Coherent Modem Research Articles

Estimation of the Noise Immunity Characteristics of Telecommunication Network

The harmful impacts of various interference sources in the communication channel of data transmission systems and an increasing noise immunity in linear distortion conditions are important problems in the development of information transmission systems. To improve reliability and the data receiving accuracy, a mathematical model is developed, which allows to assessment of the survivability of telecommunication systems to interference at incoherent receiving. In this case is assumed that system impact the unintentional sources. The mathematical model considers the communication quality, spectral efficiency, coding method, and modulation principle used during the multimedia services. Presently, the development of the corporate multiservice communication networks is in conditions where the size of user and service traffic packets is rapidly increase. To ensure transmission reliability is promising of an implementation of the coherent modem. It will allow a decrease in the failures of such systems. To increase the transmission rate used coherent modem with noise-resistant characteristics. However, the communication quality in this case is decreased, due to more influence of the inter-symbol interference to the data quality receiving. The synthesis of optimal signals based on rectangular pulses with different shapes by the coherent modem for the data transmission system aims is solved by using of matching filter that has a pulse transient response. To obtain this the frequency limiter is used at the output of the channel digital demodulator. One of the ways to properly solve the problem noted above is by applying in the development of telecommunication multiservice networks the mathematical model with correction ability. This one can be developed based on the optimal signal-receiving theory.

Performance Oriented DSP for Flexible Long Haul Coherent Transmission

In long haul optical fiber communication networks, whichcan span thousands of kilometers, bandwidth is at a premium due to the relatively low availability of optical fibers when compared with network traffic demands. Therefore, these networks require the highest performance in the physical layer, with transceivers that are capable of extracting all the available capacity from each optical fiber. Digital coherent transmitters and receivers, which enhance optical transmission systems by using digital signal processing, are essential for achieving this goal. This tutorial discusses the digital signal processing techniques that are used in the design of high performance coherent modems to compensate for adverse channel effects such as fiber impairments and optoelectronic device non-idealities.

Advances in High-Speed DACs, ADCs, and DSP for Optical Coherent Transceivers

We examine analog-to-digital and digital-to-analog converters (ADCs and DACs), as well as digital signal processing (DSP) functions for optical coherent modems.

Real-Time PMD Tolerance Measurements of a PIC-Based 500 Gb/s Coherent Optical Modem

We present real-time polarization mode dispersion (PMD) tolerance measurement results with a commercially available 500 Gb/s coherent modem. The first- and second-order PMD space is explored, showing that peak values of 500 ps of static, first-order PMD (differential group delay) have small penalties. The system was stressed using fast scrambling, with polarization change of over 10 000 rad/s, along with high mean PMD. Penalties were small with sufficient equalization.

Low-Power Based Coherent Acoustic Modem for Emerging Underwater Acoustic Sensor Networks

Smart, small, inexpensive sensor nodes are used to construct underwater acoustic sensor networks. In addition, with the recent increase in the importance of underwater applications, the need for underwater communication has become more important. Hence, an acoustic modem capable of effective underwater communications has become more necessary for the sensor nodes to obtain underwater data. To develop an acoustic modem for effective underwater communications, some limitations must be overcome, such as the very short transmission range of radio waves, limited power supply, and high cost of commercial acoustic modems. Recently, low-power, low-cost acoustic modems have been developed. However, the data rates of these modems are so slow that sensor nodes cannot perform energy-efficient protocols. The objective of this work is to develop an acoustic modem capable of supporting high data rates. We introduce a coherent acoustic modem that uses waterproof ultrasonic sensors to process acoustic waves. The proposed modem is based on a low-power, low-cost, short-range concept, and it also supports a high data rate for energy-efficient MAC and routing protocols. Underwater experiments are conducted to evaluate the performance improvements of our modem. Experimental results show that our modem has the best performance among all recently developed low-power modems and that it is preferable to develop a coherent modem able to perform effective underwater communications.

Effect of number of sub-carriers, cyclic prefix and analogue to digital converter parameters on coherent optical orthogonal frequency division multiplexing modem's transmission performance

The number of sub-carriers, cyclic prefix (CP), and the characteristics of the analogue to digital converter (ADC), are significant parameters influencing the transmission capacity and distance of an optical orthogonal frequency division multiplexing (OOFDM) modem. Numerical simulations of coherent OOFDM modems are undertaken to investigate the effect of the number of sub-carriers, the CP length and ADC associated parameters such as the sampling speed, clipping ratio and quantisation bit on the system performance over single mode fibre (SMF) links for data rates up to 80 Gb/s. The use of a large number of sub-carriers is more effective in combating fibre dispersion when compared with employing a long CP. Moreover in the presence of fibre non-linearities identifying the optimum number of sub-carriers is crucial factor for the modem performance. For various signal data rates up to 40 Gb/s, a set of data rate and transmission distance-dependent optimum ADC parameters are identified. These parameters give rise to negligible clipping and quantisation noise, moreover ADC sampling speed can increase dispersion tolerance while transmitting over SMF links.

Low-Power Based Coherent Acoustic Modem for High-Speed Communication in Underwater Sensor Networks

As the need for underwater communication has recently grown, an acoustic modem has become more necessary for the sensor nodes to perform effective underwater communication. To develop acoustic modems for effective underwater communication, some limitations must be overcome, such as the limited power supply and high cost of commercial acoustic modems. Recently, low-power, low-cost acoustic modems have been developed. However, the data rates of these modems are very slow. The objective of this work is to develop an acoustic modem capable of supporting high data rates. We introduce a coherent acoustic modem that uses waterproof ultrasonic sensors to process acoustic waves. The proposed modem is based on a low-power, low-cost, short-range concept, and it also supports a high data rate as confirmed by underwater experiments. Experimental results show that our modem has the best performance among all recently developed low-power modems.

Multiplexing Technique Based Coherent CPFSK Modem for Voice Band Data Transmission

An analog multiplexing technique based Continuous Phase Frequency Shift Keying (CPFSK) modulator design is described. A 4:1 multiplexer is proposed to be used to map the input data sequence to appropriate synchronous sinusoids, satisfying the orthogonal requirements of a CPFSK waveforms. Phase continuity and constant envelope of the modulated waveform during symbol transition is inherently retained. An all digital demodulator is proposed. Design procedure and implementation of hardware for modem has been provided.

Pilot-symbol aided coherent M-ary PSK in frequency-selective fast Rayleigh fading channels

Pilot-symbol aided coherent M-ary PSK modems in digital cellular mobile radio systems are analyzed theoretically. The error-floors caused by the Doppler spread in a fast fading channel are removed in both flat and selective fading channels. However, the error-floors caused by the delay spread are lower-bounded by those that exist in the ideal coherent detection. The systems are modeled as frequency-selective fast Rayleigh fading channels, corrupted by co-channel interference (CCI) and additive white Gaussian noise (AWGN). In the proposed scheme, pilot symbols are inserted periodically to monitor the channel characteristics. The fading processes experienced by the pilot symbols are used to estimate those suffered by the data symbols using interpolation or filtering. The estimated fade characteristics are used to compensate the random phase variation caused by the Doppler spread, so that the signals can be demodulated coherently. The theoretical performances of the fade compensated coherent modems are evaluated. The results show that the fade compensated coherent demodulation with the least redundancy achieves the same performance as the ideal differential detection in a fading channel. The performance approaches that of the ideal coherent demodulation as more redundancy is allowed. The pilot-symbol-insertion (PSI) scheme is also applicable to M-ary QAM modems and Rician channels. The residual frequency offset can also be compensated by the PSI technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pilot tone aided coherent GMSK systems for sound/data mobile broadcasting

A Gaussian minimum-shift keying (GMSK) modem with built-in fade canceler is proposed narrowband mobile digital broadcasting systems. The proposed scheme combines DC suppressing line code and high-pass filters to create a spectral null at the carrier frequency of the transmit GMSK signal spectrum. This enables an unmodulated carrier pilot tone to be transmitted with the digital broadcast signal without mutual interference. In the receiver, the recovered pilot is used to coherently demodulate the received signal without phase ambiguity. The pilot can be also be used to track and cancel out the random phase noise induced by fast fading. The scheme is applied to a 16 kb/s BT=0.25 GMSK signal with B/sub v/=800 Hz, and its performance over a fast Rayleigh fading channel is investigated using computer simulation. The results demonstrates that the proposed coherent modem is capable of significantly outperforming conventional differential detection in fast fading environments such as a broadcasting channel for mobile reception.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An Efficient DSP QPSK Modem Algorithm Extended to a Parallel Structure

A 9.6 Kbits/sec digital coherent QPSK modem implementation is described which employs single TMS320C25 DSP systems and uses all-digital phase- locked-loops to realize the receiver synchronization. Attention is directed toward an efficient algorithm, this being achieved through optimization of the assembly language program.

On‐board processing for communication satellites: Technologies and implementations

AbstractThis paper presents technologies which are key candidates for advanced on‐board processing for future communication satellites. Satellite switching centres for microwave and baseband signals, on‐board coherent modems for 120Mb/s burst‐mode TDMA systems and their technologies are discussed. A brief description of on‐board demultiplexing and demodulation of low bit rate carriers is offered and study results on investigation of on‐board unique word detectors and large buffers are presented.