Low Complexity Synchronization for Offset Tolerant DFT-Based BFSK Demodulator

Abstract

A DFT-based demodulator for BFSK is used for applications where the received signal experiences a carrier frequency offset (CFO) much larger than the data rate. It is particularly interesting for emerging ultra-narrowband communications for wireless sensor networks and IoT. The drawback is that the synchronization algorithm proposed for such a demodulator involves calculating the DFT for a window sliding over the whole preamble. This imposes a large computational load which is not desired in low power applications. To overcome CFO the sampling frequency should be larger than the signal bandwidth. Thus, the spectral range of the DFT is larger than the signal bandwidth. This means that only a subset of DFT bins have information about the signal; however, due to unknown CFO, the conventional synchronization algorithm needs all bins of the DFT. In this work, a novel synchronization algorithm is proposed which only needs a subset of DFT bins. Such algorithm can simplify implementation because efficient Single Bin Sliding DFT (SB-SDFT) algorithms can be used. Moreover, to be able to use the SB-SDFT algorithm, it is modified to incorporate zero-padding. The proposed algorithm and its implementation using the modified SB-SDFT reduce the number of complex multiplications, complex additions and memory usage by 28%, 64% and 81%, respectively, while achieving the same BER performance for the demodulator.

On the Low Complexity Implementation of the DFT-Based BFSK Demodulator for Ultra-Narrowband Communications

“…The main challenge of implementing this demodulator is the complexity of the synchronization algorithm which also increases when the tolerable frequency offset increases. In our previous work [12], a low complexity window synchronization algorithm was proposed and a Single Bin Sliding DFT (SB-SDFT) structure was introduced to efficiently implement the proposed algorithm.…”

“…This paper elaborates on our previous design, mathematical formulation and related literature while, additionally, contributes to extending and analyzing it. The stability of the SB-SDFT proposed in [12] is considered and a stabilized version (using a damping factor) is proposed. The complexity is analyzed when the new stabilized SB-SDFT is utilized for implementing the proposed algorithm.…”

“…The complexity is analyzed when the new stabilized SB-SDFT is utilized for implementing the proposed algorithm. In addition to the AWGN channel, which was considered in [12], in this work the BER performance in fading channel is also presented. Compared to our previous work, a more efficient implementation of the conventional synchronization algorithm is derived and used as the benchmark for complexity.…”

Ultra-narrowband communications : a feasibility study for the 2.4 gHz band

“…For example, typical crystal-based oscillators in the market have an accuracy of ±20 ppm [59], meaning that a simple and low-power UNB receiver has the overwhelming task of locating and decoding its 100 Hz designated signal at 868 MHz from a crowded bandwidth of 2 × 868 MHz × 20 ppm = 34 kHz [60,61]. This challenge has been addressed, e.g., recently in [62][63][64][65].…”

“…A cost-efficient channel emulator would be useful for further developing and testing the PHY of the UNB signaling scheme, such as carrier-and timing-synchronization algorithms [64], frequency-offset tolerant modulation/demodulation techniques [62,63], and interleaver-based channel-coding schemes [76]. Therefore, regarding the two UNB potential deployment scenarios in the 2.4 GHz band, the characteristics of interest of the corresponding propagation channels from the literature are summarized.…”