Cellular mobile communications system using frequencies commonly shared by neighboring cells for handoff operations

The present invention relates generally to cellular mobile communications systems, and more particularly to such a system operating during handoffs.

With conventional cellular mobile communication system, unique frequencies are allocated to each cell site. In addition to the transmitter and receivers operating at the allocated frequencies, each cell site is provided with as many receivers as there are neighbouring cell sites for receiving signals from mobile terminals for controlling handoffs. One shortcoming of the previously proposed system is that the number of such receivers is several times greater than the number of receivers operating at the allocated frequencies.

WO-A-9 107 020, a document falling under Article 54(3) EPC, discloses mobile-assisted soft-handoff in a CDMA cellular system. Handoff is initiated by the mobile when the signal strength from a neighbouring base station becomes stronger than that from the base station with which the mobile is currently connected.

A feature of a cellular mobile communication system to be described below, as an example, is that it has a smaller number of receivers for controlling handoffs.

According to one aspect of the present invention, there is provided a method of performing a handoff on a mobile station link from a first base station to a second base station, comprising the steps of: <ul id="ul0001" list-style="none" compact="compact"><li>a) assigning a common frequency to the first and second base stations;</li><li>b) detecting, at a mobile station, field strengths of signals transmitted from said first and second base stations;</li><li>c) transmitting, from the mobile station, a start-of-diversity-mode request signal when the field strength of said second base station is higher than a first threshold value, but lower than the field strength of said first base station;</li><li>d) transmitting, from the mobile station, an end-of-diversity-mode request signal when the field strength of said first base station is lower than a second threshold value;</li><li>e) transmitting, from the mobile station, signals on said common frequency during an interval between the reception of said start-of-diversity-mode signal and the reception of said end-of-diversity-mode signal; and</li><li>f) receiving, at said first and second base stations, the signals transmitted on said common frequency from said mobile station and diversity-combining the received signals.</li></ul>

The following description and drawings disclose, by means of examples, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby.

In the drawings:- <ul id="ul0002" list-style="none" compact="compact"><li>Fig. 1 is a block diagram of a base station of a cellular communications system,</li><li>Fig. 2 is a block diagram of a mobile terminal of a cellular communications system,</li><li>Figs. 3A and 3B are flowcharts describing a sequence of programmed instructions performed by the controller of the base station in one arrangement,</li><li>Fig. 4 is a flowchart describing a sequence of programmed instructions performed by the controller of the mobile terminal in the one arrangement,</li><li>Figs. 5A and 5B are schematic illustrations showing different points of a path between typical base stations travelled by mobile terminals and frequencies assigned to the mobile terminals in the one arrangement,</li><li>Fig. 6 is a flowchart describing a sequence of programmed instructions performed by the controller of a mobile terminal according to a second arrangement,</li><li>Fig. 7 is a flowchart describing a sequence of programmed instructions performed by the controller of the base station according to the second arrangement,</li><li>Figs. 8A and 8B are schematic illustrations showing different points of a path between typical base stations travelled by mobile terminals and frequencies assigned to the mobile terminals in the second arrangement.</li></ul>

The cellular communications system for mobile telephony according to this invention includes a plurality of base stations respectively located at the cell sites. As shown in Fig. 1, each base station of this invention includes an antenna 1 coupled to a diplexer 1 to which a plurality of receivers and transmitters are connected. Receivers 3-1 through 3-I operate at frequencies f_i (where i = 1, 2 ....I) which are uniquely allocated to the base station for receiving signals from mobile terminals located within the boundary of the own cell. Each signal from the mobile unit is time-division multiplexed with other mobile's signals on each of the assigned frequencies. Receivers 4-1 through 4-J are provided for use during an initial handoff operation and operate at frequencies f_j (where j = 1, 2, ....J), and receivers 5-1 through 5-K are provided for use during a subsequent handoff operation and operate at frequencies f_k (where k = 1, 2, ....K). The frequencies fj and f_k are shared by neighboring base stations. The outputs of receivers 3-1 through 3-I are coupled to a controller 6 and further to a land-line system, or mobile telephone switching office, while the outputs of receivers 4-l ∼ 4-J are coupled to controller 6 and to a diversity combiner 7, and further to neighboring base stations. The outputs of receivers 5-1 - 5-K are coupled to controller 6, land-line system and neighboring stations. Diversity signals are also received from the neighboring base stations and supplied to diversity combiner 7 to provide space diversity combining with a neighboring base station. The outputs of diversity combiner 7 are coupled to the land-line system.

Time-division multiplexed transmit signals from the land-line system are supplied to transmitters 8-1 ∼ 8-I for transmitting signals on a frequency F_i as well as to transmitters 9-1 - 9-J for transmitting TDM signals on a frequency F_j, the outputs of transmitters 8 and 9 being coupled to diplexer 2. The selection of transmit frequencies is also controlled by controller 6.

In Fig. 2, a mobile terminal comprises an antenna 11, a diplexer 12, and receivers 13-1, 13-2 to which local carriers are supplied from frequency synthesizers 14-1 and 14-2 under control of a controller 15. The outputs of receivers 13-1 and 13-2 are coupled to a diversity combiner 16 in which signals received from neighboring base stations are diversity combined in a manner as will be described. The output of diversity combiner 16 is applied to a hybrid circuit 19 and thence to a handset 20. Transmit signal from the handset is coupled through hybrid 19 to a transmitter 17 whose carrier frequency is supplied from a frequency synthesizer 18 under control of the controller 15.

Figs. 3A and 3B show details of programmed instructions to be performed by the controller 6 of the base station according to a first embodiment of this invention, Fig. 4 showing details of programmed instructions to be performed by the controller 15 of each mobile terminal according to the first embodiment.

In response to a frequency assignment request transmitted on a control (setup) channel from a mobile terminal (step 30, Fig. 3A), control branches at step 30 to step 31 to compare the field strength D of the received signal with a threshold value D_th. If D ≥ D_th, control branches at step 31 to step 32 to reset a frequency variable i to one. Step 32 is followed by a decision step 33 in which the frequency f_i is checked for availability.

Note that frequencies f_i are allocated to a base station X for assignment to mobile terminals which are located within the cell X of that base station when a call is originated from or terminated to such mobile terminals and frequencies f_m are allocated to a base station Y for assignment to mobile terminals which are located within the cell Y.

If f_i is available, control branches to step 34 to assign frequencies f_i and F_i to the requesting mobile terminal by transmitting a frequency assignment signal to it and establishes mobile-to-base and base-to-mobile channels. If f_i is not available, control branches at step 33 to step 35 to check to see if variable i is equal to a maximum value I. If the answer is negative, control branches to step 36 to increment variable i by one and returns to step 33 to check the next frequency for availability; otherwise it returns to step 30 to repeat the process. If the field strength D is determined by step 31 as being lower than threshold value D_th, control branches to step 37 to reset variable j to one and goes to step 38 to check to see if frequency f_j is available. If not, control branches to step 40 to check to see if j = J. If frequency f_j is available, exit then is to step 39 to assign frequencies f_j and F_j to the requesting mobile terminal. If variable j is not equal to J, control branches at step 40 to step 41 to increment variable j by one and returns to step 38 to repeat the process until j = J. If j = J, control branches at step 40 to the starting point of the program.

Therefore, the requesting mobile terminal is switched to voice-channel frequencies f_i and F_i if the signal level of the control channel is higher than the threshold level D_th. Otherwise, it is switched to frequencies f_j and F_j.

It is assumed that a mobile terminal is traveling in a direction away from base station X to base station Y, passing through several points A, B, C, D and E as shown in Fig. 5A. The program execution now enters a handoff routine which begins with an initialization step 42 in which variable i is set to 1. Exit then is to step 43 to check to see if frequency f_i is the one assigned to the mobile terminal. If the answer is affirmative, control branches to step 44 to compare the signal level D_i of frequency f_i with threshold level D_th.

When signal level D_i becomes lower than threshold level D_th, control branches to step 45 to switch the current frequencies of the mobile terminal to common frequencies f_j and F_j and moves to step 46. If frequency f_i is detected as being not assigned to any mobile terminal, or if level D_i is equal to or greater than threshold D_th, control branches to step 46 to check to see if i = I. If i is not equal to I, control branches to step 47 to increment variable i by one and returns to step 43 to repeat the process on the next unique frequency f_i.

Therefore, if a mobile terminal to which frequencies f_i and F_i have been assigned is crossing the point A, the signal from the mobile terminal decreases and base station X will cause it to switch to frequencies f_j and F_j.

Control then branches at step 46 to step 48 to set variable J to 1 and exits to decision step 49 to check to see if frequency f_j is the one assigned to the mobile terminal. If the answer is negative, control branches to step 50 to check to see if j = J. If j is not equal to J, control branches to step 51 to increment it by 1 and returns to step 49. If an assigned frequency f_j is detected, control branches at step 49 to step 52 to check to see if frequency f_j from the mobile terminal is being diversity-combined with a version of the signal f_jn from that mobile terminal relayed through a neighboring base station. If the answer is negative, control branches at step 52 to step 53 to compare the level D_j of signal at frequency f_j with threshold value D_th. If D_j ≥ D_th, the program execution branches at step 53 to step 54 to switch to frequencies f_i and F_i. This will occur if the mobile terminal has made a turn and is traveling backward to station X, crossing boundary point A (Figs. 5A, 5B).

If D_j &lt; D_th, control branches at step 53 to step 55 to receive field strength signals D_jn detected by and sent from neighboring base stations N indicating the level of signals at frequency f_j they are receiving from the mobile terminal. Exit then is to step 56 to determine if there is a base station Y that establishes the relation D_jy ≥ D_th. If there is one, control branches to step 57 to cause the mobile terminal to rune to frequency F_k to enable reception of signal from station Y while enabling it keep reception of signals at frequency F_j from station X. Thus, the mobile terminal is able to start reception on a frequency diversity mode by combining signals at frequencies F_j and F_k received from stations X and Y, respectively. Concurrently, diversity combiner 7 of base station X is enabled by controller 6 to start space diversity combining on a frequency-f_j signal received direct from the mobile terminal with a version of the same signal which is received by base station Y and relayed to base station X as a signal f_jy.

The execution of step 57 will occur when the mobile terminal is entering an area between points B and C (Figs. 5A, 5B). Following the execution of step 57, control returns to step 50 to repeat the process on the next one of the commonly shared frequencies f_j.

When control subsequently passes through step 52, affirmative decision will be made and it branches at step 52 to step 58 to receive the field strength signal D_jy from station Y and exits to step 59 to compare it with a diversity threshold value D_div. If D_jy &lt; D_div, control branches to step 60 to stop the diversity combining with the station Y. This will occur when the mobile terminal has made a turn on its way to station Y and crossed the boundary between points A and B, traveling backward to the starting point of the trip. If D_jy ≥ D_div, control branches at step 59 to step 61 to compare D_jy with the level D_j of the signal received by the own base station direct from the mobile terminal. If D_jy &gt; D_j, control branches at step 61 to step 62 to cause the mobile terminal to switch its transmit frequency from f_j to one of available frequencies f_k. This will take place when the mobile terminal is entering an area between points C and D (Figs. 5A, 5B).

With the mobile-to-base channel frequency being switched to f_k, the program execution now enters a subroutine in which frequency f_k is monitored while the mobile terminal continues traveling to station Y. This subroutine begins with step 63 (Fig. 3B) in which frequency variable k is set equal to 1. Exit then is to decision step 64 to check to see if frequency f_k is the one assigned to the mobile terminal. If the answer is affirmative, control branches at step 64 to step 65 to compare the signal level D_k of frequency-f_k signal with the diversity threshold level D_div. If D_k ≥ D_div, control branches to step 66 to receive field strength signals D_kn from neighboring stations N indicating the levels of signals received by these stations from the mobile terminal, and proceeds to step 67 to compare the level D_k with the level D_kn of each of these signals. If the mobile terminal has made a turn and is traveling backward, the signal level D_k will be become greater than any of levels D_kn, and control branches at step 67 to step 68 to cause the mobile terminal to switch its transmit frequency from f_k to f_j and advances to step 69 to check to see if k = K. If the answer is affirmative, control returns to the starting point of the program; otherwise it branches to step 70 to increment the k-value by one and returns to step 64 to repeat the process.

If the mobile terminal has crossed the point E, approaching base station Y, step 65 will eventually determine that the level D_k is smaller than diversity threshold D_div, and the program execution branches to step 71 to stop using frequencies f_k and F_k and proceeds to step 69, terminating a handoff procedure.

Before base station X ceases communication with the mobile terminal, a neighboring base station, which may be located close to station Y, will detect the frequency-f_k signal from the mobile terminal and establishes a situation appropriate to effect space-diversity combining at frequency f_k with station Y, while transmitting base-to-mobile signals at frequency F_j to permit the mobile terminal to continue frequency-diversity reception. Therefore, the companion base station with which base station Y has been working on space diversity mode has changed from station X to that neighboring station. This will occur when control of the neighboring station determines that frequency f_k is not the one assigned by that neighboring station, and branches at step 64 to step 72 to compare D_k with threshold D_div. If D_k ≥ D_div, control advances to step 73 to start diversity combining on frequency f_k and starts transmitting the signal to the mobile terminal at frequency F_k. Otherwise, control branches at step 72 to step 69.

Each base station is executing a set of similar handoff routines with the exception that frequency variables j and k being transposed with each other. Namely, when base station X is executing a handoff routine with frequency variables i, j and k, it is simultaneously executing another handoff routine with frequency variables j and k transposed in position with each other. Therefore, base station Y is executing a handoff routine with the transposed frequency variables j and k when an approaching mobile terminal has crossed the point E and causes it to switch from commonly shared frequencies f_k and F_k to frequencies f_m and F_m which are allocated to base station Y for assignment to mobile terminals traveling within its cell.

Referring to Fig. 4, the program execution of the mobile terminal associated with the program of Figs. 3A and 3B begins with decision step 80 which checks to see if a call request is generated by the mobile unit or received from a base station. If the answer is affirmative, control branches to step 81 to transmit a frequency assignment request. After receiving a frequency assignment signal (step 82), control executes step 83 by switching the mobile's operating frequencies to the assigned frequencies. Control determines that if there is more than one receive frequency assigned (step 84). If this is the case, control branches at step 84 to step 85 to enable diversity combiner 16 to start frequency diversity reception on signals received at different frequencies by receivers 13-1 and 13-2. Following a negative decision of step 84 or execution of step 85, step 86 is executed by checking to see if a change-of-frequencies signal is received from the base station. If the answer is affirmative, control branches at step 86 to step 87 to determine if there is only one receive frequency to be switched into operation. If the answer is negative, control branches at step 87 to step 83 to repeat the process; otherwise it branches to step 87 to cease the diversity reception mode and returns to step 83.

Before terminating the program, decision step 89 is looped as long as the answer is negative at step 86 in order to check for the end of the call.

With the previous embodiment, the decision to effect diversity or nondiversity mode of reception is made by base stations using threshold values which are determined to accommodate a sufficient amount of safety margin for using frequencies which are uniquely assigned to base stations. Therefore, diversity mode of reception accounts for a substantial proportion of each call duration, requiring a sufficient number of commonly shared frequencies for each of different bands f_j and f_k.

In a modified embodiment shown in Figs. 6 and 7, the right to make a decision on diversity or nondiversity mode of reception is on the side of mobile terminals. Since field strength measurement can be precisely detected as a factor for determining the operational mode, the time during which the mobile terminal is allowed to use uniquely assigned frequencies can be extended, reducing the time in which to use commonly assigned frequencies. Therefore, the number of commonly shared frequencies can be reduced in comparison with the previous embodiment.

Referring to Fig. 6, a mobile terminal starts executing a program with step 90 in which it checks to see if a call request is generated by the mobile terminal or received from a base station. If the answer is affirmative, control branches to step 91 co transmit a frequency assignment signal on a control channel. Exit then is to step 92 to scan control channels of surrounding base stations to detect their field strengths D_n. Control proceeds to step 93 to determine if there is a station that establishes the relation D_n &gt; D_div. If the answer is negative, control branches at step 93 to step 103 to switch its transmit and receive frequencies fi and Fi assigned by base station X, and proceeds to step 104.

If the answer is affirmative at step 93, control branches to step 94 to transmit a start-of-diversity-mode request to such base stations X and Y. As will be described, each of such base stations assigns frequencies in response to the start-of-diversity-mode request. Control exits to step 95 to check for the receipt of a frequency assignment signal. If the answer is affirmative, control exits to step 96 to switch the mobile's transmit frequency to f_j, and mobiles' receive frequencies to F_j and F_k. Exit then is to step 97 to check for an end-of-call situation. If the call is still in progress, control branches at step 97 to step 98 to detect field strength D_j and D_k of signals at receive frequencies F_j and F_k and make a comparison between D_j and D_k.

If D_j &lt; D_k, control branches at step 98 to step 99 to switch the mobile's transmit frequency from f_j to f_k and proceeds to step 100 to transpose frequency variables k to j and j to k, with control returning to step 97 to repeat the process.

If D_j ≥ D_k, control branches at step 98 to step 101 to compare the level D_j of the stronger signal with diversity threshold Ddiv. If D_j ≥ D_div, steps 97 through 100 are repeated, and if D_j &lt; D_div, control branches at step 101 to step 102 to transmit an end-of-diversity-mode request to switch the base station receive frequency from f_j to f_i. Control now exits to step 103 to switch its transmit and receive frequencies to f_i and F_i assigned by station X. Control moves to step 105 to check for an end-of-call situation and branches to step 106 if the call is in progress. Control channels are scanned and their field strengths are determined as D_n (step 106) and comparisons are made between D_n and D_div to determine if base stations are present which are establishing the relation D_n &gt; D_div. If the answer is affirmative, control branches at step 107 to step 94. Otherwise, control branches at step 107 to step 105 to repeat the process.

In Fig. 7, the program execution of a base station starts with decision step 200 in which control checks to see if a frequency assignment request is received from a mobile terminal. If the answer is negative, control branches to step 206. Otherwise, it branches to step 201 to check to see if a start-of-diversity-mode request is received. If it is, the program execution branches to step 202 to select available frequencies f_j and F_j and transmit a frequency assignment signal to the mobile terminal. If a start-of-diversity-mode request is not received within a specified period, control branches at step 201 to step 203 to select available frequencies f_i and F_i and transmit a frequency assignment signal to the mobile terminal.

Following the execution of step 202 or 203, control now enters a handoff routine which begins with step 204 to determine if a start-of-diversity-mode request is received from a mobile terminal to which frequencies f_m and F_m were assigned by base station Y and is approaching base station X. If the answer is negative, control branches at step 204 to step 206 to set variable i to one, and if affirmative, it branches to step 205 to assign frequencies f_k and F_j and starts diversity combining on the assigned frequencies, and then proceeds to step 206.

If a mobile terminal is traveling from station Y toward station X (see Fig. 8A, 8B) and crossing a point C₁, control will branch at step 204 to step 205 to assign frequencies f_k and F_j to the mobile terminal, so that it enters a diversity reception mode by combining a frequency-F_j signal of station X and a frequency-F_k signal of station Y. On the other hand, the frequency-f_k signal from the mobile terminal is received by base stations X and Y and diversity combined by station X.

Step 207 is then executed and a check is made whether a start-of-diversity-mode request is received from a mobile terminal using frequencies f_i and F_i which were assigned by station X and is traveling away from station X. When this mobile terminal crosses a point A₁ (Fig. 8A, 8B), the answer will be affirmative at step 207 and control branches to step 208 to assign frequencies f_j and F_j and starts diversity combining. In a manner similar to the mobile terminal crossing the point C₁, the mobile terminal crossing the point A₁ is switched to frequencies f_j, F_j and F_k, and proceeds to effect diversity combining on a frequency-F_j signal of station X and a frequency-F_k signal of station Y, whereas the frequency-f_j signal sent from this mobile terminal is received by base stations X and Y and diversity combined by station X.

If the answer is negative at step 207, control branches to step 209 to check for i = I. Frequency variable i is incremented by one (step 210) until variable i becomes equal to I (step 209).

If the answer is affirmative, control branches at step 209 and enters a subroutine for mobile terminals traveling between points A₁ and B₁. The subroutine begins with step 211 by setting variable j to one. Control then exits to step 212 to check to see if an end-of-diversity-mode request is received from a mobile terminal using frequencies f_j and F_j. If this request is received from a mobile terminal crossing the point A₁ in a direction approaching base station X, the answer is affirmative at step 212 and control branches to step 213 to switch the base and mobile's frequencies to f_i and F_i and discontinue the diversity reception mode.

If no end-of-diversity-mode request is received from such mobile terminals, or if step 213 is executed, control moves to step 214 to check to see if a change-to-f_k request is received from a mobile terminal operating on frequencies f_j and F_j. The decision of step 214 will be affirmative if there is a mobile terminal crossing a point B₁ in a direction away from station X (Figs. 8A, 8B), and control branches at step 214 to step 215 to switch the base and mobile's frequencies to f_j and F_j. Variable j is checked (step 216) for equality to J and is incremented by one (step 217) in a loop including steps 212 through 216.

If an affirmative decision is made at step 216, control enters a subroutine for mobile terminals traveling between points B₁ and C₁. This subroutine starts with step 218 in which frequency variable k is set to one. Exit then is to step 219 to check to see if an end-of-diversity-mode request is received from a mobile terminal operation on frequencies f_k and F_j. If there is a mobile terminal crossing the point C₁ in a direction toward station Y, an end-of-diversity-mode request will be received and an affirmative decision will be made at step 219. Control branches to step 220 to stop using frequencies f_k and F_j.

If there is no mobile terminal crossing the point C₁ in a direction approaching station Y, or if step 220 is executed, control advances to step 221 to check for the reception of a change-to-f_j request signal. This signal will be received from a mobile terminal crossing the point B₁ in a direction away from station Y. When this occurs, control branches at step 221 to step 222 to switch the base and mobile's frequencies from f_k to f_j and proceeds to step 223 to check to see if variable k is equal to K If not, control branches at step 223 to step 224 to increment variable k by one and returns to step 219 to repeat the process. If k = K, control branches at step 223 and returns to the starting point of the program.