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In recent years model and Digital Pre-Distortion dimension reduction has been widely researched. The oper- ations involved when running DPD are often far less than those needed during the training of the DPD coefficients. The proposed partitioned Least Squares (LS) adaptation allows a selected subset of DPD coefficients to be updated while the remaining coefficients are held constant. This technique allows a more adaptive training procedure, improved interpretability of the important DPD coefficient’s during training and the ability to partition the DPD function into specific groups. The Frisch-Waugh-Lovell (FWL) theorem is exploited to partition the coefficients of a DPD basis function trained using LS regression. The proposed methodology was experimentally validated with a Generalized Memory Polynomial (GMP) DPD function, used to linearize a 5W power amplifier (PA) driven by a 40MHz 5G-NR signal.

In recent years model and Digital Pre-Distortion dimension reduction has been widely researched. The oper- ations involved when running DPD are often far less than those needed during the training of the DPD coefficients. The proposed partitioned Least Squares (LS) adaptation allows a selected subset of DPD coefficients to be updated while the remaining coefficients are held constant. This technique allows a more adaptive training procedure, improved interpretability of the important DPD coefficient’s during training and the ability to partition the DPD function into specific groups. The Frisch-Waugh-Lovell (FWL) theorem is exploited to partition the coefficients of a DPD basis function trained using LS regression. The proposed methodology was experimentally validated with a Generalized Memory Polynomial (GMP) DPD function, used to linearize a 5W power amplifier (PA) driven by a 40MHz 5G-NR signal.

As the physical makeup of cellular base-stations evolve into systems with multiple parallel transmission paths the effort involved in modelling these complex systems increases considerably. One task in particular which contributes to signal distortion on each signal path, is the power amplifier. In power amplifier modelling, Recursive Least Squares has been used in the past to train Volterra models with memory terms, however instability can occur when training the model weights. This manuscript provides a computationally efficient technique to detect the onset of instability and subsequently to inform the decision when to stop adaptive training of dynamic nonlinear behavioural models and avoid the onset of instability. This technique is experimentally validated using four different signal modulation schemes.

As the physical makeup of cellular base-stations evolve into systems with multiple parallel transmission paths the effort involved in modelling these complex systems increases considerably. One task in particular which contributes to signal distortion on each signal path, is the power amplifier. In power amplifier modelling, Recursive Least Squares has been used in the past to train Volterra models with memory terms, however instability can occur when training the model weights. This manuscript provides a computationally efficient technique to detect the onset of instability and subsequently to inform the decision when to stop adaptive training of dynamic nonlinear behavioural models and avoid the onset of instability. This technique is experimentally validated using four different signal modulation schemes.

A key distinction between today's and future networks is the appetite for reliable communication to support emerging critical-communication services. In this paper, we study multi-operator connectivity as a form of redundancy to support the design of reliable networks and investigate its trade-offs. This approach is motivated by 3GPP standardisation initiatives of dual-connectivity and similar techniques in industrial wired networks. We deploy a risk awareness performance metric to assess reliability: this superquantile metric accounts for periods of connectivity shortfalls. Our analysis shows that multi-operator connectivity brings significant reliability gains, in particular when network deployments by different operators exhibit high complementarity in coverage. We also explore the effects of multi-connectivity on spectral efficiency in times of high demand for bandwidth. Our study is based on a real-world dataset comprising signal strength indicators of three mobile operators in Dublin, Ireland.

The adaptive data rate (ADR) algorithm is a key component of the LoRaWAN protocol which controls the performance of a LoRaWAN Network. This modifies the data rate parameter of a device based on the current wireless conditions. In this article, we present substantive enhancements for the End Device and Network Server which reduce the convergence time for LoRaWAN devices to reach their optimal data rate. We extend the LoRaWAN module in ns-3 by adding ADR, enabling the simulation of realistic LoRaWAN networks, and add the implementation of the new enhancements in this module. The simulations show that these modifications can result in a significant reduction of the data rate convergence time for LoRaWAN devices and lead to an increased overall packet delivery rate for the network in a dynamic network environment. Our contributions are a publicly available implementation of ADR in ns-3, an analysis of the original algorithm behavior, and a novel version of the algorithm with enhancements that improve performance in every case while remaining easily integrable into an existing LoRaWAN system.

The adaptive data rate (ADR) algorithm is a key component of the LoRaWAN protocol which controls the performance of a LoRaWAN Network. This modifies the data rate parameter of a device based on the current wireless conditions. In this article, we present substantive enhancements for the End Device and Network Server which reduce the convergence time for LoRaWAN devices to reach their optimal data rate. We extend the LoRaWAN module in ns-3 by adding ADR, enabling the simulation of realistic LoRaWAN networks, and add the implementation of the new enhancements in this module. The simulations show that these modifications can result in a significant reduction of the data rate convergence time for LoRaWAN devices and lead to an increased overall packet delivery rate for the network in a dynamic network environment. Our contributions are a publicly available implementation of ADR in ns-3, an analysis of the original algorithm behavior, and a novel version of the algorithm with enhancements that improve performance in every case while remaining easily integrable into an existing LoRaWAN system.

This paper proposes a new crest factor reduction scheme to reduce instantaneous-to-average power ratio (IAR) resulting decent peak reduction, and adjacent channel leakage ratio (ACLR) reduction, though the error vector magnitude (EVM) is significantly enhanced. In the conventional peak cancellation (CPC) technique, peak regrowth occurs as a result of finite impulse response (FIR) filter windows overlapping. The peak regrowth problem in CPC technique can be compensated, though this will lead to increased computational complexity and degradation in EVM. The proposed algorithm allocates spacing to contiguous peaks to avoid FIR filter windows overlapping. Two types of overlapping are discussed, incremental and decremental. The optimum spacing can be determined from the FIR filter impulse response characteristic. Simulations are carried out with different intra-band non-contiguous carrier aggregated OFDM signals. The results of simulations show between 3 and 10% improvement in EVM performance while maintaining equivalent IAR and ACLR performance. Applying the proposed optimum spacing peak cancellation scheme compared to the conventional peak cancellation technique.

In this paper, a novel outphasing power amplifier (PA) based on class-E amplifiers with shunt capacitances and shunt filters is proposed. The new design provides high drain efficiency for both peak and back-off power levels. A mathematical model for the class-E power amplifier with shunt capacitance and shunt filter is presented. The proposed model enables derivation of load circuit parameters that provide optimum drain efficiency for the peak and back-off power levels using closed form mathematical expressions. Based on this model, an outphasing power amplifier is designed and subsequently implemented using microstrip transmission lines and a GaN HEMT devices. The fabricated power amplifier prototype is optimized for 2.14 GHz and provides drain efficiency of over 60% for back-off power levels up to 8.5 dB. The amplifier demonstrates a 44.3% drain efficiency for 64QAM OFDM modulated signal with 20 MHz bandwidth. Adjacent channel leakage ratio (ACLR) of −39.5 dB and error vector magnitude (EVM) of 0.9 % were achieved after the application of a memory polynomial linearization algorithm.

The adaptive data rate (ADR) algorithm is a key component of the LoRaWAN protocol which controls the performance of a LoRaWAN Network. This modifies the data rate parameter of a device based on the current wireless conditions. In this article, we present substantive enhancements for the End Device and Network Server which reduce the convergence time for LoRaWAN devices to reach their optimal data rate. We extend the LoRaWAN module in ns-3 by adding ADR, enabling the simulation of realistic LoRaWAN networks, and add the implementation of the new enhancements in this module. The simulations show that these modifications can result in a significant reduction of the data rate convergence time for LoRaWAN devices and lead to an increased overall packet delivery rate for the network in a dynamic network environment. Our contributions are a publicly available implementation of ADR in ns-3, an analysis of the original algorithm behaviour, and a novel version of the algorithm with enhancements that improve performance in every case while remaining easily integrable into an existing LoRaWAN system.

This paper proposes a new crest factor reduction scheme to reduce instantaneous-to-average power ratio (IAR) resulting decent peak reduction, and adjacent channel leakage ratio (ACLR) reduction, though the error vector magnitude (EVM) is significantly enhanced. In the conventional peak cancellation (CPC) technique, peak regrowth occurs as a result of finite impulse response (FIR) filter windows overlapping. The peak regrowth problem in CPC technique can be compensated, though this will lead to increased computational complexity and degradation in EVM. The proposed algorithm allocates spacing to contiguous peaks to avoid FIR filter windows overlapping. Two types of overlapping are discussed, incremental and decremental. The optimum spacing can be determined from the FIR filter impulse response characteristic. Simulations are carried out with different intra-band non-contiguous carrier aggregated OFDM signals. The results of simulations show between 3 and 10% improvement in EVM performance while maintaining equivalent IAR and ACLR performance. Applying the proposed optimum spacing peak cancellation scheme compared to the conventional peak cancellation technique.

In this paper, a novel outphasing power amplifier (PA) based on class-E amplifiers with shunt capacitances and shunt filters is proposed. The new design provides high drain efficiency for both peak and back-off power levels. A mathematical model for the class-E power amplifier with shunt capacitance and shunt filter is presented. The proposed model enables derivation of load circuit parameters that provide optimum drain efficiency for the peak and back-off power levels using closed form mathematical expressions. Based on this model, an outphasing power amplifier is designed and subsequently implemented using microstrip transmission lines and a GaN HEMT devices. The fabricated power amplifier prototype is optimized for 2.14 GHz and provides drain efficiency of over 60% for back-off power levels up to 8.5 dB. The amplifier demonstrates a 44.3% drain efficiency for 64QAM OFDM modulated signal with 20 MHz bandwidth. Adjacent channel leakage ratio (ACLR) of −39.5 dB and error vector magnitude (EVM) of 0.9 % were achieved after the application of a memory polynomial linearization algorithm.

The adaptive data rate (ADR) algorithm is a key component of the LoRaWAN protocol which controls the performance of a LoRaWAN Network. This modifies the data rate parameter of a device based on the current wireless conditions. In this article, we present substantive enhancements for the End Device and Network Server which reduce the convergence time for LoRaWAN devices to reach their optimal data rate. We extend the LoRaWAN module in ns-3 by adding ADR, enabling the simulation of realistic LoRaWAN networks, and add the implementation of the new enhancements in this module. The simulations show that these modifications can result in a significant reduction of the data rate convergence time for LoRaWAN devices and lead to an increased overall packet delivery rate for the network in a dynamic network environment. Our contributions are a publicly available implementation of ADR in ns-3, an analysis of the original algorithm behaviour, and a novel version of the algorithm with enhancements that improve performance in every case while remaining easily integrable into an existing LoRaWAN system.

Orthogonal frequency division multiplexing (OFDM) has become an indispensable part of waveform generation in wideband digital communication since its first appearance in digital audio broadcasting (DAB) in Europe in 1980s, and it is indeed in use. As has been seen, the OFDM based waveforms work well with time division duplex operation in new radio (NR) systems in 5G systems, supporting delay-sensitive applications, high spectral efficiency, massive multiple input multiple output (MIMO) compatibility, and ever-larger bandwidth signals, which has demonstrated successful commercial implementation for 5G downlinks and uplinks up to 256-QAM modulation schemes. However, the OFDM waveforms suffer from high peak to average power ratio (PAPR), which is not desired by system designers as they want RF power amplifiers (PAs) to operate with high efficiency. Although NR offers some options for maintaining the efficiency and spectral demand, such as cyclic prefix based (CP-OFDM), and discrete Fourier transform spread based (DFT-S-OFDM) schemes, which have limiting effects on PAPR, the PAPR is still as high as 13 dB. This value increases when the bandwidth is increased. Moreover, in LTE-Advance and 5G systems, in order to increase the bandwidth, and data-rate, carrier aggregation technology is used which increases the PAPR the same way that bandwidth increment does; therefore, it is essential to employ PAPR reduction in signal processing stage before passing the signal to PA. In this paper, we investigate the performance of an innovative peak shrinking and interpolation (PSI) technique for reducing peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) based signals at waveform generation stage. The main idea behind the PSI technique is to extract high peaks, scale them down, and interpolate them back into the signal. It is shown that PSI technique is a possible candidate for reducing PAPR without compromising on computational complexity, compatibl

Next generation wireless communications such as 5G are expected to feature wide channel bandwidths on the order of hundreds of MHz. As bandwidths increase, circuit impairments caused by frequency dependent behaviour such as ripple and tilt in gain and group delay become more significant. PAPR of OFDM signals also increase with increasing number of subcarriers. Transmitter circuit characterisation for the wide-band frequency response is needed to pre-compensate the signal to be transmitted. In this article, we propose a novel scheme which uses the circuit characteristics combined with the channel response to generate the keys for encrypting signals to provide an additional tier of security at the physical layer. The modulated constellation of the signal of interest is encrypted by dispersing its phases in addition to encrypting the bits using Diffie Hellman scheme. It is also shown that the method is able to reduce the PAPR of OFDM signals. This scheme is experimentally validated from end-to-end on a millimetre wave wireless link at 28.9 GHz demonstrating security against a well-positioned eavesdropper and a reduction of PAPR by 3.5 dB in a 2048 point OFDM signal with 1664 active QPSK modulated sub-carriers.

Orthogonal frequency division multiplexing (OFDM) has become an indispensable part of waveform generation in wideband digital communication since its first appearance in digital audio broadcasting (DAB) in Europe in 1980s, and it is indeed in use. As has been seen, the OFDM based waveforms work well with time division duplex operation in new radio (NR) systems in 5G systems, supporting delay-sensitive applications, high spectral efficiency, massive multiple input multiple output (MIMO) compatibility, and ever-larger bandwidth signals, which has demonstrated successful commercial implementation for 5G downlinks and uplinks up to 256-QAM modulation schemes. However, the OFDM waveforms suffer from high peak to average power ratio (PAPR), which is not desired by system designers as they want RF power amplifiers (PAs) to operate with high efficiency. Although NR offers some options for maintaining the efficiency and spectral demand, such as cyclic prefix based (CP-OFDM), and discrete Fourier transform spread based (DFT-S-OFDM) schemes, which have limiting effects on PAPR, the PAPR is still as high as 13 dB. This value increases when the bandwidth is increased. Moreover, in LTE-Advance and 5G systems, in order to increase the bandwidth, and data-rate, carrier aggregation technology is used which increases the PAPR the same way that bandwidth increment does; therefore, it is essential to employ PAPR reduction in signal processing stage before passing the signal to PA. In this paper, we investigate the performance of an innovative peak shrinking and interpolation (PSI) technique for reducing peak to average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) based signals at waveform generation stage. The main idea behind the PSI technique is to extract high peaks, scale them down, and interpolate them back into the signal. It is shown that PSI technique is a possible candidate for reducing PAPR without compromising on computational complexity, compatibl

Next generation wireless communications such as 5G are expected to feature wide channel bandwidths on the order of hundreds of MHz. As bandwidths increase, circuit impairments caused by frequency dependent behaviour such as ripple and tilt in gain and group delay become more significant. PAPR of OFDM signals also increase with increasing number of subcarriers. Transmitter circuit characterisation for the wide-band frequency response is needed to pre-compensate the signal to be transmitted. In this article, we propose a novel scheme which uses the circuit characteristics combined with the channel response to generate the keys for encrypting signals to provide an additional tier of security at the physical layer. The modulated constellation of the signal of interest is encrypted by dispersing its phases in addition to encrypting the bits using Diffie Hellman scheme. It is also shown that the method is able to reduce the PAPR of OFDM signals. This scheme is experimentally validated from end-to-end on a millimetre wave wireless link at 28.9 GHz demonstrating security against a well-positioned eavesdropper and a reduction of PAPR by 3.5 dB in a 2048 point OFDM signal with 1664 active QPSK modulated sub-carriers.

The next generation of mobile networks will bring an appetite for reliable communication, which enables emerging critical-communication services. In this paper, we present network sharing between operators as a way to provide increased reliability while using the already existing mobile network infrastructure. Our results indicate that network sharing significantly improves network performance during periods of connectivity shortfalls, benefiting the design of reliable networks. Our conclusions are drawn from a real-world dataset of signal quality indicators for three mobile operators in Dublin, Ireland.

Next generation wireless communication systems such as 5G will feature a step increase in the speed of wireless data transmission. This requires using RF signals of wider bandwidths along with higher order modulation schemes. Such signals are subjected to frequency and power dependent impairments such as gain compression, amplitude ripple and tilt, group-delay and AM-PM conversion as they traverse through the transmitter’s hardware blocks. This paper introduces a novel calibration scheme that applies a specially designed wideband 64- QAM modulated stimulus and uses the captured response to evaluate the impairments within the transmitter’s RF chain. The needed pre-compensation is applied in a single iteration. This technique is intended for use with Doherty, Outphasing and MIMO type applications. Presented technique was experimentally validated using signal bandwidths up to 100 MHz for both weakly and strongly nonlinear systems.

In this paper, we study the sum rate maximization for a multiple-input multiple-output (MIMO) system with successive zero-forcing dirty-paper coding (SZFDPC) and per-antenna power constraint (PAPC). Although SZFDPC is a low-complexity alternative to the optimal dirty paper coding, efficient algorithms to compute its sum rate are still open problems especially under practical PAPC. The existing solution to the considered problem is computationally inefficient due to employing high-complexity interior-point method. In this study, we propose two novel low-complexity approaches to this important problem. More specifically, the first algorithm achieves the optimal solution by transforming the original problem in the broadcast channel into an equivalent problem in the multiple access channel, then the resulting problem is solved by alternating optimization together with successive convex approximation. We also derive a suboptimal solution based on machine learning to which simple linear regressions are applicable. The approaches are analyzed and validated extensively to demonstrate their superiors over the existing approach.

As the physical makeup of cellular basestations evolve into systems with multiple parallel transmission paths the effort involved in modelling these complex systems increases considerably. One task in particular which contributes to signal distortion on each signal path, is the power amplifier. In power amplifier (PA) modelling, Recursive Least Squares (RLS) has been used in the past to train Volterra models with memory terms. The Volterra model is widely used for modelling of PAs. In this paper we present a comparison of the stability performance for a PA model during training for various model memory lengths, model orders of non linearity and signal sample rates. This examination provides a technique to avoid instability occurring during the adaptive training of dynamic nonlinear behavioural models.

Efficient and linear power amplifiers (PA) are an essential part of forthcoming 5G wireless systems. Outphasing class-E PAs offer high power efficiency and an option for higher efficiency cellular networks. However, they employ signal component separators, which split the signal into two paths. In order to efficiently recombine the signal, nonlinear power combiners are used. This paper proposes a novel phase-only predistortion technique for outphasing class-E PAs. The predistortion coefficients can be extracted based on AMAM characteristics of the output signal and an analytical model of an outphasing Class E PA. The suggested technique has been validated by simulation of an outphasing power amplifier in ADS Ptolemy software. It is shown that applying this technique to a 16QAM OFDM modulated signal with 20 MHz bandwidth improves error vector magnitude (EVM) from 10.39% to 2.43% compared to the signal without predistortion.

A new technique for the estimation of coupling between array elements in MIMO transceivers is presented. The estimation method is founded on subsequent transmission and over-the-air (OTA) reception between the array elements, when the transmitter’s characteristics are unknown. The method enables accurate on-line characterization of the behavior of the array in terms of coupling for a complete system with PAs and antennas connected. The method is validated via OTA measurements on a four-element array in a MIMO transmitter configuration. The results demonstrate that the estimated couplings with the proposed technique are similar to the ones obtained from S-parameter measurements with deviations reflecting the operation with antennas and PAs connected. The proposed intra-array coupling estimation facilitates practical OTA measurement-based modeling and linearization of MIMO transmitters.

In this paper, we present a novel and effective linearization technique for nonlinear phased array antennas. For large phased arrays, linearization of the array using a single digital predistortion (DPD) is inevitable since one digital path is upconverted and feeds several RF transmission paths, each of which is connected to a power amplifier (PA) and an antenna element. However, a critical issue is that the PA characteristics can vary considerably within an array. Thus, linearizing individual PAs with one DPD is rather challenging. We formulate and solve an optimization problem that corresponds to jointly minimizing the maximum residuals between the input to the array and the output of individual PAs. We demonstrate that the proposed technique outperforms state-of-the-art linearization solutions while retaining the linear gain of the array.

In modern radio frequency (RF) transceivers the power amplifier (PA) is a central component in terms of power consumption. Achieving efficient performance in this component results in the PA output signal becoming distorted. Linearisation can be performed using techniques such as Digital Pre-Distortion (DPD). The pre-distorter operation of a DPD system involves the constant computation of a distorted signal to ensure linear operation of the nonlinear power amplifier. In this work a novel polynomial evaluation scheme is proposed to optimise the pre-distorter operation within a DPD system. Improvements to latency and hardware requirements are possible with new techniques. Validation of the proposed design was conducted using FPGA implementations and compared to incumbent pipelined solutions for both low latency and hardware efficiency. The proposed method indicated hardware savings of 67.8%, while operating 58.7% faster, compared to an existing implementation.

This work proposes a concise standalone model for Chireix power combiner. The model is based on the analysis of the nonlinear behaviour of a Chireix power combiner under amplitude imbalance. The generalised equations for input impedance of Chireix combiner branches excited by signals with different amplitudes have been derived. The results predicted by the model were validated using both commercial high frequency circuit simulations and experimentally measured results. As a result the proposed model serves two purposes. For one it can be used directly in a larger circuit simulation to predict the performance of an outphasing power amplifier. It can also be used as the basis for a linearization strategy for outphasing power amplifiers.

Power amplifiers are widely employed electronic devices in various fields such as mobile networks and radio frequency (RF) transceivers. To achieve efficient operations, power amplifiers can often suffer from nonlinearity problems. This problem can be mitigated through the use of linearization techniques, such as digital predistortion, regarded as the most promising solution to power amplifier linearization. Behavioural modeling is a substantial part of the digital predistortion, responsible for acquiring the coefficients that are necessary to linearize the power amplifier. A Complex Reduced Non-Uniform Generalized Memory Polynomial model was proposed to reach comparable performance of accuracy as Memory Polynomial Model with reduced complexities. The proposed model was tested with a 5MHz LTE signal measured at the input and output of a Doherty PA under different conditions of nonlinearities, memory effects and attenuations as well as PA working powers. It can be observed that the proposed model shows superior accuracy at low complexities, when the PA has higher levels of nonlinearity and memory depth while still maintaining low complexities. Over 60% of coefficients reduction could be reached at the same level of accuracy compared to the MP model.

In this paper, we propose a novel wideband wireless transmission scheme with spatially combined frequency interleaved transmitters of narrower bandwidths. The proposed technique enables development of seamless scalable bandwidth transmitters for next generation wireless and satellite communications with potential to reuse legacy hardware. The scheme is experimentally validated at Ka Band with instantaneous modulation bandwidths up to 400 MHz achieving transmission bit rates up to 2.039Gbps.

A new technique for the estimation of coupling between array elements in MIMO transceivers is presented. The estimation method is founded on subsequent transmission and over-the-air (OTA) reception between the array elements, when the transmitter’s characteristics are unknown. The method enables accurate on-line characterization of the behavior of the array in terms of coupling for a complete system with PAs and antennas connected. The method is validated via OTA measurements on a four-element array in a MIMO transmitter configuration. The results demonstrate that the estimated couplings with the proposed technique are similar to the ones obtained from S-parameter measurements with deviations reflecting the operation with antennas and PAs connected. The proposed intra-array coupling estimation facilitates practical OTA measurement-based modeling and linearization of MIMO transmitters.

Next generation wireless communication systems such as 5G will feature a step increase in the speed of wireless data transmission. This requires using RF signals of wider bandwidths along with higher order modulation schemes. Such signals are subjected to frequency and power dependent impairments such as gain compression, amplitude ripple and tilt, group-delay and AM-PM conversion as they traverse through the transmitter’s hardware blocks. This paper introduces a novel calibration scheme that applies a specially designed wideband 64- QAM modulated stimulus and uses the captured response to evaluate the impairments within the transmitter’s RF chain. The needed pre-compensation is applied in a single iteration. This technique is intended for use with Doherty, Outphasing and MIMO type applications. Presented technique was experimentally validated using signal bandwidths up to 100 MHz for both weakly and strongly nonlinear systems.

Power amplifiers are widely employed electronic devices in various fields such as mobile networks and radio frequency (RF) transceivers. To achieve efficient operations, power amplifiers can often suffer from nonlinearity problems. This problem can be mitigated through the use of linearization techniques, such as digital predistortion, regarded as the most promising solution to power amplifier linearization. Behavioural modeling is a substantial part of the digital predistortion, responsible for acquiring the coefficients that are necessary to linearize the power amplifier. A Complex Reduced Non-Uniform Generalized Memory Polynomial model was proposed to reach comparable performance of accuracy as Memory Polynomial Model with reduced complexities. The proposed model was tested with a 5MHz LTE signal measured at the input and output of a Doherty PA under different conditions of nonlinearities, memory effects and attenuations as well as PA working powers. It can be observed that the proposed model shows superior accuracy at low complexities, when the PA has higher levels of nonlinearity and memory depth while still maintaining low complexities. Over 60% of coefficients reduction could be reached at the same level of accuracy compared to the MP model.

In this paper, we propose a novel wideband wireless transmission scheme with spatially combined frequency interleaved transmitters of narrower bandwidths. The proposed technique enables development of seamless scalable bandwidth transmitters for next generation wireless and satellite communications with potential to reuse legacy hardware. The scheme is experimentally validated at Ka Band with instantaneous modulation bandwidths up to 400 MHz achieving transmission bit rates up to 2.039Gbps.

In modern radio frequency (RF) transceivers the power amplifier (PA) is a central component in terms of power consumption. Achieving efficient performance in this component results in the PA output signal becoming distorted. Linearisation can be performed using techniques such as Digital Pre-Distortion (DPD). The pre-distorter operation of a DPD system involves the constant computation of a distorted signal to ensure linear operation of the nonlinear power amplifier. In this work a novel polynomial evaluation scheme is proposed to optimise the pre-distorter operation within a DPD system. Improvements to latency and hardware requirements are possible with new techniques. Validation of the proposed design was conducted using FPGA implementations and compared to incumbent pipelined solutions for both low latency and hardware efficiency. The proposed method indicated hardware savings of 67.8%, while operating 58.7% faster, compared to an existing implementation.

As the physical makeup of cellular basestations evolve into systems with multiple parallel transmission paths the effort involved in modelling these complex systems increases considerably. One task in particular which contributes to signal distortion on each signal path, is the power amplifier. In power amplifier (PA) modelling, Recursive Least Squares (RLS) has been used in the past to train Volterra models with memory terms. The Volterra model is widely used for modelling of PAs. In this paper we present a comparison of the stability performance for a PA model during training for various model memory lengths, model orders of non linearity and signal sample rates. This examination provides a technique to avoid instability occurring during the adaptive training of dynamic nonlinear behavioural models.

Efficient and linear power amplifiers (PA) are an essential part of forthcoming 5G wireless systems. Outphasing class-E PAs offer high power efficiency and an option for higher efficiency cellular networks. However, they employ signal component separators, which split the signal into two paths. In order to efficiently recombine the signal, nonlinear power combiners are used. This paper proposes a novel phase-only predistortion technique for outphasing class-E PAs. The predistortion coefficients can be extracted based on AMAM characteristics of the output signal and an analytical model of an outphasing Class E PA. The suggested technique has been validated by simulation of an outphasing power amplifier in ADS Ptolemy software. It is shown that applying this technique to a 16QAM OFDM modulated signal with 20 MHz bandwidth improves error vector magnitude (EVM) from 10.39% to 2.43% compared to the signal without predistortion.

In this paper, we present a novel and effective linearization technique for nonlinear phased array antennas. For large phased arrays, linearization of the array using a single digital predistortion (DPD) is inevitable since one digital path is upconverted and feeds several RF transmission paths, each of which is connected to a power amplifier (PA) and an antenna element. However, a critical issue is that the PA characteristics can vary considerably within an array. Thus, linearizing individual PAs with one DPD is rather challenging. We formulate and solve an optimization problem that corresponds to jointly minimizing the maximum residuals between the input to the array and the output of individual PAs. We demonstrate that the proposed technique outperforms state-of-the-art linearization solutions while retaining the linear gain of the array.

This work proposes a concise standalone model for Chireix power combiner. The model is based on the analysis of the nonlinear behaviour of a Chireix power combiner under amplitude imbalance. The generalised equations for input impedance of Chireix combiner branches excited by signals with different amplitudes have been derived. The results predicted by the model were validated using both commercial high frequency circuit simulations and experimentally measured results. As a result the proposed model serves two purposes. For one it can be used directly in a larger circuit simulation to predict the performance of an outphasing power amplifier. It can also be used as the basis for a linearization strategy for outphasing power amplifiers.

In this paper, we study the sum rate maximization for a multiple-input multiple-output (MIMO) system with successive zero-forcing dirty-paper coding (SZFDPC) and per-antenna power constraint (PAPC). Although SZFDPC is a low-complexity alternative to the optimal dirty paper coding, efficient algorithms to compute its sum rate are still open problems especially under practical PAPC. The existing solution to the considered problem is computationally inefficient due to employing high-complexity interior-point method. In this study, we propose two novel low-complexity approaches to this important problem. More specifically, the first algorithm achieves the optimal solution by transforming the original problem in the broadcast channel into an equivalent problem in the multiple access channel, then the resulting problem is solved by alternating optimization together with successive convex approximation. We also derive a suboptimal solution based on machine learning to which simple linear regressions are applicable. The approaches are analyzed and validated extensively to demonstrate their superiors over the existing approach.

This paper discusses topic of high Peak to Average Power Ratio (PAPR) in Orthogonal Frequency Division Multiplexing (OFDM) based signals. An innovative technique is proposed to reduce PAPR without compromising Error Vector Mapping (EVM). The proposed technique is named Peak Shrinking and Interpolating (PSI), and results show no more than 1% EVM when the technique is operated. The hardware resource consumption of PSI technique is analyzed and compared with the simplest Crest Factor Reduction (CFR) technique known as Clipping and Filtering (CF). This analysis, together with EVM performance, and PAPR reduction performance indicates that PSI technique can be an outstanding scheme for existing and future technologies such as Long-Term Evolution (LTE) and 5th Generation of cellular mobile communications (5G), resulting in more efficient Power Amplifier (PA) operation.

This paper discusses topic of high Peak to Average Power Ratio (PAPR) in Orthogonal Frequency Division Multiplexing (OFDM) based signals. An innovative technique is proposed to reduce PAPR without compromising Error Vector Mapping (EVM). The proposed technique is named Peak Shrinking and Interpolating (PSI), and results show no more than 1% EVM when the technique is operated. The hardware resource consumption of PSI technique is analyzed and compared with the simplest Crest Factor Reduction (CFR) technique known as Clipping and Filtering (CF). This analysis, together with EVM performance, and PAPR reduction performance indicates that PSI technique can be an outstanding scheme for existing and future technologies such as Long-Term Evolution (LTE) and 5th Generation of cellular mobile communications (5G), resulting in more efficient Power Amplifier (PA) operation.

Power amplifiers are widely employed electronic devices in various fields such as mobile networks and radio frequency (RF) transceivers. To achieve efficient operations, power amplifiers can often suffer from nonlinearity problems. This problem can be mitigated through the use of linearization techniques, such as digital predistortion, regarded as the most promising solution to power amplifier linearization. Behavioural modeling is a substantial part of the digital predistortion, responsible for acquiring the coefficients that are necessary to linearize the power amplifier. A Complex Reduced Non-Uniform Generalized Memory Polynomial model was proposed to reach comparable performance of accuracy as Memory Polynomial Model with reduced complexities. The proposed model was tested with a 5MHz LTE signal measured at the input and output of a Doherty PA under different conditions of nonlinearities, memory effects and attenuations as well as PA working powers. It can be observed that the proposed model shows superior accuracy at low complexities, when the PA has higher levels of nonlinearity and memory depth while still maintaining low complexities. Over 60% of coefficients reduction could be reached at the same level of accuracy compared to the MP model.

In this paper, we present a novel and effective linearization technique for nonlinear phased array antennas. For large phased arrays, linearization of the array using a single digital predistortion (DPD) is inevitable since one digital path is upconverted and feeds several RF transmission paths, each of which is connected to a power amplifier (PA) and an antenna element. However, a critical issue is that the PA characteristics can vary considerably within an array. Thus, linearizing individual PAs with one DPD is rather challenging. We formulate and solve an optimization problem that corresponds to jointly minimizing the maximum residuals between the input to the array and the output of individual PAs. We demonstrate that the proposed technique outperforms state-of-the-art linearization solutions while retaining the linear gain of the array.

In modern radio frequency (RF) transceivers the power amplifier (PA) is a central component in terms of power consumption. Achieving efficient performance in this component results in the PA output signal becoming distorted. Linearisation can be performed using techniques such as Digital Pre-Distortion (DPD). The pre-distorter operation of a DPD system involves the constant computation of a distorted signal to ensure linear operation of the nonlinear power amplifier. In this work a novel polynomial evaluation scheme is proposed to optimise the pre-distorter operation within a DPD system. Improvements to latency and hardware requirements are possible with new techniques. Validation of the proposed design was conducted using FPGA implementations and compared to incumbent pipelined solutions for both low latency and hardware efficiency. The proposed method indicated hardware savings of 67.8%, while operating 58.7% faster, compared to an existing implementation.

In this paper, we study the sum rate maximization for a multiple-input multiple-output (MIMO) system with successive zero-forcing dirty-paper coding (SZFDPC) and per-antenna power constraint (PAPC). Although SZFDPC is a low-complexity alternative to the optimal dirty paper coding, efficient algorithms to compute its sum rate are still open problems especially under practical PAPC. The existing solution to the considered problem is computationally inefficient due to employing high-complexity interior-point method. In this study, we propose two novel low-complexity approaches to this important problem. More specifically, the first algorithm achieves the optimal solution by transforming the original problem in the broadcast channel into an equivalent problem in the multiple access channel, then the resulting problem is solved by alternating optimization together with successive convex approximation. We also derive a suboptimal solution based on machine learning to which simple linear regressions are applicable. The approaches are analyzed and validated extensively to demonstrate their superiors over the existing approach.

As the physical makeup of cellular basestations evolve into systems with multiple parallel transmission paths the effort involved in modelling these complex systems increases considerably. One task in particular which contributes to signal distortion on each signal path, is the power amplifier. In power amplifier (PA) modelling, Recursive Least Squares (RLS) has been used in the past to train Volterra models with memory terms. The Volterra model is widely used for modelling of PAs. In this paper we present a comparison of the stability performance for a PA model during training for various model memory lengths, model orders of non linearity and signal sample rates. This examination provides a technique to avoid instability occurring during the adaptive training of dynamic nonlinear behavioural models.

Next generation wireless communication systems such as 5G will feature a step increase in the speed of wireless data transmission. This requires using RF signals of wider bandwidths along with higher order modulation schemes. Such signals are subjected to frequency and power dependent impairments such as gain compression, amplitude ripple and tilt, group-delay and AM-PM conversion as they traverse through the transmitter’s hardware blocks. This paper introduces a novel calibration scheme that applies a specially designed wideband 64- QAM modulated stimulus and uses the captured response to evaluate the impairments within the transmitter’s RF chain. The needed pre-compensation is applied in a single iteration. This technique is intended for use with Doherty, Outphasing and MIMO type applications. Presented technique was experimentally validated using signal bandwidths up to 100 MHz for both weakly and strongly nonlinear systems.

In this paper, we propose a novel wideband wireless transmission scheme with spatially combined frequency interleaved transmitters of narrower bandwidths. The proposed technique enables development of seamless scalable bandwidth transmitters for next generation wireless and satellite communications with potential to reuse legacy hardware. The scheme is experimentally validated at Ka Band with instantaneous modulation bandwidths up to 400 MHz achieving transmission bit rates up to 2.039Gbps.

This work proposes a concise standalone model for Chireix power combiner. The model is based on the analysis of the nonlinear behaviour of a Chireix power combiner under amplitude imbalance. The generalised equations for input impedance of Chireix combiner branches excited by signals with different amplitudes have been derived. The results predicted by the model were validated using both commercial high frequency circuit simulations and experimentally measured results. As a result the proposed model serves two purposes. For one it can be used directly in a larger circuit simulation to predict the performance of an outphasing power amplifier. It can also be used as the basis for a linearization strategy for outphasing power amplifiers.

Efficient and linear power amplifiers (PA) are an essential part of forthcoming 5G wireless systems. Outphasing class-E PAs offer high power efficiency and an option for higher efficiency cellular networks. However, they employ signal component separators, which split the signal into two paths. In order to efficiently recombine the signal, nonlinear power combiners are used. This paper proposes a novel phase-only predistortion technique for outphasing class-E PAs. The predistortion coefficients can be extracted based on AMAM characteristics of the output signal and an analytical model of an outphasing Class E PA. The suggested technique has been validated by simulation of an outphasing power amplifier in ADS Ptolemy software. It is shown that applying this technique to a 16QAM OFDM modulated signal with 20 MHz bandwidth improves error vector magnitude (EVM) from 10.39% to 2.43% compared to the signal without predistortion.

A new technique for the estimation of coupling between array elements in MIMO transceivers is presented. The estimation method is founded on subsequent transmission and over-the-air (OTA) reception between the array elements, when the transmitter’s characteristics are unknown. The method enables accurate on-line characterization of the behavior of the array in terms of coupling for a complete system with PAs and antennas connected. The method is validated via OTA measurements on a four-element array in a MIMO transmitter configuration. The results demonstrate that the estimated couplings with the proposed technique are similar to the ones obtained from S-parameter measurements with deviations reflecting the operation with antennas and PAs connected. The proposed intra-array coupling estimation facilitates practical OTA measurement-based modeling and linearization of MIMO transmitters.