HK1076000A1 - Handover method for phs personal station - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent speech interruption at switch-back by decreasing a speech interruption time in the case of hand-over. SOLUTION: Signal reception quality is detected (104) intermittently (102) during a speech (100). In the case a high speed hand-over processing criterion is not satisfied as a result (122), while keeping radio connection with respect to a speech from a radio connection destination CS (stationary station) so far, remaining transmission reception systems not used for a speech are used to detect other CS (124) and to execute calling procedures (128-138) for establishment of the radio connection with the CS thereby reducing a speech interruption time. In the case of switch-back, the similar procedure is executed and speech interruption due to switch-back operation can be prevented during high speed movement.

Description

Cross-zone switching method for PHS personal station

Technical Field

The present invention relates to a PHS (personal handyphone system) personal station, and more particularly, to a handover method of such a personal station.

Background

In mobile communication systems which are very popular today, for example in cellular telephone systems or PHS systems, a plurality of (usually a large number of) Cell Stations (CS) are geographically distributed. Each CS covers a limited geographical area (cell). The cell stations of the system work in concert to cover the entire urban area together. System users often carry a Personal Station (PS) that can rest on the palm of the hand. When communication is required, the user controls the personal station to place or receive calls to or from other stations. For example, in a PHS system, each CS is connected to other CSs or other mobile communication systems via a Public Switched Telephone Network (PSTN) or the like. Thus, the PHS user can communicate with other PHS users, with ordinary telephone users connected to the PSTN, or with users of other mobile communication systems. In the description of the present application, the term "communication" includes not only voice communication but also other communication such as data communication.

Fig. 9 shows an example of the structure of one PS used in the PHS system. The PS shown in the figure comprises an antenna 10, a radio unit 12, a signal processor 14, a memory 16, an operating unit 18, a display 20, a microphone 22 and a speaker 24. Although not shown, there is also a power supply circuit for powering these PS components, and means for interfacing with a personal computer or with a personal digital assistant. The antenna 10 is used for transmitting or receiving signals. A radio unit 12, including a radio frequency integrated circuit or the like, for transmitting signals or receiving signals through the antenna 10. The signal processor 14, including an ASIC (application specific integrated circuit) or the like, is used to perform various digital signal processing. Memory 16, RAM or F-ROM, provides storage space for signal processor 14. An operation unit 18, on which keys and dials are mounted, for sending instructions to the signal processor 14 according to user input. A display 20, a small liquid crystal device, etc., for displaying a function menu, an operation state of the signal processor 14, and transmission and reception states of signals. The microphone 22 converts the user's voice into an electric signal, which is input to the signal processor 14. The speaker 24 converts the electric signal from the signal processor 14 into sound output.

In the signal processor 14, there are an MPU (micro processor Unit) 26, an ADPCM-CODEC 28, a TCH-CODEC30, a TDMA-TDD controller 32 and a pi/4 QPSK modem 34. The user's voice is sent to the signal processor 14 through the microphone 22. ADPCM-CODEC 28 quantizes the received speech according to ADPCM (adaptive differential pulse code modulation). The TCH-CODEC30 encodes the received quantized speech to produce TCH (traffic channel) signals. The signal generated by the encoding is sent to the pi/4 QPSK modem 34 through the TDMA-TDD controller 32. A modulator 36, a component of the pi/4 QPSK modem 34, modulates the signal in a pi/4 QPSK (quadrature phase shift keying) manner. The modulated signal is amplified and frequency converted by a transmitting circuit 38 in the radio unit 12 and is transmitted to the antenna 10 for radio transmission.

On the other hand, the receiving circuit 40 in the radio unit 12 amplifies and converts the signal received by the antenna 10. A demodulator 42, a component of the pi/4 QPSK modem 34, demodulates the frequency converted signal in a pi/4 QPSK mode. The demodulated signal is then sent to the TCH-CODEC30 through the TDMA-TDD controller 32, decoded by the TCH-CODEC30, reproduced by the ADPCM-CODEC 28 in ADPCM fashion, and output from the speaker 24. The MPU 26 serves to control the entire operation of the signal processor 14. Numeral 44 in the figure indicates an antenna switch which enables the transmit circuitry 38 and the receive circuitry 40 to share the antenna 10.

The TDMA-TDD controller 32 herein controls transmission and modulation/demodulation of signals according to a TDMA-TDD (time division multiple access-time division duplex) protocol, which is a multiplexing method used in PHS. Each carrier in PHS has four uplink (PS- > CS)/downlink (CS- > PS) time slots. In PHS, a PS that is about to make or receive a call first makes a request over a control channel. Then, a time slot is allocated to the PS by the CS closest to the PS, and the PS transmits and receives signals constituting the TCH by using the allocated time slot.

In a mobile communication system including a PHS, a PS to be connected to a CS selects the CS that will provide the PS with a signal of the best quality. For example, the PS selects one CS from all CSs from which the PS can receive a signal, whose reception electric field strength is strongest at that time. However, when the user or the PS moves during communication, the received signal quality of the signal from the CS connected may be degraded depending on the moving direction. Furthermore, when there is another CS in the moving direction of the PS, the quality of the signal received from the CS being connected is degraded, while the signal of the other CS in the moving direction is improved. In this case, the PS switches the radio connection from the CS to which the PS was just connected to the CS located in the forward direction of the PS. This is called handover. In PHS, each CS is able to cover a smaller geographical area (cell) than in other conventional systems, and handovers are more frequent than in other systems.

Fig. 10 shows a flow of the handover start algorithm, and fig. 11 shows a flow of the handover process. This processing, for example, processing within the PS shown in fig. 9, is performed by the circuits in the signal processor 14 and the radio unit 12 operating in cooperation under the control of the MPU 26. The processing is based on information from the radio unit 12 and the TDMA-TDD controller 32, as well as reference values stored in the memory 16.

First, as shown in fig. 10, it is assumed that the PS is in the process of communication. That is, the TCH has established a connection to one CS for communication (100). During the communication, the signal processor 14 checks the quality of the received signal every 1.2 seconds (102). The signal processor 14 receives an RSSI (received signal strength indicator) signal, which indicates the electric field strength of the received signal, from the receiving circuit 40 of the radio unit 12, and calculates a moving average of the latest 240 frame level to obtain the reception level L. Also, it checks the last 240 frames output of TDMA-TDD controller 32 and TCH-CODEC30 for FER (frame error Rate) (104). In summary, the higher the reception level L, the better the received signal quality, and the lower the frame error rate FER, the better the received signal quality.

After computationally checking the received signal quality, the signal processor 14 compares the frame error rate FER and the received signal level L with the corresponding handoff processing levels (106, 108). The handoff processing level THF1 of the frame error rate FER is, for example, 24. The handoff process level THL1 of the reception level L is, for example, 22dB μ V. If FER ≧ THF1 and L ≦ THL1 are satisfied at the same time, signal processor 14 adds 1(110) to variable N; otherwise, the signal processor 14 zero the variable N (112). When N is equal to 2, that is, when the above two conditions FER ≧ THF1 and L ≦ THL1 are satisfied again within the period of 1.2 seconds (114), the above sequence, i.e., the quality check sequence of 200 in FIG. 11, is completed. The control program returns to the CS check sequence, identified as 116 in fig. 10, or the CS search sequence, identified as 202 in fig. 11.

The CS search sequence takes 300 ms to receive signals from neighboring CSs and evaluates the quality of the signals received over the control channel to find a candidate CS as a new radio connection target. As a result, if a CS is found whose received signal quality is good enough to satisfy a predetermined handover target region selection level (e.g., a reception level of 24dB μ V), the signal processor 14 executes a call sequence to establish a radio connection with the found CS (118 in fig. 10). This sequence is partially omitted in fig. 11, and is performed in the following manner. A link channel request is transmitted through one of the control channels SCCH, requesting allocation of a time slot for making a radio communication connection, and in response to the request, a link channel allocation signal is received, and a synchronization burst signal is exchanged using the allocated time slot. After the UA is transmitted to the CS over the FACCH, which is one of the control channels, the call sequence is terminated and then the CC response is received from the CS over the SACCH, which is one of the control channels.

As described above, the handover process in the conventional PHS from the detection of whether the received signal is deteriorated includes the CS search sequence and the following call sequence. One problem with this approach is that the communication connection is broken during the handoff process, as shown on the right side of fig. 11.

One reason radio communication must be temporarily disconnected is that PHS employs so-called autonomous distributed dynamic channel allocation. That is, unlike the cellular system where CSs are always synchronized, in PHS these CSs are not always synchronized. Sometimes this results in the control channel time slots of one CS overlapping the TCH time slots of another CS. For this reason, the radio connection with the old radio connection target CS (handover source CS) must be disconnected first before the CS search sequence can be started so that the CS search sequence can be executed, as shown in fig. 11. On the other hand, when the call sequence is completed, a radio connection with a new radio connection target CS (handover target CS) is established. Therefore, the communication connection is disconnected during the period of time, which is 2 seconds long.

Disclosure of Invention

The present invention has been made in an effort to solve the above-mentioned problems occurring in the prior art. An object of the present invention is to reduce the time for communication disconnection during handover and reduce discomfort for PS users. It is another object of the present invention to enable PHS users to continue to communicate comfortably even when they are moving rapidly, such as on a train.

One system to which the present invention is applied is PHS. In PHS, multiple PS are employed, each carried by a user, and each PS maintains a radio connection with one of multiple interconnected (e.g., via PSTN), geographically distributed CSs in an autonomous distributed manner in accordance with a TDMA-TDD protocol. The handover method of the present invention is performed by the PS which establishes a radio communication connection with one CS. That is, the PS establishes a connection with the CS that carries the TCH.

The present invention provides two handoff sequences: a normal handoff sequence and a seamless handoff sequence. The normal handover sequence disconnects a radio communication connection with a CS currently having a radio connection (i.e., a handover source CS), searches for a CS currently having a radio connection available, and, if there is one CS currently having a radio connection available, establishes a radio connection to communicate with the CS (i.e., a handover target CS). This sequence is of the prior art. On the other hand, a seamless handoff sequence is an improvement of the present invention.

The seamless handoff sequence allows the second circuit to search for a new CS with which the personal station may quickly establish a radio connection while maintaining a radio communication connection with the CS currently having a radio connection via the first circuit. If there is such a new CS with which the personal station may soon establish a connection, the sequence establishes a new radio connection to communicate with the new CS (i.e. the handover target CS) via the second circuit. On the other hand, the sequence maintains the radio connection through the first circuit until a predetermined time before the new radio connection is established. The seamless handoff sequence maintains the current radio connection state if there is no such new CS.

The handover method of the present invention is also characterized by its determination method or level of the normal or seamless handover method. Specifically, the handover method of the present invention intermittently checks the received signal quality of the CS of the current radio connection. If the received signal quality is detected to be below a threshold level for a CS radio connection that can currently have a radio connection, the method performs a normal handover sequence. If it is detected that the received signal quality is below the seamless handoff processing level, which is set higher than the limit level for maintaining the connection, the method performs a seamless handoff sequence.

The present invention also relates to PS. The PS in the present invention is a PHS PS carried by a user and having a radio connection with one of a plurality of CSs interconnected in a geographically distributed manner in an autonomous distributed manner in accordance with a TDMA-TDD protocol. The PS of the present invention includes a plurality of circuits for modulating signals and transmitting signals to the CS, while receiving and demodulating radio signals from the CS. The PS further has a controller for controlling a first circuit for communication and a second circuit for non-communication according to the received signal quality to implement the handover method of the present invention. The first circuit and the second circuit are included in the plurality of circuits.

In the method of the invention, one of two handover sequences is performed when the intermittent received signal quality check results indicate that the received signal quality has degraded or is degrading. These two handover sequences are started or selected according to two criteria: one is a threshold value for maintaining radio communication with the currently connected CS, i.e. a (normal) handover handling level, and the other is a seamless handover handling level, which is higher than the above threshold value. That is, if it is detected that the received signal quality is lower than the threshold value, the normal handoff sequence is performed; if the received signal quality is below the seamless handoff processing level, a seamless handoff sequence is performed.

The normal handoff sequence first disconnects the radio communication connection with the currently connected CS. The sequence then searches for a CS where a radio connection can currently be made. For example, as described in the prior art, by receiving a signal for a predetermined length of time, a CS is searched for which can make the received signal quality higher (better) than a predetermined handover target selection level. If there is such a CS, the sequence establishes a new radio communication connection (i.e., a TCH connection) with the CS. The radio communication connection is established by transmitting a link channel allocation request, receiving an allocated channel through a control channel, and exchanging a synchronization burst signal through a TCH.

In order to perform radio signal transmission/reception while establishing a new connection, a seamless handover sequence employs a first circuit and a second circuit of a plurality of circuits within the PS. That is, the seamless handoff sequence employs the second circuit to search for a new CS to which the PS will soon establish a radio connection, while at the same time maintaining the radio communication connection with the CS of the current radio connection using the first circuit. If there is a new CS to which such a PS can quickly establish a connection, the sequence establishes a new radio communication connection with the new CS. Communication with the new CS, such as transmitting a link channel allocation request, is accomplished by the second circuit. While the second circuit is communicating for establishing a radio communication connection with the new CS, communication with the currently connected CS is maintained by the first circuit as much as possible, thereby ensuring substantially continuous, i.e., seamless, communication.

The normal handoff sequence differs from the seamless handoff sequence in the length of the open communication time. The normal handover sequence requires that the radio connection with the handover source CS is disconnected before the search for the handover target CS is started. On the other hand, a seamless handoff sequence allows the first circuit to maintain a radio connection and thereby continue communication through that connection until a radio connection through the second circuit is immediately established. This means that a seamless handoff sequence requires a shorter disconnect time.

Another difference between the two sequences is the start level of the sequence. The start level of the normal handoff sequence is a limit level at which communication with the currently connected CS is maintained, and the start level of the seamless handoff sequence is a seamless handoff process level which is set higher than the limit level. For this reason, the seamless handover sequence is started when the received signal quality of the currently connected CS has not decreased to the extent that communication is to be interrupted, but the signal quality is expected to decrease significantly as the PS moves. An advantage of the present invention is that a seamless handoff sequence, which is an improved version of the handoff sequence, begins before the handoff is actually needed. This enables the communication to continue even if the user carrying the PS is moving fast.

In addition, the method of the present invention combines the performance of a normal handoff sequence with the performance of a seamless handoff sequence. The start level of a normal handoff sequence may be satisfied before the start of a seamless handoff sequence because the received signal quality is intermittently checked or depending on the location of the CS. The combined execution functions are used for handover even in this case. In addition, in performing a seamless handover sequence, a start level of a general handover sequence may be satisfied. To avoid collision of the two sequences, the normal handoff sequence is not initiated, and the seamless handoff sequence continues. That is, when a seamless handover sequence is being performed, even if the received signal quality of the currently connected CS is lower than a limit level, a general handover sequence is not started. This ensures smooth and correct handover.

For example, a normal handover sequence looks for a CS that is expected to provide a received signal quality better than the selected level of the predetermined handover target region, and then selects it as the new radio connection CS. By making the seamless handover processing level higher than the handover target region selection level, it is prevented that the quality of the received signal is degraded by performing the seamless handover sequence, while the normal handover sequence can search for a target CS whose received signal quality is relatively poor. Therefore, the seamless handoff sequence of the present invention does not refer to only the handoff sequence performed before the handoff is required. In contrast, the seamless handoff sequence of the present invention also refers to a sequence that performs handoff to obtain a received signal quality better than that which can be obtained by a general handoff sequence. In this way, the seamless handoff sequence maintains the received signal quality that cannot be obtained with the normal handoff sequence even near the cell boundary.

In addition, the seamless switching sequence uses the second circuit to receive a signal for a predetermined length of time, thereby searching for a CS whose received signal quality is expected to be better than that of the currently connected CS. If there is one such CS, the sequence treats that CS as a candidate CS with which a radio communication connection is to be established over the second circuit. The sequence then establishes a new radio connection to communicate with the CS selected as the candidate CS (or one of several CSs, if there are two or more such CSs). In this case, only those CSs whose received signal quality is higher than a predetermined value are selected, ensuring that the received signal quality will be improved before and after the handover. That is, another CS is searched, which is expected to provide a received signal quality better than that of the currently connected CS, and a radio communication connection is established with the CS, which may provide a received signal quality at the cell boundary that cannot be achieved by the currently connected CS. When the number of CSs expected to provide better received signal quality than the signal quality of the currently connected CS exceeds a predetermined number, only the predetermined number of CSs with the best received signal quality are selected as candidate CSs. This option not only discards CS with relatively poor received signal quality, but also saves space within the PS to register or store candidate CS information.

If there are multiple candidates, the seamless handoff sequence repeats selecting the candidates, one at a time, for attempting to establish a radio connection through the second circuit until all candidates have been selected a predetermined number of times. If the time slot is successfully allocated, the sequence establishes a radio communication connection with the CS. This avoids using a seamless handoff sequence for too long. As a result, even if the received signal quality detected by the second circuit changes in a short time with the rapid movement of the PS, the current state is correctly reflected in the detected quality, and thus the seamless handover sequence is correctly performed.

The seamless handoff sequence may also be used to prevent interruption of communications during the switch back. Generally, when the received signal quality of a currently connected CS deteriorates, a handover starts to disconnect a radio connection with the CS and then search for another new CS. If the sequence does not find a new CS that can establish a radio connection during the search, the sequence performs an operation of re-establishing a radio connection with the CS that has just been connected by radio (i.e., an operation of switching back). However, if the sequence fails to find a CS to which a radio connection can be established while the PS is rapidly leaving the old CS with a radio connection, the received signal quality is further degraded, and a switch-back operation cannot be performed to re-establish the connection. To address this problem, a preferred embodiment of the present invention has one of the first circuit and the second circuit, e.g., the first circuit, perform a switch back operation while the other, e.g., the second circuit, performs a seamless handoff sequence. In a normal CS location, when a PS is leaving one CS, the PS is approaching another CS. Thus, by simultaneously performing the back-switching operation and the seamless handover by the first circuit and the second circuit, the communication interruption can be prevented even if the back-switching operation is performed when the PS moves rapidly.

In one embodiment of the present invention, the PS is configured to be able to perform a seamless handoff sequence. That is, a plurality of circuits corresponding to these functional parts (from the antenna or the radio unit to the signal processor) are provided. When a first circuit, i.e. one of the plurality of circuits, has a radio communication connection with a CS, the controller (corresponding to a component of the signal processor, in particular the MPU) may use a second circuit, i.e. one of the other circuits, to perform the seamless handover sequence. Providing the plurality of circuits in this manner enables a seamless handoff sequence to be easily performed, which is one of the advantages of the present invention.

The multiple transmit/receive systems may be constructed in several ways. One way is to provide at least two circuits, each having a modulation/transmission circuit and a reception/demodulation circuit (dual transmission/dual reception mode). The modulation/transmission circuit is a series of circuits for modulating a signal and transmitting the modulated signal, and the reception/demodulation circuit is a series of circuits for receiving a radio signal from the CS and demodulating the received signal. In this mode, the controller may maintain a radio communication connection with the old CS having a radio connection using the modulation/transmission circuit and the reception/demodulation circuit of the first circuit until a predetermined time before a new communication connection is established using the second circuit. This way the time for the interruption of the communication is reduced to 0 in practice.

Another method of providing at least two transmission/reception systems is that each system has a reception/demodulation circuit for receiving a radio signal of the CS and demodulating the received signal, but the two systems share a modulation/transmission circuit for modulating a signal and transmitting the modulated radio signal to the CS (single transmission/double reception mode). When communicating over one of the first and second circuits, the shared modulation/transmission circuit is used to transmit signals to communicate with the CS over the radio link. When performing a seamless handoff sequence, the shared modulation/transmission circuitry is used to maintain a radio connection with an old CS having a radio connection until a transmission signal is needed to communicate with a new CS, thereby transmitting a signal for communication with the new CS having a radio connection. The switching is done under the control of the controller. In this manner, the communication is disconnected between the time when the shared modulation/transmission circuit starts transmitting a signal to attempt to establish a radio connection with a new radio connection target and the time when a radio connection is established with a new radio-connected target CS. Therefore, although the time for disconnecting the communication is longer than that of the dual transmission/dual reception mode, it is shorter than that of the related art. Furthermore, the circuit in this manner is smaller than the circuit in the dual transmit/dual receive mode.

In addition, the single transmit/dual receive mode can be implemented in a PS configured with dual transmit/dual receive modes (with an additional modulation/transmit circuit). Under certain conditions, it is sometimes better to perform a single transmit/dual receive mode in a PS with dual transmit/dual receive mode. That is, if a time slot for a radio connection with the currently connected target CS is allocated by the new radio connection target CS in the course of performing a seamless handover sequence, the controller causes the first circuit to disconnect the radio connection. This function prevents the PS from receiving its own generated signal and solves the problem of time slot collision that occurs with a probability of 1/8 because 4 time slots are used for uplink/downlink transmission.

In the method of the present invention, the communication disconnection time for performing the seamless handover sequence is shorter than the communication disconnection time for performing the normal handover sequence. In addition, the method of the present invention performs a seamless handoff sequence before handoff is actually required, enabling communication to continue without interruption even if the user carrying the PS is moving rapidly. Further, the method of the present invention combines the ordinary handoff sequence execution function with the seamless handoff sequence execution function, so that the handoff can be performed even if the ordinary handoff sequence start condition is satisfied before the seamless handoff sequence is performed. In the preferred embodiments of the present invention, there are other effects as described above or below.

According to another aspect of the present invention, there is provided a handover method for a PHS (personal handyphone system) in which a plurality of personal stations each carried by a user and capable of establishing a radio connection with one of a plurality of geographically distributed cell stations connected to each other in an autonomous distributed manner in accordance with a TDMA-TDD (time division multiple access-time division duplex) protocol, the handover method being performed by the personal station in a state where the radio connection with one of the cell stations is established,

the handover method has a seamless handover sequence which maintains a radio connection for a call with a cell station currently having a radio connection through a first circuit while letting a second circuit search for a cell station with which a personal station can establish a radio connection soon, performs a process of maintaining a radio connection for a call with the cell station using the second circuit if the corresponding cell station exists and has a higher signal quality than the cell station currently having a radio connection, and maintains the radio connection by the first circuit until the process is finished, and continues to maintain the current radio connection state if there is no corresponding cell station,

the radio connection processing of a call with the cell station includes processing of transmitting a link channel allocation request to the cell station, receiving a link channel allocation from the cell station, and exchanging synchronization burst signals.

Drawings

Fig. 1 is a block diagram of a PS configuration in one embodiment of the invention.

Fig. 2 is a flowchart of the handover start sequence in this embodiment.

Fig. 3 is a timing diagram for seamless handoff in dual transmit/dual receive mode.

Fig. 4 is a diagram illustrating a time slot coincidence state.

Fig. 5 is a flow chart of switching between dual transmit/dual receive mode and single transmit/dual receive mode.

Fig. 6 is a timing diagram for seamless handoff flow in single transmit/dual receive mode.

Fig. 7 is a flowchart of the operation of switching back.

Fig. 8 is a timing chart illustrating the operation of switching back.

Fig. 9 is a block diagram of a general PS configuration example.

Fig. 10 is a flowchart illustrating a general handover start sequence.

Fig. 11 is a timing chart illustrating the flow of the handover current.

Detailed Description

A preferred embodiment of the present invention will be described below with reference to the accompanying drawings. Components corresponding to those in FIGS. 9-11 are numbered the same and thus a description of these same components will not be repeated.

Fig. 1 is a configuration of a PS used in a PHS system in one embodiment of the present invention. The PS in the figure has a similar hardware configuration to the general PS shown in fig. 9 except that the signal transmission/reception part is doubled. That is, there are two antennas, antenna switches, reception/transmission circuits in the radio unit 12A, and modulators and demodulators in the pi/4 QPSK modem 34A. In addition, each of the TDMA-TDD controller 32A and the TCH-CODEC 30A in the signal processor 14A includes control and processing functions for both transmit/receive systems. In this figure the numbering of the elements associated with the first transmit/receive system in the first circuit is appended with "-1", while the numbering of those elements associated with the second transmit/receive system in the second circuit is appended with "-2".

The circuit configuration and circuit function of the PS in this embodiment are different from those in the ordinary PS. However, this is not the only difference. The PS in this embodiment utilizes not only the handover processing level and the handover target region selection level of the normal PS but also a new seamless handover processing level and a seamless handover target region selection level difference, each of which is stored in the memory 16A, for example, an EEPROM, for performing the handover processing.

Fig. 2 shows a start sequence of the handover process, and fig. 3 shows a signal timing during the seamless handover process. Assume that a radio connection (100) has been established with one CS using one of two transmit/receive systems, as shown in fig. 2. In the following description, it is assumed that the communication is performed with the first transmission/reception system. One MPU 26A in the signal processor 14A checks the quality of the signal from the current radio connection target CS every 1.2 seconds (102). That is, the MPU 26A calculates a moving average of the RSSI signals of the last 240 frames, and finds the reception level L therefrom (104). It also records the number of errors in the last 240 frames and thus calculates the frame error rate FER.

Then, the MPU 26A simultaneously executes a start sequence of the normal handoff sequence and a start sequence of the seamless handoff sequence. The start sequence of the normal handoff sequence is almost the same as the conventional pass sequence shown in fig. 10. Except that when the seamless handover sequence is performed, the normal handover sequence is stopped, and control is given to step 112 (120). This process prevents the normal handoff sequence from colliding with the seamless handoff sequence. That is, even if the start sequence of the normal handoff sequence is satisfied (106, 108, 114), the seamless handoff sequence can be continued if the seamless handoff sequence is being performed. In this way, MPU 26A can select a CS having better received signal quality and complete handover earlier. The normal handoff sequence is performed by the first transmit/receive system currently in use for communication.

The start sequence of the seamless handover is performed in the following manner. First, the MPU 26A checks that the reception level L calculated in step 104 is equal to or less than the seamless handoff processing level THL2 (122). The seamless handoff processing level THL2 is set to a value, as compared to the handoff processing level THL1, that is large enough to maintain the radio communication connection. It is also necessary to make the sum of the seamless handoff processing level THL2 or the seamless handoff processing level THL2 and the seamless handoff target region selection level difference X larger than the handoff target region selection level so that the seamless handoff can improve the received signal quality as much as possible. If the handoff process level THL1 is set to 22dB μ V and the handoff target region selection level is set to 34dB μ V as described above, the seamless handoff process level THL2 should be set to, for example, 35dB μ V. If L ≦ THL2 is not satisfied, communication is performed through the current radio connection target CS.

If L ≦ THL2 is satisfied in step 122, MPU 26A selects a candidate cell or candidate CS using a second transmit/receive system that is different from the one currently involved in the communication to receive signals for 300 milliseconds (124). That is, while performing communication using the first transmission/reception system shown in fig. 3, MPU 26A searches for CS using the second transmission/reception system in step 124 in fig. 2 (202A). If the CS search process finds one or more CSs (126), each having a signal reception level higher than the signal reception level L calculated in step 104 by at least one predetermined seamless handover target region selection level difference x (db), the MPU 26 selects all the obtained CSs as handover target candidate CSs under the control of the TDMA-TDD controller 32A. Alternatively, the MPU 26 selects a predetermined number of such CSs in accordance with the level L to save memory space. The seamless handover target region selection level difference X is set to a value in the range of, for example, 6dB to 10 dB. In summary, the received signal level of the plurality of CSs is L + x (db) or higher. At the same time, the memory space for storing the candidate CS should be kept to a minimum. To this end, in step 126, the MPU 26A selects a predetermined number (e.g., 8) of CSs having the highest signal reception level from among CSs having signal reception levels equal to or higher than L + x (db). The information of the remaining CSs is cleared.

Next, the MPU 26A selects one (128) from the candidate CSs selected in step 126. For example, MPU 26A selects a candidate CS having the best received signal quality from those candidate CSs for which link channel assignment has not been completed. MPU 26A sends a link channel assignment request to the selected candidate CS through the second transmit/receive system (130). This is done using SCCH as one of the control channels. If MPU 26A successfully receives a link channel assignment, i.e., time slots for TCHs, from the candidate CS via SCCH (132), it continues the communication and establishes a radio connection with the second transmit/receive system (134). For example, it exchanges synchronization burst signals with these time slots. In contrast, if no link channel assignment is received, MPU 26A passes control to step 128 to send a link channel assignment request to the next candidate CS. It is to be noted that when the number of attempts to transmit a link channel assignment request has reached the maximum (e.g., twice (136)), or there is no next candidate CS anymore (138), the MPU 26A abandons the handover by the seamless handover sequence, and passes control to step 100.

As described above, the call sequence in which the radio connection is established before communication is a sequence of the following communication steps: sending a link channel assignment request, receiving a link channel assignment response, exchanging synchronization burst signals, and receiving a CC response after transmitting/receiving a signal not shown. When the UA is sent out or the CC response is received, a radio communication connection with a new radio connection target CS (i.e., a handover target CS) is established. Thus, with a seamless handoff sequence, communication with the old CS (the handoff source CS) continues with the first transmit/receive system until either a UA is sent or a CC response is received. In this embodiment, this means that the time to disconnect the communication during the execution of the seamless handoff sequence is very short. In the case where the time is longest, it is equal to a period of time from the time of transmitting the UA to the time of receiving the CC response. In comparison with the conventional handover sequence shown in fig. 11, in which communication is disconnected from the time when the CS (202) is found to the time when the CC response is received, the seamless handover sequence in this embodiment apparently allows the PS user to obtain a smooth handover.

It is important for implementing the seamless handover sequence that brings about the above-described advantages that the handover is performed under the control of the seamless handover process level THL2 and the seamless handover target region selection level difference X. However, the improvements brought about by this embodiment are not limited to the introduction and addition of new handoff sequences (seamless handoff sequences).

First, when the condition defined by the seamless handoff process level THL2 and the seamless handoff target region selection level difference X is satisfied, a seamless handoff sequence is performed in this embodiment. That is, the seamless handover sequence is performed only when the received signal quality in the first transmission/reception system starts to deteriorate and another CS in the vicinity is found, which is expected to ensure that the received signal quality is higher than that of the current radio connection target CS. In other words, in this embodiment, a seamless handoff sequence is performed in the case where it is not necessary to perform a normal handoff sequence, but better received signal quality is obtained if a handoff is performed. Thus, the PS in this embodiment can improve the received signal quality, particularly near the cell boundary, while at the same time enabling the user to continue communication even if the user carrying the PS is moving fast, e.g., at a speed of 80 km/hour (which is a measure.

In this embodiment, a normal handoff sequence is started instead of a seamless handoff sequence according to circumstances. For example, when the PS moves rapidly to a position near the cell boundary within a time of 1.2 seconds, which is the time interval for calculating the reception level L and the frame error rate FER, it is sometimes difficult to maintain the radio connection with the old connection target CS without judging whether the condition for performing the seamless handover sequence is satisfied (or until the condition is satisfied twice in succession). In this case, a normal handover sequence is performed in this embodiment. In this way, a seamless handover sequence and a normal handover sequence are selectively performed while intermittently checking the received signal quality.

Moreover, the PS in this embodiment has two transmit/receive systems for supporting the execution of a seamless handoff sequence. One reason for adopting this circuit configuration is that in PHS, which is an autonomously assigned dynamic channel assignment system, CS is not always synchronized in the manner in a cellular system. In a cellular system where the CSs are always synchronized, the PLL in the receiving circuit may be synchronized for a short time with some other frequency to check the received signal quality of another CS. However, in PHS, the time slots used for CS control signals in a cell may coincide or overlap in time with the time slots used by a TCH of another CS in an adjacent cell. For PS in the cell overlap region, this requires a CS search to be performed in step 116 or 124 of fig. 2 using 300 milliseconds, during which time TCH synchronization is stopped. This requirement causes the communication to be disconnected during the CS search (202) in the conventional sequence shown in fig. 11. The PS in this embodiment searches for the CS using a second transmitting/receiving system not used for communication, thereby making the disconnection time of communication shorter than that of the ordinary PS.

The PS in this embodiment communicates with one of the two transmitting/receiving systems, and the other transmitting/receiving system searches for the CS and performs a call sequence. One problem caused by this method is solved in the following manner.

This problem is illustrated in fig. 4. That is, when the time slot for communication in the first transmission/reception system almost coincides with the time slot allocated by the handover target CS in the second transmission/reception system, as shown in fig. 4, the PS receives the radio wave transmitted by itself. To avoid this, the PS in this embodiment employs the following method. When the handover target CS (i.e., the one to which the link channel allocation request is transmitted by the second transmission/reception system) allocates the time slot that has been allocated by the handover source CS (i.e., the time slot used for communication by the first transmission/reception system) (300), a single transmission/double reception mode is adopted instead of the above-described double transmission/double reception mode under the control of the MPU 26A shown in fig. 5.

In the two transmission systems and the two reception systems in this embodiment, the PS in the single transmission/double reception mode uses one system for transmission and the two systems for reception to maintain a radio communication connection with the handover source CS, searches for one CS to be used as the handover target CS, and establishes a radio connection with the handover target CS. When the PS executes step 134 in this mode, the communication is temporarily disconnected as shown in fig. 6. This is because, when the PS obtains a link channel allocation from the handover target CS, the PS is forced to transmit a synchronization burst signal in a time slot obtained by the link channel allocation using the transmission system. In other words, the transmission system used for communication up to now is dedicated to transmitting a signal to the handover target CS after the completion of the burst exchange. Therefore, the time to disconnect the communication is longer compared to the dual transmission/dual reception mode shown in fig. 3; however, the time for disconnecting the communication is short compared to the conventional handoff sequence shown in fig. 11. The PS in this embodiment uses this method to implement dual transmit/dual receive mode.

In step 116 of fig. 2, the PS searches for the CS in 300 milliseconds. If a CS is found that is expected to provide good received signal quality sufficient to meet the selected level of the handover target region, step 118 is performed to establish a radio connection with the CS. Conversely, if such a CS is not found, a switch back operation is performed. The operation of switching back is performed to re-establish a connection with the CS with which radio communication was disconnected before searching for the target CS (see fig. 11). The switch back operation is done using the sync burst signal from the CS.

In this embodiment, when the received signal quality deteriorates during communication (204 in fig. 8), and the sequence shown in fig. 2 starts the switch-back operation (206), the MPU 26A causes the transmission/reception system that has been used for communication (in this example, the first transmission/reception system) to perform the switch-back operation. If the receiving level of the synchronous pulse train signal sent by the switched target CS is high enough, the CS can be established with a radio connection; otherwise, the radio connection cannot be reestablished. Therefore, the MPU 26A causes the first transmission/reception system to receive the synchronization burst signal from the switched-back target CS. Meanwhile, the MPU 26A checks whether the reception level of the synchronization burst signal is greater than or equal to the executable level of switching back (140 in fig. 7) based on the RSSI signal or the like received from the first transmission/reception system during reception of the synchronization burst signal. If no sync burst signal is received that is greater than or equal to the switch back executable level, MPU 26A causes speaker 24 or some other sound or vibration component to sound or vibrate, giving an alarm signal that leaves the service area (142). The MPU 26A monitors the reception level of the synchronization burst signal for a maximum of 10 seconds. If a sync burst signal having a level greater than or equal to the executable level for switching back is received before the end of the 10 second period, or more than 10 seconds, the MPU 26A stops sounding or vibrating (144).

In the conventional PS, if no sync burst signal having a level greater than or equal to the executable level of the switch-back is detected within 10 seconds, the PS cannot establish a radio connection with any CS and communication is disconnected. In this embodiment, because the second transmit/receive system performs the seamless handoff sequence simultaneously with the first transmit/receive system performing the switch back operation, see fig. 7 and 8, the communication is not disconnected if the seamless handoff sequence successfully completes the handoff. In FIG. 7, the seamless handoff sequence corresponds to steps 124A-138A and 102A. The processing steps in steps 124A-138A are almost the same as those performed in steps 124-138 of FIG. 2. It is necessary to set the "recall-type handover target processing level" in step 126A to be higher than the handover target region selection level. In step 102A, measurements are taken over a 1.2 second period as in step 102. Step 202C in fig. 8 corresponds to step 124A, and so on.

In the above description, both the reception level L and the frame error rate FER are used to check and evaluate the received signal quality. Other indicators may also be used in practicing the present invention. Wherever handover is required, the present invention can be applied regardless of the type of protocol (industrial or public application). When the seamless handoff sequence is performed according to the steps in fig. 7, the sequence may be switched to the single transmit/dual receive mode shown in fig. 5.

Claims (9)

1. A hand-off method for a Personal Handyphone System (PHS) in which a plurality of personal stations, each carried by a subscriber, are capable of establishing a radio connection with one of a plurality of interconnected geographically distributed cell-stations in an autonomously distributed manner according to a time division multiple access-time division duplex (TDMA-TDD) protocol, said hand-off method being performed by a personal station in a state in which a radio connection is established with one of said cell-stations,

the handover method has a seamless handover sequence which maintains a radio connection for a call with a cell station currently having a radio connection through a first circuit while letting a second circuit search for a new cell station with which a personal station can establish a radio connection soon, if there is a corresponding new cell station and the personal station can be radio-connected to the new cell station with a higher signal quality than the cell station currently having a radio connection, performs a process of establishing a new radio connection for a call with the new cell station using a second circuit and maintains the radio connection by the first circuit before the end of the process, and if there is no corresponding new cell station with which the personal station can make a radio connection with a higher signal quality than the cell station currently having a radio connection, continues to maintain the current radio connection state without establishing a new radio connection,

the radio connection processing for the call with the new cell station includes processing for transmitting a link channel allocation request to the cell station, receiving a link channel allocation from the cell station, and exchanging synchronization burst signals.

2. A handover method according to claim 1, characterised in that the seamless handover sequence receives signals over a predetermined length of time via the second circuit to search for cell stations for which a received signal quality exceeding the received signal quality of signals from cell stations currently having radio connection is expected, and if such cell stations are successfully found, the cell station is selected as a candidate for establishing a new radio connection via the second circuit.

3. The handover method according to claim 2, wherein the seamless handover sequence searches only cell stations whose received signal quality is higher than that of a target currently having radio connection to some extent as candidates.

4. The handoff method of claim 3 wherein the seamless handoff sequence selects as a candidate the predetermined maximum number of cell stations having the highest received signal quality if the number of cell stations having received signal quality higher than the received signal quality of the signal from the currently connected cell station exceeds the predetermined maximum number.

5. A handover method according to claim 2, c h a r a c t e r i z e d in that, if there are a plurality of said candidates, the seamless handover sequence selects one candidate at a time for attempting to establish the radio connection repeatedly through the second circuit within a predetermined number of times until the plurality of said candidates are used in said attempt or a time slot is successfully allocated, and if the time slot is successfully allocated, establishes a radio connection with such a cell station.

6. A personal station for use in a Personal Handyphone System (PHS), the personal station being carried by a user and capable of establishing radio connections with one of a plurality of geographically distributed interconnected cell sites in an autonomous distributed manner in accordance with a time division multiple access-time division duplex (TDMA-TDD) protocol, the personal station comprising:

a plurality of circuits which receive radio signals from the cell station and demodulate the received signals while modulating the signals and transmitting them to the cell station by radio, respectively, and

a controller for controlling a first circuit currently used for communication and a second circuit not currently used for communication according to the received signal quality, the first circuit and the second circuit being included in the plurality of circuits, and performing handover according to the method of one of claims 1 to 5.

7. The personal station of claim 6, wherein each of the first circuit and the second circuit includes a modulation/transmission circuit for modulating signals and transmitting the modulated signals to the cell station, and a reception/demodulation circuit for receiving radio signals from the cell station and demodulating the received signals.

8. The personal station of claim 7, wherein said controller disconnects the radio connection made by the first circuit if a new cell station is assigned a time slot that is the same as the time slot being used for the radio connection with a currently connected cell station when performing said seamless handoff sequence.

9. The personal station of claim 6, wherein each of the first circuit and the second circuit has a receiving/demodulating circuit for receiving a signal from the cell station and demodulating the received signal, and the first circuit and the second circuit share a modulating/transmitting circuit for modulating the signal and transmitting the modulated radio signal to the cell station, and

when communicating via the first or second circuit, the modulation/transmission circuit is switched, under control of the controller, to transmit signals to the cell station currently having radio connection to maintain radio connection with the cell station currently having radio connection until a seamless handoff sequence is performed requiring transmission of signals for communication with a new cell station, which is then transmitted for communication with the new cell station.