Abstract:
A method and devices are provided for adapting the transmission power level of a mobile terminal during a handover phase at which the mobile terminal switches from one base station to another. The method provided comprises applying values of operating parameters that characterize both base stations and adapting the transmission power level to an acceptable level at which communications transmitted from the terminal to the second station would be properly received without applying further increase in the transmission power while ensuring that the communications transmitted by the mobile terminal to the second base station, will not cause severe interference to communications exchanged between the second base station and the other terminals communicating therewith.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to wireless communications, and in particular, to cellular wireless communication networks carrying out handover process. 
     BACKGROUND OF THE INVENTION 
     In modern multiple access systems, alternate and TX noise floor interferences generated by adjacent Terminal Stations (TS&#39;s) is one of the most significant factors in utilizing the system capacity and in affecting session quality. One common way to overcome this problem is to reduce the transmit power for each user so that the interference is eliminated or at least limited. However, it is obvious that the transmit power cannot be simply reduced without taking into considerations other factors such as that the reduced power should still be enough in order to maintain the required performances for a satisfactory traffic quality. 
     Two approaches are known in the art for carrying out adaptation of the Terminal Station (TS) transmission power when it is about to start transmitting to a Target Base Station (TBS). The first approach is by carrying out a process of pre-association with the target BS before starting the actual transmission process (while maintaining or temporary suspending the communication to SBS). By this approach, the TS would receive information about the link characteristics and/or power setup requirements by connecting to the TBS, and consequently will be able to modify its own power level accordingly. Following the pre-association period, the information is used by the TS to adjust its Tx power level when connecting to the TBS. The main disadvantage of such a method is the increase in the time required for the TS scanning process, causing significant adverse impact (interruption) on the TS active data traffic session with a Serving Base Station during scanning interval. The second approach is by using close loop power control during the preliminary stage of the communication with the TBS. By this approach, the TS sets its initial TX power for transmitting to the TBS according to predefined knowledge of the TBS requirements and/or measurements of transmission(s) received from the TBS. The main disadvantage of this approach is significant delay increase during a handover (HO), a process when a connection has to be transferred from one sector to another while the user moves between cells, during active data traffic session. 
     US 20030203742 describes wireless devices incorporated in an ad-hoc wireless network which adaptively set their transmission power levels based on locally available information. Initially, each such wireless device sets its transmission power level to a relatively low level, and gradually increases its power level up to a predetermined maximum transmission power level. As the transmission power increases, the wireless device is able to incrementally connect with additional wireless devices located at increasing distances from the wireless device. As the wireless device connects with these additional wireless devices, it checks a connectivity constraint. When the connectivity constraint is satisfied, the wireless device stops increasing its power and operates at its current power level. 
     US 20050147074 describes a method for autonomously optimizing the transmission power of an endpoint in a wireless network. The method includes monitoring the signal quality associated with data transfers between an access point in the wireless network and the endpoint at a certain transmission power and a certain transmission speed, checking whether the signal quality is acceptable and then adjusting the transmission power or the transmission speed based on whether the signal quality is acceptable. 
     US 20010012766 discloses a wireless communication apparatus that includes a variable power amplifier and a power amplifier. A variable power amplifier control unit controls the gain of the variable power amplifier for controlling the transmission power. Simultaneously therewith, a detection unit detects changes in conditions of the station and based upon the detected condition changes, a transmission power control bit controlling unit and a transmission power control period controlling unit change the control period of the transmission power control bit and the transmission power control range respectively, and the transmission power control bit is inserted into the transmission signal in be received by the other end of the link receiver. 
     In addition, one of the processes characterizing a mobile wireless network is a handover (HO) process in which a connection, currently existing between a device and a second device, needs to be transferred to another device during an active communication session. Such as in the case where a user moves from one cell to another cell while surfing the web, or in the midst of a phone call. 
     The problem with such a handover process is that it should be carried while the TS moves to an area serviced by a second BS and should communicate at the correct transmission power level, i.e. not too low and not too high. Otherwise, if it&#39;s transmission power is too low, it will not be received immediately at its first attempt and the handover process might take too long time, while on the other hand, if its transmission power is too high it might cause severe interference to other TSs that are already communicating with the second (target) BS. US 20050159176 describes a radio communication system where an associated-dedicated physical channel used for carrying out a soft hand-over and a high speed-dedicated physical control channel used for carrying out a hard hand-over, are combined. 
     Still, none of the prior art methods described provide an adequate solution to the problem of carrying out a reliable hand-over process which will be completed within a short enough time interval and will not consume too much resources of the wireless network. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a novel method for carrying out a fast power adjustment during handover process when changing the base station for communication. 
     It is another object of the present invention to provide a mobile terminal operative to adjust its power transmission level when moving from an area covered by a first base station to another area covered by another base station. 
     Other objects of the invention will become apparent as the description of the invention proceeds. 
     It is therefore provided in accordance with a first embodiment of the invention, in a wireless communications network, comprising a mobile terminal communicating with a first base station, a method for adapting the mobile terminal&#39;s transmission power level during a handover process to a second base station. The method comprises applying values of operating parameters which characterize each of the first and said second base stations and adapting the transmission power level of the mobile terminal to an acceptable level in which communications transmitted from the terminal to the second station would be properly received at the second base station preferably without applying further increase in their transmission power, and wherein the communications transmitted by the mobile terminal to that second base station will not cause severe interference to the operation of other terminals being in communication with the second base station. 
     Preferably, the method comprising the steps of:
         (i) storing dynamically updated values of one or more pre-defined parameters associated with the operation of the first base station;   (ii) retrieving values of one or more pre-defined parameters associated with the operation of said second base station, from communications received by said mobile terminal;   (iii) determining a required modification in the transmission power level of the terminal for communicating with the second base station; and   (iv) setting the terminal&#39;s transmission power level in accordance with the determination made in step (iii).       

     As will be appreciated by those skilled in the art, step (ii) of retrieving values of one or more pre-defined parameters associated with the operation of the second base station may be received at the terminal directly from communications transmitted by the second base station and received by the terminal. In addition or and/or in the alternative, such one or more pre-defined parameters that associated with the operation of the second base station, may be received indirectly through communications received at a base station other than the second base station (e.g. through the first base station, or through a third base station). 
     According to another embodiment of the invention, step (iii) comprises a step of determining whether a modification is required prior to determining the required modification. 
     By yet another preferred embodiment of the invention, the pre-defined parameters associated with the first base station are one or more members of the group consisting of: power control parameters associated with the first base station and parameters characterizing the status of a communication link extending between the first base station and the terminal. 
     According to still another preferred embodiment of the invention, the pre-defined parameters associated with the second base station are one or more members of the group consisting of: power control parameters associated with said second base station, parameters characterizing the status of a communication link extending between said second base station and said terminal, and parameters associated with power control offset. As will be appreciated by those skilled in the art, the offset can derive from internal errors experienced by the TS and/or from errors discovered during communications with other base stations. 
     In accordance with another aspect of the present invention, there is provided a mobile terminal adapted to operate in a wireless network and comprising: 
     a transceiver adapted to communicate with a base station operative in said wireless network; 
     a first processor operative to retrieve values of pre-defined parameters associated with communications conducted between the mobile terminal and a first base station; 
     a storing means adapted to store current values of the pre-defined parameters retrieved by the first processor; 
     a second processor adapted to retrieve pre-defined values associated with a second base station, with which the mobile terminal is about to start communicating; 
     a third processor adapted to calculate a transmission power level for communicating with the second base station; and 
     a control circuitry adapted to set the transmitter power level to the level calculated by the third processor prior to starting communications with said second base station. 
     Although the above described mobile terminal has been described as comprising three processors, it will be appreciated by those skilled in the art that the functions associated with these three processors could alternatively be carried by using one processor only or by any number of processors, as will be desired by the manufacturer of the mobile terminal. 
     According to still another aspect of the present invention, there is provided a base station operative in a wireless network and comprising: 
     a transmitter adapted to transmit a training signal towards a mobile terminal that is about to start communicating with the base station, and wherein the training signal is adapted to provide the mobile terminal with characteristics required to allow the mobile terminal to estimate an allowed power level for transmitting communications to the base station along a certain channel that will be used for that purpose; and 
     a receiver adapted to receive communications from the mobile terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a wireless network where a mobile station moves from a cell covered by a first base station to another cell covered by another base station; 
         FIG. 2  illustrates a block diagram of a method for carrying out an embodiment of the present invention; 
         FIG. 3  presents a schematic illustration of power control circuitry constructed in accordance with an embodiment of the present invention; and 
         FIG. 4  demonstrates a block diagram of certain components comprised in a mobile terminal according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by way of examples, and without limiting the generality of the foregoing. 
     According to a preferred embodiment provided by the present invention, a method is provided by which the output power of the transmitter of a Terminal Station (TS) is set to an initial value when the TS, which is currently serviced by a SBS, is about to access a cell serviced by a different TBS. 
     This initial value of the transmitter&#39;s power output is determined by applying a correction factor (CF) to the currently used output power of the TS transmitter that is now transmitting to the SBS. Applying this CF allows determining the initial transmission power that will be used by the TS transmitter upon starting the transmission to the TBS during the HO. 
     Let us consider now a wireless network  10  as illustrated in a schematic way in  FIG. 1 . Terminal Station  12  is currently moving in an area serviced by SBS  14 , while carrying out an active communication session where traffic is conveyed through that base station. During this session, the TS periodically measures one or more pre-defined characterizing parameters of the base station which may be used to adapt its current power transmission to the SBS (if needed), but are also preferably stored in a memory means of TS  12  for later use when TS is about to access the area serviced by TBS  16 . When reaching the area when transmissions from TBS  16  may be received by TS  12 , one or more pre-defined parameters characterizing that TBS are received by TS  12  and together with the stored parameters which relate to the SBS, the change in the TS output power level is determined. Upon effecting this change (if indeed required) TS  12  will start transmitting to TBS  16  and receive communications therefrom. 
     The method provided by the present invention is further described in  FIG. 2 . The TS acquires, preferably on a periodic basis, one or more pre-defined parameters associated with a BS which is currently communicating with the TS (step  20 ). These pre-defined parameters are stored at the TS. As will be appreciated by those skilled in the art, this step may be carried by using any one or more methods such as storing the difference from pre-defined values of the pre-defined parameter(s), measuring the parameters on a non-period basis, etc. and all these methods should be considered as encompassed by the present invention for carrying out this step. When the TS approaches an area serviced by another base station, a handover process is triggered (step  25 ). Next, the new base station, the TBS, transmits certain pre-defined parameters (step  30 ), and from knowing the values of the pre-defined parameters associated with the old base station and after receiving the values of the pre-defined parameters associated with the new base station (which preferably but not necessarily are the same parameters), the value of the power output level that will suit the conditions for transmission to the new base station is determined (step  35 ) and the TS will commence communicating with the TBS. 
     According to an embodiment of the present invention, the handover output power is calculated based on the current output power and an offset therefrom derived from any one or more members of the group consisting of: SBS power control parameters, SBS link status, TBS power control parameters, TBS link status and parameters associated with power control offset (PCoffset). 
     The determination of the power to be used for the transmission to the new BS, the TBS, is by using the following relationship:
 
 P   t   TBS   =P   t   SBS   +F   offset   (1)
 
where
     P t   TBS  is the TS power level (dBm) that will be used for transmission to TBS;   P t   SBS  is the currently used TS power level (dBm) for transmission to SBS; and   F offset  is a correction factor having (dB) dimensions.   

     The correction factor can be described for example by the following function:
 
 F   offset   =f ( P   sub   SBS   , Rx _RSS SBS   , Rx _SINR SBS , Target_RSS SBS , Target_SINR SBS , Target_NI SBS   , P   sub   TBS   , Rx _RSS TBS   , Rx _SINR TBS , Target_RSS TBS , Target_SINR tBs , Target_NI SBS )
 
where
     Target_RSS SBS  is the SBS uplink target received power criteria, or in other words the strength of a signal transmitted by the TS as received by the SBS, i.e. the SBS Receive Signal Strength (“RSS”) expected to be received from the TS;   Target_SINR SBS  is the SBS uplink target Signal to Interferences and Noise Ratio (“SINR”) criterion;   Target_NI SBS  is the SBS uplink target Noise level and Interferences (NI) criterion;   P sub   SBS —the SBS′ transmitter power level;   Rx_RSS SBS —the SBS downlink (DL) RSS as measured by the TS;   Rx_SINR SBS —the SBS downlink (DL) SINR as measured by the TS;   P t   TBS —TS required transmission power level for transmission to the TBS;   Target_RSS TBS —TBS uplink target received power criterion, i.e. the SBS target Receive Signal Strength;   Target_SINR TBS  is the TBS uplink target Signal to Interferences and Noise Ratio (“SINR”) criterion   Target_NI TBS  is the TBS uplink target Noise level and Interferences (NI) criterion;   P sub   TBS —TBS transmission power level;   Rx_RSS TBS —TBS Received DL RSS as measured by the TS; and   Rx_SINR TBS —the TBS downlink (DL) SINR as measured by the TS.   

     Let us consider the following example where F offset  is determined by the following equation:
 
 F   offset ( P   sub   SBS   , Rx _RSS SBS , Target_RSS SBS   , P   sub   TBS   , Rx _RSS TBS , Target_RSS TBS )=( P   sub   SBS  [dBm]− P   sub   TBS  [dBm])−( Rx _RSS SBS  [dBm]− Rx _RSS TBS  [dBm])+(Target_RSS SBS  [dBm]−Target_RSS TBS  [dBm])
 
     Using the estimation for the required output power for transmission to the target base station by applying the relative power control approach as provided by the present invention, allows setting the initial uplink transmission power when the TS is moving from its current base station (SBS) and accesses the Target Base Station (TBS). Thereafter, if there will be some further modifications that will be required to the output power level of the TS′ transmitter, such modification may be carried out according to any applicable method known in the art per se. 
     By yet another example of an embodiment of the present invention, the F offset  is determined by using the following equation:
 
 F   offset ( P   sub   SBS   , Rx _RSS SBS , Target_SINR SBS , Target_NI SBS , P sub   TBS   , Rx _RSS TBS , Target_SINR TBS , Target_NI TBS )=( P   sub   SBS  [dBm]− P   sub   TBS  [dBm])−( Rx _RSS SBS  [dBm]− Rx _RSS TBS  [dBm])+(Target_SINR SBS  [dB]−Target_SINR TBS  [dB])+(Target_NI SBS  [dBm]−Target_NI TBS  [dBm]).
 
       FIG. 3  presents a schematic illustration of power control circuitry  50  of a mobile terminal constructed in accordance with an embodiment of the present invention. Power control circuitry  50  comprises three main parts, transceiver  90  which comprises receiver  92  and transmitter  94 , SBS power control unit  60  which is operative to control the terminal&#39;s transmission power when the terminal is communicating with the SBS (the current base station), and TBS power control unit  80  which is operative to control the terminal&#39;s transmission power when the terminal would be communicating with the TBS (the base station with which it the terminal is about to start communicating). Receiver  92 , receives communications from the SBS, and their power characteristics are measured by SBS Rx power level measuring device  62 . The measured results are provided to SBS comparator  64 , which compares the measured values with known expected values. The results of the comparator are forwarded to SBS power control generator  66 , which provides modification commands whenever it becomes necessary to modify the transmission power level for the terminal in its communication with the SBS. In addition, the information generated by SBS generator  66  is conveyed to TBS channel estimator  84  of power control unit  80 . The information generated by SBS generator  66  is used in feedback circuitry  68  that will increase or decrease the power output level for transmitter  94  that is currently communicating with the SBS. Upon triggering a handover process, receiver  92  receives one or more signals from the TBS and forwards these signals to TBS′ Rx power level measuring device  82 , which measures the power intensity of these signals received from the TBS. The measured results are forwarded to TBS channel estimator  84 , and together with the information received from generator  66 , provides an estimate of the characteristics of the channel that will be used for communications between the terminal and the TBS. The estimation is provided to processor  86  which calculates the transmission power level that should be used for communicating with the TBS, and set the power control circuitry (not shown) of transmitter  94 , accordingly. Upon completing the process, the terminal may start communicating with the TBS. 
       FIG. 4  demonstrates a block diagram of certain components comprised in a mobile terminal according to an embodiment of the present invention. Mobile terminal  100  comprises a transceiver  102  adapted to communicate with base stations that are operative in the wireless network. From the communications received in transceiver  102  processor  104  retrieves (preferably on a periodic basis) values of pre-defined parameters associated with communications conducted between the mobile terminal and base station with which the mobile terminal is currently communicating, and the retrieved values are stored in storing means  106 . When a process of switching to another base station is about to take place, one or more signals are received at the transceiver  102  from the new base station and processor  110  then retrieves the values of certain pre-defined parameters associated with the new base station, with which said mobile terminal is about to start communicating, for example parameters that are associated with the estimation of the channel that will be used for communication between the mobile terminal  100  and the second base station. The values of the parameters associated with communicating with each of the two base stations, namely the SBS and the TBS, are stored in the storing means, e.g. database  106  and  108  respectively. Processor  112  then calculates the transmission power level required for communicating with the second base station which in return allows power control circuitry  114  to initiate a modification of the transmission power level if transceiver  102  is used to communicate with the second base station, or set the initial values if another transmitter will be used for that purpose. 
     It will be appreciated that the above-described methods may be varied in many ways, including, changing the order of steps, and the exact implementation used. 
     The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features shown in a particular figure. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise,” “include,” “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.” The scope of the invention is limited only by the following claims: