1. Field of the Invention
The present invention relates generally to a method and apparatus for detecting a frequency offset (or frequency difference) in a communication system, and in particular, to a method and apparatus for detecting a frequency offset in an Orthogonal Frequency Division Multiplexing (OFDM) system.
2. Description of the Related Art
Generally, systems using an OFDM scheme (hereinafter OFDM systems) include IEEE802.11a-based Wireless Local Area Network (WLAN), Terrestrial—Digital TV Broadcasting (Terrestrial—Digital Multimedia Broadcasting (T-DMB) and Digital Video Broadcasting—Handheld (DVB-H)), and IEEE802.16e/IEEE802.20-based Portable Internet systems. The OFDM system is an efficient system that can transmit high-speed data even in a poor multipath fading channel environment. However, one of the major drawbacks of the OFDM system is that the system is very susceptible to a Carrier Frequency Offset value (CFO) between a transmitter and a receiver. The CFO induces Inter-Carrier Interference (ICI), causing a decrease in Bit Error Rate (BER) performance of the system.
In order to solve the problem, Non Data-Aided and Data-Aided frequency detection techniques are used. The Non Data-Aided technique detects a CFO through a correlation between a Cyclic Prefix (CP) and an original sample of the CP. However, this technique is disadvantageous in that it suffers performance degradation due to fading and delay of the time-varying channels. Therefore, the CP correlation scheme is generally used in an Acquisition Mode-based system that does not require optimal accuracy.
Accordingly, a frequency offset detection method using Data-Aided pilots has been introduced. This method is classified into a Time Domain Training Block (or preamble) method and a Continuous Pilot Allocation (or frequency band allocation) method according to an allocation method of pilot signals. The former is disadvantageous to fast fading, and the latter is disadvantageous in terms of the frequency efficiency.
FIG. 1 illustrates a receiver 100 in the conventional DVB-H system. Referring to FIG. 1, the receiver 100 includes an Analog-to-Digital Converter (ADC) 101 for converting a received analog signal into a digital signal, and a Fine Carrier frequency offset Recovery block (FCR) 105 for generating a fine carrier frequency offset value depending on received mode information, coarse symbol timing, and start position information of an FFT window.
The receiver 100 further includes a function block 107 for estimating the mode information and symbol timing, an NCO mixer 103 for mixing an output of the FCR 105 with an estimated coarse carrier frequency offset value, a Fast Fourier Transform block (FFT) 109 for converting an output signal of the mixer 103 into a time-domain signal, and a Coarse Carrier frequency offset Recovery (CCR) block 113 for estimating a coarse carrier frequency depending on the signal received from the FFT 109.
The frequency synchronization method in the receiver (or terminal) 100 includes a pre-FFT process for compensating for a fine carrier frequency offset value and a post-FFT process for compensating for a coarse carrier frequency offset value before and after the FFT 109. With use of the mode information, coarse symbol timing, and FFT start position information obtained after mode detection and coarse Symbol Timing Recovery (STR), the receiver 110 estimates a fine carrier frequency offset value. Thereafter, the receiver 110 performs FFT, and performs a CCR process using the OFDM symbols output from the FFT and information on the known continual pilots.
A method for estimating a CFO in the conventional receiver 110 includes a confidence check scheme in the Continuous Pilot Allocation method. The confidence check scheme is a method for detecting a coarse frequency offset by comparing the values designated in a preset confidence check counter with the associated continual resulting values. If the predetermined values and the continual resulting values are not output, this method needs a new OFDM symbol to detect a coarse frequency offset, causing an increase in the time for detection of the coarse frequency offset. As a result, due to the failure of fast frequency synchronization, the receiver 100 suffers from ICI for a longer time, causing a reduction in the final BER performance. In addition, the logic for coarse frequency detection operates for a longer time, causing an increase in the dynamic power consumption.