Method and system for providing a soft handoff in communications in a CDMA cellular telephone system

Claims

We claim:

1. In a cellular telephone system in which a mobile system user and another system user communicate user information signals therebetween via at least one of a plurality of geographically separated cell-sites each defining a respective geographic service area, a system for directing communications between said mobile system user and said another system user via said at least one of said plurality of cell-sites as said mobile system user changes cell-site service areas, comprising:

means for, while said mobile system user is in a service area of one cell-site and communicating user information signals with said another system user via said one cell-site, determining a transition of said mobile system user from said one cell-site service area to a service area of another cell-site, and for providing a handoff request identifying said another cell-site;

means responsive to said handoff request for coupling a communication of said user information signals between said mobile system user and said another system user via said another cell-site while said mobile system user and said another system user continue in communication of said user information signals via said one cell-site such that said mobile system user and said another system user concurrently communicate said user information signals through said one cell-site and said another cell-site; and

means responsive to said coupling of communication of said user information signals between said mobile system user and said another system user via said another cell-site for terminating said communication of said user information signals between said mobile system user and said another system user via said one cell-site with said communication of said user information signals continuing between said mobile system user and said another system user via said another cell-site.

2. The system of claim 1 wherein each cell-site transmits a pilot signal indicative of said transmitting cell-site, said means for determining comprising:

scanning receiver means located at said mobile system user for when said mobile system user is in a transition from said one cell-site service are to said another cell-site service area, receiving said cell-site transmitted pilot signals, measuring signal strength of each received pilot signal, comparing pilot signal strength measurements, and providing a signal strength signal indicative of a received pilot signal of greatest measured signal strength and cell-site of origin; and

processing means located at said mobile system user for receiving said signal strength signal and for, when said signal strength signal changes from indicating said one cell-site transmitted pilot signal is of a greater signal strength than said another cell-site transmitted pilot signal to indicating said another cell-site transmitted pilot signal is of a signal strength greater than said one cell-site transmitted pilot signal, generating said handoff request.

3. The system of claim 2 wherein said mobile system user communicates said handoff request to said one cell-site for coupling to a system controller, wherein said system controller comprises said means for coupling, and said means for coupling comprises:

system processing means for receiving said handoff request and responsive thereto for generating a first switch command; and

switching means for, under control of said system processing means, coupling said communication of said user information signals between said mobile system user and said another system user via said one cell-site and responsive to said first switch command for coupling said communication of said user information signals between said mobile system user and said another system user via said another cell-site.

4. The system of claim 3 wherein said means for terminating comprises:

said processing means being further responsive to said communication of user information signals between said mobile system user and said another system user via said another cell-site for generating a handoff complete command, said mobile system user communicating said handoff complete command to said system controller via at least one of said one cell-site and said another cell-site;

said system processing means being further responsive to said handoff complete command for generating a second switch command; and

said switching means being further responsive to said second switch command for decoupling communication of said user information signals between said mobile system user and said another system user via said one cell-site.

5. In a cellular radio communication system, a method for handing off communications between a mobile system user and another system user from one cell to another cell comprising the steps of:

directing by a system controller a relay of user information signals between a mobile system user and another system user through one cell-site;

determining, by said mobile system user while said mobile system user and said another system user are communicating said user information signals through said one cell-site, a transition of said mobile system user from a service area of said one cell-site to a service area of another cell-site;

generating, by said mobile user in response to said transition determination, a first control signal identifying said another cell-site;

communicating said first control signal from said mobile system user through said one cell-site to said system controller;

directing, by said system controller in response to said first control signal, a communication of said user information signals between said mobile system user and said another system user through said another cell-site concurrently with said communication of said user information signals between said mobile system user and said another system user through said one cell-site.

6. The method of claim 5 further comprising the steps of:

generating, by said mobile system user in response to said communication of said user information signals between said another system user and said mobile system user via said another cell-site, a second control signal;

communicating said second control signal from said mobile system user to said system controller via at least one of said one cell-site and said another cell-site; and

terminating by said system controller a directing of said user information signals to said one cell-site.

7. The method of claim 5 wherein said user information signals communicated by said mobile system user, said one cell-site and said another cell-site are spread spectrum modulated according to predetermined spreading codes.

8. The method of claim 6 wherein said user information signals communicated by said mobile system user, said one cell-site and said another cell-site are spread spectrum modulated according to predetermined spreading codes.

9. In a cellular radio telephone system in which mobile users are capable of initiating and receiving calls to and from other mobile users and users in a public telephone system wherein said calls are relayed through at least one of a plurality of cell-sites under the control of a system controller, each of said calls comprised of user information signals which are spread spectrum modulated according to a different predetermined user information signal spreading code as communicated between said at least one cell-site and a corresponding mobile user, and wherein each cell-site transmits a pilot signal that is spread spectrum modulated according to a pilot signal spreading code common to all cell-sites and each cell-site pilot signal is of a different predetermined code phase with respect to a neighboring cell-site pilot signal, a method for directing the relay of communications between a mobile user and another user through said cell-sites as said mobile user changes service areas of said cell-sites, said method comprising the steps of:

communicating user information signals between a mobile user and another user through a first cell-site;

transmitting a pilot signal by each of said first cell-site and a second cell-site;

receiving at said mobile user said first and second cell-site transmitted pilot signals;

determining at said mobile user relative pilot signal strength of said pilot signals as received at said mobile user;

generating, at said mobile user in response to said determination of relative pilot signal strength, a handoff request when said second cell-site transmitted pilot signal is of a predetermined level with respect to said first cell-site transmitted pilot signal;

communicating said handoff request to said system controller via said first cell-site;

assigning by said system controller said second cell-site to relay said communication of user information signals between said mobile user and said another user; and

communicating said user information signals between said mobile user and said another user through said second cell-site wherein said mobile user and said another user concurrently communicate through both said first and second cell-sites.

10. The method of claim 9 further comprising the steps of:

detecting at said mobile user said communication of said user information signals as relayed through said second cell-site;

generating a handoff complete command at said mobile user in response to said detection of said second cell-site relayed communication of said user information signals;

transmitting said handoff complete command to at least one of said first and second cell-sites; and

terminating in response to said handoff complete command said communication of user information between said mobile user and said another system user through said first cell-site.

11. The method of claim 9 further comprising the steps of:

generating a cell-diversity mode request at said first cell-site;

relaying said cell-diversity mode request to said mobile user; and

wherein said mobile user is responsive to said cell-diversity mode request for maintaining communication with said another user through said first and second cell-sites.

12. The method of claim 9 wherein the step of communicating said user information signals through said first cell-site comprises the steps of:

receiving at said system controller an another user information signal from said another user;

coupling said another user information signal from said system controller to said first cell-site;

receiving at said first cell-site from said system controller said another user information signal;

modulating at said first cell-site said another user information signal according to a predetermined another user information signal spreading code so as to provide a first spread spectrum signal;

transmitting by said first cell-site said first spread spectrum signal;

receiving at said mobile user said first spread spectrum signal;

despreading, at said mobile user according to said predetermined another user information signal spreading code, said received first spread spectrum signal so as to provide said another user information signal to said mobile user;

modulating, at said mobile user, a mobile user information signal according to a predetermined mobile user information signal spreading code so as to provide a second spread spectrum signal;

transmitting by said mobile user said second spread spectrum signal;

receiving at said first cell-site said second spectrum signal;

despreading, at said first cell-site according to said predetermined mobile user information signal spreading code, said received second spread spectrum signal so as to provide said mobile user information signal;

coupling said mobile user information signal to said system controller from said first cell-site;

receiving at said system controller said mobile user information signal from said first cell-site; and

providing said mobile user information signal from said system controller to said another user.

13. The method of claim 9 wherein the step of determining relative pilot signal strength comprises the steps of:

measuring pilot signal strength of said received pilot signals at said mobile user;

comparing at said mobile user said pilot signal strength measurements; and

identifying one of said pilot signals of greatest signal strength.

14. The method of claim 9 wherein the step of communicating said handoff request comprises the steps of:

transmitting said handoff request to said first cell-site; and

relaying by said first cell-site said handoff request to said system controller.

15. The method of claim 9 wherein the step of assigning said second cell-site to said communication of user information signals comprises the steps of:

determining from said handoff request an identification of said second cell-site;

communicating to said second cell-site an assignment of said second cell-site in communicating said user information signals between said mobile user and said another user; and

selecting at said second cell-site one of a plurality of modems at said second cell-site for communication of said user information signals between said mobile user and said another user.

16. The method of claim 12 wherein the step of communicating said user information signals through said second cell-site comprises the steps of:

coupling said another user information signal from said system controller to said second cell-site;

receiving at said second cell-site from said system controller said another user information signal;

modulating at said second cell-site said another user information signal according to said another user information signal spreading code so as to provide a third spread spectrum signal;

transmitting by said second cell-site said third spread spectrum signal;

receiving at said mobile user said third spread spectrum signal;

despreading, at said mobile user according to said another user information signal spreading code, said received third spread spectrum signal;

combining said despread first and third spread spectrum signals so as to provide said another user information signal to said mobile user;

receiving at said second cell-site said second spread spectrum signal;

despreading, at said second cell-site according to said mobile user information signal spreading code, said second cell-site received second spread spectrum signal so as to provide at said second cell-site said mobile user information signal;

coupling said mobile user information signal to said system controller from said second cell-site;

receiving at said system controller said mobile user information signals from said second cell-site;

combining said at said system controller said mobile user information signals received from each of said first and second cell-sites; and

providing said combined mobile user information signals from said system controller to said another user.

17. The method of claim 10 wherein the step of terminating said communication of user information signals between said mobile user and said another system user through said first cell-site comprises the steps of:

relaying by said at least one of said first and second cell-sites said handoff complete command to said system controller;

decoupling by said system controller a communication of said user information signals between said another user and said first cell-site;

discontinuing by said first cell-site a communication of said user information signals from said another user to said mobile user;

generating a termination command at said first cell-site;

transmitting said termination command to said mobile user; and

discontinuing by said mobile user a communication of said user information signals from said mobile user to said first cell-site.

18. In a mobile radio telephone system in which mobile users are capable of initiating and receiving calls comprised of information signals to and from other mobile users and users in a public telephone system wherein said calls are relayed through at least one of a plurality of cell-sites under the control of a system controller and wherein each cell-site has a corresponding service area, a system for directing the relay of communications between a mobile user and another user through said cell-sites as said mobile user changes cell-site service areas, said system comprising:

means at each cell-site of a plurality of cell-sites for generating and transmitting a pilot signal indicative of each cell-site of origin;

means at said system controller for, receiving a user information signal from a user intended for a mobile user and coupling said user information signal to a first cell-site;

means at said first cell-site for, receiving said user information signal from said system controller, spread spectrum modulating said user information signals according to a first predetermined user information signal spreading code and transmitting said spread spectrum modulated user information signal to an intended recipient mobile user;

means at said mobile user for receiving and despreading said first cell-site transmitted spread spectrum modulated user information signal according to said first spreading code so as to provide a first resultant user information signal to said mobile user;

means at said mobile user for spread spectrum modulating a mobile user information signal intended for said user according to a second spreading code and transmitting said spread spectrum modulated mobile user information signal to said first cell-site;

means at said mobile user for, receiving said pilot signals transmitted by said first cell-site and a second cell site, determining relative pilot signal strength of each received pilot signal, generating a handoff request when said second cell-site transmitted pilot signal is of a predetermined level with respect to said first cell-site transmitted pilot signal, and transmitting said handoff request to said first cell-site;

means at said first cell-site for, receiving said handoff request and said spread spectrum modulated mobile user information signal, despreading said spread spectrum modulated mobile user information signal according to said second spreading code so as to provide a first resultant mobile user information signal, and for coupling said handoff request and said first resultant mobile user information signal to said system controller;

means at said system controller for, receiving said handoff request and said first resultant mobile user information signal from said first cell-site, coupling said first resultant mobile user information signal to said user, and said means for coupling said user information signal to said first cell-site being further responsive to said handoff request for coupling said user information signal to said second cell-site;

means at said second cell-site for, receiving said user information signal from said system controller, spread spectrum modulating said user information signal intended for said mobile user according to said first spreading code and transmitting said spread spectrum modulated user information signal to said mobile user;

said means at said mobile user for receiving and despreading said first cell-site transmitted spread spectrum modulated user information signal further for receiving and despreading said second cell-site transmitted spread spectrum modulated user information signal according to said first spreading code so as to provide a second resultant user information signal, combining said first and second resultant user information signals so as to provide a combined user information signal to said mobile user, and wherein said user contemporaneously communicates with said mobile user through both said first and second cell-sites;

said means at said mobile user for spread spectrum modulating said mobile user information signal further for transmitting said spread spectrum modulated mobile user information signal to said second cell-site;

means at said second cell-site for, receiving said spread spectrum modulated mobile user information signal, despreading said spread spectrum modulated mobile user information signals according to said second spreading code so as to provide a second resultant mobile user information signal, and for coupling said second resultant mobile user information signal to said system controller; and

said means at said system controller for receiving said first resultant mobile user information signal from said first cell-site further for receiving said second resultant mobile user information signal from said second cell-site, combining said first and second resultant mobile user information signals so as to provide a combined mobile user information signal, and for coupling said combined mobile user information signal to said user in place of said first resultant mobile user information signal, and wherein said mobile user contemporaneously communicates with said user through both said first and second cell-sites.

19. The system of claim 18 further comprising:

said means at said mobile user for generating said handoff request further for detecting a coupling of communications between mobile user and said user, generating a handoff complete command and transmitting said handoff complete command to at least one of said first and second cell-sites;

said means at said first and second cell-sites for receiving, despreading and coupling said first and second resultant mobile user information signals to said system controller further for receiving and coupling said handoff complete command to said system controller;

said means at said system controller for receiving and combining said first and second resultant mobile user information signals from said first and second cell-sites further for, receiving said handoff complete command from said at least one of said first and second cell-sites and responsive thereto for coupling only said second resultant mobile user information signal to said user; and

said means at said system controller for coupling said user information signal to said first and second cell-sites responsive to said received handoff command for decoupling said user information signal to said first cell-site.

20. The system of claim 18 wherein each cell-site spread spectrum modulates each pilot signal according to a pilot signal spreading code common to all cell-sites and cell-site each pilot signal is of a different predetermined code phase with respect to each neighboring cell-site pilot signal.

21. In a cellular radio telephone system in which mobile users are capable of initiating and receiving calls to and from other mobile users and users in a public telephone system wherein said calls are relayed through at least one of a plurality of cell-sites under the control of a system controller, each of said calls comprised of information signals which are spread spectrum modulated according to a different predetermined information signal spreading code as communicated between a user and a corresponding mobile user via at least one cell-site, and wherein each cell-site transmits a pilot signal that is spread spectrum modulated according to a pilot signal spreading code common to all cell-sites and each cell-site pilot signal is of a different predetermined code phase with respect to a neighboring cell-site pilot signal, a method for directing the relay of communications between a mobile user and another user through said cell-sites as said mobile user changes service areas of said cell-sites, said method comprising the steps of:

communicating user information signals between a mobile user and another user through a first cell-site;

determining at said first cell-site signal strength of said user information signals as received at said first cell-site from said mobile user;

generating, at said first cell-site in response to said determination of signal strength, a handoff request when said signal strength fall below a predetermined level;

communicating said handoff request to said system controller;

assigning by said system controller each cell-site of a plurality of cell-sites neighboring said first cell-site to determine signal strength of said user information signals as received at said each respective neighboring cell-site from said mobile user;

determining at each respective neighboring cell-site signal strength of said user information signals as received at said each respective neighboring cell-site from said mobile user; and

reporting to said system controller by each respective neighboring cell-site signal strength of said user information signals as received at said each respective neighboring cell-site from said mobile user;

communicating said user information signals between said mobile user and said another user through at least one of said neighboring cell-sites wherein said mobile user and said another user concurrently communicates through both said first cell-site and said at least one of said neighboring cell-sites.

22. The method of claim 21 further comprising the steps of:

detecting said communication of said user information signals through said at least one of said neighboring cell-sites; and

terminating said communication of user information signals between said mobile user and said another system user through said first cell-site.

23. The method of claim 21 further comprising the steps of:

generating a cell-diversity mode request at said first cell-site;

relaying said cell-diversity mode request to said mobile user; and

wherein said mobile user is responsive to said cell-diversity mode request for maintaining communication with said another user through said first cell-site and said at least one of said neighboring cell-sites.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The 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

The 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 U.S. patent application Ser. No. 06/921,261, filed Oct. 17, 1986, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", now U.S. Pat. No. 4,901,307 assigned to the Assignee of the present invention, the disclosure thereof incorporated by reference.

In 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.

In 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.

In 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.

The 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.

Handoff 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.

In 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.

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 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.

It 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.

SUMMARY OF THE INVENTION

In 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.

In 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. In 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.

When 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.

In 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. Although it is preferred 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.

In 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.

Each 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.

Upon 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.

When 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.

The 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.

When 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.

The 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 assisting in the assignment of 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 an outbound signal to the mobile unit. The mobile unit searches for this outbound signal in accordance with the signal and setup channel information provided by the new cell-site. When 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 con