Adaptive multicarrier modulation for wireless systems

In recent years, due to the rapid convergence of multimedia services, Internet and wireless communications, there has been a growing trend of heterogeneity (in terms of channel bandwidths, mobility levels of terminals, end-user quality-of-service (QoS) requirements) for emerging integrated wired/wireless networks. Moreover, in nowadays systems, a multitude of users coexists within the same network, each of them with his own QoS requirement and bandwidth availability. In this framework, embedded source coding allowing partial decoding at various resolution is an appealing technique for multimedia transmissions. This dissertation includes my PhD research, mainly devoted to the study of embedded multimedia bitstreams in heterogenous networks, developed at the University of Bologna, advised by Prof. O. Andrisano and Prof. A. Conti, and at the University of California, San Diego (UCSD), where I spent eighteen months as a visiting scholar, advised by Prof. L. B. Milstein and Prof. P. C. Cosman. In order to improve the multimedia transmission quality over wireless channels, joint source and channel coding optimization is investigated in a 2D time-frequency resource block for an OFDM system. We show that knowing the order of diversity in time and/or frequency domain can assist image (video) coding in selecting optimal channel code rates (source and channel code rates). Then, adaptive modulation techniques, aimed at maximizing the spectral efficiency, are investigated as another possible solution for improving multimedia transmissions. For both slow and fast adaptive modulations, the effects of imperfect channel estimation errors are evaluated, showing that the fast technique, optimal in ideal systems, might be outperformed by the slow adaptive modulation, when a real test case is considered. Finally, the effects of co-channel interference and approximated bit error probability (BEP) are evaluated in adaptive modulation techniques, providing new decision regions concepts, and showing how the widely used BEP approximations lead to a substantial loss in the overall performance.

A novel hybrid control strategy for reducing torque ripple in permanent magnet brushless direct current motors has been developed by combining the deterministic nature of direct torque control and the adaptive nature of fuzzy logic control. Direct torque control using space vector pulse width modulation has been developed. This technique ensures the near-sinusoidal nature of the stator and rotor current. The impact of commutation ripple corresponding to the sinusoidal current is reduced compared to square wave current in conventional direct torque control. Further, the fuzzy logic controller provides adaptive modulation in space vector to mitigate the torque ripple. The response of the proposed method has been analyzed in the time domain as well as in the frequency domain. Experimental results establish a significant reduction in torque ripple and minimization of high-frequency components. A comparative study with conventional direct torque control methods has also been presented.

In multi-access wireless communication systems, power control and adaptive modulation are two important means to increase spectral efficiency, combat the time varying fading environment, and reduce cochannel interference. The overall uplink transmitting power is optimized while the overall network throughput is fixed. Each link can select a range of targeted signal-to-interference-plus-noise ratio (SINR) according to its current channel condition and each link's time average SINR is maintained as a constant to ensure fairness. Adaptive M-QAM or M-PSK modulation with antenna diversity is applied to increase spectral efficiency. The scheme can be interpreted as "water filling" each link's SINR in the time domain and allocating network throughput to different links at each time. From simulation results, our scheme reduces about 40% of overall transmitting power and increases average spectral efficiency by about 0.9 bit/s/Hz.

In this thesis, based on transmitter specification of DVB-T, multi-antenna 4x4 MIMO-OFDM transmitter was designed to replace traditional single-antenna system. Furthermore, new generation image-coding technique H.264 was also presented to be an alternate for MPEG-2. In order to simulate the non-ideal effects of wireless communication environment, base-band channel model including Multi-path Fading effect, AWGN effect, CFO effect was took into account. Under such an environment and according to its structure and non-ideal characteristics, the receiver including four major sections was designed: synchronization, channel response estimation and compensation, adaptive modulation and channel decoding. First, in the synchronization section, initial estimation, symbol boundary estimation and CFO estimation/compensation were contained.Second, in the channel estimation, scattered pilots was estimated in two-dimensional time and frequency domain, and then compensated by the MIMO detector. Thirdly, adaptive modulation mechanism was proposed, with modulation variable α、β corresponded with different channel condition. The adaptive modulation technique classified the image data into different level, and then transmits each level through its corresponding protection mechanism. In the last, a concatenate error-correction decoder was used, where Viterbi Decoder and Reed-Solomon Decoder were corrected by the error of channel effect. In the hardware part, with the aim of reducing cost and size, 4x4 MIMO-OFDM system was reduced to 2x2 MIMO-OFDM system; the transmitter, channel model and receiver were verified by two FPGA board.

In this letter we investigate the impact of signal-to-noise ratio (SNR) estimation on the performance of adaptive modulation. The adaptive modulation technique uses an estimate of the SNR at the receiver to choose a modulation scheme so that a given performance target is met. By adapting the modulation scheme, the link is able to improve the spectral efficiency as compared to static modulation. An analytic expression for the impact of SNR estimation error is derived and it is shown that SNR estimation has minimal impact on the performance.

In this paper, adaptive modulation technique is applied to STS-based CDMA system to improve the data throughput of the downlink in different fading channels. The contribution in this paper is using new thresholds for adaptation procedure in different fading environments. The simulation results demonstrate that the proposed thresholds are appropriate for realistic wireless channels.

This paper discusses system design strategies and performance evaluation for adaptive modulation techniques used in new and up-and-coming wireless access systems. After a brief discussion on basic modulation parameter (MP) settings as well as functionalities specific for adaptive modulation systems, this paper discusses how to design a narrowband time-division multiple-access/time-division duplex (TDMA/TDD)-based adaptive modulation system and confirms through laboratory experiments that the narrowband TDMA/TDD-based adaptive modulation system dramatically enhances system robustness to multipath fading and flexibility in throughput and transmission quality control. Next, as an extension to many fields of application for adaptive modulation, this paper discusses subcarrier-level adaptive modulation for orthogonal frequency-division multiplexing (OFDM)-based one-cell reuse broadband cellular systems. One key result is that nonsubcarrier transmit power control (TPC) applied adaptive modulation systems can achieve almost the same performance as subcarrier TPC applied adaptive modulation systems provided that the required signal-to-noise-plus-interference power ratio increment in the MP sets is designed to be sufficiently small. Finally, this paper explains the dynamic parameter controlled OFDM/TDMA system as an example of a practical scheme for one-cell reuse broadband wireless access systems. The analysis confirms that this adaptive system can achieve a spatial reuse efficiency defined by (average throughput in multicell conditions)/(average throughput in single cell conditions) of 0.8 as well as an average media access control payload throughput of about 150 Mb/s using about 100 MHz of bandwidth.

Adaptive modulation techniques combat fading by employing a suitable modulation mode depending on the instantaneous channel conditions for improving the Bit Error Rate (BER) performance or the average throughput. Based on a generic model of constant-power adaptive modulation, exact closed form expressions of the average BER and the average throughput are derived, when employing square Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) as the constituent modulation modes, operating over a Nakagami fading channel. The optimum modulation-mode switching levels, achieving the highest possible throughput under the constraint of the average BER, are obtained using the Lagrangian optimisation method. Adaptive modulation employing the optimum switching levels shows a superior performance, while maintaining a constant average BER.

Low-power dc-dc converters employed in consumer applications are required to maintain a good efficiency over a wide operating range while keeping reduced footprint requirements. A digital control technique implementing an adaptive off-time modulation has been previously developed in order to extend the efficient operating range of quasi-resonant buck converter (QRBC) of the non-synchronous type. This paper addresses a corresponding adaptive off-time modulation technique for synchronous QRBCs, in which the problem of properly modulating the rectifying switch arises. Results indicate that, for a given converter design, application of the proposed approach extends the efficient operating range. More interestingly, from a design perspective, the proposed adaptive off-time modulation enables the use of smaller resonant inductance with negligible impact on the overall efficiency over a constant off-time controller. The proposed approach is described theoretically, and experimental results are reported for a 12 V-to-3.3 V, 1 A synchronous QRBC discrete prototype.

Coverage study for broadband wireless systems using link adaptation and diversity combining techniques

This paper considers the spectrum sharing multiple-input multiple-output (MIMO) cognitive radio network, in which single primary user (PU) coexists with multiple secondary users (SUs). Joint beamforming and admission control with adaptive modulation technique is designed to maximize the number of the serviced SUs and the transmit rate of the secondary links while imposing upper limits on the interference temperature to the PU by using the least transmit power. In the proposed cross-layer design, the SINR target is dynamically adapted with channel condition and bit-error rate (BER) requirement, and works as admission control implicitly, that is, the cognitive link is dropped as long as its SINR requirement is equal to zero. With the pre-fixed receiver, the multi-objective optimization problem is NP-hard, which is further transformed into single problem by introducing a utility function. The optimum solutions are achieved via second-order cone programming (SOCP) with an iterative bisection search method. Simulation results show that the proposed scheme achieves desirable spectrum and power efficiency, and the maximum number of the total active SUs are intelligently selected.

In this paper, an adaptive modulation technique is proposed in a MultiBand Orthogonal Frequency Division Multiplex Ultra Wide Band (MB-OFDM UWB) communication system, to improve the BER performance in a frequency selective fading channel. The modulating bits are varied across the sub-channels depending on the signal to noise ratio (SNR). Sub-channels experiencing severe fading employ lower or no bit-loading while subchannels with little or no fading utilise higher bit-loading to maintain a constant system data rate. Simulation results show that using the adaptive modulation to maintain a constant data rate features a better performance than using only QPSK modulation in a frequency selective fading channel.

Nowadays, LTE-based femtocell technology is a promising solution offering high-speed services, enhanced indoor coverage and increased system capacity. In such multi-user wireless communication systems, intelligently allocating resources is the substantial aim towards interference mitigation and enhancing power and spectral efficiencies. In this paper, we propose a joint power and channel allocation with Adaptive Modulation and Coding (AMC) technique for downlink OFDMA-based LTE system, namely AMC-QRAP. As trade-off between distributed and centralized architectures, we consider a clustered network architecture. Users differentiation is taken into account to ensure Quality of Service (QoS) guarantee where premium High Priority HP users and best effort BE users are served by the network. Our allocation problem is resolved as an optimization model using linear programming. Extensive simulations prove that our proposal outperforms different techniques in the literature and considerably improve two previously proposed methods QP-FCRA and Q-FCRA. We consider different metrics for performance evaluation such as spectral efficiency, throughput satisfaction rate, user outage and transmission power.

The novel class of nonbinary maximum minimum distance redundant residue number system (RRNS) codes is invoked in the context of adaptively RRNS coded, symbol-by-symbol adaptive multicarrier modulation, in order to combat the effects of frequency-selective fading inflicted by dispersive wide-band channels. The system's performance can be adjusted in order to maintain a given target bit error rate (BER) and bit per symbol (BPS) performance. The proposed adaptive RRNS scheme outperforms the convolutional constituent code based turbo coded benchmarker system for channel signal-to-noise ratios (SNR) in excess of about 15 dB at a target BER of 10/sup -4/.

Delta modulation plays a key role in data communication; the problem encountered in delta modulation is the slope over load error, which is inherent in the system. In order for the signal to have good fidelity, the slope-overload error need to be as small as possible. Adaptive delta modulation reduces the slope over load error to a greater extent. ADM attempts to increase the dynamic range and the tracking capabilities of fixed step-size delta modulation. The adaptive algorithms adjust the step size (from a range of step sizes) to the power level of the signal and thus enhance the dynamic range of the coding system appreciably. This paper suggests a novel 1-bit Adaptive Delta Modulation technique for improving the signal-to-noise ratio (SNR) of Adaptive Delta Modulators (ADM). Various step-size algorithms are discussed and also their performance comparison is made. A new technique has also been discussed with a suitable pre-filer used for improving the SNR.KeywordsAdaptive Delta ModulationDelta ModulationA/D conversionPre-processingSpeech processing