Method for controlling transmission during handoff in a communication system5491717Abstract Transmission is controlled using the present method after a handoff procedure has been entered between a subscriber unit and target transceivers. During this process, a communication link between the subscriber and at least one other transceiver is maintained. A quality factor of a signal from the subscriber is received by the target transceiver. If the quality factor meets a quality threshold, then a transmission from the target transceiver is activated. If the target transceiver is active and the quality factor drops below the quality threshold, then the transmission from the target transceiver is terminated. Claims I claim: Description FIELD OF THE INVENTION
TABLE A
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POWER ALLOCATION FOR A SECTOR
POWER
USER Handoff Factor
NUMBER POWER PERCENT
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PPS -- -- 2 WATTS
20%
1-WAY 1.00 1 USER 1 WATTS
10%
2-WAY 2.00 2 USERS 4 WATTS
40%
3-WAY 3.00 1 USER 3 WATTS
30%
TOTAL 4 USERS 10 WATTS
100%
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The distribution of the users in the system is provided in Table B below.
TABLE B
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USER DISTRIBUTION FOR SYSTEM
USER NUMBER DISTRIBUTION
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1-WAY 1 USER 10%
2-WAY 2 USERS 40%
3-WAY 1 USER 30%
TOTAL 4 USERS 100%
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The average transmit power of a cell site allocated per user is 34.3% as determined by equation (1). ##EQU1## where: Power Percentage is the percent of transmit power expended by the site for communication with that class (n-way) of user; Handoff Factor is the number of sites serving that class of user (e.g., during a two site handoff, two sites serve the subscriber the entire time); n-way Units is the number of units for that class; and Total Units is the total number of units in the system. The above calculation results in a forward capacity for a cell/sector of 2.33 users per cell/sector. This is determined by Forward Capacity equation (2). ##EQU2## where: Cell Capacity for Users is the total capacity (100%) less the capacity used for PPS (20%); and Average Allocation is that determined in equation (1). This means that even though each sector is shown addressing four subscriber units, the system capacity is only 2.33 users/sector without the present invention. This is a result of the number of handoffs in progress Referring now to FIG. 2, a flow chart, generally designated 30, illustrating a prior art handoff process is shown. Process 30 commences at start step 31 and measures a pilot from a target site, step 32. If the target pilot is not within a handoff threshold, in this example 16 dB, of the source pilot, process 30 loops back to step 32. If the target pilot is within the handoff threshold, then the subscriber enters handoff, subprocess 39. In handoff, the system activates a traffic channel from the target, step 34. The subscriber unit will continue to measure the pilots from the source and target sites, step 35. If the two signals are within a predetermined range, handoff threshold, of each other, then subprocess 39 loops back to step 35. If the two pilots are not within the predetermined range, then the system discontinues the traffic channel of the site with the weaker pilot, step 37, and the site transmitting the remaining traffic channel is designated the source site. The same process is used in a 3-way handoff as described above. In this instance, two traffic channels from two target sites will be activated as each comes within the handoff threshold. These two sites do not have to be activated simultaneously. For a subscriber, such as subscriber 21, three sites will be providing a traffic signal during the entire handoff process. Again, detracting from the overall system capability. A drawback of the prior art is that even though multiple traffic channels, one per site, will be transmitted to a user during handoff, often only one of those traffic channels will be processed by the subscriber unit. For example, subscriber 19 is currently being serviced by sites 11 and 13. If the signal (traffic channel) transmitted from site 11 is at -100 dBm at subscriber 19 and the signal transmitted from site 13 is at -110 dBm at subscriber 19, then subscriber 19 will process the signal transmitted from site 11 and not the signal from site 13. This occurs because there is enough difference in the signals such that the signal from site 13 is lower (e.g., 6 dB) as compared with the signal from site 11. As a result, the power used to transmit the signal from site 13 was wasted power in that it added to the noise level thereby removing capacity from the overall system. In FIG. 3, a system diagram, generally designated 10', illustrating a handoff technique incorporating the present invention is shown. To illustrate the present invention, the 2-way units 18-20 have each been divided into two units (e.g. 18.1 and 18.2). This is to illustrate that while both serving sites have dedicated resources to the particular subscriber unit, only one of the sites may be transmitting to the unit. Similarly, subscriber unit 21 has been separated into three units designated 21.1, 21.2 and 21.3. In operation, as subscriber 18 moves from site 13' toward site 12', a point is reached where the system will direct site 12' to open a traffic channel to subscriber 18.2. At this point, however, only site 13' is transmitting a traffic channel to subscriber 18.1, as opposed to the prior art where traffic channels would be transmitted from both serving sites. The system will continue to monitor the subscriber until a point is reached where a secondary, or quality, threshold is reached. This can be determined by having the sites monitor the traffic signal from the subscriber unit. Alternatively, the subscriber unit can monitor pilots from the cell sites and provide information to the system based upon those measurements. Once within the transmit threshold, both cell sites will transmit to the subscriber unit as before. However, by delaying the point at which the multiple transmissions to a single subscriber start, the system capacity can be increased significantly. This is illustrated by the line graph, generally designated 40, of FIG. 4. The line graph is a graph, in dB, of the difference between the pilots from the two sites (target less source). At the point where the target pilot would be 16 dBm less than the source pilot, handoff is entered and resources at the target are allocated for the handoff. However, under the present invention, no signal is transmitted or the traffic channel. In other words, the target is placed in a standby mode. Once the difference between the pilots reaches a transmit threshold (e.g. -6 dBm), then the normal 2-way service is provided by transmitting signals from both the source and target sites. As the subscriber moves closer to the target site, the pilot from the target will reach a point where it crosses the transmit threshold at the other end of the scale (6 dBm). At this point, the source site would be placed in standby. As the subscriber unit continues to approach the target, the handoff threshold, 16 dBm, will be exceeded and the source site will discontinue service. The target site will then be designated the new source site. It should be understood that since subscriber units may not move directly from one cell to another, the source and target sites may alternately move in and out of standby several times until the handoff procedure is completed. In addition, the result of the handoff may be that the original source site remains the source site. This would occur when the pilot from the source became stronger again after handoff was entered. In order to prevent a ping-pong effect where a site moves in and out of standby mode, a delay may be added to place the site in a particular mode for some minimum amount of time (e.g., 5 seconds). This can be performed by a timer or through counting transmission frames or slots. By utilizing the present invention, the capacity of the system can be improved as demonstrated by Table C below.
TABLE C
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POWER ALLOCATION FOR A SECTOR
POWER
USER Handoff Factor
NUMBER POWER PERCENT
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PPS -- -- 2 WATTS
20%
1-WAY 1.00 1 USER 1 WATTS
10%
2-WAY 1.25 2 USERS 4 WATTS
40%
3-WAY 1.50 1 USER 3 WATTS
30%
TOTAL 4 USERS 10 WATTS
100%
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The average transmit power allocated per user is 21.4% as determined using equation (1), above. Using this result in equation (2) gives a forward capacity of 3.7 users per cell/sector. This is an increase in capacity of almost 60%. The primary difference here is found in the handoff factor. Because the units are not being serviced by multiple sites during the entire time they are in handoff, there is more capacity available for the system. The handoff factors provided in Table B are an estimate of the average handoff factor that will be experienced in an average site. Actual figures may vary. Referring now to FIG. 5, a flow chart illustrating a process, generally designated 50, is shown. Process 50 commences at 31 and measures the pilot from the target site as in the prior art. Next, it is determined if the target pilot is within the handoff threshold, decision step 33. If not, process 50 loops back to step 32. If the target pilot is within the handoff threshold, then subprocess 52 is entered. Once subprocess 52 is entered, the system sets up the handoff by allocating the appropriate resources at the target site. However, at this time, the site is placed in standby and does not actually transmit, or transmits a very low power signal to reserve the resources. Next, quality factors are determined at the cell sites. These quality factors may be subscriber received E.sub.b /N.sub.o, frame error rate (FER), etc. It should be noted here that the term "frame error rate" is often used interchangeably with "frame erasure rate". The distinction is that frames in which errors exist and are detected are erased frames. These erased frames are then counted to give a frame erasure rate. Frames in which errors exist but are not detected are still frame errors, but are not counted to give a frame erasure rate. Under test conditions when a known signal is transmitted on a traffic channel, a true frame error rate can be determined. In normal operation, when an unknown signal is transmitted, the frame erasure rate is used as a substitute. Returning to process 50, once the received signal quality factors are determined, they are compared to determine if they fall within a transmit threshold range, decision step 55. If they are not within the transmit threshold range, then subprocess 52 loops back to step 54. If the quality factors are within the transmit threshold range, then the standby site is taken out of standby by activating the transmit signal, step 56. Alternatively, when the site is in standby, a very low power signal may be transmitted to have the transmit resources reserved. When the site is taken out of standby, the signal power is increased to a normal level. Once multiple signals are activated, the system continues to monitor the quality of the received signals, step 57. Subprocess 52 then determines if the received signal quality factors are still within the transmit threshold range, decision step 58. If they are within the transmit threshold, then no change is made and subprocess 52 loops back to step 57. If the quality factors are not within the transmit threshold range, then subprocess 52 determines if the pilots are still within the handoff thresholds, decision step 59. If not, then the handoff process is discontinued by de-allocating the resources at the weaker cell site, step 61. If the pilots are still within the handoff threshold, then the weaker signal is terminated (or its power reduced) placing that site in standby, step 60. Subprocess 52 then loops back to step 54. Examples of the signal information that can be utilized for received signal quality includes: reverse power control signal (available at the switch); means and standard deviation of the received signal-to-noise ratio (E.sub.b /N.sub.o); frame error/erasure rate; subscriber received pilot strength report message; subscriber received signal strength message, etc. The measurements may be made at the base sites and reported to a system controller or they can be measured by the mobile, such as in mobile assisted handoff (MAHO). If a MAHO type procedure is used, the mobile may either report measurements or request actions based upon its measurements. One particular example of a quality factor used in the present invention is the Reverse Power Control signal illustrated in FIG. 6. In this chart, the reverse power control is being sent every frame and is either an up or a down power control. When the reverse power control is being toggled up and down regularly it indicates that the subscriber unit is within a range that it can be controlled by the cell site. When the reverse power control is mostly up, it indicates that the mobile is at a distance from the cell site that is to great for good reception of the signal. As shown in FIG. 6, beginning at frame 0, site 1 has a reverse power control pattern that indicates an acceptable signal. On the other hand, sites 2 and 3 show an unacceptable received signal. Therefore, a signal on the traffic channel is transmitted from site 1 and sites 2 and 3 are placed in standby. At around frame 40, the reverse power control bits for site 2 begin to indicate that the received signal quality is approaching that of site 2. Therefore, site 2 will be taken out of standby and will begin transmitting a signal on a traffic channel to the unit. At around frame 70, the reverse power control bits for site 1 begin to indicate a poor power for the received quality signal. At this point, site 1 will be place in standby mode and stop transmitting. This type of measurement would continue until the handoff process was ended. As referred to above, starting and stopping a transmission relates only to the traffic channel for that subscriber. The site is still transmitting the PPS and other traffic signals. Another received quality factor example is provided in FIG. 7 using the frame error (erasure) rate (FER). As shown at time 0, the frame error rate for site 1 is around 1%. Therefore, only site 1 is transmitting to the subscriber unit at that time. At 4 seconds, the FER for site 2 has decreased to around 1% and, at that time, site 2 will be taken out of standby and begin transmitting. At around 7 seconds, the FER for site 1 has increased to a point where it is no longer acceptable and site 1 is placed in standby. Again, this type of measurement would continue until the handoff process was ended. In order to reduce erratic standby behavior, the received signal quality threshold can be an average or summed over a particular time period. For example in FIG. 6, the threshold used can be set to review the reverse power control signal over the last five frames. If the last five, or four of the last five, power control signals are up, then the transmitter is placed in standby, if currently active. In the same manner, a certain time delay can be added to the FER measurement of FIG. 7. In addition to the process described above, the system can be designed to take into account the individual received signal quality levels in addition to the relative received quality levels. For example, even if the received signals to sites 1 and 2 do not fall within a received signal threshold range, if the quality of both received signals is poor, the system can be designed to activate transmission from both sites. This is demonstrated in FIG. 8. In FIG. 8, the FER of all three received signals is poor. Therefore, the system will pick the two best sites and activate their transmissions. Here, the system starts out with the transmissions from sites 2 and 3 in standby. Then as the received signal quality from site 1 degrades, the second best received signal, at site 2, activates site 2 transmission in around 2 seconds. As the received signal from site 1 continues to degrade, the system will switch to sites 2 and 3 and places site 1 in standby, at around 4.5 seconds. As the received signal quality of site 1 begins to improve slightly, the system will switch to sites 1 and 3 and place site 2 in standby. This process would continue until either one received signal improved enough to serve the subscriber unit on its own or until the handoff process ended. An example of how process 50, FIG. 5, could be modified to implement this is illustrated in FIG. 9. In FIG. 9, a subprocess generally designated 90, is inserted between step 54 and decision step 55 of FIG. 5. Following step 54, the process determines if the received quality of any one signal is sufficient, decision step 91. If not, the two best sites are activated (power increased) to serve the subscriber unit, step 92. Subprocess 90 then loops back to decision step 91. This continues until the received signal from one site is sufficient. At this point, if more than one site was active, then the site with the lesser quality received signal is deactivated, step 93. From here, process 50 continues with decision step 55. Thus, it will be apparent to one skilled in the art that there has been provided in accordance with the invention, a method for controlling transmission during handoff in a communication system which fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alterations, modifications, and variations in the appended claims.
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