HK1074933B - Soft handoff in a cdma cellular telephone system - Google Patents

Description

Soft handoff in CDMA cellular telephone system

This application is a divisional application with application number 90109068.9.

Technical Field

The present invention relates to cellular (cellular) telephone systems, and more particularly to a new and improved system for controlling handoffs (handoffs) in communications between a base station (cell-site station) and a mobile station in a Code Division Multiple Access (CDMA) cellular telephone system.

Background

The use of code division multiple access modulation techniques. But one of several techniques that simplify multi-system user communications. While other techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and AM modulation techniques such as amplitude-scaled single sideband (ACSSB) are known, CDMA has significant advantages over these modulation techniques. The use of CDMA techniques in a multi-connection communication system has been disclosed in U.S. patent No. 4,901,307, entitled "spread spectrum multi-connection communication system using satellite terrestrial repeater," filed on 17.10.1986, 06/921,261, which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference.

In the above-mentioned patent, a multiple communications technique is disclosed in which a plurality of mobile telephone system users, each having a radio transceiver, communicate through satellite relay stations or ground stations (also referred to as base stations) using Code Division Multiple Access (CDMA) spread spectrum communication signals. In communications using CDMA, the spectrum can be reused multiple times, thus enabling increased system user capacity. The use of CDMA techniques makes the spectral efficiency much higher than other multipath communication techniques.

In conventional cellular telephone systems, the existing frequency band is typically divided into channels of 30KHZ bandwidth, using analog FM modulation techniques. The system service area is divided into regional units of different sizes. Existing frequency channels are divided into groups, each group typically containing an equal number of channels. The frequency groups are assigned to the units in a manner that minimizes the potential for common channel interference. For example, consider a system with seven frequency groups and cells in an equilateral hexagon. The frequency groups used in a cell are not used in the nearest six cells or the surrounding neighbors. Also, the frequency groups of its cells are not used in this nearest twelve neighboring cells.

In conventional cellular telephone systems, the handoff is performed in a manner intended to continue a call when the mobile telephone crosses the boundary between two cells. When the base station receiver controlling the call finds that the received signal strength from the mobile telephone has fallen below a predetermined threshold, a handoff is made from one cell to another. Low signal strength means that the mobile phone must be near the edge of the cell. When the signal level falls below a predetermined threshold, the base station requests the system controller to determine whether a neighboring base station receives a stronger mobile telephone signal than the current base station.

Upon interrogation by the current base station, the system controller sends a handoff request message to the neighboring base stations. The base stations adjacent to the current base station use a dedicated scanning receiver to search for signals from the mobile station on the designated channel. A neighboring base station attempts a handover if it reports to the system controller that sufficient signals have been received.

Then, an idle channel is selected in the new base station channel group for switching. Control information is transmitted to the mobile phone instructing it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the second base station.

In conventional systems, if the handoff to the new base station is unsuccessful, the call cannot continue. There are many reasons for a handover failure. If there is no free talk channel in the neighboring cell, the handoff will fail. Handover will also fail if another base station reports receipt of the mobile phone, which in fact receives a different mobile station using the same channel in a completely different cell. Such reporting errors will result in the call being transferred to the wrong unit, typically one with insufficient signal strength to maintain communications. Also, if the mobile phone does not receive a command to switch channels, the handover will fail. Practical working conditions show that switching failure frequently occurs, and the reliability of the system is doubtful.

Another common problem occurs in conventional telephone systems when the mobile telephone is near the boundary between two cells. In this case, the signal level tends to swing on both base stations. This signal level swing will cause a ping-pong operation, repeating the request, and the call is switched back and forth between the two base stations. Such additional unneeded handoff requests increase the likelihood that the mobile station will receive an erroneous channel switch command or upon receiving no command. Also, ping-ponging will increase the likelihood that a call will not continue if it is unnecessarily switched to a unit where all channels are currently in use and thus cannot be allowed to switch.

Disclosure of Invention

It is therefore an object of the present invention to improve the hand-off of calls between base stations in a cellular telephone system and to provide high service reliability.

In a CDMA cellular telephone system, the same frequency band is used in all cells. The characteristics of the CDMA waveform that provide the adjusted gain are also used to discriminate between two signals occupying the same frequency band. Thus, the mobile telephone or station does not need to switch frequencies when a call is handed off from one base station to another. Moreover, the probability that the call will not continue due to the receipt of an erroneous transition command will be significantly reduced.

In a CDMA cellular telephone system, each base station has multiple modem units or spread spectrum modems. Each modem is comprised of a digital spread spectrum transmitter modulator, at least one digital spread spectrum data receiver, and a searcher receiver. Each modem is assigned to a mobile station and can easily communicate with the specified mobile station as needed. Thus, in many cases, many modems are in use, while other modems may remain in operative association with respective mobile stations.

In the present invention, a CDMA cellular phone system using a handover scheme is in a state where a new base station modem is assigned to a mobile station and an original base station modem is assigned to a mobile station while the original base station continues to provide a call service. When a mobile station is located in a border area between two base stations, the call can be transferred back and forth between the base stations, subject to signal strength. Since the mobile station always communicates through at least one base station. So that no interruption effect or service interruption of the mobile station occurs.

The handoff techniques just described can be considered as a "soft" handoff of communications between base stations to a mobile station. Soft handoff is essentially a function of making a transition before disconnection. By comparison, conventional cellular telephone systems are believed to provide the capability to switch after disconnection.

In the CDMA cellular telephone system of the present invention, soft handoff techniques are implemented, which also allow for initiation by the mobile station. Allowing the mobile station to decide to switch the communication from the original base station to the best new base station.

Although it is preferable for the mobile station to issue a handoff request and determine the new base station, handoff procedure determination may be used as in conventional cellular telephone systems. As discussed above with respect to conventional systems, the base station determines, via the system controller, to request that the neighboring cell search for mobile station signals when a handoff is likely to be appropriate. The base station receiving the strongest signal, as determined by the system controller, then accepts the handoff.

In a CDMA cellular telephone system, each base station transmits a "pilot carrier" signal. The pilot signal is used by the mobile station to synchronize the initial system and provide reliable time, frequency and phase tracking of the base station transmitted signal.

Each base station also transmits a "setup" channel that includes spread spectrum modulation information such as base station identity, system timing, mobile paging information, and other various control signals. The pilot signal transmitted by each base station has the same spreading code but a different code phase shift. The phase shift enables the pilot signals to be distinguished from each other and, as a result, from the two base stations from which the pilot signals originate. The use of the same pilot code enables the mobile station to find system timing synchronization by searching all pilot code phases at once. The strongest pilot signal can be readily identified as determined from a corresponding process performed on each code phase. The identified pilot signals represent the pilot signals transmitted by the nearest base station.

After the strongest pilot signal is found, i.e., the mobile station initially synchronizes with the strongest pilot signal, the mobile station searches for an appropriate ready channel for that base station. The prepared channel is transmitted by the base station using one of a plurality of different predetermined spread spectrum codes. In an exemplary embodiment of the present invention, twenty-one different codes are used. However, it should be understood that more or fewer codes may be used in the preparation channel as determined by system parameters. The mobile station then begins searching for all the different codes used in the preparation channel.

When the mobile station determines the appropriate preparation code for the base station, the system information is received and processed. The mobile station further monitors the preparation channel for control information. One such control message would indicate that a call is waiting to be delivered to the mobile station.

The mobile station continues to check the code phase shift of the received pilot carrier signal code corresponding to the pilot signal transmitted by the neighboring base station. This check is to determine whether the pilot signal transmitted from the neighboring cell is stronger than the pilot signal that was first determined to be strongest. If, while the call is stationary, the pilot signal of the neighboring base station becomes stronger than the pilot signal transmitted by the original base station, then the mobile station will acquire the stronger pilot signal and the corresponding prepared channel of the new base station.

When a call is started, a Pseudo Noise (PN) code address used during the call is determined. The code address may be either determined by the base station or predetermined based on the identity of the mobile station. After the call has started, the mobile station continues to check for pilot signals transmitted by base stations located in neighboring cells. The pilot signal check is continued to determine whether any of the pilot signals transmitted by the neighboring base stations are stronger than the pilot signal transmitted by the base station with which the mobile station is communicating. When the pilot signal transmitted by a base station in a neighboring cell becomes stronger than the pilot signal transmitted by a base station in the current cell, this indicates that the mobile unit has entered a new cell and a handoff should be initiated. In response to the determination of the strength of the pilot signal, the mobile station generates and transmits a control message to the base station currently serving the call. This control signal, indicating that the pilot signal transmitted by the new base station is now stronger than the pilot signal transmitted by the current base station, is provided to the system controller. The control information also contains information that identifies the new base station and PN code. As this control signal is communicated to the system controller, i.e., interpreted as a handover of the mobile unit's communication to the determined new base station will begin.

The system controller now starts the switching step. It should be appreciated that during handoff, the PN code address of the particular mobile station undergoing the handoff step need not be changed. The system controller initiates a handoff to assign the modem at the new base station to the call. The modem is given a PN address associated with the call in communication between the mobile station and the current base station modem. A new base station modem serving the call is determined and the signal transmitted by the mobile station is searched and found. The base station modem also starts transmitting an output signal to the mobile station, and the mobile station searches for the output signal based on the new base station-provided signal and the preparation channel information.

When the signal transmitted by the new base station modem is obtained, the mobile station switches to listen to the signal. The mobile station then transmits a control signal indicating that the handover is complete. Control information is provided to the system controller by one or both of the original and new base station modem. Based on the control signal, the system controller individually switches the call to the new base station while interrupting the call through the original base station modem. The original base station modem then becomes a reassignable spare modem.

A further improvement is that the switching step can introduce a second operating state. In this regard, the second state is called the base station diversity state. Discussion of diversity status of base stations the inventor further discloses in co-pending U.S. patent application entitled "diversity receiver in CDMA cellular telephone system", serial No. 07/432, 552, filed on 7/11/1989, which is assigned to the assignee of the present invention.

In a base station diversity state; as described above with reference to calls handled by two base stations, the call is allowed to hold off in the intermediate state. In the exemplary embodiment described herein of a mobile phone according to the invention. A total of three demodulation processors or receivers are used. One receiver is used for the scanning function and the other two receivers are used as two channel diversity receivers. During operation in a single cell, the scanning receiver looks for signals transmitted by the base station that propagate along multiple paths to the mobile station. These multipath propagating signals are typically caused by reflections of the signal from ground structures and other signal obstructions. When two or more such reflections are found, the two receivers are assigned to the two, strongest channels. The scanning receiver continues to measure and calculate multipath; when the path conditions change, the two one receivers are kept synchronized with the signals on the two strongest paths.

In the base station diversity state, the strongest two paths from each base station are determined by the searcher receiver. Two receivers are selected to demodulate the signals on the strongest two of the four possible paths from the original base station and the new base station. In a diversity combining operation, the data demodulation process uses information from both receivers. As a result of this diversity combining operation, the detrimental attenuation that may occur in a multiple cell phone scenario is largely prevented.

Although different diversity combining techniques types are well known in the art, the present invention greatly improves the quality and reliability of communications in a mobile cellular telephone system using diversity combining. In the present invention, the maximum ratio combining form is applied. According to two paths. The contributions from these two paths, weighted accordingly, are combined to determine the signal-to-noise ratio. The combination is correlated since the demodulation pilot signal can determine the phase of each one path.

Path diversity reception can also be achieved by demodulating the signals transmitted by the mobile station through the two base stations in the way from the mobile station to the two base stations. The two base stations send their demodulated data signals to the system controller along with data indicative of the signal quality of the base station receiver. The system controller then combines the two modifications of the mobile station signal and selects the signal of the best quality. It will be appreciated that it is possible to pass the undecoded or even unmodulated signal to the system controller to enable a better diversity combining method to be utilised.

As discussed previously, the switching step is initiated in the cell diversity state. The mobile station determines that the signals transmitted by the neighboring base stations are strong enough to achieve good signal demodulation quality. The mobile station sends a control message to the current base station indicating the identity of the new base station and a request for a cell diversity status. The base station then forwards the base station identity and the request to the system controller.

The system controller responds by connecting the call to the modem of the new base station. Then, while the mobile station diversity combines the signals received from the two base stations, the system controller also combines the signals received from the two base stations. The cell diversity state will continue as long as the received signal levels from both base stations are sufficient for good demodulation quality.

The mobile station continues to search for signals transmitted from other base stations. If the signal transmitted by the third base station becomes stronger than one of the original two base station signals, the mobile station transmits control information to the system controller via at least one of the current base stations. The system controller then suspends the call through the weakest signal of the three base stations while providing the call through the two strongest base stations. A triple base station diversity state may be possible if the mobile station is equipped with additional receivers, such as three receivers.

The base station diversity state is discontinued when the mobile station determines that only one base station is providing a signal that is strong enough for the quality of the demodulation. The mobile station then sends a control message indicating that the base station is still communicating after the base station diversity state has been discontinued. The system controller also suspends base station diversity mode if the system becomes overloaded due to an insufficient number of modem to support all requests by the mobile station to operate in base station diversity mode. As discussed. The base station diversity state is implemented by the mobile station making a determination that it is desirable to operate in the base station diversity state. However, it should be understood that the base station diversity state can also be implemented by the system controller making a determination to operate in such a state.

The present invention provides a substantial improvement over current cellular telephone systems in terms of mobile station handoff. The switching principle before disconnection of the invention makes a remarkable improvement on the overall reliability of the system and reduces service faults. The implementation of the base station diversity state further improves conventional cellular telephone systems by providing additional system reliability and quality in communications.

Drawings

The features and advantages of the present invention will become more apparent in light of the following detailed description of the invention when taken in conjunction with the accompanying drawings.

Wherein:

FIG. 1 is a schematic diagram of an exemplary CDMA cellular telephone system of the present invention;

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

fig. 3 is a block diagram of a base station apparatus in a CDMA cellular telephone system;

fig. 4 is a block diagram of a mobile telephone switching office device.

Detailed Description

Fig. 1 illustrates a typical telephone system of one embodiment of the present invention. The system shown in fig. 1 uses CDMA modulation techniques in the communication between the mobile stations or mobile phones and the base stations of the system. Cellular systems in large cities have hundreds of base stations serving hundreds of thousands of mobile phones. The use of CDMA technology greatly increases the user capacity of a system of the same size as compared to a conventional FM modulated cellular system.

In fig. 1, a system control and switch 10, also known as a Mobile Telephone Switching Office (MTSO), generally includes interface and processing circuitry for performing system control of the base station. The controller 10 also controls the call path from the Public Switched Telephone Network (PSTN) to the appropriate base station for transmission to the appropriate mobile station. The controller 10 also controls the passage of calls from the mobile station to the PSTN through at least one base station. Since such mobile stations are generally not able to communicate directly with one another, the controller 10 may also control communications between mobile users via appropriate base stations.

The controller 10 may be coupled to the base station by various means such as a dedicated telephone line, an optical fiber line, or by a microwave communication line. In fig. 1, three such exemplary base stations 12, 14 and 16 and an exemplary mobile station 18 comprising a cellular telephone are illustrated. The arrowed lines 2 oa-20 b define possible communication lines between the base station 12 and the mobile station 18. The arrowed lines 22 a-22 b define possible communication lines between the base station 14 and the mobile station 18. Similarly, arrowed lines 24 a-24 b define possible communication links between the base station 16 and the mobile station 18.

The base station service area or cell is geographically shaped such that a mobile station is generally closest to a base station. When the mobile station is idle, i.e., not engaged in a call, the mobile station constantly monitors the pilot signals transmitted by each of the nearby base stations. As shown in fig. 1, base stations 12, 14 and 16 transmit pilot signals to mobile station 18 via communication lines 20b, 22b and 24b, respectively. The mobile station then determines which cell it is in by comparing the strength of the pilot signals transmitted by these particular base stations.

In the example shown in fig. 1, the mobile station 18 may be considered to be closest to the base station 16. When the mobile station 18 starts a call, control information is transmitted to the nearest base station, i.e., the base station 16. The base station 16 receives the call request message, sends a signal to the system controller 10, and transmits the telephone number. The system controller 10 then connects the call to the desired receiver through the PSTN.

If the call is in progress in the PSTN, the controller 10 transmits call information to all base stations in the area. In response, the base station transmits paging information to the intended mobile station receiver. When the mobile station receives the paging message. It transmits a control message to the nearest base station in return. The control information informs the system controller that the particular base station and mobile station are to communicate. The controller 10 then determines a call route to the mobile station through the base station.

If the mobile station 18 moves out of the active area of the original base station, base station 16, the call continues by determining the route of the call through another base station. During the handover, there are two methods for performing call handover, i.e. determining a route through another base station.

The first method is called base station initiated handoff and is similar to the handoff method used in the original first generation analog signal cellular telephone systems currently in use. In the base station initiated handoff method, the original base station, base station 16, notices that the signal transmitted by mobile station 18 has fallen below a certain threshold. Then. The base station 16 issues a handover request to the system controller 10. The controller 10 forwards this request to all base stations 14, 12 in the vicinity of the base station 16. The request transmitted by the controller includes information about the channel including the sequence of PN codes used by the mobile station 18. The base stations 12 and 14 adjust the frequency of the receiver to the channel used by the mobile station and measure the signal strength, typically using digital techniques. If the receivers of base stations 12 and 14 have a reported signal strength that is greater than the signal strength reported by the original base station, then a handoff is made to that base station.

A second method of initiating a handoff is called mobile initiated handoff. The mobile station is equipped with a searcher receiver for scanning the pilot signals transmitted by the neighboring base stations 12 and 14 and performing other functions. If the strength of the pilot signals of base stations 12 and 14 is found to be greater than the strength of the pilot signal of base station 16, mobile station 18 transmits a control message to the current base station, base station 16. The control signal contains information identifying the base station with the stronger signal and information requesting a handover to the base station. The base station 16 forwards the control information to the controller 1.

The mobile-initiated handoff method has various advantages over the base-station-initiated handoff method. The mobile station is able to discover changes in the paths between itself and various neighboring base stations earlier and more easily than the base station. However, to perform a mobile-initiated handoff, each mobile station must be equipped with a searcher receiver that performs the scanning function. However, in the exemplary embodiment described herein having mobile station CDMA communication capabilities, the searcher receiver has additional functions that it needs to perform.

Fig. 2 shows a block diagram of a typical mobile station cellular telephone. The mobile station includes an antenna 30 coupled through a duplexer 32 to an analog receiver 34 and a transmit power amplifier 36. The antenna 30 and the duplexer 32 are of a standard type and are capable of transmitting and receiving simultaneously via a single antenna. The transmitted signals are received by an antenna 30 and provided through a duplexer 32 to an analog receiver 34. Receiver 34 receives the RF signal from duplexer 32. duplexer 32 is typically in the 850MHz band for amplification and frequency conversion to IF frequencies. Frequency conversion is achieved using a standard-sized frequency synthesizer that enables the receiver to tune to any frequency within the receive band of all cellular telephone bands.

The IF signal is then passed through a surface acoustic wave (SAM) bandpass filter, which in the preferred embodiment has a bandwidth of about 1.26 MHZ. The characteristics of the SAM filter are selected to coincide with the signal waveform transmitted by a base station having a directional sequential spread spectrum modulated with a PN sequence timed at a predetermined frequency, which in the preferred embodiment is 1.25 MHZ. The clock frequency is selected to be an integer multiple of several common data rates, such as 16Kbps, 9Kbps, and 4.8 Kbps.

The receiver 34 also performs a power control function to adjust the transmit power of the mobile station. The receiver 34 generates an analog power control signal that is provided to the transmit power control circuit 38. Control and implementation of mobile unit power control features is disclosed in U.S. patent No. 07/433, 031 entitled "method and apparatus for controlling transmit power in a CDMA cellular mobile telephone system," filed on 7/11/1989, assigned to the assignee of the present invention.

The receiver 34 also has an analog-to-digital (a/D) converter (not shown) to convert the IF signal to a digital signal at a clock frequency of 9.216MHz, which in the preferred embodiment is exactly 8 times the frequency of the PN Chip (Chip). The digitized signal is sent to each of two or more signal processors or data receivers, one of which is a searcher receiver and the remaining of which are data receivers.

In fig. 2, the digital signal output from receiver 34 is sent to digital data receivers 40 and 42 and search receiver 44. It should be appreciated that an inexpensive low performance mobile station may have only a single data receiver, while a high performance mobile station may have two or more, capable of diversity reception.

The digitized IF signal contains many of the signals being communicated along with the pilot carriers transmitted by the current and all neighboring base stations. The function of the receivers 40 and 42 is to correlate the IF samples with the appropriate PN sequence. This correlation process provides a characteristic well known in the art called "processing gain" that will improve the signal-to-interference ratio of the signal matching the proper PN sequence without improving the other signals. The correlation output is then synchronously detected using the pilot carrier from the nearest base station as the carrier phase reference. The result of this detection is an encoded data symbol.

The characteristic of the PN sequence as in the present invention is to repel multiple signals. When a signal that has traveled more than one path reaches the mobile receiver, there is a difference in signal reception time. The receive time difference corresponds to the speed of light divided by the distance difference. If the time difference exceeds one microsecond, the correlation process will reject one path. The receiver may choose to track and receive the earlier or closer path. If there are two receivers, such as receivers 40 and 42, then two independent channels are tracked and coexist.

The searcher receiver 44 is controlled by a control processor 46 to receive other multiple multi-frequency signals from the same base station and other pilot signals transmitted singly in a continuous scan time regime around the very short time of pilot signal reception by the base station. At times other than this very short time, the receiver 44 measures any received strength of the desired waveform. The receiver 44 compares the strength of each received signal. The receiver 44 provides a signal of signal strength to the control processor representing the strongest signal

The processor 46 provides the signals to the digital data receivers 40 and 42, each processing a respective one of the strongest signals. Sometimes, the strength of the pilot signal transmitted by another base station is greater than the signal strength of the current base station. Then the control processor 46 will generate a control message to the system controller via the current base station to switch that base station to the base station corresponding to the strongest pilot signal. Receivers 40 and 42 may thus manage the call through two different base stations.

The outputs of the receivers 40 and 42 are provided to a diversity combiner and decoding circuit 48. The diversity combining circuit contained within circuit 48 simply aligns the timing of the two received signals and adds them together. This addition is performed by multiplying the two stream signal by a number corresponding to the relative signal strengths of the two streams. This operation can be considered as maximum ratio diversity combining. The resulting combined signal stream is then decoded with a forward stream error detection decoder also included in the circuit element.

In an exemplary embodiment, convolutional coding is used. The constraint length of the convolutional code is 9 and the code rate is 1/3, i.e., the code rate is. Three code symbols are generated and transmitted for each bit of information transmitted. The optimal decoder for this type of code belongs to the soft decision Viterbi algorithm decoder design. The resulting decoded information bits are provided to the subscriber digital baseband circuitry 50.

The baseband circuit 50 typically includes a digital vocoder (not shown). The baseband circuitry 50 also functions to interface with a handset or other type of peripheral device. The baseband circuitry 50 is adapted for a variety of different vocoders. The baseband circuit 50 sends an output information signal to the user in accordance with the information from the circuit 48.

The user analog voice signal, which is typically provided to the handset, is provided as its input to the baseband circuitry 50. The baseband circuit 50 includes an analog-to-digital (a/D) converter (not shown) that converts the analog signal to digital form. The digital signal is sent to a digital vocoder where it is encoded. The vocoder output is sent to forward error correction coding circuitry (not shown) for error correction. The speech digitized encoded signal is output from the baseband circuit 50 to a transmit modulator 52.

The transmit modulator 52 modulates the code signal onto a PN carrier signal whose PN sequence is selected based on an address function determined for the call. The control processor 46 determines the PN sequence based on the call preparation information transmitted by the base and decode receivers 40 and 42. In addition, the control processor 46 may also decide the PN order according to the base station by arranging in advance. The control processor 46 passes the PN sequence signal to the transmit modulator 52 and the receivers 40 and 42 for call decoding.

The output of the transmit modulator 52 is supplied to the transmit power control circuit 38. The signal transmit power is controlled by an analog power control signal provided by the receiver 34. Also, the control bits are transmitted by the base station in the form of power adjustment commands and processed by the data receivers 40 and 42. The power adjustment command is used to control the processor to establish a mobile station transmit power level. In response to the power adjustment command, control process 46 generates a digital power control signal that is provided to circuitry 38. In addition, receivers 40 and 42 control the interrelation information of processor 46 and transmit power control circuit 38. As also described further in the above-identified co-pending patent application.

The transmission power control circuit 38 outputs a power control modulation signal to the transmission power amplifier circuit 36. Circuitry 36 amplifies the IF signal and converts the IF signal to an RF frequency by mixing with the frequency synthesizer output signal, which tunes the signal to the appropriate output frequency. Circuit 36 includes an amplifier that amplifies power to the final output level, and the desired transmit signal is output from circuit 36 to duplexer 32. The duplexer 32 couples the signal to the antenna 30 for transmission to the base station.

The control processor 46 can also generate control information such as a cell diversity status request and a base station communication suspend command. These commands are sent to transmit modulator 52 for transmission. The control processor 46 makes switching and diversity combining decisions based on the data received from the data receivers 40, 42 and the searcher receiver 44.

Fig. 3 illustrates in block diagram form an exemplary embodiment of a base station apparatus. At the base station, two receiving systems are used, each with a separate antenna and analog receiver for full set reception in space, and in each receiving system, the same processing is applied to the signals before they are subjected to diversity combining processing, and the elements within the dashed lines represent the elements corresponding to the communication between the base station and one of the mobile stations. The output of the analog receiver is also provided to other units used in communication with other mobile stations.

In fig. 3, the first receiving system includes an antenna 60, an analog receiver 62, a search receiver 64, and a digital data receiver 66. The receiving system may also include an optional digital data receiver 68. The second receiving system includes an antenna 70, an analog receiver 72, a search receiver 64, and a digital data receiver 66. A base station control processor 78 is also used in signal processing and control for handover and diversity. Both receiving systems are coupled to a diversity combining and decoding circuit 80. Signals to and from the MTSO (fig. 4) use a digital connector 82 in communication with a base station transmit modulator 84 and circuitry 80 under the control of the control processor 78.

The signal received at antenna 60 is passed to an analog receiver 62. The received signal, amplified by an amplifier in the receiver 62, is converted to an IF frequency by mixing with the frequency synthesizer output signal. The IF signal is bandpass filtered and digitized in the same process as described for the mobile station analog receiver. The digitized IF signals are provided to a digital data receiver 66, a selective digital receiver 68 and a searcher receiver 64 and processed in a manner similar to that described with reference to the digital data receiver and the searcher receiver of the mobile station in fig. 2, respectively. However, the processing performed by the digital data receiver and the searcher receiver differs in several respects from the line from the base station to the mobile station in the line from the mobile station to the base station.

In the return line, i.e., the mobile station to base station line, the mobile station does not transmit a pilot signal that can be used as a correlation reference for base station signal processing. Therefore, the mobile station to base station line adopts an uncorrelated modulation and demodulation scheme using 64-array (ary) orthogonal signal transmission.

The searcher receiver 64 is also used to scan the time domain for receiver signals, ensuring that the associated digital data receiver 64 and data receiver 68 (if used) track and process the resulting strongest time domain signal. This tracking process is the same as that described for the mobile station. The searcher receiver 64 provides a signal to a base station control processor 78 and the processor 78 provides control signals to the digital data receivers 66 and 68 to select the appropriate received signal for processing.

In 64-array orthogonal signal transmission process, there are 64 different possibilities for the symbols transmitted by the mobile station. A 6-bit symbol can be encoded into one of 26, or 64, different binary sequences. The set of sequences chosen is called the Walsh function. The best acceptance function among the Walsh functions is the fast hadamard transform (FTH). In the searcher receiver 64 and the digital data receivers 66 and 68, the input signals are correlated with correlator outputs fed to the FTH processor as discussed with respect to the mobile station receivers. The FTH processor generates one stage of 64 coefficients for every 6 symbols. The 64 symbols are then multiplied by a weighting function generated in the receiver. A weighting function is associated with the measured signal strength. The weighted data is then provided as output to the diversity combining and decoding circuit 50.

The second receiving system processes the received multiple signals in a similar manner as discussed with respect to the first receiving system in fig. 3. The weighted 64 symbol outputs from the receivers 66 and 76 are provided to a diversity combining and decoding circuit 80. Circuit 80 comprises an adder that adds the weighted 64 symbols from receiver 66 to the weighted 64 symbols from receiver 76. The resulting 64 coefficients are compared with each other to determine the largest coefficient. The magnitude of the comparison result is used, along with the result itself, i.e., the largest of the 64 coefficients, to determine a set of decoding weights and symbols for the Viterbi quadrature decoder of circuit 80.

The Viterbi decoder preferably has a constraint length of 9 and a code rate of 1/2. The Viterbi decoder is used to determine the most likely information bit order. For each vocoder block, nominally 15 milliseconds long, a signal quality estimate is obtained and transmitted to the mobile station along with the data as a mobile station power adjustment command. Other aspects regarding the generation of the quality estimate are discussed in further detail in the above-identified co-pending application. The quality estimate is the average of the signal-to-noise ratio over a 15 millisecond (mste) interval.

In fig. 3, to improve system performance, a selective digital data receiver 68 may be included. This additional data reception, either by itself or in combination with another receiver, can track and receive other possible delay legs of the signal transmitted by the mobile station. The receiver is constructed and operates similar to that described with respect to the digital data receivers 66 and 76. The receiver 68 is used to obtain additional diversity states. Alternative additional digital data receivers that provide additional diversity modes are useful at base stations in crowded cities where multiple signals are likely to be generated.

The signal from the MTSO is connected to a suitable transmit modulator via a digital connector 82 under the control of a control processor 78. The transmit modulator 84 spreads the spectrum, as specified by the control processor 78, and modulates the data according to a predetermined spreading function for transmission to the desired receiving mobile station. The output of the transmit modulator 84 is fed to a transmit power control circuit 86. There, its transmit power is controlled under the control of a control processor 78. The output of circuit 86 is fed to a transmit power amplifier circuit 88.

Circuitry 88 includes an adder that adds the output of transmit modulator 84 to the output of other transmit modulators in the base station. Circuit 88 also includes an adder that adds the pilot signal output by pilot generator 90 to the summed transmit modulator output signal. Circuitry 88 also includes a digital-to-analog converter, an up-conversion circuit, and an amplifier to convert the digital signal to an analog signal, to convert the IF frequency signal output by the transmit modulator to RF frequency, and to amplify the RF signal, respectively. The output of circuit 88 is applied to antenna 92 and transmitted to mobile stations within the cell site service area.

The base station control processor 78 assumes the task of assigning data receivers and modulators to individual calls. The control processor 78 also monitors the progress of the call, signal quality and initiates the rejection of signal loss. The unit stations communicate with the MTSO via connectors 82, the connectors 82 connecting the MTSO with standard telephone lines, optical fibers or microwave lines.

Fig. 4 shows in block diagram form a device for use in an MTSO. The MTSO generally includes a system controller, i.e., system control processor 100, a digital switch 102, a diversity combiner 104, a digital vocoder 106, and a digital switch 108. Additional diversity combiners and digital vocoders are connected between digital switches 102 and 108, but are not shown.

When the cell diversity state is active, i.e. the MTSO is in a handover procedure handled by two base stations regarding the call, the signal will arrive at the MTSO from more than one base station nominally with the same information. However, because of the attenuation and interference experienced on the return path from the mobile station to the base station, the signal from one cell site may be of better quality than the signal from another cell site.

The digital switch 102 is used to switch on the information sources corresponding to a given move from one or more base stations to the diversity combiner 104 or related diversity combiners as determined by the signal from the system control processor 100 when the system is not in a cell diversity state, the diversity combiner 104 is either bypassed or the same information is fed at each input.

There are multiple sets of diversity combiners and vocoders connected in series, providing parallel connections, one set for each path of call to be processed. The diversity combiner 104 compares signal quality indicators associated with information bits from two or more base station signals. The diversity combiner 104 selects the bit corresponding to the base station signal of the best quality from the frame-by-frame information and outputs the bit to the vocoder 106.

The vocoder 106 converts the format of the digitized voice signal to a standard 61Kbps PCN telephony format, analog or any other standard format. The resulting signal is transmitted from the vocoder 106 to the digital switch 108. The call leg is connected to the PSTN under the control of the system control processor 100.

Voice signals from the PSTN intended for the mobile station are sent to a suitable digital vocoder, such as vocoder 106, via digital switch 108 under the control of system control processor 100. The vocoder 106 encodes the input digitized voice signal and sends the resulting information bit stream directly to the digital switch 102. The digital switch 102, under control of the system control processor, sends the encoded data to the base station with which the mobile station is communicating. If the mobile station is in a handoff state for communicating with multiple base stations or in a base station diversity state, the digital switch 102 transmits the call to the appropriate base station for transmission to the desired receiving mobile station by the transmitter of the appropriate base station. However, if the mobile station is communicating with only one base station or is not in cell diversity, the signal is directed to one base station.

The system control processor controls the digital switches 102 and 108 to send and retrieve data to and from the MTSO. The system control processor 100 also determines the vocoder that distributes the calls to the base stations and MTSO. The system control processor 100 communicates with each base station control processor, and assigns a PN code for individual calls between the MTSO and the base station. It should also be understood that the digital switches 102 and 108 are shown as two separate switches in fig. 4, however, this function could also be accomplished by a single structure switch unit.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. It will be readily apparent to those skilled in the art that various modifications can be made to the embodiments and the generic principles described herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles disclosed herein as novel features.

Claims (26)

1. A communication control system for controlling communication links between a mobile system unit and geographically independent base stations in a cellular telephone system in which the mobile system unit communicates information signals with another system unit via a system controller connected to the base stations, each base station defining a geographic service area, said information signals being transmitted as code division spread spectrum signals along said communication links, when the mobile system unit changes service areas, said communication control system comprising:

means for determining a transfer of said mobile system unit from the area served by said one base station to the area served by another base station and providing an indication identifying said another base station when said mobile system unit is located within the service area of said one base station and said information signal is communicated with said another system unit over the communication link of said one base station;

means for further connecting communication of said information signals between the mobile system unit and said another system unit over the communication link of said another base station in response to said indication while said mobile system unit remains in said communication of said information signals with said another system unit over said communication link of said one base station; and

means for interrupting communication of said information signals between said mobile system unit and said another system unit over said communication link of said one base station while continuing communication of said information signals between said mobile system unit and said another system unit over said communication link of said another base station in response to said information signal communication connection between said mobile system unit and said another system unit over said communication link of said another base station.

2. The communication control system of claim 1 wherein each base station transmits the same pilot signal but with a different phase than neighboring base stations, said means for determining a transition of said mobile system unit from an area served by said one base station to an area served by another base station and providing said indication comprises:

scanning receiving means, disposed in said mobile system unit, for receiving pilot signals transmitted by base stations, measuring the signal strength of each of said received pilot signals, comparing the relative signal strengths, and providing a signal strength signal indicative of the received pilot signal having the strongest signal strength; and

and a processing device, disposed in the mobile system unit, for receiving the signal strength signal, and generating a handover request command when the signal strength signal indicates that the pilot signal transmitted by the another base station is stronger than the pilot signal transmitted by the one base station, wherein the handover request command indicates the another base station and is transmitted to the one base station.

3. The communication control system of claim 2 wherein said one base station couples said handover request command to said system controller, and wherein said means for coupling communication of said information signals between said mobile system unit and said another system unit via a communication link of said another base station comprises:

a system processing device, located on the system controller, for receiving a handover request command of the one base station connection and generating a first handover command in response to the handover request command; and

switching means, located within said system controller, for connecting communications between said mobile system unit and said another system unit via said one base station unit and controlling communications between said mobile system unit and said another system unit via said another base station unit in response to said first transition command.

4. The communication control system of claim 3 wherein said means for interrupting the communication of said information signals between said mobile system unit and said another system unit over said communication link of said one base station comprises:

said processing means responsive to communication of information signals between said mobile system unit and said another system unit via said another base station for generating handover complete control information and for communicating said handover complete control information to said system controller via at least one of said one base station and said another base station;

the system processing device responds to the switching completion control information and generates a second switching command; and

the switching means, in response to the second changeover command, disconnects the communication of the information signals of the mobile system unit and the other system unit through the one base station.

5. The communication control system of claim 1 or 2, further comprising at least two receivers for demodulating code division spread spectrum signals on the communication links transmitting the strongest signals.

6. The communication control system of claim 5, comprising a diversity mixer that uses information from said at least two receivers.

7. The communication control system of claim 5, wherein each base station modulates each pilot signal in a spread spectrum manner according to the same predetermined pilot signal spreading code, the predetermined code phase of each pilot signal being different from each adjacent base station.

8. A communication control method for controlling a communication link between a mobile system unit and geographically independent base stations in a cellular telephone system in which the mobile system unit communicates information signals with another system unit via a system controller connected to the base stations, each base station defining a geographic service area, said information signals being transmitted as code division spread spectrum signals along said communication link, when the mobile system unit changes service areas, said communication control method comprising the steps of:

determining a transition of said mobile system unit from the service area of one base station to the service area of another base station and providing an indication identifying said another base station when said mobile system unit is located within the service area of said one base station and is in communication with said another system unit via the communication link of said one base station;

further connecting communication of said information signals between the mobile system unit and said another system unit over the communication link of said another base station in response to said indication while said mobile system unit remains in said communication with said another system unit over said communication link of said one base station; and

interrupting communication of the information signal between the mobile system unit and the other system unit over the communication link of the one base station while continuing communication of the information signal between the mobile system unit and the other system unit over the communication link of the other base station in response to the communication connection between the mobile system unit and the other system unit over the communication link of the other base station.

9. The method of claim 8, further comprising the steps of:

selecting, under control of said system controller, communication of information signals between said mobile system unit and said another system unit via said one base station;

generating a first control signal indicative of another base station in response to said determining a transition of said mobile system unit from the service area of said another base station to the service area of said another base station;

transmitting the first control signal to the system controller through the first base station; and

under the control of the system controller, in response to the first control signal, selecting to conduct communication of the information signal between the mobile system unit and the other system unit through the other base station while communication of the information signal between the mobile system unit and the other system unit through the one base station is still conducted.

10. The method of claim 8 or 9, further comprising the steps of:

said mobile system unit generating a second control signal in response to communication of said information signal by said another system unit with said mobile system unit through said another base station;

transmitting said second control signal from said mobile system unit to said system controller via at least one of said one base station and said another base station; and

the system controller interrupts the transmission of the information signal to the one base station.

11. The method of claim 8 wherein said mobile system unit is capable of making and receiving calls to and from other system units and units in the public telephone system through at least one of a plurality of base stations under the control of a system controller, wherein each base station transmits a pilot signal spread spectrum modulated according to the same pilot signal spreading code having a predetermined code phase different from that of the pilot signal of an adjacent base station, and wherein said method further comprises the steps of:

communicating information signals between the mobile system unit and said another system unit via the first base station;

the first base station and the second base station transmit the pilot signals;

receiving at said one mobile system unit pilot signals transmitted by said first and second base stations;

determining, at the mobile system unit, a pilot signal strength of the pilot signal as received by the mobile system unit;

generating a handoff request by said mobile system unit in response to a determination of said pilot signal strength when said second base station transmits a pilot signal that is stronger than said first base station;

sending the handover request to the system controller via the first base station;

said system controller allocating said second base station to relay said communication of information signals between said mobile system unit and said another system unit; and

the mobile system unit and the other system unit communicate the information unit through the second base station, wherein the mobile system unit and the other unit communicate simultaneously through the first and second base stations.

12. The method of claim 8, further comprising the steps of:

upon selection of communication via the second base station, the mobile system unit detecting the communication of the information signal;

generating handover complete control information at said mobile system unit in response to detection of communication of said information signal by said second base station;

transmitting the control information to the system controller via at least one of the first and second base stations; and

interrupting communication of information signals between the mobile system unit and the other system through the first base station in response to the control information.

13. The method of claim 8, wherein said step of communicating said information signal through said first base station comprises the steps of:

the system controller receiving an information signal from the other system unit;

connecting said further information signal from said system controller to said first base station;

the first base station receiving the further information signal from the system controller;

said first base station modulating said further information signal in accordance with a first predetermined information signal spreading code to provide a first spread spectrum signal;

the first base station transmitting the first spread spectrum signal;

the mobile system unit receiving the first spectrum signal;

said mobile system unit demodulating said received first spread spectrum signal in accordance with said first predetermined information signal spreading code to provide an output of said another information signal;

the mobile system unit receives an input of a mobile information signal;

said mobile system unit modulating said mobile information signal in accordance with said first predetermined information signal spreading code to provide a second spread spectrum signal;

the mobile system unit transmitting the second spread spectrum signal;

the first base station receiving the second spread spectrum signal;

said first base station demodulating said received second spread spectrum signal in accordance with said first predetermined information signal spreading code to provide said mobile information signal;

connecting said mobile information signal from said first base station to said system controller;

the system controller receiving the movement information signal from the first base station; and

providing an output of the movement information signal from the system controller to the other system unit.

14. The method of claim 11 wherein the step of determining the pilot signal strength comprises the steps of:

the mobile system unit measuring a pilot signal strength of the received pilot signal;

the mobile system unit comparing the pilot signal strength measurements; and

identifying the pilot signal with the strongest signal strength.

15. The method of claim 11, wherein the step of transmitting the handover request comprises the steps of:

transmitting the handover request from the mobile system unit to the first base station; and

the first base station relays the handover request to the system controller.

16. The method of claim 11, wherein the step of assigning comprises the steps of:

determining an identity of the second base station based on the handover request;

transmitting the allocation to the second base station; and

the second base station allocates a modem on the second base station for communication of the information signal between the mobile system unit and the other system unit.

17. The method of claim 8 wherein said step of communicating said information signal through said first and second base stations comprises the steps of:

the system controller receiving an information signal from the other system unit;

connecting said further information signal from said system controller to said first and second base stations;

said first and second base stations receiving said further information signal from said system controller;

said first and second base stations modulating said further information signal according to a first predetermined information signal spreading code; to provide a first spread spectrum signal at each of said first and second base stations;

the first and second base stations transmitting the first spread spectrum signal;

the mobile system unit receiving the first spread spectrum signal;

the mobile system unit demodulating the received first spread spectrum signal in accordance with the first predetermined information signal spreading code;

combining the predetermined first spread spectrum signal to provide the further information signal;

the mobile system unit receiving an input of mobile user information converted into a mobile information signal;

said mobile system unit modulating said mobile information signal in accordance with said first predetermined information signal spreading code to provide a second spread spectrum signal;

the mobile system unit transmitting the second spread spectrum signal;

the first and second base stations receiving the second spread spectrum signal;

said first and second base stations demodulating said second spread spectrum signal received at each of said first and second base stations in accordance with said first predetermined information signal spreading code to provide an output of said mobile information signal from each of said first and second base stations;

connecting said mobile information signal from each of said first and second base stations to said system controller;

the system controller receiving the movement information signal from the first and second base stations; and

the system controller combining the mobile information signals received from the first and second base stations; and providing an output of the movement information signal from the system controller to the other system unit.

18. The method of claim 8 wherein the mobile system unit is capable of making and receiving calls to and from other mobile system units and units in the public telephone system through at least one of the plurality of base stations under control of the system controller, each of said calls including information signals modulated by the spread spectrum to communicate between the at least one base station and the corresponding mobile system unit, wherein each base station transmits a pilot signal modulated according to the same predetermined pilot signal spreading code and having a different predetermined code phase from neighboring base stations, said method further comprising the steps of:

communicating information signals between the mobile system unit and said another system unit via the first base station;

the first base station determining a signal strength of the information signal received by the first base station from the mobile system unit;

sending a handover request to said system controller when said determined signal strength is below a predetermined level;

the system controller requesting each of a plurality of neighboring base stations to determine, to the first base station, the signal strength of the information signal received from the mobile system unit by the each neighboring base station at each neighboring base station;

determining, at each neighboring base station, a signal strength of the information signal received by said each neighboring base station from the mobile system unit;

each neighboring base station reporting to the system controller the signal strength of the information signal received by said each neighboring base station from the mobile system unit; and

communicating said information signal between said mobile system unit and said another system unit through at least one of said neighboring base stations reporting a signal strength that is stronger than a signal strength determined by said first base station, wherein said mobile system unit and said another system unit communicate through said first base station and said neighboring base station simultaneously.

19. The method of claim 18, further comprising the steps of:

detecting the communication of the information signal by at least one of the neighboring base stations; and

interrupting said communication of information signals between said mobile system unit and said another system unit through said first base station.

20. A mobile system unit for use in a system for controlling communication links between said mobile system unit and geographically independent base stations each defining a geographic service area when said mobile system unit changes service areas in a cellular telephone system, wherein said mobile system unit communicates information signals with another system unit via a system controller coupled to said base stations, said information signals being communicated along said communication links in the form of code division spread spectrum signals, said mobile system unit comprising:

transfer determining means for determining that a mobile system unit is transferred from the service area of one base station to the service area of another base station and providing an identification identifying said another base station when said mobile system unit is located in the service area of said one base station and is communicating said information signal with said another system unit via the communication link of said one base station;

connection means responsive to said identification for further connecting communication of said information signals between said mobile system unit and said another system unit over a communication link of said another base station while said mobile system unit remains in said communication with said another system unit over said communication link of said one base station; and

interrupting means, responsive to said connection of communication between said mobile system unit and said another system unit over said communication link of said another base station, for interrupting said communication of information signals between said mobile system unit and said another system unit over said communication link of said one base station while continuing to maintain said communication of information signals between said mobile system unit and said another system unit over said communication link of said another base station.

21. The mobile system unit according to claim 20, wherein said transfer determining means is adapted to receive pilot signals transmitted by said one base station and said another base station, to determine a pilot signal strength of each received pilot signal, and to generate a handoff request.

22. The mobile system unit of claim 21, comprising means for transmitting said handoff request to said one base station.

23. A mobile system unit according to any of claims 20 to 22 wherein said connecting means comprises at least two receivers for demodulating code division multiple access spread spectrum signals on said communications link conveying the strongest signals.

24. The mobile system unit of claim 23, further comprising a diversity combiner that utilizes information from said at least two receivers.

25. A mobile system unit according to any of claims 20 to 23, further comprising:

scanning receiving means for receiving the pilot signals transmitted by the base station, measuring the signal strength of each received pilot signal, comparing the relative signal strengths, and providing a signal strength signal indicative of the pilot signal with the strongest signal strength among the received pilot signals; and

processing means for receiving said signal strength signal and generating a handoff request command when said signal strength signal indicates that said another base station transmits a pilot signal stronger than said one base station transmits, wherein said handoff request command indicates said another base station and transmits the handoff request to said one base station.

26. The mobile system unit of claim 25, wherein said means for interrupting comprises:

processing means for generating handover completion control information in response to communication between the mobile system unit and the another base station.