Patent Application
20080303508
The present invention relates to an apparatus and method for estimating carrier frequency offset in a communication terminal operating in a communication system supporting Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiplexing Access (OFDMA), and more particularly, to a method of estimating carrier frequency offset in a communication terminal supporting DownLink (DL) Full Usage of SubChannel (FUSC) and DL Band-Adaptive Modulation and Coding (AMC) channel modes in a wireless communication system based on Institute of Electrical and Electronic Engineers (IEEE) 802.16d/e, Wireless Broadband (WiBro), and Worldwide interoperability for Microwave Access (WiMAX) standards, and a communication terminal performing the method.
A conventional wireless communication system requires estimating carrier frequency offset by a communication terminal so as to stably receive data. In the wireless communication system supporting one of IEEE 802.16d/e, WiBro, and WiMAX standards, a base station receives a synchronous signal from a global positioning system (GPS), and the communication terminal synchronizes with the base station. Variables existing at a transmission channel such as the rapid change of channel environments result in the inaccuracy of a carrier frequency. This influences an operation of an oscillator within the communication terminal, deteriorating reception performance of the communication terminal. Therefore, the communication terminal needs to estimate the carrier frequency offset, and compensate the carrier frequency offset according to the estimation result.
The present invention proposes a new method of estimating carrier frequency offset, for enhancing reception performance of a communication terminal using a pilot symbol of a received signal transmitted over a downlink channel for the estimation of the carrier frequency offset.
The present invention is directed to measuring carrier frequency offset for each frame in a communication terminal using a downlink pilot symbol, and compensating for a carrier frequency error of an oscillator using the measurement result, thereby preventing signal reception performance of the communication terminal from deteriorating due to the carrier frequency error.
The present invention is also directed to stably estimating carrier frequency offset in a communication terminal even in unexpected circumstances such as a rapid change of channel environments.
One aspect of the present invention provides an apparatus for estimating carrier frequency offset in a communication terminal operating in a communication system supporting Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiplexing Access (OFDMA). The apparatus includes: a phase difference calculator for calculating a phase difference between pilot symbols having the same linear phase among the pilot symbols of a received signal; a phase difference accumulator for accumulating the phase difference and generating a phase difference accumulation value; and a calculator for converting the phase difference accumulation value into a carrier frequency offset estimation value.
Another aspect of the present invention provides a method of estimating carrier frequency offset in a communication terminal operating in a communication system supporting Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiplexing Access (OFDMA). The method includes the steps of: calculating a phase difference between pilot symbols having the same linear phase among the pilot symbols of a received signal; accumulating the phase difference, and generating a phase difference accumulation value; and converting the phase difference accumulation value into a carrier frequency offset estimation value.
As described above, according to an apparatus and method for estimating carrier frequency offset in a communication terminal of the present invention, it is possible to measure the carrier frequency offset for each frame in a communication terminal using a downlink pilot symbol and compensate for a carrier frequency error of an oscillator using the measured result, thereby preventing reception performance from deteriorating due to the carrier frequency error.
According to the apparatus and method for estimating carrier frequency offset in the communication terminal, the carrier frequency offset can be stably estimated even in unexpected circumstances such as a rapid change of channel environments, thereby guaranteeing stable operation of the communication terminal.
In this specification, “Communication terminal” refers to a communication terminal supporting Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiplexing Access (OFDMA). Desirably, “communication terminal” refers to a communication terminal supporting DownLink (DL) Full Usage of SubChannel (FUSC) and DL Band-Adaptive Modulation and Coding (AMC) channel modes in a wireless communication system based on Institute of Electrical and Electronic Engineers (IEEE) 802.16d/e, Wireless Broadband (WiBro), Worldwide interoperability for Microwave Access (WiMAX) standards.
In this specification, “Wireless communication system” can represent a system based on any one of the IEEE 802.16d/e, WiBro, and WiMAX standards.
In this specification, “Symbol” refers to an OFDMA or OFDM symbol.
Hereinafter, an apparatus and method for estimating carrier frequency offset in a communication terminal operating in a communication system according to exemplary embodiments of the present invention, and a communication terminal performing the method will be described in detail with reference to the accompanying drawings.
As shown in
In a transmission stage, the serial/parallel converter receives and converts a serial data stream into parallel data streams corresponding to the number of sub-carriers. The IFFT unit performs inverse Fourier transform for each parallel data stream. The inverse Fourier transformed data is again converted into serial data, and is frequency-converted and transmitted. A reception stage receives a signal over a wire/wireless channel, demodulates the received signal inversely to the transmission stage, and outputs data.
The apparatus for estimating carrier frequency offset according to the present invention includes an FFT unit 201, a phase difference calculator 202, a phase difference accumulator 203, an arc-tangent calculator 204, a conversion calculator 205, and an averaging calculator 206. The FFT unit 201 can be excluded from the estimating apparatus according to need. In this case, the apparatus for estimating carrier frequency offset according to the present invention uses a signal undergoing a pre-processing process of performing FFT for a baseband received signal and performing transformation into a frequency domain.
The received signal whose time domain is transformed into the frequency domain using the FFT unit 201 of
The apparatus for estimating carrier frequency offset according to the present invention will be in detail described with reference to
The time domain of the baseband received signal transitions to the frequency domain through the FFT unit 201. The pilot symbol is extracted from the received signal Fourier transformed in the FFT unit 201, with reference to structure of a downlink channel. A predetermined pilot sequence can be correlated to a plurality of sub-carriers on the received signal that is an OFDM or OFDMA signal to extract the pilot symbol, and thus the pilot symbol can be acquired from a correlation value for the plurality of sub-carriers. In other words, the pilot symbol is predetermined in its transmission location depending on the downlink channel/channel mode in the communication system supporting the OFDM/OFDMA and thus, the pilot symbol can be extracted in such a manner that the predetermined pattern pilot sequence is correlated to the sub-carrier of the received signal.
The phase difference calculator 202 calculates a phase difference between the pilot symbols having the same location in the frequency domain. The reason for calculating the phase difference between the pilot symbols having the same location in the frequency domain is that the pilot symbols have the same linear phase. The calculation of the phase difference between the pilot symbols that are how distant from each other in the time domain (“inter-pilot-symbol distance” to be described later) can be different depending on the structure of the downlink channel, and variable depending on a range and a calculation quantity of the carrier frequency offset to be measured.
A basic principle of the carrier frequency offset estimating apparatus according to the present invention is to generate a carrier frequency offset estimation value using the phase difference between the pilot symbols. In the present invention, the interpilot-symbol distance for calculating the phase difference can be a matter of importance. An estimatable frequency band gets smaller as the inter-pilot-symbol distance for calculating the phase difference increases. The calculation quantity needed to acquire the phase difference gets greater as the inter-pilot-symbol distance for calculating the phase difference decreases, thereby increasing a system load. Accordingly, it is important to predetermine a suitable inter-pilot-symbol distance.
Referring to
PilotLocation=VariableSet#+6*(FUSC_SymbolNumber %2).
In the function, each parameter varies depending on an FFT size. A real value can refer to the IEEE 802.16d/e, WiBro, and WiMAX standards. The pilot symbol location of
Referring to
As described in detail with reference to
In other types of DL Band-AMC channel modes, there is a pattern suitable to a corresponding structure. A minimal interval for calculating the phase difference in the time domain is “2” in the DL FUSC channel mode. A minimal interval for calculating the phase difference in the time domain is “3” in the 2 bins by 3 symbols type DL Band-AMC channel mode.
In other words, the pilot symbol locations are different from each other within a period of a predetermined time domain in the downlink channel mode, and the pilot pattern is repeated in periods based on the time domain. Thus, it is desirable to use a periodicity of the repeated pattern to calculate the phase difference between the pilot symbols having the same position in the frequency domain. As described above, the inter-pilot-symbol distance in the time domain can be “2” in the DL FUSC channel mode. The inter-pilot-symbol distance can be “3” in the 2 bins plus 3 symbols type DL Band-AMC channel mode. In particular, the inter-pilot-symbol distance can vary depending on the type of the DL Band-AMC channel mode as described above. As such, the inter-pilot-symbol distance for calculating the phase difference can be flexibly set depending on an embodiment view, or the range of the carrier frequency offset to be measured. Thus, a value of “d” in Equation 1 can be decided as described above.
Each parameter of Equation 1 is defined as follows:
(1) j: index of pilot sub-carrier per symbol;
(2) n: symbol index within a DL zone;
(3) d: inter-pilot-symbol distance for calculating the phase difference between the two pilot symbols;
(4) fcurrent: carrier frequency offset estimation value measured at a current frame;
(5) fpre: carrier frequency offset estimation value averaged and calculated up to a pre previous frame;
(6) Gain: parameter for transitioning a phase value having a radian unit to a value having a frequency unit; and
(7) α: filter coefficient in the case of using a loop filter for averaging calculation.
The phase difference accumulator 203 accumulates and calculates the phase difference measured at each pilot symbol, and outputs a first phase difference accumulation value having a complex number unit to the arc-tangent calculator 204.
The arc-tangent calculator 204 converts the first phase difference accumulation value having the complex number unit into a second phase difference accumulation value having a radian unit. The arc-tangent calculation can, for example, use a LookUp Table (LUT) technique. In a case where the LUT technique is used, the arc-tangent calculation can be performed in such a manner that the second phase difference accumulation value having the radian unit corresponding to the first phase difference accumulation value having the complex number unit is recorded on an LUT, and the second phase difference accumulation value having the radian unit corresponding to the first phase difference accumulation value having the complex number unit is read with reference to the LUT, depending on the first phase difference accumulation value having the complex number unit that is input by the phase difference accumulator 203.
In another exemplary embodiment of the present invention, an arc-tangent calculator 204 can convert a first phase difference accumulation value having a complex number unit into a second phase difference accumulation value having a radian unit, using various arc-tangent calculation algorithms such as Cordic algorithm.
The conversion calculator 205 converts the second phase difference accumulation value having the radian unit, which is converted in the arc-tangent calculator 204, into the carrier frequency offset estimation value that is the value having the frequency unit.
In yet another exemplary embodiment of the present invention, the conversion calculator 205 can be omitted in a case where an apparatus for estimating carrier frequency offset in a communication terminal estimates carrier frequency offset using a second phase difference accumulation value having a radian unit, that is, in a case where an oscillator 207 of the communication terminal is controlled by the second phase difference accumulation value having the radian unit.
The communication terminal generates the carrier frequency offset estimation value through the FFT unit 201 to the conversion calculator 205. The carrier frequency offset estimation value is used as a criterion signal for controlling the oscillator 207 of the communication terminal.
In yet another exemplary embodiment of the present invention, the carrier frequency offset estimating apparatus can further include an averaging calculator 206 so that it can stably estimate the carrier frequency offset in the communication terminal. The averaging calculator 206 can average and calculate the carrier frequency offset estimation value measured for each frame, thereby stably estimating the carrier frequency offset even in a case where the carrier frequency offset measured by the communication terminal is inaccurate due to a rapid change in channel environments.
The averaging calculation executed by the averaging calculator 206 can use the loop filter. Alternately, the averaging calculation can use various algorithms including a method of averaging the carrier frequency offset estimation value that the communication terminal measures for a predetermined frame.
Referring to
The baseband received signal of the time domain is Fourier transformed, its time domain is transitioned to the frequency domain, and the pilot symbol is extracted from the Fourier transformed received signal (Step 701). As described above with reference to
Next, the phase difference between the pilot symbols having the same location in the frequency domain is calculated (Step 702). The reason for calculating the phase difference between the pilot symbols having the same location in the frequency domain is that the pilot symbols have the same linear phase. The inter-pilot-symbol distance for calculating the phase difference can be a matter of importance. The inter-pilot-symbol distance can be different depending on the structure of the downlink channel, and is variable depending on the range and the calculation quantity of the carrier frequency offset to be measured. A description of Step 702 is substituted with a detailed description of
The phase difference measured at each pilot symbol having the same location in the frequency domain is accumulated on a previous phase difference accumulation value (Step 703). Steps 702 and 703 can be repeated until they are performed for all the pilot symbols of the DL zone (Steps 704 and 705).
In Step 704, it is determined whether the calculation of the phase difference accumulation value for all the pilot symbols within the DL zone is completed. Then, the arc-tangent calculation is performed to convert the first phase difference accumulation value having the complex number unit into the second phase difference accumulation value having the radian unit (Step 706). A detailed description of the arc-tangent calculation of Step 706 is substituted with the description of
Step 707 can be omitted in a case where the carrier frequency offset is compensated using the second phase difference accumulation value having the radian unit, that is, in a case where the oscillator of the communication terminal is controlled by the second phase difference accumulation value having the radian unit in the estimating method according to another exemplary embodiment of the present invention aforementioned with reference to
The communication terminal generates the carrier frequency offset estimation value through Steps 701 to 707. The carrier frequency offset estimation value is used as the criterion signal for controlling the oscillator of the communication terminal (Step 708).
In yet another exemplary embodiment of the present invention, the method of estimating carrier frequency offset can further include the step of performing the averaging calculation to stably estimate the carrier frequency offset in the communication terminal. As described above, the averaging calculation enables the stable estimation of the carrier frequency offset even in a case where the carrier frequency offset measured by the communication terminal is inaccurate due to a rapid change in the channel environments.
The method of estimating carrier frequency offset by measuring the phase difference between the pilot symbols in the communication terminal according to the present invention can be realized in a program command form executable by various computer means, and can be recorded in a computer readable medium. The computer readable medium can include a program command, a data file, and a data structure singly or in combination. The program command recorded in the computer readable medium can be a command particularly designed for the present invention, or can be a command well known to and available by those skilled in the computer software art. The computer readable recording medium includes magnetic media such as a hard disc, a floppy disc, and a magnetic tape, optical media such as Compact Disc-Read Only Memory (CD-ROM) and Digital Versatile Disc (DVD), magneto-optical media such as a floptical disk, and a hardware device particularly constructed to store and execute the program command such as Read Only Memory (ROM), Random Access Memory (RAM), and flash memory, for example. The media can be transmission media such as light, a metal line, or a wave guide that include a carrier carrying a signal designating the program command and the data structure. The program command includes a machine language code compiled by a compiler as well as a high-level language code executable by a computer using an interpreter, for example. The hardware device can be constructed and operated as one or more software modules so as to execute the operation of the present invention, and vice versa.
The graph of the simulation result shown in
A graph 801 of
A graph 802 of
While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2005-0135948 | Dec 2005 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR06/05804 | 12/28/2006 | WO | 00 | 6/27/2008 |
