Source-linked AI summary
CP-Aware OFDM-Based OOK Signaling
Badr Eddine Ouakouak, Salah Eddine Zegrar, Hüseyin Arslan
TL;DR
OFDM-based OOK generation for low-power IoT must address CP-induced waveform distortion without assuming receiver-side CP knowledge. The paper designs OOK across the full CP-OFDM duration, accepts controlled orthogonality loss, and reports improved OOK detection with practical trade-offs.
Problem
CP insertion can create spurious edge transitions and mismatch in OFDM-based OOK, while receiver-side CP handling may require impractical synchronization assumptions.
Method
The paper generates the OOK waveform independently across the entire CP-OFDM symbol duration and minimizes CP mismatch through waveform design and block-level processing.
Results
The induced leakage remains bounded and practically controllable, while OOK detection improves and avoids the conventional method's clear error floor.
Takeaways & Limitations
CP-aware waveform design offers a promising direction for low-power IoT OOK systems, provided deployments account for relaxed orthogonality and processing trade-offs.
Abstract
from arXiv · showhide
This letter addresses the challenge of cyclic prefix (CP) problem in orthogonal frequency division multiplexing (OFDM)-based on-off keying (OOK) generation for low-power Internet of Things (IoT) systems. We propose a CP-aware waveform design that generates the OOK signal over the entire CP-OFDM symbol duration, potentially breaking subcarrier orthogonality after passing through the channel if the CP is not matched. To enable such a mechanism, OOK has to be generated independently and a modification to the existing OFDM-based OOK generation block is needed. Additionally, a block-wise time shift can be applied to minimize CP mismatch. Analytical discussions and simulations confirm that the resulting interference on legacy subcarriers remains controlled, while OOK detection performance significantly improves, albeit with trade-offs that should be carefully considered in practical deployments.
I. INTRODUCTION
OFDM-based OOK is motivated by low-power IoT communication, but existing waveform designs often overlook CP insertion, creating synchronization and edge-transition problems. The paper therefore proposes CP-aware waveform design at the transmitter, with bounded leakage and improved OOK detection.
- OOK suits extremely low-power IoT devices because of its simplicity and minimal power requirements.
- OFDM-based OOK can support spectral efficiency, coexistence with conventional data transmissions, and compatibility with existing infrastructure.
- Prior OFDM-compatible OOK strategies include power-level encoding, peak shaping, time-domain matching, and least-squares subcarrier optimization.
- Existing approaches often overlook CP insertion, while CP removal or mitigation may require accurate synchronization and can fail under sampling frequency offsets.
- The paper addresses CP mismatch during waveform design rather than relying on assumptions at OOK receivers.
- The proposed scheme extends OOK across the CP-OFDM symbol duration, analytically bounds leakage, and improves OOK detection in simulations.
II. SYSTEM MODEL
The system reserves subcarriers for a time-domain OOK waveform alongside conventional OFDM data and synthesizes that waveform through IDFT-based approximation. Least-squares processing provides the best approximation available from the selected subcarriers, with distortion when fewer than N subcarriers are used.
- W subcarriers generate the time-domain OOK waveform, while D = N − W subcarriers carry conventional OFDM data.
- The transmitted OOK sequence contains M binary chips per OFDM symbol and is sent in parallel with data for legacy user equipments.
- Each OOK chip is repeated c = N/M times, producing x_k = d_k ⊗ 1_c×1 before subcarrier-based synthesis.
- Least-squares coefficients yield the best W-subcarrier approximation of the desired OOK waveform.
- The selected W lowest-frequency subcarriers, including positive and negative frequencies, synthesize a real approximate OOK waveform through IDFT.
- When W = N reconstruction is exact; for W < N, the subcarrier-limited approximation incurs distortion.
A. CP-Handling Problem Description
CP insertion can disrupt OFDM-based OOK by creating mismatches and spurious edge transitions at symbol boundaries. These false transitions can alter the intended OOK sequence and undermine reliable detection.
- Standard OFDM prepends a CP of length L to the synthesized OOK waveform before transmission.
- CP insertion can create a false falling edge at the beginning of an OOK symbol.
- CP-induced discontinuities can alter the desired bit sequence and obstruct reliable OOK generation.
B. Proposed CP-Handling Method
The proposed method generates OOK across the full CP-OFDM symbol, independently appends its CP, and applies a block-wise time shift to reduce mismatch. This addresses CP-boundary inconsistency while accepting potentially bounded leakage to legacy data subcarriers.
- Orthogonality trade-off: The proposed construction can disrupt legacy-subcarrier orthogonality after a time-dispersive channel, but the resulting interference is intended to remain bounded.The trade-off is reduced CP mismatch in exchange for possible leakage over the data subcarriers.
- CP-aware waveform construction: The OOK waveform is generated across each CP-OFDM symbol, making the chip duration c = (N + L)/M samples.The block spans K OFDM symbols, with N + L samples per CP-OFDM symbol.
- CP-aware waveform construction: A valid CP waveform requires the first and last L samples to match, but arbitrary OOK sequences generally violate this CP condition.The mismatch is defined between the added CP and copied CP regions across all K symbols.
- Mismatch minimization: A circular time shift σ is selected by minimizing block-wide CP mismatch energy across the copied and added CP regions.The optimal shift is chosen through an optimization over the average mismatch energy.
- CP-aware waveform construction: The method independently generates and appends the OOK CP before superimposing the OOK waveform on the CP-OFDM data signal.This requires separate processing for the main and CP portions of the OOK waveform.
- Implementation cost: The scheme adds two OFDM-based OOK blocks, requiring two W-DFT processes and two N-IDFT operations instead of one each conventionally.The base station can handle this processing, but backward compatibility remains an open concern.
IV. PERFORMANCE ANALYSIS
The analysis links CP mismatch to interference on data subcarriers, showing that channel delays and CP size affect leakage while mismatch energy provides a channel-independent bound. Minimizing the mismatch measure Σ(σ) minimizes the corresponding interference upper bound.
- Interference analysis: The analysis relates CP mismatch size Σ(σ) to interference over the data subband and uses this relation to bound leakage.The resulting bound is designed to be independent of the channel’s specific delay values.
- Interference analysis: The interference vector φ is formed from the mismatch component after frequency-selective channel propagation and CP removal, while the matched CP component causes no interference.The interference is obtained through the DFT of the mismatch-related signal after channel processing and CP removal.
- Interference analysis: The norm of φ depends on the mismatch-related signal and channel delays, with the channel coefficients and path delays determining the interference across the block.Channel delays are random and generally outside transmitter and receiver control.
- Interference analysis: The CP size can further reduce leakage by absorbing channel delays, so larger effective CP protection improves the interference bound.The analysis identifies CP size through the CP-removal matrix as an additional factor affecting leakage.
- Interference analysis: Reducing Σ(σ) improves the interference bound, and the bound is minimized when the time-shift parameter equals σopt.The average upper bound per OFDM symbol is φup/K.
V. SIMULATION RESULTS
Simulations evaluate mismatch reduction, legacy OFDM-user impact, and OOK-receiver performance under the proposed CP-aware scheme. The scheme improves OOK detection while keeping interference bounded, with degradation depending on channel conditions and SNR.
- Mismatch reduction: Block-wise shifting significantly reduces CP mismatch for most OOK sequences, although some require no shift and others show little improvement.Mismatch depends strongly on the specific bit pattern, motivating representative worst-, moderate-to-poor-, and good-to-moderate-match sequences.
- OFDM data receivers: At SNR = 20 dB, the worst-case SINR drop is about 10 dB for a 16-tap channel, whereas leakage is quite tolerable for 4 taps.The evaluation compares the proposed scheme with a conventional orthogonal scheme and shows stronger degradation as channel taps increase.
- OFDM data receivers: Worst-case leakage does not exceed 1 dB at SNR = 4 dB, while the worst-case OFDM-user BER loss is around 0.5% at SNR = 20 dB.The interference is less evident at lower SNR values and is more pronounced for ideal rectangular OOK signals than for realistic smoother signals.
- OOK receivers: The proposed OOK receiver avoids the conventional scheme’s error floor across single-tap and 3-tap channels, with further BER gains from longer chip durations.Conventional generation suffers false-edge errors, while receiver-side CP removal improves BER but remains below the proposed system for longer chip durations.
VI. CONCLUSION
The letter presents a CP-aware OFDM-based OOK scheme that extends the waveform across the entire CP-OFDM symbol duration. Despite relaxed subcarrier orthogonality and required block-level modifications, analytical and simulation results show controlled interference and improved OOK detection, subject to practical trade-offs.
- Conclusion: The scheme eliminates CP distortion by extending the OOK waveform over the entire CP-OFDM symbol duration.This design addresses the cyclic-prefix constraint in OFDM-based OOK generation.
- Conclusion: The method relaxes subcarrier orthogonality and requires block-level modifications.These design changes accompany the CP-aware waveform construction.
- Conclusion: Analytical and simulation results show controlled interference and notably improved OOK detection, while practical adoption must account for inherent trade-offs.The conclusion frames the approach as a promising direction for low-power IoT systems.