Patent Publication Number: US-2004047309-A1

Title: Method and base station for power control in TDMA radio system

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to time division multiple access (TDMA) radio systems. More precisely it relates to power control in a TDMA multicarrier radio system.  
       BACKGROUND OF THE INVENTION  
       [0002] In conventional cellular systems, a base station is allocated a predetermined number of frequency carriers for communicating with mobile stations. Multiuser access methods in GSM systems are based on time division, in which eight users are separated by allocating to each user a unique timeslot in a TDMA frame. The TDMA frame has eight timeslots on one carrier. Depending on traffic requirements, for example based on the assumed number of users within a cell, the base station contains one or several carriers. In a conventional base station with single carrier technique multiple carriers are combined with poor efficiency and bulky implementation. A multicarrier transmitter offers better efficiency and more compact implementation.  
       [0003] A major obstacle in development of the multicarrier transmitters is linearity requirement. Linearity is the difference in the accuracy values through the expected operating range of the transmitter. Peak-to-average ratio measures the distance between a maximum instantaneous power and an average power of a multicarrier signal over a given duration. In most times the instantaneous power is close to the average power, while the maximum power occurs quite seldom. In other words, the maximum instantaneous power is close to the average power with high probability, while the occurrence of high power has a low probability. However, the transmitter linearity requirement is solely determined by peaks occurring seldom.  
       [0004] In order to fulfil the linearity requirement in a power amplifier of the multicarrier base station, adequate headroom is needed in the power amplifier, which, in turn, leads to poor efficiency. Several methods for achieving the necessary linearity performance in power amplifiers are described in WO-0105057. However, in the described methods the base station is forced to reduce the power of defined timeslots by some amount or to zero. This procedure causes impairments to received signal quality. Some peak clipping techniques have also been proposed as a solution for the problem, but they can cause signal deterioration, like growth of error vector magnitude (EVM) and spectral regrowth.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005] It is thus an object of the invention to provide a method and a base station in such a manner that the above-mentioned problems are solved. This is achieved by a method for transmit power control in a TDMA multicarrier radio system communicating over multiple time slots assigned to given user terminals, comprising: allocating a respective transmit power level for at least one of a plurality of user terminals; calculating the sum of transmit powers on each carrier for each time slot separately and allocating the time slots and carriers for the connections to the user terminals in such a way that the sum of transmit power levels in each time slot is minimized.  
       [0006] The invention also relates to a method for transmit power control in a TDMA multicarrier radio system communicating over multiple time slots assigned to given user terminals, comprising: allocating a respective transmit power level for at least one of a plurality of user terminals; calculating the sum of transmit powers on each carrier for each time slot separately; finding a time slot with the minimum sum of transmit powers when a connection is being initialized; finding a free carrier in the found time slot with the minimum sum of transmit powers and allocating the found time slot and the found carrier for the connection.  
       [0007] The invention also relates to a TDMA multicarrier base station communicating over multiple time slots assigned to given user terminals, the base station comprising: means for allocating a respective transmit power level for at least one of a plurality of user terminals; means for calculating the sum of transmit powers on each carrier for each time slot separately and means for allocating the time slots and carriers for the connections to the user terminals in such a way that the sum of transmit power levels in each time slot is minimized.  
       [0008] The object of the invention is also achieved by a TDMA multicarrier base station communicating over multiple time slots assigned to given user terminals, the base station comprising: means for allocating a respective transmit power level for at least one of a plurality of user terminals; means for calculating the sum of transmit powers on each carrier for each time slot separately; means for finding a time slot with the minimum sum of transmit powers when a connection is being initialized; means for finding a free carrier in the found time slot with the minimum sum of transmit powers and means for allocating the found time slot and the found carrier for the connection.  
       [0009] Preferred embodiments of the invention are described in the dependent claims.  
       [0010] The method of the invention provides several advantages. In a preferred embodiment of the invention the linearity requirement in the base station is achieved without the use of substantial additional hardware. All the problems caused by a large peak-to-average ratio of the multicarrier signal are avoided. There is no need to reduce the power of any timeslots in order to satisfy the linearity requirement of the base station. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0011] In the following, the invention will be described in greater detail with reference to the preferred embodiments and the accompanying drawings, in which  
     [0012]FIG. 1 illustrates an exemplary cellular radio system,  
     [0013]FIG. 2 illustrates an example of a TDMA frame,  
     [0014]FIG. 3 illustrates an exemplary block diagram of a network and a multicarrier base station in accordance with the invention,  
     [0015]FIG. 4 illustrates a method for power control according to exemplary embodiments of the present invention,  
     [0016] FIGS.  5 - 16  illustrate an example of arranging the time slots according to exemplary embodiments of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
     [0017] The essential parts of the structure of the cellular radio system may resemble those shown in FIG. 1. The cellular radio system in FIG. 1 is a GSM-based radio system, which employs for example EDGE (Enhanced Data Rates for Global Evolution) technology. The cellular radio system comprises a base station  100  and a plurality of user terminals  102 ,  104 ,  106  having a duplex connection  108 ,  110 ,  112  to the base station  100 . The base station  100  transmits the connections of the user terminals  102 ,  104 ,  106  to a base station controller  114 , BSC, which forwards the connections to other parts of the system and to a fixed network. The base station controller  114  controls the operation of one or more base stations  100 . The other tasks of the base station controller  114  are frequency administration and exchange functions. The base station controller  114  and the base station  100  together form a functional entity sometimes referred to as a base station subsystem, BSS. The base station subsystem, BSS, uses a time divisional multiple access technique (TDMA).  
     [0018] The multicarrier base station  100  includes one or more transmitters capable of producing a multicarrier signal, which comprises up to 16 carrier waves. In the GSM systems, one carrier wave usually comprises eight time slots, i.e. eight physical channels. One base station  100  may serve one cell or several sectorized cells. The cell diameter may vary from a few metres to dozens of kilometres. The transmit power of the base station  100  determines the absolute cell size.  
     [0019] When a user terminal  102 ,  104 ,  106  informs the system that it wants a channel, e.g., it wants to establish a connection, the base station controller  114  via the base station  100  assigns a traffic channel on which the exchange of user data is performed. Different types of messages and user data move on different types of channels.  
     [0020] The user terminals  102 ,  104 ,  106  perform continuous measurements on the quality and the power level of the serving cell, and of the power levels of the adjacent cells. The base station  100  itself also performs measurements on the quality and power level of the link to the user terminals  102 ,  104 ,  106 . The range of variation of the transmit power level is for instance 30 dB.  
     [0021]FIG. 2 illustrates an example of a TDMA frame. In a GSM system a time divisional multiple access (TDMA) is utilized, with which each frequency carrier is subdivided into eight different time slots numbered from 0 to 7. In a TDMA technique the users share a physical radio channel, where they are assigned time slots. All the users sharing the physical resource have their own assigned, repeating time slot within a group of time slots called a frame. FIG. 2 shows time slots, of which time slots  202 ,  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216  form a frame of 8 time slots. A time slot  200  is a part of the previous frame and time slots  218 ,  220  are parts of the next frame. Each time slot of the frame is assigned to an individual user. In order to increase the data transmission rate, it is also possible to assign several time slots to an individual user. All the users of the same frequency share a common frame. Each user uses only the time slot that has been assigned to that user and remains silent during other time slots. Thus, for example, one user always uses the second time slot of each frame. The transmission thereby comprises bursts. The duration of a time slot is 577 ms and the duration of a frame 4,615 ms.  
     [0022]FIG. 3 illustrates an exemplary block diagram of a network and a multicarrier base station in accordance with the invention. The areas marked with dashed lines in FIG. 3 illustrate parts of a network  300  and the base station  100 . The network part  300  comprises a mobile switching center  302 , MSC, which performs the switching functions and controls interworking with other networks. The mobile switching center  302  is capable of routing calls from the fixed network—via the base station controller  114  and the base station  100 —to an individual user terminal. Depending on the network size, there may be several mobile switching centers  302  or only one.  
     [0023] The base station part  100  comprises a transmission unit, TRU  304 , a baseband processor  306 , an up-converter  308 , an amplifier  310 , a controller  312  and a register  314 . In accordance with the GSM protocol, the digital data is formatted into bursts of 148 bits. The bits are rearranged so as to spread temporally adjacent bits over a larger time frame and then reassembled at the receiving station so as to reduce the effect of lost data. The digital data is processed in the baseband processor  306 . The baseband processor  306  sets the transmitted signal level, i.e. the power level, suitable for each carrier and time slot used. After baseband processing, the digital data is modulated onto a radio frequency (RF) carrier and forwarded for wireless transmission to the user terminals.  
     [0024] The controller  312  and the register  314  are alternatively a part of the baseband processor  306  although in FIG. 3 they are drawn apart. The controller  312  controls the functions of the base station  100  and is usually implemented as a processor and its software, but various hardware solutions are also feasible, e.g. a circuit built of logic components or one or more application specific integrated circuits ASIC. A combination of these different implementations is also possible. The controller  312  controls the allocation of each user transmit power levels in different time slots and carriers. The register  314  is updated with the transmit power data of the base station  100 .  
     [0025] According to one embodiment of the invention, sums of transmit powers on each carrier for each time slot separately are calculated in the controller  312 . After the calculation of the sums of the transmit powers in the controller  312 , the time slots and carriers for the connections are allocated to the user terminals in such a way that the sum of transmit power levels in each time slot is minimized.  
     [0026] According to another embodiment of the invention, sums of transmit powers on each carrier for each time slot are calculated separately in the controller  312 . After the calculation of the sums of the transmit powers in the controller  312 , the sums of the transmit powers of each time slot are updated to the register  314 . Thus the register  314  contains, besides the sums of the transmit powers, also the transmit power data of each carrier and time slot. Alternatively the sums of the transmit powers of each time slot are not updated after each time slot. Instead the sums of the transmit powers of the time slots are updated for example after a predetermined period of time. When a connection is initialized, i.e. a new connection is initialized or an ongoing connection is reallocated, a time slot with the minimum sum of transmit powers is found in the register  314  by the controller  312 . After that a free carrier is found in the found time slot with the minimum sum of transmit powers in the register  314 . Then the found time slot and the found carrier are allocated for the connection by the controller  312 .  
     [0027] It is possible that changes of said calculated sum of the transmit powers of each time slot are calculated by the controller  312  and for example updated in the register  314 . Thus for example an ongoing connection may be reallocated to the found time slot and the found carrier when the calculated sum of transmit powers of the found time slot has been the minimum sum of transmit powers for a predetermined period of time.  
     [0028] With reference to a flow diagram in FIG. 4, let us next examine a method according to one embodiment of the invention. In step  400  the sum of transmit powers on each carrier for each time slot are calculated separately. The calculation takes place in the controller of the base station. Next in step  402  a register of the calculated sums of the transmit powers is updated. If a connection is disconnected in step  404 , the process returns to step  400  in which the sums of transmit powers on each carrier for each time slot are calculated again. If in step  406  a new connection is initialized, the process moves to step  408 , wherein a specific time slot with the minimum sum of transmit powers is found in the register. Next in step  410 , a free carrier in the found time slot with the minimum sum of transmit power is found. In step  412  the found time slot and carrier are allocated for the new connection. After step  412 , the process then returns to steps  400  and  402 , wherein the sum of transmit powers on each carrier for each time slot are calculated again and updated to the register.  
     [0029] The sum of transmit powers calculated in step  400  can alternatively be a statistical sum of transmit powers for a predetermined period of time. In that case, the register of the calculated sums of the transmit powers is not necessarily updated after each time slot either, but only after a predetermined period of time. It is possible that the calculation of the sums of the transmit powers in step  400  and the updating of the register in step  402  are not performed every time a connection has been disconnected or a new connection is being initialized. Instead, steps  400  and  402  may be performed after a given number of disconnections or new connections have appeared. Steps  400  and  402  may also take place if, for some reason, the power level of a given time slot is changed during an ongoing connection. Alternatively the steps  400  and  402  are not performed every time the power level of a time slot is changed. It is feasible that after the power level of one or more time slots has changed a given order of magnitude, steps  400  and  402  are performed.  
     [0030] FIGS.  5 - 16  illustrate an example according to one embodiment of the invention. In FIGS.  5 - 16  it is shown step by step how the transmit power levels allocated for certain timeslots are arranged when the traffic is increasing. Let us take the idle time of the cell of a cellular network as a starting point to describe this embodiment of the invention. The idle time is the time during which there are no ongoing connections and the system is ready to receive incoming connections. The idle time occurs most probably at night when the traffic in the network is at minimum.  
     [0031] In FIGS.  5 - 16  the rows  601 - 608  illustrate the different carriers of the TDMA multicarrier radio system. Columns  501 - 508  illustrate the eight time slots in the TDMA frame. The base station transmits the information in bursts in different time slots  501 - 508 . In FIGS.  5 - 16  there are different symbols in each time slot and carrier for indicating respective transmit power levels to user terminals, which are assigned to particular time slots. The square symbol in all of the FIGS.  5 - 16  illustrate a control burst, which is sent repeatedly at a maximum power. In FIGS.  5 - 16  the control information is in the first time slot  501  on the physical control channel, i.e. on the carrier  601 . This timeslot and carrier is from now on referred to as  501 / 601 . The physical control channel can also be a carrier  602 - 608  any other than the carrier  601 .  
     [0032] In FIGS.  5 - 16  the spherical symbols of different sizes indicate the different transmit power levels that are required at certain time slots  501 - 508  and carriers  601 - 608  for reliable communications. The largest spherical symbols indicate a high transmit power, the medium sized spherical symbols indicate a medium transmit power and the smallest spherical symbols indicate a low transmit power. In reality there are for instance  16  different transmit power levels, in a control range of 30 dB, in which case the ratio between the highest and the lowest transmit power is 1000. The three transmit power levels described in FIGS.  5 - 16  are chosen only as a set of examples. In practice the transmit power control is accomplished for example by only one step, for instance 2 dB, at a time. If the base station discovers that a user terminal does not receive its signal at a sufficient power level for reliable communications, it may apply power control on its own RF output and transmit at different power levels in each time slot  501 - 508 . If the power level has been for example too low, it is increased. Each power control command controls only one time slot, i.e. the user terminal whose received power level has been too low. The power level on each timeslot  501 - 508  depends for example on the path loss between the base station antenna and the user terminal. Path loss has a non-linear relation to the physical distance between the parties, and in general, it can be related to the 2 nd , 3 rd  and 4 th  power of the distance. Use of the 4 th  power would be realistic in a multipath environment. User terminals typically have a uniform distribution over the cell region. In FIGS.  5 - 16  the user terminals will be referred to as users.  
     [0033] The diamond-shaped symbols in FIGS.  5 - 16  illustrate those free time slots to which the next new user connection can be allocated. The smallest diamond-shaped symbols illustrate the primary allocation time slots and the largest illustrate the secondary allocation time slots. However it is also possible to allocate the users in any of the free time slots regardless of the primary or secondary symbols. It is also possible to change the time slot of an ongoing connection to a different time slot.  
     [0034] At the beginning only the control burst occupies the time slot  501  of the carrier  601  ( 501 / 601 ). When a new connection is initialized, for example a call is made from the cell or to the cell, the carrier  602  and a time slot any other than the time slot  501  are allocated for the new connection. In FIG. 5 this new connection occupies the timeslot  502  of the carrier  602  ( 502 / 602 ). It is supposed that the carrier  601  is dedicated primarily to a control channel and therefore all the other time slots  502 - 508  on the carrier  601  are only utilised if needed. One of the reasons for that is that the control channel uses maximum power in all the time slots, although only the first time slot  501  is used for transmitting the control information. This way interference to other cells can be reduced. The base station starts to transmit for instance at the maximum power to the user. However the base station soon decreases the power based on the measured power reported by the user.  
     [0035] Next a new connection is initialized or the previous connection is disconnected. If a new connection is initialized while the first connection is ongoing, the allocated carrier is for example the carrier  602  or  603  and the time slot is any other than  501  or  502 . FIG. 6 illustrates how the second user is allocated in the time slot  503  on the carrier  602  ( 503 / 602 ). The procedure of allocating new users on different carriers and time slots may go on until different time slots are allocated on every carrier.  
     [0036] In FIG. 7 a third user is allocated on  504 / 602  and in FIG. 8 fourth to seventh users are allocated on  505 - 508 / 602 . FIG. 9 illustrates a situation where one more user is allocated on  501 / 602 , and the carrier  602  is now fully loaded. In the situation illustrated in FIG. 9 the power level for  502 / 602  is decreased. It is possible that the decreasing of the power level for  502 / 602  has happened gradually, for example at the same time as the user has moved closer to the base station. FIG. 10 illustrates a situation in which two new users are allocated in the time slots  502  and  505  on the carrier  603 . These time slots  502 ,  505  are selected because of smaller powers on the carrier  602  in these particular time slots  502 ,  505 . Thus, when the new connections are initialized, the time slots  502 ,  505  are selected based on the register of the calculated sums of the transmit powers. Because the time slots  502 ,  505  had smaller sum powers than the other time slots  501 ,  503 ,  504 ,  506 ,  507 ,  508 , the time slot selections in the situation of FIG. 10 were directed to said time slots  502 ,  505 . In FIG. 11 two new users are allocated on the carrier  603  in the time slots  504 ,  507 . Also the used transmit power level of some users on the carrier  602  is changed.  
     [0037]FIG. 12 illustrates a situation where there are seven users allocated on the carrier  603 . The time slot  501  has now one user and the control information, and the other time slots  502 - 508  have two users. In FIG. 13 there are two users allocated on the carrier  604 . Once again the time slot selection is based on the minimum sum of transmit powers in the time slots  502 ,  504  concerned.  
     [0038]FIG. 14 illustrates a situation after several users have been connected to the cell and have left it. FIG. 15 shows a later situation, where the cell is almost fully loaded. There have been several connections to and disconnections from the cell before reaching the situation in FIG. 15. FIG. 16 illustrates a situation in which the cell is almost fully loaded. There are only a few free time slots. A situation like this is possible for example during a rush hour. In FIG. 16 the users are allocated in time slots and carriers in such a way that the sum of transmit power levels in each time slot is minimized.  
     [0039] Even though the invention is described above with reference to an example according to the accompanying drawings, it is clear that the invention is not restricted thereto but it can be modified in several ways within the scope of the appended claims.