BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates schematically a cellular system;
FIG. 2 illustrates the arrangement of different types of channels in a prior art multi-carrier system;
FIG. 3 illustrates the arrangement of different types of channels in an inventive multi-carrier system;
FIG. 4 is a discrete Fourier transform spectrum of a digital signal;
FIG. 5 is a schematic drawing of a transmitter that may be used according to the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
An application of this is illustrated in FIG. 1, which shows, schematically, a cellular system comprising a number of cells. In each cell a base station is located, for communication with mobile terminals located within the cell. Each base station transmits synchronization channels for synchronization of the mobile terminals within its cell. A mobile terminal located in a particular cell will be able to receive synchronization channels from a number of base stations in adjacent cells.
FIG. 2 illustrates the arrangement of different types of channels in a prior art multi-carrier system. On the uppermost level there is a broadband physical resource, comprising the entire bandwidth in the system. This bandwidth is divided into resources for payload data and resources for broadcast information. If the system is cellular, the resources for broadcast information are in turn divided into resources for common information (if applicable) and resources for cell-specific information, i.e. information that is the same in the whole system.
FIG. 3 illustrates the arrangement of different types of channels in an inventive multi-carrier system: As in the prior art, the uppermost level is a broadband physical resource, comprising the entire bandwidth in the system. According to the invention a lower-rate physical resource is selected as a set of resources within the broadband physical resource. A comb decomposition unit is used to perform the selection of the set of resources. This lower-rate physical resource can in turn be divided into resources for payload data and resources for broadcast information. If the system is cellular, the resources for broadcast information can be divided into resources for cell- specific information and resources for common information, i.e. information that is the same in the whole system.
- The comb decomposition unit selects the set of resources by first selecting a number M of sub-carriers to be reserved, such that L=N/M is an integer and then allocating the set P of subcarriers P={n0+mL : m=0, 1, . . . ,M−1} for the specific information, which may be payload or broadcast information as described above. n0 is the offset of the lowest numbered subcarrier in P. Not all the subcarriers in P have to be used for the specific information. One or more of the subcarriers can be used for other information, as long as at least two subcarriers are used according to the invention. An alternative way of expressing P in this case becomes P={(n0+m*L) mod N 0≦m<M}.
The invention makes use of a situation as illustrated by the simplified example spectrum in FIG. 4 (only the absolute values of the frequency components are show, however, in practice they may be complex). In order to keep the size of the example manageable it is assumed that N=32 and M=8. In a communications system, of course, the values of N and M will be much higher. Thus, every fourth sub-carrier is included in the set P. The frequency bins of interest are indicated by circles instead of dots. In this example, sub-carriers i=1, 5, 9, 13, 17, 21, 25 and 29 are included in P, and are to be transmitted. An important feature of these frequency bins is that they are evenly distributed over the available frequency range. The frequency separation is L=N/M, which in this example equals 4. The distribution of the M frequency bins of interest is also characterized by a start position i between 0 and L, which in the example is equal to 1.
According to an embodiment of the invention, the sub-carriers comprised in P can be further divided into subsets Pk. The distance between the sub-carriers within a subset should be the same in each subset. For example, four subsets P0, P1, P2, P3 may be defined according to the following:
P0: subcarriers 1 and 17
P1: subcarriers 5 and 21
P2: subcarriers 9 and 25
P3: subcarriers 13 and 29
As can be seen, the sub-carriers within each subset Pk are also located at the same distance from each other.
In this example, if each of the base stations of FIG. 1 uses a different subset Pk of subcarriers for the synchronization signals the mobile terminal can select the synchronization signal from the appropriate base station and disregard the others. In this case, M is reduced to 2 and the spacing L=16.
According to the present invention, if more than one channel is used to cell-specific, these channels are arranged within the frequency range according to the following:
Assuming that the multi-carrier system has N sub-carriers, a set of M sub-carriers is reserved for potential use as carriers of cell-specific information in different cells. An offset n0 is defined, which effectively identifies the first channel in the set of reserved sub-carriers. The reserved channels are placed at equal distance from each other, the distance being L=N/M.
Thus, the M sub-carriers are selected so that they are spaced L sub-carriers apart and belong to the set P={(n0+m*L) mod N: m=0, 1, . . . ,M−1}. The number M of subcarriers reserved for the specific information and the carrier spacing L relate to the total number N of carriers as M*L=N. All sub-carriers in the set P does not necessarily carry specific information but they may do so.
Assuming that there are N=32 sub-carriers, numbered 0, . . . ,31, and that M=8 subcarriers have been reserved for potential use as carriers of cell-specific information in different cells. If n0=1, that is, sub-carrier number 1 is selected as the first reserved sub-carrier, then the set of reserved sub-carriers becomes P={1, 5, 9, 13, 17, 21, 25, 29}.
A subset of P may be selected, for example, P′{5, 9, 13, 17, 21, 25, 29}
Assuming that the number of subsets K=4 makes J=8/4=2. Then the set of reserved sub-carriers P={1, 5, 9, 13, 17, 21, 25, 29} can be divided into four disjoint subsets P0={1, 17}, P1={5, 21}, P2={9, 25} and P3={13, 29}. Each subset has the form {a+b*L*K} for an arbitrary offset a and for b=0, . . . ,J−1.
FIG. 5 is a schematic diagram of a transmitter according to the invention. is a block diagram of an exemplary embodiment of a transmitting end signal conversion appa- ratus in accordance with the present invention. This apparatus may, for example, be implemented in a mobile station of a multi-carrier communication system. However, in order to simplify the description, only elements necessary to explain the invention are shown in the figure. The samples to be sent over the channel are received in a serial/parallel converter 36 from an information source unit not shown in FIG. 5. The samples are buffered in serial/parallel converter 36, which serial/parallel converts M samples. The M samples are forwarded from the serial/parallel converter 36 to an M-point IFFT transformer 38. The resulting block of M samples x′(m) is forwarded to a calculating unit 40, which performs the transformation from x′(m) to x(n) in accordance with equation (4). Preferably the rotators are obtained from a lookup table 42. The output signals from calculating unit 40 are forwarded to parallel/serial converter 14, which transforms them into serial form for output to the channel. The operations performed in the IFFT transformer 38 and the calculating unit 40, using information from the lookup table 42, map the information onto the appropriate subcarriers, that is, subcarriers belonging to the set P, or subset Pk, respectively, as defined above.
Patent Application
20080045225
