1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiple Access (OFDMA) system. In particular, to the present invention relates a method and apparatus for detecting a cell in an OFDMA system.
2. Description of the Related Art
Mobile communication systems have been evolving into a 4th generation (4G) mobile communication system providing super high-speed multimedia service, succeeding a 1st generation (1G) analog system, a 2nd generation (2G) digital system and a 3rd generation (3G) IMT-2000 system providing high-speed multimedia service. In the 4G mobile communication system, a subscriber can access all of a satellite network, a local area network (LAN), and an Internet Protocol (IP) network using one mobile station (MS). That is, the subscriber can enjoy voice, image, multimedia, Internet data, voice mail, and instant message (IM) services using one mobile station. The 4G mobile communication system supports a data rate of 20 Mbps for super high-speed multimedia service, and uses an orthogonal frequency like an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
The OFDM scheme, a digital modulation scheme for multiplexing a plurality of orthogonal carrier signals, divides a single data stream into several low-rate streams and simultaneously transmits the low-rate streams using several low-rate subcarriers. As a result, a symbol interval increases, causing a reduction in relative dispersion in a time domain due to multipath delay spread.
An Orthogonal Frequency Division Multiple Access (OFDMA) system transmits data per symbol. Interference occurs between the symbols, and in order to compensate for the intersymbol interference, the OFDMA system inserts a cyclic prefix (CP) which is longer than a transport channel, into the symbol.
FIG. 1 is a diagram illustrating a symbol structure in an OFDMA system. Referring to FIG. 1, oblique-lined regions correspond to the CP. A rear part of the symbol is copied and then attached to a front part for a given guide time Tg. Here, a time defined by excluding the CP from the symbol is denoted by Tb, and a time comprising the full symbol is denoted by Ts.
If the number of subcarriers used is denoted by N, a reception signal obtained after CP removing and fast Fourier transform (FFT) has the following relation.z(k)=H(k)s(k)+ω(k)   (1)
In Equation (1), s(k) denotes a reception signal in a frequency domain, H(k) denotes a value obtained by performing N-point Discrete Fourier Transform (DFT) on a time-domain channel response h[n], and ω(k) denotes an N-point DFT coefficient for a white Gaussian noise ω[n] and has a dispersion of N0. Herein, [n] and (k) are factors for representing a time-domain signal and a frequency-domain signal, respectively.
A mobile station is required to estimate a channel H(k) in order to demodulate a signal received from a base station (BS), and to this end, the base station inserts pilots in a downlink data packet before transmission. With the use of the pilots, the mobile station not only performs channel estimation but also estimates signal-to-interference and noise ratio (SINR) information useful for power control in a multiple access scheme and transmits the SINR information to the base station.
In a cellular system, a mobile station should detect a cell to which it belongs in order to initiate communication. The cell detection is achieved through unique pseudo-random noise (PN) codes used in each base station and a cross-correlation between mobile station's reception signals. In a Wideband Code Division Multiple Access (WCDMA) system, the cell detection is achieved through a PN code allocated to each cell and a cross-correlation between a primary synchronization channel (P-SCH), a secondary synchronization channel (S-SCH) and a common pilot channel (CPICH) at the beginning of communication. On the other hand, in the OFDMA system, a base station sends a PN code allocated thereto using a preamble inserted at the head of a data frame and a mobile station can detect a cell through cross-correlation. However, the cross-correlation needs N2 multiplications, causing an increase in the number of calculations.
For the frequency-domain data signal of Equation (1), the time-domain reception signal z(k) that a mobile station receives can be represented by the product of a channel frequency response and a frequency-domain transmission signal as expressed in Equation (2).z[n]=h[n]⊙Ns[n]+w[n]  (2)where ⊙N· denotes N-circular convolution, h[n] denotes a channel response in the time domain, and ω[n] denotes a white Gaussian noise in the time domain.
Based on the relation of Equation (1), equalization or channel estimation is achieved through N efficient divisions. For equalization, particular data s(k) is estimated by dividing z(k) by an estimated value {tilde over (H)}(k) for H(k). However, for cell detection, s(k) is limited to several particular PN codes and a PN code allocated to a corresponding cell among the PN codes should be detected without an estimated value for H(k). In this case where there is no condition for H(k), it is impossible to detect a cell. However, in the OFDMA system, it is possible to detect a cell on condition that a channel length L in the time domain is very smaller than the number N of OFDM symbol sampling. In general system implementation, however, although information on the channel length L should be given to a mobile station, it is difficult for the mobile station to acquire such information at the beginning of communication.