Non-orthogonal Signal Transmission at 153 GHz

Non-orthogonal Signal Transmission at 153 GHz Yalin Zhou Department of Electronic and Electrical Engineering University College London Email: [email protected] Izzat Darwazeh Department of Electronic and Electrical Engineering University College London Email: [email protected] Abstract—We demonstrate, for the first time, a low-complexity spectrally efficient frequency division multiplexing (SEFDM) system at 153 GHz in the D-band, achieving 10 Gbps transmission with LDPC coding. Compared to orthogonal frequency division multi-plexing (OFDM), SEFDM improves the spectral efficiency by up to 50%gain with comparable complexity. I. INTRODUCTION Orthogonal frequency division multi-plexing (OFDM) remains the primary waveform in 5G systems due to its hardware maturity, standardization, and low implementation complexity. Looking ahead, 6G aims to support diverse applications over wider bandwidths and higher frequencies, potentially spanning 100 GHz to 3 THz [1]. While mmWave enables ultra-high data rates, spectrum availability is still limited. spectrally efficient frequency division multiplexing (SEFDM) [2], with its inherent bandwidth-saving properties, complements mmWave well—its benefits become even more pronounced at higher frequencies. The compressed bandwidth in SEFDM translates into savings in the order of GHz, offering substantial benefits for both data transmission and integrated communicationsensing applications. This poster presents the first experimental demonstration of SEFDM signal transmission in the D-band (110–170 GHz). Unlike previous experimental study [3], a low-complexity transceiver architecture is employed, relying solely on IFFT/FFT operations for modulation and demodulation. The system also incorporates standard low-density parity check (LDPC) coding to enhance error resilience. Compared to OFDM, SEFDM achieves significant bandwidth savings at the same data rate by introducing controlled inter-carrier interference through adjusting compression factor α. The resulting BER performance, measured across various αvalues, reveals a trade-off between spectral efficiency and error performance. Fig. 1: Experiment setup and DSP for signal generation and detection. II. EXPERIMENT SETUP Fig.1 shows the transceiver structure used for SEFDM signal generation and reception at 153 GHz. A pseudo-random bit sequence (length 218 −1) is encoded with LDPC (rate 1/2), mapped onto QPSK, and prepended with 15 ×1024 pilot symbols. After SEFDM IFFT modulation ,which was described in [4] and 256-point cyclic prefix (25%) is inserted. The digitally generated waveform is uploaded to a 60 GSa/s AWG (Keysight M8195A), producing a 10 ·αGHz bandwidth IF signal. A 12.75 GHz local oscillator is frequency-doubled to 25.5 GHz RF carrier, which is then applied at the input of a 6th-order harmonic upconversion moudle (VDI WR6.5CCU) to upconvert the IF signal to the D-band(center frequency of 153 GHz). A bandpass filter (135–150 GHz) isolates the lower sideband. At the receiver, downconversion is performed via harmonic mixing using a 22.9 GHz LO, followed by 50 GSa/s ADC sampling and further processing. Time and frequency synchronization are based on pilot correlation. After downsampling and CP removal, FFT demodulation, linear channel equalization and phase correction are performed using pilot-assisted estimation. No advanced detectors are used—the system relies solely on IFFT/FFT and LDPC decoding. III. RESULT AND DISCUSSION Fig. 2: Measured BER performance of QPSK SEFDM signals with varying compression factors α. Error floors are highlighted with markers and arrows. Corresponding received spectra after downconversion are shown, demonstrating the trade-off between spectral compression and BER performance. Fig. 2 shows the measured bit error rate (BER) performance of SEFDM systems using QPSK modulation under different compression factors α∈ {1,0.9,0.8,0.67}, without the application of FEC. These uncoded results highlight the raw signal quality and the intrinsic impact of spectral compression on system performance. As expected, when α= 1, the system behaves as a conventional orthogonal OFDM scheme and achieves the best BER performance. As αdecreases, the subcarriers become increasingly non-orthogonal, leading to significant inter-carrier interference (ICI). This degrades the BER, particularly at lower signal-to-noise ratios. For α= 0.8and α= 0.67, the BER curves exhibit error floors, indicating that the interference level becomes dominant and cannot be suppressed without error correction. Each BER curve is annotated with its corresponding received spectrum after downconversion. These spectrum plots demonstrate the bandwidth-saving effect of SEFDM. Lower αvalues result in compressed signal bandwidths, confirming the potential of SEFDM waveform for high spectral efficiency. However, this comes at the cost of increased ICI and degraded raw performance. It is worth noting that the system was also tested with LDPC channel coding (rate 1/2). Under the same experimental conditions and for the tested PRBS, no errors were observed after decoding, demonstrating that the LDPC coding employed effectively corrects the distortion introduced by compression. As a result, the coded BER curves are not included in the figure, as they remain below measurable error thresholds. This confirms that SEFDM, when combined with strong FEC, can achieve bandwidth compression without compromising reliability. IV. CONCLUSION This work demonstrates the feasibility of SEFDM transmission in the D-band using a low-complexity transceiver. Significant bandwidth savings are achieved without advanced detection, and error-free transmission is observed when combined with LDPC coding. SEFDM shows strong potential for future spectrally efficient mmWave systems. ACKNOWLEDGMENT This work is supported in part by the UK Engineering and Physical Sciences research Council-EPSRC funded project TRACCS under Grant EP/W026252/1, and in part by Horizon Europe project 6G MUSICAL under UKRI Grant 10093329. REFERENCES [1] Rappaport et al., “Wireless communications and applications above 100 ghz: Opportunities and challenges for 6g and beyond,” IEEE Access, vol. 7, pp. 78 729–78 757, 2019. [2] M. Rodrigues and I. Darwazeh, “A spectrally efficient frequency division multiplexing based communications system,” in Proc. 8th Int. OFDM Workshop, 2003, pp. 48–49. [3] H. Ghannam, D. Nopchinda, M. Gavell, H. Zirath, and I. Darwazeh, “Experimental demonstration of spectrally efficient frequency division multiplexing transmissions at e-band,” IEEE Transactions on Microwave Theory and Techniques, vol. 67, no. 5, pp. 1911–1923, 2019. [4] I. Darwazeh, H. Ghannam, and T. Xu, “The first 15 years of sefdm: A brief survey,” in 2018 11th International Symposium on Communication Systems, Networks Digital Signal Processing (CSNDSP), 2018, pp. 1–7.