ITC/USA 2013 Conference Proceedings / The Forty-Ninth Annual International Telemetering Conference and Technical Exhibition / October 21-24, 2013 / Bally's Hotel & Convention Center, Las Vegas, NV / This paper shows the design and testing of a test bed at Morgan State University as part of the development of a Link Dependent Adaptive Radio (LDAR). It shows the integration of variable rate QAM/OFDM modulation and a variable rate Punctured Convolutional Coder. It also shows a dynamic aeronautical channel simulator developed to capture the dynamics of these channels. Performance results are show for combinations of modulation, coding and channel variations that provide motivation for the potential of the LDAR system.

ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, California / This paper details design, development and test of the Low Cost OFDM Transceiver (LCOT) LCT2-040-2200 module at S band. The goal of the project is to provide a low cost transmit and receive unit for demonstrating OFDM communication on a flight platform. The LCOT module is built to transmit and receive OFDM signals. It transmits OFDM signals at 10W power out through a custom built high power amplifier and conforms to the IEEE 802.11.g spectral emissions mask.

ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, California / Radio transmission channel influences greatly the quality of transmitted voice and data signal in terms of data rate and robustness. This degradation is as a result of many factors, notable amongst them are having multiple replica of the transmitted signal at the receiver (multipath), changes of frequency as a result of the movement of the aircraft (Doppler shift) and noise. This paper characterizes the scattered components of the aeronautical channel in terms of delay spread. Geometric representation is used to derive expressions for the maximum delay spread using the 2-ray model and the three dimensional model of the scattered path. Furthermore, the delay and Doppler frequencies are described as a function of the horizontal distance to the specular reflection point between a ground station and a test article. The simulated results are compared to measured data of related articles and the value of the maximum delay spread is compared with the proposed intersymbol guard band for Orthogonal Frequency Division Multiplexing (OFDM) in the Integrated Network Enhanced Telemetry (iNET) program to see if this proposition can be adapted to the aeronautical channel.

The main objectives of Integrated Network Enhanced Telemetry (iNET) are increased data rate and improved spectral efficiency. In this paper we propose the transmission scheme for the physical layer to be coded Quadrature Amplitude Modulation-Orthogonal Frequency Division Multiplexing (QAM OFDM) which enables high data rates and spectrum efficiency. However in high mobility scenarios, the channel is time-varying the receiver design is more challenging. In this paper pilot-assisted channel estimation is used at the receiver, with turbo coding to enhance the performance; while the effect of inter symbol interference (ISI) is mitigated by cyclic prefix. The focus of this paper is to evaluate the performance of OFDM with QAM over an aeronautical channel. The M-QAM with OFDM provides a higher data rate than QPSK hence it is chosen in this paper. The implementation is done using Inverse Fast Fourier Transform (IFFT) and the Fast Fourier Transform (FFT). This paper considers how the performance of Coded QAM OFDM can be enhanced using equalization to compensate for inter symbol interference, and using turbo coding for error correction.

This dissertation presents four technical contributions in the theory and practice of low-density parity-check (LDPC) codes and orthogonal frequency division multiplexing (OFDM) systems withtransmission non-linearity and with interference due to high mobility.We first explore the universality of LDPC codes for the binary erasure channel (BEC), the AWGN channel, and the flat Rayleigh fading channel. Using excess mutual information as a performance measure, we demonstrate that an LDPC code designed on a singlechannel can be universally good across the three channels. Thus, a channel for which LDPC code design is simple may be used as a surrogate for channels that are more challenging.Due to fast channel variations, OFDM systems suffer from inter-carrier interference (ICI) in frequency-selective fast fading channels. We propose a novel iterative receiver design that achieves near-optimal performance while maintaining a complexity that grows only linearly with the number of OFDM carriers. Weprove that the matched filter bound for such a channel is also the maximum-likelihood sequence detection (MLSD) bound.Because of the presence of high peaks at OFDM modulator output, amplitude clipping due to amplifier saturation causes performance degradation. We show that existing analyses underestimate the capacity of OFDM systems with clipping, and we analyze thecapacity of clipped OFDM systems with AWGN and frequency-selective Rayleigh fading. We prove that for frequency-selective Rayleigh fading channels, under certain conditions, there exists an SNR threshold, above which the capacity of a clipped system is higherthan that of an unclipped system. We provide upper and lower bounds on the channel capacity and closed-form approximations of discrete-input capacities with and without clipping.We also derive tight MLSD lower bounds and propose near-optimal receivers for OFDM systems with clipping. We show that over frequency-selective Rayleigh fading channels, under certain conditions, a clipped system with MLSD can achieve better performance than an unclipped system. We show that the MLSD boundscan be achieved or closely approached by the proposed low complexity receivers in various channel types.

The presence of dispersive fading and inter-carrier interference (ICI) constitute the major impediment to reliable communications in orthogonal frequency-division multiplexing (OFDM) systems. Recently iterative (``Turbo'') processing techniques, which have been successfully applied to many detection/decoding problems, have received considerable attention. In this thesis, we first aim on the design of iterative receiver for single antenna OFDM system with frequency offset and dispersive fading. Further work is then extended to space-time block coded (STBC) OFDM system. At last, the technique is applied to STBC-OFDM system through a newly built channel model, which is based on a physical description of the propagation environment. The performance of such systems are verified by computer simulations. The simulation results show that the iterative techniques work well in OFDM systems.

This dissertation documents alternative designs of the Zero Forcing decoding algorithm with Successive Interference Cancellation (ZF-SIC) for use in Vertical Bell Laboratories Layered Space Time Architecture (V-BLAST) Multiple Input Multiple Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) systems, in an effort to reduce the computational complexity of the receiver. The development of a wireless platform utilizing this architecture intended for use in an indoor wireless multipath environment was created to analyze the multipath environment. This implementation is the result of efforts from several individuals within the CST group. My contributions are documented in this dissertation. In order to obtain channel state information (CSI), a training sequence is sent with each incoming frame. A pseudo-inverse operation is performed on the channel matrix and applied to each OFDM symbol that was received. Performing this operation on each tone and across each OFDM symbol is computationally inefficient in a MIMO configuration. If the number of pseudo-inverses can be reduced while maintaining acceptable levels of bit error, the processing time of each frame can be decreased. Traditionally, tests of the performance of ZF-SIC have been conducted with simulations modelling a multipath channel. In this thesis, CSI is observed using an open loop platform developed for MIMO-OFDM communications. The rate of change of the channel is observed for different multipath environments. The proposed methods of decoding require modifications to ZF-SIC. The suggested changes are only applicable to a MIMO OFDM based method of data transmission. The most effective method of reducing decoding complexity and maintaining an acceptable number of bit errors was observed to occur in the time domain rather than in the frequency domain. For selecting frames and averaging frames in the time domain it was determined that the optimal number of OFDM symbols per frame is 1932 and 174, respectively.

Synchronization is one of the most important issues in wireless communication systems design and implementation. The requirement for synchronization is going high as the signal bandwidth and the system complexity increases. For instance, the ultra-short pulse width in ultra-wideband (UWB) communication systems poses problems to the conventional timing synchronization methods and the multi-node transmission poses problems to the existing carrier frequency offset (CFO) synchronization methods. Moreover, the impact of imperfect synchronization in these systems on the system performance is more negative than that of the conventional communication systems. Therefore, efficient synchronization algorithms are really in need. This dissertation presents several synchronization methods aiming to either improve the synchronization performance or reduce the synchronization complexity. The focus of this dissertation is on UWB systems and cooperative systems. Both timing synchronization and carrier frequency synchronization problems have been investigated. Several different systems are considered, including the point to point block transmission based UWB communications, orthogonal frequency division multiplexing (OFDM) based one way and two way relaying communication systems and narrow band cooperative communication systems. For block transmission UWB systems, i.e., both OFDM and single carrier frequency domain equalization (SC-FDE) UWB systems, a new generic timing estimation method based on channel impulse response (CIR) estimation is proposed. The newly proposed method is superior to the existing methods not only in synchronization performance, but also in the algorithm complexity. For the multi-node cooperative communications, the CFO mitigation issue is studied with OFDM signaling. Due to the distributed nature of the cooperative system, the multiple CFO problem is inevitable and hard to solve. A two-step compensation scheme is designed to suppress the interference introduced by multiple CFO with low complexity. Moreover, timing synchronization in cooperative communications is studied, including the broadband OFDM based cooperative communication and the narrow band cooperative communication. A means of determining the optimal timing of the OFDM signal in asynchronous two way relay networks (TWRN) has been designed. A correlation based multi-delay estimation method is proposed for narrow band asynchronous cooperative communication systems. The synchronization issues covering both timing and carrier synchronization have been extensively studied in this dissertation. New synchronization methods have been proposed for the emerging transmission schemes such as high rate UWB transmission and the distributed cooperative transmission with challenges different from conventional wireless transmission schemes. / Graduate

Orthogonal Frequency Division Multiplexing, so called OFDM, has found a prominent place in various wireless systems and networks as a method of encoding data over multiple carrier frequencies. OFDM-based communication systems, however, lacking inherent diversity, are capable of benefiting from different spatial diversity schemes. One such scheme, Cyclic Delay Diversity (CDD) is a method to provide spatial diversity which can be also interpreted as a Space-Time Block Coding (STBC) step. The main idea is to add more transmit antennas at the transmitter side sending the same streams of data, though with differing time delays. In [1], the capacity of a point-to-point OFDM-based channel with CDD is derived for inputs with Gaussian and discrete constellations. In this dissertation, we use the same approach for an OFDM-based single-input single-output (SISO) two-user interference channel (IC). In our model, at the receiver side, the interference is treated as noise. Moreover, since the channel is time-varying (slow-fading), the Shannon capacity in the strict sense is not well-defined, so the expected value of the instantaneous capacity is calculated instead. Furthermore, the channel coefficients are unknown to the transmitters. Thus, in this setting, the probability of outage emerges as a reasonable performance measure. Adding an extra antenna in the transmitters, the SISO IC turns into an MISO IC, which results in increasing the diversity. Both the continuous and discrete inputs are studied and it turns out that decoding interference is helpful in some cases. The results of the simulations for discrete inputs indicate that there are improvements in terms of outage capacity compared to the ICs with single-antenna transmitters.

Master of Engineering by Research / As a multi-carrier modulation scheme, Orthogonal Frequency Division Multiplexing (OFDM) technique can achieve high data rate in frequency-selective fading channels by splitting a broadband signal into a number of narrowband signals over a number of subcarriers, where each subcarrier is more robust to multipath. The wireless communication system with multiple antennas at both the transmitter and receiver, known as multiple-input multiple-output (MIMO) system, achieves high capacity by transmitting independent information over different antennas simultaneously. The combination of OFDM with multiple antennas has been considered as one of most promising techniques for future wireless communication systems. The challenge in the detection of a space-time signal is to design a low-complexity detector, which can efficiently remove interference resulted from channel variations and approach the interference-free bound. The application of iterative parallel interference canceller (PIC) with joint detection and decoding has been a promising approach. However, the decision statistics of a linear PIC is biased toward the decision boundary after the first cancellation stage. In this thesis, we employ an iterative receiver with a decoder metric, which considerably reduces the bias effect in the second iteration, which is critical for the performance of the iterative algorithm. Channel state information is required in a MIMO-OFDM system signal detection at the receiver. Its accuracy directly affects the overall performance of MIMO-OFDM systems. In order to estimate the channel in high-delay-spread environments, pilot symbols should be inserted among subcarriers before transmission. To estimate the channel over all the subcarriers, various types of interpolators can be used. In this thesis, a linear interpolator and a trigonometric interpolator are compared. Then we propose a new interpolator called the multi-tap method, which has a much better system performance. In MIMO-OFDM systems, the time-varying fading channels can destroy the orthogonality of subcarriers. This causes serious intercarrier interference (ICI), thus leading to significant system performance degradation, which becomes more severe as the normalized Doppler frequency increases. In this thesis, we propose a low-complexity iterative receiver with joint frequency- domain ICI cancellation and pilot-assisted channel estimation to minimize the effect of time-varying fading channels. At the first stage of receiver, the interference between adjacent subcarriers is subtracted from received OFDM symbols. The parallel interference cancellation detection with decision statistics combining (DSC) is then performed to suppress the interference from other antennas. By restricting the interference to a limited number of neighboring subcarriers, the computational complexity of the proposed receiver can be significantly reduced. In order to construct the time variant channel matrix in the frequency domain, channel estimation is required. However, an accurate estimation requiring complete knowledge of channel time variations for each block, cannot be obtained. For time- varying frequency-selective fading channels, the placement of pilot tones also has a significant impact on the quality of the channel estimates. Under the assumption that channel variations can be approximated by a linear model, we can derive channel state information (CSI) in the frequency domain and estimate time-domain channel parameters. In this thesis, an iterative low-complexity channel estimation method is proposed to improve the system performance. Pilot symbols are inserted in the transmitted OFDM symbols to mitigate the effect of ICI and the channel estimates are used to update the results of both the frequency domain equalizer and the PICDSC detector in each iteration. The complexity of this algorithm can be reduced because the matrices are precalculated and stored in the receiver when the placement of pilots symbols is fixed in OFDM symbols before transmission. Finally, simulation results show that the proposed MIMO-OFDM iterative receiver can effectively mitigate the effect of ICI and approach the ICI-free performance over time-varying frequency-selective fading channels.