Soft handoff in a cdma cellular telephone system

BACKGROUND OF THE INVENTION

I. Field of the Invention

0001The present invention relates to cellular telephone systems. More specifically, the present invention relates to a novel and improved system for controlling handoff in communications of cell-site stations with a mobile unit in a code division multiple access (CDMA) cellular telephone system.

II. Description of the Related Art

0002The use of code division multiple access (CDMA) modulation techniques is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in <patcit id="pcit0001" dnum="US92126186A" dnum-type="L"><text>U.S. Patent Application Serial No. 06/921,261, filed October 17, 1986</text></patcit>, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", now <patcit id="pcit0002" dnum="US4901307A"><text>U.S. Patent No. 4,901,307</text></patcit>, assigned to the Assignee of the present invention and published on 13.02.90. <patcit id="pcit0003" dnum="EP0265178A"><text>EP-A-0265178</text></patcit> represents a pre-published family member of the just mentioned patent.

0003In the just mentioned patent, a multiple access technique is disclosed where a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell-site stations, or for short, cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.

0004In the conventional cellular telephone systems the available frequency band is divided into channels typically 30 kHz in bandwidth while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors of that cell.

0005In the conventional cellular telephone system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between two cells. The handoff from one cell to another is initiated when the cell-site receiver handling the call notices that the received signal strength from the mobile telephone falls below a predetermined threshold value. A low signal strength indication implies that the mobile telephone must be near the cell border. When the signal level falls below the predetermined threshold value, the cell-site asks system controller to determine whether a neighboring cell-site receives the mobile telephone signal with better signal strength than the current cell-site.

0006The system controller in response to the current cell-site inquiry sends messages to the neighboring cell-sites with a handoff request. The cell-site neighboring the current cell-site employs special scanning receivers which look for the signal from the mobile unit on the specified channel. Should one of the neighboring cell-sites report an adequate signal level to the system controller, then a handoff will be attempted.

0007Handoff is then initiated when an idle channel from the channel set used in the new cell-site is selected. A control message is sent to the mobile telephone commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first cell-site to the second cell-site.

0008In the conventional system a call will be discontinued if the handoff to the new cell-site is unsuccessful. There are many reasons that a failure in handoff may occur. Handoff can fail if there is no idle channel available in the neighboring cell for communicating the call. Handoff can also fail if another cell-site reports hearing the mobile telephone in question, when in fact this cell-site actually hears a different mobile unit using the same channel in a completely different cell. This reporting error will result in the call being switched to a wrong cell, typically one in which signal strength is insufficient to maintain communications. Furthermore should the mobile telephone fail to hear the command to switch channels, the handoff will fail. Actual operating experience indicates that handoff failures occur frequently which questions the reliability of the system.

0009Another 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 signal level fluctuation results in a ping-ponging situation in which repeated requests are made to hand the call back and forth between the two cell-sites. Such additional unnecessary handoff requests increase the possibility of the mobile unit incorrectly hearing the channel switch command or fail in hearing the command at all. Furthermore, the ping-ponging situation raises the possibility that the call will be discontinued if it is unnecessarily transferred to a cell in which all channels are currently in use and thus unavailable for accepting the handoff.

0010International Switching Symposium 1987, vol. 3, March 1987, Phoenix, USA, pages 747 - 752, Nakajima et al. "Advanced Mobile Communication Network based on Signalling System No. 7" discloses a small zone cellular system, wherein a mobile station often moves to a new channel zone from the present channel zone during a call. When the conversation is continued, the mobile station must change its speech channel, which is called "handoff." To execute handoff, the radio base station (MBS) that detects signal level reduction informs the mobile communciation control center (MCC) of the situation. MCC transmits level measurement demand to nearby MBSs, and each MBS carries out level measurement. Level measurement results are reported to the MCC from each MBS, and the MCC decides a new high level signal MBS and changes the wire speech path of the MBS and the radio speech channel to the mobile station.

0011This document further discloses a high speed handoff technique that has been developed in the high capacity land mobile communication system (MCS-L2) to reduce the length of the short cut conversation. This problem occurs because of the change from the present wire and radio speech path to the new one. In the automobile telephone system (MCS-L1), this cut off time consists of radio speech path change cut off time and wire speech path change cut off time. In the MCS-L2, present and new wire speech paths are multi-connected before the radio speech path is changed, so that there is no cut off time with the wire speech path change. Therefore, as soon as the radio channel change is completed, it is possible to converse by using the new MBS. To multi-connect present and new wire speech paths in the MCC, a high-speed handoff trunk (HSH trunk) which has a voice addition/distribution function has been installed in the MCC. A line from the PSTN (Public Switched Telephone Network) is connected to the distribution circuit and is distributed to present MBS and new MBS lines. Also, each line from both the present MBS and the new MBS are connected to the added circuit and an additional signal is transmitted to the PSTN. When it is confirmed that the mobile station has moved completely into a new MBS zone, the wire speech path to the old MBS and HSH trunk are released. With this technique, the short cut in the conversation is reduced to about 200ms.

0012In contrast to the disclosure cited above, the present invention as claimed in independent claims 1, 8 and 18 employs, among other features, a system or method for directing communication links between a mobile system unit and cell-sites that, among other features, communicates the user information signals along said communication links as code division spread spectrum signals, provides an indication identifying another cell-site into whose service area a mobile system unit has changed and responds to the indication by further coupling communication of user information signals between the mobile system unit and another system unit via a communication link while also remaining in communication of the user information signals via another communication link.

0013<patcit id="pcit0004" dnum="US3819872A"><text>US-A-3,819,872</text></patcit> discloses a high capacity cellular mobile communications system arranged to carry out an intracell handoff to maintain a mobile station communication path by transferring the mobile station from a shadowed serving base station to a clear base station. A multiport trunk circuit is utilized to establish the communication path between a switching central office and the mobile station through a serving base station. The deterioration of the serving base station radio carrier communication link, brought about by the shadowing of the serving base station, enables the trunk circuit to test continuity of communication channels through other base stations to the mobile station. Once continuity of a new communication channel has been established through a clear base station, the trunk transfers the communication path in order that a continuous communication path may be maintained between the switching central office and the mobile station.

0014Further to the above, the operation of a cut through relay 4C2 that connects or interrupts the tip and ring conductors T522 and R522 coupling the communication trunk to the remote base station 12 removes the idle line termination from across these conductors T522 and R522, and respectively connects them with tip and ring conductors T52 and R52 coupling the communication trunk to the mobile switching central office. The opening of break contacts 4C2-1 and 4C2-2 of relay 4C2 interrupts the hold path of cut-through relay 4C1 that now releases and that generally connects or interrupts the tip and ring conductors coupling the communication trunk to the remote base station 11. Release of relay 4C1 enables transfer contacts 4C1-1 and 4C1-2 of relay 4C1 to place the idle line termination across tip and ring conductors T512 and R512 that generally connect the remote base station 11 and the communication trunk and to open the connection with mobile switching central office T52 and R52 tip and ring conductors. In short, it is taught that continuous communications are maintained by rearranging the coupling of the switching office channel with one of the non-coupled base station channels ascertained to have transmission continuity.

0015In contrast to the disclosure cited above, the present invention as claimed in independent claims 1, 8 and 18 employs, among other features, a system or method for directing communication links between a mobile system unit and geographically separated cell-sites each defining a geographic service area, wherein, among other features, said communication links are directed as said mobile unit changes service areas, user information signals are communicated via a system controller connected to said cell-sites, said user information signals are communicated along said communication links as code division spread spectrum signals, a transition of said mobile system unit from one cell-site service area to another cell-site service area is determined, an indication is provided identifying said another cell-site, communication of said user information signals is further coupled between said mobile unit and said another system unit responsive to said indication.

0016<nplcit id="ncit0001" npl-type="s"><text>Archiv für Elektronik und Übertragungstechnik, vol. 29, no. 4, 1975, Stuttgart, Germany, pages 173-179</text></nplcit>, Herold "Das Wechseln von Funkbereichen ortsfester Stationen durch bewegliche Teilnehmer" relates to radio networks with small zones for the mobile subscriber, wherein it must be possible during an established connection to change the radio zone of the fixed stations. In contrast to a radio telephone system using frequency division multiplexing for channel separation, this problem is solved simply therein with code division multiplexing because the mobile subscriber has neither to change the transmitting and receiving band nor the communication channel. It is further taught that only neighboring fixed stations must exchange signalling information for the handoff of the subscriber. Furthermore, it must be possible for the station accepting the subscriber to establish a connection to the partner of the mobile subscriber. This problem is easily overcome through the use of a line network with decentralized connections, onto which all fixed stations are connected. This is possible since any piece of information can be extracted from an arbitrary location in such networks independent of the location of input.

0017Further to the above, since an exchange of signalling information between the fixed and the mobile station is not provided, the subscriber must switch automatically to the signal of the second station. Prerequisite for this handoff is a larger signal energy from the second station and phase equality of the code words. Thus, the condition of the code word synchronization circuit determines with which station the subscriber is connected. A prerequisite for the handoff is that the code word of the second station must arrive at the subscriber with a higher power and with the same phase as that of the first station. The power ratio depends on the propogation conditions. It cannot be directly controlled. Contrary hereto, the phase conditions can be created at least for a short time through various code word periods.

0018Also, when a second station is approached that is located a distance <i>r₀</i> from the first, the subscriber receives the transmission signal of the second station as a noise signal and the signal to noise ratio <i>E</i>/<i>n₀</i> decreases with increasing distance r from the first station accordingly. Through the use of quasi-orthogonal code words, the noise influence of the first station, with which the subscriber is synchronized, can be essentially avoided. Nonetheless, the influence of the second station remains and thus the signal to noise ratio curves of the two stations merge with increasing distance.

0019In contrast to the disclosure cited above, the present invention as claimed in independent claims 1, 8 and 18 employs, among other features, a system or method for directing communication links between a mobile system unit and geographically separated cell-sites each defining a geographic service area, wherein, among other features, the communication links between the mobile system unit and the cell-sites are directed, the user information signals are communicated via a system controller connected to the cell-sites, when the mobile system unit changes from one cell-site service area to another cell-site service area an indication is provided identifying the another cell-site, a further coupling of communication of the user information signals is effected between the mobile system unit and the another system unit responsive to the indication and via a communication link with another cell-site while the mobile system unit remains in communication with the another system unit via the communication link with the one cell-site, and a response is effected to said coupling of communication between the mobile system unit and the another system unit via the communication link with the another cell-site.

0020<patcit id="pcit0005" dnum="DE2625475A"><text>DE-A-26 25 475</text></patcit> relates to a method for an automatic radio traffic area change for a mobile radio speech unit while maintaining the connection in a radio transmission system with fixed radio units and mobile radio units, in particular in public or non-public mobile overland radio, wherein each fixed radio unit that is connected to the public telephone network via transmission set-ups is assigned to at least one particular duplex signalling channel and several particular duplex speech channels for signal and speech transmission, and wherein a mobile radio speech unit selects a first fixed radio unit with the aid of a signal level measurement on the signaling channel prior to the set-up of a connection, characterized in that the signal level of the HF signals on the busy speech channel is constantly monitored by the receiver of the mobile radio speech unit in a known manner during an existing speech connection and, when the signal level drops below a given threshold value a predetermined number of times, one or several measurement cycles are begun by the signal level evaluation circuit in the receiver of the mobile radio speech unit, in which cycles a second fixed radio unit with the best reception conditions is determined, whereby the mobile radio speech unit subsequently sends its own ID and the ID number of the radio traffic area to the second fixed radio unit, via which the connection is established, whereby a telephone connection from the first fixed radio unit to the second radio unit indicated by the radio traffic area ID number is established via the public telephone network and wherein the second fixed radio unit assigns the mobile radio speech unit a speech channel, on which the conversation can be continued. The method is further characterized in that, as soon as a connection from the first fixed radio unit to the second fixed radio unit is established, the selective call is sent out in the signalling channel to the mobile radio speech unit. Furthermore, the connection to the mobile radio speech unit from the transmission set-up 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 via the transmission set-up of the second local switching unit and, as a result of the interruption, the mobile radio speech unit switches to the signalling channel of the second fixed radio unit and receives its selective call.

0021In contrast to the disclosure cited above, the present invention as claimed in independent claims 1, 8 and 18 employs, among other features, a system or method for directing communication links between a mobile system unit and geographically separated cell-sites each defining a geographic service area, wherein, among other features, communication links are directed, the user information signals are communicated via a system contoller connecting the cell-sites, the user information signals along the communication links are communicated as spread spectrum code division signals, a further coupling of communication of the user information signals between the mobile system unit and the another system unit is effected via a communication link with another cell-site and a response is effected to the coupling of communication between the mobile system unit and the another system unit via the communication link with the another cell-site while communication of the user information signals between the mobile system unit and the another system unit via the communication link with the another cell-site is continued. In <patcit id="pcit0006" dnum="JP6458134B"><text>JP 64 58134</text></patcit> and <patcit id="pcit0007" dnum="JP63233623A"><text>JP 63 233623</text></patcit>, a communication system for mobile communication system is described in which a handoff is initiated by the system controller. Further, this system uses frequency division multiple access (FDMA) signals. <patcit id="pcit0008" dnum="US4222115A"><text>US 4,222,115</text></patcit> describes handoff in a cellular communication system using time-frequency coded (TFC) spread spectrum signals.

0022It is therefore an object of the present invention to provide in a cellular telephone system improvements in call handoff between cell-sites and thus provide greater service reliability.

0023According to the present invention this object is achieved by the system for directing communication links as defined in independent Claim 1, the mobile system unit as defined in independent Claim 18 and independent method Claim 8 referring to a respective method. Embodiments of the inventions are disclosed in the dependent claims.

SUMMARY OF THE INVENTION

0024In a CDMA cellular telephone 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 telephone or unit thus need not switch frequencies when handoff of the call is made from one cell-site to another. Furthermore, the probability that the call will be discontinued if the handoff command is received in error is substantially reduced.

0025In a CDMA cellular telephone system, each cell-site has a plurality of modulator-demodulator units or spread spectrum modems. Each modem consists of a digital spread spectrum transmit modulator, at least one digital spread spectrum data receiver and a searcher receiver. Each modem at the cell-site is assigned to a mobile unit as needed to facilitate communications with the assigned mobile unit. Therefore in many instances many modems are available for use while other ones may be active in communicating with respective mobile units.

0026In the present invention a handoff scheme is employed for a CDMA cellular telephone system in which a new cell-site modem is assigned to a mobile unit while the old cell-site continues to service the call. When the mobile unit is located in the transition region between the two cell-sites, the call can be switched back and forth between cell-sites as signal strength dictates. Since the mobile unit is always communicating through at least one cell-site, no disrupting effects to the mobile unit or in service will occur.

0027When mobile unit communications are firmly established with the new cell-site, e.g. the mobile unit is well within the new cell, the old cell-site discontinues servicing the call. The just described handoff techniques can be considered as a "soft" handoff in communications between cell-sites with the mobile unit. The soft handoff is in essence a make-before-break switching function. In contrast, conventional cellular telephone systems can be considered as providing a break-before-make switching function.

0028In a CDMA cellular telephone system of the present invention, a soft handoff technique is implemented which also permits the mobile unit to initiate a handoff. The mobile unit is also permitted to determine the best new cell-site to which communications are to be transferred to from an old cell-site.

0029Although it is claimed that the mobile unit initiate the handoff request and determine the new cell-site, handoff process decisions may be made as in the conventional cellular telephone system. As discussed previously with respect to conventional systems, the cell-site determines when a handoff may be appropriate and, via the system controller, request neighboring cells to search for the mobile unit signal. The cell-site receiving the strongest signal as determined by the system controller then accepts the handoff. However, such a cell-site initiated handoff is not part of the present invention.

0030In the CDMA cellular telephone system, each cell-site transmits a "pilot carrier" signal. This pilot signal is used by the mobile units to obtain initial system synchronization and to provide robust time, frequency and phase tracking of the cell-site transmitted signals.

0031Each cell-site also transmits a "setup" channel comprised of 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 offset. Phase offset allows the pilot signals to be distinguished from one another resulting in distinguishment between cell-sites from which they originate. Use of the same pilot signal code allows the mobile unit to find system timing synchronization by a single search through all pilot signal code phases. The strongest pilot signal, as determined by a correlation process for each code phase, is readily identifiable. The identified pilot signal corresponds to the pilot signal transmitted by the nearest cell-site.

0032Upon acquisition of the strongest pilot signal, i.e. initial synchronization of the mobile unit with the strongest pilot signal, the mobile unit searches for the appropriate setup channel of that cell-site. The setup 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 less codes could be used in the setup channel as determined by system parameters. The mobile unit then begins a search through all of the different codes used in the setup channel.

0033When the mobile unit identifies the appropriate setup code for that cell-site, system information is received and processed. The mobile unit further monitors the setup channel for control messages. One such control message would indicate a call is waiting for transfer to this mobile unit.

0034The mobile unit continues to scan the received pilot carrier signal code at the code offsets corresponding to neighboring cell-site transmitted pilot signals. This scanning is done in order to determine if the pilot signal emanating from neighboring cells is becoming stronger than the pilot signal first determined to be strongest. If, while in this call inactive mode, a neighbor cell-site pilot signal becomes stronger than that of the initial cell-site transmitted pilot signal, the mobile unit will acquire the stronger pilot signal and corresponding setup channel of the new cell-site.

0035When a call is initiated, a pseudonoise (PN) code address is determined for use during the course of this call. The code address may be either assigned by the cell-site or be determined by prearrangement based upon the identity of the mobile unit. After a call is initiated the mobile unit continues to scan the pilot signal transmitted by cell-sites located in neighboring cells. Pilot signal scanning continues in order to determine if one of the neighboring cell-site transmitted pilot signals becomes stronger than the pilot signal transmitted by the cell-site the mobile unit is in communication with. When the pilot signal transmitted by a cell-site located in a neighboring cell becomes stronger than the pilot signal transmitted by a cell-site in the current cell, it is an indication to the mobile unit that a new cell has been entered and that a handoff should be initiated. In response to this pilot signal strength determination, the mobile unit generates and transmits a control message to the cell-site presently servicing the call. This control message, indicative that a new cell-site transmitted pilot signal is now stronger than the current cell-site transmitted pilot signal, is provided to the system controller. The control message further contains information identifying the new cell-site and PN code. The control message as relayed to the system controller is interpreted that a handoff in mobile unit communications to the identified new cell-site is to begin.

0036The system controller now begins the handoff process. It should be understood that during handoff the PN code address of the particular mobile unit which is to undergo the handoff process need not change. The system controller begins the handoff by assigning a modem located in 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 service the call searches for and finds the mobile unit transmitted signal. The cell-site modem also begins transmitting outbound signals to the mobile unit. The mobile unit searches for the outbound signals in accordance with the signal and setup channel information provided by the new cell-site.

0037When the new cell-site modem transmitted signal is acquired, the mobile unit switches over to listening to this signal. The mobile unit then transmits a control message indicating that handoff is complete. The control message is provided by either or both of the old and new cell-site modems to the system controller. In response to this control message the system controller switches the call over to the new cell-site modem alone while discontinuing the call through the old cell-site modem. The old cell-site modem then enters a pool of idle modems available for reassignment.

0038As an additional improvement, the handoff process can introduce a second mode of operation. This second mode is referred to herein as the cell-site diversity mode. The subject matter on the cell-site diversity mode is further disclosed in copending <patcit id="pcit0009" dnum="US5109390A"><text>U.S. Patent 5,109,390</text></patcit> entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM", filed November 7, 1989, by the inventors hereof and assigned to the Assignee of the present invention.

0039In the cell-site diversity mode the call is allowed to linger in the in-between state as described above with reference to the call being processed by two cell-sites. In the exemplary embodiment described herein with reference to the mobile telephone of the present invention, a total of three demodulator processors or receivers are utilized. One of the receivers is used for the scanning function, while the two other receivers are used as a two channel diversity receiver. During operation in a single cell, the scanning receiver attempts to find the cell-site transmitted signal travelling upon multiple paths to the mobile unit. These multi-path signals are typically caused by reflections of the signals from terrain buildings, and other signals obstructions. When two or more such reflections are found, the two receivers are assigned to the two strongest paths. The scanning receiver continues to evaluate the multiple paths to keep the two receivers synchronized with signals on the two strongest paths as path conditions change.

0040In the cell-site diversity mode, the strongest two paths from each cell-site are determined by the search receiver. The two receivers are assigned to demodulate the signals on the strongest two paths of the four paths available from the original cell-site and from the new cell-site. The data demodulation process uses information from both of these receivers in a diversity combining operation. The result of this diversity combining operation is a greatly improved resistance to deleterious fading that may occur in the multipath cellular telephone environment.

0041Although different types of diversity combining techniques are known in the art, the present invention uses diversity combining to significantly advance the quality and reliability of communications in a mobile cellular telephone system. In the present invention a form of maximal ratio combining is utilized. The signal-to-noise ratio is determined for both paths being combined with the contributions from the two paths weighted accordingly. Combining is coherent since pilot signal demodulation allows the phase of each path to be determined.

0042In the path from the mobile unit to the two cell-sites, path diversity reception is also obtained by having both cell-sites demodulate the mobile unit transmitted signals. Both cell-sites forward their demodulated data signals to the system controller along with an indication of signal quality in the cell-sites receiver. The system controller then combines the two versions of the mobile unit signal and selects the signal with the best quality indication. It should be understood that it is possible to transmit the undecoded or even the undemodulated signals to the system controller in order to allow a better diversity combining process to be utilized.

0043The handoff process in the cell diversity mode is initiated as previously discussed. The mobile unit determines that a neighboring cell-site transmitted signal is of a signal strength great enough to allow good quality demodulation of the signal. The mobile unit transmits 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 cell-site identity and request to the system controller.

0044The system controller responds by connecting the call to a modem in the new cell-site. The system 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 signals received from both cell-sites are of a level sufficient to permit good quality demodulation.

0045The mobile unit continues to search for signals transmitted from other cell-sites. If a third cell-site transmitted signal becomes stronger than one of the original two cell-site signals, the control message is then transmitted by the mobile unit via at least one current cell-site to the system controller. The control message indicates the identity of this cell-site and a request for handoff. The system controller then discontinues the call being communicated via the weakest cell-site signal of the three while providing the call through the two strongest cell-sites. Should the mobile units be equipped with additional receivers, such as three receivers, a triple cell-site diversity mode may be implemented.

0046The cell-site diversity mode is terminated 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 indicative of the cell-site to remain in communication upon termination of the cell-site diversity mode. The cell-site diversity mode may also be terminated by the system controller if the system were to become overloaded with an insufficient number of modems available to support all mobile unit requests for this mode of operation. The cell-site diversity mode as discussed is implemented by decisions being made at 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 for operation in this mode being made at the system controller.

0047The present invention provides a substantial improvement over present cellular telephone systems with respect to mobile unit handoff. The make-before-break handoff mechanism of the present invention is a significant improvement in overall system reliability with lower service disruption. The implementation of a cell-site diversity mode provides further improvements over conventional cellular telephone systems by providing additional system reliability and quality in communications.

BRIEF DESCRIPTION OF THE DRAWINGS

0048The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters correspond throughout and wherein: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic overview of an exemplary CDMA cellular telephone system in accordance with the present invention;</li><li><figref idref="f0002">Figure 2</figref> is a block diagram of a mobile unit telephone configured for CDMA communications in a CDMA cellular telephone system;</li><li><figref idref="f0003">Figure 3</figref> is a block diagram of a cell-site equipment in a CDMA cellular telephone system; and</li><li><figref idref="f0004">Figure 4</figref> is a block diagram of a mobile telephone switching office equipment.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0049An exemplary telephone system in which the present invention is embodied is illustrated in <figref idref="f0001">Figure 1</figref>. The system illustrated in <figref idref="f0001">Figure 1</figref> utilizes CDMA modulation techniques in communication between the system mobile units or mobile telephones, and the cell-sites. Cellular systems in large cities may have hundreds of cell-site stations serving hundreds of thousands of mobile telephones. The use of CDMA techniques readily facilitates increases in user capacity in systems of this size as compared to conventional FM modulation cellular systems.

0050In <figref idref="f0001">Figure 1</figref>, system controller and switch 10, also referred to as mobile telephone switching office (MTSO), typically includes interface and processing circuitry for providing system control to the cell-sites. Controller 10 also 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 the mobile units, via at least one cell-site to the PSTN. Controller 10 may direct calls between mobile users via the appropriate cell-site stations since such mobile units do not typically communicate directly with one another.

0051Controller 10 may be coupled to the cell-sites by various means such as dedicated telephone lines, optical fiber links or by microwave communication links. In <figref idref="f0001">Figure 1</figref>, three such exemplary cell-sites, 12, 14 and 16 along with an exemplary mobile unit 18, which includes a cellular telephone, are illustrated. 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.

0052The cell-site service areas or cells are designed in geographic shapes such that the mobile unit will normally be closest to one cell-site. When the mobile unit is idle, i.e. no calls in progress, the mobile unit constantly monitors the pilot signal transmissions from each nearby cell-site. As illustrated in <figref idref="f0001">Figure 1</figref> the pilot signals are respectively transmitted to mobile unit 18 by cell-sites 12, 14 and 16 respectively upon communication links 20b, 22b and 24b. The mobile unit then determines which cell it is in by comparing pilot signal strength transmitted from these particular cell-sites.

0053In the example illustrated in <figref idref="f0001">Figure 1</figref>, mobile unit 18 may be considered closest to cell-site 16. When mobile unit 18 initiates a call, a control message is transmitted to the nearest cell-site, cell-site 16. Cell-site 16 upon receiving the call request message, signals system controller 10 and transfers the call number. System controller 10 then connects the call through the PSTN to the intended recipient.

0054Should a call be initiated within the PSTN, controller 10 transmits the call information to all the cell-sites in the area. The cell-sites in return transmit a paging message to the intended recipient of mobile unit. When the mobile unit hears a page message, it responds with a control message that is transmitted to the nearest cell-site. This control message signals the system controller that this particular cell-site is in communication with the mobile unit. Controller 10 then routes the call through this cell-site to the mobile unit.

0055Should mobile unit 18 move out of the coverage area of the initial cell-site, cell-site 16, an attempt is made to continue the call by routing the call through another cell-site. In the handoff process there are two different methods of initiating the handoff of the call or routing through another cell-site.

0056The first method, called the cell-site initiated handoff, is similar to the handoff method employed in the original first generation analog cellular telephone systems currently in use. In the cell-site initiated handoff method, the initial cell-site, cell-site 16, notices that the signal transmitted by mobile unit 18 has fallen below a certain threshold level. Cell-site 16 then transmits a handoff request to system controller 10. Controller 10 relays the request to all neighboring cell-sites, 14, 12 of cell-site 16. The controller transmitted request includes information relating to the channel, 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 cell-sites 12 and 14 receivers report a stronger signal than the initial cell-site reported signal strength, then a handoff is made to this cell-site.

0057The second method of initiating a handoff is called the mobile initiated handoff. According to the present invention, the second method of initiating a handoff is used. The mobile unit is equipped with a search receiver which is used to scan the pilot signal transmission of neighboring cell-sites 12 and 14, in addition to performing other functions. If a pilot signal of cell-sites 12 and 14 is found to be stronger than the pilot signal of cell-site 16, mobile unit 18 transmits a control message to the current cell-site, cell-site 16. This control message contains information identifying the cell-site of greater signal strength in addition to information requesting a handoff of this cell-site. Cell-site 16 transfers this control message to controller 10.

0058The mobile initiated handoff method has various advantages over the cell-site initiated handoff method. The mobile unit becomes aware of changes in paths between itself and the various neighboring cell-sites much sooner and with less effort than the cell-sites are capable of doing. However, to perform a mobile initiated handoff, each mobile unit must be provided with a searching receiver to perform the scan function. However, in the exemplary embodiment described herein of a mobile unit CDMA communications capability, the search receiver has additional functions which require its presence.

0059<figref idref="f0002">Figure 2</figref> illustrates in block diagram form an exemplary mobile unit cellular telephone. The mobile unit includes an antenna 30 which is coupled through diplexer 32 to analog receiver 34 and transmit power amplifier 36. Antenna 30 and diplexer 32 are of standard design and permit simultaneous transmission and reception through a single antenna. Antenna 30 collects transmitted signals and provides them through diplexer 32 to analog receiver 34. Receiver 34 receives the RF frequency signals from diplexer 32 which are typically in the 850 MHz frequency band for amplification and frequency downconversion to an IF frequency. This frequency translation process is accomplished using a frequency synthesizer of standard design which permits the receiver to be tuned to any of the frequencies within the receive frequency band of the overall cellular telephone frequency band.

0060The IF signal is then passed through a surface acoustic wave (SAW) bandpass filter which in the preferred embodiment is approximately 1.25 MHz in bandwidth. The characteristics of the SAW filter are chosen to match the waveform of the signal transmitted by the cell-site which has been direct sequence spread spectrum modulated by a PN sequence clocked at a predetermined rate, which in the preferred embodiment is 1.25 MHz. This clock rate is chosen to be an integer multiple of a number of common data rates such as 16 kbps, 9.6 kbps, and 4.8 kbps.

0061Receiver 34 also performs a power control function for adjusting the transmit power of the mobile unit. Receiver 34 generates an analog power control signal that is provided to transmit power control circuitry 38. The control and operation of the mobile unit power control feature is disclosed in copending <patcit id="pcit0010" dnum="US5056109A"><text>U.S. Patent 5,056,109</text></patcit> entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", filed November 7, 1989, by the inventors hereof and assigned to the Assignee of the present invention.

0062Receiver 34 is also provided with an analog to digital (A/D) converter (not shown) for converting the IF signal to a digital signal with conversion occurring at a 9.216 MHz clock rate in the preferred embodiment which is exactly eight times the PN chip rate. The digitized signal is provided to each of two or more signal processors or data receivers, one of which is a searcher receiver with the remainder being data receivers.

0063In <figref idref="f0002">Figure 2</figref>, the digitized signal output from receiver 34 is provided to digital data receivers 40 and 42 and to searcher receiver 44.

0064The digitized IF signal may contain the signals of many on-going calls together with the pilot carriers transmitted by the current and all neighboring cell-sites. The function of the receivers 40 and 42 are to correlate the IF samples with the proper PN sequence. This correlation process provides a property that is well-known in the art as "processing gain" which enhances the signal-to-interference ratio of a signal matching the proper PN sequence while not enhancing other signals. Correlation output is then synchronously detected using the pilot carrier from the closest cell-site as a carrier phase reference. The result of this detection process is a sequence of encoded data symbols.

0065A property of the PN sequence as used in the present invention is that discrimination is provided against multipath signals. When the signal arrives at the mobile receiver after passing through more than one path, there will be a difference in the reception time of the signal. This reception time difference corresponds to the difference in distance divided by the speed of light. If this time difference exceeds one microsecond, then the correlation process will discriminate against one of the paths. The receiver can choose whether to track and receive the earlier or later path. If two receivers are provided, such as receivers 40 and 42, then two independent paths can be tracked and in parallel.

0066Searcher receiver 44, under control of control processor 46 is for continuously scanning the time domain, around the nominal time of a received pilot signal of the cell-site, for other multi-path pilot signals from the same cell-site and for other cell-site transmitted pilot signals. Receiver 44 will measure the strength of any reception of a desired waveform at times other than the nominal time. Receiver 44 compares signal strength in the received signals. Receiver 44 provides a signal strength signal to control processor 46 indicative of the strongest signals.

0067Processor 46 provides signals to digital data receivers 40 and 42 for each to process a different one of the strongest signals. On occasion another cell-site transmitted pilot signal is of greater signal strength than the current cell-site signal strength. Control processor 46 then would generate a control message for transmission to the system controller via the current cell-site requesting a transfer of the cell to the cell-site corresponding to the stronger pilot signal. Receivers 40 and 42 may therefor handle calls through two different cell-sites.

0068The outputs of receivers 40 and 42 are provided to diversity combiner and decoder circuitry 48. The diversity combiner circuitry contained within circuitry 48 simply adjusts the timing of the two streams of received signals into alignment and adds them together. This addition process may be proceeded by multiplying the two streams by a number corresponding to the relative signal strengths of the two streams. This operation can be considered a maximal ratio diversity combiner. The resulting combined signal stream is then decoded using a forward stream error detection decoder also contained within circuitry 48.

0069In the exemplary embodiment convolutional encoding is utilized. The convolutional encoding has a constraint length 9 and a code rate 1/3, i.e. three encoded symbols are produced and transmitted for every information bit to be transmitted. The optimum decoder for this type of code is of the soft decision Viterbi algorithm decoder design. The resulting decoded information bits are passed to the user digital baseband circuitry 50.

0070Baseband circuitry 50 typically includes a digital vocoder (not shown). Baseband circuitry 50 further serves as an interface with a handset or any other type of peripheral device. Baseband circuitry 50 accommodates a variety of different vocoder designs. Baseband circuitry 50 provides output information signals to the user in accordance with the information provided thereto from circuitry 48.

0071User analog voice signals typically provided through a handset are provided as an input to baseband circuitry 50. Baseband circuitry 50 indudes an analog to digital (A/D) converter (not shown) which converts the analog signal to digital form. 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 voice digitized encoded signal is output from baseband circuitry 50 to transmit modulator 52.

0072Transmit modulator 52 modulates the encoded signal on a PN carrier signal whose PN sequence is chosen according to the assigned address function for the call. The PN sequence is determined by control processor 46 from call setup information that is transmitted by the cell-site and decoded receivers 40 and 42. In the alternative, control processor 46 may determine the PN sequence through prearrangement with the cell-site. Control processor 46 provides the PN sequence information to transmit modulator 52 and to receivers 40 and 42 for call decoding.

0073The output of transmit modulator 52 is provided to transmit power control circuitry 38. Signal transmission power is controlled by the analog power control signal provided from receiver 34. Furthermore, control bits are transmitted by the cell-sites in the form power adjustment command and are processed by data receivers 40 and 42. The power adjustment command is used by the control processor in setting the power level in mobile unit transmission. In response to the power adjustment commands, control processor 46 generates a digital power control signal that is provided to circuitry 38. Further information on the interrelationship of the receivers 40 and 42, control processor 46 and transmit power control 35 are also further described in the above-mentioned copending <patcit id="pcit0011" dnum="US5056109A"><text>U.S. Patent 5,056,109</text></patcit>.

0074Transmit power control circuitry 38 outputs the power controlled modulated signal to transmit power amplifier circuitry 36. Circuitry 36 amplifies and converts the IF signal to an RF frequency by mixing with a frequency synthesizer output signal which tunes the signal to the proper output frequency. Circuitry 36 includes an amplifier which amplifies the power to a final output level. The intended transmission signal is output from circuitry 36 to diplexer 32. Diplexer 32 couples the signal to antenna 30 for transmission to the cell-sites.

0075Control 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 the data received from data receivers 40, 42 and search receiver 44 for making decisions relative to handoff and diversity combining.

0076<figref idref="f0003">Figure 3</figref> illustrates in block diagram form an exemplary embodiment of the cell-site equipment. At the cell-site, two receiver systems are utilized with each having a separate antenna and analog receiver for space diversity reception. In each of the receiver systems the signals are processed identically until the signals undergo a diversity combination process. The elements within the dashed lines correspond to elements corresponding to the communications between the cell-site and one mobile unit. The output of the analog receivers are also provided to other elements used in communications with other mobile units.

0077In <figref idref="f0003">Figure 3</figref>, the first receiver system is comprised of antenna 60, analog receiver 62, searcher receiver 64 and digital data receiver 66. This receiver system may also include an optional digital data receiver 68. The second receiver system includes antenna 70, analog receiver 72, searcher receiver 64 and digital data receiver 66. Also utilized in signal processing and control for handoff and diversity is cell-site control processor 78. Both receiver systems are coupled to diversity combiner and decoder circuitry 80. Digital link 82 is utilized to communicate signals to and from the MTSO (<figref idref="f0004">Figure 4</figref>) with cell-site transmit modulator 84 and circuitry 80 under the control of control processor 78.

0078Signals received on antenna 60 are provided to analog receiver 62. Received signals amplified by an amplifier in receiver 62 are translated to an IF frequency by mixing with a frequency synthesizer output signal. The IF signals are bandpass filtered and digitized in a process identical to that described with reference to the mobile unit analog receiver. The digitized IF signals are provided to digital data receiver 66, optional data receiver 68 and searcher receiver 64 and are processed respectively in a manner similar to that as disclosed with reference to the digital data receivers and searcher receiver of the mobile unit in <figref idref="f0002">Figure 2</figref>. However, the processing by the digital data receivers and searcher receivers are different for the mobile to cell-site link from that used in the cell-site to mobile link in several respects.

0079In the inbound, or mobile unit to cell-site link, the mobile unit does not transmit a pilot signal that can be used for coherent reference purposes in signal processing at the cell-site. Thus, the mobile unit to cell-site link utilizes a non-coherent modulation and demodulation scheme using 64-ary orthogonal signalling.

0080Searcher receiver 64 is again used to scan the time domain about the receiver signal to ensure that the associated digital data receiver 66, and data receiver 68 if used, are tracking and processing the strongest available time domain signal. This tracking process is identical to that described with reference to the mobile unit. Searcher receiver 64 provides a signal to cell-site control processor 78 which provides control signals to digital data receivers 66 and 65 for selecting the appropriate received signal for processing.

0081In the 64-ary orthogonal signalling process, the mobile unit transmitted symbol has one of 64 different possibilities. A 6 bit symbol is encoded into one of 26, i.e. 64, different binary sequences. The set of sequences chosen are known as Walsh functions. The optimum receive function for the Walsh function is the Fast Hadamard Transform (FHT). In searcher receiver 64 and digital data receivers 66 and 68, the input signal is correlated as discussed with reference to the mobile unit receivers, with the correlator output fed to a FHT processor. The FHT processor produces a set of 64 coefficients for every 6 symbols. The 64 symbols are then multiplied by a weighting function generated in the receiver. The weighting function is linked to measured signal strength. The weighted data is then provided as an output to diversity combiner and decoder circuitry 50.

0082The second receiver system processes the received signals in a manner similar to that discussed with respect to the first receiver system of <figref idref="f0003">Figure 3</figref>. The weighted 64 symbols output from receivers 66 and 76 are provided to diversity combiner and decoder circuitry 80. Circuitry 80 includes an adder which adds the weighted 64 symbols from receiver 66 to the weighted 64 symbols from receiver 76. The resulting 64 coefficients are compared with one another in order to determine the largest coefficient. The magnitude of the comparison result, together with the identity or the largest of the 64 coefficients, is used to determine a set of decoder weights and symbols for use within a Viterbi algorithm decoder implemented in circuitry 80.

0083The Viterbi decoder is preferably of a constraint length 9, and of a code rate 1/2. The Viterbi decoder is utilized to determine the most likely information bit sequence. For each vocoder data block, nominally 15 msec of data, a signal quality estimate is obtained and transmitted as a mobile unit power adjustment command along with data to the mobile unit. Further information on the generation of this quality estimate is discussed in further detail in the co-pending <patcit id="pcit0012" dnum="US5056109A"><text>U.S. Patent 5,056,109</text></patcit> mentioned above. This quality estimate is the average signal-to-noise ratio over the 15 msec interval.

0084In <figref idref="f0003">Figure 3</figref>, optional digital data receiver 68 may be included for improved performance of the system. This additional data receiver alone or in combination with additional receivers can track and receive other possible delay paths of mobile unit transmitted signals. The structure and operation in this receiver is similar to that described with reference to the digital data receivers 66 and 76. Receiver 68 is utilized to obtain additional diversity modes. Optional additional digital data receivers providing additional diversity modes are extremely useful in those cell-sites which are located in dense urban areas where many possibilities for multipath signals occur.

0085Signals from the MTSO are coupled to the appropriate transmit modulator via digital link 82 under control of control processor 78. Transmit modulator 84 spread spectrum modulates, according to a predetermined spreading function as assigned by control processor 78, the data for transmission to the intended recipient mobile unit. The output of transmit modulator 84 is provided to transmit power control circuitry 86 where under the control of control processor 78 the transmission power may be controlled. The output of circuitry 86 is provided to transmit power amplifier circuitry 88.

0086Circuitry 88 includes a summer for summing the output of transmit modulator 84 with the output of other transmit modulators at the cell-site. Circuitry 88 further includes a summer for summing the pilot signal output from pilot signal generator 90 with the summed transmit modulator output signals. Circuitry 88 also includes a digital to analog converter, frequency upconversion circuitry and an amplifier for respectively converting the digital signals to analog signals, converting the IF frequency signals as output from the transmit modulators to an RF frequency and amplifying the RF signal. The output from circuitry 88 is provided to antenna 92 where it is radiated to mobile units within the cell-site service area.

0087Cell-site control processor 78 has the responsibility for assignment of digital data receivers and modulators to a particular call. Control processor 78 also monitors the progress of the call, quality of the signals and initiates teardown on loss of signal. The cell-site communicates with the MTSO via link 82 where it is coupled by a standard telephone wire, optical fiber, or microwave link.

0088<figref idref="f0004">Figure 4</figref> illustrates in block diagram form the equipment utilized in the MTSO. The MTSO typically includes a system controller or system control processor 100, digital switch 102, diversity combiner 104, digital vocoder 106 and digital switch 108. Although not illustrated, additional diversity combiners and digital vocoders are coupled between digital switches 102 and 108.

0089When the cell-diversity mode is active, or the MTSO is in the handoff process with the call processed by two cell-sites, signals will arrive at the MTSO from more than one cell-site with nominally the same information. However, because of fading and interference on the inbound 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.

0090Digital switch 102 is used in routing the information stream corresponding to a given mobile unit from one or more cell-sites to diversity combiner 104 or the corresponding diversity combiner as determined by a signal from system control processor 100. When the system is not in the cell-diversity mode, diversity combiner 104 may be either bypassed or fed the same information on each input port.

0091A multiplicity of serial coupled diversity combiners and vocoders are provided in parallel, nominally, one for each call to be processed. Diversity combiner 104 compares the signal quality indicators accompanying the information bits from the two or more cell-site signals. Diversity combiner 104 selects the bits corresponding to the highest quality cell-site signal on a frame-by-frame basis of the information for output to vocoder 90.

0092Vocoder 106 converts the format of the digitized voice signal to standard 64 kbps PCN telephone format, analog, or any other standard format. The resultant signals are transmitted from vocoder 106 to digital switch 108. Under the control of system control processor 100, the call is routed to the PSTN.

0093Voice signals coming from the PSTN intended for the mobile unit are provided to digital switch 108 to an appropriate digital vocoder such as vocoder 106 under control of system control processor 100. Vocoder 106 encodes the input digitized voice signals and provides the resulting information bit stream directly to digital switch 102. Digital switch 102 under system control processor control directs the encoded data to the cell-site or cell-sites to which the mobile unit is communicating. If the mobile unit is in a handoff mode communicating to multiple cell-sites or in a cell diversity mode, digital switch 102 routes the calls to the appropriate cell-sites for transmission by the appropriate cell-site transmitter to the intended recipient mobile unit. However, if the mobile unit is communicating with only a single cell-site or not in a cell diversity mode, the signal is directed only to a single cell-site.

0094System control processor 100 provides control over digital switches 102 and 106 for routing data to and from the MTSO. System control processor 100 also determines the assignment of calls to the cell-sites and to the vocoders at the MTSO. Furthermore, system control processor 100 communicates with each cell-site control processor about the assignment of particular calls between the MTSO and cell-site, and the assignment of PN codes for the calls. It should be further understood that as illustrated in <figref idref="f0004">Figure 4</figref> digital switches 102 and 106 are illustrated as two separate switches, however, this function may be performed by a single physical switching unit.