The present invention relates generally to broadband transmission of information and, more particularly, to time synchronization of a received OFDM signal.
Orthogonal Frequency Division Multiplexing (OFDM) is a spread spectrum technology wherein the available bandwidth is subdivided into a number of discrete channels or subcarriers that are overlapping and orthogonal to each other. Each channel has a well defined frequency. Data are transmitted in the form of symbols that encompass some number of subcarrier frequencies. The amplitude and/or phase of the subcarriers represents the encoded information. Before a received OFDM symbol can be decoded, frequency and time synchronization of the signal have to be determined. This process is referred to as synchronization.
Time synchronization may be achieved through the use of time-based correlation in the time domain or phase correlation in the frequency domain. Phase correlation has several advantages over time-based correlation. Phase correlation is more robust as it is significantly less affected by amplitude variations than time-based correlation. Also, symbols can be synchronized in the presence of a much stronger jammer by correlation in the frequency domain.
OFDM symbols can be synchronized in the frequency domain by correlating the phases of the carriers with the phases of a reference symbol. Alternatively, or in addition, autocorrelation can be performed by correlating the phases of a received symbol with those of an earlier received symbol.
Time synchronization of the received OFDM signals is more difficult to achieve, in particular, in the presence of narrow band interference in the OFDM channel. Apodizing window functions, such as Hanning windows, are typically used to deal with the effects of a narrowband interferer, such as a jammer signal, in samples that are passed to a phase correlator. While the apodizing window confines interference from the narrowband interferer to only several of the OFDM subcarriers, it introduces intercarrier interference into the subcarrier frequencies. The intercarrier interference can result in destructive canceling (i.e., frequency nulls") in transmitting subcarriers of equal amplitude and phase. In the presence of a frequency null, a phase correlator may miss a peak correlation pulse and thus provide unreliable synchronization. The intercarrier interference can also cause phase and amplitude distortion in each of the transmitted subcarriers processed by the apodizing window. Phase distortion in sample phases, as well as in the autocorrelation or reference phases with which they are compared, can result in poor phase correlation. Even using a best possible set of reference phases (typically, nonwindowed referenced phases), in conjunction with Hanning windowed phases reduces correlation by some factor. That is, it takes a much stronger signal to perform correlation than would be needed if a Hanning window was not used.