HK1055047B - Method and apparatus for performing idle handoff in a multiple access communication system - Google Patents

Description

Method and apparatus for implementing idle handoff in a multiple access communication system

The present application is a divisional application of an invention patent application having an application number of 97197151.X, application date 1997, 6.6, entitled "method and apparatus for implementing idle handoff in a multiple access communication system".

Technical Field

The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method of implementing idle handoff in a multiple access communication system. Furthermore, the present invention relates to an improved method of allocating traffic channels in a multiple access communication system. The present invention also relates to a method of reducing the number of required handovers that occur when a mobile station is queued for a traffic channel in a multiple access communication system.

Background

The communication system is normally barred from handover when the mobile station is in a system access state. In the system access state, the mobile station initiates communication by transmitting on an access channel or the base station by transmitting on a paging channel. In an exemplary embodiment, the messages are transmitted according to a Code Division Multiple Access (CDMA) communication format, see in detail U.S. patent No.4,901,307, entitled "spread spectrum multiple access communication system using satellite or terrestrial repeaters," and U.S. patent No.5,103,459, entitled "system and method for generating waveforms in a CDMA cell phone system," both assigned to the assignee of the present invention and incorporated herein by reference. Initiating a call using a paging channel and an access channel IS a well-known technique, see TIA/EIA interim standard IS-95-a entitled "mobile station-base station compatibility standard for dual-mode broadband spread spectrum cellular systems" for details.

One of the features of CDMA systems is that each cell reuses the same frequency. Diversity combining is a method used by receivers receiving signals carrying the same information to combine the signals, which travel through different paths to improve the accuracy of the estimation of the transmitted signal. U.S. patent No.5,109,390, entitled "diversity receiver in a CDMA cell phone system," details a receiver design that takes advantage of the diversity signals that carry the same information but are routed through different propagation paths or transmitted by different transmitters, which is assigned to the assignee of the present invention and is incorporated herein by reference.

In a soft handover method, a mobile station moving from one cell to another receives information transmitted from base stations serving more than two cells in the vicinity of a border area as long as it is near the border of the more than two cells. The signals transmitted by the base stations are combined in the mobile station receiver using the diversity combining method described above. A method and system for providing soft handoff in a CDMA communication system is disclosed in U.S. patent No.5,101,501, entitled "method and system for providing soft handoff in a CDMA cell phone system," and U.S. patent No.5,267,261, entitled "mobile assisted soft handoff in a CDMA cell communication system," in which multiple base stations communicate with a mobile station at or near the cell boundary, which patents are assigned to the assignee of the present invention and incorporated herein by reference. In contrast to soft handover, hard handover is the case when a mobile station enters another cell from one cell, the mobile station is released by the cell that it is leaving and then the base station is accepted by the cell that it is entering.

CDMA systems have higher capacity due to the use of the same frequency in each cell and the use of soft handoff. The repeated use of the same frequency in adjacent cells causes rapid changes in the signal-to-noise ratio of the forward link signal near the cell border. The reason for this is that the cell received by the mobile station may fade and the strength of the neighboring cell may increase (fade resistance).

In general, when a mobile station receives two cells, the following equation (1) gives the ratio of the received traffic channel energy per unit spreading sub-code to the total spectral noise density of the signal transmitted by cell 1:

the following equation (2) gives the ratio of the received traffic channel energy per unit spreading subcode to the total spectral noise density of the signal transmitted by cell 2:

in equations (1) and (2)

I_OCAs total thermal noise

2 are fractional ratios of the traffic channel power transmitted by cell 1 and cell 2 respectively,

fractional ratios of the traffic channel power received by the mobile station for cell 1 and cell 2, respectively.

_OCIs considered to be relativeAndis smaller. When cell 1 fades relative to cell 2,relative to each otherSmaller (ratio)Becomes larger). Thus, it is possible to provide1 becomes smaller. Such a change in signal-to-noise ratio may be problematic if the mobile station is not in soft handoff. However, if the mobile station is in soft handoff between adjacent cells, then the mobile station pair is from bothThe forward traffic channels of the cells are diversity combined so that variations in the signal-to-noise ratio do not cause problems. Although it is used for1 the first path given becomes smaller, butThe second path given by 2 becomes larger. Fading of one cell improves the signal-to-noise ratio of another cell.

Paging is a method of sending information to a mobile station to initiate mobile terminated service or receiving an indication of new overhead information. Methods of initiating a base station initial call are described in detail in U.S. patent No.5,392,287, entitled "apparatus and method for reducing power consumption of a mobile communication receiver" and co-pending U.S. patent application No.08/206,701, filed as a continuation of U.S. patent No.5,392,287 on 3/7 of 1994, which are assigned to the assignee of the present invention and are incorporated herein by reference. The present invention is equally applicable to mobile station initial calls, the method of which is described in detail in co-pending U.S. patent application No.08/219,867 filed on 1996, month 1, 18, which is assigned to the assignee of the present invention and is incorporated herein by reference.

In slotted paging systems, the mobile station monitors the paging channel for a short predetermined time interval and then does not monitor the paging channel until the next predetermined time interval. In IS-95-a, this method of periodically monitoring the paging channel IS referred to as slotted mode, and the mobile station may monitor the paging channel for 80 milliseconds every 1.28 seconds. The period between monitoring intervals may be extended according to the needs of the user. Before each pre-designated mobile station paging slot, the mobile station wakes up (enters an active state) and re-synchronizes with the base station or improves the synchronization state. The mobile station then monitors the time slot for pages or other messages. After a few intervals, the mobile station enters the inactive state and no longer monitors the paging channel until the next designated time slot is reached.

The state in which the mobile station is in before it actively communicates traffic information with the mobile communication system and after it has achieved timing synchronization with the communication system is referred to as an idle state. In the idle state, the mobile station may receive messages and incoming calls, initiate call initiation registration or message transmission. IS-95-a allows a mobile station to complete an idle handoff at any time other than when it IS required to monitor a designated time slot while the mobile station IS in an idle state.

However, when a mobile station originates a call or receives a page, it enters a system access state where it sends an origination message or a page response message. The IS-95-a mobile station does not operate in slotted mode when in the system access state. This is referred to as unslotted operation. Specifically, the mobile station continues to monitor the paging channel until the base station transitions it to a different state or an error condition occurs to exit the system access state. The illustrative embodiment describes the case of origination operations and origination messages, but the concept is equally applicable directly to mobile terminated call procedures and page response messages. After the mobile station sends an origination message and gets an acknowledgement, it will wait for a channel assignment message that indicates the channel on which the base station is communicating traffic to the mobile station.

After receiving the channel assignment message, the mobile station tunes to the assigned traffic channel, receives information on the forward traffic channel, and begins transmitting on the reverse traffic channel. The forward traffic channel is a channel in which a base station transmits information to a mobile station, and the reverse traffic channel is a channel in which a mobile station transmits information to a base station.

The interval between the mobile station sending the origination message and the mobile station receiving the channel assignment message depends on the implementation of each infrastructure provider. The interval ranges from 0.5 seconds to several seconds. The mobile station remains in the system access state until it receives a channel assignment message.

The paging channel generally does not support soft handoff. The above-mentioned fading problem occurs. This effect is typically counteracted by making the transmit power of the paging channel greater than that of the traffic channel. Since one paging channel can handle call origination and termination for multiple traffic channels, the capacity lost by increasing power is minimal. To support soft handoff on the paging channel, the system inevitably transmits the same information on the paging channel in all cells, thereby greatly reducing the overall capacity of the paging channel.

The mobile station may perform a handoff when the mobile station is in an idle state. In general, a mobile station may complete an idle handoff as long as the received signal strength from one cell is sufficiently greater than that of the other cells. Such idle handoff is typically completed before the mobile station begins monitoring the time slot. There may be situations where the mobile station cannot select the correct cell before the time slot begins and must continue to monitor the existing cells. When the mobile station is in the system access state, it cannot complete the idle handoff.

However, when the mobile station is in a system access state, it may be the case that the signal-to-noise ratioThe change is too fast, resulting in an increased message error rate and thus the mobile station being unable to receive signaling messages sent on the paging channel. The mobile station may not receive the channel assignment message. This means a failure of call origination. IS-95-a allows the mobile station to exit the system access state and return to the mobile station idle state when the paging channel message IS not received within one second. This means that the mobile station did not receive the channel assignment message and call origination was unsuccessful.

The same problem exists when the mobile station is first assigned a traffic channel. IS-95-a allows only one base station to be assigned to a mobile station. If the other cell is stronger or stronger, the mobile station may not successfully receive the forward traffic channel. And thus the call is lost. The problem is that the mobile station is assigned a traffic channel that is a member of a single active set and is not in soft handoff.

Under IS-95-a, in order for a mobile station to enter soft handoff, the following steps must be taken. First, the mobile station detects whether the pilot signal of another base station is greater than a predetermined energy threshold. Next, the mobile station transmits a pilot signal strength measurement message. Third, the infrastructure establishes a soft handoff and sends a soft handoff direction message to the mobile station. Depending on the circumstances and implementation, this can take anywhere from a few hundred milliseconds to more than a second.

While soft handoff IS generally supported in IS-95-a systems, soft handoff IS not supported when the mobile station IS in a system access state. There is therefore a need for a system that allows for soft handoff when a mobile station is in a system access state, thereby improving the reliability of the system access process and capturing other benefits.

Disclosure of Invention

Several improvements are described that can improve paging and access channel operation. A first feature of the present invention is to allow a mobile station to perform a handoff while in a system access state. This allows the mobile station to receive base stations with higher signal-to-noise ratios, thereby reducing the message error rate. This avoids losing call setup because the paging channel cannot be received. By allowing handover, the base station needs to send channel assignment messages on the paging channel via multiple base stations.

The invention provides a method for carrying out idle switching, which comprises the following steps: measuring received signal energy from a plurality of base stations at a mobile station in an idle state; compiling, at the mobile station, a first set of base stations having received energy exceeding a predetermined threshold; determining a second set of the plurality of base stations having sufficient resources to provide communications to the mobile station; and the mobile station in the access state is idle to switch to a base station common to the first group and the second group.

A second feature of the present invention is to allow the infrastructure to know which base station should send the channel assignment message to the mobile station. Furthermore, this ensures that the mobile station can handover to a different base station and immediately have a traffic channel to which the new base station allocates.

A third feature of the present invention is to allow the infrastructure to know which base station should belong in the mobile station's active set before allocating a channel to the mobile station. An active set is a set of base stations that provide the strongest signal to a mobile station at a given time. This allows the infrastructure to determine whether there are sufficient resources to cause the base station to enter soft handoff before allocating a traffic channel to the mobile station. This is a useful feature since mobile stations near the cell border may request to enter soft handoff immediately after the traffic channel is assigned. But it also minimizes the possibility of a call drop due to rapid changes in the signal-to-noise ratio as described above.

In addition, in connection with the third feature of the present invention, the inclusion of a plurality of active set members in the channel assignment message allows the mobile station to assign a traffic channel in a soft handoff state.

Finally, the above features also provide particular convenience to Priority Access and Channel Allocation (PACA) operations that provide users with access to limited communication resources according to a specified user priority.

Although the present invention is discussed in terms of a CDMA system, it may be used in any other cell or satellite communication system as well.

Drawings

A further understanding of the various features, objects, and advantages of the invention may be realized by reference to the following description in conjunction with the accompanying drawings. Like parts are designated by like reference numerals in the accompanying drawings, in which:

FIG. 1 is a mobile station communicating with one base station in a group of base stations;

fig. 2 shows a cell layout of a corresponding base station; and

FIG. 3 shows a pilot signal E of a mobile station moving between two base stations_c/I_o。

Detailed Description

Referring to fig. 1, after power up, the mobile station 2 enters the system acknowledge substate. A system acknowledgement processor (not shown) selects the system that completed the acquisition attempt and provides the necessary frequency information to the receiver (RCVR) 8. Although not separately shown, the system confirmation processor may be implemented within the control processor 18. The control processor 18 may be implemented as a microprocessor or microcontroller under the control of a program in memory.

In the exemplary embodiment, after the system acknowledgment is selected, mobile station 2 enters a pilot signal acquisition substate that attempts to demodulate the pilot signal based on the acquisition parameters searched for in the system acknowledgment substate.

In the exemplary embodiment, mobile station 2 attempts to acquire the CDMA pilot signal based on the acquisition parameters. The signal, if present, is received by the antenna 4 and sent via the duplexing means 6 to the receiver 8. The receiver 8 down-converts, amplifies the received signal, converts the analog signal to a digital signal and sends it to the searcher 10. The searcher 10 attempts to acquire the pilot signal by measuring the PN offset. The testing of the PN offset is done by demodulating the signal according to the PN offset hypothesis and measuring the energy of the demodulated signal. The design and implementation of the searcher hardware for CDMA acquisition is known in the art and is described in detail in the aforementioned U.S. patent No.5,109,390.

When the searcher 10 detects a pilot signal having energy greater than a predetermined threshold, the mobile station 2 enters a synchronization channel acquisition substate and attempts to acquire a synchronization channel. The synchronization channel broadcast by the base station typically includes basic system information such as System Identification (SID) and Network Identification (NID), but most importantly provides timing information to the mobile station 2. The mobile station 2 adjusts its timing based on the synchronization channel information and then enters the mobile station idle state.

Upon successful acquisition of the synchronization channel, the mobile station 2 monitors the paging channel according to a predetermined paging format. The mobile station 2 demodulates the signal according to a predetermined Walsh sequence reserved for paging channel transmission. For example, assuming that the acquired pilot signal is transmitted by the base station 26a, the mobile station 2 monitors the paging channel with a predetermined Walsh sequence based on timing information provided by the synchronization channel. The base station 26a transmits overhead information intermittently on the paging channel.

In an exemplary embodiment, the overhead information includes a list called a neighbor list. In IS-95-A, the list IS provided by the base stations 26a-26n to the mobile station 2 in a neighbor list message. This LIST is called NGHBR _ LIST _ BASE. NGHBR _ LIST _ BASE is a LIST of BASE stations 26a surrounding the BASE station that provide strong signals to the mobile station 2, and is therefore a candidate for idle handoff. In the illustrative embodiment, the base stations 26a-26k of FIG. 1 correspond to the cells 36a-36k, respectively, of FIG. 2. And therefore the base station 26a covers the cell 36 a.

Referring to fig. 1, in an exemplary embodiment, BASE stations 26b-26k are within NGHBR _ LIST _ BASE that are sent to mobile station 2. It is noted that the present invention is equally applicable to situations where certain base stations in the neighbor list are not controlled by the same Base Station Controller (BSC) 32. The base station controller 32 is responsible for providing information between the base stations 26a-26o, selectively providing information from a Master Telephone Switching Office (MTSO) (not shown) to the base stations 26a-26o, and providing internally generated messages to the base stations 26a-26 o.

If the acquired pilot signal is transmitted by the base station 26a, after receiving the overhead information, the mobile station 2 may register with the base station 26a by transmitting its Mobile Identification Number (MIN) to the base station 26 a. The mobile station 2 then enters an idle state and monitors the paging channel assigned to it in slotted paging mode after successfully registering with the base station 26 a. If registration is not complete, the mobile station also enters an idle state and monitors the assigned paging channel transmitted by the base station 26a in slotted paging mode.

In slotted paging mode, the base station 26a transmits paging or signaling information to the mobile station 2 at predetermined time intervals, referred to as slots. In the exemplary embodiment, the time slots and paging channels are determined based on a hash function of the Mobile Identification Number (MIN), which is known to the base station 26a and the mobile station 2 after registration.

In the present invention, the base station 26a transmits to the mobile station 2a list of base stations that it is allowed to complete idle soft handoff in the system access state. This LIST is called LIST _ BASE. The BASE stations in LIST _ BASE are typically a subset of the BASE stations in NGHBR _ LIST _ BASE and typically utilize the same BASE Station Controller (BSC). So, for example, in fig. 1, NGHBR _ LIST _ BASE may be comprised of all BASE stations 26b-26k, but LIST _ BASE is comprised of a subset of BASE stations 26b, 26c (not shown), 26g (not shown), and 26h (not shown).

The following description is in terms of call origination and the discussion focuses on origination messages. The same procedure can be used for call termination situations where the call response message replaces the origination message.

When the mobile station 2 originates a call, the message generator 20 generates an origination message and sends it on the access channel. The message generator 20 may be implemented with a microprocessor programmed to perform the functions described above. Although the message generator 20 is shown as a separate unit, it may also be implemented within the control processor 18. The message is received and demodulated by the base station 26a that the mobile station is currently monitoring. Each BASE station in LIST _ BASE 26a-26i, in response to receiving the origination message, sends a channel assignment message indicating the traffic channel on which the communication is to be conducted. It is worth noting that the Walsh channel used to communicate with the first BASE station in LIST _ BASE is not the same Walsh channel used to communicate with the second BASE station in LIST _ BASE. Since a plurality of BASE stations are transmitting channel assignment messages, the mobile station can optionally select idle handoff and still be able to receive channel assignment messages in idle handoff state and after transmitting origination messages to any of the BASE stations in LIST _ BASE.

In an alternative embodiment, the mobile station 2 sends an origination message to the base station 26a and waits for an acknowledgement of the origination message. The mobile station cannot complete the handover until after receiving the acknowledgement. However, after the mobile station receives the acknowledgement, the mobile station may optionally choose to switch to any BASE station in LIST _ BASE.

In an alternative embodiment, the mobile station 2 sends the origination message using the procedure described in IS-95-a, for further details in U.S. patent application No.08/412,648 entitled "random access channel" filed on 12.3.1994, which IS assigned to the assignee of the present invention and IS incorporated herein by reference. If the acknowledgment from the base station 26a is not received within the predetermined timeout interval, the mobile station increases its transmit power and sends the message again. If the mobile station 2 still fails to receive the acknowledgement from base station 26a after a certain number of attempts and the signal from another base station (e.g., base station 26b) is stronger, the mobile station 2 may be free to switch to base station 26b and begin sending origination messages again.

In one embodiment, the channel assignment message transmitted by each BASE station in LIST _ BASE only indicates the traffic channel that is communicating with a particular BASE station. In an alternative embodiment, each BASE station 26a-26i in LIST _ BASE transmits an equivalent channel assignment message that indicates not only the traffic channel used to communicate with a particular BASE station, but also the traffic channels for communicating with all BASE stations in LIST _ BASE. This requires each BASE station in LIST BASE to communicate over an available traffic channel via the BASE station controller 32. The success rate of the channel allocation process is greatly improved by providing the channel allocation messages by a plurality of base stations.

The above procedure allows the infrastructure to establish a soft handoff and include more than one active set member in the channel assignment message. Instead of first communicating with a base station and then entering a soft handoff state, the mobile station 2 may immediately enter a soft handoff state and immediately receive traffic communications from more than two base stations. This speeds up the process of the mobile station 2 entering a soft handoff state, improves system performance and minimizes call drops due to a lower forward traffic channel signal-to-noise ratio.

In one embodiment of the process, the BASE station establishes soft handoff with all BASE stations in LIST _ BASE, and in an alternate embodiment of the process, the BASE station establishes soft handoff with a subset of the BASE stations in LIST _ BASE and sends the information needed for mobile station 2 to enter soft handoff in a channel assignment message. The information includes an identifier of the subset of base stations. In IS-95-A, the pilot PN offset identifies the base station.

Paging messages transmitted by the base stations 26a-26i are received by the antenna 4 of the mobile station 2. The received message is then sent via the duplexing means 6 to the receiver 8, where the received signal is down-converted and amplified. The down-converted message is provided to demodulators 12a-12j that demodulate the received messages. The control processor 18 selects one or more paging channels on which the base station 2 will demodulate the incoming paging channel data based on the information from the searcher 10. In one embodiment, demodulators 12a-12j monitor only one base station.

The searcher 10, coupled to the control processor 18, determines another better base station. The control processor 18 then controls the modem to begin demodulating signals received from other base stations. Since the mobile station 2 will receive assignment messages from more than one base station, the mobile station 2 can perform idle handoff freely in the system access state. In another embodiment, the mobile station 2 monitors all BASE stations in LIST _ BASE and demodulates signals identified in LIST _ BASE.

In the preferred embodiment, LIST _ BASE is provided along with the neighbor LIST message, but a flag may be provided along with the neighbor LIST to indicate which of the neighbor LIST (NGHBR _ LIST _ BASE) are members of LIST _ BASE. In the illustrative embodiment, a reserved value in the overhead message is used to indicate which systems specified by the neighbor LIST message belong to LIST BASE. In the exemplary embodiment of the IS-95-a BASE station, the reserved value in the overhead message used to define LIST _ BASE members IS the NGHBR _ CONFIG value in the neighbor LIST message.

In the exemplary embodiment, the IS-95-a neighbor LIST message includes pilot PN offsets for BASE stations in NGHBR _ LIST _ BASE and a flag indicating which BASE stations in the neighbor LIST message belong to LIST _ BASE. The transmission of the pilot signal PN sequence of the current base station provides the mobile station 2 with a reference signal by which the mobile station 2 identifies the PN offsets of other base stations.

As described above, all BASE stations in LIST _ BASE request to send a channel assignment message to the mobile station 2. Although this may enable the mobile station 2 to handover and thereby improve the success rate of the channel allocation procedure, an increase in paging channel capacity is required for all call setup procedures.

The improved procedure relieves the pressure on the demand for paging channel capacity by causing the mobile station 2 to transmit a list of pilot signals above a predetermined power threshold. This LIST is called LIST _ MOBILE. In one embodiment, searcher 10 preferentially demodulates pilot signals for BASE station PN offsets in LIST _ BASE, followed by BASE station PN offsets in NGHBR _ LIST _ BASE, followed by the remaining PN offsets. Methods for providing optimized search priority are described in the aforementioned U.S. Pat. No.5,267,261.

In the exemplary embodiment, searcher 10 demodulates the received signal based on the pilot PN offset and measures the energy of the demodulated pilot signal. These energy values are provided to the control processor 18. The control processor 18 compares the energy of the demodulated signal to a threshold and compiles a list of PN offsets that are greater than the threshold. This LIST is called LIST _ MOBILE. Once LIST _ MOBILE compilation is complete, it is transmitted on the access channel and received by the base station 26a monitored by the MOBILE station 2. In the illustrative embodiment, LIST _ MOBILE is included in the origination message.

In another embodiment, the LIST _ MOBILE is received by more than one base station 26a-26 o. LIST _ MOBILE is provided to base station controller 32. In the preferred embodiment, the threshold used by the MOBILE station 2 to determine whether to include a base station within LIST _ MOBILE is sent by the base stations 26a-26o as part of the overhead message. In a preferred embodiment, the transmission threshold may be a T _ ADD value in an IS-95-A system parameters message. The T _ ADD value IS currently used by the IS-95-a mobile station to determine whether to send an IS-95-a pilot signal strength measurement message to the base station on the traffic channel indicating that the mobile station has detected a pilot signal greater than T _ ADD.

FIG. 3 shows E of the IS-95-A pilot signal channel for base stations 26a and 26b_C/I₀At this time, the mobile station 2 moves from the base station 26a to the base station 26 b. The pilot signal channel of base station 26b when mobile station 2 is fully within the coverage area of base station 26a as represented by area 38Below the T ADD level. Similarly, when the mobile station 2 is fully within the coverage area of base station 26b as represented by area 41, the pilot signal channel of base station 26a is below the T _ ADD level. When mobile station 2 is located in area 38, it does not report base station 26b in the origination message. Likewise, when the mobile station 2 is located in the area 41, it does not report the base station 26a in an origination message.

Pilot signal E of base station 26b when mobile station 2 is located in area 39_c/I₀Above T _ ADD and mobile station 2 reports base station 26b in the origination message. Similarly, when the mobile station 2 is located in the area 40, the pilot signal E of the base station 26a_c/I₀Above T _ ADD and mobile station 2 reports base station 26a in the origination message. Preferred embodiment IS IS-95-A given E_c/I₀To perform the measurement; but other signal strength or signal to noise ratio measurements known in the art are equally possible.

In the preferred embodiment, the MOBILE station 2 may idle soft handoff to only those BASE stations in LIST _ MOBILE and LIST _ BASE. Calling is the base station group in LIST _ BOTH. This has two advantages. First, the infrastructure need only send channel assignment messages in those base stations that the mobile station recognizes as an idle handoff candidate and can handoff. The set of base stations is given in LIST _ BOTH. This significantly reduces the amount of extra messages required. Second, LIST _ MOBILE provides the base station controller 32 with a LIST of pilot signals above T _ ADD, allowing the infrastructure to identify which base stations belong to members of the MOBILE station's active set. Thus, if the base station controller 32 wishes to establish a soft handoff when assigned to a MOBILE station traffic channel, it need only establish a soft handoff with the base station in LIST _ MOBILE.

In an alternative embodiment, the mobile station 2 sends an origination message to the base station in LIST _ BOTH. This reduces the amount of information transmitted by the mobile station 2.

And this allows the infrastructure to establish soft handoff and include more than one active set member in the channel assignment message. The channel assignment message will include the pilot signal PN offset belonging to the active set base station. The mobile station 2 may immediately enter a soft handoff state and immediately receive traffic from more than two base stations, rather than first switching on one base station and then entering soft handoff (which may not be achieved due to capacity or other limitations).

For example, as shown in fig. 2, if the MOBILE station is located at a position 37 near the boundary of cell 36a and cell 36b, LIST _ MOBILE identifies the pilot signal PN offset of base station 26 b. The channel assignment messages sent by the base stations 26a and 26b will identify the traffic channel used by the mobile station 2 that is dedicated to the communication of the mobile station 2 with the base stations 26a and 26 b. At least one of the demodulators 12a-12j is tuned to receive traffic channel information from the base station 26a and another one of the demodulators 12a-12j is tuned to receive traffic channel information from the base station 26 b. Several of the demodulators 12a-12j will begin demodulating the traffic channel signals transmitted by the base stations 26a and 26 b. The demodulated signals are provided to a diversity combiner 34 which combines the received signals to improve the estimate of the transmitted data.

There are several important aspects to the present invention. First, the mobile base station 2 cannot complete idle handoff until an acknowledgement of the transmitted message is received or the reception timeout expires. This enables the mobile station 2 to receive an acknowledgement of its access channel sounding signal. This also allows the acknowledgement signal that acknowledges the access channel sounding signal transmitted by the mobile station to be generated by the base station 26a rather than the base station controller 32. This advantageously reduces delay and speeds up the call setup process. Also, if the mobile station 2 is in the system access state and performs a soft handoff after the acknowledgment signal timeout expires, it must restart the access channel sounding signal transmission procedure. As is the case when the mobile station 2 sends a new origination message.

In an alternative embodiment, the mobile station 2 completes the idle handoff to the BASE station in LIST _ BASE before receiving the acknowledgement signal. But this means that all BASE stations in LIST BASE need to send acknowledgement signals and the BASE station controller 32 therefore needs to generate acknowledgement signals. In a modification of the alternative embodiment, the MOBILE station 2 may complete an idle handoff to a base station in LIST _ MOBILE before receiving the acknowledgement signal. Again, this means that all base stations in LIST _ MOBILE need to send acknowledgement signals and the base station controller 32 therefore also needs to generate acknowledgement signals.

In the preferred embodiment, the emergency provided by the present invention is for the case where the base station 26a transmits a channel assignment message but the mobile station 2 does not receive the message. The base station 26a may have received the origination message of the mobile station 2, but the mobile station 2 may not have received the channel assignment message of the base station 26a (which acknowledges the origination message). The mobile station 2 may be idle handed off to, for example, the base station 26b even if no acknowledgement message is received.

The base station 26b may send a channel assignment message to the mobile station 2 while the mobile station 2 retransmits the origination message. In an exemplary embodiment, when an acknowledgment message is sent, a tag of the acknowledged message is attached. The mobile station 2 ignores the channel assignment message unless the flag corresponds to a recently sent origination message. The present invention provides several methods to solve this problem. The obvious approach is for the base station 26b to use the same acknowledgment flag as the base station 26a received the origination message. This may be done by transmitting the validation token value from the base station 26a to the base station 26b via the base station controller 32. In an alternative embodiment, transmission of the access probe signal may be stopped if the mobile station 2 receives the channel assignment message and tunes to the channel specified by the channel assignment message.

In a modified embodiment, all BASE stations (BASE stations in LIST _ BASE) to which the mobile station 2 can be handed over are identical in the paging channel structure. BASE stations that do not support these functions will be excluded from LIST _ BASE.

The above structure may also be used to support Priority Access and Channel Allocation (PACA). PACA IS a well-known technique in the art and IS described in detail in "TIA/EIA/IS-53-a cell characterization". When calling PACA, if the traffic channel is unavailable, mobile station 2 is given priority to use the traffic channel over other mobile stations. Specifically, mobile station 2 sends an origination message that includes the PACA feature code and the dialed number. If a traffic channel is available on the spot, it is allocated to the mobile station 2. If immediate use is not available and mobile station 2 is authorized to use PACA, the base station monitoring mobile station 2 (i.e., base station 26a) places the request of mobile station 2 in the PACA queue. PACA queues may also be managed by the base station controller 32. The position in the queue depends on the priority of the PACA request and the morning and evening of the request. When a traffic channel is available, it is assigned to the top request in the PACA queue.

When the request of the mobile station 2 is in the PACA queue, the mobile station 2 may send periodic messages informing the user of the mobile station 2 of the queue status. The problem that PACA brings out is that the infrastructure needs to know the cell currently being used by the mobile station 2 in order to determine whether to release the channel. For most systems, this means that the mobile station must register or resend the origination message each time an idle handoff is completed. Since the transition between CDMA base stations is significant, the mobile station 2 may register or resend the origination message several times when crossing the boundary between base stations. A second problem with CDMA is that the mobile station 2 may need to make a soft handoff immediately after being assigned to a traffic channel. Channel allocation will not succeed unless there are resources available in the multiple base stations to support the call.

By improving on the origination message described above, the mobile station 2 indicates to the other base stations which should belong to the mobile station's active set, i.e., the set of base stations for which the mobile station 2 detected a stronger pilot signal. In an embodiment, the MOBILE station 2 sends LIST _ MOBILE and the base station determines LIST _ BOTH. In another embodiment, the mobile station 2 sends LIST _ BOTH. This allows the infrastructure to determine whether to release the resources in all base stations required for the PACA call. In order to reduce the transmission rate of the origination message, the mobile station 2 can move between the base stations in LIST _ BOTH without retransmitting the origination message. When this feature is invoked, the infrastructure needs to send queue status information to all base stations in LIST _ BOTH. If the mobile station 2 moves out of the range covered by the base station in LIST _ BOTH, the mobile station 2 retransmits the origination message.

From the above description, those skilled in the art can make various modifications to the present invention without departing from the scope and spirit of the present invention. Therefore, while the present invention has been described in terms of preferred embodiments, the spirit and scope of the invention is defined by the appended claims.

Claims (10)

1. A method for performing an idle handoff, comprising the steps of:

measuring received signal energy from a plurality of base stations at a mobile station in an idle state;

compiling, at the mobile station, a first set of base stations having received energy exceeding a predetermined threshold;

determining a second set of the plurality of base stations having sufficient resources to provide communications to the mobile station;

the mobile station in the access state is idle to handoff to a base station common to the first group and the second group.

2. The method of claim 1, further comprising: sending a message to the mobile station indicating the second set of base stations.

3. The method of claim 1, further comprising: transmitting a message from the mobile station indicating the first set of base stations.

4. The method of claim 3, further comprising: and sending a message indicating a base station common to the first group of base stations and the second group of base stations.

5. The method of claim 4, wherein the message indicating base stations common to the first set of base stations and the second set of base stations is sent from one base station.

6. The method of claim 4, wherein the message indicating the base station common to the first set of base stations and the second set of base stations is sent from a plurality of base stations.

7. The method of claim 6, wherein the plurality of base stations are base stations common to the first set of base stations and the second set of base stations.

8. The method of claim 2, wherein said step of sending a message to said mobile station indicating said second set of base stations comprises:

transmitting a message from a base station indicating the second group of base stations.

9. The method of claim 2, wherein said step of sending a message to said mobile station indicating said second set of base stations comprises:

transmitting messages from a plurality of base stations indicating the second group of base stations.

10. The method of claim 2, further comprising:

transmitting, from the mobile station, a message indicating a base station common to the first set of base stations and the second set of base stations.