Ofdm Techniques In Satellite Communications

The concept of Orthogonal Frequency Division Multiplexing was first proposed in 1960 and developed in the 70s .All bandpass modulation schemes use single carrier by adjusting frequency, phase or amplitude .As Digital transmission uses high Bandwidth (data rates), the duration of bit information becomes smaller. The system becomes more vulnerable to loss of information from impulse noise,signal reflections and other impairments. As the bandwidth used by a single carrier system, the vulnerability to interference from other continuous signal sources become greater.

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

Modulation:-A mapping of the information on changes in the carrier phase, frequency or amplitude or combination.

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Multiplexing:-Method of sharing a bandwidth with other independent data channels.

OFDM is a combination of both multiplexing and modulation. 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 can be done by using different modulation/demodulation techniques.

The FDM systems requires a guard band between modulated subcarriers to prevent the spectrum of one subcarrier from interfering with another and not using bandwidth efficiently as in fig(i).

If the FDM system uses a set of subcarriers that are orthogonal to each other. The use of orthogonal subcarriers would allow the subcarriers spectra to overlap,thus increasing spectral efficiency. As long as orthogonality is maintained, it is possible to recover the individual subcarriers signals despite their overlapping spectrums.

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Fig(1): General representation of an FDM scheme with guard bands.

Fig(2): Orthogonal FDM, eliminating the guard bands.

OFDM can be implemented using the Discrete Fourier Transform (DFT). The sinusoids of the DFT form an orthogonal basis set, and a signal in the vector space of the DFT can be represented as a linear combination of the orthogonal sinusoids. One view of the DFT is that the transform 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 transmitted. For wireless communications, such as wireless local area network (WLAN) specified 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.

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Mathematically, the OFDM signal using the DFT/IDFT pair would be as shown [4]:

DFT:

X(k)=Σn=0to(N-1) x(n) exp(-j2πnk/N)

IDFT:

x(k)=(1/N) Σn=0to(N-1) X(n) exp(j2πnk/N)

Figure (3): General block diagram of OFDM

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MULTIPATH CARRIERS

As OFDM uses Multipath carriers they cause two problems

Intersymbol 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 removed by using guard interval.

Intrasymbol Interference:-This occurs due to the interference amongst a given OFDM symbols own subcarriers.

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Fig(4): Performance evaluation of various modulation techniques with OFDM

OFDM ADVANTAGES AND DISADVANTAGES

OFDM offers many advantages over single-carrier modulations [1]:

The maximum signaling rate (Nyquist rate) for a given channel can be approached without the use of sharp cutoff filters.

It elongates the symbol period so that the signal is more robust against inter symbol interference caused by channel dispersions and multipath interference.

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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 effect of a slow frequency-selective fade is a separate complex gain 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 kept the same). In serial transmission, certain types of noise (such as time varying tone interference) may cause an entire system to fail; the parallel OFDM system can avoid this problem by adaptively reducing the data rate of the affected sub-bands or dropping them.

OFDM can be implemented 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.

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OFDM is based on a parallel data transmission scheme that reduces the effect 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 total bandwidth required is BWtotal=(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 also have DISADVANTAGES over single carrier modulation systems

The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF power amplifiers with a high peak to average power ratio.

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It is more sensitive to carrier frequency offset and drift than single carrier systems are due to leakage of the DFT.

High sensitivity to synchronization errors.

Nonlinear effects generated by the power amplifier may introduce inter carrier interference and thus destroy the orthogonality.

Larger sidelobes may result in sensitivity to frequency.

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APPLICATIONS OF OFDM

Wireless LAN Applications

HIPERLAN/2

HIPERLAN/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 below:

Parameter

Value

Sampling rate fs=1/T

20 MHz

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Symbol part duration TU

64*T 3,2 µs

Cyclic prefix duration TCP

16*T, 0,8 µs (mandatory); 8*T, 0,4 µs (optional)

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Symbol interval TS

80*T, 4,0 µs (TU+TCP) ; 72*T, 3,6 µs (TU+TCP)

Number of data sub-carriers NSD

48

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Number of pilot sub-carriers NSP

4

Sub-carrier spacing f

0.3125 MHz (1/TU)

Spacing between the two outmost sub-carriers

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16.25 MHz (NST*f)

IEEE 802.11

The IEEE 802.11 committee has a standard similar to the HIPERLAN. Its OFDM parameters are as shown below:

Data Rate

6,9,12,18,24,36,48,54 Mbps

Modulation

BPSK, QPSK, 16-QAM, 64 QAM

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Coding Rate

1/2, 2/3,3/4

No of Sub-Carriers

52

No of pilots

4

OFDM Symbol Duration

4 us

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Guard Interval

800 ns

Sub-Carrier Spacing

312.5 kHz

3 dB bandwidth

16.56 MHz

Channel Spacing

20 MHz

Satellite Communication applications

Digital Audio Broadcasting (DAB)

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Digital Audio Broadcasting is a new multimedia technology, with a good sound quality and better spectrum efficiency. The DAB system samples audio at a sample rate of 48 kHz and a resolution of 22 bits. Then the data is compressed to between 32 and 384 kbps. A rate ¼ 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 spread and has high spectral efficiency.

Digital Video Broadcasting (DVB-T)

Digital Video Broadcasting is an ETSI standard for broadcasting Digital Television over satellites, cables and through terrestrial (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 also uses pilot subcarrier for recovering amplitude and phase for coherent demodulation

The 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 help 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 original 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 OFDM

One 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 LEO 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 research and simulation results proved that the Turbo Coded (TC) OFDM performed better than CC or TCM with OFDM [6].

Satellite mobile systems are developed to provide connectivity between remote terrestrial networks, direct network access, Internet services using fixed or mobile terminals, interactive multimedia applications, and high data-rate transmissions. Most of these research and development 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 being 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 order 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 beginning 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,

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OFDM technology is a very attractive 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 division MAS (FDMA) system looses its flavor in competing with the code division 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 diversity 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 attention [7] in mobile multimedia communications for Non-Geo Stationary Satellite interface.

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