MXPA97003723A - Pilot signal search technology for a cellular communications system - Google Patents

Abstract

The present invention relates to a cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in an active list of one or more station entries, base, wherein each of a plurality of base stations within said system transmits a pilot-specific signal, a method for identifying some of said base stations from which the received signal resistance is sufficient to establish communication with said mobile station, said method it comprises the steps of: maintaining in said mobile station a candidate list of base station entries, wherein each entry in said candidate list corresponds to a base station capable of providing a signal strength sufficient to establish communication with said mobile station wherein said active list of one or more base station entries is kept within said station mobile ion and is derived from said candidate list of base station entries, maintaining in said mobile station a neighbor list of base station entries, wherein each entry in said neighboring list corresponds to a base station in a predetermined proximity of said mobile station; Measuring in said mobile station the signal resistance of each said pilot signal transmitted by each of said base stations in said neighbor list, comparing in said mobile station said signal strength measurements of the base station of each said input of the neighboring list with a predetermined first level, and removing by said mobile station a particular entry from said neighboring list having said signal strength measurement of the base station greater than said first predetermined level and placing said particular entry in a pre-candidate list maintained in said mobile station, wherein said entry into said pre-candidate list of entrad ace of station ba

Description

PILOT SIGNAL SEARCH TECHNIQUE FOR A CELLULAR COMMUNICATIONS SYSTEM BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates generally to cellular communication systems in which multiple base stations are placed, each of which transmits a characteristic pilot signal. More particularly, the present invention relates to a novel and improved technique for searching and identifying pilot signals from those base stations from which the signal strength received at a given location is sufficient to sustain communication. II. Description of the Related Art In conventional cellular telephone systems the available frequency band is divided into channels of typically 30 KHz bandwidth while using analog FM modulation techniques. The service area of the system is divided geographically into cells of varying size. The available frequency channels are divided into sets, each set usually containing an equal number of channels. The sets of frequencies are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, when considering a system in which there are seven sets of frequencies and the cells are hexagons of equal size. A set of frequencies used in a cell will not be used in the closest six or the surrounding neighbors of that cell. In addition, the set of frequencies in a cell will not be used in the twelve closest neighbors next to that cell. In the conventional cellular telephone system, the implemented transfer scheme is proposed to allow a call to continue when a mobile phone crosses the boundary between two cells. The transfer from one cell to another is initiated when the receiver at the cellular base station handling the call warns that the signal resistance received from the mobile telephone falls below a predetermined threshold value. An indication of low signal resistance means that the mobile phone must be near the cell boundary. When the signal falls below the predetermined threshold value, the base station questions the system controller to determine whether a neighboring base station receives the mobile telephone signal with better signal strength than the current base station. In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile station and more than one base station. The signals occupying the common frequency band are discriminated at the receiving station through waveform properties of spread spectrum CDMA based on the use of a high speed pseudorution (PN) code. The high-speed PN code is used to modulate the signals transmitted from the base stations and the mobile stations. Transmitting stations using different PN codes or PN codes that are out of step in time produce signals that can be received separately at the receiving station. The high-speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several different propagation paths. The common frequency band used through a CDMA cellular communication system allows the mobile station to remain in communication with more than one cellular base station during the transfer. In this environment, the communication between the mobile station and the other user is uninterrupted by the eventual transfer of the base station corresponding to the cell from which the mobile station leaves, to the base station corresponding to the cell in which the station enters. mobile . This type of transfer can be considered as a "soft" transfer in communications between the cellular base stations and the mobile where two or more base stations, or sectors of base stations, transmit concurrently to the mobile station. The techniques for a transfer between a sector of a cell and another cell, and a transfer between sectors of a same cellular base station for a sectorized cell are similar. Typically, the cellular system controller initiates the diversity of the base station or the so-called "soft transfer" process. The cellular system controller is started by assigning a modem, located in the new base station, for the call. This modem is given to the PN address associated with the call between the mobile station and the modem of the current base station. The new base station modem assigned to service the call seeks and finds the signal transmitted by the mobile station. The new base station modem also starts transmitting a forward link signal to the mobile station. The scanning element of the mobile station searches for this forward link signal according to the signal information provided by the old base station. When the mobile station acquires the signal transmitted by the new base station modem, the mobile station can continue communication through the two base stations. Another base station could be added in the same way as the first new base station above. In this case, the base station can continue communication through three base stations. This process may continue until the mobile station is in communication with a base station for each demodulation element that contains the mobile station and beyond. Since the mobile station communicates with the user through at least one base station at all times through a smooth transfer, there is no interruption in communications between the mobile station and the user. Accordingly, a smooth transfer in communications provides significant benefits in its inherent "do before interrupting" communication about conventional "break before doing" techniques employed in other cellular communication systems. In an exemplary CDMA cellular telephone system, such as that described in U.S. Patent No. 5,267,261, entitled MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM, which is assigned to the assignee of the present invention and which is incorporated in the present for reference, a particular technique is described for effecting a smooth transfer of the type described above. This technique is applicable to systems in which each base station transmits a "pilot" reference signal of broadcast spectrum. These pilot signals are emitted by the mobile stations to obtain the synchronization of the initial system and to provide a robust time, frequency and phase tracking of the signals transmitted by the base station. The pilot signal transmitted by each base station in a system may use the same PN code but with a different code phase offset, with the intention that the PN codes transmitted by the neighboring base stations are identical but asymmetric in time some with respect to others. The phase shift allows the pilot signals to be distinguished from each other according to the base station from which they originate. The system of U.S. Patent No. 5,267,261 contemplates the maintenance, within the mobile station, of several lists of base stations from which the received signal resistance exceeds predetermined levels. The search process for the pilot signals of the base station can be outlined by defining four different sets of pilot displacements: the Active Set, the Candidate Set, the Neighbor Set and the Remaining Set. The Active Set identifies the base station (s) or sector (s) through which the base station is communicated. The Candidate Set identifies the base station (s) or sector (s) for which the pilots have been received at the mobile station with sufficient signal strength to make them members of the Active Set, but they have not been placed in the Active Set by the base station (s). The Neighbor Set identifies the base station (s) or sector (s) that are possible candidates for establishing communication with the mobile station. The Remaining Set identifies the base station (s) or sector (s) that have all the other possible pilot trips in the current system, excluding those pilots that currently move in the Active, Candidate and Neighbor groups. After a call is initiated the mobile station continues to scan the pilot signals transmitted by base stations located in neighboring cells. The scanning of the pilot signal continues in order to determine whether one or more of the pilot signals transmitted by the neighboring base station rises above a predetermined threshold, a level which is indicative that communications can be supported between the base station and the mobile station. When the pilot signal transmitted by a base station located in a neighboring cell rises above the threshold, it serves as an indication to the mobile station that a transfer must be initiated. In response to this determination of resistance of the pilot signal, the mobile station generates and transmits a control message to the base station that currently serves the call. This control message depends on the system controller, which determines whether a transfer procedure should be initiated based on the availability of system resources. In the aforementioned CDMA system of U.S. Patent No. 5,267,261 the process of placing a member of the Neighbor Set base station in the Candidate Ensemble proceeds as follows. The pilot signal from the base station is first compared to a predefined threshold value. After the mobile station makes the determination that the measured value exceeds a predefined threshold, the control processor of the mobile station generates and transmits a corresponding Pilot Resistance Measurement Report Message. This Report Message is received by the base station with which the mobile station is currently in communication, and is advanced to the system controller. The decision to place a member of the Candidate Set in the Active Set is made through the system controller. For example, when the measured Candidate pilot is of a signal resistance that exceeds the signal strength of another pilot member of the Active Set by a predetermined value, it can join the Active Set. In an exemplary system there are limits placed on the number of members of the Active Set. If the addition of a pilot to the Active Set exceeds the limit of the Active Set, the weakest pilot of the Active Set may retreat to another set. However, unfortunately, conventional pilot resistance measurement techniques tend to erroneously identify insufficient energy pilot signals as they exceed the predefined signal strength threshold of the Candidate Set. Such erroneous measurements of the pilot resistance can result in "false alarms", in which a member of the Neighbor Set is inappropriately added to the Candidate Set. This improper addition can in turn lead to the occurrence of a "false" transfer, that is, a call transfer to a base station unable to establish communication with the mobile unit. Accordingly, an object of the present invention is to provide an improved method for searching and identifying only those pilot signals transmitted by viable base station transfer candidates.

SUMMARY OF THE INVENTION The present invention provides a novel and improved method and system for carrying out a pilot signal search operation prior to the communication transfer of the mobile station between base stations. The present invention is described herein in an exemplary embodiment as a cellular communication system utilizing code division multiple access modulation (CDMA) techniques. In the system, each base station transmits a pilot signal of a common PN broadcast code offset in a code phase from the pilot signals of other base stations. In system communications with the mobile station, the mobile station is provided with a list of PN offsets corresponding to neighboring cell base stations. In addition, the mobile is provided with a message identifying at least one pilot corresponding to a base station through which the base station is about to communicate. These lists are stored in the mobile station as a Neighbor Set and an Active Set of pilots. In addition to the Neighbor and Active Set of pilots, the mobile station maintains a list of Candidate and Pre-Candidate Sets of pilots. Based on the analysis of the pilot signals received at the mobile station, the base station inputs from the Neighbor Set can be assigned to the Candidate and Pre-Candidate Sets, and eventually to the Active Set. In an exemplary implementation each entry in the Neighbor Set corresponds to a base station in a predetermined proximity to the mobile station. In the mobile station, the signal resistance is measured by the pilot signals transmitted by each of the base stations within the neighboring list. The signal strength measurements corresponding to each base station entry within the Neighbor Set are compared to a first predetermined level. One or more Neighbor Set inputs having a signal strength measurement of the base station greater than the first predetermined level can be placed in the Re-Candidate Set. The resistance of the pilot signals associated with the entries in the Pre-Candidate Set are then valued additionally to determine the eligibility within the Candidate Set, from which the entries comprising the Active Set are selected. After a base station is added to the Active Set, the system controller communicates the information that instructs the aggregated base station to establish communications with the mobile station. The communications of the mobile station are thus directed through all the base stations identified by pilots in the Active Set of the mobile station. 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 to each part and wherein: Figure IA provides an exemplary illustration of a cellular telephone system in which the pilot signal search technique of the present invention can be applied; Figure IB illustratively represents the concept of time reference and calculation of PN phase shifts for pilots from a number of base stations; Figure 2 shows a generalized flow diagram representation of a conventional pilot signal search technique used to identify and place base stations within a mobile station listing the base stations of the Candidate Set; Figure 3 shows a generalized representation of an improved technique for searching for a pilot signal according to the present invention; Figure 4 illustrates in an illustrative manner a search window of an amplitude W centered around an expected arrival time in the mobile station of a pilot signal from a base station of the Neighbor Set; Figures 5A and 5B are a flowchart useful for describing the operation of the novel pilot signal search technique of the invention; Figure 6 shows an exemplary mobile station receiver in which a pilot explorer operative in accordance with the present invention can be incorporated, and Figure 7 illustrates a block diagram of the scan receiver placed to implement the pilot signal search technique of the present invention. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In a CDMA cellular communication system, the same frequency band can be used for all cells. The waveform properties of CDMA that provide processing gain are also used to discriminate between signals that occupy the same frequency band. In this way, a mobile station, or short mobile station, such as a telephone mounted on a vehicle or portable telephone, or personal communication system (PCS) handset, does not need switching frequencies when the transfer of the call is made from a base station to another. In addition, the probability that the call is discontinued if the transfer order is received by mistake is substantially reduced. In a CDMA cellular communication system, each base station has a plurality of modulator-demodulator stations or broadcast spectrum modems. Each modem consists of a digital broadcast spectrum transmission modulator, at least one digital broadcast spectrum data receiver and an explorer receiver. Each modem in the base station is assigned to a mobile as necessary to facilitate communications with the assigned mobile. Therefore, in many cases, many modems are available for use while others can be found Active in communication with respective mobiles. In a "soft" transfer scheme employed within a CDMA cellular communication system, such as a CDMA cellular telephone, the Private Bifurcation Exchange, or PCS system, a new base station modem is assigned to a mobile while the old base station continues to service the call. When the mobile is located in the transition region between the two base stations, the call can be provided through the various base stations as signal resistance commands. Since the mobile always communicates at least through a base station, no interruption effects will occur to the mobile station or the service. It should be understood that many aspects of the transfer techniques set forth herein are also applicable to transfers between sectors in a sectorized cell. When the communications of the mobile station are firmly established with the new base station, for example, the mobile is quite within the new cell, the old base station discontinues the service to the call. The resulting smooth transfer is essentially a do-before-interrupt switching function. In contrast, conventional cellular phone systems can be considered to provide an interrupt-before-doing switching function. In the present invention, a search technique of the pilot signal is implemented which reduces the incidence of "false alarms" that occur after the erroneous measurement in the mobile station of the pilot signal resistance of the base station. In particular, when it is mistakenly identified that the pilot signals exceed a predefined transfer threshold, a "false transfer" may occur after the call is transferred to the base stations from which the received pilot resistance was measured as in excess of a predefined transfer threshold, but which is currently below the threshold. Again, a false transfer corresponds to the situation in which a call is transferred to the base station unable to establish communication with the mobile station. Although it is preferred that the mobile initiates the transfer request and determines the new base station, decisions of the transfer process can be made as in the conventional cellular telephone system. As previously discussed with respect to conventional systems, the base station determines when a transfer may be appropriate and, through the system controller, requires neighboring cells to search for the mobile signal. The base station that receives the strongest signal as determined by the system controller then accepts the transfer. The pilot signal referred to above can be defined as the transmission from a given base station of a "pilot vehicle" on a corresponding pilot channel. The pilot signal is a broadcast spectrum signal, of direct, unmodulated sequence, transmitted at all times by each base station using a common pseudorandom (PN) noise broadcasting code. The pilot signal allows mobile stations to obtain initial system synchronization, i.e. synchronization, in addition to providing a phase reference for coherent demodulation and a signal strength reference for comparisons between base stations for transfer determination. The pilot signal, as it is transmitted by each base station, is from the same PN broadcast code but with a different code phase offset. For example, in the present invention the broadcast code of the pilot signal is of a PN code length of 215. In this example there are 511 different offsets from the zero offset, where the offsets are in increments of 64 PN microcircuits. . This is the phase shift that allows the pilot signals to be distinguished from one another by the mobile station, resulting in a differentiation between the base stations from which they originate. The use of the same pilot signal code allows the mobile station to find the timing of the system by a single search through all the code phases of the pilot signal. The strongest pilot signal, as determined by an integration process for each code phase, is easily identifiable. The identified pilot signal generally corresponds to the pilot signal transmitted by the nearest base station. An example illustration of a cellular telephone system, alternatively representative of a PBX or PCS system, in which the pilot signal search technique of the present invention can be applied, is provided in Figure IA. The system illustrated in Figure IA uses CDMA modulation techniques in the communication between the mobile stations of the system or the mobile telephones and the base stations. Cellular systems in large cities can have hundreds of base stations that serve hundreds of thousands of mobile phones. The use of CDMA techniques readily facilitates increases in user capacity in systems of this size compared to conventional FM modulation cellular systems. In Figure IA, the controller and switch of the system 10, also referred to as the mobile telephone switching office (MTSO), typically includes an interface and processing circuitry to provide system control to the base stations. The controller 10 also controls the address of telephone calls from the public switched telephone network (PSTN) to the appropriate base station for transmission to the appropriate mobile station. The controller 10 also controls the routing of calls from the mobile stations, through at least one base station to the PSTN. The controller 10 can direct calls between mobile users through the appropriate base station (s) since such mobile stations do not communicate directly with each other. The controller 10 can be coupled to the base stations by various means such as dedicated telephone lines, fiber optic links or through microwave communication links. In FIG. IA, three exemplary base stations 12, 14 and 16 are illustrated along with an exemplary mobile station 18, which includes a cellular telephone. The arrows 20a-20b define the possible communication link between the base station 12 and the mobile station 18. The arrows 22a-22b define the possible communication link between the base station 14 and the mobile station 18. Similarly, the arrows 24a-24b define the possible communication link between the base station 16 and the mobile station 18. The service areas of the base station or cells are designed in geographical configurations in such a way that the mobile station will normally be the closest to a station base. When the mobile station is idle, ie without calls in progress, the mobile station constantly monitors the pilot signal transmissions from each nearby base station. As illustrated in FIG. IA, the pilot signals are respectively transmitted to the mobile station 18 by base stations 12, 14 and 16 respectively on communication links 20b, 22b and 24b. The mobile station then determines in which cell it is found by comparing the pilot signal strength transmitted from these particular base stations. In the example illustrated in FIG. A, the mobile station 18 can be considered the one closest to the base station 16. When the mobile station 18 initiates a call, a control message is transmitted to the nearest base station, the base station 16 The base station 16 after receiving the call demand message, signals the system controller 10 and transfers the call number. The system controller 10 then connects the call through the PSTN to the proposed receiver. If a call is to be initiated within the PSTN, the controller 10 transmits the call information to all base stations in the area. The seasons. base transmit back a paging message to the mobile station of the proposed receiver. When the mobile station listens to a page message, it responds with a control message that is transmitted to the nearest base station. This control message indicates to the system controller that this particular base station is in communication with the mobile station. The controller 10 then directs the call through this base station to the mobile station. If the mobile station 18 moves outside the coverage area of the initial base station, base station 16, an attempt is made to continue the call by routing the call through another base station. In the transfer process there are two different methods to initiate the transfer of the call or its routing through another base station. The first method, called transfer initiated by the base station, is similar to the transfer method employed in the first original generation of analog cellular telephone systems currently in use. In the transfer method initiated by the base station, the initial base station, base station 16, notices that the signal transmitted by the mobile station 18 has fallen below a certain threshold level. The base station 16 then transmits a transfer request to the system controller 10. The controller 10 retransmits the demand to all neighboring base stations, 14, 12 of the base station 16. The demand transmitted by the controller includes information regarding the channel , including the PN code sequence used by the mobile station 18. The base stations 12 and 14 tune a receiver to the channel that is being used by the mobile station and measure the signal strength, typically using digital techniques. If one of the receivers of base stations 12 and 14 reports a signal stronger than the signal strength reported by the initial base station, then a transfer is made to that base station. The second method to initiate a transfer is called transfer initiated by the mobile. The mobile station is equipped with a search receiver that is used to scan the pilot signal transmission of the neighboring base stations 12 and 14, in addition to performing other functions. If a pilot signal from the base stations 12 and 14 is stronger, such as by a predetermined threshold, that the pilot signal from the base station 16, the mobile station 18, transmits a message to the current base station, base station 16 An interactive process between the mobile station and the base station then allows the mobile station to communicate through one or more base stations 12, 14 and 16. The transfer method initiated by the mobile has several advantages over the transfer method initiated by the base station. The mobile station is aware of changes in trajectories between itself and the various neighboring base stations long before the base stations are able to do so. However, in order to carry out a transfer initiated by the mobile, each mobile station must be provided with a search receiver to carry out the scanning function. However, in the exemplary embodiment described herein of a CDMA communications capability of the mobile station, the search receiver has additional functions that require its presence. The transfer initiated by the mobile depends on the mobile station detecting the presence or absence of pilot signals, and the signal resistance of the pilot signals. The mobile station identifies and measures the signal strength of the pilot signals it receives. This information is communicated through the base station (s) to which the mobile station communicates through the MTSO. The MTSO, after receiving this information, initiates or disarms the soft transfers. To define the piloting process, four different sets of pilot displacements are defined: the Active Set, the Candidate Set, the Neighbor Set and the Remaining Set. The Active Set identifies the base station (s) or sector (s) through which the mobile station communicates. The Candidate Set identifies the base station (s) or sector (s) for which the pilots have been received at the mobile station with sufficient signal strength to make them members of the Active Set, but have not been placed in the Set Active for the base station (s). The Neighbor Set identifies the base station (s) or sector (s) that are possible candidates for establishing communication with the mobile station. The Remaining Set identifies the base station (s) or sector (s) that have all the other possible pilot trips in the current system, excluding those pilot trips that are currently in the Active, Candidate and Neighbor groups. Although the mobile station is in a traffic channel communication mode with the base station, under the control of the mobile station control processor, the scanning receiver systematically examines the resistances of all the pilots in the four pilot sets, on the current CDMA frequency assignment. The test results are provided to the control processor of the mobile station for later use. The test results are sent to the base station (s) with which (s) the mobile station is in communication. In a preferred implementation, the test report contains a list of pilots and their measured resistances. The first pilot on the list is the pilot used to derive the time reference from the mobile station. The first-arriving multipath component that is demodulated is typically used as the time reference for the mobile station. The mobile station measures the pilot phase reported in relation to the zero-shift pilot PN sequence which uses the derived timing of the pilot used as the time reference. With each pilot reported the mobile station returns the value PILOT_PN_PHASE within the examination report, where this value is defined according to Equation (1): PILOT_PN_PHASEj = [64xPILOT_PNj + ti-tj] * module 215 (1) where: PILOT_PN_PHASE-j is the pilot phase of the base station j; and t ± and tj respectively denote the one-way delays in the PN microcircuits of the respective base stations to the mobile station. The concept of time reference and the calculation of PN phase shifts for pilots of other base stations is illustrated in figure IB. It should be noted that the synchronization in the mobile station is displaced from the synchronization in the base stations by microcircuits t ±. The required pilot PN phase, fj (obtained from figure IB when observing that: fj-fi-Pj + i-ÍPi + tJ (2) with the time reference, Pi = f ?; and where: Pi = 64xPIL0T_PNi and (3) Pj = 64xPIL0T_PNj (4) As noted above, the scout receiver systematically examines the resistances of all pilots in the four pilot sets. The search ratio for members of the Active Set and the Candidate set are preferably identical. The search range (ie, search window) for all members of the Active Set and the Candidate Set are specified in terms of a predetermined number of PN chips. For each member of the Active Set and the Candidate Set, the search window is centered around the component of usable multiple trajectories, first arriving. A multipath component is called usable if it is of sufficient strength so that the mobile station uses it to demodulate data. Turning now to Figure 2, there is shown a generalized flow diagram representation of a conventional pilot signal search technique used to identify and place base stations within the Candidate Set. As a specific example, consider the case of a member base station in the Active Set, one in the Candidate Set, and ten in the Neighbor Set. A preferred order in which the pilot signals from the base stations within the Active (A), Candidate (C) and Neighbor (N ±) Sets are searched as follows: A, C, N ?; A, C, N2; ..., A, C, N10; A, C, N?; ... For each PN microcircuit default amplitude search window centered around a pilot in the Neighbor Set, the received PN pilot signal is de-correlated using a set of "hypotheses" generated in a way local of the PN pilot signal. In an exemplary CDMA system, an identical pilot PN signal is transmitted from each base station. However, discrimination between pilot signals from different base stations becomes possible by transmitting each with a different synchronization offset. Each pilot signal hypothesis can be generated by: (i) the jump of a local PN pilot generator towards the synchronization shift of the base station pilot that is being searched in order to produce a replica of it, generated locally; and (ii) the jump of a locally generated pilot signal replica to a unique synchronization shift within the search window. However, each hypothesis of the pilot signal corresponds to an "assumption" regarding the arrival time within the search window of the pilot signal from a selected base station. When searching for the pilot from a member of the Neighbor Set, a search window is defined around the expected arrival time at the mobile station of the Neighbor pilot that is being searched. Then, in the mobile station, an initial Neighbor scenario is generated, corresponding to the arrival of the Neighbor driver at the beginning of the search window. The initial hypothesis is correlated with the received pilot signal on a first selected number (for example, 100) of PN microcircuits, and the results of the integrated correlation on the same microcircuit range (step 50). The result is then compared (step 60) with a previously predefined emptying threshold. If the result is less than the threshold of the previous emptying, the value of the received signal energy associated with the hypothesis is set to zero, or, for convenience of expression, "the hypothesis is set to zero". If the initial hypothesis is set to zero, the search moves to the next hypothesis. The following hypothesis is obtained by jumping synchronization of the local PN pilot generator (step 65) on 1/2 PN microcircuit. If the integration of the decorrelated hypothesis produces a non-zero value that exceeds the previous emptying threshold, the decorrelation / integration of the initial hypothesis with the received pilot is continued (step 68) for a second selected number (for example, 412) of PN microcircuits. The aggregate result of the first and second integrations, carried out on the exemplary number of 512 PN microcircuits of the initial hypothesis, is then stored by the mobile station controller. This process of de-correlation and integration is then repeated for each of the hypotheses within the search window. After each hypothesis has been examined through the decorrelation / integration process described above, the integration values associated with the three strongest hypotheses are combined (step 70) in a digital adder and filtered (step 75) by a filter of infinite impulse response (IIR). These three major values correspond to the energy of the three strongest multipath components of the pilot signal within the Neighbor Set currently under evaluation. In an exemplary implementation, the IIR filter is performed according to the following second order transfer function: Y (n) = 0.5xY (n-l) + 0.5xC3 (5) where: Y (n) denotes the output of the IIR filter; and parameter C3 represents the combined energy of the three strongest pilot trajectories produced by the digital adder. The filtered Y (n) output from the filter IIR is then compared (step 80) with a threshold of the Candidate Set (T_ADD). If Y (n) exceeds T_ADD, the base station from which the pilot signal is received is added to the Candidate Set. If Y (n) is less than T_ADD, the base station remains in the Neighbor Set. Although the output Y (n) of the IIR filter has been described in correspondence to the received pilot power, it is understood that the comparison with T_ADD could actually be made in terms of pilot energy received per microcircuit for the total spectral density received. (that is, noise and signal). In this case, the parameter T_ADD would correspond to a minimum predetermined Candidate level of signal and noise (S / N), with which the received pilot S / N level should be compared. Unfortunately, the conventional pilot measurement process illustrated in Figure 2 has resulted in received pilot signals that are erroneously identified with a resistance greater than T_ADD. The resulting relatively high incidence of "false alarms" can be attributed at least in part to the relatively short de-correlation and integration time of PN (eg, 512 microcircuits of PN) on which the pilot resistance comparison received is based on the threshold of Resistance Candidate T_ADD. It is conceivable that the mere lengthening of the decorrelation / integration interval can reduce the proportion of false alarms. But this adjustment would also increase the detection period required to evaluate the resistance of each Neighbor signal. Under rapidly changing channel conditions, such as those often experienced within cellular communication systems, such increased periods of pilot resistance detection could unpack the performance of the system by decreasing the process by which eligible base stations are added to the set. Candidate. As described hereinafter, the present invention provides an improved pilot search technique that provides a lower incidence of "false alarms" for a given period of pilot resistance detection. Turning now to Figure 3, a generalized representation of an improved technique for searching for a pilot signal according to the present invention is shown. The inventive pilot signal search technique contemplates the creation of a category of transient base station, called the "Pre-Candidate Set", to which the base stations from the Neighbor Set are assigned. In a preferred embodiment the Pre-Candidate Set includes a set of "N" Pre-Candidate States (ie, State # 0, State # 1, ..., State #N), which can be observed as if they comprised a string Ov Sea of Pre-Candidate States. As described below, qualifying base stations of the Neighbor Set are initially assigned to a predefined Pre-Candidate State (eg, State # 1), and are transferred to other Pre-Candidate States based on the results of the de-correlation operations / integration carried out by using the received pilot signal associated with them. After progressing through a sequence of States within the Pre-Candidate Set, each base station that enters the Pre-Candidate Set either eventually returns to the Neighbor Set (eg, as of Pre-Candidate Status # 0), or assigned to the Candidate Set (for example, from the Pre-Candidate State #N). As indicated by Figure 3, in an exemplary case a base member station is included in the Active Set, one in the Candidate Set, two in the Pre-Candidate Set, and ten in the Neighbor Set. A preferred order in which the pilot signals from the base stations are searched within the Active (A), Candidate (C), Pre-Candidate (PC ±) and Neighbor (Ni) sets is as follows: A, C, PC1 PC2, N1; A, C, PC1, PC2, N2; ..., A, C, PCx, PC2, N10; A, C, PCi, PC2, N ?; ... For each microcircuit amplitude search window of predetermined PN, centered around a pilot in the Neighbor Set, the received PN pilot signal is de-correlated using a set of locally generated hypotheses of the pilot signal from PN. Again, in an exemplary CDMA system, identical pilot PN signals are transmitted from each base station. However, discrimination between pilot signals from different base stations becomes possible by transmitting each with a different synchronization offset. As mentioned above, each pilot signal hypothesis can be generated by: (i) the jump from a local PN pilot generator to the pilot synchronization shift of the base station being searched in order to produce a locally generated replica of the same; and (ii) the jump of the locally generated pilot signal replica to a unique synchronization shift within the search window. Figure 4 illustratively depicts a search window of an amplitude W centered around the expected arrival time (TA) in the mobile station of the pilot signal from the base station N ± within the Neighbor Set. The search window is observed divided into equivalent time intervals of 1/2 microcircuit of PN, each of which corresponds to one of the "hypotheses" mentioned above regarding the arrival time of the particular pilot signal for which conduct a search. In the illustration of Figure 4, the resistances of the three multipath components (Sml, Sm2 and Sm3) of the pilot signal transmitted by the base station N¿ alternate along the horizontal axis based on the arrival time in the mobile station within the search window (TA ± - / 2 <t < TA / 1 + W / 2). The resistances of the multipath signal components Si ml, Si / tn2 and Si / m3 are obtained by de-correlation / integration of the pilot energy received in the aforementioned manner by using pilot hypotheses Hlf H2 and H3. Referring now to FIGS. 5A and 5B, a flowchart useful for describing the operation of the novel pilot signal search technique of the invention is provided. When searching for the pilot from a member of the Neighbor Set, a search window is defined around the expected arrival time in the mobile station of the Neighbor pilot that is being searched.

An initial Neighbor pilot hypothesis is generated in the mobile station, corresponding to the arrival of the Neighbor driver at the beginning of the search window. The initial hypothesis is de-correlated with the received pilot signal on a first selected number (for example, 100) of PN microcircuits, and the results of the integrated decorrelation on the same microcircuit range. The result of the integration is then compared (step 90) with a pre-defined predefined emptying threshold. If the result is less than the threshold of the previous emptying, the hypothesis is set to zero and the search moves to the next hypothesis, which is displaced in time by 1/2 microcircuit of PN from the initial hypothesis (stage 95). ). Referring again to the flow chart of Figures 5A and 5B, if the results of the integration of the previous emptying lead to the initial hypothesis that is established in a value that exceeds the previous emptying threshold, a second de-correlation / integration of the previous emptying is carried out. the initial hypothesis with the received pilot (stage 100) for a selected number (e.g., 412) of PN microcircuits. Each of the remaining hypotheses within the search window, mutually separated by 1/2 microcircuit of PN, is then de-correlated / integrated in the same way as the initial hypothesis. That is, in the preferred embodiment, each hypothesis is de-correlated for 100 PN microcircuits, it is integrated over the same PN microcircuit period, and the result of the integration is compared with a previous empty threshold. For those hypotheses in which the value of the "previous empty" integration exceeds the previous emptying threshold, the de-correlation / integration procedure continues for another 412 PN microcircuits. Among those hypotheses within a given search window for which a second de-correlation / integration is carried out (that is, for each de-correlated and integrated hypothesis during 512 PN microcircuits), the three integration results of higher value are combined ( stage 105). The combined result is then compared (step 110) with a first Pre-Candidate threshold, and if the first Pre-Candidate threshold is exceeded the associated base station is transferred from the Neighbor Set to a first Pre-Candidate Status (State # 1). Otherwise, the base station remains in the Neighbor Set. As noted above, it is assumed that in the present example there are two base stations (ie, PCX and PC2) within the Pre-Candidate Set. The process Pre-Candidate of the pilot signal associated with the base station PCi is described immediately below, it being understood that after the PCX base station is placed in the Candidate Set, or is returned to the Candidate Set, the pilot signal for the base station PC2 is process substantially identically. After the PCX base station is placed within State # 1, a Pre-Candidate search window is defined around the usable, multiple trajectory component of the closest arrival. Again, a multi-path component is called usable if it is of sufficient strength for the mobile station to use to demodulate data. A first pilot hypothesis corresponding to the arrival at the start of the Pre-Candidate search window of the pilot signal from the base station PC1 # is then generated in the mobile station. In a preferred implementation, this first hypothesis correlates with the received PCX pilot signal over 100 PN chips, and the results of the integrated correlation over the same microcircuit range. The result of the integration is then compared (step 120) with a predefined predefined emptying threshold. If the result is lower than the previous emptying threshold, the hypothesis is set to zero and the search is moved to the following hypothesis PCi pilot, which is displaced in time by 1/2 microcircuit of PN from the first hypothesis. Otherwise, the decorrelation / integration of the first hypothesis is continued for a preselected number (for example, 700) of PN microcircuits. Each of the hypotheses separated by 1/2 microcircuit of remaining PNs within the Pre-Candidate search window for the base station PC ^ is then de-correlated / integrated in the same way as the initial hypothesis. Among those hypotheses within the PCX search window for which a second decorrelation / integration is carried out (that is, for each de-correlated and integrated hypothesis for another 700 PN microcircuits), the three highest-value integration results are combined (stage 125). The combined result is then compared to a Pre-Candidate State # 2 threshold, and if this threshold exceeds the base station PCX is transferred from State # 1 to State # 2 of the Pre-Candidate Set. Otherwise, the PCX base station is placed in State # 0 of the Pre-Candidate Set (step 130). In the preferred embodiment, processing proceeds exactly as described above with reference to Pre-Candidate Status # 1 after the base station P L is transferred to any State # 2 or State # 0. For example, if the base station PC ^ is placed in the State # 2, a decorrelation / integration of the previous emptying and comparison with a previous emptying threshold (step 120a) is carried out. Next, an additional de-correlation / integration and combination of multiple trajectory (step 125a) is carried out in the same way as was done in step 125. The result of the resulting combined integration is compared to a Pre-Candidate State # threshold 3, and if this threshold exceeds the base station PCX is transferred from State # 2 to State # 3 of the Pre-Candidate Set. Otherwise, the PCX base station is returned to State # 1 of the Pre-Candidate Set (step 130a). Similarly, if the PCX base station has been transferred from State # 1 to State # 0, then a set of steps 120b, 125b and 130b analogous to steps 120, 125 and 130 would be carried out. Based on the results of the execution of steps 120b, 125b and 130b, the PCX base station would then be placed back into the State # 1, or it would be returned to the Neighbor Set. The PCX base station remains in the Pre-Candidate Set until it is returned to the Neighbor Set of State # 0, or until being transferred to the Candidate Set of State # 3 subsequent to the execution of stages 120c, 125c and 130c. After the PCX base station leaves the Pre-Candidate Set, the pilot signal from the base station PC2 is evaluated substantially identically. According to the invention, the various comparison thresholds and PN microcircuit integration ranges may be set differently within steps 12a, b, c to 130a, b, c as a means to vary the conditions under which they occur transitions between the State within the Pre-Candidate Set. In general, increasing the level of each Pre-Candidate Status threshold will increase the "probability of detection" that the base stations transferred from the Pre-Candidate Set to the Candidate Set will be able to establish communication with the mobile unit. Similarly, increasing the length of the de-correlation / integration intervals between the Pre-Candidate States will also tend to increase the probability of detection, and thereby reduce the likelihood of "false transfers" (ie, call transfers to base stations placed inappropriately within the Candidate Set). It is anticipated that the reduction of such threshold levels and integration intervals will tend to reduce the detection probability of the base station, but will favorably reduce the average acquisition time of the Candidate Set (ie, the average transition time of the Neighbor Set to Candidate Set through the Pre-Candidate Set). Turning now to FIG. 6, there is shown an exemplary mobile station receiver 200 in which a pilot scanner 210 can be incorporated according to the present invention. The mobile station receiver 200 includes an antenna 220, which is observed coupled to the analog receiver 224. The receiver 224 receives the RF frequency signals collected by the antenna 220, which are typically in the 850 MHz frequency band. , and effects the amplification and frequency downconversion to an IF frequency. This frequency translation process is carried out by the use of a standard design frequency synthesizer which allows the receiver 224 to be tuned to any of the frequencies within the receiving frequency band of the total cellular telephone frequency band. The IF signal is then passed through a surface acoustic wave (SA) bandpass filter which in the preferred embodiment is about 1.25 MHz bandwidth. The characteristics of the SAW filter are chosen to be coupled to the waveform of the signal transmitted by the base station which has been modulated by the broadcast spectrum of direct sequence by a PN sequence synchronized in a predetermined proportion, which in the Preferred embodiment is 1.2288 MHz. The receiver 224 is also provided with an analog to digital converter (A / D) (not shown) to convert the IF signal into a digital signal. The digitized signal is provided to each of the three or more signal processors or data receivers, one of which is the inventive scanning receiver 210, with the remaining data receivers being. For purposes of illustration, only the scanner receiver 210 and two data receivers 228 and 230 are shown in FIG. 6. In FIG. 6, the digitized signal outputted from the receiver 224 is provided to the digital data receivers 228 and 230. and the scanner receiver 210. It should be understood that a low-cost, low-cost mobile station could have only one data receiver while the higher-performance stations can have two or more, preferably a minimum of three, to allow reception diversity . The digitized IF signal may contain the signals of many calls in progress along with the pilot vehicles transmitted by the base stations within the Active Sets, Candidate, Pre-Candidate and Neighbor. The function of the receivers 228 and 230 is to correlate the IF samples with the appropriate PN sequence. This correlation process provides a property that is well known in the art as "processing gain" which improves the signal-to-interference ratio of a signal that compares the appropriate PN sequence but does not improve other signals. The correlation output is then detected consistently using the pilot carrier shift PN sequence used for the correlation as a carrier phase reference. The result of this detection process is a sequence of coded data symbols. A property of the PN sequence as used in the mobile station receiver 200 is that discrimination is provided against multi-path signals. When the signal reaches the mobile receiver 200 after passing through more than one path, there will be a difference in the signal reception time. This difference in reception time corresponds to the difference in distance divided by the speed of light. If this time difference exceeds a microcircuit of PN, 0.8138 msec. in a preferred implementation, then the correlation process will discriminate against one of the trajectories. The receiver 200 can choose between tracking and receiving the first or last path. If two data receivers are provided, such as receivers 228 and 230, then two independent paths can be tracked simultaneously. The scanner receiver 210, under the direction of the control processor (i.e. controller) 234, is for continuously scanning the time domain, around the nominal time of a pilot signal received from the Active base station (s). s) with which one (s) the mobile station is currently in communication. As discussed above, the multi-path pilot signals of the Active base station (s) and other pilot signals transmitted by the Candidate, Pre-Candidate and Neighbor base station are also detected and measured. The receiver 210 may be configured to utilize the proportion of the pilot received power per microcircuit and the total received spectral density, noise and signals, denoted as Ec / IO, as a measure of the pilot signal strength. The receiver 210 provides a signal resistance measuring signal to the controller 234, indicative of the pilot signal and its signal strength. The controller 234 provides signals to the digital data receivers 228 and 230 so that each one processes a different one of the stronger signals. The receivers 228 and 230 can process a multi-path signal from a single base station or signals from two different base stations. The outputs of the receivers 228 and 230 are provided to the diversity decoding and combining circuitry (not shown). The diversity combining circuitry adjusts the synchronization of the two received signal currents in alignment and adds them together. This addition process can be processed by multiplying two currents by a number corresponding to the relative signal strengths of the two currents. This operation can be considered a combination of diversity of maximum proportion. The resulting combined signal stream can then be decoded and provided to the digital baseband circuitry. Turning now to FIG. 7, a block diagram of the scanner receiver 210 is shown. In FIG. 7, an input signal 250 from an analog receiver 224 is assumed to be a signal input by Quadrature Phase Displacement (QSPK) having signal samples in phase (I) and phase in quadrature (Q). The signal samples I and Q, each being a multi-bit value, are introduced into de-diffusers of QPSK 260 and 270. The de-diffuser of QPSK 260 also receives the PN PNZ and PNQ pilot sequences from the PN 272 sequence generator. The PN 272 sequence generator generates the PN PNX and PNQ sequences identical to those used in the base station according to the synchronization. of sequence and state entered from the mobile station controller (not shown). The QPSK 260 de-diffuser removes the PN that is broadcast on row I and the Q signal samples to extract the samples of component I and Q not covered. In a similar way, the input signal 250 having the signal samples I and Q is input to the QPSK 270 de-diffuser. The QPSK 270 de-diffuser also receives the PN PN and PNQ pilot sequences from the pilot PN 272 sequence generator. through the time asymmetry 280. The asymmetry of time 280 advances and delays the PN PNX and PNQ pilot sequences. The QPSK 280 de-diffuser removes the PN that is broadcast on the I and Q signal samples to extract the samples from component I and Q before / after, "not covered". The pilot PN sequence generator 272 receives synchronization information from the control processor of the mobile station (not shown), which serves to jump the generator 272 from one hypothesis to the next within each search window. The search process starts when the generator 272 jumps to the offset associated with a given hypothesis, in which it remains during the number of specified PN microcircuits. De-diffused "on time" samples I and Q from de-diffuser 260 are provided to a first set of accumulators 290 and 292, and de-diffused "anterior / posterior" samples I and Q from de-diffuser 270 they are provided to a second set of accumulators 296 and 298. De-diffused I and Q samples are integrated for the appropriate integration interval (e.g., 100 microcircuits for an integration of the previous emptying) into the accumulators 290, 292, 296 and 298. A first insurance pair 302,304 and a second insurance pair 306 and 308, respectively, show the outputs of the first and second accumulator sets 290, 292, 296 and 298 at the end of each integration interval. As indicated by Figure 7, a multiplexer 312 alternately passes the contents of the first and second insurance pair to an energy calculation block I2 + Q2 320. If the comparator 324 determines that the output of the block 320 is less than the threshold of the previous emptying established by the control processor of the mobile station, then the control processor advances the displacement of the pilot signal generator of PN 272 to the following hypothesis. In a preferred embodiment, a demand for the previous empty is issued only if both pilot power levels at time and late, as provided respectively by the first insurance pair 302,304 and by the second insurance pair 306.308 to the I2 + Q2 320 block, they are both smaller than the threshold of the previous emptying. If the comparator 324 determines that the threshold of the previous emptying has been exceeded, then the integration operation carried out by the accumulators 290, 292, 296 and 298 is continued until the conclusion of the integration interval specified by the controller 234. This result can be provided directly via the signal line 334 (dotted) to the digital comparator 338, and compared to the specific threshold energy level (eg, a transition threshold of the Pre-Candidate Status) provided by the control processor of the mobile station. In an alternate modality, accuracy can be improved by adding energy from several integration steps that use the same pilot hypothesis and that compare the result with an aggregate threshold. As indicated by FIG. 7, this can be done by accumulating within the accumulator 342 the energy coming from a plurality (for example, from 2 to 7) of integration steps. After the specific number of integrations has been completed, the accumulated output from the accumulator 342 is transmitted to the digital comparator 338. The results of the comparison of the accumulated output with the aggregate threshold are then provided to the controller 234. In addition, the value of the maximum aggregate energy detected from the strongest of the hypotheses within a given search window is stored within the register 350. This maximum value indicated by the register 350 is provided to the controller 234 for its combination with the energy values associated with, for example, the aggregated values associated with a second and third hypothesis within a given search window. As discussed above, the resulting combined energy level can then be compared to a specified threshold energy level associated with the transition from one to the other of the Pre-Candidate States. It is understood that in this case the specified threshold energy will be based on the number of integration steps involved in the production of the energy value stored within the accumulator 342. The previous description of the preferred embodiments is provided to allow any person skilled in the matter elaborates or uses the present invention. The various modifications to these modalities will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other modalities without the use of the inventive faculty. In this way, it is not intended to limit the present invention to the modalities shown herein, but according to the broadest scope consistent with the principles and novel features set forth herein.

Claims (36)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. 1. In a cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in an active list of one or more base station entries, wherein each one of a plurality of base stations within said system transmits a single pilot signal, a method for identifying one of said base stations from which the received signal strength is sufficient to establish communication with said mobile station, said method comprises the steps of : maintaining in said mobile station a candidate list of base station entries, wherein each entry in said candidate list corresponds to a base station capable of providing a signal strength sufficient to establish communication with said mobile station wherein said active list of a or more base station entries is kept within said mobile station and is derived from said candidate list of base station entries; maintaining in said mobile station a neighboring list of base station entries, wherein each entry in said neighboring list corresponds to a base station in a predetermined vicinity of said mobile station; measuring in said mobile station the signal resistance of each said pilot signal transmitted by each of said base stations in said neighboring list; comparing in said mobile station said signal strength measurements of the base station of each of said entries in the neighboring list with a first predetermined level; and withdrawing by said mobile station a particular entry from said neighbor list having said signal strength measurement of the base station greater than said first predetermined level and placing said particular entry in a pre-candidate list maintained in said mobile station, wherein said entries in said pre-candidate list correspond to a set of base stations from which said candidate list of base station entries is derived. The method according to claim 1, characterized in that said step of removing said particular entry includes the step of placing said particular entry within a first state of said pre-candidate list, said pre-candidate list having associated therewith a plurality of states different from said first state. The method according to claim 2, characterized in that it further includes the steps of: correlating, for a first predetermined time interval in said mobile station, a replica of a particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received in said mobile station; and withdrawing said first state and placing said particular entry into a second state of said pre-candidate list based on a result of said correlation step. The method according to claim 3, characterized in that it further includes the steps of: correlating, for a second predetermined time interval in said mobile station, a replica of said particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received in said mobile station; and withdrawing from said second state and placing said particular entry into a third state of said pre-candidate list based on a result of said correlation step carried out during said second predetermined time interval. The method according to claim 4, characterized in that it further includes the steps of: correlating, for a third predetermined time interval in said mobile station, a replica of said particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received at said mobile station; and withdrawing said third state and placing said particular entry into said candidate list based on a result of said correlation step carried out during said third predetermined time interval. The method according to claim 2, characterized in that it further includes the steps of: correlating, for one or more predetermined time slots in said mobile station, a replica of a particular base station pilot signal corresponding to said particular input with said signal a particular base station pilot received at said mobile station; and withdrawing from said first state and returning said particular entry to said neighbor list based on a result of said correlation step carried out during said one or more predetermined time intervals. 7. In a code division multiple access (CDMA) broadcast cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in a list active one or more base station inputs, wherein each of a plurality of base stations within said system transmits a pilot PN code signal of a single phase, each of said PN code signals including a predefined sequence of PN microcircuits, a method for identifying one of said base stations from which the received signal strength is sufficient to establish communication with said mobile station, said method comprises the steps of: maintaining in said mobile station a candidate list of inputs of base station, wherein each entry in said candidate list corresponds to a base station capable of providing a resistance of sufficient signal to establish communication with said mobile station wherein said active list of one or more base station entries is kept within said mobile station and is derived from said candidate list of base station entries; maintaining in said mobile station a neighboring list of base station entries, wherein each entry in said neighboring list corresponds to a base station in a predetermined vicinity of said mobile station; measuring in said mobile station the signal strength of each said pilot PN code signal transmitted by each of said base stations in said neighboring list; comparing in said mobile station said signal strength measurements of the base station of each of said entries in the neighboring list with a first predetermined level; and withdrawing by said mobile station a particular entry from said neighbor list having said signal strength measurement of the base station greater than said first predetermined level and placing said particular entry in a pre-candidate list maintained in said mobile station, wherein said entries in said pre-candidate list correspond to a set of base stations from which said candidate list of base station entries is derived. The method according to claim 7, characterized in that when said particular entry is placed within said pre-candidate list, it is placed in a first state of said pre-candidate list, the method further includes the steps of: correlating, upon a first predetermined number of said PN microcircuits, a replica of a particular base station pilot PN code signal corresponding to said particular input with said particular base station pilot PN code signal received at said mobile station; and withdrawing from a first state and placing said particular entry into a second state of said pre-candidate list based on a result of said correlation step. The method according to claim 8, characterized in that it further includes the steps of: correlating, on a second predetermined number of said PN microcircuits, said replica of said particular base station pilot PN code signal corresponding to said particular input with said particular base station pilot PN code signal received at said mobile station; and withdrawing from said second state and placing said particular entry into a third state of said pre-candidate list based on a result of said correlation step carried out on said second predetermined number of said PN microcircuits. The method according to claim 9, characterized in that it further includes the steps of: correlating, on a third predetermined number of said PN microcircuits, said replica of the base station pilot PN code signal corresponding to said particular input with said PN code signal pilot of particular base station received at said mobile station; and withdrawing said third state and placing said particular entry into said candidate list based on a result of said correlation step carried out on said third predetermined number of said PN microcircuits. The method according to claim 7, characterized in that it further includes the steps of: correlating, on one or more predetermined numbers of said PN microcircuits, a replica of a base station pilot PN code signal corresponding to said particular input with said particular base station pilot PN code signal received at said mobile station; and withdrawing from a first state of said pre-candidate list and placing said particular entry within said neighbor list based on a result of said correlation step on said one or more predetermined numbers of said PN microcircuits. The method according to claim 8, characterized in that said correlation step further includes the steps of: defining a time window around an expected arrival time in said mobile station of said particular base station pilot PN code signal; and de-broadcasting, at multiple time shifts within said time window, said particular base station pilot PN code signal received at said mobile station with said replica of said particular base station pilot PN code signal; and integrating each result of said de-diffusion stage onto said first predetermined number of said PN microcircuits. The method according to claim 12, characterized in that it further includes the steps of: averaging a selected number of results of said integration step; comparing a result of said averaging step with a predefined threshold; and considering that said particular entry of said pre-candidate list is removed from said first state and placed within a second state of said pre-candidate list, if the result of said averaging step exceeds said predefined threshold. The method according to claim 8, characterized in that said correlation step further includes the steps of: defining a time window around an expected arrival time in said mobile station of said particular base station pilot PN signal; carrying out an integration of the previous emptying, in a first time displacement within said time window, of said base station pilot PN code replica with said particular base station pilot PN code signal received in said mobile station, said integration of the previous emptying being carried out in said first time displacement on a preselected number of said PN microcircuits; comparing a result of said integration of the previous emptying carried out in said first time displacement with a previous emptying threshold; and considering that said particular entry of said pre-candidate list is removed from said neighboring list and placed within said first state of said pre-candidate list if the result of said step of carrying out an integration of the previous cast in said first displacement of time exceeds said threshold of the previous emptying. The method according to claim 14, characterized in that it further includes the step of continuing the integration, on a second predetermined number of said PN microcircuits, of said base station pilot PN code replica with said code signal of PN pilot of a particular base station; and removing said particular entry from said first state of said pre-candidate list and placing said particular entry within a second pre-candidate list status if a result of said continuation step of the integration on said second predetermined number of said microcircuits. of PN code exceeds a second threshold. 16. In a cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in an active list of one or more base station entries, wherein each one of a plurality of base stations within said system transmits a unique pilot signal, an explorer device placed within said mobile station to identify one of said base stations of which the received signal resistance is sufficient to establish communication with said mobile station , said scanning apparatus comprises: a mobile station controller for maintaining a candidate list of base station entries, wherein each entry in said candidate list corresponds to a base station capable of providing a signal strength sufficient to establish communication with said mobile station wherein said active list of one or more base station entries is stored within said mobile station and derived from said candidate list of base station entries, said mobile station controller additionally maintains a neighbor list of base station entries, wherein each entry in said neighboring list corresponds to a base station in a predetermined proximity of said mobile station; a pilot signal measuring circuit for measuring the signal resistance of each said pilot signal transmitted by each of said base stations in said neighboring list; a comparison circuit for comparing said signal strength measurements of the base station of each said neighbor list inputs with a first predetermined level; wherein said mobile station controller removes a particular entry from said neighbor list having said signal strength measurement of the base station greater than said first predetermined level and places said particular entry in a pre-candidate list maintained within said mobile station , said entries in said pre-candidate list corresponding to a set of base stations from which said candidate list of base station entries is derived. The apparatus according to claim 16, characterized in that said mobile station controller includes means for placing said particular entry within a first state of said pre-candidate list, said pre-candidate list having associated therewith a plurality of states different from said first state. The apparatus according to claim 17, characterized in that it further includes a correlator to produce a correlation result by correlating, during a first predetermined time interval in said mobile station, a replica of a particular base station pilot signal corresponding to said input particular with said particular base station pilot signal received in said mobile station wherein said mobile station controller further includes means for withdrawing said first state and placing said particular entry on said basis within a second state of said pre-candidate list. correlation result. The apparatus according to claim 18, characterized in that said correlator is further to produce a second correlation result by correlating, for a second predetermined time interval in said mobile station, said replica of said particular base station pilot signal corresponding to said particular input with said particular base station pilot signal received at said mobile station, and wherein said mobile station controller further includes means for withdrawing from said second state and placing said particular entry in a third state of said pre-candidate list. basis to said second correlation. 20. The apparatus according to claim 19, characterized in that said correlator is further to produce a third correlation result, by correlating for a third predetermined time interval in said mobile station, said replica of said particular base station pilot signal corresponding to said particular input with said station pilot signal particular base received in said mobile station, and wherein said mobile station controller further includes means for removing said third state and placing said particular entry into said candidate list based on said third correlation result. The apparatus according to claim 19, characterized in that said correlator is further to correlate, during one or more predetermined time intervals in said mobile station, said replica of said particular base station pilot signal corresponding to said particular input with said pilot signal of a particular base station received at said mobile station, and wherein said mobile station controller further includes means for removing said particular entry from said first state and returning said neighbor based on a result of said correlator that correlates during said one or more predetermined time intervals. 22. In a code division multiple access broadcast spectrum (CDMA) cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in a list active one or more base station inputs, wherein each of a plurality of base stations within said system transmits a pilot PN code signal of a single phase, each of said PN code signals including a predefined sequence of PN microcircuits, a mobile station scanning apparatus for identifying one of said base stations from which the received signal resistance is sufficient to establish communication with said mobile station, said apparatus comprising: a mobile station controller for maintaining a candidate list of base station entries, wherein each entry in said candidate list corresponds to a base station capable of providing a sufficient signal strength to establish communication with said mobile station wherein said active list of one or more base station entries is stored within said mobile station and is derived from said candidate list of base station inputs, said station controller mobile also maintains a neighbor list of base station entries, wherein each entry in said candidate list corresponds to a base station in a predetermined vicinity of said mobile station; a pilot signal measuring circuit for measuring in said mobile station the signal resistance of each said pilot PN code signal transmitted by each of said base stations in said neighboring list; a comparison circuit for comparing in said mobile station said signal strength measurements of the base station of each said neighbor list inputs with a first predetermined level; wherein said mobile station controller removes a particular entry from said neighbor list having said signal strength measurement of the base station greater than said first predetermined level and places said particular entry in a pre-candidate list maintained within said mobile station , said entries in said pre-candidate list corresponding to a set of base stations from which said candidate list of base station entries is derived. The apparatus according to claim 22, characterized in that it further includes a correlator to produce a first correlation result by correlating, on a first predetermined number of said PN chips, a replica of the corresponding base station pilot PN code signal. to said particular input with said particular base station pilot PN code signal received in said mobile station wherein said mobile station controller includes means for withdrawing said first state and placing within said second state of said pre-candidate list said particular entry based on said first correlation result. The apparatus according to claim 23, characterized in that it also includes means for producing a first integration result by integrating, on a second predetermined number of said PN chips, said replica of the base station pilot PN code signal corresponding to said particular input with said particular base station pilot PN code signal received in said mobile station wherein said mobile station controller includes means for withdrawing said second state and placing said entry within a third state of said pre-candidate list. particular on the basis of said first integration result. The apparatus according to claim 23, characterized in that it further includes means for producing an integration result by integrating, on one or more different predetermined numbers of said PN chips, said replica of the corresponding base station pilot PN code signal. to said particular input with said particular base station pilot PN signal received at said mobile station and wherein said mobile station controller includes means for withdrawing said first state and placing said particular entry within said neighbor list if said result of integration is less than a predefined threshold. 26. The apparatus according to claim 24, characterized in that it further includes means for producing a second integration result by integrating, on a third predetermined number of said PN microcircuits, said replica of the base station pilot PN code signal corresponding to said particular input with said particular base station pilot PN code signal received at said mobile station wherein said mobile station controller further includes means for withdrawing said third state and placing said particular entry within said candidate list if said second result of integration exceeds a predefined threshold. The apparatus according to claim 24, characterized in that it also includes means for defining a time window around an expected arrival time in said mobile station of said particular base station pilot PN code signal. The apparatus according to claim 26, characterized in that said correlator is operative to integrate, on said first predetermined number of said PN microcircuits and at multiple time shifts within said time window, said pilot PN code signal replica of base station with said particular base station pilot PN code signal received at said mobile station. 29. The apparatus according to claim 28, characterized in that it further includes: means for averaging the results of a selected number of integrations carried out during said time window; a comparator to compare the average of the results with a predefined threshold; and wherein said mobile station controller includes means for withdrawing said first state and placing said particular entry in a second state of said pre-candidate list based on the result produced by said comparator. 30. The apparatus according to claim 23, characterized in that it further includes: means for defining a time window around an expected arrival time in said mobile station of said particular base station pilot PN code signal, said correlator includes means for carrying out an integration of the previous emptying, in a first time displacement within said time window, of said base station pilot PN code replica with said particular base station pilot PN code signal received in said mobile station, said integration of the previous emptying being carried out in said first time displacement on a preselected number of said PN microcircuits; a comparator for comparing the results of said integration of the previous emptying in said first time displacement with a previous emptying threshold; and wherein said mobile station controller includes means for removing said neighbor list and placing said particular entry within a first state of said pre-candidate list if the result of said integration of the previous emptying carried out in said first time offset exceeds said threshold of the previous emptying. The apparatus according to claim 30, characterized in that said correlator includes means for producing a continuous integration result upon continuing the integration, on a second predetermined number of said PN code chips, of said pilot PN code signal replica. of base station with said particular base station pilot PN code signal, and wherein said mobile station controller includes means for removing said particular input from said first state of said pre-candidate list and placing said particular input within one second pre-candidate list status if said continuous integration result exceeds a second threshold. 32. In a cellular communication system in which a mobile user communicates through a mobile station with other users through at least one base station included in an active list of one or more base station entries, wherein each one of a plurality of base stations within said system transmits a single pilot signal, a method for identifying one of said base stations of which the received signal strength is sufficient to establish communication with said mobile station, said method comprises the steps of : maintaining in said mobile station a candidate list of base station entries, wherein each entry in said candidate list corresponds to a base station capable of providing a signal strength sufficient to establish communication with said mobile station wherein said active list of a or more base station entries is maintained within said mobile station and is derived from said mobile station. ista base station ticket candidate; maintaining in said mobile station a neighboring list of base station entries, wherein each entry in said neighboring list corresponds to a base station in a predetermined vicinity of said mobile station; measuring in said mobile station the signal resistance of each said pilot signal transmitted by each of said base stations in said neighbor list, - comparing said signal strength measurements of the base station of each said input of said mobile station in said mobile station; the neighboring list with a predetermined first level; and placing a particular entry from said neighbor list having said base station signal strength measurement greater than said first predetermined level in a pre-candidate list maintained in said mobile station, wherein said entries in said pre-candidate list correspond to a set of base stations from which said candidate list of base station entries is derived. The method according to claim 32, characterized in that said placement step includes the step of introducing said particular entry into a first state of said pre-candidate list, said pre-candidate list having associated therewith a plurality of states different from said first state. The method according to claim 33, characterized in that it further includes the steps of: correlating, for a first predetermined time interval in said mobile station, a replica of a particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received at said mobile station; and withdrawing said first state and placing said particular entry into a second state of said pre-candidate list based on a result of said correlation step. 35. The method according to claim 34, characterized in that it further includes the steps of: correlating, for a second predetermined time interval in said mobile station, a replica of said particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received in said mobile station; and withdrawing from said second state and placing said particular entry into a third state of said pre-candidate list based on a result of said correlation step carried out during said second predetermined time interval. 36. The method according to claim 35, characterized in that it further includes the steps of: correlating, for a third predetermined time interval in said mobile station, a replica of said particular base station pilot signal corresponding to said particular input with said pilot signal of particular base station received in said mobile station; and withdrawing said third state and placing said particular entry into said candidate list based on a result of said correlation step carried out during said third predetermined time interval.