Non-orthogonality among adjacent OFDM channels creates OFDM adjacent channel interference and it heavily affects the entire system’s performance. Conventional methods to avoid OFDM adjacent channel interference are not only insufficient but also are wasting a lot of frequency resources. In this research, a method using combinations of modulation
schemes is proposed to avoid effects of OFDM adjacent channel interference. It can be obtained by modulating the sub-carriers at the outer sides of an OFDM channel with lower order modulation schemes (such as BPSK or QPSK), while modulating the sub-carriers at the inner side of the OFDM channel with higher order modulation schemes (such as 16QAM or 64QAM). Intensive simulations have been carried out to evaluate the performance of the
proposed method. The simulation results have shown an increase in the OFDM system’s resistance against adjacent channel interference while still maintain the bandwidth efficiency.
Table of Contents
1 Introduction
2 OFDM adjacent channel interference
3 Conventional Methods
3.1 Windowing
3.2 Filtering
3.3 Guard-band and virtual sub-carrier
3.4 Forward Error Correction coding
3.5 Adaptive modulation
4 Combinations of modulation schemes
4.1 Performance of OFDM modulation schemes
4.2 Proposal
4.3 Simulation model and results
4.3.1 Simulations of the proposed method
4.3.2 Optimization
5 Conclusion
Research Objectives and Core Themes
The primary goal of this research is to propose and evaluate a novel method for mitigating Adjacent Channel Interference (ACI) in Orthogonal Frequency Division Multiplexing (OFDM) systems by utilizing intelligent combinations of modulation schemes. The core objective is to enhance system resistance to ACI while maintaining optimal bandwidth efficiency and overall throughput.
- Analysis of conventional interference mitigation techniques in OFDM systems.
- Evaluation of modulation order impacts on ACI resistance and system throughput.
- Proposal of a mixed-modulation approach using low-order modulation on outer sub-carriers and high-order modulation on inner sub-carriers.
- Extensive simulation-based verification under various Carrier-to-Interference Ratio (CIR) conditions.
- Optimization of sub-carrier allocation to balance power and bandwidth efficiency.
Excerpt from the Book
4.2 Proposal
It is obvious that, a modulation scheme with a lower modulation order such as BPSK or QPSK can protect an OFDM system better from noise and interference while a modulation scheme with a higher modulation order such as 16QAM or 64QAM can provide the system a higher saturated throughput. The main idea of the proposal is to combine a modulation scheme having a lower modulation order with a modulation scheme having a higher saturated throughput within one OFDM system so that this system can obtain a higher resistance against noise and interference while still maintain its sufficient throughput.
In an OFDM system, each sub carrier can be modulated independently on other sub-carriers. Besides, adjacent channel interference on an OFDM system mainly effects on the sub-carriers at two sides of the working frequency channel.
Chapter Summaries
1 Introduction: Provides an overview of OFDM technology, its widespread applications, and the motivation for this research regarding interference mitigation.
2 OFDM adjacent channel interference: Discusses the regulatory background and the technical causes of adjacent channel interference, highlighting its impact on link performance.
3 Conventional Methods: Explains existing mitigation strategies such as windowing, filtering, and adaptive modulation, while identifying their respective advantages and limitations.
4 Combinations of modulation schemes: Details the proposed hybrid modulation method, its implementation, and the comparative results derived from intensive simulation models.
5 Conclusion: Summarizes the effectiveness of the proposed modulation combination strategy in enhancing interference resistance and preserving bandwidth resources.
Keywords
OFDM, Adjacent Channel Interference, Modulation Schemes, BPSK, QPSK, 16QAM, 64QAM, Throughput, BER, PER, Carrier-to-Interference Ratio, Simulation, Wireless Communication, Bandwidth Efficiency, Power Efficiency
Frequently Asked Questions
What is the core focus of this thesis?
The thesis focuses on improving the performance of OFDM systems by mitigating Adjacent Channel Interference (ACI) through the strategic combination of different modulation schemes.
What are the primary themes discussed?
Key themes include the impact of non-orthogonality on OFDM channels, limitations of conventional mitigation methods like filtering or guard bands, and the performance trade-offs between modulation orders.
What is the main research question or goal?
The goal is to increase an OFDM system's resistance to interference from adjacent channels while ensuring that bandwidth efficiency and overall data throughput remain at high, practical levels.
Which scientific methodology is employed?
The author uses intensive computer simulations of an OFDM-based WLAN system to evaluate the proposed mixed-modulation method against conventional single-modulation approaches.
What topics are covered in the main section?
The main section covers the theoretical background of ACI, an analysis of traditional mitigation techniques, the presentation of the proposal, and detailed result analysis from simulation parameters and scenarios.
Which keywords best characterize the work?
The most relevant keywords are OFDM, Adjacent Channel Interference, Bit Error Rate (BER), Throughput, Modulation Combination, and Power vs. Bandwidth efficiency.
How does the proposed modulation combination work?
It involves modulating the outer sub-carriers (most prone to ACI) with lower-order schemes like BPSK/QPSK for robustness, while using higher-order schemes like 16QAM/64QAM on inner sub-carriers for throughput.
Why are standard mitigation methods like IIR filters considered insufficient?
Standard methods like IIR filters or excessive guard bands are deemed insufficient because they are either complex to implement, cause ripple effects, or lead to a significant waste of frequency resources.
What is the conclusion regarding the optimal number of outer sub-carriers?
The research concludes that utilizing approximately half of the total sub-carriers for the robust low-order modulation provides the best balance between interference resistance and saturated throughput.
Why is the "near-far problem" significant in this study?
The near-far problem is a critical scenario where an MS is closer to an interfering BS, making the ACI power dominate the desired signal; the proposed method specifically aims to improve system resilience in such hostile conditions.
- Citation du texte
- Master of Science in Engineering Le Tien Anh (Auteur), 2008, Avoiding Effects of OFDM Adjacent Channel Interference by Using Combinations of Modulation Schemes, Munich, GRIN Verlag, https://www.grin.com/document/120771
