Noncoherent DHA FH OFDMA: possible solution for wireless coexistence in satellite communications Текст научной статьи по специальности «Компьютерные и информационные науки»

Таблица 2

Результат распределения фрагментов по узлам сети

^^^Фрагмент Узел 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 l 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

2 0 l 0 l 0 0 0 0 l l 0 0 0 l l 0 0 0 0 l

3 0 0 0 0 0 0 0 0 0 0 l 0 0 0 0 l 0 l l 0

4 0 0 0 0 0 0 l 0 0 0 0 l 0 0 0 0 0 0 0 0

5 0 0 l 0 l l 0 l 0 0 0 0 l 0 0 0 l 0 0 0

s 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Необходимо отметить, что предложенный комплекс в ближайшем будущем станет частью САПР корпоративных информационных систем различного назначения, разрабатываемых в ЮРГТУ (НПИ).

Список литературы:

1. Gartner. Gartner Top Ten Disruptive Technologies for 2008 to 2012 // Emerging Trends and Technologies Roadshow. - 2008.

2. Горобец В.В. Оптимизация размещения фрагментов РБД в узлах сети с произвольной топологией в рамках облачной структуры по критерию минимума стоимости трафика // Теория, методы проектирования, программнотехническая платформа корпоративных информационных систем: материалы X Междунар. науч.-практ. конф., г. Новочеркасск, 5 июня 2012 г. / Юж.-Рос. гос. техн. ун-т (НПИ). - Новочеркасск: ЮРГТУ (НПИ), 2012. - С. 28-35.

3. Коссман Д., Краска Т., Лоузинг С. Анализ альтернативных архитектур управления транзакциями в «облачной» среде [Электронный ресурс]. - Режим доступа: http://citforum.rU/database/articles/kossmann_sigmod_2010/2.shtml (дата обращения: 29.11.2012).

NONCOHERENT DHA FH OFDMA: POSSIBLE SOLUTION FOR WIRELESS COEXISTENCE IN SATELLITE COMMUNICATIONS © Zyablov V.V.*, Seitkulov Ye.N.*

Institute for Information Transmission Problems RAS (Kharkevich Institute),

Moscow

L.N. Gumilyov Eurasian National University, Kazahstan, Astana

In what follows perspectives of a system employing a DHA FH OFDMA with noncoherent threshold reception and a broadband DS CDMA system coexistence are evaluated.

* Заведующий лабораторией Информационных технологий передачи, анализа и защиты информации Института проблем передачи информации им. А.А. Харкевича РАН, доктор технических наук.

* Директор НИИ Информационной безопасности и криптологии, кандидат физико-математических наук.

Introduction

The problem of wireless coexistence has gained ever-growing interest in recent years. However in satellite communications this problem has hitherto been relatively rarely addressed (see e.g. [1-3]). Nevertheless as broadband satellite communications become more and more popular [4] the problem in question will be of significant importance in the nearest decades. Not only should satellite communications coexist with each other - the jamming-proofness of the communication link is of great importance too. A DHA FH OFDMA with noncoherent threshold seems a promising candidate to solve both problems.

Hereinafter we shall consider a DHA FH OFDMA system with noncoherent threshold reception [5]. This paper is organized as follows. In Section 2 a short overview of the classical DHA FH OFDMA model with noncoherent threshold reception is given. In Section 3 the concept of the maximum possible transmission rate (MPTR) is introduced. In Section 4 perspectives of a system employing a DHA FH OFDMA with noncoherent threshold reception and a broadband DS CDMA system coexistence are discussed. In concluding Section 5 the results of the paper are summarized.

Noncoherent DHA FH OFDMA: system description

Let us now consider the basics of the noncoherent DHA FH OFDMA in more detail. Consider a multiple access system where K active users transmit data to the base station through a channel split into Q frequency subchannels; the transmission is asynchronous and uncoordinated (i.e. neither of the users has information about the others). It is assumed that all the users transmit q-ary symbols. In the course of the transmission of each consecutive symbol the subchannel numbers generator assigned to the user under consideration chooses (in a random manner) q subchannels out of Q subchannels. Each symbol to be transmitted by the aforesaid user within the frame is mapped into the number of the subchannel, via which the signal is transmitted. Note that the vector of q subchannels is the instantaneous hopset, i.e. after each hop each user is allocated a new (randomly chosen) hopset.

In what follows we shall assume that the base station is equipped with the subchannel numbers generator synchronized with that of the active user. The latter means that within the scope of the reception of the respective symbol, the subchannel numbers generator of the base station produces the very same subchannel numbers vector that has been generated by the generator of the user under consideration. Note, that we simply replace a generator producing random numbers (which is an indispensable part of any conventional FH OFDMA system) with a generator producing random vectors.

In noncoherent DHA FH OFDMA within the scope of the reception of a certain symbol sent by a certain user the receiver measures power values for all the

ТеПеKOMMуHHKаЦHOHHLIе CHCTeMLI H KOMntKITepHLie CeTH

113

signals received through the subchannels chosen by the subchannel numbers generator of this user obtaining q statistics in that manner. Thus, the receiver is to decide, which subchannel has actually been used by the active user under consideration. To do so the receiver compares each statistic value with a certain threshold. If threshold crossing is detected in only one subchannel, the symbol corresponding to the subchannel where the threshold crossing was registered is accepted. Otherwise, an erasure decision is made. If a symbol other than the transmitted one is accepted, the situation is referred to as an error.

Maximum possible transmission rate concept

In what follows a short sketch of the concept of Maximum Possible Transmission Rate (MPTR) first introduced in [6] will be presented Let us now consider the aforementioned decision procedure in information theory terms. From the information theory point of view a DHA FH OFDMA is just a q-ary discrete channel with erasures. This channel will be described by the probability of errorpe and probability of erasure px. In order to find the channel capacity of this channel we shall use the following technique: the channel under consideration (let us designate it by C0) will be represented as a serial concatenation of a q-ary discrete symmetric channel (let us designate it by C1) and a q-ary erasure channel (let us designate it by C2). Note that since C1 and C2 are independent the probability of error pe that describes C1 is given by:

P e =

1 - Px

The channel capacity of channel C1 is given by

Ce = log2 q +

1 --

+—— log2

1 - Px

f Pe ^ 1 - Px

\

1 - Px

(

l0g2

Pe 1 - Px

1-

1 - Px

log2(q -1)

(1)

therefore the channel capacity of channel C0 is given by:

Cc =( l0g2 q •(! - Px )) + (! - Pe ) l0g2

1~Px

+ Pe l0g2

1-Px

-Pelog2(q-1) (2)

(See [7]). Note that bothpx and pe depend on the number of active users K, the overall number of subchannels available to the active users Q, the number of subchannels allocated to each user q, SNR and the value of the threshold value. Thus if all the parameters but the last one are fixed it is reasonable to choose thre-

1

V

shold value that maximizes Cc. The obtained value Rm is the maximum possible transmission rate (MPTR) of reliable data transmission (the term «reliable» has the same meaning here as in [8]) in the system under consideration and is given by:

Rm (mm, Q, q, SNR ) = max (Cc (K ,Q, q, SNR, k)) (3)

The value Rm is not to be mixed up with the channel capacity of the respective user’s uplink channel since both the transmission and the reception methods are fixed. Nevertheless, it can be used as a performance criterion. In what follows the MPTR concept will be used to evaluate perspectives of a system employing a DHA FH OFDMA with threshold reception and a broadband DS CDMA system coexistence.

Noncoherent DHA FH OFDMA and broadband DS CDMA: coexistence perspectives evaluation

In communication theory it is very typical to model broadband DS CDMA signals as additive white Gaussian noise (see e.g. [9]). Therefore if the problem of coexistence with a broadband DS CDMA system is considered, the problem in question might be treated as a problem of either partial-band noise (PBN) jamming or broadband noise (BBN) jamming influence on the performance of the system under consideration. This is exactly the way we are going to treat this problem hereinafter. Let us first consider the first case: the band available to the system under consideration overlaps with that of the broadband DS CDMA system only partially i.e. in terms of noise jamming only a certain fraction of the band is jammed (partial noise jamming). Let us further on designate this fraction with f In what follows we shall assume the worst case scenario i.e. we shall assume that the power available to the broadband satellite system is M times greater than that available to the system under consideration. In Fig. 1 simulation results are shown for the case, q = 4, Q = 4096, M = 500 and different values off

Fig. 1 shows that even if the power of the broadband signal is much greater than that of the signal transmitted by the authorized user in a DHA FH OFDMA system with noncoherent reception MPTR degrades moderately (approximately 1 bit per OFDM frame per 0,2 increase of the fractionf). Let us now consider the case when the band available to the system under consideration lies within the frequency band occupied by the broadband signal i.e. the signal transmitted by the user under consideration is totally jammed by the broadband signal (broad band nose (BBN) jamming, f = 1). Fig. 2 shows simulation results for that scenario (indicated as BBN) (we still assume M = 500 and for the sake of convenience we also present curves for the case f = 0,8 and for case were no interference is introduced f = 0).

Fig. 1. Simulation results for the case when the band in use only partially overlaps with that occupied by the broadband satellite signal

Fig. 2. Simulation results for the case when the band in use is totally jammed by the broadband signal (comparison with the previously considered scenario)

As can be seen from Fig. 2, the in this case MPTR degrades only slightly as compared to the case f= 0,8. Nevertheless e.g. for the case when 300 active user simultaneously transmit in the system under consideration and the band available to the users is completely jammed MPTR decreases only by 0.6 bit per OFDM frame (which is only 30 % of the maximum (theoretically possible) value). Thus the DHA FH OFDMA can coexist with broadband satellite DS CDMA experince-ing moderate degradation even if transmitted power of the interfering broadan / band signal is much greater.

References:

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5. Osipov D. On the probabilistic description of an asynchronous DHA FH OFDMA system with threshold noncoherent reception Multiple Access Communications // Lecture Notes in Computer Science. - 2010. - Volume 6235/2010. -Р 180-187.

6. Groshev F., Osipov D. Increasing transmission rate in a DHA FH OFDMA system (in Russian) // Proceedings of the ITAS 2010, Gelendzhik, Russia. September 20-24. - Р. 69-73.

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9. Gallager R., Residual noise after interference cancellation on fading multipath channels // Communications, Computation, Control, and Signal Processing. -1997. - Р 67-77.

ОЦЕНКА КАЧЕСТВА СВЯЗИ МНОГОЧАСТОТНОЙ АТМОСФЕРНОЙ ОПТИЧЕСКОЙ СИСТЕМЫ ПЕРЕДАЧИ © Краснов Р.П.*

Воронежский государственный технический университет, г. Воронеж

Рассмотрено влияние атмосферного канала на качество приема в многочастотной оптической линии связи. Приведена оценка влияния метеоусловий на вероятность замираний в канале, показано, что применение углекислотных излучателей при организации частотно-разнесенного приема приводит к повышению качества связи.

* Доцент кафедры Радиотехники, кандидат технических наук, доцент.