Soft handoff in a cdma cellular telephone system

SOFT HAND SWITCHING IN CDMA CELL PHONE SYSTEM

The invention relates to a cellular telephone system. More particularly, the invention relates to a new and improved system for controlling handover in cellular communications with mobile units in a code division multiple access (CDMA) cellular telephone system.

The use of code division multiple access (CDMA) modulation techniques is one of many techniques for enabling communication when a large number of system users, i.e., media, are present. While other techniques are known, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and AM modulation systems, such as amplitude companded single sideband (ACSSB), CDMA Has significant advantages over these other modulation techniques. The use of CDMA technology in a plurality of access communication systems is described in U.S. Patent Application 06 / 921,261, Oct. 17, 1986, entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS, now U.S. Pat.

4,901,301, the same as the present application, and the contents of which are incorporated herein by reference.

Said Patent Specification discloses a multiple access technique wherein mobile stations in a cellular telephone system, each having a transceiver, communicate via satellite repeater stations or ground base stations (also known as cell-site stations or briefly cellular stations) using code division multiple access (CDMA) wide bandwidth. When using CDMA communications, the frequency spectrum can be reused several times, allowing for increased system user capacity. The use of CDMA technology results in much higher spectral efficiency than can be achieved

08974 achieved by other multiple access technologies.

In conventional cellular telephone systems, the available bandwidth is channeled into channels typically having a bandwidth of 30 kHz using analog FM modulation technology. The system service area is geographically divided into cells of different sizes. The available frequency channels are divided into groups, each group usually containing the same number of channels. Frequency groups are assigned to cells in order to minimize the possibility of parallel channel interference. Consider, for example, a system with seven frequency groups and cells of equal size hexagons. The frequency group used in one cell is not used in the six nearest or surrounding neighboring cells of that cell. Further, the frequency group of one cell is not used in the twelve closest neighboring cells of that cell.

The handover system used in a conventional cellular telephone system is intended to allow 20 land calls to continue when a cellular phone crosses a border between two cells. A handover from one cell to another is initiated when a cellular receiver handling a call detects that the strength of the signal received from the cellular telephone drops below a predetermined threshold. A low signal strength indicates that the cell phone must be close to the cell boundary. When the signal level drops below a predetermined threshold, the cell site asks the system controller to determine whether the adjacent cell station receives a cellular signal having a better signal strength than the current cell site.

In response to a query of the current cell station, the system controller sends messages to adjacent cell 35 stations with a handover request. The cell site adjacent to the current cell site uses special scan receivers that look for a signal to the mobile station108974 on a specific channel. If one of the adjacent cell stations reports a sufficient signal level to the system controller, then handover is attempted.

The handover is initiated when the free channel group channel used in the new cell5 station is selected. A control message is sent to the mobile phone commanding it to switch from the current channel to the new channel. Simultaneously, the system controller switches the call from the first cell site to the second cell site.

In a conventional system, the call is terminated # j °^s handover to new cell site fails. There are many reasons why a handover failure may occur. The handover may fail if there is no free channel available to transfer the call in the adjacent cell. The handover may also fail if another cellular station reports that it is hearing the mobile phone in question, when in fact the cellular station actually hears a different mobile unit using the same channel in a completely different cell. This reporting error 20 results in a call being switched to the wrong cell, typically one in which the signal strength is insufficient to maintain the connection. Further, if the mobile phone fails to hear the command to change channels, the handover fails. Actual user experience shows that handoff failures often occur, which calls into question the reliability of the system.

Another common problem in a conventional telephone system occurs when a cellular telephone is near the border of two cells. In this situation, the signal level tends to fluctuate at both cellular positions. This signal fluctuation results in a ping-pong situation where repeated requests are made to switch a call back and forth between two cellular stations. These tarless handoff requests increase the possibility for the mobile unit to hear the handover request incorrectly or not hear the command at all. Further, the ping-pong108974 situation raises the possibility that a call will be interrupted if it is unnecessarily transferred to a cell where all channels are currently in use and thus not available to accept handoff.

It is therefore an object of the invention to provide improvements in a cellular telephone system for handover of a call between cellular stations and thereby provide greater service reliability.

In a CDMA cellular telephone system, the same bandwidth is used for all cells. The CDMA waveform characteristics that provide processing gain are also used to discriminate between signals in the same frequency band. Thus, the handset or mobile unit does not need to change frequencies when a handover of a call is made from one cell site to another. Further, the likelihood that the call will be interrupted if the handover command is received incorrectly will be substantially reduced.

In a CDMA cellular telephone system, each cell site has a plurality of modulation-demodulation units or wide-spectrum modems. Each modem includes a digital wide-spectrum transmission modulator, at least one digital wide-spectrum information receiver, and a paging receiver. Each modem at the cell site is designated to the mobile unit as needed to enable communication with the designated mobile unit. Therefore, in many cases, several modems are available while others may be operable with the corresponding mobile units of the messages.

The present invention utilizes a handover system for a CDMA cellular telephone system, in which a new cell-site modem is assigned to the mobile unit while the old cell-site continues to handle the call. When the mobile unit si35 divides in the transition area between two cellular stations, the call may be switched back and forth between the cellular stations, as dictated by the signal strength. Since the mobile unit 087 974 always communicates via at least one cell site, there is no interrupting effect on the mobile unit or service.

When mobile connections are assuredly established through 5 new cell stations, e.g., the mobile unit is inside a very new cell, the old cell station will discontinue serving the call. The handover technique described may be considered as a soft handoff in cellular communication with the mobile unit.

By nature, soft handoff is a do-as-you-go switching operation. In contrast, conventional cellular telephone systems can be considered to provide a turn-off-before-you-do switching function.

The CDMA cellular telephone system according to the invention employs soft handover technology, which also allows the mobile station to initiate handover. The mobile station is also allowed to determine the best new cell station to which connections will be transferred from the old cell station.

While it is preferable for the mobile unit to initiate a handover request and determine a new cell site, handover processing decisions can be made as in a conventional cellular telephone system.

As mentioned above with respect to conventional systems, the cell site determines when handover may be appropriate and, through the system controller, requests adjacent cells to look for a mobile signal. The cell station which receives the strongest signal specified by the system controller then accepts the handover.

In a CDMA cellular telephone system, each cell site transmits a control carrier signal. The mobile units use this control signal to achieve initial system synchronization and provide rough time, frequency and phase tracking of the signals transmitted by the cell site.

Each cell site also transmits a setup channel108974 van, which includes wide-spectrum modulated information such as cell site identification information, system timing information, mobile paging information, and a variety of other control signals. The control signal transmitted by each cell5 station has the same spreading code but different code phase offset. The phase shift allows the control signals to be separated from one another, resulting in the separation of the cell sites from which they originate. Using the same control signal code allows the mobile unit to find system timing synchronization through a single search through all control signal code steps. The strongest control signal determined by the reference process for each code step is immediately identifiable.

The detected control signal corresponds to the control signal transmitted by the nearest cellular station.

When acquiring the strongest control signal, i.e. initializing the mobile unit with the strongest control signal, the mobile unit wipes the appropriate setting channel for that cell site. The setup channel is transmitted at the cell site using one of a plurality of different predetermined wide spectrum codes. In one embodiment of the invention, twenty-one different codes are used. However, it will be appreciated that more or less codes may be used in a setup channel defined by system parameters. The mobile unit then initiates a search through all the different codes used in the setup channel.

When the mobile unit identifies a setting code suitable for that cell 30 station, the system information is received and processed. The mobile unit continues to wipe the setup channel for control messages. One such control message would indicate that the call is waiting to be forwarded to this mobile unit.

The mobile unit continues to examine the received control carrier signal code in code transitions corresponding to the control signals transmitted by the adjacent cell site. This examination is performed to determine whether the control signal from adjacent cells is becoming stronger than the control signal that is first determined to be the strongest. If, in this inactive mode of call, the control signal of the adjacent cell station becomes stronger, then the mobile unit assumes a stronger control signal and a corresponding setting channel of the new cell station.

Once the call has been initiated, the pseudoko10 tin (PN) code address is assigned to be used during this call. The code address may either be designated by the cell station or be predefined based on the identifier of the mobile unit. After the call is initiated, the mobile station continues to investigate the control signal of the cell stations in adjacent cells. Controlling the control signal continues to determine if one of the control signals transmitted by adjacent cell stations is becoming stronger than the control signal transmitted by the cell station to which the mobile station unit is communicating. When the control signal from a cell station in an adjacent cell becomes stronger than the control signal transmitted by the cell station in the current cell, it indicates to the mobile unit that a new cell has been reached and that handover should be initiated. In response to this determination of the control signal strength, the mobile unit generates and transmits a control message to the cell site just serving the call. This control message, which indicates that the control signal transmitted by the new cell site is now stronger than the control signal transmitted by the current cell site, is provided to the system controller. The control message further includes information identifying the new cell site and a PN code. The control message relayed to the system controller is interpreted as having to start handover to the identified cell site in the communication of the mobile unit.

The system controller now starts the handoff108974 process. It will be appreciated that during a handover, the PN code address of a particular mobile unit going through the handover process need not change. The system controller initiates a handoff call with a modem located on the new cell site. This modem is assigned a PN address associated with the call in connection between the mobile unit and the current cellular modem. The new cell-site modem, designated to serve the call, searches for and finds the signal transmitted by the mobile unit. The cell-site modem also begins to send outgoing signals to the mobile unit. The mobile unit looks for outgoing signals according to the signaling and setting channel information provided by the new cell site.

When the signal transmitted by the new cell-site modem is received, the mobile unit switches to listen to this signal. The mobile unit then sends a control message indicating that the handover is complete. The control message is produced by one or both of the old and new cell-site modems to the system controller. In response to this control message, the system controller alone connects the call to the new cell-site modem while it terminates the call through the old cell-site modem. The old cell-site modem returns to the list of free modems available for renaming.

A further improvement in the handover process may be another mode of operation. This second mode is referred to herein as the cell-site multipath mode. The cellular station mode is further described in a co-pending U.S. Patent Application, entitled DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM, No. 07 / 432,552, filed November 7, 1989 by the same inventors and applicant.

In the cell station multipath mode, the call is allowed to dwell in the space described above with reference to a call handled by two cell stations. In the exemplary embodiment described herein, with reference to the mobile phone of the present invention, three demodulator processors or receivers are used. One receiver is used for tracking, while the other two receivers are used as a dual channel multipath receiver. During operation in a single cell, the scanning receiver attempts to find a signal transmitted by the cell station, which passes several paths to the mobile unit. These multipath signals are typically caused by signal reflections from terrain buildings and other signal obstructions. When two or more such reflections are found, the two receivers are designated as the two strongest paths. The scan receiver continues to evaluate multiple paths to keep the two receivers in sync with the signals on the two strongest routes as route conditions change.

In cell station multipath mode, the paging receiver determines the two strongest paths from each cell20 station. The two receivers are designated to demodulate the signals on the two strongest paths of the four paths available from the original cell site and the new cell site. The data demodulation process uses information from each of these receivers in the multipath merge operation. The result of this multipath combining operation is a greatly improved resistance to harmful wobble that may occur in a multipath cellular telephone environment.

Although various types of mine-linking techniques are known in the art, the present invention makes significant use of multi-linking to enhance the quality and reliability of connections in a mobile cellular telephone system. The present invention employs a form of maximal ratio combining. The signal-to-noise ratio is determined for each of the paths connected by two paths, respectively weighted. Combining is coherent because demodulation of the control signal allows the determination of each phase of the path.

On the route from the mobile unit to the two cellular stations, multipath reception is achieved by allowing each of the two cellular stations to demodulate the signals transmitted by the mobile unit. Each cell site forwards the demodulated data signals to the system controller together with an indication of the signal quality at the cell site receiver. It will be appreciated that it is possible to send non-decoded signals or even demodulated signals to a system controller to allow for better utilization of the multipath merge process.

The handover process in cellular mode is initiated as described. The mobile unit determines that the signal transmitted by the adjacent cell site is of sufficient signal strength to allow good quality demodulation of the signal. The mobile unit transmits a control message to the current cell station indicating the identification of this new cell station and a request for the cellular multipath mode. The cell station thus relays the cell station identifier and the request to the system controller.

The system controller answers by connecting the call to the modem in the new cellular position. The system controller then performs multipath combining of signals received by two cellular stations, while the mobile unit performs multipath combining of signals received from two cellular stations. The cellular multipath mode continues as long as the signals received from each cell site are at a level sufficient to allow benign demodulation.

The mobile unit continues to retrieve signals transmitted from other cell sites. If the signal transmitted by the third cell site becomes stronger than one of the original cell site signals, then the mobile unit transmits a control message through the second ny108974 cell station at all times to the system controller. The control message indicates this cell station identifier and the handover request. The system controller interrupts a call that has been communicated from the three cell-site signals through hi-fi, giving the call through the two strongest cell-sites. If the mobile units were equipped with additional receivers, such as three receivers, triple-cell multi-mode could be used.

The cell-site multipath mode ends when the mobile unit determines that only one cell-site provides sufficient signals for quality demodulation. The mobile unit then transmits a control message indicating that the cell-site remains in communication when the cell-site monito15 is terminated. The system controller may also terminate the cell-site multipath mode if the system becomes overloaded, in which case an insufficient number of modems are available to support all mobile unit requests for this mode of operation. The disclosed cellular station 20 road code is implemented by a decision made in the mobile unit to operate in the cell station multiple cell mode. However, it will be appreciated that the cell site monitone mode may be implemented such that decisions to operate in this mode are made in the system controller.

The present invention provides a substantial improvement over existing cellular telephone systems with respect to handover of a mobile unit. The do-before-break-channel-switching mechanism of the present invention is a clear improvement in system-wide reliability along with a lower service interruption. The use of cell-site multipath mode provides further improvements over conventional cellular telephone systems, providing the system with additional reliability and quality in communication.

The features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the drawings, which have the same reference numerals throughout, in which:

Figure 1 is a schematic diagram of an exemplary CDMA cellular telephone system according to the invention;

Figure 2 is a block diagram of a cellular telephone for CDMA communication in a CDMA cellular telephone system;

Figure 3 is a block diagram of a cell site apparatus 10 in a CDMA cellular telephone system; and Figure 4 is a block diagram of a mobile switching center equipment.

Figure 1 is an example of a telephone system in which the present invention has been applied. The system shown in Figure 1 employs CDMA modulation technology between system mobile units or cellular phones and cellular stations. Cellular systems in large cities can have hundreds of cellular stations that serve hundreds of thousands of mobile phones. The use of CDMA20 technology provides easy access to the user capacity of systems of this size compared to conventional FM modulation cellular systems.

In Figure 1, a system controller and a switch 10, also referred to as a mobile switching center 25 (MTSO), include an interface and processing circuit for providing system control to cellular stations. The controller 19 also controls the routing of calls from the Public Switched Telephone Network (PSTN) to a suitable cell site for transmission to a suitable mobile communication unit. The controller 10 also controls the routing of calls from the mobile units through at least one cell site to the PSTN. Controller 10 can control calls between mobile users through appropriate cell sites, since such mobile units typically do not communicate directly with one another.

Controller 10 may be connected to cellular stations in various ways, such as through dedicated telephone lines, optical fiber links or microwave communication links. Figure 108974a1 shows three such cellular stations 12, 14 and 16 and a mobile unit 18 including a cellular telephone. The arrows 20a-20b define a possible communication link between the cell site 12 and the mobile unit 18.

The arrows 22a-22b define a possible communication link between the cell site 14 and the mobile unit 18. Similarly, arrows 24a-24b define a possible communication link between the cell site 16 and the mobile unit 18.

Cell site service areas or cells are geographically structured so that the mobile unit is normally closest to one cell site. When the mobile unit is idle, i.e. no calls are being processed, the mobile unit continuously monitors the control signal transmissions from each nearby cell site. As shown in Figure 1, cell stations 12, 14 and 16 transmit control signals to mobile unit 18 via communication links 20b, 22b and 24b. The mobile unit then determines which cell it is in by comparing the strength of the control signals transmitted from these particular cell sites.

In the example of Figure 1, the mobile unit 18 may be considered to be closest to the cell site 16. When the mobile unit 18 initiates a call, the control message is transmitted to the nearest cell site 16. Upon receiving the call request message, the cell station assigns a signal to system controller 10 and forwards the call number. The system controller 10 then connects the call via the PSTN to the desired recipient.

If the call is received in the PSTN, then the system 30 controller 10 transmits the call information to all cell stations in the area. The cellular stations, in turn, send a paging message to the desired mobile recipient. When the mobile station hears a paging message, it responds with a control message sent to the nearest cellular station. This control message gives a signal to the system controller that this particular cell site is communicating with the mobile unit. Controller 10 then routes the call through this cell 108974 station to the mobile unit.

If the mobile unit 18 moves out of the coverage of the original cell station, cell station 16, then an attempt is made to continue the call by routing the call through another cell site. The handover process has two different methods for initiating a call handover or routing through another cell site.

The first method, called handoff initiated by a cell site, is similar to the handover method used in the original first generation analog cellular telephone systems currently in use. In the handover initiated by the cell station, the parent cell station, cell station 16, notices that the signal transmitted by the mobile station 15 has dropped below a certain threshold level. The cell site 16 then transmits a handoff request to the system controller 10. The controller 10 relays the request to all adjacent cell sites 14, 12 of the cell site 16. The request sent by the controller includes

Information associated with the channel including the PN code set used by the mobile unit 18. Cell stations 12 and 14 tune the receiver to the channel used by the mobile unit and measure the signal strength, typically using digital technology. If one of the receivers of the cell stations 12 and 14 reports a stronger signal than the signal strength reported by the parent cell, then the handover is made to this cell station.

Another method of initiating a handover is called a handover initiated by a mobile station. The mobile unit is provided with a paging receiver which is used to monitor the control signal transmission of adjacent cell stations 12 and 14, among other operations. If the control signal of the cell stations 12 and 14 is found to be stronger than the control signal of the cell station 16, then the mobile unit 18 transmits a control message to the current cell site 16.

This control message includes information that identifies a cell site having a higher signal strength and further information requesting a handover of that cell station. Cell station 16 transmits this control5 message to controller 10.

The handover method initiated by the mobile station has various advantages over the handover initiated by the cell station. The mobile unit becomes aware of changes in the routes between itself and the various cell sites within the cell much more quickly and with less effort than the cell stations are able to do. However, in order to perform the handover initiated by the mobile station, each mobile unit must be provided with a paging receiver to perform the paging operation. However, the exemplary embodiment of the CDMA communication capability of a mobile station described herein has additional functions required for its existence.

Figure 2 is a block diagram showing an example of a cellular cellular telephone. The mobile unit includes an antenna 30 coupled via a gateway filter 32 to an analog receiver 34 and a transmission power amplifier 38. The antenna 30 and the gateway filter 32 are of conventional design and allow simultaneous transmission and reception through a single antenna. An antenna 30 collects the transmitted signals and transmits them through the gateway filter to the analog receiver 34. Receiver 34 receives RF frequency signals, usually in the 850 MHz frequency band, from gateway filters 34 for amplification and IF frequency for frequency downconverting. This frequency conversion process is accomplished using a conventional structure frequency synthesizer that allows the receiver to be tuned to any frequency throughout the cellular speech 35 within the receive frequency band of the bandwidth.

The IF signal passes through a surface acoustic wave (SAW) bandpass filter, which in a preferred embodiment has a bandwidth of about 1.25 MHz. The features of the SAW filter are selected to match the waveform of the signal transmitted by the cell site, which signal is rectangular wide-spread modulated by a PN series clocked at a predetermined rate, which is 1.25 MHz in the preferred embodiment. This clock rate is selected to be an integer multiple of a common data rate such as 16 Kbps, 9.6 Kbps, and 4.8 Kbps.

The receiver 34 also performs a power control operation to control the transmission power of the mobile unit. The receiver 34 generates an analog power control signal provided on the transmit power control circuit 38. The control and operation of the power control feature of a mobile unit is described in the co-pending U.S. Patent Application Applicant's and Applicant's Application, entitled "METHOD AND APPLICATION FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", No. 07 / 433,09 released 8/9/91.

The receiver 34 is also provided with an analog / digital converter (A / D) (not shown) for converting the IF signal to a digital signal at a conversion rate of 9.216 MHz at a preferred embodiment that is exactly eight times the PN25 chip rate. The digitized signal is provided to each of two or more signal processors or data receivers, one of which being a paging receiver, after all being data receivers. In Fig. 2, the digitized signal output from receiver 34 is provided to digital30 data receivers 40 and 42 and paging receiver 44. It will be appreciated that a low-cost, low-performance mobile unit may have only one data receiver, while high-performance units may have two or more multipaths.

The digitized IF signal may include the signals of a plurality of connected calls together with control carriers transmitted by the current and all adjacent cell stations. The function of the receivers 40 and 42 is to compare IF samples with the correct PN series. This comparison process yields a value known as processing gain in this technique, which increases the signal-to-interference ratio of a signal suitable for the correct PN series while not increasing other signals. The reference output is then detected synchronously using a control signal from the nearest cell site as a carrier or step comparison. The result of this detection process is a line of coded data symbols.

The value of the PN series used in the present invention is that the discrimination is arranged against multipath signals. When a signal arrives at a mobile station receiver after passing through more than one route, there is a difference in the signal reception time. This difference in reception time corresponds to the difference in distance divided by the speed of light. If this time difference exceeds one microsecond, then the comparison process discriminates against one path. The receiver can choose whether to follow and receive an earlier or later route. If two receivers, such as receivers 40 and 42, are provided, then two independent routes can be followed in parallel.

The paging receiver 44 is controlled by the control processor 46 to continuously monitor the time domain for the received cell station control signal, in the vicinity of the nominal time, for other multipath control signals from the same cell station and for other control signals transmitted by the cell station. The receiver 44 measures the intensity of any reception of the desired waveform at times other than the nominal time. The receiver 44 compares the signal strength in the received signals. The receiver 44 provides a signal strength signal 35 to the control processor 46 which indicates the strongest signals.

Processor 46 outputs signals to digital data receivers 40 and 42 so that each handles a different signal from the strongest signals. Occasionally, the control signal transmitted by the second cell site has a higher signal strength than the current cell 5 station signal strength. Then, the control processor 46 generates a control message to be transmitted to the system controller via the current cell site, requesting that the cell be switched to a cell station corresponding to a stronger control signal. Receivers 40 and 42 can then handle calls through two different cell sites.

The outputs of the receivers 40 and 42 provide the multipath combiner and decoding circuit 48. The multipath combiner circuit contained in the circuit 48 simply adjusts the timing of the two currents of the received signals and adds them together. This insertion process can be continued by multiplying the two currents by a number corresponding to the relative signal strengths of the two currents. This activity can be considered as the maximum ratio multipath connection. The resulting composite sig20 signal stream is then decoded using a downstream interference detection decoder included in circuit 48.

In the exemplary embodiment, convolutional decoding is used. Convolutional decoding has a forced length of 9 and a code number of 1/3, i.e., three encoded symbols are produced and transmitted for each bit of information transmitted. The optimizer decoder for this type of code is the soft decision Viterbi algorithm decoder. The resulting decoded information bits are applied to the user's digital baseband circuitry 50.

The baseband circuitry 50 typically includes a digital video encoder (not shown). The baseband circuitry 50 further serves as an interface to a handset or any other type of terminal. The baseband circuit 50 is adapted to many different vocoder structures. The baseband circuit 50 provides output information signals to the communication device according to the information the circuit 48 provides to it.

08974

Usually, the user's analog audio signals received through a handset are input to the baseband circuit 50. The baseband circuit 50 includes an analog / digital (A / D) converter (not shown) that converts the analog signal to a digital format. The digital signal is provided to the digital video encoder where it is encoded. The output of the vocoder is provided to a co-error correction coding circuit (not shown) for error correction. This audio digital coding signal is an output from the baseband circuit 50 to the transmission modulator 52.

The transmission modulator 52 modulates the encoded signal with a PN carrier signal having a PN series selected according to a designated address function for the call.

The PN series is determined by the control processor 46 from the call setup information transmitted by the cell site and the decoded receivers 40 and 42. Alternatively, the control processor 46 may determine the PN series cell position through pre-arrangement. control processor

46 provides PN serial information to the transmission modulator 52 and the receivers to decode the call.

The output of the transmit modulator 52 provides the transmit power control circuit 38. The signal transmission power is controlled by an analog power control signal received from the receiver 34. Further, the cell stations transmit control bits in the form of a power control command and In response to the power control 30 commands, the control processor 46 generates a digital power control signal which is output to the circuit 38. Further information on the interrelationships between the receivers 40 and 42, the control processor 46, and the transmit power control 35 is further described in the above-referenced patent application.

The transmit power control circuit 38 outputs a power controlled modulated signal to the transmit power gain circuit

36. Circuit 36 ​​amplifies and converts an IF signal to an RF frequency by mixing a frequency synthesizer with an output signal, which tunes the signal to the correct output frequency. Circuit 36 ​​includes an amplifier that amplifies power to the final output level. The intended transmission signal is output from circuit 36 ​​to gateway filter 32. Gateway filter 32 connects the signal to antenna 30 for transmission to cellular stations.

The control processor 46 is also capable of generating control messages 10 such as cellular multipath mode requests and cell-site relay termination commands. These commands are issued for transmission to the transmission modulator. The control processor 46 responds to data received from the data receivers 40, 42 and the paging receiver 44 to make decisions about handover and multipath combining.

Figure 3 is a block diagram illustrating an exemplary embodiment of a cell site apparatus. The cellular station uses two receiver systems, each with a separate antenna and an analogue receiver for space multipath reception. In both receiver systems, the signals are processed identically until the signals undergo a multipath association process. The elements within the dotted line correspond to the elements responsible for communication between the cell site and one mobile unit. The outputs of the analog receivers are also arranged in other elements used for communication with other mobile units.

In Figure 3, the first receiver system comprises an antenna 60, an analog receiver 62, a paging receiver 64 and a digital data receiver 66. This receiver system may further include a digital data receiver 68. The second receiver system includes an antenna 70, analog receiver 72, paging receiver 64, and digital data also a cell site control processor 78. Each receiver system is coupled to a multipath combiner and decoding circuit 80. Digital link 82 is used to communicate signals to the MTSO and the MTSO (Fig. 4) by the cell site transmit modulator 84 and the circuit 80 under control of the control processor 78.

The received signals to the antenna 60 are provided to an analog receiver 62. The received signals amplified by the amplifier at the receiver 62 are converted to an IF frequency by mixing with the output signal of the frequency synthesizer. The IF signals are baseband filtered and digitized by a process identical to that described for the analog receiver in the mobile unit. The digitized IF signals are provided to the digital data receiver 66, the optional data receiver 68 and the paging receiver 64 and processed accordingly, as described in connection with the digital data receivers and paging unit 20 of the mobile unit. However, the processing of digital data receivers and paging receivers is in many respects different from the mobile-to-cell link than that used in the cell-to-mobile link.

In an internal, or mobile-to-cell-to-cell link, the mobile unit does not transmit a control signal that can be used for coherent comparison purposes in signal processing at the cell-site. Thus, a cell-to-cell link utilizes a non-coherent modulation and demodulation system using 64-ary orthogonal signaling.

The paging receiver 64 is again used to monitor the time domain of the received signal to ensure that the associated digital data receiver

66, and the data receiver 68, if used, monitor and process the strongest available time domain signal. This tracking process is identical to what has been described in connection with the mobile unit. The paging receiver 64 provides a signal to the cell-site controller processor 75, which provides control signals to the digital data receivers 66 and 65 to select suitable received signals for processing.

In the 64-ary orthogonal signaling process, symbols transmitted by the mobile station have one of 64 different possibilities. The 6-bit symbol is coded10 into one 2⁶so 64, different binary series. The group of selected sets is known as Walsh functions. The optimal reception function for the Walsh function is Fast Hadamard Transform (FHT). In the paging receiver 64 and the digital data receivers 66 and 68, the input signal is compared, as described in connection with the receivers of the mobile unit, with the reference output supplied to the FHT processor. The FHT processor produces a group of 64 coefficients for each of the 6 symbols. 64 the symbol is then multiplied by the weight 20 function generated at the receiver. The weighting function is related to the measured signal strength. The weighted data is then output to the multipath combiner and decoding circuit 50.

The second receiver system processes the received signals in the same manner as described for the first receiver system of Figure 3. The output of the weighted 64 symbols from the receivers 66 and 76 is provided to the multipath combiner and decoding circuit 80. The circuit 80 includes an adder which adds the weighted 64 symbols from the receiver 66 to the weighted 64 symbol receiver 76. The resulting 64 coefficients are compared to determine the maximum coefficient. The magnitude of the comparison result, together with the maximum coefficient 64 of the identifier, is used to determine a set of decoder weight values ​​and symbols for use in the Viterbi algorithm decoder used in circuit 80.

The Viterbi decoder preferably has a forced length of 9 and a code number of 1/2. The Viterbi decoder is used to determine the most likely information bit string. For each vocoder data block, nominally 15 ms of data, a signal quality estimate is obtained and transmitted as a power control command of the mobile unit along with the data to the mobile unit. Further details on how to generate this quality rating are described in more detail in the aforementioned parallel application. This quality10 estimate is the average signal-to-noise ratio over a period of 15 ms.

In Figure 3, an optional digital data receiver 68 may be included to improve system performance. This additional data receiver, alone and in combination with other receivers, can track and receive other possible delay paths of signals transmitted by the mobile unit. The structure and operation of this receiver are similar to those described for digital data receivers 66 and 76. Receiver 68 is used to provide additional multipath modes. Optional additional digital data receivers are immensely useful in cellular stations located in densely built-up urban areas where there are many opportunities for multipath signals.

Signals from the MTSO are coupled to a suitable transmission modulator via digital link 82 under control of control processor 78. The transmission modulator 84 wide-spread modulates, in accordance with a predetermined spreading function which is oh30 in the sensing processor 78, data to be transmitted to the desired receiving mobile unit. The output of the transmission modulator 84 is provided in the transmission power control circuit 86, where, under the control of the control processor 88, the transmission power can be controlled. The output of circuit 86 is provided in transmission power amplifier circuit 88.

The circuit 88 includes an adder for summing the output of transmit transmitter 1098974 to the output of other transmit modulators in the cell site. The circuit 88 further includes an adder for summing the control signal output from the control signal generator 90 to the summed transmit 5 modulator output signals. The circuit 88 further includes a digital / analog converter, a frequency converter circuit and an amplifier respectively for converting the digital signals into analog signals, converting the IF frequency signals as output from the transmit modulator symptoms.

RF frequency and amplify the RF signal. The output of circuit 88 is provided to antenna 92, from where it radiates to mobile units in the cell site service area.

The cell site control processor 78 is responsible for designating digital data receivers and modulators15 for a particular call. The control processor 78 also monitors the progress of the call, the quality of the signals, and initiates decompression when the signal is lost. The cellular station communicates with the MTSO via link 82 to which it is connected by a conventional telephone cable, optical fiber, or microwave link.

Figure 4 is a block diagram showing the equipment used in the MTSO. The MTSO typically includes a system controller or system controller processor 100, a digital switch 102, a multipath combiner 104, a digital encoder 106, and a digital switch 108. Although not illustrated, additional multipath connectors and digital video encoders are connected between digital switches 102 and 108.

When the cellular multipath mode is active, or the MTSO is in the handover process when a call is being processed by two cell sites, signals arrive at the MTSO from more than one cell site with nominally the same information. However, due to fluctuation and interference in the internal link from the mobile station to the cell sites, the signal from one cell site may be better in quality than the signal from another cell site.

Digital switching 102 is used to route an information stream corresponding to a particular mobile unit from one or more cell stations to a multipath combiner 104 or a corresponding multipath combiner as determined by a signal from the system controller processor 100. When the system is not in cellular multipath mode, the multipath combiner can either be bypassed or input the same information at each input port.

A plurality of serially connected multipath connectors and 10 vocoders are arranged in parallel, namely one for each handled call. The multipath combiner 104 compares signal quality indicators following information bits from two or more cell site signals. The multipath combiner 104 selects bits which, on a frame-by-frame basis, receive information of the highest quality cell-site signal for output to vocoder 90.

The vocoder 106 converts the digitized audio signal format to a standard 64 Kbps PCN telephone format, analog or any other standard format. The resulting signals are transmitted from vocoder 106 to digital exchange 108. Under system controller processor 100, the call is routed to the PSTN.

The audio signals from the PSTN to the mobile unit are provided to a digital switcher 108 suitable for digital exchange 108, such as vocoder 106, under the control of a system controller processor 100. Vocoder 106 encodes the incoming digitized audio signals and outputs the resulting information bit stream directly to digital switch 102. Under the control of the system controller processor, the digital switch 102 redirects the encoded data to the cellular site (s) to which the mobile unit communicates. If the mobile unit is in handoff mode, communicating with multiple cell sites or in a cellular multipath mode, digital switch 102 routes routes to suitable cell stations by a suitable cell site transmitter to be transmitted to the receiving cellular unit desired. However, if the mobile unit is only communicating with one cellular station or is not in the cellular multipath mode, the signal is directed to only one single cellular station.

System controller processor 100 controls digital exchanges 102 and 106 to route data to and from the MTSO. The system controller processor 100 also assigns call assignments to cellular stations and vocoders in the MTSO. Further, the system controller processor 100 communicates with each cell site control processor for assigning calls between the MSTO and the cell site, and assigning PN codes for calls. It will be further understood that when the digital steps 102 and 106 shown in Figure 4 are shown as two separate gears, however, this operation may be performed by a single physical gear unit.

The above description of preferred embodiments is intended to enable one skilled in the art to make and use the present invention. Various modifications of these applications will be apparent to those skilled in the art, and the general principles defined herein may be applied to other applications without the use of the inventive ability. Thus, the present invention is not limited to the examples set forth herein, but should be afforded the broadest scope corresponding to the novel features set forth in the claims.