GB2269726A

GB2269726A - Alternative base-site transmission of a handoff command in a communication system

Info

Publication number: GB2269726A
Application number: GB9312213A
Authority: GB
Inventors: Jeffrey D Bonta; Barry J Menich
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1992-06-22
Filing date: 1993-06-14
Publication date: 1994-02-16
Anticipated expiration: 2013-06-14
Also published as: GB9312213D0; DE4320696C2; GB2269726B; DE4320696A1; FR2692739B1; FR2692739A1

Description

1 2269726 ALTERNATE BASE-SITE TRANSMISSION OF A HANDOFF COMMAND IN A

COMMUNICATION SYSTEM

Field of the Invention

The invention relates generally to cellular radiotelephone systems and more specifically to handoff of communication of a mobile in a cellular radiotelephone system.

Background ofthe Invention

Handing over from one radio-frequency (RF) channel to another in order to maintain a reasonable level of quality is a well known procedure in communications systems, specifically cellular radiotelephone systems. In today's cellular radiotelephone systems, the method of instructing a mobile to move from one RF channel to another is to transmit a analog-modulated signal in analog systems and a digital message using a frame stealing or blank-and-burst procedure and time-division multiple access (TDMA) cellular radiotelephone systems. Given the cellular radiotelephone systems multi-path fading environment, the digital message, or handoff command, is encoded and repeated in such a way that the receiving mobile has the best chance of decoding the message under adverse conditions. In addition, most cellular systems suffer from thermal noise or interference (or both) at various locations throughout the geographic area being served. At times, the noise can be quite substantial with respect to the desired signal. If the signal-to-noise (S/N) ratio is poor enough in the down-link direction (transmission from a source base-site to a mobile) then the probability of 2 successfully sending a handoff command from a source basesite--to a given mobile is significantly impaired.

In an urban micro-cellular system environment where base-site antennas are typically the 5 to 10 meters above the street, the signal path from the serving base-site on a given street to a mobile on a perpendicular street can be obstructed by buildings. The pathloss from a source basesite to the mobile can change for the worse 20- 30 dB within 10-40 meters as the mobile turns a street corner (otherwise known as the 'Street corner effecC). Reaction time to this condition is essential since the S/N ratio degrades rapidly, and hence the quality of the call degrades rapidly. A handoff to an adjacent, alternate base-site (or cell) is required immediately in order to maintain the call. Since the mobile must receive a handoff command in order to perform handoff, a reasonable S/N ratio is essential for good probability of the mobile receiving and decoding the handoff command.

Unfortunately, the conditions described here often result in a failed handoff attempt ultimately leading to a dropped call. To make matters worse, fast moving mobiles and intermittent blocking conditions (Jack of available channels for handoff) add to the failed handoff problems as pathloss from the source cell to the mobile increases. In this situation, the mobile has little chance of hearing a handoff command transmitted by the source base-site.

Thus, a need exists for a communications system, and more specifically a cellular radiotelephone system, which provides an increased probability of a mobile receiving a handoff command, and thus increasing the probability of handoff, in the adverse conditions experienced by cellular radiotelephone systems.

3 Summary of the Invention

A mobile is handed-off from a source base-site operating on a first RF channel to an alternate base-site operating on a second RF channel by choosing an alternate base-site to transmit a handoff command, tuning the alternate base-site to the first RF channel, and transmitting the handoff command from the alternate base- site on the first RF channel.

Brief Description of the Drawings

FIG. 1 generally depicts a cellular radiotelephone system which may beneficially employ the present invention.

FIG. 2 generally depicts a detailed block diagram of a base-site which may implement the present invention.

FIG. 3 generally depicts implementation of power level increase during handoff command transmission in a TDMA cellular radiotelephone system.

Detailed Description of a Preferred Embodiment

FIG. 1 generally depicts a communication system, and more specifically a cellular radiotelephone system, which may beneficially employ the present invention. As depicted in FIG. 1, base-sites 109-113 are coupled to a switch interface 102, which provides switching and interface functions between base-sites 109-113 and a public switched telephone network (PSTN). In the preferred embodiment, switch interface 102 may be either a Mobile services Switching Center (MSC) or a Base Station Controller (BSC).

4 For purposes of example, base-site 109 communicates to a mobile 108 via a signal 106. In the preferred embodiment, signal 106 is a full duplex radio frequency (RF) signal having a downlink transmission (transmission from base-site 109 to mobile 108) frequency range between 935-960 MHz and an uplink transmission (transmission from mobile 108 to base site 109) frequency range between 890-915 MHz. Also in the preferred embodiment, the cellular radiotelephone system depicted in FIG. 1 is a time-division multiple access (TDMA) cellular radiotelephone system, and consequently, signal 106 has TDMA timesiots multiplexed thereon.

As depicted in FIG. 1, base-site 109 is a source base site (i.e., a base-site in communication with mobile 108 via signal 106) while base-sites 110-113 comprise alternate base-sites. In typical cellular radiotelephone systems, many more base-sites are required to provide adequate coverage of a particular coverage area. Of alternate base-sites 110-113, base-sites 110-112 are target base-sites (i.e., a base-site that would be included in a mobile-generated list of potential handoff candidates) while base-site 113 is an umbrella base site (i.e., a base-site that would not be included in a mobile generated list of potential handoff candidates but could accommodate transmission of a handoff command in accordance with the invention). Umbrella base-site 113 is not included in a frequency reuse pattern that would be implemented with, inter alia, base-sites 109-112; instead, umbrella base-site 113 would more likely be implemented in an overlay/underlay frequency use scheme with respect to base-sites 109-112.

FIG. 2 generally depicts a detailed block diagram of base-sites 109-113 which may implement the present invention. As depicted in FIG. 2, base-sites 109-113 are generally comprised of an antenna 200 which receives and transmits signal 106 to mobile 108. Antenna 200 is coupled to a. receiver 204 and a transmitter 210 via a duplexer 202. Duplexer 202 is essentially two bandpass filters, one having a frequency range between 935-960 MHz for downlink transmission via transmitter 210, and a second having a frequency range between 890-915 MHz for uplink reception by receiver 204. When signal 106 is transmitted by mobile 108, receiver 204 receives signal 106 and, using signal strength indication (SSI) block 214, monitors the signal level of signal 106. Output from SSI block 214 is input into control/equalizer board 206, which performs all necessary channel processing and provides coupling to channel equalizers 208. When SSI block 214 indicates that the level of signal 106 is below a predetermined threshold, controllequalizer board 206 commences transmission of a handoff command to mobile 108. At this point, controllequalizer board 206 will send the appropriate handoff command to transmitter 210 for transmission of the handoff command at a first power level P1 to mobile 108 via signal 106. If base-site 109 does not receive an indication that handoff has properly occurred, control/equalizer board 206 will instruct PA control board 216 to increase the power level of signal 106 from the first power level P1 to a second power level P2 during transmission of the handoff command.

This increase may either be a single-step increase or an incremental increase. If the increase is an incremental increase, the process of increasing the level of signal 106 during transmission of the handoff command is repeated until handoff of mobile 108 is complete, or the process eventually terminates itself.

For TDMA cellular radiotelephone systems, such as the group special mobile (GSM) Pan-European Digital Cellular System, the United States Digital Cellular (USDC) system 6 described in EIA Standard IS-54, January 1991, printed in the USA, and the Japanese Digital Cellular (JDC) system described in Research and Development Center for Radio Systems (RCR) Standard 27A, January 1992, printed in Japan, a need for handoff can be detected by use of mobile 108. A need for handoff occurs when, for example, mobile 108, which is communicating to base-site 109 via signal 106, moves to a point where the quality of signal 106 (typically the power level of signal 106 as seen by mobile 108) degrades beyond a predetermined threshold. Since, when handoff is required, the quality of signal 106 has degraded beyond a predetermined threshold, transmission of a handoff command via signal 106 would likewise suffer from the same degradation. With this in mind, alternate base-sites 110 113 may be better candidates to transmit the handoff command than is source base-site 109. If one of alternate base-sites 110-113 can be instructed to transmit a handoff command when a transmission by source base-site 109 fails to reach mobile 108, an increase of the probability that mobile 108 will receive the handoff command, and a corresponding reduction of the rate of dropped calls, will occu r.

Referring back to FIG. 1, when source base-site 109 detects that mobile 108 requires a handoff, source base-site 109 transmits a handoff command to mobile 108, instructing mobile 108 to tune to an alternate base-site 110-113 where mobile 108 would be better served. The alternate base-site 110-113 may be chosen from a list of potential target base sites 110-112 generated by mobile 108, or from a list which is kept by switch interface 102. The list kept by switch interface 102 is generated by monitoring, at the alternate base-sites 110-113, a signal level transmitted by mobile 108 and then transferring the monitored signal level to 7 switch interface 102. Switch interface 102 chooses an alternate base-site 110-113 based on the monitored signal levels transferred from the alternate base-sites 110-113. Typically, alternate base-site 110-113 which measures the strongest signal level transmitted by mobile 108 is chosen as the handoff candidate.

If the handoff command transmitted by source basesite 109 on a first RF channel is unsuccessful (i.e., mobile 108 did not receive the handoff command), source base-site 109 will instruct the chosen alternate base-site 110-113 to transmit the handoff command. Source base-site 109 may trigger alternate base-sites 110-113 to transmit the handoff command by inter alia, waiting for the elapse of a predetermined time period, receiving, or the absence of receiving a reply signal transmitted by mobile 108, or notifying chosen alternate base-site 110-113 to transmit the handoff command starting in a particular frame if communication on a second RF channel at chosen alternate base-site 110-113 has not been established. In the preferred embodiment, the communication system is microcellular radiotelephone system. In microcellular radiotelephone systems, system synchronization is more tightly controlled than in macrocellular radiote ' lephone systems. System synchronization is important in the embodiment of FIG. 1, since only one base-site may transmit the handoff command to mobile 108 at a time. If there is any overlap of transmission (i.e., source base-site 109 and a chosen alternate base-site 110-113 transmitting at the same time), mobile 108 would receive conflicting, if not interfering, information from each of the two base-sites In addition, due to the size of macrocellular radiotelephone systems, the time "window" in which mobile 108 would expect to receive a transmission from a target base-site 8 110-112 may not be large enough. If mobile 108 is too far from-. target base-site 110-112, the time-delay signal 106 experiences after transmission from target base-site 110112 would be sufficient to miss the time "window" which 5 mobile 108 would expect to receive the transmission. Microcellular systems, which are sufficiently small to mitigate these excessive time-deiays, do not experience this problem.

Typically, the chosen alternate base-site 110-113 will be operating on a second RF channel. To transmit the handoff command, the chosen alternate base-site 110-113 will tune from the second RF channel to the first RF channel and transmit the handoff command on the first RF channel. After transmission of the handoff command on the first RF channel, the chosen alternate base-site 110-113 will tune back to the second RF channel, and, if mobile 108 received the transmitted handoff command, establish communication between the chosen alternate base-site 110-113 and mobile 108 on the second RF channel. In this manner, handoff of mobile 108 is accomplished even though the initial handoff command transmitted by source base-site 109 failed to reach mobile 108.

In an alternate embodiment, only target base-sites 110-112 may be configured to operate on a second RF channel. For target base-sites 110-112, the steps to transmit the handoff command and establish communication would be as described above. However, umbrella base-site 113 may only be used for handoff command transmission on the first RF channel. Depending on an operator's system configuration, umbrella base-site 113 may not be configured to communicate on the second RF channel. Consequently, umbrella base-site 113 may transmit the handoff command to mobile 108 on the first RF channel, but establishment of 9 communication by mobile 108 would have to be made to one of target base- sites 110-112.

In an alternate embodiment, alternate base-site 110113 transmission of a handoff command in accordance with the invention could be coupled with power level increase of a handoff command. Since base-sites 109-113 incorporate variable power-level power amplifiers, source base-site 109 could increase its output power by some level during the transmission of the handoff command to mobile 108 before instructing a chosen alternate base-site 110-113to transmit the handoff command. Likewise, if a chosen alternate base-site 110-113 transmits a handoff command, and if handoff is unsuccessful, chosen alternate base-site 110-113 could increase its output power by some level and again transmit the handoff command. If handoff is still unsuccessful, chosen alternate base-site 110-113 may continue to incrementally increase the power level of transmission of the handoff command, or may terminate the handoff procedure.

FIG. 3 generally depicts how power level increase during transmission of a handoff command in a TDMA cellular radiotelephone system would be performed. Source base-site 109 would perform power level increase before instructing an alternate base-site 110-113 to transmit the handoff command; alternate base-sites 110-113 would perform power level increase after being instructed by source base site 109 to transmit the handoff command. As depicted in FIG. 3, eight (8) timeslots tO-t7 comprise frames Fl-F68. Of course, other frames occur before and after frames Fl-F68, but these frames are not depicted for clarity. Continuing, timeslots W-t7 of frames F1-1768 are continuous in time, and are defined in GSM Recommendation 5.02, version 3.4.1, January, 1990. In the GSM configuration, timeslot tO is dedicated to, inter alia, control channel (CCH) and broadcast channel (BCCH) information. The remaining timeslots M-t7 of each frame Fl-F68 are generally dedicated to voice transmission. When base-site 109 communicates to mobile 108 via signal 106, the voice communication occurs during a dedicated one timeslot (indicated by 'V' shown in timeslot Q FIG. 3) of each frame Fl-F36.

Transmission of the handoff command (indicated by "HC" shown in timeslot t3, FIG. 3) to mobile 108 at an increased power level may occur after the detection of a predetermined triggering event. For example, in the embodiment depicted in FIG. 3, the predetermined triggering event is the elapse of a predetermined number of transmissions at a first power level. As depicted in FIG. 3, timeslot t3 of frame F1 depicts transmission of a handoff command HC at a power level Pl. Transmission of the handoff command HC occurs during timeslot t3 for eight consecutive frames Fl-F8. In this scenario, handoff command HC steals timesiot t3 from voice communication V.

When transmission of the handoff command HC at the first power level P1 is completed, voice communication V continues for the next 26 frames F9-F34 at power level Pl.

If base-site 109, which is transmitting the timeslots depicted in FI.G. 3, does not receive an indication that handoff has been completed (for example, a TLEAR SOURCE" command from switch interface 102), base-site 109 will automatically increase the power level of the transmission of the handoff command HC during timeslot t3 of frames F35 F42 to a second power level P2. In this scenario, the handoff command is repeated every 34 frames, thus the triggering event would be the elapse of 34 frames, where eight frames have the handoff command transmitted during timeslot t3 at a first power level Pl. At the end of transmission of handoff 1 1 command HC at the second power level P2, base-site 109 continues voice communication V during frames F43-1768 at the first power level Pl. At this point, base-site 109 may either continue increasing the level of transmission of the handoff command HC during subsequent frames, or may terminate the entire handoff procedure.

Other predetermined triggering events may also signal base-site 109 to increase power level during transmission of the handoff command. For example, after base-site 109 transmits the handoff command at a first power level, base site 109 may wait for the elapse of the predetermined time period, and then increase the power level to a second power level during transmission of a subsequent handoff command.

Likewise, base-site 109 may increase the power level of a handoff command transmission to a second power level if base-site 109 receives a reply signal transmitted by the mobile or, does not receive a reply signal transmitted by the mobile. As target base-sites 110-112 and umbrella base site 113 may perform power level increase as well, the above discussion of power level increase applies equally to these base-sites 110-113. For the corresponding base-site 109 113 that is performing power level increase, the events described above are valid triggering events for power level increase during handoff command transmission.

Thus, it will be apparent to one skilled in the art that there has been provided in accordance with the invention, a method and apparatus for providing alternate base-site transmission of a handoff command in a communication system that 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 12

Claims

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. What we claim is:

13 Claims 1 A method of handing-off a mobile from a source basesite operating on a first RF channel to an alternate base-site operating on a second RF channel, said method comprising the steps of:

choosing an alternate base-site to transmit a handoff command; tuning said alternate base-site to said first RF channel; and transmitting said handoff command from said alternate 15 base-site on said first RF channel.

14 2. The method of claim 1 further comprising the steps of: tuning said alternate base-site to a second RF channel; and 5 establishing communication between said alternate base-site and said mobile on said second RF channel.

3. A method of handoff in a communication system, the communication system having a source base-site communicating to a mobile on a radio- frequency (RF) channel, the source base-site coupled to alternate base- sites via a switch interface, the method comprising the steps of:

transmitting a handoff command from the source basesite on the RF channel; choosing an alternate base-site to transmit the handoff command; and instructing the chosen alternate base-site to transmit the handoff command on the RF channel.

16 4. The method of claim 3 further comprising the step of transmitting, at the chosen alternate base-site, the handoff command on the RF channel.

17 5. A method of handoff in a communication system, the communication system having a source base-site communicating to a mobile on a radiofrequency (RF) channel at a first power level, the source base-site coupled to alternate base-sites via a switch interface, the method comprising the steps of:

transmitting a handoff command from the source basesite on the RF channel at the first power level; increasing the power level of said transmission above the first power level during transmission of the handoff command; choosing an alternate base-site to transmit the handoff command; and instructing the chosen alternate base-site to transmit the handoff command on the RF channel.

c 18 6. The method of claim 5 further comprising the step of transmitting, at the chosen alternate base-site, the handoff command on the RF channel at a second power level.

7. The method of claim 6 further comprising the step of increasing, at the chosen alternate base-site, the power level of said transmission above the second power level during transmission of the handoff command.

19 8. A method of handing-off a mobile from a source basesite,operating on a first RF channel to an alternate base-site operating on a second RF channel, said method comprising the steps of:

tuning said alternate base-site to the first RF channel; transmitting a handoff command from said alternate base-site on said first RF channel; and establishing communication with the mobile on one of 10 either the first RF channel or the second RF channel.

P_ 9. A communication system having a source base-site com^municating to a mobile on a radio-frequency (RF) channel, the communication system comprising:

means for transmitting a handoff command to the mobile on the RF channel; means, responsive to said means for transmitting, for choosing an alternate base-site to transmit the handoff command to the mobile; and means, coupled to said means for choosing, for instructing the chosen alternate base-site to transmit the handoff command to the mobile on the RF channel.

21 10. The communication system of claim 9 further comprising means, at the chosen alternate base-site, for transmitting the handoff command on the RF channel.

11. The communication system of claim 9 wherein said alternate base-site further comprises one of either a target base-site and an umbrella basesite.

12. The communication system of claim 9 wherein said means for choosing an alternate base-site further comprises means for choosing an alternate base-site from a list of alternate base-sites generated by the mobile.

13. The communication system of claim 9 wherein said means for choosing an alternate base-site further comprises means, at an alternate base-site, for monitoring a signal level transmitted by the mobile on the RF channel and transferring said monitored signal level to the switch interface.

14. The communication system of claim 13 wherein said means for choosing an alternate base-site further comprises means, at the switch interface, for choosing an alternate base-site based on the monitored signal levels transferred from said alternate base-sites to said switch interface.

15. The communication system of claim 9 wherein said means for instructing the chosen alternate base-site to transmit the handoff command on the RF channel further comprises means for instructing the chosen alternate basesite to transmit the handoff command on the RF channel after an occurrence of a predetermined triggering event.

22 16. The communication system of claim 15 wherein said predetermined triggering event further comprises the elapse of a predetermined time period.

17. The communication system of claim 15 wherein said predetermined triggering event further comprises instructing the chosen alternate basesite to transmit the handoff command starting in a particular frame if communication on a second RF channel at the chosen alternate base-site is not established.

18. The communication system of claim 15 wherein said predetermined triggering event further comprises, at the source base-site, one of either a reception of a reply signal transmitted by the mobile or absence of a reception of a reply signal transmitted by the mobile.

19. The communication system claim 15 wherein said communication system further comprises a time-division multiple access (TDMA) communication system.

20. The communication system of claim 19 wherein said means for transmitting the handoff command to the mobile further comprises means for transmitting the handoff command to the mobile during a timeslot.

21. The communication system of claim 20 wherein said predetermined triggering event further comprises the elapse of a predetermined number of timesiots that transmission of the handoff command to the mobile occurs.

23 22. The communication system of claim 9 wherein said base'-site further comprises a code-division multiple access (CDMA) communication system.

1 24 23. A communication system having a source base-site com"municating to a mobile on a radio-frequency (RF) channel at a first power level, the source base-site coupled to alternate base-sites via a switch interface, the source base- site transmitting a handoff command to the mobile to initiate handoff, the communication system comprising:

means for transmitting the handoff command to the mobile on the RF channel at the first power level; means, coupled to said means for transmitting, for increasing the power level of said transmission above the first power level during transmission of the handoff command; means, responsive to said means for transmitting, for choosing an alternate base-site to transmit the handoff command to the mobile; and means, at the chosen alternate base-site, for transmitting the handoff command on the RF channel at a second power level.

1 24. The communication system of claim 23 further comprising means, at the chosen alternate base-site and coupled to said means for transmitting, for increasing the power level of said transmission above the second power 5 level during transmission of the handoff command.