On the Satellite Diversity in CDMA based Mobile Satellite Systems Ernestina Cianca * , Silvia De Fina * , Marina Ruggieri * , Ramjee Prasad ** (*) University of Roma “Tor Vergata”, Dpt. of Electronics Engineering Via di Tor Vergata 110, I–00133 Roma (Italy) (**) Center for PersonKommuniKation, Aalborg University (Denmark) e-mail: cianca@nausicaa.eln.uniroma2.it, ruggieri@uniroma2.it ABSTRACT - With non-GEO constellations the primary means of counteracting the shadowing and the blockage effects is through the use of the satellite diversity, i.e. multiple satellites in the constellation are simultaneously visible to user. The probability system availability is im- proved by increasing the probability at least one satellite is in clear Line Of Sight. In the selection diversity (SD) scheme the user is served by one satellite selected on the basis of the better Signal to Noise Ratio. In CDMA sys- tems the spatial diversity provided by the satellite diver- sity can be more effectively exploited by combining sig- nal replica coming from satellites in view with the aim at increasing the system capacity. Actually, the performance of signal combining/selection diversity schemes depend on the specific propagation scenario and it is not always effective to combine signal replica. In this paper hybrid selection/combining diversity schemes are proposed and a performance comparison in terms of system availabil- ity and capacity enhancement is carried out. Furthermore, an analytical methodology of the Downlink of DS-CDMA systems which takes into account the power constraints of a satellite environment is described and applied for the capacity assessment of the proposed satellite diversity schemes. 1. Introduction It is now well accepted that satellite systems will be nec- essary to complement terrestrial facilities to offer an any- time anywhere service. Most of the air-interface proposals submitted to ITU for the terrestrial IMT-2000 are based on various flavors of Wideband Code Division Multiple Ac- cess (W-CDMA) [1]-[4]. This fact constitutes an impor- tant guideline for steering the development of the satellite- component access scheme, as an air interface similarity will certainly contribute to making dual-mobile terminal more cost-effective [5]-[6]. Terrestrial systems seldom experience line-of-sight communication links; on the con- trary, the capability to provide mobile satellite commu- nications is strongly related to presence of a clear non obstructed path between satellite and user. In low-earth orbits moreover, path blockage is a very critical problem due to the low elevation angle at which satellites are seen most of the time. When there is an obstruction the systems must have a way to coping with these events. On the other hand, satellite systems are often power limited ([7],[8]) and, hence, counteracting shadowing and blockage effects by means of additional link margins may not be viable. With non-GEO constellations the primary means of cop- ing with obstruction is through the use of satellite diver- sity: when communications is to be established from user to a terrestrial gateway there are usually multiple satellites in the constellation that are jointly visible to user and gate- way and call can be simultaneously established through two different links-one on each satellites. If one of the links is obstructed the other link can carry the call. There- fore, satellite diversity greatly reduces the signal blockage probability as it has been confirmed during experimental campaigns whose results are summarized in [9]. If the re- duction in the probability of a dropped call is the greatest benefit of the satellite diversity there is a subsidiary ben- efit: the satellite power per user is reduced and, hence, given the overall available on-board power, the system ca- pacity can be increased. A meaningful quantitative as- sessment of the impact in terms of capacity of the satellite diversity exploitation needs the evaluation of the achiev- able power saving through a proper DS-CDMA downlink analysis. So far, the downlink analysis has been developed either without constraints on the limited spacecraft power [10], or under the hypothesis of full orthogonality between the spreading sequences [11]-[13]. The latter actually ap- plies only in the case of a very low satellite load, below the codebook limit (namely, less that 256 voice users for the WCDMA radio interface). In both cases, the actual sensitivity of capacity with respect to power management strategies, as power control and satellite diversity, cannot be fully appreciated. Furthermore, the full frequency reuse scheme, typically adopted in CDMA systems allows to exploit more effec- tively such a offered spatial diversity by coherently com- bining signal replica transmitted over multiple satellites or beams. With respect to a selection diversity scheme, a combining scheme provide an higher system availabil- ity and an increase or a decrease of the capacity on the basis of the combination of the following factors: 1) re- duction of the power link margin per user; 2) increase in the number of the interference users or, from another point of view, reduction of the available satellite resources per user. In fact, when each physical user is served from two satellites the effect from the interference point of view is a doubling of the number of users. In some cases, the reduc- tion of the power link margin and, hence, the reduction of average interference contribution per user could not com-