A pseudorandom postfix OFDM modulator - semi-blind channel estimation and equalization

Abstract

Abstract-This paper details a new orthogonal-frequency-division-multiplexing (OFDM) modulator based on the use of a pseudorandom postfix (PRP)-OFDM and discusses low-complexity equalization and channel estimation/tracking architectures. The main property of this new modulation scheme is the ability to estimate and track the channel variations semi-blindly using order-one statistics of the received signal. Compared with known cyclic prefix OFDM (CP-OFDM) pilot-symbol-assisted modulation (PSAM) schemes, the pilot overhead is avoided: The channel estimation is performed based on the exploitation of pseudorandomly weighted postfix sequences replacing the guard interval contents of CP-OFDM. PRP-OFDM is shown to be of advantage if the target application requires 1) a minimum pilot overhead, 2) low-complexity channel tracking (e.g., high mobility context), and 3) adjustable receiver complexity/performance trade-offs (available due to the similarities of PRP-OFDM to the zero-padded OFDM (ZP-OFDM) modulation scheme) without requiring any feedback loop to the transmitter.

Channel Estimation for Wireless OFDM Communications

“…Although the mobile communication applications mentioned above are all based on cyclicprefix OFDM (CP-OFDM) modulation techniques, several encouraging contributions have investigated some alternatives, e.g., zero-padded OFDM (ZP-OFDM) (Muquest et al, 2002;Muquet et al, 2000) and pseudo-random-postfix OFDM (PRP-OFDM) (Muck et al, 2006;2003) to replace the TD redundancy with null samples or known/pre-determined sequences. It has been found that significant improvements over CP-OFDM can be realized with either ZP-OFDM or PRP-OFDM (Muquest et al, 2002;Muquet et al, 2000;Muck et al, 2006;2003).…”

“…It has been found that significant improvements over CP-OFDM can be realized with either ZP-OFDM or PRP-OFDM (Muquest et al, 2002;Muquet et al, 2000;Muck et al, 2006;2003). In previous works, ZP-OFDM has been shown to maintain symbol recovery irrespective of null locations on a multipath channel (Muquest et al, 2002;Muquet et al, 2000).…”

“…Meanwhile, PRP-OFDM replaces the null samples originally inserted between any two OFDM blocks in ZP-OFDM by a known sequence. Thus, the receiver can use the a priori knowledge of a fraction of transmitted blocks to accurately estimate the CIR and effectively reduce the loss of transmission rate with frequent, periodic training sequences (Muck et al, 2006;2003). A more recent OFDM variant, called Time-Domain Synchronous OFDM (TDS-OFDM) was investigated in terrestrial broadcasting applications (Gui et al, 2009;Yang et al, 2008;Zheng & Sun, 2008;Liu & Zhang, 2007;Song et al, 2005).…”

“…TDS-OFDM works similarly to the PRP-OFDM and also belongs to this category of CEs assisted by TD redundancy. Several research efforts that address various PRP-OFDM CE and/or subsequent equalization problems have been undertaken (Muck et al, 2006;2003;Ma et al, 2006). However, these studies were performed only in the context of a wireless local area network (WLAN), in which multipath fading and Doppler effects are not as severe as in mobile communication.…”

“…However, these studies were performed only in the context of a wireless local area network (WLAN), in which multipath fading and Doppler effects are not as severe as in mobile communication. In addition, the techniques studied in previous works (Muck et al, 2006;2003;Ma et al, 2006) take advantage of a time-averaging method to replace statistical expectation operations and to suppress various kinds of interference, including inter-block interference (IBI) and ISI. However, these moving-average-based interference suppression methods investigated in the previous studies (Muck et al, 2006;2003;Ma et al, 2006) cannot function in the mobile environment because of rapid channel variation and real-time requirements.…”