Frequency Diversity Benefits Research Articles

An approach to PAPR Reduction in OFDM using Goppa codes

Orthogonal frequency division multiplexing (OFDM) is a widely used modulation and multiplexing technology applied in many telecommunications standards due to its spectral efficiency and robustness against multipath fading. The multicarrier aspect of OFDM signal is characterized by high peak-to-average power ratio (PAPR), which renders the power amplifier (PA) inefficient and causes distortion in the transmitted signal. Several techniques to overcome and reduce PAPR such as signal distortion, signal scrambling, coding etc. are proposed and implemented on multipath fading channels. Coding methods does not affect system complexity as forward error correction is an inherent block in any digital communication system. The frequency diversity benefits of OFDM is not utilized well unless channel coding is used. Several codes like Hamming, Golay, BCH, RS, Reed Muller etc. have been investigated so far. Goppa codes were needed to be explored in OFDM systems. In this paper, its ability to reduce the peak power is studied for conventional OFDM signal. These codes are used in computer and telecommunication based applications owing to its useful properties required for cryptography. The PAPR reduction of Goppa coded OFDM (GOFDM) is found to be satisfactory for higher order constellation mapping of binary information. A look up table can also be created to select Goppa code words on the basis of a desired threshold of PAPR in dB. A mathematical analysis of the relationship of input data and its PAPR is also executed and certain conclusions are drawn as shown in the lemmas. But coding methods solely cannot achieve substantial PAPR reduction and therefore most of the literature on PAPR reduction methods have combined coding with other techniques. In this paper, G-OFDM is extended to a hybrid method of constant amplitude modulated (CAM)-G-OFDM to completely mitigate the PAPR problem. But it is achieved at the cost of transmitter and receiver complexity and performance parameters like bit error rate (BER), dependency on modulation index etc. which can be taken care of by making some amendments in the system design.

Quality of Service Aware Inter Carrier Interference Mitigation and Antenna Selection Schemes for Beyond 4G Systems

Future broadband wireless communication systems demand high quality of service (QoS) for anytime anywhere multimedia applications. The standards which use orthogonal frequency division multiplexing (OFDM) coupled with multi input multi output (MIMO) are expected to rule the future wireless world. Time selective nature of the channel introduces inter carrier interference (ICI), which is the major performance limiting parameter in OFDM based systems. ICI causes loss in spectral efficiency and results in poor bit error rate (BER) performance, affecting the QoS of MIMO-OFDM systems. The conventional single input single output (SISO)-OFDM-flexible subcarrier spacing (FSS) system offers better performance than the fixed subcarrier spacing systems in terms of ICI mitigation. But BER and spectral efficiency performance of SISO-OFDM-FSS is not good enough to satisfy the requirements of future wireless broadband services. To improve the BER performance, SISO-OFDM system is replaced by space frequency block coded (SFBC)-OFDM system, which adds spatial and frequency diversity benefits to the conventional system. More number of antennas in the MIMO scheme increases the hardware cost, computational complexity and percentage of overhead. In the present study, to improve the spectral efficiency and to reduce the complexity and cost, optimal transmit antenna selection (OTAS) is combined with the SFBC-OFDM-FSS scheme. The simulation results prove that the proposed SFBC-OFDM-FSS-OTAS scheme offers better QoS than the conventional SISO-OFDM-FSS scheme.

Detection Performance using Frequency Diversity with Distributed Sensors

Detection using a frequency diverse (FD), distributed, radar system is investigated. Distributed sensing systems provide an inherent spatial diversity by viewing a potential target from different aspect angles. By using different frequencies at each platform, a diversity gain is obtained in addition to the advantages of spatial diversity while also avoiding mutual interference. Here, since platforms are distributed spatially, true time delay is used at each platform to align the sample look point in time. Data models for a distributed system with and without frequency diversity are developed. These models are used to analyze the corresponding signal-to-interference-plus-noise ratio (SINR) and probability of detection for the two cases in the context of space-time adaptive processing (STAP). The simulation results presented here illustrate the limitations imposed by mutual interference and the significant benefits of spatial and frequency diversity.

FD-MC-CDMA: A Frequency-Based Multiple Access Architecture for High Performance Wireless Communication

Multi-carrier code division multiple access (MC-CDMA) demonstrates good probability of error performances in frequency selective fading channels, a direct result of its ability to resolve the available frequency diversity. However, MC-CDMA performances may be limited by degradation due to large multiple-access interference (MAI). Frequency division multi-carrier code division multiple access (FD-MC-CDMA), a novel multiple access architecture proposed in this paper, exploits the available frequency diversity benefits while reducing MAI. Specifically, instead of transmitting all users' information bits over all carriers, FD-MC-CDMA employs a subset of carriers to support a subset of users (while maintaining the same overall system capacity and throughput as in MC-CDMA). By careful selection of each subset of carriers, the available frequency diversity benefits are fully exploited, while the MAI experienced by each user is reduced. Furthermore, since the number of carriers employed by each user is significantly reduced, the complexity of the truly optimal multiuser detection (MUD) receiver for FD-MC-CDMA is low. An optimal MUD based on the maximum likelihood (ML) criteria is employed to optimize the system performance of FD-MC-CDMA. The proposed FD-MC-CDMA (with MUD) provides significantly better bit error rate (BER) performances than traditional MC-CDMA systems with little increase in system complexity.

CI/FSK: Bandwidth-Efficient Multicarrier FSK for High Performance, High Throughput, and Enhanced Applicability

In traditional binary frequency-shift keying (BFSK), two distinct carrier frequencies are used to represent binary data. In this letter, we propose transmitting multiple orthogonal, in-phase subcarriers around two distinct carrier frequencies to represent binary information. The envelope of the resulting FSK signal is a carrier interferometry pattern, and hence, the name carrier interferometry (CI/) FSK. We demonstrate that this technique is spectrally efficient when compared to traditional BFSK. Performance and data-rate benefits are also demonstrated with the use of a novel coherent reception technique. Moreover, while it is difficult to employ traditional BFSK in multipath channels, we show how the new CI/FSK scheme exploits frequency-diversity benefits when used in such channels.

Merging Multicarrier CDMA and Oscillating-Beam Smart Antenna Arrays: Exploiting Directionality, Transmit Diversity, and Frequency Diversity

In this paper, a novel merger of multicarrier code-division multiple access (MC-CDMA) and smart antenna arrays is introduced. Here, a group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna array at the base station (BS). The smart antennas are located in close proximity to one another. We generate a transmit diversity gain at the receiver by carefully moving (oscillating) the antenna array's pattern. The pattern oscillation is achieved by applying appropriate time-varying phases to array elements of each smart antenna. The beam pattern oscillation ensures a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam pattern oscillation leads to a time-varying channel with a controllable coherence time; hence, a transmit diversity benefit, in the form of a time diversity benefit, is available at the receiver. Employing MC-CDMA with the proposed smart antenna at the BS, we achieve: 1) directionality which creates high network capacity via space-division multiple access; 2) a transmit diversity gain which supports high performance at the receiver in the mobile unit; and 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits, respectively.

Oscillating-beam smart antenna arrays and multicarrier systems: achieving transmit diversity, frequency diversity, and directionality

We introduce a novel merger of smart antenna arrays and multicarrier code-division multiple-access (MC-CDMA) systems. Here, each group of Q carriers in the MC-CDMA system is applied to its own M-element smart antenna at the base station (BS). The smart antennas are separated by a distance that ensures that signals generated by each smart antenna are independent. Applying proper time-varying phases to array elements of each smart antenna array, the beam pattern is carefully controlled to generate a mainlobe at the position of the intended user and small oscillations in the beam pattern. This beam-pattern oscillation creates a time-varying channel with a controllable coherence time and a time diversity benefit at the receiver. Employing MC-CDMA with the proposed antenna array at the BS, we achieve: 1) directionality, which supports space-division multiple access (SDMA); 2) a time diversity gain; 3) increased capacity and performance via MC-CDMA's ability to support both CDMA and frequency diversity benefits. Hence, merging MC-CDMA and BS antenna arrays in an innovative fashion, we achieve high performance at the mobile via joint frequency-time diversity, and high network capacity via joint space-code division multiple access.

Innovative pulse shaping for high-performance wireless TDMA

This letter introduces a novel pulse shaping scheme that enables receivers to demonstrate high performance in wireless fading environments. Called carrier interferometry pulse shaping, pulses are created by the superposition of N carriers. At the receiver, low probability-of-error performance is achieved by breaking the pulse into its frequency components and optimally recombining to create frequency diversity benefits. When implemented in a wireless TDMA system, simulations indicate 5-8-dB improvement at probability-of-error of 10/sup -2/ over traditional Gaussian pulse shaping with decision feedback equalization (DFE(6,4)) in hilly terrain (HT) and typical urban (TU) channels.

SOMARE‐99: Observations of tropospheric scattering layers using multiple‐frequency range imaging

Results from an experimental implementation of multiple‐frequency range imaging (RIM) are presented. The technique exploits the benefits of frequency diversity to improve range resolution of atmospheric radar systems. The theory has been described in the literature, and simulations have proven its usefulness. Nevertheless, experimental results have been extremely limited. Over a 5‐day period in May 1999 we conducted experiments using RIM on the sounding system (SOUSY) radar in northern Germany to observe the layered structure in the troposphere. The experiment is referred to as the SOUSY Multifrequency Atmospheric Radar Experiment 1999 (SOMARE‐99). Estimates of range brightness produced by the RIM analysis provide insight about the layered structure of the atmosphere. The RIM results show distinct similarities to previous in situ measurements, which have shown sharp refractive index discontinuities to exist throughout the troposphere and stratosphere. Examples from selected time periods show layers modulated by possible short‐period gravity waves or advection of periodic structures as well as other layers with apparent downward motion possibly caused by the progression of a warm front or large‐scale subsidence.