Abstract:
The present invention relates to a method of sending first and second signals to a plurality of user equipments, the method comprising the steps of providing of a dedicated channel for each one of the plurality of user equipments, providing of a code-multiplexed shared channel for the plurality of user equipments, assigning of an antenna of a set of antennas to each one of the user equipments, sending of one of the first signals to one of the plurality of user equipments on one of the dedicated channels on a carrier frequency by applying transmit diversity, sending of one of the second signals to one of the plurality of user equipments on the code-multiplexed shared channel on the carrier frequency by applying multi-user diversity.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to telecommunications, and more particularly, to transmit and multi-user diversity in a cellular mobile telecommunication system. The invention is based on a priority application EP 02 360 277.4 which is hereby incorporated by reference. 
     BACKGROUND AND PRIOR ART 
     The demand for data communication services has exploded with the acceptance and widespread use of the Internet. While data communications have historically been serviced via wired connections, wireless users are now demanding that their wireless units also support data communications. Many wireless subscribers now expect to be able to “surf” the Internet, access their email, and perform other data communication activities using their cellular phones, wireless personal data assistants, wirelessly linked notebook computers, and/or other wireless devices. 
     Significant performance issues exist when using a wireless network to service data communications. Wireless networks were initially designed to service the well-defined requirements of voice communications. Generally speaking, voice communications require a sustained bandwidth with minimum signal-to-noise ratio (SNR) and continuity requirements. Data communications, on the other hand, have very different performance requirements. Data communications are typically bursty, discontinuous, and may require a relatively high bandwidth during their active portions. 
     The wireless network infrastructure must support both low bit rate voice communications and the varying rate data communications. More particularly, the network infrastructure must transmit low bit rate, delay sensitive voice communications together with high data rate, delay tolerant rate data communications. 
     It is therefore desirable to provide a communication system that is capable of carrying both delay sensitive lower data rate voice communications and delay tolerant higher data rate data communications with minimal waste of spectral capacity. Further, it is also desirable to provide a communication system that also services bursty data traffic for a plurality of data users without wasting allocated spectrum. 
     One example of such a communication system is the Universal Mobile Telecommunications System (UTMS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM). UTRAN is a wideband code division multiple access (W-CDMA) system. 
     A goal of the Third Generation Partnership Project (3GPP) is to evolve further the UTRAN and GSM-based radio access network technologies. Of particular interest here is the support of variable transmission rate services in the third generation mobile radio communications system for both real time and non-real time delay tolerant services. Because users share the same radio resources, the radio access network must carefully allocate resources to individual user equipment (UE) connections based on quality of service requirements, such as variable rate services, and on the availability of radio resources. 
     For example, in a multimedia session, one bearer may carry a speech connection, another bearer carries a video connection, and a third bearer may carry a packet data connection. Connections are mapped by the UTRAN onto physical transport channels. 
     Between the UE and the UTRAN, a connection may be mapped to one or more dedicated transport channels (DCHs) or to a common transport channel such as a random access common channel (RACH), a forward access common channel (FACH), a common packet channel (CPCH), a downlink shared channel (DSCH), and a high speed-downlink shared channel (HS-DSCH). 
     Real time connections are mapped to dedicated channels. On a dedicated channel, resources may be guaranteed to provide a particular service, such as a minimum transmission rate for voice communications. 
     To provide effective multimedia capabilities in UMTS, the High-Speed Downlink Packet Access (HSDPA) scheme is being developed which facilitates transfer of packet data to a mobile station at up to e.g. 4 Mbps. 
     The concept of HSDPA has been recently standardized in 3GPP for UMTS. It considers enhancements that can be applied to UTRA to provide very high-speed downlink packet access by means of a high-speed downlink shared channel (HS-DSCH). 
     For the basic structure of HS-DSCH two architectures have been considered (R2A010010: HSDPA radio interface protocol architecture&#39;, Ericsson, Motorola), i.e. an RNC-based architecture consistent with R99 architecture and a node B-based architecture for scheduling. Moving the scheduling to the nodes B enables a more efficient implementation of scheduling by allowing the scheduler to work with the most recent channel information. The scheduler can adapt the modulation to better match the current channel conditions and fading environment. Morever, the scheduler can exploit the multi-user diversity by scheduling only those users in constructive fades. 
     To improve transmission in a fading environment diversity techniques based on the use of multiple downlink transmit antennas are well known. Second order applications of these have been applied in the UTRA R99 specifications. Such techniques exploit spatial and/or polarisation decorrelations over multiple channels to achieve fading diversity gains. 
     Multiple input multiple output (MIMO) processing employs multi antennas at both the base station transmitter and terminal receiver, providing several advantages over transmit diversity techniques with multiple antennas only at the transmitter and over single antennas systems. If multiple antennas are available at both the transmitter and the receiver, the peak throughput can be increased using a technique known as code re-use. 
     It is an object of the present invention to provide an improved transmit diversity technique, in particular for usage in HSDPA-type systems. 
     SUMMARY OF THE INVENTION 
     The invention enables to send both real time and non-real time signals to user equipment of a mobile cellular telecommunication system by applying transmit diversity at least to the real time signals while making efficient usage of the available transmission power. This is accomplished by providing of dedicated channels for each one of the user equipments for the purpose of transmitting real time signals, such as voice and/or video signals. Preferably diversity techniques based on the use of multiple downlink transmit antennas are applied to these dedicated channels. 
     Further a code-multiplexed shared channel is provided for the user equipments. The active user equipments are split into two groups one of which is assigned to a first antenna and one of which is assigned to a second antenna. Non-real time signals which need to be sent to the first group are amplified by the power amplifier coupled to the first antenna and sent out from the first antenna; likewise non-real time signals to be sent out to the second group are amplified by the power amplifier connected to the second antenna and sent out from the second antenna. 
     In the more general case n-transmit diversity is applied, for example n=4. In this case the user equipments are split into a number of n=4 groups which are assigned to corresponding antennas. 
     In accordance with a further preferred embodiment of the invention a scheduler is used to exploit the multi-user diversity by scheduling only those users in constructive fades. 
     In accordance with a further preferred embodiment of the invention an improved transmit diversity technique is provided which enables to make efficient usage of the total available transmission power of the power amplifiers, in particular for providing both real time and non-real time services. In a preferred application of the present invention real time signals, such as voice and/or video signals, are sent out by applying a transmit diversity technique with multiple power amplifiers and multi antennas. Each of the power amplifiers supports at least two carrier frequencies. The real time signals are split up into a group of signals which are sent on the first carrier frequency and into another group of signals which are sent on the second carrier frequency. 
     Non-real time signals are scheduled in order to exploit multi-user diversity by scheduling only those users in constructive fades. Because of this kind of scheduling no transmit diversity is required. In order to make symmetric usage of the power amplifiers the active user equipments within the cell are split into a group which is assigned to the first transmission frequency and into another group which is assigned to the second transmission frequency. Non-real time signals which are to be sent to the first group of user equipments are amplified by the first power amplifier and non-real time signals to be sent to the second group are amplified by the second power amplifier. Hence, in average the usage of the power amplifiers is about symmetric and efficient usage of the total available transmission power is made. 
     In accordance with a preferred embodiment of the invention the real time signals are transmitted over DPCHs and the non-real time signals over a shared HS-DSCH of a HSDPA system. The real time signals are transmitted on the DPCHs using transmit diversity and each of the non-real time signals is transmitted over HS-DSCH over only one of the transmission antennas without transmit diversity but applying multi-user diversity. This way statistical balancing of transmission power can be achieved by using multi-carrier power amplifiers. 
     In accordance with a further preferred embodiment of the invention more than two carrier frequencies are used. The multi-carrier power amplifiers needs to support these carrier frequencies. To obtain statistical balancing of the usage of the transmission power of the power amplifiers the number of carrier frequencies must be equal to the number of diversity branches. For example, instead of a two transmit diversity scheme with a two-carrier power amplifier a four-transmit diversity scheme with a four-carrier power amplifier can be used. 
     In accordance with a further preferred embodiment of the invention each user equipment is assigned to both one of the set of carrier frequencies and one of the antennas of the set of antennas. Further each of the dedicated channels is assigned to one of the set of carrier frequencies. 
     In accordance with a further preferred embodiment of the invention closed loop transmit diversity is applied to the non-real time signals in addition to the multi-user diversity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following preferred embodiments of the invention will be described in greater detail by making reference to the drawings in which: 
         FIG. 1  is a block diagram of a first embodiment of a telecommunication system of the invention based on a single carrier frequency, 
         FIG. 2  is illustrative of a diagram showing the usage of the power amplification capacity, 
         FIG. 3  is illustrative of a flow chart for operating the telecommunications system of  FIG. 1   
         FIG. 4  is a block diagram of a second preferred embodiment of a telecommunications system of the invention using at least first and second carrier frequencies, 
         FIG. 5  is illustrative of is illustrative of the statistical utilisation of the transmission power capacities of the dual carrier power amplifiers of the system of  FIG. 4 , 
         FIG. 6  is illustrative of a flow chart of a preferred embodiment of a method of the invention 
         FIG. 7  is illustrative of a third preferred embodiment of a telecommunication system, 
         FIG. 8  is illustrative of is illustrative of the statistical utilisation of the transmission power capacities of the dual carrier power amplifiers of the system of  FIG. 7 , 
         FIG. 9  is illustrative of a fourth preferred embodiment, 
         FIG. 10  is illustrative of is illustrative of the statistical utilisation of the transmission power capacities of the dual carrier power amplifiers of the system of  FIG. 9   
         FIG. 11  is illustrative of a fifth preferred embodiment of the invention, 
         FIG. 12  is illustrative of is illustrative of the statistical utilisation of the transmission power capacities of the dual carrier power amplifiers of the system of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a telecommunication system having a transmitter  1  for servicing of a plurality of user equipments UEi, UEj, . . . within a cell of a mobile telecommunication system. The transmitter  1  has a transmit diversity module  3  for applying of transmit diversity to DPCHs. The outputs of the transmit diversity module  3  are coupled to adders  5  and  7 , respectively. Adder  5  is coupled to signal carrier amplifier  9 , which is coupled to antenna  11 . Likewise adder  7  is coupled to power amplifier  13  which is coupled to antenna  15 . 
     Further transmitter  1  has code multiplexer  17  for code multiplexing of HS-DSCH. Code multiplexer  17  provides at its output signal components SUEi and SUEj, . . . for the respective user equipments UEi, UEj, . . . . 
     Each one of the user equipments UEi, UEj, . . . is assigned to one of the antennas  11  or  15 . For example the user equipment UEi is assigned to the antenna  11  and the user equipment UEj is assigned to the antenna  15 . The outputs of code multiplexer  17  are coupled to the respective adders  5  and  7 . This way the signal component SUEi is provided to adder  5  and the signal component SUEj is provided to adder  7 . 
     Scheduler  19  schedules the sending of non-real time signals via HS-DSCH in order to provide multi-user diversity by scheduling only those users in constructive fades. 
     By splitting the user equipments into groups which are assigned to different antennas the loading of the power amplifiers  9  and  13  is approximately symmetric even though only for DPCHs transmit diversity is applied. 
     The statistical balancing of the utilisation of the total available transmission power is illustrated by way of example in  FIG. 2 .  FIG. 2  shows diagrams  20  and  22  illustrating the utilisation of the transmission power of power amplifiers  9  and  13  of  FIG. 1 , respectively, in the time domain. The time axis is divided into scheduling intervals, which are referred to as transmission time intervals (TTI) in UTRA notation. As it is apparent from  FIG. 2  most of the time both power amplifiers  110  and  112  are operated at or close to their respective maximum power output capability. 
       FIG. 3  shows a corresponding flow chart. In step  30 , DPCHs are provided. In step  32  a shared HS-DSCH is provided. In step  34  an antenna and an associated power amplifier is assigned to each active user equipment. Preferably about the same number of user equipments is assigned to each one of the antennas. This can be done by an appropriate signalling scheme between the transmitter and the user equipments. 
     In step  36  real time signals are sent on the DPCHs with transmit diversity on a single carrier frequency. In step  38  non-real time signals are sent on HS-DSCH with multi-user diversity on the single carrier frequency. No transmit diversity needs to be applied because of the multi-user diversity. 
     Each active user equipment preferably monitors the channel quality for either of the propagation channels, and it reports the quality of the best channel to node B. In addition also the transmission antenna associated with the channel quality feedback is reported to node B. Different user equipments that are addressed in code multiplex can be addressed via different transmission antennas. For example a first code is transmitted via antenna  11  (cf.  FIG. 1 ) to the assigned user equipment UEi and a second code is transmitted via antenna  15  to the assigned UEj. 
     It is to be noted that this embodiment is particularly advantageous as the probability that a user equipment ‘sees’ a good channel is relatively high. Further cross-talk from Hadamard codes associated with other user equipment is reduced since the corresponding channels seen weaker by the addressed user equipment. 
     Preferably node B should balance the Hadamard codes between the transmission antennas such that the transmit power is well balanced. Further, due to a limited load balancing within each carrier, support of a flexible number of carriers is generally possible, e.g. single-carrier or three-carrier operation. 
       FIG. 4  shows a block diagram of a telecommunication system for servicing a number of mobile user equipments (UEs). By way of example the user equipments UEn UEj UEi and UEm are shown in the block diagram of  FIG. 4 ; it needs to be noted that in a practical application there can be many more UEs. 
     Each of the UEs is assigned to a first transmission frequency f 1  or a second transmission frequency f 2 . For example UEn is assigned to f 2 , UEj is assigned to f 1 , UEi is assigned to f 1  and UEm is assigned to f 2 . This way the UEs are split into a first group of UEs which are assigned to the first carrier frequency f 1  and into a second group which is assigned to the second carrier frequency f 2 . 
     Preferably the assignment of carrier frequencies to UEs is performed by appropriate signalling between the UEs and the transmitter  100  of the telecommunication system. For example carrier frequencies are assigned to UEs which become active alternatingly. For example the UEs become active in the following sequence: 
     UEi, UEn UEj, UEm, . . . . 
     The first UE which becomes active, i.e. UEi, is assigned to the first carrier frequency f 1 . The second UE which becomes active, i.e. UEn is assigned to the second carrier frequency f 2 . The next UE which becomes active, i.e. UEj, is assigned to the first carrier frequency f 1 , and so on. This way the two groups of UEs results, where each group comprises about the same number of UEs if a larger number of UEs is considered. 
     Preferably the assignment of UEs to frequencies is performed in order to balance the load of the power amplifiers. It is to be noted that this assignment can be dynamic and that  FIG. 1  is to be understood as a snap shot. 
     The transmitter  100  serves to transmit both real time and non-real time signals to the UEs. Real time signals, such as voice or video signals, are transmitted via DPCHs. Each of the DPCH&#39;s is assigned to either the first carrier frequency f 1  or to the second carrier frequency f 2 . In order to provide transmit diversity for the DPCHs the transmitter  100  has transmit diversity modules  102  and  104 . 
     Transmit diversity module  102  receives real time signals which are to be transmitted on a DPCH being assigned to the frequency f 1 . Likewise transmit diversity module  104  receives such real time signals which are to be transmitted on DPCHs being assigned to the second carrier frequency f 2 . 
     Transmit diversity module  102  is coupled via adders  106  and  108  to power amplifiers  110  and  112 , respectively. Both power amplifiers  110  and  112  are dual-carrier power amplifiers which support the carrier frequencies f 1  and f 2 . Power amplifier  110  is coupled to antenna  114  and power amplifier  112  is coupled to antenna  116 . This way any known diversity technique based on the use of multiple downlink transmit antennas can be implemented. 
     For the HS-DSCH the transmitter has code multiplexes  118  and  120 . Code multiplexer  118  has an input for receiving of non-real time signals to be sent to the first group of UEs, i.e. to UEs which are assigned to the second carried frequency f 2 . This way the signal components SUEi, SUEj, . . . to be transmitted on carrier frequency f 1  and the signal components SUEm, SUEn, . . . to be transmitted on carrier frequency f 2  are provided by the code multiplexes  118  and  120 , respectively. The signal components SUEi, SUEj, . . . which are to transmitted on carrier frequency f 1  are input into adder  106 . Likewise the signal components SUEm, SUEn, . . . are input into adder  108 . 
     Further the transmitter  100  has scheduler  124 . Scheduler  124  schedules the non-real time signals to be sent over HS-DSCH in order to provide multi-user diversity by scheduling only non-real time signals to users in constructive fades. 
     In operation, the power amplifier  110  is controlled to amplify the real time signals of the DPCHs being assigned to the frequency f 1  on carrier frequency f 1  and the real time signal components of the DPCHs assigned to the carrier frequency of f 2  on frequency f 2 . The signal components SUEi, SUEj, . . . of HS-DSCH to be sent on carrier frequency f 1  are only amplified by power amplifier  110  on carrier frequency f 1 . The same principle applies correspondingly to the operation of power amplifier  112 . 
     The statistical balancing of the utilisation of the total available transmission power is illustrated by way of example in  FIG. 5 .  FIG. 5  shows diagrams  200  and  202  illustrating the utilisation of the transmission power of power amplifiers  110  and  112  of  FIG. 1 , respectively, in the time domain. The time axis is divided into scheduling intervals, which are referred to as transmission time intervals (TTI) in UTRA notation. As it is apparent from  FIG. 5  most of the time both power amplifiers  110  and  112  are operated at or close to their respective maximum power output capability. 
       FIG. 6  illustrates an embodiment of a method of the invention by way of example. In step  300  DPCHs are provided for transmitting of real time signals. A transmission frequency of a set of transmission frequencies is assigned to each one of the DPCHs in step  302 . 
     In step  304  a HS-DSCH is provided as a shared channel for transmitting of non-real time signals. In step  306  a transmission frequency of the set of transmission frequencies is assigned to each active UE within the cell. This is done by an appropriate signalling protocol. When the UEs are capable to receive only one carrier frequency this step is not required as the carrier frequency has already been assigned in step  302 . In this instance the carrier frequency assigned in step  302  to a UE will also be used for the HS-DSCH transmission to that UE. 
     In step  308  the real time signals are sent on the DPCHs with transmit diversity. In step  310  the non-real time signals are sent on the shared HS-DSCH with multi-user diversity but without transmit diversity. Due to the assignment of transmission frequencies to user equipment in step  306  a statistical balancing of the utilisation of the power amplifiers is accomplished. 
       FIG. 7  shows a further preferred embodiment which combines the embodiments of  FIGS. 1 to 3  and  4  to  6 . Like elements in the embodiment of  FIG. 7  are designated with the same reference numerals as in the embodiment of  FIG. 4 . 
     As opposed to the embodiment of  FIG. 4  each user equipment is not only assigned to one of the available carrier frequencies f 1 , f 2  but also to one of the antennas  110 ,  112 . The assignment of user equipments to antennas is done as in the embodiment of  FIGS. 1 to 3 . This way the advantages of both embodiments are combined. 
     The multi-carrier operation on frequencies f 1  and f 2  further improves the power amplifier load balancing. In this instance, load or Hadamard code balancing can be performed jointly for all the carriers, i.e. joint multi-carrier scheduling. This has the further advantage that heavy peak loads can be avoided by means of a single-carrier or multi-carrier scheduling technique. Packets which are addressed to slowly moving user equipments are delayed in order to avoid peak loads. Such a technique can also be employed for the other preferred embodiments. 
     The corresponding time domain characteristic is shown in  FIG. 8 . 
       FIG. 9  shows a block diagram of a further preferred embodiment. Like elements of the embodiment of  FIG. 9  are designated with the same reference numerals as in the embodiments of  FIGS. 4 and 7 . 
     The embodiment of  FIG. 9  is similar to the embodiment of  FIG. 7 . In addition to the embodiment of  FIG. 7  closed loop transmit diversity is applied for HS-DSCH on good channels. Preferably this scheme is used only if the DPCH associated with the HS-DSCH is transmitted using closed loop transmit diversity, where generally any closed loop transmit diversity mode can be used, for example R99 closed loop transmits diversity mode 1 or mode 2. This scheme can be combined with any of the above disclosed preferred embodiments, e.g. HS-DSCH transmission via one antenna or using antenna selection. 
     In the example considered here two-transmit diversity is applied. Each of the user equipments monitors the channel quality for either of the propagation channels. If both channels seen by the user equipment are good, then the user equipment indicates to node B that closed loop diversity shall be used on the HS-DSCH. This means that the HS-DSCH transmission is performed via two different transmission antennas. 
     In this case the channel quality feedback shall include the anticipated and/or experienced beam forming gain obtained after combing in the user equipment. In the four-transmit diversity case closed loop transmit diversity could be applied with a flexible number of transmission antennas, for example if two (three/four) out of four propagation seen by the user equipment are good, then apply two-transmit (three-transmit/four-transmit) closed loop diversity. 
     It is to be noted that if HS-DSCH is transmitted using closed loop transmit diversity, then HS-DSCH should typically use the same antenna weights as the associated DPCH. In general, however, both HS-DSCH and the associated DPCH could also use different antenna weights. 
     As compared with the above described embodiments increased system throughput can be expected, particularly at low terminal velocities. This throughput increase is due the coherent combining gain of up to 3 db in the two-transmit diversity case. This is obtained by using closed loop transmit diversity in the case that more than one channel seen by the user equipment are good. 
     Coherent combining results in a directed transmission, similar as with beam steering. Therefore also the interference caused to other terminals can be reduced. Further this embodiment is particularly advantageous as the power amplifier load balancing can be further improved. 
     In the preferred embodiment of  FIG. 9  the closed loop transmit diversity scheme is combined with the scheme of  FIG. 4 , i.e. HS-DSCH is either transmitted via one antenna, or, if the user equipment sees several good channels, using closed loop transmit diversity. This way a coherent combining gain can be obtained with little additional signalling overhead. In the two-transmit diversity case, the additional signalling from a certain user equipment to node B is confined to switching close loop transmit diversity on/off. This way transmit diversity can be applied to HS-DSCH in addition to the multi-user diversity in order to take advantage of the presence of several good channels. This transmits diversity for the HS-DSCH is provided by transmit diversity module  122  of transmitter  100  of  FIG. 9 . 
     The corresponding time domain characteristic is shown in  FIG. 10 . 
       FIG. 11  shows a block diagram of a further preferred embodiment combing the additional transmit diversity for HS-DSCH with the embodiment of  FIG. 1 . Like reference numerals are used in  FIG. 11  to designate like elements. The transmitter  1  of  FIG. 11  in addition has a transmit diversity module  122  in order to provide 2-transmit diversity for HS-DSCH. In this case, the transmission alters between an antenna selection mode and a closed loop transmit diversity mode, depending on the quality of the propagation channels. In the example considered here, 2-transmit diversity is assumed. If a user equipment sees a bad channel, antenna selection is used. In this case the user equipment reports the quality of the best channel to node B. In addition, also the transmit antenna associated with the channel quality feedback is reported to node B. If both channels seen by the user equipment are good, user equipment signals to node B that closed loop transmit diversity shall be used for HS-DSCH transmission. 
     In the preferred embodiment considered here with respect of  FIG. 11  the single-carrier operation is illustrated. In the 4-transmit diversity case, closed loop transmit diversity can be applied with a flexible number of transmitter antennas. 
     In the example considered with respect to  FIG. 11  UEi only sees one good channel and therefore uses antenna selection. As opposed to this the user equipment UEj sees two good channels and therefore uses 2-transmit closed loop diversity by means of transmit diversity module  122 . 
       FIG. 12  shows the corresponding time domain characteristic. It is to be noted that none of the above described embodiments is restricted to 2-transmit diversity. Rather n-transmit diversity can be implemented, where n can be any number above 2. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  transmitter 
           3  transmit diversity module 
           5  adder 
           7  adder 
           9  power amplifier 
           11  antenna 
           13  power amplifier 
           15  antenna 
           17  code multiplexer 
           19  scheduler 
           20  diagram 
           22  diagram 
           100  transmitter 
           102  transmitter diversity module 
           104  transmitter diversity module 
           106  adder 
           108  adder 
           110  power amplifier 
           112  power amplifier 
           114  antenna 
           116  antenna 
           118  code multiplexer 
           120  code multiplexer 
           122  transmit diversity module 
           124  scheduler 
           200  diagram 
           202  diagram