Wireless LAN modem

Introduction

Wireless local area networks (WLANs) represent an important field in the wireless communications industry, with a projected growth rate of 100% per year for the next three years. Some of the computer and communications industries' leading vendors are introducing Personal Digital Assistants (PDAs), modems, wireless interfaces and other devices and applications in support of wireless communications. WLANs offer new appealing features as they represent an alternative for adding new users to corporate LANs and supporting workers in remote locations, a low cost alternative to cable-base systems and a near ubiquitous possibility to access to any data base or any application located in the backbone. To satisfy the needs of wireless data networking, the study group 802.11 was formed under IEEE project 802 to recommend an international standard for WLANs. Key parts of the standard are the Medium Access Control (MAC) protocols (to support asynchronous and time bounded delivery of data frames) and the DS-SS and FH-SS Physical Layer Specifications (PHY). Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) has been proposed as the basis for the IEEE 802.11 WLAN MAC protocols. The IEEE 802.11 standard adopts the unlicensed ISM due to the available free bandwidth. Interference and multipath fading are the most significant issues in WLANs operating in the ISM band, where electronic devices radiate RF energy. In such an environment the interference rejection properties of the Spread Spectrum (SS) technique are precious. Moreover, the adoption of the SS technique allows it to gain an intrinsic diversity of the signal in order to cope with the effects of multipath fading. The Multicarrier Modulation (MCM) was considered by the ETSI for the HIPERLAN standard, although Decision Feedback Equalisation (DFE) was finally adopted. The MCM approach, when combined with the SS technique, leads to some interesting technical solutions that exploit the strong issues of both techniques, such as frequency diversity, full channel equalisation by means of a Complex-valued Automatic Gain Control (CAGC) at the receiver side, and rejection of the interference. An MCM-DS-SS radio subsystem for WLAN must be designed taking account of the special issues of such an application. Since a CSMA/CA access scheme is adopted, the Spread Spectrum technique is only devoted to the rejection of interference and to provide a proper diversity order. Moreover, since a good value of throughput has to be guaranteed also for short packets and for handshake of MAC messages, a great care must be observed in the strategy adopted for the channel estimation and equalisation, by preferring simple techniques which allow to obtain small values of the processing delay.

In the following slides we will present our solutions for the implementation of parts of the system (FFT and Channel Estimation and Equalisation) with a TMS320C6x Digital Signal Processor (one of the most recent DSPs produced by Texas Instrument).

Summary:

• IEEE 802.11 Draft Standard
• IEEE 802.11 - Access technique
• MCM-DS-SS Radio subsystem - System A Modem Configuration
• Channel Estimation and Equalisation
• The FFT (OFDM demodulation block)
• FFT Radix-4 implementation
• FFT Radix-4 implementation with TMS320C6x
• Future Developments

Author: Maddalena Calzolari