CDMA versus FDMA channel capacity in mobile satellite communication

At present a major effort is under way to define the most efficient modulation/multiple access system in mobile satellite communication. Where the emphasis is on digital voice modulation, the proposed multiple access methods almost always imply frequency division multiple access (FDMA). This analysis presents a comparison between FDMA and code division multiple access (CDMA), for the operation of both multiple access methods in the mobile satellite communication environment. The mobile satellites under consideration use multiple-beam or scan-beam antennas and employ frequency reuse of the allocated L-band frequency spectrum. As CDMA is better at absorbing Doppler and multipath effects, and permits higher rate coding, in general (practical considerations aside) it appears to be the more capable system.     

A Multiple Frequency Channels Reserved Timeslots Assignment Algorithm for Real-Time Traffic in MF-TDMA Satellite System

Timeslots assignment can have a great impact on the bandwidth allocation efficiency in multi-frequency time division multiple access (MF-TDMA) satellite system. In this paper a multiple frequency channels reserved timeslots assignment algorithm is proposed for MF-TDMA user terminals (UTs) with real-time traffic. The scheme can reserve several frequency channels simultaneously for one UT and realize fast timeslots block searching within a MF-TDMA frame. Computational complexity analysis demonstrates the performance of the proposed method on computational efficiency. What’s more, simulation results show that both channel utility and connection rejection ratio can be improved compared with existing algorithm.     

Performance simulation of NC-PRMA with signatures in microcellular communication systems

Packet reservation multiple access (PRMA) improves capacity in microcellular systems compared with time division multiple access (TDMA) or frequency division multiple access (FDMA). In PRMA, when a mobile terminal has information packets to transmit, it contends with other terminals for access to a common radio channel. Therefore the main performance degradation is due to the collision of terminals simultaneously transmitting packets. In this paper we propose a non-collision PRMA (NC-PRMA) protocol with signatures to achieve a better performance than PRMA does. Two classes of duplexing schemes, frequency division duplexing (FDD) and shared time division duplexing (STDD), are explored and two speech activity models, slow and fast, in both FDD and STDD schemes are studied. From the results of a computer simulation it is observed that, with the constraint of a packet-dropping rate no greater than 0·01, NC-PRMA can support 38 (43) and 45 (49) users respectively under the FDD and STDD schemes if the slow (fast) speech activity detector is adopted. © 1998 John Wiley & Sons, Ltd.     

Cell search scheme for long-term evolution of TD-SCDMA system

Cell search is an important aspect for 3G long-term evolution （LTE）. This article deals with cell search in the time-division-synchronou code-division multiple access （TD-SCDMA） LTE system. On the basis of the synchronization channel （SCH） and cell specific reference symbols （CSRSs）, the proposed cell search procedure includes five stages： frame detection and coarse timing, coarse carrier frequency offset （CFO） estimation, fine timing, fine CFO estimation, and cell identification. The key features of the proposed method are as follows： first, the neighboring three cells＇ CSRSs are frequency division multiplexed （FD） to mitigate inter-cell interference. Second, the frequency domain differential cross-correlation, computed from CSRSs are maximally ratio combined for cell identification. Finally, the large set Kasami sequences are quadrature phase shift key （QPSK） modulated to be cell specific sequences （CSSs）, to support a large number of target cells. Simulations show that the FD method is better than the code division multiplexed （CD） method.     

Spread Spectrum and Time Division Multiple Access Satellite Communications

Although time division multiple access (TDMA) satellite communication provides the highest traffic capacity per satellite and offers efficient transmission of a wide variety of services, it suffers from network timing and ranging requirements and message security. On the other hand, spread spectrum multiple access (SSMA) satellite communication is suited for reliable random access and tactical transmission systems. Orthogonal functions such as Rademacher, Haar, and Walsh, or pseudonoise sequence coding of amplitude and phase of the analog message or code division of baseband signals, frequency and time hopping are among the several methods employed for spread spectrum communications. A combination of TDMA and SSMA can be used for asynchronous and reliable transmission of digital or analog signals.     

Multiple-access techniques for commercial satellites

The unique multipoint interconnection capabilities of satellite communication networks place a burden on the communications system designer to achieve highly efficient transmission capacity in the face of a very wide range of network requirements. The ability of various types of multiple-access systems to meet these requirements is the subject of this paper. INTELSAT, recognizing that the frequency division multiplex-frequency modulation, frequencY division multiple access (FDM-FM-FDMA) method it currently uses has advantages in some applications but is not optimum for the multiple-access environment, has supported the development of two-digital multiple-access communications techniques: single channel per carrier frequency division multiple access (SPADE) and time division multiple access (TDMA). These two techniques are discussed in detail and the paper concludes with a presentation of a promising multiple-access technique of the future which includes time division switching on board the satellite.     

Spread SCPC/FDMA,spread TDMA/FDMA and SSMA/FDMA transmission through A non-linear satellite channel with an earth-station off-axis emission constraint

This paper analyses and compares possible access schemes to be used in satellite networks with a large number of small earth-stations. Basically, frequency division (FDMA), time division (TDMA), and spread spectrum multiple access (SSMA) are considered. For TDMA, one single carrier originated in one particular earth-station is transmitted in each sub-band and therefore a single-channel-per-carrier system results (SCPC/FDMA). For TDMA and SSMA, it is assumed that groups of earth-stations using either of these access schemes share the full available band in FDMA. The corresponding systems are thus TDMA/FDMA and SSMA/FDMA. Moreover, for SCPC/FDMA and TDMA/FDMA individual carriers may be spread to the extent that the available bandwidth is always fully occupied. Spectral efficiencies for these three access modes (Spread SCPC/FDMA, Spread TDMA/FDMA and SSMA/FDMA) are computed and compared for transmission through a non-linear satellite channel and optimum operating points for the non-linear amplifier are also determined. Comparisons are also performed for the situation in which transmitting earth-stations are bounded to obey an off-axis emission constraint.     

An IP transport and routing architecture for next-generationsatellite networks

Over the past several years, a number of new satellite systems have been proposed to provide high-speed Internet and multimedia services to businesses and home users. These proposals have been driven by the desire of network operators to reach end users that do not have cost effective access to other alternatives such as fiber, DSL, and cable, and by the availability of new spectrum (Ka-band) for use by new satellite services. The proposed systems generally employ multiple high-power spot beams, an onboard fast packet switch, and a demand-assigned multiple access scheme to provision IP-based services. In this article we concentrate on a geosynchronous satellite system where packet transport and switching within the satellite system are based on ATM. We describe an IP/ATM interworking and IP routing architecture that is driven by three main requirements: (1) the ability to support ATM SVCs between hundreds of thousands of satellite terminals by a single ATM switch located onboard; (2) a scalable IP routing architecture that does not result in large volumes of routing traffic to be transported over the satellite; and (3) the ability to segment the satellite terminals for routing and administrative control by ISPs and enterprise networks     

Communication satellite processing repeaters

Two kinds of processing repeaters which may find application on communication satellites in the near future are described. The type I repeater allows access only if the transmitted signal contains a predetermined code structure. This serves three purposes: first, unauthorized users are excluded, second, co-channel interfering signals are not retransmitted on the downlink, so as not to waste a portion of the satellite transmitter power, and, third, removal of the interference signal at the satellite avoids having to do this at the ground receiving terminal. Thus such a repeater would find particular application where there are a large number of ground receiving stations. The type 2 repeater routes signals received at its N input terminals to be transmitted at N output terminals, thus acting as a "switchboard in the sky." Two purposes are served: first, the satellite capacity is more fully utilized in the presence of fluctuating traffic demands, and, second, single-frequency transmission and reception are possible for user ground stations, thus simplifying these stations and still allowing communication to any station in the network. It is established when the type 1 repeater is able to increase the satellite communication capability beyond that of a simple repeater and further that fairly simple filtering is sufficient on the satellite. For the type 2 repeater, a proposed frequency control plan minimizes the filtering required on the satellite where frequency division multiplex is used and reduces the amount of switching required on the satellite to N single-pole N-throw switches where time division multiplex is used without the requirement of any memory on the satellite. A summary of present-day translating repeaters, as used in the Intelsats III and IV and DSCS II satellites, is included.     

Personal communications via iNTELSAT Ku-band transponders

The concept of a fully meshed network of very small terminals is presented for personal communications over K_u-band satellite transponders. In this concept, undesirable double-hop delays are avoided for voice communications between these terminals. The bandwidth and power resources of the transponder are efficiently shared by users in a simple demand-assigned manner via code-division multiple access (CDMA).^1,2 Voice, data and facsimile are statistically multiplexed at each terminal. In order to minamize terminal costs, frequency-precorrected and level-preadjusted continuous wave (CW) tones are sent from the central network control station in each beam. The terminals in each down-link beam can use these pilots as references for antenna acquisition and tracking, as reliable frequency sources, and as indicators of signal fade for up-link power control (ULPC). The potential CDMA ‘near-far’ problem due to up-link fades is mitigated by using ULPC. Quasi-burst-mode transmission is employed to minimize the potential loss of clock and pseudorandom number code synchronization. When these terminals are used only to interconnect into the public switched telephony networks (PSTNs) through large, hub-like gateway stations, they can employ flat-plate antenna technology. The complete terminal can thereby be packaged into a briefcase so that it is easily transportable for personal use.     

基于多波束GEO卫星的大容量互联网接入系统

针对大容量地球同步轨道（GEO）卫星系统目的关口站距用户远、同步耗时长、下行传输效率低等问题,提出基于多波束GEO卫星的大容量互联网接入系统。所提方案星上进行非再生式信号处理、基带程控交换、闭环同步控制、信令信息处理和信道调度管理以及下行链路采用时分复用传输方式。对于支持大量分散或移动用户接入互联网的应用,业务质量显著改善,且因信道利用率大大提高,实际运行时的用户容量、数据吞吐率也可能更大。仿真结果表明多频-准正交时分复用传输方式的性能更好,且根据我们已进行过的有关单元的现场可编程门阵列设计实现经验,所提系统现实可行。      

正交频分多址联接及其关键技术

正交频分多址(OFDMA)是一种基于正交频分复用(OFDM)原理的多址技术,具有很高的带宽利用率,同时汇集了OFDM、跳频及时分多址(TDMA)技术的诸多优点,在无需使用均衡器的情况下,可有效地克服码间干扰。与传统的直扩CDMA(DS-CDMA)或多址CDMA(MC-CDMA)相比,OFDMA提供的容量更大。     

Diversity ALOHA--A Random Access Scheme for Satellite Communications

A generalization of the slotted ALOHA random access scheme is considered in which a user transmits multiple copies of the same packet. The multiple copies can be either transmitted simultaneously on different frequency channels (frequency diversity) or they may be transmitted on a single high-speed channel but spaced apart by random time intervals (time diversity). In frequency diversity, two schemes employing channel selections with and without replacements have been considered. In time diversity, two schemes employing a fixed number of copies or a random number of copies for each packet have been considered. In frequency diversity, activity factor-throughput tradeoffs and in time diversity, delay-throughput tradeoffs for various diversity orders have been compared. It is found that under light traffic, multiple transmission gives better delay performance. If the probability that a packet fails a certain number or more times is specified not to exceed some time limit (realistic requirement for satellite systems having large round trip propagation delay), then usually multiple transmission gives higher throughput.     

Analysis of new methods for broadcasting digital data to mobileterminals over an FM-channel

Two new methods using an FM-radio channel for transmission of digital data to mobile terminals are examined: 1. A modification of the radio data system (RDS). In RDS, additional digital information is multiplexed with a stereo sound signal. A new system is suggested where the data signal can be multiplexed with a mono audio signal. This causes extension to the bandwidth available for the data signal, and therefore the RDS bitrate can be increased. Error calculations are performed both for the original RDS system and for the new system. 2. Orthogonal frequency division multiplexing (OFDM). OFDM is used in the digital audio broadcasting system (DAB), which is designed to transmit digital audio in the FM band. In OFDM a signal is divided over a large number of 2- or 4-PSK modulated orthogonal subcarriers. The subcarriers of 6 different programmes are multiplexed in one beam to reduce the effects of frequency selectivity of the transmission channel. A new system based on OFDM is proposed, in which the carriers of each programme are transmitted in one FM-channel with a bandwidth of 200 kHz instead of multiplexed with the carriers of other programmes. Error calculations are performed for the subcarriers used in the OFDM modulation method     

Code-based all optical routing using two-level coding

Code-based all optical routing employing a two-level-coding scheme is proposed. The first level of coding is employed to establish connections between users within a local area network; a second level of coding provides routing/interconnectivity between networks. The limitations due to physical-layer impairments, such as relative intensity noise (RIN) of the optical source, the signal-dependent shot noise, optical beat interference (OBI), and thermal noise at the receiver, which are some of the fundamental issues in the design of practical optical code division multiple access (CDMA) systems, are analyzed for the two-level scheme. The throughput in terms of "packets/time slot" offered by the scheme is also compared with that of the wavelength division multiple access (WDMA) system. The system capacity in WDMA is limited by the number of available wavelengths; optical CDMA, on the other hand, has many codes (user addresses), but the throughput is limited by multiple user interference, OBI, and RIN. System designs that overcome these effects and thereby improve the throughput are suggested.     

Satellite systems in a VSAT environment

The next generation of satellites is discussed and a Ka-band payload is proposed which will provide mesh connectivity at high data rates (e.g. 1 Mbit/s) directly between very small aperture terminals (VSATs) using a maximum antenna diameter of 1.5 m. It is demonstrated that there are no significant differences between Ku-band and Ka-band VSAT operations apart from propagation margins, which are analysed statistically over 650 European cities. It is shown that margins required for Ka-band operation are not excessive. A Ka-band satellite payload is proposed, based on frequency division multiple access, multibeam switching with dynamically controllable transponder bandwidths and gain. The payload adapts to traffic-loading requirements in its provision of multibeam connectivity and capacity. The signal format, modulation and coding technique used for transmission is unconstrained, except in terms of bandwidth, and this will allow advances in modulation and coding to take place during the lifetime of the satellite in orbit     

Delay analysis for interlan traffic using two suitable TDMA satellite access schemes

Two demand assignment time division multiple access (TDMA) satellite access schemes that are suitable for providing flexible interconnections of local area networks are compared by means of two simulations; Fifo Ordered Demand Assignment/Information Bit Energy Adaptive (FODA/IBEA) and Combined Fixed Reservation Assignment (CFRA). A unique simulation tool was used, a meshed network of very small aperture terminals (VSATs) under the control of a master station was considered and similar networking conditions were applied to both access schemes. The generated traffic had two components with different level of burstiness; bulk traffic and interactive traffic. The performance evaluated is the delay for each traffic component. We show that CFRA is best when connecting clusters of only a few stations and even individual stations exchanging light traffic. On the other hand, FODA/IBEA is better suited to interconnect networks, or within networks with many hosts, where heavy traffic is more likely. © 1997 John Wiley & Sons, Ltd.     

RADIO AND NETWORK COMPARISONS FOR MSBN AND GSM SYSTEMS IN A GEOSTATIONARY SATELLITE ENVIRONMENT

One of the most frequently debated issues in satellite communications concerns the comparison between time division multiple access (TDMA) and code division multiple access (CDMA) systems. Although, in our view, a general answer to this problem cannot be found, this paper presents comparisons of two systems utilizing the above-mentioned access techniques, both from a radio and a network point of view. In particular, the mobile satellite business network (MSBN), (a CDMA system currently being developed by the European Space Agency (ESA) for general satellite applications) and the full integrated extension to satellite systems of the global system for mobile communication (GSM) (the well-known frequency division multiplex (FDM)/TDMA pan-European mobile cellular system) are compared by taking, as the reference payload, the L-band land mobile (LLM) satellite (a spot-beam geostationary satellite whose launch should take place at the end of 1997). Reasonable assumptions take into account the fact that, on one hand, the MSBN system is not completely defined and, on the other, the GSM is applied in a context different from the one specified by the European Telecommunication Standard Institute (ETSI). Performance comparisons are carried out in terms of flexibility and number of available channels for the same quality requirements, emitted power levels, propagation environment and interference levels coming from adjacent satellites. It is concluded that MSBN is more flexible than GSM and that, as regards capacity, an advantage of GSM over MSBN is present in the transportable mobile case, whereas, in the vehicular mobile case, MSBN shows an advantage over GSM. © 1996 by John Wiley & Sons, Ltd.     

Beat Noise Mitigation of Spectral Amplitude Coding OCDMA Using Heterodyne Detection

In order to relax the limitation of the number of multiplexed signal lights caused by beat noise between signal lights, we investigate the applicability of a heterodyne detection technique to a spectral amplitude coding optical code division multiple access scheme. In this investigation, for the first time, we found analytically that the optical frequency chips that form parts of the signal and local lights require uniform phase differences even for envelope detection. We also confirm this requirement and our theoretical analysis experimentally.     

MIMO Frequency Hopping Spread Spectrum Multi-Carrier Multiple Access: A Novel Uplink System for B3G Cellular Networks

In this paper, we propose and give the performance of a novel uplink system based on the combination of multi-carrier (MC), code division multiple access (CDMA) and multiple input multiple output (MIMO) techniques. First, we describe the interests of spread-spectrum multi-carrier multiple access (SS-MC-MA) scheme for uplink, especially compared to MC-CDMA. Classically, with SS-MC-MA, each user spreads its data symbols on a specific subset of adjacent or multiplexed subcarriers, to facilitate the channel estimation and reduce complexity at the reception. In order to compensate for the lack of frequency diversity of SS-MC-MA with adjacent subcarriers, we first combine it with an orthogonal space-time block code (STBC) and demonstrate the resulting spatial diversity gain. Then, we propose to allocate the subsets to the different users by applying a frequency hopping pattern (FH). In that case, each user benefits from the frequency diversity linked to the total bandwidth as with the multiplexed subcarriers solution, while keeping the advantages of the adjacent subcarriers solution. The gain provided by the use of the frequency hopping is stressed on. Finally, the performance of this scheme is evaluated over realistic MIMO channel with channel turbo coding for systems offering asymptotic spectrum efficiency of 1, 2, 3 and 4.5 bit/s/Hz. Thus, the efficiency of the novel proposed STBC FH SS-MC-MA system as a very promising multiple access and modulation scheme for the uplink of the future wideband wireless networks is successfully demonstrated.