Showing papers by "Barry G. Evans published in 1991"

Low earth orbit satellite systems for communications

Abstract: It may be that we are now entering the era of LEO constellation satellite communications after thirty years of domination by the GEO systems. This paper heralds the coming of the new era with a tutorial approach to the system design and trade-offs of LEO constellation system design. It discusses orbital configurations, network topologies and routeing considerations, multiple access schemes and link performance design. In so doing it brings out the major design parameters and how they interact with each other. Also considered are the service applications for the LEO constellation systems, and the important difference between real-time and delayed communication systems is highlighted. Examples of single and multi-beam (cellular) coverage system link designs are presented for L-Ka frequency bands. Future papers will consider aspects of the LEO spacecraft and launchers.

High quality multiband LPC coding of speech at 2.4 kbit/s

Abstract: Until recently good quality digital speech transmission was not possible below about 9.6 kbit/s. With the development of code excited linear prediction (CELP) and multiband excitation (MBE) vocoders,2 high quality digital speech transmission became possible at bit rates as low as 4.8 kbit/s. Below 4.8 kbit/s however, vocoders such as LPC-103 and the channel vocoder4 which can only produce synthetic and unnatural speech are still in use. A new coding scheme termed the multiband LPC (MB-LPC) vocoder is proposed, which produces good and natural quality speech at 2.4 kbit/s. Subjective performance of speech at 2.4kbit/s produced by the CELP is very close to MBE coded speech at 4.8 kbit/s. Informal listening tests have shown that in most cases people could not tell the difference between the new 2.4kbit/s MB-LPC coder and the 4.8 kbit/s MBE vocoder.

Digital frequency error detectors for OQPSK satellite modems

Abstract: Two algorithms for frequency error detection in OQPSK satellite modems are presented. The results of computer simulations in respect of acquisition and noise performance are given. These algorithms are suitable for DSP implementation and are applicable to mobile satellite systems in which significant Doppler shift is experienced.

Fault tolerance of on-board digital signal processing circuits

Abstract: The paper is organized in four sections. The first section introduces the nature of faults as well as their causes. The methods used to identify faults and the actions necessary to correct the situation are outlined. The second section identifies the different fault tolerance approaches to conventional computational circuits and the DSP circuits. Current research work in the area of fault tolerance of FFT, signal processing and VLSI circuits involving systolic arrays is reviewed. Since some of the techniques do not involve error correction, reconfiguration of the circuit after error detection becomes necessary and a brief look at the relevant reconfiguration strategies is appropriate. Software fault tolerance is introduced and some work applicable to computations in general is reviewed. The implementation of the methods and its consequences are described in the third section. Concluding remarks form the final section.

DSP implementation of low bit-rate CELP based speech orders

Abstract: Low bit-rate speech coding techniques are very important in applications where power and bandwidth are very limited. Prime requirements for speech coding are low bit-rate, high quality and low implementation cost. Since 1984 CELP coding of speech has been most popular at medium to low bit-rates. The major drawback of CELP coding, however, has been its large computational requirement. The authors report on the real-time implementation of a CELP coder in a single DSP32C floating-point processor on a four-layer single Eurocard size board with its appropriate input/output stages and 32 kbytes of external RAM. They also report on various complexity reduction techniques that they have employed to achieve a single DSP solution for both encoder, decoder and a very effective voice activity detection (VAD) devices. >

Soft RS codes for half rate GSM channel

Abstract: After the adoption in 1987 of the RPE-LTP* coder as the 22.8KBPS Pan European Mobile Speech Communication standard (full rate GSM† or F-GSM),1 the next stage in the project is the development of a CODEC (source and channel) operating at 11.4KBPS (half rate GSM or H-GSM). Many speech coding algorithms under evaluation are expected to meet the stringent quality specifications of H-GSM. However, during operation the channel perturbations are expected to be twice as bad for H-GSM as for F-GSM. Despite this, an error control scheme which uses less redundancy and provides performance which for the most part is superior to the scheme for F-GSM is expected. A high performance FEC scheme is presented which employs soft decoded Reed-Solomon (RS) codes on the H-GSM channel.

Comparison of the simulated performance of a vsat satellite link with measurements

Abstract: This paper combines a number of features. First the simulation of the transmission performance of a Ka-band VSAT system (CODE) is presented. The results are used to verify the systems design that was performed for CODE and to demonstrate that the implementation margin due to intermodulation noise, adjacent channel interference, filter ISI, amplifier non-linearities and phase noise was sufficient. In the design of VSAT systems of this type phase noise was thought to be particularly important, and a new and detailed simulation of its effects is presented in the paper. For the first time, experimental measurements of system degradations have been taken on a VSAT system which have been used to verify simulation results. This was particularly important for the new phase noise simulation results and good agreement was obtained. An overall loop-back experiment using the Ka-band VSAT terminal with the Olympus satellite shows good agreement with the simulation results. Finally, the availability of experimental results has enabled us to compare the accuracy of TOPSIM III and BOSS simulations for the VSAT system. The paper indicates some shortcomings in the TOPSIM III simulation of phase noise and highlights the generally good agreement obtained with experimental results by the BOSS simulations.

Satellite link performance for an SS-TDMA experiment via Olympus

Abstract: The simulated results and experimental measurements of the Olympus SS-TDMA link are presented. The degradations of the satellite link were simulated using semi-analytic and Monte Carlo methods. These results are compared with the experimental results. The simulated results show good agreement with the measured results. The paper includes the description of the experimental test set-up, and the operation of the acquisition and synchronization unit (ASU) along with the TDMA reference and traffic terminals used in the experiment. These tests demonstrated the reference-to-remote and remote-to-reference as well as reference station loopback tests using the SS-TDMA payload. The simulated results of multicarrier operation are given. These results were obtained in order to predict the degradation due to TWTA operating point, carrier fade and adjacent channel interference for the purpose of link budget analysis to achieve the required satellite link performance. The simulated results for the TWTA operating point degradation as well as carrier fade are also compared with the measured satellite link degradations.