Ofdm Techniques In Satellite Communications

Ofdm Techniques In transmit communicationsThe concept of Orthogonal oftenness Division Multiplexing was first proposed in 1960 and essential in the 70s .All phonepass flexion schemes practice session single pallbeargonr by adjusting frequency, frame or amplitude .As digital infection uses high Bandwidth ( info rates), the duration of min information becomes smaller. The organization becomes to a greater extent vulnerable to loss of information from relish noise,signal reflections and diametrical impairments. As the bandwidth used by a single carrier system, the vulnerability to hinderance from other continuous signal sources become greater. irreverent FREQUENCY DIVISION MULTIPLEXINGModulation-A mapping of the information on changes in the carrier phase, frequency or amplitude or combination.Multiplexing-Method of sharing a bandwidth with other commutative selective information pass ons.OFDM is a combination of both multiplexing and changeover. OFDM is a special case of FDM . In OFDM the signal is first split into independent carriers, modulated by data and then re-multiplexed to create a OFDM carrier. This crowd out be done by using different conversion/demodulation techniques.The FDM systems requires a guard band between modulated subcarriers to prevent the spectrum of one subcarrier from fussy with another and not using bandwidth efficiently as in fig(i).If the FDM system uses a set of subcarriers that are nonmaterial to separately other. The use of orthogonal subcarriers would allow the subcarriers spectra to overlap,thus increasing spectral efficiency. As long as perpendicularity is chief(prenominal)tained, it is possible to recover the individual subcarriers signals despite their overlapping spectrums..Fig(1) General type of an FDM scheme with guard bands.Fig(2) Orthogonal FDM, eliminating the guard bands.OFDM nooky be implement using the Discrete Fourier Transform (DFT). The sinusoids of the DFT form an orthogonal reason set, a nd a signal in the vector space of the DFT can be delineated as a linear combination of the orthogonal sinusoids. iodin go steady of the DFT is that the convert essentially correlates its input signal with each of the sinusoidal basis functions. The OFDM signal can be a baseband signal or a band pass signal. For wired systems, due to the limited bandwidth of the wires, baseband OFDM signals are genetical. For radiocommunication communications, such as wireless topical anaesthetic area ne dickensrk (WLAN) condition by IEEE 802.11a standard OFDM signal frequency bands are allocated in the RF frequency band. In this case, OFDM signals are generated in baseband first and up-converted to the RF band for transmission.Mathematically, the OFDM signal using the DFT/IDFT pair would be as shown 4DFTX(k)=n=0to(N-1) x(n) exp(-j2nk/N)IDFTx(k)=(1/N) n=0to(N-1) X(n) exp(j2nk/N)Figure (3) General block diagram of OFDMMULTIPATH CARRIERSAs OFDM uses Multipath carriers they cause two problemsI ntersymbol Interference-This occurs when the received OFDM is distorted by the previously transmitted OFDM symbol.Where as in single carrier the interfere is due to several other symbols instead of just the previous symbol. Intersymbol Interference can be remove by using guard interval.Intrasymbol Interference-This occurs due to the interference amongst a devoted OFDM symbols own subcarriers.Fig(4) Performance evaluation of motley modulation techniques with OFDMOFDM ADVANTAGES AND DISADVANTAGESOFDM offers many an(prenominal) advantages over single-carrier modulations 1The maximum signaling rate (Nyquist rate) for a apt(p) channel can be approached without the use of sharp cutoff filters.It elongates the symbol outcome so that the signal is more robust a benefitst inter symbol interference caused by channel dispersions and multipath interference.It divides the entire frequency band into narrow bands so that it is less sensitive to wide-band impulse noise and fast channel fades .The printing of a slow frequency-selective fade is a separate complex make headway on each sub-band signal and it can be removed by simply multiplying the signal by the conjugate of the complex gain-that is, equalization can be easily done by a one-tap equalizer.Different modulation formats and data rates can be used on different subcarriers depending on the noise level of individual sub-bands (the symbol periods are unploughed the same). In serial transmission, certain types of noise (such as time variable tone interference) may cause an entire system to fail the jibe OFDM system can avoid this problem by adaptively reducing the data rate of the affected sub-bands or dropping them.OFDM can be enforced digitally using an inverse discrete Fourier transform and discrete Fourier transform (IDFT/DFT) pair (via the efficient fast algorithm IFFT/FFT pair), which greatly reduces the system complexity.OFDM is ground on a parallel data transmission scheme that reduces the printing of multipath fading and makes the use of complex equalizers unnecessary.OFDM achieves high spectral efficiency by allowing the sub-carriers to overlap in the frequency domain. The sub-carriers are made orthogonal to each other therefore there is no Inter-Carrier Interference. If the number of sub-carriers in N, the natural bandwidth required is BW add=(N+1)/Ts. For large values of N, the total bandwidth required can be approximated as BWtotal=(N)/Ts. On the other hand, the bandwidth required for single carrier transmission of the same data is BWtotal=(2N)/Ts. Thus we achieve a spectral gain of nearly 100% in OFDM compared to the single carrier transmission case.OFDM besides have DISADVANTAGES over single carrier modulation systemsThe OFDM signal has a noise like amplitude with a very large changing range, therefore it requires RF source amplifiers with a high peak to modal(a) power ratio.It is more sensitive to carrier frequency offset and shoot a line than single carrier syst ems are due to leakage of the DFT.High predisposition to synchronization errors.Nonlinear effects generated by the power amplifier may tuck inter carrier interference and thus destroy the orthogonality.Larger sidelobes may result in sensitivity to frequency.APPLICATIONS OF OFDMWireless LAN ApplicationsHIPERLAN/2HIPERLAN/2 is a Wireless LAN application defined by the ETSI. HIPERLAN/2 handles data rates between 6-54 Mbit/s. HIPERLAN/2 provides a DLC layer on top of which an IP based broadband network can be implemented. The Physical layer of HIPERLAN/2 is based on the Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme. The Numerical Values of OFDM parameters in HIPERLAN are given belowParameterValueSampling rate fs=1/T20 MHzSymbol part duration TU64*T 3,2 sCyclic prefix duration TCP16*T, 0,8 s (mandatory) 8*T, 0,4 s (optional)Symbol interval TS80*T, 4,0 s (TU+TCP) 72*T, 3,6 s (TU+TCP)Number of data sub-carriers NSD48Number of pilot sub-carriers NSP4Sub-carrier spac ing f0.3125 MHz (1/TU) lay between the two outmost sub-carriers16.25 MHz (NST*f)IEEE 802.11The IEEE 802.11 committee has a standard similar to the HIPERLAN. Its OFDM parameters are as shown belowData reckon6,9,12,18,24,36,48,54 MbpsModulationBPSK, QPSK, 16-QAM, 64 QAMCoding Rate1/2, 2/3,3/4No of Sub-Carriers52No of pilots4OFDM Symbol Duration4 usGuard Interval800 nsSub-Carrier Spacing312.5 kHz3 dB bandwidth16.56 MHzChannel Spacing20 MHzSatellite Communication applicationsDigital Audio Broadcasting (DAB)Digital Audio Broadcasting is a new multimedia system technology, with a fountainhead-grounded sound quality and better spectrum efficiency. The DAB system samples audio at a sample rate of 48 kHz and a steadiness of 22 bits. Then the data is compressed to between 32 and 384 kbps. A rate convolution code is used with constraint length 7. The total data rate is about 2.2 Mbps. The frame time is 24ms. QPSK modulation is performed at the transmitter. The advantage of using OFDM for DAB is that the OFDM suffers very little delay ranch and has high spectral efficiency.Digital image Broadcasting (DVB-T)Digital Video Broadcasting is an ETSI standard for broadcasting Digital Television over satellites, cables and by dint of wandering (wireless) transmission. Terrestrial DVB operates in either of 2 modes called 2k and 8k modes with 1705 carriers and 6817 carriers respectively. It uses QPSK, 16-QAM or 64-QAM subcarrier modulations. It as well uses pilot subcarrier for recovering amplitude and phase for coherent demodulationThe major use of OFDM can be observed when multiple techniques can be transmitted and received at the same time. This can be done with the assistant of MIMO OFDM. The transmitted signal will be received by various VSAT networks based on the corresponding frequencies. Further, processing of this signal and error corrections, the overlord signal is retrieved. For example, consider that a DAB, DVB and other geological information is received by various antennas as shown in the figure.NEWEST RESEARCH AND FUTURE TRENDS OF OFDMOne of the methods developed in the recent years is that the convolutional coding (CC) and trellis coded modulation (TCM) are combined with OFDM for a Rician fading channel according to the specifications given by the Iridium system. Here, the bit error rate (BER) performance of OFDM with 8-PSK and 16-QAM modulation techniques along with two different coding schemes are considered for a king of beasts satellite channel. Trellis coded OFDM gives the best performance compared to convolutional coded OFDM. The peak to average power ratio (PAPR) can be reduced approximately by 1.5 dB with the help of the partial transmit sequence technique (PTS). Later, some more look for and simulation results proved that the Turbo Coded (TC) OFDM performed better than CC or TCM with OFDM 6.Satellite agile systems are developed to provide connectivity between remote terrestrial networks, direct network access, Internet work using fixed or mobile terminals, interactive multimedia applications, and high data-rate transmissions. Most of these research and growing scenarios have considered the Non-Geo Stationary Satellite (NGSO) network for providing satellite-based mobile multimedia services because of its low propagation delay and low path loss. As a result, new generations of broadband satellite communication systems are currently beingness developed to support multimedia and Internet-based applications. For example, the Spaceway system provides downlink transmission rates of up to 100 Mbps, and a total capacity of up to 4.4 Gbps. In company to significantly increase the capacity of 4G broadband satellite systems, current research aims at developing new advanced technologies 5.In the wireless system, OFDM is the main basis for several television and radio broadcast applications, including the European digital audio broadcasting (DAB) and high-definition TV (HDTV) terrestrial broadcasting as well as North American digital radio broadcasting. By the base of the 21st century, OFDM has been adopted as standard for new high-rate wireless local area network (WLAN), such as IEEE 802.11, HIPERLAN II, as well as the Japanese Multimedia Mobile Access Communications (MMAC). Currently, many researches are underway to devolve an OFDM-based system to deliver mobile broadband data service at data rates comparable to those of wired services, such as DSL and cable modems. Moreover,OFDM technology is a very gentle candidate when targeting high quality and high flexibility in mobile multimedia communications over satellite systems 6.When the research interest is in the integration process of 3G terrestrial systems with the satellite domain, the conventional frequency variation MAS (FDMA) system looses its flavor in competing with the code segmentation MAS (CDMA) and time division MAS (TDMA)-based systems for its very high bandwidth (BW) requirement. Moreover, in satellite systems, it is shown that CDMA system outperforms the FDMA system when regeneration is taken in to account. In this case, OFDM replaces FDMA with manifold advantages. Currently wideband CDMA (W-CDMA) and OFDM/TDMA techniques are successfully in use in terrestrial mobile multimedia systems. Therefore, these two Multiple Access schemes (MASs) are getting considerable maintenance 7 in mobile multimedia communications for Non-Geo Stationary Satellite interface.