Soft handoff in a cdma cellular telephone system

IS 2 142 800 T5

DESCRIPTION

Seamless switching on a CDMA cellular phone system.

Background of the invention

I. Field of the invention

The present invention relates to cellular telephone systems. More particularly, the present invention relates to a new and improved system for controlling the switching of cell-site station communications with a mobile unit, in a code division multiple access (CDMA) cellular telephone system.

II. Description of related techniques

The use of code division multiple access (CDMA) modulation techniques is only one of several techniques to facilitate communications in which a large number of users of the system are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation models are known, such as amplitude-limited single sideband (ACSSB), the CDMA has important advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in US patent application Serial No. 06 / 921,261, filed October 17, 1986, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS" ("Spread spectrum multiple access communication system using satellite or terrestrial repeaters"), which is currently US Patent No. 4,901,307, assigned to the assignee of the present invention and published on 02/13/1990. EP-A-0265178 represents a previously published member of the aforementioned patent family.

In the aforementioned patent, a multiple access technique is disclosed in which a large number of users of the mobile phone system, each of whom owns a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell-site stations, or cell-sites for short) using code division multiple access (CDMA) spread spectrum communication signals. When using CDMA communications, the frequency spectrum can be reused several times, thus allowing an increase in the user capacity of the system. The use of CDMA techniques results in much greater spectral efficiency than can be achieved using other multiple access techniques.

In conventional cellular telephone systems the available frequency band is divided into channels of typically 30 kHz bandwidth, when analog FM modulation techniques are used. The system's service area is geographically divided into cells of varying size. The available frequency channels are divided into groups, each group usually containing an equal number of channels. The frequency groups are assigned to the cells in a way that minimizes the possibility of interference on the channel itself. For example, consider a system in which there are seven groups of frequencies and the cells are hexagons of equal size. A group of frequencies used in one cell will not be used in the six neighboring cells closest to or around that cell. Furthermore, the frequency group of a cell will not be used in the twelve closest neighboring cells of that cell.

In the conventional cellular telephone system, the switching model implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between two cells. Switching from one cell to another is initiated when the cell-site receiver handling the call notices that the signal strength received from the mobile phone falls below a predetermined threshold value. An indication of low signal strength implies that the mobile phone must be close to the edge of the cell. When the signal level falls below the predetermined threshold value, the cell-site requests the system controller to determine whether a nearby cell-site is receiving the mobile phone signal with better signal strength than the current cell-site.

The system controller in response to the current cell-site request sends messages to nearby cell-sites with a switch request. The cell-site that is located near the current cell-site employs special scanning receivers that search for the signal from the mobile unit on the specified channel. If one of the nearby cell sites communicates to the system controller that the signal level is adequate, then a handover will be attempted.

The handover is then initiated when a free channel is selected from the group of channels used in the new cell-site. Then a control message is sent to the mobile phone instructing it to change from the current channel to the new channel. At the same time, the system controller passes the call from the first cell-site to the second cell-site.

In the conventional system, a call will be dropped if the handover to the new cell-site is unsuccessful. Switchover can fail for many reasons. The switch may fail if there is no free channel in the nearest cell to communicate the call. Also, switching may fail if another cell-site reports that it is listening to the mobile phone in question, when in fact this cell-site is listening to a different mobile unit using the same channel in a completely different cell. This communication error

ES 2 142 800 T5 will result in the call going to the wrong cell, usually a cell where the signal strength will be insufficient to maintain communications. Also, if the mobile phone could not hear the command to change channels, then the switching would be successful. In reality, operating experience indicates that switching failures occur frequently, which casts doubt on the reliability of the system.

Another common problem in the conventional telephone system occurs when the mobile telephone is near the border between two cells. In this situation, the signal level tends to fluctuate at both cell sites. This fluctuation in signal level results in a toggle situation where repeated requests are made to transfer the call back and forth between the two cell-sites. Such unnecessary switch requests increase the likelihood that the mobile unit will incorrectly hear the command to change channels or not be able to hear the command at all. Furthermore, the toggle situation increases the probability that the call will be dropped if it is unnecessarily transferred to a cell in which all channels are currently being used, thus not being available to accept the handover.

The document "Advanced Mobile Communication Network based on Signaling System No. 7" by Nakajima et al, International Switching Symposium 1987, vol. 3, March 1987, Phoenix, USA, pages 747-752 discloses a small area cellular system, in which a mobile station frequently travels to a new channel area from the current channel area during a call. When the conversation continues, the mobile station must change its talk channel, effecting what is called a "handover". To perform the switching, the radio base station (MBS) that detects the reduction in signal level reports the situation to the mobile communication control center (MCC). The MCC transmits a request for a level measurement to nearby MSBs, and each MBS performs a level measurement. The level measurement results are communicated to the MCC from each MBS, and the MCC decides on a new high signal level MBS and changes the wire talk path from the MBS and the radio talk channel to the mobile station.

Furthermore, this document discloses a high speed switching technique that has been developed in the High Capacity Land Mobile Communication System (MCS-L2) to reduce the duration of the speech interruption. This problem occurs due to the change from the present wire and radio talk path to the new path. In the automobile telephone system (MCS-L1), this cut-off time consists of the radio talk path change cut-off time and the talk-path change cut-off time per wire. In MCS-L2, the present and new wire talk paths have multiple connections before the radio talk path is switched, so there is no cutoff time with the wire talk path change. Therefore, as soon as the radio channel change is finished, it is possible to converse using the new MBS. To make various connections between the present and new wire talk paths, a high speed switching trunk (HSH trunk) having a voice add / distribute function has been installed in the MCC. A line from the PSTN (public switched telephone network) is connected to the distribution circuit and distributes to the present and new MBS lines. Also, each line of both the present MBS and the new MBS are connected to the add-on circuit, and an additional signal is transmitted to the PSTN. When it is confirmed that the mobile station has fully entered a new MBS area, the wire talk path to the old MBS and the HSH trunk are released. With this technique, the cutoff in conversation is reduced to around 200 ms.

In contrast to the discussion provided above, the present invention, claimed in independent claims 1, 8 and 18, uses, among other resources, a system or method to direct communication links between a mobile unit of the system and cell sites that, between other functions, it is responsible for communicating user information signals along said communication links as spread spectrum signals by code division, provide an indication that identifies another cell-site within whose service area a mobile system unit has traveled and respond to the indication by further coupling communication of user information signals between the mobile system unit and another system unit by medium of a communication link, while also remaining communicating user information signals via another communication link.

Document US-A-3,819,872 discloses a high capacity cellular mobile communication system intended to carry out a handover to maintain a mobile station communication path, transferring the mobile station from a transient base station in service to a free base station. A main circuit with multiple ports is used to establish the communication path between a central switching office and the mobile station through a serving base station. The deterioration of the radio carrier communication link of the base station in service, caused by the obscuration of the base station in service, allows the main circuit to check the continuity of the communication channels through other base stations to the mobile station . Once the continuity of a new communication channel has been established through a free base station, the main circuit transfers the communication path so that a continuous communication path can be maintained between the central switching office and the mobile station.

In addition to the above, the operation of a 4C2 traffic cut-off relay that connects or breaks the tip and loop conductors T522 and R522, which couple the main communication line with the remote base station 12, eliminates the free line termination through these conductors T522 and R522, and connects them respectively with the tip and loop conductors T52 and R52 that connect the main communication line with the central office of

ES 2 142 800 T5 mobile switching. Opening of the 4C2-1 and 4C2-2 break contacts of the 4C2 relay interrupts the holding path of the 4C1 transit cut-off relay which is now released, and which generally connects or breaks, the tip and loop conductors that couple the main line of communication with remote base station 11. Releasing the 4C1 relay allows the 4C1-1 and 4C1-2 transfer contacts of the 4C1 relay to place the free line termination across the T512 and R512 tip and loop conductors that typically connect the remote base station 11 and the main communication line, and open the connection with the mobile switching central office T52 and the tip and loop conductors T52 and R52. In summary, as described, continuous communications are maintained by rearranging the switching office channel coupling to one of the uncoupled base station channels that has been determined to have continuity in transmission.

In contrast to the discussion provided above, the present invention claimed in independent claims 1, 8 and 18, uses, among other resources, a system or method to direct communication links between a mobile unit of the system and geographically separated cell sites, each one of which defines a geographic service area, and said system or procedure, among other functions, is responsible for directing said communication links as said mobile unit changes service areas, communicating user information signals through a system controller connected to said cell sites, communicating said user information signals along such communication links as code division spread spectrum signals, determining a transition of said mobile system unit from the service area of ​​one cell-site to the service area of ​​another cell-site, providing an indication that identifies said other cell-site, effecting further coupling of the communication of said information signals between said mobile unit and said other system unit responsive to said indication.

The document "Das Wechseln von Funkbereichen ortsfester Stationen durch bewegliche Teilnehmer" by Herold, Archiv für Elektronik und Übertragungstechnik, vol. 29, n ° 4, 1975, Stuttgart, Germany, pages 173-179 refers to radio networks with small areas for the mobile subscriber, in which during an established connection it must be possible to change the radio area of ​​fixed stations. In contrast to a radio telephone system that uses frequency division multiplexing for channel separation, here this problem is simply solved with code division multiplexing, since the mobile subscriber does not have to change either the transmission band and reception or communication channel. In the document, it is also stated that only the closest fixed stations should exchange signaling information for subscriber switching. Furthermore, it must be possible for the subscriber admitting station to establish a connection with the mobile subscriber's partner station. This problem is easily overcome through the use of a network of lines with decentralized connections, to which all fixed stations are connected. This is possible because any data can be extracted from an arbitrary location on these networks regardless of the input location.

In addition to the above, since no signaling information exchange is provided between the fixed station and the mobile station, the subscriber must automatically switch to the second station's signal. A prerequisite for this switching is that the signal energy of the second station must be higher and that the phase of the code words must be the same. Therefore, the condition of the codeword synchronization circuit determines with which station the subscriber is connected. A prerequisite for switching is that the codeword of the second station must reach the subscriber with a higher power and with the same phase as that of the first station. The power ratio depends on the propagation conditions and cannot be controlled directly. Rather, phase conditions can be created for at least a short time, over various codeword periods.

Also, when the subscriber approaches a second station that is located at a distance r₀ of the first, receives the transmission signal of the second station as a noise signal, and the signal-to-noise ratio E / n₀ decreases in line with an increase in distance r from the first station. Through the use of nearly orthogonal code words, the influence of noise from the first station, with which the subscriber is synchronized, can be almost completely avoided. However, the influence of the second station remains and, consequently, the signal-to-noise ratio curves of the two stations converge with increasing distance.

In contrast to the discussion provided above, the present invention, claimed in independent claims 1, 8 and 18, uses, among other resources, a system or method to direct communication links between a mobile unit of the system and geographically separated cell sites, each of which defines a geographic service area, and said system or procedure, among other functions, is in charge of directing the communication links between the mobile system unit and the cell sites, communicating the user information signals through a system controller connected to the cell sites, when the mobile system unit changes the area of service from one cell-site to the service area of ​​another cell-site, provide an indication that identifies the other cell-site, effect a greater coupling of the communication of user information signals between the mobile system unit and the other system unit sensitive to the indication, and by means of a communication link with another cell-site, while the mobile system unit remains in communication with the other system unit via the communication link with the other cell-site, and providing a response to said communication coupling between the mobile system unit and the other system unit via the communication link with the other cell-site.

Document DE-A-26 25 475 relates to a procedure for automatic radio traffic zone change for a mobile radio talk unit while maintaining the connection in a radio transmission system

ES 2 142 800 T5 with fixed radio units and mobile radio units, in particular, in public and non-public mobile land radio, in which each fixed radio unit that is connected to the public telephone network by means of telephone establishments transmission is assigned to at least one particular duplex signaling channel and several particular duplex speech channels for signal transmission and speech, and wherein a mobile radio talk unit selects a first fixed radio unit with the aid of a measurement of the signal level on the signaling channel prior to establishing a connection, characterized in that the signal level of the HF signals of the Talk channel busy is constantly monitored by the mobile radio talk unit receiver in a known way, during an existing talk connection and, when the signal level falls below a given threshold value a predetermined number of times, the signal level evaluation circuit starts one or more measurement cycles in the receiver of the mobile radio talk unit, determining in said cycles a second fixed radio unit with the best reception conditions, by means of which the mobile radio talk unit subsequently sends, its own ID and the ID number of the radio traffic area to the second fixed radio unit, by means of which the connection is established, according to which a telephone connection from the first fixed radio unit to the second unit radio, indicated by the ID number of the radio traffic area, is established by means of the public switched telephone network, with the second fixed radio unit assigning a talk channel to the mobile radio talk unit, in which you can continue the conversation. The method is further characterized in that, as soon as a connection is established from the first fixed radio unit to a second fixed radio unit, the selective call is sent on the signaling channel to the mobile radio talk unit. Furthermore, the connection with the mobile radio talk unit since the transmission establishment of the first local switching unit is interrupted for a short time, as soon as the first local switching unit has received a termination signal by means of the establishment of transmission of the second local switching unit and, following the interruption, the mobile radio talk unit switches to the signaling channel of the second fixed radio unit and receives its selective call.

In contrast to the discussion provided above, the present invention, claimed in independent claims 1, 8 and 18, employs, among other resources, a system or method to direct communication links between a mobile unit of the system and geographically separated cell sites, each of which defines a geographic service area, said system or procedure, among other functions, is responsible for directing communication links, communicate user information signals by means of a system controller connecting cell-sites, communicate user information signals along communication links as spread spectrum signals by code division, effect greater coupling of communication of user information signals between the mobile system unit and the other system unit via a communication link with another cell-site, and providing a response to the communication coupling between the mobile system unit and the other system unit via the communication link with the other cell-site, while continuing communication of user information signals between the mobile system unit and the other system unit via the communication link with the other cell-site.

In JP 64 58134 and JP 63 233623, a communication system for a mobile communication system is described in which a handover is initiated by the system controller. In addition, this system uses frequency division multiple access (FDMA) signals. US Patent No. 4,222,115 describes switching in a cellular communication system using time frequency coded spread spectrum (TFC) signals.

Accordingly, one of the objectives of the present invention is to provide improvements in call switching between cell-sites in a cellular telephone system and thereby provide greater reliability of service.

According to the present invention, said objective is achieved by a system for directing communication links as defined in independent claim 1, a mobile system unit as defined in independent claim 18 and a method as defined in independent claim 8 , referring to a respective procedure. In the dependent claims, embodiments of the present invention are set forth.

Summary of the invention

In a CDMA cellular phone system, the same frequency band is used for all cells. The CDMA waveform properties that provide processing gain are also used to discriminate between signals that occupy the same frequency band. A mobile phone or unit therefore does not need to change frequency when switching the call from one cell site to another. Additionally, the probability that the call will be dropped if the failover command is received is greatly reduced.

In a CDMA cellular phone system, each cell-site has a plurality of modulator-modulator units or spread spectrum modems. Each modem consists of a spread spectrum digital transmission modulator, at least one spread spectrum digital data receiver, and a page receiver. Each cell-site modem is assigned to a mobile unit as needed to facilitate communications with the assigned mobile unit. Therefore, in many cases, there will be many modems available to use, although others may be active and in communication with the respective mobile units.

IS 2 142 800 T5

In the present invention, a switching model is employed for a CDMA cellular telephone system in which a modem from a new cell-site is assigned to a mobile unit, while the old cell-site continues to handle the call. When the mobile unit is located in the transition region between the two cell-sites, the call can be passed from one cell-site to another depending on the signal strength. Since the mobile unit always communicates through at least one cell-site, there will be no disruption to the mobile unit or service.

When the mobile unit's communications with the new cell-site have been firmly established, eg, when the mobile unit is well positioned within the new cell, the old cell-site stops handling the call. The switching techniques just described can be considered "programmable" handoffs in cell-site communications with the mobile unit. Programmable switching is essentially a close-to-open switching function. By contrast, conventional cellular telephone systems are considered to provide an open-before-close switching function.

In a CDMA cellular telephone system of the present invention, a programmable handover technique is implemented that also allows the mobile unit to initiate a handover. The mobile unit is also allowed to determine the best new cell-site to which communications should be transferred from an old cell-site.

Although the mobile unit is claimed to initiate the handover request and determine the new cell-site, handover procedure decisions can be made as in a conventional cellular telephone system. As noted above in connection with conventional systems, the cell-site determines when a handover may be appropriate and, via the system controller, requests the closest cells to search for the mobile unit signal. The cell-site that, according to the system controller, receives the strongest signal, will then accept the handover. However, such cell-site-initiated switching is not part of the present invention.

In the CDMA cellular phone system, each cell-site transmits a "pilot" carrier signal. This pilot signal is used by mobile units to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of signals transmitted by the cell-site.

Also, each cell-site transmits a "tuning" channel comprising spread spectrum modulated information such as cell-site identification, system timing, mobile paging information, and various other control signals. The pilot signal transmitted by each cell-site is of the same spreading code but with a different code phase shift. The phase shift makes it possible to distinguish between one pilot signal and another and, therefore, makes it possible to distinguish the cell sites from which they originate. Using the same pilot signal code allows the mobile unit to find the timing of the system by a single search through all phases of pilot signal codes. The strongest pilot signal, determined by a correlation procedure for each code phase, is easily recognizable. The recognized pilot signal corresponds to the pilot signal transmitted to the nearest cell-site.

Once the strongest pilot signal has been acquired, that is, the initial synchronization of the mobile unit with the strongest pilot signal, the mobile unit searches for the appropriate tuning channel for that cell-site. The tuning channel is transmitted by the cell-site using one of a plurality of different predetermined spread spectrum codes. In an exemplary embodiment of the present invention, twenty-one different codes are used. However, it should be understood that more or fewer codes can be used in the setting channel depending on the system parameters. The mobile unit then begins a search for all the different codes used in the setting channel.

When the mobile unit recognizes a suitable setup code for the cell-site, the system information is received and processed. The mobile unit then checks the setting channel for control messages. One of said control messages could indicate that there is a call waiting to be transferred to this mobile unit.

The mobile unit continues to scan the received pilot carrier signal code at code offsets corresponding to the pilot signals transmitted by the nearest cell-sites. This scan is performed to determine if the pilot signal from the closest cells is becoming stronger than the pilot signal initially determined to be the strongest. If, during this call idle mode, the pilot signal from a neighboring cell-site becomes stronger than the pilot signal transmitted by the initial cell-site, the mobile unit will pick up the strongest pilot signal and the corresponding setting channel of the cell-site. new cell headquarters.

When a call is initiated, a pseudo-noise code (PN) address is determined for use during the course of this call. The code address can be assigned by the cell-site or can be determined by prior agreement based on the identity of the mobile unit. Once a call is initiated, the mobile unit continues to scan the pilot signal transmitted by cell-sites located in nearby cells. The pilot signal scan continues to determine if one of the pilot signals transmitted by the nearest cell-sites becomes stronger than the pilot signal transmitted by the cell-site with which the mobile unit is communicating. When the pilot signal transmitted by a cell-site located in a nearby cell becomes stronger than the pilot signal transmitted by a cell-site of the current cell, this fact indicates to the mobile unit that a new cell has been entered and that it must initiate a switchover. In response to this determination of

ES 2 142 800 T5 pilot signal strength, the mobile unit generates and transmits a control message to the cell-site currently handling the call. This control message, indicating that there is currently a pilot signal transmitted by a new cell-site that is stronger than the pilot signal transmitted by the current cell-site, is provided to the system controller. The control message also contains information indicating the new cell-site and the PN code. Interpretation of the control message, once transmitted to the system controller, indicates that a communication switch from the mobile unit to the new recognized cell-site is to be initiated.

The system controller then begins the switching procedure. It should be understood that during the handover, it is not necessary to change the PN code address of the particular mobile unit that is to undergo the handover procedure. The system controller begins the handover by assigning a modem located at the new cell-site to the call. This modem is given the PN address associated with the call in communications between the mobile unit and the current cell-site modem. The new cell-site modem assigned to handle the call searches and finds the signal transmitted by the mobile unit. The cell-site modem also begins transmitting outbound signals to the mobile unit. The mobile unit searches for the output signals according to the signal information and the tuning channel provided by the new cell-site.

When the signal transmitted by the modem of the new cell site is picked up, the mobile unit listens to this signal. The mobile unit then transmits a control message indicating that the handover is complete. The control message is provided by the old cell-site modem, the new cell-site modem, or both to the system controller. In response to this control message, the system controller passes the call to the new cell-site modem only, while interrupting the call through the old cell-site modem. The old cell-site modem then enters a pool of free modems available for reassignment.

As a further improvement, the switching procedure can introduce a second mode of operation. This second embodiment is referred to herein as the cell-site diversity mode. The subject of the cell-site diversity modality is set forth in greater detail in US Pat. 5,109,390 of shared processing entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM" ("Diversity receiver in a CDMA cellular telephone system"), filed on November 7, 1989, by the inventors thereof and assigned to the assignee of the present invention.

In the cell-site diversity mode, the call is allowed to remain in an intermediate state as described above in connection with the call that is processed by two cell-sites. In the exemplary embodiment described here in relation to the mobile phone of the present invention, a total of three demodulation processors or receivers are used. One of the receivers is used for the scanning function, while the other two receivers are used as a two-channel diversity receiver. During single cell operation, the scanning receiver attempts to find the signal transmitted by the cell-site that travels several paths to the mobile unit. Multi-path signals are typically caused by reflections of the signals from buildings on the ground and other signal obstructions. When two or more of these reflections are found, the two receivers are assigned to the two strongest paths. The scanning receiver continues to evaluate the various paths to keep the two receivers in sync with the signals from the two strongest paths as path conditions change.

In cell-site diversity mode, the search receptor determines the two strongest paths for each cell-site. The two receivers are assigned to demodulate the signals from the two strongest paths of the four available paths from the original cell-site and from the new cell-site. The data demodulation procedure uses information from both of said receivers in a diversity combining operation. This diversity combining operation results in much better resistance to the damaging fading that can occur in the multipath cellular telephone environment.

Although different types of diversity combining techniques are known in this art, the present invention uses diversity combining to significantly enhance the quality and reliability of communications in a mobile cellular telephone system. In the present invention, a maximum ratio combining form is used. The signal-to-noise ratio for the two paths that are combined is determined with the contributions of the two paths weighted accordingly. The combination is consistent since the demodulation of pilot signals makes it possible to determine the phase of each path.

On the path from the mobile unit to the two cell-sites, path diversity reception is also performed by having both cell-sites demodulate the signals transmitted by the mobile unit. Both cell-sites send their demodulated data signals to the system controller along with an indication of the signal quality at the cell-site receiver. The system controller then combines the two versions of the mobile unit signal and selects the signal that has the best quality indication. It should be understood that it is possible to transmit the uncoded or even the unmodulated signals to the system controller to allow the use of a better diversity combining procedure.

The switching procedure in the cell diversity mode is started as above. The mobile unit determines that a signal transmitted by a nearby cell-site is strong enough to allow good quality signal demodulation. The mobile unit transmits

ES 2 142 800 T5 a control message to the current cell-site indicating the identity of this new cell-site and a request for the cell diversity mode. The cell-site then relays the identity of the cell-site and the request to the system controller.

The system controller responds by connecting the call to a modem in the new cell-site. The controller then performs diversity combining of the signals received by the two cell-sites, while the mobile unit performs diversity combining of the signals received from the two cell-sites. The cell diversity mode continues as long as the signals received from both cell-sites are of a sufficient level to allow good quality demodulation.

The mobile unit continues to search for signals transmitted from other cell-sites. If a signal transmitted by a third cell-site becomes stronger than one of the signals from the two original cell-sites, then the control message is transmitted by the mobile unit via at least one current cell-site to the controller. of the system. The control message indicates the identity of this cell-site and contains a request for handoff. The system controller then interrupts the communicating call using the weakest of the three cell-site signals, while passing the call through the two strongest cell-sites. In case the mobile units are equipped with additional receivers such as three receivers, a triple cell-site diversity mode can be implemented.

The cell-site diversity mode ends when the mobile unit determines that only one cell-site is providing adequate signals for quality demodulation. The mobile unit then sends a control message indicating that the cell-site will remain in communication after the end of the cell-site diversity mode. The system controller can also terminate the cell-site diversity mode in the event of a system overload with an insufficient number of modems available to support all requests from mobile units for this mode of operation. The described cell-site diversity mode is executed by decisions made in the mobile unit to operate in the cell-site diversity mode. However, it should be understood that the cell-site diversity mode can be implemented with the decisions made in the system controller for operation in this mode.

The present invention provides a considerable improvement over current cellular telephone systems in mobile unit switching. The close-to-break switching mechanism of the present invention is a major improvement in overall system reliability with less service disruption. Running a cell-site diversity mode provides further improvements over conventional cellular telephone systems by providing greater system reliability and quality of communications.

Brief description of the drawings

The characteristics and advantages of the present invention will be more apparent from the detailed description set forth consulted in conjunction with the drawings, in all of which corresponding equivalent reference characters are used, and in which:

Figure 1 is a schematic overview of an example CDMA cellular telephone system in accordance with the present invention;

Figure 2 is a block diagram of a mobile unit telephone configured for CDMA communications in a CDMA cellular telephone system;

Figure 3 is a block diagram of cell-site equipment in a CDMA cellular phone system; Y

Figure 4 is a block diagram of the equipment of a mobile telephone switching office.

Detailed description of the preferred embodiments

In Figure 1, an example telephone system is illustrated which constitutes an embodiment of the present invention. The system illustrated in Figure 1 uses CDMA modulation techniques in communication between the mobile units or mobile phones of the system and the cell sites. Cellular systems in large cities can have hundreds of cell-site stations serving hundreds of thousands of mobile phones. The use of CDMA techniques makes it possible to easily increase user capacity in systems of this size compared to conventional FM modulation cellular systems.

In Figure 1, the system controller and switch 10, also referred to as a mobile telephone switching office (MTSO), typically includes interface and processing circuitry to provide system control to cell-sites. In addition, controller 10 controls the routing of telephone calls from the public switched telephone network (PSTN) to the appropriate cell site, for transmission to the appropriate mobile unit. Controller 10 also controls the routing of calls from mobile units, via at least one cell-site, to the PSTN. Controller 10 can direct calls between mobile users via appropriate cell-site stations, since such mobile units do not usually communicate directly with each other.

IS 2 142 800 T5

Controller 10 can be coupled to cell-sites by various means such as dedicated telephone lines, fiber optic links, or microwave communication links. In Figure 1, three examples of such cell sites, 12, 14 and 16, are illustrated along with an example mobile unit 18 that includes a cell phone. Arrows 20a-20b define the possible communication link between cell-site 12 and mobile unit 18. Arrows 22a-22b define the possible communication link between cell-site 14 and mobile unit 18. Similarly, arrows 24a-24b define the possible communication link between cell-site 16 and mobile unit 18.

Cell-site or cell-site service areas are structured geographically so that the mobile unit will typically be closer to a cell-site. When the mobile unit is idle, that is, there is no call in progress, the mobile unit constantly monitors the pilot signal transmissions from each nearby cell-site. As illustrated in Figure 1, the pilot signals are transmitted to mobile unit 18 respectively by cell-sites 12, 14, and 16, on communication links 20b, 22b, and 24b, respectively. The mobile unit then determines which cell it is in by comparing the intensity of the pilot signals transmitted from those particular cell-sites.

In the example illustrated in Figure 1, mobile unit 18 can be considered to be the closest to cell-site 16. When mobile unit 18 initiates a call, a control message is transmitted to the nearest cell-site, the cell-site. Cell 16. Once the call request message has been received, cell-site 16 indicates this to the system controller 10 and transfers the call number. The system controller 10 then connects the call via the PSTN to the desired recipient.

In the event that a signal is initiated within the PSTN, the controller 10 broadcasts the call information to all cell-sites in the area. The cell-sites in return transmit an access message to the desired recipient of the mobile unit. When the mobile unit hears the access message, it responds with a control message that is transmitted to the nearest cell-site. This control message indicates to the system controller that this particular cell-site is in communication with the mobile unit. The controller 10 then routes the call through this cell-site to the mobile unit.

If mobile unit 18 leaves the coverage area of ​​the initial cell-site, cell-site 16, an attempt will be made to continue the call, routing the call through another cell-site. In the switching process, there are two different procedures for initiating the call switching or routing it through another cell-site.

The first procedure, called cell-site-initiated handover, is similar to the handover procedure used in the original first-generation analog cellular telephone systems that are currently in use. In the cell-site-initiated handover procedure, the initial cell-site, ie, cell-site 16, observes that the signal transmitted by mobile unit 18 has dropped below a certain threshold level. Cell-site 16 then transmits a switch request to the system controller 10. Controller 10 transmits the request from cell-site 16 to all the nearest cell-sites, 14, 12. The request transmitted by the controller includes channel-related information. , including the PN code sequence used by mobile unit 18. Cell-sites 12 and 14 tune a receiver to the channel being used by the mobile unit and measure the signal strength, typically using digital techniques. If one of the receptors at cell-sites 12 and 14 indicates the presence of a signal stronger than the signal from the indicated initial site, then a switch is made to this cell-site.

The second method of initiating a handover is called a mobile unit initiated handover. According to the present invention, the second method of initiating a handover is used. The mobile unit is equipped with a paging receiver that is used to scan the transmission of the pilot signal from the nearest cell-sites 12 and 14, in addition to performing other functions. If a pilot signal from cell-sites 12 and 14 is found to be stronger than the pilot signal from cell-site 16, mobile unit 18 transmits a control message to the current cell-site, cell-site 16. This control message control contains information indicating the cell-site with the highest signal intensity in addition to inter-cell transmission request information for this cell-site. Cell-site 16 passes this control message to controller 10.

The mobile unit initiated handover procedure has several advantages over the cell-site initiated cell handover procedure. The mobile unit checks for changes in the paths between the mobile unit itself and the various nearest cell-sites much earlier and with less effort than the cell-sites are capable of. However, to carry out a handover initiated by a mobile unit, each mobile unit must be provided with a paging receiver to perform the scanning function. However, in the exemplary embodiment described here of the CDMA communications capability of a mobile unit, the paging receiver has additional functions that require its presence.

Figure 2 illustrates in a block diagram an example of a mobile unit cell phone. The mobile unit includes an antenna 30 that is coupled through diplexer 32 to analog receiver 34 and transmitting power amplifier 36. Antenna 30 and diplexer 32 are of standard design and allow simultaneous transmission and reception over a single antenna. The antenna 30 captures the transmitted signals and provides them, through the diplexer 32, to the analog receiver 34. Receiver 34 receives RF frequency signals from diplexer 32 which

ES 2 142 800 T5 are typically found in the 850 MHz frequency band for frequency amplification and downconversion to an IF frequency. This frequency translation procedure is carried out using a standard design frequency synthesizer, which allows the receiver to be tuned to any of the frequencies of the reception frequency band included in the global frequency band of the cell phone.

The IF signal is then passed through a surface acoustic wave (SAW) bandpass filter which, in the preferred embodiment, has a bandwidth of approximately 1.25 MHz. A SAW filter is chosen with characteristics that match the waveform of the signal transmitted by the cell-site that has been modulated by direct sequence spread spectrum by a PN sequence synchronized at a predetermined frequency that, in the preferred embodiment , is 1.25 MHz. Choose a clock frequency that is an integer multiple of a group of common data flow rates, such as 16 kbps, 9.6 kbps, and 4.8 kbps.

Also, the receiver 34 performs a power control function to adjust the transmitting power of the mobile unit. Receiver 34 generates an analog power control signal that is provided to transmit power control circuits 38. Control and operation of the mobile unit power control facility is disclosed in US Pat. 5,056,109 pending entitled "METHOD AND APPARATuS FOR CONTROLLING TRANSMISSION POWERIN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM" ("Procedure and apparatus for controlling transmission power in a CDMA mobile cellular telephone system"), filed on November 7, 1989 , by the inventors thereof and assigned to the assignee of the present invention.

Receiver 34 is also provided with an analog-to-digital (A / D) converter (not shown) to convert the IF signal to a digital signal, with conversion taking place at a 9.216 MHz clock frequency, in the preferred embodiment. , which is exactly eight times the PN microprocessor frequency. The digitized signal is provided to each of two or more signal processors or data receivers, one of which is a search receiver and the rest are data receivers.

In Figure 2, the digitized signal output from receiver 34 is provided to digital data receivers 40 and 42 and search receiver 44.

The digitized IF signal can contain the signals from many incoming calls along with the pilot carriers transmitted by the current cell-site and all the nearest cell-sites. The function of receptors 40 and 42 is to correlate the IF samples with the appropriate PN sequence. This correlation procedure provides a property that is well known in the art as "processing gain" that improves the signal-to-interference ratio of a signal that matches the proper PN sequence, while not enhancing the other signals. The correlation output is then synchronously detected using the nearest cell-site pilot carrier signal as a carrier phase reference. The result of this detection procedure is a sequence of encoded data symbols.

A property of the PN sequence used in the present invention is that it provides discrimination against multipath signals. When the signal reaches the mobile receiver after having passed through more than one path, there will be a difference in the signal reception time. This reception time difference corresponds to the distance difference divided by the speed of light. If this time difference is greater than one microsecond, then the correlation process will discriminate relative to one of the trajectories. The receiver can choose whether to track and receive the first or second path. If two receivers are provided, such as receivers 40 and 42, then two independent paths can be tracked in parallel.

The search receiver 44, under the control of the control processor 46, is used to continuously scan the time domain, around the nominal time of a pilot signal received from the cell-site, to find other pilot signals from the same multipath. cell-site and pilot signals transmitted by other cell-sites. Receiver 44 will measure the strength of any reception of a desired waveform at times other than nominal time. The receiver 44 compares the signal strength of the received signals. Receiver 44 provides a signal strength signal to control processor 46 indicating the strongest signals.

Processor 46 provides signals to digital data receivers 40 and 42 for each to process a different signal from the stronger signals. On occasion, the intensity of the pilot signal transmitted by another cell-site is greater than the signal intensity of the current cell-site. Control processor 46 will then generate a control message for transmission to the system controller via the current cell-site, requesting a handoff of the cell to the cell-site corresponding to the strongest pilot signal. Consequently, receivers 40 and 42 can handle calls through two different cell-sites.

The outputs of the receivers 40 and 42 are provided to the diversity combiner and decoder 48 circuits. The diversity combiner circuits contained within the circuits 48 simply adjust the timing of the two received signal streams to align them, and the add up. This addition procedure can continue by multiplying the two currents by a number corresponding to the relative signal strengths of the two currents. This operation can be considered a combination of maximum ratio diversity. The resulting combined signal stream is then decoded using a forward current error detection decoder that is also contained in circuitry 48.

IS 2 142 800 T5

In the example embodiment, convolutional coding is used. Convolutional encoding has a limitation length 9 and a code rate 1/3, that is, three encoded symbols are produced and transmitted for each bit of information to be transmitted. The optimal decoder for this type of code is the programmable decision Viterbi algorithm decoder design. The resulting decoded information bits are passed to user 50 digital baseband circuits.

Baseband circuitry 50 typically includes a digital vocoder (not shown). Baseband circuits 50 also serve as an interface to a handset or any other type of peripheral device. Baseband circuits 50 accommodate a variety of different vocoder designs. Baseband circuits 50 provide output information signals to the user based on information provided to them from circuits 48.

Analog voice signals typically provided through a handset are provided as input to baseband circuitry 50. Baseband circuitry 50 includes an analog-to-digital (A / D) converter (not shown) that converts the analog signal into a digital signal. The digital signal is provided to the digital vocoder where it is encoded. The vocoder output is provided to a forward error correction encoding circuit (not shown) for error correction. This digitized coded speech signal is transferred from baseband circuits 50 to transmit modulator 52.

The transmit modulator 52 modulates the encoded signal into a PN carrier signal whose PN sequence is chosen in accordance with the address function assigned for the call. The PN sequence is determined by the control processor 46 from call setup information that is transmitted by the cell-site and decoded receivers 40 and 42. Alternatively, the control processor 46 may determine the PN sequence by prior agreement. with the cell headquarters. Control processor 46 provides PN sequence information to transmit modulator 52 and receivers 40 and 42 for decoding of the call.

The output of the transmit modulator 52 is provided to the transmit power control circuitry 38. The signal transmit power is controlled by the analog power control signal provided from the receiver 34. In addition, cell-sites transmit bits of control in the form of a power adjustment command, these being processed by data receivers 40 and 42. The power setting command is used by the control processor when setting the transmission power level of the mobile unit. In response to power adjustment commands, control processor 46 generates a digital power control signal that is provided to circuits 38. Further information on the interrelationship of receivers 40 and 42, control processor 46, and transmit power control 35 is provided in co-pending US Patent No. 5,056,109 mentioned above.

The transmit power control circuits 38 transfer the controlled power modulated signal to the transmit power amplifier circuits 36. The circuits 36 amplify and convert the IF signal into an RF frequency signal, mixing it with an output signal from the Frequency synthesizer that tunes the signal to the appropriate output frequency. Circuits 36 include an amplifier that amplifies power to a final output level. The desired transmission signal is transferred from circuits 36 to diplexer 32. Diplexer 32 couples the signal to antenna 30 for transmission to cell-sites.

Control processor 46 is also capable of generating control messages such as cell diversity mode requests and cell-site communication termination commands. These commands are provided to transmit modulator 52 for transmission. Control processor 46 is responsive to data received from data receivers 40, 42 and paging receiver 44 to make decisions regarding diversity switching and combining.

Figure 3 illustrates in a block diagram an exemplary embodiment of the cell-site equipment. At the cell-site, two receiver systems are used, each of which has a separate antenna and an analog receiver for spatial diversity reception. In each of the receiver systems, the signals are processed identically until the signals undergo a diversity combining process. Items surrounded by dashed lines correspond to items that correspond to communications between the cell-site and a mobile unit. The output of the analog receivers is also provided to other items used in communications with other mobile units.

In Figure 3, the first receiver system comprises an antenna 60, an analog receiver 62, a search receiver 64, and a digital data receiver 66. This receiver system may also include an optional digital data receiver 68. The second The receiver system includes an antenna 70, an analog receiver 72, a search receiver 64, and a digital data receiver 66. Cell-site control processor 78 is also used in signal processing and control for switching and diversity. Both receiver systems are coupled to the diversity combiner and decoder 80 circuitry. Digital link 82 is used to communicate source and destination signals at the MTSO (Figure 4), with the cell-site transmission modulator 84 and circuits 80, under control of control processor 78.

The signals received at the antenna 60 are provided to the analog receiver 62. The received signals amplified by an amplifier of the receiver 62 are converted to IF frequency signals by mixing them with an output signal from the frequency synthesizer. IF signals are passed through a bandpass filter and digitized in a procedure.

ES 2 142 800 T5 identical to that described in relation to the analog receiver of the mobile unit. The digitized IF signals are provided to the digital data receiver 66, the optional data receiver 68, and the search receiver 64, and are respectively processed in a manner similar to that discussed in connection with the digital data receivers and the search receiver. of the mobile unit of Figure 2. However, the processing by digital data receivers and paging receivers for the mobile unit-cell-site link differs from that used for the cell-site-mobile unit link in several respects.

On the internal destination link, or from the mobile unit to the cell-site, the mobile unit does not transmit any pilot signals that can be used for coherent reference purposes in cell-site signal processing. Therefore, the mobile unit-cell-site link uses non-coherent modulation and a demodulation model that uses 64-order orthogonal signaling.

Search receiver 64 is further used to scan the time domain around the receiver signal to ensure that associated digital data receiver 66, and data receiver 68 if used, are tracking and processing the signal. most intense time domain available. This tracking procedure is identical to that described for the mobile unit. Search receiver 64 provides a signal to cell-site control processor 78 that provides control signals to digital data receivers 66 and 65 to select the appropriate received signal for processing.

In the 64th order orthogonal signaling procedure, the symbol transmitted by the mobile unit has one of 64 different probabilities. A 6-bit symbol is encoded into a 2-bit⁶, that is, in 64 different binary sequences. The group of sequences chosen are known as Walsh functions. The optimal reception function for the Walsh function is the fast Hadamard transform (FHT). In search receiver 64 and digital data receivers 66 and 68, the input signal is correlated as described in connection with the mobile unit receivers, with the output of the correlator applied to an FHT processor. The FHT processor procedures produce a group of 64 coefficients for every 6 symbols. The 64 symbols are then multiplied by a weighting function generated at the receiver. The weighting function is related to the measured signal intensity. The weighted data is then provided as output to the diversity combiner and decoder 50 circuits.

The second receiver system processes the received signals in a manner similar to that described in relation to the first receiver system of Figure 3. The 64 weighted symbols transferred from receivers 66 and 76 are provided to the diversity combiner and decoder circuitry. 80. Circuitry 80 includes an adder that adds the 64 weighted symbols from receiver 66 to the 64 weighted symbols from receiver 76. The 64 resulting coefficients are compared to each other to determine the highest coefficient. The magnitude of the comparison result, along with the identity of the highest of the 64 coefficients, is used to determine a set of encoder weights and symbols for use in a Viterbi algorithm decoder executed on circuits 80.

The Viterbi decoder preferably has a limited length 9 and a code rate of 1/2. The Viterbi decoder is used to determine the most likely information bit sequence. For each vocoder data block, nominally 15 msec of data, a rough estimate of the signal quality is obtained and transmitted, as a mobile unit power adjustment command along with data, to the mobile unit. The aforementioned pending US Patent No. 5,056,109 provides more information about generating this quality estimate. The quality estimate is the mean signal-to-noise ratio over a 15 msec interval.

In Figure 3, the optional digital data receiver 68 can be included for better system performance. This additional data receiver, alone or in combination with additional receivers, can track and receive other possible delay paths of the signals transmitted by the mobile unit. The structure and operation of this receiver is similar to that described for digital data receivers 66 and 76. Receiver 68 is used to obtain additional diversity modes. Optional additional digital data receivers that provide additional diversity modes are extremely useful in cell-sites that are located in dense urban areas where multipath signals are likely to be produced.

The MTSO signals are coupled to the appropriate transmission modulator via digital link 82 under control of control processor 78. Transmission modulator 84 performs spread spectrum modulation according to a predetermined spread function assigned by the control processor 78, of the data for transmission to the desired recipient mobile unit. The output of the transmit modulator 84 is provided to the transmit power control circuits 86 where the transmit power can be controlled under control of the control processor 78. The output of the circuits 86 is provided to the transmit power amplifier circuits. transmission 88.

Circuitry 88 includes an adder to sum the output of transmit modulator 84 with the output of other transmit modulators at the cell-site. In addition, the circuits 88 include an adder for summing the pilot signal output from the pilot signal generator 90 with the summed output signals from the transmit modulator. Also, circuits 88 include a digital-to-analog converter, frequency overconversion circuits, and

ES 2 142 800 T5 an amplifier serving, respectively, to convert the digital signals into analog signals, convert the IF frequency signals transferred from the transmission modulators into RF frequency signals and amplify the RF signal. The output of circuits 88 is provided to antenna 92 ​​where it is radiated to mobile units in the cell-site service area.

Cell-site control processor 78 is responsible for assigning digital data receivers and modulators to a particular call. Also, the control processor 78 monitors the course of the call, the quality of the signals, and initiates the interruption when the signal is lost. The cell-site communicates with the MTSO, via link 82, to which it is coupled via standard telephone cable, fiber optic, or microwave link.

Figure 4 illustrates in a block diagram the equipment used in the MTSO. The MTSO typically includes a system controller or system control processor 100, a digital switch 102, a diversity combiner 104, a digital vocoder 106, and a digital switch 108. Although not illustrated, diversity combiners and digital vocoders Additional ones are coupled between digital switches 102 and 108.

When the cell diversity mode is active, or the MTSO is in the switching process, the call being processed by two cell-sites, the signals will arrive at the MTSO from more than one cell-site with the same information, nominally. However, due to fading and interference on the internal destination link from the mobile unit to the cell-sites, the signal from one cell-site may be of better quality than the signal from the other cell-site.

The digital switch 102 is used in the routing of the information stream corresponding to a given mobile unit, from one or more cell-sites to the diversity combiner 104 or the corresponding diversity combiner, as determined by a signal from the control processor of the mobile unit. system 100. When the system is not in cell diversity mode, diversity combiner 104 can be ignored or the same information can be applied to it at each gateway.

A large variety of serial coupled vocoders and diversity combiners are provided in parallel, nominally one for each call to be processed. The diversity combiner 104 compares the signal quality indicators that accompany the information bits of the signals from the two or more cell-sites. The diversity combiner 104 selects the bits corresponding to the best quality cell-site signal, frame by frame of the information, for transfer to the vocoder 90.

The vocoder 106 converts the digitized voice signal format to the standard 64 Kbps format, the PCN phone format, the analog format, or any other standard format. The resulting signals are transmitted from vocoder 106 to digital switch 108. Under control of system control processor 100, the call is routed to the PSTN.

The voice signals originating from the PSTN destined for the mobile unit are provided to the digital switch 108 for a suitable digital vocoder such as, for example, the vocoder 106, under control of the system control processor 100. The vocoder 106 encodes the signals digitized speech channels and provides the resulting information bit stream directly to digital switch 102. Digital switch 102, under control of the system control processor, directs the encrypted data to the cell-site or cell-sites with which the mobile unit is communicating. If the mobile unit is in switched mode and is communicating with multiple cell-sites or is in a cell diversity mode, the digital switch 102 routes the calls to the appropriate cell-sites for transmission by the site transmitter. cell phone appropriate to the desired recipient mobile unit. However, if the mobile unit is communicating with one cell-site only or is not communicating with any in cell-diversity mode, the signal is directed to one cell-site only.

System control processor 100 provides control of digital switches 102 and 106 to route data to the MTSO and vice versa. The system control processor 100 also determines the allocation of calls to the MTSO's cell-sites and vocoders. In addition, the system control processor 100 communicates with the control processor of each cell-site, regarding the allocation of particular calls between the MTSO and the cell-site and the allocation of PN codes for the calls. It should further be understood, that although in Figure 4 the digital switches 102 and 106 are shown as two separate switches, this function can be carried out by a single physical switch unit.