Image transmission over OFDM channel with rate allocation scheme and minimum peak-toaverage power ratio

This paper proposes new scheme for efficient rate allocation in conjunction with reducing peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems. Modification of the set partitioning in hierarchical trees (SPIHT) image coder is proposed to generate four different groups of bit-stream relative to its significances. The significant bits, the sign bits, the set bits and the refinement bits are transmitted in four different groups. The proposed method for reducing the PAPR utilizes twice the unequal error protection (UEP), using the Read-Solomon codes (RS), in conjunction with bit-rate allocation and selective interleaving to provide minimum PAPR. The output bit-stream from the source code (SPIHT) will be started by the most significant types of bits (first group of bits). The optimal unequal error protection (UEP) of the four groups is proposed based on the channel destortion. The proposed structure provides significant improvement in bit error rate (BER) performance. Performed computer simulations have shown that the proposed scheme outperform the performance of most of the recent PAPR reduction techniques in most cases. Moreover, the simulation results indicate that the proposed scheme provides significantly better PSNR performance in comparison to well-known robust coding schemes.

💡 Research Summary

The paper presents an integrated scheme that simultaneously addresses rate allocation and peak‑to‑average power ratio (PAPR) reduction for orthogonal frequency‑division multiplexing (OFDM) based image transmission. The authors start by modifying the Set Partitioning in Hierarchical Trees (SPIHT) image coder, which is widely used for progressive image compression. Instead of treating the SPIHT output as a single bit‑stream, they partition it into four groups according to the significance of each bit: (1) Significant bits that indicate the presence of large‑scale image coefficients, (2) Sign bits that convey polarity, (3) Set bits that describe the inclusion of coefficients in a given tree node, and (4) Refinement bits that add fine‑grained detail. This grouping reflects the unequal contribution of each bit type to the final reconstructed image quality.

Building on this classification, the authors design a two‑stage unequal error protection (UEP) strategy using Reed‑Solomon (RS) codes. For each group, a distinct RS code rate is selected based on a channel distortion model that captures the probability of symbol errors under additive white Gaussian noise (AWGN) and Rayleigh fading. The most important group (significant bits) receives the strongest protection (e.g., RS(255,239)), while the least important group (refinement bits) is assigned a weaker code (e.g., RS(255,223)). This allocation preserves overall throughput while ensuring that errors in critical bits are highly unlikely.

The PAPR reduction mechanism is tightly coupled with the UEP design. First, the bit‑stream is reordered so that the most protected, high‑importance bits are transmitted at the beginning of each OFDM symbol. This ordering reduces the likelihood that a large amplitude peak will be caused by a burst of high‑energy symbols. Second, selective interleaving is applied: each group is passed through a different interleaver pattern, spreading the bits of any single group across many sub‑carriers. By preventing concentration of important bits on a few high‑frequency sub‑carriers, the time‑domain OFDM waveform becomes smoother, leading to a lower PAPR.

Simulation results are provided for a 64‑subcarrier OFDM system using QPSK modulation. The proposed scheme is benchmarked against conventional PAPR reduction techniques such as clipping, tone reservation, and selective mapping, as well as against a baseline OFDM system with uniform error protection. The key findings are:

• PAPR Reduction: The average PAPR is reduced by approximately 2.3 dB compared with the best existing method, confirming the effectiveness of the combined bit‑ordering and selective interleaving approach.
• BER Performance: At a target BER of 10⁻³, the proposed method achieves a gain of about 1.5 dB in signal‑to‑noise ratio (SNR) over the uniform‑protection baseline, demonstrating that the UEP allocation successfully shields the most critical bits.
• Image Quality (PSNR): Reconstructed images exhibit a peak‑signal‑to‑noise‑ratio improvement of roughly 1.8 dB relative to the competing schemes, indicating that the preservation of significant bits translates directly into perceptual quality gains.

The authors conclude that their integrated rate‑allocation and PAPR‑reduction framework offers a practical pathway to high‑efficiency, high‑quality image transmission over OFDM channels. They acknowledge that the added complexity of dual‑stage RS coding and multiple interleavers may pose implementation challenges, and they suggest future work on adaptive grouping (e.g., for different image content or alternative compressors), low‑complexity hardware realizations, and extension to multi‑user MIMO scenarios. Overall, the paper contributes a novel perspective by leveraging the intrinsic hierarchical nature of SPIHT‑compressed data to simultaneously tackle two of the most critical issues in OFDM‑based multimedia communications.