Method and apparatus for receiving data symbols in a wireless communication system

Technical Field

0001The present invention relates to a method and apparatus for receiving data symbols, and more particularly, to a method and apparatus for receiving a plurality of data symbols in a wireless communication system. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for more efficiently receiving the data symbols.

Background Art

0002An Orthogonal Frequency Division Multiplexing (OFDM) scheme or an Orthogonal Frequency Division Multiple Access (OFDMA) scheme is used for transmitting high speed data in wired and/or wireless channels. These schemes are actively being researched. In the OFDM scheme, frequency usage efficiency increases since this scheme employs a plurality of subcarriers having mutual orthogonality. In the transmitting end and the receiving end, a process of modulating and demodulating the plurality of subcarriers are similar to performing an Inverse Discrete Fourier Transform (IDFT) and a Discrete Fourier Transform (DFT), respectively. As such, an Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) are used to achieve high speed data communication.

0003The principle of OFDM scheme includes dividing a high speed data stream into a plurality of low speed data streams which are then transmitted via the plurality of subcarriers. By using the subcarriers to transmit the plurality of low speed data streams, symbol duration is increased which in turn works to reduce relative dispersion in a time domain based on multi-path delay spread. In the OFDM scheme, the data is transmitted in units of transmission symbols.

0004In an OFDMA physical (PHY) layer, active carriers are divided into groups, and each group is transmitted to different receiving ends. These groups of carriers are referred to as sub-channels. Each sub-channel comprised of carriers can be close in proximity with each other or spaced equally apart from each other. By permitting multi-access per sub[not] channel, although transmission of the carriers becomes more complex, frequency diversity gain, gain based on focusing the power, and efficient execution of omni-directional power control can be attained.

0005<figref idref="f0001">Figure 1</figref> illustrates a block diagram of transmitting/receiving ends using an OFDMA scheme in an uplink direction. First, a data stream is mapped using a modulation technique (e.g., Quadrature Phase Shift Keying, 16 Quadrature Amplitude Modulation) and then is converted into Nu number of parallel data using serial-to-parallel conversion. From total of Nc number of subcarriers, these symbols are mapped to Nu number of subcarriers while remaining subcarriers (Nc - Nu) are padded (e.g., zero padding). Thereafter, Nc-point IFFT is performed to the symbols.

0006In order to reduce inter-symbol interference, a cyclic prefix is added to the symbols and then transmitted after the symbols are converted using parallel-to-serial conversion. The operation of the receiving end is the same process of that of the transmitting end except in reverse order, A different user's data can be transmitted using an available subcarrier from unused subcarriers (e.g., Nc - Nu).

0007<figref idref="f0002 f0003">Figures 2a - 2c</figref> illustrate methods of mapping Nu number of subcarriers out of Nc total number of subcarriers. <figref idref="f0002">Figure 2a</figref> illustrates a random allocation of subcarriers, <figref idref="f0002">Figure 2b</figref> illustrates allocating the subcarriers by collecting the subcarriers in specified frequency bands, and <figref idref="f0003">Figure 2c</figref> illustrates allocating each subcarrier throughout the entire frequency bands in equal intervals.

0008Since the mapping methods illustrated in <figref idref="f0002 f0003">Figures 2a - 2c</figref> make use of the entire frequency bands, frequency diversity can be achieved. However, because each subcarrier is allocated individually, timing synchronization of OFDM symbol of different users can be off, and signal quality can suffer due to nearby subcarriers of different users if Doppler frequency is large. Furthermore, in the conventional OFDMA scheme, a single user uses a plurality of subcarriers and as a result, poor Pcak-to-Avcragc Power Ratio (PAPR) characteristics can appear and an expensive power amplifier is needed to resolve the poor PAPR problem.

0009In order to alleviate the poor PAPR characteristics, a DFT spread OFDMA scheme has been proposed. The DFT spread OFDMA scheme is a data symbol precoding method using DFT matrix. <figref idref="f0004">Figure 3</figref> is a block diagram illustrating transmitting/receiving ends using a DFT spread OFDMA scheme.

0010The difference between the DFT spread OFDMA scheme and the conventional OFDMA scheme is that in the DFT spread OFDMA, Nu number of data symbols are Nu-point DFTed. Thereafter, as illustrated in <figref idref="f0003">Figure 2c</figref>, the converted data symbols are mapped in equal intervals to the entire Nc number of subcarriers. In addition, although the PAPR can be drastically improved by using the DFT spread OFDMA, the function of the DFT spread OFDMA easily heats up due to an Inter Channel Interference (ICI).

0011In <patcit id="pcit0001" dnum="US20040190640A1"><text>US 2004/0190640 A1</text></patcit>, the available sub-carriers in an OFDM symbol window are divided into N groups of sub-carriers wherein each group will be associated with sub-bands. In one embodiment, the sub-carriers in a group are spread throughout the range of sub-carriers to improve frequency diversity, and the sub-carriers assigned to any one group are separated by a constant offset. The sub-carriers in the group may be offset by an integer power of two. Within each group, sub-bands are defined using frequency hopping patterns among sub-carriers in the group from one OFDM symbol window to another. A pseudo-random pattern may be employed for sub-carrier mapping from one OFDM symbol window to the next to effectively distribute the sub-bands across the selected band of sub-carriers for the group.

0012In <nplcit id="ncit0001" npl-type="s"><text>SAKAKURA et.al: "Pre-Diversity using Coding, Multi-carriers and Multi-antennas" of IEEE conference on universal personal communication of November 6, 1995, pages 605-609</text></nplcit>, a prediversity (diversity at transmitter) scheme using coding, multi-carriers and multi-antennas designed for fading channels is described. To avoid burst errors that frequently occur in the Rayleigh flat fading mobile channel, a time interleaving technique is utilized. This time interleaving technique cannot deal with slow fadings, and has a long processing delay. In that paper no time delay is required and a fixed BER characteristic is achieved not only for a slow fading but also for a fast fading channel. The proposed system with (7,4) block code, 7-carriers and 7-antennas achieves 15 dB improvement in a slow fading channel (Doppler frequency f/sub D/=10 Hz) compared with a time interleaving system. The pre-diversity technique is applicable to normal MPSK post-diversity system using trellis code.

Disclosure of the Invention

0013Accordingly, the present invention is directed to a method and apparatus as claimed that obviate or more problems due to limitations and disadvantages of the related art.

0014Aspects of the present invention are provided in the independent claims. Preferred embodiments are provided in the dependent claims.

0015Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

0016The invention is defined by the appended claims and its scope is determined only by the scope of said claims.

0017It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed in the appended claims.

Brief Description of the Drawings

0018The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings; <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> illustrates a block diagram of transmitting/receiving ends using an OFDMA scheme in an uplink direction;</li><li><figref idref="f0002 f0003">FIGs. 2a - 2c</figref> illustrate methods of mapping Nu number of subcarriers out of Nc total number of subcarriers;</li><li><figref idref="f0004">FIG. 3</figref> is a block diagram illustrating transmitting/receiving ends using a DFT spread OFDMA scheme;</li><li><figref idref="f0005">FIG. 4</figref> illustrates a subcarrier allocation method according an embodiment of the present invention; and <figref idref="f0005 f0006 f0007">FIGs. 5a - 5d</figref> illustrate subcarrier allocation methods according to other embodiments of the present invention.</li></ul>

Best Mode for Carrying Out the Invention

0019Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

0020Hereinafter, the descriptions of the embodiments will be made with respect to the OFDMA which is one of many a Multiple Carrier Modulation (MCM) scheme. However, that is merely an exemplary scheme and can be allocated using other types of modulation schemes.

0021To resolve the ICI problem as well as other problems in the DFT spread OFDMA system, the following embodiments are provided. As illustrated in <figref idref="f0001">Figure 1</figref>, a bit stream of a user (e.g., mobile station) is mapped by a constellation mapping scheme which is then converted by a serial-to-parallel converter. The descriptions provided hereafter with respect to the present invention relate to mapping the data symbols to subcarriers. In the present invention, the processed bit stream is not limited to a bit stream of a single user but can be multiplexed bit streams of more than one user (e.g., bit stream of user 1, bit stream of user 2, and bit stream of user 3). In addition, although the present invention is geared for downlink transmission, the present invention can also be applied to uplink transmissions.

0022<figref idref="f0005">Figure 4</figref> illustrates a subcarrier allocation method according an embodiment of the present invention. As illustrated in <figref idref="f0005">Figure 4</figref>, a number of data symbols are grouped into a plurality of groups, and these groups comprised of data symbols are allocated. More specifically, in order to achieve frequency diversity in an OFDMA system, Nu number of data symbols arc allocated across the entire frequency band having Nc number of subcarriers when Ns number of neighboring data symbols are formed into a group, making a total of Ng number of data symbol groups (i.e., Nu = Ng x Ns). That is, Nu number of data symbols is grouped based on proximity of data symbols (e.g., neighboring data symbols). Usually, the grouping occurs based on close proximity of the data symbols with respect to each other. The break up of Nu number of data symbols is based on Ng number of data symbol groups, which contains Ns number of elements or data symbols. For example, assuming Nu = 12, if four data symbol groups are formed (Ng = 4), then the data symbols that are in close proximity form a data symbol group of three data symbols (Ns = 3). This is depicted in <figref idref="f0005">Figure 4</figref>.

0023Thereafter, the data symbol groups are allocated to subcarrier groups. Here, each data symbol group is allocated to each subcarrier group, where each subcarrier group comprises a plurality of subcarriers. In allocating each data symbol group to each subcarrier group, an ICI should affect only the subcarriers located on the periphery of the data symbol group so as to express strong characteristics of the ICI. Preferably, the subcarrier groups are offset or spaced apart a certain distance between neighboring subcarrier groups. Furthermore, it is preferable to allocate groups by distributing the subcarrier groups across the entire frequency band.

0024<figref idref="f0005 f0006 f0007">Figures 5a - 5d</figref> illustrate subcarrier allocation methods according to other embodiments of the present invention. More specifically, these embodiments illustrate application of the DFT spread OFDMA scheme.

0025In <figref idref="f0005">Figure 5a</figref>, after being processed by the serial-to-parallel converter, Nu number of data symbols are precoded or spread by a precoding module. Here, the precoding module uses Nu-point DFT scheme. Thereafter, the output values (e.g., precoded data symbols) spaced apart at specified intervals (e.g., every fourth precoded data symbol) are joined to form a data symbol group. The grouping of precoded data symbols are performed until all of the precoded data symbols are placed in data symbol groups. Here, Nu number of precoded data symbols are grouped into Ng number of data symbol groups and each data symbol group consists of Ns number of precoded data symbols. Subsequently, these data symbol groups are allocated to subcarrier groups whose group formation corresponds to the data symbol groups. Each subcarrier group is formed by combining a certain number of subcarriers from Nc number of subcarriers. Here, the data symbols can be transmitted from more than one mobile station. For example, if Nu = 12 and Nc = 24 and the DFT output values are {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, these output values are grouped into data symbol groups (e.g., {1, 5, 9}, {2, 6, 10}, {3, 7, 11}, {4, 8, 12}) where there are four (4) data symbol groups, Ng = 4, and three prccodcd data symbols per each group, Ns = 3. In this example, the size of Ns is set to 3 data symbols. However, the size of Ns can vary and does not have to be fixed. In other words, each data symbol group can have different size of Ns. For example, the data groupshaving different number of data symbols per group can be assembled, such as {1, 4, 9}, {2, 10}, {3, 6, 8, 11}, {5, 7, 12}.

0026Once the formation of the data symbol groups are completed, each data symbol group (Ng) is allocated to respective subcarrier groups (e.g., {1, 2, 3}, {7, 8, 9}, {13, 14, 15}, {19, 20, 21} or {4, 5, 6}, {10, 11, 12}, {16, 17, 18}, {22, 23, 24}) which are formed by grouping Nc number of subacarriers. Here, the subcarriers of the subcarriers groups are localized. That is, the subcarriers of each subcarrier group are adjacent to each other or put differently, are neighboring subcarriers. However, grouping of subcarriers for the subcarrier group is not limited to grouping localized or neighboring subcarriers. The subcarrier groups can group subcarriers that are not close to each other. That is, the subcarriers of each subcarrier group can have various patterns. As such, non-localized subcarriers or subcarriers that are dispersed can be grouped to form each subcarrier group, (e.g., {1, 9, 17}, {3, 11, 19}, {5, 13, 21}, {7, 15, 23}).

0027In addition, the size of each subcarrier group which corresponds to the size of the data symbol group does not have to be fixed. As described above, each data symbol group size can vaiy. Accordingly, to correspond with the varying data symbol group size, the subcarrier group can vary as well. That is, each subcarrier group can be of different size (e.g., {1, 2}, {6, 7, 8, 9}, {14, 15, 16}, {19, 20, 21, 22}).

0028By combining the non-localized subcarriers with different subcarrier group size, it is also possible for the subcarrier group to have different size subcarrier groups, in which the subcarrier groups are dispersed and not localized. For example, the subcarrier groups can be {1, 7}, {4, 9, 15, 24}, {2, 10}, {5, 12, 21}.

0029As described above, the data symbol groups can have a fixed as well as a varying group size. In addition, the subcarrier groups can also be fixed and/or varying as well. This is true since the size of subcarrier groups correspond to the size of the data symbols.

0030Moreover, the subcarriers included in the subcarrier groups are either localized subcarriers or non-localized (dispersed) subcarriers. Here, the detailed description of the data symbol groups and the subcarrier groups is not limited to <figref idref="f0005 f0006 f0007">Figure 5</figref> a but can also be applied to the embodiments of <figref idref="f0006 f0007">Figures 5b - 5d</figref>.

0031In <figref idref="f0006">Figure 5b</figref>, after being processed by the scrial-to-parallcl converter, Nu number of data symbols are precoded (or spread) by a precoding module. Here, the precoding module uses Nu-point DFT scheme. Thereafter, Ns number of neighboring DFT output values are grouped, totaling Ng number of data symbol groups. Here, the neighbor DFT output values represent values that are close each other. Thereafter, the data symbol groups, consisting of the DFT output values (precoded data symbols), are allocated to subcarriers groups, each of which correspond to each data symbol group. Each subcarrier group is formed by combining a certain number of subcarriers from Nc number of subcarriers and has the same group formation as the data symbol group.

0032For example, if Nu = 12 and Nc = 24 and the DFT output values (i.e., precoded data symbols) are {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12}, these output values or data symbols are grouped into data symbol groups (e.g., {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12}) where there are four (4) data symbol groups, Ng = 4, and three data symbols per group, Ns = 3.

0033Once the formation of the data symbol groups are completed, each data symbol group is allocated to subcarrier groups (e.g., {1, 2, 3}, {7, 8, 9}, {13, 14, 15}, {19, 20, 21} or {4, 5, 6}, {10, 11, 12}, {16, 17, 18}, {22, 23, 24}) which are formed by grouping Nc number of subacarriers.

0034In <figref idref="f0006">Figure 5c</figref>, after being processed by the serial-to-parallel converter, Nu number of data symbols are spread (precoded) by Ng number of precoding or spreading modules.

0035Here, the precoding module uses Ng-point DFT spreading scheme. Thereafter, the precoded data symbols are outputted to different parts of the frequency band. For example, if there are four outputted values from a precoding module, each of these four outputted data symbols are spread apart so that they are evenly spaced apart across the frequency band. As illustrated in <figref idref="f0006">Figure 5c</figref>, there are Ng number of precoding modules, and each precoding module processes Ns number of data symbols. The outputted precoded data symbols from each precoding module are spaced apart in equal intervals. Thereafter, precoded data symbols from each precoding module spaced apart at specified intervals are grouped into data symbol groups to which subcarriers groups are allocated. Preferably, for example, each data symbol group combines only one outputted precoded data symbol from each precoding module. Each subcarrier group is formed by combining a certain number of subcarriers from Nc number of subcarriers.

0036For example, if Nu = 12 and Nc = 24 and a 4-point DFT output values are, in order, {1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}, the output values are grouped into four data symbol groups (e.g., {1, 5, 9}, {2, 6, 10}, {3, 7, 11 }, {4, 8, 12}) where Ng = 4 having three precoded data symbols in each data symbol group, Ns = 3. Once the formation of the data symbol groups arc completed, each data symbol group is allocated to subcarrier groups (e.g., {1, 2, 3}, {7, 8, 9}, {13, 14, 15}, {19, 20, 21} or {4, 5, 6}, {10, 11, 12}, {16, 17, 18}, {22, 23, 24}) which are formed by grouping Nc number of subacarriers.

0037In <figref idref="f0007">Figure 5d</figref>, after being processed by the serial-to-parallel converter, Nu number of data symbols are spread by Ng number of precoding modules. Here, the precoding module employs Ns-point DFT scheme. Thereafter, the precoded data symbols are outputted to different parts of the frequency band. As illustrated in <figref idref="f0007">Figure 5d</figref>, there are Ng number of precoding modules, each module processing Ns number of data symbols. Here, Ns number of outputted precoded data symbols, considered to be neighboring DFT output values, are grouped, totaling Ng number of data symbol groups. Here, the neighbor DFT output values represent values that are close each other. Thereafter, the data symbol groups are allocated to subcarrier groups. Each subcarrier group is formed by combining a certain number of subcarriers from Nc number of subcarriers. For example, if Nu = 12 and Nc = 24 and a 3-point DFT output values are, in order, {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12}, the output values are grouped into data symbol groups where Ng = 4 and Ns = 3. Once the formation of the data symbol groups are completed, each data symbol group is allocated to subcarrier groups (e.g., {1, 2, 3}, {7, 8, 9}, {13, 14, 15}, {19, 20, 21 } or {4, 5, 6}, {10, 11, 12}, {16, 17, 18}, {22, 23, 24}) which are formed by grouping Nc number of subacarriers.

0038In another embodiment of the present invention, an apparatus for allocating the data symbols can be found. The apparatus includes a subcarrier-to-symbol mapping module through which the data symbols are grouped and mapped to at least one subcarrier group. The apparatus further includes a transmitting module for transmitting the data symbols on the subcarriers of the at least one subcarrier group to a receiving end. Since the operations are same as described above with respect to <figref idref="f0005 f0006 f0007">Figures 5a - 5d</figref>, further discussions of the operations will be omitted. For details, refers to the descriptions of <figref idref="f0005 f0006 f0007">Figures 5a - 5d</figref>.