Abstract:
The invention relates to a method for controlling output power of a radio transmitter, the radio transmitter operating on a radio channel. The method includes determining requested output power level, and deriving output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.

Description:
FIELD OF THE INVENTION 
   The invention relates to power control and especially to transmission power control between a mobile device and some other radio network equipment such as a base station. 
   BACKGROUND OF THE INVENTION 
   Maximum link range of a mobile device operating in a mobile communication network is strongly dependent on the maximum available output power of such device. Typically, the maximum available output power of mobile device has an effect on the base station density in the network and also on the network&#39;s operating expenses and capital expenses. 
   The maximum output power of a mobile device is dependent on the maximum available DC power, signal waveforms, and minimum possible link distance to adjacent base stations operating on adjacent channels. The minimum distance to the base stations operating on adjacent channels is important because power amplifiers (PA) are non-linear elements and thereby generate so-called adjacent channel power (ACP). RF (Radio Frequency) specifications of systems typically set restrictions to the power that is radiated on the adjacent channels. The reason for setting such restrictions is that ACP interferes with reception of the adjacent base stations if the ACP level is too high. 
   The restrictions of the specifications are conventionally derived on the basis of the minimum attenuation from the mobile device to the adjacent base stations and on the basis of integrated noise+noise figure over the channel bandwidth. That is, the restrictions are derived on the basis of the worst-case scenario. 
   Especially in systems, which use signals with high PAPR (Peak-to-Average-Power Ratio), ACPR (Adjacent Channel Power Ratio) requirements of specifications restrict the maximum transmission power level of the mobile devices and PAs of the mobile devices have to continuously operate with high backoff and with very small efficiency in order to maintain sufficient linearity. For example, in OFDM (Orthogonal Frequency Division Multiplexing) PA backoff can be almost 8 dB. 
   PA backoff is defined as follows: backoff is the ratio of power that is transmitted from the PA to the −1 dB compression point of the PA in dBs. 
   Because the ACPR requirements are derived on the basis of the worst-case scenario, the whole system is actually operating with low power efficiency and the idle and operation times or ranges of the system remain small. 
   Typically the upper bound of the output power is determined so that it is possible to implement required TX (transmission) spectrum mask with reasonable power consumption. If the requested output power is less than the available maximum, it is possible to save DC power by tuning for example the supply voltage with DC-DC converters and reducing the output power this way. Current specifications do not allow higher values of ACP than the value derived from the minimum attenuation to receivers operating on adjacent channels. Typically, power control is designed to take into account only in-band power or SINR (Signal-to-Interference and Noise Ratio) requirements. 
   PAPR reduction techniques can be used for reducing the power amplifier backoff. A technique that is called narrow band or soft clipping reduces probability of high amplitude peaks and in this way reduces the PA backoff one to two dBs. It is possible to clip the signal even more if the required signal quality is not high. 
   In addition to these measures, there is a need to develop more efficient power control algorithms. 
   SUMMARY OF THE INVENTION 
   Now a new power control algorithm has been invented. 
   One of the basic ideas of the invention is to divide power control algorithm into two areas.
         In the first area the requested output power is less than or equal to the maximum output power derived in a conventional way (that is, on the basis of worst-case minimum attenuation to adjacent base stations). There the output power to be used is derived in the conventional way and conventional radiation mask has to be met.   In the second area the requested output power exceeds the maximum output power derived in the conventional way. In this area an extended power control scheme is used. The mobile device measures actual minimum attenuation to the adjacent base stations operating on adjacent channels and derives output power to be used on the basis of the measured minimum attenuation. That is, the output power can be higher than the maximum derived in the conventional way.       

   According to a first aspect of the invention, there is provided a method for controlling output power of a radio transmitter, the radio transmitter operating on a radio channel, wherein the method comprises:
     determining requested output power level, and   deriving output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.   

   According to a second aspect of the invention, there is provided a radio communication device comprising a radio transmitter, the radio transmitter operating on a radio channel, wherein the radio transmitter comprises:
     processing means,   said processing means being arranged to determine requested output power level, and   said processing means being arranged to derive output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.   

   According to a third aspect of the invention, there is provided a transmitter module, the transmitter module operating on a radio channel, wherein the radio transmitter comprises
     processing means,   said processing means being arranged to determine requested output power level, and   said processing means being arranged to derive output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.   

   According to a fourth aspect of the invention, there is provided a system comprising
     at least a first and a second radio receiver, which operate on adjacent radio channels, and   at least one radio transmitter communicating with said first radio receiver on the operating channel of the radio transmitter, wherein said radio transmitter comprises   processing means,   said processing means being arranged to determine requested output power level, and   said processing means being arranged to derive output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.   

   According to a fifth aspect of the invention, there is provided a computer program for controlling output power of a radio transmitter, the radio transmitter operating on a radio channel, wherein the computer program comprises:
     program code for determining requested output power level, and   program code for deriving output power, which is to be used, on the basis of a power control algorithm having at least a first and a second power control area, maximum output power of the first area being derived at least on the basis of theoretical minimum attenuation to adjacent receivers, and maximum output power of the second area being derived at least on the basis of actual minimum attenuation to adjacent receivers, wherein an adjacent receiver is a receiver operating on an adjacent channel with respect to the operating channel of said radio transmitter.   

   Dependent claims contain some embodiments of the invention. The subject matter contained in dependent claims relating to a particular aspect of the invention is also applicable to other aspects of the invention. 
   The power control method of the invention suits well for controlling uplink power in mobile communication networks. 
   The invention provides a possibility to use higher maximum output power with higher efficiency than tolerated by conventional specifications. For example, it is possible to increase efficiency of systems that use multi-carrier signals like OFDM and that thereby have low power efficiency in the mobile transmitter in conventional systems. 
   By means of some embodiments of the invention it is possible to increase the maximum link distance in the area of low BTS density. That is, in the rural area the base station separation can be increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
       FIGS. 1A and 1B  illustrate systems wherein the invention may be employed and a basic idea of the invention; 
       FIG. 2  is a flow diagram illustrating principles of power control according to an embodiment of the invention; 
       FIG. 3  is a flow diagram illustrating operation of an extended power control algorithm according to an embodiment of the invention; 
       FIG. 4  is a flow diagram illustrating the use of a look-up table in an extended power control algorithm according to an embodiment of the invention; 
       FIG. 5  is a flow diagram illustrating operation of a conventional power control algorithm; 
       FIG. 6  illustrates a look-up table according to an embodiment of the invention; 
       FIG. 7  illustrates backoff of a PA as a function of signal EVM and ACPR; 
       FIG. 8  illustrates efficiency of a PA as a function of signal EVM and ACPR; 
       FIG. 9  illustrates schematically a transceiver implementation according to an embodiment of the invention; and 
       FIG. 10  shows a block diagram illustrating a device according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIGS. 1A and 1B  illustrate systems wherein the invention may be employed and a basic idea of the invention. Both Figures comprise a first mobile station (MS 1 )  101 , a second mobile station (MS 2 )  104 , a first base station (BTS 1 )  102  and a second base station (BTS 2 )  103 . BTS 1  and BTS 2  operate on adjacent channels. MS 2  transmits to BTS 2  on channel number N and BTS 2  receives signals on channel number N. Correspondingly, MS 1  transmits to BTS 1  on channel number N+1 (or N−1) and BTS 1  receives signals on channel N+1 (or N−1). 
   In  FIG. 1A , MS 2  is in proximity of BTS 1 . The emission mask of the MS 2  has to be such that BTS 1  is able to receive signals from MS 1  even when MS 2  is this close to BTS 2 . That is, ACP of MS 2  on channel N+1 (or N−1) may not be so high that it would interfere with the signals of MS 1  at BTS 1 . In conventional power control algorithms this ACP requirement has to be met over the whole area of the system; that is, irrespective of the distance between MS 2  and adjacent base stations. 
   In  FIG. 1B , MS 2  is located further away from BTS 1 . Thereby ACP of MS 2  on channel N+1 (or N−1) at BTS 1  is lower than in the case of  FIG. 1A . Thus, ACP of the MS 2  transmitter can be increased from the conventional maximum by the amount of the attenuation between MS 2  and BTS 1 . This is one of the basic ideas of the invention. The increase in allowable ACP can be used for increasing efficiency and output power of MS 2 . 
     FIG. 2  is a flow diagram illustrating principles of power control according to an embodiment of the invention. In step  201 , requested output power P_out_req and requested modulation and coding MCS_req (Modulation and Coding State) are read. P_out_req is obtained from basic uplink power control and MCS is obtained from AMC (Adaptive Modulation and Coding). These are known to persons skilled in the art and thus they need not be discussed any further herein. Further, any suitable method for deriving required uplink power may be used in connection with the invention. 
   Then requested P_out_req is compared to P_threshold in step  202 , wherein P_threshold is maximum output power derived on the basis of theoretical minimum attenuation to adjacent receivers. (That is, P_threshold is the maximum output power used in conventional methods.) If P_out_req is below or equal to P_threshold, output power P_out is derived by means of conventional techniques and the method proceeds to step  203 . 
   In step  203 , P_out_req and MCS_req are given as an input to a conventional power control algorithm having adaptive clipping and returning achievable P_out, MCS and clipper setting. In step  204 , TX (transmission) state is set in accordance with the output of the conventional power control algorithm by setting P_out and clipper settings. 
   If P_out_req is found to be higher than P_threshold in step  202 , output power P_out is derived by means of an extended power control algorithm and the method proceeds to step  205 . In step  205 , P_out_req and MCS_req are given as an input to the extended power control algorithm, which returns achievable P_out, MCS and clipper setting. In step  206 , TX (transmission) state is set in accordance with the output of the extended power control algorithm by setting P_out, MCS and clipper settings. 
   Then in step  207 , achievable P_out and MCS derived by means of conventional power control algorithm or extended power control algorithm are returned. 
     FIG. 3  is a flow diagram illustrating operation of an extended power control algorithm according to an embodiment of the invention. This is one possible implementation of step  205  in  FIG. 2 . In step  300 , the algorithm receives requested output power P_out_req and requested modulation and coding MCS_req as an input. In step  301 , DC bias of the amplifier is set to a nominal value. 
   Then in step  302 , RSSI (Received Signal Strength Indication) on adjacent channels is measured. That is, maximum of RSSI on channels number N+1 and N−1 is measured, wherein N is the number of the channel used by the transmitter. Also the transmission power levels of the base stations P(BTS) operating on the adjacent channels are received. On the basis of these values minimum attenuation between the transmitter and the adjacent base station is calculated in step  303 . 
   In step  304 , decision logic and a look-up table are used for deriving achievable P_out, MSC and clipper setting on the basis of the attenuation calculated in step  303 , P_out_req and MCS_req. Then in step  305 , the algorithm returns achievable P_out, MCS and clipper setting. 
     FIG. 4  is a flow diagram illustrating the use of a look-up table in an extended power control algorithm according to an embodiment of the invention. This is one possible implementation of step  304  in  FIG. 3 . In step  400 , the algorithm receives minimum attenuation between the transmitter and adjacent base stations, requested output power P_out_req and requested modulation and coding MCS_req as an input. 
   In step  401 , maximum allowed output power P_out_max is checked from the look-up table on the basis of the attenuation. If requested output power P_out_req exceeds the maximum allowed output power P_out_max, the value output power P_out is set to value of P_out_max in step  402 . Otherwise, the value of output power P_out is set to the value of the requested output power P_out_req in step  402 . 
   In step  403 , maximum allowed modulation and coding MCS_max is checked from the look-up table on the basis of P_out. If requested modulation and coding MCS_req exceeds the maximum allowed modulation and coding MCS_max, the value of MCS is set to value of MCS_max in step  404 . Otherwise, the value of MCS is set to the value of the MCS_req in step  404 . 
   Then in step  405 , clipper setting is checked from the look-up table on the basis of P_out and MCS and in step  406 , the algorithm returns achievable P_out, MCS and clipper setting. 
     FIG. 5  is a flow diagram illustrating operation of a conventional power control algorithm. This is one possible implementation of step  203  in  FIG. 2 . In step  500 , the algorithm receives requested output power P_out_req and requested modulation and coding MCS_req as an input. In step  501 , clipper setting is checked from a look-up table on the basis of P_out_req and MCS_req. Then in step  502 , the algorithm returns achievable P_out, MCS and clipper setting. 
     FIG. 6  illustrates a look-up table according to an embodiment of the invention. The look-up table is divided into first and second power control areas  600  and  601 . The look-up table in the first area  600  corresponds to conventional look-up tables. The look-up table comprises settings (illustrated by black dots) for plurality of TX states as a function of requested power level and modulation and coding state. This look-up table is used when the requested output power level of a mobile transmitter is below or equal to P_threshold, which is maximum output power derived on the basis of theoretical minimum attenuation to adjacent receivers. The look-up table of this first power control area is used for minimizing or reducing used DC power and ACPR. ACPR is reduced by setting soft clipper backoff appropriately. 
   In other words, in the first power control area conventional power control scheme defines output power and adaptive modulation and coding defines the used modulation and coding combinations. In this area, conventional radiation mask specification has to be met. An adaptive limiting function may be added on top of this conventional power control method. Such adaptive limiting function minimises locally the used DC power. It can be used also for minimizing ACPR. The limiter backoff is adjusted according to the used modulation so that with low order modulations the signal is clipped more than with high order modulations.  FIG. 7  below presents as an example the behaviour of the PA as a function of the backoff of the limiter and PA. 
   The look-up table in the second area  601  is an extended look-up table according to an embodiment of the invention. The look-up table comprises settings (illustrated by grey dots) for plurality of TX states as a function of requested power level and attenuation to adjacent receivers. This look-up table is used when the requested output power level exceeds P_threshold. The look-up table of this second power control area is used for maximizing efficiency of the radio transmitter. 
   It must be noted that the power control can be arranged to continuously follow the attenuation to the adjacent BTS. The terminal should have possibility to measure the attenuation to the adjacent BTS frequently enough, so that no large errors are generated. The rate of the attenuation measurement can be related to the power control rate of the system and/or attenuation measurement can be set to follow slow fading of the adjacent channel. 
   One of the targets of the extended power control in addition to the conventional power control scheme is to maximise the power efficiency of a mobile device transmitter in cases, where higher ACP and higher error vector magnitude (EVM) are allowable. As is known to persons skilled in the art, ACP is dominating the PA backoff when low order modulation is used and if high order modulation like 64 QAM is used the required in-band EVM restricts the PA backoff. Thus, in cases where actual ACP and EVM requirements are not as strict as conventional requirements, transmitter efficiency can be increased. 
   The fact that the requested output power level exceeds P_threshold in the second power control area means that the requested output power is higher than the power, which is achievable in conventional techniques with reasonable DC power consumption and with a typical power amplifier. In the second power control area the mobile transmitter measures attenuation to the closest BTS operating on an adjacent channel and calculates, on the basis of this value, the maximum allowed ACP at the mobile transmitter. If the maximum allowed attenuation is the same or smaller than conventional maximum defined for the threshold power (nominal power), the device does not increase the output power. If the maximum allowed ACP is higher than the conventional maximum ACP, the output power can be increased. 
   Increase of output power in the second power control area means that also the radiated ACP is increased. Because of this difference, radiation mask for the second power control area may be required to be specified for each extra power step separately. Also because of smaller backoff of the PA the EVM is higher and thus it may not be possible to use all the higher order modulations any more. On the basis of this, look-up table points can be defined for each extra power level and attenuation to the adjacent channel BTS with associated allowed modulations and clipping levels. 
   If the adaptive modulation and coding requests modulation, for which it is not possible to use the requested output power, the extended power control algorithm may inform the physical layer functionalities that lower order modulation should be used. This way it is possible to operate with small PA backoff without interfering adjacent channel operation. This method will increase the maximum link distance in the area of low BTS density. Additionally, the method allows an increase in the maximum output power without increasing the DC power consumption of the mobile transmitter. 
   The design of the PA should be such that transmitter can meet the conventional radiation mask requirements with nominal DC power consumption. If the requested output power is less than the threshold power, the DC power consumption can be reduced. Also when the requested output power is higher than the threshold power the DC power consumption should be kept at the nominal level, but efficiency of the transmitter can be increased in that case. Nevertheless, the radiation masks defined for the power levels exceeding the threshold power should be met. Following table presents an example of possible transmitter emission mask for RF specifications. 
   
     
       
             
             
             
             
             
             
           
         
             
                 
             
             
                 
                 
                 
                 
                 
               TX 
             
             
                 
                 
               EVM 
                 
               Attenuation, 
               noise 
             
             
               P_out 
               MCS 
               requirements 
               ACP, dBm 
               dB 
               floor 
             
             
                 
             
           
           
             
               P_min − 
               MCS1- 
               −25 dB-−5 dB 
               ACP spec 
               Min SA 
               PTX 
             
             
               P_thres 
               MCS9 
                 
                 
               (Specified 
               noise 
             
             
                 
                 
                 
                 
               Attenuation) 
               floor 
             
             
               P_thres + 
               MCS3- 
               −19 dB-−5 dB 
               ACP spec + 
               SA + 6  
               PTX 
             
             
               2 dB 
               MCS9 
                 
               6 
                 
               noise 
             
             
                 
                 
                 
                 
                 
               floor 
             
             
               P_thres + 
               MCS5- 
               −16 dB-−5 dB 
               ACP spec + 
               SA + 12 
               PTX 
             
             
               4 dB 
               MCS9 
                 
               12 
                 
               noise 
             
             
                 
                 
                 
                 
                 
               floor 
             
             
               P_thers + 
               MCS7- 
               −10 dB-−5 dB 
               ACP spec + 
               SA + 18 
               PTX 
             
             
               6 dB 
               MCS9 
                 
               18 
                 
               noise 
             
             
                 
                 
                 
                 
                 
               floor 
             
             
               P_thers + 
               MCS9 
               −5 dB 
               ACP spec + 
               SA + 24 
               PTX 
             
             
               8 dB 
                 
                 
               24 
                 
               noise 
             
             
                 
                 
                 
                 
                 
               floor 
             
             
                 
             
           
        
       
     
   
     FIG. 7  illustrates backoff of a PA as a function of signal EVM and ACPR for multi-carrier signals having more than ten sub-carriers. In other words  FIG. 7  illustrates simulation of the operation of a PA in the first power control area of an embodiment of the invention. Black curves represent different soft-clipper backoffs and grey, dashed curves represent different PA backoffs. 
   Curves are shown for soft-clipper backoffs of 2 dB, 3 dB, 4 dB, 5 dB and 6 dB, the leftmost curve representing soft-clipper backoff of 6 dB and the rightmost curve representing soft-clipper backoff of 2 dB. With regard to PA backoff, curves are shown for PA backoffs of 2 dB, 4 dB, 6 dB, 8 dB, 10 dB, 12 dB, 14 dB, 18 dB and 22 dB, the uppermost curve representing PA backoff of 2 dB and the lowermost curve representing PA backoff of 22 dB. 
     FIG. 8  illustrates efficiency of a PA as a function of signal EVM and ACPR for multi-carrier signals having more than ten sub-carriers and for a class AB PA. In other words  FIG. 8  illustrates simulation of the operation of a PA in the second power control area of an embodiment of the invention. Black curves represent different soft-clipper backoffs and grey, dashed curves represent different PA efficiency figures. 
   Also herein, curves are shown for soft-clipper backoffs of 2 dB, 3 dB, 4 dB, 5 dB and 6 dB, the leftmost curve representing soft-clipper backoff of 6 dB and the rightmost curve representing soft-clipper backoff of 2 dB. With regard to PA efficiency, curves are shown for PA efficiency figures of 45%, 31%, 20%, 13%, 8%, 5%, 3%, 1% and 0.3% dB, the uppermost curve representing PA efficiency of 45% and the lowermost curve representing PA efficiency of 0.3%. 
   The invention can be employed in systems that use synchronised TDD (Time Division Duplex), FDD (Frequency Division Duplex), or FDD/TDD hybrid. The measurement of the attenuation between a mobile device and adjacent BTSs is more accurate if TDD is used because in TDD uplink and downlink channels are at the same frequency. In the case of FDD the attenuation measurement might not be as accurate because uplink and downlink channels are at different frequencies. The accuracy can, however, be improved with longer time averaging of the attenuation measurement and with some safety margins. In FDD system, it is also possible to arrange attenuation measurements so that the mobile device sends a measurement request to the BTSs operating on the adjacent channels and the BTSs send to the mobile device information about the received power level, which is then used for attenuation calculations at the mobile device. In systems that use OFDM, a BTS transmits a training sequence (TS) in the beginning of each downlink MAC frame. By measuring the power of the TS at the mobile device it is possible to calculate the attenuation if the transmission power of the TS is known. If the transmission power is not known, the mobile device can find it out by synchronizing to the channel and decoding the broadcast information, which has to include the BTS transmission power. 
   The invention may be implemented for example by means of a suitable combination of hardware and software components. 
     FIG. 9  illustrates schematically a transceiver implementation  900  according to an embodiment of the invention. The transceiver may be for example a part of a radio communication device or it may be produced as a separate transceiver module. 
   The transceiver  900  comprises a transmit branch  901  and a receive branch  902 , which are connected to power control module  903 . The power control module  903  comprises physical layer functionalities including AMC power control and extended power control and controls transmission power level by means of soft clipping module  905  and DC-DC converter  906 . The power control module  903  is also connected to higher layer modules  904 . 
   It must be noted that the implementation of  FIG. 9  is only an example and that various other implementations are possible as is obvious to persons skilled in the art. 
     FIG. 10  shows a block diagram illustrating a radio communication device  1000  according to an embodiment of the invention. Such device may be for example a mobile terminal, PDA or some other device comprising communication capabilities. 
   The device  1000  comprises a processing unit  1001  and a user interface module  1002  coupled to the processing unit  1001 . The user of the device may give commands through the user interface module  1002 . The processing unit  1001  is coupled to an RF (radio frequency) module  1003  as well. The RF module may comprise for example the transceiver shown in  FIG. 9 . 
   The processing unit controls, in accordance with software stored in the radio transmission device, the RF module to provide transmission power control algorithm, wherein output power to be used is derived on the basis of a power control algorithm having at least a first and a second power control area according to the invention. 
   Particular implementations and embodiments of the invention have been described. It is clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention. The scope of the invention is only restricted by the attached, patent claims.