REACQUISITION AND HANDOFF IN A SLOTTED MODE COMMUNICATION SYSTEM

Abstract: In a remote unit (50) of a slotted mode wireless communication system, the handoff performance is improved. Search parameters for a preferred base station, and a desired number of neighboring base stations, are passed to a search engine during a preparation period (46) of an active state (42) of the remote unit prior to its assign slot. While in the preparation period (46), the search engine performs (70) searches and the controller evaluates the results. If one of the neighboring base stations' pilot signal is sufficiently stronger than the preferred base station (74) then a handoff is performed, assigning (76) the neighboring base station as the preferred base station. The handoff is completed before the remote unit enters its assigned slot (48).

Description: 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 10 A, 10B and IOC 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 IOC 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 TL /El A 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 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 in 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 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 δ with a phase shift of ® radians, and the second path has an attenuation factor of X dB, a time delay of δ with a phase shift of ® + π 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. 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. 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 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 in 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 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 2 15 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 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 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 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 communication link for conducting subsequent voice communication in 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, 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 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, in 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 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 in 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. 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 in 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 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 in 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. 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 in 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. 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. 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 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 comprises a controller 52 in communication with a search list 54 stored in 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 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, 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 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 in 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 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 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 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 changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. We claim:

Claims: CLAIMS 1. A method of communication in a slotted mode wireless communication system, comprising: selecting search parameters for a preferred base station and at least one neighboring base station- searching using the search parameters during a preparation period to determine search results wherein the preparation period precedes in time an assigned slot period; evaluating the search results to determine if a selected neighboring base station pilot signal is stronger than the preferred base station pilot signal by at least a specified amount; and if the neighboring base station signal is stronger than the preferred base station pilot signal by at least the specified amount, initiating a handoff of the communication to the selected neighboring base station during the preparation period. 2. The method of Claim 1 wherein the at least one neighboring base station comprises two neighboring base stations. 3. The method of Claim 1 wherein the two neighboring base stations had the strongest pilot signal strengths of all neighboring base station signals measured during a previous active state. 4. The method of Claim 1 further comprising monitoring a paging channel signal from the selected neighboring base station during at least a portion of the assigned slot period. 5. The method of Claim 1 further comprising: refraining from monitoring wireless link signals during an inactive state; and transitioning from the inactive state to the preparation period in order to be prepared to monitor wireless link signals during the assigned slot period. 6. A remote unit for use in a slotted mode communication system comprising: means for selecting search parameters for a preferred base station and at least one neighboring base station; means for searching using the search parameters during a preparation period to determine search results wherein the preparation period precedes in time an assigned slot period; means for evaluating the search results to determine if a selected neighboring base station pilot signal is stronger than the preferred base station pilot signal by at least a specified amount during the preparation period; and means for initiating a handoff of the communication to the selected neighboring base station during the preparation period when the neighboring base station signal is stronger than the preferred base station pilot signal by at least the specified amount. 7. The system of Claim 6 further comprising means for monitoring a paging channel signal from the selected neighboring base station during at least a portion of the assigned slot period. 8. The system of Claim 6 further comprising: means for refraining from monitoring wireless link signals during an inactive state; and means for transitioning from the inactive state to the preparation period in order to be prepared to monitor wireless link signals during the assigned slot period. 9. A remote unit for use in a slotted mode wireless communication system comprising: a search engine configured to accept search parameters, and to output search results; a data array coupled to the search engine, said data array configured to accept and store search result from the search engine; a search list configured to accept search entries; and a controller coupled to said search list and configured to pass search parameters to the search engine, and to accept search results from the data array; wherein during a preparation period, the controller evaluates search entries received from the search list to determine a desired order of searching neighboring base stations and passes selected search parameters corresponding to a desired neighboring base station to the search engine, and the search engine performs searches using the selected search parameters and stores first search results in the data array and wherein, while still in the preparation period, the controller evaluates the first search results and determines if a handoff to the desired neighboring base station should be made prior to entering an assigned slot period. 10. The remote unit of Claim 9 wherein the controller is a microprocessor. 11. The remote unit of Claim 9 wherein the controller is an application specific integrated circuit. 12. The remote unit of Claim 9 wherein the set of search parameters comprises: a PN offset; a number of noncoherent passes; and an integration interval. 13. The remote unit of Claim 9 wherein the controller is further configured to reduce power consumption of the search engine during an inactive state which precedes the preparation period. 14. The remote unit of Claim 9 wherein the controller is further configured to reduce power consumption of a portion of the remote unit so as to inhibit performance of a first set of functions in an inactive state which precedes the preparation period. 15. The remote unit of Claim 14 wherein the controller is further configured to control power consumption of the remote unit so as to allow performance of the first set of functions in the assigned slot period. 16. A method of communication by a remote unit, in a wireless communication system, comprising: entering a transition state and applying power to a search engine; transiting into an active state comprising a preparation period during which the method comprises: selecting search parameters for a preferred base station and at least one neighboring base station, performing searches according to the search parameters to determine search results, evaluating the search results, and initiating a handoff if the pilot signal strength of a suitable neighboring base station is sufficiently stronger than the pilot signal strength of a preferred base station. 17. The method of Claim 16 wherein the active state further comprises an assigned slot period during which the method comprises: selecting additional search parameters, passing the additional search parameters to the search engine, and performing searches; and wherein at the end of the assigned slot period the method comprises transiting into an inactive state wherein the method comprises: evaluating the search results, and building a search list comprising pilot signal strengths of base stations.

Inventors: HUGHES ROBBIN, HUGHES, ROBBIN
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2001
Main classification: H04Q7
IPC codes: H04B1, H04W36, H04W74