Multi-Code TDMA (MC-TDMA) for Multimedia Satellite Communications用于多媒体卫星通信的MC--TDMA（多码时分多址复用）R. Di Girolamo and T. Le-NgocDepartment ofa Electricl and Computer Engineering - Concordia University1455 de Maisonneuve Blvd. West, Montreal, Quebec, Canada, H3G 1M8 ABSTRACT摘要In this paper, we propose a multiple access scheme basedon a hybrid combination of TDMA and CDMA,在这篇文章中，我们提出一种基于把时分多址复用和码分多址复用集合的多址接入方案。
referred toas multi-code TDMA (MC-TDMA). 称作多码—时分多址复用The underlying TDMAframe structure allows for the transmission of variable bitrate (VBR) information,以TDMA技术为基础的帧结构允许传输可变比特率的信息while the CDMA provides inherentstatistical multiplexing.和CDMA提供固有的统计特性多路复用技术The system is studied for a multimediasatellite environment with long-range dependentdata traffic,and VBR real-time voice and video traffic研究这个系统是为了在远程环境下依赖数据传输和可变比特率的语音和视频传输的多媒体卫星通信系统 . Simulationresults show that with MC-TDMA, the data packetdelay and the probability of real-time packet loss can bemaintained low. 仿真结果表明：采用MC-TDMA的多媒体卫星通信，数据包延时和实时数据丢失的可能性可以保持很低。
The above advantages are achieved at the expense of soft blocking, which occurs when many packetsare transmitted simultaneously, but on different spreadingcodes.上述的优点是以软阻塞为代价达到的，当很多数据包以不同的扩频码同时传输时会发生软阻塞。
Similar conclusions are drawn for simulationswhere the channel propagation conditions, adjacent beaminterference, and imperfect power control are considered.当仿真时考虑到信道传播条件，相邻的电波干扰和非理想功率控制等因素也会有相似的结论。
1 INTRODUCTION介绍The nature of satellite channels make them ideally suitedfor broadcast and multipoint transmissions. 卫星信道的性质非常适合广播和多点传输The envisionedsatellite systems do not only provide a backup tothe terrestrial wire and wireless networks, but in many situationsthey are the only feasible alternative.未来卫星系统不只是给地面上有线和无线网络提供一个备份，在很多状况他们还是唯一可行的替代This is particularlytrue in cases where the infrastructure基础设施to support theterrestrial systems is not in place. 在支持地面的系统的基础设施不到位的情况下上面的说法就更正确了In such environments,在这种情况下the satellite provides a means of communication which isinsensitive to the location and the distance between thecommunicating users.卫星会提供一种通信方式，这种通信方式对在通信的用户所在的位置和他们之间的距离不敏感With the increased emphasis placed on multimedia, 现在重点放在多媒体上，any proposed systemmust be able to support a mixture of voice,video, and data. 任何被提议的系统都必须可以支持语音，视频和数据的混合Typical applications could include videoconferencing, LAN interconnection, medical imaging, remotevideo monitoring, and the like典型的技术包括视频会议，局域网互联，医学影像，远程视频监控和类似于这类的技术。
. As a consequence, thesatellite system must be able to support all these services，and any future services which may become popular.所以，卫星系统必须可以支持所有这些服务和未来可能很受欢迎的服务。
The problem lies in the characterization of this multimedia trafficand in the different quality of service (QoS) requirementsfor each of these. 问题就在于这种多媒体交换的描述和服务质量的不同要求不同。
Packetized data, for instance, is generallyjitter tolerant but loss sensitive, 比如，封包化的数据通常可以容错但是不够敏感。
and often requires the retransmissionof packets which are received incorrectly.而且经常要求重新发送在接收端被错误接受的数据包In addition,此外recent traffic studies have shown that data traffic exhibits long-range dependence [ll. 最近在通信交换的研究表明数据的交换存在一种长期的依赖，On the other hand,另一方面voice and video are delay sensitive (real-time), 语音和视频信号对延迟是敏感的，but normallycan sustain a certain level of packet loss.但是一般情况下他可以支撑到数据包丢失的某个电平Furthermore,此外asa result of the coding techniques used,作为编码技术被利用的结果the real-time trafficcan be of variable bit rate (VBR). 它可以实现可变比特率的实时交换Any designed systemshould address the above issues.任何一个设计的系统都应该考虑上述问题。
The second issue to be resolved is the method of multipleAccess.第二个要解决的问题就是多址接入的方法How to geographically dispersed earth stations(or users) supporting multimedia traffic, 在地理上怎么划分可以支持多媒体交换的区域或者用户efficiently share the available fixed uplink bandwidth怎么有效的分享可利用的固定的上行带宽. One of thesimplest approaches is to use a form of demand assignedTDMA or multi-frequency TDMA (MF-TDMA),where anearth station periodically requests capacity, 最简单的方法就是使用一种按需求分配的形式的TDMA或者多频TDMA。
地面的无线站周期性的请求信道带宽，有一个中心程序机来响应这些请求and a centralscheduler honors these requests [2]. A second broad classof technique which has recently been gaining momentum,has CDMA as a basis. 最近已经蓄势待发的以CDMA为基础的第二大类技术，These spread spectrum approachesrequire very high transmission chip rates, and are hamperedwith problems dealing with synchronization and unequalpower control.这些扩频方法要求很高的传输芯片率，也受到处理同步和不平均的功率控制等问题的限制However,然而它们拥有很多优点包括固有的统计复用，they possess a number of advantages,including inherent statistical multiplexing, a gracefuldegradation in system performance as load increases,当负荷增加时在系统性能上有一个故障弱化的优点coexistence with narrowband analog systems in the samefrequency band, etc.在相同的频带内可以和窄带模拟系统共存。