Wireless multicarrier communication method with dynamic allocation of frequency width and number of subbands

0001The invention relates to a method for communication between radio stations of a radio communication system using at least one frequency band divided into a number of subbands and a method for the transmission of data between a transmitter and a receiver in a radio communication system, the data on a number of subbands in one Subbands divided frequency band are transmitted.

0002Furthermore, the invention relates to a device in a radio communication system for determining the division of a frequency band into subbands, as well as a device for sending data and a device for receiving data in a radio communication system.

0003In radio communication systems, information (for example voice, image information, video information, SMS (Short Message Service) or other data) is transmitted with the aid of electromagnetic waves via a radio interface between the transmitting and receiving radio station. The electromagnetic waves are emitted at carrier frequencies that lie in the frequency band provided for the respective system. A radio communication system can include radio stations such as subscriber stations, for example mobile stations, and base stations, for example node B's or other radio access devices, and, if appropriate, further network-side devices.

0004The access of radio stations to the common radio resources of the transmission medium, such as time, space, frequency, code, power is regulated in radio communication systems by multiple access methods (Multiple Access, MA).

0005In order to ensure the most efficient transmission of data, the entire available frequency band is broken down into several subbands (multi-carrier method). The idea underlying the multicarrier systems is to convert the initial problem of the transmission of a broadband signal into the transmission of a set of narrowband signals. This has the advantage, among other things, that the complexity required at the receiver can be reduced. Furthermore, the division of the available bandwidth into several narrowband subbands enables a significantly higher granularity of the data transmission with regard to the distribution of the data to be transmitted to the different subbands, ie the radio resources can be distributed with great fineness to the data to be transmitted or to the receivers . The available bandwidth can be used efficiently by assigning a number of subbands to different receivers, particularly in the case of transmissions with variable data rates or in the case of burst-based data traffic.

0006In OFDM (Orthogonal Frequency Division Multiplexing), pulse shapes that are approximately rectangular in time are used for the subbands. The frequency spacing of the subbands is selected such that in the frequency domain at that frequency at which the signal of one subband is evaluated, the signals of the other subbands have a zero crossing. The subbands are thus orthogonal to one another. A spectral overlap of the subbands and, as a result, a high packing density of the subbands is permitted, since the orthogonality ensures that the individual subbands can be distinguished. Therefore, better spectral efficiency is achieved than with simple FDM (Frequency Division Multiplexing), in which the orthogonality condition of the subbands is not taken into account and therefore larger subband spacings are required to avoid possible interference between the individual subbands, which negatively affect the transmission quality . The mostly very small spacing of the subbands and the narrowband nature of the signals transmitted on the individual subbands in OFDM are intended to ensure that the transmission within the individual subbands is generally not frequency-selective. This simplifies signal equalization at the receiver.

0007A disadvantage of OFDM systems is their high sensitivity to poor time and frequency synchronization, as well as to Doppler effects caused by movements of radio stations. These unfavorable phenomena occur primarily in the upward direction, ie noticeable in the case of data transmissions from subscriber-side radio stations to a network-side device, the data from different subscriber-side radio stations being distributed over different subbands in the sense of FDMA (Frequency Division Multiple Access). In this case, the effects mentioned above can cause interference between the subbands and thus also between the different transmitting subscriber-side radio stations.

0008The invention is based on the object of demonstrating an effective method for communication in a multi-carrier radio communication system which is improved compared to conventional multi-carrier methods, as well as a network-side device for supporting the method and a transmitter and a receiver for carrying out the method.

0009With regard to the method, this object is achieved by a method having the features of claim 1.

0010Advantageous refinements and developments are the subject of dependent claims.

0011In the method for communication between radio stations of a radio communication system using at least one frequency band divided into a number of subbands, the type of division of the at least one frequency band into subbands varies according to the invention over time.

0012According to the use of a frequency band divided into a number of subbands, the radio communication system is a multicarrier system, the temporary division of the frequency band into only one subband being possible; the communicating radio stations can be, for example, mobile stations and base stations, the communication can concern both the upward and the downward direction. The method is preferably used in one or more radio cells of the radio communication system. The division of a frequency band into subbands is not constant, as is the case, for example, with a division prescribed by radio network planning, but is changed over time. The change in time usually does not take place continuously, but rather as a reaction to changed conditions within the radio cell. Variations in the division can take place both during communication between two radio stations, as well as after this has ended and before the start of further communication. Changes in the division into subcarriers can be decided and arranged in particular by a suitable network-side device.

0013In a development of the invention, the variation relates to the number of subbands of the frequency band and / or the frequency width of at least two subbands. In the case of equidistant subbands, a change in the number of subbands automatically also includes the frequency width of all subbands. However, it is also possible to change the frequency width of some or all of the subbands without changing the total number of subbands. In general, any changes to a division into sub-bands can be used.

0014Advantageously, filter functions used for data transmission on subbands can vary with time in addition to varying the type of division into subbands. A filter function is a filter in the frequency range or - obtainable by a Fourier transformation - in the time domain. The variation of filter functions affects at least one filter function used by one transmitter, but can also affect several filter functions used by several transmitters. Eg it is possible that all filter functions of subbands that are affected by a variation in the type of division are changed. A variation of a filter function means in particular an adaptation to the changed division of the frequency band; the bandwidth of a filter function can be varied by changing the sampling rate of a data stream. It is particularly cost-effective if the bandwidth of all filter functions and thus the overlap of neighboring subbands are varied to the same extent by the choice of the sampling rate of a data stream.

0015Furthermore, an inexpensive implementation consists in using the same filter function per subband.

0016It is possible that at least temporarily at least two filter functions used on subbands differ from one another. In this way, filter functions can be changed independently of one another, whereby an efficient use of the scarce radio resources in connection with a relatively low complexity of the filter implementation can be achieved by the selection of suitable filters.

0017In one embodiment of the invention, the type of division is varied and, if appropriate, the filter functions are varied depending on radio channel properties, such as effects caused by multipath propagation, and / or parameters of at least one radio station, such as the degree of mobility of a transmitting radio station, and / or the desired granularity of a data transmission. A high level of granularity is then desirable or required if a large number of radio stations want to communicate at the same time, so that a large number of subbands is advantageous. A high level of granularity requires a fine division of the available radio resources into subbands. The desired granularity is influenced, for example, by the number of communicating radio stations and properties of the data to be transmitted.

0018The above-mentioned object with regard to the method is further achieved by a method having the features of claim 7.

0019In the method for transmitting data between a transmitter and a receiver in a radio communication system, the data being transmitted on a number of subbands of a frequency band divided into subbands, the frequency width used for transmission by the transmitter varies from at least one subband of the number of subbands and if necessary additionally the number of subbands during the transmission of the data.

0020The transmission process of an amount of data between a transmitter and a receiver is considered here. During this transmission, the radio resources in the form of the subbands used for this purpose on the transmitter side are not constant over time. For example possible that a first bit is transmitted on a first subband with a first frequency width and a second bit, which is subsequently transmitted, on a second subband with a second frequency width, which differs from the first. The frequency position of the center of the second subband can coincide with that of the first subband or be shifted against this.

0021The above object is achieved with regard to the device in a radio communication system by a device with the features of claim 8.

0022The device according to the invention in a radio communication system has means for determining the division of at least one frequency band used for communication between radio stations into subbands such that the type of division of the frequency band into subbands varies with time.

0023The device is suitable for carrying out the method steps described above in accordance with the method for communication between radio stations and can have further means for this. The device can be structurally connected to other network-side devices, such as base stations or devices for radio access control. The arrangement is preferably determined by the device for exactly one or also for a plurality of radio cells of the radio communication system.

0024The above-mentioned object is achieved with regard to the device for sending data by a device with the features of claim 9.

0025The device for sending data in a radio communication system has means for sending data on a number of subbands of a frequency band divided into subbands. The means are designed such that the frequency width used by the device of at least one subband of the number of subbands and, if appropriate, additionally the number of subbands used varies with time. The variation can take place both during the sending of data to a recipient and between different sending of data.

0026The above-mentioned object is achieved with respect to the device for receiving data by a device having the features of claim 10.

0027The device for receiving data in a radio communication system has means for receiving data transmitted on an optionally time-varying number of subbands with at least partially frequency-varying frequency width of a frequency band divided into subbands.

0028The invention is explained in more detail below using an exemplary embodiment. Show<dl id="dl0001"><dt>Figure 1:</dt><dd>a section of a radio cell of a radio communication system,</dd><dt>Figure 2:</dt><dd>a frequency band divided into subbands,</dd><dt>Figure 3:</dt><dd>a transmitter-side arrangement for performing the method.</dd></dl>

0029FIG. 1 shows a radio cell of a radio communication system, which comprises a base station BS and two mobile stations MS1 and MS2. The base station BS is connected to a network-side device NE, which connection can also be established via a number of other devices. Furthermore, it is also possible for the network-side device NE to be part of the base station BS.

0030The data transmission by radio between the two mobile stations MS1 and MS2 and the base station BS will be considered below. For this purpose, a frequency band F is available in the radio cell, which is shown in FIG. 2. The frequency band can, for example, be the band for the upward direction of an FDMA system. This frequency band is divided into subbands. The use of narrow-band subbands has the advantage that these subbands have a lower frequency selectivity than the broad frequency band F. This frequency selectivity comes about through multipath propagation of signals between the transmitter and receiver. Despite this frequency selectivity over the entire frequency band F, with a suitable choice of the width of the subbands, it can be achieved that the individual transmission channels on the subbands are not frequency-selective, ie have a constant transfer function. The narrower a subband, the greater the probability that the transfer function of the subband is constant in the frequency domain. The effort required for equalization, which the receiver generally has to carry out due to the multipath propagation, can thus be significantly reduced by a suitable choice of the sub-bandwidths.

0031If the two mobile stations MS1 and MS2 move during the transmission of radio signals, there is a frequency shift due to the Doppler effect. This can have the consequence that different subbands - separated without the Doppler effect - overlap and thus interfere with the signals of these subbands (ICI, Inter Carrier Interference). As a result, the symbols transmitted on different subbands can no longer be clearly distinguished in the receiver or the reception of the symbols transmitted on the different sub-bands is disturbed by mutual interference. In order to reduce such interference due to the Doppler effect in moving mobile stations MS1 and MS2, it is advantageous to use subbands as broadband as possible.

0032According to the invention, the type of division of the frequency band F into subbands is varied over time. FIG. 2 shows three different ways of dividing the frequency band F into subbands. In the upper illustration in FIG. 2, the frequency band F is divided into four subbands SB1, SB2, SB3 and SB4 of the same width. These subbands SB1, SB2, SB3 and SB4 are assigned to the mobile stations MS1 and MS2 for communication with the base station BS. For example, the mobile station MS1 ​​can use the two subbands SB1 and SB2 for communication, and the mobile station MS2 the two subbands SB3 and SB4. A further division of the frequency band F into subbands is shown in the middle of FIG. 2. The frequency band F is divided into three subbands SB1, SB2 and SB3, which each have the same frequency width. In the lower representation in FIG. 2, the frequency band F is likewise divided into three subbands SB1, SB2 and SB3, which differ from one another in terms of their frequency width.

0033However, once the frequency band F has been divided into subbands, it is not constant, but is changed over time. Such a variation is arranged by the network-side device NE of FIG. 1, which has suitable means M1 for this. This change can take place depending on several influencing variables. Examples of this are channel properties, such as the maximum delay or the coherence bandwidth as a measure of the frequency selectivity of the channel, and the degree of mobility of the mobile stations. In the following, a frequency band with a bandwidth of 20 MHz at a frequency of 5.5 GHz will be considered as an example.

0034The first scenario represents a rural environment in which mobile phone subscriber speeds of up to 250 km / h can occur. In a rural environment, a mobile radio channel generally has a low delay due to multipath propagation of, for example, 2.5 μs, which corresponds to a coherence bandwidth of approximately 1.52 MHz. The maximum Doppler shift at the specified maximum speed is 1.27 kHz. In this rural scenario, the type of division of the 20 MHz frequency band can be determined such that there are only 16 subbands with a frequency spacing of 1.25 MHz. Despite the width of the subbands, the frequency selectivity is relatively low. On the other hand, the frequency band can also be divided into 128 equidistant subbands, in which case, in spite of the large number of subbands, the maximum Doppler shift has less than one percent of the subband spacing.

0035The second scenario is to consider an urban environment, whereby a maximum delay of 10 µs due to the strong multipath propagation can be assumed, as well as a maximum speed of the mobile radio subscriber, which is not higher than 60 km / h. The coherence bandwidth is therefore 380 kHz and the maximum Doppler shift is 305 Hz. A division of the frequency band into, for example, 64 subbands with a frequency spacing of 312 kHz is possible, or a division into 512 subbands, in which case the Doppler shift is again less than one percent of the frequency spacing of the 512 subbands.

0036Due to the variable division of a frequency band into subbands, a compromise can be found between the mutual interference of the individual subbands due to the Doppler effect and a temporal distortion due to the multipath propagation. This compromise is made possible by the fact that in the decision about the division of the frequency band into subbands the propagation conditions, ie the channel properties and the mobility scenario in the considered cell are included.

0037FIG. 3 shows an arrangement for sending data in accordance with a filter bank multi-carrier method which can be used to carry out the transmitter-side method steps of the method described above. The data a₁, a₂ etc. to aN are distributed to the individual subbands SB1, SB2 etc. to SBN, the division of the frequency band used into the subcarriers SB1, SB2 etc. to SBN not being constant over time. The data streams are sampled at the rate M, ie each bit of the data stream is sampled M times. Then a filter function H₁, H₂ etc. H_N applied, whereby this filter function for the individual subbands can differ from one another. For example, for a subband that is used by a fast moving mobile station, it is advantageous to use a filter function that is strongly localized in the frequency range in order to establish good spectral separation from the neighboring subbands. On the other hand, with regard to the subbands of slowly moving mobile stations, a strong spectral restriction of the filter function in the frequency domain means a loss of available bandwidth, and thus an inefficient use of the available radio resources. In addition, the narrower the filter function in the frequency domain, the more complex the processing of the signal at the receiver. Following the application of the filter function to the data stream, the data stream is modulated onto the respective subbands SB1, SB2 etc. to SBN. The summed signal is then transmitted to the receiver.

0038In the multi-carry method described, the sampling rate M can be set differently. If the sampling rate M exceeds the number of subbands N, the distance between the subbands is<maths id="math0001" num=""><math display="inline"><mrow><mfrac><mrow><mtext mathvariant="italic">M</mtext></mrow><mrow><mtext mathvariant="italic">N · T</mtext></mrow></mfrac></mrow></math><img file="EP1478148A1_D0001.tif" /></maths> , where T is the duration of a bit of data a₁, a₂ etc. to a_N represents. As a result, the actual utilization of the available bandwidth of the subbands depending on the channel properties and the subscriber mobility can additionally be improved or possibly reduced.

0039Are H₁, H₂ etc. to H_N If the same functions are used, the described method can be implemented in the event that the sampling rate corresponds to the number of subbands, using an exponential function as a filter function in the frequency domain or a rectangular function in the time domain as an OFDM method. Furthermore, when using any but the same filter functions H₁, H₂ etc. to H_N cost-effective implementations of the multicarrier transmission method according to the invention can be realized either by implementing only one filter and modulating all filtered data streams to be transmitted on different subbands onto different subcarriers, or by filtering using a single prototype filter shifted in the frequency domain. The change in the sampling rate M should then be the same for all subbands and accordingly has the same effect on all subbands. The use of a root raised cosine (rrcos) filter has proven to be particularly advantageous. This leads to a particularly efficient use of the frequency band.

0040A mobile station can be assigned both a single subband for communication and a plurality of subbands. This makes it possible for both simple mobile stations that can only communicate on a single subband and more complex mobile stations that can communicate on several subbands to communicate in a radio cell at the same time.