Look-up Table Predistortion Research Articles

Low-Complexity Multi-Symbol Multi-Modulus Weighted Lookup Table Predistortion in UWOC System

Lookup table (LUT) is an efficient digital predistortion technique for mitigating the pattern dependent distortion in the field of underwater wireless optical communication (UWOC) system. However, the complexity of LUT grows geometrically when applied with high-order QAM modulation formats with multi-pattern length sequences. The complexity reduction of LUT is essential for practical applications. In this paper, we study the patterning effects of LUT predistortion and propose to reduce the LUT complexity by pruning its table entries aided with multi-modulus weighted strategy. The size of LUT has been reduced significantly in experiments, which shows that the complexity of the proposed MMW-RLUT only accounts for 17.6%, 9.8%, 4.9% and 14% of full-size LUT for 64QAM, normal 32QAM, square geometrical shaping (SGS) 32QAM and 16QAM modulated UWOC system, without degradation of performance.

Digital Signal Processing for High‐Speed THz Communications

The use of advanced digital signal processing (DSP) technology in the high-speed terahertz (THz) communication can effectively compensate the linear and nonlinear effects of the system and further improve the transmission performance of the system. This paper introduces the principle and application of advanced DSP algorithms such as probability shaping (PS) technology, Volterra series nonlinear compensation algorithm, Kramers-Kronig receiver, look-up table (LUT) pre-distortion compensation technology, pre-equalization and decision-directed least-mean-square equalization algorithm. Combined with DSP algorithms such as PS and LUT pre-distortion, using photon-assisted technology to successfully realize the wireless transmission of vector THz signals higher than 1 Tbit/s on the sub-THz band (D-band) 4 × 4 multiple input multiple output system.

Hybrid Predistorter for Broadband Power Amplifiers Linearization With Relaxed DAC Speed in the Signal Transmit Path

This paper proposes a novel predistorter architecture suitable for hybrid baseband/RF implementation. The distortion function is split into a memory polynomial and an AM/AM only look-up table (LUT). The memory polynomial DPD is to be applied in the baseband digital domain whereas the LUT predistorter is suitable for low complexity implementation in either the baseband digital or the RF analog domains. The proposed model was compared to benchmark models using 20MHz and 40MHz wide 5G test signals, and found to be superior in terms of adjacent channel leakage ratio performance and complexity. Experimental results showed that the model can achieve ACLR levels of better than 50dBC with a relatively low number of coefficients. The proposed model requires around 20% less coefficients than the reverse twin-nonlinear two-box model, and achieves slightly better ACLR while easing the analog implementation of the static nonlinearity predistortion function. Most importantly, the low-complexity hybrid implementation of the proposed model can drastically reduce the speed requirements in the digital to analog converter of the signal transmit path by up to 80% when compared to traditional DPD systems.

Amplifier-less transmission of beyond 100-Gbit/s/λ signal for 40-km DCI-Edge with 10G-class O-band DML

In this article, we experimentally demonstrate the amplifier-less transmission of beyond 100-Gbit/s signal with advanced modulation formats including discrete multi-tone (DMT) and pulse amplitude modulation (PAM) in an intensity modulation direct detection (IM-DD) system for 40-km datacenter interconnects edge (DCI-Edge) utilizing a low cost commercial 10G-class O-band directly modulated laser (DML). During the transmission of DMT signal, Chow Cioffi Bingham algorithm-based bit-loading (CCB-BL) and discrete-Fourier-transform spread (DFT-spread) techniques are both discussed in the bandwidth-limited communication system. For the PAM single carrier communication system, look-up table (LUT) pre-distortion and time-domain pre-equalization schemes are jointly employed at the transmitter side to mitigate nonlinear distortions and insufficient bandwidth induced impairments, respectively. The results show that 105-Gbit/s DFT-spread DMT, 115-Gbit/s CCB-BL DMT, and 120-Gbit/s PAM-8 signals are successfully transmitted over 40-km single mode fiber (SMF) with the bit-error-rate (BER) below hard decision forward error correction (HD-FEC) threshold of 3.8 × 10–3. 120-Gbit/s DFT-spread DMT and CCB-BL DMT signals allocated within 20-GHz bandwidth can be successfully transmitted over 20-km SMF, and the receiver sensitivity of DFT-spread DMT outperforms CCB-BL DMT by 1.5-dB at the BER of 3.8 × 10–3. Due to the flexibility of bit-loading technique, when the allocated bandwidth of 120-Gbit/s CCB-BL DMT signal extends from 20-GHz to 25-GHz, the required received optical power (ROP) of CCB-BL DMT can be lower than DFT-spread DMT by 1-dB at the BER of 3.8 × 10–3 over 20-km SMF transmission. In 120-Gbit/s PAM-8 system, LUT pre-distortion technique improves the system receiver sensitivity by more than 1-dB over 40-km SMF transmission. This is the first time to discuss and implement different advanced modulation formats to achieve 100-Gbit/s/λ transmission in a 40-km amplifier-less IM-DD system with a low-cost O-band 10G class DML. Both CCB-BL DMT and LUT pre-distortion assisted PAM-8 can achieve a BER lower than HD-FEC threshold of 3.8 × 10–3 when the net rate per optical carrier is higher than 100-Gbit/s, which are promising solutions for 40-km four-lane 400-GbE DCI-Edge applications.

Spectrally efficient single carrier 400G optical signal transmission

In this paper, the recent progress on spectrally efficient single carrier (SC) 400G optical signal transmission was summarized. By using quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16QAM) and 64QAM, we can realize transmission distance over 10000, 6000 and 3000 km, respectively, with large area fiber and all-Raman amplification. To improve the system performance and generate high-order QAM, advanced digital signal processing algorithms such as probabilistic shaping and look-up table pre-distortion are employed to improve the transmission performance.

PAM-4 delivery based on pre-distortion and CMMA equalization in a ROF system at 40 GHz

In this paper, we proposed a PAM-4 delivery in a ROF system at 40-GHz. PAM-4 transmission data can be generated via look-up table (LUT) pre-distortion, then delivered over 25km single-mode fiber and 0.5m wireless link. At the receiver side, the received signal can be processed with cascaded multi-module algorithm (CMMA) equalization to improve the decision precision. Our measured results show that 10Gbaud PAM-4 transmission in a ROF system at 40-GHz can be achieved with BER of 1.6 × 10−3. To our knowledge, this is the first time to introduce LUT pre-distortion and CMMA equalization in a ROF system to improve signal performance.

High Symbol Rate Signal Generation and Detection With Linear and Nonlinear Signal Processing

Digital signal processing is the core functional block in beyond-100G transmission systems with a high symbol rate and/or high spectral efficiency. We review recent progress on high symbol rate signal generation and detection, and corresponding advanced algorithms for signal equalization and compensation at the transmitters (Tx) and the receivers (Rx), respectively. Thanks to the joint Tx-side optical pre-equalization (pre-EQ) and Rx-side partial response maximum likelihood sequence estimation, significant performance improvement can be achieved by reducing the penalty caused by bandwidth limitations of the optical and electronic components. A joint pre-EQ and look-up table predistortion technique is also reviewed in this paper, which is demonstrated to be an effective method for mitigating devices' nonlinearity impairment. We exemplify how these advanced algorithms enable high symbol rate beyond-100G transmission systems with signal generation based on either electronic time-division multiplexing or digital-to-analog converters.

Nonlinear Look-Up Table Predistortion and Chromatic Dispersion Precompensation for IM/DD PAM-4 Transmission

In this paper, we propose and experimentally demonstrate 10 GBaud intensity modulation direct detection pulse amplitude modulation 4 transmission over 48-km single mode fiber (SMF). Look-up table predistortion and chromatic dispersion precompensation are employed to mitigate channel bandwidth constraint and nonlinear impairment. At the receiver side, decision-directed least mean square and cascaded multimodule algorithm are employed to improve the decision precision. By combining these techniques, link budgets of 5.3 and 4 dB are achieved at bit error rate of 1 × 10−3 for the back-to-back case and after transmission over 48-km SMF, respectively.

Linear and Nonlinear Compensation for 8-QAM SC-400G Long-Haul Transmission Systems

Digital signal processing (DSP) with both linear equalization and nonlinear compensation for the single-carrier 400G system based on 84-GBaud polarization-division multiplexing 8QAM signals is studied in this paper. The 8QAM signals are generated and preprocessed with pre-equalization and lookup table predistortion at the transmitter (Tx)-side. The implementation principles of training-based adaptive equalization and predistortion are presented in this paper with experimental studies on the tap and memory length. At the receiver (Rx)-side, fiber-nonlinearity compensation based on digital backward propagation is also utilized to further improve the transmission performance. Thanks to the Tx-side and Rx-side DSPs, eight channels of 400G are successfully transmitted over 2125 km SSMF with EDFA-only amplification.

On the Robustness of Look-Up Table Digital Predistortion in the Presence of Loop Delay Error

Look-up table (LUT) predistortion is the most promising technique to linearize RF power amplifiers (PAs). Before it can be employed successfully, loop delay errors must be estimated and compensated accurately. In this paper, the effect of loop delay errors on LUT predistortion is analyzed. It is shown that LUTs fluctuations happen to both amplitude and phase tables. Then, a novel loop delay estimation algorithm, with a high degree of robustness and accuracy, is presented. To further improve the robustness of LUT predistortion to loop delay errors, the smoothing filter (SMF) method to polish LUTs fluctuations is proposed here. LUT predistortion can suffer bigger loop delay errors because of the proposed SMF method, but retains good linearization performance close to the optimal one. The performance of the proposed loop delay estimation algorithm is investigated by comparative study with a state-of-the-art algorithm. Its robustness is also demonstrated under a noisy feedback path by simulations. The effectiveness of the SMF method is assessed by both simulations and experiments. Comparison among different predistortion systems is also presented. The results clearly show that the proposed techniques are useful for LUT predistortion.

A novel distortion elimination method of power amplifier in wideband OFDM system

The nonlinear, memory and saturate distortion caused by power amplifier lead to low BER performance and low power efficiency in wideband OFDM system. To solve the related problems, a novel power amplifier based on combined amplification with multi power amplifiers is proposed in this paper. In addition, a novel simplified filter look-up table predistorter is proposed. The novel predistorter connects to a memoryless predistorter subsystem with an adaptive filter subsystem in cascade, and can eliminate the nonlinear memory distortion effectively. The simulation results indicate that the power efficiency and BER performance will be improved by using the novel amplifier structure together with the novel predistorter.

Efficient Lookup Table-Based Adaptive Baseband Predistortion Architecture for Memoryless Nonlinearity

Digital predistortion is an effective means to compensate for the nonlinear effects of a memoryless system. In case of a cellular transmitter, a digital baseband predistorter can mitigate the undesirable nonlinear effects along the signal chain, particularly the nonlinear impairments in the radiofrequency (RF) amplifiers. To be practically feasible, the implementation complexity of the predistorter must be minimized so that it becomes a cost-effective solution for the resource-limited wireless handset. This paper proposes optimizations that facilitate the design of a low-cost high-performance adaptive digital baseband predistorter for memoryless systems. A comparative performance analysis of the amplitude and power lookup table (LUT) indexing schemes is presented. An optimized low-complexity amplitude approximation and its hardware synthesis results are also studied. An efficient LUT predistorter training algorithm that combines the fast convergence speed of the normalized least mean squares (NLMSs) with a small hardware footprint is proposed. Results of fixed-point simulations based on the measured nonlinear characteristics of an RF amplifier are presented.

Optimal Spacing of a Linearly Interpolated Complex-Gain LUT Predistorter

Modern spectrally efficient modulation techniques produce radio-frequency signals with varying envelopes. Transmitting these signals through nonlinear amplifiers results in spectral regrowth and degradation of the bit error rate. Lookup table (LUT) predistorters have been widely used to mitigate the effects of nonlinear amplifiers. This paper theoretically proves that the use of linearly interpolated LUT predistorters results in superior rejection of residual nonlinear distortions, as compared with conventional noninterpolated LUT predistorters. The refinement of LUT performance with linear interpolation results in 12-dB additional rejection for every bit of precision, as opposed to 6 dB for noninterpolated LUTs. It is also shown that linearly interpolated LUT predistorters benefit more from optimal spacing than their noninterpolated counterparts. Besides improved mapping fidelity, potential benefits also include reduced gate count and faster convergence of adaptive LUT training algorithms.

Digital Baseband Predistortion Based Linearized Broadband Inverse Class-E Power Amplifier

A newly introduced inverse class-E power amplifier (PA) was designed, simulated, fabricated, and characterized. The PA operated at 2.26 GHz and delivered 20.4-dBm output power with peak drain efficiency (DE) of 65% and power gain of 12 dB. Broadband performance was achieved across a 300-MHz bandwidth with DE of better than 50% and 1-dB output-power flatness. The concept of enhanced injection predistortion with a capability to selectively suppress unwanted sub-frequency components and hence suitable for memory effects minimization is described coupled with a new technique that facilitates an accurate measurement of the phase of the third-order intermodulation (IM3) products. A robust iterative computational algorithm proposed in this paper dispenses with the need for manual tuning of amplitude and phase of the IM3 injected signals as commonly employed in the previous publications. The constructed inverse class-E PA was subjected to a nonconstant envelope 16 quadrature amplitude modulation signal and was linearized using combined lookup table (LUT) and enhanced injection technique from which superior properties from each technique can be simultaneously adopted. The proposed method resulted in 0.7% measured error vector magnitude (in rms) and 34-dB adjacent channel leakage power ratio improvement, which was 10 dB better than that achieved using the LUT predistortion alone.

An Improved Look-Up Table Predistortion Technique for HPA With Memory Effects in OFDM Systems

It is well known that HPAs (High Power Amplifiers) are inherently nonlinear devices and many researches have focused on the predistortion for memoryless HPAs. However, memory effects of HPAs can no longer be ignored when communication systems have wider bandwidth. Memoryless predistortion techniques proposed previously seldom have satisfactory effectiveness for typical wideband applications such as OFDM (Orthogonal Frequency Division Multiplexing) systems. In this paper, an improved adaptive predistortion method called 2D LUT (2-dimension look-up table) with different accuracy levels is presented to linearize HPAs with memory effects. Simulation and experimental results show that 2D LUT implements excellent performance in mitigating the signal deterioration caused by memory effects, both rectifies the signal constellation distortion and suppresses the spectrum emission. Large scale matrix computation is also avoidable in these adaptive algorithms, which makes them feasible when a real-time system is necessary.