Performance evaluation of MC-CDMA techniques for variable bit-rate transmission in LEO satellite networks

Abstract

This work1 is aimed at investigating the use of Multi Carrier CDMA (MC-CDMA) techniques in variable bitrate transmission over low-earth-orbit (LEO) satellite channels by means of realistic simulations. It is known by literature that MC-CDMA techniques are much more resilient with respect to multi-user interference effects in multipath fading channels than single-carrier DS/CDMA ones. Moreover, MC-CDMA exhibits a natural capability to deliver multirate services simply by assigning to each user a variable-cardinality set of subcarriers. The achieved simulation results clearly confirmed the expected improved robustness of MC-CDMA techniques transmitting multirate data streams in frequency selective LEO satellite channels, with respect to state-of-the-art DS/CDMA transceivers. INTRODUCTIONIn these last years, the a considerable amount of R&D activities were carried on about the actual applications of Spread Spectrum and CDMA techniques to satellite communication systems working both on geo-stationary (GEO) and low-earth-orbit (LEO) constellations [1]. The advantages involved by the use of CDMA techniques in wireless communications are well known. The main problem involved by the use of CDMA in the satellite environment consists in the heavy limitation of capacity due to multi-user interference (MUI). This kind of disturbance is particularly severe in the upstream direction of low-earth-orbit (LEO) satellite channels, which are affected by propagation impairments, multipath fading, and near-far degradation quite similar to the one affecting the terrestrial pointed out a significant improvement of the BER performances and capacity yielded by a MC-CDMA system, with respect to a DS/CDMA system using a rake receiver, working in the same condition of asynchronous transmission over an indoor wireless multipath channel and absence of power control. This improvement is due to the orthogonality inherent to MC-CDMA techniques, which is retained also in multipath fading channels [4]. This fact is not verified for DS/CDMA systems, because multipath fading destroys the orthogonality of user codes and the resulting MUI may render symbol recovery impossible. In [9] an actual example of multi-code DS/CDMA satellite modem working over the Ka-band is dealt. Even though, the geostationary Ka-band channel exhibits a slow nonselective fading, the capacity of the system in terms of number of codes allowed to simultaneous transmission is strongly reduced by multi-user interference. The solution considered in [9] for improving system capacity was the introduction of a trained LMSE multi-user detection algorithm. Another solution considered in literature is to

Situation-aware radio resource management for multi-rate MC-CDMA wireless networks targeted at multimedia data exchanges in local areas

“…When user performances in terms of FER are not satisfactory, the AP issues a "class downshift" for users penalized by heavy FERs. Such a downshift mechanism, introduced in [2], substantially consists in a decrease of the data rate, with a consequential increase of the processing gain and transmission SNR, achieved in the context of the VBR MC-CDMA PHY-layer design detailed in [3]. The effectiveness of the class downshift mechanism has been already proven in [2].…”

“…MC-CDMA techniques are regarded as the "natural" spread spectrum extension of OFDM. MC-CDMA is robust against multipath fading distortions and exhibits a natural inclination to variable-bit-rate (VBR) transmission as shown in [3]. The provision of efficient and asynchronous multiple access at different data rates guaranteed by MC-CDMA is a very interesting feature, especially if applications like multimedia data exchange in local areas, characterized by different QoS requests coming from connecting users, are considered (see e.g.…”

“…The adopted strategy is substantially a joint MAC-PHY layer approach derived by the one shown in [2] which relies on the variable-bit-rate (VBR) MC-CDMA physical layer design proposed in [3]. The overall transmitting users are partitioned into different VBR classes, each one characterized by a set of PHY-layer parameters consisting of: a) a variable-cardinality subcarrier set and b) a variable-length spreading code, each one depending on the bit-rate class.…”

Maximum likelihood estimation of carrier offset in a variable bit rate orthogonal multicarrier CDMA

“…It's worth noting that there is a rapid convergence in the estimation process for the lowest bit rate user, whereas for the highest bit rate user some additional iterations are required. This due to subcarriers strategy allocation chosen [5]. Figure 6 shows numerical results for the highest bit rate user in case of K=8, K=12 and K=20 users.…”

“…When a MC-CDMA system is employed to provide Variable Bit Rate (VBR) services the same transmission philosophy is kept but a strategy allocation for different variable rate users has to be given. In this work, the use of Orthogonal Variable Spreading Factor codes to provide VBR capabilities is considered [4] [5]. MC-CDMA, similar to OFDM scheme, suffers a great deal of performance degradation from the presence of frequency offset in the subcarriers [6] [7].…”