Baseband Digital Signal Research Articles

Adaptive suppression of passive intermodulation in digital satellite transceivers

For the performance issues of satellite transceivers suffering passive intermodulation interference, a novel and effective digital suppression algorithm is presented in this paper. In contrast to analog approaches, digital passive intermodulation (PIM) suppression approaches can be easily reconfigured and therefore are highly attractive for future satellite communication systems. A simplified model of nonlinear distortion from passive microwave devices is established in consideration of the memory effect. The multiple high-order PIM products falling into the receiving band can be described as a bilinear predictor function. A suppression algorithm based on a bilinear polynomial decorrelated adaptive filter is proposed for baseband digital signal processing. In consideration of the time-varying characteristics of passive intermodulation, this algorithm can achieve the rapidness of online interference estimation and low complexity with less consumption of resources. Numerical simulation results show that the algorithm can effectively compensate the passive intermodulation interference, and achieve a high signal-to-interference ratio gain.

Low Power Gated-Clock Design for Multi-core DSP Based SDR Platform

With the rapid development of wireless communication systems, multi-core DSP with high computing performance is an important part of SDR (Software-Defined Radio) platform. Research has focused on low-power design in SDR platform since the SDR platform is sensitive to power consumption. Following the baseband digital signal processing features of SDR application, this paper proposes a data-driven and task-driven gated-clock architecture. The DSP cores in the multi-core DSP can be turned on and turned off with this architecture at the appropriate time. Experiments show that the proposed low power gated-clock architecture can provide effective low-power performance for multi-core DSP in SDR platforms.

A Fully Integrated IEEE 802.15.7 Visible Light Communication Transmitter With On-Chip 8-W 85% Efficiency Boost LED Driver

This paper presents an IEEE 802.15.7 Physical Layer I (PHY-I) standard compliant visible light communication (VLC) transmitter system-on-a-chip (SoC), enabling the use of ordinary white LED lights as beacons or broadcasters for location-based applications. The mixed-signal SoC integrates a baseband digital signal processing (DSP) unit, a VLC modulator, and a switching boost LED driver with an on-chip power MOSFET. The DSP unit supports various coding schemes as well as both the on-off keying (OOK) modulation and the variable pulse-position modulation specified in the aforementioned standard. The boost LED driver is designed to power a matrix of 4 × 5 LEDs using Li-ion batteries for portable applications. The SoC simultaneously supports VLC and LED illumination with nine dimming levels. Implemented in a 0.35-μm CMOS process with 20-V devices, the SoC achieves a measured VLC data rate of up to 266 kb/s at a package error rate (PER) < 10% using a 211 −1 PRBS (pseudo random binary sequence) input. The communication distance measured is more than 2 m using no optical lens with the link efficiency of 5 nJ/bit. From an input at 3–5 V, the boost converter generates an output at 6–20 V with a 92% peak efficiency and an 8-W rating.

Concurrent Multiband Digital Outphasing Transmitter Architecture Using Multidimensional Power Coding

All-digital outphasing transmitter architecture using multidimensional power coding (MDPC) is proposed for noncontiguous concurrent multiband transmission with a high power efficiency. MDPC transforms multiband digital baseband signals into multibit low-resolution digital signals that drive switching-mode PAs. A prototype digital outphasing transmitter consists of two 1-GHz bandwidth GaN Class-D PAs and a Chireix power combiner. The two GaN PAs are driven by bipolar radio frequency (RF) pulse-width modulation (PWM) signals, which are transformed from a concurrent dual-band LTE signal by MDPC. The dual-band LTE signal with 15-MHz aggregate channel bandwidth at 240 and 500 MHz frequency band is transmitted with -30 and -37 dBc out-of-band emissions, respectively. Digital outphasing achieves more than two times higher coding efficiency than conventional concurrent dual-band digital transmitters with the same PAs in Class-S operation. Measured power coding efficiencies of 35.4% and 47.1% are observed with outphasing bipolar and 3-level RF PWM signals respectively, which are encoded from the dual-band LTE signal.

Toward a Novel Digital Electronic Conditioning for the GMI Magnetic Sensors: The Software Defined Radio

A new high frequency and fully digital electronics was implemented for the conditioning of giant magnetoimpedance (GMI) sensors. The electronics are based on the use of a direct digital synthesizer as a high frequency source for the excitation of the sensing element and a software defined radio (SDR) that performs a direct analog-to-digital conversion, with over-sampling, and quadrature digital down conversion (DDC) of GMI voltage. This DDC includes digital mixers and digital decimation filters that ensure a low-pass filtering and a processing gain. The DDC outputs the in-phase and quadrature baseband digital signals. Except for an optional preamplifier and a voltage-to-current converter, the whole system is digital and fully programmable in real time using a high performance digital signal processor. The architecture of the developed electronics is given and the various aspects of the design are addressed. The performance and potential of the SDR are discussed throughout this paper. The dynamic range for the digital full scale output was between 95 and 120 dB. The out-of-band noise rejection was beyond our measurement instruments. The concept has been successfully applied and used for a GMI sensor and the first measurements were conducted.

The Research on Phantom-Bit Technology of Digital Phase Shifters for Satellite Tracking System of Phase Array

Phased array satellite platform self-tracking system is for the stability between the missiles and other high-speed movement of the platform and the relay satellite two-way information transmission needs to carry out the self-tracking technology research of onboard platform-dimensional active phased array satellite. The system uses a sub-array correlation method for accurate measurement of the angle of the satellite signal. Receiving array is divided into four 4 * 4 sub-array, each antenna signal combining unit 4 sub array for 4-way A / D to be converted, through down-conversion, filtering, extraction and other processes to get a digital baseband signal, the baseband digital signal processing is to extract the angle error information into digital beam orientation system for tracking filtering operation, thereby ensuring that the transceiver has been aligned with the satellite antenna beam direction. In this paper ,phantom-bit technology for satellite tracking system under the condition of minimum beam displacement is researched for satellite tracking system.

2FSK Modem Based on the Microcontroller AT89C51

In order to simplify the device of modulation and demodulation of 2FSK, a 2FSK modem based on the microcontroller AT89C51 was designed. This modem is mainly composed of an AT89C51 microcontroller, a simple two order passive low-pass filter, and a shaping circuit. Modulation is achieved on the basis of the principle of pulse width modulation (PWM). By executing PWM signal generating program, a digital base band signal is converted into the corresponding PWM signal after it is input into an I/O port of the microcontroller. Then the PWM signal is outputted from another I/O port and sent into the two order passive low-pass filter. On the output side of the filter, the 2FSK signal can be obtained. For demodulation, the 2FSK signal shaped by the shaping circuit is input into microcontroller from its I/O port. Then the widths of positive and negative half-waves are measured by the microcontroller internal timer. By comparing the value of each width with the appropriate threshold value, the binary digital signal is gotten and outputted from another I/O port of microcontroller. This modem makes full use of the resources of the microcontroller. As a result, it is simple, easy to use, economical and reliable. Furthermore, it can generate and demodulate the 2FSK signal with high good accuracy.

Simulating GPS radio signal to synchronize network--a new technique for redundant timing.

Currently, many distributed systems such as 3G mobile communications and power systems are time synchronized with a Global Positioning System (GPS) signal. If there is a GPS failure, it is difficult to realize redundant timing, and thus time-synchronized devices may fail. In this work, we develop time transfer by simulating GPS signals, which promises no extra modification to original GPS-synchronized devices. This is achieved by applying a simplified GPS simulator for synchronization purposes only. Navigation data are calculated based on a pre-assigned time at a fixed position. Pseudo-range data which describes the distance change between the space vehicle (SV) and users are calculated. Because real-time simulation requires heavy-duty computations, we use self-developed software optimized on a PC to generate data, and save the data onto memory disks while the simulator is operating. The radio signal generation is similar to the SV at an initial position, and the frequency synthesis of the simulator is locked to a pre-assigned time. A filtering group technique is used to simulate the signal transmission delay corresponding to the SV displacement. Each SV generates a digital baseband signal, where a unique identifying code is added to the signal and up-converted to generate the output radio signal at the centered frequency of 1575.42 MHz (L1 band). A prototype with a field-programmable gate array (FPGA) has been built and experiments have been conducted to prove that we can realize time transfer. The prototype has been applied to the CDMA network for a three-month long experiment. Its precision has been verified and can meet the requirements of most telecommunication systems.

IF Modulation in the Millimeter-Wave Transmitter End

This article describes a method of IF modulation used in millimeter-wave transmitter, the method is AD9857 combined with FPGA, choose the EP1C6 as a FPGA chip, and use this FPGA chip complete the base-band digital signal processing, and implement the control to the AD9857. AD9857 is a kind of the orthogonal frequency converter based on DDS technology, complete the functions of base-band digital signal interpolation filter, orthogonal modulation, and D/A conversion in the digital domain and complete QDPSK modulation signal output of 70MHz IF carrier. This method can eliminate phase and gain imbalances and distortion which caused by analog modulation, also can simplify the system structure, reduce cost and improve the performance and reliability of the system.

Optical-wireless-optical full link for polarization multiplexing quadrature amplitude/phase modulation signal transmission

We propose and experimentally demonstrate an optical wireless integration system at the Q-band, in which up to 40 Gb/s polarization multiplexing multilevel quadrature amplitude/phase modulation (PM-QAM) signal can be first transmitted over 20 km single-mode fiber-28 (SMF-28), then delivered over a 2 m 2 × 2 multiple-input multiple-output wireless link, and finally transmitted over another 20 km SMF-28. The PM-QAM modulated wireless millimeter-wave (mm-wave) signal at 40 GHz is generated based on the remote heterodyning technique, and demodulated by the radio-frequency transparent photonic technique based on homodyne coherent detection and baseband digital signal processing. The classic constant modulus algorithm equalization is used at the receiver to realize polarization demultiplexing of the PM-QAM signal. For the first time, to the best of our knowledge, we realize the conversion of the PM-QAM modulated wireless mm-wave signal to the optical signal as well as 20 km fiber transmission of the converted optical signal.

Research on Performance Analysis of Digital Baseband Signal Transmission

Nowadays, in order to improve the quality of signal transmission and reduce the signal attenuation and noise interference, the transmission digital baseband signal modulation become mainstream in today's signal transmission. For the digital baseband signal transmission performance of research helps us to understand the process of digital signal transmission. Firstly, using shift registers to produce m sequence, used to simulate the data signal and the pseudo random noise signal. Then, to design a low pass filter, used to simulate the channel. Data signal transmission in the channel was carried with additive random noise interference, we can measure signal interference by observing eye pattern at the receiving terminal. If to Manchester encoding of data signal, the receiver decoding output data signal can be extracted using the synchronous clock circuit to get the clock signal to synchronize, and display the eye pattern, which is used to evaluate the performance of the transmission system. And we put forward the optimization scheme of the digital baseband signal transmission performance through the research.

2FSK Signal Generator Based on AT89C51 Microcontroller

In order to simplify the devices used for generating the 2FSK signal, a 2FSK signal generator based on AT89C51 microcontroller is proposed in this paper. A digital base band signal is inputted to the microcontroller through one of I/O ports. Then microcontrollers program is executed, which are designed for generating PWM signals according to the sine waveforms corresponding with 2FSK signals required. Consequently, an appropriate PWM signal is outputted from another I/O port. This PWM signal is sent into a simple second order passive low-pass filter, and the 2FSK signal can be obtained on the output side of the filter. The 2FSK signal generator makes full use of the resources of the microcontroller, and as a result, it is simple, easy to use, economical and reliable.

Test Signal Development and Analysis for OFDM Systems RF Front-End Parameter Extraction

Testing radio frequency (RF) transceivers requires the measurement of a diverse set of specifications, requiring multiple testing setups. This complicates load board design, debug, and diagnosis, as well as results in long testing time. In this paper, we present a single setup testing solution for orthogonal frequency-division multiplexing systems RF front-ends based on a loop-around scheme. With this technique, it is possible to determine gain and phase mismatch, inphase-quadrature time skew, and dc offset. Linear gain and IIP3 decouple the transmitter parameters from the receiver parameters. Although loop-around has been used in many forms, the basic challenge is to determine what input conditions will lead to accurate measurement and what form of modeling will yield this accuracy. To this end, we develop test signal design and multistep extraction techniques. Experimental results indicate that IIP3 can be extracted with 0.6 dB maximum error while phase mismatch and gain mismatch can be extracted with 0.3° and 0.6% maximum error. Our method is able to de-embed the characteristics of transmitter from those of receiver while it requires the analysis of only low-frequency digital baseband signals (I and Q branches) and eliminates the need for RF testers.

Digital predistortion of 75–110 GHz W-band frequency multiplier for fiber wireless short range access systems

We present a W-band fiber-wireless transmission system based on a nonlinear frequency multiplier for high-speed wireless short range access applications. By implementing a baseband digital signal predistortion scheme, intensive nonlinear distortions induced in a sextuple frequency multiplier can be effectively pre-compensated. Without using costly W-band components, a transmission system with 26 km fiber and 4 m wireless transmission operating at 99.6 GHz is experimentally validated. Adjacent-channel power ratio (ACPR) improvements for IQ-modulated vector signals are guaranteed and transmission performances for fiber and wireless channels are studied. This W-band predistortion technique is a promising candidate for applications in high capacity wireless-fiber access systems.

Digital compensation of transmitter leakage in FDD zero‐IF receivers

ABSTRACTTransmitter leakage (TxL) has a significant impact on the performance of frequency division duplex devices using zero‐IF receivers and thus requires a suitable compensation. In contrast to analog approaches, digital TxL compensation approaches can easily be reconfigured and therefore are highly attractive for multiband mobile devices. In this contribution, we present a system model, describing the TxL impact on the received digital baseband signal. Furthermore, we show analytically that TxL causes an additional, irreversible signal‐to‐noise loss due to the quantisation in the analog‐to‐digital converter, which limits the applicability of a digital TxL compensation. However, this signal‐to‐noise ratio loss remains negligibly small as long as the TxL interference does not exceed significantly the desired received signal. For TxL to be compensated digitally, the TxL channel needs be estimated. In this work, we present two TxL channel estimation approaches suitable for frequency flat and frequency selective channels. It is shown that the usage of these approaches mitigates effectively the TxL impact on the system performance. Thus, the system designer can allow much stronger TxL, which significantly relaxes the requirements on the transmitter–receiver isolation of the duplexer and the linearity of the analog receiver frontend. Copyright © 2011 John Wiley &amp; Sons, Ltd.

100-GHz Wireless-Over-Fiber Links With Up to 16-Gb/s QPSK Modulation Using Optical Heterodyne Generation and Digital Coherent Detection

In this letter, a novel technique for direct conversion of an optical baseband quadrature phase-shift keying (QPSK) signal to a millimeter-wave wireless signal and subsequent signal demodulation is reported. Optical heterodyne mixing of the optical baseband QPSK signal with a free-running unmodulated laser for the wireless signal generation is employed. To correct for the phase and frequency offset originating from the heterodyne mixing of the two free-running lasers, wireless signal demodulation based on optical coherent detection in combination with baseband digital signal processing is implemented. As a proof of concept, 5-Gb/s amplitude-shift keying and up to a 16-Gb/s QPSK wireless signal in the band of 75-110 GHz was generated and successfully demodulated. All-photonic millimeter-wave wireless signal generation and digital coherent detection at baud-rate are employed without complex optical phase-locked loop.

Radio-Frequency Transparent Demodulation for Broadband Hybrid Wireless-Optical Links

A novel demodulation technique which is transparent to radio-frequency (RF) carrier frequency is presented and experimentally demonstrated for multigigabit wireless signals. The presented demodulation technique employs optical single-sideband filtering, coherent detection, and baseband digital signal processing. Multigigabit wireless signal demodulation of 1.25-Gbaud quadrature phase-shift-keying modulated data at 40- and 35-GHz RF carrier frequency is experimentally demonstrated using the proposed demodulation scheme.

Power Amplifiers and Transmitters for Next Generation Mobile Handsets

As a wireless handset deals with multiple application standards concurrently, RF transmitters and power amplifiers are required to be more power efficient and reconfigurable. In this paper, we review the recent advances in the design of the power amplifiers and transmitters. Then, the systematic design approaches to improve the performance with the digital baseband signal processing are introduced for the next generation mobile handset.

Wireless Communications Transmitter Performance Enhancement Using Advanced Signal Processing Algorithms Running in a Hybrid DSP/FPGA Platform

This paper deals with digital base band signal processing algorithms, which are seen as enabling technologies for software-enabled radios, that are intended for the correction of the analog front end. In particular, this paper focuses on the design, optimization and testability of predistortion functions suitable for the linearization of narrowband and wideband transmitters developed with a hybrid DSP/FPGA platform. To select the best algorithm for the identification of the predistortion function, singular value decomposition, recursive least squares (RLS), and QR-RLS algorithms are implemented on the same digital signal processor; and, the computation complexity, time, accuracy and the required resources are studied. The hardware implementation of the predistortion function is then carefully performed, in order to meet the real time execution requirements.

Efficient Design of OFDMA-Based Programmable Wireless Radios

With the increasing demand for efficient spectrum management, programmable wireless radios can potentially play a key role in shaping our future spectrum use. In this paper, we consider the design of low-power programmable wireless radios based on orthogonal frequency division multiple access (OFDMA). To meet the demands of higher data rate communications, we split OFDMA symbols carrying multiuser data across several noncontiguous bands of available spectrum. To relax power consumption in analog-to-digital and digital-to-analog converters, we use a programmable narrowband RF front end comprising of programmable synthesizers and fixed low-pass filters. To perform digital baseband signal processing in an energy efficient manner, we propose efficient designs for the fast Fourier transform (FFT) and inverse FFT (IFFT) modules. Our designs of the FFT/IFFT modules reduce power consumption and chip area, and are capable of handling the dynamic nature of spectrum in programmable radios. To recover data that falls within the transition band of the filters, we propose a combiner similar to maximal ratio combiner. We also present the complete design of programmable wireless radios in accordance with the IEEE 802.22 (draft) standard.