Maximum Achievable Gain Research Articles

A 201- and 283-GHz Dual-Band Amplifier in 65-Nm CMOS Adopting Dual-Frequency $G_{\max }$-Core With Dual-Band Matching

This work reports a concurrent dual-band amplifier with an extensive spacing between the two bands by adopting a proposed dual-frequency maximum achievable gain ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$G_{max}$</tex-math></inline-formula> ) core with a dual-band matching. The proposed dual-frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$G_{max}$</tex-math></inline-formula> -core can expand the difference between the two target frequencies by focusing on satisfying dominant gain-boosting condition and adopting a linear, lossy, and reciprocal based design approach. Implemented in a 65 nm CMOS process, a 5-stage dual-band amplifier shows a peak power gain of 23.6 and 13.7 dB, 3 dB bandwidth of 5 and 17 GHz, saturated output power ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P_{sat}$</tex-math></inline-formula> ) of -1.2 and -2.2 dBm, and peak power added efficiency (PAE) of 2.1 and 1.5 % at 201 and 283 GHz, respectively, while consuming a DC power of 34.5 mW. The proposed amplifier is the first demonstration of the concurrent dual-band amplifier operating at G- (140-220 GHz) and H- (220-325 GHz) bands.

Analysis and Design of Power-Efficient H-Band CMOS Frequency Doubler Employing Gain Boosting and Harmonic Enhancing Techniques

This article presents a power-efficient frequency doubler employing gain boosting and harmonic-enhancing techniques. With a single transistor only, the gain boosting technique can reach the maximum achievable gain ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) by adding embedded passive components, thereby obtaining high voltage swings. Then, the transistor’s nonlinearity is essential, which is maximized by the harmonic transition scheme of the transistor operation along with high voltage swings. In addition, a harmonic reflector and a harmonic leakage canceller are employed for the second harmonic enhancement. The harmonic reflector prevents unwanted harmonic mixing by minimizing the incoming second harmonic current fed back to the input. The harmonic leakage canceller suppresses the leakage loss of the second harmonic current present at the output. Furthermore, thanks to a proposed dual-band output matching network, the output impedance is conjugately matched to achieve the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> at the fundamental frequency while it is matched to extract the second harmonic output power simultaneously. To verify the proposed techniques, the prototype was designed as a single-stage circuit that does not require additional amplifying stages, which led to higher power efficiency and lower chip area. Implemented in a 65-nm CMOS process, the measurement results show a saturated output power of 0.9 dBm and 3-dB bandwidth of 26 GHz (237-263 GHz), respectively, while requiring a chip area of 0.071 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Total power efficiency, including the effect of injected signal power, is 2.87 % while consuming only 37 mW dc power.

Simulation, Fabrication, and Testing of a Position-Sensitive Proportional Gas Chamber with Equipotential Parallel Wires with Similar Behavior as Micromegas Detectors

Different strategies exist for electron multiplication in a proportional radiation gas detector. In this work, the amplification region is formed by an array of equipotential stainless steel wires that were fixed at equal distance from a bared silver flat ribbon cable as the anode of the detector. The wires in this structure have the same role as the micromesh in Micro-Mesh Gaseous (Micromegas) detectors. Its fabrication method is simple and low cost. In this work, the amplification gain of the fabricated sample at different anode voltages was extracted, and the maximum achievable gain without electric discharge was measured to be M = 315 at 700 V. The proposed detector has an inherently two-dimensional positioning capacity, and the position sensitivity of the detector in one dimension was tested, the results of which show a good discrepancy with theoretical expectation. For the fabricated detector, the maximum number of charges before electric discharge was extracted (1 × 107) and compared with the best-designed Micromegas detector (6 × 107). These results clearly show that the proposed detector, despite its simplicity and cost-effective process, has a reasonable quality in comparison to the Micromegas detector.

Parametric Amplification via Superconducting Contacts in a Ka Band Niobium Pillbox Cavity

Superconducting parametric amplifiers are commonly fabricated using planar transmission lines with a nonlinear inductance provided by either Josephson junctions or the intrinsic kinetic inductance of the thin film. However, Banys et al. (J Low Temp Phys, 2020) reported nonlinear behaviour in a niobium pillbox cavity, hypothesising that below T_c, the pair iris-bulk resonator would act as a superconducting contact surface exploiting a Josephson-like nonlinearity. This work investigates this effect further by applying Keysight Technologies’ Advanced Design System (ADS) to simulate the cavity using an equivalent circuit model that includes a user defined Josephson inductance component. The simulations show that for a resonance centred at nu _0=30.649 GHz, when two tones (pump and signal) are injected into the cavity, mixing and parametric gain occur. The maximum achievable gain is explored when the resonator is taken to its bifurcation energy. These results are compared to cryogenic measurements where the pump and the signal are provided by a Vector Network Analyzer.

A New Efficient Array Architecture for Small L-Band Secondary Surveillance Radars with Reduced Number of Elements

ABSTRACT In this paper, a new and more efficient array architecture for small Secondary Surveillance Radar (SSR) antennas is presented. The architecture uses a side element for the generation of the Control (CTRL) beam as a new proposed method and is applied on a 7-element 1.5-m SSR antenna array. All three required patterns for secondary surveillance radar containing SUM, DIFF, and CTRL are designed and implemented in an integrated one-sheet feed network. The array antenna uses Taylor distribution for SUM port, obtaining 16 dBi and 16.7 dBi of gain at 1030 MHz and 1090 MHz, respectively, and a side lobe level of 23 dB at the measurements. It is demonstrated that the last side element at the edge is the best choice for CTRL pattern with respect to the final pattern shape, maximum achievable gain, and easy implementation in this small array architecture, which results in more than 60% of aperture efficiency. The simulation and measurement results are in acceptable agreement.

Improved Receivers for Optical Wireless OFDM: An Information Theoretic Perspective

We consider performance enhancement of asymmetrically-clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and related optical OFDM schemes, which are variations of OFDM in intensity-modulated optical wireless communications. Unlike most existing studies on specific designs of improved receivers, this paper investigates information theoretic limits of all possible receivers. For independent and identically distributed (IID) complex Gaussian inputs, we obtain an exact characterization of information rate of ACO-OFDM with improved receivers for all SNRs. It is proved that the high-SNR gain of improved receivers asymptotically achieve 1/4 bits per channel use, which is equivalent to 3 dB in electrical SNR or 1.5 dB in optical SNR; as the SNR decreases, the maximum achievable SNR gain of improved receivers decreases monotonically to a non-zero low-SNR limit, corresponding to an information rate gain of 36.3%. For practically used constellations, we derive an upper bound on the gain of improved receivers. Numerical results demonstrate that the upper bound can be approached to within 1 dB in optical SNR by combining existing improved receivers and coded modulation. We also show that our information theoretic analyses can be extended to Flip-OFDM and PAM-DMT. Our results imply that, for the considered schemes, improved receivers may reduce the gap to channel capacity significantly at low-to-moderate SNR.

Overcoming the Transimpedance Limit: A Tutorial on Design of Low-Noise TIA

Noise probably the single most important performance metric of the high-speed transimpedance amplifier (TIA), which directly sets the sensitivity of optical receiver. The transimpedance limit which dictates the maximum achievable transimpedance gain of the TIA also turns out to fundamentally limit the TIA noise performance. In this tutorial, we analyze and explore two circuit design approaches to overcome the transimpedance limit. The first approach (Type I) realizes a divide-and-conquer methodology to separate the noise-bandwidth problem and solve them individually. The second approach (Type II) employs a multi-stage stagger-tuned amplifier. Both approaches can overcome the transimpedance limit, forming an effective toolkit for the design of low-noise high-speed TIA for high-sensitivity CMOS optical receivers in current and future applications.

Maximum Achievable Gain of Annular-Shaped Beams

The maximum achievable gain and the corresponding source distribution for an annular or “donut”-shaped beam produced by a circular aperture are studied. Expressing the source in terms of orthogonal basis functions and then applying Lagrange’s multiplier method, the optimum source distribution is determined. The residue calculus is then invoked for the closed-form expressions. Numerical results are shown and discussed. In applications such as direction finding and satellite tracking systems on moving vehicles, a high-gain annular beam is suitable for maintaining uninterrupted connectivity.

A low power CMOS programmable gain amplifier employing positive feedback technique

AbstractThis paper presents a low power CMOS programmable gain amplifier (PGA) utilizing a novel gm‐boosting technique to increase the maximum achievable DC gain. The structure increases the effective transconductance of the circuit without any additional elements. The gain enhancement is accomplished without increasing the power consumption. The variable gain function is achieved by varying the effective transconductance of the circuit and its output resistance. The proposed PGA achieves a wide gain range of 47.5 dB from 2.1 to 49.6 dB with a constant bandwidth. According to the post‐layout simulation results of the proposed circuit in 0.18‐μm TSMC CMOS process the bandwidth and the input referred noise voltage at the maximum gain are 11.3 MHz and 10.76 nV/sqrt (Hz), respectively. The second and third harmonic distortions of the proposed PGA at the minimum gain are less than −72.4 and −40.7 dB, respectively. The core circuit of proposed PGA draws only 164 μA from a 1.2‐V supply voltage occupying an area of 29 μm * 26 μm.

Practical Hybrid Beamforming for Millimeter Wave Massive MIMO Full Duplex With Limited Dynamic Range

Full Duplex (FD) radio has emerged as a promising solution to increase the data rates by up to a factor of two via simultaneous transmission and reception in the same frequency band. This paper studies a novel hybrid beamforming (HYBF) design to maximize the weighted sum-rate (WSR) in a single-cell millimeter wave (mmWave) massive multiple-input-multiple-output (mMIMO) FD system. Motivated by practical considerations, we assume that the multi-antenna users and hybrid FD base station (BS) suffer from the limited dynamic range (LDR) noise due to non-ideal hardware and an impairment aware HYBF approach is adopted by integrating the traditional LDR noise model in the mmWave band. In contrast to the conventional HYBF schemes, our design also considers the joint sum-power and the practical per-antenna power constraints. A novel interference, self-interference (SI) and LDR noise aware optimal power allocation scheme for the uplink (UL) users and FD BS is also presented to satisfy the joint constraints. The maximum achievable gain of a multi-user (MU) mmWave FD system over a fully digital half duplex (HD) system with different LDR noise levels and numbers of the radio-frequency (RF) chains is investigated. Simulation results show that our design outperforms the HD system with only a few RF chains at any LDR noise level. The advantage of having amplitude control at the analog stage is also examined, and additional gain for the mmWave FD system becomes evident when the number of RF chains at the hybrid FD BS is small.

On the Performance of Video Resolution, Motion and Dynamism in Transmission Using Near-Capacity Transceiver for Wireless Communication.

This article investigates the performance of various sophisticated channel coding and transmission schemes for achieving reliable transmission of a highly compressed video stream. Novel error protection schemes including Non-Convergent Coding (NCC) scheme, Non-Convergent Coding assisted with Differential Space Time Spreading (DSTS) and Sphere Packing (SP) modulation (NCDSTS-SP) scheme and Convergent Coding assisted with DSTS and SP modulation (CDSTS-SP) are analyzed using Bit Error Ratio (BER) and Peak Signal to Noise Ratio (PSNR) performance metrics. Furthermore, error reduction is achieved using sophisticated transceiver comprising SP modulation technique assisted by Differential Space Time Spreading. The performance of the iterative Soft Bit Source Decoding (SBSD) in combination with channel codes is analyzed using various error protection setups by allocating consistent overall bit-rate budget. Additionally, the iterative behavior of SBSD assisted RSC decoder is analyzed with the aid of Extrinsic Information Transfer (EXIT) Chart in order to analyze the achievable turbo cliff of the iterative decoding process. The subjective and objective video quality performance of the proposed error protection schemes is analyzed while employing H.264 advanced video coding and H.265 high efficient video coding standards, while utilizing diverse video sequences having different resolution, motion and dynamism. It was observed that in the presence of noisy channel the low resolution videos outperforms its high resolution counterparts. Furthermore, it was observed that the performance of video sequence with low motion contents and dynamism outperforms relative to video sequence with high motion contents and dynamism. More specifically, it is observed that while utilizing H.265 video coding standard, the Non-Convergent Coding assisted with DSTS and SP modulation scheme with enhanced transmission mechanism results in gain of 20 dB with reference to the Non-Convergent Coding and transmission mechanism at the objective PSNR value of 42 dB. It is important to mention that both the schemes have employed identical code rate. Furthermore, the Convergent Coding assisted with DSTS and SP modulation mechanism achieved superior performance with reference to the equivalent rate Non-Convergent Coding assisted with DSTS and SP modulation counterpart mechanism, with a performance gain of 16 dB at the objective PSNR grade of 42 dB. Moreover, it is observed that the maximum achievable PSNR gain through H.265 video coding standard is 45 dB, with a PSNR gain of 3 dB with reference to the identical code rate H.264 coding scheme.

Analysis and Design of Lossy Capacitive Over-Neutralization Technique for Amplifiers Operating Near f MAX

This paper proposes a technique to enhance the maximum achievable power gain ( G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> ) of the two-port active network (2PAN) in the near- f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> region. This technique is based on using the optimized passive-linear-lossy-reciprocal (PLLR) embedding to increase the unilateral power gain ( U) of the 2PAN and accordingly the G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">MAX</sub> . Due to the possibility to increase U, it shows the potential to improve the conventional gain-boosting approach which relies on the passive-linear-lossless-reciprocal (PLLLR) embedding and keeps the U constant. The concept itself is demonstrated on pseudo-differential NMOS pair using lossy capacitive feedback as the PLLR. This structure can serve as a basis for further gain optimization by implementing additional PLLLR embedding on top of it. Finally, by using the mentioned technique, a 190GHz amplifier is implemented in 28nm bulk CMOS technology, achieving 14.3dB of gain with 1.5dBm of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SAT</sub> and 2.6% of maximum PAE.

A D-Band High-Gain and Low-Power LNA in 65-nm CMOS by Adopting Simultaneous Noise- and Input-Matched G max-Core

This article proposes a high-gain and low-power low-noise amplifier (LNA) by adopting a simultaneous noise- and input-matched (SNIM) maximum achievable gain ( G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) core. The G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core is implemented by adjusting the infinite combinations of embedding networks with three passive elements. Based on the proposed two-port noise analysis for implementing the G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core, the input stage G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core can achieve a simultaneous power gain and noise matching. The adoption of the G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core in the input stage can maximize the amount of gain per stage, leading to higher total power gain and lower noise figure (NF). The two-stage 150-GHz LNA adopting the SNIM G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core is implemented in a 65-nm CMOS process. The measurement results show a peak gain of 17.9 dB at 152.2 GHz, 3-dB bandwidth of 11 GHz, NF of 4.7 and 6.2 dB at 148 and 150 GHz, respectively, and a peak power added efficiency (PAE) of 7.7% while dissipating only 13.73 mW. This work shows the highest gain per stage and the lowest NF with the lowest dc power consumption among other reported CMOS D-band amplifiers.

A 250-GHz 12.6-dB Gain and 3.8-dBm P sat Power Amplifier in 65-nm CMOS Adopting Dual-Shunt Elements Based G max-Core

This letter proposes a high output power 250-GHz power amplifier adopting dual-shunt elements in the implementation of maximum achievable gain (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) core. By the adoption of the dual-shunt-element-based G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core, the output transistor size can be increased, which leads to higher output power. Implemented in a 65-nm CMOS, the measurement results show a peak gain of 12.6 dB, P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> of 3/3.8 dBm, OP1 dB of 1.4/2.3 dBm, and peak power-added efficiency (PAE) of 4.8/3.2% at 248.6 GHz while dissipating 40/62.4 mW from a 1/1.2-V supply, respectively.

A D-Band Power Amplifier in 65-nm CMOS by Adopting Simultaneous Output Power-and Gain-Matched Gmax-Core

This paper proposes a simultaneous output power- and gain-matching technique in a sub-THz power amplifier (PA) design based on a maximum achievable gain ( G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) core. The optimum combination of three-passive-elements-based embedding networks for implementing the G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> -core is chosen considering the small- and large-signal performances at the same time. By adopting the proposed technique, the simultaneous output power- and gain-matching can be achieved, maximizing the small-signal power gain and large-signal output power simultaneously. A 150 GHz single-ended two-stage PA without power combining circuit is implemented in a 65-nm CMOS process based on the proposed technique. The amplifier achieves a peak power gain of 17.5 dB, peak power added efficiency (PAE) of 13.3 and 16.1 %, saturated output power ( P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) of 10.3 and 9.4 dBm, and DC power consumption of 86.3 and 52.4 mW, respectively, under the bias voltage of 1.2 and 1 V, which corresponds to the highest PAE, gain per stage and P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">out</sub> per single transistor among other reported CMOS D-band PAs.

On Performance of Ultra-Dense Neighborhood Small Cell Networks in Urban Scenarios

The neighborhood small cell (NSC) network, which employs indoor small base stations (SBSs) to serve both indoor and outdoor users, is considered a cost-effective solution to address the increasing wireless traffic demand. In this letter, a novel evaluation scheme is proposed to predict the performance of a pure NSC network. A network model composed of indoor ultra-dense small cells (UDS) with coordinated multi-point (CoMP) transmissions is employed to address the maximum achievable performance gain in a given neighborhood. A closed-form analytical model is proposed to approximately compute the signal strength received by a specific outdoor user from the SBSs in a complex-shaped building and verified by the Monte Carlo simulations. The performance prediction of the ultra-dense NSC network is demonstrated in two practical urban scenarios. The proposed evaluation scheme facilitates the tractability of NSC network performance predictions in practical scenarios, which can be employed in future NSC embedded neighborhoods design.

Synthesis of contoured beam multifeed reflector antenna for optimum coverage

AbstractA synthesis procedure for realizing contoured beams in multifeed reflector antenna is proposed. The proposed procedure solves the synthesis problem so that the maximum gain and low ripple together with low cross polarization are achieved. To this end, first, the relation between the physical parameters of the antenna and the gain loss over the contour is investigated. Then, the maximum achievable gain is approximated, which is needed to study the next issue, that is the optimization procedure. A new optimization procedure is presented, through which the desired contoured beam is achieved. The synthesis procedure employs physical optic (PO) method, which is time‐consuming. To reduce the computational time, which is another focus of the analysis, we introduce an efficient method for computing the PO integral. Through this method, the 3D PO integral is mapped to a 2D integral. The procedure is applied to Brazil coverage. Results show that the cross‐polarization is below −20 dB and the gain deviation is less than 1 dB over the contour, which is suitable for satellite communications.

Diversity Gain Analysis of Distributed CDD Systems in Non-Identical Fading Channels

This paper investigates the diversity gain of a distributed cyclic delay diversity (dCDD) scheme for cyclic-prefixed single carrier systems in non-identical fading channels. Non-identical small-scale fading is assumed in the environment, in which non-identical line-of-sight and non-line-of-sight fading coexist. A condition for dCDD resulting in intersymbol interference free reception at the receiver, is extended to this new channel environment. For an overpopulated system setup, a generalized performance analysis, is not available from existing works, is conducted after developing closed-form expressions for the distribution of the signal-to-noise ratio (SNR) realized at the receiver. Since the order statistics are involved in the statistical properties of the SNR, the corresponding spacing statistics are utilized to derive feasible closed-form expressions. The finalized closed-form expressions are shown to provide very reliable outage probability and spectral efficiency of dCDD for underpopulated and overpopulated systems. An asymptotic performance analysis verifies the maximum achievable diversity of the dCDD even in the overpopulated case within the considered channel environment. Link-level simulations are conducted and these verify the maximum achievable diversity gain.

Performance analysis of the matched subspace detector in the presence of signal-dependent interference for MIMO radar

For a statistical multiple-input multiple-output (MIMO) radar, we investigate the maximum achievable diversity gain and the minimum possible probability of miss detection and the conditions of achieving those at sufficiently high signal-to-noise ratio (SNR) when the matched subspace detector is employed in the presence of signal-dependent interference. In addition to linear dependence between target returns at each receiver, we show that linear dependence between target and interference subspaces reduces the diversity gain when the matched subspace detector is employed. We derive the approximated form of the probability of miss and introduce the signaling loss concept based on it. We show that the probability of miss is minimized at high SNR when not only is the diversity gain maximized, but the signaling loss is also minimized. Then, we use the signaling loss corresponding to the maximum diversity gain as a novel design metric for designing signals and minimizing the probability of miss at sufficiently high SNR in statistical MIMO radar. Due to the fact that designing the optimal signals is challenging, we derive analytically the detection performance loss of quasi-optimal signals by employing the signaling loss concept.

Energy-transfer upconversion and excited-state absorption in KGdxLuyEr1-x-y(WO4)2 waveguide amplifiers

We perform a systematic spectroscopic study in channel waveguides of potassium gadolinium lutetium double tungstate doped with different Er3+ concentrations. Transition cross sections of ground-state absorption (GSA) and excited-state absorption (ESA), as well as stimulated emission (SE) at the pump wavelength around 980 nm are determined. ESA is directly measured by the pump-probe technique. Evaluation of GSA and ESA spectra indicates that ESA may be diminished by an appropriate choice of pump wavelength near 980 nm. Besides, GSA and SE at the signal wavelength around 1.5 µm are measured and the wavelength-dependent gain cross section as a function of excitation density is determined. Non-exponential luminescence decay curves from the 4I13/2 and 4I11/2 levels are analyzed and the probabilities of the energy-transfer-upconversion (ETU) processes (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and (4I11/2, 4I11/2) → (4I15/2, 4F7/2) are quantified. Despite the large interionic distance between neighboring rare-earth sites in potassium double tungstates, the probability of ETU is comparatively large because of the large cross sections of the involved transitions. A rate-equation analysis of the influence of ETU and ESA on gain at ∼1.5 µm is performed, revealing that ETU from the 4I13/2 amplifier level strongly limits the gain when the doping concentration increases above ∼6at.%. The calculated maximum achievable internal net gain per unit length amounts to ∼15 dB/cm for an optimized Er3+ concentration of ∼4 × 1020 cm−3 and a launched pump power of 300 mW at a pump wavelength of 984.5 nm, in reasonable agreement with recent experimental results.