Modeling of Orthogonal Frequency Division Multiplexing (OFDM) for Transmission in Broadband Wireless Communications

Orthogonal Frequency Division Multiplexing (OFDM) is a multi carrier modulation technique that provides high bandwidth efficiency because the carriers are orthogonal to each other and multiple carriers share the data among themselves. The main advantage of this transmission technique is its robustness to channel fading in wireless communication environment. This paper investigates the effectiveness of OFDM and assesses its suitability as a modulation technique in wireless communications. Several of the main factors affecting the performance of a typical OFDM system are considered and they include multipath delay spread, channel noise, distortion (clipping), and timing requirements. The core processing block and performance analysis of the system is modeled usingMatlab

The Bit Error Rate (BER) Performance in Multi-Carrier (OFDM) and Single-Carrier

The spectacular growth of wireless communication tools has escalated the number of mobile subscribers from almost 700 million in 2000 to more than 4 billion in 2009. The huge number of subscribers has led to several issues with how service is provided. The high user demand has forced developers to overcome the problems of the old analog systems and to introduce OFDM as a promising technique that can fulfill users\u27 high demands. This technique matches well with high data rate connection and provides a higher capacity for the subscribers\u27 usage. The OFDM, as a multi-carrier, is more complex than the single-carrier transmission scheme. However, the OFDM technique maintains better performance for high data rate in terms of bit error rate (BER). In this thesis a comparison has been presented between the multi-carrier OFDM and the single-carrier to prove, in a simulation form, the theoretical point of view. Despite the advantages of using the OFDM scheme, there are several drawbacks. One of these negatives is the high peak to average power ratio (PAPR). To overcome this problem, there are power reduction techniques that can be applied to the signal to reduce the high power. One of these techniques is the clipping and filtering technique. A maximum level is sited for the transmitted signal to reduce the power and afterward, the signal goes through a filter to remove the influence of the in-band distortion and out-of-band radiation

Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance

In massive MIMO, most precoders result in downlink signals that suffer from high PAR, independently of modulation order and whether single-carrier or OFDM transmission is used. The high PAR lowers the power efficiency of the base station amplifiers. To increase power efficiency, low-PAR precoders have been proposed. In this article, we compare different transmission schemes for massive MIMO in terms of the power consumed by the amplifiers. It is found that (i) OFDM and single-carrier transmission have the same performance over a hardened massive MIMO channel and (ii) when the higher amplifier power efficiency of low-PAR precoding is taken into account, conventional and low-PAR precoders lead to approximately the same power consumption. Since downlink signals with low PAR allow for simpler and cheaper hardware, than signals with high PAR, therefore, the results suggest that low-PAR precoding with either single-carrier or OFDM transmission should be used in a massive MIMO base station

Two novel nonlinear companding schemes with iterative receiver to reduce PAPR in multi-carrier modulation systems

Companding transform is an efficient and simple method to reduce the Peak-to-Average Power Ratio (PAPR) for Multi-Carrier Modulation (MCM) systems. But if the MCM signal is only simply operated by inverse companding transform at the receiver, the resultant spectrum may exhibit severe in-band and out-of-band radiation of the distortion components, and considerable peak regrowth by excessive channel noises etc. In order to prevent these problems from occurring, in this paper, two novel nonlinear companding schemes with a iterative receiver are proposed to reduce the PAPR. By transforming the amplitude or power of the original MCM signals into uniform distributed signals, the novel schemes can effectively reduce PAPR for different modulation formats and sub-carrier sizes. Despite moderate complexity increasing at the receiver, but it is especially suitable to be combined with iterative channel estimation. Computer simulation results show that the proposed schemes can offer good system performances without any bandwidth expansion

Performance of Spatial Diversity DCO-OFDM in a Weak Turbulence Underwater Visible Light Communication Channel

The performance of underwater visible light communication (UVLC) system is severely affected by absorption, scattering and turbulence. In this article, we study the performance of spectral efficient DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in combination with the transceiver spatial diversity in turbulence channel. Based on the approximation of the weighted sum of lognormal random variables (RVs), we derived a theoretical exact bit error rate (BER) for DCO-OFDM systems with spatial diversity. The simulation results are compared with the analytical prediction, confirming the validity of the analysis. It is shown that spatial diversity can effectively reduce the turbulence-induced channel fading. The obtained results can be useful for designing, predicting, and evaluating the DCO-OFDM UVLC system in a weak oceanic turbulence condition

Peak-to-Average-Power-Ratio (PAPR) Reduction Techniques for Orthogonal-Frequency-Division- Multiplexing (OFDM) Transmission

Wireless communication has experienced an incredible growth in the last decade. Two decades ago,the number of mobile subscribers was less than 1% of the world\u27s population. As of 2011, the number of mobile subscribers has increased tremendously to 79.86% of the world\u27s population. Robust and high-rate data transmission in mobile environments faces severe problems due to the time-variant channel conditions, multipath fading and shadow fading. Fading is the main limitation on wireless communication channels. Frequency selective interference and fading, such as multipath fading, is a bandwidth bottleneck in the last mile which runs from the access point to the user. The last mile problem in wireless communication networks is caused by the environment of free space channels through which the signal propagates. Orthogonal Frequency Division Multiplexing (OFDM) is a promising modulation and multiplexing technique due to its robustness against multipath fading. Nevertheless, OFDM suffers from high Peak-to-Average- Power-Ratio (PAPR), which results in a complex OFDM signal. In this research, reduction of PAPR considering the out-of-band radiation and the regeneration of the time-domain signal peaks caused by filtering has been studied and is presented. Our PAPR reduction was 30% of the Discrete Fourier Transform (DFT) with Interleaved Frequency Division Multiple Access (IFDMA) utilizing Quadrature Phase Shift Keying (QPSK) and varying the roll-off factor. We show that pulse shaping does not affect the PAPR of Localized Frequency Division Multiple Access (LFDMA) as much as it affects the PAPR of IFDMA. Therefore, IFDMA has an important trade-off relationship between excess bandwidth and PAPR performance, since excess bandwidth increases as the roll-off factor increases. In addition, we studied a low complexity clipping scheme, applicable to IFDMA uplink and OFDM downlink systems for PAPR reduction. We show that the performance of the PAPR of the Interleaved-FDMA scheme is better than traditional OFDMA for the uplink transmission system. Our reduction of PAPR is 53% when IFDMA is used instead of OFDMA in the uplink direction. Furthermore, we also examined an important trade-off relationship between clipping distortion and quantization noise when the clipping scheme is used for OFDM downlink systems. Our results show a significant reduction in the PAPR and the out-of-band radiation caused by clipping for OFDM downlink transmission system

Reduction of peak to average power ratio in OFDM and OFDM-CDMA system

High Peak to Average Power Ratio is a main problem in Orthogonal Frequency Division Multiplexing(OFDM) system and Orthogonal Frequency Division Multiplexing -Code Division Multiple Access (OFDM-CDMA) system. Considering a compensated High Power Amplifier(HPA), in order to keep linearity for amplification, Input Back-Off(IBO) of the HP-A, has to be increased for handling a high PAPR. Higher 1130 will make HPA less efficient. Additionally, a signal with high PAPR will suffer clipping when PAPR is larger than 1130 of the HPA. Clipping can cause the distortion of the signal and deteriorate the performance of the system. Reduction of PAPR is a main issue for OFDM and OFDM-CDMA application. In this thesis, some coding schemes are discussed to reduce PAPR. Of all possible code words for transmission, some code words with lower PAPR are chosen for transmission. Some redundancies are added to the end of the original code words to form these lower PAPR code words, though the net bit rate will decrease as a tradeoff. To reduce PAPR block coding scheme and cyclic coding scheme are discussed first. An odd parity bit is added as a redundancy bit. Also the code rate is 3/4. A novel block coding scheme with bit position control is discussed, where the position of the redundancy bit in the original code words is chosen by a feedback selection. For multiuser application, OFDM-CDMA is discussed. Walsh- Hadard (WH) and Complementary (CP) and Gold code sequences are used as the spreading sequences. These coding schemes used in OFDM system are also used before spreading to reduce PAPR in OFDM-CDMA system