Method and system for performing a handoff in a wireless communication system, such as a hard handoff

ES 2 328 079 T3

DESCRIPTION

Procedure and system for carrying out a handoff in a wireless communication system such as a hard handoff.

I. Field of the invention

The invention relates to wireless communication systems and, more specifically, to methods and apparatus for providing hard handoffs between cells in such systems.

Background of the invention

In a code division multiple access (CDMA) system, the vast majority of handoffs take place between cells on the same CDMA channel and use soft handoff procedures. Sometimes mobile stations need to perform a handoff between cells on different CDMA channels, where those channels are at different radio frequencies (RF), often referred to as hard handoff between frequencies. Such situations typically include, but are not limited to, handoffs between different operators, handoffs between different assigned RF channels for capacity reasons, or handoffs between different signal modulation technologies.

Before carrying out a hard handover between frequencies, the mobile station receives from the base station the command to tune in to the new destination frequency, measure the radio environment (e.g., the pilot signal strength of the signals received, etc.) and report the measurement to the base station. Such a procedure is specified in the TIA / EIA-95-B standard and greatly improves the probability of success of an interfrequency transfer.

An essential requirement of measurement on the destination frequency, often referred to as "search excursion", is to minimize the disturbance of current service on the originating frequency. Transfers to a second frequency without adequate pre-sampling could result in poor signal performance. On the other hand, sampling for long periods of time can cause total loss of the signal on the first frequency. The procedure described below enables the mobile station to minimize the search time and limit the disruption of the service.

Attention is also called to document WO 97/29611 A which describes a method and apparatus for determining the fidelity of one or more communication channels with high efficiency and resolution, with a particularly advantageous application to digital communications in a radio communications environment where portable radios roaming through multiple RF site coverage areas are striving to monitor the optimal control channel having the most fidelity. signal high under current reception conditions. The radio collects the signal fidelity information for the alternate communication channel in very short time intervals without losing important messages on a communication channel that that radio is currently monitoring. Alternate communications channel signal fidelity is calculated without the radio being synchronized at a bit or digital frame level with the alternate communications channel. The fidelity calculation provides information about both the accuracy (that is, the digital value of a received digital pulse) and the quality (that is, the distortion level of the received pulse), of the received signal.

Summary of the invention

According to the present invention there is provided a method for measuring the intensity of signals in a wireless communication system, as stated in claim 1, an apparatus for measuring the intensity of signals in a wireless communication system, as stated in claim 7, and a mobile station adapted to measure signal strength in a wireless communication system, as stated in claim 13. Preferred embodiments of the invention are described in the dependent claims.

The invention overcomes the limitations described above and offers additional benefits by providing a method and apparatus that minimize search time at another frequency and limit service disturbance. This procedure is applicable to all types of services (voice, packed data, circuit data, signaling) to which the mobile station connects, and does not depend on the number of dedicated code channels allocated on the forward link and the reverse link. .

One aspect of the invention involves receiving a frequency change command at a user station, such as a mobile station, to switch from receiving a signal on a first frequency to receiving a signal on a destination frequency; tuning the mobile station to the destination frequency and collecting and storing signal samples; tuning the mobile station to the first frequency and processing the signal samples; and transmitting the results of the signal sample processing to a base station.

According to another embodiment of the invention, herein is disclosed a wireless communication system that includes a user station, such as a mobile station, having at least one transmitter circuit, one receiver circuit, and one temporary memory. The mobile station is configured to receive from a base station

ES 2 328 079 T3 a frequency shift command, to switch to a target frequency, to tune to the target frequency and collect and store signal samples in the buffer, to retune to the first frequency and process the stored signal samples, and to transmit the results of sample processing to the base station. The mobile station may also be configured to minimize the loss of symbols on the forward link and on the reverse link during the switch to the destination frequency by increasing the magnitude of the power assigned to the other symbols or to the frame affected by the switch to the destination frequency. target frequency. The additional amount of power that must be allocated to symbols that have not been affected by switching to the destination frequency for the frame to be demodulated is a function of the time the mobile station is on the destination frequency.

Brief description of the drawings

In the Figures, equivalent reference numerals are used to identify similar elements. For the purpose of facilitating the display of any specific item, the most significant digit of an item's reference number refers to the number in the Figure in which that item first appears (for example, item 204 first appears in Figure 2 and described in connection therewith).

Fig. 1 illustrates a typical wireless communication system that may employ the present invention.

Fig. 2 is a block diagram of typical components found in the wireless communication system of Fig. 1 that may employ the present invention.

Fig. 3 is a timing diagram of an interfrequency search excursion.

FIG. 4 is a flow chart of a method for performing an inter-frequency search excursion in accordance with an embodiment of the present invention.

FIG. 5 is a power-time graph illustrating the forward link power level sequence in relation to inter-frequency search excursions.

FIG. 6 is a power-time graph illustrating an increase in reverse link power during the search excursion.

FIG. 7 is a flow chart of a method for performing an interfrequency search excursion, minimizing service disturbances, in accordance with another embodiment of the present invention.

Detailed description of the preferred embodiments

This document describes in detail a wireless communication system and, in particular, a method and apparatus for minimizing the search excursion time of a destination frequency and the disturbances of the current service on a source frequency. In the following description, numerous concrete details are provided to enable a thorough understanding of the present invention. Those skilled in the related art, however, will readily understand that the present invention can be practiced without these particular details or with alternative elements or steps. In other cases, well-known structures and procedures are not depicted in detail so as not to hinder understanding of the present invention.

FIG. 1 illustrates a subscriber cellular communication system 100 in which multiple access techniques, such as code division multiple access (CDMA), are used for communication between users of user stations (eg. , mobile phones) and cell offices or base stations. In FIG. 1, a user's mobile station 102 communicates with a base station controller 104 via one or more base stations 106a, 106b, and the like. Similarly, a user's fixed station 108 communicates with the base station controller 104, but only through one or more nearby predetermined base stations, such as base stations 106a and 106b.

Base station controller 104 typically includes and is coupled to interface and processing circuitry to allow base stations 106a and 106b to control the system. Base station controller 104 may also be docked and communicate with other base stations, and possibly even other base station controllers. The base station controller 104 is coupled to a mobile switching center 110 which, in turn, is coupled to a base position register 112. During the registration of each user station at the beginning of each call, the base station controller 104 and the mobile switching center 110 compare the registration signals received from the user stations with data contained in the home location register 112, from the shape known within the art. Handovers can take place between base station controller 104 and other base station controllers, and even between mobile switching center 110 and other mobile switching centers, as is well known to those skilled in the art.

When system 100 processes voice or data traffic calls, base station controller 104 establishes, maintains, and breaks the wireless link with mobile station 102 and fixed station 108, while mobile switching center 110 establishes, maintains, and interrupts. communications with the public switched telephone network

ES 2 328 079 T3 (PSTN). Although the following description focuses on the signals transmitted between base station 106a and mobile station 102, it will be understood by those skilled in the art that the description is equally applicable to other base stations and to fixed station 108. In the present document, the terms "Cell" and "base station" are generally used interchangeably.

Referring to FIG. 2, mobile station 10 includes an antenna 202 that transmits signals to and receives signals from base station 106a. A duplexer 203 provides a forward link channel or signal from base station 106a to mobile receiver system 204. Receiver system 204 performs down-frequency, demodulation, and decoding of the received signal. The receiver system 204 then provides a predetermined parameter or set of parameters to a quality measurement circuit 206. The parameters may include, for example, the measured signal-to-noise ratio (SNR), the measured received power, or the decoder parameters, such as the symbol error rate, the Yamamoto metric, or the parity bit control indication. A buffer 207 may be included for use in conjunction with the present invention described herein. For additional information about the operation of mobile station 102 (and base station 106a), for example, see US Patent No. 5,751,725, entitled "METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM" transferred to the assignee of the present invention and which is incorporated herein by reference.

The quality measurement circuit 206 receives the parameters from the receiver system 204 and determines a quality measurement signal or the power level of the received signal. The quality measurement circuit 206 can generate energy measurements per bit (E_b) or energy per symbol (E_s) from parts or windows of each frame. Preferably, energy-per-bit or energy-per-symbol measurements are normalized (e.g., E_b/ N_OR) or are normalized and include interference factors (for example, E_b/ N_t), in a manner known within the art. Based on these measurements, the quality measurement circuit 206 generates a power level signal.

A power control processor 208 receives the power level signal from the quality measurement circuit 206, compares the signal to a threshold, and generates a power control message based on the comparison. Each power control message can indicate a power change in the forward link signal. On the other hand, the power control processor 208 generates power control messages representing the absolute power of the received forward link signal, in a manner known within the art. Power control processor 208 preferably generates multiple (eg, sixteen) response control messages in response to multiple power level signals per frame. Although the quality measurement circuit 206 and the power control processor 208 are generally described herein as separate components, said components can be integrated monolithically, or the operations carried out by said components can be carried out by a single microprocessor. .

A mobile transmission system 210 performs the encoding, modulation, amplification, and frequency raising of the power control messages, via the duplexer 203 and antenna 202. In the illustrated embodiment, the system 210 of mobile transmission 210 provides the power control message at a predetermined location in a reverse link output frame.

The mobile transmission system 210 also receives reverse link traffic data, such as voice data or general computer data, from the user of the mobile station. The mobile transmission system 210 requests a particular service (including power / speed) from the base station 106a, based on the traffic data to be transmitted. In particular, the mobile transmission system 210 requests a suitable bandwidth allocation for the particular service. Base station 106a then organizes or allocates bandwidth resources (power / speed), based on requests from mobile station 102 and other users to take full advantage of said resource allocation, given system power limitations. In this way, by effectively managing the transmission power of the system, a more productive use of the bandwidth will be achieved.

Base station 106a includes receiving antenna 230 that receives reverse link frames from mobile station 102. A receiving system 232 of base station 106a performs frequency reduction, amplification, demodulation, and decoding of the traffic from the reverse link. A base interconnect transceiver 233 receives and forwards reverse link traffic to the base station controller 104. Receiver system 232 also separates the power control messages from each reverse link traffic frame and provides the power control messages to a power control processor 234.

Power control processor 234 monitors power control messages and transmits a forward link transmitter power signal to a forward link transmitter system 236. In response to this, the forward link transmitter system 236 increases, maintains, or decreases the power of the forward link signal. The forward link signal is then transmitted via a transmitting antenna 238. In addition, the power control processor 234 analyzes the quality of the reverse link signal from the mobile station 102 and provides appropriate feedback control messages to the forward link transmitter system 236. In response to these, forward link transmitter system 236 transmits the feedback control messages, via transmitting antenna 238 and through the forward link channel, to mobile station 102. Transmitter system 236 also receives forward link traffic data from base station controller 104 via transceiver.

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233 interconnection of bases. Forward link transmitter system 236 encodes, modulates, and transmits forward link traffic data via antenna 238.

Unless otherwise indicated, the various blocks and elements depicted in Figs. 1 and 2 and the rest of the figures are built and function according to a conventional design. For that reason, it will not be necessary to provide those skilled in the related art with a more detailed description of said blocks or elements. Any additional description is omitted so that the detailed description of the present invention is clear and concise. Those skilled in the related art will be able to easily make any necessary modifications to the blocks of the communication system 100 of Figs. 1 and 2 or the other illustrated systems, based on the detailed description provided herein.

The closed-loop power control system for user stations, including mobile station 102 and base station 106a, dynamically adjusts the transmit power for each user, based on the user's propagation conditions, and provides each user the same Frame Error Rate (FER) for voice services (eg 1% FER). However, as noted above, transmission may not be requested by many users for voice services, but rather for data services, such as fax, e-mail, and general computer data, all of which are insensitive to delay. although they require a lower FER or a lower Bit Error Rate (BER). A user can even request video services that, apart from requiring a lower FER, are sensitive to delay. The base station 106a dynamically assigns the transmission rates, based on the requests of each user, according to known techniques.

According to a CDMA standard, described in the document “TIA / EIA-95-A Mobile Stations-Base Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System” [“TIA / EIA-95-A Standard for Compatibility between Mobile Stations and Base Station for a Dual Mode Broadband Spread Spectrum Cellular System ”] of the Telecommunications Industry Association, each base station transmits pilot channels, synchronization, radio paging and direct traffic to your users. The pilot channel is an unmodulated direct sequence spread spectrum signal that is permanently transmitted by each base station. The pilot channel allows each user to obtain the timing of the channels transmitted by the base station, and provides a phase reference for coherent demodulation. Also, the pilot channel provides means for making signal strength comparisons between base stations to determine when a handoff should take place between base stations (eg, when moving from one cell to another). Recent CDMA modulation techniques have been proposed, using dedicated time multiplexed pilot symbols ("DTMP"). According to the DTMP approach, the separate pilot symbols are time multiplexed on each user's traffic channel. Each user sequentially de-spreads the pilot symbols (and information symbols). There is also an alternative common code multiplexed pilot symbol ("CCMP") approach, in which a common channel is available dedicated to transmitting a pilot signal. No pilot symbols are multiplexed with dedicated channels, and all users de-spread both pilot symbols and modulated information signals in parallel. Such systems are described in greater detail in US Patent Application No. 09 / 144,402, filed August 31, 1998, entitled "METHOD AND APPARATUS FOR REDUCING AMPLITUDE VARIATIONS AND INTERFERENCE IN COMMUNICATION SIGNALS, SUCH AS WIRELESS COMMUNICATION SIGNALS EMPLOYING INSERTED PILOT SYMBOLS ”(“ Procedure and apparatus for reducing amplitude variations and interference in communication signals, such as wireless communications signals using embedded pilot symbols ") and transferred to the same assignee of the present invention.

Interfrequency search

Referring now to Fig. 3, a diagram of the different timings involved in conducting a search excursion is depicted. Although for those skilled in the related art Fig. 3 will be self explanatory, a brief description of it is provided below. The reference t_search (t_search) corresponds to the time required to collect the N samples at the frequency f2. The total time will be equal to t_search plus the time it takes to process the samples after returning to the original frequency f1. The times t_synth (Lint) and t_settle (t_was) correspond to the time required to switch and settle on a new frequency, respectively. The time period of N_s x T_c represents the sampling time for N_samples, and you_process (t_process) represents the time to process the samples.

A procedure to minimize the search time for another frequency can be described as follows:

First of all, currently the mobile station is demodulating an original frequency or first frequency f1. It may be necessary to perform a hard handover between frequencies towards a target frequency f2, as in the case where certain measurements of signal quality (eg, those listed above) fall below predetermined thresholds. When such a drop in quality is communicated to base station 106a, the base station instructs mobile station 102 (for example, via a Candidate Frequency Search Request / Control message ("CFSCM")) to perform a search excursion to a target frequency f2.

The mobile station tunes to frequency f2 and collects N segment samples (one segment being a 1024 b / s pseudo noise bit, for example, for orthogonally encoded symbols). The samples are stored in a temporary memory; The mobile station does not carry out pilot signal searches or intensity measurements.

ES 2 328 079 T3 the pilot signal while at frequency f2. The mobile station retunes to the original frequency f1, resumes forward link reception and reverse link transmission, and processes the N collected samples on frequency f2, simultaneously.

The mobile station processes the collected samples at frequency f2 using a finder that processes the stored samples, while simultaneously processing the received signal at the original frequency f1. The mobile station communicates to the base station the corresponding intensity measurements of the pilot signal on frequency f2. The person skilled in the art will know how to recognize and will have the required capacity to provide or obtain the aforementioned search engine.

The foregoing procedure is illustrated in Fig. 4 as a routine 400 starting at step 410, in which base station 106a transmits a frequency change command to mobile station 102, in accordance with the control / request message. Candidate Frequency Search Defined in the TIA / EIA-95-B standard, included here for reference. In response to this command, mobile station 102 tunes to destination frequency f2 in step 420.

In step 430, mobile station 102 collects the signal samples at the destination frequency f2 and stores the samples locally in buffer 207. In step 440, mobile station 102 retunes to the first frequency f1 and processes the signal samples stored in buffer 207 in step 450. It should be noted that steps 440 and 450 can be performed simultaneously.

Once the signal samples have been processed in the manner described above, the mobile station 102 transmits, in step 460, the results of the signal sample processing to the base station 106a.

Minimizing the impact of the search excursion on the current plot

When the mobile station tunes to another frequency f2 to perform an inter-frequency search, the forward link symbols transmitted by the base station during the time period t_search (t_search) cannot be received by the mobile station. Similarly, the mobile station does not make any transmission during t_search(t_search) and the base station loses reverse link symbols during time period t_search (t_search). To minimize the impact of this loss on both current forward link frames and current reverse link frames, mobile stations and base stations increase the amount of power assigned to the other symbols in the symbol frame subjected to Interlaced and forward-looking error-correcting encoding that has been affected by search excursion. For the frame to be demodulated correctly, the additional magnitude of power required for symbols unaffected by the search excursion must be a function of the search excursion time t_search (t_search), as indicated.

Forward link power control during search visit

To compensate for the loss of forward link symbols during the time period t_search (t_search), the mobile station applies an increment of A_targetdB (A_destinydB) to E value_b/ N_or destination of the forward link closed loop fast power control system.

This new Target Eb / No. value is set to K Power Control Groups (PCG) prior to the search excursion. The necessary number K of affected anterior GWPs before the search excursion and the necessary increase in E_b/ N_OR (TO_target(TO_destiny)) depend on the duration of the search excursion t_search(t_search); the longer t_search(t_search), the larger will be K. As a consequence of the increase in the E value_b/ N_or destination, the forward link power will rise before the interfrequency search.

In Fig. 5, a series of forward link power levels are illustrated in relation to an interfrequency search excursion. Although Fig. 5 will be self-explanatory to those of ordinary skill in the related technology, a brief description of the technology is provided. After the search excursion, mobile station 102 resumes demodulation of the forward link symbols of the current frame. At this stage, mobile station 102 knows the total symbol energy received in the current frame and can compare it to the energy per frame required to achieve the desired frame error rate. Mobile station 102 may use this metric to increase or decrease the destination Eb / No value for the rest of the power control groups in the frame. If the search excursion exceeds the limit of a frame, the mobile station 102 can increase its value E_b/ N_or during the next frame to compensate for the loss of symbols in the first part of the frame. More details about closed-loop power control can be found, for example, in US Patent Applications No. 6075974 and No. 5982760, entitled "METHOD AND APPARATUS FOR ADJUSTING THRESHOLDS AND MEASUREMENTS OF RECEIVED SIGNALS BY ANTICIPATING POWER CONTROL COMMANDS YET TO BE EXECUTED" ”) And“ METHOD AND APPARATUS FOR POWER ADAPTATION CONTROL AND CLOSED-LOOP COMMUNICATIONS ”(“ Procedure and apparatus for power adaptation control and loop communications closed ”), filed on November 20, 1996 and June 20, 1997, respectively, and transferred to the assignee of the present invention.

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Checking the reverse link power during the search visit

While searching on destination frequency f2, base station 106a will lose communication with mobile station 102 and will not receive any symbols during time period ts_earch(t_search). To compensate for this loss of symbols, mobile station 102 may apply an increment of A_searchdB (A_searchdB) to the total transmit power of the reverse link. Quantity A_searchdB (A_searchdB) depends on the duration of t_search(t_search), and corresponds to the additional symbol energy needed throughout the rest of the frame, to compensate for the loss of symbols during ts_earch(t_search) and still allow base station 106a to demodulate the frame correctly. Base station 106a may indicate to mobile station 102 increment A_searchdB (A_searchdB) maximum tolerable in the message instructing the mobile station to carry out an inter-frequency search (for example, in the command "FCSM"). This value may depend on the maximum tolerable interference currently determined by base station 106a.

Fig. 6 illustrates the sequence of reverse link power increases during a search excursion. Although Fig. 6 will be self-explanatory for those of ordinary skill in the related technology, a brief explanation of this is provided. During the interfrequency paging frame, transmitted with a power increase, the base station 106a will send down commands in which it instructs the mobile station 102 to reduce its power. Mobile station 102 simply ignores these download commands until the end of the interfrequency page frame, as seen in FIG. 6. These upload and download commands are represented by the long dark arrows 602 and 604, respectively, in the Fig. 6. If the search excursion exceeds the boundary of one frame, mobile station 102 can increase its total transmit power during the next frame, in a manner similar to that noted above to compensate for the loss of initial symbols in the next frame. Normal power control resumes once the frame boundary is crossed, as illustrated in Fig. 6.

Therefore, the procedure described above with respect to Fig. 4 can be modified to ensure uninterrupted communication during a search excursion. The stages of the modified procedure are depicted in Fig. 7, beginning with step 710, during which base station 106a transmits the frequency change command (FCSM) to mobile station 102.

Before mobile station 102 tunes to the destination frequency, the value E_b/ N_or The target of the forward link closed loop fast power control system is increased and passed from a first level to a second level as described above. Mobile station 102 applies an increment of A_searchdB (A_searchdB) to the total reverse link transmit power, as also described above and illustrated in step 720.

The mobile station then tunes to the destination frequency and collects signal samples from the destination frequency, such as segment sample data, and stores the signal samples in memory 207, in steps 730 to 740.

At step 750, mobile station 102 retunes to the first frequency when it has finished collecting signal samples. Mobile station 102 processes the signal samples in the buffer and resumes communication with base station 106a at the first frequency f1. Upon resuming communications, mobile station 102 adjusts the E value_b/ N_or destination of the rest of the power control groups in the frame, and then applies a reduction of A_target (TO_destiny) to E value_b/ N_or destination and normal control of the reverse link total transmit power resumes, as illustrated in step 760.

Finally, in step 770, the results of signal sample processing, such as pilot signal strength measurements, are transmitted to the base station.

Base station 106a and mobile station 102 can be configured to carry out the above procedure. The source code to carry out the above procedure can easily be generated by those of ordinary skill in this matter, based on the detailed description provided here.

While a preferred embodiment of the present invention has been illustrated and described in the foregoing, it should be understood that various changes may be made therein without departing from the spirit and scope of the invention. For example, mobile station 102 may use the status of the long code mask in order to select an initial position in a frame to perform the inter-frequency search. Mobile station 102 may select a scrambling period such that the interfrequency search typically does not exceed one frame. Scrambling the search excursion position between different mobile stations will reduce reverse link interference and reduce the total forward link power requirement. Accordingly, the present invention will only be limited by the scope of the appended claims.

Although specific embodiments and examples of the present invention are described herein for illustrative purposes, various modifications equivalent to these can be made without departing from the scope of the present invention, as will be recognized by those skilled in related technology. For example, embodiments are generally represented and described as implemented in software and executed by means of a processor. Such software can be stored on any type of suitable computer-readable medium, such as a microcode stored on a semiconductor chip or a computer-readable disk, or it can be downloaded and stored from a server.

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The present invention could equally be implemented in hardware, for example a DSP (Digital Signal Processor) or an ASIC (Application Specific Integrated Circuit).

The information on the present invention provided herein may be applied to other communication systems, which are not necessarily like the communication system illustrated and described above. For example, while the present invention has been generally described as being employed in CDMA communication system 100, the present invention is equally applicable to other analog or digital cellular communication systems. The present invention may be modified to employ aspects of the systems, circuits, and concepts of the various patents and standards described above, all of which have been incorporated by reference.

In light of the detailed description above, it is possible to make these and other changes to the present invention. In general, the terms of the appended claims should not be construed as limiting the present invention to the particular embodiments disclosed in the specification and claims. Accordingly, the scope of the present invention is not limited by the present disclosure, but is instead fully determined by the appended claims.