Pilot tone configurations for PAPR reduction in OFDM systems.

摘要

Orthogonal Frequency Division Multiplexing (OFDM) has emerged as an efficient multicarrier modulation scheme for wireless communication channels subjected to harsh multipath conditions. Ease of implementation, high spectral efficiency, resilience to impulse noise are a few advantages of OFDM systems. However, in order to use these systems even more effectively, the high peak-to-average-power ratio (PAPR) inherent in OFDM signals has to be curtailed further than what is achievable with existing methods. In particular, high PAPR limits the efficiency of the non-linear power amplifiers specific to wireless communications by forcing them to operate at lower average power.; In this thesis, we introduce a novel PAPR reduction methodology in OFDM systems. The proposed techniques deploy the pilot tones for PAPR reduction in configurations that are unique to this thesis. Conventionally, the pilot tones are used for channel estimation, frequency synchronization and carrier phase tracking. In this thesis, in addition to maintaining their conventional functions, pilot tones, namely their signaling points and positions, are carefully chosen to "balance" the envelope peaks in the time domain OFDM frame. Significant reductions in PAPR are achieved in the proposed schemes at the expense of increased computational complexity of the OFDM systems.; By augmenting the features of pilot tones, two unique approaches to reduce PAPR are proposed with: (a) pilot tones at pre-determined positions; and (b) pilot tones at, pseudo-random positions. The common feature of both methods is that the pi lot symbol on a given subcarrier is allowed to come from multiple signaling point representations. In the first approach, the traditional pilot position structure is preserved, i.e., the positions of pilot subcarriers are known at the receiver. In the second approach, the receiver no longer has the pilot tone index information and estimates it based on the increased power of pilot subcarriers. In the proposed methodology, the PAPRs for alternative pilot symbol configurations are calculated, and the OFDM frame that results in the lowest PAPR is transmitted. When reducing the PAPR, the use of pilot tones at pseudo-random positions adds additional degree of freedom and has potential of reducing the PAPR more than the approach with the deterministic location of subcarriers. However it requires (i) power shaping on pilot tones at the transmitter and (ii) more complex receivers.