Method of transmitting pilot bits in a wireless communication system

Abstract

A method of allocating pilot bits in a wireless communication system using a multiple carrier modulation (MCM) is disclosed. The method includes allocating a plurality of precoded data symbols precoded by a precoding matrix module and a plurality of non-precoded pilot bits to a plurality of subcarriers, and transmitting the allocated precoded data symbols and the allocated non-precoded pilot bits.

Claims

1. A method for a User Equipment (UE) operating in a wireless communication system, the method comprising: allocating a plurality of data symbols to a first plurality of subcarriers after applying transform precoding by the UE; transmitting, at a first point of time, a first plurality of pilot signals without the plurality of data symbols using a second plurality of subcarriers, wherein the first plurality of pilot signals are allocated to the second plurality of subcarriers without applying transform precoding by the UE, and the first plurality of pilot signals are allocated by a symbol-to-subcarrier mapping module to reserved locations where the plurality of data symbols are not allocated; transmitting, at a second point of time, the plurality of data symbols without the first plurality of pilot signals using the first plurality of subcarriers; and transmitting, at a third point of time, both the plurality of data symbols and a second plurality of pilot signals, wherein the second plurality of pilot signals are allocated to a third plurality of subcarriers without applying transform precoding by the UE, both the plurality of data symbols and the second plurality of pilot signals are allocated by the symbol-to-subcarrier mapping module, and the second plurality of pilot signals are allocated to locations that are not reserved in consideration of locations of the plurality of data symbols, wherein channel estimation for the plurality of data symbols is performed by a receiver based on the first plurality of pilot signals that are not transmitted with the plurality of data symbols.

2. The method according to claim 1, wherein applying the transform precoding comprises: using a Discrete Fourier Transform (DFT) scheme.

3. A User Equipment (UE) operating in a wireless communication system, the UE comprising: a Radio Frequency (RF) module; and a processor configured to: control the RF module; allocate a plurality of data symbols to a first plurality of subcarriers after applying transform precoding by the UE; transmit a first plurality of pilot signals without the plurality of data symbols using a second plurality of subcarriers, wherein the first plurality of pilot signals are allocated to the second plurality of subcarriers without applying transform precoding by the UE, and the first plurality of pilot signals are allocated by a symbol-to-subcarrier mapping module to reserved locations where the plurality of data symbols are not allocated; transmit the plurality of data symbols without the first plurality of pilot signals using the first plurality of subcarriers; and transmit both the plurality of data symbols and a second plurality of pilot signals, wherein the second plurality of pilot signals are allocated to a third plurality of subcarriers without applying transform precoding by the UE, both the plurality of data symbols and the second plurality of pilot signals are allocated by the symbol-to-subcarrier mapping module, and the second plurality of pilot signals are allocated to locations that are not reserved in consideration of locations of the plurality of data symbols, wherein channel estimation for the plurality of data symbols is performed by a receiver based on the first plurality of pilot signals that are not transmitted with the plurality of data symbols.

4. The UE according to claim 3, wherein the processor is further configured to apply the transform precoding using a Discrete Fourier Transform (DFT) scheme.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a pert 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;

(2) FIG. 1 illustrates a block diagram of transmitter/receiver ends using an OFDMA scheme in an uplink direction;

(3) FIG. 2a illustrates a random allocation of subcarriers;

(4) FIG. 2b illustrates allocating the subcarriers by collecting the subcarriers in specified frequency bands;

(5) FIG. 2 illustrates allocating each subcarrier throughout the entire frequency bands in equal intervals;

(6) FIG. 3 is a block diagram illustrating transmitting/receiving ends using a DFT spread OFDMA scheme;

(7) FIG. 4 illustrates a block diagram of a transmitter and a receiver applying a DFT spread OFDMA scheme according an embodiment of the present invention;

(8) FIG. 5 illustrates a pilot bit transmission method and formation of subcarriers associated with the method according to a first embodiment of the present invention; and

(9) FIG. 6 illustrates allocation of non-precoded pilot bits to subcarriers.

BEST MODE FOR CARRYING OUT THE INVENTION

(10) Reference 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.

(11) As discussed above with respect to conventional DFT spread OFDMA system, the bit stream data is spread by the DFT matrix and thereafter transmitted via subcarriers which are allocated to specific user(s) in equal intervals. In the transmission of data according to the conventional system, data symbol and pilot bit are transmitted after being spread together. In other words, the pilot bit and the data symbol are spread by the same method More specifically, the pilot bits is spread by the DFT matrix and mapped to subcarriers thereafter. At this time, the pilot bit is spread to the entire frequency domain having Nc number of subcarriers. That is, the pilot bit-mapped subcarriers are distributed across the entire frequency domain and therefore causes inconsistent power levels, making channel estimation difficult. To address this problem, the present invention proposes transmitting a pilot in a DFT spread OFDMA system.

(12) In the present invention, the term “symbol” can also be referred to as “bit,” “signal,” and a like. As such, a term ‘pilot bit’ can also be referred to as ‘pilot symbol’ or simply, ‘pilot.’

(13) FIG. 4 illustrates a block diagram of a transmitter and a receiver applying a DFT spread OFDMA scheme according an embodiment of the present invention. In addition, FIG. 5 illustrates a pilot bit transmission method and formation of subcarriers associated with the method according to a first embodiment of the present invention. Hereafter, the first embodiment will be described with respect to FIGS. 4 and 5.

(14) In the first embodiment, a pilot bit is not spread by the DFT matrix. Rather, the pilot bit is allocated to Nc number of subcarriers directly (without being processed by the DFT matrix) and then transmitted. Because the pilot bit is not spread by the DFT matrix, the pilot bits are allocated to a specified number of subcarriers from available Nc number of subcarriers. With this, the power level of the pilot signal on the frequency domain can better maintain power levels, making simpler channel estimation by the receiver.

(15) As illustrated in FIG. 4, the pilot bits is mapped to subcarriers without going through the DFT matrix spreading process. However, the data symbol is mapped to subcarriers after going through the DFT matrix spreading process.

(16) A more detailed description of FIG. 4 is as follows. Data, in bits, is transmitted from a transmitting end and is passed through a constellation mapping module 401. Here, the data can be associated with more than one source. Thereafter, the data symbols ace then passed through a serial-to-parallel (S/P) converter 402 in which the serially-processed data symbols are converted into parallel data symbols. The data symbols are then precoded or spread by a Nu-Point DFT Spreading module 403. Here, Nu refers to a number of data symbols that are precoded by a DFT matrix. After the data symbols have been spread, they are then processed by a symbol-to-subcarrier mapping module 404. In the symbol-to-subcarrier mapping module 404, the precoded data symbols am allocated to subcarriers. Here, the allocation takes place in a frequency domain.

(17) It is at this point the pilot bits 405 can be allocated as well. In this embodiment as well a other embodiments, as mentioned above, the pilot bits can be referred to as pilot symbols or pilots as well. As discussed above, unlike the data symbols, the pilot bits 405 are not preceded. As such, the pilot bits 405 can be allocated along with the precoded data symbols to the subcarriers at the symbol-to-subcarrier mapping module 404. Alternatively, the pilot bits 405 can be allocated after the data symbols have been allocated to the subcarriers. In such a case, the symbol-to-subcarrier mapping module 404 would allocate the data symbols and reserve certain subcarriers for the pilot bits 405. For example, the symbol-to-subcarrier mapping module can reserve every fifth subcarrier for allocating the pilot bits.

(18) After the precoded data symbols and the pilot bits are allocated in the frequency domain, these symbols are physically allocated in Nc-Point IDFT module 406. Here, Nc refers to a number of subcarriers which can be more than the number of the symbols. After the symbols are processed by the Nc-Point IDFT module 406, each symbol is added a cyclic prefix 407. The cyclic prefix 407 is a repeat of the end of the symbol at the beginning whose purpose is to allow multipath to settle before the main data arrives for orthogonality reasons. Thereafter, the cyclic prefix-added data and pilot bits are processed through a parallel-to-serial (P/S) converter 408 and then transmitted to a receiving end.

(19) At the receiving and, the processed symbols, as described above, can be decoded by a reverse process. For example, since the symbols were processed by the P/S converted when transmitted, the receiving end can apply the S/P converter 410, followed by a Nc-Point DFT module 411 to counter Nc-Point IDFT 406. Thereafter, the symbols can be processed through a subcarrier-to symbol mapping module 412 and omitting a few processes, ending with processing the symbols using a constellation demapping module 415. Thereafter, the symbols can be processed through a subcarrier-to symbol mapping module 412 and omitting a few processes, ending with processing the symbols using a constellation demapping module 415. After processing the symbols through this reverse process, then the symbols can be properly decoded.

(20) FIG. 5 illustrates data symbols and independently (non-precoded) pilot bits allocated to subcarriers. Data (D) of FIG. 5 corresponds to the data symbols after they are processed by a Nu Point DFT Spread module in which the data symbols are spread by the DFT matrix. Pilot (P) corresponds to pilot bits which are not processed through the Nu Point DFT Spread module but allocated independently into an Inverse DFT (IDFT). In short, FIG. 5 represents the spread (precoded) data symbols and the pilot bits, which have been not spread, allocated to subcarriers by a symbol-to-subcarrier mapping module. As illustrated in FIG. 5, Ps are allocated to the subcarriers on a specific frequency out of Nc number of available subcarriers. Through this, the pilot signals are transmitted from a frequency having a constant or reliable power level to make channel estimation by the receiver simpler.

(21) There are two means by which the pilot symbols/bits, which have not been precoded, can be allocated to subcarriers. First, when the precoded data symbols are allocated to subcarriers, certain subcarriers can be reserved or put differently, pre-allocated for allocation of the pilot bits. Therefore, when the pilot bits are allocated, they are allocated to the reserved subcarriers. For example, these reserved subcarriers can be located between the subcarriers for the data symbols and/or can be assigned at specified intervals, as illustrated in FIG. 5.

(22) Second, the non-precoded pilot bits and the precoded data symbols can be allocated at the same time by the symbol-to-subcarrier mapping module. Here, although the source of the symbols may be different, the allocation of these symbols can be performed by the symbol-to-subcarrier module, without the need for reserving certain subcarriers.

(23) Further, as described above, FIG. 4 illustrates a block diagram of a transmitter and a receiver applying a DFT spread OFDMA scheme according an embodiment of the present invention. Hereafter, the second embodiment will be described with respect to FIGS. 4 and 5. In the second embodiment of the present invention, the pilot and the data are transmitted separately. As illustrated in FIG. 5, the bit stream transmitted to a specified user includes symbols, each of which contain only the pilot or the data. In this second embodiment, as illustrated in FIG. 4, the DFT spreading is not performed on the pilot bits.

(24) The second embodiment has superior PAPR characteristics than those of the first embodiment. In the first embodiment, the pilot does not perform DFT spreading, and the pilot and data symbols are not separated. Rather, the data and the pilot are transmitted simultaneously in the first embodiment. As a result, an advantage of the DFT spread OFDMA scheme, which reduces the PAPR of the conventional OFDMA scheme, cannot be fully utilized.

(25) In the second embodiment also, the pilot is mapped to a specific subcarrier on the frequency domain and does not go through the DFT spreading procedure. Consequently, the PAPR characteristics can be deteriorated by the pilot bit being not spread by the DFT as opposed to being spread by the DFT. To combat this problem, as illustrated by second embodiment, if the symbols for the data and the symbols for the pilot are separated and then transmitted, the transmission of the pilot bit, like the transmission of the data symbol, can experience reduction in the PAPR.

(26) In the second embodiment, information related to channel estimation in certain parts of the frequency domain are not included since the symbol for the pilot and the symbol for the data are transmitted independently. Here, the pilot is allocated to a specific area of the frequency domain and not to the entire frequency domain, and in the channels of other frequency area (area where pilot is not allocated), interpolation is performed thereon to estimate the status of the channel. Furthermore, since the pilot is not transmitted to the entire frequency domain, the subcarrier, which is not used for transmitting the pilot, can be used to transmit other user data. FIG. 5 illustrates the pilots allocated to the subcarriers of specified frequencies. As described above, the non-pilot transmitting subcarriers can be used to transmit data. Furthermore, for channel estimation in frequency areas where the pilot is not transmitted, interpolation can be performed to make channel estimation.

(27) Alternatively, FIG. 6 illustrates allocation of non-precoded pilot bits to subcarriers. In the discussion above, the non-precoded pilot bits 601 are allocated separately from the precoded data symbols. It is possible that either only the non-precoded pilot bits 601 are allocated to subcarriers or only the precoded data symbols are allocated to subcarriers. That is, the pilot bits 601 can be allocated to subcarriers even in the absence of the data symbols. If the pilot bits 601 are the only symbols being allocated to the subcarriers, the actual mapping takes place at Nc-Point IDFT module 602. Here, a size (length) of IDFT can be different when the precoded data symbols are passed through compared to when the non-precoded pilot bits are passed through. In this embodiment, only the pilot signals are transmitted while the data signals are not transmitted. In abort, the size of the IDFT varies depending on whether the data symbols are passed through or the pilot bits are passed through. As mentioned above, a cyclic prefix 603 is added to each symbol before being transmitted. After being added, the symbol is processed through the P/S converter 604 and then transmitted to the receiving end.

(28) It will be apparent to those skilled in the at that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.