HK1080628A - Method and apparatus for coordinating transmission of short messages with hard handoff searches in a wireless communications system - Google Patents

Description

Method and apparatus for coordinating short message transmission with hard handoff search in a wireless communication system

The present application is a divisional application of the invention patent application entitled "method and apparatus for coordinating short message transmission with hard handoff search in a wireless communication system" filed 30/4 1999, application number 99808324.0.

Background

I. Field of the invention

The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for hard handoff in different wireless communication systems.

Description of the related Art

In a Code Division Multiple Access (CDMA) spread spectrum communication system, a common frequency band is used to communicate with all base stations in the system. In TIA/EIA provisional Standard IS-95-A, an example of such a system IS described under the name "Mobile station-base station compatibility Standard for Dual-mode wideband spread Spectrum cellular systems", the contents of which are incorporated herein by reference. Generation and reception of CDMA signals is disclosed in U.S. patent 4,401,307, entitled "spread spectrum multiple access communication system using satellite or terrestrial repeaters," and U.S. patent 5,103,459, entitled "system and method for generating waveforms in a CDMA cellular telephone system," both assigned to the assignee of the present invention and incorporated herein by reference.

Signals occupying the common frequency band are distinguished at the receiving station by a high-speed pseudo-noise (PN) code. The PN code modulates signals transmitted by the base station and the mobile station. At the receiving station, signals from different base stations are separately received by distinguishing the unique time shift assigned to each base station in the PN code. High-speed PN modulation also allows a receiving station to receive a signal from a single transmitting station, where the signal is transmitted from the base station to the receiving station via several different propagation paths (commonly referred to as "multipaths"). Modulation of multipath signals is disclosed in U.S. patent 5,490,165, entitled "demodulation element assignment in a system capable of receiving multiple signals," and in U.S. patent 5,109,390, entitled "diversity receiver in a CDMA cellular telephone system," both assigned to the assignee of the present invention and the contents of which are incorporated herein by reference.

The use of a common frequency band by all base stations within a particular system allows communication between a mobile station and more than one base station at the same time. This is commonly referred to as "soft handoff. U.S. patent No. 5,101,501, entitled "soft handoff in a CDMA cellular telephone system," and U.S. patent No. 5,267,261, entitled "mobile-assisted soft handoff in a CDMA cellular communication system," both assigned to the assignee of the present invention and incorporated herein by reference, provide embodiments of a soft handoff method and apparatus. Likewise, a mobile station may be communicating with two sectors of the same base station at the same time. This is referred to as a "softer handoff" as described in co-pending U.S. patent application 08/405,611. The filing date of U.S. patent application 08/405,611, entitled "method and apparatus for performing handoff between sectors of a common base station," is 3/13/1995, is assigned to the assignee of the present invention and is incorporated herein by reference. An important feature is that both soft and softer handoffs establish a new connection before the active connection is broken.

If a mobile station moves outside the boundaries of the system with which it is currently communicating, it is desirable to maintain the communication link by placing a call to a neighboring system (if any). The neighboring systems may use any radio technology such as CDMA, NAMPS, Advanced Mobile Phone Service (AMPS), Time Division Multiple Access (TDMA), or Global System for Mobile (GSM). If the neighboring system uses CDMA on the same frequency band as the current system, a soft handoff between systems can be performed. In the event that an intersystem soft handoff is not possible, the communication link is transferred by a hard handoff, where the hard handoff interrupts the current connection before a new connection is established. Examples of typical hard handoffs include the following: (1) the mobile station is moving from an area serviced by the CDMA system to an area serviced by a system using another technology; and (2) transferring a call between two CDMA systems using different frequency bands (inter-frequency hard handoff).

Inter-frequency hard handoffs may also occur between base stations of the same CDMA system. For example, areas with greater demand, such as high density urban areas, may require more frequencies to meet these demands than surrounding suburban areas. It may not be cost effective to always use all available frequencies in the system. When a subscriber moves to a region of lower density, calls placed on frequencies used only in high density regions must be handed off. Another example is that the system encounters interference from another service operating on the interfering frequency within the system boundary. When a subscriber moves into an area that is interfered with by another service, his call may require a handoff to a different frequency.

The handoff may be initiated using various techniques. Co-pending U.S. patent application 08/322,817 discloses several handover techniques including those that use signal quality measurements to initiate a handover. The filing date of said patent application, entitled "method and apparatus for performing a handover between different cellular communication systems", is 1994, 10, 16, and is assigned to the assignee of the present invention and the content of which is incorporated herein by reference. Co-pending U.S. patent application 08/652,742, entitled "method and apparatus for hard handoff in a CDMA system," filed 5/22 1996, which is assigned to the assignee of the present invention and is incorporated herein by reference, further discloses a handoff technique. Co-pending U.S. patent application 08/413,306 (the' 306 application) discloses a handoff from a CDMA system to another technology system. The filing date of this patent application, entitled "method and apparatus for hard handoff from mobile-assisted CDMA to another system," 3/30/1995, is assigned to the assignee of the present invention and is incorporated herein by reference. In the' 306 application, pilot beacons are placed at the boundaries of the system. These beacons are transmitted in the frequency band being monitored by the approaching mobile, allowing the mobile to monitor the pilot beacon without retuning to another frequency band. When the mobile station reports these pilot beacons to the base station, the base station knows that the mobile station is approaching a boundary and, in response, is ready for possible inter-system hard handoff.

When the system determines that a call should be transferred to another system by hard handoff, a message is sent to the mobile station directing it to do so and parameters enabling the mobile station to connect with the target system. The system from which the mobile station is leaving only has an estimate of the actual location and environment of the mobile station and so there is no guarantee that the parameters sent to the mobile station are accurate. For example, with beacon assisted handoff, measurement of pilot beacon signal strength can be an effective trigger for handoff. However, it is not necessary to know those base stations in the target system that are capable of effectively communicating with the mobile station. However, those base stations with which the mobile station has made active communications and those base stations that are considered good candidates according to additional criteria are maintained in a list of mobile stations. The base stations included in the list depend on the allocation of the forward link resources by the base station in question. Since, in general, only a relatively few candidate base stations are required, allocating forward link resources with all possible candidate base stations is a waste of system resources and reduces the available system capacity.

Co-pending U.S. patent application 08/816,746 discloses a method of increasing the probability of successfully completing a hard handoff. The filing date of this patent application, entitled "method and apparatus for mobile station assisted hard handoff between communication systems" is 1997/2/18, assigned to the assignee of the present invention and is incorporated herein by reference. In most current systems, a mobile station has only one Radio Frequency (RF) front-end circuit. Therefore, only one band can be received at a time. Therefore, in order for the mobile station to communicate with the target system, contact with the originating system must be ceased. In the' 746 application, the mobile station temporarily tunes to the frequency of the hard handoff target system and searches for available pilot signals on that frequency for inclusion of the associated base station in the active set. After the search task is completed, the mobile station will retune to the origination frequency to resume the current communication. Any data frames generated by the mobile station or transmitted by the base station will be corrupted when tuned to the alternate frequency. Typically, the base station provides only a subset of the possible offsets (commonly referred to as an "allow list") for the mobile station to search.

Co-pending U.S. patent application 09/013,413 discloses a method of minimizing the duration of a search. The filing date of this application, 1998, 26, entitled "method and apparatus for mobile station assisted hard handoff using offline search," is assigned to the assignee of the present invention and is hereby incorporated by reference. In this application, the receiver stores information received on the frequency band used by a possible hard handoff candidate base station. The signal is not processed until the receiver is tuned back to the frequency band used by the originating base station. The receiver can be tuned to the frequency of the originating base station for a longer period of time by storing the signal for processing after the receiver has retuned back to the frequency used by the originating base station. Therefore, less information is lost. However, when the originating base station transmits at a relatively high data rate, information is lost. When information is lost, the base station must retransmit the information, or in the absence of information, the receiver waits for data to arrive. Therefore, there is a need for a method and apparatus that can further reduce the amount of information lost when tuning to other frequencies, such as when attempting to identify potential hard handoff candidates.

Disclosure of Invention

The methods and apparatus disclosed herein may minimize the amount of "down time" in the communication link between a mobile station and an "originating" base station when searching for an appropriate system to perform mobile station assisted hard handoff.

In one example of the disclosed method and apparatus, a mobile station tunes to an alternate frequency and samples incoming data, storing the samples in memory. During the period of time that the mobile station is tuned to the alternate frequency, all data sent to the mobile station along the forward link is lost. The reverse link data transmitted by the mobile station will be transmitted on the alternate frequency. Therefore, the originating base station does not receive these reverse link data. When a sufficient number of samples are stored, the mobile station tunes back to the origination frequency. At this time, the mobile station receives the forward link data again, and successfully transmits the reverse link data to the originating base station. After retuning to the origination frequency, a searcher in the mobile station will sequentially search for pilot signal offsets using stored data received from alternate frequencies. According to the method and apparatus disclosed herein, the active communication link is not disrupted because the time required to sample and store the signal at the alternate frequency is relatively short. The active communication link is also unaffected by subsequent offline searches. Another approach is to do real-time processing as the receiver tunes to the alternate frequency. However, such real-time processing typically increases the time that the receiver tunes to the alternate frequency, thereby also increasing the amount of information that the receiver cannot receive over the originating frequency.

According to the method and apparatus disclosed herein, error correction coding used by the receiver allows information that cannot be received because the receiver is tuned to an alternate frequency to be determined from information received over the originating frequency. The method and apparatus disclosed herein also improves the receiver by increasing the transmit power when transmitting information that the receiver will use to determine the information content transmitted when the receiver is tuned to an alternate frequency. On the other hand, conventionally redundant information is sent over the originating frequency when a lower data rate is being used, but the present invention removes this redundant information to provide a window during the time period in which the receiver can be tuned to the alternate frequency.

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. The attached drawings are as follows:

fig. 1 is a schematic diagram illustrating a spread spectrum CDMA communication system in accordance with the present invention;

FIG. 2 illustrates the amount of time a mobile station receiver is tuned to an alternate frequency;

FIG. 3 is a conceptual timing diagram illustrating the operation of the method and apparatus according to the invention;

FIG. 4 is a time line illustrating operation in a boost mode;

FIG. 5 is a block diagram illustrating operation of a base station, including coding and modulation on a forward link communication channel, and including an up mode operation;

FIG. 6 is a flow chart illustrating base station operation in a boost mode; and

fig. 7 is a flowchart illustrating the operation of the mobile station 5 in the lift mode.

Detailed description of the preferred embodiments

A method and apparatus that includes embodiments of the present invention are described in detail below. Fig. 1 illustrates a communication system in which a mobile station 5 actively communicates with a fixed communication system on the forward and reverse links through an "origination" base station 10. The originating base station 10 is part of an "originating" system that transmits and receives information over a forward link and a reverse link, respectively, using a first frequency f 1. In the figure, the mobile station 5 is moving from an originating system to a "target" system, which is transmitting and receiving information on a second frequency f 2. The target system includes "target" base stations 20 and 22, which are now not in communication with the mobile station 5. However, if the mobile station 5 is tuned to the frequency f2, the mobile station 5 can receive the pilot signal from the target base station 20. Both the originating system and the target system are part of the fixed communication system, which allows the mobile station to communicate with other communication devices, such as conventional telephones or other wireless communication devices connected to a public switched telephone network. It should be understood that a fixed communication system may include any device or combination of devices capable of providing wireless communication between a mobile system and other communication devices.

In accordance with one example of the methods and apparatus disclosed herein, a mobile station is triggered to tune to an alternate frequency. For example, the originating base station 10 may use the mobile station 5 for mobile-assisted inter-frequency hard handoff. An example of a mobile assisted inter-frequency hard handoff is disclosed in co-pending U.S. patent application 08/816,746. The title of this patent application, entitled "method and apparatus for mobile assisted hard handoff between communication systems," 1997, 2/18, is assigned to the assignee of the present invention. In such mobile assisted inter-frequency hard handoffs, the originating base station 10 sends a "tune message" to the mobile station 5. The tuning message instructs the mobile station to tune to an alternate frequency (f 2 in this example) and to search for a set of available pilot signals (e.g., the pilot signals of the target base stations 20 and 22).

Alternatively, the mobile station may be triggered by other events to search for hard handoff candidates. For example, a mobile station detects a signal, such as a beacon signal transmitted by a base station in another system. The beacon signal may be transmitted in a frequency band monitored by the mobile station. The beacon signal indicates to the mobile station that there is a hard handoff candidate in the vicinity. In response, the mobile station will tune to an alternate frequency associated with the detected signal.

When triggered to tune to the alternate frequency f2, the mobile station 5 tunes to the alternate frequency f2 and performs an activity appropriate to the trigger signal. For example, if the trigger is a tune message, the mobile station 5 will tune to an alternate frequency and search for hard handoff candidates. Once the activity is over, the mobile station 5 retunes to the frequency f1 and resumes communication with the originating base station 10. If the activity performed by the mobile station 5 produces information to be transmitted, such as a search result for a pilot signal of a candidate hard handoff system, the mobile station 5 transmits a message indicating the result to the originating base station 10 of the originating system. The originating system determines from the results whether further action should be taken. Other devices or systems may also be involved in determining whether additional action is required. For example, if the mobile station 5 is searching for hard handoff candidates, it is determined by the originating system along with the target system whether to perform a hard handoff, and if a handoff is required, which target base station in the target system to handoff to.

When the mobile station 5 tunes to the frequency f2, all forward link traffic from the originating base station 10 is lost. In addition, in most conventional systems, the same local oscillator is used by both the transmitter and receiver portions of the mobile station. Therefore, any attempt to transmit reverse link data to the originating base station while the receiver is tuned to f2 will be ineffective. That is, since these transmissions use frequency f2 and the originating base station 10 does not monitor frequency f2, the originating base station 10 does not receive these transmissions.

In one example of the method and apparatus disclosed herein, when the originating base station 10 commands the mobile station 5 to tune to frequency f2, the mobile station does not process the information in real time as in the prior art. Instead, samples of the signal are recorded at frequency f2 and stored in memory. It should be appreciated that any storage device capable of storing information for later processing, such as Random Access Memory (RAM), may be used. Once the desired number of samples have been taken, the mobile station 5 retunes to the frequency f1 and resumes communication with the originating base station 10 on the forward and reverse links 12 and 14. In this manner, the amount of time it takes for the receiver to tune to other frequencies, different from the frequencies used by the mobile station to communicate with the originating base station, is greatly reduced.

The information transmitted over the forward link is arranged into frames, with a frame transmission time of approximately 20 milliseconds. The information within the frames is arranged into one or more error correction blocks based on the rate at which the originating base station transmits data in accordance with well-known conventional techniques for transmitting information over a digital wireless communication network. Each such block is encoded to produce an error correction sequence. If any information within the sequence is corrupted or lost (i.e., collectively referred to as "received in error"), the remaining information within the sequence may be used to derive the portion of the sequence that was received in error (i.e., "correct" the error). The amount of information that can be corrected depends on the particular error correction coding algorithm used. Wireless communication systems typically rely on convolutional coding schemes and viterbi decoders for error correction. In addition, the information within a block is typically interleaved to improve the error correction capability of the error correction scheme for relatively long sequences of erroneously received information. Interleaving is a process of spreading out (i.e., scrambling) adjacent information within an error correction sequence across the sequence. For example, if the sequence 13245 is an error correction sequence, then the interleaved error correction sequence may be 41235, so that any two adjacent digits in the original sequence are not adjacent in the interleaved sequence. Algorithms for interleaving information are well known in the art. In some cases, several error correction code blocks may be transmitted together within one 20ms frame. Generally, this occurs at relatively high data rates. However, each block is independently coded. In general, the resulting error correction sequences are also independently interleaved.

Fig. 2 illustrates the time taken for the mobile station 5 to tune to frequency f2 versus the duration of a frame in accordance with one example of the disclosed method and apparatus. Since the time it takes for the mobile station receiver to tune to the alternate frequency is relatively short, the interleaving and error correction coding means can derive information that was not received by the mobile station when it tuned to the alternate frequency.

Once the data is acquired, an offline search is conducted (at which time the mobile station 5 tunes to frequency f 1). Therefore, the communication between the mobile station 5 and the originating base station 10 is resumed faster than the time taken for the receiver to process the received information while maintaining the frequency f 2. The duration of the erasure caused by tuning to frequency f2 in the present invention is significantly less than the corresponding duration in the prior art methods. In an IS-95 system, tuning and retuning may be completed in approximately 4 milliseconds. The memory size requirement in such a system allows 512-chip data to be sampled at twice the chip rate (4 bits/sample for both I and Q channels). This results in a storage requirement of 1024 bytes. It will be apparent to those skilled in the art that other values may be substituted for the above values, each with a trade-off between complexity and performance. The capture time in this embodiment of the invention is approximately 0.5 ms. The duration of the IS-95 data frame IS 20 ms. Thus, for this example, the total erasure time is about 5ms, which is not enough to corrupt the entire frame.

According to an embodiment, the search for the alternate frequency f2 involves lower rate frames, such as 1/8 rate frames. In this example, the amount of data erased is generally not important and can therefore be corrected by encoding and interleaving without errors.

In another embodiment, to reduce storage requirements, a smaller sample length from frequency f2 may be recorded. These results may be used to compute partial results for an offline search. The mobile station 5 does not return to the frequency f2 until the search is completed. An example of the search is described below.

The method and apparatus are improved since offline searching need not be performed in real time. The search is performed as fast as the circuit operating speed allowed by current technology, or within the power budget, a trade-off common in the art. In this way, the system can be designed to greatly reduce the erasure rate and search time compared to prior art methods.

Since the received signal may change rapidly due to changes in the environment in which the mobile station 5 is located, it is desirable to be able to repeat the process of sampling the alternate frequency f2 multiple times if a large number of offsets are to be searched. The iterative process allows new data to be used while the improvement provided by the present invention can reduce the frame error cost associated with the risk of repetition to other frequencies.

This method of sampling and storing information allows the mobile station 5 to begin establishing contact with the target base station while the originating base station is communicating user data over the origination frequency. In addition, the mobile station 5 can identify a time offset when the mobile station receives a multi-path signal from the target base station before actually performing the hard handoff. Thus, the amount of time required to obtain the target base station when performing hard handoff is greatly reduced.

Fig. 3 is a conceptual timing diagram illustrating the operation of the apparatus and method according to the present invention. During time period 210, energy is transferred through the originating frequency. During time period 212, the receiver is retuned from the originating frequency to the target frequency band and the signal received at that frequency is sampled and stored. The receiver is then tuned back to the originating frequency. During time period 212, the mobile station 5 does not receive any data with the origination frequency. The mobile station receiver may tune to other frequencies and store information from these other frequencies multiple times, resulting in sufficient information being stored to allow the mobile station receiver to identify a desired number of hard handoff candidates, or to determine that no such candidates exist. In fig. 3, the above process is repeated two more times during time periods 214, 216, 218, and 220. During time period 222, the mobile station receiver receives data using the origination frequency. During time period 224, a handoff is performed from the origination frequency to the target frequency. At the beginning of time period 226, search data is collected with a target frequency. During the acquisition process in time period 224, no user data is transmitted, resulting in a service outage period 230.

Because the information received at the target frequency is collected and stored during time periods 212, 216, and 220, the duration of the acquisition process performed after handoff is reduced and, in some cases, eliminated. A shortened capture process is performed with the collected preliminary data. For example, the mobile station 5 may use the collected information to substantially narrow the search window used by the mobile station 5 to find assignable multipath signals. In some cases, the mobile station receiver will know the true offset of each multipath signal of interest within the target frequency band.

In accordance with one example of the method and apparatus disclosed herein, data is transmitted over an origination frequency in a manner effective to increase the instantaneous data rate relative to a nominal selected data rate, immediately before and after the mobile station receiver is tuned to an alternate frequency. Increasing the data rate relative to the nominal data rate may prevent information from being lost during reception interruptions when the mobile station 5 is not tuned to the originating frequency. That is, by increasing the amount of data transmitted before and after the receiver tunes to the alternate frequency, a window may be established during which the receiver in the mobile station may stop receiving information using the origination frequency without reducing the total amount of data transmitted by the origination base station 10 to the mobile station 5. This window is used to collect data at other frequencies of interest. The data rate may be increased above the nominally selected rate by various means. The example given below IS preferred because it can be implemented within the constraints of the IS-95 system.

One limiting factor in determining the data rate of a system is the desired link performance. The required link performance is generally determined by the amount of error that the resulting received signal can tolerate. The bit error rate is the energy per bit and the power density of the noise at the time of signal reception (e)_b/N₀) As a function of the ratio of. Energy per bit e_bIs the total received signal power over the duration of one bit. For example, the energy per bit is the same in both cases, the first case is that a bit is received at-50 decibels of power (dBm) versus one milliwatt for a microsecond of time; the second case is to receive one bit at-47 dBm of power in 500 nanoseconds. Noise power density (N)₀) Is a measure of the background noise experienced by the bit energy. If the magnitude of the background noise remains the same, but the power at the time of receiving the bit is doubled, then at the same e_b/N₀So that the time taken to transmit the same data can be halved for the same link performance. This relies on the principle of up mode operation and adds additional flexibility to the channel.

The boost mode is a means and method that can temporarily increase the system data rate. The boost mode operates under the limitations of the IS-95 system, but IS generally applicable to many systems. Fig. 4 is a time line illustrating the operation of the boost mode. Fig. 4 shows 5 frames, running time from left to right. When the base station determines that up mode is required, the base station issues an up mode command during frame 240. The up mode command specifies a pair of push frames. In this example, the base station has elected the second and third frames that follow the frame that received the command. During frame 242, data is transmitted in a standard manner. Also during frame 242, the mobile station 5 processes the up mode command. During frames 244 and 246, the up mode command is executed. In the first half of the frame 244, the base station transmits data to the mobile station 5 in the up mode. During the boost mode, the effective data rate is increased. In the second half of frame 244, mobile station 5 is freed from implementing a frequency offset function such as the acquisition segmentation process described above. Also, in the first half of the frame 246, the mobile station 5 is free to continue performing the frequency offset function. In the latter half of frame 246, the base station transmits data to the mobile station 5 in the up mode. During frame 248, normal data transmission may resume.

According to IS-95, the duration of each frame IS 20 milliseconds. Thus, the idle time 250 established in this way is approximately 20 milliseconds. Typically, the mobile station 5 requires approximately 3 milliseconds to switch to the target frequency band and approximately 3 milliseconds to switch back, thus requiring approximately 14 milliseconds to remain in order to implement the frequency offset function. If the system is acquiring, several up mode frames may be performed in succession. Captured data that is not used in a timely manner becomes obsolete as field conditions change over time.

The specific format of the up mode command depends on the frequency offset operation that can be achieved. If the up mode command specifies that the mobile station 5 perform an acquisition segment, the up mode command may have the following format: frequency designation, pilot signal designation, search window size. The frequency designation is used to designate the frequency band or channel on which the mobile station 5 performs the acquisition segment. The pilot signal specifies a sequence that should be used by the mobile station 5 during the search. The search window size is used to specify the set of time offsets that the mobile station 5 uses to correlate the sequence with the incoming data. The up mode command may also specify the selected frame pair if the relationship between receiving the up mode command and the selected frame pair is not inherent in the message. In fig. 4, it is assumed that when the up mode command is received, the mobile station 5 performs the specified tasks in the second and third frames after receiving the up mode command.

The boosted mode data transmission may work well under the limitations of IS-95. Increasing the power at which the base station transmits the forward link signal during the boost mode may have two different uses. First, by transmitting with greater power, the duration of one symbol can be shortened, so that more information can be transmitted in the same time. Second, by transmitting with greater power, the integrity of the received information will be better and thus the received information will be less erroneous. This is especially true when attenuation occurs within a frame. If the frame is transmitted with more power, the probability of attenuation errors is reduced. Therefore, even if the rate at which data is transmitted is not increased, the rate at which error-free data is received is significantly increased. By reducing the likelihood of errors in the transmission, the error correction capability of the receiver can be used to derive the content of the frames lost when the mobile station 5 is tuned to an alternate frequency. Both of the above advantages can be used independently or together (i.e., power up and information sent at the same rate to reduce errors, or power up to support transmission at a greater rate)

Fig. 5 is a block diagram illustrating operation of a base station, including coding and modulation performed on a forward communication channel, and including a boost mode operation. In contrast to the prior art method of operation shown in fig. 4, three inputs determine the amplitude applied to the signal: a forward link power control index, a data rate multiplier, and a boost mode multiplier. The forward link power control index is determined by a forward link power control mechanism. The data rate multiplier is determined by the data rate of the current frame. In addition, the new multiplier 126 exerts a rising pattern multiplier effect on the control signal that determines the final relative output level. The up mode multiplier is used to raise the level of the transmitted data to an elevated level during a portion of the up mode frame. The switch 125 is used to interrupt the transmission of signals on the forward link channel during the frequency offset portion of the up mode frame. Another approach is to simply set the gain of the forward link channel to zero.

The multiplier 126 and the switch 125 may be implemented in various media including software and hardware. Typical embodiments of the present method and apparatus include computer software executing on a standard microprocessor, or an Application Specific Integrated Circuit (ASIC). Accordingly, the methods and apparatus disclosed herein are relatively easy to implement.

The mobile station 5 decodes the data in the elevated mode frame in the same manner as the standard frame. The valid data is generated by the method used for data encoding. If the up mode frame includes full rate data, half of the symbols are not transmitted. For example, during the first selected frame 244 of fig. 4, the second set of eight power control groups is not transmitted. Note, however, that due to the pattern of the block interleaver 114, the first set of eight transmitted power control groups contains all odd-numbered symbols and the second set of eight power control groups contains all even-numbered symbols.

As will be appreciated by those skilled in the art, if symbols corresponding to only one output of the encoder 110 are present, the original bit sequence can be recovered using a standard convolutional decoder, such as a viterbi decoder, and the structure of the mobile station 5 need not be modified to operate in the boost mode. However, the redundancy added by the encoding process is lost, or even immunity to data loss (such as due to fading). If the mobile station 5 does not intervene in the standard data reception process, the energy of the untransmitted symbols will produce lower noise values which will be input to the decoding process, but which, by virtue of the decoding process, will not affect the decoding output too much. Another method is for the mobile station 5 to intervene in and decode symbols that are not transmitted as erasure content at the mobile station 5. In either of the above manners, if raising the mode multiplier raises the forward link signal level, the full rate data bits can be restored to a performance comparable to standard data transmission, thereby overcoming the redundancy penalty.

As described above, in accordance with IS-95, power control commands typically puncture the forward link channel. In this way, the forward link channel carries the power control subchannel with a loss in performance of the forward link channel. If the symbols are also punctured by the power control, the mobile station 5 cannot decode data from only even-numbered symbols or odd-numbered symbols due to the loss of redundancy. Thus, when the full-rate frame undergoes the ramp-up process, the MUX 118 no longer punctures the power control commands on the forward link channel. In addition, the mobile station 5 interprets each symbol it receives as data, rather than replacing the power control bits with erasures, before sending the symbol to the decoding process.

The base station does not puncture the forward link channel with power control commands, but simply delays transmitting the power control commands. For example, referring again to fig. 4, the base station punctures the power control command transmitted within frame 244 and sends the command in frame 248 immediately following the second frame 246 of the selected frame pair. Similarly, the power control command that has punctured the second frame 246 of the selected pair will puncture the frame following frame 248. This operation is advantageous because the frequency offset task also interrupts the reverse link channel, so that power control commands generated by the base station for the reverse link frames corresponding to the selected frame pair will not generate valid power control information. Thus, the base station may discard power control commands generated from a reverse link selected frame pair and apply delayed but valid power control commands on subsequent frames in place of invalid commands.

Low rate operation is even better. For the 1/2 rate frame of IS-95, the first set of eight power control groups includes all symbols from 1-192. Note that the second field is simply a repetition of the first field. Therefore, the mobile station 5 can receive all symbol data even if the energy of half a frame is not transmitted. If the up multiplier increases the forward link signal level to overcome the loss of half the signal energy, then the mobile station 5 can decode the half rate data with the same performance as if the entire frame was sent.

Also, note that for the 1/4 rate frame of IS-95, the first set of 4 power control groups includes all of the symbols from 1-96, and the symbols in the first set of 4 power control groups simply repeat over the remaining 12 power control groups. Note that for the IS-95 1/8 rate frame, the first set of 2 power control groups includes all symbols from 1-48, and the following 14 power control groups repeat the same symbol 7 times. Thus, if the up multiplier increases the forward link signal level, overcoming the loss of half the signal energy, then the mobile station 5 can decode 1/4 and 1/8 rate data with the same performance as if the entire frame was sent. For lower rate data frames, the base station may also not operate the power control subchannel.

Multiplying the data by the boosted pattern increases the interference to the remaining mobile stations during at least one half of the frame. During the other half of the frame, no interference is added to the system. Thus, the average interference added by the boost mode is the same as the interference added under normal operating conditions.

Ideally, the output power on the forward link channel is doubled during the boost mode frame. However, in some cases, such operation may not be necessary or possible. In addition, in some cases, a power increase of less than two times may be sufficient to achieve the desired system performance. In other cases, depending on the operating parameters of the current system (including the mobile station forward link power control index), the base station may choose that the rejection will double the forward link channel power of the current mobile station 5 entirely to facilitate reducing the interference generated to the remaining mobile stations. For example, typical base station designs limit the forward link power control range to about 3dB below and about 6dB above the nominal level. If the up mode multiplier command changes outside the allowable range, it may be desirable to limit the effect of the up mode multiplier.

Fig. 6 is a flow chart illustrating the operation of the base station in the boost mode. Flow begins at start block 260. In block 262, the base station sends a message to the mobile station 5 identifying the frame being elected. For example, the selected frames may correspond to the selected frame pairs 244 and 246 in fig. 4. As shown in block 264, the base station increases the forward link power level when it transmits the first frame of the selected frame pair. Also at block 264, the base station disables the power control subchannel by ceasing power control puncturing on the forward link channel. At block 266, the base station transmits the first half of the first selected frame. In block 270, the base station discontinues transmitting the second half of the first selected frame and the first half of the second selected frame with the forward link. For example, referring again to fig. 5, the base station may open switch 125. In block 270, the base station transmits a second half of the second selected frame. At block 272, the base station resets the forward link power control to a nominal level by removing the effect of the boosting multiplier and operates the power control subchannel. The process ends at block 274.

Fig. 7 is a flowchart illustrating the operation of the mobile station 5 in the up mode. Flow begins at start block 280. In block 282, the mobile station 5 receives the up mode command identifying the selected pair. For example, in fig. 6, the up mode command transmitted in frame 240 designates frames 244 and 246 as the selected frame pair. In block 284, the mobile station 5 receives the first half of the first selected frame. The processing of the frame occurs in parallel with the remaining steps shown in fig. 7. In block 286, mobile station 5 performs the task of frequency offset. In block 288, the mobile station 5 receives the second half of the second selected frame and decodes the frame as described above. The process ends at end block 290.

In general, the invention can be implemented in a system that arranges the symbols such that each bit of information is duplicated during a subdivision of a standard data unit. For example, in the above description, the interleaving pattern would be the first set of symbols (which includes an encoded replica of each information bit) from a half-rate convolutional encoder. According to the base station/mobile station system described above, either the forward or reverse link, or both, can operate in an up mode. For example, in an ideal case, the forward and reverse link channels may enter the boost mode at the same time, since the boost mode will not lose data on either link.

Several other embodiments of the above general principles will be readily apparent to those skilled in the art. For example, from the above explanation, it is clear that the boost mode works better when data is transmitted at a rate less than full rate. Thus, in one embodiment, the base station imposes a restriction on the data source, forcing the data rate to be less than full rate during the selected frame. For example, the base station may impose a limit on the variable rate vocoder or reduce the amount of digital data retrieved from the queue. In another embodiment, the base station sends the up mode command after checking the selected frame and detecting that the selected frame is less than full rate. For example, the up mode command may specify that a selected pair of frames that the base station already knows is comprised of frames less than full rate. In yet another embodiment, the base station may attempt to predict the occurrence of low rate frames. For example, digitized speech is statistical model-compliant. In digital voice, some full-rate frame bursts are typically interspersed in a series of low-rate frames. When a series of low rate frames are detected, the base station may predict that a selected frame comprises a low rate frame. During high rate data, the base station may choose to delay issuing the up mode command. Thus, the base station may predict that a frame may include less than full rate data.

In addition, the up mode command does not necessarily consume system resources. For example, in fig. 4, it can be seen that the up mode command occupies frame 240, since user data cannot be transmitted during this frame. However, just as power control commands are punctured into the power control sub-channel on the forward link channel, up mode commands may also be punctured into the forward link channel. Alternatively, the up mode command may be sent to the mobile station 5 over a separate control channel.

The boost mode may also be performed for other reasons than the task of temporary frequency offset at the mobile station 5. For example, the system may establish a boost mode for a period of time during which the mobile station 5 may receive messages on a different channel operating on the same frequency, such as a control channel. Another approach is to implement the assistance function in the base station with idle time. If the assistance function is implemented within the base station, the base station need not inform the mobile station 5 of the up command.

In another example, the boost mode may be used to provide additional time to perform a permanent transition to a target frequency band. For example, referring now to FIG. 3, note that during time period 222, data is transmitted at the originating frequency at the higher elevated mode data rate. Under normal operating conditions, data transmitted during time period 222 has continued to be transmitted over the original channel during the time indicated by dashed area 228. Thus, the service interruption period 230 begins at the right edge represented by the dashed area 228, rather than the right edge of the time period 222. During the time period represented by dashed area 228, the receiver of mobile station 5 may change the incoming frequency to the target frequency band and begin the acquisition or simplified acquisition process. In this example, the base station sends an up mode transition command to the mobile station 5 specifying a selected frame and a hard handoff band. The base station transmits the up mode data on a first half of the selected frame and stops transmitting during a second half of the selected frame.

In yet another example, the up mode may be used to provide information about the effective handoff target frequency. When the mobile station 5 moves around the coverage area of the system, the actual position of the mobile station 5 is not known by the system. To determine whether the mobile station 5 is located at a location where a hard handoff should occur, the mobile station 5 may collect data samples at the target frequency in a manner similar to the acquisition segmentation process described above. The samples are detected to determine if the mobile station 5 is receiving a valid signal level from the target base station.

In some cases, such as the handoff determination application just described, it may be advantageous to perform the up mode frames in a periodic or model-like manner. In this case, the up mode command may specify a start time, a mode or period, and an end time.

In some cases, the mobile station 5 itself may determine when the up mode frame should be performed. For example, the mobile station 5 may make such a decision based on reverse link data characteristics or forward link performance characteristics. In this case, the mobile station 5 sends an up mode command to the base station specifying one or more selected frames.

Also, the up mode need not include a pair of selected frames. The up mode may be performed during a single frame or over a series of frames. The selected frame pair need not be two consecutive frames. If the frequency offset task requires more time than is established within the selected pair of frames, the base station may execute a first boosted mode frame, suspend transmission of the forward link channel for a number of frames, and then execute a second boosted mode frame.

In addition, the present invention may be implemented so as to establish an idle time above a half frame or below a half frame. For example, if the selected frame is carrying 1/8 rate data in the up mode, the data may be sent at a level of approximately 8 times the nominal level, resulting in an idle time equal to 7/8 for one frame.

In one embodiment of the system incorporating the present invention, a command from the originating base station 10 is used to determine the time at which the mobile station 5 ceases to receive forward link signals from the originating base station 10 using the originating frequency and tunes to another frequency in order to search for signals transmitted using the other frequency. The time of day may be clearly identified within the command or a period of time may be identified within the command, where the period of time is relatively long relative to the amount of time required for the search. If a relatively long period of time is identified (e.g., 80 milliseconds), the mobile station may correctly select when to search within this identified period of time. Preferably, the command is sent on the originating frequency. In another system, the mobile system will tune to other frequencies only at predetermined times relative to the start or end of a frame or relative to another reference point where the originating base station 10 and mobile station 5 are allowed to coordinate the times at which the mobile station 5 will stop receiving transmissions from the originating base station 10. Then, the timing of the search is coordinated with the time at which the originating base station 10 transmits the short message using the originating frequency.

In addition, once the mobile station 5 completes the alternative frequency search, the mobile station 5 reports the search result to the originating base station 10. Since the originating base station 10 cannot receive information from the mobile station until the mobile station 5 returns to the origination frequency, the mobile station 5 must also ensure that these report messages are only sent when the mobile station 5 has returned to the origination frequency.

For example, messages (such as control signaling messages) having a duration of less than 5ms are typically sent to the mobile station using the origination frequency. In accordance with one embodiment of the system, the originating base station 10 ensures that short messages are sent only during the first part (such as the second half) of a 20ms frame. Thus, the originating base station 10 commands the mobile station 5 to tune to other frequencies only during other portions of the 20ms frame (such as the first half) that are not the first portion, so that this first portion does not overlap with other portions of the frame that are used to send short messages from the base station or report messages from the mobile station. This is particularly important in the case where a frame is divided into a plurality of subframes.

For example, there is a proposal currently being considered by standards organizations in the communications industry in which a conventional 20ms frame is divided into a plurality of 5ms frames for transmission on a dedicated control channel. These 5ms frames are then aggregated into one 20ms frame. However, each such subframe is encoded with an error correction code so that the subframe can be error corrected according to the content of the particular subframe. The error correction for the particular subframe is only possible when a sufficient amount of correct data within the particular subframe is received. In this example, tuning the mobile station to another frequency for only 3ms makes it impossible to recover information transmitted during a particular 5ms subframe because the information contained within such subframes is independently encoded (i.e., the size of the data block used for error correction encoding is equal to the amount of data transmitted in the 5ms subframe). Thus, by ensuring that the time at which the origination base station 10 sends the short message coincides with the time at which the mobile station 5 does not tune to the origination frequency, it can be ensured to both the mobile station 5 and the origination base station 10 that the mobile station 5 can successfully receive the short message that was intended for the mobile station. In addition, by matching the time at which the mobile station 5 transmits the report message with the time at which the mobile station 5 tunes to the origination frequency, the search itself, or any subsequent search, does not interrupt the transmission of the report message by the originating base station 10.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. 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.

Claims (6)

1. A base station apparatus for minimizing data loss in a communication link between a mobile station and a base station when searching for a suitable system for use in subsequently performing mobile station assisted hard handoff, the apparatus comprising:

a forward link control mechanism for determining a forward link power control index;

a boost mode multiplier for increasing the relative output power level at which data is communicated during a portion of the current data frame to produce a boost mode data frame having a power level determined by the forward link power control index;

a data rate multiplier for increasing the data rate during a portion of the current data frame to produce an elevated mode data frame having an increased data rate determined by the data rate multiplier;

a block interleaver for error correction coding the elevated mode data frame;

a convolutional encoder for error correction encoding the elevation mode data frame;

a symbol repeater for establishing symbol repetition within a less than full rate elevated mode data frame; and

a multiplexer for controlling the puncturing of power control commands into the boosted mode data frames on the forward link channel.

2. The apparatus of claim 1, wherein the multiplexer for controlling puncturing of power control commands on a forward link channel is configured for delaying puncturing of power control commands.

3. A method at a base station for boosting the output power level of a portion of a pair of data frames to ensure that an error correction code is able to determine information transmitted at a first frequency during tuning of a mobile station receiver to a second frequency, the method comprising the steps of:

transmitting a message from the base station to the mobile station identifying a selected frame pair comprising the first frame and the second frame;

increasing a forward link power control index;

transmitting a portion of the first frame in the identified selected pair of frames at a power level determined by the increased forward link power control index;

interrupting forward link transmission during the second portion of the first frame in the selected frame pair and the first portion of the second frame in the selected frame pair;

transmitting a second portion of a second frame in the selected frame pair at a power level determined by the increased forward link power control index; and

the forward link power control index is reset.

4. The method of claim 3, wherein the step of increasing a forward link power control index comprises deactivating a power control subchannel.

5. The method of claim 3, wherein the step of resetting a forward link power control index comprises operating a power control subchannel.

6. A method at a base station for increasing the data rate of a portion of a pair of data frames to ensure that an error correction code is able to determine information transmitted at a first frequency during tuning of a mobile station receiver to a second frequency, the method comprising the steps of:

transmitting a message from the base station to the mobile station identifying a selected frame pair comprising the first frame and the second frame;

transmitting a portion of the first frame of the identified selected pair of frames at the increased data rate determined by the data rate multiplier;

interrupting forward link transmission during the second portion of the first frame in the selected frame pair and the first portion of the second frame in the selected frame pair;

a second portion of the second frame in the selected frame pair is transmitted at the increased data rate determined by the data rate multiplier.