Browsing by Subject "Modulation (Electronics)"
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Item Channel equalization to achieve high bit rates in discrete multitone systems(2004) Ding, Ming; Evans, Brian L.Multicarrier modulation (MCM) techniques such as orthogonal frequency division multiplexing (OFDM) and discrete multi-tone (DMT) modulation are attractive for high-speed data communications due to the ease with which MCM can combat channel dispersion. With all the benefits MCM could give, DMT modulation has an extra ability to perform dynamic bit loading, which has the potential to exploit fully the available bandwidth in a slowly time-varying channel. In broadband wireline communications, DMT modulation is standardized for asymmetric digital subscribe line (ADSL) and very-high-bit-rate digital subscriber line (VDSL) modems. ADSL and VDSL standards are used by telephone companies to provide high speed data service to residences and offices. In an ADSL receiver, an equalizer is required to compensate for the channel’s dispersion in the time domain and the channel’s distortion in the frequency domain of the transmitted waveform. This dissertation proposes design methods for linear equalizers to increase the bit rate of the connection. The methods are amenable to implementation on programmable fixed-point digital signal processors, which are employed in ADSL/VDSL transceivers. A conventional ADSL equalizer consists of a time-domain equalizer, a fast Fourier transform, and a frequency domain equalizer. The time domain equalizer (TEQ) is a finite impulse response filter that when coupled with a discretized channel produces an equivalent channel whose impulse response is shorter than that of the discretized channel. This channel shortening is required by the ADSL standards. In this dissertation, I first propose a linear phase TEQ design that exploits symmetry in existing eigen-filter approaches such as minimum mean square error(MMSE), maximum shortening signal to noise ratio (MSSNR) and minimum intersymbol interference (Min-ISI) equalizers. TEQs with symmetric coefficients can reach the same performance as non-symmetric ones with much lower training complexity. Second, I improve Min-ISI design. I reformulate the cost function to make long TEQs design feasible. I remove the dependency of transmission delay in order to reduce the complexity associated with delay optimization. The quantized weighting is introduced to further lower the complexity. I also propose an iterative optimization procedure of Min-ISI that completely avoids Cholesky decomposition hence is better suited for a fixed-point implementation. Finally I propose a dual-path TEQ structure, which designs a standard singleFIR TEQ to achieve good bit rate over the entire transmission bandwidth, and designs another FIR TEQ to improve the bit rate over a subset of subcarriers. Dualpath TEQ can be viewed as a special case of a complex valued filter bank structure that delivers the best bit rate of existing DMT equalizers. However, dual-path TEQ provides a very good tradeoff between achievable bit rate vs. implementation complexity on a programmable digital signal processor.Item Joint diversity combining technique and adaptive modulation in wireless communications(2006) Nam, Haewoon; Womack, Baxter F., 1930-Wireless communications has become a major economic sector with an unprecedented growth rate over the past decade. This phenomenal growth rate has increased even faster in the 21st century due to the success of wireless cellular systems and wireless local area networks. Furthermore, a variety of applications for high quality media content running on mobile devices have also fueled this phenomenon. In order to maintain the rapid growth rate and satisfy such a high demand from users, the next generation communication systems must achieve both reliability and high data rate using a limited spectrum, power, and complexity budget. Unfortunately, a harsh and unpredictable wireless radio propagation environment, with issues such as multipath, shadowing effects, and frequency selectivity, makes this goal very challenging. There are several techniques in wireless communication systems to combat, or even exploit, such a detrimental effect of fading channels. The most popular technique is the diversity combining technique, where multiple replicas of the same signal are used to reduce the amount of fading. By coherently combining these multiple copies of the transmitted signal, this technique provides reliability of the communication link and offers a higher dynamic range. Among other techniques is adaptive modulation, which attempts not to mitigate the fading effect but to take advantage of it by adaptively adjusting the modulation constellation to the instantaneous channel quality. Thus, this technique aims at achieving a high spectral efficiency given a certain level of bit error rate (BER). This dissertation examines diversity combining techniques and adaptive modulation with an emphasis on how these two different techniques can jointly operate in various wireless systems to achieve both reliability and high spectral efficiency. After a brief introduction to the conventional diversity combining schemes, the adaptive diversity combining schemes are first discussed including a performance analyis. Embedded with a target signal-to-noise ratio (SNR), which may be pre-determined based on the quality of service (QoS) for an application, the adaptive diversity combining schemes achieve a reduced complexity while satisfying the target performance. Second, a joint diversity combining and adaptive modulation technique in multi-carrier systems is proposed and presented with analytical results. Since the fourth generation (4G) wireless cellular system standards adopt orthogonal frequency division multiplexing (OFDM) as a basic transmission technology, techniques to improve the performance at the cell edge in such multi-carrier systems are becoming very important. Exploiting a diversity combining technique, the proposed scheme offers an improved spectral effi- ciency in the low SNR region. Finally, a simple and practical system based on a switched diversity scheme with adaptive modulation is presented. This system provides a reduced number of channel estimation while satisfying the optimum spectral efficiency compared to a selection diversity system. In addition, the switching threshold is easily manipulated so as to make an efficient use of the trade-off between spectral efficiency and the number of channel estimation. An extension of this scheme into a multiuser scenario is considered. This switch-based multiuser access scheme results in an average feedback load that is lower than using the optimal selection-based multiuser scheme. Numerical results show we can obtain a trade-off between spectral efficiency and the feedback load by choosing the switching threshold appropriately.Item Maximizing data rate of discrete multitone systems using time domain equalization design(2003) Milošević, Miloš; Evans, Brian L. (Brian Lawrence), 1965-Asymmetric Digital Subscriber Line in its standardized versions G.DMT and G.Lite uses discrete multitone modulation (DMT) for data transmission. Orthogonal Frequency Division Multiplexing (OFDM) is a similar modulation standard for wireless transmission that has been adopted in IEEE 802.11a wireless local area network, Digital Video Broadcasting and HYPERLAN/2. The transmission channel induces inter-symbol (ISI) interference and other noise sources. The traditional DMT or OFDM equalizer is a cascade of a time domain equalizer (TEQ) as a single finite impulse response filter (FIR), a fast Fourier transform (FFT) multicarrier demodulator, and a frequency domain equalizer as a one-tap filter bank. The time domain equalizer shortens the transmission channel impulse response to mitigate ISI. Previous TEQ design methods optimize objective functions not directly tied to system bit rate. I present the equalizer design that maximizes the bit rate of a DMT system at the output of the FFT demodulator. I develop a subchannel Signal-to-Noise Ratio (SNR) model where the desired signal is formed as the circularly convolved data symbol and the channel impulse response at the input of the FFT and noise is the difference between the received and the desired signal. The received signal also includes the near-end crosstalk, additive white Gaussian noise, analog-to-digital converter quantization noise and the digital noise floor due to finite precision arithmetic. Using the subchannel SNR model, I arrive at the optimal time domain per-tone equalizer filter bank (TEQFB) that maximizes a measure of the ADSL system bit rate. I propose a novel receiver architecture that uses TEQFB and a Goertzel filter bank demodulator at the receiver during data transmission. I also present the design of single FIR equalizer that on average achieves more than 99% of the performance of the TEQFB for the tested standard ADSL carrier serving area loops. Simulation results show that the TEQFB and single FIR outperform the bit rate achieved by the minimum mean-squared error design methods, maximum bit rate approach, and minimum ISI design. The TEQFB also outperforms the leastsquares initialized per-tone equalizer (LS PTE) method while the single FIR closely matches LS PTE performance.