New technique for channel synchronisation in time division multiplexing

A channel synchronisation technique is described for t.d.m. systems carrying voice or teletype signals. According to this method, no extra sync signal is transmitted and synchronisation is extracted from the information signal. Furthermore, some encryption of the signal is achieved in this way. Tests had quite satisfactory results.     

Apodized chirped fiber Bragg grating for postdispersion compensation in wavelength division multiplexing optical networks

In this paper, a postdispersion compensation unit is proposed leading to a better performance for the optical communication systems. This unit utilizes a chirped fiber Bragg grating (CFBG). For enhanced performance of the CFBG, a proper apodization function is chosen to improve the quality factor (Q‐factor) and the bit error rate (BER) of the system. A 110‐km wavelength division multiplexing (WDM) optical link is investigated. The system performance is evaluated through its Q‐factor, eye diagram, and BER showing best performance when using the Hamming apodization function.     

Prime sequences: quasi-optimal sequences for OR channel code division multiplexing

A new family of multiplexing sequences suitable for low duty cycle pulse position modulation schemes and for frequency hopped stread spectrum systems is introduced. The sequences have maxima in their crosscorrelation functions of two or one.     

（英）Orthogonal frequency division multiplexing for improving the performance of terahertz channel

Terahertz (THz) communication is considered to be one of the demanding technology for the upcoming 5G standards. The incredible demand for high rate through wireless channel necessitates the use of THz frequency for communication. The development of communication systems in this frequency band possess technical challenges as the characteristic of THz band is very much different from the present wireless channel. However, the advancements in the development of transceiver and antenna systems are rapidly bringing the THz communication into reality. The high path loss in THz band limits the communication range of this channel. Even, for a distance of few meters (>5m), the absorption coefficient is very high and hence the performance of the system is poor. Performance over this frequency channel can be enhanced by considering transmission windows over this band instead of the entire band. The transmission windows are the frequencies over which the absorption is relatively low. Though there is an improvement in the performance with this adaptive modulation scheme, but not sufficient for longer distance. Apart from path loss, the frequency selective nature of this high bandwidth channel is also a major reason for the poor performance of THz channel. Orthogonal Frequency Division Multiplexing (OFDM) is a promising solution to mitigate the effects of frequency selective nature of the wireless channel. OFDM has been exploited in this paper to improve the performance of terahertz channel. The results show that the Bit Error Rate (BER) of the terahertz channel is considerably improved with OFDM.     

A channel estimation algorithm for optical orthogonal frequency division multiplexing systems

A simple channel estimation （CE） scheme, which is pilot-aided with just a little number of pilots inserted in the first half part of the subcarriers, is proposed and simulated for resisting the fiber dispersion in optical orthogonal frequency division multiplexing （OFDM） systems. The simulation results verify that the receiver sensitivity is improved by 2-3 dBm for bit error rate （BER） of 10^-3 with the proposed CE algorithm than that with other kinds of CE algorithms based on linear square principle. The good constellation performance for a 40 Gbit/s transmission system can be also obtained by the proposed scheme.     

新的信道复用技术：素数复用

为了提高信道利用率,根据素数积的惟一分解特性,提出一种新的信道复用技术:素数复用,并阐述了素数复用的工作原理和优缺点。素数复用特别适用于终端用户复用或终端功能复用。     

Fixed-lag smoother for digital channel equalisation

A new equaliser for digital communication channels, which carries out a discrete-time fixed-lag smoothing of the state vector of the channel, modelled as a tapped delay line of finite length, is proposed. The smoothed estimate uses and improves the filtered estimate of the conventional Kalman-filter equaliser.     

Integrated three channel laser and optical multiplexer fornarrowband wavelength division multiplexing

The fabrication and characterisation of a monolithically integrated three channel narrowband wavelength multiplexer and DBR laser are reported. The multiplexers include Bragg gratings with an extinction ratio of >20 dB and a bandwidth of ~1 nm to give channel spacings of <10 nm     

TD-HSPA控制信道空分复用技术创新应用

TD-SCDMA系统对于要支持4倍空分复用功能需至少配置相应4对数的HS-SCCH/HS-SICH控制信道,这使得HSPA带宽降低或接入用户数下降,影响HSPA业务信道空分复用的效果容量.为此,提出了控制信道空分复用技术,它有效的解决了该问题.     

Optical-fiber filters for wavelength division multiplexing

The author briefly discusses Fabry-Perot etalon filters, multilayer thin-film filters, and fiber Bragg grating filters. Filter technology and applications are outlined     

Sparse channel estimation and tracking for cyclic delay diversity orthogonal frequency division multiplexing systems

In cyclic delay diversity orthogonal frequency division multiplexing systems, the excessive channel delay spread and corresponding high frequency selectivity makes channel estimation a challenging task. In this paper, we propose a two‐stage scheme to estimate and track the highly frequency selective channel. At the preamble reception stage, least squares channel estimation with L₀ norm regularization is proposed to exploit the channel sparsity. At the data demodulation stage, an expectation–maximization algorithm with the most significant tap selection is developed to track channel variations by using the channel order obtained from the first stage. Compared with other estimation methods, the proposed scheme requires no prerequisite knowledge of delay parameter settings, which leads to more flexibility. Furthermore, the scheme can exploit the channel sparse structure by detecting the nonzero taps and, consequently, has better mean squared error performance. Simulation results show that the proposed estimation scheme can retain the provided diversity gain of cyclic delay diversity effectively in time‐varying fading channels. Copyright © 2011 John Wiley & Sons, Ltd.     

Optoelectronic time division multiplexing

The concept of using optoelectronic (photoconductive) switches as the sampling element in time division multiplexing is introduced in the context of VLSI off-chip data transmission. A 4:1 multiplexer was fabricated in Cr : GaAs, activated by a GaAs laser via optical fibre delay lines and operated at 2.5 Gbit/s.     

Space division multiplexing holography

A technique is proposed for multiple information storage in a sampled hologram in long-wavelength holographies. An experiment on the proposed technique is conducted to construct a hologram using 15 kHz sound waves and to reconstruct images using laser light.     

Overlapped frequency-time division multiplexing

A technique named overlapped frequency-time division multiplexing (OVFTDM)) is proposed in this article. The technique is derived from Nyquist system and frequency-time division multiplexing system. When the signals are compactly overlapped without the orthogonality in time domain, the technique is named overlapped time division multiplexing (OVTDM), whereas when signals are compactly overlapped without the orthogonality in frequency domain, the technique is called overlapped frequency division multiplexing (OVFDM). To further improve spectral efficiency, the OVFTDM in which signals are overlapped both in frequency domain and in time domain is explored. OVFTDM does not depend on orthogonality whatever in time domain or in frequency domain like Nyquist system or OFDM system, but on the convolutional constraint relationship among signals. Therefore, not only the spectral efficiency but also the reliability is improved. The simulations verify the validity of this theory.     

Sequential algorithm for channel equalisation employing a stack

A sequential algorithm based on a stack, and applicable to the problem of recovering digital data after degradation by intersymbol interference and additive noise, is described. Results are presented showing a tradeoff between performance and equipment complexity, with performance asymptotic to the maximum-likelihood-sequence (Viterbi) estimate when the storage is large.     

Bayesian channel equalisation and robust features for speechrecognition

The use of a speech recognition system with telephone channel environments, or different microphones, requires channel equalisation. In speech recognition, the speech model provides a bank of statistical information that can be used in the channel identification and equalisation process. The authors consider HMM-based channel equalisation, and present results demonstrating that substantial improvement can be obtained through the equalisation process. An alternative method, for speech recognition, is to use a feature set which is more robust to channel distortion. Channel distortions result in an amplitude tilt of the speech cepstrum, and therefore differential cepstral features provide a measure of immunity to channel distortions. In particular the cepstral-time feature matrix, in addition to providing a framework for representing speech dynamics, can be made robust to channel distortions. The authors present results demonstrating that a major advantage of cepstral-time matrices is their channel insensitive character     

Optical phase division multiplexing for local communicationsapplications

A local fiber bus system is described wherein many users sequentially phase-modulate the output of a single light source. Since the modulated signal has a constant envelope, optical amplifiers can be used to boost the signal power along the bus to a level sufficient for reliable detection. Also, since a single laser source is used, a higher quality device (for example, with small linewidth) is more readily affordable. This system is analyzed to determine the limits of performance due to noise. The analysis indicates that many wideband signals could be accommodated if noise sources only, rather than signal distortions caused by nonideal optical devices, were the limiting factor. For example, with optical amplifiers distributed along the bus, this system can support 1000 10-MHz users     

Differential modulation techniques for a 34 MBit/s radio channel using orthogonal frequency division multiplexing

The orthogonal frequency division multiplexing (OFDM) technique has been proposed for terrestrial digital transmission systems due to its high spectral efficiency, its robustness in different multipath propagation environments and the ability of avoiding intersymbol interference (ISI). Our studies consider a radio channel bandwidth of 8 MHz and a data rate of 34 Mbit/s.In the case of the OFDM transmission system a coherent 64-QAM requires a channel estimation process and a channel equalization in frequency-selective interference situations [4]. The equalization process can be realized by a multiplier bank at the FFT output in the receiver, a so-called frequency-domain equalizer. Alternatively, a multilevel differential modulation technique, the so-called differential amplitude and phase shift keying (64-DAPSK) considering the phase and simultaneously the amplitude for differential modulation, is proposed and presented in this paper. Differential modulation/demodulation techniques do not require any explicit knowledge about the radio channel properties in the differential channel equalization. It is therefore not necessary to implement a frequency-domain equalizer in an OFDM/64-DAPSK receiver, which reduces the computation complexity. The performance of both modulation techniques has been analysed in the uncoded and coded case referring to Gaussian and frequency-selective Rayleigh fading channels. Simulation results are presented in this paper.The OFDM signal has a non-constant envelope with large instantaneous power spikes possible primarily resulting in an overdriving of the high power amplifier (HPA) at the transmitter. This leads to nonlinear distortion causing intermodulation noise and spectral spreading. Both effects can be limited by introducing an appropriate input backoff (IBO). In this paper the performance of OFDM signals in the presence of nonlinearities is analysed quantitatively.     

A highly sustainable multi-band orthogonal wavelet code division multiplexing UWB communication system for underground mine channel

The underground mine channel is highly hostile for communication. A sustainable communication system is required to be integrated as a multipurpose system capable of transferring all types of information such as data, voice and video on identical infrastructure. With its large bandwidth, the Ultra Wide Band (UWB) provides a promising solution to satisfy these requirements. The Multi-Band Orthogonal Frequency Division Multiplexing (MBOFDM) UWB system provides high resilience to Inter-Symbol Interference (ISI) caused by multipath fading channels. This study compares the performance of various existing multi-band UWB systems in the IEEE UWB channel and underground mine channel: MBOFDM, Multi-Band Orthogonal Frequency Code Division Multiplexing (MBOFCDM) and Multi-Band Orthogonal Wavelet Division Multiplexing (MBOWDM). Further, the application of 2D spreading to the MBOWDM UWB system is attempted, and as a result, the Multi-Band Orthogonal Wavelet Code Division Multiplexing (MBOWCDM) UWB system is proposed. The performance of the MBOWCDM system in the IEEE UWB channel and underground mine channel is investigated. The performance of the MBOWCDM UWB system is observed to be superior to those of other multi-band UWB systems. Moreover, the MBOWCDM UWB system supports a higher data rate with low complexity and lower overheads.