bination of different forms (adaptive modulation, adaptive power-allocation, adaptive coding etc.). Another important question which this thesis seeks to answer is what ben-efits can be extracted from the hardware implementation aspects of the link-adaptation algorithms especially with the availability of adaptivity in the implementation platforms as well (FPGAs, configurable processors etc.)
1.4 Our Contributions and Thesis Presentation
However, the main objective of this research was/is to reduce the complexity of the ex-isting adaptive algorithms for multicarrier systems, both, from a theoretical/algorithmic perspective as well as taking advantage from the adaptive nature of the state-of-the-art underlying implementation platforms
Therefore, to reach this main objective, several sub-objectives have been achieved in this dissertation, namely:
• The first insight into the inherent pattern, which exists in the optimal bit-allocation, and based upon this pattern the design of a Novel Optimal Discrete Bit Loading algorithm which has a complexity significantly lower than existing algorithms for discrete bit-loading
8 Chapter 1 : Introduction
• Theoretical developments for an optimal power-allocation including the Peak-Power Constraint (explained in Chapter3) and the design of a novel algorithm for Peak-Power Constrained Optimal Peak-Power Allocation which involves a complexity signifi-cantly lower than the classical method of Iterative- Waterfilling, which is conven-tionally employed for such constrained power allocations.
• The proposal of the novel idea ofWave-Quantization in Irregular-LDPC Codes, the theoretical developments in this regard leading to a simplistic calculation method of theelite-effect in Irregular LDPC-codes which is not possible with classical methods.
Finally a proposal of the design and optimization of Finite-Length Irregular LDPC Codes based on thisWave-Quantization methodology.
• The optimization of a flexible implementation architecture for the algorithms of link-adaptation, where a methodology to tune the hardware resources at run-time based upon the needs of the channel and system is proposed. This results in the use of the link-adaptation algorithms at those ranges of doppler-frequencies/user-mobility, which are not possible to operate upon, otherwise.
Chapter 2 introduces the reader to the proposed bit loading algorithm. An extensive state-of-the-art on the loading algorithms is presented along with the basic concepts of the theoretical foundations of the algorithm. The algorithm’s performance in terms of complexity is done with respect to other proposed algorithms, both theoretically and in terms of exact number of execution cycles over a Superscalar architecture.
Chapter 3 introduces the reader to the proposed power loading algorithm. A state-of-the-art on different power optimization methods with different goals and constraints is given. Our concerned problem is formulated for Bit Error Rate (BER) minimization, for a given throughput and with a peak-power constraint using the lagrange optimization method. Then, Iterative Surplus Re-distribution (ISR) based power-allocation algorithm is presented as a simplification to the classical Iterative-Waterfilling algorithm, and its complexity analysis is presented.
Chapter 4 presents our developments regarding the development of a simplified methodology to update the irregularity profile of an irregular LDPC code based upon the channel-gains. State-of-the-art on irregularity profile optimization is presented along with the our proposed idea of Wave-Effect Quantization. Corresponding developments
1.4 Our Contributions and Thesis Presentation 9 to make it computationally feasible and finally theProgressive-Irregularity-Construction (PIC) based methodology for the design and optimization of finite-length irregular codes.
Chapter 5 will present our work related to the implementation aspects of different algorithms. The set-up used for performing an exhaustive search over a configurable processor architecture for the bit-loading algorithm is presented. Then, genetic algorithms were employed to converge faster toward the optimal hardware configuration, for a given algorithm. A methodology to link the run-time tuning of the processor architecture parameters based upon the time-varying channel needs is finally proposed, to enhance the operating doppler-range of a wireless system.
Finally, in chapter 6, the research achievements of this work are outlined, conclusions are drawn from this research and perspectives of interest regarding this work are presented which may be taken as research-directions in the future.
10 Chapter 1 : Introduction
Chapter 2
Bit-Loading Algorithm for Multicarrier Systems
2.1 Introduction
We explained in chapter 1 that an adaptive selection of system parameters (modulation, power, coding etc.) with respect to the channel response results in an overall improve-ment in system performance. This adaptation can be done in the dimension of time (corresponding to different channel conditions at different time-slots), in the dimension of frequency (corresponding to channel gains at different frequencies), in the dimension of space (corresponding to channel conditions at different branches of a multi-antenna system) or any given combination of the above three possibilities. Adaptation in the dimension of frequency is greatly facilitated by means of a Multi-carrier system as we explained in the previous chapter because of the presence of a large number of subcar-riers at unique frequencies, which eventually allows fine-tuning of system parameters at different frequencies. Adaptive Modulation generally refers to the adaptation policy where modulation type/constellation size ( BPSK, QPSK, QAM-16 etc.) is allowed to be varied with respect to the channel conditions, i.e. the better the channel, the higher the modulation size. In multicarrier systems, since a different modulation size may be allowed for each sub-carrier, the phenomenon of adaptive modulation with respect to each sub-carrier is termed asbit-loadingwhen constellation-size is only allowed discrete number of bits as shown in figure 2.1
Since the multicarrier transmission technique was first employed in the wireline DSL
12 Chapter 2 : Bit-Loading Algorithm for Multicarrier Systems Figure 2.1Adaptive Modulation : Continuous and Discrete (Bit-Loading) Constellation Size
technology, much of the literature regarding the ’bit-loading’ algorithm comes under the paradigm of Discrete Multitone (DMT), which is the variant of multicarrier transmission with bit-loading applied to the DSL technology. The task of optimizing the bit-profile i.e. how many discrete bits should be allocated to each subcarrier for a given channel condition and a given number number of sub-carriers has been of great research interest in the past under the title of Discrete Bit-Loading Problem.It has been found that the optimized bit-profile for the same channel and sub-carriers is different for different goals (maximizing the system throughput, margin, etc) and constraints (maximum available power, maximum no. of bits to be allocated, peak-power constraint etc.) associated with the optimization problem. Whereas a large number of optimization techniques from convex optimization to combinatorial methods (as will be indicated in the next state-of-the-art section), the aim has been to achieve the optimal-profile with minimal involved complexity. Complexity of the bit-loading optimization problem is of utmost importance because of the fact that in the fast-varying wireless channel, the optimization process would be required to be re-performed each time channel conditions are varied and hence be required to finish under a hard time-constraint. Hence, the basic aim of this chapter is to introduce to the reader the bit-loading problem in detail, the existing solutions in this regard and our proposed 3-dB subgrouping based bit-loading method which claims to achieve the optimal profile with complexity lesser than any other available algorithm.