Downlink transmission of broadband OFCDM systems-part IV: soft decision

In this paper, the performance of turbo-coded orthogonal frequency and code-division multiplexing (OFCDM) systems is investigated with soft multicode interference (MCI) cancellation and minimum mean-square error (MMSE) detection for downlink transmission in future high-speed wireless communications. To regenerate the soft interference signal, the conventional turbo decoding algorithm must be modified to provide log-likelihood ratio (LLR) values for all coded bits. Based on the LLR outputs of turbo decoder, two soft-decision functions are proposed, called LLR-soft-decision and Gaussian-soft-decision functions. The Gaussian assumptions used for deriving these two soft functions are verified by simulation results, and simple methods are proposed to estimate parameters used in the soft functions in practical systems. By means of computer simulations, the performance of soft MCI cancellation is studied extensively and compared with that of hard ones. It is shown that in a highly frequency-selective channel, soft MCI cancellation and MMSE detection can significantly improve the performance of turbo-coded OFCDM systems. Two iterations in turbo decoding are sufficient for both hard and soft-decision functions. The proposed soft-decision functions outperform the hard-decision function with various channel conditions and system parameters, such as the channel correlation, the quality of channel estimation, the number of iterations in turbo decoding and the frequency-domain spreading factor (N/sub F/). Furthermore, the Gaussian-soft-decision function provides better performance than the LLR-soft-decision function. Finally, although frequency diversity gain is saturated for large channel correlation when N/sub F/ is large as in , the gain increases further with increasing N/sub F/ for small channel correlation even when N/sub F/ is large.     

Downlink transmission of broadband OFCDM Systems-part I: hybrid detection

The broadband orthogonal frequency and code division multiplexing (OFCDM) system with two-dimensional spreading (time and frequency domain spreading) is becoming a very attractive technique for high-rate data transmission in future wireless communication systems. In this paper, a quasianalytical study is presented on the downlink performance of the OFCDM system with hybrid multi-code interference (MCI) cancellation and minimum mean square error (MMSE) detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. The effects of channel estimation errors and sub-carrier correlation are also studied. It is shown that the hybrid detection scheme performs much better than pure MMSE when good channel estimation is guaranteed. The power ratio between the pilot channel and all data channels should be set to 0.25, which is a near optimum value for the two-dimensional spreading system with time domain spreading factor (N/sub T/) of 4 and 8. On the other hand, in a slow fading channel, a large value of the channel estimation window size /spl gamma/N/sub T/, where /spl gamma/ is an odd integer, is expected. However, /spl gamma/=3 is large enough for the system with N/sub T/=8 while /spl gamma/=5 is more desirable for the system with N/sub T/=4. Although performance of the hybrid detection degrades in the presence of the sub-carrier correlation, the hybrid detection still works well even the correlation coefficient is as high as 0.7. Finally, given N/sub T/, although performance improves when the frequency domain spreading factor (N/sub F/) increases, the frequency diversity gain is almost saturated for a large value of N/sub F/ (i.e., N/sub F/ /spl ges/ 32).     

Downlink transmission of broadband OFCDM Systems-part II: effect of Doppler shift

The orthogonal frequency and code division multiplexing (OFCDM) system with 2-D spreading (time- and frequency-domain spreading) is becoming a promising candidate for future broadband wireless communication systems. OFCDM is more attractive than orthogonal frequency-division multiplexing (OFDM) both by introducing frequency-domain spreading for frequency diversity provision and time-domain spreading for flexible data rate provision. To provide high-speed mobile services, multicode transmission is employed in conjunction with OFCDM. In a Gaussian or flat-fading channel, multicode channels are orthogonal. However, in a realistic wireless channel, the orthogonality no longer maintains. Thus, multicode interference (MCI) is caused. This paper focuses on the investigation of the effect of Doppler shift on the downlink transmission of high-speed mobile OFCDM systems. A practical channel estimation algorithm based on a code-multiplexed pilot channel is employed to track the variations of fading channels. Hybrid MCI cancellation and minimum mean-square error (MMSE) detection proposed by the authors is employed as an efficient way to eliminate the MCI in the frequency domain. The system performance is analytically studied with imperfect channel estimation to show how it is affected by parameters such as the window size in the channel estimation, Doppler shift, the number of stages of the hybrid detection, the power ratio of pilot to data channels, spreading factor, and so on.     

OFCDM: a promising broadband wireless access technique

Future mobile communication systems aim to provide extremely high-speed data transmission, especially in the downlink. The broadband orthogonal frequency- and code-division multiplexing system with two-dimensional spreading (time and frequency domain spreading) is becoming a very promising technique for highspeed wireless communications due to its advantages over OFDM. This article presents the basic structure and main functions of the OFCDM system. A nonsequential code assignment scheme is introduced. The novel detection method for the OFCDM, called hybrid multi-code interference cancellation and minimum mean square error detection, is described. The application of advanced techniques to the OFCDM, such as turbo coding and MIMO, is also discussed. It is shown in this article that OFCDM is superior to OFDM.     

宽带E波段无线传输系统

本实验采用74 GHz本振源与一对矩形喇叭天线实现了非归零开关键控(NRZ-OOK)信号在室内环境下的传输。在实验中还研究了不同传输距离和速率对系统性能的影响。结果表明误码率性能随着距离和速率的增加而下降,使用前向纠错编码(FEC)的5 Gbit/s和6 Gbit/s信号可以在3.6 m内实现无误码传输,而7 Gbit/s信号只能在2.4 m内实现无误码传输。     

ALCATEL 4400宽带用户交换机

ＡＬＣＡＴＥＬ４４００是为宽带交换设计的新一代用户交换（ＰＡＢＸ）。该设计满足在一个或多个场所要求１００个以上终端的中型和大型商业机构的通信需求。它的结构建立在关键系统功能分散的基础上。还使用一种创新的技术来连接各接口。其软件结构建立在提供Ｕｎｉｘ实时处理和分配的操作系统基础上。这种新一代ＰＡＢＸ把传统的通信服务与现代的数据传输业务结合在一起。     

Digital video transmission for broadband networks

The information society to come is characterized by a large information flow in the integrated broadband network. Today it is the distribution of TV programmes that requires high transmission capacity. This leaves the cable operator with a problem. How should the CATV network of today be constructed? It must fulfil the need for distribution now and it must be prepared for new services to come. Some aspects of the above problem are discussed in this paper. It is emphasized that digital techniques as well as optical fibre transmission should be included in a modern cable network. The Danish DOCAT network concept is presented as one way to accomplish this. DOCAT combines optical fibre transmission in the trunk network with conventional coaxial techniques in the subscriber network.     

实用化宽带模拟光纤传输系统

介绍了一种基于半导体激光器调制特性的实用化宽带模拟光纤传输系统及其结构组成和实测性能。该系统的-3dB带宽为30kHz～1.1GHz,非线性小于±5%的动态范围不小于40dB,输出噪声峰峰值小于5mV。具有频带宽、动态范围大、输出噪声低、抗电磁干扰能力强等特点,可用于复杂电磁环境下的亚纳秒脉冲信号传输。     

Multi-rate group-orthogonal OFDMA-CDMA for broadband mobile transmission

In this work, the problem of multi-rate Multi-Carrier (MC) Code Division Multiple Access (CDMA) wireless transmission is addressed. In particular, we investigated the possibility of exploiting subcarrier grouping, already considered in literature for constant bit-rate MC-CDMA, in order to reduce mutual interference among different rate users and to allow the use of theoretically-optimum Maximum-Likelihood Multi-User Detection (ML-MUD) with affordable computational burden. We propose a multi-code Group Orthogonal (GO) OFDMA-CDMA system where the available subcarriers are subdivided into fixed-cardinality orthogonal subcarrier groups. The user’s data stream is selectively multiplexed into a variable number of substreams, which depends on the data-rate. Then, these substreams are transmitted over an orthogonal subcarrier group, univocally assigned to a user rate class. Experimental results obtained by adopting linear multi-user detection show that the proposed GO-OFDMA-CDMA outperforms state-of-the-art Variable Spreading Length (VSL) and multi-code MC-CDMA as far as higher data rate users are concerned. On the other hand, BER performance of lowest-rate users is slightly worse. Orthogonal subcarrier grouping allows to greatly increasing BER performance when using ML-MUD operated over small subcarrier groups. In such a case, the tradeoff to be managed is between achievable performance and computational complexity.     

Design of bit-interleaved turbo-coded modulations

It is shown that the bit error rate performance of bit-interleaved turbo-coded modulations can be optimised by using carefully selected M-ary signal constellations. When the parameter M is in the form M=2^2p where p is an integer, the use of rectangular QAM constellations leads to optimal performance     

BER performance of multiple-antenna OFCDM with imperfections

This letter investigates the performance degradation due to the imperfect parameters in orthogonal frequency- and code-division multiplexing (OFCDM) systems with multiple antennas. For the performance evaluation, the average bit error rate (BER) expression is generalized to the multiple-antenna case in the presence of the channel estimation error and frequency offset. Derived results show that the multiple-antenna OFCDM systems experience a severe BER degradation regardless of the number of users.     

一种适合低轨卫星通信的宽带传输技术

针对低轨卫星通信过程中功率受限的约束以及宽带业务需求的不断增长,研究了一种宽带传输技术。首先,分析了离散傅里叶变换扩频正交频分复用技术的宽带传输能力以及低峰均比特性;其次,进行了低轨卫星运动场景下的多普勒频移及采样偏差的分析,并在此基础上提出了一种低轨宽带通信的帧结构。仿真结果表明,该技术可有效抗低轨场景下的多普勒残留频偏且能完成高速率的传输任务。     

Experiments on long-haul broadband WDM transmission with Ramanamplification

It is experimentally confirmed that Raman amplification with multi-wavelength pumping sources enables the pass bandwidth to be expanded to 33 nm even over transoceanic distances. It is also shown that the transmission performance of Raman amplifiers, which is dominated by not only OSNR but nonlinear effects, is superior to that of EDFAs     

Variable bit-rate video transmission in the broadband ISDNenvironment

Many compensative measures have been proposed recently the make the cell loss in ATM (asynchronous transfer mode) networks subjectively imperceptible. These schemes include the simple automatic repeat quest (ARQ) scheme, error concealment by command refreshment, appropriate queueing disciple and priority switching design and layered source coding schemes. The authors summarize these efforts and, in particular, elaborate on different layered source coding schemes. Four types of signal priority classification schemes are identified: bit-plane separation (FDS) combined with bit-plane-frequency separation (CBFS), and feature plane separation (FPS). Different layered coding techniques are discussed and compared. Some open questions and recommendations are also presented for further research and study     

基于网络编码的配电网络下行数据路由算法

文中讨论了利用网络编码技术和使用冗余链路来提高配电网络下行监控网络端对端吞吐量的算法,并通过仿真结果说明了建立多条链路的方法能以较小的网络延迟代价,显著地增强监控网络下行链路的吞吐量,可以提高网络链路的使用。     

Design of practical broadband matching networks with commensurate transmission lines

To design broadband matching networks for microwave communication systems, commercially available computer aided design (CAD) tools are always preferred. But these tools need proper matching network topology and element values. Therefore, in this paper, a practical method is proposed to generate distributed-element matching networks with good initial element values. Then, the gain performance of the designed matching network can be optimized employing these tools. The utilization of the proposed method is illustrated by means of the given example. It is shown that proposed method provides very good initials for CAD tools.     

A compact broadband MMIC sub-harmonic mixer using quasi-lumped transmission lines

A compact broadband monolithic microwave integrated circuit (MMIC) sub-harmonic mixer using an OMMIC 70 nm GaAs mHEMT technology is demonstrated for 60 GHz down-converter applications. The present mixer employs an anti-parallel diode pair (APDP) to fulfill a sub-harmonic mixing mechanism. Quasi-lumped components are employed to broaden the operational bandwidth and minimize the chip size to 1.5×0.77 mm². The conversion gain is optimized by a quasi-lumped 90° phase shift stub. Experimental results show that from 50 GHz to 70 GHz, the conversion gain varies between −12.1 dB and −15.2 dB with a LO power level of 10 dBm and 1 GHz IF. The LO-to-RF, LO-to-IF and RF-to-IF isolations are found to be greater than 19.5 dB, 21.3 dB and 25.8 dB, respectively. The second harmonic component of the LO signal is suppressed. The proposed mixer has an input 1 dB compression point of -2 dBm and exhibits outstanding figure-of-merits.     

SVD iterative detection of turbo-coded multiantenna unitary differential modulation

A new suboptimal demodulator based on a singular value decomposition for estimation of unitary matrices is introduced. Noncoherent communication over the Rayleigh flat fading channel with multiple transmit and receive antennas, where no channel state information is available at the receiver is investigated. Codes achieving bit-error rate (BER) lower than 10/sup -4/ at bit energy over the noise spectral density ratio (E/sub b//N/sub 0/) of 1.6-1.9 dB from code restricted capacity limit were found. At higher data rates, computation of code restricted capacity is impractical. Therefore, the mutual information upper bound of the capacity attaining isotropically random unitary transmit matrices was used. The codes achieve BER lower than 10/sup -4/ at E/sub b//N/sub 0/ of 3.2-6 dB from this bound, with coding rates of 1.125-5.06 bits per channel use, and different modulation decoding complexities. The codes comprise a serial concatenation of turbo code and a unitary matrix differential modulation code. The receiver employs the high-performance coupled iterative decoding of the turbo code and the modulation code. Information theoretic arguments are harnessed to form guidelines for code design and to evaluate performance of the iterative decoder.