Analysis of physical-layer network coding over asymmetric two-way relay channel with carrier frequency offset

In this chapter, two-way relaying networks using Physical-Layer Net- work Coding (PLNC) are considered with practical constraints. In a Multiple Access (MA) phase, two users transmit messages simultaneously on the same physical resources. The relay constructs a relay message based on the superimposed receive signal of both users. This message is broadcasted to the users which are able to extract the desired data from the other user. However, as the relay is not able to resolve shifts in time and frequency for the individual channels, the transmission scheme needs to be adjusted, if perfect synchronization cannot be ensured. This chapter deals with the proper selection of waveforms for multi-carrier systems in the Two-Way-Relay Channel (TWRC) using PLNC. We address Generalized Frequency Division Multiplexing (GFDM) compared to Orthogonal Frequency Division Multi-plexing (OFDM) under practical constraints like Carrier Frequency Offsets (CFOs) and Timing Offsets (TOs) within double dispersive channels. For OFDM rectangular transmit filters are used, which have a broad spectrum shape and are sensitive to shifts, especially in frequency domain. GFDM applies Gaussian waveforms that are inherently less susceptible to synchronization offsets than theirs rectangular counterparts, but additional interference is introduced even in a perfectly synchronized and flat fading channel. Thus, interferences caused by the channel or by the non-orthogonal Gaussian impulse shape are treated by an equalizer at the relay. The simulation results show that the Gaussian waveform outperforms the conventional OFDM system if the nodes are not perfectly synchronized.

Traditional communication system typically separates the configuration of the physical layer from the network traffic load and topology. In this paper, we study how to apply physical-layer network coding considering the locations and traffic loads of multiple nodes in multiple-input multiple-output (MIMO) networks. We propose the heterogeneous-modulation physical-layer network coding (HePNC) design for MIMO networks with asymmetric two- way relay channel (TWRC), where all nodes are equipped with multiple antennas. Comparing to the single-antenna case, we study how to ensure the goodput with a fixed per-bit-energy can be scaled up w.r.t. the number of antennas, and also achieve performance gains in terms of end-to-end bit error rate (BER). The MIMO HePNC transmission includes the multiple access (MA) and broadcast (BC) stages. As the global channel state information (CSI) may be too costly to obtain, we propose two practical MIMO HePNC protocols based on maximum likelihood (ML) multi-user detector (MUD) that do not rely on global CSI. The first protocol is a heuristic one evolved from the single-input single-output (SISO) HePNC, and the second protocol upgrades the design and performance of both of the MA and BC stages. Analytical and extensive simulations demonstrated that, with two antennas each, the proposed MIMO HePNC protocols can not only double the goodput, but also achieve a substantial reduction on error rate, which indicates that combining HePNC and MIMO is a very promising cross-layer solution. We further discuss the impacts of the bottleneck link and provide guidelines on the relay location selection.

Wireless communication algorithms are implemented using a wide spectrum of building blocks such as: source coding; channel coding; modulation; multiplexing in time, frequency and code domains; channel estimation; time and frequency domain synchronization and equalization; pre-distortion; transmit and receive diversity; combat and take advantage of fading and multi-path channels; intermediate frequency (IF) processing in software defined radio, etc.

OFDM (orthogonal frequency division multiplexing) systems are very sensitive to carrier frequency offset. Although frequency offset is generally compensated for by an automatic frequency control (AFC), residual frequency offset remains after the AFC. Moreover, OFDM signals are also affected by phase noise. Thus, a kind of phase tracking is needed to compensate for these effects. In an OFDM system such as IEEE 802.11a, a phase tracking scheme that uses pilot subcarriers to detect the phase rotation caused by residual frequency offset and phase noise is employed. However, the spectrum of the AFC output signal is shifted by residual frequency offset. That is, the detected phase is the composite phase of the pilot subcarrier and the adjacent subcarrier because of inter-carrier interference (ICI). The compensation based on the composite phase sometimes overcorrects or undercorrects the phase rotation. In this paper, we propose a phase tracking scheme using iterative estimation of residual frequency offset in OFDM systems. The proposed phase tracking scheme estimates residual frequency offset in the frequency domain, and compensates for it in the time domain. The compensation in the time domain mitigates the spectrum shift of the AFC output. Therefore, the proposed scheme can reduce ICI, and track phases more accurately than the conventional phase tracking scheme.

In this paper, an integrated Orthogonal Frequency Division Multiplexing (OFDM) system is developed to improve Bit Error Rate (BER) performance of communication systems in order to meet the requirements of advanced communication systems like long term evaluation advanced (LTE –A). The proposed OFDM system consists of channel estimation using comb type pilot arrangement and convolution encoder with the best known Coding Bounds and Maximum A Posteriori (MAP) decoder. Using pilot symbols, the channel quality is estimated. The BER performance of OFDM system is improved using pilot carriers, channel estimation schemes and MAP decoder at the best known coding bounds. The system performance is analyzed with recursive systematic and non-systematic convolution codes, ML and MAP decoders. From the results, it is evident that the proposed OFDM system with channel estimation and MAP decoder at the best known coding bounds improves the system performance significantly.

Orthogonal Frequency Division Multiplexing (OFDM) is a advance bandwidth efficient digital modulation technique highly adopted for the broadband wireless services because it gives high performance in the multipath fading environments. It is very robust against narrowband interference, but it is not totally free from drawbacks. There are timing and frequency synchronization error for which this technique is very sensitive. These errors are quite detrimental to the mutual orthogonality among the subcarriers which is main condition of the OFDM technique. If it is not resolved properly, it creates inter-symbol interference (ISI) and inter-channel interference (ICI). ISI and ICI increase the Bit Error Rate (BER) and creates packet loss in the transmission. Means, OFDM System has no proper use without precise synchronization technique to assure reasonable system performance. This paper provides an overview on the causes and the effects of these synchronization errors. It focuses on various symbol timing and carrier frequency acquisition schemes including coarse and fine synchronization techniques, using cyclic prefix or training symbols. It has also discussed the attention to implement different standards in the same wireless device using common frequency synchronization structure.

Based on superposition of signals in the time domain, we propose a new low-complexity scheme for the reduction of the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme first generates signal candidates by combining an OFDM signal and its cyclically delayed versions with different delays and phases, and then selects the candidate signal with minimum PAPR. It is shown that the complexity of the proposed scheme is significantly lower than that of the selective mapping (SLM) at the cost of some degradation in the bit error rate (BER) performance and, at similar PAPR and coded BER performance, is lower than that of the low-complexity SLM proposed recently.

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.

Artificial noise (AN) is one of physical layer security techniques, which can realize secure communication. AN is a noise signal which is intentionally transmitted from transmitter along with data signals. Since AN is generated to be canceled out at legitimate receivers, it can prevent eavesdroppers from eavesdropping even if transmitters do not know the existence of eavesdroppers. In spite of its usefulness, most of the AN generation techniques cannot be applied to multiple-input multiple-output (MIMO) systems when the number of antennas at the transmitter is less than that at the legitimate receiver. In this paper, we consider time-domain AN generation techniques for MIMO orthogonal frequency division multiplexing (OFDM) systems. A time-domain AN generation technique is originally proposed for single-input single-output (SISO) OFDM systems. Thus, we first consider to apply the conventional time-domain AN generation technique to MIMO-OFDM systems, and show that this technique requires a large number of antennas at the transmitter and/or a longer cyclic prefix (CP). Next, to solve this problem, we propose another time-domain AN generation technique. Our proposed time-domain AN is generated to be canceled out at the legitimate receiver in the frequency domain, while the conventional time-domain AN is canceled out in the time domain. As a result, the proposed time-domain AN generation technique can be applied to MIMO-OFDM systems, irrespective of the number of antennas and the length of CP. Simulation results can show that the proposed AN achieves the same secrecy rate as the conventional time-domain AN. Moreover, the proposed technique achieves a higher secrecy rate when a MIMO-OFDM system is employed with the antenna configuration to which the conventional time-domain AN generation technique is hard to be applied.

Differential space-time coding was proposed recently in the literature for multi-antenna systems, where neither the transmitter nor the receiver knows the fading coefficients. Among existing schemes, double differential space-time (DDST) coding is of special interest because it is applicable to continuous fast time-varying channels. However, it is less effective in frequency-selective fading channels. This paper’ls authors derived a novel time-frequency double differential space-time (TF-DDST) coding scheme for multi-antenna orthogonal frequency division multiplexing (OFDM) systems in a time-varying frequency-selective fading environment, where double differential space-time coding is introduced into both time domain and frequency domain. Our proposed TF-DDST-OFDM system has a low-complexity non-coherent decoding scheme and is robust for time-and frequency-selective Rayleigh fading. In this paper, we also propose the use of state-of-the-art low-density parity-check (LDPC) code in serial concatenation with our TF-DDST scheme as a channel code. Simulations revealed that the LDPC based TF-DDST OFDM system has low decoding complexity and relatively better performance.

In this paper, we consider out-of-band radiations caused by high sidelobes in orthogonal frequency division multiplexing (OFDM) system. Suppression of sidelobes in OFDM system allows higher spectral efficiency and less interference with existing legacy systems in OFDM based overlay system. We combine different sidelobe suppression techniques to suppress the sidelobes in OFDM system. The multiple-choice sequence (MCS) is combined with cancellation carriers (CCs) and also MCS is combined with subcarrier weighting (SW) technique. MCS is applied prior to CCs or SW techniques. Simulation results show that combining MCS with CCs and MCS with SW techniques the sidelobes in OFDM system can be significantly reduced, which allow a successful co-existance between OFDM overlay system and an existing legacy system.

The nonlinearity of high power amplifiers (HPAs) exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers in orthogonal frequency division multiplexing (OFDM) systems, and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. Due to the ICI is a random process and has unknown distribution function, it is difficult to evaluate the theoretical BER when the ICI is taken into account. This paper evaluates the BER analytically using sequence development approach (SDA) for OFDM systems perturbed by the HPA. In comparison to the state-of-the-art technique, the evaluation of the BER using this approach achieves superior approximation at the expense of additional computational complexity.

Orthogonal frequency-division multiplexing (OFDM) can achieve remarkable performance in terms of spectral efficiency, but its power amplifier efficiency becomes poor because of its high peak-to-average power ratio (PAPR) nature of transmitted signals. Trellis shaping is an effective method to reduce peak and average power without signal distortion at the cost of transmitter complexity and its performance depends on the metric used for the trellis decoder. Several metrics, defined in either time or frequency domain, have been proposed in the literature. In many practical OFDM systems, however, slight nonlinearity is tolerable as long as the resulting distortion meets the system requirements such as bit error rate (BER) and spurious level of transmitted signals. In this paper, we make a thorough performance comparison of trellis-shaped uncoded OFDM systems with different metrics in terms of complexity, achievable PAPR, and resulting BER as well as power spectrum in a practical channel where the signal envelope is distorted by a power amplifier (PA). Our study shows that the metric designed in frequency domain is mostly favorable in such a scenario.

Orthogonal Frequency Division Multiplexing (OFDM) is one of the promising contendents in the race of very high data rate delivery, for 4G wireless systems. The conventional OFDM systems, so far deployed make use of Fast Fourier Transform (FFT) as their basic building block. Besides the numerous advantages, the restriction over the data rate imposed by the FFT in the form of cyclic prefix may be intolerable for the next generation OFDM systems. So, researches are on the way to replace FFT with a more suitable transform which is redundant free. DWT OFDM is one such system in which FFT in the conventional OFDM is replaced with Discrete Wavelet Transform (DWT). Using simulations, already it has been established that in ideal conditions, the performance of DWT OFDM is better than that of FFT OFDM. As phase noise can be considered as the major RF impairment and one of the leading performance limiting factors of the wireless systems, this paper analyses the Bit Error Rate (BER) performance of the DWT OFDM in presence of phase noise. BER performances of both DWT OFDM and FFT OFDM are analysed and compared in presence of phase noise. For this, Matlab Simulink model have been developed for both types of OFDM, and simulation is carried over in presence of phase noise for different modulation schemes. In all cases the BER performances of DWT OFDM is found to be fairly ahead that of FFT OFDM.

The rapid growth and development of wireless data transmission brings new trends, challenges and constraints in the telecommunications field. Novel multicarrier techniques are proposed to meet these requirements. The orthogonal frequency division multiplexing (OFDM) is an attractive technique that achieves high data rate and high spectral efficiency and, at the same time, suppresses multipath fading in wireless communications. However, the OFDM is very sensitive to mobility of the transmitter, receiver or surrounding objects. The mobility produces interchannel interference which significantly deteriorates transmission performance. The interchannel interference suppression represent research challenge. Carrier frequency offset (CFO) estimation and its compensation represent a way to improve OFDM system's performance. In this paper, we consider CFO estimation in the frequency domain by using well known iterative algorithms for precise estimation of spectral peak's central peak frequency. Simulations show that the proposed method outperforms standard CFO estimation methods in terms of accuracy. In simulations, both AWGN channel and multipath channel are considered.