An apparatus and a method for distributing a transmission power in a cellular communications network

0001This invention relates to the field of power control in mobile communications networks. More particularly, but not exclusively, the invention relates to control of the uplink transmission power.

0002A user equipment (UE) for the use in cellular communications networks such as the Universal Mobile Telecommunications System (UMTS) has a limited amount of power resources available for transmitting user and control data in the uplink direction to the base stations of the network. Also, the network may limit the UE power allowed to transmit in the uplink direction in order to ensure that the signal achieves a predetermined signal-to-noise ratio.

0003The allowed or available uplink transmission power may be shared by a number of channels transmitted at the same time. In macro cell coverage scenarios, under deep fades or at the edge of the cell, the UE may often have to operate at or near maximum transmission power.

0004In case the UE detects that it may have a power problem such as it is required to serve multiple channels with a total transmission power exceeding the available or allowed uplink transmission power, there are two mechanisms foreseen in UMTS to handle such power "shortages".

0005The first mechanism is the so-called long-term behaviour. By applying the long-term mechanism, the UE is controlling the data rate used for uplink transmission power.

0006In UMTS, the network allocates a range of suitable bit rates or transport formats to the UE. The UE selects an appropriate transport format from the allowed set according to its buffer occupancy and power availability.

0007If the UE is running low on power, then the UE will reduce its data rate by selecting a lower data rate transport format. For example, if a UE is running low on power at the edge of a cell, it will eliminate certain allowed Transport Format Combinations (TFCs) from the set of allowed TFCs given by the network. In this manner, the UE will try to avoid a power problem by selecting an appropriate transport format corresponding to a lower data rate at the beginning of the next transmission frame.

0008The long term mechanism used in UMTS is described in more detail in the 3^rd Generation Partnership Project (3GPP) Specifications, see section 11.4 of the 3GPP TS 25.321, <i>"Technical Specification Group Radio Access Network; Medium Access Control (MAC) protocol specification"</i> and section 6.4 of 3GPP TS 25.133, <i>"Technical Specification Group Radio Access Network; Requirements for Support of Radio Resource Management (FDD)".</i>

0009The second mechanism is the so-called short-term behaviour. The UE will apply the short-term behaviour when the UE is already experiencing a power problem such as described above. In this mechanism the uplink transmission power is scaled down such that the maximal transmission power is not exceeded. This mechanism can be applied directly for transmission in the next slot rather than at the beginning of the next transmission frame as in the case of the above described long term behaviour.

0010The introduction of multiple parallel services such as those available in UMTS has made things even more challenging. In Release 5 of the 3GPP specifications, a High Speed Downlink Packet Access (HSDPA) feature is introduced which supports high data rate transmission in downlink direction. See the 3GPP specification TS 25.308, <i>"High Speed Downlink Packet Access (HSDPA), Overall Description"</i> for more details. However, this service requires transmission of signalling data in the uplink direction using a new physical channel, the so-called High Speed-Dedicated Physical Control Channel, HS-DPCCH. The impact of this could be a significant addition of power strain on the UE. The power requirement for transmission on the HS-DPCCH could be as high as 20-30% of the total available power. This means that HSDPA capable UE's will have higher probability of experiencing a power problem.

0011It is an object of the present invention to improve the mechanisms to control uplink transmission power of a UE in a cellular communications network.

0012According to a first aspect of the present invention, there is provided a mobile terminal for use in a cellular communications network the terminal being adapted to: i) if the terminal transmits signals at a power below the maximum uplink transmission power, distributing the available uplink transmission power between different channels according to a first scheme; and ii) if the terminal transmits signals at a power exceeding or about to exceed the maximum uplink transmission power, distributing the available uplink transmission power between different channels according to a second scheme different from said first scheme.

0013In this way the uplink transmission power of a UE can be controlled such that a loss of performance, particularly for some channels, is less likely or avoided.

0014Preferably, the uplink transmission power is distributed according to the priorities of the channels.

0015In this way, important or "high priority" channels are less affected by an uplink transmission power problem of a UE. For example, on important channels, such as the Dedicated Physical Data Channel (DPDCH) or the Dedicated Physical Control Channel (DPCCH), no or only reduced loss of performance is expected for higher layer 3 Radio Resource Control (RRC) signalling or high priority application such as a voice call.

0016According to the prior art procedures, equal compression throughout all served channels is applied if a UE is experiencing power problems. This could, for example, result in loss of DCCH data, which is undesirable, as the overall impact on system performance due to loss in performance on the DCCH is greater than loss of data on, for example, the dedicated traffic channel (DTCH) or HS-DPCCH.

0017Preferably, in the first and second scheme gain factors are used to define the distribution of uplink transmission power between different channels and at least one of the gain factors of the second scheme is different to a corresponding gain factor of the first scheme.

0018In this way the existing mechanisms and "infrastructure" for distributing the available uplink transmission power between channels can be re-used for the second scheme.

0019According to a further aspect of the present invention, there is provided a network element of a cellular communications network, the network element being adapted to determine one or more parameters defining the distribution of uplink transmission power between different channels if the maximum uplink transmission power is exceeded or about to being exceeded, wherein at least one of said parameters are different to parameters defining the distribution if the maximum uplink transmission power is not exceeded.

0020According to a further aspect of the present invention, there is provided A method for an uplink data transmission in a communication system, comprisin.g steps of: distributing transmission powers of each channels transmitted by a first transceiver side; monitoring whether a total transmission power of the first transceiver exceeds a maximum transmission power of the first transceiver or not, where the total transmission power of the first transceiver is a summation value of the transmission powers of each channels; re-distributing transmission powers of the each channels by scaling down a transmission power of a channel having a low priority, when the total transmission power of the first transceiver exceeds the maximum transmission power of the first transceiver; and transmitting the each channels to a second transceiver side through the re-distributed transmission powers.

0021Embodiments of the present invention will now be described, by example only, with reference to the accompanying figures, whereby <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a schematic outline of a mobile telecommunications network, in which the present invention can be incorporated;</li><li>Figs. 2A and 2B are schematic diagrams illustrating the distribution of uplink transmission power according to the prior art;</li><li>Fig. 2C is a schematic diagram illustrating the distribution of uplink transmission power according to one embodiment of the present invention; and</li><li>Fig. 3 is a flow chart diagram illustrating the process of controlling uplink transmission power according to one embodiment of the present invention.</li></ul>

0022In Figure 1 a schematic outline of a mobile telecommunications network according to the Universal Mobile Telecommunications System (UMTS) standard is shown. The typical architecture of such a network comprises mobile user equipments (UEs) 8, a UMTS Terrestrial Radio Access Network (UTRAN) 3 and one or more core networks (CNs) 1. UMTS is a third generation radio network using wideband code division multiple access (W-CDMA) technology.

0023The core network 1 may comprise Mobile Switching Centre (MSC) or Serving GPRS (General Packet Radio Services) Support Nodes (SGSN). The core network is connected via communication links to a number of Radio Network Controllers (RNCs) 4. The RNCs are dispersed geographically across areas served by the core network 1. Each RNC 4 controls one Radio Network Subsystems (RNSs) 5, including one or more base stations 6 such as "Nodes B" located remote from, and connected by further communication links to, the RNC 4. Each base station 6 transmits radio signals to, and receives signals from, user equipment or terminal 8 which is in an area served by that base station 6. The area is referred to as a "cell". A UMTS network is provided with a large number of such cells, which are ideally contiguous to provide continuous coverage over the whole network territory. See the UTRAN Overall Description, 3GPP TS 25.401, by 3GPP for more details.

0024With reference to Figures 2A and 2B, the setting of the uplink power difference between the DPDCH, the DPCCH and the HS-DPCCH will be explained by a simplified example. More details may be found in the 3GPP <i>"Technical Specification Group Access Network; Spreading and Modification. (FDD) "</i> TS 25.213 and the specification <i>"Physical Layer Procedures (FDD) "</i> TS 25.214.

0025In case of the 3GPP specifications, UE can simultaneously transmit a Dedicated Physical Data Channel (DPDCH) and a Dedicated Physical Control Channel (DPCCH) in Release 99, a High Speed- Dedicated Physical Control Channel (HS-DPCCH) in Release 5, and a Enhanced uplink Dedicated Physical Data Channel (E-DPDCH) and a Enhanced uplink Dedicated Physical Control Channel (E-DPCCH) in Release 6.

0026At this time, the total transmission power for the channels may exceed the maximum allowed uplink transmission power.

0027In this case, a UE must scale the total transmission power to level of the maximum allowed uplink transmission power.

0028In present invention, the UE monitors whether the total transmission power for the channels exceed the maximum allowed uplink transmission power or not. If the total transmission power for the channels exceed the maximum allowed uplink transmission power, The UE scales the total transmission power with reference to priorities of each channels. That is, the UE maintains the transmission power for the channels having relatively high priority and scales the transmission power for the channels having relatively low priority. A rule of priority assignment is as follow. That is, a priority of a voice channel is higher than a priority of a packet data channel, and a priority of a control channel is higher than a priority of a packet data channel.

0029In the present invention assumes that a DPCCH and a DPDCH have a first priority, a HS-DPCCH has a second priority, a E-DPCCH has a third priority, and a E-DPDCH has a lowest priority. Therefore, if the total transmission power for the channels exceeds the maximum allowed uplink transmission power, the UE scales the transmission power for the E-DPDCH or E-DPDCH and E-DPCCH having relatively high priority, while the UE maintains the transmission power for the other channels having relatively high priority. Herein, the mechanism can be operated per slot or sub frame or frame.

0030A detail operation of the present invention will be specified by referring to figures. In a following detail specification, the present invention assumes that the UE transmits simultaneously the DPCCH, the DPDCH and the HS-DPCCH, the priorities of the DPCCH and DPDCH are higher than the priority of the HS-DPCCH, the transmission power scaling is performed by resetting gain factors for each channels, a period of the transmission power scaling is a slot.

0031The maximum UE transmitter power is defined as the minimum of the maximum allowed uplink transmission power as set by the network and the maximum transmit power of the UE, see the 3GPP specification TS 25.133. In order to distribute the maximum UE transmitter power between DPDCH, DPCCH and HS-DPCCH, so-called "gain factors" β_C, β_D and β_HS are used.

0032The UPLINK power ratio between the DPCCH and the DPDCH is defined by the gain factors β_C and β_D (i.c. by β_C/β_D), whereas the gain factor β_HS defines the UPLINK power ratio between the HS-DPCCH and DPCCH.

0033These gain factors are generally determined by the network. β_C is either signalled from the network clement to the UE or calculated by the UE based on settings for a TFC, which are again signalled from network elements to the UE.

0034More details about the computation of the gain factors from TFC setting may be found in the 3GPP specification TS 25.214.

0035β_HS is calculated by the UE from so-called offset-values Δ_HS-DPCCH, which are again signalled by the network elements to the UE.

0036The HS-DPCCH carries acknowledgement and channel quality indication (CQI) signals. Usually three different power offset parameters are determined by the network, i.e. Δ_ACK and Δ_NACK for the acknowledgement signals and Δ_CQI for the quality indication signals; thus three different β_HS factors may be computed by the UE from the power offset parameters signalled by the network; i.e. two for the acknowledgement messages and one for the CQI signals.

0037Referring now to Fig. 2A, a simplified example is illustrated. Here it is assumed that the maximum UE transmission power is 1 Watt, and the gain factors β_C and β_HS are set to:<maths id="math0001"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>C</mtext></mrow></msub><mtext> = 0.33;</mtext></mrow></math><img file="EP1564905A2_D0001.tif" /></maths><maths id="math0002"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>D</mtext></mrow></msub><mtext> = 1.0</mtext></mrow></math><img file="EP1564905A2_D0002.tif" /></maths><maths id="math0003"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>HS</mtext></mrow></msub><mtext> = 2.0.</mtext></mrow></math><img file="EP1564905A2_D0003.tif" /></maths>

0038Accordingly, the UE allocates the available uplink transmission power such that the DPDCH, DPCCH and HS-DPCCH transmits with the following powers :<maths id="math0004"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>DPDCH</mtext></mrow></msub><mtext> = 0.5 Watt;</mtext></mrow></math><img file="EP1564905A2_D0004.tif" /></maths><maths id="math0005"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>DPCCH</mtext></mrow></msub><mtext> = 0.17 Watt;</mtext></mrow></math><img file="EP1564905A2_D0005.tif" /></maths><maths id="math0006"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>HS-DPCCH</mtext></mrow></msub><mtext> =0.33 Watt.</mtext></mrow></math><img file="EP1564905A2_D0006.tif" /></maths>

0039If now the UE experiences, for example, a deep fade situation, the power required to transmit signals in all three channels with the same quality as before the deep fade situation would require an uplink power which is greater than the maximum UE transmission power (see Fig. 2B). However, as the maximum UE transmission power is limited as described above, the UE needs to limit the total UE transmission power to the maximum, UE transmission power.

0040According to one embodiment of the present invention, the network provides for additional gain factors β' which the UE uses if it is about to exceed the maximum UE transmission power. The UE can apply the gain factors β' at the beginning of the next slot.

0041By using a single set of gain factors and applying scaling if the UE is experiencing power problems, transmission power is suppressed equally for all served channels, possibly resulting in a loss of performance for all channels. However, the different channels have typically different priorities. For example, the priority of logical channels like DCCH carrying higher layer signalling is usually higher than for the other channels like DTCH or HS-DPCCH.

0042By the use of additional gain factors, the available uplink transmission power can be redistributed among the different channels if the UE experiences a power problem, and priorities of different channels can be taken into account.

0043In this way an improvement in performance of higher priority channels can be achieved.

0044In this case, the network provides the UE with a second set of offset values Δ_HS-DPCCH, i.e. Δ'_ACK, Δ'_NACK and Δ'_CQI. The UE computes the gain factors β'_HS in the same manner as β_HS but using the offset-value Δ'_HS-DPCCH instead of Δ_HS-DPCCH.

0045Alternatively, the network may signal a single offset-value Δ'_HS-DPCCH, which is then used by the UE for transmitting acknowledgement and channel quality indication signals for the HSPDA service.

0046Referring now to Fig. 2C, a simplified example of the use of gain factor β'_HS is described.

0047The simplified example described above, is now extended and an additional gain factor β'_HS is considered. It is again assumed that the maximum UE transmission power is 1 Watt, and the gain factors are set to:<maths id="math0007"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>C</mtext></mrow></msub><mtext> = 0.33;</mtext></mrow></math><img file="EP1564905A2_D0007.tif" /></maths><maths id="math0008"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>D</mtext></mrow></msub><mtext> = 1.0</mtext></mrow></math><img file="EP1564905A2_D0008.tif" /></maths><maths id="math0009"><math display="block"><mrow><msub><mrow><mtext>β</mtext></mrow><mrow><mtext>HS</mtext></mrow></msub><mtext> = 2.0;</mtext></mrow></math><img file="EP1564905A2_D0009.tif" /></maths><maths id="math0010"><math display="block"><mrow><msub><mrow><mtext>β'</mtext></mrow><mrow><mtext>HS</mtext></mrow></msub><mtext> = 0.5.</mtext></mrow></math><img file="EP1564905A2_D0010.tif" /></maths>

0048Accordingly, if the UE experiences a power problem, it allocates the available uplink transmission power using the additional gain factor β'_HS.

0049Thus, the UE allocates the available uplink transmission power such that the channel transmits with the following powers:<maths id="math0011"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>DPDCH</mtext></mrow></msub><mtext> = 0.66 Watt;</mtext></mrow></math><img file="EP1564905A2_D0011.tif" /></maths><maths id="math0012"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>DPCCH</mtext></mrow></msub><mtext> = 0.22 Watt;</mtext></mrow></math><img file="EP1564905A2_D0012.tif" /></maths><maths id="math0013"><math display="block"><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>HS-DPCCH</mtext></mrow></msub><mtext> = 0.11 Watt.</mtext></mrow></math><img file="EP1564905A2_D0013.tif" /></maths>

0050In this way the available power is distributed between the different channels such that the "high priority" channels receive more power compared to the lower priority channels. In case of a power problem in the uplink direction, the UE can thus use an additional power of 0.16 Watts and 0.06 Watts for uplink transmission on the DPDCH and DPCCH, respectively, compared to the case where no additional gain factor is provided.

0051It is noted that, by the use of the additional gain factor β_HS', the power available for transmission on the HS-DPCCH is significantly reduced. Therefore, a possible loss in HSDPH performance may arise following the improvement in performance of higher priority channels. However, usually the above described long-term mechanism using TFC selection is applied as soon as power problems are foreseeable and thus the number of occasions in which the maximal uplink transmission power is to be exceeded is small. Hence, the overall loss in HSDPA performance is not expected to be significant.

0052Also, any possible effects of the UE transmitting with a reduced power for the HS-DPCCH are expected to be minor, as the mechanism for acknowledgement in the HSDPA service is very stable and the network may for example increase the number of repetitions in cases where it is known that the UE may often experiences a power problem.

0053With reference to Fig. 3, the process of applying the different set of gain factors is now described.

0054The process starts in step 102, and the UE transmits uplink signals in different channels as required. Regularly, the UE determines whether it exceeds the maximal UE transmission power or whether it is about to exceed the maximal UE transmission power (step 104).

0055This is done by the UE measuring its transmitted power for a predetermined measurement period, usually for the duration of one slot. From this measurement the UE estimates whether it has reached the maximum allowed uplink transmission power, taking into account tolerances as defined for the UE transmitted power measurement accuracy in the 3GPP specification TS 25.133.

0056However, if the UE output power is outside the range covered by the UE transmitted power measurement, the UE determines whether it exceeds the maximum allowed uplink transmission power with more than the tolerances specified for the Open Loop Power Control in the 3GPP specification TS 25.101.

0057If it is determined in step 104 that the maximum UE transmission power is not exceeded, the UE applies the "normal" gain factors including β_HS to set the power ratio between the different channels (step 106). If, on the other hand, it is detected in step 104 that the maximum UE transmission power is exceeded or is about to be exceeded, the process continues in step 108 by applying the gain factors provided for power problem situations. In order to set the power ratio between the DPCCH and the HS-DPCCH, the UE uses β_HS' instead of β_HS. The UE uses the gain factors β_HS' from the beginning of the current or next DPCCH slot.

0058In step 110 the UE transmits data using transmission power set on the step 106 or the step 108 and then again monitors the uplink transmission power on the step 104.

0059Whilst in the above mentioned embodiments transmission power control for UMTS systems has been described, it is appreciated that the present invention can be applied to other systems like for example GMS or other WCDMA systems in a similar way.

0060It is to be understood that the embodiments described above are preferred embodiments only. Various features may be omitted, modified or substituted by equivalents, without departing from the scope of the present invention.