There are two ways to transmit training symbols: preamble or pilot tones. Preambles entail sending a certain number of training symbols prior to the user data symbols. In the case of OFDM, one or two preamble OFDM symbols are typical. Pilot tones involve inserting a few known pilot symbols among the subcarriers.
Channel estimation in MIMO-OFDM systems can be performed in a variety of ways, but it is typical to use the preamble for synchronization6 and initial channel estimation and the pilot tones for tracking the time-varying channel in order to maintain accurate channel estimates.
In MIMO-OFDM, the received signal at each antenna is a superposition of the signals transmitted from the transmit antennas. Thus, the training signals for each transmit antenna need to be transmitted without interfering with one another in order to accurately estimate the channel. Figure shows three MIMO-OFDM patterns that avoid interfering with one another: independent, scattered, and orthogonal patterns.
The independent pattern transmits training signals from one antenna at a time while the other antennas are silent, thus guaranteeing orthogonality between each training signal in the time domain. Clearly, an channel can be estimated over training signal times. The scattered-pilot pattern prevents overlap of training signals in the frequency domain by transmitting each antenna’s pilot symbols on different subcarriers, while other antennas are silent on that subcarrier. Finally, the orthogonal pattern transmits training signals that are mathematically orthogonal, similar to CDMA.
The independent pattern is often the most appropriate for MIMOOFDM, since the preamble is usually generated the in time domain. For transmitting the pilot tones, any of these methods or some combination of them can be used. In MIMO-OFDM, frequency-domain channel information is required in order to detect the data symbols on each subcarrier .
Since the preamble consists of pilot symbols on many of the subcarriers the channel-frequency response of each subcarrier can be reliably estimated from preamble with simple interpolation techniques. In normal data OFDM symbols, there are typically a very small number of pilot tones, so interpolation between these estimated subchannels is required.
The training-symbol structure for the preamble and pilot tones is shown in Figure , with interpolation for pilot symbols. One-dimensional interpolation over either the time or frequency domain or two-dimensional interpolation over both the time and frequency domains can be performed with an assortment of well-known interpolation algorithms, such as linear and FFT.