Abstract:
A method synchronizes a radio communication system divided into radio cells. Data is transmitted in the radio communication system by a multiple access method. Each radio cell thus has a base station for the radio feed of several mobile stations, allocated to the radio cell. A synchronization of the base station is carried out using the received signals from mobile stations in the same radio cell and also in adjacent radio cells. The base station determines at least one pilot signal and signals said pilot signal to the allocated mobile stations in a downwards direction. The allocated mobile stations transmit the signaled pilot signal in an upwards direction to the base station. The base station determines a synchronization value, from the received pilot signals from the radio cell thereof and also from the adjacent radio cells, fro a time synchronization and/or for a frequency synchronization on which the bas station is synchronized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on and hereby claims priority to PCT. Application No. PCT/EP2004/051395 filed Jul. 7, 2004, and German Application No. 10336312.2 filed on Aug. 7, 2003, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a method for synchronizing a radio communication system divided into radio cells a base station, a mobile station and a radio communication system. 
   Cellular radio communication systems, in particular mobile radio systems, are subdivided into synchronized and unsynchronized radio communication systems. 
   In the case of the former, base stations of adjacent radio cells are synchronized with each other in respect of time and/or carrier frequency. For the purposes of synchronization, in particular for time synchronization, GPS receivers are for example deployed at the base stations or base stations are synchronized by synchronization signals that are complex to exchange. The transmission of synchronization signals also takes up radio transmission resources, which are then no longer available for chargeable user data transmissions (payload). 
   In the case of unsynchronized radio communication systems, base stations of adjacent cells are not synchronized with each other. 
   Synchronization methods are particularly important in mobile radio networks in particular, when so-called Orthogonal Frequency Division Multiplexing or OFDM transmission methods are used. Services requiring high data speeds, for example video transmissions, can be transmitted in a cost-effective manner by OFDM data transmissions. OFDM data transmission takes place by so-called subcarriers, which are formed by subdividing an available bandwidth. As these subcarriers are particularly advantageously used in a multiple manner in adjacent cells, resulting co-channel interference should be taken into account during planning and operation. 
   So that radio transmission resources can be allocated or managed (Radio Resource Management RRM) in an optimum manner in respect of data transmission, frequency and/or time synchronization must be precise, depending on the radio transmission methods used in each instance. The two synchronization methods mentioned by way of example, because they are based on received mobile station signals, are largely dependent both on the quality and number of received mobile station signals, as far as precision is concerned. 
   SUMMARY OF THE INVENTION 
   One possible object of the present invention is therefore to specify a synchronization method with a low level of complexity for a radio communication system with a cellular structure, in particular a mobile radio system with OFDM data transmission. The method allows synchronization to be achieved in respect of time and/or frequency in a cellular radio communication systemin a simple manner with the aid of pilot signals. 
   The use of cost-intensive GPS receivers in particular is thereby dispensed with, as is the transmission of additional signaling information for synchronization purposes, as had to be exchanged previously on a higher protocol level between base station and mobile station. 
   Synchronization is implemented independently and exclusively by receive-side signal processing and subsequent adjustment of the synchronization status of the base stations or mobile stations. 
   Pilot signals are used, for example, so that pilot signal overlay takes place within each radio cell, advantageously resulting in a high signal-to-noise ratio of the pilot signal at the base station in question. Receive-side evaluation of the pilot signals for synchronization purposes therefore takes place, even if the receive conditions are unfavorable. 
   The method advantageously allows pilot signal overlay with a high signal-to-noise ratio to be achieved at the base station in question, said high signal-to-noise ratio allowing the receive-side evaluation of pilot signals for synchronization purposes to take place, even if the receive conditions are unfavorable. 
   With the method the proposed pilot signal structure and the in particular randomly occurring selection of the pilot signal mean that no central control device is required to emit the respective pilot signals. 
   The pilot signal selection and assignment, which take place for example in a radio-cell-specific manner and optionally by random selection, allow radio-cell-dependent weighting of received mobile station signals and base station signals. When determining the synchronization value for the frequency and/or time synchronization to be implemented, it is therefore possible to weight the synchronization value estimation in respect of the radio cells, so that even weakly received mobile station and base station signals ae taken into account. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  shows a synchronization method according to one embodiment of the invention, in a radio communication system with a cellular structure, 
       FIG. 2  shows a pilot signal subcarrier receive situation at a base station as illustrated in  FIG. 1 , 
       FIG. 3  shows a frame, with the aid of which both useful data and pilot signals are transmitted, with reference to  FIGS. 1 and 2 , 
       FIG. 4  shows the overlaying of mobile station signals from a common radio cell at a base station and 
       FIG. 5  shows the overlaying of mobile station signals from adjacent radio cells at a base station. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
     FIG. 1  shows one embodiment of a synchronization method in a radio communication system with a cellular structure. 
   A cellular radio communication system is considered, which represents other mobile radio systems, in which a data transmission is implemented by an OFDM transmission method, such that the data transmission takes place by a frame structure based on time slots and Frequency Division Multiple Access FDMA. 
   An available bandwidth is subdivided into so-called subcarriers and different users transmit in time slots on different subcarriers. 
   Base stations of adjacent radio cells share the use of a stock of radio transmission resources, formed by subcarrier time slots. Adjacent radio cells therefore have a frequency repetition factor of one in respect of the subcarriers. 
   Three adjacent radio cells FZ 1  to FZ 3  each have a base station BTS 1  to BTS 3 . Each individual base station BTS 1  to BTS 3  covers a number of mobile stations MT 11  to MT 33  assigned to the respective radio cell FZ 1  to FZ 3 . A total of four mobile stations MT 11  to MT 14  is thereby assigned to a first base station BTS 1  for radio coverage purposes, a total of five mobile stations MT 21  to MT 25  is thereby assigned to a second base station BTS 2  and a total of three mobile stations MT 31  to MT 33  is thereby assigned to a third base station BTS 3  for radio coverage purposes. 
   The first base station BTS 1 , representing all the others, selects two pilot signal subcarriers TS 11  and TS 12  from the available subcarriers for a pilot signal transmission based on OFDM data transmission. The first base station BTS 1  notifies the assigned mobile stations MT 11  to MT 14  for example of the selected pilot signal subcarriers TS 11  to TS 12  by direct signaling. 
   These selected pilot signal subcarriers TS 11  to TS 12  are used in parallel with other subcarriers, which are assigned to a useful data transmission, to transmit a data frame to be sent in a downlink. 
   In contrast to direct signaling of the pilot signal subcarriers used, it is also possible to use tables disposed on the transmit side and receive side, in which pilot signal subcarrier pairs to be used are stored respectively. In this instance the base station notifies the assigned mobile stations of a corresponding subcarrier pair by reference to a table input. 
   Random selection of pilot signal subcarrier pairs can also take place with the aid of a hopping pattern with a defined number of pilot signal subcarriers. The assigned mobile stations are then notified of the hopping pattern for example. 
   The signaled pilot signal subcarriers TS 11 , TS 12  are also used by the mobile stations MT 11  to MT 14  for pilot signal transmission in an uplink to the first base station BTS 1 . 
   Selection of the pilot signal subcarriers by the base station BTS 1  is advantageously random and is carried out in an alternating manner frame by frame, as a result of which the pilot signal subcarriers alternate frame by frame both in the uplink and in the downlink. 
   The same applies to the second base station BTS 2  and the third base station BTS 3  of the adjacent radio cells FZ 2  and FZ 3 . The second base station BTS 2  for example selects two pilot station subcarriers TS 21  and TS 22 , which it signals in the downlink to the assigned mobile stations MT 21  to MT 25 , while the third base station BTS 3  for example selects two pilot station subcarriers TS 31  and TS 32  and signals them correspondingly to the mobile stations MT 31  to MT 33  assigned to it. 
   For their part the mobile stations MT 21  to MT 25  use the pilot signal subcarriers TS 21  and TS 22  assigned to them for a pilot signal transmission in the uplink to the second base station BTS 2 , while the mobile stations MT 31  to MT 33  use the pilot signal subcarriers TS 31  and TS 32  assigned to them for a pilot signal transmission in the uplink to the third base station BTS 3 . 
   Synchronization is described in more detail with reference to the first radio cell FZ 1 , which also represents the adjacent radio cells FZ 2 , FZ 3 . Synchronization here refers to a time synchronization of time slots and/or a frequency synchronization of the subcarriers used. 
   The first base station BTS 1  of the first radio cell FZ 1  receives both the pilot signals TS 11 , TS 12  of the mobile stations MT 11  to MT 14  assigned to it and for example the pilot signals TS 21 , TS 22 , TS 31 , TS 32  from the mobile stations MT 21 , MT 22 , MT 31 , MT 32  of the adjacent radio cells FZ 2  and FZ 3  in the uplink UL. On the basis of the received pilot signals TS 11 , TS 12 , TS 21 , TS 22 , TS 31  and TS 32  the first base station BTS 1  determines a first time deviation and/or a first frequency deviation and uses these values to derive an appropriate synchronization value for time and/or frequency synchronization, to which the first base station BTS 1  is synchronized. 
   A third mobile station MT 13  of the first radio cell FZ 1 , which represents all the mobile stations, receives both pilot signals TS 11 , TS 12  from the base station BTS 1  of its own radio cell FZ 1  and pilot signals TS 21 , TS 22 , TS 31 , TS 32  from the adjacent base stations BTS 2  and BTS 3  of the radio cells FZ 2  and FZ 3  in the downlink. On the basis of the received pilot signals TS 11 , TS 12 , TS 21 , TS 22 , TS 31  and TS 32  the third mobile station M 13  now determines a second time deviation and/or a second frequency deviation and uses these values to derive an appropriate synchronization value for time and/or frequency synchronization, to which the mobile station MT 13  is synchronized. 
   Synchronization is repeated frame by frame, as a result of which a precise, automatically organized time and/or frequency synchronization is achieved on average over time. 
   With reference to  FIG. 1 ,  FIG. 2  shows a pilot signal subcarrier receive situation at the first base station BTS 1 . Subcarrier frequencies f are thereby plotted on the horizontal axis and symbols SYMB on the vertical axis. 
   The first base station BTS 1  receives both the pilot signal subcarriers TS 11  and TS 12  from the mobile stations MT 11  to MT 14  that can be assigned to the first radio cell FZ 1  and the pilot signal subcarriers TS 21  and TS 22  from the mobile stations MT 21  and MT 22  that can be assigned to the second radio cell FZ 2  and the pilot signal subcarriers TS 31  and TS 32  from the mobile stations MT 31  and MT 32  that can be assigned to the third radio cell FZ 3 . 
   During pilot signal transmission no symbols SYMB—shown here as circular markings on the horizontal axis—are transmitted by the further available subcarriers. 
   The subcarrier pairs TS 11  and TS 12 , TS 21  and TS 22 , TS 31  and TS 32  are separated by an unused subcarrier band GB, which prevents intercarrier interference ICI for the duration of the pilot signal. 
     FIG. 3  shows a frame Fr, which is used to transmit both useful data Data and pilot signals Test, with reference to  FIGS. 1 and 2 . 
   Available subcarriers sub are thereby plotted on the vertical axis, while a pattern over time Time of the frame fr is shown on the horizontal axis. 
   The frame Fr has a first block Data used to transmit useful data, the useful data transmission being implemented with the aid of an OFDM data transmission not described in more detail here. A second block Test follows the first block Data, said second block Test being used for pilot signal transmission. 
   In a preferred embodiment two directly adjacent subcarriers TS 11  and TS 12  or TS 21  and TS 22  or TS 31  and TS 32  respectively are selected as pilot signals by each base station. A time deviation, for example from the first base station BTS 1  to the mobile stations MT 11  to MT 14 , MT 21 , MT 22 , MT 31  and MT 32  is determined by estimation on the basis of two adjacent pilot signal subcarriers, each transmitting the same symbols. 
   At least two successive symbols SYM 1  and SYM 2  or SYM 2  and SYM 3  of the pilot signal subcarriers TS 11  and TS 12  or TS 21  and TS 22  or TS 31  and TS 32  are used to determine a frequency synchronization value for a frequency synchronization. The use of three symbols SYM 1  to SYM 3  improves the precision of the estimation carried out to generate the frequency synchronization value, as this prevents so-called intersymbol interference ISI during the evaluation for synchronization purposes. 
   The pilot signal subcarriers TS 11  and TS 12  or TS 21  and TS 22  or TS 31  and TS 32  of a radio cell are ideally directly adjacent, but it is also possible for there to be an interval between the two pilot signal subcarriers TS 11  and TS 12  or between the two pilot signal subcarriers TS 21  and TS 22  or between the two pilot signal subcarriers TS 31  and TS 32 . This interval should be selected such that the individual pilot signal subcarriers are separated from each other according to a predefinable minimum phase deviation. 
     FIG. 4  shows a segment of the overlay of mobile station signals from the mobile stations MT 11  to MT 12  of the common radio cell FZ 1  at the base station BTS 1 . A transmit power TX Power is thereby plotted on the vertical axis with a pattern over time Time of a frame on the horizontal axis, while a third axis is used to show subcarrier frequencies Frequency. 
   The three mobile stations MT 11  to MT 13  each simultaneously use pilot signal subcarriers testsub with the same structure within an area Test in the uplink. 
   In contrast in an area Data the respective useful data transmission from the respective mobile station MT 11  to MT 13  takes place with the aid of subcarriers datasub. 
   Additive overlaying of the mobile station signals sent from the mobile stations MT 11  to MT 13  takes place at the base station BTS 1 , with a significant rise in the signal level being achieved in the area Test, without an increase in transmit power being required on the part of the mobile stations MT 11  to MT 13  for this purpose. 
   To summarize, standard symbols of the pilot signals subcarriers testsub are transmitted simultaneously within a radio cell from all mobile stations MT 11  to MT 13 , thereby achieving a rise in the receive level of the cumulative signal at the base station BTS 1  in question. A maximum frequency interval is ideally used between the subcarrier pairs used for the pilot signal transmission. A phase-continuous pilot signal is transmitted in an advantageous embodiment. 
   An OFDM symbol symb and an OFDM subcarrier sub are input at the mobile station MT 12 , representing the mobile stations MT 11  to MT 13  and the base station BTS 1 . 
     FIG. 5  shows a segment of the overlaying of mobile station signals of adjacent radio cells FZ 1  to FZ 3  at a receiving base station BTS 1 . 
   A transmit power TX Power is thereby plotted on the vertical axis and a pattern over time Time of a frame is plotted on the horizontal axis, while a third axis is used to show subcarrier frequencies Frequency. 
   A useful data transmission again takes place in the area Data, while a pilot signal transmission with corresponding pilot signal subcarriers again takes place in the area Test. 
   Mobile station signals from the three radio cells FZ 1  to FZ 3  are cumulatively overlaid with respectively assigned pilot signal subcarrier pairs TS 11  and TS 12 , TS 21  and TS 22  and TS 31  and TS 32  at the base station BTS 1 . 
   Every received subcarrier pair TS 11  and TS 12 , TS 21  and TS 22  and TS 31  and TS 32  can be assigned to a radio cell FZ 1  to FZ 3 . This means that the time and frequency deviations occurring in adjacent radio cells can be determined individually in each base station. 
   The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).