WO1998059448A2

WO1998059448A2 - Prioritized pilot searching in a code-division multiple access communication system

Info

Publication number: WO1998059448A2
Application number: PCT/US1998/012979
Authority: WO
Inventors: Scott M. Owen
Original assignee: Qualcomm Incorporated
Priority date: 1997-06-20
Filing date: 1998-06-19
Publication date: 1998-12-30
Also published as: WO1998059448A3; AU8159698A

Abstract

A method for prioritized searching of pilot signals of neighboring base stations in a CDMA communication system, in which the frequency of revisitation of a high priority neighbor's pilot is increased with respect to the lower priority neighboring pilots. The mobile station receives a neighbor list message (200) from the base station. The neighbor list message includes pilot signal parameters for a plurality of neighboring pilot signals transmitted by neighboring base stations. A plurality of priority indications, each corresponding to one of the plurality of pilot signals is also included. In a preferred embodiment, the first of the plurality of priority indications corresponds to a high priority and the second of the plurality of priority indications corresponds to a low priority. The mobile station stores these pilot signal parameters and plurality of priority indications in a neighbor list table (202).

Description

PRIORITIZED PILOT SEARCHING IN A CODE-DIVISION MULTIPLE ACCESS COMMUNICATION SYSTEM

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a novel and improved method for scanning prioritized neighboring pilot signals in a code-division multiple access (CDMA) communication system.

II. Description of the Related Art In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled "Mobile Station - Base Station Compatibility Standard for Dual- Mode Wideband Spread Spectrum Cellular System." Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled "Personal Station - Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems." Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.

The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Patent No. 4,901,307, issued February 13, 1990, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell- sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Patent No. 5,101,501, issued March 31, 1992, entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM", and U.S. Patent No. 5,109,390, issued April 28, 1992, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.

The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Patent No. 5,056,109, issued October 8, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", Serial No. 07/433,031, filed November 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Patent No. 5,103,459, issued April 7, 1992, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. Patent Application Serial No. 08/509,721, entitled "METHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM," assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Patent No. 5,103,459 and will not be discussed in detail herein.

The viable base station candidates can be divided into four sets. The first set, referred to as the Active Set, comprises base stations which are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations which have been determined to be of sufficient strength to be of use to the mobile station. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined threshold T_ADD. The third set is the Neighbor Set which is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set which consists of all other base stations.

The above standards and patents describe, among other things, the manner in which a mobile station is to execute a "handoff" between neighboring base stations as it travels between their respective geographic coverage areas. For example, in the CDMA-based standards IS-95 and J-STD- 008, the base station sends a message to the mobile station listing many of the system parameters of its neighboring base stations, including such information as would assist the mobile station in executing an "autonomous" handoff between base stations. An autonomous handoff is one that is not initiated or directed by the base station, but rather is initiated by the mobile station itself.

An example of one such neighbor list message is the "Extended Neighbor List Message" of J-STD-008. When the base station sends an Extended Neighbor List Message to the mobile station, it uses the format of Table I.

TABLE I

Zero or more occurrences of the following record:

The above table is taken from Section 3.7.2.3.2.14 of J-STD-008, and indicates the various fields transmitted in an exemplary Extended Neighbor List Message. Of particular concern to the present invention are the following fields:

NGHBR_PN - the base station sets this field to the pilot PN sequence offset for this neighbor, in units of 64 PN chips;

NGHBR_FREQ - the base station sets this field to the CDMA channel number corresponding to the CDMA frequency assignment for the CDMA channel containing the paging channel that the mobile station is to search; and

SEARCH_PRIORITY - the base station sets this field to the search priority for the pilot channel corresponding to NGHBR_PN, according to TABLE II below:

TABLE II

Thus, according to J-STD-008, the mobile station is given the frequency and PN offset of each neighboring base station. This gives the mobile station enough information to make a more focused search for neighbor pilots, rather than having to search all possible PN offsets on all possible CDMA frequency assignments. For example, the mobile station may keep a table of all the neighbors that were passed to it in the neighbor list message or extended neighbor list message, plus all of the neighbors that it detected on other frequencies during its own independent searching. Such a table might resemble Table III below.

TABLE III

These priority bits provide a reference for differentiating among the relative priorities of various neighboring pilots. This is useful when there are many neighbors to be scanned because it may take several seconds to search for all neighbor's pilot signal's when there is long neighbor list. During the time that the mobile station is using one of its limited number of demodulation resources (i.e. one searcher receiver) to search for a first neighbor's pilot signal, a second neighbor's pilot signal may be causing significant interference. For example, if the neighbor pilot signals were searched sequentially as listed in TABLE III above, the mobile station might first search for a first neighbor with a PN offset of 12 chips on frequency f(l), and then search for a second neighbor on the same frequency f(l) with a PN offset of 24 chips, and so on. If the mobile station is travelling throughout the network coverage area, for example in a car, it may travel into an area where one particular neighbor's pilot is very strong, whereas other neighbor's pilots are relatively weak. If the mobile station is spending time and resources searching for the relatively weak neighboring pilots at a time when a very strong neighboring pilot coincidentally appears, there is a high likelihood that the significant interference caused by the very strong neighboring pilot would cause unacceptable communications with the existing base station, and perhaps even loss of the forward link. The very nature of a CDMA system would allow for the mobile station to acquire and then initiate a soft handoff to this very strong neighbor if its searcher receiver were not busy searching for another neighbor. Thus, it is clear that a novel and improved method for scanning prioritized neighbors in a CDMA communication system is needed, in which the frequency of revisitation of a higher priority neighbor's pilot is increased with respect to the lower priority neighboring pilots.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method for prioritized searching of pilots signals of neighboring base stations in a CDMA communication system, in which the frequency of revisitation of a high priority neighbor's pilot is increased with respect to the lower priority neighboring pilots. The mobile station receives a neighbor list message from the base station. The neighbor list message includes pilot signal parameters for a plurality of neighboring pilot signals transmitted by neighboring base stations, for example the extended neighbor list message of TABLE I and EL A plurality of priority indications, each corresponding to one of the plurality of pilot signals is also included. In the preferred embodiment, the first of the plurality of priority indications corresponds to a high priority and the second of the plurality of priority indications corresponds to a low priority. The mobile station stores these pilot signal parameters and plurality of priority indications in a neighbor list table.

In response to this neighbor list message, the mobile station determines a search schedule in accordance with the plurality of priority indications, wherein the neighboring pilot signals having a first one of the priority indications are scheduled to be searched more frequently than neighboring pilots having a second of the priority indications. The mobile station then searches for the plurality of neighboring pilot signals according to the search schedule. In the preferred embodiment, the mobile station also schedules a search of an active pilot signal associated with a currently assigned paging channel. The duty cycle of searching this active pilot is preferably a fifty percent duty cycle. However, other embodiments use other duty cycles.

In accordance with another aspect of the present innovation, the mobile station calculates a scan rate for the plurality of neighboring pilot signals in response to a total number of the plurality of neighboring pilot signals and a distribution of the priority indications of said neighboring pilot signals. 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:

FIG. 1 is a block diagram of the apparatus of the present invention; and

FIG. 2 is a block diagram of the method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates mobile station 2 of the present invention. Mobile station 2 continuously or at intermittent intervals measures the strength of pilot signals of neighboring base stations. Signals received by antenna 50 of mobile station 2 are provided through duplexer 52 to receiver (RCVR) 54 which amplifies, downconverts, and filters the received signal and provides it to pilot demodulator 58 of searcher subsystem 55. In addition, the received signal is provided to traffic demodulators

64A-64N. Traffic demodulators 64A-64N, or a subset thereof, separately demodulate signals received by mobile station 2. The demodulated signals from traffic demodulators 64A-64N are provided to combiner 66 which combines the demodulated data, which in turn provides an improved estimate of the transmitted data.

Mobile station 2 measures the strength of pilot channels. Control processor 62 provides acquisition parameters to search processor 56. Specifically, control processor 62 provides such acquisition parameters to execute the method described below with reference to FIG. 2. Control processor 62 builds a Neighbor List Table (not shown in FIG. 1) similar to Table III above from the Extended Neighbor List Message sent by the base station and described above in Tables I and H Control processor 62 then accesses the neighbor list table to determine the scheduling of neighbor pilot searches to be executed by searcher subsystem 55. Control processor 62 may be a conventional microprocessor as is known in the art, and associated memory. In the exemplary embodiment of a CDMA communication system, control processor 62 provides a PN offset to search processor 56 in accordance with the next neighbor pilot signal to be searched. Search processor 56 generates a PN sequence which is used by pilot demodulator 58 to demodulate the received signal. The demodulated pilot signal is provided to energy accumulator 60 which measures the energy of the demodulated pilot signal, by accumulating the energy for predetermined lengths of time as is known in the art. The measured pilot energy values are provided to control processor

62. In the exemplary embodiment, control processor 62 compares the energy values to thresholds T_ADD and T_DROp. T_ADD is threshold above which the received signal is of sufficient strength to effectively provide communications with mobile station 2. T_DROp is a threshold value below which the received signal energy is insufficient to effectively provide communications with mobile station 2.

Control processor 62 provides the identities of the pilots and their corresponding measured pilot energies to message generator 70. Message generator 70 generates a Pilot Strength Measurement Message containing the information. The Pilot Strength Measurement Message is provided to transmitter (TMTR) 68, which encodes, modulates, upconverts and amplifies the message. The message is then transmitted through duplexer 52 and antenna 50.

Referring now to FIG. 2, a flowchart illustrating the method of the present invention is shown. The flow begins at block 200 with the mobile station 2 in Idle Mode, monitoring its assigned paging channel, and receiving an Extended Neighbor List Message. In response to the received Extended Neighbor List Message, the mobile station 2 builds the neighbor list table. The flow continues to block 204 where a variable MASK is set equal to 0. The variable MASK corresponds to the priority of the neighbors as defined in the extended neighbor list message and listed above in TABLE III. In block 206, the variable i is set to be equal to the number of entries in the neighbor list table which have a priority which is greater than or equal to the present value of the variable MASK. For example, in the neighbor list table of TABLE m, when MASK=0, i would be set equal to 5 at block 206 because there are five entries with a priority greater than or equal to zero. As another example, when MASK=3, i would be set equal to 2 at block 206 because there are only two entries with a priority greater than or equal to 3.

At block 208 the control processor 62 provides the relevant search parameters to searcher subsystem 55, including the PN offset and frequency of the i-th entry in the neighbor list table that has a priority greater than or equal to the current value of MASK. The searcher subsystem 55 then scans the scheduled neighbor pilot, and the flow proceeds to block 210 where the control processor 62 then directs the searcher subsystem 55 to track the active pilot (i.e. the pilot signal associated with the currently assigned paging channel). In an alternate embodiment, block 210 may be skipped. However, in the preferred embodiment, the scanning of the prioritized neighbors in the neighbor list table (represented by block 208) is alternated with tracking of the active pilot (represented by block 210) according to a duty cycle which is dependent on the number of neighbor pilots with the same priority, the total number of neighbor pilots in the neighbor list table, as well as the distribution of the neighbor pilots among various priorities, as will be discussed later herein. After the mobile station 2 has scanned the i-th neighbor with a priority greater than or equal to the current MASK value in block 208, and also the active pilot in block 210, a decision is made at diamond 212 whether this is the last neighbor pilot at the current MASK value to be scanned by comparing the variable i to 1. If i does not yet equal 1, then there remain additional neighbor pilots in the neighbor list table which have priorities greater than or equal to the current value of MASK. In this case, the flow proceeds to block 214 where the variable i is decremented, and the flow then returns to block 208 to scan the next neighbor pilot with a priority greater than or equal to the current value of MASK. When all neighbor pilots with the a priority greater than or equal to the current value of MASK have been scanned, the result of decision 212 is YES, and the flow proceeds to decision diamond 216 where it is determined whether MASK=3. If MASK does not yet equal 3, then the flow continues to block 218 where MASK is incremented and returns to block 206 where the variable i is reset to the number of neighbor pilots in the neighbor list table with a priority greater than or equal to the current value of MASK. However, if MASK=3 at decision 216, then the flow returns to block 204 where the variable MASK is reset to 0, and the process repeats. It should be noted that at any time the mobile station 2 may receive another Extended Neighbor List Message from the base station, and restart the process at block 200.

As can be seen from the operation of the exemplary embodiment of FIG. 2, the mobile station cycles through the variable MASK in the sequence [0, 1, 2, 3]. For each value of MASK, the mobile station 2 then scans each neighbor pilot in the neighbor list table with a priority greater than or equal to the current value of MASK. Thus, those neighbor pilots with a priority of 3 are scanned four times more often as those of priority 0, three times more often as those of priority 1, and twice more often as those of priority 2. This ensures that higher priority neighbor pilots are revisited more often, allowing mobile station 2 the opportunity to acquire them earlier than would be the case if the search was performed in a simple serial manner.

In accordance with a further aspect of the present invention, the scan rate, and therefore the duty cycle of scanning of the prioritized neighbors is determined according to the priority of the particular neighbor pilot, how many fellow neighbor pilots are at the same priority, as well as the distribution of neighbor pilots at other priorities. In the preferred embodiment, the pilot signal of the active base station(s) is always searched on every other scan, resulting in a duty cycle of 50% for the active pilot. The remaining 50% of the time, the mobile station is cycling through the various neighbor pilots in the neighbor list table according to the method of FIG. 2. Therefore, if the raw search rate capability of searcher subsystem 55 in searches per second is equal to a value R, then the active pilot is scanned with a scan rate of R/2. The scan rate for neighbors would then be determined by the current value of the variable MASK, and the value of the variable i from FIG. 2, according to the relationship F_n(MASK) = R/2L

It should be noted that the present invention is not limited by the duty cycle associated with the revisitation of any particular pilot signal. A person of ordinary skill in the art could devise a different scan rate and duty cycle other than 50% without departing from the spirit of the present invention. For example, in alternate embodiments, the MASK variable may not follow the sequence [0, 1, 2, 3] continuously, but may mix up the order of searching or spend extra time at one or more priorities. It also should be noted that the present invention is applicable to other standards besides J- STD-008. The present invention is applicable to any wireless communication system wherein there is a need to revisit certain pilot signals more often then others according to a prioritized schedule.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

I CLAIM:

Claims

1. A method for prioritized searching of pilots signals of neighboring base stations, the method comprising the steps of: receiving a neighbor list message from said base station, said neighbor list message including pilot signal parameters for a plurality of neighboring pilot signals transmitted by said neighboring base stations and a plurality of priority indications, each corresponding to one of said plurality of pilot signals; determining a search schedule in accordance with said plurality of priority indications, wherein said neighboring pilot signals having a first of said priority indications are scheduled to be searched more frequently than said neighboring pilots having a second of said priority indications; and searching for said plurality of neighboring pilot signals according to said search schedule.

2. The method of claim 1 further comprising the step of storing said pilot signal parameters and said plurality of priority indications in a table.

3. The method of claim 1 wherein said determining step further comprises scheduling a search of an active pilot signal associated with a currently assigned paging channel, and wherein said searching step further comprises searching for said active pilot signal according to said search schedule.

4. The method of claim 3 wherein said active pilot signal is searched with a fifty percent duty cycle.

5. The method of claim 3 wherein said determining step further comprises calculating a scan rate for said plurality of neighboring pilot signals in response to a total number of said plurality of neighboring pilot signals and a distribution of said priority indications of said neighboring pilot signals.

6. The method of claim 1 wherein said first of said plurality of priority indications corresponds to a high priority and said second of said plurality of priority indications corresponds to a low priority.