LTE-Advanced Signals Research Articles

Performance Analysis of Carrier-Aggregated Multiantenna 4 × 4 MIMO LTE-A Fronthaul by Spatial Multiplexing on Multicore Fiber

In this paper, we experimentally propose and evaluate the performance of multiantenna LTE-Advanced (LTE-A) systems implementing multiple-input-multiple-output (MIMO) space division multiplexing on multicore fiber, compared to single-antenna single-antenna system (SISO) transmissions. Fully standard 3GPP LTE-A cellular signals are transmitted with MIMO and carrier aggregation in radio-over-fiber over a four-core fiber in different configurations. The processing capabilities of in-built 3GPP MIMO processing are evaluated experimentally in two-antenna and four-antenna LTE-A configurations and compared with single-antenna SISO performance. The robustness of 3GPP MIMO processing is analyzed over different optical paths in a four-core fiber and the optical power margin available between the four optical paths is calculated for each configuration. Finally, the transmission performance of carrier-aggregated LTE-A signals is evaluated in the four-antenna system implementing 4 × 4 MIMO spatial multiplexing with different carrier separation and center frequency configurations, including regulated cellular frequencies of frequency division duplex bands 7 and 20.

Comparison of a genetic programming approach with ANFIS for power amplifier behavioral modeling and FPGA implementation

Accurate modeling of power amplifiers (PA) is of upmost importance in the design process of wireless communication systems where a high linearity and efficiency is required. To deal with the nonlinear behavior of PAs effectively a linearization stage is applied to minimize the distortions of in-band and adjacent transmission channels, which translate to an improvement of the signal integrity and the operation cost of the transmitter system. This paper presents a method based on genetic programming with a local search heuristic (GP-LS) to emulate the electrical memory effects by using the characteristic conversion curves of the radio frequency (RF) PA NXP Semiconductor of 10 W GaN HEMT working at 2.34 GHz. This method is compared with an Adaptive Neuro-Fuzzy Inference Systems (ANFIS) through several performance metrics (NMSE, MAE and correlation coefficient), with GP-LS achieving a better modeling accuracy. Moreover, the models produced by GP-LS permit a reduction in the required hardware resources, when it is implemented on a Field-Programmable Gate Array through the DSP Builder tool. The models are derived using a data-driven approach, posed in two different ways. Firstly, experiments are performed using a testbed Arria V GX for a flexible vector signal generation that provides the raw data of the PA characterization using an LTE-Advanced signal with 10-MHz bandwidth. Secondly, the modeling is derived from a filtered version of the data and then adding a high-frequency signal as a post processing step to approximate the true behavior of the system. In both cases, the models are generated with ANFIS and GP-LS, performing extensive logic-based simulations and implementing the models on a Cyclone III development board. Both approaches are compared based on accuracy and required hardware resources, with GP-LS substantially outperforming ANFIS. These results suggest that the GP-LS models can be implemented in a digital predistortion chain and used in the linearization stage for a RF-PA.

Next-Generation Optical Fronthaul Systems Using Multicore Fiber Media

This paper proposes and investigates the use of multicore fiber (MCF) media performing space-division multiplexed transmission for next-generation optical fronthaul systems. We report the experimental demonstration of combined radio-over-fiber (RoF) transmission of full-standard LTE-Advanced (LTE-A) and WiMAX signals providing fronthaul connectivity in 150 m of 4-core fiber (4CF), transmitting simultaneously fully independent wireless services. Operating in linear and nonlinear optical power regimes, the experimental evaluation verifies that the error vector magnitude (EVM) is not degraded when intercore and intracore Kerr nonlinearities are excited in MCF with high input power levels. As a result, nonlinear regime is proposed as a key factor to reduce the temporal EVM fluctuation induced by the random nature of the intercore crosstalk in MCF. In addition, MCF fronthaul applied to converged fiber-wireless polarization multiplexed passive optical networks is demonstrated to transmit LTE-A and WiMAX signals over two orthogonal optical polarizations. The polarization-multiplexed signal is transmitted in RoF over 25.2 km of standard single-mode fiber and then demultiplexed and injected in different cores of the 4CF to provide fronthaul connectivity. Finally, the extension of multicore optical fronthaul capacity is proposed using MIMO LTE-A signals. The tolerance of the MIMO LTE-A RoF transmissions to in-band crosstalk is reported and compared to single-input single-output (SISO) configuration. The experimental results indicate that MIMO configuration is more tolerant than SISO to in-band crosstalk considering both internal and external interferences. MIMO and SISO configurations are compared when transmitted in RoF over a 4CF operating in linear and nonlinear regimes and core interleaving nonlinear stimulation is proposed to reduce the temporal and spectral EVM fluctuation when the same wireless standard is propagated in each core.

Statistical model of OFDM and its application in nonlinearity analysis of LTE-Advanced systems

In this letter, to study the nonlinearity of RF power amplifiers (PAs), we showed that LTE-Advanced signals with large number of subcarriers can be expressed as a white Gaussian noise statistical model with flat power spectrum. This characteristic enables us to express the spectrum regrowth of LTE-Advanced signals up to an arbitrarily high order in an explicit form, in terms of the traditional PA nonlinearity parameter intercept points without complicated convolution. This general analytic expression would help RF designers to predict the PA distortion with simple intercept point expression, and provide insight for pre-distortion.

Highly Linear Fully Integrated Wideband RF PA for LTE-Advanced in 180-nm SOI

A highly linear fully integrated RF power amplifier (PA) for advanced long-term evolution (LTE-advanced) is fabricated in a 180-nm standard silicon-on-insulator process. To improve the linearity, several harmonic traps are introduced to minimize the second- and third-order harmonics, and hence, the third-order intermodulation distortion (IMD3). The impact on the linearity of the RF PA of each of the harmonic controls is studied and simulated. Finally, both the linearity of a standalone RF PA and an RF PA with proposed linearity enhancement circuitry is investigated by comparing their measured IMD3. For an LTE uplink signal with 20-MHz signal bandwidth and a 64-QAM 8.7-dB peak-to-average power ratio, the RF PA achieves 21.7% power-added efficiency (PAE) with 11-dB gain while delivering an average output power of 22.4 dBm in LTE-band VII. Simultaneously, the RF PA obeys the stringent spectral mask and the $-$ 30-dBc adjacent channel leakage ratio linearity requirement and achieves an error vector magnitude of only 4.05%. The linearized wideband RF PA is also verified for LTE-band I and achieves similar performance from 1.9 to 2.8 GHz. Thanks to the applied linearization technique, the proposed RF PA is able to transmit an LTE-advanced (release 12) signal with a carrier-aggregated bandwidth up to 60 MHz while satisfying the linearity requirements. For a 40-MHz (2 $\times$ 20 MHz) and 60-MHz (3 $\times$ 20 MHz) LTE-advanced signal at band VII, the RF PA produces an average output power of 19.2 and 17.6 dBm while achieving a PAE of 17.2% and 13.7%, respectively.

DESIGN OF ASYMMETRICAL DOHERTY POWER AMPLIFIER WITH REDUCED MEMORY EFFECTS AND ENHANCED BACK-OFF EFFICIENCY

This paper presents the design of an asymmetrical Doherty power amplifier (DPA) with improved linearity and efficiency performance. Resonator-type drain bias networks, providing high impedances at the carrier frequency and low impedances with small variation at the envelope frequency, are introduced to reduce the DPA's memory effects when transmitting wideband signals. The general criteria for DPA design are summarized, and the approach to obtain optimum fundamental and harmonic impedances is proposed to achieve back-off efficiency enhancement. For experimental validation, the asymmetrical DPA is designed and implemented using two identical GaN HEMTs. Measured with continuous wave (CW), the proposed DPA delivers a saturation power greater than 49.3 dBm from 3400 to 3600 MHz, along with high drain efficiency of over 62% and 48% at peak and 8-dB back-off power, respectively. Driven with 100-MHz LTE-advanced signals, the adjacent channel leakage ratio (ACLR) asymmetry of the DPA at 20-MHz offset is lower than 1-dB. After digital predistortion (DPD) linearization, the proposed DPA achieves an ACLR of better than �48 dBc at an average output power about 41 dBm and the drain efficiency over 45% across the frequency band.

Radio-Over-Fiber Optical Polarization-Multiplexed Networks for 3GPP Wireless Carrier-Aggregated MIMO Provision

This paper proposes and demonstrates experimentally the application of optical polarization-multiplexed radio-over-fiber wireless backhauling of fully standard 3GPP carrier-aggregated multiple-input multiple-output (MIMO) signals. The experimental work demonstrates successful long-reach optical transmission of 3GPP carrier-aggregated LTE-Advanced (LTE-A) signals using 2×2 MIMO spatial diversity. The suitability of MIMO provision using radio-over-fiber optical links is demonstrated over different E-UTRA frequency division duplex frequency bands. The performance of electrical carrier aggregation is evaluated in different configurations comprising one, three and five LTE-A component carriers of 10 and 20 MHz bandwidth each. The experimental results demonstrate successful 2×2 MIMO radio-over-fiber polarization-multiplexed transmission of five LTE-A carriers over 25 km, three LTE-A carriers over 75 km and an LTE-A carrier over 100 km of standard single mode fiber to provide pervasive MIMO wireless service to a large number of users.

A Linearized 2–3.5 GHz Highly Efficient Harmonic-Tuned Power Amplifier Exploiting Stepped-Impedance Filtering Matching Network

This letter presents the analysis, implementation and experimental results of a broadband highly efficient power amplifier (PA) using harmonic-tuned method for long-term evolution (LTE)-advanced application. Load-pull simulation has been employed to obtain the optimal load impedances, especially the second harmonic impedances, for high efficiency and output power over wide bandwidth. A stepped-impedance low-pass matching network was then designed to achieve the desired impedance transformation. Experimental results show that the PA achieves 64%-76% efficiency and higher than 40 dBm output power over the frequency band from 2 to 3.5 GHz. When driven by a 100 MHz LTE-advanced signal at an average output power of 33.5 dBm, the average efficiency of 33.8%, 34.9%, and 37.8% was measured at 2.15, 2.55, and 3.45 GHz, respectively. The corresponding adjacent channel leakage ratio can be linearized to about -48 dBc by using digital pre-distortion technique.

A Wideband Doherty Power Amplifier With 100 MHz Instantaneous Bandwidth for LTE-Advanced Applications

In this letter, a 698-862 MHz wideband Doherty power amplifier (DPA) with 100 MHz instantaneous bandwidth is presented. The electrical memory effect of the wideband DPA is reduced by the LC resonant circuits employed in the drain bias networks. The influence of the integrated Doherty combiner is compensated by the broadband output matching networks, which were designed by applying the simplified real frequency technique. Driven with 100 MHz LTE-advanced signals, the adjacent channel leakage ratio (ACLR) and ACLR asymmetry of the proposed DPA at 20 MHz offset are improved by about 4 dB and 3 dB respectively, compared to the wideband DPA with conventional bias networks. After digital predistortion (DPD) linearization, the proposed DPA shows an ACLR of lower than -48 dBc at an average output power of 42.5 dBm, and high drain efficiency of over 42% across the operating frequency range.

A 2.5GHZ-2.7GHz Unsymmetrical Doherty Power Amplifier with Digital Predistortion for LTE-Advanced Applications

An unsymmetrical GaN based Doherty power amplifier (DPA) operating from 2.5GHz to 2.7GHz is presented in this paper. To achieve a good tradeoff among the output power, efficiency and bandwidth, the ladder-type multisection output matching networks are optimized for the carrier amplifier and the peaking amplifier, respectively. Measured with continuous wave (CW) signal, the broadband DPA provides more than 49dBm saturation power in the operating band. The drain efficiency is greater than 44% over 7dB back-off power. For a LTE-Advanced signal with 100MHz bandwidth, the drain efficiency is higher than 42% at an average output power of 41dBm, along with an adjacent channel leakage ratio (ACLR) of better than-49.9dBc after digital predistortion (DPD).

Design of Unsymmetrical Doherty Power Amplifier for 460MHz LTE-Advanced Applications

An unsymmetrical Doherty power amplifier (DPA) at 460MHz is presented in this paper. The carrier and peaking amplifier of the DPA, which base on two equal-sized devices, are matched with different networks to mitigate the performance degradation caused by the limited load modulation. Measured with continuous wave (CW), the unsymmetrical DPA saturates at an output power of 49.2dBm and achieves a drain efficiency of 51% at 6dB back-off. Using a one-carrier long term evolution advanced (LTE-Advanced) signal with 20MHz bandwidth, the unsymmetrical DPA exhibits a drain efficiency of 48.7% at an average output power of 42.1dBm, along with adjacent channel leakage ratio (ACLR) of-34.1dBc and-53.3dBc before and after digital pre-distortion (DPD), respectively.

High-Efficiency GaN Doherty Power Amplifier for 100-MHz LTE-Advanced Application Based on Modified Load Modulation Network

This paper presents a high-efficiency GaN Doherty power amplifier (PA) with 100-MHz instantaneous bandwidth for 3.5-GHz long-term-evolution (LTE)-advanced application. A modified load modulation network, employing an enlarged peaking amplifier to carrier amplifier power ratio and moderately increased load impedance of the carrier amplifier, is proposed for enhancing efficiency and achieving improved load modulation. To increase the power ratio and alleviate the influence of slight impedance mismatch, a proposed load impedance strategy and corresponding stepped-impedance matching network are adopted for high-efficiency and wideband operation. By tuning the carrier offset line, the inconsistency of efficiency, gain, and output power in the operation band can be alleviated. Measurement results show that the Doherty PA has a drain efficiency of approximately 40% with gain fluctuation less than 0.5 dB at 9-dB back-off power, and maximum efficiency of about 60% at saturation in the signal band of 3.4-3.5 GHz. By using the digital pre-distortion (DPD) technique, the Doherty PA achieves adjacent channel leakage ratio of about -48 dBc at an average output power of 40.4 dBm with efficiency of 42.5%, when driven by 100-MHz LTE-advanced signal. To the best of the authors' knowledge, this is the first high-performance result of linearization using conventional DPD technique with 100-MHz bandwidth signals for the GaN Doherty PA at 3.5-GHz frequency band thus far.

Obtaining polynomial coefficients from intercept points of RF power amplifiers

The RF power amplifier is a critical component in 4G wireless communication systems. One of the main concerns of RF power amplifier design is its nonlinearity, which is more severe in OFDM modulation widely used in LTE and WiMAX systems. Nonlinearity is typically described in terms of the intercept points. It is very useful for the design to obtain coefficients of the memoryless polynomial of the power amplifier model from those intercept points. Previously, the expression of the fifth-order coefficient from the fifth intercept point IP5 was derived. This expression is low generalised to arbitrarily high order. A downlink LTE advanced signal is used for experimental verification. This result could be used for the design of power amplifiers or predistorters.

Evolution of reference signals for LTE-advanced systems

3GPP LTE Release 10 standards (also known as LTE-Advanced) adopted some of the state-of-the-art radio access technologies that include carrier aggregation, eight-layer downlink spatial multiplexing, and four-layer uplink spatial multiplexing. For facilitating these enhancements, reference signals have significantly evolved in LTE-Advanced. This article examines underlying design principles of the LTE-Advanced reference signals. Specifically, newly introduced dedicated demodulation reference signals and channel state information reference signals for downlink and improvements of demodulation reference signals and sounding reference signals in uplink are discussed.