370 IEEE TRANSACTIONS ON BROADCASTING, VOL. 58, NO. 3, SEPTEMBER 2012
A Comparative Study Between SC-FDMA andOFDMA Schemes for Satellite Uplinks
Vassilis Dalakas, Member, IEEE, P. Takis Mathiopoulos, Senior Member, IEEE, Filippo Di Cecca, andGennaro Gallinaro
Abstract—This paper presents a detailed comparative study oftwo single-carrier frequency-division multiple access (SC-FDMA)schemes, namely localized FDMA scheme (LFDMA) and inter-leaved FDMA scheme (IFDMA), versus orthogonal FDMA scheme(OFDMA), for a satellite uplink. The air-interface of the latter isbased on the digital video broadcasting (DVB) family of standards.Considering two state-of-the-art high power amplifiers (HPAs),operating in the K- and S-bands, the performance of synchronousand asynchronous LFDMA, IFDMA and OFDMA is evaluatedin a multi-user environment. Systematic comparison results showthat although for synchronous reception IFDMA outperformsthe other two schemes, for asynchronous reception it is the mostsensitive to degradation caused by inter-block interference (IBI).Furthermore, due to its relatively large envelope fluctuations,OFDMA is the most sensitive scheme to non-linear distortion.Although for synchronous reception LFDMA shows only slightlyinferior performance as compared to IFDMA, it outperforms theother two schemes for the asynchronous reception considered,especially for increased IBI distortion.
Index Terms—Digital video broadcasting via satellite,inter-block interference, orthogonal frequency-division mul-tiple access, satellite uplink, single-carrier, total degradation.
I. INTRODUCTION
I N RECENT years, the increasing commercial demand forhigher data rates has led to the utilization of Orthogonal Fre-
quency-Division Multiplexing (OFDM) in several well knownstandards, including the 2nd generation of Terrestrial DigitalVideo Broadcasting (DVB-T2) [1], Digital Video BroadcastingSatellite Handheld (DVB-SH) [2] and 3rd Generation Partner-ship Project (3GPP) [3]. The main technical advantage for sucha choice is OFDM’s robustness in the presence of frequencyselective fading channels commonly encountered in wirelessbroadband communication systems [4]. However, OFDM tech-nology has a major drawback, since it suffers from high peak-to-average power ratio (PAPR) of the transmitted signals [4, Chap.
Manuscript received May 16, 2010; revised January 31, 2012; accepted Feb-ruary 12, 2012. Date of publicationMay 11, 2012; date of current versionAugust17, 2012. This work was supported by ESA under Contract 21072/07/NL/AD.V. Dalakas is with the Network Operations Center, Harokopio University of
Athens, 176 71 Athens, Greece (e-mail: [email protected]).P. T. Mathiopoulos is with the Institute for Space Applications and Remote
Sensing, National Observatory of Athens, 15236 Penteli, Greece (e-mail:[email protected]).F. Di Cecca and G. Gallinaro are with Space Engineering, 00155 Rome, Italy
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TBC.2012.2193494
6]. Such signals require linear power amplifiers to avoid exces-sive signal distortion. Hence, the transmit power amplifiers haveto operate with a large input power back-off, (IBO), from theirpeak power which leads to poor power efficiency [4, Chap. 6].Orthogonal Frequency-Division Multiple Access (OFDMA) isthe multiple access scheme that naturally extends OFDM to si-multaneously serve multiple users.Single-Carrier Frequency-Division Multiple Access
(SC-FDMA) schemes are employed as alternative accessschemes, which offer reduced PAPR as compared to OFDMA’shigh PAPR [5]. A signal with lower PAPR is desired, as itimproves the power efficiency of the employed non-linearamplifier [6]. Although SC-FDMA utilize single-carrier mod-ulation at the transmitter and frequency domain equalizationat the receiver and typically achieve lower PAPR, they havesimilar transmitter structure and Bit Error Rate (BER) perfor-mance, as compared to an OFDMA system. The key differencein the transmitters of the two schemes is the presence of anadditional discrete Fourier transform (DFT) in SC-FDMA.Thus, SC-FDMA is sometimes referred to as DFT-spread orDFT-precoded OFDMA [5]. Among the various SC-FDMAschemes, the most popular are: (i) Localized FDMA (LFDMA);and (ii) Interleaved FDMA (IFDMA) [5].Because of its lower PAPR, a SC-FDMA scheme has been
adopted for the uplink as the multiple access scheme in 3GPP[3]. Since 3GPP focuses on the Long Term Evolution (LTE)of UMTS (Universal Mobile Terrestrial Systems) radio access,it has led to a renewed interest for performance comparisonsstudies between SC-FDMA and OFDMA for terrestrial broad-band communications systems. For example, a comparisonstudy between SC-FDMA and OFDMA has confirmed thatSC-FDMA has better PAPR performance in terms of its com-plementary cumulative distribution function (CCDF) [7]. Thesame study has also investigated the effects of pulse shapingon the PAPR performance of both access schemes. SC-FDMAschemes appear to have an additional advantage over OFDMAscheme since they offer increased spectrum efficiency betweendifferent users [8]. In [9], it has been shown that LFDMA hashigher throughput than the IFDMA. A turbo equalization tech-nique that improved SC-FDMA performance over OFDMAhas been presented in [10]. A performance analysis of OFDMAversus several variants of DFT-precoded OFDMA, as wellas SC-FDMA in its time- and frequency-domain implemen-tations, has been presented in [8] confirming the advantagesof SC-FDMA in terms of performance and spectrum effi-ciency, especially for frequency selective fading channels.More recent studies comparing the performance of OFDMA
DALAKAS et al.: COMPARATIVE STUDY BETWEEN SC-FDMA AND OFDMA SCHEMES FOR SATELLITE UPLINKS 371
versus SC-FDMA have been published in [11] and [12]. In[11], a International Mobile Telecommunications—Advanced(IMT-A) system operating in a linear channel was considered,while more recently in [12] limited BER performance eval-uation results have been presented using a soft-limiter as anapproximation for non-linear channel. In general it appearsthat, while SC-FDMA is an attractive alternative to OFDMAaccess scheme, the PAPR advantage of SC-FDMA is mainlyrelevant for the uplink. For example, for the 3GPP, SC-FDMAhas been adopted for its uplink, while OFDMA is used for itsdownlink [3].It is important to underline that the previously mentioned
studies have dealt exclusively with terrestrial 3GPP systems.Although OFDM techniques are established in terrestrial mo-bile networks they have not found up to now significant use inspace based communication systems. To the best of our knowl-edge, similar to terrestrial communications systems studies forstate-of-the-art satellite based systems, such as the DVB familyof standards (e.g., DVB-S2, DVB-SH), are not available in theopen technical literature. An important difference of multipleaccess schemes for satellite based systems, which distinguishthem from their terrestrial counterparts, is their inherent diffi-culty in obtaining accurate synchronization between users [13,Chap. 6]. Although it may be common for terrestrial communi-cation systems to assume ideal synchronization [5], for a satel-lite based system due to its long and varying signal propaga-tion delays such assumption is not always easy to achieve. Er-roneous synchronization will affect the orthogonality of the sub-carriers, leading to significant performance degradation due tothe presence of Inter-Block Interference (IBI) between succes-sive symbol blocks in a multi-user scenario [14]. It is there-fore important to study the performance of such access schemesfor satellite communication systems in the presence of synchro-nization error. Another difference is the High Power Amplifier(HPA) used in satellite systems. As well known, HPAs used insatellite communication systems have highly non-linear transferfunctions, including non-linear phase characteristics [13, Chap.7]. Typically, this is not the case for the power amplifiers em-ployed in terrestrial communications systems where simplifiedmodels, such as the Rapp model [8] or the soft limiter [12] havebeen used.Motivated by the above, in this paper we consider the appli-
cation of -FDMA1 schemes for state-of-the-art satellite multi-user systems. In particular, we present a thorough and detailedcomparison study of -FDMA access scheme performances, fora satellite uplink operating in dual frequency bands (K- andS-Bands) in the presence of non-linear HPAs and synchroniza-tion error. The organization of this paper is as follows. After thisintroduction, the system model, together with a short overviewof the -FDMA schemes, will be presented in Section II. Per-formance evaluation results can be found in Section III, wherethe multi-user performances with and without synchronizationerror are presented and compared. Concluding remarks can befound in Section IV.
1For the conciseness of the presentation, from now on and unless otherwisestated, the notation -FDMA will be used as a common rep-resentation of the three multiple access schemes considered in this work, i.e.,LFDMA, IFDMA and OFDMA.
II. SYSTEM MODEL
The block diagram of the system model under consideration,as shown in Fig. 1, represents the uplink of a typical multi-usersatellite communication system. It consists of users thatmay transmit simultaneously, each generating independent-FDMA signals, which after passing through a non-linearHPA, are transmitted through the channel. Corrupted by addi-tive white Gaussian noise (AWGN) with single-sided powerspectral density, , the signals arrive at the satellite, in thegeneral case asynchronously, i.e., with different time offsets
. Since both asynchronous, i.e., ,and synchronous reception, i.e., , , will be considered,the time delay blocks shown in Fig. 1 are denoted as optional.As the multiple access scheme used for downlink is typicallydifferent from the access scheme employed for the uplink [15],the system model under consideration assumes that the usersignals are demultiplexed and fully recovered at the satellitefrom the aggregated signal , observed with a common timeinterval using -FDMA receivers2. In the uplink, each userterminal multiplexes only its own signal before its HPA (oneper user). This procedure is presented with details in nextsection, and the main difference from downlink is that at thesatellite the users signals are multiplexed and amplified by asingle HPA. Throughout this paper the following notations willbe used. The -th symbol for the -th user
is denoted with , while its corresponding vectoris denoted with bold as . When only one user is referred, theindex will be omitted.
A. -FDMA
The detailed block diagram of the -FDMA signal generatorrepresenting its discrete baseband equivalent is illustrated inFig. 2. It consists of a Turbo Encoder (TE), aModulator (MOD),an -ary Discrete Fourier Transform ( -DFT), used only forSC-FDMA signal generation, a Sub-carrier Mapping (SM), an-ary Inverse DFT ( -IDFT) and a Cyclic Prefix (CP) adder.
The input to the TE are random and equally probable bits, ,where they are encoded by a turbo code with rate, . The codedbits, , are modulated into the following time domain vector
where denotes transposition and the -th element, , is acomplex symbol, which can be selected from a variety of mod-ulation formats. Two of the most popular modulation formats,namely Quadrature Phase Shift Keying (QPSK) and 16 Quadra-ture Amplitude Modulation (16-QAM), will be considered inthis paper. Further processing of will vary according to thespecific access scheme used, as will be presented next.Firstly, for a SC-FDMA scheme, the use of a -point DFT is
required, in order to produce a frequency domain vector
(1)
2Although these receivers depend on the particular access scheme used, ageneric receiver structure for all three access schemes considered in this paperwill be presented in Section II-A.
372 IEEE TRANSACTIONS ON BROADCASTING, VOL. 58, NO. 3, SEPTEMBER 2012
Fig. 1. Block diagram of the system under consideration.
Fig. 2. Block diagram of the -FDMA signal generator representing its discrete baseband equivalent.
with
Each of the -DFT outputs, , is then mapped to one oforthogonal sub-carriers (or tones) resulting in a new
frequency vector
where only of its elements are non-zero. The sub-car-rier mapping determines which part of the spectrum is used fortransmission. On the one hand, for the LFDMA scheme, iszero padded to form a localized mapping, i.e.,
On the other hand, for the IFDMA scheme, , is up-sampledto form a new vector with distributed mapping,
Secondly, for the OFDMA scheme, the -DFT shown inFig. 2 is omitted, so that . As each user employsonly of the available tones, the remaining tonesare set to zero, e.g.,
For the three access schemes by applying an -point IDFTto , the -FDMA symbol block is formed as
(2)
where is the -FDMA symbolblock of time duration , is the DFT matrix oper-ator and denotes Hermitian transposition of . As shownin Fig. 2, prior to transmission, a CP of length and dura-tion is inserted per symbol block in order toprovide guard time, which eliminates IBI between successivesymbol blocks. Discarding this CP at the receiver, converts thelinear convolution between the transmitted sequence and thechannel impulse response into circular convolution, facilitatingthe equalization of the signal. The symbol block after CP addi-tion can be conveniently expressed in matrix form as a vector
(3)
of length , where is the matrix resulting bythe concatenation of the last rows of an identity matrix
(denoted as ) with itself. The HPA output signal, ,can be mathematically presented as
(4)
where denotes the HPA transfer functions presented by Fig. 4.In general, the signals from each user are corrupted by AWGN
DALAKAS et al.: COMPARATIVE STUDY BETWEEN SC-FDMA AND OFDMA SCHEMES FOR SATELLITE UPLINKS 373
Fig. 3. Block diagram of the -FDMA receiver.
Fig. 4. AM/AM and AM/PM characteristics: (a) for the linearized ; (b) for .
and arrive at the satellite with different time offsets denoted with.
A straightforward generalization of the signal notation forusers is presented in Fig. 1. The -FDMA signal arriving at thesatellite can be expressed as
(5)
where is identical to in the case of synchronous recep-tion. The more complex case of asynchronous reception is de-scribed in a following section. The block diagram of the generic-FDMA receiver is depicted in Fig. 3. It consists of a CP cutter,an -DFT, a Sub-carrier DeMapping (SDM), an -IDFT, usedonly for SC-FDMA, a Demodulator (DEMOD) and a Turbo De-coder (TDEC), leading to an estimation, , of the originallytransmitted bits, .
B. Non-Linear Amplification
In this study two distinct HPAs operating in two different fre-quency bands are considered. The first one, is a linearized HPAoperating in the K-Band, which will be denoted as . ItsAM/AM and AM/PM transfer functions have been excerptedfrom the DVB-S2 standard [15] and are shown in Fig. 4(a). Thesecond amplifier is another state-of-the-art HPA, which operatesat the S-Band and will be denoted as . Its AM/AM andAM/PM transfer functions are illustrated in Fig. 4(b). In bothfigures, the AM/AM and AM/PM characteristics are presented
as output versus input back off (OBO and IBO). It is noted thatthe linearized exhibits fairly constant AM/PM charac-teristics for IBO signal levels ranging from 12 dB to 3 dB be-fore saturation. Furthermore, comparing their transfer functions,it is noted that although their AM/AM transfer functions aresimilar, their AM/PM differ with the being significantlynon-linear. It should be also noted that although these HPAs aretypically used on-board, without any loss of generality on ouranalysis, they could be also used for the uplink.
C. Classes of Users
Fig. 5 illustrates the general case of asynchronous receptionwhere the following three distinct classes of users are shown.1) “User A”: This class of users represents the informationcarrying users and will be referred to asUseful Users (UU).
2) “User B”: This class of users represents the asynchronous,with respect to the UU, users and will be referred to asOther Users (OU).
3) “User C”: This class of users represents the users con-tributing IBI and will be referred to as IBI Users (IBIU).
The signals from each of these classes of users consist of se-quences of -FDMA symbols with duration , each of thempreceded by a CP with duration . In the same figure, the re-ceiver’s observation time interval and its timeoffsets from the signals of the three classes of users are also in-dicated. As shown in Fig. 5 since for User C, the timing offsetexceeds the duration of the CP , IBI will occur.
Clearly, as increases, the amount of IBI will also increase.
374 IEEE TRANSACTIONS ON BROADCASTING, VOL. 58, NO. 3, SEPTEMBER 2012
Fig. 5. Schematic of asynchronous reception. Three user classes are illustrated, namely, Users A, B and C. Sequences from -FDMA symbols prefixed with CPare illustrated in relation to the common DFT window of the receiver.
As the other two time offsets ( or ) do not exceed the du-ration of the CP, i.e., , they do not introduceIBI. Since in our system model the discrete baseband equiva-lent signal representation is used, these time differences mustbe represented in discrete form. Hence, the number of sub-car-riers , referred to as IBI factor, for which the offset exceeds CP,is used to present the amount of IBI introduced by User C. Sincethe time period corresponding to one sub-carrier is , thefollowing relation for the delay and the sub-carriers can bederived:
(6)
where denotes integer part.
III. PERFORMANCE EVALUATION RESULTS AND DISCUSSION
The -FDMA multi-user communication system underconsideration has been implemented in software employing amaximum of 15 users. Its performance has been evaluated bymeans of Monte Carlo computer simulations for synchronousand asynchronous reception. On the one hand, for synchronousreception, the Total Degradation (TD) performance has beenused to evaluate the degradation caused by the two amplifiers,
and . The TD was introduced in [16] as perfor-mance criterion indicating how, as compared to the AWGNchannel, the BER performance degrades in the presence of apower amplifier as a function of its operating point (i.e., IBO orOBO). For this reason, considering an AWGN channel, a targetBER level must be selected as a point of reference. Only thefirst user belongs to the class of User A (UU) and the remaining14 users belong to the class of User B (OU). On the otherhand, for asynchronous reception, to investigate the influenceof multi-user interference on the system performance the BERcriterion3 was used. For the sake of clarity, but without any loss
3It is noted that here the TD is not a meaningful performance criterion sincethe signal-to-noise ratio (SNR) should be kept constant.
of generality, only three users, each belonging to one of thethree class of users, UU, OU and IBIU, have been selected.Performance evaluation results have been obtained with a
common air-interface for both frequency bands. In particular,for the TE a Duo-Binary Convolutional Turbo Code from theDVB-RCS standard was selected [17]. The frame size was set212 bit couples4 (424 total bits or ATM packets of 53 bytes)and the code rate . The modulator formed QPSKor 16-QAM symbols, while the Max-Log-MAP algorithm[18] with 8 iterations was used to perform decoding. Theuser block consists of sub-carriers, the length of theIDFT, sub-carriers and the length of CP issub-carriers.
A. Synchronous Reception
This subsection presents TD performance evaluation resultsfor each -FDMA scheme assuming ideal synchronizationamong users. Since , 15 (out of 16 possible) userswere selected so that the remaining 8 sub-carriers have beenused as guard tones. An expression for TD is given by [19]:
(7)
where is the required signal-to-noise ratio (SNR) toachieve a target BER in an AWGN channel, is the re-quired SNR for the same target BER, taking into account thedistortion caused by the HPA at a certain IBO. While in liter-ature (7) is commonly used for obtaining TD, in practice it ismore useful to determine, instead of the best operating IBO, thecorresponding best operating OBO, which allows direct use ofTD for link budget calculations. Expressing OBO as a functionof the nominal IBO, (7) becomes:
(8)
It is noted that since OBO is not only a function of IBO, but alsodepends on the modulation format and the particular -FDMAaccess scheme, it must be calculated for each case, individu-ally. For the schemes under consideration the values of OBO
4Bits are encoded by couples and not individually as in the classical approachand the resulting Turbo Code is labeled Duo-Binary Turbo [17].
DALAKAS et al.: COMPARATIVE STUDY BETWEEN SC-FDMA AND OFDMA SCHEMES FOR SATELLITE UPLINKS 375
TABLE IIBO AND OBO CORRESPONDENCE PER ACCESS SCHEME
AND MODULATION FORMAT
Fig. 6. TD versus OBO for coded QPSK with rate 1/2 obtained at : (a) linearized ; (b) .
Fig. 7. TD versus OBO for coded 16-QAM with rate 1/2 obtained at : (a) linearized ; (b) .
as a function of IBO have been obtained via simulation and arepresented in Table I.The TD performance versus OBO has been obtained by
means of computer simulations setting the target .The best operating point for an HPA is the one that presents thelowest TD, .The obtained results can be classified according to the em-
ployed modulation schemes and HPAs. Fig. 6(a) and (b) illus-trates the TD for the and amplifiers, respectively,when QPSK is used. For the amplifier [see Fig. 6(a)],the occurs at about 1 dB for both LFDMA and IFDMAand at about 2 dB for the OFDMA scheme. When is em-ployed [see Fig. 6(b)], the occurs a bit higher, i.e., for
both LFDMA and IFDMA have a and for theOFDMA .The TD performance for the and amplifiers
using 16-QAM are illustrated in Fig. 7(a) and (b), respectively.For the amplifier [see Fig. 7(a)], for bothLFDMA and IFDMA schemes, while for the OFDMA scheme
. Similar to the QPSK case, for the am-plifier [see Fig. 7(b)], occurs higher than the one for the
. Both LFDMA and IFDMA have aand for the OFDMA scheme .Clearly, these performance evaluation results have shown
that, independent of the choice of amplifier and/or modulationscheme, IFDMA exhibits consistently the lowest , while
376 IEEE TRANSACTIONS ON BROADCASTING, VOL. 58, NO. 3, SEPTEMBER 2012
Fig. 8. Schematic of the 3 users scenario assuming synchronous reception: (a)LFDMA; (b) IFDMA; (c) OFDMA.
the performance of the LFDMA is only slightly worst. Dueto their lower PAPR [5], which make them less sensitive tonon-linearities [6], both schemes outperform OFDMA.
B. Asynchronous Reception
This subsection presents BER performance evaluation resultsfor the three access schemes in an AWGN channel and the pres-ence of interference which is caused by the asynchronous recep-tion. For this case, a fundamental scenario consisting of threedistinct users, each belonging to a set of users, i.e., Users A,B and C, was adopted. As illustrated in Fig. 5, User C, i.e., aIBIU type user, will introduce different levels of IBI dependingupon the selected value of the IBI factor (see (6)). User A isconsidered as the UU and User B is an OU type of user. For il-lustration purposes and assuming ideal synchronous reception,the -FDMA signals are graphically shown in Fig. 8. However,for asynchronous reception, due to the complex DFT processsuch a graphical representation is not easily illustrated.Due to the significant performance degradation caused by
IBI, only QPSK modulation with (before satura-tion) were considered. The symbol energy to noise power spec-tral density ratio was dB per subcarrier, where isthe QPSK symbol energy. The various performance evaluationresults which have been obtained are summarized in Figs. 9–11where the BER performance on a per user and per access schemebasis are presented as a function of the IBI factor, . Fig. 9can serve as a reference since these BER performance resultshave been obtained for a linear, i.e., without any HPA, channel.Figs. 10 and 11 present equivalent performance results in thepresence of the and amplifiers, respectively.
Fig. 9. BER performances on a per user and per access scheme basis as a func-tion of the IBI, introduced by User C without an HPA. Solid lines were used todepict the performance of User A, dash-dotted lines for User B and dashed linesfor User C.
Fig. 10. Same as Fig. 9 but also using the amplifier.
Fig. 11. Same as Fig. 9 but also using the amplifier.
As it can be seen from the obtained performance results,in all three cases and independent of the access scheme, theBER of User C is rapidly deteriorating with increasing IBI.The performance of User B, whose sub-carriers are next infrequency to those of User C, is better, while User A exhibitsthe best performance, since its sub-carriers are located evenfurther away. An interesting observation is made by notingthat the performance of User A does not change significantly
DALAKAS et al.: COMPARATIVE STUDY BETWEEN SC-FDMA AND OFDMA SCHEMES FOR SATELLITE UPLINKS 377
TABLE IIADVANTAGES AND DISADVANTAGES FOR EACH ACCESS SCHEME IN A
NON-LINEAR CHANNEL AND IN THE PRESENCE OF IBI. AMEANS ADVANTAGE, A MEANS DISADVANTAGE,AND MEANS ALMOST EQUAL TO THE BEST
when varies. It is also noted that the OFDMA exhibits similarperformance with LFDMA in absence of non-linearity (seeFig. 9), while in the presence of HPA the LFDMA schemeoutperforms OFDMA.Clearly, an IBIU type user, such as User C in Fig. 5, will
affect its own performance as well as the performance of otherusers located in its neighborhood. For example, the smaller itsfrequency distance is from a UU type user, such as User A inFig. 5, the higher will be the degradation. Further experimentshave shown that the degradation of a UU will also increase byincreasing the number of IBIUs. It is thus recommended that thenumber of IBIUs should be kept to a minimum.Table II presents a qualitative comparison of the perfor-
mances of the three access schemes for specific values of theIBI factor. Clearly, the IFDMA access scheme, although itprevailed in performance for synchronous reception, is themost sensitive to IBI. For a non-linear channel both LFDMAand IFDMA have an advantage over OFDMA. However, whenIBI is present, combined with a non-linearity, the performancesof the three access schemes depend on amount of IBI, i.e., as. LFDMA and IFDMA outperform OFDMA for lower IBIlevels. For low , the performance of LFDMA and IFDMA isnearly identical, while LFDMA slightly outperforms IFDMAas IBI increases. Furthermore, the performance of LFDMAdoes not change significantly as increases, in contrast to theperformance of the IFDMA scheme that degrades.
IV. CONCLUSION
This paper presented a thorough comparison study for twoSC-FDMA schemes, LFDMA and IFDMA versus OFDMA,for a satellite uplink, the air-interface of which is based on theDVB-family of standards. Both synchronous and asynchronoussignal reception has been considered for two state-of-the-artHPAs operating in the K- and S-band. Performance evaluationresults have shown that for a synchronous system IFDMA out-performs OFDMA and is slightly better than LFDMA in TDperformance, although, for asynchronous reception it is themostsensitive to degradation. OFDMA, due to its large PAPR, hasbeen found as the most sensitive to non-linearity. On the con-trary, LFDMA had only slightly inferior performance as com-pared to IFDMA for synchronous reception while it outper-formed the other two access schemes in the asynchronous sce-nario examined, i.e., in the presence of IBI.
ACKNOWLEDGMENT
The authors would like to thank the anonymous reviewers fortheir valuable comments that helped improve the presentation ofthe paper.
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Vassilis Dalakas (M’01) received the B.Sc. degreein physics, the M.Sc. degree (with honors) withspecialization in digital signal processing, and thePh.D. degree with specialization in digital commu-nications, all from the University of Athens (UoA),Greece, in 1998, 2002, and 2010, respectively. Themain part of his Ph.D. thesis research was carried outat the Institute for Space Applications and RemoteSensing (ISARS) of the National Observatory ofAthens (NOA).Since 2001 he has been affiliated also with
the Harokopio University of Athens (HUA), Greece, as a Research Fellow(2001–2007 in the Department of Geography and since 2008 in the Departmentof Informatics and Telematics) and as a network and system administrator since2005. His research interests include wireless digital communications for satel-lite and terrestrial system applications, digital signal processing techniques, aswell as modeling and simulation standardization methods. In these areas, hehas co-authored several papers and a book chapter. He has been a technicalexpert in Information Technology for the Greek Government and a reviewer ofseveral scientific journals and conferences.Dr. Dalakas was a co-recipient of the 2006 Best Paper Award in Proceedings
of the 15th International Conference on Software Engineering and Data Engi-neering (SEDE). Through his affiliation with ISARS/NOA he has participatedin a number of R&D projects funded by the European Commission and the Eu-ropean Space Agency (ESA).
P. Takis Mathiopoulos (SM’94) received the Ph.D.degree in digital communications from theUniversityof Ottawa, Canada, in 1989.From 1982–1986, he was with Raytheon Canada
Ltd., working in the areas of air navigational andsatellite communications. In 1988, he joined theDepartment of Electrical and Computer Engineering(ECE), University of British Columbia (UBC),Canada, where he was a faculty member until 2003,holding the rank of Professor since 2000. He iscurrently Director of Research at the Institute for
Space Applications and Remote Sensing (ISARS), National Observatory ofAthens (NOA), where he established the Wireless Communications ResearchGroup. As ISARS’ Director (2000–2004), he has led the Institute to a signifi-cant expansion R&D growth, and international scientific recognition. For theseachievements, ISARS has been selected as a national Centre of Excellence forthe years 2005 to 2008. Since 2003 he also teaches part-time at the Departmentof Informatics and Telecommunications, University of Athens, where he wasrecently elected Professor of Digital Communications. In 2008 and for a periodof 5 years he has been appointed Guest Professor at the Southwest JiaotongUniversity, China. For the last 20 years he has been conducting research mainlyon the physical layer of digital communication systems for terrestrial and satel-lite applications, including digital communications over fading and interferenceenvironments. He co-authored a paper in GLOBECOM’89 establishing for thefirst time in the open technical literature the link between MLSE and multiple(or multi-symbol) differential detection for the AWGN and fading channels. Heis also interested in channel characterization and measurements, modulationand coding techniques, SIMO/MIMO, UWB, OFDM, software/cognitiveradios, and green communications. In these areas, he has co-authored morethan 80 journal papers, mainly published in various IEEE and IET journals, 4book chapters and more than 110 conference papers. He has been PI for morethan 40 research grants and has supervised the thesis of 11 Ph.D. and 23 Masterstudents.
Dr. Mathiopoulos has been or currently serves on the editorial board ofseveral scientific journals, including the IET Communications, and the IEEETRANSACTIONS ON COMMUNICATIONS (1993–2005). He has regularly acted asa consultant for various governmental and private organizations. Since 1993,he has served on a regular basis as a scientific advisor and a technical expertfor the European Commission (EC). In addition, since 2001 he has served asthe Greek representative to high level committees in the European Commission(EC) and the European Space Agency (ESA). He has been a member of theTPC of more than 50 international conferences, as well as TPC Vice Chairfor the 2006-S IEEE VTC and 2008-F IEEE VTC as well as Co-Chair ofFITCE2011. He has delivered numerous invited presentations, includingplenary lectures, and has taught many short courses all over the world. Hewas an ASI Fellow, a Killam Research Fellow and a co-recipient of two bestpaper awards (2nd International Symposium on Communication, Control, andSignal Processing in 2008 and 3rd International Conference on Advances inSatellite and Space Communications in 2011). Further details are available athttp://www.space.noa.gr/ mathio/.
Filippo Di Cecca received the Dr. Ing. degree inTLC engineering in 2007 from the University ofRome “Tor Vergata” with a thesis on radars. Hereceived in 2009 the M.Sc. degree in “advancedsatellite systems of telecommunications and naviga-tion” from the University of Rome “Tor Vergata”.Since 2007 he has been working with Space En-
gineering in TLC department. He has been involvedin several ESA projects and his main interests are insatellite TLC area (mainly physical and MAC layersanalyses and simulations).
Gennaro Gallinaro received the Doctoral degree inelectronic engineering (magna cum laude) from Uni-versity of Rome in 1979 with a thesis on TV signalsdigital compression techniques.From 1979 to 1980 he worked at Fondazione Bor-
doni (Government Research Center on AdvancedTelecommunications) on Teletex signal simula-tions. After serving in the Italian Navy he joinedTelespazio in 1981 where he was first involved insystem planning studies, and then in the analysisand simulation of RF transmission links, payload
hardware assessment, new modulation access techniques and analog/digitalsignal processing technologies. Since 1989 he has been with Space EngineeringS.p.A. where he was involved in several space communications related projectsand studies. He has got in-depth experience in the analysis, computer-aideddesign and simulation of transmission systems (modulation, coding, etc.)and digital signal processing hardware (on-board multi-carrier demodulators(MCDs), digital beam forming, etc.). He is co-author of several papers, onSignal Processing and satellite communication techniques.Dr. Gallinaro was a co-recipient of the 2003 and 2009 IEEE Vehicular
Technology Society Jack Neubauer Memorial Awards which recognize thebest systems paper published in the IEEE TRANSACTIONS ON VEHICULARTECHNOLOGY.
