卫星会议控制信道TDM/TDMA关键技术的研究

主要针对会议电视在卫星信道传输时,为实现会场连接、会议控制及勤务联络而设置的TDM/TDMA信道的关键技术进行了相应研究。其研究内容不仅对专用的低速TDM/TDMA网络有一定的借鉴,对于通用的TDM/TDMA网络也有一定的参考价值。     

卫星会议电视系统及网控技术

介绍了一种卫星会议电视系统，它利用IBS／IDR技术进行业务信息传输，利用TDM／TDMA技术进行网控信息传输。重点介绍了卫星会议电视系统中的关键技术之一－－网控技术，包括会议网管系统、卫星会议电视多点控制器和远端控制器、网控信道TDM／TDMA调制解调器、TDM／TDMA帧结构等，并对网控信道性能进行了分析。     

某卫星地面站数据终端与计算机接口部件的设计

本数据终端是卫星地面站非标准的二种终端之一。其总数率为19.2kb/s,与信道机接口采用19.2kb/s全占空信码和半占空位定时信号,接口误码率规定不大于10~(-4)。本终端采用TDM制度,具有多路多种(多种数率)数据复接的功能,其中包括与计算机数据复接。信道质量主要由衰落和干扰二个因素决定其总效果由误码率表现出来。据文献报道,卫星信道误码特点是;主要是随机性误码,突发性误码较少(主要由地面站附近环境所致),但突发-旦发生其持续     

基于星上交换的卫星ATM系统设计

简要介绍了一个区域性的专用卫星ATM网的总体结构和TDMA/TDM多址方式.这个网络由一颗地球同步轨道卫星、若干个地球站和一个主控制站组成.重点论述了为每个地球站配置的多业务卫星ATM接口单元(SAIU)的设计方法,指出了各个环节中应注意解决的一些技术问题.     

视频压缩与信号复接系统设计

为了满足视频信号传输的高实时性,提高无线信道的利用率,设计并实现了一种以专用视频压缩芯片ADV202实现视频信号压缩,并采用FPGA为信号复接单元的视频压缩与信号复接系统。系统中通过ADV202实现了基于JPEG2000的视频数据压缩,保证了信号传输的高实时性;并通过FPGA实现对压缩数据的复接,提高了无线信道的利用率。     

Ad-hoc网络中一种低控制开销的多信道MAC协议

针对Ad-hoc网络中多信道MAC机制引起的控制信道瓶颈问题和隐终端问题,该文提出一种低控制开销MAC协议(LCO-MAC)。与基于信道使用表一类的MAC机制不同,LCO-MAC参考Meshhadany提出的RTS/CTS(Request To Send/Clear To Send)信道分配机制,将数据信道映射为帧中时隙,但不同的是LCO-MAC不限制RTS的发送时间,且一旦申请信道后即可发送数据。仿真结果表明,LCO-MAC无需传输太多的控制信息用于预约信道,有效缓解了控制信道瓶颈问题和多信道隐终端问题,网络吞吐量也得到明显提升。     

正码速调整异步复接中的码速调整特性

<正> 一、引言复接可以提供多路信号组合的方法,以便经济地利用信道的有效传输容量。在数字传输的TDM体系中,数字复接是把若干个低速数字流,周期地排列成一个较高速率的合路数字流。数字复接分同步复接和异步复接两种,同步复接是复接相同速率的数字序列,要求待复接的各支路信源间相互同步,因而需要有个同步网;异步复接可复接若干个相互独立的异步支路信息,其方法是通过塞入脉冲进行码速调整,从而变异步为同步,然后再进行同步复接。     

应用在INTELSAT系统中的TDMA/DSI技术(续)

2.TDMA/DSI系统的帧结构及帧的作用前面已介绍了突发的形成和还原,这里进一步介绍由各突发时隙组成的帧结构,从而了解TDMA/DSI系统的卫星传输原理。为便于信息传送和网路的控制管理,INTELSAT的TDMA/DSI系统帧结构是分级组成的(由图4上半部分可见),除基本帧外,还有复帧、控制帧和超帧,共四级。现分别加以说明。(1)基本帧     

无线Ad hoc网络动态TDMA信道接入协议的研究

文章提出了一种新的基于动态时分多址接入(TDMA)方案适用于无线Ad hoc网络信道接入控制协议,分别为突发业务,实时业务和非实时业务提供相应顺序接入信道的优先级,并且在传输的过程中避免碰撞和保证突发业务有限的时延。最后通过仿真比较了动态TDMA、CSMA及MACA三种信道接入协议的性能。     

专用移动通信的技术进步与应用前景

本文回顾了专用移动通信的发展历程,概述了模拟专用移动通信中的集群技术;通过分析FDMA和TDMA方式下的信道共用,指出在数字专用移动通信中集群的技术特征已经消亡;对几种典型的数字专用移动通系统的分析,得出专用移动通信和公众移动通信将会在技术和网络上进一步融合的发展趋势,并探讨了专用移动通信的应用前景。     

Channel Borrowing Without Locking for Asynchronous Hybrid FDMA/TDMA Cellular Communications

The use of real time channel borrowing in digital cellular asynchronous hybrid FDMA/TDMA systems is considered. These systems, which are exemplified by North American digital cellular (IS-54), are asynchronous in the sense that time slots in different cells are not aligned. CBWL (channel borrowing without locking) techniques can be applied but (without cell-to-cell synchronization) borrowing individual time slots from adjacent cells would violate co-channel interference constraints. Instead, frequency carriers can be borrowed. In IS-54, a carrier supports three TDMA slots. So if only one TDMA slot is needed in the borrowing cell, two TDMA slots are unnecessarily transferred. We devised an appropriate carrier borrowing scheme and an analytical model to determine the traffic performance of TDMA/CBWL. Fast carrier returning is used to increase channel utilization by returning borrowed carriers as soon as possible. An efficient computational method that uses macro-states, decomposition, combinatorial analysis and the convolution algorithm is devised to find blocking probabilities. The results show that in comparison with FCA, the new CBWL scheme can significantly improve system performance of asynchronous TDMA cellular systems that use FDMA/TDMA multiplexing.     

The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation

Frequency-selective fading caused by multipath time delay spread degrades digital communication channels by causing intersymbol interference, thus resulting in an irreducible BER and imposing a upper limit on the data symbol rate. In this paper, a frequencyselective, slowly fading channel is studied by computer simulation. The unfiltered BPSK, QPSK, OQPSK, and MSK modulations are considered first to illustrate the physical insights and the error mechanisms. Two classes of modulation with spectral-shaping filtering are studied next to assess the tradeoff between spectral occupancy and the performance under the influence of time delay spread. The simulation is very flexible so that different channel parameters can be studied and optimized either individually or collectively. The irreducible BER averaged over fading samples with a given delay profile is used to compare different modulation/detection methods, while the cumulative distribution of short-term BER is employed to show allowable data symbol rates for given values of delay spread. It is found that both GMSK and QPSK with a raised-cosine Nyquist pulse are suitable for a TDM/TDMA digital portable communications channel.     

Spread SCPC/FDMA,spread TDMA/FDMA and SSMA/FDMA transmission through A non-linear satellite channel with an earth-station off-axis emission constraint

This paper analyses and compares possible access schemes to be used in satellite networks with a large number of small earth-stations. Basically, frequency division (FDMA), time division (TDMA), and spread spectrum multiple access (SSMA) are considered. For TDMA, one single carrier originated in one particular earth-station is transmitted in each sub-band and therefore a single-channel-per-carrier system results (SCPC/FDMA). For TDMA and SSMA, it is assumed that groups of earth-stations using either of these access schemes share the full available band in FDMA. The corresponding systems are thus TDMA/FDMA and SSMA/FDMA. Moreover, for SCPC/FDMA and TDMA/FDMA individual carriers may be spread to the extent that the available bandwidth is always fully occupied. Spectral efficiencies for these three access modes (Spread SCPC/FDMA, Spread TDMA/FDMA and SSMA/FDMA) are computed and compared for transmission through a non-linear satellite channel and optimum operating points for the non-linear amplifier are also determined. Comparisons are also performed for the situation in which transmitting earth-stations are bounded to obey an off-axis emission constraint.     

Joint data and channel estimation for TDMA mobile channels

One of the main problems in time-division multiple-access (TDMA) digital mobile communications is the poor performance of the receiver for fast-fading channels. We propose to use a novel joint data and channel estimation (JDCE) technique to improve performance. The basic idea of this method originates from the so-called generalized likelihood ratio (GLR) test of classical detection theory. The technique was combined with the well-known Viterbi algorithm and applied to blind equalization by Seshadri [1]. In this paper, we introduce this technique to TDMA mobile communications. We find that the problems of delayed channel updating and divergence caused by error decision feedback in conventional maximum likelihood sequence estimation with decision delay (MLSE/DD) can be completely solved. An improvement of 4.5 dB can be obtained compared with the MLSE/DD method when it is applied to the Northern American Digital Cellular (NADC) system. The specified IS-54 requirement of 19 dB signal-to-noise ratio (SNR) at a bit error rate (BER) of 3% and a vehicle speed of 100 km/h can be met with a remarkable margin of 8 dB. When the intersymbol interference (ISI) of a channel is not severe, the required computation is moderate. For example, in the NADC system, the computational burden of the JDCE method is twice that of the MLSE/DD method if the least-mean-squares (LMS) algorithm is used for channel tracking.This work was presented in part in the Third International Symposium on Personal, Indoor and Mobile Radio Communications, Boston, MA, October 1992.     

An assessment of alternative wireless access technologies for PCSapplications

The results of studies on three important wireless access technology issues in the personal communications services (PCS) arena are described. They are signal duplexing, multiplexing on the downlink, and channel frequency assignment. Each of these issues was addressed through the comparison of two competing technologies. In the area of duplexing, frequency division duplexing (FDD) was compared to time division duplexing (TDD) with the result that FDD is preferred if appropriate spectrum is available. Multiplexing on the downlink examined time division multiplexing (TDM) versus a burst downlink (TDMA). TDM is preferred for the wide-area local-exchange environment, but in certain indoor, office applications, TDMA may prove advantageous. The two methods of frequency channel assignment investigated are quasi-state autonomous frequency assignment (QSAFA) and dynamic channel assignment (DCA). QSFA is generally preferred, although indoor, multitenent environments with low port loading may benefit from the use of DCA. These analyses assume that the system uses time-division multiple access (TDMA)     

Power efficient asynchronous multiplexer for X-ray sensors in medical imaging analog front-end electronics

This paper presents a new power efficient asynchronous multiplexer (MUX) for application in analog front-end electronics (AFE) used in X-ray medical imaging systems. Contrary to typical synchronous MUXes that have to be controlled by a clock, this circuit features a simple structure, as the clock is not required. The circuit dissipates power only while detecting the active signals and then automatically turns back to the power down mode. Medical imaging systems usually consist of several dozen to even several hundreds of channels that operate asynchronously. The proposed MUX enables an unambiguous choice of the active channel. In case of two or more channels that become active at the same time the MUX serializes the reading out data from particular channels. This characteristic leads to 100% effectiveness in data processing and no impulses’ loss. The proposed MUX together with an experimental readout ASIC has been implemented in the CMOS 0.18 μm process and occupies 1100 μm²/channel area. It works properly in a wide range of the voltage supply in between 0.8 and 1.8 V. Energy consumed during the detection of one active channel is below 1 pJ, while the detection time is about 1 ns.     

Hybrid Dynamic Channel Assignment in Clustered Wireless Multihop CDMA/TDMA Ad Hoc Networks

A good channel assignment scheme in a multihop ad hoc network should not only guarantee successful data transmissions without collisions, but also enhance the channel spatial reuse to maximize the system throughput. It becomes very inefficient to use fixed channel assignment when the network size grows. Therefore, spatial reuse of channels become more important in a large multihop ad hoc network. In this paper, we consider an ad hoc network with an overlaid CDMA/TDMA structure. We divide each code into time slots to form the channels. A dynamic channel assignment (DCA) strategy called Hybrid-DCA is proposed in a clustered ad hoc network. This DCA strategy is designed to make the best use of available channels by taking advantage of the spatial reuse concept. In Hybrid-DCA, the increase in spatial reuse is achieved by adding certain control overhead. We show that the bandwidth saving due to channel spatial reuse is higher than the additional bandwidth spent on the control overhead.     

A Network-Based Dynamic Channel Assignment Scheme for TDMA Cellular Systems

Network-based dynamic channel assignment (DCA) schemes can be used to increase the capacity of TDMA cellular systems. In this paper, a new distributed network-based DCA scheme, known as DCA with interference information, DCA-WI, is proposed and its performance is studied. In this scheme, a base station (BS) assigns a channel in such a way as to minimize the effect on the availability of channels for use in its interfering cells. To accomplish this, each BS maintains an interference information table which contains information about the local cell and its interfering cells. DCA-WI does not require system-wide information. Channel reassignment for new and completed calls are used to further reduce the call blocking probability. Simulation results show that DCA-WI provides a lower call blocking probability compared to other existing schemes in both uniform and nonuniform traffic distributions.     

Application of NB/WB AMR speech codecs in the 30-kHz TDMA system

A new system enhancement method is proposed for the EIA/TIA-136 system offering both channel operational range extension and improved performance within the current operational range. The existing time-division multiple-access (TDMA) (136) speech codec, the IS-641 enhanced full rate vocoder, operates at a fixed bit rate and does not allow the reallocation of bits to channel error protection as channel conditions degrade. The research presented here investigates the application of the narrow-band adaptive multirate (NB-AMR) speech codec and the wide-band AMR (WB-AMR) codec, both originally designed for the 200 kHz GSM channel, in the TDMA (TIA/EIA-136) 30-kHz system. In particular, we investigate adaptively allocating bits between NB/WB speech coding and error control coding within the limited channel bandwidth. Four modes out of 17 have been carefully chosen for the new TDMA/AMR system. Switching between codec rates as channel conditions change produces range extension below a C/I of 15 dB while also improving performance in the existing operational range above 15 dB. We keep the time slot formats unchanged so that our method is completely compatible with existing 136 systems.