Improving power efficiency of coded OFDM with feedback

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission scheme. OFDM transmits data by using a large number of narrow bandwidth carriers. In an OFDM transmission system, each subcarrier is attenuated individually under the frequency-selective and fast fading channel if the same fixed transmission scheme is used for all OFDM subcarriers thus it results in highest attenuation and hence poor performance. The purpose of this paper is to introduce the adaptive modulation to get an understanding of the differences between fixed and adaptive modulations schemes. In this work adaptive modulation is implemented by dividing whole subcarriers into blocks of adjacent subcarriers. Based on calculated average instantaneous signal to noise (SNR) same modulation scheme is applied to all subcarriers of same block. Here average bit error rate (BER) performance of OFDM system under fixed modulation and adaptive modulation is observed. Average BER performance of these modulation techniques is observed with various inverse fast Fourier transform (IFFT) size and using simpler adaptive quadrature amplitude modulation (QAM) schemes. The simulation results show that BER performance of OFDM system using adaptive modulation is better than fixed modulation.

Orthogonal Frequency Division Multiplexing (OFDM) is a very attractive technique for high data rate transmission with good bandwidth efficiency. OFDM system utilize the cyclic prefix (CP) to mitigate the effect of inter symbol interference (ISI) produced by the multipath effect of wireless channel. The CP insertion only mitigates the effect of inter block interference of OFDM symbols, whereas the intra block interference remain present in the OFDM symbol. The intra block OFDM symbol interference degrades the error performance of the OFDM system. In order to improve the error performance of the OFDM system forward error correction scheme can be utilized. Many FEC schemes for single carrier and multicarrier systems have been proposed in the literature. Convolution and LDPC codes are two popular coding schemes for FEC. In this paper we have considered Convolution and LDPC codes for improving the error performance of OFDM system. The BER performance of convolution ally coded a LDPC coded OFDM is evaluated for various code rates in AWGN and fading channel. We have also compared the performance of coded OFDM system with an OFDM system without FEC.

Orthogonal frequency division multiplexing (OFDM) is a type of multicarrier modulation (MCM) technique in which larger bandwidth is divided into parallel narrow bands each of which is modulated by different subcarriers. All the subcarriers are orthogonal to each other and hence it reduces the interference among various subcarriers. OFDM technique is an efficient modulation technique used in certain wired and wireless application.In a wireless communication channel, the transmitted signal can travel from transmitter to receiver over multiple reflective paths. This results to multipath fading which causes fluctuations in amplitude, phase and angle of arrival of the received signal. For example, the signal which is transmitted from BTS (base transceiver station) may suffer multiple reflections from the buildings nearby, before reaching the mobile station. Such multipath fading channels are classified into slow fading/fast fading and frequency-selective/flat fading channels. This paper discusses the performance of OFDM system using various fading channels and channel coding. The parameter which is known as Bit error rate (BER) is calculated under different fading channels (AWGN, Rayleigh and Rician) for different digital modulation (BPSK, QPSK and QAM) and Channel coding (linear/Cyclic coding). Matlab Simulink tool is used to calculate the BER parameter.

Orthogonal frequency division multiplexing (OFDM) is a block transmission technique with bandwidth efficient signalling scheme for wideband digital communications. In OFDM the spectrum of the individual carriers mutually overlaps. Nevertheless, the OFDM carriers exhibit orthogonality on a symbol interval if they are spaced in frequency exactly at the reciprocal of the symbol interval, which can be accomplished by utilizing the discrete time Fourier transform (DFT). In the baseband, complex-valued data symbols modulate a large number of tightly grouped carrier waveforms. The transmitted OFDM signal multiplexes several low-rate data streams each data stream is associated with a given subcarrier. The main advantage of this concept in a radio environment is that each of the data streams experiences an almost flat fading channel. In slowly fading channels, the inter-symbol interference (ISI) and inter-carrier interference (ICI) within an OFDM symbol can be avoided with a small loss of transmission energy using the concept of a cyclic prefix. With the development of modern signal processing technology, OFDM has become practical to implement and has been proposed as an efficient modulation scheme for applications ranging from modems, digital audio broadcast, to next generation high speed wireless data communications. The high speed wireless LAN standard IEEE 802.11a is based on OFDM. Also OFDM is been used for WIMAX and a candidate for 4G technology. This paper is an attempt to understand multiple effects of varying length of cyclic prefix for Additive White Gaussian Noise (AWGN) channel and with Rayleigh fading channel.

This paper compares the forward link radio link capacity between variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) with one-cell frequency reuse and orthogonal frequency division multiplexing (OFDM) with conventional three-cell frequency reuse or with one-cell frequency reuse utilizing symbol repetition, considering the effect of sectorization and the adaptive modulation and channel coding (AMC) scheme in a multi-cell environment in a broadband multipath fading channel. Simulation results demonstrate that, although the achievable radio link capacity in VSF-OFCDM is approximately 1.5 fold greater compared to that in OFDM with three-cell frequency reuse, almost identical radio link capacity under full channel-load conditions is realized in VSF-OFCDM and OFDM with one-cell frequency reuse applying elaborate symbol repetition transmission to suppress other-cell interference. Meanwhile, to achieve a lower data rate channel than the peak rate, higher-quality transmission is possible in VSF-OFCDM employing two-dimensional spreading compared to OFDM, by utilizing the frequency diversity effect in frequency domain spreading. Consequently, we conclude that VSF-OFCDM is superior to OFDM in broadband wireless access, since the former provides higher-quality transmission and more flexible accommodation of the lower data rate channels, along with the flexibility in designing essential channels such as the control and pilot channels.

Orthogonal frequency division multiplexing (OFDM) has been widely regarded as an effective modulation technique for mitigating the effects of ISI in a frequency selective fading channel and for providing reliable high data rate transmission over wireless links. Adaptive antenna arrays at the base and mobile stations can achieve further increases in system's capacity and bandwidth efficiency, as well as in QoS improvement in conventional OFDM systems. The conventional adaptive antenna arrays based OFDM systems always use the sub-carriers characterized by the first largest eigenvalues to transmit the OFDM block symbols. In this paper and in contrast to previous work, we propose dynamic sub-channel allocation with adaptive beamforming for broadband OFDM wireless transmission systems. The proposed system adaptively selects the eigenvectors associated with the relatively large sub-channel eigenvalues to generate the beamforming weights at the mobile and base stations and then dynamically assigns the corresponding best sub-channels to transmit the OFDM block symbols. It is shown by simulation that, without adding much complexity, the proposed system can achieve a better performance than an adaptive antenna arrays based OFDM system without dynamic sub-channel allocation over multipath fading channels. Simulation results also reveal that the proposed system is not too sensitive to channel estimation errors.

We propose a sub-band spreading technique for adaptive modulation (AM) in orthogonal frequency division multiplexing (OFDM) systems in order to reduce signaling overheads and to average frequency selective fading channels causing different signal-to-noise ratio (SNR) values for subcarriers in each sub-band. The conventional sub-band based AM schemes can also reduce signaling overheads and complexity for allocating a resource per sub-band at a time. However, they may suffer from the channel variation in a sub-band when the sub-band size is larger than the channel coherence bandwidth (BW). The sub-band spreading at the transmitter enables the received symbols in each sub-band to have an identical reliability even in a frequency selective fading channel. We rigorously analyze the averaged SNR value at the receiver of the sub-band spreading system and the analyzed average SNR in a sub-band is used for an adaptation criterion. The proposed AM scheme outperforms the conventional sub-band based OFDM scheme without spreading, and it can yield better throughput performance than the conventional subcarrier based AM schemes when we consider the signaling overheads.

The asymptotic error rate performance of orthogonal frequency division multiplexing (OFDM) signals in frequency-selective Nakagami-m fading channels is considered. A novel approach is provided to study the asymptotic error rate performance of a large class of modulations in general fading channels. This approach is applied to an OFDM signal on two-tap multipath Nakagami-m channels. Our analysis and numerical results show that the asymptotic error performance of an OFDM signal is similar to that of a single carrier signal transmitted over a Rayleigh fading channel. Our analysis further shows that a channel with two constant taps has similar asymptotic error rate performance to that of a one-sided Gaussian fading channel.

It is well known that Orthogonal Frequency Division Multiplexing (OFDM) systems suffer from intercarrier interference (ICI) in mobile environment due to loss of orthogonality among subcarriers caused by Doppler shifts. There exist many ICI mitigation techniques in the literature to improve the performance of OFDM systems. However, most of the existing ICI mitigation techniques assume the OFDM transmission bandwidth is narrow enough that the frequency offsets on all subcarriers are identical. In a wideband OFDM transmission or a non-contiguous OFDM spanning over large bandwidth, the Doppler shifts on different subcarriers are different, especially in high speed aerial vehicle communication systems. In this paper, we analyze the wideband OFDM system in high mobility environment where the frequency offsets vary from subcarrier to subcarrier. We then propose a novel ICI cancellation scheme to eliminate the ICI effect and offer the wideband OFDM system significantly improved BER performance. Simulation results in AWGN channel and multipath fading channel confirm the effectiveness of the proposed scheme in the presence of frequency offset and time variations in the channel, offering the best BER performance available which matches the BER performance of wideband OFDM system without ICI. To our knowledge, this paper is the first to address the ICI problem of varying frequency offsets across subcarriers in wideband OFDM system.

This paper considers OFDM (orthogonal frequency division multiplexing) based broadband wireless communications in high mobility with rapid time-varying Doppler shifts, where the received signal is significantly degraded by Doppler distortion, such as in a scenario of low-earth-orbit satellite communications. Since existing Doppler rate estimation methods are mainly designed for single carrier systems, two new CP (Cyclic Prefix) based Doppler rate estimators are proposed for OFDM based systems with discrete pilots, i.e. SOD (second order difference) estimator and ML (maximum likelihood) estimator. Furthermore in a time-dispersive channel a modification is conducted to improve both estimators accuracy in a fading channel. In the simulation part proposed estimators are compared with existing single-carrier Doppler rate estimators to verify the effectiveness and efficiency of the proposed estimators in an OFDM system. Specifically simulation results verify the low complexity of SOD estimator, the high accuracy of ML estimator, the insensitivity to initial Doppler shifts of both estimators and the accuracy improvement of modified estimators in a fading channel.

Different attempts have been made to compare the performance of coded orthogonal frequency division multiplexing (COFDM) and a corresponding single carrier system employing a linear equalizer often by comparing the average bit error rates. The random coding technique is used to analyze the performance of these systems for signaling over piecewise constant channels. The probabilities that the cutoff rates are less than a desired rate are calculated and compared. It is shown that for low SNRs and low code rates, the cutoff rates of both systems are approximately the same. With an increasing SNR the cutoff rate for the linearly equalized system, however, converges faster to its maximum value than the cutoff rate for the COFDM system. Simulation results for the outage probability are presented for the two-ray Rayleigh fading channel with QPSK as the fundamental modulation technique. For code rates close to one or for uncoded transmission, the linearly equalized single carrier system substantially outperforms the COFDM system. For low to medium code rates COFDM and a linearly equalized single carrier system perform approximately identically.

Orthogonal frequency division multiplexing (OFDM) system has been widely used for the high speed digital communication area and it is very promising for the next generation communication standard because of high data rate capability. However, OFDM system has inherent serious problem of high peak-to-average power ratio (PAPR) that makes the OFDM signal be distorted in the nonlinear HPA (high power amplifier). In this paper, CI (carrier interferometry)-OFDM system is considered to overcome this problem and is newly analyzed in the selective fading and narrow-band interference channel. CI-OFDM system distributes each parallel data to all sub-carriers in differently phase-rotated form. So, PAPR becomes lowered and it shows the frequency diversity effects. BER (bit error rate) performance is analyzed in the selective fading and narrow-band interference environment. From the simulation results, it is confirmed that CI-OFDM system calms down the high PAPR problem and CI-OFDM system is more robust than the ordinary OFDM system in the selective fading and narrow-band interference channel.

Jamming behavior in fading channels is not well understood when compared to additive white Gaussian noise channels. Specifically, it is not clear how or if a jammer should perform power control in the presence of a time or frequency-selective fading channel between itself and the victim receiver. Such a problem is relevant in the context of a) orthogonal frequency division multiplexing (OFDM)-based signaling systems, where the jammer must optimize its power allocation across a frequency-selective channel and b) single-carrier systems where the jammer must optimize its power allocation across a time varying fading channel in order to optimally attack the victim receiver. In this paper, we address this problem by studying jammer power allocation across a fading channel in order to maximize the error probability of a victim receiver that uses orthogonal frequency division multiplexing (OFDM)-based signaling. We propose an iterative algorithm to determine a jammer power allocation that performs up to 4 dB better, in terms of the victim signal power required to attain the same error rates, than other commonly used power allocation strategies. We also show that as the jammer's knowledge about the victim decreases, the gains obtained by using the proposed jamming strategy reduce but still performs better than the commonly used jamming strategies.

Recent wireless communications systems have adopted orthogonal frequency-division multiplexing (OFDM) due to its robustness against fading channels with simple receiver structure. However, it results in poor power amplifier (PA) efficiency or otherwise suffers from severe nonlinear distortion that causes adjacent channel interference (ACI) because of its high peak-to-average power ratio (PAPR) property of transmit signals. To mitigate this issue, DFT-precoded OFDM has been adopted in the LTE-A standard, which virtually forms single-carrier signals and thus generates lower PAPR than the original OFDM. Nevertheless, as the information rate increases for better spectral efficiency, the performance of DFT-precoded OFDM with simple linear detection becomes inferior to that of the original OFDM over frequency-selective fading channels when both the systems are protected by high-rate channel coding. In this work, we propose an adaptive use of DFT-precoding for OFDM where the DFT operation is applied only when the generated OFDM symbol has large dynamic range and thus causes severe nonlinear in-band distortion after clipping and filtering. It is shown that a good trade-off in terms of bit error rate (BER) performance can be achieved while maintaining the specified level of the PA efficiency and the amount of out-of-band radiation.

Orthogonal frequency division multiplexing (OFDM) is the contemporary technology adopted for digital audio/video broadcasting as well as wireless local-area and metropolitan-area networks. Since the wireless multimedia services often have different quality-of-service requirements and their performance is sensitive to the channel conditions, the conventional fixed OFDM modulation scheme might not be a satisfactory solution nowadays. In this paper, we introduce a novel pilot-free adaptive modulation scheme, which is bandwidth-efficient and allows variable data rates, for the future robust OFDM systems. We design a number of modulation modes in a combination of different constellation sizes and different polynomial cancellation coding methods (PCC) to combat the crucial intercarrier interference problem. Instead of estimating the channel quality based on the overhead pilot symbols, we propose to directly estimate the signal-to-noise ratio (SNR) without using any pilot. Besides, our scheme offers more modulation modes than some other existing adaptive modulation methods which are simply based on different constellation sizes. According to the Monte Carlo simulations, the empirical results show that our adaptive modulation scheme, in most channel conditions (SNR/spl ges/15 dB), not only can satisfy the predetermined bit error rate (BER) requirement (BER/spl les/10/sup -4/) but also can dynamically enhance the throughputs in the rather clean environments with high SNR values.