ISSN:0737-2884    

DOI:10.1002/sat.4600070502

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThe possibilities of finer longitude control of a geostationary satellite are studied with a view to providing insight into the problem of co‐location of satellites. It is shown that for a three‐axis stabilized satellite such as INSAT‐1B, placed close to an equilibrium longitude, the propellant expenditure for satellite attitude control is considerably larger than that required for longitude drift control. An experiment was conducted on INSAT‐1B in which the forces arising from the attitude control thrusters are optimally used to counteract the force due to the solar radiation pressure thereby reducing the perturbations on the orbital eccentricity vector. This is accomplished by suitably switching between the redundant attitude control thrusters located in the positive and negative roll faces of the satellite. The extent to which the effects of the solar radiation pressure are nullified is quantitatively estimated from the observed behaviour of the eccentricity vector. Without any recurrent propellant expenditure it is shown that it is possible to restrict the longitudinal excursions to nearly half of the normally allocated ±0·1° long

ISSN:0737-2884    

DOI:10.1002/sat.4600070503

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractAn analytical expression, previously derived in a companion paper published in Volume 7, Issue No. 1 of theInternational Journal of Satellite Communications(pp. 3–6), was used to generate signal suppression data for 3, 5, 10, 20, 40 and 55 input signal scenarios in the presence of Gaussian noise assuming an ideal hard‐limiter for a wide range of input signal ratios. Additionally, the signal suppression effect has been quantified for up to 200 input signals with input SNRs of ∞, 6 dB and 0 dB for the case in which a single higher‐power input signal exists in the presence of lower, equal‐powered signals. The data support the argument that the signal suppression effect is significantly reduced as the number of input signals is increased for the scenarios investigated. Also, the generated data show that the signal suppression effect decreases asymptotically to a value less than 1 dB for the case of a single higher‐power signal in the presence of varying degrees of input Gaussian noise and up to 200 total signals. Finally, a Gaussian approximation of the input signal ensemble was verified for the full range of input signal sets i

ISSN:0737-2884    

DOI:10.1002/sat.4600070504

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThe paper studies a digital modem that transmits a convolutionally encoded eight‐phase‐shift keyed (CE8PSK) signal over a satellite channel and employs a near‐maximum‐likelihood decoder at the receiver. Several different earth‐stations are assumed to have simultaneous access to a given transponder in the satellite as an arrangement of frequency‐division multiple access. Adjacent channel interference (ACI) is neglected here. The high‐power amplifier (HPA) at the earth‐station transmitter and the satellite transponder are assumed to be linear. The channel distorts the data signal, through the bandlimiting introduced by the equipment filters, and it adds stationary white Gaussian noise to the data signal at the receiver input. The resulting noisy and distorted data signal is coherently demodulated. Four different bandwidths of the transmitted CE8PSK signal are considered, together with three different demodulation processes. The decoder uses the Viterbi algorithm, together with one of six different distance measures to determine the costs of the stored vectors (survivors).The paper describes the various techniques involved in the modem, and presents the results of an extensive series of computer‐simulation tests to measure the performances of the system with all appropriate combinations of the signal bandwidth, demodulation process and distance measure. The results of the tests are then used to determine the prefe

ISSN:0737-2884    

DOI:10.1002/sat.4600070505

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThis paper investigates the effects of additive white Gaussian noise (AWGN), adjacent‐channel interference (ACI) and co‐channel interference (CCI) combined with flat fading on the performance of bandlimited 16‐ary differentially encoded quadrature amplitude modulation (16‐ary DEQAM) signalling transmitted through a non‐linear high‐power amplifier (HPA). It is assumed that the HPA exhibits both AM‐AM and AM–PM conversion distortions. This situation may be applied to the up‐link or down‐link of a regenerative satellite system where the HPA is the earth‐station travelling‐wave tube (TWT) or the on‐board TWTA, respectively. The signal‐to‐noise ratio (SNR) degradations at a specific bit error rate (BER) are evaluated by means of a series of computer simulation tests for various values of channel roll‐off factor α (channel bandwidth) and different degrees of output backoff (OBO) of the HPA in the presence of various combinations of the channel impairments. It is shown that by employing a baseband or RF predistortion technique for the HPA and selecting the best arrangement of channel roll‐off factor, OBO value of the HPA and channel spacing, a good performance can be achieved regarding the most efficient use of bandwidth together with a minimum SNR degradation due to the presence of channel impairments. Thus, the use of 16‐ary DEQAM for satellite communica

ISSN:0737-2884    

DOI:10.1002/sat.4600070506

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThis paper studies a digital modem for use over a regenerative satellite link. The modem transmits a convolutionally encoded 16‐ary quadrature‐amplitude‐modulated (CE16QAM) signal. The convolutional code has a three‐bit constraint length and a coding rate of 3/4. The high power amplifier (HPA) on board the satellite transmitter and at each station may introduce non‐linear distortion into the CE16QAM signal, but only a single‐link transmission is considered. Several different earth‐stations are assumed to be used in an arrangement of frequency division multiple access. Thus adjacent‐channel interference (ACI) can be introduced into the signal by the signals that occupy the immediately adjacent frequency bands.Extensive computer‐simulation tests have been carried out to determine the effects of bandlimiting, non‐linear distortion and ACI, on the tolerance to additive white Gaussian noise of the modem, using an equivalent baseband model of the transmission system. The results are used to select the preferred system arrangement. It has been shown that, with 4 dB output back‐off (OBO) in the HPA, a bandwidth efficiency of 2·4 b/s Hz can be achieved, with an improvement in perforamnce of 2·5 dB, relative to that of an ideal uncoded eight‐phase‐shift keyed (8PSK) signal,

ISSN:0737-2884    

DOI:10.1002/sat.4600070507

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

摘要:

AbstractThis paper proposes and evaluates selective reject (SREJ) ALOHA/FCFS, a highly efficient, flexible and robust multiaccess protocol for very small aperture terminal (VSAT) applications. SREJ‐ALOHA with locally synchronous FCFS retransmission scheduling is a ‘semi‐compatible’ enhancement of asynchronous SREJ‐ALOHA, which has earlier been shown to be an attractive satellite channel sharing protocol for low volume bursty terminals with variable length messages. The FCFS retransmission scheduling policy proposed here broadens the applicability of SREJ‐ALOHA by providing higher maximum throughput, particularly in the important source traffic scenario with bimodal message length distributions. Improved channel‐sharing efficiency and delay are achieved by exploiting time‐of‐arrival feedback naturally embedded in an unslotted SREJ‐ALOHA channel to develop an implicit time‐of‐arrival based (FCFS) ordering for conflict‐free, locally synchronous retransmission of most contending messages. An analytical procedure for evaluation of stability, throughput and delay for a general finite source, variable message length traffic model is described. The model is used to demonstrate that random access SREJ‐ALOHA/FCFS typically achieves a maximum throughput in the region of 0·4–0·45 along with excellent stability and delay properties. In addition, a natural extension of the protocol which supports implicit reservation for long messages is described and shown to offer a high maximum throughput (>0·6), comparable to that achieved by highly co‐ordinated slotted systems, for example mixed interacti

ISSN:0737-2884    

DOI:10.1002/sat.4600070508

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY

ISSN:0737-2884    

DOI:10.1002/sat.4600070501

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1989

数据来源: WILEY