OFDM Receiver Design

Othogonal Frequency Division Multiplex (OFDM) has gained considerable attention in recent years. It has been adopted for various standards include the 802.11a wireless LAN standard. In this project, we implemented an OFDM receiver based 802.11a standard. Furthermore, since spatial diversity is the ultimate way to increase system capacity in bandwidth-cautious wireless applications, the SVD antenna-array processing algorithm is also implemented and will be integrated with the OFDM receiver. Key system blocks including Cordic, FFT, Viterbi decoder, and SVD are implemented in both Simulink and Module Compiler. Simulink simulation of the OFDM receiver is performed and BER is determined. Total chip area of the OFDM system in 0.25mm process is 430mm and dissipates about 2.6W of power, dominated by the SVD array. 1. Overview 1.1 Background Orthogonal Frequency Division Multiplex (OFDM) system has inherent advantage over single carrier system in frequency-selective fading channel. It has been adopted by various standards in recent years including DSL and 802.11a wireless LAN standards. 1.2 Project goal The goal of the project is to: 1. Implement an OFDM digital receiver that conforms to the 802.11a standard 2. Integrate antenna-array processing module into the OFDM system. The antenna-array processing module implements the SVD algorithm proposed in the TFS radio project. 1.3 Report organization The report is organized into six sections. The second section discusses the basics of OFDM and various practical problems with OFDM. The system architecture of 802.11a is introduced in the third section. The synchronization and channel estimation schemes are discussed. Section four discusses the system Simulink simulation including the detailed implementation of individual blocks. Section five talks about the VHDL implementation of several key blocks of the OFDM receiver as well as the testing and simulation results for these blocks. The reported is concluded by section six. 2. Introduction to OFDM 2.1 Signal representation In an OFDM system, data is carried on narrow-band sub-carriers in frequency domain. Data was transformed into time-domain using IFFT at the transmitter and transformed back to frequency-domain using FFT at the receiver. The total number of sub-carriers translates into the number of points of the IFFT/FFT. Suppose the data set to be transmitted is ) 1 2 / ( , ), 1 2 / ( ), 2 / ( − + − − N U N U N U K where N is the total number of sub-carriers. The discrete-time representation of the signal after IFFT is