CDMA soft handover method using uplink measurements

Abstract: The invention concerns a method of selecting a base station of a CDMA type network with which a terminal is to perform a soft handover. According to the invention, the base station which requires the lowest transmission power on the part of the terminal is selected.

Description: The present invention relates to a soft handover method used in a CDMA mobile communication system. In a cellular communication system, a mobile terminal is free to roam and connect from one base station to another within the mobile communication system. When a terminal is handling a call during a cell crossover, the call must also be switched to the new base station by a way which causes as little disturbance to the call as possible. The cell crossover process during an ongoing call is called a handover. In a CDMA mobile communication system, a mobile terminal is able to communicate simultaneously with different base stations, these base stations being those registered in the active set of the mobile terminal. Each base station transmits data on a dedicated broadcast channel, e.g. on the broadcast channel called BCCH (Broadcast Control Channel) in the GSM system, this channel containing some information related notably to the load of this base station. The broadcast channels of the different base stations of the CDMA system are sent with a same given power. Each mobile terminal of the network receives these different broadcast channels and demodulates them in order to determine a ratio Eb/I, where Eb is the energy level per bit received and I the spectral interference density (or Ec/I with Ec the energy level per chip) for each base station. This ratio corresponds to the quality with which the mobile terminal receives the broadcast channel of the corresponding base station. Alternatively, the received power level from the BCCH channel can be used as a quality criterion. The mobile terminal can therefore determine a signal quality level for each base station and select the base stations that it receives with the best quality for soft handover purposes. This selection consists in registering the best received base stations in its active set. Such a process is performed on a regular time basis in order to ensure that the mobile terminal is always in communication with the best received base stations. Such communications with different base stations is called macrodiversity situation. This allows to lower the interference level of the system in comparison with a communication established with a single base station. Macrodiversity is also a means of improving signal quality for mobiles at cell border. The goal of a soft handover criterion is to anticipate cell changes and detect potentially interesting macrodiversity situations. In this manner, interference is reduced in the network while maintaining quality. Prior art solutions then consist in downlink-oriented soft handover criteria. The mobile terminal carries out measurements (signal strength, Eb/I,...) over the downlink broadcast channel of each base station and compares the outcome of these measurements with fixed/dynamic thresholds or with the measurements realized on the broadcast channel of other base stations. Eventually, based on this criterion, the Radio Network Controller (Controller of the base stations) or the mobile terminal selects the base stations to be part of the mobile's active set, i.e. the base stations with which the mobile is simultaneously communicating. The active set contains preferably at least two or three base stations because the quality of the communications can change very rapidly and the active set is changed on a much lower basis. However, with downlink-oriented handover criteria, situations can occur where uplink macrodiversity is wrongly unused (and therefore interference could be further minimized) because the handover criterion has not detected the need for macrodiversity in the uplink. Such a situation occurs when a base station that was not selected in the active set receives signals from a mobile terminal with a better quality than the base stations that are already in the active set. Since the uplink transmission path is not taken in consideration for building the active set, such a base station cannot be put in the active set, although the power of the signals sent by the mobile terminal could have been lowered. It is known from US-5,487,174 a method for handling a bidirectional connection involving a mobile station in a cellular mobile radio communication system having adjacent cells, wherein decisions on which base station should be responsible for an uplink channel of a bi-directional connection are based on uplink parameters including the estimated strength of signal received at base stations. In this document the base station responsible for the uplink is chosen thanks to measures at the different base stations potentially candidate to be responsible for the uplink channel. At call set up the base station requiring the lowest output power of radio signals from the mobile station is selected. The problematic to minimize the uplink transmitted power during the handoff procedure for selecting a new base station is not considered. Indeed, this document discusses the choice of only one base station to be responsible for the uplink channel and doesn't take into account specific problems concerning macrodiversity. It is also known from EP-0,671,819 that a mobile terminal selects, for sending its next frame, the base station which has so far received the highest average power signal from this terminal. In this document also, only the signal power received by the base station is taken into account. More precisely, comparing the received power at the base station comes down to comparing the attenuation experienced by the mobile signal, such as seen by different base stations. This attenuation is not sufficient to account for the needs in transmit power of the mobile. In reality, the required transmit power at the mobile depends on the loading status of the cell, which is expressed by the total power received at the base. This quantity represents the total interference to be overcome by the uplink signal to assure a good reception at the base. The required transmitted power depends also on the signal quality required by the different base stations. Therefore, the solution proposed in this document is not optimal. A particular object of this invention is to take into account the uplink transmitted power that would be required from the part of the mobile terminal if a certain base station were to communicate with the mobile terminal (e.g. enter its active set). As a consequence the object of this invention is not to choose the base station receiving the highest transmission power level from a mobile terminal but the base station receiving signal from the mobile terminal with a required quality and requiring the lowest transmission power from the part of the terminal. This object is obtained with a method according to claims 1 and with a device according to claim 7. The basic idea of the invention is to supplement an uplink-oriented handover criterion to be possibly combined with known downlink-based criteria, so that the needs of each direction can be fulfilled independently. An optimal uplink soft handover criterion, derived from a generalized open-loop power control procedure, is introduced. More precisely, in order to reduce interference in the uplink, it is desirable that the most favorable link (in terms of required transmit power) controls the transmit power of the mobile. This is normally ensured by the fast closed-loop power control procedure, provided that all relevant base stations for macrodiversity actually belong to the active set. This power control procedure consists in sending to the mobile terminal instructions to increase or to lower its transmission power, on the basis of the quality of the signal sent by this mobile terminal and received by the base stations of the active set. This means that all the base stations of the active set send instructions to the mobile terminal, these instructions being related to the output power required from the terminal. The terminal listens to these instructions and modifies its output power such that its output power corresponds to the lowest power requested by the base stations of the active set, in order that at least one base station of the active set receives the signals sent by the terminal with a good quality. This ensures that the terminal always sends signals with the lowest possible output power. The invention therefore consists in at least selecting the base station which requires the lowest transmission power on the part of the terminal. Several implementations of this concept are possible. For example, based on information normally used for open-loop power control, the mobile terminal evaluates the required transmit power towards several "candidate" base stations to achieve the target Eb/I. If one of these power values proves to be close enough to the corresponding required transmit power calculated for base stations of the active set, then the "candidate" base station bearing this low power requirement should be added to the active set, i.e. soft handover should occur with this base. To put it in a nutshell, the mobile terminal runs several open-loops in parallel with base stations of the active set and the candidate set. The transmit power estimates of the candidate set are compared with those of the active set. In the sequel, this handover scheme is described in more detail. Calculation of the required transmit power can be realized in the following way: Open-loop power control relies on measurements made over the broadcast channel (e.g. the BCCH) and on the information broadcast on this channel. In each cell, the BCCH periodically sends the following information: required Signal to Interference Ratio at the base, noted SIR req , total power measured at the base P r,tot , transmit power of the BCCH channel P t,BCCH . The broadcast information is periodically updated on a slow basis (about every 100ms in UTRA-FDD system for CDMA). For every base station belonging to the candidate set and the active set, the mobile is capable of demodulating the BCCH channel and thus the preceding information is available. In addition, the power received on the BCCH channel of candidate and active (i.e. in communication with the mobile terminal) base stations can be measured. It should be noted that this measurement is already needed to perform conventional downlink-driven soft handover. The power received on the BCCH channel is averaged over such a period that fast-fading effects are removed while slow-fading effects are still accounted for. As a result, the required transmit power P r,UL to achieve target SIR vis-a-vis an active or candidate base (if the mobile were in communication with it) is evaluated as follows: P r,UL = (SIR req * P r,tot * P t,BCCH ) / (PG * P r,BCCH ) where PG denotes the processing gain for the said communication (i.e. the ratio between the spreading bandwidth and the bit rate), P r,tot embodies uplink cell loading information and the ratio P t,BCCH / P r,BCCH the pathloss (with P r,BCCH the received power of the BCCH channel). Uplink cell loading conditions depend essentially on the number of users in the cell while downlink cell loading is highly dependent on mobiles' positions. Since uplink and downlink loading conditions do not generally coincide, there is a clear need for distinct uplink and downlink handover criteria. In a preferred embodiment, the selection of the base station requiring the lowest transmission power on the part of the terminal consists in: determining, for each communication with a candidate base station, the transmission power required in the uplink direction; and selecting the base station for which the corresponding transmission power required in the uplink direction is lower than a threshold value plus the power allocated by said terminal to communication in progress with another base station. This threshold value corresponds to a margin meant to avoid the well known ping-pong effect. The suppression of a base station in the active set of the terminal can be realized when this base station requires uplink transmission power that is greater than that of a candidate base station minus a threshold value. The threshold value can be null. This corresponds to the situation where the maximum size of the active set is reached, and the suppressed base station is to be replaced by a candidate base station. Removal of a base station from the active set can also occur when this base station requires uplink transmission power that is greater than that of an active base station plus a threshold value. A simplified embodiment of an uplink-based decision algorithm for soft handover is provided hereafter. Determine best (resp. worst) active cell as the one with the lowest (resp. highest) P t,UL , say P t,best (respectively P t,worst ) For each cell in the candidate set, IF (P t,cand < P t,best + M enter ) THEN IF (active set size not exceeded) THEN candidate cell enters active set ELSEIF ((P t,cand < P t,worst - M repl ) THEN candidate cell replaces worst cell in active set If (P t,act > P t,best + M exit ) then remove active cell from active set where: P t,act: required transmit power for one of the active base stations M enter: hysteresis margin for entering the active set M repl: hysteresis margin for replacing a base station in the active set M exit : hysteresis margin for removing a base station from the active set The decision to put a base station in the active set of the terminal can be taken at the level of the terminal or at the level of the controller of the base stations (RNC). More precisely, in case of mobile-assisted handover, the decision algorithm is to be implemented in both the mobile and the RNC: the mobile detects potential changes in the active set, but it is up to the RNC to make the final decision of modifying the active set. RNC decisions are based on BCCH power measurements reported by the mobile to the RNC. Using P r,BCCH values, the RNC can readily reconstitute P t,UL values according to relation (1), and apply the preceding decision algorithm. An alternative embodiment of the solution where evaluation of P t,UL is carried out solely in the RNC could also be envisaged (RNC-evaluated / RNC-executed handover strategy). The chosen handover strategy is assumed to be the same for up- and downlink. Concerning a combination of uplink and downlink handover criteria, the invention consists in the following. Since disjoint handover (independent execution of uplink and downlink handover) is impractical because of closed-loop power control constraints, one must find an appropriate combination of up- and downlink handover criteria. From the uplink viewpoint, augmenting the active set can only help decrease mobile transmit power, without any further penalty for the system. However, in the downlink, augmenting the active set implies sending more signals, which causes in principle more interference. Nevertheless, this negative effect can be largely suppressed if efficient power distribution techniques are used. For example, one can add a base station to the active set to help reduce uplink interference, and assign a very low power fraction to this base (a minimum is required for the signaling part to enable the transmission of power control commands) in the downlink, if no significant advantage is to be expected from the use of this base in the downlink. In this context, if either of uplink/downlink criteria for addition is verified, the active set should be augmented. On the contrary, when it comes to removing/replacing a base station, more restrictive criteria should be used: The decision is taken if both uplink and downlink conditions are satisfied. As a conclusion, the following combination scheme can be implemented: logical OR for "add" operation, logical AND for "remove"/"replace" operations. The present invention also concerns a device for performing soft-handover in a CDMA type network comprising at least two base stations dedicated to communicate with a terminal performing macrodiversity, each base station broadcasting a signal, the device having access to following information for each base station: a total power received at the base station, and a power with which said base station broadcasts said signal. The device comprises means for receiving an indication relating to the strength, experienced at the terminal, of said broadcast signal for each of said base stations. The device further comprises means for: determining for each base station a target SIR or a target Eb/l required by the base station; for each base station, evaluating an uplink transmission power level to be transmitted by the terminal to said base station to fulfill said target SIR or said target Eb/I required by the base station, the uplink transmission power level depending on the measured strength of said broadcast signal, on the total power received at the base station, on the power with which the base station broadcasts the signal, and on the target SIR or the target Eb/I; selecting the base station which requires the lowest uplink transmission power level to be transmitted by the terminal. Such a device can be located at the level of the terminal or at the level of the controller of the base stations. As compared to existing solutions, the invention presents the following benefits: improved uplink interference management: thanks to the invention, uplink interference is reduced (+ load balancing effect), resulting in an uplink capacity increase; increased flexibility in cell transition handling: optimal uplink criterion and customized downlink criterion (instead of neutral pathloss-based handover criterion), leading as well to a gain in downlink interference management; no additional reporting from mobile to network: using reports of BCCH received power measurements, the RNC can infer P t,UL values; possible extensions to disjoint handover and emergency power regulation. The present invention can be applied to several cellular or trunked mobile communication system, such as the Pan-European mobile communication system GSM, DCS1800 (Digital Communication System), PCN (Personal Communication Network), UMC (Universal Mobile Communication), UMTS (Universal Mobile Telecommunication System), FPLMTS (Future Public Land Mobile Telecommunication System), etc.

Claims: A method for performing soft-handover in a CDMA type network comprising at least two base stations dedicated to communicate with a terminal performing macrodiversity, each of said base stations broadcasting a signal, each of said base stations being associated a power with which said base station broadcasts said signal and a total power received at said base station, said method comprising the steps of measuring at said terminal the strength of said broadcast signal for each of said base stations, said method being characterized in that it further comprises the steps of: determining for each of said base stations a target SIR or a target Eb/I required by said base station; for each of said base stations, evaluating an uplink transmission power level to be transmitted by said terminal to said base station to fulfill said target SIR or said target Eb/I required by said base station, said uplink transmission power level depending on said measured strength of said broadcast signal, on said total power received at said base station, on said power with which said base station broadcasts said signal, and on said target SIR or said target Eb/I; selecting the base station which requires the lowest uplink transmission power level to be transmitted by said terminal; and performing soft-handover with said selected base station. A method according to claim 1, characterized in that it also comprises a step of selecting a base station for which the corresponding uplink transmission power required is lower than a threshold value plus the power allocated by said terminal to a communication in progress with another base station. A method according to any of claims 1 to 2, characterized in that said selection method is implemented in said terminal, said signal broadcast by each of said base stations comprising said target SIR or said target Eb/I required by said base station, said power with which said base station broadcasts said signal and said total power received at said base station. A method according to any of claims 1 to 2, in which said selection method is implemented in the controller of said base stations. A method according to any of claims 1 to 4, characterized in that it is used to manage an active set of a terminal operating in a CDMA transmission network. A method according to claim 5, characterized in that it consists also in eliminating from the active set a base station requiring an uplink transmission power level greater than that of said selected base station plus a second threshold value. A device for performing soft-handover in a CDMA type network comprising at least two base stations dedicated to communicate with a terminal performing macrodiversity, each of said base stations broadcasting a signal, said device having access to following information for each of said base stations: a total power received at said base station, and a power with which said base station broadcasts said signal, said device comprising means for receiving an indication relating to the strength, experienced at said terminal, of said broadcast signal for each of said base stations, said device being characterized in that it further comprises means for: determining for each of said base stations a target SIR or a target Eb/I required by said base station; for each base station, evaluating an uplink transmission power level to be transmitted by said terminal to said base station to fulfill said target SIR or said target Eb/I required by said base station, said uplink transmission power level depending on said measured strength of said broadcast signal, on said total power received at said base station, on said power with which said base station broadcasts said signal, and on said target SIR or said target Eb/I; selecting the base station which requires the lowest uplink transmission power level to be transmitted by said terminal. A device according to claim 7, characterized in that said device is comprised in said terminal, said signal broadcast by each of said base stations comprising said target SIR or said target Eb/I required by said base station, said power with which said base station broadcasts said signal and said total power received at said base station. A device according to claim 7, characterized in that said device is comprised in the controller of said base stations.

Inventors: CORDIER CHRISTOPHE, DE HOZ GARCIA BELLIDO ALEJANDR, CORDIER, CHRISTOPHE, DE HOZ GARCIA BELLIDO, ALEJANDRO
Applicants: CIT ALCATEL, ALCATEL
Application year: 1999
Main classification: H04Q7
IPC codes: H04J13, H04W36, H04W52