rt preamble Long preamble Data/payload Figure 5.8 An FBMC data packet with short and long preambles. 5 Filter Bank Multicarrier Modulation transmission. Clearly, an attempt to reduce the length of the preamble improves the band- width efficiency. Research in the above cited works has led to the following observations. (i) A preamble with a set of pilot symbols that are well separated in frequency is needed to assure detection and correction of large CFO values. (ii) A longer preamble that allows a more accurate estimate of CFO may be also necessary. On the other hand, one may real- ize that to obtain the CFO and symbol timing estimates, it is not necessary to send pilots on all the subcarriers. The number of parameters that characterize the channel response (determined by the length of the channel impulse response) is usually much smaller than the number of subcarriers. This means, a fraction of subcarriers, whose number is greater than or equal to the length of the channel impulse response, would suffice for pilot subcar- riers in the preamble. This, in turn, implies that one may propose to combine the short and long preambles into one preamble, and by taking such an approach, obviously, a shorter preamble will result. More recent research into channel estimation methods for FBMC has noted that the above mentioned works may be classified as frequency-domain approaches. They assume frequency flat gain over each subcarrier band, which may not be a good approximation in some cases. It has been noted that this problem may be resolved by adopting a time domain approach, e.g. see [27]. In these works, the channel is estimated by comparing the transmit- ted signal and received signal in the time domain. Extension of these methods to multiuser cases where multiple users transmit pilots simultaneously and base station (BS) estimates their channels simultaneously has also been made; see [28] and the references therein. This method obviously leads to high spectral efficiency since multiple users share the same time slots f