most significant 16 bits of the ESN;
D is a number 6 times the least significant 12 bits of the ESN;
X represents the largest integer less than or equal to X ; represents a
bitwise exclusive-OR operation; and all other operations are integer
arithmetic.
Equation 1 may be solved for PGSLOT, which represents the time at which the
assigned slot occurs as measured from the beginning of the slot cycle of
maximum length. Equation 2, below, relates this time to system time.
Receiver 12 uses ESN 16 to calculate its PGSLOT and receiver 14 uses ESN
18. Note that PGSLOT has a maximum value of 5.times.2.sup.MAX --.sup.SSI
slots (2.sup.MAX --.sup.SSI seconds). However, receivers 12 and 14 may
choose shorter slot cycles, as exemplified by FIG. 1 where both slot cycle
24 and 26 are 10 slots (2 seconds) in length.
Active slots 40 and 42 occur periodically within slot cycles 24 and 26,
respectively. Equation 2 below may be used to determine when active slots
40 and 42 occur relative to system time.
(N-PGSLOT)mod(5.times.2.sup.SSI)=0 (2)
In Equation 2, N is the current slot. As discussed above, the first slot of
all possible slot cycles occurs at the beginning of system time, i.e.,
when N equals zero. Receivers 12 and 14 each substitute slot cycle indices
20 and 22 respectively for SSI in Equation 2. The value of PGSLOT is also
unique to each receiver 12 and 14 because it is derived from ESN 16 and
18, respectively. Receivers 12 and 14 each may compute Equation 2 once
each slot cycle and, if true, monitor the channel for incoming messages
because the current slot is active slot 40 or 42, respectively. Of course,
receivers 12 and 14 need not compute Equation 2 each slot cycle. Receivers
12 and 14 may compute Equation 2 at some initial point in time and, upon
Equation 2 being true, may thereafter monitor the channel periodically at
intervals of slot cycle 24 and slot cycle 26.
The computations discussed above in reference to mobile station receivers
12 and 14 are also performed by base station transmitter 10. For example,
when a caller dials a telephone number associated with a mobile station,
the MTSO routes the call to a base station in the vicinity of the mobile
station. The base station retrieves the ESN and slot cycle of the mobile
station by providing a lookup table with the telephone number. The base
station computes the assigned slot in which it must transmit to the mobile
station using Equations 1 and 2. When the base station slot timing
generates the assigned slot, the transmitter sends a message that
indicates the presence of an incoming call to the mobile station.
When mobile station receiver 12, for example, selects slot cycle index 20,
it transmits the value selected to the base station on another channel
(not shown). The base station acknowledges the selection by transmitting
an acknowledgement message to mobile station receiver 12. Transmitter 10
begins using the newly selected slot cycle index after transmitting the
acknowledgement. However, if receiver 12 does not receive such an
acknowledgement because of a transmission error, receiver 12 will continue
to use the old slot cycle index. Messages may be lost if transmitter 10
does not compute the active slots of receiver 12 using the same slot cycle
index that receiver 12 uses to compute its active slot. To facilitate
recovery from such an error, receiver 12 selects a default slot cycle
index of "1" if it does not receive an acknowledgement. A slot cycle index
of "1" ensures that an active slot as computed by transmitter 10 will
coincide with an assigned slot as computed by receiver 12. Actually all
that is required is that the receiver uses a slot cycle index less than or
equal to that of the transmitter for the slots to line up.
Slot timing 28 of base station transmitter 10 is synchronized to slot
timing 32 during transmission of messages to mobile station receiver 12
and to slot timing 36 during transmission of messages to mobile station
receiver 14. Transmitter 10 synchronizes slot timing 28 to its system time
counter (not shown).
The timing relationship between a base station transmitter and a mobile
station receiver is shown in FIGS. 2a-2d. FIGS. 2a-2d represent successive
"snapshots" in time and show a portion of the transmitter and receiver
signals at these successive points in time. Note that the arrow 72 is
simply a fixed point in time that serves as a common reference point for
facilitating comparison of the signals throughout FIGS. 2a-2d. The signals
can be thought of as moving in time from left to right towards arrow 72,
as though on conveyor belts.
In FIG. 2a, a base station transmitter, such as base station transmitter 10
of FIG. 1, transmits a pilot signal 50, synchronized to the system clock,
on a separate pilot channel. Base station transmitter 10 synchronizes
transmitter slot signal 52, which has slots 54, 56, and 58, to pilot
signal 50. Although pilot signal 50 is shown as having the same period as
slots 54, 56, and 58, it may be any type of signal from which such a
periodic signal could be derived. Slot 54 has messages 60, 62, 64, and 66.
Although at least one message must be transmitted in each slot, the
maximum number of messages that may be transmitted in a slot is limited
only by the transmission rate and slot length.
FIG. 2a shows the signals at a point in time during which the receiver is
in the inactive state. Receiver slot signal 68 is shown in broken lines to
represent the inactive state because in the inactive state the receiver
may conserve power by removing power from circuitry (not shown) that
monitors the channel for messages. It may also remove power from circuitry
(not shown) that tracks pilot signal 50. It is emphasized that the
receiver may perform any action in the inactive state that does not
require coordination with the transmitter.
As shown in FIG. 2a, receiver slot signal 68 may not be precisely aligned
with transmitter slot signal 52 because in the inactive state the receiver
is not tracking pilot signal 50 to which it could otherwise synchronize
slot signal 68. However, the maximum time by which these signals may drift
apart is substantially less than one slot.
Slot 70 is the active slot of the receiver and may correspond to active
slot 40 or 42 of FIG. 1. The transmitter will send a message at the point
in time when the first message, message 66, reaches arrow 72. The
transmitter timing may determine this point by counting slots of the slot
cycles from the beginning of system time. For example, slot zero occurred
for the first time at the beginning of system time and repeats with a
periodicity of the slot cycle. Although the receiver timing may have
drifted slightly from the transmitter timing during the preceding inactive
state, they are synchronized long before the occurrence of the next slot.
Typically the drift is only about 2 microseconds for a receiver using a
slot cycle of 2 seconds. Therefore, the receiver can determine the point
in time at which it may expect to receive a message, i.e., arrow 72, with
a precision well within a single slot. It can thus begin to transition to
the active state shortly before this occurrence.
FIG. 2b shows the same signals at a point in time later than that of FIG.
2a. At a point in time between that of FIG. 2a and that of FIG. 2b, the
receiver began the transition to the active state and applied power to the
circuitry that tracks pilot signal 50. It is preferred that the transition
begin after the beginning of slot 74, the slot preceding active slot 70,
has reached arrow 72. However, the transition may begin at an earlier
time. During the transition state, the receiver may apply power to
circuitry, perform hardware resets, perform initialization routines,
require pilot signal 50, synchronize signals, or perform any action
necessary to prepare it to receive messages in active slot 70 at arrow 72.
The transition state 80 is shown in FIG. 4 beginning in slot.sub.4, the
slot preceding the active slot, slot.sub.5. The receiver is in the
inactive state 81 before this time. During slot.sub.5, the receiver in is
the active state 82, and returns to inactive state 84 at the end of
slot.sub.5. In the absence of conditions discussed below, a receiver is in
the active state only during its active slot.
Returning to FIG. 2c, which shows the signals at a point in time later than
that of FIG. 2b, receiver slot signal 68 is completely synchronized to
transmitter slot signal 52. The receiver has re acquired and is tracking
pilot signal 50. The receiver is in the active state because it is
prepared to receive a message in active slot 70 at arrow 72.
At the point in time represented by FIG. 2d, the receiver is receiving
message 60. It has already received messages 62, 64, and 66. Each message
may have several fields, for example, fields 90, 92, 94, and 96 of message
62. The fields contain the address of the receiver and instructions for
the receiver. The field may contain system parameters for use by the
receiver. Alternatively, the message field may contain the phone number
when the transmitter is "paging" the receiver. The receiver decodes each
message and may perform one or more actions according to the values
contained in the fields.
FIG. 3 shows a block diagram of a system for generating the signals
described in FIGS. 2a-2d. The system comprises base station transmitter
120 and mobile station receiver 122. A user (not shown) may, for example,
initiate a call to the mobile station having receiver 122. In a cellular
telephone system, such a call is received at the MTSO (not shown) and
includes the telephone number of the mobile station being called. The MTSO
routes the call to a base station. The MTSO obtains the mobile station ESN
and slot cycle in response to the telephone number of the mobile station.
The MTSO then provides the base station with input information 124, which
includes the ESN and slot cycle of the mobile station. Information 124 is
received by the transmitter processor 126, which may be a microprocessor
or other control circuitry. Processor 126 may use the hash function of
Equations 1 above to obtain the assigned slot of the mobile station.
Transmitter slot signal generator 130 generates an active slot signal 129,
which may interrupt processor 126 when processor 126 must provide messages
128, i.e., a short time before the active slot. Transmitter slot signal
generator 130 may have a counter for maintaining a slot count.
Alternatively, the count may be maintained by processor 126. Transmitter
slot signal generator 130 synchronizes messages 128 to the system clock
138, which is generated by transmitter clock source 140. Pilot signal
generator 136 generates pilot signal 134, which is also synchronized to
system clock | | |
