A REVIEW ON ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING 1 Awadhesh Kumar, 2 Mr. Kuldeep Sharma

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1 A REVIEW ON ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING 1 Awadhesh Kumar, 2 Mr. Kuldeep Sharma 1 Research Scholar, Electronics & Communication Engineering Department, Monad University, U.P., INDIA 2 Assistant Professor & Head, Electronics & Communication Engineering Department, Monad University, U.P., INDIA Abstract- Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmission technique where a single set of data is transmitted over a number of sub-carrier. OFDM takes the advantage of multi-path propagation and reduces the fading effect. The idea of OFDM is to split the total transmission bandwidth into a number of orthogonal subcarriers which reduces the inter-symbol-interference, power consumption and increases the capacity and efficiency of the system. In this paper, we present a comprehensive survey on OFDM for wireless communications. We address basic OFDM and related modulations, as well as techniques to improve the performance of OFDM for wireless communications. The applications of OFDM in current systems and standards have also been discussed. 1. Introduction In OFDM system a set of subcarrier are used that are orthogonal to each other in order to achieve a high spectral efficiency. The subcarriers in OFDM system will allow subcarrier spectra to overlap with each other, hence increasing spectrum efficiency. With the increase of communications technology, the demand for higher data rate services such as multimedia, voice, and data over both wired and wireless links is also increased. New modulation schemes are required to transfer the large amount of data which existing techniques cannot support. These techniques must be able to provide high data rate, allowable Bit Error Rate (BER), and maximum delay. Orthogonal Frequency Division Multiplexing (OFDM) is one of them. OFDM has been used for Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) in Europe and for Asymmetric Digital Subscriber Line (ADSL) high data rate wired links. OFDM has also been standardized as the physical layer for the wireless networking standard HIPERLAN2 in Europe and as the IEEE a, g standard in the US, promising raw data rates of between 6 and 54Mbps. The concept of using parallel data transmission and FDM was published in the mid-1960s [1], [2]. and modulation scheme can be individually controlled for each carrier. However in broadcast systems these are fixed due to the one-way communication. The basic principle of OFDM is to split a high-rate data stream into a number of lower rate streams that are transmitted simultaneously over a number of subcarriers. Fig. 1. OFDM Transmitter The block diagram showing a simplified configuration for an OFDM transmitter and receiver is given in Fig.1 & Fig. 2. OFDM is a special case of multi-carrier modulation. Multicarrier modulation is the concept of splitting a signal into a number of signals, modulating each of these signals to several frequency channels, and combining the data received on the multiple channels at the receiver [3]. In OFDM, the multiple frequency channels, known as sub-carriers, are orthogonal to each other [4]. 2. OFDM Systems The OFDM signal generated by the system in Figure 1 & 2 is at baseband in order to generate a radio frequency (RF) signal at the desired transmit frequency filtering and mixing is required. OFDM allows for a high spectral efficiency as the carrier power Fig. 2. OFDM Receiver In digital communications, information is expressed in the form of bits. The term symbol refers to a collection, in various sizes, of bits. OFDM data are generated by taking symbols in the spectral space using M-PSK/ QAM and convert the spectra to time 38 P a g e

2 domain by taking the Inverse Discrete Fourier Transform (IDFT). Since Inverse Fast Fourier Transform (IFFT) is more cost effective to implement, it is usually used instead. The main features of a practical OFDM system are as follows: makes them orthogonal over each symbol period; this property is expressed as: Data pre-processing for correcting errors, interleaving and mapping of bits onto symbols. The symbols are modulated onto orthogonal sub-carriers using IFFT. Orthogonality is maintained during channel transmission by adding a cyclic prefix to the OFDM frame to be sent. Synchronization: cyclic prefix can be used to detect the start of each frame. Demodulation of the received signal using FFT. Channel equalization: the channel can be estimated either by using a training sequence or sending known so-called pilot symbols at predefined sub-carriers. Decoding and de-interleaving. where (.)* denotes the complex conjugate operator and, is the Kronecker delta. To avoid intersymbol interference in multipath fading channels, a guard interval of length, is inserted prior to the OFDM block. During this interval, a cyclic prefix is transmitted such that the signal in the interval equals the signal in the interval cyclic prefix is thus:. The OFDM signal with The number of carriers in an OFDM system is not only limited by the available spectral bandwidth, but also by the IFFT size, which is determined by the complexity of the system [5]. The more complex the OFDM system is the higher IFFT size it has; thus a higher number of carriers can be used, and higher data transmission rate achieved. The choice of M-PSK modulation varies the data rate and Bit Error Rate (BER). The higher order of PSK leads to larger symbol size, thus less number of symbols needed to be transmitted, and higher data rate is achieved. But this results in a higher BER since the range of degrees of phases will be divided into more sub-regions, and the smaller size of sub-regions is required, thereby received phases have higher chances to be decoded incorrectly. OFDM signals have high peak-to-average ratio, therefore it has a relatively high tolerance of peak power clipping due to transmission limitations. The low-pass signal above can be either real or complex-valued. Real-valued low-pass equivalent signals are typically transmitted at baseband wire line applications such as DSL use this approach. For wireless applications, the low-pass signal is typically complex-valued; in which case, the transmitted signal is upconverted to a carrier frequency. In general, the transmitted signal can be represented as: 3. Mathematical Representation Consider N sub-carriers are used, and each sub-carrier is modulated using M alternative symbols, the OFDM symbol alphabet consists of combined symbols. The low-pass equivalent OFDM signal is expressed as: where { } are the data symbols, N is the number of sub-carriers, and T is the OFDM symbol time. The sub-carrier spacing of 1/T 4. Channel Estimation In OFDM systems, channel can be estimated using training symbols known at both the transmitter and the receiver. The training symbols may be inserted at different sub-channels of different OFDM blocks, as shown in Fig. 3. These training symbols are more often called pilots. The channel corresponding to the pilot sub-channels is first estimated and then that corresponding to the data-bearing sub-channels is obtained by interpolation. In addition to interleaving the training symbols and the informative symbols by such frequency-division multiplexing, they may also be superimposed, which can be regarded as a 39 P a g e

3 special form of pilots [6]. This kind of training symbols are usually called superimposed pilots, which were first proposed to phase synchronization and originally called spread-spectrum pilots [7] and were later applied for channel estimation. On the other hand, all training symbols may be arranged at the first OFDM blocks. Fig. 3. OFDM channel estimation The training blocks in this case are sometimes called preamble. The channel corresponding to the training blocks are first estimated, and that corresponding to the subsequent data blocks can be tracked and further improved with the help of the demodulated data. This is called decision-directed channel estimation (DDCE) [8], [9]. 5. Impairment Mitigation In this section, we will address time- and frequency-varying impairment mitigation. Frequency-varying impairments are caused by the timing offset between the transmitter and the receiver or the delay spread due to a multipath of wireless channels. The impact of delay spread is a multiplicative channel distortion on the demodulated signal if the CP or guard interval is long enough, which can easily be mitigated once CSI is estimated. The impact of timing offset is much simpler than that of delay spread. If the timing offset τ is less than the CP, then it will cause a phase rotation of 2πk.Δf.τ to the symbol at the kth sub-channel [10]. If the timing offset exceeds the CP, then IBI will be generated, in addition to the phase rotation. The phase rotation due to the timing offset is different for different subchannels. This property can be exploited to estimate the timing offset. The mitigation process can be divided into following steps: sub-channel when we cancel the impact of the delay spread of wireless channels [11]. Frequency-Offset Estimation and Correction: From the perspective of its impact and signal processing, the CFO can be divided into integer and fractional parts. The integer part of the CFO is a multiple of the sub-channel space Δf, which will cause a symbol or sub-channel shift, that is, the transmitted symbol in one sub-channel is shifted to another at the receiver. The fractional part results in the loss of orthogonality among sub-channels and generates ICI. Once the CFO is estimated, its impact can completely be canceled in the time domain by multiplying the received signal x(t) by the frequency shift factor e j2πδft [12]. Mitigation of ICI Caused by the Doppler Spread: ICI may be caused by the CFO, phase noise, timing offset, and Doppler spread [13], [14], [15], [16]. However, ICI induced by the first three impairments can completely be compensated or corrected. Since the Doppler spread or shift is random, we can only mitigate its impact. The existing ICI mitigation techniques include frequency equalization, ICI selfcanceling, time-domain windowing, coding, extended kalman filter, unscented kalman filter etc. PAPR Reduction: As indicated before, the OFDM signal has a large PAPR. A traditional method dealing with the large PAPR is to back off the operating points of nonlinear power amplifiers; however, it severely reduces the efficiency of the power amplifiers. Therefore, by exploiting the special characteristics of the OFDM signal, various approaches have been proposed to cope with the issue. They include clipping and filtering [17], [18], selected mappings (SLM) [19], [20] and [21], partial transmit sequence (PTS) [22], etc. To reduce the PAPR of an OFDM signal, a clipper can directly be used. However, such nonlinear processing will cause in-band distortion and out-of-band radiation. If the out of band interference is filtered out, then the PAPR of the clipped signal will regrow [23]. Therefore, if clipping and filtering are repeated several times, then both the PAPR and out-ofband radiation will be reduced, as proposed in [24]. However, the clipping and filtering techniques are unable to remove the in-band distortion. The technique is improved in [25] by limiting the distortion of each sub-channel. 6. Modulation Techniques Timing-Offset Estimation and Correction: The timing offset can be estimated with pilot- and non-pilot aided techniques. After the timing offset is estimated, its integer part, which is a multiple of the sampling interval, is used to adjust the starting position of the FFT window, and its fractional part will generate a phase offset and can be compensated at each There are many other modulation or access techniques related to OFDM. MC modulation is a general category of modulation to which OFDM belongs. A single-carrier system with frequencydomain equalization (SC-FDE) and energy spreading transform (EST)-based modulation are two block transmission schemes that 40 P a g e

4 exploit the CP to mitigate the delay spread of wireless channels, which share the same spirit as OFDM. Furthermore, based on OFDM, many access techniques have been developed. MC- CDMA and OFDM access (OFDMA) are two of the examples. In this section, we will briefly describe MC modulation, SC-FDE, EST-based modulation, MC-CDMA, and OFDMA. 7. MIMO Techniques MIMO techniques or space time processing can be used in wireless communications for diversity gain and capacity improvement [26], [27]. Recent literature in [28], [29] have given a comprehensive introduction of MIMO techniques. Here, we focus on special issues when MIMO techniques are used with OFDM. Most of MIMO techniques are developed for flat fading channels. However, multipath will cause frequency selectivity of broadband wireless channels. Therefore, MIMO-OFDM, which has originally been proposed to exploit OFDM to mitigate ISI in MIMO systems, turns out to be a very promising choice for future high-data-rate transmission over broadband wireless channels. The earliest work in MIMO-OFDM can be found in [30] and [31]. Since that time, MIMO-OFDM has become a very popular area in wireless communications, particularly in the past several years [32]. In this section, we only very briefly provide an introduction of the topic. 8. Advantages, Disadvantages and Applications OFDM offers advantages like multipath delay spread tolerance, immunity to frequency selective fading channels, efficient modulation and demodulation, high transmission bit rates, cancellation of fading affected channel, access to all sub-carriers within a cell layer, easy equalization, high spectral efficiency, resilient to RF interference and lower multi-path distortion. Everything has good and bad aspects and OFDM is no exclusion. Various problems involved with OFDM are distortion problem, multi-path propagation must be avoided to preserve orthogonality, complex than single-carrier modulation, requires more linear power amplifier, presence of noise, sensitive to drift and offsets, capacity and power loss due to large interval and significant bandwidth and power loss due to guard interval. During the past decade, OFDM has been adopted in many wireless communication standards, including European digital audio broadcasting, terrestrial digital video broadcasting, and satellite terrestrial interactive multiservice infrastructure in China. In addition, OFDM has been considered or approved by many IEEE standard working groups, such as IEEE a/g/n, IEEE a, and IEEE d/e. The applications include wireless personal area networks, wireless local area networks, and wireless metropolitan networks. Currently, OFDMA is being investigated as one of the most promising radio transmission techniques for LTE of the 3rd Generation Partnership Project (3GPP), International Mobile Telecommunications Advanced Systems. 9. Conclusion The demand for high data rate wireless communication has been increasing drastically over the last decade. One way to transmit this high data rate information is to employ well known conventional single carrier systems. Since the transmission bandwidth is much larger than the coherence bandwidth of the channel, highly complex equalizers are needed at the receiver for accurately recovering the transmitted information. Multi-carrier techniques can solve this problem significantly. In this paper we have discussed about the basic idea behind the OFDM, the most emerging technology of this era. Here we take a review on its concept, its properties in terms of its advantages and disadvantages, its limitations and also its applications in different fields. 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