AU4667499A - System for elimination of audible effects of handover - Google Patents

Description

WO 99/65266 PCT/SE99/00947 SYSTEM FOR ELIMINATION OF AUDIBLE EFFECTS OF HANDOVER BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to cellular radio telecommunications systems and, more particularly to a method and system for eliminating the audible effects of handing over a cellular radio subscriber terminal from one cell to another. Description of the Related Art In a cellular radio telecommunications system. the area within which a plurality of mobile stations are served by the system is divided into cells. Each idle and active mobile station within the cell remains in radio contact with a base station serving that cell. In conventional operation, when a mobile station that is engaged in active communication moves from one cell to another cell the cellular system performs a hand-off (handover) in which the moving mobile station is instructed to retune its radio to a new traffic channel that is served by the base station of the cell it is then entering. The principal reason for performing handover is the deterioration of signal quality experienced by the mobile for its serving cell and the existence of a signal from a base station of a neighboring cell which would provide it with better signal quality. A cellular system must know the approximate location of each of the mobile stations engaged in active communication in order to provide efficient hand-off and traffic management functionality. Location information is generally provided by the measurement of signal strength of radio signals passing between the mobile station and its serving base station as well as radio signals of neighboring base stations serving the cells in the surrounding geographic area. The basic criteria defining a good radio air interface connection between a mobile terminal and a base station in a cellular system are: a carrier signal strength above a preselected threshold value; a relatively high speech quality on the connection as defined by carrier-to-interference ratio (C/I); and a sufficient number of available channels in the serving cell. As soon as any of these criteria can no longer be met by the base station with which a subscriber terminal is in communication, the system will try to hand-off the connection to a more suitable cell within the system. As system WO 99/65266 PCT/SE99/00947 -2 operators strive to increase the capacity of their networks to serve the increasing number of mobile subscribers, the geographic size of the cell within the system is being continually decreased. This is to enable the system operator to reuse the limited number of available frequency channels more often to increase the capacity of the system. This increase in frequency reuse also dictates that there will be a higher number of handovers per call as a result of the smaller size of the cells. The handover of a mobile radio terminal from one cell to another is performed by sending the necessary signaling between the mobile terminal and the cellular network on the old traffic channel within the presently serving cell after which the radio connection of the call is transferred directly to a new traffic channel belonging to the target cell. This necessarily produces a short speech interruption during the actual transfer of the traffic channel from one cell to another which is typically on the order of a few hundred milliseconds. Such traffic channel interruptions are audible both parties to the conversation and can be extremely annoying when many handovers in a row occur during a single call. The system of the present invention provides a solution to the problem of audible effects of handover which will, in most cases, eliminate the perceived speech interruptions to either one or both parties to the conversation. BRIEF SUMMARY OF THE INVENTION One aspect the present invention relates to reducing the audible effect of handover in at least one link in the traffic channel of a radio air interface of a cellular radio system. Discontinuous transmission (DTX) is implemented within the link within which the audible effects of handover are to be reduced by detecting whether each traffic frame in the link consists of speech, non-transparent data or background noise. A silence descriptor (SID) frame is sent containing information related to the background noise within the link from the transmitter toward the receiver in response to detecting that neither speech nor non-transparent data are present in the link. Incoming traffic frames in the receiver are monitored and when a SID frame is detected in the receiver, the comfort noise characteristics are updated and comfort noise is generated within the receiver. When it is determined that a handover of a mobile station on the link should be performed in response to signal measurements WO 99/65266 PCT/SE99/00947 -3 within the system, the performance of that handover is delayed until a preselected number of SID frames have been sequentially received at the receiver. This indicates that the party at the transmitter is not generating either speech or non-transparent data and that the receiver instead is receiving comfort noise. In response to this condition, handover of the mobile station from one BTS to another is performed which eliminates the audible effect of the handover to the party at the receiver, as comfort noise will still be generated throughout the handover process. In another aspect of the invention, the audible effect of handover is reduced in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system. DTX interrupts radio transmissions from a party on the link which is not at that moment sending a communication and to instead send regular periodic indications to the other party indicating that comfort noise should be generated in the receiver of that party. The transmission of the periodic indications sent by the party which is not at that moment sending a communication is monitored along with the need by the system to perform a handover from one cell to another of a mobile station which is a party to that communication. The performance of the needed handover is delayed until a preselected number of regular periodic indications have been received from the party which is not at that moment sending a communication to ensure that the other party is at that moment receiving only comfort noise and, thus, will not hear the audible effects of the handover being performed. Once that condition exists, the needed handover is performed. BRIEF DESCRIPTION OF THE DRAWINGS For an understanding of the present invention and for further objects and advantages thereof, reference can now be had to the following description, taken in conjunction with the accompanying drawings in which: FIG. 1 is a diagrammatic depiction of a cellular radio system including a plurality of cells and base stations with which an embodiment of the present invention may be used; WO 99/65266 PCT/SE99/00947 -4 FIG. 2 is a functional block diagram representative of a cellular communications system employing sectorized antennas within which an embodiment of the present invention may be incorporated; FIG. 3 is an illustrative diagram of the manner in which signal strength measurements are performed and a mobile subscriber terminal is handed off from one cell to another in a cellular radio system; FIG. 4 is a flow chart illustrating certain aspects of an algorithm used in the system of the present invention; FIG. 5 is a timing diagram illustrating the elimination of audible handover effects in accordance with one aspect of the invention; FIG. 6 is a timing diagram illustrating the elimination of audible handover effects in accordance with another aspect of the invention; and FIG. 7 is a timing diagram illustrating the elimination of audible handover effects in accordance with yet another aspect of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To provide a context within which the details of the present invention may be set forth, the general construction and operation of a cellular mobile radio system will first be described. Fig. 1 is a diagrammatic representation of such a system which contains ten cells, C 1-C 10. Of course, a cellular radio system would contain many more cells than ten but for purposes of this illustration, the system shown in Fig. 1 may be considered to be an isolated portion of a larger system, only ten cells of which are shown. Within each of the cells C1-C10 there is a corresponding respective base station B 1-B 10 serving that cell. In the exemplary embodiment of Fig. 1, the base stations B 1-B 10 are shown as having omni-directional antennas and being located in the vicinity of the center of the cells, however, a person skilled in this art will recognize that base stations may typically be located in other areas of the cells, for example, at the intersection of three adjacent cells and have sectorized antennas so that a single base station may serve three cells. Also shown in Fig. 1 are a plurality of mobile stations, M1-M10 which are subject to movement within each cell and from one cell to another cell. Of course, a typical cellular system would have many more WO 99/65266 PCT/SE99/00947 -5 mobile stations operating within it than ten. A mobile switching center MSC1 is connected to each of the base stations B1-BO10 by means of a cable or microwave and to a switched telecommunication network (PSTN). Each cellular radio telecommunication system is assigned a particular frequency band within which it must operate. This frequency band is subdivided into units called frequencies, groupings of which are allocated to the cells located in a particular area. Because of the limited radio frequency spectrum as the need for numbers of traffic channels within each cell increases due to increased traffic within the system, system operators tend to continually reconfigure their systems generally reducing the geographic extent covered by each cell in order to increase the frequency reuse within the system. This decrease in the size of the cell means that there will be an increased number of handovers during each call in which a mobile station is moving through the system from cell to cell. Fig. 2 illustrates another exemplary cellular radio telecommunications system which employs base stations each having three sectorized antennas. The system, shown generally at 10, comprises a cellular network 12 which includes a plurality of fixed site base stations 16 positioned at spaced-apart locations throughout a geographic area. In the illustration of Fig. 2, each of the base stations 16 defines a cell 18 and groups of three base stations 16 are located together and each of the co-located base stations serves a coverage area which defines three adjacent but separate cells 18-18. Each base station 16 typically includes a fixed site transceiver which utilize a sectorized antenna covering approximately 120' and is located in the periphery of the cell which it is serving. Each of the base stations 16 permits wireless radio communication to be effectuated with remotely positioned mobile subscriber terminals, such as the subscriber terminal 22, when the mobile is positioned within communication range of one of the base stations. The cells 18-18 defined by the coverage areas of the base stations are shown to be hexagonal in shape purely for purposes of illustration. In an actual system, the communication range, i.e., the coverage area, of a base station would differ from that shown in the figure. That is to say, cells 18-18 may well be non-symmetrical with respect to the base station and the cells defined by different ones of the base stations 16-16 would likely differ in shape from one another. Also. within an actual cellular WO 99/65266 PCT/SE99/00947 -6 radio communication system, the coverage areas (such as adjacent base stations) would overlap with one another to a certain degree. Groups of the base stations 16-16 are coupled to a base station controller (BSC) 24. Communication lines 26 couple the base stations 16-16 to the BSCs 24 which are in turn coupled to mobile switching centers (MSCs i. such as MSC 27, by way of communication lines 28. As shown in Fig. 2, when the mobile subscriber terminal 22 communicates with another mobile communications station connected to the network, those communications are effected by way of a base station 16 within the network 12. When the mobile terminal 22 moves between cells 18-18 of the network 12. ongoing communications are handed over between successive ones of the base stations 16-16. If the mobile terminal is in the idle mode, cell selection. such as that which occurs during registration procedures, is performed when the mobile moves from cell to cell. Referring to Fig. 3, there is shown how a mobile station 22 which is in communication with a first base station serving cell A, has its radio air interface connection subjected to periodic measurements for quality criteria by both its serving cell and its neighboring cells. Once it is decided that one or more of the quality criteria have fallen below preselected minimum values, the network then causes the radio air interface to be transferred from Cell A to a base station which is serving mobiles within cell B. During this transfer of the radio connection from Cell A to Cell B, there is a brief interruption in the traffic channel which is normally audible to one or both of the parties to the conversation and creates a distraction and annoyance. A number of different prior art techniques have been proposed to attempt to solve the problem of long speech interruption times. One of these has been to employ various procedures for reducing the handover times as much as possible or to smooth out the effect of those handovers and the audible impact of them. One method which has been used is that, during an intra-BSC handover, one transcoder and rate adapter (TRA) is connected to both BTSs and the switching is done in the TRA. This leads to a significant improvement of the audible effects of handover on the uplink in which the interruption is reduced to approximately 20 to 40 milliseconds. Another method of eliminating the effects of handover is shown in U.S. Patent No. 5.640,679 to Lundqvist et al. in which the time offset between a mobile WO 99/65266 PCT/SE99/00947 -7 station in the serving cell and the target cell are measured and then the offset value is sent to the mobile station so that it can shift its timing during hand-off and to be in closer synchronization with the target cell and thereby eliminate the circuit interruption time usually necessary for resynchronization. It is said that this technique allows the disruption to be masked by the speech decoder and to be restricted to less than 40 milliseconds. Still another attempt to solve this problem has been the use of muting to comfort noise level in order to reduce the audible effects on the uplink instead of muting to silence. An improved algorithm is used to gradually mute toward comfort noise level to render the handover less audible to the listener. Additionally, SID update interpolation provides an increasingly smooth transition between consecutive SID update frames and thereby improves subjective quality of the comfort noise at the receiving end during silent periods. From the point of view of speech quality one of the principal problems with handover is the speech interruption times and, in tight frequency reuse networks, the importance of minimizing the speech interruption times during handover is significant. Nevertheless. none of the prior techniques mentioned above totally eliminates the perceived speech degradation during hand-off but rather only reduces the apparent speech interruption time and the apparent affect of them. The method and system of the present invention employs a technique which has been implemented by many cellular radio networks and is called discontinuous transmission (DTX). TDMA cellular radio systems which employ frequency hopping such as the global system for mobile communication (GSM) or the American PC 1900 TDMA systems supplied by Ericsson Radio Systems benefit particularly from the addition of DTX. The DTX functionality is defined and described, for example, in the GSM specifications GSM 06.31 and is embodied in the European Telecommunications Standard ETS300 580-5, both of which are hereby expressly incorporated by reference herein. In general, the DTX functionality is based upon the proposition that during a conversation, each participant is traditionally silent on average for about 50% of the time. If nothing is said into the microphone of a mobile radio terminal, there is no point of sending anything in the way of radio transmission into the air and, thus. transmissions are made from the radio only when speech is detected over the WO 99/65266 PCT/SE99/00947 -8 connection. This DTX procedure decreases the power consumption of the mobile subscriber terminal (MS) as well as in the base station transceiver system (BTS) and also reduces the amount of radio power emitted into the air. Since the power level used when actually transmitting is unaffected, the C/I ratio is raised for all connections when DTX is implemented. DTX is used to decrease both the MS and BTS power consumption as well as to reduce the interference within the system and is only used on the transmission of traffic channel (TCH) frames. DTX can be separately implemented on either uplink (UL) traffic channels (from mobile station to base station) or downlink (UL) traffic channels (from base station to mobile) or both. An integral part of the DTX functionality is the inclusion of a voice activity detector (VAD) in the transmitter of the BTS and/or MS. The general function and operation of a VAD is defined and described, for example in the global system for mobile communication (GSM) specification GSM 06.32 and is embodied in the ETSI Standard ETS 300 580-6, both of which are hereby expressly incorporated by reference herein. In general, the VAD in the transmitter of either a BTS or MS detects whether a traffic frame consists of speech, non-transparent data or background noise. If the frame consists only of noise, the transmitter sends one silence descriptor frame (SID), and then the transmission is stopped. Following the initial SID frame, one new SID frame is sent during each SACCH period until either speech or non-transparent data is again detected within a traffic frame. The signal quality measure reports are sent as usual on the SACCH. Each of the SID frames contains information about the background noise of the established connection which is being monitored by the VAD. In the receiver of the MS or BTS in which DTX functionality is in operation, a SID frame detector checks all incoming frames. The SID frame detector is able to separate SID frames from speech or non-transparent data frames and, when a SID frame is detected, the comfort noise characteristics within the receiver are updated and comfort noise is generated in accordance with those characteristics. Comfort noise generation is interrupted when a speech frame is detected and the speech is decoded for the listener. Thus, SID frames are sent between the transmitter and receiver during periods of silence primarily for two reasons: (1) to update the comfort noise characteristics on the receiving side and (2) to enable signal strength and signal quality WO 99/65266 PCT/SE99/00947 -9 measurements to be made and transmitted. The VAD must be operating at all times in order to assess whether or not the input signal contains speech or non-transparent data. The mobile station and/or the BTS send information in the measurement report every 480 milliseconds telling whether it has used the DTX sometime during that interval or not. In most contemporary networks DTX in the uplink (L) is often used to save the battery consumption of mobile stations. However, DTX in the downlink (DL) is rarely used as most operators perceive that the degradation of speech caused by the use of the DTX functionality to be worse than the power saving and benefits in terms of the reduced interference. The method and system of the present invention functions in a somewhat different way from the several prior techniques discussed above. That is, rather than reducing the interruption times of the traffic channels, the technique of the present invention "hides" or "masks" the effects of those interruptions from audible perception by the end user. The system of the present invention incorporates the existing DTX functionality within cellular radio systems described above and employs that functionality by performing the handover during the silent periods of a conversation when only comfort noise is being created for the listener. With the present technique the handover is delayed until several SID frames (an operator selectable number) have been received in a row indicating that the user is at that moment listening to the other person rather than simply breathing between two sentences in a conversation. When the DTX functionality is in use, only one SID frame is sent during the entire SACCH period thus. when several sequential SID frames have been detected indicating that several SACCH periods have passed without any speech from that party, the handover is quickly performed in less than 200 milliseconds. By using this technique to time the handover the audible affects of the speech interruption is eliminated for the one of the two parties to a communication which is at that moment receiving comfort noise in the receiver. Moreover, the handover between two cells will, in most cases, take place when the signal strength of the target cell is stronger than the currently serving cell within a certain number of dBs. Usually the handover can be delayed substantially without any severe quality problems to the specific radio air interface of that mobile or the WO 99/65266 PCT/SE99/00947 -10 network as whole. However, implementation of control of the proposed functionality which incorporates the delay of the handover for sometime also employs a timer which is operator selectable to a certain value. In that case, if either no or an insufficient number of sequential SID frames have been detected before the expiration of the preselected time period after the system has determined that a handover is necessary, the handover is performed anyway to avoid inadvertent loss of the connection. The actual implementation algorithm for performing the handover in accordance with the present technique also preferably includes the monitoring of the quality of the radio air interface of the connection. If the signal quality parameters, such as signal strength or bit error rate (BER) fall below preselected values or the rate of decrease of those parameters exceeds a preselected threshold rate, a handover is performed regardless of the data being monitored in terms of the DTX parameters such as the SID periods. However, most handovers are performed at relatively good signal quality and signal strength and in such cases a 5-10 second delay prior to actual handover has no impact upon that quality. Referring next to Fig. 4, an exemplary implementation algorithm is illustrated in the form of a flow chart. At 31 the system begins and moves to 32 at which it determines whether or not the handover criteria have been fulfilled. If not, the current radio connection is maintained. If, however, at 32 handover criteria have been fulfilled, the system moves to 33 and determines whether or not the quality and/or signal strength of the connection has degraded below a predetermined threshold or is degrading at a rate which is greater than a selected threshold value. If the answer is no, the system moves to 34 and queries whether a sufficient number of SID frames have been detected on the operator chosen link, i.e., UL or DL, to conclude that the radio connection is currently in a silent period and only comfort noise is being generated. If the answer is yes, the system moves to 35 and the handover is performed. If a determination is made at 33 that the signal quality and/or signal strength has degraded below a preselected threshold or the rate of degradation is faster than a threshold value, the system does not further delay a handover to determine whether or not the radio air interface is in a silent period but, rather, moves immediately to 35 and performs the handover to avoid the risk of losing the connection.

WO 99/65266 PCT/SE99/00947 -11 The technique of the present invention is applicable to a radio air interface connection between two parties only one link at a time. Which link is chosen for application of the technique is based upon the choice of the operator and is applied to either the uplink (UL) or downlink (DL). The solution of the present invention has been shown to be highly significant with regard to eliminating the audible effects of a handover and essentially eliminates completely the reception of audible handover effects on the chosen one of the links. Referring next to Fig. 5, there is shown a timing diagram illustrating a situation wherein DTX functionality is enabled for the uplink of a radio air interface connection but not for the downlink thereof. In general, for those cellular system operators who today implement DTX, 90% implement the functionality in only the uplink as assumed in Fig. 5. The reason for this is that the real economic gain produced by interrupting the transmitter when the user is not speaking occurs in the mobile, which saves battery power, not the BTS. In addition, the slowness of operation of the VAD in the DTX system slightly degrades the quality of plosives (sounds such as "p"; "t" and "k") and the DTX function prevents signal quality measurements by the mobile and the BTS from being performed as frequently as normal. In the uplink of Fig. 3, the condition of DTX "off' means that the speech is being transmitted between the mobile and the base station and the condition of DTX "on" means that there is a silent period by the user of the mobile station and only SID frames are being sent over the uplink between the mobile and the base station. To simplify the example, the diagram of Fig. 5 also represents a call between a mobile user and a person on a fixed phone, however, the following reasoning would also hold true for a MS to MS call. The dotted time line labeled "HO1" represents the time at which a handover takes place regardless of the silent periods and "HO2" is the time at which a handover timing has been adjusted in accordance with the silent periods on the uplink in accordance with the principles of the present invention. If all of the handovers that would have happened during the speech periods of the mobile user are instead delayed to occur during the silent periods, as illustrated at HO2, none of these handovers would be audibly detected by the "fixed" user. The handovers would in this case always happen when the "fixed" user is speaking and receiving comfort noise in the receiver since the mobile user is not speaking and DTX is on. Thus, the effects of WO 99/65266 PCT/SE99/00947 -12 the handover would be inaudible to the fixed party of the communication. However, the mobile user would in this case hear the handover interruption since DTX functionality is not being implemented on the downlink (DL). Other prior art techniques. such as those discussed above, might be used to minimize the speech interruption time on this link to minimize the audible effect. Referring next to Fig. 6, there is shown another timing diagram depicting the opposite, although relatively unusual in present day practice, situation to that illustrated in Fig. 5. In Fig. 6, the DTX functionality is enabled for the downlink but not for the uplink of a radio air interface connection between two parties. With this example it is better to delay the handover based upon the downlink since the uplink is always transmitting. In this situation each of the handovers that will happen during the speech periods of the assumed to be fixed user on the downlink are delayed from HO1 (when there is no DTX on either the uplink or downlink) until HO2 when DTX is operational during the silent periods of the fixed user on the downlink. Thus, none of the handovers will be audibly detected by the mobile user who is then receiving only comfort noise. Instead, the handovers would always happen when the mobile user is speaking (or at least the person on the other end of the connection is not speaking) and the mobile user would never hear them. The "fixed" user would in this case always hear the handover interruptions as the DTX functionality is not in operation on the uplink. Under these circumstances, it is possible for the cellular system operator to market its network as being superior with respect to quality aspects since no speech interruptions are being detected by mobile subscribers connected to its network. Referring finally to Fig. 7, a still further aspect of the present invention is also depicted in the form of a timing diagram. In this situation if it is assumed that both users on the uplink and the downlink are neither silent nor speaking at the same time, every handover will happen during a silent period for one of the two parties to the communication. In the example of Fig. 7, the handover event noted by the time line "HO l" is performed when the mobile user is silent and DTX is functional on the uplink, and. thus, not noticeable to the "fixed" user. The handover illustrated by the dotted time line "HO2" occurs when the "fixed" user is silent and DTX is active on the downlink. This handover is therefore not audibly noticeable to the mobile user. Since WO 99/65266 PCT/SE99/00947 -13 such handovers often happen randomly both the mobile user and the "fixed" user will hear on average, about 50% of the handover interruptions. By delaying handovers in accordance with the silent periods of one of the links, the perceived handover interruptions are totally eliminated for either the mobile user or the "fixed" user, but not both. It would be indeed a rare circumstance when both parties were silent for an extended period (and DTX was functioning on both the uplink and downlink simultaneously) and a handover was needing to be performed. The system operator selects a particular parameter within the present system to choose the direction in which the system is optimized i.e., either uplink or downlink when DTX is operational in both directions. For example, if all handovers are performed during the silent periods of the "fixed" user, the quality of the network will appear from the mobile user's perspective to be enhanced since a mobile user will never experience any audible interruptions whatsoever. For a MS to MS call in the same network one mobile user would hear other mobile's handovers but not their own handovers if the same strategy of performing handovers during the silent periods of the downlink were used. It can be seen from the above description of the invention, that the method and system of the present invention may be implemented in different ways. In either case. the audible effect of the handover to one or the other of two parties to a conversation across a radio air interface in a cellular radio system may be totally masked from any audible perception. Thus, the system contains great advantages to system operators all of which desire to enhance their respective claims to signal quality in their systems. As increasing amounts of frequency reuse within the cellular system occurs, the method and system of the present invention becomes increasingly important in reducing the annoying audible effects of handover. Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying drawings and described in the foregoing description, it is understood that the invention is not limited to the embodiment( s) disclosed but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined in the following claims.

Claims (32)

1. A method of reducing the audible effect of handover in at least one link in the traffic channel of a radio air interface of a cellular radio system, comprising: implementing discontinuous transmission (DTX) within the one link other than the link within which the audible effects of handover are to be reduced by detecting whether each traffic frame in said one link consists of speech, non transparent data or background noise; sending a silence descriptor (SID) frame containing information related to the background noise within the one link from the transmitter toward the receiver min response to detecting that neither speech nor non-transparent data are present on said link: monitoring all incoming traffic frames in the receiver: detecting a SID frame in the receiver and. in response thereto. updating the comfort noise characteristics and generating comfort noise within the receiver; determining that a handover of a mobile station on said link should be performed in response to signal measurements within the system; delaying the performance of said handover until a preselected number of SID frames have been sequentially received at the receiver to indicate that the party at the receiver is generating either speech or non-transparent data and is receiving comfort noise and then, in response thereto, performing handover of the mobile station from one BTS to another.

2. The method set forth in claim 1 which also includes: monitoring the quality of the connection during the time period within which said SID frames are being detected and counted; and performing said handover regardless of the number of sequential SID frames which have been received in response to either the signal quality or bit error rate (BER) of the connection deteriorating below a preselected value or at a rate greater than a preselected rate.

3. The method as set forth in claim 1 which also includes the step of: WO 99/65266 PCT/SE99/00947 -15 triggering a timer within said system having a preselected timeout value in response to the receipt of a first SID frame; and performing said handover regardless of the number of SID frames which have been received in response the said timer reaching its timeout value.

4. The method as set forth in claim 1 wherein the detection of whether each traffic frame in said link consists of speech, non-transparent data or background noise is performed by a voice activity detector (VAD).

5. The method as set forth in claim 1 wherein DTX is implemented on only the uplink of the cellular system.

6. The method as set forth in claim 1 wherein DTX is implemented on only the downlink of the cellular system.

7. The method as set forth in claim 1 wherein DTX is implemented on both the uplink and the downlink of the cellular system.

8. A method of reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system to interrupt radio transmissions from a first party on said link which is not at that moment sending a communications and to instead send regular periodic indications to a second party indicating that comfort noise should be generated in the receiver of the second party, said method comprising: monitoring the transmission of said periodic indications sent by the first party which is not at that moment sending a communication; monitoring within the system the need to perform a handover from one cell to another of a mobile station which is one of said parties to said communications; delaying the performance of said needed handover until a preselected number of regular periodic indications have been received from the first party which is not at that moment sending a communication to ensure that the second party is WO 99/65266 PCT/SE99/00947 -16 receiving only comfort noise and will not hear the audible effects of the handover being performed and then performing said handover.

9. A method of reducing the audible effect of handover in a digital cellular radio system as set forth in claim 8, wherein the communication being had between a mobile on one of said links and another party is a voice call and wherein said step of monitoring the need to perform a handover of said mobile station further comprises: monitoring the signal strength and bit error rate (BER) on both the uplink and downlink of said mobile along with the rate at which signal quality and BER are deteriorating; comparing the monitored signal strength and rate of deterioration of signal strength to respective preselected threshold values: performing said handover regardless of the receipt of said preselected number of regular periodic indications in response to either said monitored signal strength deteriorating below said preselected threshold value or said rate of deterioration of said monitored signal strength becoming greater than said threshold value.

10. A method of reducing the audible effect of handover in a digital cellular radio system as set forth in claim 9, wherein said step of monitoring the need to perform a handover of said mobile station further comprises: comparing the BER and rate of deterioration of BER to respective preselected threshold values; performing said handover regardless of the receipt of said preselected number of regular periodic indications in response to either said monitored BER deteriorating below said preselected threshold value or said rate of deterioration of said monitored BER becoming greater than said threshold value.

11. A method of reducing the audible effect of handover in a digital cellular radio system as set forth in claim 8, wherein DTX is enabled only on the uplinks of said system. the communication being had between a mobile on one of said uplinks and another party is a voice call, and wherein said needed handover is delayed until WO 99/65266 PCT/SE99/00947 -17 said mobile on the uplink is not speaking and the other party to the conversation with said mobile is receiving only comfort noise to eliminate the audible effect of handover to said other party.

12. A method of reducing the audible effect of handover in a digital cellular radio system as set forth in claim 8, wherein DTX is enabled only on the downlinks of said system. the communication being had between a mobile on one of said uplinks and another party is a voice call, and wherein said needed handover is delayed until said mobile on the uplink is speaking, the other party to the conversation is not speaking and said mobile is receiving only comfort noise to eliminate the audible effect of handover to said mobile.

13. A method of reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 8, wherein the regular periodic indications sent by the party not at that moment sending a communication are in form of one SID frame during each SACCH period and wherein said step of monitoring the transmission of said periodic indications includes monitoring and counting the number of successive SID frames which are received from said party.

14. A method of reducing the audible effects of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 8, wherein the regular periodic indications sent by the party not at that moment sending a communication are in form of one SID frame during each SACCH period and wherein said delaying step includes: counting the number of successive SID frames received from said party; comparing said number to a preselected value; and performing the hand-off when said number exceeds said preselected value. WO 99/65266 PCT/SE99/00947 -18

15. A method of reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 8 wherein said radio transmissions from a party on said link which is not at that moment sending a communications are interrupted by a voice activity detector (VAD) which instead causes the sending of regular periodic indications to the other party indicating that comfort noise should be generated in the receiver of that party.

16. A method of reducing the audible effect of handover in a digital cellular radio system as set forth in claim 9. wherein the communication being had between a mobile on one of said links and another party is a non-transparent data call.

17. A system for reducing the audible effect of handover in at least one link in the traffic channel of a radio air interface of a cellular radio system, comprising: means for implementing discontinuous transmission (DTX) within the one link other than the link within which the audible effects of handover are to be reduced including means for detecting whether each traffic frame in said one link consists of speech, non-transparent data or background noise: means for sending a silence descriptor (SID) frame containing information related to the background noise within the one link from the transmitter toward the receiver in response to detecting that neither speech nor non-transparent data are present on said link; means for monitoring all incoming traffic frames in the receiver; means for detecting a SID frame in the receiver and, in response thereto, updating the comfort noise characteristics and generating comfort noise within the receiver: means for determining that a handover of a mobile station on said link should be performed in response to signal measurements within the system: means for delaying the performance of said handover until a preselected number of SID frames have been sequentially received at the receiver to indicate that the party at the receiver is generating either speech or non-transparent WO 99/65266 PCT/SE99/00947 -19 data and is receiving comfort noise and then, in response thereto, performing handover of the mobile station from one BTS to another.

18. The system set forth in claim 17 which also includes: means for monitoring the quality of the connection during the time period within which said SID frames are being detected and counted; and means for performing said handover regardless of the number of sequential SID frames which have been received in response to either the signal quality or bit error rate (BER) of the connection deteriorating below a preselected value or at a rate greater than a preselected rate.

19. The system set forth in claim 17 which also includes the step of: means for triggering a timer within said system having a preselected timeout value in response to the receipt of a first SID frame; and means for performing said handover regardless of the number of SID frames which have been received in response the said timer reaching its timeout value.

20. The system set forth in claim 17 wherein the detection of whether each traffic frame in said link consists of speech, non-transparent data or background noise is performed by a voice activity detector (VAD).

21. The system set forth in claim 17 wherein DTX is implemented on only the uplink of the cellular system.

22. The system set forth in claim 17 wherein DTX is implemented on only the downlink of the cellular system.

23. The system set forth in claim 17 wherein DTX is implemented on both the uplink and the downlink of the cellular system.

24. A system for reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on WO 99/65266 PCT/SE99/00947 -20 at least one of the uplink or downlink radio channels of the system to interrupt radio transmissions from a first party on said link which is not at that moment sending a communications and to instead send regular periodic indications to a second party indicating that comfort noise should be generated in the receiver of that party, said system comprising: means for monitoring the transmission of said periodic indications sent by the first party which is not at that moment sending a communication; means within the system for monitoring the need to perform a handover from one cell to another of a mobile station which is one of said parties to said communications; means for delaying the performance of said needed handover until a preselected number of regular periodic indications have been received from the party to the communication which is not at that moment sending a communication to ensure that the other party is receiving only comfort noise and will not hear the audible effects of the handover being performed and then performing said handover.

25. A system for reducing the audible effect of handover in a digital cellular radio system as set forth in claim 24, wherein the communication being had between a mobile on one of said links and another party is a voice call and wherein said means for monitoring the need to perform a handover of said mobile station further comprises: means for monitoring the signal strength and bit error rate (BER) on both the uplink and downlink of said mobile along with the rate at which signal strength and BER are deteriorating; means for comparing the monitored signal strength and rate of deterioration of signal strength to respective preselected threshold values; means for performing said handover regardless of the receipt of said preselected number of regular periodic indications in response to either said monitored signal strength deteriorating below said preselected threshold value or said rate of deterioration of said monitored signal strength becoming greater than said threshold value. WO 99/65266 PCT/SE99/00947 -21

26. A system for reducing the audible effect of handover in a digital cellular radio system as set forth in claim 25, wherein said means for monitoring the need to perform a handover of said mobile station further comprises: means for comparing the BER and rate of deterioration of BER to respective preselected threshold values; means for performing said handover regardless of the receipt of said preselected number of regular periodic indications in response to either said monitored BER deteriorating below said preselected threshold value or said rate of deterioration of said monitored BER becoming greater than said threshold value.

27. A system for reducing the audible effect of handover in a digital cellular radio system as set forth in claim 24, wherein DTX is enabled only on the uplinks of said system, the communication being had between a mobile on one of said uplinks and another party is a voice call, and wherein said needed handover is delayed until said mobile on the uplink is not speaking and the other party to the conversation with said mobile is receiving only comfort noise to eliminate the audible effect of handover to said other party.

28. A system for reducing the audible effect of handover in a digital cellular radio system as set forth in claim 24, wherein DTX is enabled only on the downlinks of said system, the communication being had between a mobile on one of said uplinks and another party is a voice call, and wherein said needed handover is delayed until said mobile on the uplink is speaking, the other party to the conversation is not speaking and said mobile is receiving only comfort noise to eliminate the audible effect of handover to said mobile.

29. A system for reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 24. wherein the regular periodic indications sent by the party not at that moment sending a communication are in form of one SID frame during each SACCH period and wherein said means for monitoring the transmission of said periodic indications WO 99/65266 PCT/SE99/00947 -22 includes means for monitoring and counting the number of successive SID frames which are received from said party.

30. A system for reducing the audible effects of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 24, wherein the regular periodic indications sent by the party not at that moment sending a communication are in form of one SID frame during each SACCH period and wherein said delaying step includes: means for counting the number of successive SID frames received from said party; comparing said number to a preselected value; and performing said hand-off when said number exceeds said preselected value.

31. A system for reducing the audible effect of handover in a digital cellular radio system in which discontinuous transmission (DTX) is implemented on at least one of the uplink or downlink radio channels of the system as set forth in claim 24 wherein said radio transmissions from a party on said link which is not at that moment sending a communications are interrupted by a voice activity detector (VAD) which instead causes the sending of regular periodic indications to the other party indicating that comfort noise should be generated in the receiver of that party.

32. A system for reducing the audible effect of handover in a digital cellular radio system as set forth in claim 25, wherein the communication being had between a mobile on one of said links and another party is a non-transparent data call.