Abstract
A new method to reduce the computational complexity of the turbo decoding in ultrawideband (UWB) orthogonal frequency division multiplexing (OFDM) system is proposed. Existing stopping techniques for turbo decoding process using constrained decoding assume fixed signal-to-noise ratio (SNR) for all the OFDM symbol bits so they fail to yield an acceptable bit-error rate (BER) performance in multicarrier systems. In this paper, we propose a bit-level stopping technique for turbo decoding process based on the constrained decoding method. In this technique, we combine the cyclic redundancy check (CRC) with an adaptive threshold on the log likelihood ratio (LLR) on each subcarrier to detect for convergence. The threshold is adaptive in the sense that the threshold on the LLR of a bit is determined by the average SNR of the OFDM symbol and the channel gain of the transmission subcarrier. Results show that when the channel state information (CSI) is used to determine the threshold on LLR, the stopping technique can reduce the computational complexity by about 0.5–2.5 equivalent iterations compared to GENIE turbo without degradation in the BER performance.
Highlights
A leading candidate for fast data transfer is the ultrawideband (UWP), a wireless technology designed for short-range Personal area networks (PANs)
The main reason why the proposed method is more effective at low signal-to-noise ratio (SNR), that is, that the stopping technique saves more computations with no degradation in performance, is that the cyclic redundancy check (CRC) detection technique takes fewer decoding stages to decide that the decoded sequence is correct since there are more errors at low SNR, but the thresholding technique still can detect the bits sent over high SNR subcarriers and clamp these bits, satisfying the likelihood ratio (LLR) condition
We show that the existing early stopping constrained decoding methods for turbo decoder are not Average effective number of iterations appropriate in multicarrier systems with different fading coefficients for each subcarrier since they give equal weights for bits with variable SNR
Summary
A leading candidate for fast data transfer is the ultrawideband (UWP), a wireless technology designed for short-range Personal area networks (PANs). The main challenge in implementing turbo codes in the UWB systems is the consequent complexity consideration in spite of the high bit-error rate (BER) performance and savings in transmission power consumption as pointed out above. All previously proposed stopping techniques were based on single-carrier systems, where the signal-to-noise ratio is assumed to be constant for all frame bits. These approaches may not be directly implemented for turbo decoder in International Journal of Digital Multimedia Broadcasting multicarrier systems like UWB OFDM, since each bit (or group of bits) undergoes different signal-to-noise ratios (SNRs) due to the different fading parameters of different subcarriers.
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