Note: Descriptions are shown in the official language in which they were submitted.
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REACQUISITION AND HANDOFF IN A SLOTTED MODE
COMMUNICATION SYSTEM
FIELD OF THE INVENTION
The invention relates to wireless communications systems. In
particular, the invention relates to reacquisition and handoff by a remote
unit in a wireless communication system.
BACKGROUND OF THE INVENTION
A wireless communication system may comprise multiple remote
units and multiple base stations. Figure 1 exemplifies an embodiment of a
terrestrial wireless communication system with three remote units 10A,
10B and 10C and two base stations 12. In Figure 1, the three remote units
are shown as a mobile telephone unit installed in a car 10A, a portable
computer remote 10B, and a fixed location unit 10C such as might be
found in a wireless local loop or meter reading system. Remote units may
be any type of communication unit such as, for example, hand-held
personal communication system units, portable data units such as a
personal data assistant, or fixed location data units such as meter reading
equipment. Figure 1 shows a forward link 14 from the base station 12 to
the remote units 10 and a reverse link 16 from the remote units 10 to the
base stations 12.
Communication between remote units and base stations, over the
wireless channel, can be accomplished using one of a variety of multiple
access techniques which facilitate a large number of users in a limited
frequency spectrum. These multiple access techniques include time
division multiple access (TDMA), frequency division multiple access
(FDMA), and code division multiple access (CDMA). An industry
standard for CDMA is set forth in the TIA/EIA Interim Standard entitled
"Mobile Station - Base Station Compatibility Standard for Dual-Mode
Wideband Spread Spectrum Cellular System", TIA/EIA/IS-95, and its
progeny (collectively referred to here as IS-95), the contents of which are
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incorporated by reference herein in their entirety. Additional information
concerning a CDMA communication system is disclosed in U.S. Patent No.
4,901,307, entitled SPREAD SPECTRUM MULTIPLE ACCESS
COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL
REPEATERS, (the '307 patent) assigned to the assignee of the present
invention and incorporated in its entirety herein by reference.
In the '307 patent, a multiple access technique is disclosed where a
large number of mobile telephone system users, each having a transceiver,
communicate through base stations using CDMA spread spectrum
communication signals. The CDMA modulation techniques disclosed i n
the '307 patent offer many advantages over other modulation techniques
used in wireless communication systems such as TDMA and FDMA. For
example, CDMA permits the frequency spectrum to be reused multiple
times, thereby permitting an increase in system user capacity.
Additionally, use of CDMA techniques permits the special problems of the
terrestrial channel to be overcome by mitigation of the adverse effects of
multipath, e.g. fading, while also exploiting the advantages thereof.
In a wireless communication system, a signal may travel several
distinct propagation paths as it propagates between base stations and
remote units. The multipath signal generated by the characteristics of the
wireless channel presents a challenge to the communication system. One
characteristic of a multipath channel is the time spread introduced in a
signal that is transmitted through the channel. For example, if an ideal
impulse is transmitted over a multipath channel, the received signal
appears as a stream of pulses. Another characteristic of the multipath
channel is that each path through the channel may cause a different
attenuation factor. For example, if an ideal impulse is transmitted over a
multipath channel, each pulse of the received stream of pulses generally
has a different signal strength than other received pulses. Yet another
characteristic of the multipath channel is that each path through the
channel may cause a different phase on the signal. For example, if an ideal
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impulse is transmitted over a multipath channel, each pulse of the
received stream of pulses generally has a different phase than other
received pulses.
In the wireless channel, the multipath is created by reflection of the
signal from obstacles in the environment such as, for example, buildings,
trees, cars, and people. Accordingly, the wireless channel is generally a
time varying multipath channel due to the relative motion of the
structures that create the multipath. For example, if an ideal impulse is
transmitted over the time varying multipath channel, the received stream
of pulses changes in time delay, attenuation, and phase as a function of the
time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal
received by the remote unit and result in, among other things, fading of
the signal. Fading is the result of the phasing characteristics of the
multipath channel. A fade occurs when multipath vectors add
destructively, yielding a received signal that is smaller in amplitude than
either individual vector. For example if a sine wave is transmitted
through a multipath channel having two paths where the first path has an
attenuation factor of X dB, a time delay of 8 with a phase shift of 0
radians, and the second path has an attenuation factor of X dB, a time delay
of 8 with a phase shift of ~ + ~ radians, no signal is received at the output
of the channel because the two signals, being equal amplitude and opposite
phase, cancel each other. Thus, fading may have a severe negative effect
on the performance of a wireless communication system.
A CDMA communication system is optimized for operation in a
multipath environment. For example, the forward link and reverse link
signals are modulated with a high frequency pseudonoise (PN) sequence.
The PN modulation allows the many different multipath instances of the
same signal to be separately received through the use of a "rake" receiver
design. In a rake receiver, each element within a set of demodulation
elements can be assigned to an individual multipath instance of a signal.
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The demodulated outputs of the demodulation elements are then
combined to generate a combined signal. Thus, all of the multipath signal
instances must fade together before the combined signal experiences a deep
fade.
In a communication system based on the industry standard for
CDMA, IS-95, each of the multiple base stations transmits a pilot signal
having a common PN sequence. Each base station transmits the pilot
signal offset in time from neighboring base stations so that the signals can
be distinguished from one another at the remote unit. At any given time,
the remote unit may receive a variety of pilot signals from multiple base
stations. Using a copy of the PN sequence produced by a local PN
generator, the entire PN space can be searched by the remote unit. Using
the search results, the controller distinguishes pilot signals from multiple
base stations based on the time offset.
In the remote unit, a controller is used to assign demodulation
elements to the available multipath signal instances. A search engine is
used to provide data to the controller concerning the multipath
components of the received signal. The search engine measures the
arrival time and amplitude of the multipath components of a pilot signal
transmitted by the base stations. The effect of the multipath environment
on the pilot signal and the data signal transmitted by a common base
station is very similar because the signals travel through the same channel
at the same time. Therefore, determining the multipath environment's
effect on the pilot signal allows the controller to assign demodulation
elements to the data channel multipath signal instances.
The search engine determines the multipath components of the
pilot signals of base stations in the proximity of the remote unit by
searching through a sequence of potential PN offsets and measuring the
energy of the pilot signal received at each of the potential PN offsets. The
controller evaluates the energy associated with a potential offset, and, if it
exceeds a threshold, assigns a signal demodulation element to that offset.
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A method and apparatus of demodulation element assignment based on
searcher energy levels is disclosed in U.S. Patent No. 5,490,165 entitled
DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE
OF RECEIVING MULTIPLE SIGNALS, (the '165 patent) assigned to the
5 assignee of the present invention.
Figure 2 shows an exemplifying set of multipath signal instances of
a single pilot signal from a base station arriving at a remote unit. The
vertical axis represents the power received in decibels (dB). The horizontal
axis represents the delay in the arrival time of a signal instance due to
multipath delays. The axis (not shown) going into the page represents a
segment of time. Each signal spike in the common plane of the page has
arrived at the remote unit at a common time but has been transmitted by
the base station at a different time. Each signal spike 22-27 has traveled a
different path and therefore exhibits a different time delay, a different
amplitude, and a different phase response. The six different signal spikes
represented by spikes 22-27 are representative of a severe multipath
environment. A typical urban environment produces fewer usable paths.
The noise floor of the system is represented by the peaks and dips having
lower energy levels. The task of the search engine is to identify the delay,
as measured by the horizontal axis, and amplitude, as measured by the
vertical axis, of signal spikes 22- 27 for potential demodulation element
assignment.
Note, as shown in Figure 2, each of the multipath peaks varies i n
amplitude as a function of time as shown by the uneven ridge of each
multipath peak. In the limited time shown, there are no major changes in
the multipath peaks. Over a more extended time range, multipath peaks
disappear and new paths are created as time progresses. Multipath peaks
are likely to merge together or blur into a wide peak over time.
Typically, the operation of the search engine is overseen by a
controller. The controller commands the search engine to step through a
set of offsets, called a search window, that is likely to contain one or more
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multipath signal peaks suitable for assignment to a demodulation
element. For each offset, the search engine reports the energy it found
offset back to the controller. Demodulation elements may then be
assigned by the controller to the paths identified by the search engine (i.e.
the timing reference of their PN generators is aligned with the timing of
the identified path). Once a demodulation element has locked onto the
signal, it then tracks that path on its own without controller supervision,
until the path fades away or until the demodulation element is assigned to
another path by the controller.
As noted above, each base station in a given geographical area is
assigned a sequence offset of a common PN pilot sequence. For example,
according to IS-95, a PN sequence having 215 chips and repeating every
26.66 milliseconds (ms) is transmitted by each base station in the system at
one of 512 PN sequence offsets as a pilot signal. According to IS-95
operation, the base stations continually transmit the pilot signal which can
be used by the remote unit to identify the base station as well as other
functions, such as for example, determining the multipath environment
the remote unit is operating in and synchronization of remote unit timing
to the base station timing.
During initial power on, or any other situation when the remote
unit has lost a pilot signal such as when performing a hard handoff to a
different operating frequency , the remote unit evaluates all possible PN
offsets of the pilot PN sequence. Typically, a search engine measures the
pilot signal strength at all possible PN offsets, proceeding at a
measurement rate that produces an accurate measure of the pilot signal
present at the corresponding offset. Proceeding in this manner, the search
engine determines the PN offset of base stations which are geographically
near the remote unit. Searching each PN offset in this manner can take
anywhere from hundreds of milliseconds to a few seconds depending on
the channel conditions during acquisition. This amount of time for the
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remote unit to reacquire a pilot signal is detrimental to the remote unit
operation, and may be annoying to the user of the remote unit.
Figure 3 shows an extended portion of PN space on the horizontal
axis. The groups of peaks 30, 32 and 34 represent transmissions from three
different base stations. As shown, the signal from each base station signal
experiences a different multipath environment. Also, each base station
has a different PN offset from the PN reference 36. Thus, the controller
may select a set of PN offsets corresponding to search windows for any of
the identified base stations. This allows the remote unit to simultaneously
demodulate signals from multiple base stations by assigning
demodulation elements appropriately.
In a typical CDMA communication system, remote units
sporadically establish bi-directional communications with a base station.
For example, a cellular telephone remains idle for significant periods of
time when no call is in process. However, to ensure that any message
directed to a remote unit is received, the remote unit continuously
monitors the communication channel, even while it is idle. For example,
while idle, the remote unit monitors the forward link channel from the
base station to detect incoming calls. During such idle periods, the cellular
telephone continues to consume power to sustain the elements necessary
to monitor for signals from - the base stations. Many remote units are
portable and are powered by an internal battery. For example, personal
communication system (PCS) handsets are almost exclusively battery-
powered. The consumption of battery resources by the remote unit in idle
mode decreases the battery resources available to the remote unit when a
call is placed or received. Therefore, it is desirable to minimize power
consumption in a remote unit in the idle state and thereby increase battery
life.
One means of reducing remote unit power consumption in a
communication system is disclosed in U.S. Patent No. 5,392,287, entitled
APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION
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IN A MOBILE COMMUNICATION RECEIVER (the '287 patent), assigned
to the assignee of the present invention and hereby incorporated in its
entirety herein by reference. In the '287 patent, a technique for reducing
power consumption in a remote unit operating in an idle mode (i.e. a
remote unit which is not engaged in bi-directional communication with a
base station) is disclosed. In idle, each remote unit periodically enters an
"active" state during which it prepares to and receives messages .on a
forward link communication channel. In the time period between
successive active states, the remote unit enters an "inactive" state. During
the remote unit's inactive state, the base station does not send any
messages to that remote unit, although it may send messages to other
remote units in the system that are in the active state.
As disclosed in the '287 patent, a base station broadcasts messages
which are received by all remote units within the base station coverage
area on a "paging channel." All idle remote units within the base station
coverage area monitor the paging channel. The paging channel is divided
in the time dimension into a continuous stream of "slots." Each remote
unit operating in slotted mode monitors only specific slots which have
been assigned to it as assigned slots. The paging channel continually
transmits messages in numbered slots, repeating the slot sequence, such as
for example, every 640 slots. When a remote unit enters the coverage area
of a base station, or if a remote unit is initially powered on, it
communicates its presence to a preferred base station. Typically the
preferred base station is the base station which has the strongest pilot
signal as measured by the remote unit. .
The preferred base station, along with a plurality of geographically
near neighboring base stations, assign a slot, or a plurality of slots, within
their respective paging channels, for the remote unit to monitor. The base
station uses the slots in the paging channel to transmit control
information to a remote unit, if necessary. The remote unit may also
monitor a timing signal from the preferred base station allowing the
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remote unit to align, in the time dimension, to the base station slot
timing. By aligning in the time dimension to the 'preferred base station
slot timing, the remote unit can determine when a paging channel slot
sequence begins. Thus, knowing when the paging channel slot sequence
begins, which slots are assigned for it to monitor, the total number of slots
in the repetitive paging channel sequence of slots, and the period of each
slot, the remote unit is able to determine when its assigned slots occur.
Generally, the remote unit is in the inactive state while the base
station is transmitting on the paging channel in slots which are not within
the remote unit's assigned set. While in the inactive state, the remote unit
does not monitor timing signals transmitted by the base station,
maintaining slot timing using an internal clock source. Additionally,
while in the inactive state the remote unit may remove power from
selected circuitry, such as, for example, circuits which monitor pilot signals
transmitted by base stations to detect changes in the wireless channel
including the search engine. Using its internal timing, the remote unit
transits to its active state a short period of time before the next occurrence
of an assigned slot.
When transiting to the active state, the remote unit applies power
to circuitry that monitors the wireless channel, including the search
engine. The search engine is used to reacquire the preferred base station's
pilot signal and to detect changes in the wireless channel which may have
occurred due to the movement of the remote unit or to the movement of
objects within the coverage area of the base station. In addition to
reacquiring the pilot signal, the remote unit may perform any other
actions or initializations in preparation for receiving a message at the
beginning of its assigned slot.
When the remote unit enters the active state, it may receive
messages in its assigned slots in the paging channel and respond to
commands from the base station. For example, the remote unit may be
commanded to activate a "traffic" channel to establish a bi-directional
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communication link for conducting subsequent voice communication i n
response to an incoming call. If there is no message from the base station,
or no command requesting the remote unit to remain active, at the end of
the assigned slot the remote unit returns to the inactive state. In addition,
5 the remote unit returns to the inactive state immediately if commanded to
do so by the base station.
During its active state, the remote unit's search engine measures the
pilot signal strengths of the preferred base station as well as the pilot
signal
strength of neighboring base stations. If the remote unit relocates from the
10 coverage area of one base station to another neighboring base station's
coverage area, the remote unit needs to "handoff" communication to the
neighboring base station. A handoff occurs when the transmitted pilot
signal strength of a neighbor base station becomes sufficiently stronger
than the preferred base station. When this occurs, the neighboring base
station is assigned as the preferred base station. Following a handoff, i n
the next active state, the remote unit monitors the paging channel of the
new preferred base station to receive messages and commands.
In addition to providing data for determining when a handoff
should occur, searches of the preferred base station's pilot signal allow the
remote unit to make adjustments to compensate for changes in the
multipath environment. For example, if one of the multipath signal
instances weakens to the point that it is unusable, the remote unit may
reassign demodulation elements accordingly.
Knowing the nominal PN offset of the preferred base station as well
as a neighboring set of base stations, typically, the controller passes a set
of
search parameters to the search engine specifying PN offsets at which
multipath signal instances of pilot signals are likely to be found. At the
completion of the search, the search engine passes the search results to the
controller. The controller analyzes the search results and selects a set of
search parameters for the next search. Following selection of the new
search parameters, the controller passes the parameters to the search
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engine and the search process is repeated. This process is repeated until
the remote unit once again enters the inactive idle state.
Generally, the preferred base station's pilot signal is stronger than
any of the measured neighbor pilot signals when the remote enters its
inactive state. Therefore, when the remote unit enters the next active
state, it monitors the paging channel of the preferred base station.
However, while the remote unit is in its inactive state, the remote unit
may relocate from the coverage area of the preferred base station into the
coverage area of a neighboring base station. When the remote unit is i n
the inactive state, it does not monitor the signal strengths of the preferred
and neighboring base stations. Thus, even if the neighboring base station
signal has increased to a signal strength sufficiently greater than the
preferred base station for a handoff to occur, the remote unit does not
perform a handoff in the inactive state. Thus, when the remote unit
returns to the active state, it may not be monitoring the optimum base
station, i.e. the base station with the strongest pilot signal strength. In
fact,
the signal strength from the preferred base station may be so low that the
remote unit is unable to properly decode the information therein. If the
remote unit is unable . to decode the information, the remote unit cannot
detect the information carried on the paging channel. Thus, the remote
unit operation is unreliable in these circumstances, causing delays,
retransmissions and increasing power consumption.
Therefore, there is a need in the art for a method and apparatus for
improved handoff in a slotted mode communication system.
SUMMARY OF THE INVENTION
The invention improves the handoff performance of a remote unit
in a slotted mode wireless communication system. During a previous
active state of the remote unit, a search list is built comprising pilot
signal
strengths of the preferred base station and neighboring base stations. The
search list may be built in accordance with the above-referenced U.S.
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Patent Application Serial No. 09/540,802, entitled PRIORITIZATION OF
SEARCHING BY A REMOTE UNIT IN A WIRELESS COMMUNICATION
SYSTEM (Attorney Docket No. QUALB.010A; Qualcomm Reference No.
PD990251) assigned to the assignee of the present invention and
incorporated in its entirety herein by reference, or the listing may be built
using other techniques well known in the art.
A controller in the remote unit evaluates the search list, selecting a
desired number of the strongest neighboring base stations. The controller
then passes search parameters corresponding to the selected base stations
to the search engine after entering the preparation period, prior to entering
the assigned slot period. The search engine, using the search parameters,
performs searches. The search parameters may comprise a range of PN
offsets, a number of noncoherent passes and an integration interval i n
accordance with the U.S. Patent Application Serial No. 09/346,368, entitled
DYNAMIC CONTROL OF SEARCH DURATION IN A WIRELESS
COMMUNICATION DEVICE (Attorney Docket No. QUALB.014A;
Qualcomm Reference No. PD990257) assigned to the assignee of the
present invention and incorporated in its entirety herein by reference.
The results of the searches are evaluated by the controller, prior to
entering the assigned slot period, which makes a determination if a
handoff should occur. If one of the neighboring base stations' pilot signal
is sufficiently stronger, such as, for example, at least 3dB greater than the
preferred base station, a handoff is performed. The handoff is completed
prior to the beginning of the assigned slot period.
In one embodiment search parameters for the preferred base station,
and a desired number of neighboring base stations, are passed to the search
engine during a preparation period, after the remote unit has transited
from an inactive state to an active state. While in the preparation period,
the search engine performs searches, the results of which are analyzed by
the controller. If one of the neighboring base stations' pilot signal is
sufficiently stronger than the preferred base station, a handoff is
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performed, assigning the neighboring base station as the preferred base
station. If the remote unit determines a handoff should occur, the handoff
is executed before the remote unit enters its assigned slot period.
Determining if a handoff is warranted prior to the remote unit entering its
assigned slot period helps ensure that the remote unit is monitoring the
strongest pilot signal before beginning to receive messages on the paging
channel.
In one embodiment, the searches performed during the preparation
period are of the preferred base station and the two strongest neighboring
base stations. In other embodiments, the number of neighboring base
stations searched may be one, or more than two.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects and advantages of the present invention will
become more apparent from the detailed description set forth below when
taken in conjunction with the drawings in which like reference characters
identify correspondingly throughout, and wherein:
Figure 1 is a representative diagram showing a typical modern
wireless communication system.
Figure 2 is a graph showing an exemplifying set of multiple signal
instances of a pilot signal from a single base station arriving at a remote
unit.
Figure 3 is a graph showing an exemplifying set of multiple signal
instances of pilot signals from multiple base stations arriving at a remote
unit.
Figure 4 is a representative diagram illustrating the transition from
the inactive state to the active state at the assigned slot of a remote unit i
n
a slotted mode communication system.
Figure 5 is a block diagram of an embodiment of a remote unit.
Figure 6 is a flow chart illustrating the method of operation of one
embodiment of the invention.
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DETAILED DESCRIPTION OF THE INVENTION
Figure 4 shows a time line presented in two separate portions. An
upper potion 41 represents a continual sequence of slots which flow i n
time from left to right. The lower portion 42 represents events occurring
during a transition between active and inactive states of a remote unit in a
slotted mode communication system in which slot 5 is an assigned slot.
The time scale for the lower portion has been expanded so that the
transition can be shown in more detail.
In particular, the lower portion 43 of Figure 4 shows the transition
from an inactive state 40 to an active state 42. In the active state 42, the
remote unit monitors the base station signal during at least a portion of
slot 5. Prior to the start of slot 5, the remote unit transits from the
inactive
state 40 to the active state 42 through a transition state 44. As described
above, in the inactive state 40, selected circuitry in the remote unit is
unpowered, reducing power consumption and extending battery life of the
remote unit. For example, power may be removed from the search engine
during the inactive state 40.
During the transition state 44, power is reapplied to the selected
circuitry of the remote unit. For example, if the search engine is
unpowered, power is reapplied in the transition state 44. The duration of
the transition state 44 is sufficient to allow the remote unit to power on
circuits and initialize functions so that the remote unit is functional,
allowing it to perform searches at the end of the transition state 44.
Following the transition state 44, the remote unit enters the active
state 42. The active state 42 is made up of two parts: a preparation period
46 and an assigned slot period 48. During the preparation period 46, an
initial search is performed reacquiring the pilot signal of the preferred base
station so that the remote unit is prepared to monitor the paging channel
during the assigned slot period 48. The assigned slot period 48 begins at
the beginning of slot 5.
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During the assigned slot period 48, the remote unit receives
messages on the paging channel from the preferred base station.
Nominally, at the completion of slot 5, the assigned slot period 48 and the
active state 42 terminate and the remote unit enters the inactive state 40.
5 In order to further reduce the power consumption of the remote unit, the
base station may command the remote unit to enter the inactive state 40
before the completion of slot 5. Alternatively, if the base station cannot
complete the transfer of messages during slot 5, the base station may
command the remote unit to remain in the assigned slot period 48 after
10 the completion of the slot. 5. Subsequently, the base station commands the
remote unit to enter the inactive state 40. Searching terminates upon
entering the inactive state 40 and power can be removed from the search
engine. Figure 5 is a block diagram of an embodiment of a remote
unit which can be used to implement the invention. The remote unit 50
15 comprises a controller 52 in communication with a search list 54 stored i n
memory. The controller 52 also has a control port 55 in communication
with a search engine 56 so as to pass search parameters to the search engine
56. The search engine 56 has an output port 57 in communication with a
data array 58 so as to store search results. The controller 52 also has a data
port 59 in communication with the data array 58 providing the controller
52 access to the search results stored therein. In one embodiment, the
controller 52 is a microprocessor. In other embodiments, the controller 52
may be an Application Specific Integrated Circuit (ASIC), a Field
Programmable Gate Array (FPGA), discrete logic, analog circuitry, or other
control circuitry.
When a remote unit is initially powered on, no entries are in the
search list 54. The remote unit may perform searches in accordance with
the technique disclosed in the above-referenced U.S. Patent Application
Serial No. 09/540,128 entitled FAST ACQUISITION OF A PILOT SIGNAL
IN A WIRELESS COMMUNICATION DEVICE (Attorney Docket No.
QUALB.012A; Qualcomm Reference No. PD990253), or other well known
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techniques, to evaluate pilot signal strength. At the completion of
searching, the search results are stored in the data array 58.
After the remote unit 50 has reacquired the preferred base station
signal according to well-know techniques, the base station transmits
nominal PN offsets for neighboring base stations to the remote unit 50
according to IS-95. The remote unit 50, using these offsets, searches the
neighboring base stations and measures their pilot signal strengths. The
controller 52 builds a search list 54 comprising the neighboring base station
identification, measured pilot signal strength and measurement time.
During subsequent searching by the remote unit 50, entries in the search
list 54 are updated. Thus, the search list 54 contains the most current
measured pilot signal strength of neighboring base stations and an
indication of when the measurement was made.
When the remote unit is in the preparation period 46, the controller
52 evaluates the entries in the search list 54. The controller 52 selects
entries from the search list 54. The controller 52 generates search
parameters corresponding to the selected base station according to standard
searching techniques and forwards them to the search engine 56. In one
embodiment, the selected entries from the search list 54 correspond to the
preferred base station and the two base stations with the strongest pilot
signal strength. In other embodiments, the search parameters correspond
to the preferred base station and any desired number of neighboring base
stations.
Not all base stations can be searched, due to the limited duration of
the preparation period 46. Therefore, the selected entries of the search list
54 are chosen to correspond to the most likely base stations to which the
remote unit may perform a handoff. However, it is also advantageous to
limit the duration of the preparation period 46 in order to retain the
power-saving benefits of slotted mode operation.
The search engine 54 performs searches using the search parameters
passed to it by the controller 52. While still in the preparation period 46,
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the controller 52 evaluates the search results determining if a handoff to a
neighboring base station should occur. A handoff may occur, for example,
if one of the base station's pilot signals is sufficiently stronger than the
preferred base station, such as, for example, at least 3dB stronger. If a
handoff occurs, it is executed during the preparation period 46 prior to the
remote unit entering the assigned slot period 48. Performing searches and,
if desired, handoffs during the preparation period 46 prior to entering the
assigned slot period 48 improves the remote unit's performance, allowing
the remote unit to monitor the paging channel of the base station with the
strongest signal level during the assigned slot.
Following the preparation period 46, the remote unit enters the
assigned slot period 48. During the assigned slot period 48, the remote unit
receives messages on the paging channel from the preferred base station.
Also during the assigned slot period 48, the remote unit measures pilot
signal strength of the preferred base station as well as neighboring base
stations. The preferred base station may also command the remote unit to
search for the pilot signal from other base stations not currently in the
search list 54. Data from these searches are used to update the search list
54.
Figure 6 is a flowchart illustrating operation of one embodiment of
the invention for determining if a handoff should occur during the
preparation period. Flow begins in block 60. In block 62, the remote unit is
initially powered and performs an initial search of a PN space. Following
completion of the initial search, flow continues to block 64 wherein the
controller, using the search results from the initial search, builds a search
list and selects a preferred base station according to well-known
techniques. Flow then waits until the remote unit's assigned slot
approaches. In block 66, prior to the assigned slot, the remote unit enters a
transition state 44. In the transition state 44, the remote unit applies power
to selected circuitry which had power removed during an inactive period
including, for example, the search engine. Following initial power up of
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18
the selected circuitry, flow continues to block 68 where the remote unit
enters the preparation period 46 of the active state 42. In block 70, while i
n
the preparation period 46 of the active state, prior to the assigned slot
period, the controller passes search parameters corresponding to the
selected entries from the search list to the search engine. The search
engine performs searches using these search parameters. Flow then
continues to block 72 where, while the remote unit is still in the
preparation period, the controller analyzes the search results.
In block 74, the controller determines if one of the neighboring base
stations' pilot signal strength is sufficiently stronger than the preferred
base station's pilot signal strength, warranting a handoff to the
neighboring base station. For example, in one embodiment, the controller
determines whether the signal strength of the neighboring base station is
at least twice a strong as (3 decibels greater than) the signal strength of
the
preferred base station. If the neighboring base station's pilot signal
strength is strong enough, flow continues to block 76 wherein the remote
unit assigns the neighboring base station to be the preferred base station.
Flow then continues to block 78. Referring again to block 74, if it is
determined that no neighboring base station is sufficiently strong enough
to warrant a handoff, the assignment of the preferred base station remains
unchanged. Flow then continues to block 78.
In block 78, the remote unit enters the assigned slot period 48 of the
active state 42 and monitors the paging channel of the preferred base
station to receive commands. Flow then continues to block 80 wherein
the remote unit measures the pilot signal strength of neighboring base
stations. Flow remains in block 80 while the remote is in the active state.
At the end of the assigned slot, or when commanded by the preferred base
station, flow continues to block 82. In block 82, the remote unit leaves the
active state 42 and re-enters the inactive state 40. While in the inactive
state 40, the remote unit updates the search list entries with the measured
pilot signal strengths of the base stations. Following update of the search
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list, flow continues to block 66 and the remote unit waits to enter the
transition state 44.
More information concerning the searching process, demodulating
element assignment and search engines can be found in:
(1) U.S. Patent Number 5,644,591, entitled METHOD AND
APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA
COMMUNICATIONS SYSTEM;
(2) U.S. Patent Number 5,805,648, entitled METHOD AND
APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA
COMMUNICATIONS SYSTEM;
(3) U.S. Patent Numbers 5,867,527 and 5,710,768, entitled
METHOD OF SEARCHING FOR A BURSTY SIGNAL;
(4) U.S. Patent Number 5,764,687, entitled MOBILE
DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEM;
(5) U.S. Patent Number 5,577,022, entitled PILOT SIGNAL
SEARCHING TECHNIQUE FOR A CELLULAR COMMUNICATIONS
SYSTEM;
(6) U.S. Patent Number 5,654,979, entitled CELL SITE
DEMODULATION ARCHITECTURE FOR A SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEMS;
(7) Application Number 08/987,172, entitled MULTI CHANNEL
DEMODULATOR, filed on December 9, 1997; and
(8) Application Number 09/283,010, entitled PROGRAMMABLE
MATCHED FILTER SEARCHER, filed on March 31, 1999;
each of which is assigned to the assignee hereof and incorporated herein by
reference, in its entirety.
Thus, the invention overcomes the longstanding problem in the
technology of unreliable reception of the paging channel by allowing the
remote unit to perform a handoff before beginning to monitor an assigned
slot on the paging channel. In this way, the remote unit monitors the best
available signal during the assigned slot, increasing the probability of
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reliable reception, reducing the number of required retransmissions and
reducing the power consumption of the remote unit.
The foregoing description details certain embodiments of the
invention. It will be appreciated, however, that no matter how detailed
5 the foregoing appears, the invention may be embodied in other specific
forms without departing from its spirit or essential characteristics. The
described embodiment is to be considered in all respects only as illustrative
and not restrictive and the scope of the invention is, therefore, indicated
by the appended claims rather than by the foregoing description. All
10 changes which come within the meaning and range of equivalency of the
claims are to be embraced within their scope.
We claim:
