A METHOD OF TRANSMITTING PILOT BITS IN A WIRELESS

COMMUNICATION SYSTEM

TECHNICAL FIELD

The present invention relates to a method of transmitting pilot bits, and more

particularly, to a method of transmitting pilot bits in a wireless communication system.

BACKGROUND ART

Figure 1 illustrates a block diagram of transmitter/receiver ends using an OFDMA

scheme in an uplink direction. First, data stream sent to users are digitally modulated using

modulation techniques such as Quadrature Phase Shift Keying and 16 Quadrature

Amplitude Modulation. After the modulation, a. constellation mapping is performed on the

modulated data streams which are then passed through serial-to-parallel converter and

converted into Nu number of parallel symbols. Here, Nu represents a number of subcarriers

allocated to a mobile station (MS). From a total of Nc number of subcarriers, these symbols

are mapped to Nu number of subcarriers while remaining subcarriers (Nc - Nu) are mapped

to zero, or put differently, the remaining subcarriers are padded (e.g., zero padding). Here,

Nc represents a total number of subcarriers before a cyclic prefix is added. That is, the

remaining Nc - Nu number of subcarriers are zero padded and then is applied Nc-point

Inverse Fast Fourier Transform (IFFT).

Furthermore, in order to reduce inter-symbol interference, a cyclic prefix is added to

the symbols and passed through a parallel-to-serial converter, which is then transmitted to

channels. The symbols transmitted via channels are mapped to subcarriers in amount of Np

+ Nc according to IFFT and the cyclic prefix.

As illustrated in Figure 1 , the procedures of an OFDMA receiver are same as that of

the transmitter except in reverser order.

Figures 2a - 2c illustrate methods of mapping Nu number of subcarriers out of Nc

total number of subcarriers according to conventional art. Figure 2a illustrates a random

allocation of subcarriers, Figure 2b illustrates allocating the subcarriers by collecting the

subcarriers in specified frequency bands, and Figure 2c illustrates allocating each subcarrier

throughout the entire frequency bands in equal intervals.

Since the mapping methods illustrated in Figures 2a - 2c 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, Peak-to- Average Power Ratio (PAPR) characteristics

can get worse.

The OFDMA signals in a time domain comprises a large number of subcarriers

modulated independently. Consequently, if these subcarriers are added on the same phase, a

problem of a large size signal being generated can occur. Even though average power

remains fixed at a certain level, a maximum power can increase drastically which in turn

increases the PAPR. If the PAPR is increased, Analog-to-Digital Conversion (ADC) and

Digital-to- Analog Conversion (DAC) becomes more complex while efficiency of a Radio

Frequency (RF) power amplifier is reduced. In order to resolve these types of PAPR heating

problem, several resolution encoding methods have been proposed including using signal

distortion or a special forward error correction symbol set (excluding OFDM symbol)

having a large PAPR.

In order to improve the PAPR characteristics, as illustrated in Figure 3, a DFT

spread OFDMA scheme has been proposed. Figure 3 is a block diagram illustrating

transmitting/receiving ends using a DFT spread OFDMA scheme.

The 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 Figure 2c, the converted data symbols are mapped

in equal intervals to the entire Nc number of subcarriers. Compared to the conventional

OFDMA, the DFT spread OFDMA has improved PAPR characteristics.

Because data in the DFT spread OFDMA system are allocated in the frequency

domain, it is necessary to perform channel estimation on the channels of the frequency

domain. In the conventional OFDMA system, data and pilot are simultaneously allocated in

the frequency domain, in the DFT spread OFDMA, the data is spread via the DFT matrix.

As such, if the pilot is simultaneously processed with the remaining data, the spreading

operation is also performed on the pilot, making channel estimation difficult.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention is directed to a method of transmitting pilot bits

in a wireless communication system that substantially obviates one or more problems due to

limitations and disadvantages of the related art.

An object of the present invention is to provide a method of allocating pilot bits in a

wireless communication system using a multiple carrier modulation (MCM).

Additional 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.

To achieve these objects and other advantages and in accordance with the purpose of

the invention, as embodied and broadly described herein, 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.

In another aspect of the present invention, 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 non-precoded pilot bits to a plurality of

subcarriers, and transmitting the allocated non-precoded pilot bits.

It 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The 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;

FIG. 1 illustrates a block diagram of transmitter/receiver ends using an OFDMA

scheme in an uplink direction;

FIG. 2a illustrates a random allocation of subcarriers;

FIG. 2b illustrates allocating the subcarriers by collecting the subcarriers in specified

frequency bands;

FIG. 2c illustrates allocating each subcarrier throughout the entire frequency bands

in equal intervals;

FIG. 3 is a block diagram illustrating transmitting/receiving ends using a DFT

spread OFDMA scheme;

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;

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

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

BEST MODE FOR CARRYING OUT THE INVENTION

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.

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.

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.'

Figure 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,

Figure 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 Figures 4 and 5.

hi 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.

As illustrated in Figure 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.

A more detailed description of Figure 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 are 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-P oint 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 are allocated to subcarriers.

Here, the allocation takes place in a frequency domain.

It is at this point the pilot bits 405 can be allocated as well. In this embodiment as

well as 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 precoded. 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.

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 othogonality

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.

At the receiving end, 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.

Figure 5 illustrates data symbols and independently (non-precoded) pilot bits

allocated to subcarriers. Data (D) of Figure 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,

Figure 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 Figure 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.

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 be located between

the subacarriers for the data symbols and/or can be assigned at specified intervals, as

illustrated in Figure 5.

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.

Further, as described above, Figure 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 Figures 4 and

5. In the second embodiment of the present invention, the pilot and the data are transmitted

separately. As illustrated in Figure 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 Figure 4, the DFT spreading is not performed on the pilot bits.

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.

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.

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 channels. 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. Figure 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.

Alternatively, Figure 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 short, 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.

It will be apparent to those skilled in the art 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.