SIMULATION STUDY ON COPY DEMULTIPLEXING *

Transcription

1 SIMULATION STUDY ON COPY DEMULTIPLEXING * Minglu JIN Dept. of Electronic Shenyang Institute of Aeronautical Engineering Shenyang , China Qishan ZHANG Beijing University of Aero. & Astr. P.O. Box , Beijing , China ABSTRACT In this paper, by using computer simulations, the interference of channel data in the SDM telemetry system is investigated, the performance of the copy demultiplexing is examined, and finally the selection rule of Walsh functions is recommended. KEY WORDS Demultiplexing, Wa1sh functions, Copy analysis, Multichannel Telemetry Processing INTRODUCTION To transmit multi-channel data in the telemetry systems, there are three kind of methods, i.e Frequency-Division Multiplex, Time-division Multiplex and Sequency-Division Multiplex( SDM). The last method was first proposed by labratory in BUAA[1], based on Wa1sh functions. In the SDM telemetry system, the demultiplexing scheme is based on the orthogonality of the Wa1sh functions. While Walsh functions has its particular property copy feature, and based on this property a new demultiplexing scheme, named as copy demultiplexing, is proposed by JIN and ZHANG [2]. In the following sections, we shall discuss this new method of demultiplexing. * The Project was supported by the National Natural Science Foundation and Aducation Committee Foundation of China

2 SDM TELEMETRY SYSTEM For the comparison purpose, we first review the SDM Telemetry system. The SDM Telemetry System is illustrated in the Figure.1 Figure.l SDM Telemetry System In this system design and analysis, the calculation of the cross-correlation Walsh functions is very important,and by using copy theory of Walsh functions,this can be done vary easly[3].to calculate this cross-correlation functions, two paremeters must be considered, i.e., waveform distortion and frame asynchronization. By theoretical and computer computation, ZHANG etc. have recommended the selection of the Walsh functions in the SDM Telemetry Systems, as Table.1 shows. Table.2 Walsh functions used in SDM

3 COPY DEMULTIPLEXING In terms of the copy feature of Walsh functions and copy analysis [4] [5], we proposed Copy demultiplexing scheme as Figure.2 shows. Figure.2 Copy Demultiplexing In this copy demultiplexing system, the copy analysis plays very important role, and its performance affect th whole performance of the system directly. The copy analysis is a kinds of orthogonality analysis, and thus it can decompose diffenrent Walsh functions, while Walsh functions can be generated by the copy method as Figure.3 indicates. Figure. 3 Copy Generation of Walsh functions Also the Walsh functions in the Figure.3 can be copy analyzed as Figure.4 shows. Figure.4 Copy analysis of Walsh function

4 For the simple explanation bellow, we put all of the copy analyzed components in the Fig.4 to one figure as Figure.5, and named it as Copy Analyzed Spectrum. Figure.5 Copy analyzed spectrum In the real applications, the Walsh function waveforms are not an ideal waveform, because of the band limitation of the real systems. The waveform can be distorted as shown Figure.6. Figure.6 distorted Walsh function waveform Thus the copy analyzed spectrum is not the case as Figure.4, while it is the case as Figure.7 Fig.7 Copy analyzed spectrum of distorted Walsh function In the Figure.7 the nth component is called as main lobe, and the others are called as sub lobe, and the appearance of sub lobe is caused by the waveform distortion, and more larger the waveform distortion is, the more lager is the sub lobe. Thus it can be

5 used to indicate the interference of the mth channel to other channels. In the computer simulation, we supposed as bellow: -$t (1) The distortion is a kind of e waveform (2) The distortion change is very quick, i.e. its transition completed in one bit period. The results of simulation are given in the Table.2. Table.2 Normalized copy analyzed spectrum when there is only distortion In addition to the waveform distortion, there is another factor to affect the performance, i.e., the synchronization error. In the simulation, we modeled this error as time shift of waveform, as Fig.9 shows. The simulation condition is : Fig.9 time shift error (1) 0<t <T o (2) t is independent from functions order m o Table.3 shows the simulation computation results.

6 Table.3 Normalized copy analyzed spectrum when there is only asynchronization In the real applications, there are both waveform distortion and time shift, while the two factors is independent from each others. Table.4 shows this case. CONCLUSION From above simulation results, there are several conclusions as bellow: (1) The effect of waveform distortion to copy demultiplexing is small. (2) The effect of asynchronization to copy demultiplexing is critical. (3) When there are both distortion and asychronization, the effect of asychronization is dorminant. (4) The selection rule of Walsh functions is that to minimize the sub lobe in copy analyzed spectrum. (5) The Walsh functions in the Table.l are also suitable in the copy demultiplexing. (6) For the copy multiplexing, there are other combimation of Walsh functions which are not in the Table.l result in the same performance.

7 The idea and material presented in this paper will find more applications in the telemetry areas. Table. 4 Normalized copy analyzed spectrum when there are both distortion and asynchronization REFERENCE [l] Minglu JIN and Qishan ZHANG "Symmetry Copy Analysi of Signals" ICNNSP'93,Guangzhou, China, Nov [2] Minglu JIn and Qishan ZHANG "Shift Copy Analysis of Signals" ICT'94, Dubai, England, Jan [3] Minglu JIN and Qishan ZHANG " Copy Feature of Walsh Functions and its Applications" ISPC'93, Nanjing, China, Oct [4] Qishan ZHANG etc "Telemetry Systems Based on Walsh Functions" ITC Proceeding XVI [5] Minglu JIN and Qishan ZHANG " a New Method of Demultiplexing " NAECON'94, Dayton, May, 1994.